Multiple basin filling and excavation cycles and fault reactivation
Fault systems in the broken foreland of NW Argentina are associated with isolated seismicity, deformation, and uplift, and they pose a major problem in understanding the spatiotemporal characteristics of individual earthquakes and long-term deformation patterns. Range uplift in these environments is highly disparate in time and space, without a clear deformation front as in the foreland fold-and-thrust belt to the north. Some ranges constitute large anticlines that have formed over blind thrusts, such as the growing anticlines west of the town of Salta. On geological time scales, the erratic tectonic behavior of fault-bounded intermontane basins has caused a disturbance of the fluvial systems and resulted in multiple episodes of basin filling and sediment removal to the foreland. Importantly, there appears to be a relationship between sediment removal and ensuing changes in crustal stresses in these environments. For example, fault reactivation within and along basin margins is observed to follow the removal of the sedimentary load on time scales of several 105 years. To elucidate the mechanisms by which such fault arrays in broken forelands organize, activate, and deactivate over time, we will compare geomorphic and geologic records that integrate the activity of faults over multithousand-year timescales (i.e., by dating deformed geomorphic features such as fans and terraces using U/Pb dating) to million-year timescales (i.e., by applying geochronologic and thermochronologic methods). These types of observations will enable the determination of the spatiotemporal faulting history and help to assess how deformation may be transferred between fault fault systems over long timescales (PhD1). We will furthermore use the Gale geodynamic model 87 to study how topographic construction and changing constitutive properties of fault zones may moderate the transfer of deformation between such arrays (PhD2). As deformation accrues and mountain ranges are built, body forces in the crust may cause one set of structures to become less susceptible to failure relative to surrounding structures. Deformation may then systematically migrate from areas of higher to lower elevations. However, coeval erosional processes export mass from the basins and ranges, and so may buffer the accumulation of body forces in the crust. Gale allows application of surface-processes rules, enabling us to couple erosion at the surface to changes in the deformation field within the upper crust. Combining field observations, geochronology and modeling is expected to help us ascertain plausible causal mechanisms of spatiotemporal patterns of deformation.