BASIN EVOLUTION AND ITS EFFECTS ON PREDICTED FORELAND BASIN THERMOCHRONOMETERS USING SEQUENTIAL FORWARD KINEMATIC AND THERMAL MODELS OF THE NORTHERN BOLIVIAN ANDES

Sequential kinematic and thermochronometric modeling indicates different exhumation magnitudes are necessary to reset the same thermochronometer based on the location of the sample in the fold-thrust-belt-foreland basin system. In this study a cross section through northern Bolivia was sequentially deformed in 10 km steps with each step taking onto account isostatic loading by thrust faults and subsequent erosion. This kinematic process links the growth of hinterland structures to the developing foreland basin (FB) adjacent to the fold-thrust belt (FTB). The modeling goal is to match the depth of the foreland and hinterland basins, geology present at the surface, the depth and angle of the decollement, and the shape of the modern observed topography. Once the model sufficiently duplicates geologic constraints, a grid of unique points is deformed with the model and used to determine displacement vectors for each shortening step. A velocity field is created by assigning an age to each displacement vector, and is used in a modified version of the advection diffusion modeling software Pecube to predict thermochronometer cooling histories across the modeled surface. This combined modeling method highlights the spatiotemporal aspects of sedimentary wedge propagation, identifies necessary external negative buoyancy effects, and reproduces depressed isotherms in the foreland and compressed isotherms in the hinterland. In northern Bolivia, flexural isostasy alone inadequately reproduces the thick FB sediments and the hinterland high topography, necessitating imposed foreland subsidence and hinterland uplift. Long-wavelength subsidence and uplift were imposed in the second half of the model resulting in increased sediment accumulation, depression of isotherms, and an increase in sample exhumation magnitude in the FB. Uplift in the hinterland only affected the absolute elevation of model and did not change sample exhumation. Subsidence experienced by FB samples necessitates an increase in the magnitude of exhumation needed to reset thermochronometers in FB thrust sheets when compared to hinterland samples in the FTB. The effects are amplified in models with rapid sediment accumulation rates, due to more significant depressions of isotherms, and minimized in models with slow sediment accumulation rates.