NEW CONSTRAINTS ON THE TEMPORAL AND SPATIAL EVOLUTION OF THE CENTRAL ANDEAN HUARINA BACKTHRUST BELT SOUTH OF LA PAZ, BOLIVIA

The west-vergent Huarina fold-thrust belt in the central Andes of Bolivia forms the boundary between the Altiplano plateau, with an average elevation of ~4 km, and the high peaks (>6 km) of the Eastern Cordillera. A better understanding of the evolution of this fold-thrust belt can provide important insights into how and when the Eastern Cordillera and Altiplano tectonic provinces were created. New geologic mapping in the Salla region, ~100 km south of La Paz, reveals previously undocumented structures in the Huarina fold-thrust belt and allows for a more detailed kinematic reconstruction of upper crustal shortening. The study area is characterized by a series of NNW-SSE trending folds and east- and west-vergent thrust faults. Extensive folding and faulting of pre-Eocene strata coupled with the presence of growth strata within the Luribay Conglomerate indicate significant deformation occurred during Late Eocene-Oligocene time. The Salla Formation (29-24 Ma), which has been interpreted as post-dating tectonic activity in the region, is in fact strongly folded in areas and cut by numerous thrust faults. Deformation within the Huarina fold-thrust belt was associated with eastward displacement of a basement thrust sheet, which carried Paleozoic and Mesozoic cover rocks over a west-dipping ramp. These relationships, and information from newly constructed cross-sections, indicate the majority of crustal shortening in the Huarina fold-thrust belt occurred during Eocene-Oligocene time, with the waning stages of deformation having continued through Early Miocene time. If Airy isostatic balance was maintained, the majority of surface uplift in the region should have taken place before 20 Ma.