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.

Most flat-slab subduction regions are marked by an absence of arc volcanism, which is consistent with closure of the hot mantle wedge as the subducting plate flattens below the continent. Farther inland, low surface heat flow is observed, which is generally attributed to cooling of the continent by the underlying flat slab. However, modern flat slabs have only been in place for <20 Ma, and it is unclear whether there has been sufficient time for cooling to occur. We use numerical models to assess temporal variations in continental thermal structure during flat-slab subduction. Our models show that the flat slab leads to continental cooling on timescales of tens of millions of years. Cool slab temperatures must diffuse through the continental lithosphere, resulting in a delay between slab emplacement and surface cooling. Therefore, the timescales primarily depend on the flat-slab depth with shallower slabs resulting in shorter timescales. The magnitude of cooling increases for a shallow or long-lived flat slab, old subducting plate, and fast convergence rates. For regions with flat slabs at 45–70 km depth (e.g., Mexico and Peru), shallow continental cooling initiates 5–10 Ma after slab emplacement, and low surface heat flow in these regions is largely explained by the presence of the flat slab. However, for the Pampean region in Chile, with an ~100-km-deep slab, our models predict that conductive cooling has not yet affected the surface heat flow. The low heat flow observed requires additional processes such as advective cooling from the infiltration of fluids released through dehydration of the flat slab.

<p>Numerical modeling methods and supplemental Figures S1–S4, Movies S1 and S2, and Table S1 (model material properties). </p>

<p>Numerical modeling methods and supplemental Figures S1–S4, Movies S1 and S2, and Table S1 (model material properties). </p>