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VAN DER HILST, ROB, Leeds University, Department of Earth Sciences, Leeds, United Kingdom, and PAUL MANN,* University of Texas, Institute for Geophysics, Austin, TX

ABSTRACT: Tectonic Implications of Tomographic Imgaes of Subducted Lithosphere Beneath Northwestern South America

Tomographic images and deep seismicity of the mantle beneath northwestern South America support the existence of extensive and continuous subducted slabs which may be important controls on the neotectonic deformation of the overriding South America plate. Using tomographic data, we have estimated lateral continuity, regional dip, downdip length and approximate convergence direction of two, largely aseismic slabs. Tomographic data suggests that the Cauca slab, previously defined using earthquakes beneath southern Colombia, extends 300 km into the mantle beneath northern Colombia, has an average dip of 50 degrees, and is the downdip extension of Eocene? to Miocene Panama island arc and Oligocene-Miocene oceanic crust of the Nazca plate. The Maracaibo slab of northern Colombia and wester Venezuela extends up to 450 km into the mantle at an average angle of 16 degrees and is the downdip extension of Late Cretaceous oceanic plateau crust of the Caribbean Sea. The boundary between the two slabs is less than 50 km in width. The Maracaibo and northern Cauca slabs are generally aseismic and not associated with an overlying volcanic arc. Instead, both slabs are overlain by broad zones of diffuse active faulting and high topography in the northern Andes. The shallow dip, angle of oblique subduction and thickness of the Maracaibo slab may produce crustal deformation including northward tectonic escape of the Maracaibo block, a triangular wedge of continental crust in the overriding South America plate.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.