Deep Seismic Structure of Aleutian Subduction Zone Using Teleseismic PP and SS Precursor Functions

Abstract:

Subduction of the ancient Kula Plate started at the Aleutian subduction zone during Jurassic times and by 48 Mya, the Kula plate was replaced by the subduction of the Pacific Plate. Currently, 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 lithospheric and upper mantle structure across the Aleutian subduction zone using PP and SS functions. In this project, we leveraged the US Transportable Array –a highly dense seismic survey- that provided us with higher quality seismic data and made seismic images with the Wavefield Iterative Deconvolution (WID) stacking method[A. Ainiwaer and Gurrola, 2018]. We tested the robustness of our results by comparing them with a 3D GyPSuM Earth Model [Simmons et al., 2010]. 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 35 km. We interpreted 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 ferripericlase, and has a negative Claperyon slope. Also in the transition zone, the 520 discontinuity 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 Lithosphere Asthenosphere Boundary (LAB), the 220 discontinuity, and geophysical evidence of possible remnants of the source for Bowers Ridge, which we identified as an Island Arc System.