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 crust-mantle 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, Curaçao, and Bonaire (Aruba: Helmers and Beets, 1977; Curaçao: 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 700°C essentially the crystallization temperature; the minerals commonly investigated by 40Ar/39Ar dating give information about a range of temperatures with amphibole at about 500°C, micas at 370 to 300°C and K-feldspar at 300 to 150°C; fission-track dating of zircon and apatite gives information about the passage through ~225 and~100°C, respectively; (U+Th)/He dating of apatite gives information about the lowest temperatures, in the range of 80 to 60°C. 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, Curaçao, La Orchilla, and La Blanquilla.
Venezuelan Metamorphic Belts.
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 (Avé Lallemant and Sisson, 1993; Sisson et al., 1997; Smith, 1996; Smith et al., 1999; Maresch,1975; Guth, 1991; Stockhert et al, 1995; Avé 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.
Avé Lallemant will undertake kinematic (deformation) studies with the cooperation of Dr. Marino Ostos of the Universidad Central de Venezuela and 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). 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 Français du Pétrole 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. Vander 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 arrayto 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). 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 6–14°N and 61–71°W (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. Avé 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 five 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 sound source signals makes this margin attractive for wide-angle investigations at relatively low cost.
MCS Profiles, Profile Planning, and Shooting Arrangements:
We will acquire ~7,000 km of marine reflection data with the R/V Maurice Ewing. The R/V Ewing has a 480 channel, 6 km digital streamer, with hydrophone groups at 12.5m group spacing. The Ewing's sound source is a 140 liter 20 sound source array (theoretically equivalent to ~1 ton of explosive; Diebold, personal communication). Firing every 20s, at ~50m, produces 60 fold MCS data in 6.25m common midpoint (CMP) bins. We will likely re–bin the data to 12.5m with 120 fold.
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). 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 muchsmaller; 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.
OBS Operation:
The OBSs will be deployed from a second ship that has the capability of handling 50 OBSs and an endurance of at least 39 days (e.g., the R/V Melville) to allow full use of the R/V Ewing for MCS acquisition. Our plan is to deploy ~50 OBSs along each of the four N/S profiles. Figs. 12–13 shows tracklines and deployment locations. In the following description it is assumed that shooting starts in the west and proceeds to the east; this may be reversed depending on the Ewing's port city preceding our cruise. In each N/S corridor, shooting generally starts inthe center to acquire the northern section of the 20s MCS data from south to north. During this time, the OBS ship deploys 40–50 OBSs along the N/S lines. When the Ewing starts shooting the 100-s OBS line from the north, the OBS ship returns to the northern end of the line and starts recovery of the OBSs. At this time, the Ewing will be over 150 km away from the northernmost OBS. We do not anticipate to record any arrivals except Pn at that source-receiver offsets. Therefore, we will not lose any information by starting the OBS recovery while the Ewing is still shooting but we will save valuable time. We estimate about two hours of recovery time per instrument including transit, during which the instruments can be refurbished (Babcock, personal communication). The transit times between two lines offers an additional contingency of about 18 hours for refurbishing of the instruments.
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. Avé Lallemant, is a professor at the Universidad Central de Venezuela at Caracas and will participate in the research (see supporting letter). Avé 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.
Avé 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 Avé 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.
Ocean Bottom Seismograph Instrument Pool, a PASSCAL–like facility for OBS instruments
*OBSIP: Ocean Bottom Seismograph Instrument Pool, a PASSCAL–like facility for OBS instruments
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