EROSION, CLIMATE, AND EARLY GROWTH OF THE CENTRAL ANDEAN PLATEAU, NORTHERN BOLIVIA: INSIGHTS FROM LOW-TEMPERATURE THERMOCHRONOLOGY

Orogenic plateaus are important topographic features that are invoked to force global climate, terrestrial sediment fluxes, and ocean chemistry. Quantifying the evolution of plateaus has been hampered by limited age constraints on their deformation and erosion histories. We use low-temperature thermochronology and regional climate data to constrain the erosion history of the Andean plateau margin between 15-16ºS. New apatite fission track (AFT), zircon (U-Th)/He (ZHe), and existing zircon fission track (ZFT) cooling ages were measured from igneous and meta-sedimentary bedrock samples exposed along a 200 km across-strike transect of the Bolivian thrust belt. Fission track ages are interpreted by combining a grain-age deconvolution algorithm with inverse modeling of track lengths to delimit possible sample cooling histories. Interpretations focus on the youngest cooling component recorded in each sample associated with the most recent exhumation related to erosion and deformation in the thrust belt.

We group observations and interpretations by region from the plateau margin eastward. At the margin, ZFT data records cooling from ~25-50 Ma, ZHe records cooling from 16-33 Ma, and AFT records cooling from ~5-20 Ma. In the Interandean zone, AFT data records cooling from ~5-40 Ma. In the Subandes, AFT ages record mostly 4-19 Ma cooling, with 11-40 Ma cooling along its westernmost margin. Subandean ZHe ages record pre-Andean cooling and provide an important limit to the erosion magnitude. Mean annual precipitation rapidly increases eastward from <0.5 m/yr on the plateau to >4 m/yr on the frontal structures of the Subandes.

Taken together, the previous spatial variations in cooling ages and precipitation suggest (1) initial erosion associated with deformation migrated eastward from the plateau from 5-50 Ma, (2) the entire region experienced distributed erosion from 5-20 Ma, and (3) the magnitude of erosion decreases eastward from the plateau margin to the foreland. We infer initial plateau development equivalent to its modern width by 20 Ma, correlated with basement megathrust emplacement. Furthermore, estimated erosion magnitudes and precipitation are anti-correlated, suggesting a week feedback between climate and tectonics.