A Synthesis of Denudation Rates in the Bolivian Andes: Tracking the Erosion Gradient in an Active Fold-Thrust Belt

Dramatic along strike contrasts in the tectonics, topography, and climate of the Bolivian Andes (14-22 S) have been used to argue the importance of erosion in the evolution of the fold-thrust belt. Even though erosion has been invoked as a controlling factor for the along-strike variability in the kinematic evolution of the fold-thrust belt, current denudation rates are sparse. New and existing material-flux and denudation-rate estimates were synthesized in an attempt to more quantitatively describe the spatial and temporal variations in erosion across the Bolivian fold-thrust belt. The variety of sources used to calculate these estimates include structural cross-sections and their restorations, thermochronology, foreland basin sediment volumes, basin fill, erosion surfaces, mass balance and DEM analysis, landslide mapping, and sediment-flux data. Material-flux rates derived from structural cross-sections and their restorations of the entire thrust-belt range from 32.4-56.4 km2/Ma in the north to 31.6-58.9 km2/Ma in the south over the last 40 Ma. Apatite and zircon cooling ages in the northern Eastern Cordillera yield exhumation rates of 0.1-0.75 mm/yr. Approximated exhumation rates from reported apatite cooling ages in the southern central Eastern Cordillera range from 0.09-0.17 mm/yr over the last 45-25 Ma. Material-flux rates from the Subandean portion of the fold-thrust belt averaged over the last 10-20 Ma range from 21.3-41.8 km2/Ma in the north to 12.8-28.8 km2/Ma in the south. Approximated exhumation rates from apatite cooling ages in the southern Interandean zone are 0.19-0.42 mm/yr over the last 10-22 Ma. In the south, material-flux and denudation-rate estimates from erosion surfaces, DEM analysis, and Tertiary basin fill range from 11-82 km2/Ma and from 0.13-0.2 mm/yr over the last 2-10 Ma. Modeled fluvial incision in the northern Eastern Cordillera predicts variable local rates of up to 5.0 mm/yr over the last 7.5 Ma. Short-term (last 2-35 years) denudation-rate averages from sediment load data and landslide mapping range from 1.0-8.0 mm/yr in the north to 0.3-0.4 mm/yr in the south. The longest-term averaged rates are similar across both northern and southern regions. In general, the northern estimates begin to exceed their southern counterparts starting at rates averaged over 10-20 Ma. The southern region rates remain similar in magnitude over all timescales, whereas the northern rates appear to increase to the present. The disparity may be a reflection of the biases in the methods or the feedbacks associated with uplift, climate, and erosion. These rates and their contrasts constrain geomorphic process rates in an active fold-thrust belt over a wide range of timescales that may provide calibration for models that simulate uplift-climate-erosion feedbacks in mountain building.