INTERACTIONS BETWEEN A SUB-HORIZONTAL SUBDUCTING PLATE AND THE OVERLYING CONTINENTAL LITHOSPHERE

It is well-known that flat-slab subduction has a profound effect on the overlying continent, causing the termination of arc magmatism and triggering deformation far inboard from the plate boundary. However, the mechanisms driving these processes are debated. Here, we present 2D thermal-mechanical models to study the development of flat-slab subduction and its effects on the overlying continent. The models show that slab-flattening initiates as an oceanic plate with a buoyant aseismic ridge subducts below a trenchward-moving continent. As the slab flattens, it displaces (bulldozes) the lowermost ~20-50 km continental mantle lithosphere (CML), with greater thicknesses for relatively weak (hydrated) CML. No models show displacement of the full thickness of CML, owing to the temperature-dependent mantle rheology. This suggests that areas where the flat-slab is found immediately below the continental Moho (e.g., Mexico, Peru) must have had an initially thin CML. As the flat-slab advances, the displaced CML fills the mantle wedge above the slab, terminating arc magmatism. If the CML has a primitive composition, it is entrained by the slab and sinks. If the CML is compositionally buoyant (i.e., depleted), it forms a growing keel ahead of the flat-slab. The bulldozed keel may survive for millions of years after flat-slab removal. Several seismic and gravity anomalies in the shallow mantle of the western United States may represent fossil keels from Late Cretaceous Farallon flat-slab subduction. The models also show that little stress is transferred into the continent through shear of the weak, basal CML. Instead, slab-flattening induces a compressional state of stresses in the continent through end-loading. The models provide an interpretive framework for modern flat-slab segments in Alaska, Mexico, and South America.