When did the Altiplano Form? A Coupled Thermochronometer and Numerical Model Test

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Understanding the timing, rates, and deformation associated with mountain belt plateau formation is central to quantifying relationships between plateaus, plate tectonics, and climate. Computer models of plateau development invoke variations in crustal temperatures and strength as the main mechanism of plateau formation. Strength variations are important because as tectonic processes shorten and thicken the crust the lower crust weakens and flows laterally, preferentially supporting plateau formation over a narrow mountain belt. However, tests of these computer models are hampered by a lack of knowledge about the kinematic history of plateau formation. This project is using rock cooling histories, field-constrained structural analysis, and computer models to delineate the tectonic evolution of the Bolivian fold and thrust belt to infer the formation process and surface uplift chronology of the Andean Plateau.Apatite fission track (AFT) and zircon (U-Th)/He (ZHe) ages are being collected and analyzed along two transects in the Bolivian fold and thrust belt to determine rock cooling histories. The AFT and ZHe data are being used to define the timing and rates of uplift along each sampling transect. Coupled two dimensional thermal, tectonic, and erosion computer models are being used to quantify the history of the Bolivian fold and thrust belt by predicting cooling ages and comparing them with the AFT and ZHe data. The coupled computer models aid in the interpretation of data through several steps: (1) the tectonic history of the thrust belt is prescribed using the 2DMove software, restored structural cross-sections, and geologic maps along both profiles, (2) the tectonic model then drives advective heat transfer in the thermal model and surface uplift in the erosion model, (3) for each tectonic and erosion history simulated across the fold and thrust belt the thermochronometer ages exposed at the surface are computed using apatite fission track annealing and zircon He diffusion algorithms. Model predicted and observed ages are compared with statistical methods to identify the range of permissible tectonic and erosion histories for Andean Plateau formation and surface uplift. The data and computer models are being integrated to quantify the timing, rate, and duration of motion on each thrust fault across each sampling transect. Combining the deformation and erosion history of all the faults across each transect will provide constraints on (1) the timing and history of Andean Plateau formation, and (2) temperature and strength dependent computer models of plateau formation.