T33B-1361 INVITED 1340h
BOLIVAR: The SE Caribbean Continental Dynamics Project
Post-Archean continental growth is presumed to result from accretion of island arcs to older continental masses. Along the Caribbean-South American (CAR-SA) plate boundary zone (pbz) the Antilles Arc has obliquely collided with the northern SA margin since the Late Cretaceous. Neogene motions across the pbz combine right-lateral strike-slip faulting along the coastal faults (El Pilar-Oca), underthrusting of CAR seafloor beneath SA, and folding and thrusting and foredeep sedimentation at the southern edge of the pbz. We provide a progress report on the components of the NSF Continental Dynamics BOLIVAR project ("Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region"). The focus of this study is the mechanism for accretion of arc-related terranes to the northern SA continent. The project includes geological, geochemical, and geophysical investigations involving more than 30 scientists at 9 institutions in the U.S. and Venezuela. The study area extends from the Atlantic Ocean to the 71$^{o}$W meridian, and from the Guyana Shield (60$^{o}$N) into the eastern Caribbean basin (14$^{o}$N). This immense area ($>$0.7M km$^{2}$) is comparable in size to California and its continental margin, and rivals the San Andreas plate boundary zone in geologic complexity. Geologic studies have included mapping and age dating of igneous rocks of the Leeward Antilles from Aruba to Los Testigos, mapping of brittle deformation structures on the ABC islands, and mapping in the Villa de Cura blueschist belt in Venezuela, basin analysis and analysis of uplift and subsidence patterns in the onshore and offshore region, and 3-D reconstruction of palegeographic evolution. The BOLIVAR passive seismology group has completed installation of a network of 35 PASSCAL broadband seismographs in central and eastern Venezuela, and 14 long-term OBSIP recorders along the Leeward Antilles arc and in the southeastern Caribbean basin. The PASSCAL and OBSIP instruments complement the 35 satellite-telemetered broadband stations operated by FUNVISIS, giving a broadband array of 84 stations. In June, 2004, the BOLIVAR active seismology group completed marine reflection profiling, and joint land refraction, wide-angle onshore-offshore and OBS recording. We acquired $\sim$6000 km of reflection data, including 5 principal reflection/wide-angle profiles. Four of these are along meridians (64$^{o}$W, 65$^{o}$W, 67$^{o}$W, 70$^{o}$W), extending from the Caribbean basin, and crossing the South Caribbean deformed belt, the Oca-El Pilar fault system, and most of the fold-thrust belt onland in Venezuela. The fifth profile was oriented northwest and extended from the Atlantic Ocean east of Trinidad and Tobago to the eastern Caribbean Sea and spanned the Lesser Antilles arc and associated structures. The R/V Ewing provided the sound source for much of the wide-angle program as well as acquired the reflection data. Dense wide-angle data were recorded using 49 OBSIP instruments deployed 169 times from the R/V Seward Johnson II, and 550 Reftek Texans deployed along each profile on the SA mainland. We also recorded reflection profiles and wide-angle data along the length of the Leeward Antilles arc. We summarize the progress made in interpreting the geological and geophysical data, with a goal toward developing a unified model for development of the southeastern Caribbean plate boundary zone.
http://earthscience.rice.edu/BOLIVAR.html
T33B-1362 1340h
Onshore observations of land and sea seismic sources in northern Venezuela
As part of the BOLIVAR and GEODINOS projects, active seismic measurements were done during the months of April and May 2004 in northern Venezuela between 64 and 70 degree W. Four deep seismic reflection and refraction profiles were acquired perpendicular to the Caribbean-South America plate boundary. To the north, the plate boundary zone with complex plate interactions is marked by the Oca-Ancon, San Sebastian and El Pilar fault zones, a 1000 km long dextral strike-slip fault system, which accommodates a relative movement of 2 cm/year between both plates. From the Caribbean Mountain System in the north, the seismic profiles run into the Falcon Basin in the west and the Guarico and Oriental Basins in central and eastern Venezuela, divided by Frontal Thrust Systems from the Caribbean Mountain System. Further south, the Guayana Craton corresponds to the stable South American plate. Up to now, little is known about the crystalline crustal structure. In western Venezuela, the overall crustal thickness decreases from 45 km in the craton area to around 35 km along the coastline. Air gun shots from the marine refraction lines, recorded by the broadband stations of the Venezuelan national seismic network as well as the BOLIVAR project temporal seismological network, were used to build seismic sections in which first arrivals can be observed at offsets larger than 200 km. This data will be modeled and interpreted using two different approaches: 2D traveltime inversion using a layered model, and 3D tomography using seeking a smoth velocity model. Along the four main profiles, 2 land shots with chemical explosions were fired on each profile in order to provide information on the crustal and upper mantle structures. Data quality is good for the land shots to at least 200 km distance from the shotpoints. We will provide information on the crustal structure and thickness on the northern edge of the South American continent. The use of seismic recorders from IRIS-PASSCAL Instrument Centre and technical support is acknowledged. Contribution to project G-2002000478.
T33B-1363 1340h
Proyecto Caribe: Preliminary Results From an Ongoing Broadband, Passive Seismic Array Experiment in Venezuela
Proyecto Caribe is an ongoing experiment to investigate the large scale structure and tectonic framework of the northern boundary of South America. A 77 station, broadband array is currently operational in Venezuela with instruments running from the craton south of the Orinoco River to OBS instruments located on oceanic crust of the Caribbean plate. Initial work on seismicity suggests this experiment will improve detection and location of events in the Orinoco basin and southern Venezuela where national network coverage is sparse. The data from the experiment suggests there are large variations in the propagation characteristics of regional phases across Venezuela. In particular, we note that high amplitude, impulsive Sn phases are observed in the Maturin Basin, but Lg is not observed there. In contrast, stations on the craton south of the Orinoco River commonly record both Sn and Lg. This indicates a major difference in lower crust and upper mantle structure for the two regions. Initial measurements of P wave residuals from teleseismic earthquakes indicate a large signal is present. We currently are working with the first 7 months of data collected from the array to develop a preliminary P wave tomographic model to gain insights on the three-dimensional geometry of the Caribbean-South American plate boundary.
T33B-1364 1340h
BOLIVAR: Structure of the Leeward Antilles "Arc" and Growth of the South American Continent
It is generally believed that in the Post-Archean continents have grown by accretion of island arcs to older continental masses. The bulk composition of sediments and compilations of global seismic refraction measurements suggest that average continental crust has an intermediate composition ($\sim$~58-62$%$ Si0$_{2}$) whereas seismic velocity measurements in the Aleutians and the Sierra Nevada show that neither Aleutian-type island arcs, nor Sierran-type continental arcs are chemically similar to average continental crust, the former being too mafic, and the latter being too felsic. Without considerable mechanical and/or chemical refining, neither of these types of arcs can lead to continental crust. Several models have been proposed to refine the crust, including delamination of the lower-most crust to reduce its mafic composition. We are examining the seismic structure of the Leeward Antilles islands, which have been accreting to the South American continent throughout the Cenozoic in a transpressive tectonic setting. Mapping on Aruba, Bonaire, and Curacao shows that this element of the arc is composed of a basalt-grabbo-tonalite core, whereas the islands to the east have been mapped as island arc quartzdiorites and granodiorites. During the 2004 BOLIVAR active source experiments we acquired wide-angle data along the strike direction of the arc using 15 PASSCAL seismographs and on 5 dip direction profiles crossing the arc using numerous OBSIP instrument deployments. These data are being used to measure average crustal velocities, from which we can infer the bulk composition of the arc, as well as that of the voluminous sedimentary accumulations surrounding the arc. The development of the large-scale strike-slip system along the margin to which the Antilles arc is accreting and the associated basin development adjacent to the arc during accretion allow us to study the margin's evolution over time. The sediment mass can modulate the composition of the resulting continental crust as it is metamorphosed, shortened, and mixed with arc and oceanic rocks during accretion.
http://earthscience.rice.edu/BOLIVAR.html
T33B-1365 1340h
Origin of Caribbean Plateau Basalts, the Arc-Arc Caribbean-South America Collision, and Upper Level Axis Parallel Extension in the Southern Caribbean Plate Boundary Zone
Three new aspects of the Pacific-origin model for Caribbean evolution are proposed as part of the BOLIVAR program. First, mid-Cretaceous emplacement of the Caribbean Plateau Basalt Province is NOT due to the often-cited idea that the plate rode over Galapagos hot spot, whose proven age is less than Eocene and whose hypothetical Cretaceous mantle reference position was well west of Caribbean lithosphere, but rather due to progressive Cretaceous subduction of the Proto-Caribbean Spreading Center (a western arm of the Atlantic) beneath the Caribbean Plate as the Plate was engulfed between the American continents. A sub-Caribbean subduction shadow approaching the size of the Basalt Province is permissible on plate kinematic grounds, suggesting a Basin and Range [continental] analogue for Late Cretaceous extension, thinning and volcanism in/on Caribbean crust. Second, plate kinematic and tomographic data suggest that Cenozoic North America-South America convergence was accommodated at a N-facing Proto-Caribbean subduction zone along northern South America prior to the E-wardly diachronous collision of the Caribbean Plate with South America. Further, incision depths to Albian and normal fault offsets reaching 500m on the regional Paleogene northern South American unconformity are too great to be explained by a migrating forebulge model (traditional arc-continent view of Caribbean-South America collision). Thus, Caribbean-South America interaction was an E-ward younging dextral oblique island arc-Andean arc collision, although the latter was amagmatic due to minimal rate and amount of Proto-Caribbean subduction, which took place above a vertically-interring Proto-Caribbean lithosphere. Deep, S-ward lessening Paleogene incision in northern South American basins is described as forearc uplift, ie thrusting of the South American hanging wall onto the flexurally arched and buttressed Proto-Caribbean lithosphere between the converging Caribbean and South American plates, prior to Caribbean collision. In addition, facies and sand/conglomerate mineralogy in the Scotland Fm of Barbados suggest that it represents part of the Proto-Caribbean, not Caribbean, Paleogene accretionary prism, and was accreted late (in the Mid-Miocene) into the Caribbean prism. Third, the highly-oblique Caribbean-South America collision above the Proto-Caribbean slab requires strong axis-parallel extension in the plate boundary's basement rocks and sedimentary cover. Seismic examples and Middle Miocene sedimentary facies development in Trinidad's Central Range demonstrate how such extension occurs in upper structural levels of the orogen to complement axis-parallel extension at basement levels described elsewhere.
T33B-1366 1340h
Re - interpretation of magnetic signatures from parts of Colombian and Rancheria basins, Caribbean Sea, north of Colombia
Boundaries of Caribbean are major structural features of earth's surface. Structural features within these boundaries are of great importance in understanding the tectonic history of Caribbean regions. An attempt has been made to re interpret the magnetic signatures with a new technique for better understanding of the area. The paper presents a new technique based on the finite element analysis approach for separating regional and residual components of a magnetic field response. The method is discussed and its efficacy is brought out with the help of field data. The proposed technique is employed to computer the regional and residual components of magnetic anomaly for the area from parts of Colombian and Rancheria basins, Caribbean sea, north of Colombia. The paper discusses the behaviour of residual anomaly map. Given below are some of the observations. Images bring out structural features more clearly than contour maps. Magnitude of the anomalies obtained using this method is greater than obtained using other methods. Northern part of Colombian basin shows EW trend where as the southern part NE - SW trend. Deep basin represents images and contours extending firmly in EW direction where as the marginal anomalies trend roughly parallel to coastline. Contact between ocean and continent can be drawn. Structure in the extended direction of Beata Ridge is seen in the new map. Possible extension of Cuiza fault (or may be some other structure) extending in the west is seen in the new map. It is shown that these maps prepared using the new technique help in identifying and demarcating structures in the area and hence in better understanding of offshore tectonics and onshore tectonics.
T33B-1367 1340h
Aruba and Curacao: Remnants of a Collided Pacific Oceanic Plateau? Initial Geologic Results from the BOLIVAR Project
The oldest rocks exposed on Aruba are amygdaloidal pillow basalts and lesser basaltic sandstone. The pillow lavas are geochemically similar to MORB and OIT. Locally, there is a distinctive unit of pebbly mudstone, argillite and basaltic tuff interstratified with the pillow lavas. Turonian(?) ammonites have been recovered from argillites. Both the pillow lava unit and argillaceous sediments were then intruded by a diabase complex. The intrusive rocks are geochemically indistinguishable from the pillow lavas. White and others (1999) interpret the lavas and diabase as having been formed in an ocean plateau. Following emplacement of the diabase unit, Aruba became emergent. There is abundant evidence for weathering and erosion of the pillow lavas and diabase unit. Basaltic magmatism continued as witnessed by the accumulation of accretionary lapilli tuff. Accretionary lapilli only form in a subaerial environment. The accretionary lapilli tuff was shortly thereafter reworked by local stream(s) which produced a coarse conglomerate composed entirely of clasts of basalt, diabase and lesser amounts of accretionary lapilli. The acreetionary lapilli and conglomerate units are almost always found together and are interpreted to have been deposited in a paleo-valley. Following this, the entire sequence underwent low grade regional metamorphism accompanied by folding and cleavage formation. Available data constrain this event to be post Turonian (93.5 $\pm$ 4 Ma) but pre emplacement of the Aruba batholith at 89 $\pm$ 1 Ma (SHRIMP-RG U-Pb zircon date). Thus, Aruba went from being emergent to undergoing regional metamorphism and batholith emplacement in a very short time period. We suggest that the pre-batholithic rocks were partially subducted prior to batholith emplacement. The metamorphic complex and batholith were exhumed and exposed by at least the Eocene/Miocene as they are overlain by shallow marine limestone of that age. On Curacao, the Curacao lava formation is geochemically similar to and broadly correlative with pre-batholithic basalts and diabase on Aruba (e.g. White and others, 1999). Following accumulation of the Curacao lava formation the area became emergent as there is widespread evidence for weathering and erosion of the basalts (Beets, 1972). Thus, both Curacao and Aruba became emergent in the Late Cretaceous. However, the subsequent geologic history of Curacao is distinct from Aruba, as it was not affected by Turonian/Coniacian deformation and metamorphism, but instead subsided and developed as a sedimentary basin, leading to the accumulation of extensive deposits of Late Cretaceous (Santonian-Maastrichtian) to Early Paloecene age. Late Cretaceous turbidites on Curacao contain a significant population of euhedral zircons in the range of 70-87 Ma (older Cretaceous grains in the range of 103-125 Ma are also present) along with angular hornblende and plagioclase mineral fragments. These arc derived grains are accompanied by older Mesozoic, Paleozoic and Precambrian zircon and undated coarse detrital muscovite, all presumably derived from the South American continental margin. Deformation (folding) on Curacao occurred between the Danian and middle Eocene (Beets, 1972) presumably during accretion to the Venezuelan continental margin. These data complicate models which interpret Aruba and Curacao to represent remnants of a Pacific oceanic plateau whose collision with a west-facing island arc produced subduction polarity reversal in the Late Cretaceous. Alternatively, we suggest that Aruba and Curacao formed on the subducting plate of an already east-facing arc system. Preliminary work on the island of Bonaire suggests a similar overall tectonic setting.
T33B-1368 1340h
Chronology of Tectonic Events in Western Venezuela Based on Integration of BOLIVAR MCS Data with Wells and Onland Geology
Newly acquired BOLIVAR MCS lines combined with existing GULFREX lines collected in 1975 document a complex chronology of tectonic events affecting the Paraguana Peninsula, Falcon coast of Venezuela, and the offshore Bonaire basin. Three basin phases can be identified: 1) Paleogene backarc opening of the 3 km-thick Bonaire-Grenada basin occurred along low-angle normal fault systems that have locally been inverted by later tectonic phases. Normal faults trend EW and parallel the trend of the Bonaire basin; 2) WNW to NW-striking normal faults crosscut this older trend and form deep offshore rifts with up to 4 km of fill that separate the islands of the Dutch Antilles. One EW-trending rift with 4 km of fill continues onshore to the northern Paraguana Peninsula. Offshore well data and the age of onland sediments indicate that rifting occurred in Oligocene through middle or late Miocene. GPS data from Falcon and the Paraguana Peninsula indicate east-west extension of the area is occurring at rates of 10-15 mm/yr; and 3) A Miocene-recent accretionary prism parallels the coast of Falcon for about 175 km, forming a boundary between the Falcon and Bonaire basins. Restoration of imbricate thrusts seen on seismic lines indicate that a minimum of 7 km of shortening has occurred. Onland wells show that detachment occurs at the level of Oligocene shale units. Shortening between the Falcon coast of Venezuela and the offshore Bonaire basin is related to the NE displacement of the Maracaibo block along the Bocono fault.
T33B-1369 1340h
Geologic-Structural and Geophysical Study of Cabo San Roman-Barquisimeto Section, Falcon and Lara State.
The main objective of this investigation is the geologic-structure interpretation of the Falcon Basin inverted along the Cabo San Rom Barquisimeto Section, by means of an integrated Geology and Geophysics study starting from several gravity-magnetics models of the section based on geophysical and geologic data. The area of study is located inside a corridor, that it have 10 km wide and 244 km long, with an area of 2440 km2. The profile, with the corresponding corridor, begins on north of the Paraguana Peninsula in the Punta Barco town and finishes at the south of Barquisimeto City in the Lara State, near the town of Las Cuibitas, with a trend of N 14°W (346°). For the geophysical analyses we considered a much more extensive window defined by the parallels 12°20'N and 10°N, and the meridians 70°W and 69°W. Begining from the analysis of the gravity-magnetics models proposed with different alternatives of slab subduction, we appreciated that shallow slab subduction model is of better adjustment, with the presence of the Bonaire Block. The structural map exhibits three well differentiated areas, so much in the geologic environments as in the structural styles, which is show in a decisive way on the geomorphology of the whole region. From the maps analyses and gravity-magnetics models presented, it is evidenced a cortical thinning at the center of the Falcon Basin inverted, defining a graben, which is limited southward for the Oca-Ancon faults system, while northward the limit is not expressed in surface. This geometry evidences that Falcon Basin was formed during a distensive regime. The balanced section demonstrates two tectonics settings and a third that includes both first regimens: a) compressional "pulses" at the beginning of the Tertiary, b) a relax and extension phase in Middle to Late Tertiary and c) a new compressive phase post-Tertiary. CONTRIBUTION TO PROJECT G-2002000478
T33B-1370 1340h
BOLIVAR: Tectonic History Based on Brittle Deformation Features of the Netherlands Leeward Antilles Island Arc
As part of the BOLIVAR project, we performed structural analysis of the brittle deformation features on the Netherlands Leeward Antilles island arc. The arc is located offshore Venezuela and from west to east includes Aruba, Cura\c{c}ao, and Bonaire (ABCs). The ABCs trend approximately WNW-ESE parallel to the obliquely convergent Caribbean and South American plate boundary zone. The goal of the research project is to characterize deformation structures in relation to plate motions to improve understanding of regional tectonic history. Structural analysis of faults, joints, veins, and bedding planes has identified several deformation events on the ABCs. The oldest (D1) structures are megascopic folds that trend WNW-ESE. Some of these folds display a weak axial plane cleavage. Thrust faults trend NW and are thought to be related to the D1 event. These structures are crosscut by NE-SW trending joints, veins, and normal faults (D2). Dextral strike-slip faults (D3) trend WNW-ESE to NE-SW. Normal faults trending WNW may be the youngest structures (D4). Newly acquired MCS lines are interpreted; megascopic structures in these lines are compared to the mesoscopic structural data on the islands. We attribute all of these features to two deformation mechanisms: 1) oblique plate convergence accompanied by displacement partitioning and 2) escape tectonics of the Maracaibo block. The D1 - D4 structures result from displacement partitioning and can be separated into three components. The arc-normal component of the plate convergence vector is accommodated with margin parallel folds and thrusts. Joints and veins correspond to the arc-parallel stretching component. The parallel component is characterized by dextral strike-slip faults. The tectonic escape of the Maracaibo block is associated with the clockwise rotation of the ABCs from EW to WNW. These are preliminary results and future study includes zircon and apatite fission-track dating and fluid inclusion analysis to determine the relationship between uplift of the ABCs and plate boundary dynamics.
T33B-1371 1340h
BOLIVAR: Crustal Structure across the Caribbean-South American plate Boundary at 67.5$\deg$W: results from Multichannel Seismic Reflection data
In April-June 2004 $\sim$6000 km of marine reflection profiles were collected as part of the NSF-funded BOLIVAR experiment (Broadband Ocean and Land Investigation of Venezuela and the Antilles arc Region). These profiles complement 169 OBS deployments along 5 transects, four $\sim$200 km long offshore-onshore refraction profiles, seismic recording on several islands in the Leeward Antilles arc and geologic field work in some of the Leeward Antilles islands and on the Venezuelan Caribbean (see Levander et al., this session). A main goal of this multidisciplinary investigation is to understand the processes of continental crustal growth that have been taking place along this boundary since the late Cretaceous and during the oblique collision between the South American plate and the Caribbean plate. We present results from one of the 5 main multi-channel marine reflection transects, trending approximately N-S at the longitude of 67.5$\deg$W. The $\sim$400 km long profile extends from latitude 10.5$\deg$N to 14.5$\deg$N, crossing from south to north the E-W trending Oca-Moron-El Pilar Fault strike slip system, the Bonaire basin, the Aves Ridge, the South Caribbean deformed belt and the southern edge of the Venezuela basin. Along the same transect wide angle seismic data were recorded by 42 Ocean Bottom Seismometers and by 550 Reftex Texans deployed at 329 sites onshore (see Zelt et al., this session). The high-quality marine reflection profile images with great clarity all the main geologic features of this area. In the south we have imaged a crossing of the Moron Fault, a segment of the continental strike-slip system boundary. The geometry of the sediments in the upper portion of the profile suggests modern deformation along the fault. Compressive structures, likely related to the movement along the Moron fault, are also visible in Paleogene sediments $\sim$40 km north of the fault trace. The Bonaire basin is characterized by Paleogene rift structures buried under 6 km of Cenozoic sediments. The platform of the Aves ridge separates the Bonaire basin to the south from the Los Roques basin to the north. This narrow and relatively deep basin filled with ~6 km of sediments is bounded to the north by compressive features of the Cenozoic accretionary wedge of the Southern Caribbean Deformed Belt. The 75 km wide belt has a typical melange seismic signature with the thrust fault geometry difficult to identify. A bottom simulating reflector (BSR) is traceable along the top of the northern portion of this feature. To the north, the accretionary wedge overrides the basement of the Caribbean plate and the sediments of the Venezuelan basin. The subducting plate is visible 25 km south of the frontal thrust of the accretionary prism. The only deep reflectors that we interpret as Moho along this profile are observed at the base of this plate at 10-11.5 s and they define a crustal thickness for the Caribbean plate of $\sim$6.5 km suggesting that the plate consists of normal oceanic crust.
http://earthscience.rice.edu/BOLIVAR.html
T33B-1372 1340h
BOLIVAR: Crustal Structure Across the Caribbean-South American Plate Boundary at 67.5$\deg$W: Results From Wide-Angle Seismic Data
The active-seismic component of the multi-disciplinary NSF Continental Dynamics BOLIVAR project (Broadband Ocean and Land Investigations of Venezuela and the Antilles arc Region) was completed in June, 2004. The goal of this project is to image the Caribbean-South American plate boundary and study continental growth by accretion of the Leeward Antilles island arc to the South American continent. The arc has been obliquely colliding with the passive margin of northern South America since the late Cretaceous. Motions across the plate boundary are transpressive and include right-lateral strike-slip faulting, underthrusting of Caribbean seafloor beneath the South Caribbean deformed belt, and the formation of a fold and thrust belt and adjacent foreland basin. In addition to the active-seismic studies, other workers on this project in the U.S. and Venezuela are involved in geological, geochemical and passive seismological studies. We present preliminary results of the wide-angle data along a north-trending profile $\sim$600 km long at 67.5$\deg$W longitude, between 8.5$\deg$ to 14$\deg$N latitude. This profile extends offshore from the Venezuela basin to the fold and thrust belt onland in Venezuela. In between the profile crosses the South Caribbean deformed belt, the Aves Ridge, the Bonaire Basin, and the Oca-Moron-El Pilar fault system. High-quality multi-channel seismic reflection data were acquired along the 390 km offshore portion of the profile using the R/V Ewing (see Magnani et al., this session). The Ewing airgun shots were also recorded by 42 ocean bottoms seismometers deployed from the R/V Seward Johnson II. In addition, onshore-offshore data were acquired by recording the airgun shots using Reftek Texans at 329 stations extending from the coast to $\sim$200 km inland. Two large land shots were also recorded by the land stations to provide reversed coverage of the onland portion of the profile. The wide-angle data and a preliminary 2-D velocity model from traveltime inversion along the 600 km profile using the OBS, onshore-offshore, and land shot data will be presented.
http://earthscience.rice.edu/bolivar.html
T33B-1373 1340h
BOLIVAR: Deformation History of the Villa de Cura Blueschist Belt, Venezuela
As part of the BOLIVAR project ("Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region") we conducted an on-land geological study of the high pressure-low temperature (HP-LT) Villa de Cura blueschist belt (VdC) in Venezuela. The belt is part of the Caribbean Mountain system (CMS) that dominates the South American-Caribbean plate boundary zone in central Venezuela. The CMS is characterized by south-vergent thrust faults and east trending dextral strike-slip faults. We performed a structural analysis in the western and central parts of the VdC belt. The Villa de Cura Group consists of coherent blueschist-facies meta-volcanic and meta-sedimentary rocks that display island-arc affinities. It overlies middle- to upper-greenschist facies meta-volcanics of the Las Hermanas Formation. The VdC belt is in fault contact with non- to lower-greenschist facies metamorphosed lithologies of the Paracotos belt to the north and with the non-metamorphosed sedimentary units of the Serran$\'{i}$a del Interior foreland fold-thrust belt to the south. Most authors consider the VdC belt to be a klippe that in Cenozoic times was thrust over the Venezuelan passive margin and foreland basin during the diachronous collision (progressively younger plate interaction from west to east) of the Antilles arc terrane with the South American plate. The main objectives of this research are: (1) to test the hypothesis that diachronous collision of the island arc with the continent lead to diachronous deformation, uplift and exhumation of HP-LT metamorphic rocks, and (2) to provide a full deformation-time path for the VdC to test previous geothermobarometric, geochronological, and geochemical studies. Meso- and microstructural analysis and petrography have led to the recognition of three ductile, two brittle-ductile and at least two brittle deformation phases. Samples were collected for $^{40}$Ar/$^{39}$Ar radiometric age dating and for fluid inclusion microthermometry. In most active margins around the world studies of HP-LT metamorphosed rocks have provided constraints on the development of the margin, both in structural styles as well as age. We propose that the ductile deformation at near peak-metamorphic conditions took place during the final stages of subduction and early uplift of these rocks under a volcanic arc. The brittle-ductile folding and faulting is then associated with the collision of the arc with the South American continental margin and early stages of thrust emplacement of the CMS. The brittle structures are associated with deformation of the margin and final emplacement and exhumation of the VdC.
http://earthscience.rice.edu/BOLIVAR.html
T33B-1374 1340h
Tectonic reconstruction of sedimentary basins associated with the proto-Maracaibo and proto-Orinoco Rivers: New constraints from BOLIVAR and GULFREX seismic data
The proto-Orinoco River has long been proposed as the single, fluvial source for thick Cenozoic basins along northern Venezuela. The location of these buried and deformed depocenters are significant both for their use as piercing points to reconstruct the amount of Caribbean-South America right-lateral plate motion and because they are excellent source and reservoir rocks for hydrocarbons onland. We have integrated MCS lines of selected BOLIVAR-2004 and GULFREX-1975 data to produce maps of the major depocenters offshore and reconstruct their tectonic history. The two offshore basins in the study area are the Falcon-Bonaire-Grenada basin, a Paleogene backarc basin, and the Tobago trough basin, a Paleogene forearc basin. A plate reconstruction restoring ~1000 km of east-west, right-lateral strike-slip displacement between these Caribbean arc-related basins and the continental margin of South America reunites a ~5-km-thick lobe of marine sediment in the Tobago trough basin with the distal end of a Paleogene river system in western Venezuela that we name the proto-Maracaibo River. It is significant that all sediment associated with the proto-Maracaibo river was confined to the margin and did not reach the deepwater Venezuelan basin west of the Tobago basin. Late Miocene Andean uplift diverted most drainage from the proto-Maracaibo to the proto-Orinoco River south of the Venezuelan coastal ranges. The proto-Orinoco prograded eastward in post-late Eocene time along its E-W-trending corridor between late Eocene to recent time and formed a separate fluvial-deltaic system from the slightly older paleo-Maracaibo system to the north.
T33B-1375 1340h
BOLIVAR: Onshore-Offshore Seismic Transect of the Caribbean/South American Plate Boundary at Barcelona, Venezuela
In April-June, 2004, scientists from Venezuela and the USA acquired a series of marine multi-channel, ocean bottom seismograph, land seismic recorder profiles across the boundary between the Caribbean (CAR) and South American (SA) plates on and off shore northern Venezuela. The ocean bottom seismographs and land recorders recorded the marine airgun-array shots as well as two land explosive shots. This presentation focuses on results from a transect acquired along a roughly north-south line which passes near Barcelona, Venezuela, and extends 175 km inland and 330 km seaward from the coast. Features imaged along the transect (from the seaward end) include the Venezuela Basin, the South Caribbean Deformed Belt, a zone of underthrusting north of the ABC Islands, the Los Roques Canyon, an incipient subduction zone that extends eastward to Grenada, the El Pilar strike-slip fault zone and the associated Cariaco pull-apart basin, the Pilar strike slip fault, the coast, the Serrania Del Interior fold-thrust belt and the Maturin foreland basin. The location of the transect corresponds to a number of important changes in the tectonics of the plate boundary. It lies just to the west of the intersection of the Aves Ridge and the CAR/SA boundary. Associated with this intersection is a roughly 100 km southward shift in the location of the zone of underthrusting marking the southern extent of the Venezuela basin. The shift appears to be a key to understanding the process by which volcanic arc crust of the Aves Ridge is transferred to the SA plate. A carbonate platform, including the Venezuelan islands Isla Blanquilla and Isla la Orchila and possibly overlying Aves Ridge volcanic rocks, lies between the 2 branches of the subduction system. This area of thickened crust may be choking off the eastern branch of subduction, causing subduction to step 100 km northward. We present marine reflection seismic data, and marine and land long-offset seismic refraction data, together providing structure and velocity control on the geology along the transect.
http://earthscience.rice.edu/BOLIVAR.html
T33B-1376 1340h
Tectonic Evolution of the Central Venezuela Margin From Integration of BOLIVAR and GULFREX MCS Data, Venezuelan Margin
The Lesser Antilles arc obliquely collided with the passive margin of central Venezuela in Miocene and younger time. The offshore basement arc high now trends directly into outcropping arc and passive margin rocks of the 2-km-high Cordillera de la Costa. We combine a newly acquired, deep-penetration BOLIVAR MCS line with GULFREX lines and published data to constrain the following tectonic features of the area: 1) Paleogene back-arc basin opening northwest of the arc basement high is shown by half-grabens filled by sedimentary wedges; these features are contemporaneous with similar half-grabens observed to the west in the Bonaire basin and to the east in the Grenada basin; the degree of fault inversion is much less in this area than in the western and eastern areas 2) the strong basement reflector of the extinct arc is known from published well data to correspond to Late Cretaceous andesite, basalt, and diorite formed in the Lesser Antilles volcanic arc; 3) the NW-striking Margarita fault zone is an active right-lateral strike-slip fault connecting dying thrust faults at the eastern end of the South Caribbean deformed belt with active, EW-striking right-lateral strike-slip faults of the El Pilar fault system; the Tuy basin, a Miocene-recent depocenter may have formed at a stepover in this fault system; and 4) the Cariaco basin is a large, active pull-apart basin formed at a stepover in the San Sebastian-El Pilar fault system; on BOLIVAR line 19, the basin-controlling El Pilar fault is downthrown to the south and controls an asymmetric wedge of Plio-Pleistocene sediment.
T33B-1377 1340h
Structural, Thermochronological, Topographic, and Precipitation data of the Transpressional Orogen of the Venezuelan Paria Peninsula, SE Caribbean-South American Plate Boundary
New structural and thermochronological data as well as published structural, topographic, and precipitation data from the Paria Peninsula suggest a general model for the exhumation and deformation of rocks in the SE Caribbean-South American plate boundary. The mountain belt of the Paria Peninsula in eastern Venezuela is composed of deformed and metamorphosed sediments that were deposited on the northern South America passive margin in early Mesozoic time. This belt is currently bounded by the active right-lateral strike-slip El Pilar fault to the south and the Coche/North Coast fault to the north. Metamorphic grade increases from sub-greenschist facies in the south to amphibolite facies in the north. In eastern Paria Peninsula, the focus of our study, foliation (S1) dips steeply to the south along the southern coast and gradually shallows to the north to $25-30\deg$ dip. In general, S1 strikes approximately 060-075\deg subparallel to oblique to the general trend of the metamorphic belt. Elongation lineations (L1) are defined by the preferred orientation of elongated quartz grains that plunge subhorizontally to the SW. Quartz c-axes fabric patterns from samples along and across the region show top-to-SW or oblique-normal sense of shear, and suggest activation of the basal$<$a$>$, prism $<$a$>$, and rhomb $<$a$>$ slip systems under relatively low temperature ($300-400\deg$C). Apatite and zircon fission-track ages obtained for individual samples plot within 2 sigma error of each other, which we interpret as due to rapid cooling. Apatite and zircon FT ages range from 29 Ma in the south to 4-5 Ma in the north. The rapid cooling rates in the temperature range for zircon and apatite ($350 \deg$C -$120 \deg$C) are similar across the peninsula, but the event was diachronous from south to north. The topography of the Paria Peninsula and its precipitation pattern are both asymmetric (approximately 1 m/yr on the southern flank, and 2-3 m/yr on the northern flank). Rivers in the northern flank flow perpendicular to the main divide with steep gradients, short distances, and high discharge rates while drainage patterns in the southern flanks are characterized by lower gradients, with fluvial plains and alluvial fans. This orogen shows significant similarities to the Southern Alps orogen in New Zealand, in terms of internal deformation, exhumation patterns, topography, erosion rates, and precipitation. Therefore, a general model for a transpressional double wedge is proposed for both orogens. In this model, material is extruded toward one side of the orogen by "channel-like" flow. This model accounts for the asymmetric profiles of deformation and erosion rates in both orogens. Internally, the direction of material accretion is the main variable that controls the orogen's internal particle path and deformation. Externally, the configuration of the orogen is controlled by erosion that "facilitates" localization of exhumation and deformation. Interaction among exhumation, deformation, and erosional processes characterize this type of setting and possibly dictate orogenic evolution.
T33B-1378 1340h
BOLIVAR: Backstepping of Subduction and Accretion of New Continental Crust Along the SE Caribbean Plate Margin at 64$\deg$W
BOLIVAR, the Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region, an NSF and Venezuelan funded, collaborative active source seismic experiment conducted in April and May of 2004, acquired multi-channel seismic (MCS), ocean-bottom seismic (OBS), and concurrent land seismic data in the southeast Caribbean region (for details, see Levander et al.). The purpose of the BOLIVAR project is to look at the oblique collision of the Caribbean Plate with the South American Plate, where the Leeward Antilles arc is accreting onto the South American craton. The 275-km-long MCS profile BOL-28 trends 5$\deg$ W of N, at 64$\deg$W longitude, from 10.8$\deg$N to 13.1$\deg$N. A 2-sec-thick (twt; 3 km at 3 km/s) package of relatively flat-lying, recently thrust-faulted sediments overlies complex crustal structure and associated infill in the Venezuela Basin. At the southern end of the Venezuela Basin, a 17-km-wide, southward-thickening accretionary wedge abuts a carbonate platform overlying the thickened remnant arc crust of the Aves Ridge. Subduction cannot be traced beneath the Aves Ridge platform; this may be due to either poor seismic penetration beneath the carbonate, or relatively recent onset of subduction limiting the extent of slab penetration. However, south of the Aves Ridge, another 25-km-wide accretionary wedge thickens to the south, underlain by a strong top of Caribbean crust reflector that can be traced from the seafloor at 0.5 sec twt to the bottom of the accretionary wedge at 4.2 sec twt, 27 km to the south. This reflector may continue to the south, beneath the accreted Leeward Antilles arc crust east of Margarita Island; sub-crustal reflectors are apparent, but further processing is necessary to improve the image enough to infer continuity. We interpret this to be the original subduction zone, choked by the thickened Aves Ridge crust. Subduction has now migrated to the north, effectively accreting the Aves Ridge crust onto the South American plate and providing a mechanism for continental crustal growth. Modeling of the velocity structure from the coincident OBS data and onshore-offshore data along the same profile will further constrain the interpretation.
http://earthscience.rice.edu/BOLIVAR.html
T33B-1379 1340h
Crustal Structure of the Southern Lesser Antilles Arc: Integration of BOLIVAR Wide-angle and MCS Data
In April-June 2004 we conducted an active-source seismic survey in the SE Caribbean as part of the NSF Continental Dynamics BOLI-VAR study. Here we present data from a coincident seismic reflec-tion and wide-angle profile extending 550 km from the Aves Ridge in the northwest to the Atlantic Ocean southeast of Trinidad and To-bago. In this region the Lesser Antilles island arc is currently collid-ing with northern South America; further to the west this collision has resulted in the accretion of the Leeward Antilles island arc to the South American continent. Geologic features crossed by the profile include the Aves Ridge, the Grenada back-arc basin, the Lesser Antilles island arc, the Tobago Trough, the Trinidad and Tobago region, and the Lesser Antilles ac-cretionary prism. The Aves Ridge and Lesser Antilles island arc have similar crustal structure: upper basement velocities are 4 km/s, velocities of 6 km/s are reached at depths of 6-7 km, and crustal thicknesses are 18-24 km. The Grenada back-arc basin, which formed when the present-day Lesser Antilles island arc rifted away from the Aves ridge during the Paleocene-early Eocene, contains approximately 9.5 km of sediment overlying crust with a thickness of 9-11 km. This crustal thickness suggests that rifting did not proceed to the generation of oceanic crust. The Tobago trough, which lies south-east of the Lesser Antilles island arc, has similar sediment and crustal thickness to the Grenada basin. Seismic penetration is poor through the metamorphic rocks comprising much of the Trinidad and Tobago system; however our data does indicate that velocities of 6 km/s are reached at depths of 4-5 km. Southeast of Trinidad and Tobago the accretionary prism has sediment thickness greater than 10 km due to its position near the Orinoco Delta.
T33B-1380 1340h
BOLIVAR and GULFREX MCS Data Constrain Closure of the Grenada Backarc Basin During Oblique Collision Between the Lesser Antilles Arc System and Northern South America
The Lesser Antilles evolved from the mid-Cretaceous as an extensional arc system formed above a steeply dipping slab of Atlantic oceanic crust. The arc became extensional during the Paleocene - early Eocene along normal faults at the eastern edge of the basin as the present-day Lesser Antilles chain rifted away from the Aves Ridge. Backarc rifting ceased during the early Eocene, leaving the 140 km wide 3 km deep Grenada backarc basin. Sediment accumulation reached nearly 8 km during the Paleogene with another 1.5 km of sediments accumulating during the Neogene. In this presentation, we combine newly acquired MCS lines from the BOLIVAR study with existing GULFREX data collected in 1975 to document the structural and stratigraphic effects of closure of the Grenada backarc basin because of the progressive, oblique collision between the Lesser Antilles arc system and northern South America. The southern end of the Grenada basin has been narrowed from an undeformed width of approximately 100 km to a deformed width of 30 km, and rotated nearly 90 degrees to the west as the arc system obliquely collides with the South American margin. Shortening of the colliding backarc basin is mainly accommodated by inversion of Paleogene normal faults on the eastern edge of the basin, folding, low-angle thrust faults, and possibly shale diapirism. We propose that this closure in the area is a backthrusting response during Oligocene - late Miocene closure along the leading edge of the oblique arc-continent collision in the Eastern Venezuelan basin.
T33B-1381 1340h
Tectonics Of Eastern Offshore Trinidad Based On Integration Of BOLIVAR 2D Seismic Lines With Industry 3D Seismic Surveys
New MCS lines in the eastern offshore area of Trinidad augmented by existing 3D seismic surveys by industry provide new insights into complex, strain partitioning produced along this segment of the South America-Caribbean plate boundary. Two major tectonosequences are imaged separated by a Middle Miocene angular unconformity known from wells and mapping in Trinidad. A thick section of deep-marine carbonate and clastic rocks are cleanly truncated by the Middle Miocene unconformity and are chaotically deformed along vertical to northwest-dipping thrust faults. This shortening event reflects a major pulse of pre-Middle Miocene southeastward overthrusting of the Caribbean arc over the passive margin of South America. An upper 2-7-km-thick tectonosequence consisting of late Miocene-Quaternary shelf-related sandstone and shale was deposited by the nearby Orinoco delta. This section is folded to lesser degree and deformed by the sub-vertical, right-lateral Central Range fault zone (CRFZ), known from GPS studies to accommodate 12 mm/yr, of the total 20 mm/yr of interplate motion. Deep, continuous reflec-tors are observed at a depth of 12-17 km beneath eastern Trinidad are correlated with authochthonous, late Cretaceous-early Tertiary carbonate and clastic rocks of the South American passive margin. The Darien fault southeast of the CRFZ accommodates active shortening, elevates passive margin rocks to the surface in Trinidad, and forms the northeastern limit of a large, 12-km-thick foreland basin (Columbus basin) that extends onshore.
T33B-1382 1340h
Structure and Stratigraphy of the Barbados Accretionary Prism and the Tobago Forearc Basin
The relationship between the Lesser Antilles island arc, the Tobago forearc basin, and the Barbados accretionary prism shows classic convergent margin geometry. Barbados is the only emergent part of the accretionary prism with 80% of the island's land area being covered by Pleistocene limestone. Erosion of the limestone cap in the northeastern part of the island exposes older rocks of the prism. A 450-km2 2-D seismic data volume allows extension of these stratigraphic units offshore and definition of a regional structural framework. The relationship between the unit identified onshore as the Early Eocene to Middle Miocene Oceanic Formation and the basal unit, the intensively folded and faulted Eocene prism rocks of the Scotland Group, has long been debated. Previous proposals claim that the Oceanic Formation, consisting of pelagic clays with some ash beds, is allochthonous and has been thrusted into its present position above accreted sediments of the Scotland Group. However, seismic data show no evidence of nappes-the basis for the overthrusting hypothesis. Seismic interpretation presented here supports the opposing view that the Oceanic Formation and its offshore equivalent in the offshore Tobago forearc basin was deposited in situ and onlap the older, more highly deformed rocks of the accretionary prism. Previous workers proposed that the region's extensive mud diapirism (identified onshore as the Joes River Formation) has caused the emergence of Barbados, which continues to rise 0.44 mm/yr. However, seismic lines suggest that the island's emergence and present-day uplift is related to footwall uplift along a large, NE-striking normal fault off the east coast of the island.
T33B-1383 1340h
Texas Teacher at Sea on the BOLIVAR Project Geophysical Cruise
UTIG provides K-12 teachers with research experiences in field programs that involve UTIG scientists. I am a 6th-9th grade science teacher in Van Vleck, Texas and in April 2004 I sailed to the southeastern Caribbean aboard the R/V Maurice Ewing as a member of the BOLIVAR Project alongside scientists from the U.S. and Venezuela. Our goal was collect seismic data to image the crust and mantle beneath the Caribbean as part of a study of the tectonic processes accompanying different stages of the Caribbean arc/South America continental collision process. Throughout the 52-day cruise I worked as a watch stander, interpreted newly collected seismic reflection data, helped deploy the streamer, maintained a cruise blog (a chronological journal weblog documenting personal thoughts about my experience), spoke with students in Texas with the telephone on loan from Iridium Satellite Solutions, and responded to email inquiries from shore-based students. It was hard work, but most importantly, a voyage of discovery. With guidance from scientists and GK-12 Fellows at UTIG, I am using my experience and the data collected as the basis for K-12 curriculum resources, including learning activities and a video documentary. Support for my participation and post-cruise activities was provided by the NSF and the Trull Foundation in Texas. If given the opportunity to do this again, I would, without reservation!
http://www.ig.utexas.edu/outreach/ttif/BOLIVAR/
T33B-1384 1340h
Early Seafloor Ridge Jumps in the Central Atlantic Ocean
Five Mesozoic Chrons from M28 to M40, in the Jurassic Magnetic Quiet Zone (JQMZ), have been mapped between Atlantis and Fifteen-Twenty fracture zones on the North American flank, and between Atlantis and Kane fracture zones on the African flank, of the central Atlantic Ocean. We interpret two seafloor spreading ridge jumps to have occurred during the early evolution of the central Atlantic: one ~170 Ma on the western flank, the other between 164 Ma and 159 Ma on the eastern flank. These jumps may be related to changes in plate motions as North America separated from Gondwana. Chron 40 (167.5 Ma) anomalies are mapped about 65 km outboard of the Blake Spur Magnetic Anomaly (BSMA) of North America, and the conjugate S1 anomaly of Africa. The East Coast Magnetic Anomaly (ECMA), which coincides with seaward-dipping reflectors, is located about 180 km inboard of the BSMA. In contrast, anomaly S3 which is interpreted as the conjugate of ECMA on the African side is located only 30 km inboard of S1. Therefore the long-hypothesized ridge jump to the east between BSMA and ECMA anomalies (~170 Ma), by Vogt in 1971, is supported by this study. The width of the JMQZ between Atlantis and Kane fracture zones is about 70 km greater (i.e., ~22%) on the African side than on the North American side. Inspection of magnetic anomalies in the JMQZ reveals additional correlatable features over Africa that suggest to us a second ridge jump occurred, in this case to the west. Modeling results indicate that this jump occurred between 164 Ma and 159 Ma, approximately the same time that some workers have suggested for the onset of seafloor spreading in the Gulf of Mexico.
T33B-1385 1340h
Assessing Acoustic Sound Levels Associated with Active Source Seismic Surveys in Shallow Marine Environments
The potential effect of active source seismic research on marine mammal populations is a topic of increasing concern, and controversy surrounding such operations has begun to impact the planning and permitting of academic surveys [e.g., Malakoff, 2002 Science]. Although no causal relationship between marine mammal strandings and seismic exploration has been proven, any circumstantial evidence must be thoroughly investigated. A 2002 stranding of two beaked whales in the Gulf of California within 50 km of a R/V Ewing seismic survey has been a subject of concern for both marine seismologists and environmentalists. In order to better understand possible received levels for whales in the vicinity of these operations, modeling is combined with ground-truth calibration measurements. A wide-angle parabolic equation model, which is capable of including shear within the sediment and basement layers, is used to generate predictive models of low-frequency transmission loss within the Gulf of California. This work incorporates range-dependent bathymetry, sediment thickness, sound velocity structure and sub-bottom properties. Oceanic sounds speed profiles are derived from the U.S. Navy's seasonal GDEM model and sediment thicknesses are taken from NOAA's worldwide database. The spectral content of the Ewing's 20-airgun seismic array is constrained by field calibration in the spring of 2003 [Tolstoy et al., 2004 GRL], indicating peak energies at frequencies below a few hundred Hz, with energy spectral density showing an approximate power-law decrease at higher frequencies (being ~40 dB below peak at 1 kHz). Transmission loss is estimated along a series of radials extending from multiple positions along the ship's track, with the directivity of the array accounted for by phase-shifting point sources that are scaled by the cube root of the individual airgun volumes. This allows the time-space history of low-frequency received levels to be reconstructed within the Gulf of California. At various times or positions along the ship's track, the predicted mean and maximum sound level in the water column are contoured. By reconstructing the possible positions of the whales during the survey, based on the time of their stranding and reasonable swim velocities, we constrain the sound levels that they may have been subjected to for a series of scenarios. It is hoped that this work will facilitate a better understanding of acoustic propagation during future airgun experiments in similar environments.
http://www.ldeo.columbia.edu/res/pi/tphase/
T33B-1386 1340h
Arc-continent collision on the southern Margin of North America: Cuba and Hispaniola
Comparison of the geology of Cuba and Hispaniola demonstrates how different tectonic styles can be produced in closely adjacent parts of the accretion arc. Cuba is a laterally extensive fold-and-thrust belt that includes a narrow belt of Early Cretaceous to Campanian arc rocks, ocean crust, sub-oceanic mantle and metamorphosed continental crust. Hispaniola represents a broader and more complete (although deformed) Early Cretaceous to Eocene arc structure with a single accreted, metamorphosed continental terrane. We suggest that Cuba's preset arc terrane is essentially a fore-arc that was underplated by continental crust in the Campanian collision of the Greater Antilles arc with southern Yucatan. Following this collision, the proto Caribbean lithosphere detached from the eastern margin of Yucatan resulting in rapid, north directed rollback. This, in turn, developed extension of the over-riding Cuban lithosphere and exhumed the underplated continental crust as extensional metamorphic core complexes. The present Cuban fold belt was formed as a result of collision with the southern margin of North America in the mid-Eocene. In contrast, Hispaniola was to the east of Cuba and was not detectably involved in the Campanian collisional event. At present, it is not clear if the extensional rollback event that affected Cuba had any significant effect on Hispaniola. Hispaniola's collision with North America began in the late Eocene(?) and culminated in the Oligocene and was highly oblique. This event resulted in highly oblique, south-verging thrusting that telescoped the back arc basin. It also produced a pop-up structure that reveals evidence of an earlier, mid-Cretaceous collisional event within the arc. This oblique collision persists until the present.
T33B-1387 1340h
Crustal Thickness Along the Central American Volcanic Arc
Crustal thickness of volcanic arcs in subduction zones has been used to infer the time-integrated rate of mantle melting and the consequent rate of continental growth. Subduction of the Cocos Plate under the Caribbean Plate results in active volcanism in Central America. Tomography Under Costa Rica And Nicaragua (TUCAN) is an eighteen month broadband seismic experiment that began in the summer of 2004 with the goal of seismically imaging the upper mantle and subducting slab. Forty-eight IRIS/PASSCAL instruments were installed in two densely spaced cross-arc lines and two arc-parallel lines. Initial analysis has focused on three permanent and one pilot station in the region. Receiver functions calculated from teleseismic events recorded on these stations have strong signals from the Moho as well as a signal from a dipping slab, exhibiting a strong tangential component. The P-to-S conversions from the Moho seen in receiver functions in Costa Rica have approximately a two second delay relative to those in Nicaragua, indicating crustal thickening from Nicaragua into Costa Rica. A station in Costa Rica has an estimated crustal thickness of thirty to thirty-one kilometers whereas a station in Nicaragua has a thickness between forty and forty five kilometers depending on the velocity models used. This is consistent with previous crustal thickness estimates from Bouguer gravity and regional elevation, assuming Airy isostasy, meaning that elevation is controlled by crustal thickness. The difference in crustal thickness between Nicaragua and Costa Rica could be the result of different rates of crustal growth, or more likely, tectonic changes to crustal thickness (as a result of extension or compression) in one or both places.
T33B-1388 1340h
The Interaction Between the Caribbean Plate and the Products of the Galapagos Hotspot in Southern Costa Rica: A Transition from Subduction Accretion to Erosion
The Costa Rica portion of the Middle America Trench (MAT) is characterised by active tectonic erosion, a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward. Offshore studies demonstrate accelerated subduction erosion starting at the Miocene-Pliocene boundary, as the result of subduction of thickened/Galapagos related crust, as presently represented by the Cocos Ridge. The subduction of the Cocos Ridge also caused the uplift of the outer forearc and its exposure in the Osa peninsula, which offers a window to explore the tectonic evolution of the area. The rocks outcropping on Osa Peninsula are of a Middle Eocene-Late Oligocene melange dominated by basalt, chert and limestone. The igneous elements of the melange and the coherent igneous rocks present inboard the melange, still on the Osa peninsula, have a volcanic island chemical affinity, that together with the younger age, demonstrate they belong to an event different than the one which produced the Caribbean plate. The melange and the coherent igneous rocks represent a tectonic rock assemblage accreted prior to the arrival of the Cocos Ridge, during subduction of seamounts. The accretion-dominated period of the MAT evolution ended at the Miocene Pliocene boundary, when continuously thickened crust produced at the Galapagos hotspot arrived at the trench. The thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the Osa melange. The change for accretion to erosion caused the outer forearc to be offset along sub-vertical faults that define small (km-size) blocks that are going through differential vertical movements in response to the morphology of the subducting ridge. Subduction accretion and erosion are two processes that can alternate in time or coexist along the same margin, so that mass removal can develop on a previously growing margin and completely remove an accretionary prism.
T33B-1389 1340h
Implications for the Petrogenesis of Distinct Silicic Magma Types from the Lower Pleistocene Guachipelin Caldera, NW Costa Rica
Lower Pleistocene pyroclastic ash-flow deposits in NW Costa Rica represent sequential eruptions of high-silica (69-79%\ SiO2) magmas from the Guachipelin Caldera. These high silica eruptions are not common in areas void of continental crust. The stratigraphic order of seven distinct units is identified by primary mineralogy and bulk chemical composition. Initial distinctions among separate stratigraphic units are defined based on pumice size, mineralogy, physical breaks, and color. First, six major units are identified based on field observations including mineralogy: glomerophyric plagioclase-amphibole (GPA), white biotite (WB), pink biotite (PB), amphibole (A), green unit rich in amphibole (GA), and plagioclase (PR). Further subdivisions are characterized by discrete chemical heterogeneities of trace elements within the macroscopic units. Most of the units identified in the field also have discrete ratios of trace elements (e.g. Nb/Ta): GPA (13.3-19.3), WB (7.6-14.6), PB (3.8-5.0), GA: (23.4-29.4); PR: (7.2-10.4). The amphibole unit (A) is the only one that presents two discrete ranges (6.5-9.5 and 11.5-13.0), which can be interpreted as an indication that the pumice fragments belong to two distinct units instead of one. These collective variations within the sequence provide the basis for petrogenetic interpretation. Differences in the incompatible element ratios behavior are consistent with partial melting (or melt segregation) of several different sources and/or partial melting of same source crust at varying degrees. Melt segregation (partial melting) from several different sources would require a complex plumbing system linking spatially distant crustal sources to a single shallow magma chamber or multiple magma chambers in the same area. In contrast, varying degrees of partial melting from a single crustal source could provide magma for recharge into a shallow chamber from a central conduit re-tapping the same source periodically. Considering the temporal ($<$0.5Ma) and spatial (single caldera) constraints of this sequence of eruptions, significant chemical variations of the magmas have occurred, which require processes to operate on relatively short time scales.
T33B-1390 1340h
Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador
Whole-rock geochemical data on basaltic-to-rhyolitic samples from 12 Late Pleistocene-Holocene volcanic centers are used to constrain the role of continental crust in the genesis of melts formed beneath the anomalously wide arc in Ecuador. Primitive and relatively homogeneous isotopic compositions observed across the arc imply that Ecuador lavas were produced largely within the subduction zone and not by extensive melting of crustal rocks similar to those upon-which the volcanoes were built. Mixing calculations limit the quantity of assimilated crust to less than approximately 10 percent. Cross-arc geochemical variation (e.g., in 143/144Nd, Ba/Nb, La/Yb) indicates that assimilation of crustal rock is most active in the center-arc area, around the Inter-Andean Graben, at distances of 330-360 km from the trench. Characteristically `adakitic' features (low Y, Yb, high La/Yb), which are observed in virtually all andesites and dacites in Ecuador, appear to be strongly influenced by crystal fractionation (e.g., as indicated by compatible element behavior for Y) and are most clearly developed in lavas that, based on isotopic compositions, appear to have assimilated the most crust. A subset of andesites, which display an unusual combination of high Sr ($>$900 ppm) and non-radiogenic isotopes (epsilonNd $>$4.1, delta7/4Pb $<$6.0), appear to be regionally distinctive in Ecuador (i.e., not observed among modern lavas in the Andean central and southern volcanic zones or in Colombia). Only these lavas, which are well represented at Imbabura Volcano, appear to have geochemical features that may be consistent with a slab-melt interpretation associated with the subduction of relatively young, over-thickened oceanic crust of the subducting Carnegie Ridge.