Lowermost Mantle Anisotropy Beneath Africa From Differential SKS‐SKKS Shear‐Wave Splitting

Abstract

We investigate seismic anisotropy in the lowermost mantle in the vicinity of the African large low shear velocity province (LLSVP) using observations of differential SKS‐SKKS shear‐wave splitting. We use data from 375 permanent and temporary stations in Africa which enable us to map the spatial distribution of the anisotropic regions of the lowermost mantle in unprecedented detail. Our results corroborate previous findings that anisotropy is most clearly observed at the margins of the LLSVP, indicating strong deformation at its border, and they are generally consistent with a mostly isotropic LLSVP interior. We find that most discrepant SKS‐SKKS measurements sample the lowermost mantle close to what is inferred to be the root of the Afar plume. We also identify strongly discrepant splitting in the vicinity of a previously mapped ultralow velocity zone (ULVZ) at the base of the LLSVP, beneath Central Africa. This represents an unusual observation of lowermost mantle anisotropy that is spatially coincident with a ULVZ and may reflect a unique anisotropic mechanism such as alignment of partial melt or the presence of strongly anisotropic magnesiowüstite. We interpret discrepant measurements outside of the LLSVP as likely reflecting a change in flow direction from the horizontal plane to a more vertical direction, which may be caused by deflection at the steep LLSVP border. We propose that our observations of D″ anisotropy associated with the African LLSVP can be explained by a mantle flow regime that maintains passive thermochemical piles with slab‐driven flow and allows for the formation of upwellings at their edges.

A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

“…At the shortest epicentral distances we consider (∆ = 105°) SKS and SKKS exit the core approximately 700 km apart, this distance increases with ∆. This significant deviation leads to the assertion that discrepant splitting between these two phases is best explained by anisotropy in D (e.g., Niu and Perez, 2004;Long, 2009;Reiss et al, 2019). Adapted from Nowacki et al (2011).…”

“…Studies of D typical use either near-horizontally propagating phases (S, ScS, Sdiff) (e.g., Lay and Young, 1991;Wookey et al, 2005a;Maupin et al, 2005;Thomas et al, 2007;Nowacki et al, 2010) or more steeply incident, on the order of ∼ 20°∼ 60°92 depending on epicentral distance, phases (SKS, SKKS) (e.g., Niu and Perez, 2004;Restivo and Helffrich, 2006;Vanacore and Niu, 2011;Ford et al, 2015;Reiss et al, 2019) (Fig. 1).…”

“…SKS-SKKS shear-wave splitting are uncommon, with only ∼ 5% of cases showing discrepancy attributable to D anisotropy in global studies (Niu and Perez, 2004;Restivo and Helffrich, 2006). Discrepant SKS-SKKS shear-wave splitting has been observed near the margin of the African (Wang and Wen, 2007;Lynner and Long, 2014;Ford et al, 2015;Long and Lynner, 2015;Grund and Ritter, 2018;Reiss et al, 2019) and…”

“…We review current methods for identifying discrepant shear-wave splitting, testing existing approaches based on comparing estimated 2σ measurement uncertainties (e.g., Lynner and Long, 2014) and splitting intensity (Deng et al, 2017;Grund and Ritter, 2018;Reiss et al, 2019). We identify and demonstrate clear improvements that can be made to these methods using a set of synthetic split shear-waves and develop a new, multiparameter, approach to identifying discrepant shear-wave splitting.…”

Upper‐Mantle Anisotropy and Dynamics Beneath Northeast Asia: Insight From SKS and Local S Splitting Analysis

“…However, the delay time of the local S wave splitting (0.97 s) is larger than that of the SKS splitting (0.70 s). Compared with the local S waves, the SKS waves have extra raypaths in the lower mantle, whose bottom (D″ region) is expected to be strong anisotropic (e.g., Grund & Ritter, 2019;Lynner & Long, 2014;Reiss et al, 2019). If we attribute the difference to the Figure 5.…”