Late Cenozoic uplift of the Eastern Cordillera, Bolivian Andes

Abstract

This paper analyses Late Cenozoic uplift in the Bolivian Andes, using the morphology of well preserved regional paleosurfaces in the Eastern Cordillera that define three axially draining braided river catchments that formed between ∼ 12 and ∼ 9 Ma. Rock uplift since the formation of the paleodrainage systems, which has been quantified using four different methods, is 1705 ± 695 m, with a mean erosion of 230 ± 90 m as a consequence of entrenchment of the drainage systems. The lack of faulting or tilting in the regions immediately farther west strongly suggest that rock uplift of the paleodrainage systems also extends to the western margin of the Eastern Cordillera. Balanced structural cross-sections require crustal deformation at depth beneath the Eastern Cordillera, in order to accommodate underthrusting of the Brazilian Shield beneath the thin-skinned fold and thrust belt on the eastern margin of the Bolivian Andes. In this case, the observed uplift of the Eastern Cordillera is easily explained by sliding up a ramp in the major decollement, dipping in the range 4°–16°W, as a consequence of the observed 60–110 km of shortening farther east, in the Subandean zone. Contemporaneous uplift of the western margin of the Eastern Cordillera is easily explained in terms of crustal thickening as a result of ductile squeezing in the lower crust accommodating at depth the Subandean shortening. It remains unclear how this uplift relates to that of the regions farther west, in the Altiplano and volcanic arc, except that uplift in the Eastern Cordillera coincides with a phase of intense shortening in the northern Altiplano, commencing at ∼ 9.5 Ma and continuing to ∼ 2.7 Ma and possibly younger.