Mechanisms for Slab-edge Volcanism Along the Pacific Rim of Fire: A case study of Alaska

Abstract:

The Pacific Rim of Fire is a tectono-geographic term that delineates the geographic perimeter of the Pacific Ocean and the tectonic activity associated with the subduction zones that outwardly flank the Pacific Ocean. Seismic studies that delineate the location of submerged (subducted) plates in the upper mantle show that slab geometries along the Pacific Rim of Fire are complex, containing discontinuous segments that vary in depth, dip, and radius of curvature, and can intersect. Although arc volcanoes track the subducted slabs at depth along most subduction zones along the Pacific Rim of Fire, volcanoes with anomalous morphology and geochemistry also systematically occur, not above a slab, but distal to the slab edge forming a pattern not explained by a two-dimensional subduction paradigm. For example, the northeastern corner of the Pacific Rim of Fire is comprised of the eastern Alaska subduction zone that contains a suite of anomalous volcanics. This Wrangell Volcanic Field is spatially separated from the main Aleutian-Alaska volcanic arc and located above a short (< ~100 km) -to-seismically indistinguishable Wrangell slab, comprising the eastern edge of the main Aleutian-Alaska slab. High-resolution, three-dimensional numerical models are presented of this subduction zone to examine how the slab edge geometry plays a role in producing the anomalous slab-edge volcanism. The geodynamic models show that the pivoting motion of the eastern edge of the subducting Aleutian-Alaska slab produces dynamic flow in the mantle containing counterclockwise toroidal flow and a locus of mantle upwelling situated within the toroidal flow and spatially below the Wrangell Volcanic Field. The predicted vertical mantle upwelling beneath the Wrangell volcanics produced by the geodynamic models is also consistent with the occurrence of null shear wave splitting observations from the region. The results imply that slab-edge driven volcanism may be a common phenomenon associated with plate tectonics on a sphere and the subduction of discontinuous slabs.