Influence of Upper Plate Structure on Flat-Slab Depth: Numerical Modeling of Subduction Dynamics

Western North and South America have been affected by flat-slab subduction where a segment of the subducting plate becomes horizontal below the overlying continent. Modern observations and constraints on past geometries show that the depth of the flat-slab varies from just below the continental Moho to >100 km depth. Thus, in some areas, there is little to no continental mantle lithosphere (CML) above the flat-slab, whereas other flat-slab areas are overlain by CML thicknesses >50 km. The mechanisms causing different slab depths are unclear. We examine flat-slab subduction dynamics and slab depth through 2-D thermal-mechanical modeling. The models investigate plate structures and velocities similar to those of the Cretaceous western United States and present-day South America. Models show that flat-slab depth is primarily determined by continental structure. A deep flat-slab occurs if the continent is initially thick, as its mantle lithosphere is cool and thus too strong to be displaced by the flat-slab. The CML rheology plays a secondary role. A weak, hydrated lithosphere is easily displaced, leading to a shallower slab. The flat-slab can displace up to 50% of the thickness of the CML, and no model exhibits displacement of the full CML thickness. This suggests that shallow flat-slabs below Mexico and Peru require a thin continent prior to slab flattening. The models also show that a flat-slab is deflected downward when it encounters thick craton lithosphere, with a larger depth for a chemically depleted craton. This has implications for modification of the Wyoming craton during Farallon flat-slab subduction.