Download Cenozoic basin evolution of the Virgin Islands basin and Anegada

Cenozoic basin evolution of the Virgin Islands basin and Anegada Passage,
northeastern Caribbean
A Thesis Presented to the
Faculty of the Department of Earth and
Atmospheric Sciences
University of Houston
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
By
Patrick Loureiro
December 2014
Cenozoic basin evolution of the Virgin Islands basin and Anegada Passage,
northeastern Caribbean
Patrick Loureiro
APPROVED:
Dr. Paul Mann
Dr. Michael Murphy
Dr. Pete Emmet
Dean, College of Natural Sciences and Mathematics
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ACKNOWLEDGEMENTS
I thank Dr. Paul Mann, for suggesting this topic as a master’s project and his
willingness to supervise this study relatively late in my master’s career at the University
of Houston. I also thank the industry sponsors of the Caribbean Basins, Tectonics and
Hydrocarbons (CBTH) project at the University of Houston for funding my support as a
graduate research assistant that allowed me to finish the project quickly. Special thanks
go to Dr. Michael Murphy and Dr. Pete Emmet for their service on my thesis committee
and suggestions for improving this work.
I would like to thank fellow grad students of the CBTH project for their support
and insights on my project. Bryan Ott, Murad Hassan, and Kyle Reuber provided many
useful suggestions for improving my thesis figures and for helping to utilize different
software. I would also like to thank Luan Nguyen for his assistance in helping me create
the gravity and magnetic profiles.
Finally, I would like to thank my friends and family members for their continued
and unwavering support over the years.
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Cenozoic basin evolution of the Virgin Islands basin and Anegada Passage,
northeastern Caribbean
An Abstract of a Thesis
Presented to the
Department of Earth and Atmospheric Sciences
University of Houston
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
By
Patrick Loureiro
December 2014
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ABSTRACT
The Virgin Islands basin (VIB) is a 1 to 4.5 km‐deep, fault‐bounded marine
passage that forms the southwestern and deepest segment of the Anegada Passage
connecting the Atlantic and Caribbean Seas. A variety of models have been proposed to
explain the tectonic origin of the VIB that range from right‐ and left‐lateral pull-apart
basins, a rotational-type basin, and a rift basin formed orthogonally to the direction of the
North America plate beneath the Caribbean plate This study integrates several geological
and marine geophysical data types to better understand the Miocene to recent kinematics
of a VIB opening and its present-day tectonics. A grid consisting of 400 km’s of 2D
seismic lines, provided courtesy of the 2006 Danish Galathea 3 expedition, reveals the
geometry of faults underlying the VIB to a depth of 7.5 seconds two-way time (~8 km),
and a 68 km-long deep-penetration line from Shell shows the basin structure to a depth of
15 km.
The VIB is an asymmetrical half-graben with greater throw along its southeastern
bounding normal fault than along the normal fault on its northwestern edge.
The
elongate, 40-km-long island of St. Croix is the uplifted footwall of the southeastern,
larger-throw normal fault, while the elongate island of Vieques, Puerto Rico, is the
uplifted footwall of the normal fault bounding the northwestern edge of the basin. A
linear, active, oblique-slip fault system can be traced for a distance of 80 km along the
axis of the basin. High-resolution bathymetric data reveals a previously unrecognized
linear, fault scarp offsetting the deep basinal sediments and seafloor of the VIB and
extending 66 km to the west into the whiting basin southeast of Puerto Rico. Gravity and
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magnetic transects across the Virgin Islands basin and Anegada Passage constrain the
depth to basement, ranging from 8.5 km below sea level in the VIB to 3.5 km below sea
level in the Anegada Passage to the northeast. I present a tectonic model for the VIB
involving: 1) Early Miocene opening of the basin as part of the Kallinago intra-arc basin;
2) Middle Miocene oblique collision of the Bahama carbonate platform and early leftlateral shear, basin opening, and offset of the Kallinago basin; and 3) counterclockwise
rotation and right-lateral shear along the Virgin Islands strike-slip fault system through
the VIB.
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TABLE OF CONTENTS
ABSTRACT
v
TABLE OF CONTENTS
vii
LIST OF FIGURES
x
CHAPTER 1: Introduction to thesis
Rationale for the study of the basin evolution of the Virgin Islands
basin
1
History and development of this thesis
2
Future plans
3
CHAPTER 2: Cenozoic basin evolution of the Virgin Islands basin and
Anegada Passage, northeastern Caribbean
Introduction and major structural features
4
4
Active tectonic setting of the northeastern Caribbean plate boundary 6
Previous tectonic interpretations of faulting and basin formation in
the Anegada Passage-VIB area
10
Data and methods used in the thesis
12
Bathymetric and topographic data
12
Earthquake focal mechanism data
12
GPS vectors of plate motions
13
Gravity, magnetic, and refraction data
13
Deep-penetration seismic reflection and interval velocity data
13
Onland geologic data
15
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Shallow-penetration seismic reflection data
15
Visualization and structural restorations using MOVE software 15
Objectives of the thesis
16
Crustal structure of the VIB and Anegada Passage using gravity
and magnetic transects
16
Constraints on deeper crustal structure of the VIB from potential
fields
16
Gravity methods used for crustal structure
17
Magnetic methods used for picking top of crystalline basement 20
Combined gravity and magnetic results from three
gravity/magnetic transects across the VIB
21
Structure of the Virgin Islands basin from deep- and shallowpenetration seismic reflection and bathymetry
Deep-penetration multi-channel seismic data
22
22
Simplified geologic cross section of the VIB based on the
Shell line and seismic velocities
25
Stratigraphic column of the VIB based on correlations to the
Shell line and onshore outcrops in St. Croix and Vieques
25
Active seafloor faulting in the Virgin Islands basin from highresolution bathymetric data
27
Methods
27
Extent of the active faulting from the Mona rift to the VIB
27
Seafloor faulting in the Whiting basin
29
Main fault systems of the VIB
31
VIRGIN ISLANDS FAULT ZONE
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31
TORTOLA RIDGE FAULT ZONE
31
ST. CROIX FAULT ZONE
34
Overall fault interpretation of the VIB
34
Cross-sectional structure of the Virgin Islands and St. Croix
basins and Anegada Passage from shallow-penetration singlechannel seismic data
35
Results of shallow seismic profiling from Jany et al. (1990)
35
Structure of the deep Virgin Island basin on Jany et al. (1990)
lines
35
Structure of Anegada Passage and St. Croix basin on Jany et al.
(1990) lines
37
Structure of the deep Virgin Island basin on Raussen et al. (2013)
lines
40
Discussion and Conclusions
Isochron map of the VIB based on Jany et al. (1990) data
45
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Restoration of extension that formed the Virgin Islands basin using
the Shell line
50
Tectonic model for the evolution of the VIB and Puerto Rico
area
52
REFERENCES CITED
57
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LIST OF FIGURES
Figure 1: Plate tectonic setting of the northeastern Caribbean-North America plate
boundary shown on a basemap of free-air gravity from Smith and Sandwell
(1997). Hot colors indicate areas of thicker crust while cool colors are areas of
thinner crust. The active line of volcanoes of the Lesser Antilles arc (red
triangles) are separated from the inactive line of pre-Miocene volcanoes of the
Limestone Caribees (open triangles) by the Kallinago intra-arc rift basin. Radial
normal faults affect the forearc area of the Lesser Antilles arc southward from the
Anegada passage. Seamounts (black squares) have been proposed in the area of
the Virgin Islands basin (VIB). Box shows more detailed map of the VIB area
shown in Figure 2A.
p. 5
Figure 2: A. Active tectonic setting of Puerto Rico, VIB, Anegada Passage, and
surrounding features using high-resolution, multibeam bathymetric data from
Taylor et al. (2008) and Grothe et al. (2012). A) Tectonic map of the Puerto RicoVirgin Islands region with GPS vectors showing that the Anegada Passage is an
active, oblique-slip fault separating northeastern vectors in Puerto Rico from a
more easterly vectors on St. Croix and the Lesser Antilles arc (all vectors are
relative to a fixed North America plate) purple vectors are from Benford et al.,
2013; black vector from Manaker et al., 2008). Slip vectors from subductionrelated earthquakes from Hippolyte et al. (2005) and Doser et al. (2005) show a
radial pattern of thrusting around the strongly curved subduction plate boundary.
Earthquake focal mechanisms from the Mona rift and in the western VIB from
Mondziel et al. (2010) show active extension along with earthquake swarms in the
VIB from Frankel et al. (1980). B) Oblique view of bathymetry using the same
data in A showing prominent Anegada Passage fault zone and its abrupt
truncation of the Kallinago basin. Profile in inset shows a cross section of the
Kallinago basin formed by a large normal fault along its eastern edge (upthrown
side of the normal fault now covered by Miocene and younger shallow-water
limestone).
p. 8
Figure 3: Three previous tectonic models for the opening of the Virgin Islands basin that
all predict different opening mechanisms. A) Mann and Burke (1984) and
Raussen et al. (2013) proposed left-lateral shear similar to other faults in the
Caribbean-North America plate boundary zone. B) Jany et al. (1990) predicted
right-lateral shear based on their inferred geometry of pull-apart basins within the
VIB; their proposed driving mechanisms for right-lateral shear included: 1)
tectonic escape; 2) east-west shortening and bending (Stephan et al., 1986); and 3)
counterclockwise rotation of the Puerto Rico microplate that was later developed
by Masson and Scanlon (1991). C) Speed and Larue (1991) and Feuillet et al.
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(2002) proposed that the VIB opened by orthogonal rifting with negligible left- or
right-lateral shear.
p.11
Figure 4: Structural map of the VIB using high-resolution bathymetric data from Grothe
et al. (2012) and Taylor et al. (2008) and topographic DEM from GEBCO
(2013). All seismic reflection lines used in the study are labeled: Lines 16, 17,
18, and 19 in black are single-channel lines from Raussen et al. (2013) and lines
9, 10, 11 and 12 in blue are single-channel lines from Jany et al. (1990). Line JShell Shell is a multichannel Shell line with velocity information that was
published in Jany et al. (1990) and reinterpreted in this thesis in Figure
6. Seamount features are labeled as black squares: I reinterpret most of these
seamounts as uplifted footwall blocks whose sharp and conical summits resemble
seamounts on 2D seismic profiles. Yellow dots represent locations of
stratigraphic columns shown in Figure 12. Faults shown on this map are
interpreted from the grid of all seismic lines shown.
p.14
Figure 5: A) Bouguer gravity anomaly map from compilation by Decade of North
American Geology (DNAG) (1994) showing locations of three gravity transects
crossing the VIB and Anegada Passage in B, C, and D. Inverted white triangles
on each of the three lines show refraction stations by Officer et al. (1959) that
were used in this model to constrain crustal thicknesses. Black dots on the gravity
profiles are Werner deconvolution solutions from coincident magnetic profiles
that were used to more precisely locate the top of crystalline basement. These
values were consistent with depth conversions performed on the seismic profiles
using Midland Valley’s Move 2014.2 software. B) Bouguer gravity profile B-B’
crossing the Anegada Passage in an east-west direction shows 20-km-thick crust
of the Virgin Islands platform thinning to the east in the area of the St. Croix
basin and Kallinago trough. Basement is elevated to a depth of 600 m below sea
level (BSL) west of the Anegada basin likely as the result of the uplift of the
footwall of a normal fault. C) Bouguer gravity profile C-C’ crossing the deepest
part of the VIB (4500 m BSL) showing crust of equal thickness to the north
beneath the Virgin Islands basin and to the south beneath St. Croix; the profile
also shows an overall basin asymmetry consistent with the presence of the largest
normal fault on the steeper, south side of the basin with St. Croix island uplifted
as its footwall block. Basement is predicted to be at a depth of 7 km beneath the
center of the VIB and about 1 km beneath its uplifted flanks. D) Bouguer gravity
profile D-D’ crossing the VIB in an east-west direction shows 22-km-thick crust
of southeastern Puerto Rico thinning to the east with a thickness of 18 km in the
area of the St. Croix basin and Kallinago trough. Basement is most elevated to a
depth of 600 m below the surface west of the Anegada basin likely as the result of
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the uplift of a footwall block along a normal fault. Basement is calculated about 8
km deep beneath the deepest part of the VIB and is consistent with my
interpretation shown for the Shell multi-channel seismic line shown Figure 6.
p.19
Figure 6: A) My interpretation of the multi-channel Shell seismic line published as Figure
4 in Jany et al. (1990). I used the velocity information published in the Jany et al.
(1990) paper in the B part below and integrated outcrop information from the rift
flanks of St. Croix and Vieques islands to derive a lithologic interpretation shown
on the stratigraphic column to the right and also shown as the same-colored units
on the 1:1 scale cross section. Note that there are no major normal faults along
the north side of the basin. B) Original Shell seismic section and interpretation by
Jany et al. (1990).
p.24
Figure 7: Stratigraphic columns showing known lithologies of sedimentary rocks and
basement rock from Vieques and St. Croix flanking the VIB and inferred
lithologies to a depth of about 10 km within the VIB using the velocity
information from the Shell line as shown in Figure 6. Onland stratigraphic data
from Vieques and St. Croix are from Hubbard et al. (2004).
p.26
Figure 8: The trace of the Puerto Rico-Virgin Islands fault zone (PRVIFZ) forms a
continuous and linear, northwest-to-southeast-striking fault over a distance of 200
km. I digitized fault segments making up the PRVIFZ fault zone from the
following sources: 1) Mona rift (Mondziel et al., 2010); 2) Cerro Goden fault
zone (Mann et al., 2007a); 3) Great Southern Puerto Rico fault zone (Glover,
1971); 3) Great Southern fault zone on southern shelf of Puerto Rico (Mann et al.,
2007b); and 4) faults in the VIB mapped in this thesis using high-resolution,
multibeam bathymetry from Grothe et al. (2012) and Taylor et al. (2008) as
shown in Figure 3. Faults in the Whiting basin are inferred from seafloor scarps
on the basin floor that are continuous with the Great Southern fault zone in Puerto
Rico. Most workers have thought that the Great Southern fault zone is a leftlateral fault including Glover (1971) who identified left-lateral offsets of 10 km
and Erikson et al. (1991) who inferred left-lateral shear combined with thrusting.
However, the continuity of the PRVIFZ would require right-lateral shear to
maintain extension in the Oligocene to recent Mona rift. Mondziel et al. (2010)
has shown a total of 6.1 km of opening on the Mona rift with no evidence for a
past history of either compression or inversion of normal faults.
p. 28
xii
Figure 9: A) View of multibeam bathymetry in the western VIB showing the
morphologic expression of active strike-slip faults passing from the VIB into the
Whiting basin to the west and up the slope of southeastern Puerto Rico to
continue as the onland Great Southern Puerto Rico fault zone mapped by Glover
(1971). B) Zoom of eastern edge of Whiting basin showing ~3.5 km of rightlateral offset on the PRVIFZ. C) Zoom of canyon along northern slope of the VIB
showing 1.5 km of right-lateral offset on the PRVIFZ.
p.30
Figure 10: Oblique view of the VIB looking east using high-resolution bathymetry from
Grothe et al. (2012) and Taylor et al. (2008) and displayed using Midland Valley
MOVE version 2014.2 (2014). Lines controlling the location of the faults in the
VIB are shown and include lines from Raussen et al. (2013) and Jany et al.
(1990). This view shows well the asymmetrical, half-graben structure of the VIB
with the steeper scarp of the St. Croix fault and the more gently sloped, largely
unfaulted, and thinly sedimented basement surface on the north side of the basin
(compare this view to the Shell seismic line shown in Figure 6). Seismic lines
used to constrain the fault locations are shown.
p.32
Figure 11: Oblique view of the VIB looking east using high-resolution bathymetry from
Grothe et al. (2012) and Taylor et al. (2008) and displayed using Midland Valley
MOVE version 2014.2 (2014). Lines controlling the location of the faults in the
VIB are shown and include lines from Raussen et al. (2013) and Jany et al.
(1990). Four major faults are identified in the VIB and include: 1) St. Croix
normal fault forming the major north-dipping, listric normal fault controlling the
main phase of basin opening and best imaged on the Shell seismic lines shown in
Figure 6; note that this large normal fault tilts its footwall, the island of St. Croix,
to the south; the eastward extension of the St. Croix normal fault along the
northern edge of Saba bank does not appear to be active as the Saba bank is
horizontal and sits at a greater depth (~60 m BSL) than the more active St. Croix
footwall block that is tilted and elevated up to 350 m ASL; 2) the Tortola normal
fault forms a major south-dipping normal fault controlling the uplift and
northwest tilt of Tortola ridge (footwall block) that forms the southeastern edge of
the Anegada Passage; the Tortola fault can be traced on seismic lines about
halfway across the VIB; 3) Virgin Islands fault zone can be traced as flower zone
structure across the VIB and northeatward into the Anegada Passage.
p.33
Figure 12: A) Uninterpreted seismic line from Jany et al. (1990, their figure 5) across the
central VIB. B) Interpreted line showing the north-dipping St. Croix normal fault
along the southern edge of the VIB and the positive flower structure formed by
xiii
the Virgin Islands fault zones (basement is shown in red, sedimentary section is in
purple). Sediment wedging along the southern edge of the VIB shows that the St.
Croix normal fault is the major fault controlling the VIB opening with greatest
amount of normal fault throw as supported by the interpretation of the Shell
multi-channel seismic line shown in Figure 6. Flower structure formed by Virgin
Islands fault zones has a relatively minor structural effect on the top basement
surface and is consistent with its proposed strike-slip character and its likely
younger age than the St. Croix normal fault.
p.36
Figure 13: A) Uninterpreted seismic line from Jany et al. (their figure 6) across the
central VIB. B) Interpreted line showing the north-dipping St. Croix normal fault
along the southern edge of the VIB and the positive flower structure defined by
the Virgin Islands fault zones (basement is shown in red, sedimentary section is in
purple). Sediment wedging to south shows that St. Croix normal fault is the
major fault controlling the VIB opening with greatest amount of fault throw as
supported by the Shell multi-channel seismic line shown in Figure 6. Flower
structure formed by Southern and Virgin Islands fault zones has a minor effect on
the top basement surface consistent with its strike-slip character and likely
younger age than the St. Croix normal fault.
p.38
Figure 14: A) Uninterpreted seismic lines from Jany et al.(1990) that combines two of
their lines 10 and 11B across several normal fault blocks in the Anegada Passage,
Tortola Ridge, St. Croix basin, and into the Kallinago Trough. B) Interpretation
shows the steep-sided Tortola ridge is a horst block bounded by faults in the
Anegada passage to the west and the St. Croix basin to the east. The St. Croix
basin at a depth of 2850 m BSL is asymmetrical with greater throw on the Tortola
fault to the west than the Saba bank fault to the east. The Kallinago basin to the
east sits at a shallower depth (610 m BSL) and exhibits a full graben geometry on
this line.
p.39
Figure 15: A) Uninterpreted seismic lines from Jany et al., 1990, their figure 11A, across
several normal fault blocks in the Anegada Passage, Tortola Ridge, St. Croix
basin, and into the Kallinago Trough. B) My interpretation proposes that the
steep-sided Swordfish and Shark seamounts are uplifted footwall blocks adjacent
to normal faults, rather than seamounts of volcanic origin as previously
interpreted. The Tortola ridge footwall is the dominant vertical footwall uplift in
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this area compared to the Anegada Passage that forms a narrow, symmetrical
valley with more gently-dipping
p.41
Figure 16: A) Uninterpreted single-channel line 3 from Raussen et al. (2013) (location
shown in Figure 3). B) Zoom of VIB center showing the Virgin Islands fault zone
with flower zone structure interpreted as undergoing a right-lateral sense of shear
and the sub-parallel St Croix normal fault along the southern basin margin
showing normal offset.
p.42
Figure 17: A) Uninterpreted, single-channel line 1 from Raussen et al. (2014) (location
shown in Figure 3). B) Zoom of VIB center showing Virgin Islands fault zone
with flower zone structure interpreted as undergoing an active, right-lateral sense
of shear and the St Croix normal fault along the southern basin margin showing
normal offset.
p.43
Figure 18: A) Uninterpreted single-channel line 6 from Raussen et al. (2014) (location
shown in Figure 3). B) Zoom of line in VIB center showing Virgin Islands fault
zone with active, flower zone structure interpreted as undergoing a right-lateral
sense of shear and sub-parallel St Croix fault along the southern basin margin
showing normal offset. Note the presence of a south-dipping normal fault north
of the St. Croix fault that I propose as the western extension of the Tortola normal
fault that is well expressed in the eastern part of the VIB and bounding the St.
Croix basin (see Figure 11).
p. 44
Figure 19: A) Uninterpreted single-channel line 11 from Raussen et al. (2014) (location
of line is shown in Figure 3). B) Zoom of Anegada Passage showing Virgin
Islands fault zone exhibiting flower zone structure typical of strike-slip faults and
interpreted as undergoing a right-lateral sense of shear; the sub-parallel St Croix
fault along the southern basin margin shows normal offset. The Virgin Islands
basin faults have extended to the northeast through the Anegada passage and
bound the edges of the narrow but deep Anegada passage.
p. 46
Figure 20: Fence diagram showing the trace of the Virgin Islands, St. Croix and Tortola
fault zones passing down the long axis of of the VIB. Fence diagram is based on
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selected, interpreted seismic sections from Jany et al. (1990), and Raussen et al.
(2013).
p.47
Figure 21: A) Virgin Islands basin during the late Cretaceous to the middle Eocene when
VIB has not yet undergone shearing and only the asymmetrical, half-graben
geometry of the basin is apparent. Floor of VIB is exposed igneous basement
with no overlying sedimentary infill. B) During the middle Eocene to present the
PRVI block rotates counter-clockwise causing the Virgin Islands fault zone to
move to the north-northeast, and activating the Tortola Ridge normal fault. Major
depocenters are the VIB and the Anegada Passage. C) Present day GPS vectors
from Benford et al. (2012) indicate the PRVI block continues to undergo CCW
rotation. Major depocenters of the Anegada Passage include the VIB and the St.
Croix basin.
p. 48
Figure 22: Isochron map in milliseconds of the central VIB based on mapping the Jany et
al. (1990) seismic lines - including the Shell line - as these were the only lines
where the top basement surface beneath the VIB was visible. Sediments thicken
towards the St. Croix normal fault confirming that it was the main structure
responsible for the asymmetrical opening of the half-graben as seen on the Shell
line in Figure 6.
p. 49
Figure 23: Structural restoration of Shell seismic line presented by Jany et al. (their figure
4 shown as figure 6 in this thesis). I reconstructed the line using Midland Valley
MOVE version 2014.2 (2014). My conversion of this section from time to depth
using the velocity structure from this line was consistent with the results from the
Werner deconvolution shown in Figure 5. A) Digitized line from Jany et al., 1990.
B) Inferred lithologic layering with ages based on velocity information provided
on Figure 4 of Jany et al. (1990). B) Restored 3.1 km on normal fault and restored
Quaternary unlithified muds and clays layer. C) Restored 8.4 km on normal fault
and restored middle Eocene to Quaternary lithified sandstone; D) Restored 10.1
km on normal fault and restored late Cretaceous to middle Eocene sand and marl
layer; E. Restored 15.8 km on normal fault and restored late Cretaceous igneous
layer.
p. 51
Figure 24: Indentation and rotational model of the Puerto-Rico Virgin Islands microplate
from early Miocene to present (reconstructed positions of Bahamas platform
modified from Grindlay et al., 2007). A) The Virgin Islands basin was originally
the western extension of the Kallinago intra-arc basin that underwent left-lateral
xvi
offset in the early Miocene as a result of the oblique collision and indentation of
the arc of Puerto Rico and the Virgin Islands by Bahamas Platform. This may
explain why older volcanic arc rocks in Puerto Rico are found on the
southwestern side of the island and to the southwest of the proposed western
extension of the Kallinago trough (Cerro Goden-Virgin Islands fault zone) and
younger arc rocks are found on the northeastern side of the island (Glover, 1971).
B) As the Bahamas platform moved to the west, the PRVI microplate became free
to rotate counter-clockwise along the right-lateral Puerto Rico-Virgin Islands fault
zone. C) Modeling of GPS vectors from the PRVI microplate by Benford et al.
(2013) shows that counterclockwise of the PRVI microplate presently occurs
along right-lateral strike-slip faulting of the Cerro Goden-Virgin Islands fault
zone.
p. 55
xvii
CHAPTER 1: Introduction to the thesis
Rationale for the study of the Virgin Islands basin
This master’s thesis was motivated by a 30-year-long debate about on the tectonic
origin and geologic evolution of the Virgin Islands basin along the northeastern edge of the
Caribbean plate. While several previous studies have been conducted in the region looking
to constrain the origin of the basin, each previous study has relied on different types of data
that have led to different results. For example, interpretations range from left-lateral strike
slip faulting as proposed by Raussen et al. (2013), to right-lateral strike-slip faulting with
pull-apart basins as proposed by Jany et al. (1990), to orthogonal rifting as proposed by
Speed and Larue (1991) and Feuillet et al. (2002). These models are reviewed in detail in
Chapter 2.
My goal for the thesis was to make use of all data relevant to the basin origin rather
than restricting the thesis study to particular data types. My study integrates: 1) gravity
and magnetic data to constrain deep crustal structure beneath the basin; 2) offshore seismic
reflection, refraction, and velocity data to constrain the top of igneous basement and the
thickness, lithology and faulting of the sedimentary fill; 3) onland outcrop data to compare
to offshore geophysical data, high-resolution bathymetric data to map recent faults that
break the seafloor; and 4) GPS results to compare to the orientation and nature of mapped
faults, and earthquake data to assess the activity of faults. I have combined and visualized
the various data types using ArcGIS and Midland Valley MOVE v. 2014.2 (2014).
1
History and development of this thesis
I earned my BS degree in geology in 2011 from the University of Wisconsin,
Madison. Towards the end of my time at Wisconsin, I realized my desire to pursue a career
in the oil and gas industry. I was accepted into the Geology MS program at the University
of Houston in the fall of 2011, and started my master’s degree work in January of 2012.
My main interest during my BS degree was carbonate rocks so I pursued this interest during
my UH MS by taking all the carbonate courses offered.
I started the UH MS program as a non-thesis student with no financial support from
UH. I spent my first three semesters in search of an advisor to work with no success. In the
fall of 2012 I took Dr. Paul Mann’s Basin Analysis for Petroleum Exploration course. Two
of the teaching assistants for the course were graduate students Bryan Ott and Jordan
Sayers. Both had been members of the previous spring’s UH Imperial Barrel Award
competition. They often described to me and the basins class about their experiences on
the team and the level of knowledge and expertise that they had gained from the
competition. Dr. Mann planned a final project in the basin analysis class that provided a
condensed version of the IBA class including a team-based oral presentation. It was at this
point that I decided I was interested in competing on the IBA team and representing UH
for the next competition in the spring of 2013.
The following spring (Spring 2013) I was chosen to be on the team along with Kyle
Reuber, Dutch Crews, Altay Sansal, and Lindsey Aldrich, under the supervision of Drs.
Mann and John Castagna. The team repeated the previous year’s performance, coming in
third place in the Gulf Coast region. For my own interests, the course exposed me to a
2
larger area of basin studies than carbonates alone; this included regional tectonics,
structure, paleogeography, petroleum systems, and basin modeling. After the competition
was over, Dr. Mann hired me as a research assistant with his Caribbean Basins, Tectonics
and Hydrocarbons (CBTH) group and suggested the idea of a MS thesis working on a
problem in the northeastern Caribbean region.
During the summer of 2013, I interned at BP in Houston in their Campos and Santos
basins/Brazil exploration team working on carbonate well log analysis. Dr. Mann also
presented me with this idea for a MS project on the Virgin Islands basin after his reading
of the paper by Raussen et al. (2013) in the journal Terra Nova. I requested and received
the segy versions of the seismic lines collected by Raussen et al. as part of the Galathea 3
Danish cruise to the area in 2006. Dr. Mann and CBTH funded me as a research assistant
with the CBTH project for the fall (2013), spring (2014), and summer (2014) during which
time I developed this MS thesis project.
Future plans
In the fall of 2014, I was accepted into the PhD program at UH to start a new project
that combining my original interests in carbonate rocks with my newly developed interests
in tectonics, structure, and rift formation along the Equatorial and South Atlantic conjugate
margins. Carbonate basins formed in sag basins above rifts form a major setting for recent,
large oil and gas discoveries and my project will use seismic, well, and core data to better
understand this economically important tectonic and depositional setting.
3
CHAPTER 2: Cenozoic basin evolution of the Virgin Islands basin and Anegada
Passage, northeastern Caribbean
Introduction and major structural features
The Virgin Islands basin (VIB) is located between the islands of Puerto Rico,
Vieques, and St. Croix, U.S. Virgin Islands, on the northeastern edge of the Caribbean
plate; it is roughly 75 km long, 20 km wide, and has water depths of 4500 m of water at its
deepest area in the basin center (Fig 1). The Virgin Islands basin is situated within the
strongly curved part of the plate boundary zone between the Caribbean and North
American plates where the north-south-trending Lesser Antilles active volcanic arc and
subduction zone transitions into the east-west-trending left-lateral Caribbean-North
America plate boundary, which lacks an active volcanic arc (Fig. 1).
The regional free-air gravity map in Figure 1 shows the plate tectonic setting of the
northeastern Caribbean- North America plate boundary using a basemap of free-air gravity
from Smith and Sandwell (1997). The large, elongate gravity high forming both the Lesser
Antilles and Aves Ridge (Lesser Antilles) and continuing to the west as the Greater Antilles
in the Virgin Islands, Puerto Rico, and Hispaniola, has been interpreted by Burke et al.
(1988) - and later authors like Aitken et al. (2011) - as the Great Arc of the Caribbean
(GAC). The GAC can be traced as a semi-continuous structural and gravity high from
northern South America to western Cuba.
The gravity image in Figure 1 reveals the high degree of complexity in basement faulting,
especially in the area east of Puerto Rico and in the northern Lesser Antilles.
In these
areas, a pattern of faults is radial with respect to the overall arcuate trend of the arc
extending from the Anegada Passage in the northwest to the forearc area of the northern
4
Figure 1: Plate tectonic setting of the northeastern Caribbean-North America plate boundary shown on a basemap of free-air gravity
from Smith and Sandwell (1997). Hot colors indicate areas of thicker crust while cool colors are areas of thinner crust. The active
line of volcanoes of the Lesser Antilles arc (red triangles) are separated from the inactive line of pre-Miocene volcanoes of the
Limestone Caribees (open triangles) by the Kallinago intra-arc rift basin. Radial normal faults affect the forearc area of the Lesser
Antilles arc southward from the Anegada passage. Seamounts (black squares) have been proposed in the area of the Virgin Islands
basin (VIB). Box shows more detailed map of the VIB area shown in Figure 2A.
5
Lesser Antilles (Feuillet et al., 2002). The Anegada Passage-VIB is unique amongst all
of these radial faults, as this rift structure passes as a deepwater passage down to 4 km
water depth, penetrates through the entire GAC basement high, and connects the Atlantic
Ocean to the Caribbean Sea (Fig. 1).
Another interesting feature in this rift is the 30-km-wide and linear Miocene and
younger Kallinago trough that is an intra-arc rift separating the active volcanic chain of the
Lesser Antilles from the parallel and pre-Miocene volcanic chain to the east, now covered
by limestone platforms called the “Limestone Caribees” (Bouysse, 1984). The age of the
Kallinago trough is not well known as it - like the VIB - has never been drilled.
The Kallinago trough is not a typical rift basin related to back-arc spreading because
the active volcanic line jumped westward away from the trench and not toward the trench
as commonly found in most back-arc basins (Bouysse, 1984). The 2-km-deep Kallinago
trough is considerably shallower than the 4-km-deep VIB. The sharp break seen on the
gravity map parallel to the Anegada Passage and VIB abruptly terminates the active trend
of the Lesser Antilles arc and Kallinago trough (Fig. 1). An important question is whether
the Kallinago trough ever extended into or west of the present-day Anegada Passage-VIB
and was subsequently modified by deformation along the trend of the Anegada PassageVIB - or whether the Kallinago trough naturally terminated east of that feature.
Active tectonic setting of the northeastern Caribbean plate boundary
GPS vectors and earthquake slip vectors from previous workers compiled on Figure 2A
provide additional constraints for the present-day active tectonic setting of the area of the
Anegada Passage and VIB in the northeastern Caribbean. GPS vectors from Mann et al.
6
Figure 2: A) Tectonic map of the Puerto Rico-Virgin Islands region with GPS vectors
showing that the Anegada Passage is an active, oblique-slip fault separating
northeastern vectors in Puerto Rico from a more easterly vectors on St. Croix and the
Lesser Antilles arc (all vectors are relative to a fixed North America plate) purple
vectors are from Benford et al., 2013; black vector from Manaker et al., 2008). Slip
vectors from subduction-related earthquakes from Hippolyte et al. (2005) and Doser
et al. (2005) show a radial pattern of thrusting around the strongly curved subduction
plate boundary. Earthquake focal mechanisms from the Mona rift and in the western
VIB from Mondziel et al. (2010) show active extension along with earthquake
swarms in the VIB from Frankel et al. (1980). B) Oblique view of bathymetry using
the same data in A showing prominent Anegada Passage fault zone and its abrupt
truncation of the Kallinago basin. Profile in inset shows a cross section of the
Kallinago basin formed by a large normal fault along its eastern edge (upthrown side
of the normal fault now covered by Miocene and younger shallow-water limestone).
7
8
(2002), Manaker et al. (2008), and Benford et al. (2013) in Puerto Rico, the Virgin Islands,
and the northern Lesser Antilles, all show that the Anegada Passage-VIB is an active,
oblique-slip fault separating northeastern vectors relative to the North America plate in
Puerto Rico from a more easterly vectors on St. Croix and the Lesser Antilles arc.
Earthquake slip vectors on subduction-related events compiled from Hippolyte et al. (2005)
and Doser et al. (2005) on Figure 2A show a more continuous, radial pattern around the
strongly curved plate boundary in this same region. GPS studies by Jansma et al. (2005)
showed that the rate of motion across the Anegada Passage-VIB was absent or within the
error of GPS measurements at the time of their publication (<2 mm/yr). Bob Wang
(University of Houston, personal communication, 2014) proposes that the rate of oblique
opening of the VIB based on continuous GPS sites is on the order of 2 mm/yr.
Earthquake focal mechanisms with normal solutions in the Mona rift and in the
western VIB from Mondziel et al. (2010) show active extensional deformation, including
areas of earthquake swarms within the VIB from Frankel et al. (1980). These zones of
shallow earthquake activity indicate that the plate boundary is broad and could include
active faulting within the Mona rift, Puerto Rico, and the Virgin Islands. A devastating
earthquake that affected St. Croix in 1867 that was accompanied by a 6.1-m-high tsunami
shows that large historical earthquakes have affected the area in the past (McCann, 1985).
An oblique view of the region shown in Figure 2B using bathymetry shows the
abrupt truncation of the Kallinago intra-arc basin by the Anegada Passage-VIB as observed
on gravity data in Figure 1. The seismic profile from the University of Texas Institute for
9
Geophysics (UTIG) in the inset of Figure 2B shows a cross section of the Kallinago basin
formed by a large, active normal fault along its eastern edge. The upthrown, eastern
footwall of the normal fault is the extinct arc, now covered by Miocene and younger,
shallow-water Limestone Caribees.
Previous tectonic interpretations of faulting and basin formation in the Anegada
Passage-VIB area
Previous groups of workers have proposed different tectonic mechanisms to explain
the origin of basins and faults along the trend of the Anegada Passage and VIB (Fig. 3).
These previous models can be organized into three main groups. The first group, including
Mann and Burke (1984) and Raussen et al. (2013), proposed left-lateral shear along the
length of the bathymetric feature similar to other major fault systems of the North AmericaCaribbean plate boundary zone in areas east of Puerto Rico such as the Cayman trough
(Fig. 3A).
The second group of workers proposed that right-lateral, rather than left-lateral
shear, was the mechanism for forming the observed faults and basins in the Anegada
Passage and VIB (Fig. 3B). In this group, Jany et al. (1990) proposed northeastward escape
of the Puerto Rico block into the Puerto Rico trench area along a right-lateral shear zone
(Fig. 3B, inset). Stephan et al. (1986) proposed that this right-lateral shear was the result
of an east-west shortening of the GAC “frame”, or crustal block (Fig. 3B, inset). Finally,
Masson and Scanlon (1991) proposed right-lateral shear and basin opening as the result of
a large, counterclockwise rotation of the Puerto Rico microplate (Fig. 3B, inset). The third
group of workers that included Speed and Larue (1991) and Feuillet et al. (2002) proposed
10
Figure 3: Three previous tectonic models for the opening of the Virgin Islands basin
that all predict different opening mechanisms. A) Mann and Burke (1984) and
Raussen et al. (2013) proposed left-lateral shear similar to other faults in the
Caribbean-North America plate boundary zone. B) Jany et al. (1990) predicted rightlateral shear based on their inferred geometry of pull-apart basins within the VIB;
their proposed driving mechanisms for right-lateral shear included: 1) tectonic escape;
2) east-west shortening and bending (Stephan et al., 1986); and 3) counterclockwise
rotation of the Puerto Rico microplate that was later developed by Masson and
Scanlon (1991). C) Speed and Larue (1991) and Feuillet et al. (2002) proposed that
the VIB opened by orthogonal rifting with negligible left- or right-lateral shear.
11
that extension orthogonal to the trend of the Anegada Passage and VIB was the mechanism
responsible for the opening of the basins (Fig. 3B, inset). Feuillet et al. (2002), proposed
that radial extension of the Lesser Antilles arc may provide the tectonic mechanism for the
observed pattern of radial arc extension as observed on the gravity map in Figure 1.
Data and methods used in the thesis
I have integrated several types of geologic and geophysical data into this thesis that
were not available when most of the previous models, summarized in Figure 3, were
proposed.
Bathymetric and topographic data
As a basemap and tool to interpret the traces of active faults that form scarps on the
seafloor, I combined high-resolution bathymetric data from Grothe et al. (2012) and Taylor
et al. (2008) with a topographic DEM from GeoMapApp (2014) using Midland Valley
MOVE v. 2014.2 (2014). The 3D viewer in MOVE allowed me to examine seafloor faults
from multiple viewing directions and angles.
Earthquake focal mechanism data
I compiled shallow-focus earthquake focal mechanisms from compilations by
Doser et al. (2005) and Hippolyte et al. (2005) to overlay onto fault maps prepared from
interpretations of seismic lines. As recorded earthquakes in the VIB tend to be small
(M<5.0), there are few published focal mechanisms in the study area.
12
GPS vectors of plate motions
There are several groups with GPS data from the Puerto Rico-Virgin Islands area.
I compiled published GPS vectors by Mann et al. (2003), Manaker et al. (2008), and
Benford et al. (2013) (Fig. 2). Ongoing GPS work by Dr. Bob Wang (University of
Houston) using baselines between continuously recording sites in Puerto Rico and St. Croix
has not yet been published and was not included in this study.
Gravity, magnetic and refraction data
This part of the study used the 1987 Geological Society of America (GSA) Decade
of North America (DNAG) gravity and magnetic grids as compiled by the National
Geophysical Data Center. These grids are based entirely on ship tracks in the area and
therefore are not subject to smoothing and other artifacts that can affect grids collected by
satellites. I used a previous refraction study in the area by Officer et al. (1959) to better
constrain the crustal structure used in the gravity and magnetic models. To analyze these
data, I used Oasis Montaj software.
Deep-penetration seismic reflection and interval velocity data
One of the most useful pieces of data for this study was the single multi-channel
Shell seismic line that crossed the VIB and was recorded to a depth of 15 km (Fig. 4). This
line included velocity information and was published by Jany et al. (1990). I used velocity
information to help infer the type of sedimentary rocks and basement types beneath the rift.
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Figure 4: Structural map of the VIB using high-resolution bathymetric data from Grothe et al. (2012) and Taylor et al. (2008) and
topographic DEM from GEBCO (2013). All seismic reflection lines used in the study are labeled: Lines 16, 17, 18, and 19 in
black are single-channel lines from Raussen et al. (2013) and lines 9, 10, 11 and 12 in blue are single-channel lines from Jany et
al. (1990). Line J-Shell Shell is a multichannel Shell line with velocity information that was published in Jany et al. (1990) and
reinterpreted in this thesis in Figure 6. Seamount features are labeled as black squares: I reinterpret most of these seamounts as
uplifted footwall blocks whose sharp and conical summits resemble seamounts on 2D seismic profiles. Yellow dots represent
locations of stratigraphic columns shown in Figure 12. Faults shown on this map are interpreted from the grid of all seismic lines
shown.
I also used this line as the basis for my structural restoration in MOVE as it provided the
best image of the St. Croix normal fault bounding the southern edge of the basin.
Onland geologic data from the flanks of the VIB
Detailed onland geologic studies conducted on St. Croix by Gill et al. (1999) were
used to infer rock types beneath the rift as well as the timing of major rift events. There are
few published geologic studies available from the other rift flank on Vieques (Fig. 2).The
most notable publication on Vieques was an abstract published by Batbayar et al. (2013).
Shallow-penetration seismic reflection data
Shallow-penetration seismic data included two sources: 1) published lines in Jany
et al., 1990, which in some cases imaged the top of basement; and 2) published lines by
Raussen et al. (2013) for which I obtained the segy versions that I was able to reinterpret
as part of this thesis. None of the Raussen et al. (2013) lines recorded to a depth of only 8
seconds TWT revealed clear images of the top basement surface; so were mainly used to
map shallow faults. An isochron map of total basin sedimentary thickness was prepared
from only those lines from Jany et al. (1990) that clearly imaged the top basement surface.
Visualization and structural restorations using MOVE software.
MOVE v. 2014.2 (2014) software provided a useful tool to import features digitized
in GIS (including bathymetry and seismic lines) and then to visualize them in three
dimensions. Fault mapping compared all fault surface expression with their subsurface
structure as seen on seismic lines.
15
Objectives of the thesis
The main objective of the thesis was to reexamine the VIB area in light of the newer data
sets and tools that were not available to previous workers compiled on Figure 3. For
example, the nature and continuity of faults within the VIB varied widely from all of the
previous studies summarized on Figure 3 and therefore led to the many different
interpretations on the tectonic origin of the basin as shown on Figure 3. Improved seismic
line spacing achieved by combining the lines in Jany et al. (1990) with those in Raussen et
al. (2013), and the use of 3D viewing tools for the high-resolution bathymetry from Grothe
et al. (2012) and Taylor et al. (2008), allowed many improvements in fault mapping and
better establishing fault continuity over the previous studies that used less data. Another
objective of the study was to better quantify the amount of basin opening for the VIB as
done in a previous study by Mondziel et al. (2010) for the Mona rift off the northeast coast
of Puerto Rico (Fig. 2). This thesis uses the opening amounts for both the VIB and Mona
rift to improve our understanding of the rotation of the Puerto Rico microplate and the
regional fault systems along which the rotation occurs (Masson and Scanlon, 1991;
Benford et al., 2013) (Fig. 3).
Crustal structure of the VIB and Anegada Passage using gravity and magnetic
transects
Constraints from potential fields on deeper crustal structure of the VIB
16
The crustal structure of the Anegada Passage and Virgin Islands basin (VIB) has
proven difficult to image because of: 1) the cover of up to 2 km of overlying carbonate
strata of the Puerto Rico-Virgin Islands carbonate platform north of the basin (van Gestel
et al., 1988); 2) the deep-water VIB with an up to 8-km-thick fill; and 3) the smaller, 1.5km-thick carbonate platform capping the St. Croix ridge south of the VIB (Fig. 4). As a
result of the difficulty imaging the crust using seismic reflection profiles and the existence
of only one deep-penetration multichannel profile across the VIB (Jany et al., 1990) (Fig.
4), my study uses gravity and magnetic data, combined with existing refraction data from
the VIB area by Officer et al. (1959), to establish the deeper crustal structure below the
Anegada Passage and Virgin Islands basin. To my knowledge, this is the first gravity and
magnetic modeling attempted in this area.
Gravity methods used for crustal structure
Gravity modeling consists of defining layer thicknesses based on a certain layer’s
density and its corresponding gravity signature. This study used six different layer
densities to depict the subsurface structure of the region on transects selected from a
basemap and imported to Geosoft’s GM-SYS modeling module (Fig. 5).
This study uses the Geological Society of America (GSA) Decade of North
American Geology (DNAG) (1987) Bouguer gravity anomaly grid as well as the DNAG
(1987) magnetic anomaly grid obtained from the National Geophysical Data Center. These
data consist entirely of ship-track information and did not include any satellite data.
The upper mantle was given a value of 3.2 g/cm3. A two-layer crustal structure is
assumed in which a homogeneous upper crust overlies a homogeneous lower crust. In this
17
Figure 5: A) Bouguer gravity anomaly map from compilation by Decade of North
American Geology (DNAG) (1994) showing locations of three gravity transects crossing
the VIB and Anegada Passage in B, C, and D. Inverted white triangles on each of the
three lines show refraction stations by Officer et al. (1959) that were used in this model to
constrain crustal thicknesses. Black dots on the gravity profiles are Werner deconvolution
solutions from coincident magnetic profiles that were used to more precisely locate the
top of crystalline basement. These values were consistent with depth conversions
performed on the seismic profiles using Midland Valley’s Move 2014.2
software. B) Bouguer gravity profile B-B’ crossing the Anegada Passage in an east-west
direction shows 20-km-thick crust of the Virgin Islands platform thinning to the east in
the area of the St. Croix basin and Kallinago trough. Basement is elevated to a depth of
600 m below sea level (BSL) west of the Anegada basin likely as the result of the uplift
of the footwall of a normal fault. C) Bouguer gravity profile C-C’ crossing the deepest
part of the VIB (4500 m BSL) showing crust of equal thickness to the north beneath the
Virgin Islands basin and to the south beneath St. Croix; the profile also shows an overall
basin asymmetry consistent with the presence of the largest normal fault on the steeper,
south side of the basin with St. Croix island uplifted as its footwall block. Basement is
predicted to be at a depth of 7 km beneath the center of the VIB and about 1 km beneath
its uplifted flanks. D) Bouguer gravity profile D-D’ crossing the VIB in an east-west
direction shows 22-km-thick crust of southeastern Puerto Rico thinning to the east with a
thickness of 18 km in the area of the St. Croix basin and Kallinago trough. Basement is
most elevated to a depth of 600 m below the surface west of the Anegada basin likely as
the result of the uplift of a footwall block along a normal fault. Basement is calculated
about 8 km deep beneath the deepest part of the VIB and is consistent with my
interpretation shown for the Shell multi-channel seismic line shown Figure 6.
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19
study the Cretaceous to Eocene crystalline crust of the GAC in the Puerto Rico and Virgin
Islands area was subdivided into an upper, middle, and lower layer with densities of 2.7,
2.9, and 3.1 g/cm3, respectively. An average depth to top of mantle of 20 km - typical for
intra-oceanic island arcs like the GAC (Christenson et al., 2008) - is taken from a previous
regional refraction study by Officer et al. (1959), refraction stations are inverted white
triangles shown on the three transects in Figure 5.
Refraction and gravity data are combined to control the depth to different layers of
the model (water, sediment, crystalline crust, and top mantle) (Fig. 5). In this modeling,
the isostatic condition assumes that deep crust corresponds to a deeper Moho and shallow
crust corresponds to an elevated Moho.
Magnetic methods used for picking top of crystalline basement
Determining depth to basement in this area is problematic due to the thick carbonate
platforms overlying crystalline basement rocks of the GAC (Fig. 4). For this reason, I used
the same Werner estimation as Dale (2013) in his potential fields study of the depth to top
of basement beneath the 7-km-thick Bahama carbonate platform. Werner magnetic source
depth estimation is an automated profile-based technique that solves a system of overdetermined linear equations to calculate magnetic source position (x and z), dip, and
magnetic susceptibility. The method calculates edge solutions from the horizontal gradient
of the total field profile, and dike solutions from the total field profile. In practice many
solutions are calculated and a source location is picked by examination of the solution
distribution and used in conjunction with wells penetrating the top of basement and/or
results from seismic refraction experiments.
20
Test Werner profiles were generated over areas with seismic refraction control from
Officer et al. (1959) while changing the program parameters. These changes also altered
the pattern of depth solutions such that they can be “tuned” to be consistent with nearby
refraction control points. The patterns of depth solutions were analyzed for each magnetic
profile and final depth locations are superimposed as clusters of black dots on the gravity
profiles of Figure 5 where the densest concentrations of dots are assumed to reflect the best
estimation of the top basement surface. Twenty one final top basement depth picks were
selected from 3 straight-lined profiles from the GEODAS ship-track data (Fig. 5).
This study uses the Geological Society of America (GSA) Decade of North
American Geology (DNAG) (1987) Bouguer magnetic anomaly grid acquired from the
National Geophysical Data Center. These data consist entirely of ship-track information.
Combined gravity and magnetic results from three gravity/magnetic transects across the
VIB
Bouguer gravity profile B-B’ crossing the Anegada Passage in an east-west
direction shows 20-km-thick island arc crust of the GAC underlying the Virgin Islands
platform thinning to the east in the actively rifting area of the St. Croix basin and Kallinago
trough. Basement is most elevated to a depth of 600 m below the surface west of the
Anegada, basin likely as the result of a footwall uplift along the Anegada fault zone.
Bouguer gravity profile C-C’ crossing the deepest part of the VIB (4500 m BSL)
shows crust of equal thickness to the north beneath the Virgin Islands basin and to the south
beneath St. Croix (Fig. 5). The profile also shows a cross-sectional asymmetry consistent
21
with the presence of the largest normal fault on the steeper, south side of the basin, with
the island of St. Croix uplifted as a footwall block. Basement is predicted to be at a depth
of 7 km beneath the VIB and about 1 km beneath the flanks.
Bouguer gravity profile D-D’ crossing the VIB in an east-west direction shows 22km-thick crust of typical GAC thickness beneath southeastern Puerto Rico thinning to the
east to thickness of 18 km in the actively rifting area of the St. Croix basin and Kallinago
trough. Basement is most elevated to a depth of 600 m below the surface west of the
Anegada basin, likely as the result of a footwall uplift. Basement is calculated about 8 km
deep beneath the VIB, and is consistent with the Shell multi-channel seismic line shown in
Figure 4.
Structure of the Virgin Islands basin from deep-and shallow-penetration seismic
reflection and bathymetry
Deep-penetration multi-channel seismic data
Jany et al. (1990) used a single deep-penetration seismic reflection profile collected
by Shell in order to better characterize the structure of the Virgin Islands basin (Fig. 6).
They correlated the line shown in depth with the seismic refraction results of Officer et al.
(1959), and proposed that the units with compressional velocities of 5.06 to 5.76 km/sec
correlate to the crystalline basement rocks of the GAC. They proposed on the basis of this
line that the VIB formed as a rift basin in post-Miocene time, and that that the basin was a
half-graben with the main normal fault along the south side of the basin where crystalline
22
basement rocks have been dredged from the steep submarine scarp (Fig. 6). The steeper,
northern side of the island of St. Croix is controlled by this large normal fault with 600 m
of southward tilting of the island/footwall block during the late Miocene, documented from
biostratigraphic studies (Lidz, 1988).
The normal fault imaged on the line is listric, with the fault soling out into a
horizontal decollement which is correlated with a compressional seismic velocity
determined from the refraction study of Officer et al. (1959) to be as high as 7.33 km/sec
(Fig. 6). The high velocity layer associated with the decollement, correlated with the late
Cretaceous volcanic basement of St. Croix, is overlain by rocks with a compressional
velocity of 5.77 km/sec.
The northern slope of the VIB shows no evidence of major normal faults, in contrast
to the prominent normal fault forming the southern slope of the basin. The slope shows
large canyon features that feed large debris wedges in the deep basin. The crust beneath
the deep part of the VIB is thin and almost oceanic in character (Officer et al., 1959.
Outcrops on St. Croix provide some clues on the age of rifting. A prominent rift
trending northeast contains 2 km of Eocene to early Miocene clastic and carbonate rocks,
and corresponds to rifts of similar trend and age described on the St. Croix ridge west of
the island by Holcombe (1989). Jany et al. (1990) infer that these onshore and offshore
rifts of the St. Croix ridge can be correlated to 2-km-thick and tilted lower basinal fill
observed on the Shell line. If this correlation is correct, the VIB would have formed
initially in Eocene time. Jany et al. (1990) note that the more recent rift has a more eastwest orientation than these older northeast-trending rifts.
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Figure 6: A) My interpretation of the multi-channel Shell seismic line published as Figure 4 in Jany et al. (1990). I used the
velocity information published in the Jany et al. (1990) paper in the B part below and integrated outcrop information from the
rift flanks of St. Croix and Vieques islands to derive a lithologic interpretation shown on the stratigraphic column to the right
and also shown as the same-colored units on the 1:1 scale cross section. Note that there are no major normal faults along the
north side of the basin. B) Original Shell seismic section and interpretation by Jany et al. (1990).
The north-dipping Muertos trench is present on the south end of the Shell line (Fig.
6). The velocity structure in that area shows the northward underthrusting Caribbean
oceanic plateau and a wide accretionary prism. Granja et al. (2010) concluded that
subduction in this easternmost area of the trench is inactive due to a lack of seismicity and
recent seafloor deformation, and therefore should have no present-day tectonic control on
the VIB.
Simplified geologic cross section of the VIB based on the Shell line and seismic velocities
Using standard seismic velocities from Mavko (2014), I assigned lithologies to the
velocity values in a profile with igneous basement (6.81 km/s) overlain by
limestones/dolomites (5.77 km/s), that are in turn overlain by sandstones (3.07 km/s), and
capped by unconsolidated muds and clays (1.65 km/s) (Fig.6 A, inset with stratigraphic
column). I digitized all the units shown on the Shell line and assigned lithologies to them.
I used this section to restore the offset on the large normal fault later in the thesis.
Stratigraphic column of the VIB based on correlations to the Shell line and onshore
outcrops in St. Croix and Vieques
Figure 7 summarizes stratigraphic information from the Shell line and onshore
areas. Volcanic basement on St. Croix is known to be Cretaceous to Paleocene in age,
while volcanic basement on Vieques is slightly younger and extends into the Eocene
(Renken et al., 2002). The overlying sedimentary rocks on both islands are mainly
carbonate rocks that show a general shallowing upward trend from several hundred meters
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26
Figure 7: Stratigraphic columns showing known lithologies of sedimentary rocks and basement rock from Vieques
and St. Croix flanking the VIB and inferred lithologies to a depth of about 10 km within the VIB using the velocity
information from the Shell line as shown in Figure 6. Onland stratigraphic data from Vieques and St. Croix are from
Hubbard et al. (2004).
below sea level to the current shallow carbonate banks on the rift flanks of the evolving
VIB. Sedimentary facies on the rift flanks are marly in the early and middle Miocene and
more limestone rich in the later period of uplift in the late Miocene and Pliocene (Gill et
al., 1999). I have reflected the facies changes from rift-shoulder carbonate marl and
limestone facies and rift-center sandstones in the simplified basin cross section taken from
the Shell line and shown in Figure 6A.
Active seafloor faulting in the Virgin Islands basin from high-resolution bathymetric
data
Methods
I combined the high-resolution bathymetry from Grothe et al. (2012) and Taylor et
al. (2008) into a single land-sea map of the study area and then used Midland Valley’s
MOVE v. 2014.2 (2014) to generate high-resolution maps to map and compare faults with
surface expression to subsurface faults seen on seismic profiles.
Extent of the active faulting from the Mona rift to the VIB
The trace of the Mona rift-Cerro Goden-Great Southern Puerto Rico and Virgin
Islands basin fault zone forms a continuous, linear, northwest-to-southeast-striking fault
over a distance of 160 km (Fig. 8). I digitized the traces of these faults onto Figure 8 using
several sources: 1) active normal faults of the Mona rift (Mondziel et al., 2010); 2) active
right-lateral strike-slip faulting along the Cerro Goden fault (Mann et al., 2007a); 3) active
faulting along the Great Southern Puerto Rico fault zone (Glover, 1971); 4) active, seafloor
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28
Figure 8: The trace of the Puerto Rico-Virgin Islands fault zone (PRVIFZ) forms a continuous and linear, northwest-to-southeaststriking fault over a distance of 200 km. I digitized fault segments making up the PRVIFZ fault zone from the following sources:
1) Mona rift (Mondziel et al., 2010); 2) Cerro Goden fault zone (Mann et al., 2007a); 3) Great Southern Puerto Rico fault zone
(Glover, 1971); 3) Great Southern fault zone on southern shelf of Puerto Rico (Mann et al., 2007b); and 4) faults in the VIB mapped
in this thesis using high-resolution, multibeam bathymetry from Grothe et al. (2012) and Taylor et al. (2008) as shown in Figure 3.
Faults in the Whiting basin are inferred from seafloor scarps on the basin floor that are continuous with the Great Southern fault
zone in Puerto Rico. Most workers have thought that the Great Southern fault zone is a left-lateral fault including Glover (1971)
who identified left-lateral offsets of 10 km and Erikson et al. (1991) who inferred left-lateral shear combined with thrusting.
However, the continuity of the PRVIFZ would require right-lateral shear to maintain extension in the Oligocene to recent Mona
rift. Mondziel et al. (2010) has shown a total of 6.1 km of opening on the Mona rift with no evidence for a past history of either
compression or inversion of normal faults.
flower structures marking the southeastward extension of the Great Southern fault zone on
the southern shelf of Puerto Rico (Mann et al., 2005b); these twin, onland faults were
named the Rio Jueyes and Esmeralda fault zones by Glover (1971) who proposed 10 km
of left-lateral offset on the Rio Jueyes fault zone based on an offset of older, geologic
contacts; Mann et al. (2005b) mapped both faults using a high resolution Chirp system; and
5) faults in the VIB mapped in this thesis using high-resolution, multibeam bathymetry
from Grothe et al. (2012) and Taylor et al. (2008) and shown on Figure 4.
Seafloor faulting in the Whiting basin
Faults crossing the 2,000-m-deep Whiting basin off the southeast coast of Puerto
Rico are inferred form straight lineaments on the basin floor that are continuous with the
twin faults of the Great Southern fault zone in the coastal zone of Puerto Rico (Fig. 9). The
continuity of these faults with mapped faults in Puerto Rico and the VIB makes it unlikely
that these lineaments are artifacts of the multibeam data. Unpublished seismic lines from
Western Geco confirm the locations of the faults through the Whiting basin, despite their
appearance as seams/artifacts on the DEM. These lineaments correspond to faults that show
two apparent right-lateral offsets of 3.5 and 1.5 km in the sill area that separates the Whiting
basin from the VIB (Fig. 9). In addition to these right-lateral offsets, the geometry of faults
in the western VIB exhibits a right-lateral step over geometry in the Virgin Islands basin
fault zone that exhibit either flower structure or normal fault profiles on seismic crossings
in this area.
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30
Figure 9: A) View of multibeam bathymetry in the western VIB showing the morphologic expression of active strikeslip faults passing from the VIB into the Whiting basin to the west and up the slope of southeastern Puerto Rico to
continue as the onland Great Southern Puerto Rico fault zone mapped by Glover (1971). B) Zoom of eastern edge of
Whiting basin showing ~3.5 km of right-lateral offset on the PRVIFZ. C) Zoom of canyon along northern slope of the
VIB showing 1.5 km of right-lateral offset on the PRVIFZ.
Main fault systems of the VIB
The northern of the two faults strands in the Whiting basin extends along the
northern, channeled slope of the VIB, whereas the southern fault extends across a saddle
into the deepest part of the VIB where it bifurcates into the Northern and Southern Virgin
Islands oblique-slip faults. In this area I have good seismic coverage of this fault system
(Figs. 10, 11).
The main active seafloor faults of the VIB are summarized on the maps in Figures
11 and 12. Four major parallel fault systems are present in the deep basin of the VIB:
1)
VIRGIN ISLANDS FAULT ZONE forms a set of oblique-slip strike-slip
faults that maintain their parallelism and spacing of about 6 km and
extend along the northern edge of the basin that turn to the northeast and
enter the Anegada Passage (Fig. 11). These traces are locally buried at
the seafloor by debris shed off the southern margin of the basin,
especially in the eastern part of the basin near the Anegada Passage. The
vertical throw on these two faults is negligible in comparison to the larger
St. Croix normal fault to the south.
2)
TORTOLA RIDGE FAULT ZONE can be traced for 85 km and forms
the east flank of the Tortola ridge, a steep sided horst block that separates
the narrow Anegada Passage from the 2.9-km-deep St. Croix basin in the
eastern part of the study area. The Tortola normal fault can be traced into
the deep basin of the VIB as a south-dipping fault, but ends about halfway
along the axis of the basin (Fig. 11). The south-dipping Tortola fault and
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Figure 10: Oblique view of the VIB looking east using high-resolution bathymetry from Grothe et al. (2012) and Taylor
et al. (2008) and displayed using Midland Valley MOVE version 2014.2 (2014). Lines controlling the location of the
faults in the VIB are shown and include lines from Raussen et al. (2013) and Jany et al. (1990). This view shows well
the asymmetrical, half-graben structure of the VIB with the steeper scarp of the St. Croix fault and the more gently sloped,
largely unfaulted, and thinly sedimented basement surface on the north side of the basin (compare this view to the Shell
seismic line shown in Figure 6). Seismic lines used to constrain the fault locations are shown.
33
Figure 11: Oblique view of the VIB looking east using high-resolution bathymetry from Grothe et al. (2012) and Taylor et al.
(2008) and displayed using Midland Valley MOVE version 2014.2 (2014). Lines controlling the location of the faults in the VIB
are shown and include lines from Raussen et al. (2013) and Jany et al. (1990). Four major faults are identified in the VIB and
include: 1) St. Croix normal fault forming the major north-dipping, listric normal fault controlling the main phase of basin opening
and best imaged on the Shell seismic lines shown in Figure 6; note that this large normal fault tilts its footwall, the island of St.
Croix, to the south; the eastward extension of the St. Croix normal fault along the northern edge of Saba bank does not appear to
be active as the Saba bank is horizontal and sits at a greater depth (~60 m BSL) than the more active St. Croix footwall block that
is tilted and elevated up to 350 m ASL; 2) the Tortola normal fault forms a major south-dipping normal fault controlling the uplift
and northwest tilt of Tortola ridge (footwall block) that forms the southeastern edge of the Anegada Passage; the Tortola fault can
be traced on seismic lines about halfway across the VIB; 3) Virgin Islands fault zone can be traced as flower zone structure across
the VIB and northeatward into the Anegada Passage.
the adjacent and much larger St. Croix north-dipping normal fault
together bound a full-graben structure in this part of the VIB (Fig. 11).
3)
ST. CROIX FAULT ZONE can be traced for 164 km, it forms the steep
scarp along the southern edge of the VIB and continues to the east along
the northern margin of the Saba carbonate bank (Fig. 11). Unlike the
island of St. Croix, Saba bank appears as a horizontal carbonate platform
and therefore indicates that normal throw along the St. Croix fault
decreases to the east. One interpretation is that normal slip along the St.
Croix fault is transferred to the Tortola fault which exhibits a very
prominent and steep fault scarp along the southeastern edge of the
Tortola ridge (Fig. 11).
Overall fault interpretation of the VIB
The pattern of faults indicates a general right-step with inferred right-lateral strikeslip motion along the Northern and Southern Virgin Islands fault either passing into the
Anegada Passage or transferring to the Tortola and St. Croix normal faults. This right-step
would produce transtension consistent with the extreme depth of the basin (4400 m) and
steep scarp of the St. Croix normal fault. The large scarp of the Tortola fault suggest that
motion of the Northern and Southern Virgin Islands fault may step right onto this fault.
Using the DEM and bathymetry in conjunction with published works from
Mondziel et al. (2010), Mann et al. (2005b), and Grindlay et al. (2005b), I mapped the
Mona Rift and the Great Southern Puerto Rico fault zone (GSPRFZ) showing their
connection (Figure 7). The Mona Rift opens as the Puerto Rico-Virgin Islands block pulls
34
away from Hispaniola, undergoing 6.1 km of extension. As a result of the opening of the
Mona Rift, the GSPRFZ undergoes a right-lateral sense of shear.
Cross-sectional structure of the Virgin Islands and St. Croix basins and Anegada
Passage from shallow-penetration single-channel seismic data
Results of shallow seismic profiling from Jany et al (1990)
The single-channel lines of Jany et al. (1990) summarized in map view on Figure
11 provide better imaging of the top crystalline basement surface in the VIB and adjacent
St. Croix basin than the Raussen et al. (2013) lines used from the Galathea 3 cruise that are
summarized on Figure 10. I describe the Jany et al. (2010) lines moving from west to east
across the VIB and into the St. Croix basin (Fig. 11).
Structure of the deep Virgin Island basin on Jany et al. (1990) lines
Line 5 of Jany et al. (1990) shown in Figure 12 reveals the top basement surface at
a depth of over 7 seconds in the deepest part of the VIB, consistent with the interpretation
of the Shell line shown in Figure 6. It is important to note that the vertical exaggeration of
the Jany lines (8:1) is higher than either the Shell line in Figure 6 (1:1) or the Raussen et
al. (2013) lines (3:1), so features on the Jany lines appear in relatively higher vertical relief.
The St. Croix fault can be seen as the southernmost fault on line 5 in Figure 12.
This large normal fault has exhibited the greatest amount of offset, as supported by the
wedge-shape of the sedimentary infill of the basin with the thickest portion of the wedge
(i.e. the layers undergoing the most growth) to the south and consistent with observations
35
36
Figure 12: A) Uninterpreted seismic line from Jany et al. (1990, their figure 5) across the central VIB. B) Interpreted line
showing the north-dipping St. Croix normal fault along the southern edge of the VIB and the positive flower structure formed
by the Virgin Islands fault zones (basement is shown in red, sedimentary section is in purple). Sediment wedging along the
southern edge of the VIB shows that the St. Croix normal fault is the major fault controlling the VIB opening with greatest
amount of normal fault throw as supported by the interpretation of the Shell multi-channel seismic line shown in Figure 6.
Flower structure formed by Virgin Islands fault zones has a relatively minor structural effect on the top basement surface and
is consistent with its proposed strike-slip character and its likely younger age than the St. Croix normal fault.
from the Shell line (Fig. 6). Jany line 5 in Figure 12 also shows the two sub-vertical faults
with seafloor expression running along the northern edge of the VIB and exhibiting a
positive flower structure inferred on the basis of offsets observed to the west in the Whiting
basin to have formed by a right-lateral sense of shear. The positive flower structure marked
by the Virgin Islands fault zone has a minor effect on the top basement surface consistent
with its strike-slip character and likely younger age than the St. Croix normal fault.
Jany line 6 shown on Figure 13 shows the fault geometry as seen on line 5 in Figure
12 with the prominent St. Croix normal fault and the twin Northern and Southern Virgin
Islands faults again bounding a positive flower structure. Jany lines 5 and 6 show a thin
sedimentary cover on the north slope of the basin (much of which is likely landslides) and
no major faults on the north flank of the VIB as seen on the Shell line in Figure 6.
Structure of Anegada Passage and St. Croix basin on Jany et al. (1990) lines
Jany seismic lines 10 and 11B are combined into a single 75-km-long transect on
Figure 14 to reveal several recently-active, normal fault blocks in the Anegada Passage,
Tortola ridge, St. Croix basin, and into the Kallinago Trough. My interpretation shows the
steep-sided Tortola ridge is a horst block bounded by faults in the Anegada passage to the
west and the St. Croix basin to the east. The St. Croix basin at a depth of 2900 m BSL is
asymmetrical with greater throw on the Tortola fault to the west than the Saba bank fault
to the east. The Kallinago basin to the east sits at a shallower depth (2000 m BSL) and
exhibits a full graben geometry on this line.
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38
Figure 13: A) Uninterpreted seismic line from Jany et al. (their figure 6) across the central VIB. B) Interpreted line showing the
north-dipping St. Croix normal fault along the southern edge of the VIB and the positive flower structure defined by the Virgin
Islands fault zones (basement is shown in red, sedimentary section is in purple). Sediment wedging to south shows that St. Croix
normal fault is the major fault controlling the VIB opening with greatest amount of fault throw as supported by the Shell multichannel seismic line shown in Figure 6. Flower structure formed by Southern and Virgin Islands fault zones has a minor effect
on the top basement surface consistent with its strike-slip character and likely younger age than the St. Croix normal fault.
39
Figure 14: A) Uninterpreted seismic lines from Jany et al.(1990) that combines two of their lines 10 and 11B across
several normal fault blocks in the Anegada Passage, Tortola Ridge, St. Croix basin, and into the Kallinago Trough.
B) Interpretation shows the steep-sided Tortola ridge is a horst block bounded by faults in the Anegada passage to
the west and the St. Croix basin to the east. The St. Croix basin at a depth of 2850 m BSL is asymmetrical with
greater throw on the Tortola fault to the west than the Saba bank fault to the east. The Kallinago basin to the east sits
at a shallower depth (610 m BSL) and exhibits a full graben geometry on this line.
The 50-km-long Line 11A from Jany et al. (1990) is shown as Figure 15 and shows
the steep-sided Swordfish and Shark so-called “seamounts” as footwall uplift features
adjacent to normal faults, rather than of a true volcanic origin. The Tortola ridge has the
dominant vertical throw in this area, in contrast to the Anegada Passage that forms a
narrow, symmetrical valley with more gently-dipping sides and interpreted as a strike-slipcontrolled feature. These areas to the east have significantly less sedimentary fill than the
VIB to the west, as they are farther away terrigenous sediment supply from the larger
islands of Puerto Rico, Vieques, and St. Croix. These lines also indicate that the uppermost
unit is a transparent pelagic carbonate rather than the terrigenous Quaternary unit seen in
the VIB (Fig. 7).
Structure of the deep Virgin Island basin on Raussen et al. (2013) lines
This section reviews the structures seen on the Raussen et al. (2013) lines in a westto-east direction with all line locations shown on Figure 11. The west-to-east change
shown on these lines includes an eastward decrease in vertical throw and prominence of
the inferred right-lateral Virgin Islands fault zone, as well as the eastward increase in
normal throw on the Tortola fault. On line 3 shown on Figure 16, the St. Croix fault zone
is shown as a well-developed 1.5-km-wide flower structure with slight positive relief at the
seafloor. The Tortola fault is not present to the south, as it does not extend this far to the
west in the VIB (Fig. 17). On line 6 shown on Figure 18 to the west, the Virgin islands
right-lateral strike-slip fault is less prominent, and only 0.5 km wide and the Tortola fault
has appeared as a south-dipping normal fault parallel to the larger St. Croix normal fault.
40
41
Figure 15: A) Uninterpreted seismic lines from Jany et al., 1990, their figure 11A, across several normal fault blocks in the
Anegada Passage, Tortola Ridge, St. Croix basin, and into the Kallinago Trough. B) My interpretation proposes that the steepsided Swordfish and Shark seamounts are uplifted footwall blocks adjacent to normal faults, rather than seamounts of volcanic
origin as previously interpreted. The Tortola ridge footwall is the dominant vertical footwall uplift in this area compared to the
Anegada Passage that forms a narrow, symmetrical valley with more gently-dipping sides and interpreted as a right-lateral,
strike-slip-controlled feature.
42
Figure 16: A) Uninterpreted single-channel line 3 from Raussen et al. (2013) (location shown in Figure 3). B) Zoom of VIB
center showing the Virgin Islands fault zone with flower zone structure interpreted as undergoing a right-lateral sense of shear
and the sub-parallel St Croix normal fault along the southern basin margin showing normal offset.
43
Figure 17: A) Uninterpreted, single-channel line 1 from Raussen et al. (2014) (location shown in Figure 3). B) Zoom
of VIB center showing Virgin Islands fault zone with flower zone structure interpreted as undergoing an active, rightlateral sense of shear and the St Croix normal fault along the southern basin margin showing normal offset.
44
Figure 18: A) Uninterpreted single-channel line 6 from Raussen et al. (2014) (location shown in Figure 3). B) Zoom of line in
VIB center showing Virgin Islands fault zone with active, flower zone structure interpreted as undergoing a right-lateral sense
of shear and sub-parallel St Croix fault along the southern basin margin showing normal offset. Note the presence of a southdipping normal fault north of the St. Croix fault that I propose as the western extension of the Tortola normal fault that is well
expressed in the eastern part of the VIB and bounding the St. Croix basin (see Figure 11).
On line 6 shown on Figure 19 in the Anegada Passage, there appears to be little
active faulting that has extended into the passage from the VIB. I interpret the eastward
decline of the Virgin Islands fault zone and Anegada Passage fault zone and the eastward
increase in fault activity on the Tortola fault zone as the right-step of motion from the
Virgin Islands to the Tortola fault zone (Fig. 20). This is consistent with the prominence
of Tortola ridge as the hanging wall of the Tortola fault in the eastern part of the study area,
and the extreme water depth and crustal thinning observed beneath the VIB (Fig. 5). The
Virgin Islands fault zone was likely once located where the Tortola fault is currently. The
zone of active faulting likely stays in the same location, but as the PRVI block undergoes
counter-clockwise rotation the old fault traces are shifted to the NNE (Fig. 21).
DISCUSSION AND CONCLUSIONS
Isochron map of the VIB based on Jany et al. (1990) data
The lines from Jany et al. (1990) have a more clearly imaged basement than the
Raussen et al. (2013) lines. I created an isochron map using these lines to confirm the basic
observation, also observed on the Shell line (Fig. 6), that the thickest package of sediments
(7.5 seconds) is found banked against the St. Croix normal fault along the southern edge
of the basin (Figure 22). The thickest package of sediment is also found in the southern
part of the basin along the St Croix fault, supporting the view that this fault formed earlier
and controlled the opening of the rift. The location of the strike-slip Virgin Islands fault
zone has no effect on the thickness trends in the basin, indicating that this fault has minor
45
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Figure 19: A) Uninterpreted single-channel line 11 from Raussen et al. (2014) (location of line is shown in Figure 3). B) Zoom
of Anegada Passage showing Virgin Islands fault zone exhibiting flower zone structure typical of strike-slip faults and
interpreted as undergoing a right-lateral sense of shear; the sub-parallel St Croix fault along the southern basin margin shows
normal offset. The Virgin Islands basin faults have extended to the northeast through the Anegada passage and bound the edges
of the narrow but deep Anegada passage.
47
Figure 20: Fence diagram showing the trace of the Virgin Islands, St. Croix and Tortola fault zones passing down the long
axis of of the VIB. Fence diagram is based on selected, interpreted seismic sections from Jany et al. (1990), and Raussen et
al. (2013).
Figure 21: A) Virgin Islands basin during the late Cretaceous to the middle Eocene when
VIB has not yet undergone shearing and only the asymmetrical, half-graben geometry of
the basin is apparent. Floor of VIB is exposed igneous basement with no overlying
sedimentary infill. B) During the middle Eocene to present the PRVI block rotates
counter-clockwise causing the Virgin Islands fault zone to move to the north-northeast,
and activating the Tortola Ridge normal fault. Major depocenters are the VIB and the
Anegada Passage. C) Present day GPS vectors from Benford et al. (2012) indicate the
PRVI block continues to undergo CCW rotation. Major depocenters of the Anegada
Passage include the VIB and the St. Croix basin.
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49
Figure 22: Isochron map in milliseconds of the central VIB based on mapping the Jany et al. (1990) seismic lines - including the
Shell line - as these were the only lines where the top basement surface beneath the VIB was visible. Sediments thicken towards
the St. Croix normal fault confirming that it was the main structure responsible for the asymmetrical opening of the half-graben
as seen on the Shell line in Figure 6.
effect on the basement surface and likely formed more recently than the prominent St.
Croix normal fault.
Restoration of extension that formed the Virgin Islands basin using the Shell line
One objective of this study was to quantify the amount, timing, and direction of
extension that formed the VIB, and to compare these results to the previously proposed
models compiled in Figure 3. Using the digitized Shell line from Jany et al. (1990) in
Figure 6, I restored the Shell section using Midland Valley’s Move 2014 in four steps back
to its configuration in late Cretaceous time (Figure 23). The restoration consisted of
flattening to the various horizons shown. The basin has undergone a total of 15.8 km of
extension since the late Cretaceous with 10.1 km of extension since the middle Eocene and
8.4 km of extension since the Quaternary.
The restored Cretaceous top crystalline surface has a ridge-like shape that may
reflect that this area was once the axis of the Great Arc of the Caribbean (GAC) that was
continuous with the late Cretaceous Aves ridge shown on Figures 1 and 2. The pre-rift
section is interpreted to be late Cretaceous to middle Eocene limestone and dolomite
associated with the subsiding GAC. Rifting is thought to have begun in the middle Eocene
with a sandstone facies observed in the deeply buried VIB that is not found on the two rift
flanks of St. Croix and Vieques, which are mainly characterized by carbonate facies
ranging from deepwater marls to coral limestone (Gill et al., 1999). According to
Holcombe (1989) the earlier phase of rifting in the basin was in a more southwest direction
that evolved in time into a more north-south direction. The presence of the right-lateral
Virgin Islands fault zone within the deep VIB and its continuity with fault systems to the
50
Figure 23: Structural restoration of Shell seismic line presented by Jany et al. (their figure
4 shown as figure 6 in this thesis). I reconstructed the line using Midland Valley MOVE
version 2014.2 (2014). My conversion of this section from time to depth using the
velocity structure from this line was consistent with the results from the Werner
deconvolution shown in Figure 5. A) Digitized line from Jany et al., 1990. B) Inferred
lithologic layering with ages based on velocity information provided on Figure 4 of Jany
et al. (1990). B) Restored 3.1 km on normal fault and restored Quaternary unlithified
muds and clays layer. C) Restored 8.4 km on normal fault and restored middle Eocene
to Quaternary lithified sandstone; D) Restored 10.1 km on normal fault and restored late
Cretaceous to middle Eocene sand and marl layer; E. Restored 15.8 km on normal fault
and restored late Cretaceous igneous layer.
51
west and east of the VIB indicates that there is a significant shear component affecting the
more recent history of the basin.
Tectonic model for the evolution of the VIB and Puerto Rico area
I propose a regional tectonic model for the northeastern Caribbean that attempts to
explain the following constraints presented in this thesis:
1) Wedging in the deep VIB indicates that the basin formed as a rift as early as the
middle Eocene. In Puerto Rico, the Great Southern fault zone reactivates a linear
basin, called the Cerrillos basin by Dolan et al. (1991), that was an active zone of
clastic deposition and volcanism in the Eocene and separates the younger part of
the GAC to the northeast from the older part of the GAC to the southwest. These
two features are in rough alignment with the Kallinago trough and St. Croix basin
that separates the active volcanic arc of the Lesser Antilles to the west from the
inactive, pre-Miocene volcanic arc to the east (Figs. 1, 2). My proposal is that all
of three of these features once formed a continuous, intra-arc basin during the
Eocene, and that this basin was disrupted by post-Eocene deformation.
2) The significance of these three features is that they formed a linear zone of
weakness that cut through the GAC and was subject to reactivation during later
events, including the oblique subduction of the southeastern end of the Bahamas
platform beneath Puerto Rico in the middle Miocene (Grindlay et al., 2005). On
Figure 22, I have shown the east-to-west displacement of the Bahamas platform
from Grindlay et al. (2005).
52
3) My model predicts an early Miocene oblique collision of the Bahama platform with
the Virgin Islands area that acted to indent the arc along the Anegada Passage and
the reactivated Kallinago trough-VIB-Cerrillos basin (Fig. 24A).
Indentation
produced the initiation of subduction along the Muertos trench and left-lateral shear
along the Anegada Passage and VIB that led to a left-lateral offset of the Kallinago
trough and its western continuation in the St. Croix basin, the VIB, the Whiting
basin, and the Cerrillos belt of Puerto Rico.
4) By middle Miocene time the Bahama platform had migrated to the east and relieved
pressure on the Puerto Rico block at the Puerto Rico trench. This allowed the
Puerto Rico block to rotate in a counterclockwise direction along the obliquely
subducting plate boundary.
Reid and Plumley (1991) documented 25 degrees of counterclockwise rotation of
the Puerto Rico block during the late Miocene and Pliocene. This sense of rotation
of the Puerto Rico block as shown on Figure 21 would have led to right- lateral
transtension along the southern edge of the block in Puerto Rico and the VIB.
5) Transtension at the western edge of the rotating block would have created the Mona
basin with 10 km of extension starting in the Oligocene (Mondziel et al., 2010), and
16 km of extension in the VIB accelerating in Miocene to recent time (Fig. 23).
6) Most workers have thought that the Great Southern fault zone is a left-lateral fault,
including Glover (1971), who identified left-lateral offsets on piercing points of
older rocks of 10 km, and Erikson et al. (1991), who inferred left-lateral shear
combined with thrusting. However, the continuity of the Great Southern-Cerro
Goden-Mona rift faults would require right-lateral shear to keep the Oligocene to
53
Figure 24: Indentation and rotational model of the Puerto-Rico Virgin Islands
microplate from early Miocene to present (reconstructed positions of Bahamas
platform modified from Grindlay et al., 2007). A) The Virgin Islands basin was
originally the western extension of the Kallinago intra-arc basin that underwent leftlateral offset in the early Miocene as a result of the oblique collision and indentation
of the arc of Puerto Rico and the Virgin Islands by Bahamas Platform. This may
explain why older volcanic arc rocks in Puerto Rico are found on the southwestern
side of the island and to the southwest of the proposed western extension of the
Kallinago trough (Cerro Goden-Virgin Islands fault zone) and younger arc rocks are
found on the northeastern side of the island (Glover, 1971). B) As the Bahamas
platform moved to the west, the PRVI microplate became free to rotate counterclockwise along the right-lateral Puerto Rico-Virgin Islands fault zone. C) Modeling
of GPS vectors from the PRVI microplate by Benford et al. (2013) shows that
counterclockwise of the PRVI microplate presently occurs along right-lateral strikeslip faulting of the Cerro Goden-Virgin Islands fault zone.
54
55
recent Mona rift from closing since its inception in the Oligocene. Mondziel et al.
(2010) has shown a minimum total of 6.1 km of opening on the Mona rift with no
evidence for a past history of either compression or inversion.
56
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