Mechanisms for Layered Anisotropy and Anomalous Magmatism of Alaska Subduction System Revealed by Ambient Noise Tomography and the Wave Gradiometry Method

Abstract Seismic anisotropy can provide valuable constraints for the study of subduction zone dynamics. This study presents a high‐resolution 3‐D azimuthally anisotropic shear wave velocity model down to 230 km beneath Alaska via ambient noise tomography and wave gradiometry method. The model reveals layered anisotropy patterns related to subduction tectonics. The shear wave's fast directions in the Aleutian fore‐arc region exhibit trench‐parallel, trench‐normal, trench‐parallel, and trench‐normal variation relative to the trench trend with increasing depth. This anisotropic pattern may be attributed to the strike of fractures or faults in the overlying North American plate, subduction‐driven mantle wedge corner flow, preexisting fabrics in the subducting Pacific Plate, and entrained flow in the sub‐slab mantle, respectively. The depth‐dependent anisotropy pattern in the back‐arc mantle wedge reflects subduction‐induced corner flow, altered by the subducting slab's changing geometry. Moreover, the model provides new insights into the anomalous magmatism in the Alaskan subduction system. The 3‐D isosurface clearly shows the relatively high mantle wedge velocities beneath the Denali Volcanic Gap (DVG), suggesting a relatively dry and cold mantle wedge for the flat‐slab subduction of Yakutat slab. The absence of a magma source likely caused the DVG. The Wrangell Volcanic Field (WVF) is characterized by a similar depth‐dependent anisotropy pattern to the Aleutian‐Pacific subduction system, providing additional evidence for the presence of Wrangell Slab. The formation of WVF may be the result of combined effects of toroidal mantle upwelling around the edge of the Wrangell Slab and melting due to dehydration of the Wrangell Slab.