A POTENTIAL MECHANISM FOR SLAB UN-FLATTENING

Current efforts at geodynamically modelling flat subduction support the roles of continental obduction, suction forces derived from mantle flow, and subduction of buoyant oceanic lithosphere in driving slab flattening. Yet, these efforts also tend to show that, once initiated, flat subduction systems may remain stable indefinitely. Given that geologic evidence indicates instances of past flat subduction that are no longer extant, this result appears to be unrealistic. This study examines a possible mechanism by which a flat slab system may fail. Such systems, when well established, derive much of their stability from the suction force acting between the the two plates. The strength of this suction force is dependent on the viscosity of the material at the region of slab down-turn as well as on the subduction velocity. Preliminary modelling shows that as the viscosity adjacent to the hinge region of the flat slab decreases, the suction force weakens, and the slab can destabilize and retreat. There are at least two plausible mechanisms by which to reduce viscosity such that the suction force acting on the slab is dramatically denatured. The first involves chemistry. Introducing volatiles and hydrated material at the hinge point can greatly lower the viscosity. Another mechanism invokes the mechanical erosion of the continental lithosphere. At the point of slab downturn, the slab abrades against high viscosity lithosphere. A counterflow develops in the shear region, slowly entraining continental lithosphere into the slab's flow. Due to the high viscosity, the erosion rate is very slow. However, enough material is eventually eroded such that a low viscosity asthenospheric “finger” may reach the hinge region, thereby weakening the suction force. Simple simulations of this second mechanism suggest that flat slab rollback may be triggered via this process. Flat subduction remains stable until a slowly growing asthenospheric finger approaches the hinge point. The flattened slab then rapidly peels away from the over-riding lithosphere. Slab rollback eventually proceeds beyond the original continental margin resulting in extension and rifting of the over-lying continent. Though this process only very loosely emulates the observed extension and rifting of North America as the Farallon flat slab retreated, it may bear some relevance to other rifted continental margins.