Combining the evidence: Crustal and upper mantle structure above the flat slab in central Chile and Argentina

We combine the results from a range of seismic studies, which used data collected during the Chile Argentina Geophysical Experiment (CHARGE) to present an up to date model of the crustal and upper mantle structure in central Chile and Argentina. This area of central Chile and Argentina is of particular interest due to the presence of the central Chilean flat slab, a region in which the downgoing Nazca plate descends normally to a depth of 100 km, and then flattens, traveling horizontally for several hundred kilometers before resuming its descent into the mantle. The flat slab region is characterized by an absence of modern arc volcanism and the presence of the inland basement cored uplifts of the Sierras Pampeanas, which have been considered to be modern analogues to the Laramide uplifts in the western United States (Allmendinger et al. (1986)). The combined results of these studies, that include tomography, receiver functions, SKS and local S-wave splitting, local earthquake relocations and mechanisms, and regional waveform modeling, indicate an unusually cold and brittle crust underlain by a cold, dry, and depleted sliver of lithosphere, which is trapped between the crust and the horizontally traveling flat slab. The western Sierras Pampeanas has thick crust (55km) with a high velocity, high density lower crust that explains the relatively low average elevation. In contrast, the eastern Sierras Pampeanas crust is much thinner (35-40 km) and has lower P-wave velocities and a lower Vp/Vs ratio. The change in crustal character correlates with the Cuyania terrane composed of mafic basement and the Pampian terrane composed of more quartz-rich basement.