Sedimentary and Geomorphic Record of Crustal Shortening, Thickening, and Surface Uplift in the Eastern Cordillera of Bolivia

A variety of sedimentary basins and preserved geomorphic surfaces chronicle the Cenozoic history of subduction-related deformation in the central Andes. In Bolivia (the region of maximum horizontal shortening, crustal thickening, and surface uplift in the Andes), most recent studies indicate that the crust has thickened principally by east-west shortening. Assuming Airy isostatic balance, which applies to most of the modern Altiplano and Eastern Cordillera, the geologic record of shortening should serve as a proxy for surface elevation through time. The structural histories of Cenozoic sedimentary basins distributed across the Eastern Cordillera of Bolivia between 17° and 20° S clearly indicate shortening from roughly 40 Ma to 20 Ma, with limited shortening thereafter, suggesting most surface uplift in the Eastern Cordillera prior to 20 Ma. This conclusion, however, is in direct conflict with paleobotanical estimates of Miocene paleoelevation. The problem may be addressed, in part, through stratigraphic, sedimentologic, and structural analyses of three distinctive categories of sedimentary basins across the Eastern Cordillera: Eocene-Oligocene foreland basins; early Miocene intermontane basins; and late Miocene-Pliocene basin fill capped by subhorizontal geomorphic surfaces. Whereas foreland basin deposits represent syn-contractional sedimentation at low elevation, the other stratigraphic packages represent largely post-contractional accumulation at higher elevations. Considerations of the stratigraphic ages, depositional systems, and structural settings within the context of regional kinematic reconstructions provide a template for evaluating the temporal variations in surface elevation. Most significant are sedimentologic and structural similarities between early Miocene and late Miocene-Pliocene basin-fill packages, suggesting limited surface uplift in the Eastern Cordillera during this time frame, consistent with paleoelevation predictions based on the shortening record.