Seismic anisotropy has been documented in many portions of the lowermost mantle, with particularly strong anisotropy thought to be present along the edges of large low shear velocity provinces (LLSVPs). The region surrounding the Pacific LLSVP, however, has not yet been studied extensively in terms of its anisotropic structure. In this study, we use seismic data from southern Peru, northern Bolivia and Easter Island to probe lowermost mantle anisotropy beneath the eastern Pacific Ocean, mostly relying on data from the Peru Lithosphere and Slab Experiment and Central Andean Uplift and Geodynamics of High Topography experiments. Differential shear wave splitting measurements from phases that have similar ray paths in the upper mantle but different ray paths in the lowermost mantle, such as SKS and SKKS, are used to constrain anisotropy in D. We measured splitting for 215 same station-event SKS-SKKS pairs that sample the eastern Pacific LLSVP at the base of the mantle. We used measurements of splitting intensity(SI), a measure of the amount of energy on the transverse component, to objectively and quantitatively analyse any discrepancies between SKS and SKKS phases. While the overall splitting signal is dominated by the upper-mantle anisotropy, a minority of SKS-SKKS pairs (∼10 per cent) exhibit strongly discrepant splitting between the phases (i.e. the waveforms require a difference in SI of at least 0.4), indicating a likely contribution from lowermost mantle anisotropy. In order to enhance lower mantle signals, we also stacked waveforms within individual subregions and applied a waveform differencing technique to isolate the signal from the lowermost mantle. Our stacking procedure yields evidence for substantial splitting due to lowermost mantle anisotropy only for a specific region that likely straddles the edge of Pacific LLSVP. Our observations are consistent with the localization of deformation and anisotropy near the eastern boundary of the Pacific LLSVP, similar to previous observations for the African LLSVP.

Observations of seismic anisotropy near the core‐mantle boundary may yield constraints on patterns of lowermost mantle flow. We examine seismic anisotropy in the lowermost mantle beneath Australia, bounded by the African and Pacific Large Low Shear Velocity Provinces. We combined measurements of differential splitting of SKS‐SKKS and S‐ScS phases sampling our study region over a range of azimuths, using data from 10 long‐running seismic stations. Observations reveal complex and laterally heterogeneous anisotropy in the lowermost mantle. We identified two subregions for which we have robust measurements of D″‐associated splitting for a range of ray propagation directions and applied a forward modeling strategy to understand which anisotropic scenarios are consistent with the observations. We tested a variety of elastic tensors and orientations, including single‐crystal elasticity of lowermost mantle minerals (bridgmanite, postperovskite, and ferropericlase), tensors based on texture modeling in postperovskite aggregates, elasticity predicted from deformation experiments on polycrystalline MgO aggregates, and tensors that approximate the shape preferred orientation of partial melt. We find that postperovskite scenarios are more consistently able to reproduce the observations. Beneath New Zealand, the observations suggest a nearly horizontal [100] axis orientation with an azimuth that agrees well with the horizontal flow direction predicted by previous mantle flow models. Our modeling results further suggest that dominant slip on the (010) plane in postperovskite aggregates provides a good fit to the data but the solution is nonunique. Our results have implications for the mechanisms of deformation and anisotropy in the lowermost mantle and for the patterns of mantle flow.

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.

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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.

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Seismic anisotropy has been detected at many depths of the Earth, including its upper layers, the lowermost mantle, and the inner core. While upper mantle seismic anisotropy is relatively straightforward to resolve, lowermost mantle anisotropy has proven to be more complicated to measure. Due to their long, horizontal raypaths along the core-mantle boundary, S waves diffracted along the core-mantle boundary (Sdiff) are potentially strongly influenced by lowermost mantle anisotropy. Sdiff waves can be recorded over a large epicentral distance range and thus sample the lowermost mantle everywhere around the globe. Sdiff therefore represents a promising phase for studying lowermost mantle anisotropy; however, previous studies have pointed out some difficulties with the interpretation of differential SHdiff-SVdiff travel times in terms of seismic anisotropy. Here, we provide a new, comprehensive assessment of the usability of Sdiff waves to infer lowermost mantle anisotropy. Using both axisymmetric and fully 3D global wavefield simulations, we show that there are cases in which Sdiff can reliably detect and characterize deep mantle anisotropy when measuring traditional splitting parameters (as opposed to differential travel times). First, we analyze isotropic effects on Sdiff polarizations, including the influence of realistic velocity structure (such as 3D velocity heterogeneity and ultra-low velocity zones), the character of the lowermost mantle velocity gradient, mantle attenuation structure, and Earth’s Coriolis force. Second, we evaluate effects of seismic anisotropy in both the upper and the lowermost mantle on SHdiff waves. In particular, we investigate how SHdiff waves are split by seismic anisotropy in the upper mantle near the source and how this anisotropic signature propagates to the receiver for a variety of lowermost mantle models. We demonstrate that, in particular and predictable cases, anisotropy leads to Sdiff splitting that can be clearly distinguished from other waveform effects. These results enable us to lay out a strategy for the analysis of Sdiff splitting due to anisotropy at the base of the mantle, which includes steps to help avoid potential pitfalls, with attention paid to the initial polarization of Sdiff and the influence of source-side anisotropy. We demonstrate our Sdiff splitting method using three earthquakes that occurred beneath the Celebes Sea, measured at many Transportable Array (TA) stations at a suitable epicentral distance. We resolve consistent and well-constrained Sdiff splitting parameters due to lowermost mantle anisotropy beneath the northeastern Pacific Ocean.

The exact mechanism for lowermost mantle seismic anisotropy remains unknown; however, work on the elasticity and deformation of lower mantle materials has constrained a few possible options. The most probable minerals producing anisotropy are bridgmanite, postperovskite, and ferropericlase. While there is an extensive literature on the elasticity and deformation of lower mantle minerals, we create a comprehensive uniform database of D″ anisotropy scenarios. In order to characterize a range of the possible fabrics for D″ anisotropy, we carry out VPSC (visco-plastic self-consistent modeling) to predict textures for each proposed mineral and dominant slip system. We numerically deform each mineral under different geometrical scenarios: simple shear, pure shear, and extension. By using published single crystal elasticity values, we produce a library of 336 candidate elastic tensors. We used the elastic tensor library to revisit previously published D″-associated seismic anisotropy studies for crossing raypaths (Siberia, North America, the Afar region of Africa, and Australia). While we cannot identify a single, unique mechanism that explains all of these data sets, we find that postperovskite (dominant slip on [100](010) or [100](001)) and periclase (dominant slip on {100}<011>) provide the best fit to the observations and suggest reasonable shear directions for each region of interest. Bridgmanite generally provides a poor fit to the observations; however, we cannot completely rule out any particular model. As the number of anisotropy observations for D″ increases, this elastic tensor library will be helpful for observational seismologists in identifying possible mechanisms of anisotropy and shear directions at in the lowermost mantle.Plain-Language Summary At 2,800 km below the Earth's surface, minerals are being deformed under the high pressures, temperatures, and stresses of the deep mantle. A seismic phenomenon (seismic anisotropy) has been observed within this region, likely due to the deformation of some unknown mineral. There have been three proposed minerals based on experimental and theoretical work. As a result, we create a library of proposed mechanisms of this anisotropy by numerically calculating a series of plausible deformed rocks with a code called VPSC (viscoplastic self-consistent modeling). We have established an open-source library of elastic tensors (plausible deformed rocks) for all of the proposed minerals. We compare this library to data that have been previously observed near the core-mantle boundary (Siberia, North America, the Afar region of Africa, and Australia). We find that some of the tensors cannot consistently fit all of the available data sets (such as bridgmanite-the most abundant mineral in the lower mantle). However, two of the minerals can fit all of the data quite well (postperovskite and ferropericlase). Postperovskite is a result of bridgmanite changing its structure due to the high pressures and temperatures, approximately 200 km above the core-mantle boundary, and the lowe...