Seismic Imaging of the Alaska Subduction Zone: Implications for Slab Geometry and Volcanism

摘要

Alaska has been a site of subduction and terrane accretion since the mid-Jurassic. The area features abundant seismicity, active volcanism, rapid uplift, and broad intraplate deformation, all associated with subduction of the Pacific plate beneath North America. The juxtaposition of a slab edge with subducted, overthickened crust of the Yakutat terrane beneath central Alaska is associated with many enigmatic volcanic features. The causes of the Denali Volcanic Gap, a 400-km-long zone of volcanic quiescence west of the slab edge, are debated. Furthermore, the Wrangell Volcanic Field, southeast of the volcanic gap, also has an unexplained relationship with subduction. To address these issues, we present a joint ambient noise, earthquake-based surface wave, and P-S receiver function tomography model of Alaska, along with a teleseismic S wave velocity model. We compare the crust and mantle structure between the volcanic and nonvolcanic regions, across the eastern edge of the slab and between models. Low crustal velocities correspond to sedimentary basins, and several terrane boundaries are marked by changes in Moho depth. The continental lithosphere directly beneath the Denali Volcanic Gap is thicker than in the adjacent volcanic region. We suggest that shallow subduction here has cooled the mantle wedge, allowing the formation of thick lithosphere by the prevention of hot asthenosphere from reaching depths where it can interact with fluids released from the slab and promote volcanism. There is no evidence for subducted material east of the edge of the Yakutat terrane, implying the Wrangell Volcanic Field formed directly above a slab edge. Summary We present new images of the Alaskan subduction zone that reveal the three-dimensional structure of the upper mantle and crust. Our study leverages data from a new array of high-quality seismometers, the Transportable Array, which has been deployed across the entire state. We combine multiple geophysical techniques with comple