Origin of the high plateau in the central Andes, Bolivia, South America

Abstract

The Bolivian Altiplano, in the Central Andes of South America, is part of the second largest high plateau on Earth. It is an elongate region of subdued relief, ∼1.2 × 105 km2 and ∼4 km above sea level, bounded by the Eastern Cordillera and volcanic arc (Western Cordillera). Here the crust is up to ∼75 km thick. We describe the Cenozoic geological evolution of this region, using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure. Crustal shortening and magmatic addition and, locally, sedimentation are the main mechanisms of Cenozoic crustal thickening, leading to nearly 4 km of surface uplift since the Paleocene. Addition of mafic melts appears to be a first‐order mechanism of Cenozoic crustal growth, contributing ∼40% of the crustal thickening beneath the volcanic arc. Removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind‐arc mafic volcanism, at ∼23 Ma and 0 – ∼5 Ma, contributing to the surface uplift. The Altiplano originated as a sedimentary basin, several hundred kilometers wide, between the proto‐Western Cordillera and a narrow zone of uplift (proto‐Eastern Cordillera) farther east. The latter zone formed by inversion of the center of a wide lacustrine or marine Cretaceous ‐ Paleocene basin close to sea‐level at ∼45 Ma. A thickness of 2–4 km of Paleogene continental elastics accumulated in the proto‐Altiplano basin. Subsequently, in the Oligocene, we estimate that this region and the western margin of the Eastern Cordillera were technically shortened ∼22% (∼65 km), resulting in ∼9 km of average crustal thickening. The Altiplano basin was rejuvenated at ∼25 Ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting Eastern and Western Cordilleras. Between ∼25 and 5 Ma, folding and thrusting in the western margin of the Eastern Cordillera migrated westward into the center of the Altiplano basin, essentially terminating deposition, except in local subbasins, and accommodating ∼13% (∼30 km) of shortening and an estimated ∼7 km of average crustal thickening. Subsequently, there has been strike‐slip deformation and limited local thrusting (< 5 km of shortening). Geomorphological and geochronological evidence for 1.5–2 km of surface uplift of this region since the Late Miocene suggests ∼14 km of lower crustal thickening beneath an essentially rigid “lid”, and can be explained by ∼100–150 km of underthrusting of the Brazilian shield and adjacent regions beneath the eastern margin of the Central Andes. The present subdued relief in the Altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin.