K. M. Fischer

This study probes the lithosphere‐asthenosphere system beneath 155 Ma Pacific seafloor using teleseismic S‐to‐p receiver functions at the Pacific Lithosphere Anisotropy and Thickness Experiment project ocean‐bottom‐seismometers. Within the lithosphere, a significant velocity decrease at 33–50 km depth is observed. This mid‐lithospheric discontinuity is consistent with the velocity contrast between the background mantle and thin, trapped layers of crystallized partial melt, in the form of either dolomite or garnet granulite. These melts possibly originated from deeper asthenospheric melting beneath the flanks of spreading centers, and were transported within the cooling lithosphere. A positive velocity increase of 3%–6% is observed at 130–155 km depth and is consistent with the base of a layer with partial melt in the asthenosphere. A shear velocity decrease associated with the lithosphere‐asthenosphere boundary at 95–115 km depth is permitted by the data, but is not required.

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.