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Abstract

We discuss results from a large-scale investigation of the southeastern Caribbean (CAR) plate boundary conducted in Venezuela and the Leeward Antilles by Venezuelan and U.S. scientists. The project, known as BOLIVAR in the U.S. and GEODINOS in Venezuela, included offshore reflection, onshore-offshore wide-angle reflection/refraction profiling, teleseismic body and surface wave tomography, structural geology, and geochemistry. The various types of seismic imaging of the crust and upper mantle in northern South America provide a number of important constraints on the evolution of the Caribbean-South American plate boundary, and identify substantial modifications to crust and upper mantle structure of coastal South America resulting from plate boundary tectonics.

As the Atlantic subducts, the southern end of the descending slab applies a load to the adjacent South American lithosphere, depressing the South American lithosphere and providing space for the leading edge of the Caribbean to overthrust northern South America. This has a number of consequences that extend from the surface to the base of the lithosphere: (1) As suggested by others, the descending Atlantic tears from the SA continental margin, producing an isolated nest of intermediate depth earthquakes. This lithospheric tear is associated with, but offset from the eastern end of the South American-Caribbean strike-slip fault boundary. (2) Lithospheric flexure at the South American coast creates space to accommodate overthrusting of South American passive margin deposits and Caribbean island arc and prism terranes, aiding in development of the coastal mountain belts and exhuming HP-LT rocks. (3) Flexure and overthrusting deepens the South American Moho to ~50 km in northeastern Venezuela, and over thrust terranes occupy the upper 25-50% of this thickness. (4) The depressed South American Moho is substantially offset (~15-20 km deeper) from that of the adjacent Antilles arc terranes. (5) Lastly, the subducting Atlantic plate viscously removes the base of the South American passive margin continental lithospheric mantle at least 100 kilometers south of the plate boundary and destabilizes the continental lithosphere farther inland, triggering convective instabilities in the lithosphere south of the plate boundary.

Once subduction has migrated eastward from a given point along the margin, the load caused initially by Atlantic subduction is removed, allowing the crust to rebound, shallowing the South American Moho, reducing the Moho offset between coastal South America and the offshore terranes, and enhancing erosion of accreted terranes. We observe that the continental lithosphere west of the subduction zone is thinner than expected between the Guayana shield and the plate boundary. We hypothesize that the subducting Atlantic has viscously removed the mantle lithosphere beneath the South American continental margin and destabilized the lithosphere farther inland, everywhere west of the current subduction zone. Although modulated by the paleogeography of South America and preexisting lithospheric structures on both the South America and Caribbean plates, this simple time transgressive model of subduction, lithospheric loading, flexure, and viscous removal of mantle lithosphere can account for much of the lithospheric structure of northern South America as far west as the Boconó Fault. Development of the Boconó and Santa Marta-Bucaramanga faults has added an additional layer of tectonic complexity in western Venezuela in the past ~10-20 m.y. that overprints, but does not completely destroy, the effects of the migrating Atlantic subduction zone.

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