A 3-D Shear Velocity Model of the Crust and Uppermost Mantle Beneath Alaska Including Apparent Radial Anisotropy

摘要

This paper presents a model of the 3-D shear velocity structure of the crust and uppermost mantle beneath Alaska and its surroundings on a ~50-km grid, including crustal and mantle radial anisotropy, based on seismic data recorded at more than 500 broadband stations. The model derives from a Bayesian Monte Carlo inversion of Rayleigh wave group and phase speeds and Love wave phase speeds determined from ambient noise and earthquake data. Prominent features resolved in the model include the following: (1) Apparent crustal radial anisotropy is strongest across the parts of central and northern Alaska that were subjected to significant extension during the Cretaceous. This is consistent with crustal anisotropy being caused by deformationally aligned middle to lower crustal sheet silicates (micas) with shallowly dipping foliation planes beneath extensional domains. (2) Crustal thickness estimates are similar to those from receiver functions by Miller and Moresi (2018, https://doi.org/10.1785/0220180222). (3) Very thick lithosphere underlies Arctic-Alaska, with high shear wave speeds that extend at least to 120-km depth, which may challenge rotational transport models for the evolution of the region. (4) Subducting lithosphere beneath Alaska is resolved, including what we call the "Barren Islands slab anomaly," an "aseismic slab edge" north of the Denali Volcanic Gap, the "Wrangellia slab anomaly," and Yakutat lithosphere subducting seaward of the Wrangell volcanic field. (5) The geometry of the Alaskan subduction zone generally agrees with the slab model Alaska_3D 1.0 of Jadamec and Billen (2010, https://doi.org/10.1038/nature09053) except for the Yakutat "slab shoulder region," which is newly imaged in our model.