GEODYNAMIC MODELS OF SHORT-LIVED, LONG-LIVED AND PERIODIC FLAT SLAB SUBDUCTION

Flat slab subduction is characterized by a subducting slab that lies (sub)horizontally for several hundred kilometres below the base of the overriding plate. It has been documented at several subduction zones on Earth and has often been ascribed to subduction of buoyant ridges/plateaus or to forced trench retreat. There are a number of settings in nature, however, with subduction of a buoyant ridge/plateau that lack a flat slab, and flat slab settings that lack a buoyant ridge/plateau. Additionally, geodynamic subduction models with forced trench retreat require forces to reproduce flat slab subduction that can be higher than available in nature. In this contribution I will present the first buoyancy-driven geodynamic models of flat slab subduction, demonstrating that it can occur in the absence ridge/plateau subduction, in the absence of forced subduction, and during negligible trench migration. The geodynamic models, combined with a global compilation of subduction zones, indicate that flat slab formation requires subduction zones to be very wide (large trench-parallel extent) and old. Indeed, flat slab subduction starts only after a prolonged period (> ~100 Myr) of progressive decrease in slab dip angle, which enhances suction in the mantle wedge, and disappears due to overriding plate thickening, which forces the mantle wedge to open up. Such thickening provides a potential explanation for flat slab termination at the end of the Laramide orogeny in western North America and termination of the South China flat slab in the Jurassic. Depending on overriding plate strength and subducting plate thickness, the geodynamic models can produce both short-lived (<20 Myr) and long-lived (>50 Myr) flat slab subduction, and also demonstrate the possibility of periodic flat slab subduction during which the flat slab segment periodically changes its dip angle due to slab folding at the 660 km discontinuity.