EXPLORING THE RELATIONSHIP BETWEEN UPPER CRUSTAL DEFORMATION, SEDIMENTATION, AND SURFACE UPLIFT IN THE ALTIPLANO OF THE CENTRAL ANDES, BOLIVIA

The west-vergent Huarina backthrust belt in the Central Andes of Bolivia extends along the western margin of the Eastern Cordillera and forms the eastern margin of the ~ 4km high Altiplano Plateau. The timing and magnitude of deformation in this region have important implications for understanding the relationship between upper crustal deformation and surface uplift in orogenic belts. Here we present new structural, stratigraphic, and geochronologic data that help constrain the deformational and depositional history of the area. The Puchuni region of Bolivia is located ~100 km south of La Paz and is characterized by west-vergent folds and thrust faults, some newly discovered as a result of this study. These structures involve Silurian through Cretaceous strata and have detachment horizons in the Silurian Uncia Formation. Clastic strata of the Cenozoic-age Salla Beds overlie these structures and infill mini-basins that form in structural depressions. Thrust-faults that cross-cut the Salla Beds are largely east-vergent, whereas most of those that pre-date the Salla Beds are west-vergent. The majority of the region is overlain by an undeformed succession of clastic strata, which we hereby name the Puchuni Beds. Detrital zircon ages from the Salla Beds yield a maximum depositional age of 25 Ma, whereas those of the Puchuni Beds yield a maximum depositional age of 2.5 Ma. Based on these data, we are able to reconstruct the deformational and depositional history of the region. Primary upper crustal deformation in the area was accommodated by west-vergent folds and thrust faults and occurred prior to 25 Ma. After 25 Ma, and following the majority of the deformation in the area, sediments of the Salla Beds infilled structural and paleotopographic lows. At ca. 11 Ma, the Salla Beds were folded into broad folds as preexisting faults were reactivated during an episode of minor deformation. Since ca. 2.5 Ma, sedimentation has resumed in the area and no deformation has occurred. These data demonstrate the majority of deformation in the region occurred prior to 25 Ma and suggests the region was topographically high by this time. However, paleoelevation data indicate the area did not attain high elevations until ca. 8 Ma, suggesting processes other than upper crustal deformation may have been important in the topographic evolution of this area.