Implications of Flat-Slab Subduction on Hydration, Slab Seismicity, and Arc Volcanism in the Pampean Region of Chile and Argentina

The Pampean flat slab in central Chile and Argentina is characterized by the inland migration and subsequent cessation of arc volcanism since the mid-Miocene. Slab flattening also affects the distribution and number of intermediate-depth earthquakes and the evolution of the overlying continental thermal structure. In this study, we combine thermal-mechanical models with petrological models to examine temporal changes in pressure, temperature, and composition during flat-slab subduction and estimate water carrying capacity, predicted melt distributions and predicted changes in melt composition. Model results indicate that the present-day flattened Nazca plate carries water to similar to 700 km inland from the trench and could cause flux melting if the material above the slab remains fertile. Observed slab seismicity matches areas where hydrated materials have similar to>3 wt% H2O in the oceanic crust and mantle lithosphere. Seismicity increases as slab water carrying capacity decreases (slab dehydration). As P-T conditions and compositions of the rock trapped above the slab change during slab flattening, flux melting switches from a peridotite-dominated early phase to a combined mid-ocean ridge basalt/eclogite and peridotite melting at similar to 8 Ma. The results provide broad consistency with known earthquake distributions, seismic velocities, and observed temporal and spatial changes in volcanic patterns above the Pampean flat slab and point toward the role of melt depletion in the decrease and ultimate cessation of arc volcanism in this region.