Mineralogy, fabric and deformation domains in D″ across the southwestern border of the African LLSVP

Abstract

Summary Recent advances in seismic anisotropy studies that jointly use reflections and shear wave splitting have proven to place tight constraints on the plausible anisotropic and deformation scenarios in the D″ region. We apply this novel methodology to a large area of the D″ region beneath the South Atlantic, in proximity to and within the African large low seismic velocity province (LLSVP). This area of the mantle is characterized by a transition from fast to slow seismic velocity anomalies and it is thought to be the location of deep-seated plumes responsible for hotspot volcanism. Attempting to probe mantle composition and deformation along the LLSVP borders may provide key information on mantle dynamics. By analyzing seismic phases sampling this region, we detect a D″ discontinuity over a large area beneath the South Atlantic, with inferred depth ranges ∼170 to ∼240 km above the core mantle boundary. We find evidence for a D″ reflector within the area of the LLSVP. Shear wave splitting observations suggest that anisotropy is present in this region of the mantle, in agreement with previous studies that partially sampled this region. We model the observations considering lattice- and shape-preferred orientation of materials expected in the D″ region. A regional variation of mineralogy, phase transition boundaries, and deformation direction is required to explain the data. We infer two distinct domains of mineralogy and deformation: aligned post-perovskite outside the LLSVP and aligned bridgmanite within the LLSVP. The scenario depicted by this study agrees well with the current hypotheses for the composition of the LLSVP and with the prevalence of vertical deformation directions expected to occur along the LLSVPs borders.

Global Compilation of Deep Mantle Anisotropy Observations and Possible Correlation With Low Velocity Provinces

“…Because our aim is to illustrate which regions are well-sampled by commonly used splitting methods and where seismic anisotropy has and has not (yet) been detected, we do not include PdP and SdS polarity measurements here. Such measurements are usually used as an additional constraint along with complementary splitting data (e.g., Pisconti et al, 2019Pisconti et al, , 2023 rather than being interpreted as uniquely indicative of deep mantle anisotropy. We estimate this ray coverage by considering all events with moment magnitudes 6 or larger (according to the International Seismological Center Bulletin, International Seismological Centre ( 2023)) that occurred between January 1990 and March 2023, and the station distribution covered by most common data request clients (see Acknowledgments).…”

“…Because our aim is to illustrate which regions are well-sampled by commonly used splitting methods and where seismic anisotropy has and has not (yet) been detected, we do not include PdP and SdS polarity measurements here. Such measurements are usually used as an additional constraint along with complementary splitting data (e.g., Pisconti et al, 2019Pisconti et al, , 2023 rather than being interpreted as uniquely indicative of deep mantle anisotropy.…”

“…This assumption is generally valid if the difference in splitting due to deep mantle anisotropy is sufficiently large (e.g., Tesoniero et al, 2020), although it has been shown that small differences in SKS-SKKS splitting can be due to upper mantle structure (e.g., Lin et al, 2014). Wookey et al (2005) developed the S-ScS differential splitting technique, which has been widely used since then (e.g., Asplet et al, 2023;Creasy et al, 2017;Nowacki et al, 2010;Pisconti et al, 2023). This method exploits the fact that S and ScS waves have very similar raypaths in the upper mantle beneath the source and receiver.…”

“…Later studies expanded polarity analyses to consider D′′-reflected SdS and PdP waves (Figure 2b; e.g., Pisconti et al, 2019;Thomas et al, 2011). Some of these studies show that changes in polarity are likely caused by the presence of seismic anisotropy (e.g., Garnero, Maupin, et al, 2004;Maupin et al, 2005), and others use polarities of reflected waves as an additional constraint on the nature of an anisotropic D′′ region along with a different method (Pisconti et al, 2019(Pisconti et al, , 2023. Wang and Wen (2004) and Niu and Perez (2004) were among the first studies to interpret differences in SKS and SKKS splitting in terms of deep mantle anisotropy.…”

Lowermost mantle structure beneath the central Pacific Ocean: ultra-low velocity zones and seismic anisotropy

“…A plausible scenario is that the inferred lowermost mantle anisotropy can be explained by lattice-preferred orientation (LPO) in the lowermost mantle within or outside the LLVP (Figure 9a). In theory, measurements of deep mantle anisotropy splitting parameters can be used to constrain plausible flow scenarios if the anisotropy is due to LPO (e.g., Ford et al, 2015;Creasy et al, 2021;Wolf & Long, 2022;Pisconti et al, 2023).…”