Mid-mantle discontinuities beneath subduction zones linked to depth-dependent deformation of bridgmanite.

Bridgmanite, the main mineralogical phase in the Earth's lower mantle, is intrinsically anisotropic and may therefore generate seismic anisotropy in actively-deforming regions, particularly subduction zones. The bulk of the lower mantle is widely regarded as well-mixed but recent seismic observations suggest structures at mid-mantle depths whose origin remains unclear. Here, we propose a mechanism that could generate seismic discontinuities beneath subduction zones with depth distributions consistent with some of the observed mid-mantle reflectors. By coupling thermo-mechanical subduction models with pressure-dependent fabric calculations, we show that gradual slip transitions in bridgmanite with depth can manifest anisotropic discontinuities near 1000 km, which can deepen depending on the extent of accumulated deformation. This discontinuity is most pronounced in regions of strong slab-driven flow as revealed by its reflection coefficients, the strength of which varies with distance and azimuth. While our models predict  ~ 1.5% contrast in fast versus slow shear wave velocities in terms of radial anisotropy, our full waveform simulations show that the discontinuity is detectable with array seismological methods. These results suggest that mid-mantle reflectors near subduction zones may be strain-induced, providing an additional mechanism that links mantle flow, mineral physics, and seismic observations.