Onshore-offshore shear wave splitting observations beneath the Alaska subduction zone: Mantle flow constraints from the Alaska Amphibious Community Seismic Experiment

Abstract:

Subduction zones recycle oceanic lithosphere and help drive convection, however, the asthenosphere's dynamic response is not uniform. Evidence for plate motion driven entrainment, toroidal flow due to slab rollback, and complex mantle wedge dynamics have been observed globally, and are critical to understanding volcanism, margin evolution, and lithosphere-asthenosphere coupling. The easternmost Alaska subduction zone has been well-studied, showing evidence for large-scale toroidal flow around the slab edge. Westward however, near the Shumagin Gap, observations are lacking, particularly beneath the incoming Pacific plate. Along-strike changes in oceanic plate fabric, steepening slab dip, distance from the slab edge, plate motions, and the hydration state of the mantle may all influence anisotropy and mantle flow in this region. To evaluate the likely drivers of mantle flow beneath southwestern Alaska, we constrain seismic anisotropy using shorecrossing data from the Alaska Amphibious Community Seismic Experiment. Here, we present an analysis of SK(K)S splitting measurements and interpret patterns in the mantle wedge and beneath the oceanic plate. We obtain an average delay time of 1.38 s for 58 stations with usable measurements. Onshore analysis reveals trench-parallel anisotropy and average individual delays of 1.14 s, consistent with previous studies. Average delays offshore are 1.57 s and are more variable than the onshore measurements, with many trench-parallel observations perpendicular to Pacific absolute plate motion. The offshore data is noisy, particularly the horizontal channels, making measurements difficult. We average 4.1 usable events per station onshore compared to 1.6 offshore. Onshore fast-axis directions are inconsistent with 2D wedge flow but are consistent with a B-type fabric or trench-parallel flow as suggested by previous studies. Fast-axes do not vary significantly across opposing oceanic plate fabrics, suggesting the contribution from frozen lithospheric anisotropy is low. Offshore results are largely inconsistent with plate driven entrainment, as suggested beneath parts of central Alaska. These results highlight the importance of offshore mantle flow constraints, which increasingly differ from simple plate motion driven predictions.