Patterns of Seismic Anisotropy Around Subduction Zones: Model Predictions and Implications for Subduction-Induced Mantle Flow

Abstract:

Subduction zones are sites of large lithospheric slabs sinking into the Earth’s mantle inducing complex 3D flow. Seismic anisotropy generated by strain-induced lattice/crystal preferred orientation (LPO/CPO) of intrinsically anisotropic minerals is commonly used to study the patterns of mantle flow and the associated plate motions at convergent margins. Here, we computed the upper mantle fabric due to strain-induced LPO in 3D thermo-mechanical models of dynamic subduction. Overall, strong fabrics develop in the upper and mid mantle around the subduction zone. We find that the mantle fabric occurrence depends on the distribution and amount of the deformation, whereas it is independent of the rate of subduction. As a consequence, distinctive fabric patterns are formed in the upper mantle below, aside and above the slab. Additionally, synthetic seismograms of teleseismic waves propagating sub-vertically were computed to estimate SKS splitting, which are sensitive to the upper mantle anisotropy. The results are remarkably comparable with observations from different subduction settings (i.e., Cascadia, Calabria, Aegean), yielding strong constraints on the recent dynamics of these margins. Concluding, we discuss the potential bias the seismic anisotropy introduces, which might affect isotropic seismic tomographies imaging subduction zones and eventually leading to (mis)interpreation of artificial seismic anomalies.