A dipping, thick segment of the Farallon Slab beneath Central US

Abstract

It has been hypothesized that much of the crustal deformation attributed to the Laramide orogeny of the southwest North American Cordillera was caused by dynamic effects induced by the flat subduction of a large oceanic plateau that was embedded within the Farallon plate. Previous studies have identified within the North American mantle a seismic velocity anomaly that plausibly represents the remnants of the subducted plateau. Coupled plate kinematic and dynamic modeling of the anomaly, as well as surface geological findings identify this anomaly as the subducted conjugate to the Shatsky Rise. Here, we find clear evidence for a northeastward dipping (35° dip), thick (up to 400 km thick) slab-like seismic velocity anomaly within the top of the lower mantle below the central United States. Using a deep focus earthquake below Spain, we find that the observed seismic waveforms recorded with the dense USArray display multi-pathing indicative of sharp surfaces. Plate tectonic reconstructions in which the anomaly is migrated back in time suggest strong coupling of the plateau-thickened slab segment to the overriding North America Plate as it was subducted. In combination with the reconstructions, we interpret the structure as arising from eastward dipping Farallon subduction at the western margin of North America during the Cretaceous, in contrast with some recent interpretations. It appears that the plateau area of the slab has been further thickened, which might undergo a combination of pure shear bulk shortening during flat-slab subduction and/or by a folding instability during penetration into the lower mantle.