Foundering lithosphere triggers transient basins and backarc magmatism at subduction zones?

Abstract:

Many upper-plate processes at subduction zones cannot be directly explained by traditional subduction mechanisms. In the Central Andes, the crust is shortened and thickened by the subduction of Nazca plate, but the lower lithosphere is anomalously thin at present. Within the plateau, localized, transient basins have formed since the Miocene. These basins have experienced subsidence, internal shortening, and then inversion. One hypothesis is these basins are related to the formation and foundering of dense eclogite rocks in the lithosphere. Along the eastern plateau, there are sites of basaltic magmatism which show a gradual westward migration. Geochemistry studies suggest that these magmas are mainly caused by upwelling asthenosphere, indicating lithosphere thinning beneath this area. However, the magmas are landward of the basins, and therefore the formation and removal of the dense anomaly is spatially and temporally offset from the region of lithosphere thinning. In this study, 2D numerical models are used to investigate lithosphere removal within a subduction zone. A dense root is placed in lower crust of the upper plate to simulate the eclogitization process and initiate gravitational removal. The model evolves in three phases: 1) As the root becomes denser, the overlying surface subsides and a basin forms; 2) once the root is denser than mantle, it sinks and decouples from the upper plate. During this period, the basin inverts and uplifts. 3) Meanwhile, the mantle lithosphere landward of the root is sheared by the corner flow in the mantle wedge. As the lithosphere is carried trenchward, a gap forms at the landside of plateau which widens over time. Hot asthenosphere upwells to fill the gap and undergoes decompression melting. The model results are consistent with observations from the Central Andes and could have implications for other subduction regions with enigmatic transient basins and backarc magmatism, such as those in North America and Eastern China.