Lowermost Mantle Anisotropy Research Articles

Seismic anisotropy in the lowermost mantle near the Perm Anomaly

AbstractThe lower mantle is dominated by two large structures with anomalously low shear wave velocities, known as Large Low‐Shear Velocity Provinces (LLSVPs). Several studies have documented evidence for strong seismic anisotropy at the base of the mantle near the edges of the African LLSVP. Recent work has identified a smaller structure with similar low‐shear wave velocities beneath Eurasia, dubbed the Perm Anomaly. Here we probe lowermost mantle anisotropy near the Perm Anomaly using the differential splitting of SKS and SKKS phases measured at stations in Europe. We find evidence for lowermost mantle anisotropy in the vicinity of the Perm Anomaly, with geographic trends hinting at lateral variations in anisotropy across the boundaries of the Perm Anomaly as well as across a previously unsampled portion of the African LLSVP border. Our observations suggest that deformation is concentrated at the boundaries of both the Perm Anomaly and the African LLSVP.

Lowermost mantle anisotropy and deformation along the boundary of the African LLSVP

AbstractShear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we present SKS and SKKS splitting measurements for 233 event‐station pairs at 34 seismic stations that sample D″ beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D″ region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed).

Lowermost mantle anisotropy beneath the northwestern Pacific: Evidence from PcS, ScS, SKS, and SKKS phases

[1] In this study, we utilize data from broadband seismic stations of the Japanese F-net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern Pacific. The comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle, such as PcS/ScS or SKS/SKKS, can yield constraints on anisotropy in the D″ region. We measured splitting for SKS, SKKS, PcS, and ScS phases at F-net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region. We observed many examples of discrepant SKS/SKKS splitting and splitting of ScS, PcS, SKS, and SKKS phases that does not agree well with the known upper mantle anisotropy beneath individual stations. The most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle. In particular, for SKS/SKKS phases that sample the lowermost mantle just to the south and east of the Kamchatka peninsula, we observed generally N-S fast directions with delay times between 0.5 and 1.5 s. These data suggest the presence of a fairly large, coherent region of deformation in the lowermost mantle beneath the northwestern Pacific. Our preferred model for these observations is that solid-state flow at the base of the mantle induces a lattice-preferred orientation of lowermost mantle minerals, generating a seismically anisotropic fabric.

Constraints on lowermost mantle mineralogy and fabric beneath Siberia from seismic anisotropy

Seismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle ( D″). Here we present analysis of the seismic anisotropy beneath Siberia from shear-wave splitting measured in ScS phases. Data come from two near-perpendicular raypaths (Hindu Kush to Northern Canada and the Kuril Arc to Germany; both at ~ 80° epicentral distance) with close ScS reflection points on the Core–Mantle boundary (CMB). We apply differential S–ScS splitting to minimise contamination from the source and receiver-side upper mantle. The two raypaths show different ScS splitting times and fast shear-wave orientations, incompatible with the VTI style of anisotropy inferred for much of the lowermost mantle. The availability of data at two azimuths give us an opportunity to better understand D″ anisotropy than in previous studies. For example, the results provide the first accurate measurement of the dip of the symmetry plane. Several mechanisms have been suggested to explain lowermost mantle anisotropy, including the lattice-preferred orientation of lower mantle minerals such as perovskite or post-perovskite, or the shape-preferred orientation of inclusions of melt. In order to infer the flow regime implied by these mechanisms we use elasticities from published deformation experiments to forward model shear-wave splitting. Tomography of the region suggests a north–south trend in the geodynamics, and a model incorporating post-perovskite with a [100](010) slip system or aligned melt inclusions are most naturally compatible with such a trend. This may suggest a connection with remnant slab material from past subduction in the north Pacific.

Complex seismic anisotropy at the border of a very low velocity province at the base of the Earth's mantle

We constrain the anisotropy associated with a very low velocity province (VLVP) at the base of the Earth's mantle using the SKS and SKKS waves sampling the region. Our selected high‐quality data sets consist of 415 SKS and 111 SKKS waveforms for 127 deep earthquakes recorded at distances between 90° and 150° by the seismic stations in three temporary broadband PASSCAL seismic arrays: the Kaapvaal seismic array (1997–1999), the Tanzania seismic array (1994–1995), and the Ethiopia/Kenya seismic array (1999–2001), as well as the permanent stations in the Global Seismographic Network. These seismic data provide good sampling coverage for some portion of the VLVP and its surrounding areas. Our results show, when the SKS or SKKS phases sample the regions away from the border of the VLVP (inside or outside the VLVP), the apparent splitting parameters inferred from the SKS phases are consistent with those inferred from the SKKS phases, and their variations strongly correlate with seismic stations but not with the exit points at the core‐mantle boundary of these seismic phases. However, when the SKS or SKKS phases sample near the border of the VLVP, the apparent splitting parameters inferred from the SKS phases and SKKS phases are different, and their variations no longer correlate with seismic stations. These features indicate that part of the shear wave splitting for the seismic data sampling the border of the VLVP has to originate from deep mantle, most likely near the border of the VLVP. We assume that the anisotropy in the shallow mantle beneath seismic stations has a horizontal hexagonal symmetry axis and infer the splitting parameters associated with the shallow anisotropy beneath the seismic stations using the SKS and SKKS waveforms for the seismic data sampling the regions away from the border of the VLVP. We then obtain the splitting parameters associated with the lowermost mantle anisotropy using the SKS and SKKS waveforms corrected for the inferred shallow anisotropy beneath seismic stations, assuming that the medium in the lowermost mantle also has a horizontal hexagonal symmetry axis. Our results reveal a complex anisotropy pattern near the border of the VLVP. Such a complex anisotropy pattern may be explained by lattice‐preferred orientation of anisotropic mantle aggregates aligned by a complex mantle flow near the VLVP margins. The complex flow pattern near the VLVP margins may indicate strong interactions between the VLVP, a compositional anomaly, and the surrounding normal mantle, and may provide an explanation to the concentration of some hot spots geographically near the borders of the VLVPs in the lowermost mantle.

Whole mantle discontinuity structure beneath Hawaii

We examined mantle structure beneath the southeast Hawaiian Islands using multiple ScS reverberations from four earthquakes from the island of Hawaii and recorded at station KIP on the island of Oahu. We find an unusually deep 410‐km discontinuity and a transition zone thickness of 227 km, corresponding to a temperature increase of 87 K above the global average. Other reflectors include a lid‐low‐velocity zone boundary, a weak 520‐km discontinuity, and smaller discontinuities at 224 km, 288 km, and 1000 km. Whole mantle travel time is near the global average, which we attribute to an inclined or branching plume, lowermost mantle anisotropy, and estimate bias due to a possible ultra‐low velocity zone atop the core.

Lowermost mantle anisotropy beneath the north Pacific from differentialS-ScSsplitting

Seismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle (D″). We introduce differential S–ScS splitting as a tool for removing the effect of near-source and near-receiver anisotropy to estimate splitting accrued in the D″ region. This is applicable to events recorded at epicentral distances between 60° and 85°. Near-source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy. We apply differential S–ScS splitting to records from Canadian National Seismic Network stations of western Pacific earthquakes; these sample the lowermost mantle beneath the north Pacific. The residual splitting in ScS, which we attribute to D″, shows lag times between 1.0 and 3.9 s. Given the near horizontal ray path of ScS in D″, we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non-VTI (transverse isotropy with a vertical axis of symmetry) style of anisotropy. The largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core–mantle boundary towards an upwelling region beneath the central Pacific. Non-VTI symmetry and the many possible contributions to D″ anisotropy from lower-mantle minerals, melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy.

Development of finite strain in the convecting lower mantle and its implications for seismic anisotropy

Seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle. A combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics. Specifically, it is investigated whether lattice‐preferred orientation (LPO) can explain the anisotropy observed in regions of paleosubduction. Since LPO is caused by dislocation creep and is destroyed by diffusion creep, we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric. Strain is then calculated in these regions and is related to mineral physics experiments combined with high‐pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy. Uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range. The effect of variations in transition stress, activations parameters, and strength of slabs on fabric development is investigated. It is shown that LPO is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction.

Lowermost mantle anisotropy beneath the Pacific: Imaging the source of the Hawaiian plume

We utilized recordings of seismic shear phases provided by several North American broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central Pacific Ocean. Using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms, we examined relative travel times of teleseismic S and Sdiff that sample a large area of lowermost mantle structure. The results of this study provide evidence for small-scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the Pacific Ocean. In particular, we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of D″ beneath Hawaii. Our results, combined with a previous study of V P/ V SH ratios, indicate that ancient subducted slab material may be responsible for observations beneath the northern Pacific, while lenses or layers of core–mantle boundary reaction products or partial melt, oriented by horizontal inflow of mantle material to the Hawaiian plume source, can explain observations beneath the central Pacific.