Lithospheric velocity structure of Alaska revealed by double difference tomography

Alaska region is formed by the northward accretions of terranes from different geological periods and has experienced extensive internal deformation and metamorphism, and the geological structure is complex. The Array Network Facility （ANF） website recently provides new observational data from the seismic network of USAarray, filling the observation gap in the west and north of Alaska. Based on P and S wave arrivals of 5 638 events recorded by 345 stations from ANF, this study relocates earthquakes and images the 3D lithospheric P-wave velocity structure beneath Alaska simultaneously by regional double difference tomography. The results reveal larger dip angle of subducting Pacific Plate and low-v_P anomalies in the mantle wedge beneath western Alaska. These observations reflect the subduction process, during which the dehydration of the subduting Pacific Plate releases fluids into the mantle wedge, triggers partial melting, and generates melts, which was then transported to the surface by the upwelling flow so as to form Aleutian volcanic arc. In central Alaska, the coupling between the Yakutat terrane and the Pacific Plate reduces the subduction dip. On the one hand, the shallow subduction of the Yakutat terrane increases the compressive stress of the crust, causing the crustal thickening and uplifting of Chugach mountains. On the other hand, it cools the mantle wedge reducing magma generation, which are then combined with the closure of crust fractures resulted from the increase of the crustal stress, blocking the supply of melt to the surface, and finally leading to the formation of Denali volcanic gap. In addition, there is a clear boundary between Yakutat terrane and Wrangell volcanic field in the east, and the low velocity zone corresponding to magmatic activity in the region is concentrated in the northwest. The magma source may be related to upwelling of the toroidal mantle flow around the Pacific-Yakutat slab edge. These results suggest that the complex geodynamic processes in deep Alaska lead to the complex geological structure on the surface.