HIGH-LEVEL SURFACES, PLATEAU UPLIFT, AND FORELAND DEVELOPMENT, BOLIVIAN CENTRAL ANDES

Newly dated Tertiary strata in the Bolivian central Andean plateau and synthesis of the Tertiary record in the adjacent Subandean fold-thrus t belt constrain the age of deformation in both regions. Age of deform ation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high-level surface known as th e San Juan del Oro surface. New Ar-40-Ar-39 dates on undeformed strata above the high-level surface preclude significant upper-crustal short ening within the Eastern Cordillera after 10 Ma. Tertiary strata withi n the adjacent Subandean region demonstrate that formation of the fold -thrust belt occurred after 10 Ma. On the basis of these data, we prop ose a two-stage model of late Cenozoic Andean growth that links platea u uplift to the development of the fold-thrust belt. In the first stag e, early plateau uplift occurred in response to widespread compression al deformation of the plateau (Eastern Cordillera and Altiplano). Duri ng the second stage, beginning after 10 Ma, upper-crustal deformation within the plateau terminated, and the Subandean fold-thrust belt deve loped. Crustal-scale eastward thrusting along the eastern margin of th e Eastern Cordillera drove Subandean folding and thrusting; the Easter n Cordillera served as the ''bulldozer'' for the deforming Subandean w edge.