Recycling the Andes: Surface expressions of lithosphere removal

Abstract:

The central Andes is the type example of a Cordilleran orogen where mountain-building coincides with Nazca plate subduction. Crustal thicknesses of 50-70 km are due primarily to shortening of the South American crust and magmatic addition from below. The orogen has been further modified by processes that remove mass, such as subduction erosion, ablation, surface erosion, and convective removal of the deep lithosphere. Here, we address mantle dynamics below the Puna plateau in the southern central Andes. Seismic tomography and other geophysical studies show that the mantle lithosphere is thin (<30 km) despite the thickened crust. This region is also characterized by localized (<100 km wide) areas of anomalous surface uplift/subsidence and magmatism that is derived from both lithospheric and asthenospheric sources. These can not be explained by orogenic shortening, as paleoelevation studies indicate that this region achieved its modern elevation by 40 Ma, and structural studies show that most current shortening is localized on the eastern side of the plateau. We use numerical models to demonstrate that the localized dynamics may be related to small-scale lithospheric removal induced by the presence of dense pyroxenite. To create a significant surface deflection, the pyroxenite must originate within, or just below, the crust. If the pyroxenite is removed as small drips (diameter <10 km), the drips can melt during descent. This is followed by decompression melting of the upwelling asthenosphere. The models also highlight the role of the crust in modulating the surface expressions of removal. If the crust is thick and warm (Moho >600C), flow within the deep crust can be triggered during removal, resulting in surface uplift prior to pyroxenite removal. In addition, flow of hot crust toward the dripping pyroxenite results in significant local crustal heating on timescales <20 Ma.