GEODYNAMIC MODELING OF FLAT SLAB SUBDUCTION

It has been suggested that slab flattening in South America results from subduction of buoyant aseismic ridges or trenchward motion of the overriding plate. In the case of South America flat slab subduction, numerical models suggest that, given the plate velocities for Nazca and South America, the actual ridge dimensions are too small to induce flattening of such large slab segments. Here flat subduction is localized where thick cratonic lithosphere is close to the trench. In order to study the influence of over thickened lithosphere on the subduction style, we develop time-dependent numerical experiments and show that flat subduction can result from a combination of trenchward motion of thick cratonic lithosphere accompanied by trench roll-back. If the trench retreats as the craton approaches it, the asthenospheric wedge progressively closes and the suction forces increase enough to generate, in some cases, flat subduction. We model the last 30 Myr of subduction in the Chilean flat slab segment and demonstrate that trenchward motion of cratonic lithosphere, 200-300 km thick, presently ~700-800 km away from the trench, reproduces a flat-slab geometry that fits many observables as the stress pattern, seismicity distribution, and the temporal and spatial evolution of deformation and volcanism in the region.