Basal continental mantle lithosphere displaced by flat-slab subduction

During the subhorizontal (flat) subduction of an ocean-lithosphere plate, the overlying continental plate is deformed far inland of the plate boundary. In addition, arc magmatism, which is caused by melting in the asthenospheric wedge during steep subduction, wanes or ends when the subduction is subhorizontal. The observed upper-plate deformation patterns have been explained by an end load exerted at the plate boundary or by stress transmitted into the overlying plate along the top of the flat slab. Here we present numerical thermal-mechanical models of flat-slab subduction that show the flattening of the slab results in a compression of the continental plate through end loading. The advancing flat slab scrapes off the lowermost 20-50 km of continental mantle lithosphere. The displaced continental mantle lithosphere fills the asthenospheric wedge, ending arc-type melting. If the displaced material is buoyant, it accumulates in a growing keel that migrates ahead of the slab; if it is dense, the displaced material sinks with the slab. Flat-slab removal renews the asthenospheric wedge and arc magmatism, and leaves a step in the lithosphere-asthenosphere boundary and/or a keel that consists of displaced continental mantle lithosphere. A fossil keel and a fossil step, formed during Laramide flat subduction, are preserved below the western United States.