Upper mantle structure beneath the Caribbean-South Americanplate boundary from surface wave tomography

摘要

We have measured shear wave velocity structure of the crust and upper mantle of theCaribbean-South American boundary region by analysis of fundamental mode Rayleighwaves in the 20- to 100-s period band recorded at the BOLIVAR/GEODINOS stationsfrom 2003 to 2005. The model shows lateral variations that primarily correspond totectonic provinces and boundaries. A clear linear velocity change parallels the platebounding dextral strike-slip fault system along the northern coast of Venezuela, illustratingthe differences between the South American continental lithosphere, the Venezuelanarchipelago, and the Caribbean oceanic lithosphere. At depths up to 120 kin beneath theVenezuelan Andes and the Maracaibo block, there is evidence of underthrusting of theCaribbean plate, but there is no other evidence of subduction of the Caribbean platebeneath the South American plate. In eastern Venezuela, linear crustal low velocities areassociated with the fold and thrust belts whereas as higher, crustal velocities are imaged inthe Guayana shield lithosphere. The subducting oceanic part of the South American plateis imaged beneath the Antilles arc. The surface wave images combined with seismicitydata suggest shear tearing of the oceanic lithosphere away from the buoyant continentalSouth American plate offshore of northeastern Venezuela. The continental lithospheresouth of the slab tear is bent down toward the plate boundary in response to thepropagating tear in the lithosphere. We interpret a nearly vertical low-velocity "column"west of the tear centered beneath the Cariaco Basin, with three-dimensionalasthenospheric flow around the southern edge of the subducting oceanic lithosphere, withthe asthenosphere escaping from beneath continental South America and rising into theplate boundary zone. The complex plate boundary structure is best examined in threedimensions. We discuss the new surface wave tomographic inversion in the context ofresults from other researchers including local seismicity, teleseismic shear wave splits, andinterpretations from active source profiling.