Evidence of Mid-Mantle Deformation From Shear-Wave Splitting - fig. 2

Evidence of Mid-Mantle Deformation From Shear-Wave Splitting - fig. 2 Figure 2. Possible scenarios of mid-mantle anisotropy. Three postulate the lattice-preferred orientation (LPO) of mid-mantle minerals, in a global boundary layer (Panel A), by stress-fields associated with subduction (Panel B) or by an upwelling in the lower mantle (C). The final scenario is the shape-preferred orientation (SPO) of subducted material (for example, basaltic melt) (Panel D), in a ‘megalith’ in the uppermost lower mantle.

Until recently, the mid-mantle region of the Earth (the transition zone and the uppermost-lower mantle) has been considered isotropic. In this study we present evidence of anisotropy in the mid-mantle region near the Tonga-Kermadec subduction zone, inferred from shear-wave splitting in teleseismic S phases recorded at Australian seismic stations. We analyse the seismograms after applying a wavefield decomposition method to remove the effects of shear-coupled P waves. Lag times of between 0.7-6.2 s are observed, and given the absence of splitting in SKS phases at the Australian stations, we infer the presence of significant anisotropy in the mid-mantle. Forward modeling suggests that the uppermost 100 km or so of the lower mantle is the most likely location for this anisotropy, and we propose several scenarios that might account for this (Figure 2). These involve the lattice-preferred orientation of anisotropic lower-mantle minerals such as MgSiO3 perovskite, or the shape-preferred orientation of inclusions of a subducted phase such as basaltic melt.

Wookey, J., Kendall, J-M. and Barruol, G., Mid-mantle deformation inferred from seismic anisotropy, Nature, 415, 777-780, 2002.

Wookey, J., and Kendall, J-M., Evidence of mid-mantle anisotropy from shear wave splitting and the influence of shear-coupled P waves, J. Geophys. Res., 109, doi:10.1029/2003JB002871, 2004.


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