Mid-mantle anisotropy in subduction zones and deep water transport

Abstract

The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source-side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe. We find significant amounts of splitting ( 2.4 s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice-preferred orientation development in wadsleyite, ringwoodite, or MgSiO 3 -perovskite. We invert for the orientation of several mechanisms-subject to uncertainties in mineralogy and deformation-and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth.

Mid-mantle seismic anisotropy beneath southwestern Pacific subduction systems and implications for mid-mantle deformation

“…10. Following our own previous work and that of others (e.g., Russo and Silver, 1994;Di Leo et al, 2012;Nowacki et al, 2015), we interpret our measurements as mainly reflecting anisotropy in the mid-mantle near the earthquake Foley and Long (2011) and Lynner and Long (2015). Beneath Tonga, the dominant observed fast directions are mainly trench-parallel for both up-dip and down-dip ray paths.…”

“…We compare these results to previously published measurements for Tonga: Foley and Long (2011) documented mainly trench-parallel to trench-oblique fast directions for a set of raypaths measured in western North America, but their study did not have coverage for the central portion of the Tonga-Kermadec subduction zone. In contrast, Nowacki et al (2015) reported convergence-parallel fast directions from northern Tonga for a set of raypaths measured in western North America. It is not clear why these measurements disagree with those of Foley and Long (2011), but the discrepancies may reflect complex anisotropy beneath western North America, which would make accurate receiver-side corrections difficult.…”

“…Well-documented patterns of mid-mantle anisotropy near subducting slabs would provide valuable constraints on patterns of deformation in the mid-mantle and could potentially constrain global models for mantle convection and material transport. Here we build on previous work in Tonga and elsewhere (e.g., Foley and Long, 2011;Di Leo et al, 2012;Kaneshima, 2014;Long, 2014, 2015;Nowacki et al, 2015) and present measurements of source-side shear wave splitting from deep earthquakes originating in the Tonga-Kermadec, Sumatra, New Britain, New Herbides, and Philippines subduction systems. We interpret these measurements as reflecting anisotropy in the transition zone and uppermost lower mantle.…”

“…Several studies have documented seismic anisotropy at mid-mantle depths using both body waves (Wookey et al, 2002;Chen and Brudzinski, 2003;Wookey and Kendall, 2004;Foley and Long, 2011;Di Leo et al, 2012;Lynner and Long, 2014;Nowacki et al, 2015) and surface waves (Trampert and van Heijst, 2002;Yuan and Beghein, 2013), although they are not plentiful compared to studies of upper mantle anisotropy. Global models by Trampert and van Heijst (2002) using dispersion measurements of Love wave overtones suggested the global presence of azimuthal anisotropy in the transition zone.…”

“…Using a similar technique but a different set of events and raypaths, Kaneshima (2014) argued that anisotropy in the upper-most lower mantle just to the north of Tonga is negligible. Finally, Nowacki et al (2015) also applied the source-side splitting technique and argued for the presence of significant anisotropy at mid mantle depths in northern Tonga, suggesting that the anisotropy originates from hydrous phases within the slab.…”