NEW INSIGHTS ON THE FORMATION AND CONSEQUENCE OF FLAT-SLAB SUBDUCTION (Invited Presentation)

Flat-slab subduction represents one of the most effective ways plate tectonics shape Earth’s surface geology, including the formation of severe back-arc deformation and widespread basin subsidence. However, questions remain on both mechanisms of flat-slab subduction and its dynamic interaction with the overring plate. We present recent studies of subduction modeling in South America and North America where flat slabs with different sizes and durations occur.

We find that the narrow (<300 km wide) Late Cenozoic South American flat slabs are due to a combination of several mechanisms including the presence of a thick and strong overriding plate and the subduction of several buoyant oceanic ridges, with the latter playing a more important role. An interesting result is that all these flat slabs are internally stretched to form slab holes due to viscous necking caused by buoyancy-induced strain localization. In contrast, the broad (>500 km wide) Late Cretaceous Farallon slab flattens mostly due to the fast motion of the overriding continent, with additional lift from the buoyant Shatsky Conjugate Plateau. For all the models analyzed, we find that a weak mantle wedge or weak near-trench lithosphere is necessary to allow initial slab advance and subsequent slab flattening.

We further analyze the surface responses during the Farallon flat-slab subduction, and find that the slab itself mostly causes dynamic subsidence that is consistent with the observed Western Interior Seaway, as is due to its overall negative buoyancy. Contemporaneous surface uplift occurs mostly due to upper plate isostacy where either the low-density portions of the continental lithosphere (e.g., the crust and the upper mantle lithosphere) thickens or the high-density portion (e.g., lowermost mantle lithosphere) delaminates. This isostatic uplift correlates well with the observed Laramide Orogeny.We also observe other phases of dynamic subsidence and uplift associated with the repeated formation and removal of flat slabs at the base of the North American lithosphere during the Cenozoic that may reconcile the apparently conflicting observations for the uplift history since the Late Cretaceous.