TIMESCALES OF ANDEAN FLAT-SLAB SUBDUCTION, CORDILLERAN MAGMATISM, AND GEODYNAMIC EVOLUTION

The effects of flat-slab subduction on arc magmatism, contractional orogenesis, and basin evolution are fundamental to convergent-margin tectonics, with implications for feedbacks among geodynamic, magmatic, and surface processes. Potential drivers of flat-slab subduction are numerous but, the spatial-temporal link among flat-slab subduction and upper-plate processes remains less clear. This study focuses on coupled Mesozoic-Cenozoic detrital zircon U-Th-Pb and Hf isotopic results from sediments throughout the Pampean flat-slab subduction system of Chile-Argentina, which is characterized by Miocene to present-day shallow subduction angle (5°), a spatial gap in active volcanism (27-33˚S), and foreland basement uplifts of the Sierras Pampeanas potentially driven by subduction of the Juan Fernandez ridge. Integrated detrital and bedrock geochronology provide valuable insights into the temporal and spatial distribution of continental arc magmatism and associated isotopic patterns during major shifts in Cordilleran plate dynamics along the western margin of South America prior to and during the Neogene phase of flat-slab subduction. Progressive continental arc broadening and subduction angle shallowing initiated by the Late Cretaceous, and corresponds with an increase in arc magmatism tempo from Mesozoic 45-60 Myr phases of high arc activity and 25 Myr lulls, to Cenozoic 25 Myr magmatic flare ups and negligible magmatic lulls (<5 Myr). The initiation of shallow to flat-slab subduction is synchronous with evolved (negative) εHf values, low (< 0.5) zircon Th/U ratios, and eastward advance of magmatism. Changes to continental arc magmatism is indicative of increased plate coupling, as expressed in the intensification of Neogene volcanism, hinterland exhumation, and foreland basin subsidence over numerous 5-7 Myr cycles. New temporal, spatial, and isotopic constraints on Miocene flat-slab subduction indicate a complex geodynamic evolution that includes (1) accelerated relative convergence following the Jurassic to Cretaceous opening of the South Atlantic; (2) far field effects from the subduction of positively buoyant oceanic lithosphere; with (3) implications for Cordilleran cyclicity and prolonged (>200 Myr) slab anchoring into the lower mantle.