BOLIVAR: INVESTIGATING ISLAND ARC ACCRETION ALONG THE SOUTHEASTERN CARIBBEAN PLATE BOUNDARY

Continental growth is thought to result from accretion of island arcs to older continental masses, but whether continents are still growing today is unclear. Along the Caribbean-South American (CAR-SA) plate boundary zone, the Antilles Arc has collided obliquely with the northern SA margin since the Paleocene. Neogene motions combine right-lateral strike-slip faulting along the coastal faults, underthrusting of CAR beneath SA, and development of fold and thrust belts at the southeastern corner of the plate boundary.

We describe the ongoing NSF Continental Dynamics project BOLIVAR and its Venezuelan counterpart, GEODINOS. The projects include geological, geochemical, geodynamic, and geophysical investigations involving ~30 scientists at 9 U.S. and Venezuelan institutions. The study area extends from the Atlantic Ocean to the 71oW meridian, and from the Guayana Shield (6oN) into the eastern Caribbean basin (14oN). This area (>0.7M km2) is comparable in size to California and its continental margin, and rivals the San Andreas plate boundary in complexity. Although analysis of the dataset is still in progress, we have made a number of important discoveries.

Ages of igneous rocks in the Leeward Antilles show a progressive west to east decrease along the plate boundary from 89 Ma on Aruba to 59 Ma on La Blanquilla. Strain markers in the rocks on the ABCs demonstrate three deformation stages and are compatible with 90o clockwise rotation of the islands. Active source seismic data shows that the Caribbean plate progressively underthrusts SA more from east to west, with underthrusting just commencing at 64oW. Wide-angle data image high compressional velocity (~ 6.5 km/s) bodies in the shallow to mid-crust along the major strike-slip faults along the plate boundary, suggesting exhumation mechanisms for HP/LT rocks from the Lesser Antilles subduction zone. Receiver functions images of the Moho agree with coincident active source data, and document downward flexing of the SA lithosphere near the modern arc. The crust shows a high degree of heterogeneity, with thickness varying from 38-40 km beneath the Guayana Shield, and 10-15 km beneath the Caribbean LIP, to as much as 50 km under the Sierra del Interior and Maturin basin, with variations from 25-40 km elsewhere beneath the Venezuelan coastal mountain belts.