WHAT IS THE MECHANISM OF UPPER-PLATE DEFORMATION AS A RESULT OF FLAT-SLAB SUBDUCTION?

In ancient and modern locations of subhorizontal subduction below continents, deformation of the upper plate occurs far away from the plate boundary. Mechanisms that explain these observations are debated, and can be divided into: (1) end-loading mechanisms where a compressional force at the plate boundary is responsible for this deformation, and (2) basal shear traction along the horizontal plate interface transferred to the upper plate. We eveloped thermal-mechanical models to study slab-overriding plate interactions during slab-flattening and flat-slab subduction. Our models show that flattening of the slab causes a compressional state of stress of the overriding plate through end-loading of the continent. Little stress is transferred into the upper plate through shear of the weak, basal continental mantle lithosphere. Our models predict that the advancing flat slab scrapes off (bulldozes) the lowermost ~20-50 km of weak continental mantle lithosphere. This has implications for arc magmatism: when this material is buoyant, it is bulldozed ahead of the flat slab, filling the asthenospheric wedge, and ending arc magmatism. (B)The bulldozed keel may survive for millions of years after slab rollback: examples under the western U.S. are identified. When the removed material is dense, it is entrained by the slab and sinks.