Tectonic control of topography, rainfall patterns, and erosion during rapid post-12 Ma uplift of the Bolivian Andes

We integrate analysis of present-day topography with a synthesis of current knowledge of the geology, deformation history, exhumation history, and the pattern of erosion rates to address the controversies surrounding the surface uplift history of the Bolivian Andes and the relative roles of climate and tectonics in the evolution of this mountainous landscape. Using metrics of channel steepness (k_sn, a measure of channel slope normalized by drainage area), local relief (over a 2.5 km radius), and hillslope gradient we identify and map a suite of previously unrecognized perched, low-relief upland landscape patches in northern Bolivia that define a long-wavelength (~300 km) topographic ramp with an ~3.5 km elevation drop from SE to NW. We interpret these low-relief patches as the remnants of a formerly continuous lowrelief landscape formed on grade with the foreland that has been uplifted and warped since formation. The 11-7 Ma Cangalli Formation onlaps the northern end of this ramp and suggests a shared history and common baselevel with the well-known ca. 12-9 Ma San Juan del Oro erosion surface in southern Bolivia. Patterns of rock and surface uplift rate implied by this interpretation are consistent with those inferred independently from analysis of channel steepness (k_sn), the distribution of fluvial hanging valleys, reconstruction of channel profiles, and the distribution of published low temperature thermochronometric ages. These data reinforce earlier interpretations for 2-3 km surface uplift in the Bolivian Andes in the last 12 Ma. The marked contrast in topography across the Santa Cruz bend (~18°S) appears largely controlled by differences in tectonics conditioned by inherited geologic contrasts and we show that post-12 Ma erosion and exhumation in northern Bolivia are controlled primarily by tectonics, not climate. However, the pattern and style of uplift in many locations suggests warping over deep structures in absence of significant shortening, consistent with the observation that exhumation patterns cease to be coupled to patterns of contractile deformation post-11-15 Ma. Tectonically controlled topography strongly focuses rainfall and may enhance erosional efficiency and thus erosion rates in zones of high rainfall, but no data from this study area demands this. Moreover, we find no evidence for a tectonic response to the modern rainfall pattern. We speculate that differences in the pattern of deformation and uplift across the Santa Cruz bend appear to be accommodated by a crustal-scale relay ramp.