Abstract

Newly dated Tertiary strata in the Bolivian central Andean plateau and synthesis of the Tertiary record in the adjacent Subandean fold-thrust belt constrain the age of deformation in both regions. Age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high-level surface known as the San Juan del Oro surface. New 40Ar-39Ar dates on undeformed strata above the high-level surface preclude significant upper-crustal shortening within the Eastern Cordillera after 10 Ma. Tertiary strata within the adjacent Subandean region demonstrate that formation of the fold-thrust belt occurred after 10 Ma. On the basis of these data, we propose a two-stage model of late Cenozoic Andean growth that links plateau uplift to the development of the fold-thrust belt. In the first stage, early plateau uplift occurred in response to widespread compressional deformation of the plateau (Eastern Cordillera and Altiplano). During the second stage, beginning after 10 Ma, upper-crustal deformation within the plateau terminated, and the Subandean fold-thrust belt developed. Crustal-scale eastward thrusting along the eastern margin of the Eastern Cordillera drove Subandean folding and thrusting; the Eastern Cordillera served as the "bulldozer" for the deforming Subandean wedge.

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