High-altitude palaeosurfaces in the Bolivian Andes: evidence for late Cenozoic surface uplift

Abstract

Abstract Low relief palaeosurfaces are widely preserved in the Cordillera Oríental of the Bolivian Andes at altitudes between 2000 m and 4000 m in a region up to 100 km wide and 600 km long. Remnants of these nearly horizontal or gently undulating erosion surfaces, from 5 to 2000 km 2 in area, cut across folded Palaeozoic to Cenozoic bedrock. They appear to be erosional pediments formed between 12 and 3 Ma along broad, low gradient, valleys, which are generally undeformed. In detail, the morphology of surface remnants can be used to construct two distinct palaeodrainage basins. Very little of the material eroded during surface cutting appears to have remained within the Cordillera Oriental and must have been carried into a foreland basin, now preserved in the Subandean fold and thrust belt. The palaeovalleys are generally parallel to the dominant north-south structural grain, forming a tortuous low gradient drainage system which flowed into the foreland basin in an east-west distance of < 150 km. Reconstructions of the drainage systems suggest that they have been uplifted c. 2 km since their formation. These estimates are consistent with plausible models of uplift of the Cordillera Oriental as a consequence of intense deformation in the Subandean fold and thrust belt since c. 10 Ma. However, only since 3 Ma have the surfaces been deeply dissected by drainage cutting more directly across the structural grain. The timing of deep dissection may be partly related to a climate change to wetter and colder conditions.