T13C-1467
New K/Ar ages on Martinique Island: A Record of the Lesser Antilles Island Arc Evolution
We present here a combined geochronology, geochemistry and geomorphology study of Martinique Island based on carefully selected lava flows and domes sampled during two field trips in spring 2006 and 2007. Our aim is to better constrain the whole volcanic evolution of the island, with a special emphasize on the timing of its initiation in Oligocene, and its westward migration during Miocene. The Lesser Antilles island arc results from the westward subduction of the Atlantic plate beneath that of the Caribbean. Whereas in the southern part of the arc volcanic activity occurred along a single SW-NE axis, the northern part, which experienced a westward jump of the volcanic front, is divided in two distinct branches. Due to its central position where the two northern arcs merge, Martinique is the Lesser Antilles island where the most complete history of the arc can be found. Volcanism has there been almost continuous and outcrops from east to west, without superimposition neither apparent hiatus such as observed for other islands of the arc. Our first results using the K-Ar Cassignol-Gillot technique, which has demonstrated its suitability for Antilles lavas at La Guadeloupe Island (Samper et al., 2007), show that the oldest products are older than 23 Ma. They are situated in the eastern and south-eastern peninsulas and interpreted as the onset of subaerial volcanism in the Lesser Antilles island arc. The NW-SE trending Vauclin- Pitault chain, in the central southern part, is constituted of hyaloclastites attesting for a submarine emplacement. Within the whole arc, it is though to be the only outcropping products emitted during the westward arc migration. Our ages constrain its activity between 17 and 7 Ma. During Miocene, volcanism pursued to the southwest with emplacement of small monogenetic volcanoes aligned along active faults between 6 Ma and 340 ka. In the meantime, the northern compartment has been constructed with the Morne Jacob volcano (5.5 – 1.5 Ma), Pitons du Carbet (1 Ma – 340 ka), Mont Conil (550 – 200 ka) and Mount Pelée, the active volcano. This northern compartment experienced flank collapses on the western flank of Mount Pelée at about 100 ka, 25 ka and 9 ka (Le Friant et al., 2003). Our new ages confirm that the western flank of Pitons du Carbet was affected by a collapse event at 337 ka, and that the eastern flank of Morne Jacob also, between 2.11 and 2.04 Ma, from dated lava flows sampled outside and inside the structure, respectively. Strong geochemical variations in major and trace elements occurred between pre and post collapse volcanism, suggesting that conditions within the superficial magmatic chamber was modified by this event. Finally our set of about 80 new ages, ranging between 23 Ma and 10 ka, allow us to revise the chronology of the different volcanic phases of Martinique island, and to identify episodes of high production rate magmatism associated with either regional geodynamic changes and/or flank collapse events.
T13C-1468
Late Cretaceous Arc Initiation on the Edge of an Oceanic Plateau (Southern Central America)
The Caribbean Plate comprises one or several late Cretaceous oceanic plateaus imbricated between the Northern and Southern Americas. Uplifted portions of plateau(s) along plate boundaries have been recognized in many sites, including that underlying the south Central American Volcanic Arc. We provide new constraints for the role of the plateau in the evolution of this arc obtained by mapping of the uplifted forearc area between southern Costa Rica and western Panama. An oceanic plateau, accreted seamounts and arc rocks were identified, and a new tectono-stratigraphy defined. The arc basement is composed of a Coniacian oceanic plateau. In the outer margin, late Cretaceous-Eocene accreted seamounts are in contact with the plateau along tectonic mélanges and active faults. Campanian-Maastrichtian primitive arc rocks are found 40-110 km to the trench on the top of -or as dykes within- the plateau. The location of these rocks correlates to previous observations and indicates that the arc front migrated away from the trench during the late Cretaceous, potentially in response to subduction erosion or slab flattening [Lissinna et al., EGU 2006]. The first island arc lavas were deposited under sea level, over a broad area. They were quickly followed by more evolved intrusives and lavas, which were emplaced along a volcanic front during the late Cretaceous-Paleocene. Detrital and volcanic records along the Central American isthmus indicate that a continuous volcanic arc extended between eastern Panama and northern Costa Rica in this time. In southern Costa Rica (Golfito complex) and western Panama (Sona-Azuero-Coiba complex), the oceanic plateau consists mainly of pillowed and massive low Fe (tholeiitic) basalts. These rocks have a highly consistent geochemistry characterized by flat, primitive upper mantle-normalized incompatible element patterns with low Pb and high Nb-Ti contents. Primitive arc igneous samples are low-medium Fe basalts to trachyandesites found as pillow lavas, massive lava flows and intrusives. These rocks have incompatible element patterns similar to plateau basalts. However, whole rock and unaltered clinopyroxene compositions indicate variable Pb enrichments and Nb-Ti depletions that are due to slab-derived fluids in the source. Differentiation trends of the primitive island arc differ compared with the plateau in terms of Si, Al, Fe, Mg, Ca and alkalis. The source of the primitive arc magmas corresponds to the lithospheric mantle initially associated with the plateau. Beginning of melting is linked to slab-derived fluids and was likely facilitated by high thermal gradients under the plateau. As a consequence, incompatible elements of the primitive arc lavas mimic plateau affinities and show variable arc signature overprints. In some cases, the resemblance between the early arc and plateau rocks is very high, making a distinction between the two types of rocks almost impossible.
T13C-1469
Provenance And Tectonomagmatic Setting Of The Santa Marta Schists, Northern Colombia Caribbean Region: Insights On The Styles Of Growth And Approach Of Caribbean Intra- Oceanic Domains To The Continental Margin
The life cycle of an intra-oceanic terranemincludes different phases and styles of magmatic growth, accretion with other terranes and translation before reaching a continental margin. In order to unveil the nature of these phases in crystalline rocks from northern Colombia, U/Pb LA-MC-ICP-MS detrital geochronology and whole rock geochemical data were obtained from stacks of intercalated metavolcanic-sedimentary rocks of the Santa Marta Schists in the Sierra Nevada de Santa Marta. Immobile elements whole rock geochemistry from greenschist to amphibolite facies units are characterized by low to moderate LREE/HREE, variable Th enrichment and weakly negative Nb and Ti anomalies, which are similar to island arc and MORB signatures. The intercalated metasedimentary rocks show a REE pattern similar to the PAAS and high Zr/Sc vs Th/Sc ratios, which suggest a felsic and highly diferentiated upper crust sources for the protoliths. Detrital zircons from three different units were obtained, The maximum depositional age for the northwestern unit is limited to the late Cretaceous, with a major peak of 83 Ma. Variable input of older crustal sources with Jurassic (153 Ma), Permo-Triassic (250-290 Ma), Cambrian to Late Neoproterozoic (520-560 Ma) and Middle Mesoproterozic (1000-1500 Ma) ages which are clearly recognized in older units of the Sierra Nevada de Santa Marta massif and the northern South American basement are also recorded. This type of volcano-sedimentary record within an intra-oceanic arc bears strong similarities with the modern Lesser-Antilles and the Tonga-Kermadec arcs, where continentally derived sediments can be transported houndred of kilometers to the fore-arc, back-arc or the accretionary prism of the active intra-oceanic arc. This record also suggests that this arc has an intra-Americas position, near to its final accretionary stop. Although the metamorphic overprint has obliterated the stratigraphic relations, apparent variations of the LREE/HREE in the metavolcanics and the preservation of continentally derived sediments, can be ascribed to variation in the sediment subduction vs accretion sediment budget in an evolving back-arc basin. Similar lithostratigraphic associations within the Caribbean metamorphosed and unmetamorphosed magmatic and sedimentary units can be related to a similar detrital continental input, whereas, the existence of coherent metamorphosed pre-Mesozoic crustal elements within subduction-accretion complexes, may reflect the existence of continental islands that were detached during the initial proto-Caribbean rifting phases and mixed within intra-oceanic accretionary prisms.
T13C-1470
Late Cretaceous and Paleogene evolution of the Greater Antilles fold- and thrustbelt: structure and stratigraphy in the Camagüey region, Cuba
The northern Caribbean margin underwent arc-continent collision in the late Cretaceous and Paleogene. On Cuba this les to the stacking of tectonic slices that comprise from top to bottom a volcanic arc unit, an ophiolite complex, a deformed belt of sedimentary rocks (the Camajuaní and Placetas belts) and finally rocks correlative to the Bahamas platform on the southern North American continental margin. On south-central and western Cuba, HP-LT metasedimentary rocks, on the Isle of Pines including a HT-LP overprint, were exhumed in the course of the late Cretaceous, probably at least partly added by extensional unroofing. These metamorphic rocks are exhumed in tectonic windows in the ophiolite and volcanic arc tectonic slices. Their exhumation quite surprisingly coincided with the arrest in arc volcanism in the Cuban periphery. Here, we present an integrated structural geological and stratigraphic study of the sedimentary units incorporated in the basal parts and underlying the ophiolite unit in the Camagüey province in northern central Cuba. Aim of this study was to constrain the direction and timing of compressional deformation contemporaneous with and following the exhumation and possibly extension in the southern internal parts of the Cuban fold-and thrust belt, and with the arrest in arc volcanism. Our results indicate that the Placetas belt in the Camagüey region consist of tightly, polyphase folded deep marine upper Jurassic to upper Cretaceous limestones, forming isolated blocks incorporated in a tectonic mélange at the base of the ophiolite unit. Timing of their deformation is likely late Cretaceous and younger. The Bahamas platform-related carbonates in the Sierra de Cubitas at the base of the Cuban nappe stack are characterized by a single, open folding phase trending sub-parallel to the main NW-SE trending structural grain of the fold- and thrust belt. This deformation marks the arrest in emplacement of the Cuban nappe stack onto the southern North American continental margin in the Mid-Eocene. (Under-)thrusting hence continued after the late Cretaceous into the Mid-Eocene, i.e. well after the arrest of arc volcanism and exhumation of metamorphic rocks in the internal parts of the Cuban collage. We will discuss the implications of this study for the northern Caribbean subduction history http://www.geologist.nl
T13C-1471
Southern Costa Rica: an Island-arc Segment That Behaves Like a Doubly Vergent Orogen
Southern Central America is a Mesosoic/Cenozoic island-arc that evolved from the subduction of the Farallón Plate below the Caribbean Plate. The southern Costa Rican land-bridge comprises deformed fore-arc and back-arc basins in the west and east, respectively, separated by the up to 3.8 km high Talamanca Range. The structure of the southern Central American island-arc is similar to doubly vergent and asymmetric orogens. The deformed fore-arc basin in the west and the Limon fold-and-thrust belt in the east can be interpreted as pro-wedge and retro-wedge, respectively. The Talamanca Range represents the uplifted block in between. The pro-wedge is wider and has a lower slope angle than the retro-wedge. The uplift of the Talamanca Range is probably related to a system of conjugate shear zones. Precipitation is unevenly distributed, with orographic effects concentrating precipitation in SW Costa Rica, which has caused pro-wedge denudation, leading to exhumation of granitic rocks at in the interior of the mountain range. The large-scale structure of the Central American island-arc in southern Costa Rica can be described using models of continental collision zones. Previous studies attributed the deformation and uplift pattern to the subduction/collision of the Cocos Ridge. Another reasonable driving mechanism for the evolution of such an orogen in an oceanic island-arc setting is the basal traction due to long-term subduction of the Cocos Plate at a very low angle.
T13C-1472
Coulomb stress evolution in NE Caribbean over the past 250 years due to coseismic, postseismic and interseismic deformation
The northeastern Caribbean region marks the active boundary between the North American and Caribbean plates, accommodating ~20 mm/yr of oblique relative plate convergence distributed between the subduction interface and major strike-slip faults within the overriding plate. This heavily populated region has experienced eleven large (M≥7.0) earthquakes over the past 250 years. In an effort to improve our understanding of the location and timing of these earthquakes and to understand where current seismic hazards may be greatest, we calculate the evolution of Coulomb stress in the region since 1751 due to coseismic slip from all known large earthquakes, associated postseismic viscoelastic relaxation of a mobile mantle, and long-term interseismic deformation. The deformation field is calculated using spheroidal and toroidal modes of a spherically stratified viscoelastic earth, with interseismic deformation constrained by a block model based on geodetic, geologic and seismic observations. We find that the observed progressive westward propagation of earthquakes on major fault systems (i.e., the Septentrional and Enriquillo strike-slip faults and the megathrust) was encouraged by coseismic stress changes associated with prior earthquakes. For the strike-slip faults, the loading of adjacent segments was further amplified by postseismic relaxation of a viscoelastic mantle in the decades following each event. Furthermore, earthquakes on the Septentrional fault relieve a small level of Coulomb stress on the parallel Enriquillo fault to the south (and vice-versa), perhaps explaining the anti-correlated timing of events on these respective fault systems. The greatest net build-up of Coulomb stress changes over the past 250 years occurs along the central and eastern segment of the Septentrional and the Bowin strike-slip faults, as no recent earthquake has relieved stress in these regions. For oblique thrust faults, net stress build-up over the past 250 years is largest on the North American/Caribbean megathrust west of 70.5°W. High Coulomb stress has also developed east of 65.5°W, where no historic events have been inferred to have relieved stress, though limited GPS coverage in this region may have led to over-estimation of interseismic slip deficit rates in this area.
T13C-1473
Imaging the Southeast Caribbean Plate Margin with Teleseismic P-wave Tomography
As part of the Bolivar Project, the Passive Array Group (authors plus Funvisis; University of the West Indies; University of California, San Diego; and Rice University) operated a 113 station, broadband array in Venezuela and the southern Antilles. The combined array ran from December 2003 to May 2005 with stations running from the craton south of the Orinoco River to OBS instruments located on oceanic crust of the Caribbean plate. This collaborative geological and geophysical study of the Caribbean-South American Plate boundary was designed to investigate the large scale structure and tectonic framework of this northern boundary of South America. We measured P wave residuals from 382 teleseismic events with a two-step procedure. First, we used a new array processing method to cross-correlate vertical component data with an array beam computed through a robust, nonlinear stacking method. Second, we reviewed the cross-correlation results adding picks manually for stations that did not correlate reliably with the array beam. We used these P wave residuals to construct a series of P- wave tomographic models of this region. Robust features seen in these models include: (1) a high velocity upper mantle under the craton grading to lower velocities under the Orinoco basin; (2) higher velocities are seen in western Venezuela that correlate with the subducting Nazca plate; and (3) the subduction of the Atlantic at the Antilles Arc is marginally resolved in the northwest corner of the study area.
T13C-1474
BOLIVAR: the Caribbean-South America plate boundary between 60W and 71W as imaged by seismic reflection data
We present the results of ~6000km of marine multi-channel seismic (MCS) reflection data collected offshore Venezuela as part of the Broadband Ocean Land Investigation of Venezuela and the Antilles arc Region project (BOLIVAR). The imaged area spans almost 12 degrees of longitude and 5 degrees of latitude and encompasses the diffuse plate boundary between South America (SA) and the SE Caribbean plate (CAR). This plate boundary has been evolving for at least the past 55My when the volcanic island arc that borders the CAR plate started colliding obliquely with the SA continent: the collision front has migrated from west to east. BOLIVAR MCS data show that the crustal architecture of the present plate boundary is dominated by the eastward motion of the Caribbean plate with respect to SA and is characterized by a complex combination of convergent and strike-slip tectonics. To the north, the reflection data image the South Caribbean Deformed Belt (SCDB) and the structures related to the thrusting of the CAR plate under the Leeward Antilles volcanic arc region. The data show that the CAR underthrusting continues as far east as the southern edge of the Aves ridge and detailed stratigraphic dating of the Venezuela basin and trench deposits suggests that the collision began in the Paleogene. The amount of shortening along the SCDB decreases toward the east, in part due to the geometry of plate motion vectors and in part as a result of the NNE escape of the Maracaibo block in western Venezuela. South of the SCDB the MCS profiles cross the Leeward Antilles island arc and Cenozoic sedimentary basins, revealing a complex history of Paleogene-Neogene multiphase extension, compression, and tectonic inversion, as well as the influence of the tectonic activity along the right-lateral El Pilar – San Sebastian fault system. East of the Bonaire basin the MCS data image the southern end of the Aves Ridge abandoned volcanic island arc and the southwestern termination of the Grenada basin, characterized here by middle Miocene inverted structures, likely related to the WNW-ESE transpression between CAR and SA. The easternmost MCS profile crosses the ongoing arc-continent collision of the Lesser Antilles arc with SA and the backarc (Grenada Basin) and forearc (Tobago Basin) basins as well as the suture between the Caribbean arc and the passive margin of the continental SA plate near eastern Trinidad.
T13C-1475
Two-stage Extensional Opening of the Mona Passage as Revealed by New Multibeam Bathymetry and Seismic Reflection Data
The Mona Passage, located between Puerto Rico (P.R.) and the Dominican Republic (D.R.), has long been considered a region of significant extensional deformation resulting from the oblique subduction system active along the northern Caribbean plate boundary. Published analyses of GPS data show that extension in this region continues today, with P.R. moving northeasterly away from Hispaniola at a rate of between 3-4 mm/yr, maintaining the active extensional environment between these islands. High-resolution EM 1002 swath bathymetry and multi-channel seismic reflection profiles collected in the Mona Passage in March 2007 and October 2006 respectively have for the first time clearly revealed the nature of extensional deformation within this seismically active region. These new data, combined with data from several recent bathymetry surveys in the region show a complex morphostructural terrane, comprising a mix of fault-, erosional-, and landslide-related morphologies. Flat-lying and northward- and southward-backtilted fault blocks, capped by Oligocene to Holocene aged carbonate units, dominate the seafloor and sub-seafloor landscape. At the seafloor, the fault blocks are in places eroded by the powerful bottom currents passing through the passage, while in the subsurface, they form extensional basins, covered by variable thicknesses of young sediments. The most prominent of these fault blocks is the north-dipping Desecheo Ridge, where as much as 700 m of carbonate and Cretaceous-lower Oligocene volcanic and basement units are exposed on its southern side along the multi-stranded southward- dipping Desecheo Ridge fault. The Desecheo Ridge fault extends from near Punta Higuero on the west coast of P.R. to Desecheo Island, where is steps approximately 1.5 km south and continues westward. Exposed on the seafloor and in the shallow subsurface are two dominant fault sets of differing orientations; a N- to NNW-oriented set and a W- to WNW-oriented set. The N- to NNW-striking faults are typically shorter, deeply eroded, and in many places such as at the southwest corner of Mona Canyon, curve along strike. Faults in the W- to WNW-oriented set, cross-cut the N- to NNW-set and in many places cut the overlying sediment cover, indicating that they are the younger, active faults. To date, no definitive piercing-points or other evidence of lateral or oblique movement have been identified along any of the faults in the passage, suggesting that most if not all of the current motion on these faults may be extensional. The perpendicular direction to the active fault set (NNE-SSW), while similar to the relative motion between the eastern D.R. and western P.R. given from GPS, is at odds with the E-W extension direction implied by the Mona Rift. Several of the long E-W striking faults extend along much of the approximately 125 km width of the passage, some of which appear to be continuations of sub-aerially exposed faults identified in western P.R., such as Cerro Goden and Mayaguez faults. The length of these faults and the presence of other major, and likely active, faults in close proximity to the coasts of P.R. and the D.R., make an understanding of these individual faults and wider structural fabric of the region important for seismic hazard analysis. In terms of the tectonic history of the region, the presence of two distinct fault orientations of differing ages in the passage may reflect a moderate shift in plate motion during the current stage of extensional opening or a record of the current and an earlier (Miocene-Pliocene?) stage of deformation.
T13C-1476
Insights Into Caribbean Lithospheric Structure From S Wave Receiver Functions
BOLIVAR (Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region) was aimed at investigating the interplay between the lithospheric and asthenospheric mantle of the Caribbean and the South America plates. The oblique collision of the Caribbean plate migrating eastwards has created a complicated deformation zone with strike-slip, compressional and extensional structures along the Caribbean and South America boundary. Earlier results with P receiver functions revealed strong variations in crustal thickness ranging from 15 km beneath the Caribbean Sea to 55 km beneath Venezuela. However, one of the fundamental questions not yet resolved concerns the thickness of the lithosphere in this region. Using the S wave receiver function technique, we analyzed seismograms from some 100 events at epicentral distances of 55-125 degree. The seismograms were rotated and deconvolved to isolate S-to-P conversions from the incident S wave. These were subsequently stacked after their respective conversion points and mapped into the subsurface. A strong negative phase is associated with the S-to-P conversion from the base of the lithosphere. Analysis of these data is ongoing, but we expect to see large variation in lithospheric thickness as the BOLIVAR array spans the transition from the Caribbean with OBS stations to the interior of South America (Guyana Shield).
T13C-1477
BOLIVAR: Crustal structure of the Caribbean-South America plate boundary between 60W and 70W from wide-angle seismic data
We present the results from five wide-angle seismic profiles collected onshore and offshore Venezuela in 2004 as part of the Broadband Ocean Land Investigation of Venezuela and the Antilles arc Region project (BOLIVAR). The study area is the diffuse plate boundary between South America (SA) and the SE Caribbean plate (CAR) covering roughly 1000 km by 500 km. Over the past 55 My the Leeward Antilles island arc that borders the CAR plate has been colliding obliquely with the SA continent resulting in a collision front that has migrated from west to east. The five wide-angle profiles sample different stages of the time-transgressive margin from west to east, each crossing the margin roughly perpendicularly. The main purpose of this presentation is to contrast and compare the crustal velocity structure along these profiles to better understand the tectonic processes that are responsible for the evolution and present-day configuration of the plate boundary. Each of the wide-angle profiles is about 500 km in length and includes both onshore and offshore shots and receivers, except the easternmost profile, which is entirely offshore. The dense wide-angle data were modeled in the same way along each profile using a two-step, layer-stripping approach: (1) the first-arrival times were tomographically inverted for a smooth velocity model, and (2) the lower crust, Moho, and uppermost mantle were determined by simultaneous inversion of the PmP refection and Pn refraction phases while keeping the upper and middle crust from the first step fixed. The five models show tremendous lateral heterogeneity, as they cross features such as normal oceanic crust, oceanic plateau crust, an accretionary wedge, active and remnant island arcs, forearc and foreland basins, a major strike-slip system, a fold and thrust belt, and the edge of cratonic continental crust. Two of the main contributions of the wide-angle models to the BOLIVAR project, and the focus of this presentation, are the Moho structure, which is generally not imaged in the coincident reflection data (see Magnani et al., this session), and the 1-D velocity structure of the tectonic units that make up the diffuse plate boundary. The first-order results are (1) a general smoothing and flattening of Moho topography from the youngest to oldest parts of the margin (east to west), suggesting a "relaxation" of the Moho, (2) an offset in the Moho roughly beneath the strike-slip fault system, more pronounced in the east, suggesting that the deformation is not confined to the crust, and (3) high crustal velocities at all depths within the island arc.
T13C-1478
BOLIVAR: Crustal Structure of the Caribbean-South America Plate Boundary at 65W
We describe a ~550 km long, N-S oriented, onshore-offshore profile that crosses the SE Caribbean plate boundary at approximately 65W longitude. The profile is one of the principal seismic reflection and refraction transects acquired in 2004 as part of the Broadband Ocean and Land Investigation of Venezuela and the Antilles arc Region (BOLIVAR) experiment. The transect starts ~330 km offshore northern Venezuela and crosses the plate boundary from the stable Caribbean (CAR) plate to the stable South America (SA) continent crossing the El Pilar continental strike-slip fault system. High quality multi-channel seismic (MCS) reflection data were acquired along the 330 km offshore portion of the profile. The airgun shots from the MCS vessel and two land shots were also recorded by 7 OBS's and 514 single channel land receivers for wide-angle analysis. We have analyzed the wide-angle data using a tomographic inversion of first arrival travel times to obtain upper and mid-crustal P wave velocities, followed by simultaneous inversion of PmP and Pn to obtain Moho depth and lower crustal and upper mantle P wave velocities. The P-Wave velocity model was converted to density using a 5th order polynomial fit to global experimental data, and the calculated gravity response was compared to an observed gravity curve resulting from averaging a swath 50 km wide about the profile. The calculated gravity response fits the observed locations and wavelengths of local highs and lows well, although the anomaly amplitudes do not fit exactly using the automatic velocity-density conversion. The model suggests a Caribbean crust that is over 10 km thick and shows South American crust of variable thickness ranging from 30 to 40 km. Structures imaged in the model include the Cariaco trough offshore, a sedimentary basin that reaches a depth of ~9 km; and the Espino Graben onshore, that reaches a maximum depth of ~8 km. The MCS data image the stable CAR plate and the thrusting of the CAR plate under the South Caribbean Deformed Belt (SCDB) prism. The accretionary prism is highly deformed although the seismic data suggest limited recent tectonic activity: The frontal thrust faults are draped by an apron of recent sediments, mostly deformed by slumping above the steepest slope of the prism. Wide-angle data corroborate a limited amount of thrusting of the CAR plate under the Leeward Antilles arc region. To the south, the reflection data cross the El Pilar continental strike-slip system along the northern edge of the Cariaco basin. At this longitude the dextral strike-slip system accommodates ~50% of the strike-slip displacement between the CAR and SA plate and is characterized by transtensional motion. The Cariaco basin appears as a narrow, deep trough, bounded to the north by the nearly vertical El Pilar strike slip fault, which juxtaposes crustal rocks with an average velocity of 5.7 km/s against sedimentary sequences with an average velocity of 3.0 km/s.
T13C-1479
Onset of subduction in eastern Cuba (Sierra del Convento melange): geochemical evidences for a partially melted young slab.
Geochronological and regional geological arguments indicate the onset of subduction of the Protocaribbean (Atlantic) lithosphere during the Aptian (ca. 120 Ma). Exotic blocks of MORB-like amphibolite and associated tonalite-trondhjemite from Sierra del Convento melange in eastern Cuba document this early stage of subduction. Major and trace element geochemical signatures and Sr-Nd isotope data of these blocks indicate that oceanic crust underwent partial melting processes during subduction. Theoretical melting models suggest that the trondhjemites are not oceanic plagiogranite formed after differentiation or partial melting of MORB at oceanic ridges, but instead that they formed by 1-30 wt % water-saturated melting of epidote±garnet amphibolite. REE patterns of the trondhjemites are fractionated (1 < (La/Yb)n < 16.8), with LREE enrichment and flat or slightly depleted HREE patterns which compare well with adakitic melts formed by partial melting of mafic material at moderate to high pressure. Indeed, trondhjemites from Sierra del Convento have some of the geochemical features of Cenozoic adakites, including SiO2>56%, 3.5%<Na2O<7.5%, (K2O/Na2O) ~ 0.42, Mg# ~ 50, Ni =20-40 ppm, Cr =30-50 ppm, LREE enrichment, HREE depletion, and high La/Yb and Sr/Y. However, the trondhjemites from Sierra del Convento are more comparable to acid rocks from Catalina Schist melange (California) considered to be primary slab melts which did not react with the mantle wedge. In agreement with major and trace element geochemistry, the Sr-Nd isotope systematic of selected amphibolites and trondhjemites can be explained by a two stage model involving a) generation of MORB-like basalts and b) partial melting of subducted MORB. Partial melting of amphibolite in the Sierra del Convento melange is consistent with the onset of subduction of young oceanic lithosphere of the Protocaribbean.
T13C-1480
Capacity Building for Caribbean Tsunami Warnings: A Regional Training Course
Between June 25 and June 30 the Seismic Research Unit (SRU) of the University of the West Indies (UWI) hosted a Caribbean regional training program in Seismology and Tsunami Warnings. A total of 43 participants from 21 countries and territories, representing meteorological, emergency management, and seismological institutions in the region, attended this training aimed at developing their understanding of the science of tsunamis, hazard and risk assessment, preparedness, education, and outreach, and operational best practices. As an outcome of the course the participants drafted six recommendations (outlined on the poster) that they felt were priority action items for expeditious realization of a Tsunami Early Warning and Mitigation System. The program was conducted under the UNESCO IOC banner in response to a call for such a training program at the Second Session of the Intergovernmental Coordination Group for Tsunami and Other Coastal Hazards Warning System for the Caribbean and Adjacent Regions (ICG/CARIBE-EWS II), held in Cumanã, Venezuela, March 12-14, 2007. The majority of funding for the course was provided by the Office of Foreign Disaster Assistance (ODFA) of the US Agency for International Development (USAID), the United Nations Development Programme (UNDP), the Disaster Reduction Center of the UWI, and the US Geological Survey (USGS).
T13C-1481
Oxygen isotope constraints on the origin of island arc granitoids
Granitic intrusions in island arcs constitute additions of juvenile crust from oceanic environs that ultimately get accreted to continents. The genesis of island arc granitoids is thus important to studies of the growth of oceanic and continental crust. Puerto Rico (USA) is a composite island arc terrane that preserves a record of plutonism from 85 Ma to 38 Ma (Cavosie et al., 2007 AGU). Mid-crustal granitoid plutons are exposed (~1 to 500 km2), but their origins are unknown, as no suspected parental magmas associated with the plutons (e.g., gabbro) are exposed. This study uses petrography, WR major elements, and oxygen isotopes of WR and zircon from granitoids and xenoliths to place better constraints on the origin of granitoid in the Greater Antilles island arc. WR δ18O analyses were made with laser fluorination by IRMS at the Univ. of Wisconsin (uncertainties = 0.10 to 0.20‰, 2sd). The main plutons (Caguas, Rio Blanco, San Lorenzo, Utuado, Vieques) yield primitive δ18O(WR) values, ranging from 6.24 to 7.72‰ over a range of wt.% SiO2= 58.03 to 66.54. Smaller stocks (<20 km2) yield higher δ18O(WR) values, ranging from 7.47 to 10.27‰. Qualitative petrographic analysis reveals that granitoids with δ18O(WR) >~7.5‰ are partially to pervasively altered. Zircon preserves magmatic δ18O and is used here to quantitatively evaluate the measured δ18O(WR) values. If δ18O(Zrc) and wt.% SiO2 are known, a comparison of measured vs. predicted δ18O(WR) can be made (Valley et al., 2005, CMP). The measured δ18O(WR) values record variable amounts of alteration, ranging from virtually undetectable, to WR δ18O elevations of ~4‰, indicative of low-T subsolidus alteration. The Δ18O (WR-Zrc) values using calculated δ18O(WR) yield the following fractionations: -1.57 to 1.00‰ for granodiorites (wt.% SiO2=66 to 57); -0.85‰ for diorite (wt.% SiO2=55); and -0.56‰ for the only gabbro analyzed (wt.% SiO2=50). Mafic xenoliths (53-57 wt.% SiO2) from 4 granitoids yield Δ18O(WR granitoid –WR mafic xenolith) as follows: Rio Blanco (0.58‰), San Lorenzo (0.55‰), Utuado (0.42‰), and Vieques (0.06‰). Petrographic analysis of the xenoliths reveals igneous textures (e.g., oscillatory zoning in plag) and assemblages dominated by hbl + plag, indicative of an igneous origin in a hydrous melt. The xenolith results (with the exception of Vieques) are consistent with models for the derivation of the granitoid plutons from gabbroic parental melts represented by the xenoliths. The small fractionations of Δ18O(WR granitoid – WR mafic xenolith) are permissible for magmatic processes such as partial melting or fractional crystallization. Thus, samples of the elusive gabbroic source(s) for the granitoids in Puerto Rico have tentatively been identified.
T13C-1482
Seismic Pattern of the Guerrero-Oaxaca, Mexico Region and its Relationship to the Continental Margin Structure
The purpose of this article is to make aware seismic evidences that suggest a possible segmentation of the continental margin at the Guerrero-Oaxaca, Mexico region. A survey of microseismicity using a portable seismographs network and the analysis of the aftershocks distribution of 1982 and 1995 major earthquakes permit to infer the characteristics of the seismic patterns of the Acapulco-Pinotepa segment of the Middle America subduction region. Two different seismic regimes are apparent, one in the Acapulco-Marquelia and another in the Marquelia-Pinotepa areas. In the Acapulco-Marquelia segment the seismicity is disperse and broader it starts at the coast up to 160 km inland approximately. Seismicity in the Marquelia-Pinotepa segment on the other hand, is narrower and concentrates near the coast. The seismic zones are separated by two narrow bands of low seismic activity nearly perpendicular to the coast and trench axis. These two low activity strips, located near the northern tip of the Ometepec submarine canyon and Punta Maldonado respectively, are interpreted in the present paper as corresponding to disruptions of the continental margin. Supplementary geophysical information in this area seems to additionally substantiate this interpretation. This result is important to understand the mechanics of the major earthquakes that frequently occur in this region.
T13C-1483
Active Faults in Eastern Hispaniola: The Hispaniola-Puerto Rico Microplate Boundary?
An extensive tract of limestone of mostly Pleistocene-Recent age covers the Eastern part of the Dominican Republic. Numerous distinctive marine terraces outcrop along the southern and eastern coast, the lowest of which has been dated at about 125Ka. In the eastern area, the highest terrace is about 50m asl, is very variable in elevation, and correlates with a terrace of about 50 m asl along the southern coast. This feature might correlate with a feature of similar height on the Island of Marie Galante in the Lesser Antilles, dated at 250Ka. Manipulation of 3 arc-sec grid of SRTM land data and a 12 arc-sec grid of marine data reveals the location of the upper marine terrace as well as numerous scarps with 10's of meters of relief tending WNW across the region. The 2nd derivative of the relief grid is used to objectively identify the location of the upper terrace, which is compared to the elevation grid to develop an along escarpment profile of terrace elevation. If undisturbed, this feature should be contour parallel, that is all at the same elevation. Systematic elevation changes along profile suggest titling and numerous abrupt vertical (~30-50m) and at least one horizontal offset (375m) of this feature. Terrace displacing scarps can be traced many kilometers from offshore, across the coast paralleling marine terraces, and continuing inland as linear features that I interpret as active normal faults cutting the limestone platform. Five systems of normal faults have been identified in this manner, the longest of which may be capable of generating earthquakes of about magnitude 7-7 1/4. If the age of the upper terrace is roughly about 250Ka, then the observed horizontal displacements of about 375 meters suggest a rate of fault motion on the order of mm"s/yr for each of the5 faults. This total rate of deformation of several mm/yr is similar to the rate of deformation calculated from GPS studies for the rate of motion between the Hispaniola and Puerto Rico microplates, suggesting that much of the inter-microplate motion is not contained to the offshore regions of the Mona Passage, but rather passes on shore in the eastern part of the Dominican Republic.
T13C-1484
Tsunami Hazard for the Bay of Honduras Posed by Roatan Island Faulting
Uplifted and warped coastal landforms (fossil coral reef and beachrock, wave-cut and beach terraces) on the western part of Roatan Island off the northern Honduran coast record at least two late Holocene earthquakes that we estimate to have been >M7. Uplift has been primarily across a fault following the south coast of western Roatan, herein termed the "Flowers Bay fault". We constrain the ages of the displaced landforms with ESR and radiocarbon dates of carbonate and a sea level elevation curve compatible with Caribbean data. The fossil reef grew between 37 and 34 Ka, and the beachrock horizon and lowest terrace developed between 1700 and 1000 AD. One earthquake that raised the south coast up to 5 m post-dates 1700 AD. We interpret this event as the great earthquake of August 1856 off the northern Honduran coast that generated a tsunami that ran as much as 24 km onto the mainland. Another prehistoric earthquake circa 900 AD caused similar vertical displacement as the later event and likely generated an equivalent tsunami. The age and elevation of the fossil reef require an average uplift rate of 3 mm/yr, consistent with a recurrence interval of ~1000 years for these large earthquakes.