3D Lithospheric Structure beneath the Caribbean – South American Plate Boundary from Surface Wave Tomography

The shear wave velocity structure of the crust and upper mantle beneath the Caribbean – South American boundary was determined by analysis of fundamental mode Rayleigh waves in the 20-100s band that were recorded at the 84 BOLIVAR/GEODINOS stations in the SE Caribbean between 2003-2005. Earthquakes in the distance range 20° to 120°, with magnitudes greater than 5.1, were used to invert for the upper mantle and lithospheric structure using a two plane wave approach. The model contains lateral variations that primarily correspond to tectonic provinces and plate boundaries. A clear linear velocity change parallels the plate bounding strike-slip fault systems along the northern coast of Venezuela, illustrating the differences between the continental lithosphere of the South American plate and the lithosphere of the Caribbean Large Igneous Province. Shallow low velocities associated with the basins near the coast are clearly imaged, as are higher velocities in the crust and upper mantle associated with the Guayana shield in southeastern Venezuela. The subducting oceanic portion of the South American plate is imaged beneath the Antilles Arc as a shear tear where the subducting slab tears away from the buoyant continental lithosphere of South America at the eastern end of the Caribbean-South American plate boundary. A low velocity “column” at the edge of the tear may be associated with asthenospheric flow from behind the subducting Antilles slab. The complex structure of the plate boundary is best imaged and interpreted with three-dimensional modeling. We have combined the surface wave model, receiver functions, relocated local seismicity, and interpretations from active source profiling to more completely understand the complex geometry and lithospheric structure of the region.