Mapping the Lithosphere and Asthenosphere beneath Alaska with Sp Converted Waves

We obtained a 3D image of crust and mantle seismic velocity gradients beneath the state of Alaska using common-conversion point (CCP) stacking of S-to-p converted body waves recorded by hundreds of stations from the NSF EarthScope Transportable Array and other portable arrays and permanent networks. Moho depths delineate the thick crust of the underthrust Yakutat terrane and the crustal root beneath the Brooks Range. The North American lithosphere is particularly thin close to the subducting lithosphere in the Alaska subduction zone, consistent with thinning of the upper plate by subduction zone flow and melt rising from the mantle wedge. The lithosphere remains relatively thin far to the northwest and north, including the Seward Peninsula and regions to the south of the Brooks Range where lithospheric extension and foundering may have played a role. The lithosphere dramatically thickens beneath the Brooks Range and northern Arctic Alaska terrane where it appears to be both cold and highly viscous. The CCP stack also revealed a pronounced positive velocity gradient at depths of 130-230 km that represents the base of a layer within the asthenosphere whose low velocities are best explained by the presence of partial melt. Although this gradient is present close to the subducting lithosphere, where partial melting is enabled by slab-derived fluids, it is strongest beneath the Seward Peninsula and northeast of the Wrangell volcanic field, suggesting the presence of partial melt in the asthenosphere hundreds of kilometers away from the slab, likely due to decompression melting in upwelling asthenosphere.