Friday Seminar (14 October 2022) Hejun Zhu (UT Dallas)

Title: Mapping subduction-induced mantle flows

Laboratory experiments and geodynamic flow simulations demonstrate that poloidal- and toroidal-mode mantle flows develop around subduction zones, driven by the downdip motion and rollback of subducting slabs. To date, inferring deep mantle flow circulation patterns in actual subduction environments using shear wave splitting or surface wave tomography remains elusive, due to limited depth or lateral resolution of these techniques. In this study, we use a new 3-D azimuthal anisotropy model constrained by full waveform inversion to infer deep subduction-induced mantle flows underneath the Middle American and Cascadian subduction zones. At depths shallower than 150~km, poloidal-mode mantle flow is observed perpendicular to the trajectory of the Middle American and Cascadian Trenches. From 300 to 450~km depth, return flows surround the lateral edges of the descending Rivera, Atlantic and Gorda slabs. Furthermore, at 700~km, the study region is dominated by the Farallon anomaly, with fast axis directions perpendicular to its strike, suggesting the development of latticepreferred orientations by substantial strains. These observations provide depthdependent constraints on seismic anisotropy for future mantle flow simulations, and call for further investigations about the deformation mechanisms and elasticity of minerals in the transition zone and uppermost lower mantle.