Bulldozing of Continental Mantle Lithosphere by Flat-Slab Subduction: Implications for Upper Plate Deformation and Arc Magmatism

It is well known that subhorizontal subduction below continents causes deformation of the upper plate far away from the plate boundary and arc magmatism to wane or end, but mechanisms that explain these observations are debated. We developed 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. The advancing flat slab scrapes off (bulldozes) the lowermost 20-50 km of continental mantle lithosphere. When this material is buoyant, it is bulldozed ahead of the flat slab, filling the asthenospheric wedge, and ending arc-magmatism. The bulldozed keel may survive for millions of years after slab rollback. When the removed material is dense, it is entrained by the slab and sinks. We show that in the western US, Laramide fossil keels have been preserved. This model provides an interpretive framework for modern flat-slab segments in Alaska, Mexico, and South America.