Structural Development of the Bolivian Orocline and Implications for Evolution of the Altiplano Plateau

Abstract

In the central Andes of Bolivia, tectonic shortening due to subduction of the Nazca plate below South America has compressed the crust below the Altiplano plateau to thicknesses over 65-70 km, some of the highest on the planet. The modern ~4 km elevation of the Altiplano and ~6 km peak elevations in the Bolivian Andes are proposed to have developed gradually as a response to crustal thickening since ~50 Ma. However, paleoelevation estimates from Altiplano carbonates suggest that high elevations may have developed as recently as ~20-10 Ma. Rapid, recent uplift is proposed to result from the removal of thickened mafic lower crust and mantle lithosphere. In this dissertation, I present new structural data from the Bolivian Andes integrated with existing timing and deformation constraints to model the development of the cordillera and Altiplano plateau. I reconstruct deformation since 50 Ma to track the evolution of crustal thickening and independently determine viable locations, times, and magnitudes of lower crustal loss.

Deformation at the Bolivian Andes is complicated by the curvature of the mountain belt. Referred to as the Bolivian orocline, the curvature is the combined result of thrust-faulting, strike-slip displacements parallel to orogenic trend, and vertical axis rotations. Presented here is new detailed geologic mapping, thermochronology, balanced cross sections, and strain data from Bolivia that fills gaps in data coverage. Mapping revealed the exact location and offset of strike-slip faults that partially accommodate the development of orogenic curvature. Restoring deformation based on these constraints predicts ~13° regional rotations in the orocline limbs and orogen parallel displacements up to 70 km on strike-slip faults. Using the reconstructed displacement field to forward model crustal thickening predicts a 5-8% volume excess compared to modern crustal structure. However, the timing and magnitude of volume excess is insufficient to account for regional removal of >10 km of lower crust from below the entire plateau at 10-6 Ma as proposed by paleoelevation proxies. Localized lower crustal loss of < 5 km is possible in northern Bolivia by <20 Ma but excess volume does not develop in southern Bolivia until <5 Ma.