IMPLICATIONS FOR CENOZOIC EXHUMATION OF THE CENTRAL ANDEAN PLATEAU BASED ON THERMOCHRONOLOGY, GEOCHRONOLOGY AND UPPER CRUSTAL STRUCTURE OF THE CORDILLERA REAL, BOLIVIA

The type example of a subduction-related noncollisional mountain belt, the Andes represent the largest active orogenic-plateau system after the Himalayan-Tibetan orogen. Despite their global tectonic significance, the timing, tempo, and manner of mountain building remains poorly understood. This study addresses these uncertainties through structural geologic mapping, thermochronology, and geochronology in the Cordillera Real and Quimsa Cruz ranges in the Eastern Cordillera of Bolivia, and sedimentary provenance analysis of adjacent basin deposits. New zircon U-Pb, ⁴⁰Ar/³⁹Ar, and apatite fission track results for granites along three mapped transects provide a framework for interpreting the structural and cooling history of the high elevation (>4-6 km) central Andean backthrust belt. Cross-cutting relationships, composite mineral cooling histories, and growth stratal deposits sourced from the thrust belt provide constraints on timing, rates, and mechanisms of exhumation. These data indicate crustal shortening across the backthrust system prior to 26 Ma. Cooling ages record two distinct episodes of rapid exhumation from 45-40 to 26 Ma and from ~11 Ma onward. These findings suggest that crustal shortening played a principal role in middle Eocene-late Oligocene exhumation of the central Andes. However, for late Miocene exhumation, the importance of alternative, nonshortening mechanisms is difficult to ascertain due to a poor understanding of subsurface structures. We speculate that greater precipitation impinging on the eastern edge of the central Andean plateau north of ~17.5° S was a major factor in driving rapid, youthful exhumation of the Cordillera Real.