Proposed
Research
The research we are proposing relies on geological studies, structural analysis, thremochronology, and thermobarometry to establish the age and sequencing of different margin processes. The active and passive source seismic studies establish detailed 2-dimensional cross-sections of the crust (active source) and 3-dimensional models of the lithosphere and sub- lithospheric mantle (passive studies), as well as determine modern mantle flow fields (passive studies). The geologic to active source to passive source studies provide resolution of crust-mantle features at very fine to increasingly large scales, all at the maximum resolution possible with existing technologies. The present day seismic geometry and the age of different deformation events can be interpreted quantitatively with 2-D and 3-D geodynamic models using realistic crustal and mantle rheologies for understanding the arc accretion, metamorphic rock exhumation processes and the development of depocenters along the margin. The geometry provided by the seismic data, and the age and deformational information from the geologic studies constrain the geodynamic modeling to provide a margin-wide understanding of continental crustömantle interactions along this complex transpressional/transtensional plate margin.
Geological Studies: Leeward
Antilles Arc, Venezuelan Metamorphic Belts, & Cenozoic Basins: The
geologic studies are essential to the project to ascertain timing and
sequencing of all of the important plate boundary zone processes. The geologic
studies proposed here fall into three broad categories: 1) Reconstruction of
the along strike, west to east, plate boundary evolution (cessation of
magmatism, arc accretion to continent, and extensional dismembering) of the
Leeward Antilles arc; 2) Reconstruction of the exhumation history of the
high-pressure metamorphic rocks buried during previous subduction and 3)
Evolution and depositional history of the Cenozoic basins as a function of
margin evolution. Below we outline our plan of geologic mapping, structural,
metamorphic, and geochronologic studies for each of these broad
categories.
Leeward Antilles Arc. Our primary goals are to test the
hypotheses that the Leeward Antilles arc from Aruba in the west to Los Testigos
in the east 1) was shut off because of increasing plate convergence obliquity
(see Fig. 11) from mid-Cretaceous time to the Present, 2) was accreted
diachronously to the Venezuelan continental margin from the Eocene to the
Present, and 3) that the arc is indeed felsic-intermediate in composition (Figs
2-3).
The first hypothesis has been put forward by a
number of workers, but is based primarily on K-Ar dates determined from
volcanic and plutonic rocks of the arc terrane which indicate that arc
magmatism progressively shuts off from about 85 Ma in the west (Aruba) to 45 Ma
in the east (Los Testigos; Priem et al., 1979; Santamar’a and Schubert,
1974). We propose to test this as
yet poorly substantiated hypothesis by beginning a program of geologic and
geochronologic studies along the length of the Leeward Antilles arc. One of our primary goals will be to
establish the youngest age of igneous activity in a west-to-east transect along
the length of the arc. If the
Leeward Antilles arc shuts off diachronously (as proposed in Fig. 11), then a
pattern where the youngest magmatic activity recorded becomes progressively
older from east to west should be evident. Geologic maps and general field
relations have been fairly well established for the western islands (Figs. 2-3,
Netherlands Antilles) of Aruba, Cura?ao, and Bonaire (Aruba: Helmers and Beets,
1977; Cura?ao: Beets, 1977; Bonaire: Beets et al., 1977). Although perhaps not as well known, the
general geologic features of the more easterly Venezuelan offshore islands such
as the Aves Islands, Los Roques Islands, La Orchila, La Blanquilla, Los
Hermanos, and Los Testigos have also been generally established. For example, intrusive igneous rocks
occur on most of these islands and published descriptions suggest that they
should be good candidates for either U-Pb zircon or
40Ar/39Ar
geochronology (e.g., Bellizzia and Dengo, 1990).
The application of several geochronologic
techniques to these samples will not only establish the age of formation of
various plutonic and volcanic rocks but also the subsequent cooling history.
Since most tectonic processes involve the transfer of thermal energy, often by
advective mass transfer, a detailed understanding of the thermal history of a
rock can provide valuable information in assessing its tectonic history. We
have the capacity to determine the thermal history of a rock over essentially
the entire range of temperatures of geologic interest, given the proper
mineralogy. The techniques we use will be U-Pb, 40Ar/39Ar,
fission track, and (U+Th)/He dating. The U-Pb method on minerals such as zircon
and monazite can establish the last time a rock was at a temperature at or
above 700oC essentially the crystallization temperature; the
minerals commonly investigated by 40Ar/39Ar dating give information about a range
of temperatures with amphibole at about 500oC, micas at
370 to 300oC
and K-feldspar at 300 to 150oC; fission-track dating of zircon and
apatite gives information about the passage through ~225 and~100oC,
respectively; (U+Th)/He dating of apatite gives information about the lowest
temperatures, in the range of 80 to 60oC. By applying several
techniques to the same rock in many locations, a detailed understanding of the
thermal history of region can be established. Knowing when rocks crystallized
and when and how fast they cooled is extremely important in establishing under
what conditions they were formed and how they were subsequently modified. These
data will provide precise constraints on the timing and rate of exhumation of
the rocks on these island which is also a key component of the predictions of
the diachronous-collision hypothesis for the South America Caribbean plate
margin.
Another goal will be to evaluate the possibility
that the structural geology of the islands also records the diachronous
accretion of the arc to the Venezuelan continental margin. There is little or
no discussion in the literature concerning the Mesozoic structural evolution of
the Leeward Antilles. We plan detailed field geologic analyses in order to put
our geochronological investigations into a field context and also to determine
if there is any evidence for diachronous deformation along the strike of the
arc that might be correlative with progressive accretion of the arc from west
to east.
Wright and Copeland will be in charge of this part of the
project. Wright has many years of
experience in deciphering the magmatic and structural evolution of oceanic arc
terranes in the western U. S. Cordillera. Copeland has several years experience
in interpreting the thermal history of rocks as it relates to the exhumation in
convergent plate settings in the Himalaya and other mountain belts. The U-Pb
work will be done by Wright using the Stanford SHRIMP-RG. The Ar/Ar
thermochronology will be supervised by Copeland. Fission-track thermochronology
is being contracted to Ann Blythe from USC.
During year 1 we plan to initiate reconnaissance studies
along the length of the arc in order to decide which islands will be the best
candidates for detailed work in years 2 and 3. We plan to collect samples for thermochronology during
year 1 particularly from the better described plutonic complexes exposed on
Aruba, Cura?ao, La Orchilla, and La Blanquilla.
Venezuelan Metamorphic Belts.Ê Two of the E-W trending metamorphic belts contain
HP/LT assemblages of Cretaceous metamorphic age. They are believed to be
related to subduction of the South American-Atlantic plate beneath the
Proto-Caribbean plate in the Cretaceous (Figs. 2-3). Their burial, uplift,
exhumation, and ultimately obduction onto the South American continent have
resulted from the Cretaceous to Recent interactions of the two plates (Fig. 4).
In north-central Venezuela these belts have been recently studied by faculty
and students at Rice as well as others previously (Ave Lallemant and Sisson,
1993; Sisson et al., 1997; Smith, 1996; Smith et al., 1999; Maresch,1975; Guth,
1991; Stockhert et al, 1995; Ave Lallemant, 1997). We note that the areas between western north-central
Venezuela, and the east (Margarita Island and the Araya Peninsula) have not
been studied at all, and are important for establishing the history of the
boundary wide processes.
The hypothesis that we are testing is that the exhumation of the high pressure rocks
progresses from west to east in two stages: During the first stage (mid
Cretaceous to Present) these rocks ascended to shallower depths due to strain
partitioning: combined strike-slip, and arc parallel extension in the
accretionary wedges (Fig. 11). The longer the rocks have been in the plate
boundary the deeper the level exposed at the surface: it appears that the
deepest rocks are found in the west, shallowest in the east, however this
pattern has yet to be unambiguously confirmed. The second stage (Eocene to
Present) is related to the obduction of the Leeward Antilles arc and its
accretionary wedge onto the South American margin, a process that also formed
the foreland fold and thrust belt. We propose to carry out a program including
structural analysis, metamorphic petrology (thermobarometry), and geochronology
in order to establish the spatial and temporal patterns of exhumation to test
whether exhumation of the HP/LT metamorphic belts is diachronous with the
eastward migration of the arc as preliminary dating and geobarometry
suggest. We will focus in unmapped areas, and
focus extra attention on the corridors to be investigated by active source
seismic methods. We will sample these and the previously studied areas for
geothermo-barometry, deformation, and timing. By carrying out these P-T-d-t
studies along an EW transect we will be able to relate the burial and
exhumation history as a function of the west to east migration of the Caribbean
plate boundary, as well as the north to south obduction of these
belts.
Ave Lallemant will undertake kinematic
(deformation)
studies with the cooperation of Dr. Marino Ostos of the Universidad Central de
Venezuela snf Dr. Franklin Yoris of the Universidad Simon Bol’var. The P-T conditions of the rocks from
great to shallow depths will be determined from electron microscopy of suitable
mineral assemblages and by fluid inclusion studies of quartz veins that have
intruded these rocks at various points in their history. These studies will be
carried out by Dr. Enci Ertan, a Rice University post-doc. Timing of events
will be determined by 40Ar/39Ar dating by Peter Copeland
at the University of Houston, and by fission track studies under contract to
Ann Blythe at the University of Southern California. The thermal history of the
unmetamorphosed rocks of the Serran’a del Interior foreland fold and
thrust belt will be determined by fission track analysis as well. Selected
samples will be sent to commercial laboratories for vitrinite reflectance
analysis.
Cenozoic Basins:
Mann has extensive experience in the tectonics of the Caribbean (e.g., Mann et
al., 1991; Mann, 1995, 1999). He will analyze the 3000 km of 24 fold digital
MCS data from offshore Venezuela and Trinidad which were donated to UT by Gulf
Oil (Fig. 17).Ê He will make time-depth maps for key horizons including
the top of the accretionary wedge, a prominent Middle Miocene (~12 Ma) angular
unconformity surface, and the base of several small, mainly fault or
fold-controlled late Quaternary basins.
Maps of faults and folds affecting the upper part of the crust will also
be completed and compared with previous regional mapping studies of the
offshore area including Ladd et al. (1984), Robertson and Burke (1989), and
Ysaccis (1998). Maps of offshore
structures will be integrated with previous and proposed on-land structural and
isotopic work on deformed metamorphic and arc-related rocks of the Leeward
Antilles. In some cases, these
islands are exposed as positive flower structures along active strike-slip
faults or as footwall uplifts along normal faults. The structural character and age of these faults is more
easily resolved on offshore lines than by the limited onland exposures in older
rocks.
Age
control of horizons is constrained by limited wells reported previously
(Biju-Duval et al., 1978; 1982).
Mann has spent part of 1999 and 2000 in Nice, France, under a
"Professeur de l'Academie" fellowship, where he has been examining
French seismic data and well information from the Venezuelan offshore area that
is archived at the Insitut Fran?ais du PŽtrole in Malmaison, France.
Pindell will examine a significant portion of 6,000 km of
onshore and offshore petroleum seismic reflection data in northern Colombia.
(2/3 of it digital, donated to him in 1998 by Ecopetrol, Fig 17). These MCS
data will constrain the behavior of the South American autochthon during the
onset of subduction of Caribbean crust at the beginning of accretion history,
how South America behaved vertically as the Aves/Leeward Antilles Arc rounded
the northern corner of South America in the Paleogene, and the earliest stages
of arc-polarity reversal (Paleocene?) in the western corridor. Pindell will
also work with the geophysicists to assess the inter-relationships between
plate kinematics, dynamic processes recorded in the sedimentary basins, and the
processes identified by the deeper geophysics. Pindell will compile and
integrate existing seismic profiles, existing gravity data, seismicity data,
Landsat and radar imagery, and heat flow data.
These studies will allow more quantitative definition of
the structural geometry and dynamics of the Cenozoic basins, and their
mechanisms of formation, history of strain partitioning in the plate boundary
zone, and the diachronous regional subsidence patterns within the
Caribbean-South American plate boundary zone. Synthesis of these data will
produce a revised terrane and paleogeographic history for the plate boundary
zone, to better understand and interpret the deep geophysical data and
uplift/exhumation studies, and provide constraint for geodynamic models.
Geophysical Investigations: Passive Seismology: Tomography, Receiver Functions and Seismicity
Studies: In regions of good data coverage, tomographic imaging is a
powerful tool in investigations of mantle structure for building a structural
framework for the reconstruction of past plate motions in the region and for
establishing the relationship between deformation in the crust and mantle.
Travel time tomography has produced spectacular new insight in the structure of
the mantle beneath the Americas on scales of several hundreds to thousands of
kilometers (Grand, 1994; Grand et al., 1997; Van der Hilst et al., 1997).
However, the northern edge of the South American craton is poorly resolved, as
is the plate boundary region under central and eastern Venezuela (Figs 8). For
studies of the crust and upper mantle high resolution imaging has been
restricted to a few regions with sufficient numbers of stations and
earthquakes, for instance the northwestern part of South America (van der Hilst
and Mann, 1994). Tomographic images of the Caribbean-South American plate
boundary only resolve mantle features at best at about 100 km vertically and
300 km horizontally (Figs 8; Van der Hilst, 1990; Van der Hilst and Mann,
1994). Van der Hilst's (1990) tomography employed recordings from
only a handful
of stations in Venezuela and Trinidad, and virtually none in the Leeward
Antilles and eastern Venezuela.
Higher resolution tomographic studies using portable arrays to record
local and teleseismic earthquakes and more precise earthquake location studies
are a natural addition to complement the active source investigations of the
crust and upper mantle across the plate boundary zone. They also complement the
seismic investigations already underway further south in the continent (Beck et
al., 1996), as well ongoing instrumentation of the Venezuelan mainland by
FUNVISIS using broadband and short-period permanent and portable recorders (see
below). The flip in subduction polarity along the length of this boundary
necessarily produces a complex mantle structure that high resolution tomography
of the mantle will help to understand. This flip also undoubtedly results in
unusual asthenospheric flow patterns around the subducting slabs under the
plate margin that should be recognizable in shear wave splitting
studies.
On the scale of the Caribbean plate in general, and
our study region in particular, the current data coverage is insufficient to
constrain shallow mantle structure or the crust-mantle coupling, which severely
hampers our efforts to understand the tectonic development of the region. Van
der Hilst (1990) and van der Hilst and Engdahl (1991) demonstrated that lateral
variations in seismic structure of the upper mantle can be reasonably well
resolved directly beneath the plate boundary zones where most earthquakes and
recording stations are located.
However, they also concluded that the uneven data coverage precluded the
imaging of structure in the shallow mantle beneath the inter-plate regions
(e.g., the Caribbean sea, the Gulf of Mexico, and continental South America),
and that locally significant "smearing" occurred in the down dip
direction of subducting slabs. For
example, with the data from permanent stations we cannot conclusively determine
the maximum depth to which the Atlantic sea floor subducts beneath the southern
part of the Lesser Antilles arc, the southward extent of the deep slab beneath
continental South America, or the northern extend of the South American craton
(Figs 8). To improve this situation we propose to teleseismically image part of
the Caribbean plate boundary using portable broadband land recorders and ocean
bottom seismographs deployed in a continental margin-southern Caribbean array
to complement the Venezuelan upgrade of their seismographic network.
This proposal includes a temporary deployment of 25
broadband PASSCAL and 15 OBSI. seismic stations in Venezuela, on
the southern
Caribbean islands, and in the southern Caribbean by Wallace and
Vernon.
The portable broadband and OBS stations will be deployed for 15 to
18 months, and will collect high quality data from more than 1000
teleseismic,
regional and local seismic events. The teleseismic and regional events will
be used to supplement the tomography, broadband waveform modeling
and receiver
function analysis to constrain and image the gross crustal and
mantle structure.
The broadband deployment will also provide a valuable data source for the
seismicity studies, and the modeling of regional waveforms can be used to
determine moment tensors of small and moderate sized events to define the
Benioff zones at both the east and west ends of the plate
boundary. Previous
passive seismic experiments have proven to be extremely
successful in providing
tectonic models in other parts of South America (e.g. Beck et
al., 1996).Ê
The internal road system in Venezuela is adequate for deployment of an EW
and a NS profile. Local, regional and teleseismic seismicity would provide
an extensive waveform archive. The EW profile will cross Venezuela parallel
to the arc, the NS deployment will extend to the Guyana shield and coincide
with the 65W active
source corridor.
The NS and EW arrays will include stations on the Caribbean
islands to extend
then from 6-14oN and 61-71oW (Fig. 18). FUNVISIS has
agreed to deploy 7 portable seismographs in central / western Venezuela
along
a second NS profile extending to the shield. An alternate passive
seismology
deployment plan is shown in Figure 18B. The deployment and
service teams will
include geophysics students from Simon Bolivar University.
Modeling regional waveforms: Shear-couple
P waves recorded at near-regional distances provide important information about
lithospheric structure. Analysis and modeling of shear-couple P waves, in
conjunction with tomography can be used to determine details of
crustal thickness,
variations in crustal velocities and crust and mantle Vp/Vs ratios.
Receiver Function Analysis: We
will use broadband records from teleseismic and deep regional earthquakes for
receiver function analysis to constrain average crustal thickness and crustal
Vp/Vs rations in northern Venezuela.
Average P-wave crustal velocities determined from the regional waveforms
allows the Receiver Function Analysis (RFA) to provide details of impedence
contrasts in the crust and mantle.
Similar experiments we have deployed in Bolivia produced RFAs that
allowed the mapping of structure throughout the crust and upper mantle,
including the 400 and 670 km discontinuities.
The passive array archive will be used to: (1) Develop detailed tomographic models of the mantle at resolution at the ~50km scale. This is extremely important in understanding the nature of the Caribbean-South America plate boundary, and will provide the linkage between the subcrustal lithosphere and the crustal and surface features imaged by the active source experiment and investigated geologically. (2) Develop shear-wave splitting to constrain the flow within the mantle across the length of this boundary to test the hypothesis that the Caribbean plate is the result of flow around the subducting Nazca plate. (3) Develop receiver function profiles and whole crustal Poisson's ratio values from the Leeward Antilles arc to the South American craton. (4) Catalogue the local seismicity (to magnitudes less than 1.5) to map the plate boundary and the complex active tectonic processes in the Gulf of Paria. (5) Model the regional distance wave forms to constrain the lithospheric mantle structure of the South American plate.
The station log for the IRIS GSN station SDV (Santo
Domingo, Venezuela) shows that an 18 month portable array deployment would
record more than 1000 local and regional events.
Geophysical Investigations: Active Seismic MCS, OBS, and Onshore-Offshore
Studies: Shallow
reflection seismic profiles, but no deep crustal data, have been acquired by
the petroleum industry both offshore Venezuela and in the foreland basin south
of the Serran’a del Interior foreland fold and thrust belt. A large grid
of marine lines has been compiled by Mann and Pindell (Fig. 17), while other
reflection lines are available at Rice, and have been used in student theses
(supervised by A. W. Bally, P. Vail, and H. G. Ave Lallemant). No seismic
reflection lines are available for the Caribbean Mountain system and the few
lines through the Serran’a del Interior fold and thrust belt are shallow
and difficult to interpret. Onshore reflection profiling through the
Serran’a is extremely expensive (~$500,000-$1,000,000) and not central to
the hypotheses we are testing here. However, FUNVISIS, in collaboration with
this project, is requesting funds from PDVSA-INTEVEP for a
reflection/refraction profile across the Serran’a, which will tie to and
complement the proposed onshore-offshore profiles (Figs 12-13).
We are proposing 5 major marine
reflection/onshore-offshore/OBS profile corridors (Figs 12-13), all of which
are complemented by MCS profiles providing spatial coverage between the main
transects. The four N-S transects and ancillary MCS profiles will provide a
time transgressive view of the plate boundary development with the crossings of
the plate boundary 50Ma, 30 Ma, 15Ma, and 0Ma since arc-continent collision.
The goals of the four N-S transects are to image:
1) the accretionary
geometry of Southern Caribbean Deformation Belt, the Leeward Antilles arc and
the Caribbean Mountain system onto the South American continent,
2) the deep structure of
the broad and complex strike-slip and thrust fault system forming the
Caribbean-South American plate boundary, including the detachment surfaces
under the hinterland of the fold and thrust belt, the Caribbean Mountains and
possibly the arc, and
3) the structures of the
incipient fold and thrust belt at the front of the transpressional regime in
the Trinidad region and Gulf of Paria.
The 70W/50Ma,
67.5W/30Ma and
65W/15Ma profiles are designed to address 1) and 2), and the
Trinidad/0Ma profile is designed
to address 3). We note that the irregular coast line permits a marine
active source investigation of the hinterland and portions of the foreland
thrust belt to examine exhumation of the high pressure rocks exposed on
Margarita Island and on the Araya peninsula, as well as determine the northward
extent of the South American cratonic crust.
The 12N/Arc
profile along the 12th parallel will examine the deep structure of the arc,
determine its seismic velocity structure to the upper mantle, ascertain if the
upper half of the crust is indeed felsic to intermediate, and examine the
extensional dismemberment of the Leeward Antilles arc as it is accreted to
South America.
All of the profiles will determine uppermost mantle
structure along the length of the boundary as subduction polarity changes. The
five transects jointly will provide a time transgressive view of the
development of the transpressional margin.
Onshore, the Caribbean Mountain system and
Serran’a del Interior fold and thrust belt have a system of primary and
secondary highways and roads adequate for onshore-offshore profiling. This will
permit construction of an uninterrupted, integrated seismic profile across the
entire plate boundary from the Caribbean plate, through the hinterland of the
fold and thrust belt, into the northern edge of the foreland fold and thrust
belt. The availability of island and peninsular recording sites (the Leeward
Antilles, Margarita Island, Trinidad, and the Araya and Paria peninsulas) for
onshore-offshore recording of air gun signals makes this margin attractive for
wide-angle investigations at relatively low cost.
Each of the 4 NS profiles will be shot twice, once
with 20s shooting for MCS recordings, and a second time with 100s intervals for
recording OBS profiles to reduce previous shot noise (Christeson et al., 1996;
Zelt, personal communication). This shooting arrangement has a practical
advantage for the onshore-offshore profiles, as each line is recorded twice,
and will therefore be recorded at night. Night recording greatly reduces
cultural and wind noise, dramatically improving data quality. The 100s interval
line will be slightly offset from the 20s line and recorded as a lower fold MCS
profile (24 fold in 12.5m bins), enabling us to identify out of plane
reflections easily. The Arc profile will be acquired at 40s shot interval, a
compromise between the optimal requirements of the MCS and wide-angle
acquisition; time does not allow shooting this profile twice. This will produce
nominal 60 fold in 12.5m bins. While the OBS ship is retrieving the ~50 OBS
instruments, the MCS vessel will continue acquiring MCS data between the main
transects.
The extensive grid of shallow marine reflection
data acquired by Mann and Pindell (Fig. 17) will be examined carefully to
optimize the location of the main trasnects with respect to shallow structure
well before the experiment (see below). We will have a scientific party of 8, 5
watch standers, 2 Chief Scientists (Levander and Mann), and 1 seismic
processor. We will take one of our Omega processing computers with us aboard
the R/V Ewing, or use the Omega system now available for this purpose through
IRIS.
Onshore-Offshore Profiles: The onshore-offshore experiments will be recorded by 640 of the Texan
seismographs operated by UT El Paso and PASSCAL. We will occupy 320 sites with
two instruments each. Stations will be sited ~250m to 1km apart to produce
100-150km long profiles. The Texans are capable of recording continuously for
25 hours at 8 msec sampling from a programmed turn-on time, two
instruments/site gives a 50 hour recording window. These instruments are ideal
for dense land deployments because of their light weight, low power
requirements
(2 D-cells), and easy programming and downloading. A team of 2 deployers can
easily deploy 80 instruments at 40 locations in a day. Each deployment team in
Trinidad and Venezuela will be composed of a US and a Venezuelan geophysics
student, to ensure smooth deployments in the Spanish speaking countries. All
sites will be pre-permitted.
In addition we will deploy 10 Refteks equipped with
large disk drives and powered by car batteries in 2 deployments on 5 islands of
the Leeward Antilles arc for each deployment (Figs 12-13). The Refteks will
provide wide-angle velocity control along the Arc profile. The islands are easily accessible by commercial and charter
air.
Zelt, assisted by a post-doc, will be responsible
for onshore-offshore recording. We will deploy the 640 Texans on the mainland
in profiles of 100-320 stations each (Figs. 12-13).Ê Similar shooting
geometries, and instrument density in the Mendocino Triple Junction Seismic
Experiment produced high resolution images of the crust and uppermost mantle
across the continental margin in the San Andreas transform system (Figs. 19-20;
Henstock et al., 1997; Henstock and Levander, 2000). The dense shot and
receiver spacing permits excellent crustal tomography images (e.g. Zelt and
Smith, 1992; Zelt et al., 1999), and migrating the data with wide-angle
Kirchhoff depth migration techniques (e.g. Holbrook et al., 1992; Lafond and
Levander, 1995; Henstock and Levander, 2000). Wide-angle migration permits
undershooting complex near surface structures, such as those along the coast in
the metamorphic belts of the Caribbean Mountains. Wide-angle migration takes
advantage of large reflection coefficients at wide-angles to produce
reflectivity images with resolution comparable to normal-incidence shooting,
and allows for amplitude vs. offset studies directly from the migrated
wavefield (Fig. 19-20). Onshore-offshore data are particularly useful for
investigating the velocity and reflectivity structure of the lower crust and
crust-mantle interface (Henstock et al., 1997; Henstock and Levander, 2000).
Fig. 21 shows that large traveltime variations are expected for the OBS and
land stations across this margin.
Use of the Texans rather than Refteks will speed
the redeployment of instruments as we follow the Ewing along the South American
coast. [A Reftek weighs about 20 kg with batteries for 48 hour recording,
whereas two Texans weigh 3.0 kg with batteries. Similarly the Texans are much
smaller; 80 can be deployed from a 4WD sport utility vehicle,
whereas 20
Refteks require a cargo van]. Venezuela has an adequate highway and secondary
road system for traversing the length of the margin and deploying away from
heavily used roads. Venezuelan students in each deployment team will minimize
navigational errors onshore while deploying, and smooth interactions with the
rural population. The only potential problem we see is arranging customs
clearance to transport half of the Texans to Trinidad for the final profile.
Fortunately we have excellent co-investigators in Venezuela, as well as good
relationships with a number of Trinidad earth scientists to help us smooth this
process, and we have a slightly longer time between MCS transects to deploy in
Trinidad.
The coordination between OBS and MCS operations is
shown in Fig 22. Underlying assumptions are 300-km long lines, 50 OBSs (spacing
of 6.1 km; we are planning a denser spacing on the landward side and relatively
sparse spacing on the oceanic segment however, this doesn't affect time
estimates), deployment time of 1 hr. per instrument including transit, 5 knots
profiling speed of the Ewing, 10 knots transit speeds of both ships, 18 hrs.
transit time between two lines.
One OBS line would then be completed in 7 days. Including 2 days each for contingency
and transit between OBS deployments, 2 days of transit from/to port, and 1 day
in port, the entire cruise would last 39 days. Our plan leaves ~120 hours between OBS deployments for the
EW MCS profiling and additional NS MCS profiles and equipment maintenance by
the Ewing between the main transects.
Alternatively, this study could be
conducted entirely
from the Ewing in two legs. One leg would be dedicated to the OBS
operations. Total Ewing shiptime
is ~62 days.
Using two-ships has a number of scientific
advantages, notably it improves the quality of the onshore-offshore recording
and allows for additional MCS lines between the main transects as the OBS
vessel is retrieving and redeploying instruments. Assuming two-ship operation,
Levander and Mann will sail on the Ewing, Pecher will be responsible for the
OBS operations, and Zelt will supervise the on-shore operations.
Data processing and interpretation: Initial OBS data reduction will begin on board
immediately following each recovery.
As soon as the shot time and navigation data are obtained from the
Ewing, the OBS field data will be transformed into standard SEG-Y. Two of the
main MCS/OBS/onshore-offshore transects will be processed at Rice, the other
two at UTIG. Data processing
will depend on characteristics of the signal and the noise. Depending on the data, we plan to apply
several newly developed techniques, such as an approach for a successful
suppression of water multiples based on wavefield separation. Water multiples may potentially cause
problems in records from shallower water depths. Once data processing is completed, we will interpret records
and identify arrivals. We will
determine the velocity structure using combined refraction and reflection
inversion (e.g. Zelt and Smith, 1992; Zhang et al., 1998). We will then interpret the resulting
velocity image together with the coinciding reflection lines to obtain the
geologic structure of the Venezuelan margin and to improve our understanding of
continental growth. We plan to
deploy several OBSs densely (ca. 2-3 km) spaced between the landward edge of
the MCS lines and the shore. This,
together with the dense spacing of the land station, should enable us to
perform wide-angle migration and to extend the MCS reflection images landward
(see Henstock and Levander, 1999, for a similar approach in an onshore/offshore
experiment in northern California)
Levander, Zelt, Pecher, and Mann have extensive
experience in MCS, onshore-offshore, and active source OBS acquisition,
processing, and interpretation. Levander has conducted MCS/OBS/onshore-offshore
experiments in central California and the Mendocino Triple Junction region.
Zelt has participated in OBS/onshore-offshore cruises off Chile and Iberia.
Pecher has experience with OBS data collection and analysis. In addition, Paul
Mann will be involved in interpretation of about 3000 km of 24 fold shallow
penetration MCS data from offshore Venezuela and Trinidad which were donated to
UT by Gulf Oil (Fig. 17). Mann
will work with Levander, Zelt, and Pecher on the geologic interpretation of
OBS/onshore-offshore and MCS data acquired as part of this program, based on
his knowledge of the shallow structure of the area gained from the study of the
Gulf data. He will also merge the shallow and deep structure into synthetic
regional cross sections.
Geodynamic Modeling: Several
of the hypothesis testing aspects of this proposal, particularly those
involving crust-mantle interactions, will benefit from geodynamic modeling. The
combination of seismology, geology, and petrology that forms the bulk of this
proposal will lead to several data sets. By finding the connections between
these data sets, the geologic evolution of the region can be tied to the
evolution of the mantle below (both in terms of mantle lithospheric structure
and shallow mantle flow patterns). Quantitative geodynamic models will allow us
to systematically map physically plausible evolution scenarios that are in line
with the range of data to be acquired.
Using geodynamic modeling as a tool to help explore
crust-mantle interactions within an arc-continent collision zone requires good
data constraints, of course, but it also requires bridging the gap between
mantle convection modeling and continental tectonics modeling. Mantle
convection is ultimately what drives all large-scale tectonics on our planet.
The connection between mantle convection and oceanic tectonics has been
explored through mantle convection modeling [e.g., Moresi and Solomatov, 1998;
Tackley, 1998; Trompert and Hansen, 1998]. However, the interaction between
mantle dynamics and continental tectonics has yet to be addressed in depth
within the mantle convection community.
At the other end of the spectrum, continental
tectonics models have become very advanced incorporating crustal rheologies
that allow for depth variable deformation mechanisms [e.g., Bird, 1989;
Fullsack, 1995] and for lateral variations [e.g., Dunbar and Sawer, 1989]. The
bulk of such models, however, do not deal directly with mantle convection or
evolving mantle flow patterns. Rather, the tectonic driving force associated
with mantle flow is only indirectly accounted for by prescribing a velocity
boundary condition at the base of a crustal layer [e.g., Willett et al., 1993;
Royden, 1996] or at the edge of a lithospheric layer [e.g., England and
Houseman, 1986].
For geodynamic modeling to be of true benefit to
this study, the coupling of continental tectonics and mantle convection must be
directly addressed. As an example of why this is so, one need only recall that
most thoughts about subduction polarity reversal, a key issue in the evolution
of the Caribbean margin, revolve around the idea that surface tectonics leads
to thick crustal regions which congest a subduction zone and cause first-order
changes in mantle flow patterns which then feedback and alter subsequent
tectonic deformation, i.e., the system involves strong coupling. The Co-PI
Lenardic, together with Dr. Louis Moresi of CSIRO Australia, has been involved
in developing models of coupled continental tectonics and mantle dynamics over
the last year [Moresi and Lenardic, 1999; Lenardic et al., 2000]. The major
tool that has been employed is a version of the CITCOM finite element code
developed by Dr. Moresi [Moresi and Solomatov, 1995]. The most recent version
of the code specifically combines insights from the continental tectonics
modeling community with those from the mantle convection modeling community.
This tool is already well suited to exploring issues pertinent to the Caribbean
margin. To show this, we have performed some 20 test case numerical simulations
focused on arc-continent collision and subduction polarity reversal.
Figure 23 shows a numerical simulation of coupled mantle convection and continental tectonics applied to arc-continent collision. The simulation allows for the formation of both continental and oceanic plates. The incorporation of plate-like behavior into mantle convection models has experienced a great surge in the last several years [e.g., Moresi and Solomatov, 1998; Tackley, 1998; Trompert and Hansen, 1998]. Much of the knowledge gained over this time has been incorporated into the model of Figure 23 that allows for the formation of localized shear zones that represent plate margins. This is accomplished through the use of a visco-plastic rheology akin to that used by Moresi and Solomatov [1998] but also allowing for a strain-rate dependent weakening along the plastic rheologic branch, a factor that has been found to be important for generating truly plate-like behavior in mantle convection models [Bercovici, 1996; Tackley, 1998].
The simulation of Figure 23 allows for continents
comprised of chemically distinct crust and sub-crustal mantle lithosphere. Each
component can have its own rheologic law and the laws employed are based
directly on the rock deformation experiments [e.g., Kirby, 1985; Kohlstedt et
al., 1995]. This means that the crust, for example, can either flow in a
ductile regime under warm thermal conditions or fail as a coulomb material
under cool thermal and high stress conditions. The formulation used for
modeling crustal deformation is based on established techniques used in the
continental tectonics community. To show this, Figure 24 compares an early
evolution frame from the model of Figure 23 to a classic model of a doubly
vergent orogen [Willett et al., 1993]. Crustal deformation patterns are similar
at the time shown because the driving force for crustal deformation, subcrustal
subduction, is the same and because both approaches use a similar rheologic
framework. The approaches differ in that subcrustal subduction in the Willett
et al. [1993] model is prescribed as a static velocity boundary condition on
the lower crust while in our model it results through the dynamics of mantle
convection. For tectonic evolution problems spanning on the order of a million
years, the classic approach has proven very powerful and Figure 24 simply shows
that we have confirmed that our simulations give consistent results. As our
simulation evolves, deformation patterns do change as crustal thickening feeds
back and alters the specifics of subcrustal subduction. In effect, the crust
does not experience a static velocity condition at its base for evolution times
approaching ten million years.
It is precisely for evolution times of ten million
years or more that our modeling approach becomes most useful as no modeling
study has explored the interaction of continental tectonics and mantle dynamics
over such times scales. By doing so, with a focus on the Caribbean margin, we
will be able to add a geodynamic compliment to testing several of the
hypotheses laid out in this proposal. Refinement of the modeling approach will
be carried out throughout the funding cycle so as to allow for greater
explorations of parameter space. Much of the early modeling can begin even
before data is acquired. This will allow us to get a physical feeling for how
an analog system works and map a range of parameter effects. Doing so early and
building physical intuition will prove useful in initial data interpretation
and will allow us to more quickly rule out unlikely models once the data starts
flowing in at a high rate.
Figure 23 assumes upper mantle convection and a 2-D
geometry. This will not be a restriction for production models. Whole mantle
convection models require added numerical resolution and added physical effects
[e.g., Tackley, 1995; Bunge et al, 1996]. The added physical effects can
already be handled within CITCOM. Added resolution means that powerful
computers are required to allow for a practical exploration of parameter space.
Workstations now in place at Rice can deal with this in 2-D. Although 2-D
models will be useful for some parameter space exploration, the transpressional
nature of the study region makes 3-D modeling a must. CITCOM already has 3-D
capability as shown in Figure 25. However, 3-D simulations using complex
crustal and mantle rheologies of the type employed in Figure 23 can not be
performed practically on single processor workstations. For production modeling
this will not be constraint as a recent MRI award has allowed the Rice Center
for Computational Geophysics to acquire a large multiprocessor machine. Dr.
Moresi is already funded to visit Rice to help parallelize a version of the
CITCOM code, and tune it to the specific machine architecture.
International Collaboration : Attachments in Appendix 1: We have a
large-scale collaboration planned with a number of Venezuelan scientists and
organizations including FUNVISIS, PDVSA-INTEVEP, and a number of Venezuelan
Universitites. (Letters of intent are included in the appendix). Dr. Marino
Ostos, a former student of Hans G. Ave Lallemant, is a professor at the
Universidad Central de Venezuela at Caracas and will participate in the
research (see supporting letter). Ave Lallemant has worked in Venezuela for the
last 12 years with help from faculty of the Universidad Central de Venezuela.
Professors J. Rigueiro and J. Castillo, geophysicists at Simon Bol’var
University have up to 20 students to help with the passive and active
seismology deployments (see attachments). They will be included in the
interpretation of the data. We have requested lab space, aid in permitting, and
interpreter assistance both universities. We have a long tradition of educating
Venezuelan, Trinidadian, and other South American students at Rice, UH, UT
Austin, and Arizona, and have many contacts in academia and the South American
petroleum industry as a result. We anticipate that a number of South American
or Caribbean geology and geophysics students will enroll in graduate programs
at our universities to work on the data from this project. In the
past these students
have been at least in part funded by their home countries, and thus we will
have a larger group of students working on this project than is reflected in
the request to NSF.
Collaborative Research: The research we are proposing requires examining the structures of the
Caribbean plate boundary across a range of scales form the 50-100km scale in
the mantle to the 100m to 1 km scale in the crust, to the 1-100m outcrop scale
at the surface: Deep mantle flow, sub-crustal lithospheric plate
structure, whole
crustal and intra-crustal structure, outcrop scale sampling, and
thermobarometry and thermochronology on hand samples and thin sections. The
research is inherently multidisciplinary. Faculty at Rice and UT Austin have
supervised a number of theses on Venezuelan structure, stratigraphy, and
tectonics. This body of work provides a strong background in the tectonics of
this area, the structure of the Caribbean Mountain system, and the structure
and development of the fold and thrust belt-foreland basin system which is
invaluable for the proposed research. As we have mentioned, we have acquired
several dense grids of shallow petroleum industry data (Fig 17) that will be
used to help design the marine acquisition program, and which will be
invaluable for tying together the active source corridors. The active source
experiments tie the outcrop scale studies to the mantle observations made by
the passive seismology. Hopefully, the geodynamic modeling produces predictive
models relevant to all scales.
Management Plan:
The project time lines are given in
Table 2. This proposal is the outgrowth of a workshop on the Caribbean-South
American plate boundary held at Rice in October, 1997. For this project the PIs
and Venezuelan scientists will hold a science-planning workshop in Houston in
2001. A number of the PI’s will participate in a reconnaisance
survey/liason workshop in Venezuela in 2001 (Levander, Zelt, Pecher, Wallace,
Vernon). After the active source work is completed and intermediate results are
available, we will have a second workshop in Arizona in late 2002. A third,
synthesis workshop will be held in Venezuelan in mid-late 2003, after the
passive seismic investigations are completed, and final results are available
from the active source investigations. In addition to our 3 workshops, we will
also hold working group meetings at the fall AGU.
Levander will coordinate the project with the aid
of the Rice post-doc and the Rice Geophysics Program secretary.
Wallace will coordinate the land and marine passive
source seismic deployments. Vernon is responsible for field deployment of
passive OBS instruments with help from the OBSIP* support group. Wallace,
Vernon, 2 Arizona graduate students, and a UCSD research associate, and
undergrad are responsible for analysis of the passive source data. Wallace and
Vernon have a long history of collaboration. Vernon will assemble the entire
passive land and marine dataset at UCSD and deliver it to the IRIS DMC.
Zelt will coordinate the active source
seismic investigations.
Zelt, Levander, the Rice post-doc, and Pecher will reconnoiter
the onshore-offshore
seismic lines on the islands and mainland so that the MCS, OBS and onshore
groups are fully cognizant of the operational limitations faced
by each group.
Pecher will run the active source OBS operation with two experienced field
personnel from UTIG in addition to the personnel from the OBSIP
support group.
Levander and Mann will be chief scientists aboard the R/V Ewing. Zelt and
the Rice seismology post-doc will run the onshore-offshore programs on the
islands and mainland. Communication will be maintained through INMARSAT and
cellular telephone.
The combined MCS/onshore-offshore/OBS datasets from
the active source experiments will be distributed to Rice and UT following the
field programs. Rice will archive the data at the IRIS DMC. Rice and UT divide
the dataset for analysis, so that the data are analyzed in a timely fashion.
The short distance between UT and Rice will ensure that the active source data
are processed and interpreted in a unified manner. Levander, Zelt, a post-doc,
Mann, Pecher, and 3 grad students at Rice and UT are responsible for active
source data analysis and interpretation.
Ave Lallemant will coordinate the geological
studies on the Venezuelan mainland and the Leeward Antilles arc. He will
conduct field studies on the Venezuelan mainland with 1 Rice graduate student.
He will direct Ertan’s microprobe analyses, and is responsible for
choosing samples for apatite fission track analysis. Pindell has included
travel to conduct field studies of the basins in Venezuela, interpret industry
seismic and satellite data, and visit Houston frequently to work with Ave
Lallemant. Wright, Copeland, and 2 graduate students are responsible for most
of the field studies on the Leeward Antilles and the age dating.
All of the investigators have primary collaborators
from FUNVISIS or the Venezuelan Universities with whom they will work, as
described above.
Education and Human Resources : A total of 8 graduate students and 1.25 post-docs
will be funded by NSF. One
post-doc will work on the project at no cost to NSF. Rice has 3 graduate
students and 1.25 post-docs in its budget, UAz has 2 graduate students, and
UTIG, UH, UGa each have 1 graduate student. [Two Rice graduate students and 1
post-doc are currently working in Venezuela on related problems]. Arizona will
have one post-doc working on the project at no charge to NSF. As we noted
above, we also expect a number of Venezuelan and Trinidadian students to enroll
at our institutions with support from their home countries.
PDVSA-INTEVEP has already
offered to fund 2 Venezuelan Ph.D. students in the U.S.
Lastly we intend to include undergraduates in both
the field programs and the processing and analysis of the seismic data. Field
work for undergraduate participation in this project is funded through this
grant. Analysis and meeting related travel will be funded through departmental
sources and REU requests to NSF as the students engage in the project. We
expect at least 8 undergraduates to be involved in the project through field
work and REU projects.
OBSIP: Ocean Bottom Seismograph Instrument Pool, a PASSCAL-like facility for OBS instruments