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

Abstract

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 ∼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 of40Ar/39Ar and fission track modeling help constrain the moderate to low‐temperature (<350°C) cooling history, revealing two phases of rapid cooling from 45–40 Ma to 26 Ma and from ∼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 ∼7.5 km of upper crust is estimated on the basis of thermochronologic data. Rapid late Miocene and younger cooling involved an estimated ∼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 ∼17.5°S was a key factor in driving rapid, youthful exhumation of the Cordillera Real.