UNSTEADY EXHUMATION OF THE EASTERN MARGIN OF THE CENTRAL ANDEAN PLATEAU, NORTHERN BOLIVIA

The Cordillera Real (4-6.4 km elevation) in northern Bolivia defines the eastern topographic margin of the central Andean plateau and the structural boundary between the Altiplano and Eastern Cordillera. As the principal drainage divide, the range has influenced orographic precipitation, regional climate, and geomorphic evolution of the plateau. New mapping, basin analysis, and ⁴⁰Ar/³⁹Ar, fission track, and U-Pb thermo/geochronology reveal rapid cooling during middle Eocene to late Oligocene and late Miocene to Pliocene exhumation. In the NW trending Cordillera Real, numerous granites and SW directed thrust structures define the central Andean backthrust belt. U-Pb zircon analyses indicate Triassic and late Oligocene granitic magmatism. Mapping reveals low magnitudes of slip (<2-5 km) for most faults on the basis of unit thicknesses, stratigraphic separation, and cutoff relationships. These results suggest that a deeper structure was responsible for exhumation of rocks from >5 km depth. The 26 Ma Quimsa Cruz granite postdates most structures, suggesting that upper crustal shortening in the region was complete by late Oligocene time. Results of ⁴⁰Ar/³⁹Ar and fission track modeling reveal two phases of rapid cooling from 45-40 to ~26 Ma and from ~11 Ma onward. Initial cooling coincided with middle Eocene to late Oligocene deformation in the backthrust belt and associated deposition of coarse clastic sediments in growth structures of the Luribay-Salla Formation. Eocene-Oligocene exhumation of ~7.5 km of upper crust is estimated on the basis of thermochronologic data. Rapid late Miocene and younger cooling involved ~3.5 km of exhumation and occurred in the apparent absence of upper crustal shortening. These findings suggest that crustal shortening and resulting exhumation of middle Eocene–late Oligocene age played a major role in construction of the central Andean plateau. The importance of alternative, nonshortening mechanisms during late Miocene exhumation is difficult to ascertain due to a poor understanding of subsurface structures. We speculate that greater precipitation on the eastern plateau margin north of ~17.5ºS helped induce rapid, youthful exhumation.