ALLMENDINGER, R. W., and T. L. GUBBELS, Department of Geological Science, Cornell University, Ithaca, NY

ABSTRACT: Controls on Andean Thrust Belt Development, 15 to 30 degrees South

The Andean foreland thrust belt would appear to be the perfect test locality for models of climatic control on thrust belt geometry and evolution. To evaluate this possibility, we examine three transects across the thrust belt: the Subandean belt at (approx.) 15 degrees and 20 degrees S, and the Precordillera at 30 degrees S, where current precipitation is 1000 to 2400 mm/yr, 400 to 800 mm/yr, and 200 mm/yr, respectively.

Thrust belt width is directly correlated with average topographic slope and shortening. The northern Subandean belt and the Precordillera are both narrow (70 and 50 km), have high surface slopes of 2.5 to 3.5 degrees, and have net shortening >60% whereas the intervening southern Subandean belt is 90 to 110 km wide, has a slope of 0.5 to 1 degrees, and <50% shortening. Additional non-thrust belt shortening in the north occurs as major strike-slip faults and in the south as basement uplifts. The formation of the Altiplano plateau west of the Subandean thrust belt required both Neogene and Quaternary magmatism and large magnitude shortening; neither alone would be sufficient to form the Altiplano. Second order plateau width variations may be climatically controlled.

There are two significant controls on thrust belt geometry: the pre-existing basin architecture (i.e. width, thickness, number of competent units, pre-existing structures) and the Andean plate setting (arc magmatism, oblique shortening and strike-slip faulting, and basement uplifts). The climatic influence is most clear

in drainage and denudation patterns which determine the locus of sediment redistribution (i.e. piggyback basins, provenance of foreland basin fill), but otherwise climate remains an elusive control.

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