Cooling of the Continental Plate During Flat-slab Subduction

Abstract:

Since the Late Cretaceous, several segments of western North and South America have been affected by flat-slab subduction where the subducting plate becomes horizontal below the overlying continent. Flat-slab subduction is typically marked by the cessation of arc volcanism, caused by the closure of hot mantle wedge displacing by the cool oceanic plate. In western North America, thermochronological data shows evidence of widespread cooling during the Cretaceous Farallon flat-slab event (~40 Ma duration). Modern flat-slab regions in South America and Mexico are also inferred to be relatively cool, based on observations of low surface heat flow. The cooling is typically attributed to the presence of the cold slab immediately below the continental lithosphere. However, modern flat-slabs have only been in place for ~10 Ma. Thus, it is unclear whether there has been sufficient time for the continent to cool. In this study, we use 2D thermal-mechanical numerical models to assess temporal variations in the continental thermal structure during flat-slab subduction. The models show that continental cooling occurs through conduction over 10’s of Ma with 5-15 Ma delay after flat-slab emplacement. The cooling rate and lag time are mainly controlled by the flat-slab depth, with more rapid cooling and a decreased lag time where the flat-slab is at a shallow depth. For a slab at ~90 km depth, as in the Pampean region of central Chile, there is little decrease change in surface heat flow (<1 mW/m²) and the Moho (~48 km depth) temperature drops only by ~13 ˚C during ~10 Ma flat-slab duration. Therefore, conductive cooling is not sufficient to explain the observed low surface heat flow here and additional factors need to be considered, such as cooling by the infiltration of fluids released from the slab. Alternately, the cool lithosphere may reflect the pre-existing continental structure. In the Mexican and Peruvian subduction zones, the flat-slabs are at shallow depths (45-65 km), and thus can cause substantial conductive cooling within 10-15 Ma. For the western North America, the flat-slab was deeper (~110 km), but the longer flat-slab duration and older oceanic plate may have enhanced continental cooling. More work is needed to assess whether conductive cooling in this region can explain the thermochronological data.