Deep seismic structure of Aleutian subduction zone using teleseismic PdP and SdS precursor functions

The Aleutian subduction zone is a site of active volcanism with back-arc spreading and island arc formation. The goal of this project was to investigate upper mantle structure across the Aleutian subduction zone using PdP and SdS functions. The Aleutian subduction zone was chosen as our study area because of its significant tectonic activity coupled with high data density PdP/SdS midpoints. In this project, we leveraged the US Transportable Array –a highly dense seismic survey- that provided us with higher quality seismic data. Seismic images were made using the Wavefield Iterative Deconvolution (WID) stacking method. We tested the robustness of our results by comparing them with a 3D GyPSuM Earth Model. The results of our investigation show that where the Pacific plate passes through the transition zone, the 410 discontinuity is elevated by up to 20 km, and the 660 discontinuity is depressed by up to 30 km. We interpreted the variations in the boundaries of the transition zone in terms of phase changes and Claperyon slope, where the 410 discontinuity (phase change) represents olivine to wadsleyite and has a positive Claperyon slope, while the 660 discontinuity (phase change) represents ringwoodite to perovskite and ferropericlase, and has a negative Claperyon slope. Also in the transition zone, the 520 discontinuity, which does not show up globally in seismic data, is observed in regions close to the cold subducting slab and we suggest this observation is a result of “mantle chilling effect”, where the cold subducting Pacific slab cools the mantle near the 520 discontinuity, leading to a sharp 520. We infer that the Pacific slab pools atop the 660 discontinuity and undergoes dehydration, and the release of water contributes to the 660 depression observed. Other significant upper mantle features observed from our results were the presence of a Lithosphere Asthenosphere transition zone (LATZ), between discontinuities observed between depths of 90 to 220 km, and geophysical evidence of possible remnants of the source for Bowers Ridge, which we identified as an Island Arc System.