BOLIVAR: Crustal Structure across the Caribbean-South American plate Boundary at 67.5°W: results from Multichannel Seismic Reflection data

M.B. Magnani, C.A.Zelt, A. Levander, D.S. Sawyer and the BOLIVAR Study Group
Department of Earth Science, Rice University, Houston, TX, USA

In April-June 2004 ~6000 km of marine reflection profiles were collected as part of the NSF-funded BOLIVAR experiment (Broadband Ocean and Land Investigation of Venezuela and the Antilles arc Region). These profiles complement 169 OBS deployments along 5 transects, four ~200 km long offshore-onshore refraction profiles, seismic recording on several islands in the Leeward Antilles arc and geologic field work in some of the Leeward Antilles islands and on the Venezuelan Caribbean (see Levander et al., this session). A main goal of this multidisciplinary investigation is to understand the processes of continental crustal growth that have been taking place along this boundary since the late Cretaceous and during the oblique collision between the South American plate and the Caribbean plate.

We present results from one of the 5 main multi-channel marine reflection transects, trending approximately N-S at the longitude of 67.5°W. The ~400 km long profile extends from latitude 10.5°N to 14.5°N, crossing from south to north the E-W trending Oca-Moron-El Pilar Fault strike slip system, the Bonaire basin, the Aves Ridge, the South Caribbean deformed belt and the southern edge of the Venezuela basin. Along the same transect wide angle seismic data were recorded by 42 Ocean Bottom Seismometers and by 550 Reftex Texans deployed at 329 sites onshore (see Zelt et al., this session).

The high-quality marine reflection profile images with great clarity all the main geologic features of this area. In the south we have imaged a crossing of the Moron Fault, a segment of the continental strike-slip system boundary. The geometry of the sediments in the upper portion of the profile suggests modern deformation along the fault. Compressive structures, likely related to the movement along the Moron fault, are also visible in Paleogene sediments ~40 km north of the fault trace. The Bonaire basin is characterized by Paleogene rift structures buried under 6 km of Cenozoic sediments. The platform of the Aves ridge separates the Bonaire basin to the south from the Los Roques basin to the north. This narrow and relatively deep basin filled with ~6 km of sediments is bounded to the north by compressive features of the Cenozoic accretionary wedge of the Southern Caribbean Deformed Belt. The 75 km wide belt has a typical mélange seismic signature with the thrust fault geometry difficult to identify. A bottom simulating reflector (BSR) is traceable along the top of the northern portion of this feature. To the north, the accretionary wedge overrides the basement of the Caribbean plate and the sediments of the Venezuelan basin. The subducting plate is visible 25 km south of the frontal thrust of the accretionary prism. The only deep reflectors that we interpret as Moho along this profile are observed at the base of this plate at 10-11.5 s and they define a crustal thickness for the Caribbean plate of ~6.5 km suggesting that the plate consists of normal oceanic crust.