On the Origin of Radial Anisotropy Near Subducted Slabs in the Midmantle

Recent seismic studies indicate the presence of seismic anisotropy near subducted slabs in the transition zone and uppermost lower mantle (mid-mantle). In this study, we investigate the origin of radial anisotropy in the mid-mantle using 3-D geodynamic subduction models combined with mantle fabric simulations. These calculations are compared with seismic tomography images to constrain the range of possible causes of the observed anisotropy. We consider three subduction scenarios: (i) slab stagnation at the bottom of the transition zone; (ii) slab trapped in the uppermost lower mantle; and (iii) slab penetration into the deep lower mantle. For each scenario, we consider a range of parameters, including several slip systems of bridgmanite and its grain-boundary mobility. Modeling of lattice-preferred orientation shows that the upper transition zone is characterized by fast-SV radial anisotropy anomalies up to −1.5%. For the stagnating and trapped slab scenarios, the uppermost lower mantle is characterized by two fast-SH radial anisotropy anomalies of ∼+2% beneath the slab's tip and hinge. On the other hand, the penetrating slab is associated with fast-SH radial anisotropy anomalies of up to ∼+1.3% down to a depth of 2,000 km. Four possible easy slip systems of bridgmanite lead to a good consistency between the mantle modeling and the seismic tomography images: [100](010), [010](100), [001](100), and (Formula presented.). The anisotropy anomalies obtained from shape-preferred orientation calculations do not fit seismic tomography images in the mid-mantle as well as lattice-preferred orientation calculations, especially for slabs penetrating into the deep lower mantle.