The Southeast Caribbean Plate Boundary: New Insights from the Bolivar Project

Prof. Gary Pavlis
Indiana University

Abstract:
The modern Caribbean and South American plates have a relative motion of approximately 20 mm per year with the Caribbean translating eastward relative to South America. The plate boundary is defined by diffuse deformation in an east-west mountain belt in northern Venezuela extending as far east as Trinidad. To the west the deformation splits into a V-shaped pair of mountain belts that form the edges of the Maracaibo Block. The Maracaibo Block is translating northward from South America at approximately 10 mm/yr. The western edge of the Maracaibo Block overlies the subduction zones linked to the Nazca plate in Columbia. This complicated tectonic region has been the focus of a recent continental dynamics project called BOLIVAR (Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region). The Bolivar project included active source seismic, passive array seismic, and a series of geologic investigations focused on blueschists found at various locales along the plate boundary. I will provide a brief review of related work by others in the project, but the bulk of the presentation will focus on recent results from the passive array study. Receiver functions have been used to estimate crustal thickness throughout the region. Crustal thickening is observed in northern Venezuela beneath the mountain belts. The crust thins offshore into anomalously thick crust of the Caribbean. Shear-wave splitting resuls show predominately east-west fast directions. The magnitudes of observed splitting times in an area centered on Margarita Island are some of the largest ever observed. This combined with results or teleseismic P wave tomography, and structures inferred from onshore-offshore refraction data confirm a model of hinge faulting originally proposed in 1969 by Molnar and Sykes. These data are consistent wih a model of eastward rollback of the Antilles arc that has formed a vertical, strongly anisotropic shear zone in the upper mantle at the northern boundary of South America.