Deformation of the lowermost mantle from seismic anisotropy

Abstract

isotropy, VTI). With this assumption, SH leads SV here, corresponding to "'=±90 " in our notation (Fig. 1c). A further limitation is using only one azimuth of rays in D!: this cannot distinguish VTI from the case of an arbitrarily tilted axis of rotational symmetry in which wave speed does not vary (tilted transverse isotropy, TTI) when the axis dips towards the receivers or stations. An improvement on this situation can be made by utilising crossing ray paths in D! 10 , but this relies on having the correct source-receiver geometry, which is not possible beneath North America using only deep earthquakes.We address this issue beneath the Caribbean by incorporating measurements from shallow earthquakes in our dataset, and thus reduce the symmetry of the anisotropy which must be assumed.We measure anisotropy in D! using differential splitting in S and ScS phases using an approach described by refs. 10,11 . Both phases travel through the same region of the upper mantle (UM), but only ScS samples D! (Fig. 1a). As the majority of the lower mantle (LM) is relatively isotropic 12 , by removing the splitting introduced in the UM we can measure that which occurs only in D! (see Supplementary Information).Earthquakes in South and Central America, Hawaii, the East Pacific Rise (EPR) and the Mid-Atlantic Ridge (MAR), detected at North American stations, provide a dense coverage of crossing rays which traverse D! beneath southern North America and the Caribbean (Fig. 1b). Three distinct regions are covered (Fig. 2), each sampled along two distinct azimuths. The Caribbean (region ÔSÕ) has been previously well studied 1,4,8 , but the northeast (ÔEÕ) and southwest (ÔWÕ) United States have not. Hence nowhere are our measurements compatible with VTI, because we do not find "«=±90 " within error in both directions for any region.A likely mechanism for the production of anisotropy in D! is the lattice-preferred orientation (LPO) of anisotropic mineral phases present above the CMB such as (Mg,Fe)O, and MgSiO 3 -perovskite (pv) and -postperovskite (ppv). These may give rise to styles of anisotropy more complicated than TTI with lower symmetries, which are compatible with our two-azimuth measurements. We investigate the possibility of LPO in ppv leading to the observed anisotropy rather than other phases because of its likely Our results can differentiate between these candidate mechanisms if we assume that most of the measured anisotropy in D! is a result of deformation-induced LPO in ppv, and we have an accurate estimate of the mantle flow where we measure anisotropy.At present, such models of mantle deformation are in their infancy, but we can nonetheless make inferences from broad-scale trends in subduction and global V S models. We calculate the orientations of the shear planes and slip directions which are compatible with our measurements for the three slip systems in ppv. Aggregate elastic (001) system. These planes and directions are plotted in Fig. 3. We also produce the shear planes predicted for cases of pv and MgO ( Suppl...

The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

“…We show results in terms of the observed fast orientation in the ray frame (φ ′ ), but also as the inferred dip. This is relevant because many previous studies (see Nowacki et al 2011) have attempted to interpret the texture or structure in D ′′ in terms of the orientation of the fast shear wave, in some cases inverting for orientations (e.g., Wookey & Kendall 2008;Nowacki et al 2010;. The ray-theoretical lines and finite-frequency points should overlap in Figures 6, 7 and 10 if the ray-theoretical calculations are appropriate.…”

“…As an introductory aside, we first compare our ray-theoretical results to the frequently made assumption that the ScS ray is horizontal in D ′′ for its whole path in the anisotropic layer (Wookey et al 2005;Wookey & Kendall 2008;Nowacki et al 2010;Ford et al 2015, amongst others). This approximation simplifies the interpretation of results since no epicentral distance dependence arises, and raypaths need not be calculated in an Earth model.…”

“…However, in general φ ′ is not the same for the two methods, and δt is smaller for the finite-frequency methodthough this δt relationship is not universally true, notably for the S2 path. This immediately implies that previous interpretations of flow direction or likely mechanisms for anisotropy in D ′′ using ray methods (e.g., Wookey et al 2005;Wookey & Kendall 2008;Long 2009;Nowacki et al 2010;He & Long 2011;Nowacki et al 2013;Cottaar et al 2014;) may need to be revisited, even in areas of apparently 'weak' and simple anisotropy.…”

“…Furthermore, shear wave splitting measurements do not assume any particular symmetry to the Earth's elasticity, such as radial anisotropy, and hence may yield more information about flow directions than comparisons of the vertically-(SV) and horizontally-polarised (SH) shear arrivals. Several studies (Wookey et al 2005;Wookey & Kendall 2008;Nowacki et al 2010; have used splitting observations to examine D ′′ flow.…”

Strong inheritance of texture between perovskite and post-perovskite in the D′′ layer

“…We compressed flux--grown single crystals of NaNiF 3 suggesting dominant slip on {110} or (100) 6,8 , (010) 7,9 and, recently, (001)tomography 18,19 yields models of the lowermost mantle with horizontally polarized shear waves with a higher velocity than vertically polarized shear waves in presumed cold regions such as the circum--Pacific.…”