Big Mantle Wedge and Intraplate Volcanism in Alaska: Insight From Anisotropic Tomography

We determine high-resolution tomographic models of isotropic P-wave velocity (Vp) and tilting-axis anisotropy of the Alaska subduction zone using a large number of local and teleseismic data recorded at many portable and permanent network stations in and around Alaska. We find a flat high-Vp slab in the mantle transition zone (410-670 km depths) beneath western Alaska, which is connected with the subducting Pacific slab at 0-410 km depths, suggesting that a big mantle wedge has formed under western Alaska. Our tilting-axis anisotropy model reveals complex mantle flows in the asthenosphere. Corner flow in the mantle wedge above the subducting Pacific slab and toroidal flow in the big mantle wedge are revealed, which may cause the Cenozoic intraplate volcanoes in western Alaska and the Bering Sea. In central Alaska, the mantle wedge beneath the Denali volcanic gap is characterized by high-Vp and subhorizontal fast velocity directions normal to the volcanic arc, which may reflect a remnant of the subducted Yakutat slab. In SE Alaska, the shallow subduction of the Wrangell slab is visible above 150 km depth, and hot mantle upwelling through the Wrangell-Yakutat slab gap may contribute to the Wrangell volcanic field. Alaska is home to over 100 Cenozoic volcanoes, but their formation mechanism is still puzzling. Here we use a large number of seismic data and a novel tomographic technique to obtain high-resolution images of isotropic P-wave velocity (Vp) and anisotropy down to 800 km depth beneath Alaska. Our results show that a flat high-Vp slab exists in the mantle transition zone (410-670 km depths) beneath western Alaska, where a big mantle wedge has formed above the flat slab. Fluids in the big mantle wedge and large-scale mantle flow around it have caused Cenozoic intraplate volcanoes near the Bering Sea. The Denali volcanic gap, different from its neighboring Buzzard Creek-Jumbo Dome volcanoes, has a high Vp mantle wedge with arc-normal fast anisotropy, which may reflect a cool fluid barrier from the Yakutat slab. The high-Vp Wrangell slab is revealed down to similar to 100 km depth beneath the Wrangell volcanic field, Mount Churchill and Prindle volcano. Hot upwelling flows near the Wrangell slab edge may cause these volcanoes. A big mantle wedge (BMW) has formed above a flat slab in the mantle transition zone under western AlaskaIntraplate volcanism in western Alaska and the Bering Sea is caused by hot upwelling flows in the BMWThe Wrangell volcanic field and Denali volcanic gap are caused by subduction of the Wrangell and Yakutat slabs, respectively