12:00 PM - 12:15 PM
[SIT17-12] Origin of intraplate volcanism and big mantle wedge beneath western Alaska and the Bering Sea
Keywords:intraplate volcanism, subduction zone, mantle flow, seismic tomography
Many active arc volcanoes exist in south-central Alaska and the Aleutian Islands, which are caused by active subduction of the Pacific plate beneath the North American plate. In addition, some Cenozoic intraplate volcanoes exist in western Alaska and the Bering Sea, but their formation mechanism is still unclear. Here we determine high-resolution 3-D tomographic models of isotropic P-wave velocity (Vp) and tilting axis anisotropy of the Alaska subduction zone down to 800 km depth by inverting a large number of local and teleseismic travel-time data recorded at many portable and permanent network stations in and around Alaska. Our results reveal a flat high-Vp anomaly in the mantle transition zone (410–670 km depths) beneath western Alaska, which is connected with the subducted dipping Pacific slab at 0–410 km depths, suggesting that a big mantle wedge (BMW) has formed above the Pacific slab beneath western Alaska and the Bering Sea. 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 BMW 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.