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GUBBELS, T. L., and B. L. ISACKS, Cornell University, Department of Geological Sciences, Ithaca, NY, and R. W. KOCH, Chevron Overseas Petroleum Inc., San Ramon, CA

ABSTRACT: A New Tectonic Model for the Development of the Eastern Cordillera, Altiplano, and Subandean Zones, Bolivian Central Andes, 20 degrees S Latitude

Construction of a regional transect across the central Andes at 20 degrees S sheds new light on the relationship between the Altiplano, Eastern Cordillera (EC), and Subandean zones and allows us to refine the two-stage model of Isacks (1988) for the growth of the Central Andes. This new model is based on examination of the

regional geology and geophysics, coupled with field investigations, satellite image analysis, and new Ar-Ar geochronology.

In this model, widespread Oligocene to mid-Miocene compressional deformation in the Altiplano and EC was followed in the late-Miocene and Pliocene by thrusting localized east of the EC within the Subandean fold-thrust belt. During the first stage of deformation, the Altiplano basin underwent important subsidence and internal deformation. The EC was both deformed internally and thrust westwards over the Altiplano basin, while the present Subandean zone was the site of an early, broad foreland basin which received material eroded from the EC. During the second stage, beginning at (approx.) 10 ma, deformation terminated within the EC and became concentrated within the fold-thrust belt in response to large scale overthrusting of the EC above the Brazilian shield; this resulted in major th usting along the Cabalgamiento Frontal Principal (CFP), which soles into the master Subandean decollement, and (approx.) 100 km of telescoping within the early, broad foreland basin. In the EC, this second stage is marked by the elaboration of a regionally extensive erosion surface, ponding of gravels in shallow basins, and the emplacement of giant ignimbrite sheets. The Eastern Cordillera can thus be thought of as a crustal-scale wedge which has been extruded upward and outward on alternate sides during successive stages of late Cenozoic deformation. This motion has served to drive subsidence in both the Altiplano and Subandean foreland basins, as well as shortening in the fold-thrust belt.

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