Download TECTONIC ANALYSIS OF NORTHWESTERN SOUTH AMERICA

TECTONIC ANALYSIS OF NORTHWESTERN SOUTH
AMERICA FROM INTEGRATED SATELLITE, AIRBORNE
AND SURFACE POTENTIAL FIELD ANOMALIES
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the
Graduate School of The Ohio State University
By
Orlando Hernandez, B.S., M.S.
*****
The Ohio State University
2006
Dissertation Committee:
Approved by
Dr. Ralph R. B. von Frese, Adviser
Dr. Hallan C. Noltimier
Dr. Michael Barton
Dr. Douglas E. Pride
Adviser
Graduate Program in
Geological Sciences
c
°Copyright
by
Orlando Hernandez
2006
ABSTRACT
Northwestern South America is one of the most populated regions of the Americas
with more than 80 million people concentrated along the Andes Mountains. This region
includes a complex and dangerous mosaic of tectonic plates that have produced devastating earthquakes, tsunamis, volcanic eruptions and landslides in the last decades.
The region’s economic development has also seriously suffered because the region is
poorly explored for natural resources. To more effectively assess the tectonic hazards
and mineral and energy resources of this region, we must improve our understanding
of the tectonic setting that produced them.
This research develops improved tectonic models for northwestern South America
from available satellite, airborne and surface gravity and magnetic data integrated with
global digital topography, seismic, and GPS plate velocity data. Reliable crustal thickness estimates that help constrain tectonic stress/strain conditions were obtained by
inverse modeling of the magnetic anomalies and terrain compensated gravity anomalies. Correlated positive Terrain Gravity Effects (TGE) and Free Air Gravity Anomalies (FAGA) suggest that the crust - mantle interface under the northwestern Andes
is closer to the surface than expected, indicating that these mountains are not isostatically compensated. Correlated negative FAGA anomalies observed along western
South America and the Greater and Lesser Antilles islands are associated with isostatically disturbed mantle displaced by subducting oceanic plates. Subtracting TGE
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from the terrain-correlated FAGA (TCFAGA) yield compensated terrain gravity effect (CTGE) anomalies that characterize the Andes Mountains with deep roots of low
density crust displacing denser underlying mantle and thickening the local crust.
FAGA and TGE correlate at all levels of compensation, but the correlations are
especially strong where the compensation is less than 100%. Correlated first vertical
derivative FAGA (FVD(FAGA)) and differentially reduced−to-pole total field (DRTP)
magnetic anomalies show crustal thickness variations and states of magneto-isostatic
compensation. Continental crustal thickness estimates for the North Andes are in the
range from roughly 34 km to 55 km, conforming well to and extending regional seismic constraints. The analysis highlights crustal deformation from plate collision and
subduction in Northwestern South America.
Inversely correlated FVDFAGA and DRTP magnetic anomalies suggest thickness
variations in the lower crust and thermal effects in terms of the Curie isotherm. Directly
correlated FVDFAGA and DRTP magnetic anomalies indicate thickness variations of
the upper crust due to the formation of recent topography.
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To Olga Lucia, Camilo Andres, Carolina and Alejandra
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ACKNOWLEDGMENTS
I thank the Government of The Unites States of America, through the FULBRIGHT Fellowship Program and The Ohio State University for this opportunity
to pursue my doctorate at OSU. I also thank the Government of Colombia and The
Universidad Nacional de Colombia for endorsing my application and for granting me
authority to study abroad.
I am deeply and indebted grateful to my advisor Pr. Dr. Ralph R. B. von Frese for
his tutelage and academic advice over the years. I also thank Drs. Hallan Noltimier,
Michael Barton, Douglas Pride and Terry Wilson for their guidance, encouragement,
review and criticism of my research work.
I want to extend my thanks to Mohammad Asgharzadeh, Timothy Leftwich, Laramie
Potts, Luis Gaya-Pique and Hyung Rae Kim for their assistance and cooperation during my stay at the Ohio State University.
This work has been partially supported by grants from FULBRIGHT - LASPAU,
COLFUTURO and Sociedad Minera Kedahda S.A. I also thank the Planetary Geodynamics Branch at NASA’s Goddard Space Flight Center for providing access to
satellite gravity and magnetic data.
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VITA
October 26, 1962 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Born - Pacho, Colombia
December, 1988 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..B.S. Geological Sciences,
Universidad Nacional de Colombia
August, 1995 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .M.S. Exploration Geophysics
ITC, Delft, The Netherlands.
1989-1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Geologist, Geophysicist
INGEOMINAS, Colombia
1997-2006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Associate Professor
Universidad Nacional de Colombia
PUBLICATIONS
Hernandez, O., R. R. B. von Frese, Crustal modeling of Northwestern South America from spectrally correlated free-air and terrain gravity, Journal of South American
Earth Sciences (in review).
Acosta J. E., Hernandez, O., Analisis neotectonico al Este de Pasca, Cundinamarca.
Geologia Colombiana, Vol 23. 2000
Romero, O. F., Hernandez, O., Exploracion geologica y analisis mineralogico de los
depositos de esmeraldas de San Antonio de Yacopi, Cundinamarca. Geologia colombiana, Volumen 22, 1999.
vi
FIELDS OF STUDY
Major field: Geological Sciences
Studies in:
Geomathematics
Geophysics
Dr. Ralph. R. B. von Frese
Drs. Hallan Noltimier
and Ralph. R. B. von Frese
Drs. Michael Barton
and Phil Westerhoff
Drs. Michael Barton
and Loren E. Babcock
Drs. Terry Wilson
and Hans Diederix
Drs. Colin V. Reeves
Sally Barritt and
Rob Sporry
Drs. Ekkehard Jordan and
Cees van Westen
Petrology
Historical Geology
Structural Geology
Exploration Geophysics
Geographic Information Systems
vii
TABLE OF CONTENTS
Page
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ii
Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iv
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
v
Vita . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vi
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xi
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xii
Chapters:
1.
2.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1 Nature and Significance of the Problem .
1.2 Objectives . . . . . . . . . . . . . . . . . .
1.3 Description of procedures . . . . . . . . .
1.3.1 Data Compilation . . . . . . . . .
1.3.2 Data processing . . . . . . . . . . .
1.3.3 Data interpretation and integration
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THEORETICAL GRAVITY AND MAGNETIC MODELING . . . . . . .
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with
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geology
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . .
2.2 Forward Modeling of Gravity and Magnetic Anomalies
2.2.1 Isostatic models of mountainous topography . .
2.2.2 Isostatic model of depressed topography . . . .
2.2.3 Isostatic model of an oceanic ridge . . . . . . .
2.2.4 Local body gravity effects . . . . . . . . . . . .
viii
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2.2.5 Sedimentary basin gravity effects . . . . . . .
2.2.6 Dipping crustal interface . . . . . . . . . . . .
2.2.7 Generalized oceanic - continental subduction
2.2.8 Estimating the angle of subduction . . . . . .
2.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . .
3.
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ISOSTATICALLY DISTURBED TERRAINS FROM SPECTRALLY CORRELATED FREE-AIR AND TERRAIN GRAVITY DATA . . . . . . . . .
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Abstract
3.1
3.2
3.3
3.4
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Introduction . . . . . . . . . . . . . . . . . . . . . .
Spectrally correlated free-air and terrain gravity . . .
Analysis of anomaly correlations . . . . . . . . . . .
Discussion of results . . . . . . . . . . . . . . . . . .
3.4.1 Digital terrain model (DTM) . . . . . . . . .
3.4.2 Free-air gravity anomalies (FAGA) . . . . . .
3.4.3 Terrain gravity effects . . . . . . . . . . . . .
3.4.4 Terrain-correlated free-air gravity anomalies .
3.4.5 Terrain-decorrelated free-air gravity anomalies
3.4.6 TCFAGA and FCTGE anomaly correlations .
3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . .
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CRUSTAL THICKNESS AND DISCONTINUITY ESTIMATES . . . . . .
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Abstract
4.1
4.2
4.3
4.4
4.5
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crustal modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison with Airy MOHO estimates . . . . . . . . . . . . . . . .
Comparison with seismic MOHO estimates . . . . . . . . . . . . . .
Crustal cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.1 Pacific subduction zone - Andes Mountains - Guiana Craton
4.5.2 Guiana Craton-Andes Mountains-Caribbean subduction zone
4.5.3 Middle American Trench - Caribbean Plate - North American
Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 Crustal discontinuities of the North Andes Microplate . . . . . . . .
4.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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CRUSTAL MODELING FROM MAGNETIC DATA . . . . . . . . . . . .
95
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Core Field and external field reductions . . . . . . . . . . . . . . . .
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5.3 Differential Reduction to the Pole (DRTP) . . . . . . . . . . . . . . .
5.4 First vertical derivatives of FAGA (FVD(FAGA)) . . . . . . . . . . .
5.5 Spectral correlations of CHAMP DRTP (TMFA) and FVD (FAGA)
estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Aeromagnetic anomalies of the North Andes Microplate . . . . . . .
5.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.
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CRUSTAL THICKNESS VARIATIONS AND SEISMICITY . . . . . . . . 119
Abstract
6.1
6.2
6.3
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Introduction . . . . . . . . . . . . . . . . . . . . . . . .
Zero Curvature of crustal thickness variation . . . . . . .
Seismic data compilation . . . . . . . . . . . . . . . . . .
6.3.1 ANSS Catalog . . . . . . . . . . . . . . . . . . .
6.3.2 RSNC historical and instrumental seismic catalog
6.3.3 Recent seismicity in the RSNC catalog . . . . . .
6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1 Ocean-continent collision zones . . . . . . . . . .
6.4.2 Ocean-ocean collision zones . . . . . . . . . . . .
6.4.3 Continent-continent collision zones . . . . . . . .
6.4.4 Oceanic spreading ridges . . . . . . . . . . . . . .
6.4.5 Transform faults . . . . . . . . . . . . . . . . . .
6.4.6 Cratonic zone . . . . . . . . . . . . . . . . . . . .
6.5 Focal mechanism solutions . . . . . . . . . . . . . . . . .
6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .
7.
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CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . . . . . . 143
Appendices:
A.
TWO DIMENSIONAL GRAVITY AND MAGNETIC ANOMALIES DUE
TO A POLYGON BY GM-SYS . . . . . . . . . . . . . . . . . . . . . . . . 151
A.1 Gravity Anomaly due to a Polygon . . . . . . . . . . . . . . . . . . . 153
A.2 Magnetic Anomaly due to a Polygon . . . . . . . . . . . . . . . . . . 155
B.
FRACTIONAL VERTICAL DERIVATIVES . . . . . . . . . . . . . . . . . 160
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
x
162
LIST OF TABLES
Table
1.1
2.1
2.2
2.3
2.4
2.5
2.6
3.1
5.1
Page
Compiled geographical, geological and geophysical databases and sources
of information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Correlation coefficients of gravity effects for an isostatically compensated mountain range, for observations at the surface, 20 km, 50 km,
100 km, 150 km and 200 km altitude, and 100% and 50% isostatic compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
Densities (gm/cm3 ) and magnetic susceptibilities (cgs) for intrusive
bodies and host rocks modeled in Figures 2.6 and 2.7. . . . . . . . .
26
Correlation coefficients between FAGA and TFMA for an intrusive body
with different rock composition affecting a sedimentary host rock . . . .
28
Correlation coefficients between FAGA and TFMA for a sedimentary
basin hosted within mafic and metasedimentary basements . . . . . . .
28
Correlation coefficients between FAGA and TFMA for a layer dipping
at 15o , 30o , 45o , 60o , 75o and 90o inclination. . . . . . . . . . . . . . . .
28
Correlation coefficients between FAGA and TFMA for a subduction
zone model at 0 km, 20 km , 50 km, 100 km, 200 km, and 350 km
altitudes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
Correlation coefficients (CC) between the gravity anomalies at 20 km
altitude for the area from −8o S to 23.5o N latitude and from −90o W to
−58.5o W longitude in northwestern South America. . . . . . . . . . . .
50
Correlation coefficients (CC) between the FAGA in Figure 5.6, FVI(FAGA)
in Figure 5.5, CHAMP total field magnetic anomalies (TFMA) in
Figure 5.2 and the CHAMP DRTP TFMA in Figure 5.3 at 400 km
altitude for northwestern South America from −8o S to 23.5o N latitude
and from −90o W to −58.5o W longitude. . . . . . . . . . . . . . . . . . 109
xi
LIST OF FIGURES
Figure
1.1
2.1
2.2
2.3
Page
Tectonic framework of northwestern South America (Trenkamp et al.,
2004) showing the plate boundaries and major faults (black lines) . . .
3
Airy model of an idealized isostatically compensated mountain range
with gravity effects, evaluated at 0 km, 20 km, 50 km, 100 km, 150 km,
and 200 km altitude. (a) Geologic model showing topography, densities
and crustal-mantle interface. (b) Anomaly profile conventions. (c) TGE
at 100% isostatic compensation. (d) Edge effects in TGE and FAGA at
100% isostatic compensation. (e) FAGA at 100% isostatic compensation. (f) TGE, FAGA and RGE and FAGA at 100% isostatic compensation. (g) RGE at 100% isostatic compensation (h) TGE, FAGA and
RGE at 50% isostatic compensation. . . . . . . . . . . . . . . . . . . .
17
Flexure model of an idealized isostatically compensated mountain range
with gravity effects evaluated at 0 km, 20 km, 50 km, 100 km, 150 km,
and 200 km altitude. (a) Geologic model showing topography, densities
and crustal-mantle interface. (b) CC between Airy and flexural gravity
anomalies. (c) TGE at 100% isostatic compensation. (d) Edge effects in
TGE and FAGA at 100% isostatic compensation. (e) FAGA at 100%
isostatic compensation. (f) TGE, FAGA and RGE and FAGA at 100%
isostatic compensation. (g) RGE at 100 % isostatic compensation. (h)
TGE, FAGA and RGE at 50% isostatic compensation. . . . . . . . . .
18
Airy gravity model of an isostatically overcompensated mountain range
evaluated at 20 km altitude. (a) Geologic model. (b) Gravity Anomalies:
RGE overcompensates TGE making FAGA negative. In this case, the
equilibrium could be reached by isostatic rebound to increase TGE and
diminish RGE to make FAGA approach zero. . . . . . . . . . . . . . .
21
xii
2.4
Isostatic overcompensated topographic depression with lateral crustal
density variation and mantle upwelling evaluated at 20 km altitude.
(a) Geologic model. (b) Gravity anomalies: TGE are negative, “anti”
RGE overcompensates TGE so that FAGA are positive. In this case,
the isostatic state of equilibrium could be reached by crustal subsidence
lowering the upwelled mantle and diminishing RGE to make FAGA
approach to zero. TGE and FAGA are negatively correlated . . . . . .
22
Gravity effects of isostatically overcompensated oceanic ridge with underplated material evaluated at 20 km altitude. (a) Geologic model of
a volcanic ridge (b) Gravity anomalies: TGE are positive and RGE are
strongly negative making FAGA negative. The model shows how FAGA
can also be affected by density variations in the mantle . . . . . . . . .
24
Gravity and magnetic responses evaluated at 20 km altitude of igneous
intrusions affecting a sedimentary sequence. (a) Geologic model. (b)
Gravity anomalies: TGE is equal to zero. FAGA is positive. (c) Total magnetic field anomalies. The gravity and magnetic anomalies are
positively correlated. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
Gravity and magnetic anomalies for a sedimentary basin estimated at 20
km altitude and 50 km station interval. (a) Geologic model. (b) Negative
FAGA for a sedimentary basin with different basement rock types. (c)
Negative TFMA for basin thicknesses of 1 km, 2 km, 3 km, 4 km, and
5 km within mafic basement. . . . . . . . . . . . . . . . . . . . . . . . .
27
Gravity and magnetic anomalies for a dipping interface beneath thin
overburden evaluated at 20 km altitude at a 10 km station interval. (a)
Negative FAGA for 15o , 30o , 45o , 60o , 75o and 90o inclination.(b) TFMA
for 15o , 30o , 45o , 60o , 75o and 90o inclination . . . . . . . . . . . . . . .
29
Gravity and magnetic anomalies of a subduction zone evaluated at 20
km altitude and a 100 km station interval. (a) Geological model with
an oceanic plate subducting under a continental plate. (b) The FAGA
at 0 km, 20 km, 50 km, 100 km, 200 km, and 350 km altitude and (c)
corresponding TFMA. . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
2.10 Subduction zone modeling with 15o , 30o , 45o and 60o angles of inclination estimated at 20-km altitude at a 100-km station interval. (a)
Geologic model. (b)FAGA and (c) corresponding TFMA. . . . . . . . .
33
2.5
2.6
2.7
2.8
2.9
xiii
3.1
3.2
3.3
3.4
3.5
3.6
Topography/bathymetry of northwestern South America with superposed regional tectonic features between −8o S to 23.5o N latitude and
from −90o W to −58.5o W longitude. Map annotations include the amplitude range (AR) of (min; max) values, the amplitude mean (AM)
and standard deviation (SD). SNSM= Sierra Nevada of Santa Martha,
W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection. . . . . . . . . . . . . . . . . . . . .
38
Free-air gravity anomalies (FAGA) at 20 km elevation for the study
region (Lemoine, et al., 1998). Map annotations include the amplitude
range (AR) of (min; max) values, the amplitude mean (AM) and standard deviation (SD). SNSM= Sierra Nevada of Santa Martha, W-Mid
= Western-Central ranges. This map was produced using the Albers
equal-area conic projection. . . . . . . . . . . . . . . . . . . . . . . . .
39
Terrain Gravity Effects (TGE) at 20 km elevation for the study region
by Gauss-Legendre quadrature integration. Map annotations include the
amplitude range (AR) of (min; max) values, the amplitude mean (AM)
and standard deviation (SD). SNSM= Sierra Nevada of Santa Martha,
W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection. . . . . . . . . . . . . . . . . . . . .
41
Terrain-correlated FAGA (TCFAGA) at 20 km elevation for the study
area. Map annotations include the amplitude range (AR) of (min; max)
values, the amplitude mean (AM) and standard deviation (SD). SNSM
= Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection. .
43
Terrain-decorrelated FAGA (TDFAGA) at 20 km elevation for the study
area. Map annotations include the amplitude range (AR) of (min; max)
values, the amplitude mean (AM) and standard deviation (SD). SNSM
= Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection. .
44
FAGA-correlated TGE (FCTGE) at 20 km elevation for the study area.
Map annotations include the amplitude range (AR) of (min; max) values, the amplitude mean (AM) and standard deviation (SD). SNSM =
Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges. This
map was produced using the Albers equal-area conic projection. . . . .
45
xiv
3.7
FAGA-decorrelated TGE (FDTGE) at 20 km elevation for the study
area. Map annotations include the amplitude range (AR) of (min; max)
values, the amplitude mean (AM) and standard deviation (SD). SNSM
= Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection. .
46
Summed local favorability indices (SLFI) for TCFAGA and FCTGE
at 20 km elevation for the study region showing TCFAGA-peak to
FCTGE-peak correlations for SLFI ≥ 8.7226. SNSM= Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . .
48
Summed local favorability indices (SLFI) for TCFAGA and FCTGE
at 20 km elevation for the study region showing the TCFAGA-trough
to FCTGE-trough correlations for SLFI ≤ −8.7226. SNSM= Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . .
49
3.10 Differenced local favorability indices (DLFI) between TCFAGA and
FCTGE at 20 km elevation for the study region showing the TCFAGApeak to FCTGE-trough associations for DLFI> 4.8904. SNSM= Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . .
51
3.11 Differenced local favorability indices (DLFI) for TCFAGA and FCTGE
at 20 km elevation for the study region showing the TCFAGA-trough to
FCTGE-peak associations for DLFI<- 4.8904. SNSM= Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . .
52
3.8
3.9
4.1
4.2
Compensated terrain gravity effects (CTGE) at 20 km elevation for
the study region obtained by subtracting TGE from TCFAGA. Map
annotations include the amplitude range (AR) of (min; max) values,
the amplitude mean (AM) and standard deviation (SD). SNSM = Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . .
65
Gravity MOHO obtained from the inversion of CTGE showing deep
MOHO beneath the mountains and oceanic ridges. Map annotations
include the amplitude range (AR) of (min; max) values, the amplitude
mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of
Santa Martha, W-Mid = Western-Central ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . .
66
xv
4.3
Seismic MOHO from the CRUST 2.0 model (Bassin et al., 2000) showing deeper MOHO below the mountains and oceanic ridges. Map annotations include the amplitude range (AR) of (min; max) values and
amplitude mean (AM) and standard deviation (SD). SNSM = Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . .
67
Airy MOHO obtained the hydrostatic equilibrium principle. Map annotations include the amplitude range (AR) of (min; max) values and
amplitude mean (AM) and standard deviation (SD). SNSM = Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . .
69
MOHO differences by subtracting gravity MOHO from Airy MOHO estimates. Map annotations include the amplitude range (AR) of (min;
max) values and amplitude mean (AM) and standard deviation (SD).
SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central
ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
MOHO differences by subtracting gravity MOHO from seismic MOHO
estimates. Map annotations include the amplitude range (AR) of (min;
max) values and amplitude mean (AM) and standard deviation (SD).
SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central
ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
MOHO differences by subtracting the seismic MOHO from the Airy
MOHO estimates. Map annotations include the amplitude range (AR)
of (min; max) values and amplitude mean (AM) and standard deviation
(SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = WesternCentral ranges. This map was produced using the Albers equal-area
conic projection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
Gravity, seismic and Airy MOHO estimates at 5o N latitude from −90o to
−58.5o longitude. MOHO differences are greatest at the volcanic ridges
and Andes Mountains. . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
Gravity, seismic and Airy MOHO estimates at the Guiana Craton and
North Andes − Caribbean subduction zone along (Oo N, −78o W) to
(23.5o N, −75o W). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
4.10 Gravity, seismic and Airy MOHO estimates at the Caribbean − North
American plates along (12.5o N, −90o W) , (16o N, −58.5o W). . . . . . .
78
4.4
4.5
4.6
4.7
4.8
4.9
xvi
4.11 Crustal profile at 5o N latitude along −90o W to −79o W across the Cocos
and Malpelo Ridges that is consistent with TCFAGA. Oceanic crustal
thickening partially compensates TGE. The complete matching of observed and calculated gravity anomalies required the inclusion of underplated material that was less dense than the mantle. . . . . . . . . . . .
79
4.12 Two dimension P-wave velocity versus depth model across the Malpelo
Ridge and Eastern Panama Basin showing an ophiolitic sequence with
thinned oceanic crust and less dense underplated material in the upper
mantle. The dashed line indicates a change in velocity gradient (Trummer, 2002). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
80
4.13 Crustal profiles showing the different angles of subduction along the
Nazca-North Andes plate boundary zone that are required to satisfy
the TCFAGA. The TCFAGA map is also given in F igure 4.14. The
TCFAGA map and crustal profiles were produced in Cartesian coordinates with origin (1’000.000 N, 1’000.000E) at the National Observatory
of Bogota. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
4.14 TCFAGA of northwestern South America at 20 km altitude and interpreted plate boundaries and intracrustal discontinuities. This map
was produced in Cartesian coordinates with the origin (1’000.000 N,
1’000.000 E) at the National Observatory of Bogota. . . . . . . . . . . .
83
4.15 Plate boundaries and intracrustal discontinuities of northwestern South
America interpreted from TCFAGA at 20 km altitude. This map was
produced in Cartesian coordinates with the origin (1’000.000 N, 1’000.000
E) at the National Observatory of Bogota. . . . . . . . . . . . . . . . .
84
4.16 Profiles (W-E) of the northern Andes Microplate showing the subducting oceanic slab under the continental crust as interpreted from
TCFAGA at 20 km altitude. (a) Profile A-A: Two main continental
roots were modeled below the west-central and eastern ranges, respectively. Smooth MOHO variations are observed below the Guiana Craton. (b) Profile B - B: A single continental root was modeled below
the west-central ranges. No clear continental root below the incipient
eastern range is evident and the MOHO below the Guiana Craton is
relatively flat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
4.17 Surface FAGA and interpreted crustal discontinuities. This map was
produced in Cartesian coordinates with the origin (1’000.000 N, 1’000.000
E) at the National Observatory of Bogota. . . . . . . . . . . . . . . . .
87
xvii
4.18 Plate boundaries and intracrustal discontinuities interpreted from surface FAGA. This map was produced in Cartesian coordinates with the
origin (1’000.000 N, 1’000.000 E) at the National Observatory of Bogota. 88
4.19 The tectonic realms and blocks of northwestern South America interpreted from geological and geophysical data (Cediel et al., 2003). . . .
89
4.20 Surface complete Bouguer anomalies and interpreted plate boundaries
and intracrustal discontinuities. This map was produced in Cartesian
coordinates with the origin (1’000.000 N, 1’000.000 E) at the National
Observatory of Bogota. . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
4.21 Plate boundaries and intracrustal discontinuities interpreted from surface Bouguer anomalies. This map was produced in Cartesian coordinates referred to an origin (1’000.000 N, 1’000.000 E) at the National
Observatory of Bogota. . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
5.1
Core magnetic intensity (color), inclination (white contours) and declination (black contours) for northwestern South America from IGRF10 (NOAA, 2006). SNSM = Sierra Nevada of Santa Martha, W-Mid
= Western-Central ranges. This map was produced using the Albers
equal-area conic projection. . . . . . . . . . . . . . . . . . . . . . . . . 100
5.2
CHAMP total magnetic field anomalies (nT) at 400 km altitude for
northwestern South America. Map annotations include the amplitude
range (AR) of (min; max) values and amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa Martha, W-Mid
= Western-Central ranges. This map was produced using the Albers
equal-area conic projection. . . . . . . . . . . . . . . . . . . . . . . . . 102
5.3
Differentially reduce-to-pole CHAMP magnetic anomalies DRTP(TFMA)
at 400 km for Northwestern South America. Map annotations include
the amplitude range (AR) of (min; max) values and amplitude mean
(AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . . . . . . . 104
5.4
Crustal magnetic susceptibility values for northwestern South America
calculated by linear inversion of DRTP CHAMP magnetic anomalies.
Map annotations include the amplitude range (AR) of (min; max) values
and amplitude mean (AM) and standard deviation (SD). SNSM = Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . . 106
xviii
5.5
First vertical derivative (FVD) of the free-air gravity anomalies (FAGA)
at 400 km for northwestern South America. Map annotations include the
amplitude range (AR) of (min; max) values and amplitude mean (AM)
and standard deviation (SD). SNSM = Sierra Nevada of Santa Martha,
W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection. . . . . . . . . . . . . . . . . . . . . 107
5.6
Free-air gravity anomalies (FAGA) at 400 km elevation for northwestern
South America. Map annotations include the amplitude range (AR) of
(min; max) values and amplitude mean (AM) and standard deviation
(SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = WesternCentral ranges. This map was produced using the Albers equal-area
conic projection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.7
Peak-to-peak correlations between FVD(FAGA) and CHAMP DRTP
(TFMA) at 400 km altitude for northwestern South America. SNSM
= Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection. . 110
5.8
Trough-to-trough FVD(FAGA) and CHAMP DRTP (TFMA) at 400
km altitude for northwestern South America. SNSM = Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . . 111
5.9
FVD(FAGA) peak-to- CHAMP DRTP (TFMA) trough correlations
at 400 km altitude for northwestern South America. SNSM = Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection. . . . . . . . 112
5.10 FVD(FAGA) trough-to-CHAMP DRTP (TFMA) peak correlations at
400 km altitude for northwestern South America. SNSM = Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the Albers equal-area conic projection. . . . . . . . . . . . 113
5.11 Total magnetic field anomalies at 20 km altitude for the North Andes
Block with interpreted crustal discontinuities superposed (blue lines).
This map was produced in Cartesian coordinates with the origin (1’000.000
N, 1’000.000 E) at the National Observatory of Bogota. . . . . . . . . . 115
xix
6.1
ANSS seismic catalog for 1966-2006 for three categories of earthquake
magnitudes: M=3-4 (minor earthquakes), M= 4.1-5.5 (intermediate earthquakes), and M ≥ 5.5 to 10 (major earthquakes). Crustal discontinuities
interpreted from gravity anomalies and the zero curvature contours of
crustal thickness variations are also displayed. Yellow and blue zones
correspond to thicker and thinner crust, respectively. This and succeeding figures were created using the capabilities of Oasis Montaj (Geosoft,
2006). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6.2
RSNC historical seismic catalog for 1643-1991 of earthquakes with magnitudes from M=3.0 to 4.0. Crustal discontinuities interpreted from
gravity anomalies and the zero curvature contours of crustal thickness
variations are also displayed. The yellow and blue zones correspond to
thicker and thinner crust, respectively. . . . . . . . . . . . . . . . . . . 126
6.3
RSNC historical and instrumental seismic catalog for 1643-1991 of earthquakes with magnitudes from M=4.1 to 5.0. Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of
crustal thickness variations are also displayed. The yellow and blue zones
correspond to thicker and thinner crust, respectively. . . . . . . . . . . 127
6.4
RSNC historical seismic catalog for 1643-1991 of earthquakes with magnitudes M > 5.0. Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are
also display. The yellow and blue zones correspond to thicker and thinner crust, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
6.5
RSNC modern seismic catalog for 1992-2006 of earthquakes with magnitudes from M=3.0 to 4.0. Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. The yellow and blue zones correspond to thicker
and thinner crust, respectively. . . . . . . . . . . . . . . . . . . . . . . . 130
6.6
RSNC modern seismic catalog for 1992-2006 of earthquakes with magnitudes from M=4.1 to 5.0. Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. The yellow and blue zones correspond to thicker
and thinner crust, respectively. . . . . . . . . . . . . . . . . . . . . . . . 131
6.7
RSNC modern seismic catalog for 1992-2006 of earthquakes with magnitudes M > 5.0. Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are
also displayed. The yellow and blue zones correspond to thicker and
thinner crust, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . 132
xx
6.8
RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 0 km to 10 km. Crustal discontinuities interpreted from
gravity anomalies and thezero curvature contours of crustal thickness
variations are also displayed. The yellow and blue zones correspond to
thicker and thinner crust, respectively. . . . . . . . . . . . . . . . . . . 134
6.9
RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 10 km to 30 km. Crustal discontinuities interpreted from
gravity anomalies and the zero curvature contours of crustal thickness
variations are also displayed. The yellow and blue zones correspond to
thicker and thinner crust, respectively. . . . . . . . . . . . . . . . . . . 135
6.10 RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 30 km to 70 km. Crustal discontinuities interpreted from
gravity anomalies and the zero curvature contours of crustal thickness
variations are also displayed. The yellow and blue zones correspond to
thicker and thinner crust, respectively. . . . . . . . . . . . . . . . . . . 136
6.11 RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths to the hypocenter from 70 km to 300 km. Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours
of crustal thickness variations are also displayed. The yellow and blue
zones correspond to thicker and thinner crust, respectively. . . . . . . . 137
6.12 Main stress axes determined by the Reches Method (Reches, 1983;
Reches, 1987) from 94 focal mechanism solutions of the Central Moment
tensor catalog (CMT) reported by Harvard University (CTM, Harvard,
2006) for the period 1976-2000 and event magnitudes M = 5.0 (Vargas
et al., 2005). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
xxi
CHAPTER 1
INTRODUCTION
1
1.1
Nature and Significance of the Problem
The plate tectonic setting of northwestern South America (Figure 1.1) is characterized by the convergence of the North Andes microplate and the Nazca, Caribbean
and South American plates (Bird, 2003 ; Cediel et al., 2003; Trenkamp et al., 2001;
Pindell, 2006). This is one of the most populated regions of the Americas with more
than 80 million people concentrated in urban areas along the Andes Mountains. The
region is also subject to devastating earthquakes, volcanic eruptions, landslides and
other tectonic hazards due to the complex interactions of the crustal plates. However,
the dense vegetation that covers most of the rock exposures seriously limits conventional geological mapping in this region. Thus, the tectonic setting is much more poorly
known for northwestern South America than it is for the mountainous backbone of the
Americas north to Alaska and south to Patagonia (Ramos, 2006).
Kellogg et al. (1985) defined the Panama block in southeastern Central America
using seismic and geodetic data and inferred that it is moving northwards relative
to the Caribbean plate and eastwards relative to the South American plate. Mann
and Kolarsky (1955) suggested that the converging boundary between Panama and
North Andes microplates is near the Panama-Colombia border. Kellogg et al. (1985)
defined the North Andes block within the western, central and eastern Cordilleras of
Colombia, Ecuador and Venezuela, respectively. From geologic and seismicity data they
inferred that it moves about 10 mm/a towards 55o E with respect to South America
or about 17 − 19 mm/a northwestwards with respect to the Caribbean plate. Dewey
and Pindell (1985) presented a contrasting tectonic model interpreted from geologic
data whereby during the last 9 ma the Central block moved about 275 km at 31 mm/a
northeastwards with respect to South America, and the Maracaibo block moved even
further northwards about 105 km at12 mm/a. Perez et al. (1997) studied the Benioff
2
Figure 1.1: Tectonic framework of northwestern South America (Trenkamp et al., 2004)
showing the plate boundaries and major faults (black lines)
3
zone of seismicity within this slab, and showed that it terminates eastwards near
Curacao-Aruba.
The CASA project (Kellogg, 1985) compiled GPS data and quantified the relative
velocities of the Panama and North Andes microplates. Van der Hilst and Mann (1994)
from seismic data interpreted the Caribbean plate as subducting beneath the South
American Plate to a depth of 275 km with 17o of eastwards inclination. Moore and
Sender (1995) mapped the triple-junction of the Cocos-Nazca-Panama plate and the
complex relative plate movements. Trenkamp et al. (1996) defined the motion of North
Andes block with respect to South America for a 78 mm/a dextral component and
a 45 mm/a compressive component on their common boundary. Perez et al. (1997)
interpreted the Caribbean-North Andes and South America triple-junction as lying
at the intersection of the Bocono fault (North Andes-South America), San Sebastian
- El Pilar fault (Caribbean - South America), and a seismically active dextral fault
trending NW (ND-CA).
Bird (2003) presented a comprehensive compilation of the local tectonics of the
area, with a set of plate boundaries interpreted from topography, volcanism, and seismicity, taking into account relative plate velocities from magnetic anomalies, moment
tensor solutions, and geodesy. Compiling observations from seismic, gravity, magnetic,
tomography and satellite techniques, Cediel et al. (2003) presented the tectonic assembly of the North Andes block defining four tectonic realms including the Guiana
Shield, the Maracaibo sub-plate, the central continental sub-plate and the western
tectonic realms. Collot et al. (2005) described the boundary between the North Andes
microplate and the South American plate as a mega thrust that is apparently reactivated in an oblique dextral-normal sense in Ecuador and continues into Colombia along
the Borde Llanero fault system to the Merida Andes in Venezuela. Although much work
has been done, the tectonic model for the area is mostly supported from data of the
4
uppermost crust. To advance our understanding of the hidden deeper geology and tectonic forces of the North Andes microplate and its surroundings, I developed the first
comprehensive crustal model of northwestern South America from integrated satellite,
airborne and surface gravity and magnetic anomalies. Satellite gravity and magnetic
observations mapped the major anomalies measured in hundreds of kilometers providing unique insights on plate boundary zones from the surface to the crust-mantle
interface at depths up to about 56 km.
Airborne and surface gravity and magnetic data provided regional and local anomalies respectively measured in tens of kilometers to kilometers for constraining shallow tectonic features at the plate boundary zones and intraplate regions. Therefore,
the integration of gravity and magnetic observations from multi-altitude observations
also integrates interpretations at different depths providing a comprehensive threedimensional crustal model from the crustal surface to the interface of the crust with
the mantle or the MOHO.
Crustal magnetic anomalies have been widely associated with crustal magnetization variations along plate tectonic boundaries (e.g., von Frese et al., 1996). Thus,
I analyzed magnetic data from the Ørsted and CHAMP (Challenging Minisatellite
Payload) satellite missions, as well as gravity observations from EGM96 to model
regional composition variations of the crust and the upper mantle. I also estimated
crustal thickness variations and thermal perturbations that help control earthquakeproducing stresses along the boundaries of the Nazca, Caribbean and South American
Plates and Panama,Costa Rica and North Andes Microplates.
5
Critical geological problems that I addressed include:
Assessing stress controlling crustal thickness variations and crustal discontinuities.
Establishing the tectonic features that control the region’ s earthquake and
volcanic threats.
Optimizing the use of multi-altitude geophysical data in the exploration of
mineral and energy resources.
1.2
Objectives
The aim of this project is to use available gravity and magnetic observations from
satellite, airborne and surface surveys in conjunction with new developments in crustal
isostatic modeling to develop new insights on the crustal tectonic features and evolution of Northwestern South America. Improved knowledge of the crustal thickness
variations can yield important constraints on the local stress field (e.g., Kim, et al.,
2000) that in combination with regional plate tectonicstress fields provide new insight
on earthquake hazards within northwestern South America. In addition, these results
may contribute to exploration of the region’s mineral and energy resources to promote
the economic development.
1.3
Description of procedures
Initially, hypothetical isostatic models were constructed to show examples of isostatic states of compensation for idealized mountain ranges, volcanic ridges, subduction zones and crustal discontinuities. Then, observed regional potential field anomalies
were interpreted in terms of the crust-mantle interface and related to isostatic disturbances of the crust from tectonic, thermal, and other geological forces. The methodology applied in this research project includes data compilation, data processing, qualitative and quantitative interpretation, and integration of the results with geological
data.
6
1.3.1
Data Compilation
Disparate satellite, airborne and surface potential field data sets were compiled
and standardized for regional analysis and modeling of northwestern South America
. The compilation included geological, seismological and geodetic data sets. Satellite
data offered regional spatial coverage, whereas the coverage from airborne and surface
surveys was much less complete. Regional satellite geopotential data helped augment
or extend the analysis between local near-surface surveys that provided important local
constraints on the tectonic features of the study area. Table 1 summarizes sources and
types of datasets compiled for this study.
1.3.2
Data processing
The geophysical and geological data sets were converted into a common digital format to facilitate interpolation, gridding, spectral analysis, inversion, trend surface analysis, and other quantitative processing objectives (von Frese, 1980; Reeves, 1991; Kim,
2000; Leftwich, 2005). The geophysical and geological data sets were integrated using GIS tools including digitization, rasterization, reclassification, data overlaying and
overlapping, conditional retrieving, logical boolean sorting, analytical signal modeling,
and neighborhood assessments for visualization purposes (Hernandez, 1995; Borrough,
et al., 1998; Kessler, 2006). Software was developed and integrated with commercial
software to facilitate the quantitative analysis and digital display of the analytical
results.
The crustal thickness map for northwestern South America was calculated using
spectral correlation analysis between terrain and free air gravity anomalies. Terrain
7
ANALOG AND DIGITAL DATA
SOURCE OF INFORMATION
Satellite EGM (Earth Gravity Model)
The Ohio State University.
96
Satellite GRACE (Gravity Recovery
The Ohio State University, NASA.
and Climate experiment) data
Ørsted
The Ohio State University, ØRSTED.
Satellite CHAMP (Challenging Min-
The Ohio State University, NASA.
isatellite Payload) Magnetic data
Topography 96 Digital elevation model
The Ohio State University, NOAA.
Digital Elevation Model
Universidad Nacional de Colombia
Free Air Gravity Anomaly map of
Colombian geological survey (INGE-
Colombia
OMINAS).
Complete Bouguer Gravity Anomaly
INGEOMINAS.
map
Crustal thickness seismic database
http://mahi.ucsd.edu
Geologic map of Colombia
INGEOMINAS.
Aeromagnetic data of Colombia
INGEOMINAS, ECOPETROL, USGS.
Seismological databases
INGEOMINAS, ANSS, USGS.
Table 1.1: Compiled geographical, geological and geophysical databases and sources of
information.
8
gravity effects were modeled by spherical coordinate Gauss Legendre quadrature integration of topography data. Gravity and magnetic data were combined by the application of the spectral correlation theory to separate core, lithospheric and external
field components in CHAMP magnetic anomalies. Crustal discontinuities and crustal
thickness variations were mapped and integrated with seismological data using GIS
overlapping and superposition operations in the spatial domain.
1.3.3
Data interpretation and integration with geology
Crustal models were developed using geophysical, geological and geodetic datasets.
These data sets included seismic data, GPS stress and strain models, and maps of the
geology, ore deposits, sedimentary basins, and volcanic and earthquake activities.
In Chapter 2, I investigated the gravity and magnetic responses of hypothetical
geological models and their isostatic responses using available forward modeling techniques. Based on these models, I evaluated the contributions of spatial variation of
density and magnetic susceptibility contrasts in the respective gravity and magnetic
anomalies and their spectral correlations. This new modeling was focused on free-air
gravity and terrain gravity anomalies for analyzing and interpreting observed data
from northwestern South America.
In Chapter 3, I examined the spectral correlation of free-air and terrain gravity
anomalies for their isostatic compensation state using Gauss-Legendre quadrature integration. By spectral correlation analysis, terrain-correlated and -decorrelated anomalies were obtained for separating gravity anomalies associated with the core-mantle and
crust-mantle interfaces and intra-crustal density variations. Normalization and local
favorability indices were developed to facilitate qualitative and quantitative interpretation of the complex spatial distribution of gravity anomalies.
9
In Chapter 4, I investigated the terrain-correlated gravity effects for Moho estimates
and crustal thickness variations. I obtained comprehensive MOHO estimates for northwestern South America that are consistent with the gridded topography, bathymetry
and gravity observations. I compared the gravity MOHO against sparse Seismic MOHO
to show the remarkable improvement of the gravity MOHO estimates. I also compared
the gravity and seismic MOHOs against the theoretical Airy Moho. Crustal discontinuities from satellite and surface data were related to the tectonic processes and
evolution of the North Andes microplate.
In chapter 5, I focused on extracting the crustal components in the CHAMP magnetic data and their spectral correlation with the first radial derivative FAGA anomalies at satellite altitudes. Satellite, airborne and surface magnetic data were integrated
and combined with gravity models to investigate the magnetic effects of the crustal
thickness variations. The analysis focused on the magnetic interpretation of crustal
discontinuities of the North Andes microplate.
In chapter 6, I investigated the edges of crustal thickness variations calculated
from the curvature of the crustal thickness estimates and their associated seismic and
magnetic responses. Zero curvature values were used to separate the thicker and thinner
crust. These models were combined with historical and instrumental seismic catalogs
for stress/strain modeling of the region and evaluating the seismicity of local crustal
discontinuities and plate boundaries.
In Chapter 7, I summarized the methodologies and results from the hypothetical
models and spectral analysis of observed gravity and magnetic data for applications
to seismic hazard analysis and exploring for minerals and fossil fuels. I also considered
recommendations for expanding this research.
10
CHAPTER 2
THEORETICAL GRAVITY AND MAGNETIC MODELING
11
ABSTRACT
The phenomenon of isostasy concerns the equilibrating response of the crust to
the imposition and removal of loads. The isostatic condition of the crust and the terrain were evaluated using the modeled gravity effects of idealized crustal models. The
idealized crustal models represented major tectonic features of the study area and included the mountain range, volcanic ridge, topographic depression, igneous intrusion,
sedimentary basin, and fault and subduction zone models. These models were implemented by interactive forward modeling of gravity data to better understand how
free-air, terrain and crustal root gravity anomalies interact for various states of isostatic compensation. The models facilitated interpreting spectrally correlated terrain
and free-air gravity anomalies for isostatic disturbances of the crust.
The analysis suggested that the isostatic condition of the crust strongly affects
correlations between the free-air gravity anomalies and the gravity effects of the terrain.
Additionally, the magnetic response for some of the idealized models was obtained
and correlated with gravity anomalies. These models were useful for improving the
interpretation of gravity and magnetic anomalies at multi-altitude observations.
The idealized models were also used to investigate the non-uniqueness condition
of the potential field anomalies, in particular where anomaly superposition and inverse correlations occur. The results indicated that isostatic disturbances of the terrain greatly enhance correlations between the gravity effects of the terrain and free-air
anomalies. Furthermore, these correlations are relatively insensitive to the differences
12
in models of crustal isostasy at altitudes that are large compared to the relief of the
crustal terrain.
2.1
Introduction
The main purpose of this section is to develop theoretical geological models that
provide insight on the free-air gravity and magnetic responses of isostatically disturbed
terrain. The geologic models represent simplified geologic features that are relevant for
the tectonic analysis of Northwestern South America and the isostatic conditions of
the North Andes mountains.
Simple idealized geologic models commonly help to explain the basis of isostatic
compensation. For example, Keary and Vine (1996) graphically described the inverse
correlation of gravity anomalies with topography showing free-air, Bouguer and isostatic anomalies over an idealized mountain range for various levels of isostatic equilibrium ranging from 100% to 0% compensation. However, these models did not include
the associated terrain gravity effects or the multi-altitude variations of the gravity
anomalies. Watts (2001) examined isostatic properties of crustal structures using the
Airy and Pratt models in explaining crustal structures, and introduced examples of the
flexure model of isostasy. Turcotte and Schubert (2002) also presented compensation
models for observed geoid anomalies and estimated the forces required to maintain
topography and the geoid. Apart from these studies, however, few examples of gravity modeling of free-air and terrain gravity effects at multi-altitude elevations and
continental scales are available.
In the section below, I develope theoretical crustal models applicable to northwestern South America to illustrate possible relationships between terrain and free-air
13
gravity and magnetic observations. These results extend to multi-altitudes the surface relationships usually described by the Bouguer anomaly in common textbooks on
geophysics (e.g., Reeves, 1991; Keary and Brooks, 1992; Telford, et al., 1994).
2.2
Forward Modeling of Gravity and Magnetic Anomalies
Gravity anomalies of the hypothetical geological models were calculated by forward
modeling using the software capabilities of GM-SYS. This software for 2.75 D forward
and inverse gravity and magnetic modeling allows intuitive, interactive manipulation
of the geologic model (Northwest Geophysical Associates, 1999). GM-SYS is based
on published algorithms (Talwani et al., 1959; Talwani and Heirtzler, 1964; Won and
Bevis, 1987; Parker, 1994) that are described in more detail in Appendix A.
For a constant density crust overlying a constant density mantle, the free-air gravity
anomaly (FAGA) in mGals may be expressed by:
F AGA = T GE + RGE,
(2.1)
where
TGE = Terrain Gravity Effects (mGals) and
RGE = Root Gravity effects (mGals).
Estimating the gravity anomalies of the idealized models involved the following
procedures:
1. The TGE were estimated using a single density contrast.
2. The MOHO interface was estimated using an effective density contrast between
the mantle and crust and varying the MOHO relief until the TGE were nullified.
This approach assumes that terrain loads on the crust defined by the topography
14
and its density contrast to air are compensated by mass differentials across the
MOHO.
Simple and complex geological models were evaluated to obtain their superimposed
gravity effects at 0 km, 20 km, 50 km, 100 km, 150 km and 200 km altitude. The magnetic responses were also determined for some of the models using simplistic magnetic
susceptibility values for the crustal materials and considering the mantle to be nonmagnetic. The TGE, FAGA and RGE were graphically displayed using MATLAB
(Mathworks, 2005). Procedures were developed to extend the GM-SYS forward modeling capabilities to do isostatic analysis and to integrate the GM-SYS and MATLAB
capabilities.
The correlation coefficient (CC) such as between TGE and FAGA (Davis, 1986;
Isaaks and Srivastava, 1989) was calculated by:
·
#
σ 2 (T GE, F AGA)
CC (T GE, F AGA) = 2
,
σ (T GE)σ 2 (F AGA)
(2.2)
where σ 2 (T GE, F AGA) is the covariance between the two signals with variances
σ 2 (T GE) and σ 2 (F AGA). CC varies between +/- 1.0, where CC = 1.0 implies positive
or direct correlation between the signals, CC= -1.0 implies negative or inverse correlation, and CC ≈ 0 is generally taken to indicate no correlation between the signals,
but the interpretation is not unique (von Frese, et al., 1997).
2.2.1
Isostatic models of mountainous topography
Figure 2.1 (a) shows the basic terrain model that consists of positive topographic
relief over a flat crustal surface. The mean crustal thickness is 35 km with a varying
crustal root into the mantle. The densities used were 2.85 gm/cm3 for the continental
15
CC
0 km
20 km
50 km
100 km
150 km
200 km
FAGA(Airy,F lex)100%
0.7235
0.8008
0.8435
0.868
0.8719
0.8803
FAGA(Airy,F lex)50%
0.9968
0.9974
0.998
0.9987
0.9991
0.9993
RGE(Airy,F lex)100%
0.9931
0.9947
0.9962
0.9977
0.9985
0.9989
RGE(Airy,F lex)50%
0.8736
0.8863
0.9035
0.9245
0.9398
0.9514
TGE(Airy,F lex)
0.8565
0.8706
0.8874
0.9076
0.923
0.9354
Table 2.1: Correlation coefficients of gravity effects for an isostatically compensated
mountain range, for observations at the surface, 20 km, 50 km, 100 km, 150 km and
200 km altitude, and 100% and 50% isostatic compensation
crust and 3.3 gm/cm3 for the mantle. The gravity effects were modeled at a station
interval of 10 km at the surface and 20 km, 50 km, 100 km, 150 km, and 200 km altitude
along a 5000 km profile oriented perpendicular to the mountainous topography.
The TGE, RGE and FAGA were calculated for the Airy and flexure isostatic models
in Figures 2.1 and 2.2, respectively. In the absence of MOHO relief (e.g., RGE=0), the
models equal the Pratt model where the terrain is compensated by intracrustal mass
differentials. This situation tends to be limited to continental crust that is dominated
by high heat flow such as occurs west of Rockies in North America (Watts, 2001). In
this study, I focused on the isostatic properties of the Airy and flexure models that
are more appropriate for representing the geology of northwestern South America.
Figures 2.1 and 2.2 give the gravity responses for the Airy and flexural isostatic
models, respectively, at the compensation levels of 0%, 50% and 100%. Table 2.1
shows how these gravity components correlate with each other at the various altitudes
of observation for the varying levels of isostatic compensation.
The isostatic condition of the crust defines the correlation between TGE and FAGA.
For instance, a topographic high can be partially or completely compensated by a
16
Figure 2.1: Airy model of an idealized isostatically compensated mountain range with
gravity effects, evaluated at 0 km, 20 km, 50 km, 100 km, 150 km, and 200 km altitude. (a) Geologic model showing topography, densities and crustal-mantle interface.
(b) Anomaly profile conventions. (c) TGE at 100% isostatic compensation. (d) Edge
effects in TGE and FAGA at 100% isostatic compensation. (e) FAGA at 100% isostatic
compensation. (f) TGE, FAGA and RGE and FAGA at 100% isostatic compensation.
(g) RGE at 100% isostatic compensation (h) TGE, FAGA and RGE at 50% isostatic
compensation.
17
Figure 2.2: Flexure model of an idealized isostatically compensated mountain range
with gravity effects evaluated at 0 km, 20 km, 50 km, 100 km, 150 km, and 200 km
altitude. (a) Geologic model showing topography, densities and crustal-mantle interface. (b) CC between Airy and flexural gravity anomalies. (c) TGE at 100% isostatic
compensation. (d) Edge effects in TGE and FAGA at 100% isostatic compensation.
(e) FAGA at 100% isostatic compensation. (f) TGE, FAGA and RGE and FAGA at
100% isostatic compensation. (g) RGE at 100 % isostatic compensation. (h) TGE,
FAGA and RGE at 50% isostatic compensation.
18
crustal root (RGE) into the mantle. The isostatic anomaly can be estimated from
TGE plus RGE. The isostatic anomaly is equal to FAGA over the central part of the
flat topographic high, representing a simple method for assessing the mountain’s state
of isostatic equilibrium. Thus, the correlation between TGE and FAGA can help to
assess the isostatic condition of topographic features.
Gravity anomaly analysis can not distinguish between the Airy and flexure models
(Watts, 2001). This is because the compensation occurs at depth and the differences
in the anomalies produced by the root are negligible. Thus, gravity solutions require
augmentation by magnetic, seismic reflection and refraction and other data (Keary
and Vine, 1990).
FAGA is close to zero in Figures 2.1(e) and 2.2(e) at the central part of the
mountain range where the RGE essentially cancels the TGE. However, FAGA and
RGE still show a positive regional correlation because the RGE can not fully negate
the edge effects of the mountain in the TGE that are thus present in the FAGA. These
edge effects are reduced with altitude, so that the RGE increasingly negate the TGE
and the correlation coefficient between FAGA and TGE approaches zero. Thus, at
altitudes that are large compared to the terrain relief, such as 20-km altitude, the
correlation between FAGA and RGE or TGE for 100% isostatic compensation is close
to zero.
The gravity curves obtained by these models in Figures 2.1 and 2.2 are useful
for interpreting the correlations between FAGA, TGE and RGE. They are applicable
for assessing the isostatic condition of the Andes Mountains and isolated topographic
highs of Northwestern South America.
The curves in figures 2.1 and 2.2 show that TGE decreases more rapidly than
FAGA from the surface to satellite altitudes. Thus, the higher altitude observations
are most effective for resolving regional isostatic disturbances of the terrain. However,
19
as the altitude of observations increases the spatial resolution of the potential field
anomalies decreases.
Figure 2.3 shows the Airy gravity modeling for an overcompensated mountain
range. Here, the crustal root is too thick and overcompensates for the topographic load
so that the resultant FAGA is negative. TGE and FAGA are negatively correlated. The
isostatic equilibrium will be achieved by crustal rebound reducing RGE as is invoked
in glacial rebound and erosional processes (Turcotte and Schubert, 2002).
2.2.2
Isostatic model of depressed topography
Figure 2.4 models the gravity components for a topographic depression underlain
by a root composed of metamorphic and mantle rocks. For this case, the root was
designed to overcompensate for the excess terrain mass.
The TGE are negative for the topographic depression whereas mantle components
produce a positive RGE. Lateral crustal variations represented by the metamorphic
rocks produce an additional local positive crustal anomaly. The combined effects produce a large positive ”anti-root” effect that makes FAGA positive.
TGE and FAGA are negative or inversely correlated. This example of anomaly
superposition may help to interpret positive FAGA over trenches, subduction zones,
cratonic areas and isostatically overcompensated topographic lows. It also can be useful
to model possible impact craters and mass concentrations or ”mascons” in other regions
of the world (Watts, 2001).
2.2.3
Isostatic model of an oceanic ridge
Figure 2.5 shows the crustal model of an overcompensated oceanic ridge along
with the related gravity components. The oceanic ridge produces a positive TGE but
the anomalous underplated material that is less dense than the mantle produces strong
20
Figure 2.3: Airy gravity model of an isostatically overcompensated mountain range
evaluated at 20 km altitude. (a) Geologic model. (b) Gravity Anomalies: RGE overcompensates TGE making FAGA negative. In this case, the equilibrium could be
reached by isostatic rebound to increase TGE and diminish RGE to make FAGA
approach zero.
21
Figure 2.4: Isostatic overcompensated topographic depression with lateral crustal density variation and mantle upwelling evaluated at 20 km altitude. (a) Geologic model.
(b) Gravity anomalies: TGE are negative, “anti” RGE overcompensates TGE so that
FAGA are positive. In this case, the isostatic state of equilibrium could be reached by
crustal subsidence lowering the upwelled mantle and diminishing RGE to make FAGA
approach to zero. TGE and FAGA are negatively correlated
22
negative RGE so that FAGA are negative. The TGE are negatively correlated with
FAGA and RGE. This example shows how mass variations in the mantle can also
be recorded in FAGA. This model may be useful to interpret the gravity response of
the Cocos, Carnegie, Malpelo and Beata volcanic ridges and related mantle density
reductions.
2.2.4
Local body gravity effects
Figure 2.6 gives the gravity and magnetic effects of a local body represented by
a granitic intrusion that is supported by the rigidity of the crust. Table 2.2 gives
the mean density and magnetic susceptibility values used to evaluate the geopotential anomalies at 20 km altitude. The magnetic anomalies were calculated with the
induction inclination of 90o .
The flat topography yields TGE that are equal to zero. The intrusive igneous rock
affecting the host sedimentary sequence produces positive FAGA and positive total
field magnetic anomaly (TFMA). The FAGA and TFMA are positively correlated as
listed in Table 2.3. The amplitudes of these anomalies vary with the sign of the density and magnetic susceptibility contrasts between the igneous intrusions of granitic,
granodioritic and gabbroic rocks and the host sedimentary rock. This model will be
useful to detect shallow igneous intrusions affecting sedimentary basins in the study
area.
2.2.5
Sedimentary basin gravity effects
Figure 2.7 shows the negative FAGA and TFMA for sedimentary infill at the
oceanic trenches and sedimentary basins. These anomalies reflect the lower density
and magnetization of the sediments relative to the basement rocks. The amplitudes of
the negative FAGA and TFMA increase with the increase in thickness of the basin.
23
Figure 2.5: Gravity effects of isostatically overcompensated oceanic ridge with underplated material evaluated at 20 km altitude. (a) Geologic model of a volcanic ridge (b)
Gravity anomalies: TGE are positive and RGE are strongly negative making FAGA
negative. The model shows how FAGA can also be affected by density variations in
the mantle
24
Figure 2.6: Gravity and magnetic responses evaluated at 20 km altitude of igneous
intrusions affecting a sedimentary sequence. (a) Geologic model. (b) Gravity anomalies:
TGE is equal to zero. FAGA is positive. (c) Total magnetic field anomalies. The gravity
and magnetic anomalies are positively correlated.
25
Rock Type
Density gm/cm3
Mag. suscep (cgs)
Granite (felsic rocks)
2.75
0.0025
Sedimentary rocks
2.35
0.0001
Granodiorite
2.85
0.035
Gabbros (mafic rocks)
3.1
0.070
Meta
2.85
0.0003
2.9
0.03
3.2
0.08
(inter-
mediate)
sedimentary
basement
Intermediate
base-
ment
Mafic basement
Table 2.2: Densities (gm/cm3 ) and magnetic susceptibilities (cgs) for intrusive bodies
and host rocks modeled in Figures 2.6 and 2.7.
The asymmetry of the magnetic anomalies shows that this model was evaluated with
the inclination of 40o . The FAGA and TFMA are positively correlated as shown in
Table 2.4. The correlation can be increased by reducing the TFMA to the pole which
is equivalent to evaluating the models with the geomagnetic field inclination of 90o .
The strongest negative FAGA and TFMA occur for the sedimentary basin within mafic
basement. The negative anomaly amplitudes increase in magnitude as the thickness
of the sedimentary sequence increases. These models are useful for interpreting the
gravity and magnetic effects for trenches, sedimentary basins and tectonic grabens.
2.2.6
Dipping crustal interface
Figure 2.8 shows the gravity and magnetic responses for a horizontal slab with
a dipping edge at various angles of inclination. The FAGA and TFMA are positively
26
Figure 2.7: Gravity and magnetic anomalies for a sedimentary basin estimated at 20
km altitude and 50 km station interval. (a) Geologic model. (b) Negative FAGA for a
sedimentary basin with different basement rock types. (c) Negative TFMA for basin
thicknesses of 1 km, 2 km, 3 km, 4 km, and 5 km within mafic basement.
27
CC
Granite
Granodiorite
Gabbro
FAGA, TFMA
0.9301
0.9240
0.9234
Table 2.3: Correlation coefficients between FAGA and TFMA for an intrusive body
with different rock composition affecting a sedimentary host rock
CC
Felsic Rock
Mafic rock
Meta sedim
FAGA, TFMA
0.8249
0.8249
0.8225
Table 2.4: Correlation coefficients between FAGA and TFMA for a sedimentary basin
hosted within mafic and metasedimentary basements
correlated for the six dip angles of the slab model listed in Table 2.5. These correlations increase as the angle of inclination of the crustal interface decreases. This model
is useful for interpreting crustal discontinuities associated with plate boundaries and
major fault systems.
CC
90o
75o
60o
45o
30o
15o
FAGA,
0.7576
0.7590
0.7609
0.7645
0.7726
0.8032
TFMA
Table 2.5: Correlation coefficients between FAGA and TFMA for a layer dipping at
15o , 30o , 45o , 60o , 75o and 90o inclination.
28
Figure 2.8: Gravity and magnetic anomalies for a dipping interface beneath thin overburden evaluated at 20 km altitude at a 10 km station interval. (a) Negative FAGA
for 15o , 30o , 45o , 60o , 75o and 90o inclination.(b) TFMA for 15o , 30o , 45o , 60o , 75o and
90o inclination
29
CC
0
20 km
50 km
100 km
200 km
350 km
FAGA,
0.0074
-0.2108
-0.2358
-0.2683
-0.2786
-0.2216
TFMA
Table 2.6: Correlation coefficients between FAGA and TFMA for a subduction zone
model at 0 km, 20 km , 50 km, 100 km, 200 km, and 350 km altitudes.
2.2.7
Generalized oceanic - continental subduction
Figure 2.9 gives the gravity and magnetic responses for an oceanic-continental
collision model. This model includes a subducting oceanic plate, a trench filled with
soft sediments, and continental crust with a mountain range and crustal root into the
mantle.
The positive FAGA over the oceanic crust become negative over the trench. Both
positive and negative FAGA are observed over the mountain range. The FAGA decrease
in amplitude and frequency from the surface to satellite altitudes.
The strongly positive TFMA over the continental crust become negative over the
trench. Negative TFMA are observed at the edges of the crustal root and above the
descending oceanic plate. The TFMA decrease in amplitude and frequency from the
surface to satellite altitudes. Table 2.6 shows that the gravity and magnetic anomalies
are inversely but weakly correlated with the CC decreasing from 20 km to 200 km
satellite elevations and increasing again at 350 km altitude.
2.2.8
Estimating the angle of subduction
Figure 2.10 gives the geopotential anomalies at 20-km altitude for the subduction
where the descending oceanic plate is dipping at 15o 30o , 45o and 60o . At the trench,
the negative FAGA and positive TFMA are attenuated at low angles of inclination.
30
Figure 2.9: Gravity and magnetic anomalies of a subduction zone evaluated at 20 km
altitude and a 100 km station interval. (a) Geological model with an oceanic plate
subducting under a continental plate. (b) The FAGA at 0 km, 20 km, 50 km, 100 km,
200 km, and 350 km altitude and (c) corresponding TFMA.
31
Over the continental crust, regions of relative thicker crust are associated with negative FAGA and positive TFMA. Table 2.6 shows that the inverse correlations between
gravity and magnetic anomalies become somewhat stronger with increasing inclination.
These models are useful for interpreting the gravity and magnetic responses of a subducting slab at low (flat slab) and intermediate angles of inclination. The models for
subducting slabs at angles greater than 45o are not realistic, especially for the study
area where relatively low angle (5o -20o ) subduction is applicable.
2.3
Conclusions
Simple theoretical geological models were evaluated for their gravity and magnetic
responses and provide the necessary backgound for signature recognition and interpretation of complex multi-altitude potential field anomalies. The theoretical geological
models also provide insights on the non-uniqueness of gravity and magnetic model
solutions and the need to evaluate their geological feasibility by comparing the results
with seismic, borehole and other geological and geophysical data.
Gravity and magnetic anomalies modeled at low altitudes exhibit amplitudes and
frequencies that are attenuated at satellite altitudes to the point they may not be
detected. Terrain gravity effects are more attenuated than root gravity effects from the
surface to satellite altitudes. Thus, at altitudes that are large compared to the relief
of the crustal terrain, FAGA provides an increasingly effective estimate of the crustal
isostatic anomaly of isostatically disturbed terrain. At these altitudes, morever, the
differences in the gravity responses of the various isostatic models (e.g., Airy, flexure,
Pratt) are quite similar. Thus, inversions of FAGA for isostatic mass differentials may
implement the most convenient isostatic model which essentially complete generality.
32
Figure 2.10: Subduction zone modeling with 15o , 30o , 45o and 60o angles of inclination
estimated at 20-km altitude at a 100-km station interval. (a) Geologic model. (b)FAGA
and (c) corresponding TFMA.
33
CHAPTER 3
ISOSTATICALLY DISTURBED TERRAINS FROM
SPECTRALLY CORRELATED FREE-AIR AND TERRAIN
GRAVITY DATA
34
ABSTRACT
Recently revised models on global tectonics describe the convergence of the North
Andes, Nazca, Caribbean and South American Plates and their seismicity, volcanism,
active faulting and extreme topography. The current plate boundaries of the area
are mainly interpreted from volcanic and seismic datasets with variable confidence
levels. New insights on the isostatic state of the northwestern Andes Mountains can be
obtained from the spectral analysis of recently available gravity and topography data.
Isostatically disturbed terrain produces free-air anomalies that are highly correlated
with the gravity effects of the terrain. Spectrally correlated the terrain gravity effects
(TGE) and free air gravity anomalies (FAGA) of the Andes mountains confirms that
these mountains are isostatically disturbed. Strong negative terrain-correlated FAGA
along western South America and the Greater and Lesser Antilles are consistent with
anomalously deepened mantle displaced by subducting oceanic plates.
Inversion of the gravity anomalies reveals plate subduction systems with alternating shallower and steeper subduction angles. The gravity modeling highlights crustal
deformation from plate collision and subduction and other constraints on the tectonism
of the plate boundary zones for the region.
3.1
Introduction
The tectonic setting of northwestern South America is poorly understood because
of the complex interaction of numerous tectonic plates, and the scarce and incomplete
35
geologic mapping due to the dense vegetation cover and limited accessibility to remote areas. International scientific interest in the northwestern Andes Mountains is
intense because it is the key to improve our understanding of the geological evolution
of the backbone of the Americas and the Caribbean and related volcanic and seismic hazards. The ability of current physical models to predict neotectonic and other
intra-plate lithospheric stresses and strain is limited because the structures and geometry of plate boundaries are largely hidden and poorly understood. However, the
deployment of geodetic sensors including the GPS-derived ground velocities and high
resolution topography has substantially improved the modeling of the lithosphere to
analyze surface mass dynamics (Kellogg, et al., 1985).
In this chapter, spectral correlation theory (e.g., Leftwich, et al., 2005; von Frese
and Tan, 1999) is applied to terrain and free-air gravity anomalies for new constraints
on the boundary zones of the North Andes and Panama Microplates, and the Cocos,
Nazca, Caribbean and South American Plates. Normalization and local favorability
indexes are implemented to facilitate the visualization and interpretation of gravity
anomalies. This methodology has been applied and validated in other crustal studies
such as East Asia (Tan and von Frese, 1997), Antarctica (von Frese et al., 1999), Greenland (Roman, 1999), Ohio (Kim et al., 2000), and Iceland (Leftwich, et al., 2005).
Isostatically disturbed terrain produces free-air anomalies that are highly correlated with the gravity effects of the terrain. Spectrally correlating the terrain gravity
effects (TGE) and free-air gravity anomalies (FAGA) suggested that the Andes mountains are isostatically disturbed with an anomalous underlying crust-mantle interface
(MOHO). Strong negative terrain-correlated FAGA along western South America and
36
the Greater and Lesser Antilles are consistent with anomalously deepened mantle displaced by subducting oceanic plates. Crustal sections from inversion of gravity anomalies showed waves of shallowing and steeping subduction systems that are also supported by seismic data. However, gravity estimates regionally augment and extend the
seismic estimates to highlight crustal deformation from plate collision and subduction
in Northwestern South America. The gravity anomalies reveal a complex evolution for
northwestern South America involving the accretion of an oceanic plateau with strong
Caribbean affinities, obduction of the oceanic crust, over thrusting and strike-slip faulting. These tectonic characteristics are not readily accounted for by the conventional
model of “Andean Type” orogenesis of the Central Andes (Liu, et al., 2002).
3.2
Spectrally correlated free-air and terrain gravity
The isostasy of northwestern South America was investigated considering the topography/bathymetry data from National Imagery and Mapping Agency (NIMA)
from −8o S to 23.5o N latitude and from −90o W to −58.5o W longitude. Surface and
bathymetry elevations from the JGP95E terrain data base (Smith and Sandwell, 1994,
1997) were processed to produce the Digital Elevation Model (DEM) in Figure 3.1 for
the water and rock terrain gravity components at 0.5o nodal spacing. Free-air gravity
anomalies (FAGA) were estimated from the EGM96 spherical harmonic Earth Gravity
Model to degree and order 360 (Lemoine, et al., 1998) at 20 km altitude over the 32o
x 32o area at 0.5o nodal spacing in Figure 3.2. The altitude of 20 km was chosen to
help minimize the effects of local density errors in the terrain gravity modeling (e.g.,
Leftwich et al., 2005).
37
Figure 3.1: Topography/bathymetry of northwestern South America with superposed
regional tectonic features between −8o S to 23.5o N latitude and from −90o W to
−58.5o W longitude. Map annotations include the amplitude range (AR) of (min; max)
values, the amplitude mean (AM) and standard deviation (SD). SNSM= Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection.
38
Figure 3.2: Free-air gravity anomalies (FAGA) at 20 km elevation for the study region
(Lemoine, et al., 1998). Map annotations include the amplitude range (AR) of (min;
max) values, the amplitude mean (AM) and standard deviation (SD). SNSM= Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map was produced
using the Albers equal-area conic projection.
39
The terrain gravity effects were modeled in spherical coordinates at 20 km altitude
by Gauss-Legendre Quadrature integration in Figure 3.3 (von Frese, 1980). The terrain gravity modeling used densities of 2.8 gm/cm3 for the crust and 1.03 gm/cm3 for
oceanic water.
Spectral correlation theory was used to analyze the co-registered FAGA and TGE
for their anomaly correlations. Specifically, the Fourier transforms T and F of TGE
and FAGA, respectively, were used to obtain their correlation spectrum (von Frese
et al., 1997a, Kim, et al., 2000) given by:
"
#
F (k) |T (k)|
CC(k) = cos(∆θk) = Re
,
T (k) |F k)|
(3.1)
where CC(k) is the correlation coefficient between the k th wavenumber components
F(k) and T(k), and denotes taking the real parts of the wavenumber components.
Usually, CC(k) is evaluated from the cosine of the phase difference (∆θk) between the
two k th wavenumber components.
Using the correlation spectrum between FAGA and TGE, spectral correlation filters
were designed to extract terrain-correlated free-air gravity signals. Those wavenumber components showing intermediate to high positive (CCp (k) ≥ 0.3) and negative
(CCn (k) ≤ 0.3) correlations were identified. The cut off values for the correlation filter
were determined to minimize correlative features between the terrain-decorrelated freeair and compensating terrain gravity components. Inversely transforming positively
and negatively correlated free-air wavenumber components according to the selected
cut off values yielded the terrain-correlated free air gravity anomalies (TCFAGA) in
Figure 3.4. The residual terrain-decorrelated free-air gravity anomalies (TDFAGA)
in Figure 3.5 were calculated by subtracting TCFAGA from FAGA, so that
40
Figure 3.3: Terrain Gravity Effects (TGE) at 20 km elevation for the study region by
Gauss-Legendre quadrature integration. Map annotations include the amplitude range
(AR) of (min; max) values, the amplitude mean (AM) and standard deviation (SD).
SNSM= Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection.
41
F AGA = T CF AGA + T DF AGA
(3.2)
TCFAGA are explained by anomalies associated with the topography while TDFAGA
include the gravity effects of sources within the crust (i.e., local bodies) and the subcrust.
Similarly, spectral correlation filters were used to extract FAGA-correlated TGE
(FCTGE) in Figure 3.6 and FAGA-decorrelated TGE (FDTGE) in Figure 3.7. The
FCTGE sharpen up the TGE components that may be isostatically disturbed, whereas
the FDTGE involve relatively marginal amplitudes that appear to reflect mostly noise
from data processing and other non-geological effects.
3.3
Analysis of anomaly correlations
TCFAGA and FCTGE were normalized to facilitate recognizing the anomaly correlations between them (von Frese et al., 1997). To enhance the visual perception of
the anomaly correlations, the following transformation was applied:
Ã
Zi (X) = σZ
x i − µX
σX
!
+ µZ ,
(3.3)
where µX and σX represent the values of the mean and standard deviation, respectively,
of the signal X. The expression in parentheses standardizes the xi coefficients to zero
mean and unit standard deviation and is dimensionless. However, the values for mean
σZ and standard deviation of µZ of the normalized signal Z can be specified by the
user to facilitate the visual analysis.
The transformations of TCFAGA and FCTGE used the normalization of σZ = 10 to
facilitate plotting the two datasets with common plotting parameters (von Frese et al.,
42
Figure 3.4: Terrain-correlated FAGA (TCFAGA) at 20 km elevation for the study
area. Map annotations include the amplitude range (AR) of (min; max) values, the
amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers
equal-area conic projection.
43
Figure 3.5: Terrain-decorrelated FAGA (TDFAGA) at 20 km elevation for the study
area. Map annotations include the amplitude range (AR) of (min; max) values, the
amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers
equal-area conic projection.
44
Figure 3.6: FAGA-correlated TGE (FCTGE) at 20 km elevation for the study area.
Map annotations include the amplitude range (AR) of (min; max) values, the amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers
equal-area conic projection.
45
Figure 3.7: FAGA-decorrelated TGE (FDTGE) at 20 km elevation for the study area.
Map annotations include the amplitude range (AR) of (min; max) values, the amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers
equal-area conic projection.
46
1997). Local favorability indices (Merriam and Sneath, 1966) were used to highlight
the various anomaly correlations in the normalized TCFAGA and FCTGE. Positively
correlated features were mapped out by summed local favorability indexes (SLFI)
obtained by:
SLF Ii =
(zi (X)) − µZ ) (zi (Y ) − µZ )
+
,
σZ
σZ
(3.4)
where Z(X) and Z(Y) were the normalized TCFAGA and FCTGE coefficients,
respectively. The peak-to-peak correlations between the two data sets were mapped
out by SLF Ii ≥ 8.7226 in Figure 3.8, whereas trough-to-trough correlations were
mapped out by the coefficients satisfying SLF Ii ≤ 8.7226 in Figure 3.9. The SLFI
coefficients brought out the positively or directly correlated features, while suppressing
the negatively and null correlated features between TCFAGA and FCTGE.
To enhance the perception of inversely correlated features obtained from wavenumbers with negative correlation coefficients, the normalized and scaled datasets were
subtracted cell by cell for differenced local favorability indexes (DLFI) by:
DLF Ii =
(zi (X)) − µZ ) (zi (Y ) − µZ )
−
σZ
σZ
(3.5)
Positive features in TCFAGA which were correlative with negative FCTGE features (peak-to-trough) were mapped out by DLFI ≥ 4.8904 in Figure 3.10, whereas
negative TCFAGA that were correlative with positive FCTGE were mapped out by
DLFI ≤ −4.8904 in Figure 3.11. The DLFI coefficients emphasized the inversely correlated features, while suppressing the positively and null correlated features between
TCFAGA and FCTGE. Table 3.1 summarizes the correlations between FAGA and
TGE and their correlated filtered components.
47
Figure 3.8: Summed local favorability indices (SLFI) for TCFAGA and FCTGE at 20
km elevation for the study region showing TCFAGA-peak to FCTGE-peak correlations
for SLFI ≥ 8.7226. SNSM= Sierra Nevada of Santa Martha, W-Mid = Western-Central
ranges. This map was produced using the Albers equal-area conic projection.
48
Figure 3.9: Summed local favorability indices (SLFI) for TCFAGA and FCTGE at
20 km elevation for the study region showing the TCFAGA-trough to FCTGE-trough
correlations for SLFI ≤ −8.7226. SNSM= Sierra Nevada of Santa Martha, W-Mid =
Western-Central ranges. This map was produced using the Albers equal-area conic
projection.
49
CC
TGE
FCTGE
FDTGE
FAGA
TCFAGA
TDFAGA
TGE
1
0.97
0.24
0.52
0.59
0
FCTGE
0.97
1
0
0.56
0.6
0.05
FDTGE
0.24
0
1
-0.09
0
-0.2
FAGA
0.52
0.56
-0.09
1
0.89
0.45
TCFAGA
0.59
0.6
0
0.89
1
0
TDFAGA
0
0.05
-0.2
0.45
0
1
Table 3.1: Correlation coefficients (CC) between the gravity anomalies at 20 km altitude for the area from −8o S to 23.5o N latitude and from −90o W to −58.5o W longitude
in northwestern South America.
The increase in the CC between TCFAGA and TGE (CC= 0.59) relative to the CC
between the raw FAGA and TGE (CC=0.52) facilitates the isostatic analysis of the
tectonic features of the study area. However, the increase in the CC between TCFAGA
and FCTGE (CC=0.6) with respect to the CC between TCFAGA and TGE (cc=0.59)
is negligible and the physical meaning of FCTGE and FDTGE is obscured.
The CC between TDFAGA and FAGA (CC= 0.45) is significant in showing the
strong influence of the gridded data in the anomaly decomposition of FAGA in TCFAGA
and TDFAGA. Therefore, further analysis of TDFAGA are required.
3.4
Discussion of results
Isostatically disturbed terrains have gravity effects that directly correlate with
FAGA. The complementary TDFAGA reflect the effects of lateral density variations
in the crust, mantle and core, as well errors in the data and data processing.
50
Figure 3.10: Differenced local favorability indices (DLFI) between TCFAGA and
FCTGE at 20 km elevation for the study region showing the TCFAGA-peak to
FCTGE-trough associations for DLFI> 4.8904. SNSM= Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers
equal-area conic projection.
51
Figure 3.11: Differenced local favorability indices (DLFI) for TCFAGA and FCTGE
at 20 km elevation for the study region showing the TCFAGA-trough to FCTGE-peak
associations for DLFI<- 4.8904. SNSM= Sierra Nevada of Santa Martha, W-Mid =
Western-Central ranges. This map was produced using the Albers equal-area conic
projection.
52
In the subsections below, the gravity effects of the DTM and spectral correlations
with free-air gravity anomalies are interpreted for constraints on the tectonic features
of the study region.
3.4.1
Digital terrain model (DTM)
The DTM in Figure 3.1 presents major morphotectonic features that produce
strong terrain gravity effects in the gravity anomalies. The topography data are increasingly being integrated into the techniques to study plate boundary zones (Stein
& Freymueller, 2002). One approach is to use digital terrain models (DTM) showing
the forms of mountain ranges, oceanic trenches, volcanic ridges and island arcs, and
oceanic basins to constrain models of the processes that produced them. The DTM
of northwestern South America suggests a complex tectonic setting with mountain
building and uplift as a consequence of the plate convergences that have created the
North Andes Mountains, the Sierra Nevada of Santa Marta and the Cordillera Central Mountains. Flat low lands are associated with the stable cratonic terrains of the
Guiana Shield. Deep Pacific Ocean trenches along the Peru - Ecuador - Colombia
coastline and the Middle America Trench are associated with subduction zones. The
Puerto Rico Trench in the Caribbean is associated with the Caribbean − North American subduction zone. The submarine Carnegie, Cocos and Malpelo volcanic ridges are
associated with the Galapagos hot spot. The volcanic arc islands of the Lesser and
Greater Antilles reflect intensive geodynamics in the region, including high seismicity
and volcanic activity.
3.4.2
Free-air gravity anomalies (FAGA)
The free air gravity anomalies in Figure 3.2 include the superposed gravity effects
of the terrain and subsurface gravity variations. The FAGA amplitudes are smaller than
53
the TGE amplitudes reflecting partial isostatic compensation of the terrain. However,
the mean FAGA of 17 mGals indicates the mean terrain is isostatically disturbed.
The largest negative anomalies are over the subduction zones, whereas the strongest
positive FAGA are over the Andes Mountains and Sierra Nevada of Santa Marta. Moderate positive and negative FAGA are distributed along the oceanic volcanic ridges,
Guiana Craton and oceanic plates. The positive correlation between TGE and FAGA
(CC = 0.52) reflects the predominant condition of isostatically disturbed terrain in
the tectonically complex study region.
3.4.3
Terrain gravity effects
The terrain gravity effects (TGE) in Figure 3.3 produced by crust - air (2.8
gm/cm3 ) and crust - water (1.82 gm/cm3 ) interfaces are the highest density contrasts that can be defined by the lithosphere. Therefore, the minimum and maximum
TGE values of -452.12 mGals and 285.02 mGals, respectively, are also the extreme
values that we can expect FAGA to take on in a case of 0% isostatic compensation.
Negative TGE predominate (T GEmean = −51.21 mGals) and are stronger over the
Caribbean Sea than the Pacific Ocean. Negative TGE overlie the Puerto Rico Trench,
Cayman Trough, and Colombian and Venezuelan Basins. The Beata Ridge, Lesser and
Greater Antilles have relatively positive TGE. In the Pacific, negative TGE overlie the
Panama Basin and the Peru Trench, and positive TGE overlie the Carnegie, Cocos
and Malpelo Ridges. At the continent, positive TGE reflect the presence of the mountainous Central America, Andes Mountains, Sierra Nevada of Santa Martha and the
Guiana Craton. Relatively negative anomalies are associated with the Vichada Plain
and Amazon River Aulacogens.
54
3.4.4
Terrain-correlated free-air gravity anomalies
TCFAGA in Figure 3.4 have a more complex anomaly pattern than TGE inferring a wide range isostatic disturbances of the terrain. TCFAGA varies from -190.28 to
131.66 mGals, with a mean value of zero. Thus, the TCFAGA reflect both negative and
positive disturbances of continental and oceanic features. Arcuate positive and negative
TCFAGA can be observed along the Greater and Lesser Antilles, along the Caribbean
- North American plate boundary. Volcanic activity and mountain building are associated with positive TCFAGA, whereas negative TCFAGA characterize trenches and
troughs. Therefore, the subduction of the North American plate under the Caribbean
Plate is well mapped from the TCFAGA. Negative TCFAGA beneath the trenches are
due to the negative density contrast (∼
= - 0.37 gm/cm3 ) of the subducting oceanic slab
into the mantle. Positive TCFAGA are explained by oceanic crustal thickening and the
recently formed volcanic islands which are not fully compensated. The TCFAGA are
smaller than TGE, suggesting partial isostatic compensation from the thickening of
the oceanic crust into the mantle. Negative TCFAGA are located along the Colombian
- Venezuelan Basins and the Caribbean − North Andes subduction zone (Bird, 2003,
CASA, 1998). The low TCFAGA amplitudes suggests that this subduction is incipient
with respect to the Caribbean - North American or Nazca - North Andes subduction
zones. In Central America, the positive TCFAGA from Panama to Costa Rica (60-80
mGals) and from Nicaragua to Honduras (20-40 mGals) are less prominent than the
TCFAGA in the Andes (80-100 mGals) showing that Central America is relatively
more fully compensated than the Andes Mountains. The converging plate boundary
between the Panama and North Andes microplates proposed by Kellogg (1985) is not
supported by the TCFAGA which shows no distinct anomaly between them.
55
At the Pacific Ocean, negative TCFAGA clearly define the Middle America Trench.
Relatively positive TCFAGA are associated with the flat slab subduction of the Cocos
ridge under the Panama − Costa Rica Microplate. Negative TCFAGA along the western margin of South America vary from -10 to -60 mGals, being comparatively smaller
in the Colombian Trench. These results reflect the lower angle of subduction of the
Nazca plate in contrast to the steeper angles of subduction and negative amplitudes at
the Chilean - Peruvian subduction zone. Relative TCFAGA highs are associated with
the flat slab subduction of the Carnegie Ridge under the North Andes Microplate.
At the continent, positive TCFAGA at the Andes Mountains (90 to 130 mGals)
show that these mountains are isostatically disturbed. A remarkable isolated positive
TCFAGA anomaly is located in northern Colombia, in the Sierra Nevada of Santa
Marta (130 mGals). Here, the highest elevations of northwestern South America have
also the maximum TCFAGA, showing that these mountains, reaching elevations of
5800 m, are even less compensated than the Andes mountains suggesting that the
Sierra Nevada of Santa Marta has been intensively deformed and uplifted after the
Andean orogenesis.
It is not clear how these mountains were formed and why they occupy their present
position. These mountains maybe are perhaps allochthonous tectonic blocks that were
moved from the east along a left lateral fault (Dewey and Pindell, 1985). The TCFAGA
also support this explanation, which implies that lateral stress caused crustal shortening and uplift of the Sierra Nevada of Santa Marta without significant isostatic
compensation.
TCFAGA at the Guiana Craton are close to zero suggesting nearly complete isostatic compensation. Local TCFAGA highs and lows may reflect the occurrence of lateral mass variations related to intrusive bodies without topographic expression (TGE
56
= 0). Here, the TCFAGA that are greater than the TGE give an example of overcompensated terrain. The negative TCFAGA along the Eastern Andes Mountains are
associated with continent - continent convergence between the North Andes Microplate
and the South American Plate with regional thrust faults dipping to the west and intensive compressional and transpressional deformation (Cediel et al., 2003). Therefore,
the negative TCFAGA along the eastern cordilleran area appear to reflect the thickening of the continental crust with thick sedimentary sequences that are known to
contain oil and gas deposits (Stein & Sella, 2002).
3.4.5
Terrain-decorrelated free-air gravity anomalies
The TDFAGA in Figure 3.5 are very poorly correlated with TGE (CC = −0.0575)
and mostly show gridded patterns due likely to data processing noise. Thus, I do not
consider the geological implications further for the TDFAGA.
3.4.6
TCFAGA and FCTGE anomaly correlations
Tectonic analysis of enhanced correlations between the TCFAGA and FCTGE
are visualized better using summed local favorability indices with (SFLI ≥ 8.7226)
in Figure 3.8. High amplitude peak-to-peak zones along the Andes Mountains overlay mapped mineralized batholithic intrusions (INGEOMINAS, 1987). According to
Kerr, et al. (1997), subduction related batholiths and extrusive rocks found around
the margin of the Caribbean - Colombian cretaceous igneous province are of two distinct events: one suite represents the pre-plateau, collision-related volcanism, whereas
the other suite, which is slightly younger than the plateau, may be associated with
subduction. Therefore, peak-to-peak zones along the western Cordillera may be associated with subduction, whereas peak-to-peak zones along the eastern Cordillera are
associated with the pre-plateau collision suite of igneous rocks.
57
Intermediate amplitude peak-to-peak correlations at the Guiana Craton also indicate isolated topographic highs on relatively denser intrusive rocks of the cratonic
basement. These inferred intrusions may be the source rocks of known gold and diamond placer deposits of the Guainia and Orinoco rivers (Romero et al., 1996).
The SLFI ≤ −8.7226 in Figure 3.9 show trough-to-trough correlations between
TCFAGA and FCTGE, respectively. They clearly mark the Puerto Rico, Colombian
and Peruvian Trenches, while the Middle American Trench and the Pacific Colombian
Trenches are relatively poorly defined. The oceanic trenches are the most isostatically
disturbed features in the area and also the most seismically active features. Of special
interest are the intermediate trough-to-trough correlations of TCFAGA and FCTGE
in the Colombian and Venezuela Basins. These areas are undergoing intensive exploration for oil and gas and have productive fields associated with delta deposits of the
Magdalena River, the major supplier of sediments to the Caribbean Sea. Therefore, the
SLFI anomaly correlations may mark strong prospective areas for locating sedimentary
basins in the Caribbean plate.
The differenced local favorability indices with DLF I ≥ 4.8904 show TCFAGA-peak
to FCTGE-trough correlations in Figure 3.10. They mark topographic depressions
that are overcompensated by local denser bodies along the Lesser Antilles, Colombian
basin, western and central ranges, Sierra Nevada of Santa Marta and at the North
American Plate. The TCFAGA-trough to FCTGE-peak correlations with DLF I ≤
4.8904 in Figure 3.11, on the other hand, show the distribution of low density zones
along the eastern Andean Mountains, the upper Amazon River and lower Orinoco
river. The negative DLFI zones mark mapped sedimentary basins including the upper,
middle and lower Magdalena valley (UMV, MMV and LMV, respectively), and the
Orito, Llanos, Maracaibo, Guajira and Orinoco basins with known productive oil and
58
gas fields. Thus, The negative DLFI zone outlined at the upper Amazon river aulacogen
may also infer a prospective sedimentary basin for oil and gas exploration.
3.5
Conclusions
The central parts of the Nazca, Cocos and Caribbean Plates are mostly in isostatic
equilibrium with TCFAGA close to zero. This condition changes at the plate boundary
zones and volcanic ridges. Positive TCFAGA are associated with the Malpelo, Cocos,
Carnegie and Beata Ridges. Thickening the oceanic crust by crustal roots into the
mantle partially compensates for the TGE. In the case of the Malpelo Ridge, this
volcanic range is partially compensated with a crustal root, in agreement with the
model proposed by Watts (2001) for isostatic compensation of seamounts and oceanic
islands. Depth to the MOHO estimated from gravity data is supported by seismic data
(Trummer, 2002). The Andes Mountains are isostatically disturbed. The Airy-type isostatic model is consistent with the thickening of the crust at the Andes Mountains,
but lateral density variations between the oceanic and continental crusts must be involved to model TCFAGA. The Pratt-type isostatic model is not appropriate to model
TCFAGA of the Andean Mountains. This model can not account for the continental
roots shown in the seismic data.
Locally positive TCFAGA anomalies in the Guiana Craton are associated with relatively denser intrusive bodies affecting the host cratonic rocks. These intrusive bodies
do not have a significant surface expression and may be related to mass variations in
the crust and upper mantle.
Compressional tectonics has played an important role in the mountain building of
the North Andes Microplate as a consequence of the relative movements among Nazca,
South America and Caribbean Plates and Panama and North Andes Micro plates.
The shortening associated with plate motion, based on GPS data only accounts for
59
less than 50% of the mountain building (Lamb, 2004). Magmatic underplating and
crustal doubling due to overthrusting may also contribute to the formation of the
Andes Mountains as inferred by the positive TCFAGA.
Considering that isostatic equilibrium is the ideal state that any uncompensated
mass tends to reach through time, implies that any uncompensated mass of the North
Andes Mountains is under pressure to adjust vertically and horizontally to reach equilibrium. Therefore, a direct correlation is expected between disturbed and seismically
active terrains whereas minimal seismic activity is expected for isostatically compensated terrains.
SLFI and DLFI enhance TCFAGA and FCTGE anomaly correlation, providing
new criteria to locate areas for prospecting mineral and fossil fuel deposits.
60
CHAPTER 4
CRUSTAL THICKNESS AND DISCONTINUITY
ESTIMATES
61
ABSTRACT
A new model for the crustal evolution of northwestern South America (−8o S to
23.5o N, −90o W to −58.5o W) was developed from gravityderived MOHO depth estimates and tectonic features interpreted from correlative geopotential anomalies and
seismic data. Crustal thickness estimates provide important constraints on the distribution of volcanic and seismic hazards, and mineral and energy deposits. Gravity
crustal thickness estimates are also useful for interpreting crustal magnetic anomalies
and thermal gradients. Crustal thickness estimates were obtained by inversion of the
compensated terrain gravity effects (CTGE) and compared against theoretical Airy
MOHO and compiled seismic MOHO estimates.
Crustal thicknesses between 45 to 55 km at the continent are related to the presence
of andesitic batholiths of economic interest. Major batholiths in the Central Andes,
are related to gravity-inferred crustal thicknesses between 55 km to 60 km. Therefore,
these results suggest that exploration of mineral deposits associated with batholithic
intrusions in the Andes Mountains can be extended to crustal thicknesses from 45 km
to 60 km.
Oceanic crustal thicknesses between 15 km to 20 km at the Caribbean are expected
to have relatively low thermal gradients. These crustal thicknesses are ideal for hosting
local oil and gas producing sedimentary basins that are associated with the delta
deposits of the Magdalena and Orinoco Rivers.
62
New insights into the tectonic setting of northwestern South America are presented
that establish the relationship between the crustal thickness variations and the distribution of magmatic bodies, active volcanoes, seismic hazards and economic mineral
and energy deposits. They also show that the northwestern Andes are different from
the Central Andes Mountains that exhibit the typical “Andean subduction zone.”
4.1
Introduction
Terrain-correlated free-air anomalies can be compared with the gravity effects of
the terrain for estimating the relief of the crust-mantle boundary (MOHO) that nongravity methods like seismic surveys image. Thus, new insights into crustal tectonic
features and their evolution can result from the analysis of recently available gravity
observations from satellite, airborne and surface survey.
Improved crustal thickness estimates are useful to advance understanding of the
isostatic state of the Andes Mountains and the mapping of seismically active crustal
discontinuities and plate boundary zones, the distribution of ore deposits and sedimentary basins for oil and gas exploration. This chapter presents the crustal thickness
modeling for estimating the MOHO topography from the spectral correlation analysis
of the free-air and terrain gravity effects. Crustal discontinuities are also interpreted
from the gravity anomalies on a 5 km x 5 km grid of the North Andes Microplate.
4.2
Crustal modeling
A new crustal thickness model for northwestern South America was developed
using the compensated terrain gravity effects (CTGE) in Figure 4.1 that resulted
when the TGE (Figure 3.3) were subtracted from TCFAGA (Figure 3.4).The CTGE
represent isostatically adjusted complete Bouguer anomalies that correspond to the
gravity effects of the terrain in isostatic equilibrium. This approach is feasible because
63
90% of the earth is in equilibrium with the mean global free-air gravity anomaly being
zero (Heiskanen and Moritz, 1967). MOHO and related crustal thickness variations
were modeled from the CTGE (Figure 4.1) by inversion as shown in Figure 4.2,
assuming the constant nominal density contrast of 0.4 gm/cm3 of the mantle relative
to the crust. This methodology has been successfully applied in studies of the mantlecrust interface for East Asia (Tan and von Frese, 1997), Antarctica (von Frese et al.,
1999), Greenland (Roman, 1999), Ohio (Kim et al., 2000), and Iceland and the North
Atlantic (Leftwich, et al., 2005, Leftwich, 2006).
Negative CTGE are located along the eastern Andes Mountains suggesting some
degree of partial compensation and thickening of the crust. The CC between TCFAGA
and CTGE is -0.3377 showing that most of the topography/bathymetry is isostatically
disturbed.
Gravity MOHO estimates were compared against the seismic MOHO in Figure 4.3
compiled by Bassin et al. (2000). This model presents crustal thickness estimates on
a 2o x2o grid was regridded to the 0.5o interval of the gravity MOHO.
4.3
Comparison with Airy MOHO estimates
The density compensation required for isostatic equilibrium from topographic loading is commonly assessed from the Airy compensation (Turcotte and Gerald, 2002)
assuming constant densities of the crust (ρc ) and mantle (ρm ). The displacement d of
the bottom of the crust in response to a crustal load of height h is:
d=
ρc h
,
ρm − ρc
(4.1)
where d and h are relative to the crust of constant thickness H. If the height of the
topography is negative and covered by water, then
64
Figure 4.1: Compensated terrain gravity effects (CTGE) at 20 km elevation for the
study region obtained by subtracting TGE from TCFAGA. Map annotations include
the amplitude range (AR) of (min; max) values, the amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = WesternCentral ranges. This map was produced using the Albers equal-area conic projection.
65
Figure 4.2: Gravity MOHO obtained from the inversion of CTGE showing deep MOHO
beneath the mountains and oceanic ridges. Map annotations include the amplitude
range (AR) of (min; max) values, the amplitude mean (AM) and standard deviation
(SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection.
66
Figure 4.3: Seismic MOHO from the CRUST 2.0 model (Bassin et al., 2000) showing
deeper MOHO below the mountains and oceanic ridges. Map annotations include the
amplitude range (AR) of (min; max) values and amplitude mean (AM) and standard
deviation (SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central
ranges. This map was produced using the Albers equal-area conic projection.
67
"
#
ρc − ρw
d=
h
ρm − ρc
(4.2)
The Airy MOHO for northwestern South America was calculated (Figure 4.4)
assuming ρc = 2.8 gm/cm3 , ρm = 3.3 gm/cm3 , ρw = 1.03 gm/cm3 and H = 35 km.
This model shows the continental and oceanic crustal thickness variations, including
the roots and anti-roots required to reach the isostatic compensation. The calculations
used a crustal thickness (35 km) that corresponds to the mean seismic MOHO estimate.
Figure 4.5 gives the differences when the gravity MOHO is subtracted from the
Airy MOHO. The mean difference is -2.2 km. Crust-mantle density contrasts of 0.37
gm/cm3 at the oceans and 0.45 gm/cm3 at the continent were used for the Airy MOHO
estimates, whereas 0.4 gm/cm3 was used for the gravity MOHO estimates, respectively.
4.4
Comparison with seismic MOHO estimates
Seismic MOHO estimates from the CRUST 2.0 model in Figure 4.3 were gathered from seismic experiments and averaged globally from similar geologic and tectonic
settings (e.g., Archean, early Proterozoic, rifts etc.). These averages were used to infer MOHO depths in regions lacking seismic information (e.g., most of Africa, South
America). Bathymetry and topography data were compiled from ETOPO5. The global
model is composed of 360 one dimensional profiles that were gridded at 2o x 2o .
The comparison of Figure 4.2 and Figure 4.3 shows that the seismic MOHO has
lower resolution than the gravity MOHO. The seismic MOHO also shows a thinner
oceanic crust and a thicker continental crust with deeper crustal roots beneath the
Andes Mountains.
The methodology applied in the CRUST 2.O model to average global seismic responses for similar geological and tectonic settings and assign these averages to regions
68
Figure 4.4: Airy MOHO obtained the hydrostatic equilibrium principle. Map annotations include the amplitude range (AR) of (min; max) values and amplitude mean
(AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa Martha, W-Mid
= Western-Central ranges. This map was produced using the Albers equal-area conic
projection.
69
Figure 4.5: MOHO differences by subtracting gravity MOHO from Airy MOHO estimates. Map annotations include the amplitude range (AR) of (min; max) values and
amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of Santa
Martha, W-Mid = Western-Central ranges. This map was produced using the Albers
equal-area conic projection.
70
without seismic information is not effective for representing the tectonic features of
northwestern South America. For example, the seismic and gravity Moho differences
in Figure 4.6 are especially severe at the Cocos, Nazca, Malpelo and Beata oceanic
volcanic ridges, the Guiana Craton, Puerto Rico Trench and Pacific Subduction zone.
The mean difference is -3.9 km which indicates that the seismic MOHO estimates
do not take into account effects related to thermal expansion and the resulting lower
density of underplated mantle materials included in the gravity MOHO estimates.
The differences between the Airy and seismic MOHO estimates in Figure 4.7 are
less severe as indicated by the mean value of 1.7 km. However, the differences are
between 5 to 10 km at the oceanic ridges, the Greater and Lesser Antilles, the Sierra
Nevada of Santa Marta, and the northernmost Andes Mountains.
4.5
Crustal cross-sections
Airy, gravity and seismic MOHO profiles were constructed across the Pacific subduction zone, the Caribbean subduction zone and the Caribbean plate from Central
America to the Lesser Antilles. In the sections below, these profiles are compared at
the major tectonic features of the study area to investigate the veracity of the gravity
MOHO solutions.
4.5.1
Pacific subduction zone - Andes Mountains - Guiana
Craton
The profile in Figure 4.8 was constructed along the 5o N latitude from −90o W to
−58.5o W longitudes. It includes the Cocos and Malpelo Ridges, the Pacific subduction zone, the Andes Mountains and Guiana Craton. The correlation of the gravity
MOHO with the Airy MOHO is maximum(CC = 0.9701), but minimum with the seismic MOHO (CC = 0.8799) and somewhat higher (CC = 0.8936) between the Airy
71
Figure 4.6: MOHO differences by subtracting gravity MOHO from seismic MOHO
estimates. Map annotations include the amplitude range (AR) of (min; max) values
and amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of
Santa Martha, W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection.
72
Figure 4.7: MOHO differences by subtracting the seismic MOHO from the Airy MOHO
estimates. Map annotations include the amplitude range (AR) of (min; max) values
and amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada of
Santa Martha, W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection.
73
MOHO and the seismic MOHO. The three MOHO solutions are consistent with a
thinner oceanic crust, low angle subducting slab, and thicker continental crust at the
Andes Mountains and an average thickness crust for the Guiana Craton. The differences between gravity and Airy MOHO solutions vary from 0 to 2.5 km at the Cocos
and Malpelo Ridges and between 5 and 7.5 km at the continental roots of the Andes Mountains, reflecting partial isostatic compensation. The differences between the
gravity and seismic MOHO estimates vary from 7.5 to 17.5 km at the volcanic ridges
and from 2.5 to 5 km at the continental roots of the Andes Mountains, respectively,
reflecting the reduced mantle density due to hotter than normal mantle at the ridge
(Leftwich et al., 2005).
4.5.2
Guiana Craton-Andes Mountains-Caribbean subduction
zone
The profile in Figure 4.9 was extracted from (Oo N −78o W) to (23.5o N −75o W),
across to the Caribbean - North Andes subduction zone . It includes the Guiana
Craton, the Venezuelan Andes, Caribbean subduction zone, Caribbean Plate, Greater
Antilles, Cayman Trough and North American Plate. The Gravity MOHO and Airy
MOHO are positively correlated (CC 0.9062) showing differences of 7.5 km at the
Venezuelan Andes mountains, 5 km at the Caribbean plate and 7 km at the Cayman
Trough. The gravity and seismic MOHO estimates are also poorly correlated (0.5348)
with differences between 5 to 7.5 km at the Caribbean subduction zone and about 2 km
at the Cayman Trough. The Airy and seismic MOHOs are likewise poorly correlated
(0.6464) with differences of 5 km at the Venezuelan Andes and 7 km at the Cayman
Trough.
74
Figure 4.8: Gravity, seismic and Airy MOHO estimates at 5o N latitude from −90o to
−58.5o longitude. MOHO differences are greatest at the volcanic ridges and Andes
Mountains.
75
Figure 4.9: Gravity, seismic and Airy MOHO estimates at the Guiana Craton and
North Andes − Caribbean subduction zone along (Oo N, −78o W) to (23.5o N, −75o W).
The three MOHO estimates show a thicker continental crust at the Andean Mountains. The North Andes - Caribbean subduction zone presents different angles of inclination at each MOHO solution. The Caribbean-North America plate boundary zone is
clearly marked by a symmetrical anomaly suggesting vertical inclination of the plate
boundary in agreement with proposed tectonic models for the Caribbean Plate (Pindell, et al., 2006).
76
4.5.3
Middle American Trench - Caribbean Plate - North
American Plate
The profile in Figure 4.10 was constructed along (12.5o N, −90o W) to (16o N,
−58.5o W), across to the Middle American Trench and Caribbean-North America subduction zone. It includes The Middle American Trench, continental Central America,
the Caribbean Plate and the Caribbean-North America subduction zone. The gravity
and Airy MOHO estimates are positively correlated (CC = 0.9427) showing the largest
differences only at the Central America-Caribbean plate boundary zone (5 km), and
at the Caribbean-North American subduction zone (6 km). The gravity and seismic
MOHO estimates are also positively correlated (CC = 0.8774). There are differences
as large as 5 km at the Caribbean-North American subduction zone and 4 km at the
Caribbean-Central America plate boundary zone, respectively. The Airy and seismic
MOHO estimates are positively correlated (CC = 0.8815) with differences as large
as 5 km at the Caribbean plate. The three MOHO estimates show a thicker continental crust beneath Central America, a relatively thinner Caribbean oceanic crust,
and thicker oceanic crust for the Lesser Antilles. The subduction zones are relatively
poorly defined in all the MOHO estimates. Figure 4.11 gives a more comprehensive
crustal model of TCFAGA along 5o N latitude from −90o to −79o W, including the
Cocos and Malpelo Ridges of Figure 4.9. The Malpelo Ridge was studied by a high
resolution seismic survey providing detailed MOHO estimates (Trummer, 2002). The
modeling of TCFAGA was performed using GM-SYS and assigning density values of
1.03 gm/cm3 for sea water, 2.93 gm/cm3 for oceanic crust, and 3.3 gm/cm3 for the
mantle. The model also included the presence of underplated material with the density of 3.0 gm/cm3 that was required to match observed and calculated TCFAGA.
The Gravity Moho is deeper beneath the Cocos and Malpelo Ridges reaching depths
77
Figure 4.10: Gravity, seismic and Airy MOHO estimates at the Caribbean − North
American plates along (12.5o N, −90o W) , (16o N, −58.5o W).
78
Figure 4.11: Crustal profile at 5o N latitude along −90o W to −79o W across the Cocos
and Malpelo Ridges that is consistent with TCFAGA. Oceanic crustal thickening partially compensates TGE. The complete matching of observed and calculated gravity
anomalies required the inclusion of underplated material that was less dense than the
mantle.
of 20-25 km depth, and is positively correlated with the high resolution seismic MOHO
of the Malpelo ridge (Trummer; 2002) in Figure 4.12, where an ophiolitic sequence
overlaying an ultramafic upper mantle was inferred. The mantle seismic velocities
show variations from 4.0 to 4.8 km/s reflecting mantle anisotropy (Trummer,(2002).
The Cocos and Malpelo ridges are considered to be the tracks of the Galapagos hot
spot, which is currently located beneath the Galapagos Archipelago (Trummer, 2002).
Therefore, the reduction in seismic velocities suggest the reduction of mantle density
due to hotter than normal mantle at the ridge.
79
Figure 4.12: Two dimension P-wave velocity versus depth model across the Malpelo
Ridge and Eastern Panama Basin showing an ophiolitic sequence with thinned oceanic
crust and less dense underplated material in the upper mantle. The dashed line indicates a change in velocity gradient (Trummer, 2002).
80
The crustal features of the seven west-east oriented TCFAGA profiles in Figure 4.13
were inversely modeled using GM-SYS to estimate the angle of inclination along the
Pacific subduction zone. The densities used were 2.85 gm/cm3 for continental crust,
2.93 gm/cm3 for oceanic crust and 3.3 gm/cm3 for the mantle. Subducting slabs with
inclinations between 10o and 12o in profiles L1-L1’, L2-L2’, and L4-L4’ reflect relatively
thinner subducting oceanic crust. The subducting slabs with an inclination < 10o (“flat
slabs”) in profiles L3-L3’, L5-L5’, L6-L6’ and L7-L7’ are related to a relatively thicker
subducting crust associated with the subduction of the Carnegie and Malpelo Ridges
underneath the North Andes Microplate.
4.6
Crustal discontinuities of the North Andes Microplate
Gravity anomalies that require offsets or discontinuities may reflect the effects of
tectonic lineaments that truncate the anomaly patterns. To interpret the TCFAGA
gradient trends for crustal discontinuities, the digitizing tools from Oasis Montaj were
used to infer major and local lineaments.
TCFAGA gradient trends in Figures 4.14 and 4.15 show the interpreted crustal
discontinuities. The results clearly delineate the plate boundaries between the CaribbeanNorth Andes, South American-North Andes and Caribbean-North American Plates.
However, the boundaries between the Caribbean-Costa Rica-Panama microplates are
not well defined. Plate boundaries at subduction zones tend to be better defined than
continent-continent collisional plate boundaries. The results locate the crustal discontinuities between the Eastern and Central Andean ranges and the crustal discontinuities
limiting the Sierra Nevada of Santa Marta, delimiting various tectonic blocks in the
North Andes Microplate.
Figure 4.16 gives further crustal details from the inversion of two TCFAGA profiles oriented perpendicular to the North Andes Mountains. The TCFAGA inversions
81
Figure 4.13: Crustal profiles showing the different angles of subduction along the
Nazca-North Andes plate boundary zone that are required to satisfy the TCFAGA.
The TCFAGA map is also given in F igure 4.14. The TCFAGA map and crustal profiles were produced in Cartesian coordinates with origin (1’000.000 N, 1’000.000E) at
the National Observatory of Bogota.
82
Figure 4.14: TCFAGA of northwestern South America at 20 km altitude and interpreted plate boundaries and intracrustal discontinuities. This map was produced in
Cartesian coordinates with the origin (1’000.000 N, 1’000.000 E) at the National Observatory of Bogota.
83
Figure 4.15: Plate boundaries and intracrustal discontinuities of northwestern South
America interpreted from TCFAGA at 20 km altitude. This map was produced in
Cartesian coordinates with the origin (1’000.000 N, 1’000.000 E) at the National Observatory of Bogota.
84
were obtained in Cartesian coordinates using GM-SYS capabilities. Profile A-A’ shows
how the continental crust varies from west to east. The MOHO estimates show two
main continental roots below the western-central and eastern ranges reaching depths
of 36 km and 47 km, respectively. The subducting oceanic slab with an initial inclination of 10o becomes steeper (15o to 20o ) at 30 km depth. Profile B-B’ shows a
single continental root below the western-central ranges reaching a depth of 47 km.
The subducting oceanic slab is dipping 14o E.
Figures 4.17 and 4.18 show the surface free-air gravity anomalies and interpreted
crustal discontinuities, respectively, in Cartesian coordinates. These maps were created
using data reduction and processing procedures described in the Gravity National Network Catalog (IGAG, 1998) and classical geophysical textbooks (Telford, et al., 1994;
Keary Brooks, 1984). The randomly distributed point data data were interpolated at
a 5 km x 5km grid using Geosoft Oasis Montaj. FAGA varies from -190 mGals to 131
mGals, with a mean zero value and standard deviation of 32 mGals. Crustal discontinuities inferred from FAGA at 20 km in Figures 4.14 and 4.15 are further detailed
in surface FAGA including minor offsets. Additional crustal discontinuities that were
not readily detected at 20 km altitude are suggested in the surface FAGA to limit the
regional tectonic blocks.
These new crustal discontinuities are associated with shallower structures, showing a regional trend oriented N20o − 30o E with a secondary trend oriented N 70o W
that delimits elongated tectonic blocks of varying sizes. Major crustal discontinuities
coincide with the limits of the tectonic realms proposed by Cediel et al. (2003) in
Figure 4.19. The crustal discontinuities interpreted from surface gravity anomalies
reveal a complex pattern of rectangular tectonic blocks for the area.
The complete Bouguer anomalies at the surface in Figures 4.19 and 4.20 vary
from -182 mGals to 249 mGals. Positive Bouguer anomalies are associated with the
85
Figure 4.16: Profiles (W-E) of the northern Andes Microplate showing the subducting oceanic slab under the continental crust as interpreted from TCFAGA at 20 km
altitude. (a) Profile A-A: Two main continental roots were modeled below the westcentral and eastern ranges, respectively. Smooth MOHO variations are observed below
the Guiana Craton. (b) Profile B - B: A single continental root was modeled below
the west-central ranges. No clear continental root below the incipient eastern range is
evident and the MOHO below the Guiana Craton is relatively flat.
86
Figure 4.17: Surface FAGA and interpreted crustal discontinuities. This map was produced in Cartesian coordinates with the origin (1’000.000 N, 1’000.000 E) at the National Observatory of Bogota.
87
Figure 4.18: Plate boundaries and intracrustal discontinuities interpreted from surface
FAGA. This map was produced in Cartesian coordinates with the origin (1’000.000 N,
1’000.000 E) at the National Observatory of Bogota.
88
Figure 4.19: The tectonic realms and blocks of northwestern South America interpreted
from geological and geophysical data (Cediel et al., 2003).
89
oceanic Caribbean and Nazca Plates. Intermediate positive anomalies are located at
the Guiana Craton and Sierra Nevada of Santa Marta and remarkable negative Bouguer
anomalies are found along the Andes Mountains. Relative to FAGA analysis, only a
few of the discontinuities can be outlined in the Bouguer anomaly map.
4.7
Discussion
The Degree of isostatic imbalance is directly related to the magnitude of FAGA.
Removing TCFAGA from FAGA reduces the effect of isostatic disturbances in the
TGE. Regional crustal thickness variations are supported by the gravity and seismic
MOHO estimates. The gravity MOHO estimates show higher resolution than the seismic MOHO estimates. Gravity components like CTGE, TGE and FAGA provide relatively continuous coverage for producing a 0.5o x 0.5o grid, while the seismic MOHO
estimates were compiled from a 2o x 2o grid from the interpolation of sparse point
depth solutions.
According to Watts (2001), local mass loads with wavelength less than 100 km
(∼
= 1o in the spherical coordinates of the study area) are supported by the rigidity
of the crust. Thus, our results which are based on a 0.5o array has limited resolution;
therefore, the contribution of local body gravity effects in the free-air gravity anomalies
is minimal.
The gravity MOHO is shallower than the seismic MOHO of the continent and
deeper than the seismic MOHO of the oceans. At the continent, seismic Moho estimates are systematically shallower (0 -12 km) than the gravity MOHO, suggesting
that the mean density value assumed for the inverse modeling was higher, requiring
less thickenning of the continental crust to match TCFAGA. At the oceans, seismic
MOHO estimates are systematically shallower 0 to 14 km systematically shallower
than the gravity MOHO estimates.
90
Figure 4.20: Surface complete Bouguer anomalies and interpreted plate boundaries
and intracrustal discontinuities. This map was produced in Cartesian coordinates with
the origin (1’000.000 N, 1’000.000 E) at the National Observatory of Bogota.
91
Figure 4.21: Plate boundaries and intracrustal discontinuities interpreted from surface
Bouguer anomalies. This map was produced in Cartesian coordinates referred to an
origin (1’000.000 N, 1’000.000 E) at the National Observatory of Bogota.
92
The advantage of using gravity data at 20 km altitude relies on enhancing the
effects of any regional disturbances of the terrain that may be related to the crustmantle interface. Regional profiles of TCFAGA and gravity MOHO are useful for
mapping the tectonic features of the oceanic and continental crust.
4.8
Conclusions
The tectonic analysis of northwestern South America shows a complex tectonic
setting with regional crustal discontinuities that can be interpreted from satellite and
surface gravity anomalies, and local discontinuities that can be only interpreted from
the surface gravity anomalies.
The thickness of the continental crust varies from 35 km to 55 km and shows that
the mountain ranges are partially compensated by continental roots. Thicker continental crust provides a better scenario for magmatic segregation of parental magmas
generated at converging continental margins. Those magmas can be contaminated by
the crust, forming mineralization of economic interest.
Major deposits of base and precious metals of the Andean Mountains are related to
intermediate to felsic intrusions. Porphyry copper-molybdenum deposits coincide with
Mesozoic - Cenozoic orogenic belts and calc-alkaline volcanism (Guilbert and Park,
1997). The crustal thickness variations yield important constraints on the process of
magma generation and the formation of ore deposits. Regions with crustal thickness
between 50 km and 60 km are associated with the batholithic intrusions of economic
interest in the Central Andes. In the North Andes Microplate, crustal thicknesses
greater that about 45 km were identified as promising zones for locating batholiths of
economic interest.
In any destructive plate margin environment (oceanic or continental) the nature
and distribution of magmatic activity in the overriding plate is directly linked to
93
the geometry of the subducted slab (Wilson, 1989). The volumetric proportions of
erupted rock types and geochemical characteristics likely are strongly correlated with
the thickness and chemical characteristics of the crust through which the rising magmas
travel.
Comparison of Figures 4.18 and 4.20 show that crustal discontinuities are more
comprehensively defined in TCFAGA anomalies than in the Bouguer anomalies. This
can be explained by the fact that some crustal discontinuities also have topographic
expression and thus they will be expressed in the TGE and TCFAGA.
The pacific margin of the North Andes microplate shows varying angles of subduction which may control the shortening and the formation of the fold and thrust belt
of the North Andes and its related magmatism.
The Caribbean margin of the North Andes microplate shows an incipient subduction of the Caribbean plate under the North Andes microplate.
94
CHAPTER 5
CRUSTAL MODELING FROM MAGNETIC DATA
95
ABSTRACT
A new crustal magnetic anomaly map for northwestern South America was produced from CHAMP observations using enhanced reduction procedures. Magnetic field
effects due to crustal thickness variations were separated from magnetic components of
the core field. Enhanced removal of external field variations and other non-lithospheric
noise resulted from modeling ionospheric effects, spectral correlation filtering of neighboring tracks, and constructing a spectrum with improved signal-to-noise properties
from the dusk and dawn maps.
The crustal magnetic anomaly map shows enhanced resolution of tectonic anomaly
features to better constrain interpretations of the crustal geology. Additional constraints on the geological interpretation of magnetic data were developed by comparing them to corresponding first vertical derivative of gravity anomalies using spectral
correlation theory.
Positive magnetic anomalies and negative TCFAGA anomalies characterize the
oceanic trenches at subduction zones. Positive TCFAGA and negative magnetic anomalies over the Andes Mountains may reflect thermally demagnetized crust. Negative
magnetic and negative gravity anomalies tend to reflect thickened oceanic crust associated with non-magnetic sedimentary basins and geothermal variations.
96
5.1
Introduction
Terrestrial, marine, airborne and satellite CHAMP magnetic anomaly maps were
analyzed and compared with the first vertical derivative gravity anomalies and proposed plate boundary zones for northwestern South America. Accurate representation
of the crustal magnetic map was necessary to better understand the crustal structures associated with the anomaly features. Satellite-derived magnetic anomalies (e.g.,
POGO, MAGSAT, CHAMP) for South America have shown good correlation with tectonic features (e.g., Longacre, 1981, Maus et al., 2002, Maus et al., 2003 Maus et al.,
2004, Lesur and Maus, 2005).
Regional variations of lithospheric magnetization are emphasized in the satellite
magnetic anomalies. These variations can be interpreted for changes in crustal lithology, thermal properties, and thicknesses that result from the tectonic evolution of the
lithosphere (von Frese et al., 1989). Therefore, analysis of satellite magnetic data can
provide unique and important information on geology and tectonics of the lithosphere.
Four global crustal field models have been derived (MF1, MF2, MF3 and MF4)
from the first four years of CHAMP data (Lesur and Maus, 2005). For each model,
data were expanded to spherical harmonic degree and order 90. To estimate the crustal
anomalies, the total field measurements were corrected for an internal field model to
degree 15, an external field model to degree two, and the predicted magnetic field
signatures for the eight main ocean tidal constituents. The remaining external field
components were evaluated and removed on a track-by-track basis.
The CHAMP models were estimated from a subset of “magnetically quiet” data
that were observed when the external field activities were minimum. The resulting
MF models provided a good representation of the crustal field down to an altitude of
about 50 km at mid latitudes, but with reduced accuracy in the polar regions. Crustal
97
features came out significantly sharper in the model MF3 (Maus, et al., 2004) than
in previous models MF1 (Maus et al., 2002) and MF2 (Maus et al., 2003). Bands of
magnetic anomalies along the subduction zones became visible for the first time in the
satellite observations.
The model MF4 (Lesur and Maus, 2005) was produced for representing the crustal
magnetic field. That model was derived from almost 5 years of CHAMP measurements. The model was given as a spherical harmonic expansion of the scalar magnetic
potential to degree 90. Coefficients 1 − 15 were set to zero, since these coefficients were
taken to represent mostly the main magnetic field with its source in the Earth’s outer
core. Relative to the model MF3, the model MF4 involved better calibrated CHAMP
vector data, improved corrections for polar electro-jets and better elimination of data
disturbed by ionospheric F-Region currents.
The above studies have provided mixed results regarding the degree that should
be used to account for the core field. If the cut-off is too low, residual core field
components might be left in the lithospheric anomaly field. Although, the remaining
core field components may be small compared to those of the total core field, they are
large relative to the crustal magnetic field components. On the other hand, if the cut-off
is too high, substantial crustal components may be removed as core field components.
Clearly, the extraction of core field components represents a significant source of error
in estimating lithospheric anomalies in regional magnetic observations.
In this study, an alternate approach for effectively separating core and crustal field
components in the satellite magnetic data was applied using spectral correlation theory
(Tan, 1998; von Frese, et al., 1997a; von Frese et al., 1999). Spatially and temporally
static crustal magnetic anomalies are contaminated by static core effects above spherical harmonic degree 12 and the dynamic, large amplitude external fields. Crustal
thickness variations produce magnetic effects that spectrally overlap the core field
98
components (von Frese et al., 1999). Thus, we can use the correlation spectrum between the pseudo magnetic effects of the crustal thickness variations and the CHAMP
core field components to separate the possible crustal thickness and core field effects
in the CHAMP observations. Spectral correlation filters also can be used to separate
the static crustal and dynamic external field components in the CHAMP data.
5.2
Core Field and external field reductions
Figure 5.1 shows the intensity, inclination, and declination properties of the core
field for study area at 400 km, from the International Geomagnetic Reference Field
(IGRF-10) obtained from NOAA (2006). Core field intensities range from 21087 nT
to 36475 nT, with magnetic declinations between −15o to 6.3o and magnetic inclinations from 0.75o to 53.79o . External fields vary typically from ±200 nT whereas the
lithospheric anomalies are relatively weak, ranging commonly from ±20 nT. Hence,
ignoring instrumental noise, the satellite magnetometer observation includes the core
field component of roughly 98.5%, the external field component of about 0.6%, and
the lithospheric component of approximately 0.06%.
To obtain a crustal CHAMP magnetic anomaly map for the area (−8o S to 23.5o N,
−90o W to −58.5o W), an extended area (−10o S to 30o N, −100o W to −40o W) was
considered to minimize edge effects in data processing. Efforts focused on isolating the
external and core fields by wavenumber filtering correlation according to the procedures
of Kim et al. (2005).
These procedures basically involved removing the IGRF10 up to degree 13 to eliminate the core effects from each orbital data track. A 2nd − order polynomial was
removed from each orbital data profile to help account for the ionospheric field effects.
The passes were next sorted by local magnetic time into subsets of ascending and descending orbital datasets. The tracks were then correlation filtered against each other
99
Figure 5.1: Core magnetic intensity (color), inclination (white contours) and declination (black contours) for northwestern South America from IGRF-10 (NOAA, 2006).
SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges. This map
was produced using the Albers equal-area conic projection.
100
to extract the static crustal components. The correlation filtered tracks were gridded
next by least squares collocation at 400 km altitude and the two maps were correlation
filtered against each other to suppress external field noise further. The correlation filtered maps were then subjected to spectral reconstruction to suppress track-line noise.
The resulting CHAMP total magnetic field anomalies for the study region are shown
in Figure 5.2
5.3
Differential Reduction to the Pole (DRTP)
To minimize spatial distortions between satellite magnetic anomalies and their geological sources caused by core field variations, the CHAMP crustal anomaly estimates
in Figure 5.2 were differentially reduced to the pole (DRTP). Reducing the magnetic anomalies to vertical polarization enhances the ability to relate them to geologic
features and helps to reduce ambiguities in interpretation. The DRTP operation recalculates total magnetic intensity data as if the inducing magnetic field had a 90o
inclination.
This procedure transforms dipolar magnetic anomalies to monopolar anomalies
centered over their causative bodies which can simplify the interpretation of the data.
For this study, the reduction to the pole makes the simplifying assumption that the
rocks in the survey area are all magnetized parallel to the Earth’s magnetic field. This
is only true in the case of rocks with an induced magnetization so that remanent
magnetization will not be correctly dealt with if the direction of remanence is different
than the direction of the Earth’s magnetic field.
Lithospheric sources of satellite magnetic anomalies may have both inductive and
remanent components of magnetization. These sources may be predominantly in the
lower crust that is believed to be substantially more magnetized than the upper crust
101
Figure 5.2: CHAMP total magnetic field anomalies (nT) at 400 km altitude for northwestern South America. Map annotations include the amplitude range (AR) of (min;
max) values and amplitude mean (AM) and standard deviation (SD). SNSM = Sierra
Nevada of Santa Martha, W-Mid = Western-Central ranges. This map was produced
using the Albers equal-area conic projection.
102
(Wasilewski and Mayhew, 1992). As crustal depth increases conditions for coherent regional magnetization are enhanced. Remanent and thermal overprints are diminished
and viscous magnetization and initial susceptibility are enhanced with increasing temperature, especially within about 100o C - 150o C of the Curie point. The thickness
of the crust affected by this thermal regime may be 5 km to 20 km depending on
the steepness of the geothermal gradient. Hence, deep magnetic sources are related to
lateral variations of petrological factors or Curie isotherm topography. Viscous remanent magnetization of the lower crust is in-phase with the induced component. Thus,
reducing the regional satellite magnetic anomalies from deep crustal sources to vertical
polarization is warranted.
Remanently magnetized sources within the magnetically weaker upper crust produce high frequency signals which are substantially attenuated at satellite altitudes.
Hence, the errors in the DRTP estimates due to the upper crustal magnetic remanence
is relatively small. In basins containing non-magnetic sediments, remanence is also not
a problem. Close to the magnetic equator (≤ 10o declination), the DRTP procedures
can become unstable. Errors in the DRTP transform usually appear as narrow anomalies elongated parallel to the declination of the Earth’s magnetic field (Telford, et al.,
1994; Merrill et al., 1996), but appropriate stabilizing procedures can be implemented
to mitigate the effects (e.g., von Frese et al., 1988). Taking into account these considerations the CHAMP magnetic anomalies in Figure 5.2 were differentially reduced to
the pole as shown in Figure 5.3.
The DRTP magnetic anomalies were obtained by the inversion of the total field
magnetic anomalies in Figure 5.2 for the magnetic susceptibilities on an equivalent
source array of prisms making up the crust (von Frese et al., 1981; Kim et al., 2005).
Figure 5.4 gives the crustal susceptibilities that when evaluated under the inducing
103
Figure 5.3: Differentially reduce-to-pole CHAMP magnetic anomalies DRTP(TFMA)
at 400 km for Northwestern South America. Map annotations include the amplitude
range (AR) of (min; max) values and amplitude mean (AM) and standard deviation
(SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection.
104
field conditions of vertical inclination, zero declination and the constant intensity of
45000 nT yield the DRTP anomalies in Figure 5.3.
5.4
First vertical derivatives of FAGA (FVD(FAGA))
By Poisson’s theorem, the DRTP anomaly of a source that also represents a variation in density has the same spatial properties as the source’s first vertical derivative gravity anomaly (e.g., von Frese et al., 1982). Thus, the DRTP magnetic anomalies can be correlated against the first vertical derivative free-air gravity anomalies
(FVD(FAGA)) for additional insights on the crustal properties of the study region.
For this study, the FVD(FAGA) were computed numerically from the EGM96
coefficients evaluated at 390 km and 410 km by
F V D(F AGA)400km =
(F AGA390km − F AGA410km )
20 km
(5.1)
as shown in Figure 5.5. The polarity convention in Eq. 5.1 makes the sign of the
FVD(FAGA) estimate the same as the sign of the density contrast of the underlying
source’s. For comparison, Figure 5.6 shows the FAGA at 400 km altitude.
5.5
Spectral correlations of CHAMP DRTP (TMFA) and FVD
(FAGA) estimates
Spectral correlation analysis is applied to isolate gravity and magnetic anomaly
correlations related to crustal thickness, petrological, and thermal variations. Gravity
anomalies tend to be negative over thickened crust and positive over thinned crust,
whereas magnetic anomalies tend to be inversely correlated with these gravity anomalies. Increased heat flow in the crust also can produce inversely correlated geopotential
105
Figure 5.4: Crustal magnetic susceptibility values for northwestern South America
calculated by linear inversion of DRTP CHAMP magnetic anomalies. Map annotations
include the amplitude range (AR) of (min; max) values and amplitude mean (AM)
and standard deviation (SD). SNSM = Sierra Nevada of Santa Martha, W-Mid =
Western-Central ranges. This map was produced using the Albers equal-area conic
projection.
106
Figure 5.5: First vertical derivative (FVD) of the free-air gravity anomalies (FAGA)
at 400 km for northwestern South America. Map annotations include the amplitude
range (AR) of (min; max) values and amplitude mean (AM) and standard deviation
(SD). SNSM = Sierra Nevada of Santa Martha, W-Mid = Western-Central ranges.
This map was produced using the Albers equal-area conic projection.
107
Figure 5.6: Free-air gravity anomalies (FAGA) at 400 km elevation for northwestern
South America. Map annotations include the amplitude range (AR) of (min; max)
values and amplitude mean (AM) and standard deviation (SD). SNSM = Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection.
108
CC
FAGA)
FVD(FAGA)
TFMA
DRTP
FAGA
1
0.9101
-0.1264
0.361
FVD(FAGA)
09101
1
-0.00979
0.216
TFMA
-0.1264
-0.0979
1
-0.0453
DRTP
0.1361
0.216
-0.0453
1
Table 5.1: Correlation coefficients (CC) between the FAGA in Figure 5.6, FVI(FAGA)
in Figure 5.5, CHAMP total field magnetic anomalies (TFMA) in Figure 5.2 and
the CHAMP DRTP TFMA in Figure 5.3 at 400 km altitude for northwestern South
America from −8o S to 23.5o N latitude and from −90o W to −58.5o W longitude.
anomalies, that involve magnetic lows where the Curie isotherm is raised and gravity
highs from the inflated crust. Petrological variations may result in positively correlated
anomalies that broadly reflect the distribution of low-density and -magnetization felsic
rocks or high-density and -magnetization mafic rocks.
Local favorability indices (LFI) were obtained between the CHAMP DRTP (TFMA)
and FVD (FAGA). Figure 5.7 gives the SLFIs that highlight the peak-to-peak anomaly
correlations, whereas the trough-to-trough associations are presented in Figure 5.8.
The DFLIs highlighting the FVD(FAGA) peak-to- DRTP (TFMA) trough and FVD(FAGA)
trough-to-DRTP (TFMA) peak correlations are given in Figures 5.9 and 5.10, respectively. The geological implications of these LFIs are considered in section 5.7. The
correlation coefficients between the various regional gravity and magnetic data sets of
this study area are given in Table 5.1.
5.6
Aeromagnetic anomalies of the North Andes Microplate
To facilitate the geological interpretation of satellite geopotential field anomalies,
regional aeromagnetic data were compiled from INGEOMINAS (1998). Figure 5.11
109
Figure 5.7: Peak-to-peak correlations between FVD(FAGA) and CHAMP DRTP
(TFMA) at 400 km altitude for northwestern South America. SNSM = Sierra Nevada
of Santa Martha, W-Mid = Western-Central ranges. This map was produced using the
Albers equal-area conic projection.
110
Figure 5.8: Trough-to-trough FVD(FAGA) and CHAMP DRTP (TFMA) at 400 km
altitude for northwestern South America. SNSM = Sierra Nevada of Santa Martha,
W-Mid = Western-Central ranges. This map was produced using the Albers equal-area
conic projection.
111
Figure 5.9: FVD(FAGA) peak-to- CHAMP DRTP (TFMA) trough correlations at 400
km altitude for northwestern South America. SNSM = Sierra Nevada of Santa Martha,
W-Mid = Western-Central ranges. This map was produced using the Albers equal-area
conic projection.
112
Figure 5.10: FVD(FAGA) trough-to-CHAMP DRTP (TFMA) peak correlations at 400
km altitude for northwestern South America. SNSM = Sierra Nevada of Santa Martha,
W-Mid = Western-Central ranges. This map was produced using the Albers equal-area
conic projection.
113
shows these anomalies continued upward to 20 km for comparison with the EGM96
gravity anomalies of Figure 3.2.
Total magnetic field anomalies vary from −123.6 nT to 110.8 nT, with a mean value
of 3.86 nT and standard deviation of 26.27 nT. Although magnetic data coverage is
poor in the Andes Mountains, the upward continued anomalies provide a geologically
useful regional view of the main magnetic zones. Positive magnetic anomalies are associated with the Baudo volcanic arc, and the Sierra Nevada of Santa Martha, whereas
linear trends of magnetic highs and lows characterize the Guiana Craton. Negative total magnetic field anomalies characterize the Andes Mountains and local sedimentary
basins. Only a few discontinuities seem to be outlined due to the low resolution of the
data.
Negative magnetic anomalies along the Colombian Trench and Lesser Antilles reflect the Caribbean-North Andes plate boundary zone. The Pacific subduction along
the Colombian trench does not appear to be well defined magnetically.
5.7
Discussion
The CHAMP magnetic observations were processed for crustal anomalies by removing possible contaminating core and external field components. Deep crustal magnetic
sources were revealed that are probably related to the MOHO and Curie isotherm
topography and lateral petrological variations.
In general, crustal sources of satellite magnetic anomalies can have both inductive
and remanent magnetizations. These sources are predominantly in the lower crust that
is believed to be more magnetic than the upper crust and mantle (Wasilewski et al.,
1979; Wasilewski and Mayhew, 1982, 1992; Mayhew et al., 1985).
In Chapter 4, the crustal thickness estimates for the Andean Mountains revealed
the need for additional continental roots between 5 km to 10 km on average to reach
114
Figure 5.11: Total magnetic field anomalies at 20 km altitude for the North Andes
Block with interpreted crustal discontinuities superposed (blue lines). This map was
produced in Cartesian coordinates with the origin (1’000.000 N, 1’000.000 E) at the
National Observatory of Bogota.
115
complete compensation. This “missed” root would have produced a strong positive
magnetic anomaly from the lower crustal magnetic rocks. Therefore, a partial isostatic
compensation also implies a partial magneto-isostatic compensation, where the magnetic field associated with the isostatically disturbed crust has lower or higher intensity
depending on whether the crust is over or under compensated by the mantle.
Positive DRTP magnetic anomalies in Figure 5.3 suggest enhanced magnetization
for the crust along Central America, North of Cuba and Española, The Vichada Plain
and the Andes Mountains. Negative DRTP magnetic anomalies imply decreased magnetization for the crust in the Caribbean plate, northern Venezuela, Nazca and Cocos
Plates and the Amazon River Aulacogen.
The positive FVD(FAGA) estimates in Figure 5.5 suggest higher density crust in
the Andean Mountains and Central America. Intensely negative FVD(FAGA) imply
lower density crust along the Lesser and Greater Antilles related presumably to the
effects of the deep trench and subducting crustal slab displacing high density mantle.
The peak-to-peak correlations between FVD(FAGA) and CHAMP DRTP (TFMA)
in Figure 5.7 reflect higher density and magnetization crust along Central America
and the Andean Mountains. These areas show positive magnetic anomalies associated
with a thicker continental crust that coincide with the positive FVD(FAGA) of the
isostatically disturbed mountain ranges.
The trough-to-trough correlations in Figure 5.8 imply lower density and magnetization crust along the Lesser Antilles, the Caribbean-North American plate boundary
and portions of the Ecuadorian Andes and the Guiana Craton. The lower density and
magnetization can result from various sources such as sediment fill, the depth of the
oceanic trench in the Caribbean, and the possible presence of granitic intrusions with
lower density and magnetization than the mafic host rocks.
116
The FVD(FAGA) peak-to DRTP (TFMA) trough correlations in Figure 5.9 reflect higher density, lower magnetization crust in the Central Andes of Colombia and
eastern Ecuador, Northern Venezuela and around the Cayman trough. Sources for
these correlations include enhanced crustal heat flow and anomalously thinned crust.
The FVD(FAGA) trough-to- DRTP (TFMA) peak correlations in Figure 5.10
suggest lower density higher magnetization crust along the Lesser and Greater Antilles
(north of Cuba) and the Middle American Trench related to the subducting crustal
slab displacing higher density and lower (non magnetic) mantle. Features south of the
Carnegie ridge reflect lower density, higher magnetization, thickened oceanic crust.
The features at the Guiana Shield reflect lower density, higher magnetization sources
associated with sedimentary basins or a tectonic graben.
The aeromagnetic data in Figure 5.11 show an arcuate positive anomaly along
the western margin of the North Andes block associated with the presence of mafic
rocks of oceanic affinity. These results also show a highly magnetizated body in the
eastern cordillera and the Guiana craton.
5.8
Conclusions
The CHAMP magnetic observations were processed for effective crustal components. The processing involved spectral correlation to suppress external magnetic magnetic field effects, and to separate the overlapping magnetic effects of the core and
crustal thickness variations using the pseudo magnetic effects of the crustal thickness
variations derived from the free-air and terrain gravity effects.
The CHAMP DRTP (TFMA) anomalies generally reflect the regional crustal features of Central America, the northwestern Andes and the Caribbean. Prominent magnetic minima overlie the thinned crust of marine basins. Low magnetic anomalies of
the northern Andes Mountains may be related to demagnetizing effects of the high
117
thermal gradients of the volcanic regions, as well as the non-magnetic sedimentary
rocks of the Eastern Cordillera. Negative magnetic anomalies of the Guiana Craton
at the Amazon River Aulacogen are difficult to relate to known features of the crust.
However, they might reflect crustal thinning, enhanced heat flow, and the presence of
non-magnetic sedimentary basins.
Magnetic maxima that overlie the Central American continental crust, the Andean
Mountains at the latitude of the Carnegie Ridge and the Guiana Craton between the
Vichada Plain and Amazon River Aulacogen appear to reflect mostly enhanced crustal
thickness effects. The positive magnetic anomalies immediately northeast of Cuba and
The Española Islands also show relative thickening of the crust. The interpretation of
these magnetic anomalies are mostly consistent with their correlative satellite altitude
free-air gravity anomalies.
The aeromagnetic data of the North Andes provide limited coverage for constraining geological interpretations of the CHAMP crustal magnetic anomalies. However,
they show a striking arc-shaped positive anomaly in the western margin of the North
Andes block that may represent the dismembered remnants of an oceanic plateau
which formed the eastern Pacific Ocean at 97-90 Ma (Kerr et al., 1997). Shortly after
the plateau formed, its northern half was pushed between North and South America
to form most of the Caribbean Plate.
As the southern part of the plateau approached the continental margin of northwestern South America, it was too buoyant to totally subduct and so its topmost parts
accreted onto the continental margin (Kerr et al.,1997).
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CHAPTER 6
CRUSTAL THICKNESS VARIATIONS AND SEISMICITY
119
ABSTRACT
Isostasy is the ideal state that any uncompensated mass seeks to achieve in time
(Turcotte and Schubert, 2002). Thus, any uncompensated mass of the northern Andes
Mountains is presumably under pressure to adjust within the Earth to its ideal state of
isostatic equilibrium. These pressures interact with the relative motions between adjacent plates that give rise to earthquakes along the plate boundaries. By combining the
gravity MOHO estimates and crustal discontinuities with historical and instrumental
seismological catalogs it is possible to establish the correlation between isostatically
disturbed terrains and seismicity.
The thinner and thicker crustal regions were mapped from the zero horizontal
curvature of the crustal thickness estimates. These boundaries or edges of crustal
thickness variations were compared to crustal discontinuities inferred from gravity and
magnetic anomalies and the patterns of seismicity that have been catalogued for the
last 363 years.
The seismicity is very intense along the Nazca-North Andes, Caribbean-North
American and North Andes-South American collision zones and associated with regional tectonic compressional stresses that is locally increased and/or diminished by
compressional and tensional stress, respectively, due to crustal thickness variations.
High seismicity is also associated with the Nazca-Cocos diverging plate boundary
whereas low seismicity is associated with the Panama-Nazca Transform Fault and
the South American Plate.
120
6.1
Introduction
The increased number of seismological networks established during the last century led to the discovery that earthquakes are not randomly distributed, but tend to
occur along well defined earthquake belts (e.g., Shearer, 1999). These belts are largely
concentrated along the margins of tectonic plates that shift slowly over geologic time.
The relative motions between adjacent plates give rise to earthquakes along the plate
boundaries that include spreading oceanic ridges, converging subduction zones, collisional continental plate boundaries, and transform faults along which they shear past
each other. For northwestern South America, the improvement in instrumentation and
expansion of seismological networks has led to the production of relatively complete
and accurate catalogs of earthquake locations and ground motions.
The response of ground during an earthquake is commonly attributed to elastic
rebound theory whereby the low accumulation of shear stress at a point along a fault
builds up until the elastic strength of the rock is exceeded and it fractures releasing
energy to produce the earthquake (Reid, 1906 in Shearer, 1999). Crustal thickness variations also can contribute compressional stress where the crust is thinner than normal
and tensional stress where it is thicker than normal (Artyushkov, 1973). Therefore,
the crustal thickness estimates derived from gravity anomalies can be included in the
analysis of earthquake behavior. For instance, areas where TCFAGA are excessively
negative or positive are more prone to seismic activity than areas where the TCFAGA
are closer to zero (Song and Simons, 2003).
This chapter analyzes and compares seismic data from the Advanced National Seismic System (ANSS) and the Red Sismologica Nacional de Colombia (RSNC) catalogs
121
with crustal thickness estimates from gravity anomalies from improved understanding of the large-scale dynamic models of earthquakes and tectonics. Gravity anomalies along trenches, continental converging margins, crustal discontinuities, mountain
ranges and cratonic areas are compared with their seismic signatures from earthquake
data collected over the last 363 years.
6.2
Zero Curvature of crustal thickness variation
The terrain-correlated free-air anomalies (TCFAGA) in Figure 3.4 mark regions of
isostatically disturbed crustal features. For instance, the zero anomalies mark areas of
crustal equilibrium so that the positive and negative anomalies reflect crust in compression and tension, respectively. In other words, positive and negative TCFAGA can mark
crust that is isostatically too thin (under-compensated) or thick (over-compensated),
respectively, and hence under pressure to equilibrate by the compressive in-flow or
expansive out-flow of crustal material (e.g., Artyushkov, 1973). Thus these anomalies can reflect lithosphere in subsidence or uplift (e.g., Kim et al., 2000; von Frese
et al., 1999a), or alternatively dynamic surface topography that is too high or too low,
respectively, to be in isostatic equilibrium.
Figure 6.1 gives the crustal thickness variations for northwestern South America obtained by adding the gravity MOHO estimates in Figure 4.2 to the DTM of
Figure 3.1.
The zero curvature contour in Figure 6.1 that estimates the edges of thickness
variations was obtained using the tools in the Geosoft Oasis Montaj package (Appendix
B; Geosoft, 2006). The intervening yellow and blue regions of Figure 6.1 reflect the
thicker and thinner crustal components, respectively.
122
6.3
Seismic data compilation
For comparison with the crustal thickness estimates, the regional seismic data of
northwestern South America were compiled from the Advanced National Seismic System catalog (ANSS; USGS, 2006). A more local study was also considered using the
historic and instrumental seismic catalogs of the Red Sismologica Nacional of Colombia (RSNC; INGEOMINAS, 2006).
6.3.1
ANSS Catalog
The ANSS catalog is a world-wide earthquake catalog created by merging the
master earthquake catalogs from contributing ANSS institutions and then removing
duplicate solutions (USGS, 2006). The ANSS catalog currently consists of earthquake
hypocenters, origin times, and magnitudes. The ANSS database was searched for data
from 1966 to 2006 with Ritcher magnitudes M of 3-10.
The earthquake epicenters were converted from geographic to Cartesian coordinates
in Figure 6.1 with the proportional symbols for Ritcher magnitudes given in the
legend. Only those events with hypocenters from 0 to 60 km depths were considered
in this study. The crustal discontinuities interpreted from TCFAGA in Figure 4.15
are also superposed in Figure 6.1.
The seismic events in Figure 6.1 are concentrated along the plate boundary zones
of the Nazca, Caribbean, North American and South American Plates, the North
Andes, Panama and Costa Rica Microplates, the Cocos-Nazca spreading system, and
the intraplate discontinuities. They are predominantly located in the thicker crustal
sections and along the inferred edges of the thickness variations. However, the thicker
123
Figure 6.1: ANSS seismic catalog for 1966-2006 for three categories of earthquake
magnitudes: M=3-4 (minor earthquakes), M= 4.1-5.5 (intermediate earthquakes), and
M ≥ 5.5 to 10 (major earthquakes). Crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also
displayed. Yellow and blue zones correspond to thicker and thinner crust, respectively. This and succeeding figures were created using the capabilities of Oasis Montaj
(Geosoft, 2006).
124
oceanic sections of the Nazca, Malpelo and Cocos Ridges in the Pacific and the Beata
Ridge in the Caribbean are aseismic.
Comparatively few earthquakes are concentrated in the thinner crustal regions
with relatively negative TCFAGA. The Guiana Craton and the interior of Nazca and
Caribbean oceanic Plates also lack significant seismic events and are seismically quiescent. To account for the intraplate earthquakes, gravity and magnetic anomalies that
infer crustal discontinuities are useful to analyze deformation in the plate interior .
6.3.2
RSNC historical and instrumental seismic catalog
The RSNC catalog was created by merging the master earthquake catalogs from the
Observatorio Sismologico del Sur Occidente (OSSO), Centro Regional de Sismologia
Para America del Sur (CERESIS), Instituto Geofisico de los Andes, Red Sismologica
Nacional and regional institutions of northwestern South America and the Caribbean
(INGEOMINAS, 2006). This catalog currently consists of earthquake epicenters, origin
times, and magnitudes. The RSNC database compiled data from 1643 to 1991 for
Ritcher magnitudes from M ≥ 3.0 to M = 10.
The RSNC catalog contains an enormous number of events. Thus, the events were
sorted by magnitude for the minor earthquakes (M = 3 to 4), moderate earthquakes
(M= 4.1 to 5.0), and strong earthquakes (M > 5.0) as shown in Figures 6.2, 6.3 and
6.4, respectively.
Minor and moderate seismic events in Figures 6.2 and 6.3, respectively, are concentrated along the Pacific subduction zone and intracrustal discontinuities of the
North Andes Microplate. The concentration of events in the northern part of the eastern cordillera is known as the “Bucaramanga Nest.” Few events are recorded for the
Sierra Nevada of Santa Martha, Guiana Craton, and the northwestern flat lands of
Colombia north of the Andes Mountains. At the continent, areas with thicker crust
125
Figure 6.2: RSNC historical seismic catalog for 1643-1991 of earthquakes with magnitudes from M=3.0 to 4.0. Crustal discontinuities interpreted from gravity anomalies
and the zero curvature contours of crustal thickness variations are also displayed. The
yellow and blue zones correspond to thicker and thinner crust, respectively.
126
Figure 6.3: RSNC historical and instrumental seismic catalog for 1643-1991 of earthquakes with magnitudes from M=4.1 to 5.0. Crustal discontinuities interpreted from
gravity anomalies and the zero curvature contours of crustal thickness variations are
also displayed. The yellow and blue zones correspond to thicker and thinner crust,
respectively.
127
Figure 6.4: RSNC historical seismic catalog for 1643-1991 of earthquakes with magnitudes M > 5.0. Crustal discontinuities interpreted from gravity anomalies and the
zero curvature contours of crustal thickness variations are also display. The yellow and
blue zones correspond to thicker and thinner crust, respectively.
128
are more seismically active, while areas of thinner crust are less seismically active. An
exception is the Sierra Nevada of Santa Martha with a thick crust and low seismic
activity.
Major earthquakes in Figure 6.4 are located along the converging Nazca-North
Andes and South America-North Andes continental boundaries. The major earthquakes are located further landward from the Pacific subduction zone relative to the
minor and intermediate earthquakes that are concentrated closer to the coastline. The
few seismic events of the Guiana Craton are mostly localized along local crustal discontinuities and are not associated with crustal thickness variations.
6.3.3
Recent seismicity in the RSNC catalog
The RSNC started operating since 1992 and has amassed accurate instrumental information that consists of earthquake epicenters, hypocenters, origin times, and
magnitudes. The epicenters were sorted by magnitude for minor events (M= 3 to
4) in Figure 6.5, moderate events (M= 4.1 to 5) in Figure 6.6, and major events
(M > 5.0) in Figure 6.7.
The minor and moderate earthquakes in Figures 6.5 and 6.6, respectively, define
seismic corridors oriented along the major crustal discontinuities of the North Andes
Microplate. The Sierra Nevada of Santa Marta also shows more intensive seismic activity than previously recorded by the older data. Major earthquakes in Figure 6.7
are located along the major crustal discontinuities and plate boundary zones of the
North Andes Microplate. In general, seismic events are associated within thicker crust,
and therefore can be associated with tensional stress. (Artyushkov, 1973).
Hypocenter depths from the modern RSNC were sorted into shallow earthquakes
(from 0 to 10 km) in Figure 6.8, intermediate earthquakes (10 km to 30 km) in
129
Figure 6.5: RSNC modern seismic catalog for 1992-2006 of earthquakes with magnitudes from M=3.0 to 4.0. Crustal discontinuities interpreted from gravity anomalies
and the zero curvature contours of crustal thickness variations are also displayed. The
yellow and blue zones correspond to thicker and thinner crust, respectively.
130
Figure 6.6: RSNC modern seismic catalog for 1992-2006 of earthquakes with magnitudes from M=4.1 to 5.0. Crustal discontinuities interpreted from gravity anomalies
and the zero curvature contours of crustal thickness variations are also displayed. The
yellow and blue zones correspond to thicker and thinner crust, respectively.
131
Figure 6.7: RSNC modern seismic catalog for 1992-2006 of earthquakes with magnitudes M > 5.0. Crustal discontinuities interpreted from gravity anomalies and the zero
curvature contours of crustal thickness variations are also displayed. The yellow and
blue zones correspond to thicker and thinner crust, respectively.
132
Figure 6.9, deep earthquakes (30 km to 70 km) in Figure 6.10, and very deep earthquakes (70 km to 300 km) in Figure 6.11. The tendency of the deepening hypocenters
to migrate landward of the Pacific subduction zone is indicative of the oceanic plate
subducting under the Andes at an angle that is less than vertical.
6.4
Discussion
Gravity and topography are related to seismic activity by the frictional behavior of
plate boundary zones and crustal discontinuities. When two plates push against each
other, friction between the plates builds up causing the plates to lock. The continued
build up of tectonic stress also causes the plates to deform, creating variations in
topography and gravity. In addition to deforming the plates, the rocks of the sticking
point can break when the stress build up exceeds the breaking strength of the rocks.
The resulting strain released at the locking point produces the ground shaking of an
earthquake. For northwestern South America, plate tectonic stress plays out notably
in the ocean-continent, and ocean-ocean and continent-continent plate collision zones,
and also along oceanic spreading ridges and transform faults. In the next subsections,
these plate boundaries and associated earthquakes zones areas are analyzed further.
6.4.1
Ocean-continent collision zones
In Figures 6.1 to 6.11 the subduction of the Nazca plate under the North Andes
microplate is clearly parallel to the the Andes Mountains. The collision caused the
creation of the Andes Mountains by stacking thrust slices of crust. The lateral movement of these thrust slices in the welded continent has continued after the collision
generating minor, moderate and major earthquakes with shallow, intermediate and
deep hypocenter depths defining seismic corridors that are oriented along the the subduction zone and the Andes Mountains. The general pattern of seismicity and focal
133
Figure 6.8: RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 0 km to 10 km. Crustal discontinuities interpreted from gravity anomalies and thezero curvature contours of crustal thickness variations are also displayed.
The yellow and blue zones correspond to thicker and thinner crust, respectively.
134
Figure 6.9: RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 10 km to 30 km. Crustal discontinuities interpreted from gravity
anomalies and the zero curvature contours of crustal thickness variations are also displayed. The yellow and blue zones correspond to thicker and thinner crust, respectively.
135
Figure 6.10: RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 30 km to 70 km. Crustal discontinuities interpreted from gravity
anomalies and the zero curvature contours of crustal thickness variations are also displayed. The yellow and blue zones correspond to thicker and thinner crust, respectively.
136
Figure 6.11: RSNC modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths to the hypocenter from 70 km to 300 km. Crustal discontinuities interpreted
from gravity anomalies and the zero curvature contours of crustal thickness variations
are also displayed. The yellow and blue zones correspond to thicker and thinner crust,
respectively.
137
mechanism solutions of earthquakes are in accord with the easterly subduction of the
Nazca Plate beneath the Andes.
The intensive seismicity along the Middle American Trench (MAT) has a broad
pattern reflecting the low angle of inclination of the subducting Nazca and Cocos plates
under the Central American Continent. The seismicity along the Caribbean-North
Andes plate boundary is relatively low, with few minor (Figures 6.2, 6.5 and 6.8),
moderate (Figures 6.3, 6.6 and 6.7), and major (Figure.6, 6.7 and 6.10) historical
seismic events. This seismicity reflects the predominantly dextral lateral movement of
the Caribbean Plate with respect to the North Andes Microplate and South American
Plate that may include minor subduction that is incipient when compared with the
subduction of the Nazca Plate underneath the Andes Mountains.
6.4.2
Ocean-ocean collision zones
Figure 6.1 shows a broad arcuate seismic zone along the Lesser Antilles. A large
number of events occurs along the Caribbean-North American plate boundary, which
dips to the west away from the underthrusting oceanic plate. Shallow, intermediate and
deep focus earthquakes occur at progressively greater distances from the site of underthrusting. The topographic expression of the trench in Figure 3.1 is largely obscured
by thick sediment fill derived from the Orinoco River of Venezuela, but its presence
is indicated by the belt of negative TCFAGA in Figure 3.4. The seismic events are
located in thicker oceanic crust associated with the doubling of the collisional oceanic
crust. Focal mechanism solutions of the Lesser Antilles reflect regional compressional
stress. However, the relatively thicker oceanic crust reflects local tensional stress.
138
6.4.3
Continent-continent collision zones
The continent-continent collision boundary zone between the North Andes Microplate and the South American plate in Figures 6.1 to 6.11 is characterized by
intensive seismicity. Minor, moderate and major historical and recent seismic events
in Figures 6.2 to 6.7 have occurred along the Llanos Front Fault. Shallow events in
Figure 6.8 can be associated with tensional stress of the sub-Andean thrust belt or
fault zone, which is located on a relatively thicker continental crust that implies the
local presence of additional tensional stress.
6.4.4
Oceanic spreading ridges
Minor and moderate earthquakes occur along the Galapagos spreading ridge and
the Panama fracture zone in Figure 6.1. Focal mechanism solutions indicate regional
tensional events associated with plate accretion and strike-slip events where the ridges
are offset by transform faults. However, the Panama fracture zone is located on relatively thinner oceanic crust that may be locally associated with compressional stress.
6.4.5
Transform faults
The Southern Panama Fault Zone constitutes a conservative plate boundary between the Nazca Plate and Panama Microplate where the plates are in tangential
contact and are affected by limited or no subduction or accretion. Therefore, events
along this fracture are rare as shown in Figure 6.1.
6.4.6
Cratonic zone
The intraplate areas of the South American Plate are relatively aseismic. However,
a few minor, moderate and major historical and recent seismic events in Figures 6.2
to 6.7 have occurred at shallow, intermediate, deep and very deep hypocenter depths
139
in Figures 6.8 to 6.11. These events are located along some of the crustal discontinuities interpreted from the surface FAGA in Figure 4.18 and the total magnetic field
anomalies at 20 km altitude in Figure 5.11. Their occurrence suggest the presence
of continental sutures in the South American plate, with infrequently seismic activity. Although, these earthquakes are rare, they are important because they indicate
the nature of directions of stress within the South American Plate. The instrumental
seismic events in the above plate boundary zones are analyzed in the next section for
their focal mechanisms.
6.5
Focal mechanism solutions
Figure 6.12 gives the compressional stress regime for the study region from stress
inversions of the focal mechanisms of the instrumental earthquakes of northwestern
South America (Vargas et al., 2005), these solutions infer the type of faulting or focal
mechanisms from the radiated seismic energy. The Costa Rica-Panama-North Andes converging margins show compressional stress oriented west-east, generating the
relative movement of local tectonic blocks of the North Andes Microplate. Shallow
earthquakes associated with the Pacific subduction zone show compressional stress
oriented NW-SE. Vectors of displacement and compressional stress in southwestern
Colombia are oriented west-east. The deep seismicity in the Bucaramanga nest reflects
compressional stress oriented NNW - SSE, associated with the subducting Caribbean
Plate under the North Andes microplate. Complementary examples of global focal
mechanisms for the study area can also be obtained from the Harvard Centroid Moment Tensor catalog (CMT; Harvard, 2006). These results reflect the inversions of
long-period body and surface waves for the source moment tensors and best-fitting
double-couple solutions.
140
Figure 6.12: Main stress axes determined by the Reches Method (Reches, 1983; Reches,
1987) from 94 focal mechanism solutions of the Central Moment tensor catalog (CMT)
reported by Harvard University (CTM, Harvard, 2006) for the period 1976-2000 and
event magnitudes M = 5.0 (Vargas et al., 2005).
141
6.6
Conclusions
Seismicity in northwestern South America is mainly restricted to the plate boundary zones of the Caribbean, North America, and South America Plates and the Panama,
Costa Rica and North Andes Microplates. Seismic corridors are oriented predominantly
SW-NW, showing their association with compressional stress generated by the convergence of the Nazca and South American plates and the shortening of the continental
crust and mountain building of the Andes Mountains. The seismicity is extremely high
along the northern margin of the Pacific subduction zone.
The hypocenter depths show the tendency of the earthquakes to migrate from the
coastline eastwards as the hypocenter depths increase, confirming the eastward motion
of the Nazca Plate under the North Andes Microplate.
The spatial distribution of earthquakes is consistent with crustal discontinuities
interpreted from the gravity and magnetic anomalies. Seismic events are preferentially
concentrated in zones of of relatively thicker continental crust. This crust is inherently
under tension by virtue of its enhanced thickness, but it is also being subjected to
overwhelming compression from plate subduction forces.
The analysis of the isostatic state of compensation from terrain and gravity anomalies of northwestern South America help to differentiate probable earthquake zones.
Integrating the local stress of the crustal thickness variations with regional tectonic
stresses may help to predict local ground motions. The focal mechanisms reflect regional stress and more local stress from the thickness variations of the crust.
142
CHAPTER 7
CONCLUSIONS AND RECOMMENDATIONS
143
Spectral correlation theory was used to investigate regional topographic, gravity
and magnetic observations in spherical Earth coordinates for the crustal properties of
northwestern South America. Moho depth estimates were modeled from the inversion
of compensated terrain gravity effects, which required accurate gravity models of the
rock and water components of the terrain.
Gauss Legendre quadrature integration (GLQ) was used to model the gravity effects of the topography and bathymetry. The terrain gravity effects (TGE) were spectrally correlated with free-air gravity anomalies (FAGA) obtaining terrain-correlated
(TCFAGA) and -decorrelated (TDFAGA) free-air gravity anomalies. The TCFAGA
were removed from TGE to obtain the compensated TGE components (CTGE). The
gravity effects that annihilate the CTGE were assumed to be generated by the density
contrast at the mantle - crust interface or MOHO. Therefore, the MOHO was estimated from the CTGE by inversion with the design matrix coefficients modeled from
GLQ integration using the sparsely distributed seismic MOHO estimates.
Seismic studies have provided only limited point estimates of the MOHO and
related crustal thickness variations. Thus, the more continuous gravity MOHO and
crustal thickness estimates help to put the limited seismic point estimates into an effective regional context for enhanced tectonic analysis of northwestern South America.
Terrain-correlated free-air gravity anomalies include evidence of incomplete isostatic
compensation of the crust that when combined with the gravity MOHO constrain the
MOHO uplift and subsidence required for isostatic equilibrium.
Crustal discontinuities inferred from gravity, magnetic and other geological and
geophysical data of the North Andes Microplate were used to outline local tectonic
blocks. These blocks showed varying isostatic compensating states and therefore varying isostatic adjustments that can be related to the variations in the seismic activity
144
of the area. Thus, these results are useful for characterizing crustal stress conditions
and to interpret the historical and instrumental seismic catalogs of the study area.
The gravity MOHO and gravity and magnetic anomalies suggest a new model for
the tectonic evolution of northwestern South America. The results are consistent with
the tectonic style involving both rigid and non-rigid blocks. Where high-rigidity blocks
are present, the style is dominated by overthrusting of the more rigid block. Where
rigid blocks are largely absent, the style appears to be dominated by lower crustal flow
and continuous deformation. Within the North Andes Microplate, the rigid blocks
appear to be separated by weaker areas which correlate with crustal discontinuities.
The near-zero FAGA of the Nazca, Cocos and Caribbean Plates suggests that they
are almost completely compensated. The Cocos, Carnegie and Malpelo Oceanic Ridges
in the Pacific and the Beata Ridge in the Caribbean are associated with local positive TGE. The superposition of surface loads on the oceanic crust disturbs the local
isostatic balance. TCFAGA of these volcanic ridges is lower in magnitude than their
corresponding TGE suggesting partial isostatic compensation. Detailed analysis of the
Malpelo Ridge shows partial isostatic compensation by thickening the oceanic crust
with underplated material of lower density than the mantle. The gravity MOHO estimates for the Malpelo ridge also support interpretations of detailed seismic refraction
data (Trummer, 2002).
The large TCFAGA over the Andes Mountains (Andes Microplate) and the Sierra
Nevada of Santa Martha reveal them to be isostatically disturbed. The strong positive TCFAGA suggest the continental roots below these topographic features are not
deep enough to annihilate the TGE. The Sierra Nevada of Santa Marta, rising above
5800 m just 42 km from the Caribbean coast is the world’s highest coastal peak. Its
TCFAGA of 121 mGals clearly suggests that it is out of isostatic balance. It is one
of the most spectacular features of the northwestern South America. Topographically,
145
it is an isolated high, rising to glacier-capped peaks at nearly 5800 m elevation and
surrounded by arid lowlands near sea level. In the Caribbean Sea, 85 km to the north,
water depths exceed 3500 m, making the total topographic relief in excess of 9000 m.
The Sierra Nevada of Santa Marta and the Andes Mountains are clear examples of
topographic highs supported by lateral stress.
The gravity MOHO of the Guiana Craton locally differ from the regionally interpolated seismic Moho estimates. Local positive FAGA anomalies are associated with
more dense intrusive bodies affecting the host Paleozoic and Precambrian sequences.
These intrusive bodies do not have surface expressions and therefore are associated
with lateral density variations within the continental crust.
An improved crustal CHAMP magnetic anomaly map at 400 km altitude was
obtained using enhanced procedures for separating the external, core, and crustal
components. To separate the overlapping effects of the core field and crustal thickness
variations, the spherical harmonic expansion of IGRF10 was evaluated to degree 13.
Long wavelength features related to crustal thickness variations were extracted from
the CHAMP data by correlating differentially reduced to pole magnetic anomalies with
the pseudo magnetic anomalies of the crust that involved the first vertical derivative
of the crustal thickness gravity effects.
Feature correlations between crustal gravity and magnetic anomaly estimates were
isolated by the application of correlation filters in the wavenumber domains of the
data sets. By normalization and summing local favorability indices, positively correlated features were resolved and visualized. Differencing the normalized local favorability indices yielded the negatively correlated features between gravity and magnetic
anomalies. These correlation results further constrain the non-unique analysis of the
lithospheric anomaly components from satellite magnetic data and gravity observations.
146
The gravity and magnetic anomalies of northwestern South America are consistent with a region that is not of typical “Andean Type” orogenesis. Compressional
tectonics has played an important role in the mountain building of the North Andes
as a consequence of the relative movements among the Nazca, South American and
Caribbean Plates and the Panama and North Andes Microplates. The crustal shortening associated with plate motion has produced relative movement of thrusting faults
and crustal doubling.
The western margin of the continental crust provides clear testimony of the importance of allochthonous terranes, with contrasting rock properties and driving mechanism that have generated crustal deformation, magmatism, uplift and sedimentation
patterns. The Lesser and Greater Antilles represent the sites of subduction of the
North American Plate under the Caribbean Plate. The characteristic arcuate shape of
the gravity anomalies are clear evidence of the doubling of the oceanic crustal thickness as indicated by the positive TCFAGA. Sediments forming the upper layer of the
oceanic crust were scraped off the subducting plate as it descended and formed the
accretionary wedges of fore-arc regions that generate the negative TCFAGA.
A volcanic arc developed an uplifted surface of Precambrian and Paleozoic rocks
along much of the Pacific margin of the Americas by late Triassic or early Jurassic
(Daziel, 1986) and volcanic activity has been essentially continuous to the present
along different segments of the plate margin.
The tectonic setting shows the progressive accretionary continental growth of northwestern South America, attaching oceanic terrains to the Guiana Craton and producing shortening of the crust by orogenic events from the late Paleozoic through Cenozoic. The tectonic model also includes dextral-oblique accretion of the allochthonous
oceanic terrains along the Pacific margin with strong Caribbean Plate affinities capable of obduction of oceanic crust, the northwest migration of the tectonic bocks to
147
the Caribbean Plate, and the overthrusting of tectonic blocks with various internal
deformational styles. The geodynamic model is composed of highly oblique converging
plates, accretion, and subduction of allochthonous oceanic crust as the main types of
deformation.
Thus, accretion does not occur at the extreme northern and Southern Andes Mountains, where due to the absence of the Precambrian terrains, the unique tectonic setting
facilitated the migration of the Caribbean and Scotia Oceanic Plates of Pacific affinity
into the Atlantic Ocean. This lack of crustal accretion limited the longitudinal extend
of the Andes Mountains.
The gravity and magnetic anomalies of the western margin of northwestern South
America indicate the migration of the Caribbean Plate. It seems to have a Pacific
provenance, with the trajectory from SW to NW along the western and northern
margins of the North Andes Microplate, into the Caribbean.
The northwestern Andes involve isostatic disturbances and shortening of the continental crust that are consistent with horizontal compression maintaining vertical
loads of uncompensated masses. These masses are under pressure to adjust spatially
to greater levels of isostatic equilibrium and these adjustments generally tend to correlate with the seismicity variations of the study area.
The ANSS and RSNC historical and instrumental catalogs reveal the distinct concentrations of earthquakes along plate boundary zones and crustal discontinuities.
Shallow, intermediate, and deep earthquakes follow a pattern that infers the landward
deepening of focal depths from the Pacific subduction zone, indicating the inclination
eastward of the subducting oceanic slab under the continent.
The results of this study show the power of crustal thickness variations to sustain
regional topography. Areas with crustal thickness between 45 km and 55 km in the
148
North Andes mountains are associated with the presence of batholithic intrusions of
economic interest for exploration for porphyry base, and precious metals.
Crustal thickness variations also are fundamental for the evaluation of hydrocarbon generation in oceanic basins and adjacent continental margins. Defining the type
of crust underlying an accretionary wedge is a fundamental constraint for estimating
the heat flow present in areas of exploration. This information can contribute significantly in modeling the thermogenic generation of hydrocarbons, including areas of
the subsurface known as “source kitchens, oil kitchen or gas kitchen” (Ceron, 2005;
Schlumberger, 2006) where the source rocks reach appropriate conditions of pressure
and temperature to generate hydrocarbons. The anomalous thickened Caribbean Plate
is overlain by thick sedimentary sequences that are favorable for the exploration of oil
and gas deposits.
The interpretations and deductive processes presented here involved a judicious
balance between the mathematical and geostatistical rigour of the geopotential data
processing and the descriptive aspects of geological interpretation. Together they form
a legitimate intellectual exercise which should contribute to the geologic knowledge of
northwestern South America to help mitigate seismic and volcanic hazards and the
risk factors in energy and mineral resource exploration ventures.
The integration of satellite, airborne and surface geopotential field data and future
data from satellite missions like GOCE (Gravity field and steady-state ocean circulation explorer; ESA, 2006) and SWARM (The Earth’s magnetic field and environmental
explorers; ESA, 2006) certainly will lead to new insights of the tectonic processes of
northwestern South America. Additional recommendations for extending this research
include:
To improve TGE estimates by assigning density values from available mapped
lithological units instead of using a single density value.
149
To calculate and interpret the TCFAGA and CHAMP DRTP (TFMA) gradients
to enhanced local crustal density and magnetic susceptibility variations.
To model the thermal structure of the study area from surface and borehole observations to support the interpretation of potential field anomalies and stress modeling.
150
APPENDIX A
TWO DIMENSIONAL GRAVITY AND MAGNETIC
ANOMALIES DUE TO A POLYGON BY GM-SYS
151
The methods used by GM-SYS to calculate the gravity and magnetic model response are based on the methods of Talwani et al, (1959), and Talwani and Heirtzler,
(1964), and make use of the algorithms described in Won and Bevis, (1987). Two and
one-half-Dimensional calculations are based on Rasmussen and Pedersen, (1979).
The GM-SYS inversion routine utilizes a Marqardt inversion algorithm (Marqardt,
1963) to linearize and invert the calculations. GM-SYS uses an implementation of
that algorithm for gravity and magnetics developed by the USGS and used in their
computer program, SAKI (Webring, 1985).
GM-SYS uses a two-dimensional, flat-earth model for the gravity and magnetic
calculations; that is, each structural unit or block extends to plus and minus infinity
in the direction perpendicular to the profile. The earth is assumed to have topography
but no curvature. The model also extends plus and minus 30,000 kilometers along the
profile to eliminate edge-effects.
In GM-SYS, stations (points at which gravity or magnetic values are observed and
calculated) should be outside of the source material (i.e., in an area of the model with
a density, magnetization, and susceptibility equal to zero).
Won and Bevis(1987) present two algorithms for computing the gravitational and
magnetic anomalies due to an n-sided polygon in a two-dimensional space.
Talwani et al, (1959), and Talwani and Heirtzler, (1964) presented a method for
computing the gravitational attraction due to an n-sided polygon. Their algorithm has
been used in computer programs for two-dimensional gravity modeling. because any
2-D body of arbitrary shape can be approximated by a polygon, and any 3-D density
distribution can be modeled as an ensemble of juxtaposed constant-density polygons.
Won and Bevis (1987) modified algorithm for computing the gravitational acceleration
due to a polygon, applying Poissons relation to expressions for gravitational acceleration. They derived a second algorithm for computing the magnetic anomaly due to
152
a polygon magnetized by an external field. In this manner, the gravity and magnetic
anomalies can be correctly determined for any point inside, outside, above, or below
the polygon.
A.1
Gravity Anomaly due to a Polygon
Hubbert (1948) showed that the gravitational attraction due to a 2-D body can be
expressed in terms of a line integral around its periphery. Talwani et al (1959) considered the case of an n-sided polygon and broke the line integral up into n contributions,
each associated with a side of the polygon. Won and Bevis (1987) followed Talwani
et al (1959) by placing the point at which the gravity anomaly is to be computed (i.e.,
the station) at the origin of the coordinate system and expressing the vertical and
horizontal components of the gravity anomaly as:
∆gz = 2Gρ
n
X
Zi
(A.1)
Xi
(A.2)
i=1
and
∆gx = 2Gρ
n
X
i=1
where Zi and Xi are line integrals along the ith side of the polygon, G is the
gravitational constant, and ρ is the density of the polygon. Talwani et al,(1959) derived
expressions for Zi and Xi that make extensive references to trigonometric functions.
Grant and West (1965) reformulated the expression for Zi , by making more references to the vertex coordinates:
153
³
xi , z i
´
i
= 1.n
(A.3)
And fewer references to angular quantities, and thus reduced the number of trigonometric expressions involved in the computation. Won and Bevis (1987) followed Grant
and Wests approach producing a formula for Xi as well as Zi compacting the notation
by eliminating i . Any two successive vertices were labeled 1 and 2, and each neighboring pair of vertices was treated as vertices 1 and 2 in turn. Thus the following
algorithms were obtained:
h
Z = A (θ1 − θ2 ) + Bln
h
r2 i
,
r1
(A.4)
r2 i
,
r1
(A.5)
X = A (−θ1 − θ2 ) B + ln
A=
(x2 − x1 ) (x1 z2 − x2 z1 )
(x2 − x1 )2 + (z2 − z1 )2
(A.6)
z2 − z1
x2 − x1
(A.7)
r12 = x21 + z12 ,
(A.8)
r22 = x22 + z22 ,
(A.9)
B=
154
The computation of (−θ1 − θ2 ) requires some care if the algorithm is to be valid for
any local station. −θ1 and −θ2 are obtained using the relationship:
θj = tan−1
A.2
³z ´
j
xj
∀j=1,2.
(A.10)
Magnetic Anomaly due to a Polygon
Talwani and Heirtzler (1964) introduced a method for computing the magnetic
anomaly due to an infinite polygonal cylinder. by combining the anomalies due to an
ensemble of semiinfinite sills each of which was bounded by one of the polygons sides.
Alternatively, the magnetic anomaly due to an polygonal cylinder can be derived using
Poissons relation, from the previous expressions for the associated gravity anomaly.
Won and Bevis (1987) assumed that the magnetization of the cylinder was induced
solely by the ambient earths magnetic field. Then
∆H =
kHe δ
∆g
Gρ δα
(A.11)
where
∆H = magnetic anomaly vector,
∆g = gravity anomaly vector,
k = magnetic susceptibility of polygon,
ρ = polygon density,
He = ambient scalar earth magnetic field strength, and
α = direction of induced magnetization.
Unlike the gravity anomaly, the magnetic anomaly depends additionally on the
strike of the cylinder,
Where:
155
I = Magnetic inclination, and
β = The strike of the cylinder measured counterclockwise from magnetic
north to the negative y-axis,
It can be shown that
∂
∂
∂
= sin I
+ sin β cos I
∂α
∂z
∂x
(A.12)
From equation (A.10), vertical and horizontal components of the magnetic anomaly
can be derived as
∆Hz =
kHe ∂
∆gz
Gρ ∂α
(A.13)
∆Hx =
kHe ∂
∆gx
Gρ ∂α
(A.14)
and
where expressions for gz and gx , are given by equations (A.1) and (A.2). Substituting equations (A.l), (A.2), and (A.12) into equations (A.13) and (A.14), obtaining
³
∂Z
∂Z ´
+ sin β cos I
, (A.15)
∂z
∂x
³
∂X
∂X ´
+ sin β cos I
, (A.16)
∂z
∂x
∆Hz = 2kHe sin I
And
∆Hx = 2kHe sin I
156
Once ∆Hz and ∆Hx are known, the total field scalar anomaly anomaly the total field
scalar anomaly ∆H may be computed by
∆H = ∆Hz sin I + ∆Hx sin β cos I.
(A.17)
The derivatives in equations (a.15 and A.16) are
∂Z
(x2 − x1 )2 h
z2 − z1
r2 i
=
(θ
−
θ
)
+
ln
−P
1
2
∂z
R2
x2 − x1
r1
(A.18)
∂Z
−(x2 − x1 )(z2 − z1 ) h
z2 − z1
r2 i
=
(θ
−
θ
)
+
ln
+Q
1
2
∂x
R2
x2 − x1
r1
(A.19)
∂X
(x2 − x1 )2 h z2 − z1
r2 i
=
(θ
−
θ
)
−
ln
+Q
1
2
∂z
R2
x2 − x1
r1
(A.20)
∂Z
(x2 − x1 )(z2 − z1 ) h z2 − z1
r2 i
=
(θ
−
θ
)
−
ln
+P
1
2
∂x
R2
x2 − x1
r1
(A.21)
R2 = (x2 − x1 )2 + (z2 − z1 )2
(A.22)
x1 z2 − x2 z1 h x1 (x2 − x1 ) − z1 (z2 − z1 ) x2 (x2 − x1 ) − z2 (z2 − z1 ) i
−
R2
r12
r22
(A.23)
Where
P =
157
and
Q=
x1 z2 − x2 z1 h x1 (z2 − z1 ) + z1 (x2 − x1 ) x2 (z2 − z1 ) + z2 (x2 − x1 ) i
−
R2
r12
r22
(A.24)
Special cases 1 and 2 shown previously for the gravity problem also apply in the
same way for the magnetic anomaly. In addition, a fourth case is as follows:
if x1 = x2 , then
∂Z
= −P
∂z
(A.25)
∂Z
−(z2 − z1 )2 r2
=
ln + Q
∂x
R2
r1
(A.26)
∂X
=Q
∂z
(A.27)
∂X
(z2 − z1 )2
=
(θ1 − θ2 ) + P
∂x
R2
(A.28)
Won and Bevis (1987) developed the FORTRAN subroutine m − poly computing
the x-component, the z-component and the total anomalous magnetic field strength
due to an infinite polygon cylinder magnetized by an external magnetic field. It was
assumed that the cylinder strikes parallel to the y-axis in a right-handed coordinate
system [x, y, z]. The vertical, horizontal and total field strength anomalies depend
upon the relative locations of the polygon and station in the [x, y] plane, the magnetic susceptibility of the cylinder, the inclination of the Earth’s magnetic field,the
158
total field strength of the earth’s magnetic field, and the geomagnetic azimuth (strike)
of the polygon. This quantity (β) is the angle from magnetic north to the negative
x-axis measured in the horizontal plane. The angle is positive when measured counterclockwise from magnetic north. The x,y coordinates of the polygon’s vertices must
be specified clockwise when the x,z plane is viewed toward the negative y-axis.
The routine computes the anomalies at any specific number of stations, and these
stations may be specified in any sequence. In the event that the Earth’s magnetic field
is vertical (Inclination = 90o ), the strike (β) is undefined and irrelevant and can be set
to any value. The algorithm does not include the effects of demagnetization (Grant
and West, 1965) and thus it is not suited for modeling the anomalies due to bodies
whose magnetic susceptibility exceeds 0.01 e.m.u. Magnetic susceptibility values must
be assigned with care, even though magnetic susceptibility is a dimensionless quantity,
it differs by a factor of 4π between the SI and emu system (kemu = 4πkSI ). H units
are gammas. If the SI is used units are in NanoTeslas (nT). Spatial coordinates may
be given in any unit of length, provided the unit chosen consistently.
The Fortran codes can be consulted in the journal of Geophysics, volume 52 No. 2
published by the Society of Exploration Geophysics (http://www.segdl.org/geophysics/).
159
APPENDIX B
FRACTIONAL VERTICAL DERIVATIVES
160
The use of vertical derivatives has been always a standard method of enhancing
high frequency features in potential field data. Second order vertical derivatives were
computed using convolution filters and Laplace’s equation (Sheriff, 1994).
∇2 f = 0
Ã
∂2f
∂z 2
=−
!
∂2f
∂x2
+
∂2f
∂y 2
(B.1)
By Fourier filtering it is possible to compute the nth order vertical derivatives by using
the following relationship:
∂ 2f
F( n ) = k n .F(f )
∂z
(B.2)
Where F is the Fourier representation of the field and K is the wavenumber or frequency. The derivation of equation 6.2 from spatial representation in equation 6.3 is
based on Blakeley, (1955).
∂f
f (x, y, z) − f (x, y, z − ∆z)
= lim
∆z→0
∂z
∆z
(B.3)
Where f is the potential field (or it’s derivative) and a z positive down sign convention implies that the data is upward continued an infinitesimal distance and this
is subtracted from the original data. The Simple for of equation 2 allows the use of
non-integer values for n, providing fractional vertical derivatives which have an intermediate frequency content as compared to integer order vertical derivatives.
161
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