Basement control on thermochronometer cooling ages: an example from the Andean fold-thrust belt in Bolivia

Abstract:

Understanding the style, geometry and kinematics of basement deformation where it is not exposed is an ongoing challenge in orogenic belts. Basement deformation must be inferred based on the resulting surface observables. Geologic map patterns, thermochronometer cooling signals, and synorogenic sediment distribution in the northern Bolivian Andes strongly argue for displacement on large-scale basement thrust sheets. To show the relationship between basement deformation and surface observables such as mapped geology and thermochronometer cooling ages, we link a forward modeled balanced cross section to a 2-D thermokinematic model to compare predicted to measured cooling ages. Applying isostasy and erosion to sequentially deformed balanced cross sections links the growth of hinterland structures to the developing foreland basins and highlights subsurface controls on surface geology. The cross section kinematics become velocity vectors by assigning ages to displacement amounts. A range of potential velocity vectors is then used to calculate heat transport, erosion, and rock cooling. Predicted cooling ages highlight that large basement ramps impart a significant and tractable cooling signal. This signal is the most visible in the across strike pattern of cooling ages for each system (zircon fission track, apatite fission track and apatite (U-Th)/ He). The pronounced, predicted cooling age pattern generated by large basement thrust sheets match measured ages across the northern Bolivian Andes strongly suggesting basement deformation similar to that show in balanced cross sections.