Mantle Transition Zone Structure Beneath The Alaska-Aleutian Subduction Zone And Its Surroundings

The topographies of the 410 km and 660 km discontinuities beneath the Aleutian-Alaska subduction zone and adjacent areas are imaged by identifying SS precursors from teleseismic data recorded by the National Digital Seismic Network and a regional seismic array deployed in the northeastern Tibetan plateau operated by "ChinArray" project. We use the moveout correction and common midpoint stacking to enhance the SS precursory arrivals. Results show that the 410 km and 660 km discontinuities appear to be positively correlated beneath the areas adjacent to the Aleutian-Alaska subduction zone, such as the Bering Sea, Alaska Peninsula, and the southern and eastern Alaska. Thus a normal transition zone is inferred beneath these areas, indicating that the North Pacific plate has not yet subducted into the upper mantle transition zone. Beneath western Alaska, the 660 km discontinuity is strongly depressed and a thicker transition zone can be mapped, while the overlying 410 km discontinuity appears to be normal compared to the global model. Such observations suggest the presence of the remnant Kula plate lies just above the 660 km discontinuity beneath western Alaska.