NUMERICAL MODELS OF FLAT-SLAB SUBDUCTION: HOW FAST DOES THE OVERLYING CONTINENTAL PLATE COOL? (Invited Presentation)

Several regions of western North and South America have been affected by episodes of sub-horizontal (flat) subduction since the Cretaceous. Flat-slab subduction is typically associated with the termination of arc magmatism and refrigeration of the overlying continent, as the hot asthenospheric mantle wedge is replaced by the cool oceanic plate. Indeed, the segments of South America above the modern Peruvian and Pampean flat-slabs are marked by the absence of an active arc and appear to have a low geothermal gradient compared to adjacent steep-slab regions. However, these flat-slabs have only been in place for 10-15 Myr and are at depths of ~70 km (Peruvian) and ~100 km (Pampean). Thus, it is unclear whether there has been sufficient time for cooling of the continent. In western North America, thermochronologic data also suggest widespread cooling during the Laramide flat-slab event, with a lag time of ~10 Myr following slab emplacement.

We use numerical models to assess temporal variations in continental thermal structure as a subduction zone evolves into a flat-slab geometry. Models are based on the structure and plate velocities for western North and South America, and flat-slab subduction is induced by trenchward continental motion and subduction of a buoyant oceanic plateau. The models demonstrate that the rate of continental cooling increases as the depth of the flat-slab decreases. However, even for a relatively shallow slab (~60 km depth), it takes >20 Myr to produce an observable decrease in shallow crustal temperatures and surface heat flow, owing to the slow rate of thermal diffusion. This is in agreement with the conductive thermal time constant of 30-60 Myr for 70-100 km thick lithosphere. Observations of low continental temperatures for the Peruvian and Pampean regions may reflect either thick, cool lithosphere prior to the flat-slab or enhanced cooling by fluids released during oceanic plate metamorphism.