TECTONICS OF THE WESTERN MARGIN OF THE ALTIPLANO IN SOUTHERN PERU

The Cotahuasi and Colca rivers of southern Peru have their headwaters on the Altiplano and carve two of the deepest canyons in the world as they flow down to the Pacific Ocean. By studying the incision histories of these canyons and the exhumation histories of bedrock exposed in them, we are working toward a better understanding of the late Cenozoic (post-14 Ma) uplift history of the western Altiplano.

The western margin of the Altiplano is a sharp physiographic transition that appears to have different structural manifestations at different places along strike. Following the terminology of previous workers, we refer to this transition as the Western Andean Escarpment (WAE). Where the Cotahuasi Canyon crosses the WAE (16°S; 71°W), it takes the form of a steep monocline; c. 14 Ma volcanic units are draped across the transition with no apparent fault disruption. (U-Th)/He apatite thermochronologic data from the bedrock exposures in the canyon indicate > 2 km of incision since c. 8 Ma. We have mapped numerous perched lava flows at various elevations within the canyon and are conducting high-precision Ar/Ar dating studies of these flows in order to reconstruct the incision history of the Cotahuasi River; ostensibly, this history will serve as a valuable proxy for the uplift history of the western Altiplano at ~16°S.

Seventy-km to the southeast, where the Colca Canyon transects the WAE, the physiographic transition is marked by a steep (60-70°) normal fault system dipping toward the ocean. Based on the Ar/Ar ages of volcanic units that are cut by and that drape over this fault system, the latest stages of normal faulting along this part of the Altiplano margin occurred between 0.2 and 14 Ma, but studies are underway to "bracket" the age of faulting more precisely. We speculate that, in the regions near 16°S, uplift of the Altiplano relative to regions farther toward the coast was accommodated by a complex pattern of faults and subordinate folds as the WAE makes the transition from normal faulting near Colca Canyon to monoclinal folding near Cotahuasi Canyon. This hypothesis will be tested in the course of future mapping.