Numerical models of Farallon plate subduction: Creating and removing a flat slab

Abstract

Flat-slab subduction has affected parts of North America, South America, and Asia over the past 250 m.y. In these areas, reconstructions show that the subducting plate became subhorizontal below the continent for ~5 to >30 m.y., followed by foundering of the slab and resumption of steep-angle subduction. Using two-dimensional numerical models, we examine the factors that control the development and removal of a flat slab. Models are based on the Late Cretaceous to Oligocene Farallon flat slab below the southwestern United States. We find that the primary control on subduction geometry is the oceanic plate density structure. Subduction of a buoyant oceanic plateau creates a flat-slab segment that moves inboard at approximately the rate of continental trenchward motion (4–5 cm/yr). Steepening is initiated with eclogitization of the oceanic plateau crust. Once the plateau density exceeds that of the mantle, the slab undergoes rollback through progressive trenchward-directed detachment from the continent at a rate of 2–10 cm/yr. Rollback is enhanced by: (1) weakening of the overlying continental mantle lithosphere, inferred to result from slab-derived hydrous fluids, and (2) a slowdown in plate velocities; the rate and amount of oceanic eclogitization are second-order effects. Conversely, rollback is hindered by a strong oceanic plate and interactions between the slab and high-viscosity lower mantle. For the ~2000-km-long Farallon slab, the Conjugate Shatskey Rise plateau must have remained buoyant for 20–30 m.y. after subduction. This was followed by rapid rollback caused by both plateau eclogitization and continental weakening, leaving an area of thinned and hydrated continental lithosphere.