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.