MECHANICAL EFFECTS OF FLAT-SLAB SUBDUCTION ON BASAL CONTINENTAL LITHOSPHERE: INSIGHTS FROM THE SUBDUCTION OF THE NAZCA RIDGE IN PERU

Flat-slab subduction places a subducting plate in direct contact with the lithosphere of an overriding plate, extending the plate interface far inland of the megathrust and potentially promoting significant deformation of the base of an overriding plate’s lithosphere. This process has been hypothesized to explain the removal of part of the North America plate lithosphere and the emplacement of forearc-derived materials during Cretaceous flat subduction in the southwest U.S. We identify an analogous process in central Peru where the ongoing subduction of the Nazca Ridge has led to the removal of the Peruvian mantle lithosphere and ~10 km of the lower continental crust over ~50,000 km², requiring the displacement of a volume of upper plate lithosphere on the order of 500,000 km³. The flat slab itself fills much of the volume previously occupied by Peruvian lithosphere. We find evidence for active deformation in the forearc adjacent to the subducting ridge and extending north. This may relate to the displacement of materials derived from the forearc, which has been shown to be tectonically eroding at a high rate in association with the ridge’s subduction.

Teleseismic receiver functions and tomography along with local earthquake studies indicate that the flat slab is shallowest along the projection of the Nazca Ridge. The top of the subducted ridge is at 50 to 60 km depth and lies in contact with thinned continental crust to ~400 km inland. South of the subducted ridge, a thickened welt of continental crust is likely material displaced by the ridge.

Observed slow shear wave speeds, a complex continental Moho, and seismicity likely above the megathrust in the onshore forearc near the ridge contrasts with observations away from the ridge. The presence of slow wave speed material near the forearc Moho likely represents material eroded from the marine forearc. Seismicity in this location may be relate to the emplacement of this material in the wake of the subducted ridge.