Abstract

Recent studies show that the western margin of the Andean plateau was uplifted during Neogene time in the form of a proto-Western Cordillera through a system of high angle reverse faults. In this study, we use XRD and microprobe data of heavy minerals (HM) contained in Neogene sandstones of the Central Depression and Copaquilla Depression, to the west of the Andean plateau, to better understand the paleogeographical evolution along this border. The composition of detrital metamorphic HM assemblages, e.g. garnet-staurolite and actinolite-epidote, found in the Neogene sandstones, correlates well with that of the HM in the basement. Based on this correlation, we outline the different stages of exhumation and surface exposure of the metamorphic and crystalline basement rocks that crop out along the eastern part of the Copaquilla Depression (Belén area). In addition, the vertical stratigraphic distribution of detrital volcanic HM assemblages, e.g. olivine-pyroxenes and amphibole-biotite, records in detail the evolution of the Neogene volcanism, showing the stages in which volcanics in the Precordillera covered the basement. A first stage of basement exposure took place before 26 Ma. This was followed by Early to Late Miocene (ca. 26-10 Ma) intermediate to andesite-basalt volcanism which led to basement burial. Later on, ca. 10-2.7 Ma, a deep incision of the Precordillera associated with a generalized uplift of the plateau exposed the basement again. © 2006 Elsevier B.V. All rights reserved.