EXHUMATION AND FOREARC DRAINAGE REORGANIZATION IN RESPONSE TO FLAT SLAB SUBDUCTION, PERUVIAN FLAT SLAB

Abstract

Flat slab subduction is a key process in Cordilleran orogenesis, associated with arc shutoff, rock uplift, and crustal exhumation. While modern flat slabs occupy ~10% of Earth’s subduction zones and are readily identified with seismicity, our understanding of the kinematic responses in the overriding plate remains limited. The Peruvian flat slab is the largest active zone of flat slab subduction, making it an ideal place to evaluate its role on rock uplift and exhumation, as well as to test the surficial response by river drainage networks. We address these questions using regional compilations of thermochronometric results (apatite and zircon (U-Th)/He and fission track ages), geochronologic results (U-Pb, Ar-Ar, and K-Ar ages), 10Be erosion rates from forearc catchments, and geomorphic indices between 2°S and 18°S. Geochronologic results suggests that arc shutoff initiated at ~10 Ma in the north, and propagated southward through time. Thermochronometric results record cooling related to flat slab subduction, with Miocene and younger apatite (U-Th)/He cooling ages above the flat slab, and older ages to the south. The young cooling ages could be the product of increased exhumation and/or a lower geothermal gradient during flat slab subduction. However, geomorphic indices including hypsometric profiles and catchment-averaged channel steepness on forearc drainages suggest steeper, more youthful drainage networks above the flat slab, and generally higher erosion rates. The geomorphic indices are consistent with renewed exhumation and surface uplift driven by slab flattening. Moreover, many of the young cooling ages are in regions that lack recently active (Miocene and younger) upper crustal faults. We use these findings to evaluate potential processes that may enhance exhumation during flat slab subduction, including upper crustal shortening, dynamic topography, ridge buoyancy, and displacement of mid-to-lower crust via basal shear.