Download A density model of the Andean subduction zone

A density model of the Andean
subduction zone
T. V. ROMANYUK, Institute of Physics of the Earth, Moscow, Russia
H.-J. GÖTZE, Freie Universität Berlin, Germany
P. F. HALVORSON, consulting geophysicist, Boulder Creek, California, U.S.
D ensity changes in “basement”
rock or the deeper crust tend to cause
gravity anomalies that dominate the
observed data. Now that oil companies have begun using airborne gravity in their exploration of remote
regions of the Andes, interpreters are
building detailed earth models,
which necessarily involve geologically constrained gravity “regional
fields” to account for the overwhelming signal from deep crustal
structure.
The tectonic model in this paper
crosses Bolivia’s hydrocarbon production and its epithermal silver-tin
deposits. In northern Chile, the
model passes through the Quebrada
Blanca and Collahuasi porphyry copper deposits, just north of the
Chuquicamata porphyry copper
deposit along the Domeyko Cordillera. With the stakes high, budgets
low, and exploration costs soaring,
the less expensive airborne geophysical techniques, gravity and
magnetics, are being utilized in
advance of the hugely expensive seismic crews.
During the last decade a project
titled “Deformation Processes in the
Andes” has undertaken new geo-
physical investigations on the Andean subduction zone. A goal of the
project is to validate a regional,
crustal model for the Andes as thoroughly as possible. The main components in these studies are: (1)
seismic refraction; (2) gravity; (3)
reflection seismic; and (4) electromagnetics. These, along with other
existing data, have been integrated
with geologic data to compile a 2-D
model combining geologic, geophysical, and tectonic information
along a line at 21° S. This paper concerns part of the project: compiling
density parameters for the model.
Most convergent zones of the
earth are conceived as oceanic plates
subducting beneath immovable or
stable continents. However, we
believe that the Andean subduction
zone is an entire region (both oceanic
and continental) that is moving
downward into the earth. The movement of the material is strongly asymmetrical. The oceanic Nazca Plate is
moving horizontally about 10 cm/y
near the trench. The cratonic central
part of South America (the Brazilian
Craton) is estimated to move at about
1 cm/y. The subduction inland and
downward of continental lithosphere
Figure 1. Geophysical model of the Andean crust. Seismic velocities are
shown by numbers, densities by color.
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can explain the nature of the thickened
(up to 70 km) Andean crust. The
Andean mountain system has been
forming under a strong compressive
regime due to large-scale deformation
and the accumulation of light crustal
material over a sinking root that consists of dense lower-continental crust
and upper mantle of the Brazilian
Craton and Nazca Plate.
The surface geology and geodynamics of the Andean mountain belt
differ dramatically from neighboring
regions (Figure 1). The westernmost
oceanic crust, the trench, the coast
range, and the Chaco Plain are characterized by subhorizontal boundaries
in the upper crust, negligible vertical
movement, and low heat flow. The
coastal range seems to have been stable since the Jurassic, and the Brazilian
Craton since the Proterozoic. On the
other hand, the Andes, which have
had active tectonics since the Miocene,
display near-vertical and oblique
faults, geologic discontinuities, high
vertical movement, high heat flow, and
active volcanism.
Compilation of the model and gravity modeling. New geologic and seismic data have been combined with
previous density models to compile
an initial layer-block scheme for the
Andean crust (Figure 1). A model of
crustal and mantle density (Figure 2)
is produced with a linear gravity
inversion technique. Densities are
constrained where possible by seismic velocities and acoustic logs. We
assume that the lithosphere is close
to isostatic equilibrium at a depth of
300 km in the deep ocean and beneath the Eastern Cordillera.
The main features of the Andean
lithosphere model are:
1) The upper crust was formed on or
near Proterozoic basement, as a
broad, back-arc marine sedimentary basin during Ordovician and
Devonian time. Between Ordovician and Oligocene time, the basin
was affected by short episodes of
orogenesis (rifting, minor sedimentation, and segmentation).
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Figure 2. Density model of the Andean subduction zone. Densities are shown by numbers in each block; dual numbers indicate a vertical gradational density within the block. Observed (Bouguer anomaly over continent regions
and free-air anomaly over the ocean) and calculated gravity curves are shown; the axis is located at left. Relative
hydrostatic pressure curves at the depth of 300 km for ocean west of the trench and for continent beneath the
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The upper crust is shaped like a
lens, thickening up to 20-25 km
beneath the Cordillera mountain
belt and thinning at the edges near
the coast and the Subandean
thrust. The marine sediments
which have been affected by tectonic deformation are seen in the
Eastern Cordillera and Subandean
ranges with density = 2.6-2.7
g/cm3. Metamorphism and magmas of the volcanic arc affect the
sediments that make up the coast,
Western Cordillera, and Altiplano,
with density = 2.65-2.8 g/cm3.
2) The high-velocity layer (HVL)
(6.4-6.8 km/s; 5 km thick) at the
base of the upper crust can be
interpreted as a relict of the
basaltic-gabbroic oceanic crust
with a density of 2.9-3.0 g/cm3.
This layer is thought to be much
more rigid and less deformable
than others. This is why it plays
an important role in the redistribution of the deformation and the
accumulation of stresses.
3) A part of the sedimentary basin
between the ocean and Western
Cordillera was underthrust by a
volcanic island arc complex, presumably in the Late Jurassic,
creating a double layer of volcanosedimentary crust. It has high seis-
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mic velocity (about 6.4 km/s) and
high density (about 2.9 g/cm3).
4) The structure of the crust at the
transition from the Subandean
ranges to the Chaco Plain is not
imaged by seismic data. Thus, the
model has a very simple conventional layered crust. The bottom of
the sedimentary basin is estimated
from density modeling.
5) Beginning in the Oligocene, the
crust beneath the Eastern Cordillera and Subandean ranges has
been thickening due to underthrusting of the Brazilian Craton.
The oblique contact between the
top of the Brazilian Craton and
the HVL is prominent in seismic
reflection data. We believe that the
rigid end of the HVL acts as a
plow, scraping off the sediments
from the moving Brazilian Craton.
These sediments mix and deform
together with the Ordovician and
Silurian marine sediments, producing thin-skin tectonic structures of the Subandean ranges.
6) The dominant deformation
regime is crustal shortening. This
is estimated, beginning from Late
Cretaceous, at 300 km, including
150 km of post-Oligocene shortening. This means that the
Brazilian Craton should have
moved westward at least up to the
Altiplano. We propose that the
Brazilian Craton moves like a
rigid indivisible body beneath the
Subandean ranges, but beneath
the Eastern Cordillera the layers
of the crust move independently.
The upper crust continues to
move like a rigid body (it is
approximately outlined by a concentration of seismic reflections).
The dense high-pressure metamorphic fraction implies that the
lower part of the crust is sinking
into the mantle. The middle layers begin to behave in a fluidlike
manner, folding into 30-60 km of
material.
7) We propose that the bottom of the
underthrusting Brazilian Craton
does not coincide with the present-day seismic refraction Moho
between the Altiplano Plateau and
the Eastern Cordillera but is
located lower. The lack of a refraction Moho beneath the Eastern
Cordillera is due to a partial
eclogitization or other high-pressure metamorphic transformations which increase density and
seismic velocity up to mantle values in the lower layer of the downwardly mobilized Brazilian
Craton.
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Additional information concerning the geophysical database,
topographic grids, geologic maps, abstracts, and the organization and
services of Berlin’s Collaborative Research Center 267 is at:
http://userpage.fu-berlin.de/~data/Welcome.html or
http://userpage.fu-berlin.de/~geoinfhb/Welcome.html
The USGS National Earthquake Information Center map of seismicity
of South America from 1975 to 1995 includes color-coded depths of
earthquakes:
http://wwwneic.cr.usgs.gov/neis/general/seismicity/s_america.html
Current earthquakes (within the last five days) may be viewed on a
similar map of South America with uncluttered shaded terrain as a
“base” map:
8) The present-day volcanic arc in the
Western Cordillera is bounded by
near-vertical fault zones. Together
with other inner faults and deep
reflective boundaries, they outline
a so-called “flower structure.” No
marked horizontal density changes
were obtained inside this structure.
The nature of this structure can be
interpreted as a squeezing out of
material from the inner parts of the
“flower” to the surface. The base of
the flower structure is expected to
be a point of relaxation of the
stresses.
9) The geologic styles in the Western
and Eastern Cordilleras and the
crustal structures beneath them
differ. The common element is an
accumulation of the material in
the upper crust over the rigidly
subsiding HVL. Although the
Altiplano stands 3.7 km above sea
level, it is thought to be a sedimentary basin between these
mountain systems. It has been
forming due to the subsidence of
the HVL and the relative uplift of
the neighboring Western and
Eastern Cordillera.
10) A refraction seismic Moho was
not observed beneath the present-day Western Cordillera or
Altiplano. Moreover, seismic
investigations do not show any
layered structures in the middle
and lower crust. Electromagnetic
investigations show a sharp
decrease in resistivity. Gravity
modeling did not show any
marked horizontal density
changes. The average density
was .1 g/cm 3 higher than the
density of the Brazilian Craton’s
middle crust. Thus, we propose
that the dominant process here
is modern magmatic activity,
which has erased earlier structures. We interpret the middle
and lower crust and the uppermost mantle as an accumulation
of material strongly affected by
magma, reworked by metamorphism, and deformed in a fluidlike manner. A depth of 70 km (an
average of estimates for quartzcoesite transformation) has been
adopted as a conventional crust/
mantle boundary. This is supported by the calculation of the
isostatic Moho.
The model of the subducted Nazca
Plate and the mantle edge. The
oceanic crust of the subducting Nazca
Plate (~30-40 million years in age) is
approximated by two layers: a
basaltic layer 2 with density 2.8
g/cm3 and a gabbroic layer 3 with
density 2.95 g/cm3. At 20 km, layer
2 is believed to be wedged, because
pores in the basalts are thought to be
closed due to high pressure, so the
density must be close to that of the
gabbros. The mantle of the Nazca
Plate is believed to be harzburgite
grading with depth into spinel her-
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zolite with an average density of 3.34
g/cm3. The boundary of the asthenosphere with the Nazca Plate near
the western coast of South America
is not imaged seismically, probably a
result of the similar densities and
velocities.
The most active area of continental and oceanic plate contact is considered to be between the trench and
the volcanic arc. Dehydration and
metamorphism (most importantly
basalt-eclogite) in the subducted
oceanic crust are proposed here. Two
processes have the greatest effect on
the gravity modeling: (1) increasing
density in the downgoing slab, and
(2) distortion of the mantle edge by
the release of slab fluid, causing wet
melting of the mantle edge peridotites, forming a volcanic arc.
The upper mantle beneath the
Andes has three parts: the light mantle cones (3.2 g/cm3), the dense, sinking root of the Andean lithosphere
(3.4 g/cm3), and the “normal” continental mantle of the Brazilian Craton
(3.36 g/cm3). The densities in the
mantle in the 200-670 km depth range
are fixed in accordance with the
standard columns proposed by
Dziewonski and Anderson (1981)
and Lerner-Lam and Jordan (1987).
Further work. This paper constitutes
a “parts list” for a rigorous geophysical model of Andean tectonics.
Beyond this gravity work our
research will strive to understand
crustal dynamics by tracking down
the mechanism (stress data) that is
gleaned from the directional aspects
of the recorded seismicity. Lithospheric stresses will be calculated
from the integrated gravity model
we have described, and they will be
compared with observed seismicity.
This helps back up the gravity model
concerning the forces involved in the
plate motions.
Suggestions for further reading. The
articles cited in this text, and many
others, can be found on the Web site
organized by SEG’s Gravity and
Magnetics Committee at http://seg.org/
comm-info/grav_mag/.
Acknowledgments: We thank Regina
Patzwahl who provided results of gravity
group D3 of the Collaborative Research Center
267. “Deformation Processes in the Andes”
is a project funded under grants to Deutsche
Forschungsgemeinschaft (DFG) and Freie
Universität Berlin.
Corresponding author: T. Romanyuk,
[email protected]
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