Imaging Oceanic Upper Mantle of the Alaskan-Aleutian Subduction Zone from Pn Tomography: Initial Insights Into the Regional Slab Structure and State of Hydration

Abstract:

The extent and causative factors of slab hydration at subduction zones and its influence on variations in megathrust behavior and seismicity are poorly understood. Previous studies have linked surficial slab fabric and geomorphic expressions of bending faults to spatial variability in slab hydration and the resulting abundance of seismicity along the South American, Cascadia, and Alaskan margins. Yet recent Pn tomography of the Cascadia subduction zone (VanderBeek & Toomey, 2019) suggests a more complex relationship between oceanic mantle hydration and seafloor fabric. Here, we use travel-times of Pn waves to image isotropic and anisotropic P-wave velocities to characterize shallow mantle structure of the Alaskan-Aleutian subduction zone. The Pn phase is well-suited for constraining shallow lithospheric structure given its high frequency content and efficient propagation. From these results, we can constrain the extent and heterogeneity of mantle hydration to shed light on the possible relationship between regional seafloor fabric and the state of hydration. Previous studies focused on the Shumagin seismic gap and the Semidi regions (e.g. Shillington et al., 2015) observed variations in seismicity and hydration along strike of the subduction zone. The recent Alaska Amphibious Community Seismic Experiment (AACSE), deployed between May 2018 to October 2019, has created an extensive dataset for imaging the 3-D structure of the Aleutian subduction zone. We have identified over 250 events in the outer-rise region and picked 6400 arrivals recorded on the AACSE OBS array. The events are relocated and hypocenters predominantly shift seaward by upwards of 10s of kilometers. Our initial imaging is focused seaward of the trench to constrain oceanic mantle structure prior to subduction where bending-faults may provide pathways for mantle alteration via interaction with seawater. Our preliminary results indicate significant heterogeneity in the oceanic mantle as it enters the Alaska-Aleutian subduction zone.