Knowledge of deep mantle deformation is based on seismic anisotropy: the variation of seismic wave speed and polarization with direction. Measuring this directional dependency requires azimuthal seismic coverage at D″ depth-the bottom few hundred kilometers of the mantle-which is often a limit in retrieving the style of anisotropy. Shear wave splitting is the standard technique for probing mantle anisotropy, and recently, reflections from the D″ region have been used to infer azimuthal anisotropy. Here we combine observations and modeling of D″ reflections with shear wave splitting along a given raypath direction in order to constrain mineralogy and dynamics of the lower mantle. From our modeling, a clear distinction between different anisotropic media is possible by using both types of observations together but only one directional path. We focus on the lowermost mantle beneath the central Atlantic Ocean by using South-Central American earthquakes recorded in Morocco. We find complex azimuthal and distance variation for both polarities of D″ reflections and shear wave splitting parameters, which rules out a simple style of anisotropy-such as vertical transverse isotropy-for the region. Our preferred model consists of a phase transition from a randomly oriented bridgmanite to lattice-preferred orientation fabric in postperovskite, developed in a tilted plane sheared along a roughly SW-NE deformation direction.

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SUMMARY Iceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

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.

Observations of seismic anisotropy are a powerful tool for mapping deformation within the Earth (e.g., Long & Becker, 2010), and are often used to study deformation and flow in the upper mantle (e.g., Skemer & Hansen, 2016). The lowermost mantle, also known as the D″ layer, also clearly exhibits seismic anisotropy (e.g., Garnero & Lay, 1997;Silver, 1996, and references within Nowacki et al., 2011;Romanowicz & Wenk, 2017), and seismic anisotropy observations can be used to map deformation at the base of the mantle. However, this requires thorough knowledge of the mechanism for D″ seismic anisotropy and the relationship between deformation and strain, which can be established by experiments and theoretical modeling. There are several proposed mechanisms for D″ seismic anisotropy, including the shape preferred orientation (SPO) of melt inclusions (or other elastically distinct material) and the crystallographic preferred orientation (CPO) of bridgmanite (bm), ferropericlase (fp), post-perovskite (ppv), or some mixture of these minerals (e.g., Nowacki et al., 2011).A major obstacle in the interpretation of D″ seismic anisotropy measurements is our imprecise knowledge of the mechanism responsible, along with the non-uniqueness of data sets that are based on a limited number of measurements in a given region. A recent synthetic modeling study (Creasy et al., 2019) demonstrated that tighter constraints on seismic anisotropy at the base of the mantle can be obtained by combining different types of observations of body wave anisotropy than by a single type of data alone. Specifically, Creasy et al. ( 2019) examined reflection polarity measurements (P or S waves that reflect off the D″ discontinuity-PdP and SdS) and shear wave splitting (seismic wave birefringence). In this study, we apply the insights gained from the synthetic modeling of Creasy et al. (2019) and combine observations of shear wave splitting (for both ScS and PKS phases) due to D″ seismic anisotropy with observations of PdP/SdS polarities (and their variation with direction) to obtain tight constraints on geometry in a single target region. We apply a novel modeling approach that is based on previous work (Creasy et al., 2017;Ford et al., 2015) and has been