ASSESSING THE STRUCTURAL GEOMETRY OF THE ANDES ABOVE THE PERUVIAN FLAT SLAB (10-12° S) USING KINEMATIC MODELLING AND COOLING AGES

We present a new geologic map and cross-section from field-based structural observations, orientation measurements, lithologic data, geomorphic indicators, and preliminary cooling ages. The stratigraphy exhibits significant stratigraphic changes due to a pre-compression extensional environment. The thickness of the section increases from ~4 kilometers (km) in the east to ~12 km in the west. Preliminary cooling ages are from three mineral systems: Apatite (U-Th)/He (AHe), Apatite Fission Track (AFT), and Zircon (U-Th)/He (ZHe). AHe ages range from 2 to 65 million years (Ma) with the largest cluster of ages between 5 to 9 Ma and are located between 180 to 210 km from the deformation front. Reset ZHe ages range from 8 to 30 Ma with 11 samples indicating partial reset ages of 43 to 181 Ma. The youngest ages for AHe and ZHe are located 180-200 km and 189 km from the deformation front (respectively) and show a sharp increase in ages (older) to the southwest. These young ages are indicative of active uplift and exhumation. The increase in ages to the southwest strongly suggests exhumation over a basement ramp. Geomorphic indicators of active uplift include high topography and high K_SN. The topographic mean reaches elevations of 4 km along this section at 205 km and the maximum K_SN value is located at 180 km from the deformation front. We propose that the high topographic mean is a function of the lower edge of an active basement footwall ramp while the maximum K_SN is a function of the upper edge. These edges set the boundaries for the region with the youngest AHe ages and the youngest ZHe age. The associated hanging wall ramp is located ~60 km to the east where K_SN drops to background values. Our balanced cross-section exhibits multiple basement structures. Exposed basement west of 205 km requires a western basement thrust to promote structural elevation between 205-290, an active basement thrust between 205-120 km, and a broken foreland basin with basement exposures in the Subandean Zone <10 km from the deformation front. We propose an active basement ramp in the region of high topography, high K_SN, and young cooling ages between 180 and 205 km from the deformation front.