HR: 17:50h
AN: S44A-07    [Abstracts]
TI: Wide-angle velocity modeling and receiver functions imaging a lithospheric shear tear in the southeast Caribbean-South America plate boundary
AU: * Clark, S A
EM: stoney@rice.edu
AF: Dept. of Earth Science, Rice University, MS-126 6100 Main St., Houston, TX 77007, United States
AU: Levander, A
EM: alan@rice.edu
AF: Dept. of Earth Science, Rice University, MS-126 6100 Main St., Houston, TX 77007, United States
AU: Zelt, C A
EM: czelt@rice.edu
AF: Dept. of Earth Science, Rice University, MS-126 6100 Main St., Houston, TX 77007, United States
AU: Niu, F
EM: niu@rice.edu
AF: Dept. of Earth Science, Rice University, MS-126 6100 Main St., Houston, TX 77007, United States
AU: Sobiesiak, M
EM: polar@gfz-potsdam.de
AF: GeoForschungsZentrum (GFZ) Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AB: Two models for slab detachment have been proposed for the lithospheric structure of the northern South America plate boundary with the southeast Caribbean, where westward subduction of oceanic South America transitions to east-west transform between continental South America and the Caribbean plate. In the tensile tear model, oblique convergence causes northwest-dipping subduction, and tension on the subducting slab results in detachment orthogonal to the motion vectors. Conversely, the shear tear model predicts detachment parallel to the motion vectors along a vertical plane, with shear stress focused on the edge of the propagating transform boundary. We present new active-source onshore-offshore wide-angle tomography, integrated with new passive-source receiver function analysis, from profile 64W of the BOLIVAR (Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region) project. Profile 64W is a 460 km-long, north-south, onshore-offshore reflection/refraction/teleseismic transect located approximately at 64 deg W longitude that extends from the southeastern Caribbean across the Serrania del Interior and into the Maturin basin. The various datasets image deep crustal and upper mantle structure across the entire diffuse plate boundary zone. The active-source components of profile 64W include 33 OBSs and 344 land seismic stations which recorded 7500 offshore airgun shots and 2 chemical explosive land shots. Receiver functions along 64W were picked from hundreds of events at 18 temporary and permanent broadband stations. Close agreement exists between the wide-angle inversion of first arrivals, PmP, and Pn, and receiver function analysis for the Moho conversion, indicating that the Moho deepens northward from 35 km beneath the Guiana shield craton (from receiver functions only) to 45 km beneath the Serrania del Interior; to the north, Moho abruptly shoals to a depth of 25 km. We interpret this step change in Moho depth to be the lower crustal plate boundary between the Caribbean and South America, and coincident with a shear tear through the South American lithosphere. North of the step, Moho depth increases slightly to 28 km beneath Margarita Island, then gradually shallows to 20 km and flattens north of La Blanquilla Island. Thrust faults beneath La Blanquilla and the Serrania del Interior bound the 250 km-wide diffuse plate boundary, the former marking Caribbean underthrusting beneath the remnant arc, the latter marking the frontal thrust of the fold and thrust belt. We hypothesize that isostatic rebound following slab detachment may contribute significantly to the uplift of the Serrania del Interior, which reaches 2500 m elevation, and infer a role in the exhumation of HP/LT metamorphic rocks found along the strike- slip system. We conclude that the Serrania del Interior and Maturin basin do not represent a collisional orogeny and foreland basin, respectively, but are instead resultant from the geodynamic response to the propagating lithospheric shear tear.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8106 Continental margins: transform
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8138 Lithospheric flexure
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Seismology [S]
MN: 2007 Joint Assembly