CRUSTAL STRUCTURE ACROSS THE CARIBBEAN SOUTH AMERICA PLATE BOUNDARY AT 70°W: RESULTS OF THE BOLIVAR PROJECT

The plate interactions in the southwestern Caribbean are poorly understood. Other studies have concluded that the Caribbean Plate is obliquely underthrusting the South American Plate at a rate of ~ 20 mm/yr with a south-east dipping slab attached to the Caribbean. In western Venezuela, the complex motion across the plate boundary is complicated by the tectonic escape of the Maracaibo block -displaced northward along the Bocono and Santa Marta strike-slip faults- as a consequence of the subduction of the Nazca plate beneath South America in the west.

The BOLIVAR project (Broadband Ocean and Land Investigations of Venezuela and the Antilles arc Region) is studying the Caribbean-South America diffuse plate boundary, where collision between these two plates has been taking place for the last 55 m.y. We hypothesize that this is a site of likely continental growth by island arc accretion of the Leeward Antilles to older South American crust.

The active-seismic component of the project was completed in June 2004. We present results of analyses of reflection and refraction seismic data along a 450 km long onshore-offshore profile at 70oW, extending from 10oN to 14.3oN. The refraction data include 40 Ocean Bottom Seismometer (OBS) and 348 Reftek Texan land seismometers that recorded the R/V Ewing airgun shots. The land stations also recorded two large landshots to provide reversed refraction coverage onshore.

A 2-D velocity model obtained from travel time tomography of first arrivals and PmP reflections shows that the Caribbean crust is anomalously thick, ~ 15 km, typical of an oceanic plateau.The velocity structure from wide-angle data is well correlated with the structures interpreted in the reflection data; in particular in the upper and middle crust of the Southern Caribbean Deformed Belt, the Falcon Basin and the Aruba Rise. Low-velocity sediments on the Caribbean plate are observed subducting beneath the South-Caribbean Deformed Belt over a distance of 75-100 km. This subduction may be interpreted as A-type (i.e. Alpine) due to the lack of a volcanic arc and the thickness of the Caribbean plateau. A localized shallow high compressional velocity body onshore Venezuela is spatially associated with the largely inactive Oca Fault, formerly part of the San Sebastian-El Pilar right-lateral strike-slip system to the east.