Stephen A. Clark, Dale S. Sawyer, Alan Levander, M. Beatrice Magnani, Colin A. Zelt, Michael Schmitz, and the BOLIVAR Working Group, Dept. of Earth Science, Rice University, Houston, TX
BOLIVAR, the Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region, an NSF and Venezuelan funded, collaborative active source seismic experiment conducted in April and May of 2004, acquired multi-channel seismic (MCS), ocean-bottom seismic (OBS), and concurrent land seismic data in the southeast Caribbean region (for details, see Levander et al.). The purpose of the BOLIVAR project is to look at the oblique collision of the Caribbean Plate with the South American Plate, where the Leeward Antilles arc is accreting onto the South American craton.
The 275-km-long MCS profile BOL-28 trends 5°W of N, at 64°W longitude, from 10.8°N to 13.1°N. A 2-sec-thick (twt; 3 km at 3 km/s) package of relatively flat-lying, recently thrust-faulted sediments overlies complex crustal structure and associated infill in the Venezuela Basin. At the southern end of the Venezuela Basin, a 17-km-wide, southward-thickening accretionary wedge abuts a carbonate platform overlying the thickened remnant arc crust of the Aves Ridge. Subduction cannot be traced beneath the Aves Ridge platform; this may be due to either poor seismic penetration beneath the carbonate, or relatively recent onset of subduction limiting the extent of slab penetration. However, south of the Aves Ridge, another 25-km-wide accretionary wedge thickens to the south, underlain by a strong top of Caribbean crust reflector that can be traced from the seafloor at 0.5 sec twt to the bottom of the accretionary wedge at 4.2 sec twt, 27 km to the south. This reflector may continue to the south, beneath the accreted Leeward Antilles arc crust east of Margarita Island; sub-crustal reflectors are apparent, but further processing is necessary to improve the image enough to infer continuity. We interpret this to be the original subduction zone, choked by the thickened Aves Ridge crust. Subduction has now migrated to the north, effectively accreting the Aves Ridge crust onto the South American plate and providing a mechanism for continental crustal growth. Modeling of the velocity structure from the coincident OBS data and onshore-offshore data along the same profile will further constrain the interpretation.