Evaluating Drivers of Mantle Flow and Sources of Seismic Anisotropy in the Alaskan Subduction Zone: Observations from Offshore/Onshore Shear-Wave Splitting

Subduction zones are essential for mantle convection through the recycling of oceanic lithosphere, however, asthenospheric flow at convergent margins is not uniform. Deformation of the asthenosphere can be driven by subduction processes such as, viscous entrainment to plate motions, slab rollback-induced toroidal flow, and mantle wedge dynamics. These mechanisms are critical to understanding volcanism, margin evolution, and lithosphere-asthenosphere coupling. The easternmost Alaska subduction zone has been extensively studied showing evidence from seismic anisotropy for large-scale toroidal flow around the slab edge. Westward however, near the Shumagin Gap, few observations have been made. Along-strike changes in oceanic plate fabric, steepening slab dip, proximity to the slab edge, plate motion, and hydration of the mantle may all influence anisotropy and mantle flow in this region. Here, we evaluate models using independent offshore shear-wave splitting measurements acquired using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We compare our splitting observations to forward models that consider the distribution of anisotropy and the backazimuthal dependence of observations. The models we test include viscously entrained flow due to oceanic plate motion (~310° CW North), anisotropic fabric variations, anisotropy related to bending faults and mantle serpentinization, and changes in frozen anisotropy in the oceanic lithosphere. Onshore shear-wave splitting observations show fast-axis directions ~55° CW North, inconsistent with 2D mantle wedge flow, assuming A-type olivine, but it is consistent with B-type fabric or trench-parallel flow as suggested by previous studies. Offshore splitting observations appear to vary along-strike. Here, two distinct oceanic plate fabrics exist, one developed from a northeast-spreading direction and the other from an east-spreading direction. Frozen anisotropy in the oceanic lithosphere may play a significant role in the splitting signal produced offshore and may be an important contributor to the distribution of seismic anisotropy. By synthesizing onshore and offshore observations here with our understanding of flow at the eastern slab edge, we can build a more complete model of mantle dynamics for the Alaskan subduction zone.