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

Abstract

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 <110>false{true1¯10false}. 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.

SASSY21: A 3-D seismic structural model of the lithosphere and underlying mantle beneath Southeast Asia from multi-scale adjoint waveform tomography

“…The absolute, depth-averaged v SH and v SV values can be found in Figure 7, while Figure 9 presents lateral variations at 150 and 250 km depth. These results reveal negative radial anisotropy along the slabs (which is in good agreement with Sturgeon et al, 2019) and beneath Sundaland, and positive radial anisotropy around the Celebes Sea, Sulawesi and the Banda Sea. We believe this to be the result of two different mechanisms: (a) the oceanic (Indo-)Australian plate consists of horizontally aligned minerals, which then rotate into subvertical orientations during subduction and/or entrain the surrounding mantle and induce vertical flow, thus accounting for negative values along the slab (Song & Kawakatsu, 2012) and (b) negative frozen-in anisotropy of continental-lithosphere roots during formation (Priestley et al, 2021), thus explaining negative values beneath the Sundaland block.…”

Geodynamic tomography: constraining upper-mantle deformation patterns from Bayesian inversion of surface waves

“…Forward models such as this assist further in the interpretation of seismic tomography models in terms of mantle circulation patterns. To cite an example, CPO-induced anisotropy resulting from to 3-D numerical simulations of subducting slabs shows consistency with radial anisotropy patterns inferred from global tomographic images (Ferreira et al 2019;Sturgeon et al 2019). However, most studies rely on visual comparisons between CPO obtained from numerical simulations and tomographic images.…”

Quantifying Intrinsic and Extrinsic Contributions to Radial Anisotropy in Tomographic Models

“… with the intrinsic radial anisotropy CPO E  computed from a geodynamically based CPO model (Becker et al, 2003(Becker et al, , 2006Ferreira et al, 2019;Sturgeon et al, 2019). The comparison should be done instead with an effective model * CPO E  , which is difficult to estimate without access to any elastic homogenization tools.…”

“…For instance, tomographic imaging has revealed the presence of strong azimuthal and radial anisotropy in the upper 250 km of the mantle (refer to Long and Becker [2010] for a comprehensive review). Long‐wavelength seismic anisotropy is also prevalent in the transition zone (e.g., Trampert & van Heijst, 2002; Wookey & Kendall, 2004) although its origin is still highly debated (Chang & Ferreira, 2019; Chen & Brudzinski, 2003; Sturgeon et al., 2019). Probing deeper depths, the lower mantle appears to be isotropic (e.g., Meade et al., 1995) barring the D” layer where enough evidence have shown it to be anisotropic (e.g., Beghein et al., 2006; Kendall & Silver, 1998; McNamara et al., 2002; Panning & Romanowicz, 2006).…”