Deformation within continental plates: Surface deflection and magmatism induced by lithosphere removal

English Abstract:

Many localized pulses of deformation have been found in the interior of continental plates, and in some cases, these can not be readily linked to regional tectonics. Further, beneath many regions, the lithosphere is found to be abnormally thin, and there is evidence for detached lithosphere material located at 100-200 km depth. One hypothesis is that the lower lithosphere has been gravitationally removed and been replaced by upwelling asthenosphere. This sudden removal abruptly changes the stress and temperature field in crust and mantle, which may generate localized transient basins/orogens and pulses of volcanism. Thus, studying this surficial evidence provides a window to investigate the deep mantle dynamics. 2D thermal-mechanical numerical models are used to explore the surface deflection and magmatism induced by gravitational lithosphere removal. We find that topography and magmatism are primarily controlled by lithosphere structure. In hot lithosphere (e.g., a back arc), the lithosphere is removed through small wavelength (<50 km) instabilities. Magmas can be generated through conductive heated of sinking lithosphere and through decompression melting of upwelling asthenosphere. Lithosphere destabilitization causes thickening of warm and weak crust, producing in isostatic uplift. The resulting surface topography either has small subsidence (<0.5 km) or generate a topographic high during the removal. In warm lithosphere (e.g., average Phanerozoic lithosphere), lithosphere foundering occurs on longer wavelengths (50-100 km). In this case, the only melts are from asthenosphere. The surface subsides, followed by a partial uplift as the instability detaches. In cold lithosphere (e.g., craton), the lithosphere removal has the longest wavelength (>100 km), and the surface can subside >1 km; however, no magmas are generated. In most models, lithosphere removal and the associated surface deflection and magmatism occur on time scales of a few Myr. After this, stress relaxation and surface cooling erase these expressions. Observational data from a number of regions (e.g., Sierra Nevada in California, Tibet in China, Puna plateau in the central Andes) are consistent with our models, suggesting that pulses of deformation and magmatism in these areas are related to local lithosphere removal.