Thermochronology, geochronology, and upper crustal structure of the Cordillera Real: Implications for Cenozoic exhumation of the central Andean plateau

[1] Structural mapping, 40Ar/39Ar and fission track thermochronology, U-Pb geochronology, and basin analysis reveal rapid cooling during middle Eocene-late Oligocene and late Miocene-Pliocene exhumation in the central Andean plateau of Bolivia. In the 4-6 km high Cordillera Real, numerous granites and SW directed fold-thrust structures define the central Andean backthrust belt along the Altiplano-Eastern Cordillera boundary. U-Pb zircon analyses indicate Permo-Triassic granitic magmatism, with less extensive magmatism of late Oligocene age. 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 probably involved in exhumation of rocks from >5 km depth. The similar to 26 Ma Quimsa Cruz granite postdated most thrust structures, suggesting that upper crustal shortening in the Cordillera Real had largely ceased by late Oligocene time. Results of 40Ar/39Ar and fission track modeling help constrain the moderate to low-temperature (<350 degrees C) cooling history, revealing two phases of rapid cooling from 45-40 Ma to 26 Ma and from similar to 11 Ma onward. Initial cooling coincided with middle Eocene-late Oligocene deformation in the backthrust belt and associated deposition of coarse clastic sediments in the Altiplano basin. Eocene-Oligocene exhumation of similar to 7.5 km of upper crust is estimated on the basis of thermochronologic data. Rapid late Miocene and younger cooling involved an estimated similar to 3.5 km of exhumation and occurred in the apparent absence of upper crustal shortening. These findings suggest that crustal shortening and resultant exhumation of middle Eocene-late Oligocene age played a major role in construction 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 on the eastern edge of the central Andean plateau north of similar to 17.5 degrees S was a key factor in driving rapid, youthful exhumation of the Cordillera Real.