Seismic anisotropy in subduction zones: numerical modeling and infinite strain axis calculations

Subduction zones show significant variability in shear wave splitting, both above and below the slab, and along-strike. For example, in the Peruvian flat slab region, where the slab has a 3D morphology, shear wave splitting shows complex patterns that indicate that the anisotropic structure varies both laterally and vertically. Above the flat-slab, fast directions measured on shear wave splitting of local S phases are dominantly trench-parallel north of the Nazca Ridge, but become more complicated towards the south as the slab steepens. For SKS phases that sample the whole subduction zone, there is an intriguing and pronounced region of nulls measurements, which could indicate either an apparent lack of mantle anisotropy or seismic anisotropy with a vertically aligned symmetry axis. Seismic anisotropy is often interpreted as the result of lattice preferred orientation (LPO) of olivine due to flow in the mantle. We provide some insight into the relationship between mantle flow and seismic anisotropy using 3D numerical modeling and infinite strain axis (ISA) calculations. Our models include a slab with along-strike variable geometry that changes rapidly with time. The changing geometry of the slab induces a complex mantle flow with both vertical and trench-parallel components. Trench-parallel flow is intense in broad regions in the mantle wedge ( 1 cm/yr) and in the sub-slab mantle (>2 cm/yr). We obtain some regions of trench-parallel ISA in the supra-slab mantle, compatible with observations. However, below the slab, the ISA is mostly vertical, with trench-parallel ISA present only in narrow regions. Vertical ISA in broad regions could potentially offer an explanation for the null SKS splitting measurements found below the Peruvian flat-slab region.