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BAHLBURG, HEINRICH, Department of Geosciences, Pennsylvania State University, University Park, PA

ABSTRACT: Paleogeography and Geodynamic Evolution of Early Paleozoic Marginal Basins in the Southern Central Andes of Northwest Argentina and North Chile

In the Andes of Argentina and Chile, the mid-Cambrian Pampean Orogeny marked the collision between eastern North America and western South America during the assemblage of the Late Proterozoic-Early Cambrian supercontinent. Its subsequent dispersal started here in the Late Cambrian with the formation of a west-facing passive margin, recorded in the Cordillera Oriental by tidally influenced quartz-sandstones and shales. Ongoing extension is marked by a Latest Cambrian erosional unconformity (sequence boundary), above which the sea had transgressed eastward by the beginning of the Ordovician. Restricted to the Early Ordovician, a west-facing volcanic arc was active to the west in North Chile. While shallow marine clastic sedimentation continued on the platform in the east, thick volcano lastic apron deposits formed in the back-arc basin. At the Early-Middle Ordovician transition, the extinct arc was thrust eastward over part of its

back-arc basin causing tectonic subsidence peaking at 600 m/Ma. Volcanoclastic turbidite deposition continued during the Middle Ordovician in a foreland basin, whereas formation of a flexural bulge led to emergence of the eastern platform. Modeling indicates that the underthrust lithosphere had a flexural rigidity of 1023 Nm (equivalent elastic thickness: 25 km). Elevations of the thrust terrane reached 3 km above sea-level, pointing to high relief. This coincides with the compositional immaturity, and facies patterns of the clastic units. Numerical experiments support interpretations that the observed cyclicity of the turbidite units formed in response to tectonic events. The described basins offer no prospect for petroleum exploration as the units are overmature due to the overprint by various magmatic cycles.

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