Shear Wave Splitting Observations Research Articles

Insights From Layered Anisotropy Beneath Southern New England: From Ancient Tectonism to Present‐Day Mantle Flow

AbstractSeismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area.

Seismic Azimuthal Anisotropy of Northeastern Tibetan Plateau From Ambient Noise Double Beamforming Tomography: Implications for Crustal Deformation

AbstractIn this study, we obtain a three‐dimensional (3‐D) high resolution seismic anisotropy model of NE Tibet using ambient noise double beamforming (DBF) tomography with ChinArray‐Himalaya II data. DBF tomography technique allows for the simultaneous derivation of local phase velocity and azimuthal anisotropy from ambient noise cross‐correlations without performing tomographic inversion. Based on the DBF tomography results, we have constructed a high resolution 3‐D azimuthal anisotropic shear wave velocity model of NE Tibet. Our model indicates the presence of prominent low velocities and strong azimuthal anisotropy in the mid‐to lower crust beneath the Songpan‐Ganzi belt, while the mid‐to lower crust of the Qilian belt exhibits relatively weak low velocity and azimuthal anisotropy. We propose that crustal channel flow may be the primary mechanism driving crustal deformation in the Songpan‐Ganzi belt, which is in the mature part of the plateau. As the Qilian belt is at an early stage of plateau formation, it is characterized by crustal shortening with the development of a sub‐vertical foliation. Additionally, the misalignment between predicted and observed shear wave splitting reveals decoupled crust and upper mantle deformation west of the Ordos block where eastward mantle flow is deflected by the lithospheric keel of Ordos.

Heterogeneous seismic anisotropy beneath Madeira and Canary archipelagos revealed by local and teleseismic shear wave splitting

SUMMARYMid-plate upward mantle flow is a key component of global mantle convection, but its patterns are poorly constrained. Seismic anisotropy is the most direct way to infer mantle flow as well as melt distribution, yet the convection patterns associated with plume-like mantle upwelling are understudied due to limited seismic data coverage. Here, we investigate seismic anisotropy beneath the Madeira and Canary hotspots using a dense set of shear wave splitting observations and combining teleseismic and local events recorded by three-component broad-band and short-period seismic stations. Using a total of 26 stations in the Madeira archipelago and 43 stations around the Canary Islands, we obtain 655 high-quality measurements that reveal heterogeneous flow patterns. Although local event results are sparse around most islands, we can observe a small average of S-wave splitting times of 0.16 ± 0.01 s, which significantly increase with source depth beneath El Hierro (>20 km) and Tenerife (>38 km) up to 0.58 ± 0.01 and 0.47 ± 0.05 s. This suggests an influence of melt pocket orientation in magma reservoirs developed at uppermost-mantle depths. Likewise, anisotropy increases significantly beneath the islands with shield stage volcanism (up to 9.81 ± 1.78 per cent at El Hierro, western Canaries, against values up to 1.76 ± 0.73 per cent at Lanzarote, eastern Canaries). On average, teleseismic SKS-wave splitting delay times are large (2.19 ± 0.05 s), indicating sublithospheric mantle flow as the primary source for anisotropy in the region. In the Canaries, the western islands show significantly smaller average SKS delay times (1.93 ± 0.07 s) than the eastern ones (2.25 ± 0.11 s), which could be explained by destructive interference above the mantle upwelling. Despite complex patterns of fast polarization directions throughout both regions, some azimuthal pattern across close stations can be observed and related to present-day mantle flow and anisotropy frozen in the lithosphere since before 60 Ma. Additionally, we infer that the current presence of a mantle plume beneath the archipelagos leads to the associated complex, small-scale heterogeneous anisotropy observations.

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

SUMMARYRecent 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 analysing 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.

Mantle flow under the Central Alps: Constraints from shear-wave splitting for non-vertically-incident SKS waves

Shear-wave splitting measurements map the orientation of geodynamically induced fabrics in the upper mantle. However, due to the limited depth resolution of shear-wave splitting investigations from core-mantle refracted phases, it has so far been difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To improve this differentiation, we take advantage of backazimuthal variations of the fast orientation ϕ and delay time Δt that is expected for an olivine-mantle block with vertical b-axis as compared to vertical c-axis. This suggests that shear-wave splitting observations can in principle provide constraints that have so far not been recognized. The two end member cases can occur depending on the type of deformation, for example, a subhorizontal foliation in the case of a Simple Asthenospheric Flow and a subvertical foliation when considering transcurrent/transpressional circumstances during Vertical Coherent Deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Fast orientations ϕ show a spatially coherent and relatively simple mountain-chain-parallel pattern without large azimuthal variations per station, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the entire study area, our procedure indicates b-up olivine and thus favors an asthenospheric source of the anisotropy, with a horizontal flow plane of deformation. We also test the influence of position relative to the (European) slab under the Central Alps, distinguishing northern and southern subarea based on vertically-integrated travel times calculated from a tomographic model. Differences in the statistical distribution of splitting parameters ϕ and Δt, and in the backazimuthal variation of their variation δϕ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of planar Poiseuille flow around the Alps, the origin in the southern subarea remains uncertain, but it is likely to be affected by the slab itself.

Nature and structural heterogeneities of the lithosphere control the continental deformation in the northeastern and eastern Iranian plateau as revealed by shear-wave splitting observations

To investigate the intracontinental deformation during the early stage of Arabia-Eurasia collision, we studied the seismic anisotropy of the northeastern and eastern Iranian plateau via teleseismic shear-wave splitting analysis using a dense array of 65 stations. While the average delay times of ∼0.75 s for most stations suggest a moderate magnitude of anisotropy, small-scale variations in fast polarization directions (FPDs) indicate complex anisotropic features that may be related to the nature and structural heterogeneities of the lithosphere. The variations in FPDs are correlated with tectonic boundaries, including the northeastern plateau around the Paleo-Tethys suture and large-scale strike-slip faults bounding the Lut block in the eastern plateau. This suggests that the weak boundary zones may have significantly impacted the lithospheric deformation and anisotropy in response to the ongoing continental collision. Within the Lut block, the insignificant surface deformation suggests that the upper crust might not be noticeably affected by collision, and the fault-induced deformation mainly occurs at depth. The regional northeastward asthenospheric flow driven by the lithospheric thickness contrast across the collisional front could contribute but to different extents among areas. The asthenosphere-induced anisotropy has a magnitude (delay time) comparable to that from lithospheric deformation in central Lut, consistent with the two-layer model observed there. It dominates the seismic anisotropy in northern Lut where the thin lithosphere may contribute less but probably facilitates melt concentration on the top of the asthenosphere and thus enhances anisotropy through aligned melt-filled pores. The complex deformation pattern of the study region suggests that the intracontinental deformation may be controlled by not only the collisional processes at the plate boundary but also the nature and structural heterogeneities of the continental lithosphere itself, and the Arabia-Eurasia convergence may be more preferentially accommodated along tectonic boundaries and at depth.

Diurnal expansion and contraction of englacial fracture networks revealed by seismic shear wave splitting

Fractures contribute to bulk elastic anisotropy of many materials in the Earth. This includes glaciers and ice sheets, whose fracture state controls the routing of water to the base and thus large-scale ice flow. Here we use anisotropy-induced shear wave splitting to characterize ice structure and probe subsurface water drainage beneath a seismometer network on an Alpine glacier. Shear wave splitting observations reveal diurnal variations in S-wave anisotropy up to 3%. Our modelling shows that when elevated by surface melt, subglacial water pressures induce englacial hydrofractures whose volume amounts to 1-2 percent of the probed ice mass. While subglacial water pressures decrease, these fractures close and no fracture-induced anisotropy variations are observed in the absence of meltwater. Consequently, fracture networks, which are known to dominate englacial water drainage, are highly dynamic and change their volumes by 90-180 % over subdaily time scales.

Shear wave splitting measurements in northeastern Uganda and southeastern Tanzania: corroborating evidence for sublithospheric mantle flow beneath East Africa

SUMMARY Several interpretations of shear wave splitting measurements in East Africa have been proposed, but a comprehensive understanding of seismic anisotropy is lacking vis-à-vis geodynamic processes affecting the African Plate, in part because of geographically limited data coverage. Here, we report new shear wave splitting observations using PKS, SKS and SKKS phases recorded on 17 temporary AfricaArray seismic stations in northeastern Uganda and southeastern Tanzania that help fill key gaps in data coverage in non-volcanic regions of East Africa. The stations in northeastern Uganda straddle the Aswa shear zone, which separates the Neoproterozoic Saharan Metacraton (to the east) and the Palaeoproterozoic Northern Uganda Terrane (to the west). The stations in southeastern Tanzania are located mostly in Karoo basins formed within the Neoproterozoic Mozambique Belt. Our results show NNE fast polarization directions in northeastern Uganda, with splitting times between 1.0 and 1.5 s, and NE directions in southeastern Tanzania, with splitting times between 0.6 and 1.3 s. The NNE fast polarization directions in Uganda cannot be explained with oriented melt pocket (OMP) anisotropy in the lithosphere, as the stations are far from any volcanic fields, or fossil anisotropy resulting from the Neoproterozoic East African orogeny. They are, however, parallel to the flow direction of the African superplume and corroborate previous measurements from northern Uganda that were used to argue that the overall northerly orientation of fast polarization directions across East Africa can be attributed to sublithospheric mantle flow associated with the African superplume. The NE fast polarization directions in Tanzania cannot be explained with SE-oriented fossil anisotropy expected from the Mesozoic rifting of Madagascar from Africa or OMP anisotropy in the lithosphere. However, they are consistent with the overall northerly orientation of fast polarization directions in East Africa and can be attributed to either fossil anisotropy in the Mozambique Belt lithosphere resulting from the East African Orogeny or sublithospheric mantle flow associated with the African superplume.

Shear-wave splitting observations: Implications toward the variable anisotropic structure of the upper mantle below the Kachchh rift zone, Gujarat, India

The shear-wave splitting parameters (i.e. fast axis orientation (ψ) and delay time (δt)) are measured at twelve broadband stations in the Kachchh rift zone (KRZ), Gujarat, India, using SKS/SKKS core phases. The average splitting parameters (ψ, δt) vary from (49.41 ± 5.88, 1.63 ± 0.28) at Bhachau (i.e., 4 km SE of the 2001 Bhuj mainshock location) to (103.78 ± 12.82, 1.91 ± 0.35) at Mandvi near the coast. The present study reveals a mean ψ of (76.91o ± 4.57o) with a mean δt of 1.84 ± 0.30 s, for the KRZ. The observed large mean delay 1.84 ± 0.30 s for the region suggests a combined anisotropic contribution from both the lithospheric frozen anisotropy as well as the asthenospheric flow induced anisotropy. Inferred thicknesses of the anisotropic layers from the estimated δt vary from 160 km (in the central KRZ) to 269 km (in the surrounding unrifted zones (SURZ)). The measured deviation of ψ from the absolute plate motion (APM) direction of the Indian plate varies from 8.4o (in the central KRZ) to 62.8° (in the SURZ). The large δt and large deviation of ψ from the APM (>35°) are found to be associated with those stations (e.g. MND, KNM, VGH, NGR, VJP, TPM), which lie near to the geological boundaries in the SURZ. The main finding of the present modeling study depicts a thick highly anisotropic upper mantle underlying the KRZ, where mantle anisotropy parallel to the Kachchh rift axis direction is caused by both frozen lithospheric anisotropy and asthenospheric flow-induced anisotropy. The geographical and azimuthal variations of the measured splitting parameters detect three different anisotropic continental fragments, which might have preserved an ENE-WSW trending fossilized olivine fabric, which was formed before the assemblage of these continental fragments.

Constraining Ice Shelf Anisotropy Using Shear Wave Splitting Measurements from Active‐Source Borehole Seismics

Observations of ice shelf anisotropy on borehole seismic data are presented. Hot‐water‐drilledboreholes were made by the Aotearoa New Zealand Ross Ice Shelf Programme through a grounding‐line proximal site at Windless Bight and the central Ross Ice Shelf site HWD‐2. The boreholes were used to freeze seismometers into the ice at different depths. Seismic observations of shear wave splitting were made on the borehole seismometers using active sources deployed at the surface. These shear wave splitting data were used to constrain anisotropic ice crystallographic preferred orientations (CPO) within the ice column. Forward models of seismic properties from different CPO geometries are compared to the observations and a best fitting CPO model is found to explain the seismic anisotropy at HWD‐2. This model consists of a vertical girdle of ice c axes that constitute 80% of the CPO in combination with tight horizontal clusters, which contribute 20% of ice c axes. The origin of the modeled CPO is discussed with regard to calculated strain rates at the site and found to be indicative of the current shear kinematics with vertical shear plane and horizontal shear direction. At HWD‐2 the 370 m thick ice shelf is calculated to consist of at least 197 m of anisotropic ice.

Subduction dynamics and structural controls on shear wave splitting along the South American convergent margin

Observations of shear wave splitting have provided constraints on patterns of deformation in the mantle in a variety of tectonic settings, including those with complex dynamics such as subduction zones. The South American subduction system is characterized by the longest laterally continuous subducting slab present today. As such, it represents an excellent natural laboratory to study variability in anisotropy along thousands of kilometers of a subduction zone. We perform shear wave splitting analyses at 59 stations along the Chilean coast spanning over 2500 km of the subducting Nazca slab. This dataset specifically targets stations that sample minimal amounts of thickened South American crust and mantle wedge material making it easier to attribute splitting to the sub-slab mantle. We observe a stark transition in splitting from the central Bolivian Orocline to southern Chile indicative of a transition in mantle deformation beneath the subducting slab from convergent flow beneath the Orocline to trench parallel flow farther south. This supports previous conceptual models of mantle dynamics in the region where a stagnation point exists beneath the Bolivian Orocline resulting in plate motion driven flow with trench parallel escape flow to both the north and south. Our results as well as previous measurements largely agree with a recent numerical modeling study that has proposed similar large-scale deformation patterns as well as local deviations due to slab structures like tears. The agreement between model predictions and our observations, however, is not universal along strike. We see several regions where measured splitting deviates significantly from predictions. These areas warrant further study and will aid in refining future geodynamic modeling efforts.

Shear-wave splitting beneath Fennoscandia — evidence for dipping structures and laterally varying multilayer anisotropy

SUMMARYThe geodynamic evolution of Fennoscandia in northern Europe (Finland, Sweden and Norway) is coined by ca. 3 Ga history of tectonic processes including continental growth in its central and eastern parts and Neogene uplift processes of the Scandinavian mountains (Scandes) located along its western edge. Many details are still under debate and we contribute with new findings from studying deep-seated seismic anisotropy. Using teleseismic waveforms of more than 260 recording stations (long-running permanent networks, previous temporary experiments and newly installed temporary stations) in the framework of the ScanArray experiment, we present the most comprehensive study to date on seismic anisotropy across Fennoscandia. The results are based on single and multi-event shear-wave splitting analysis of core refracted shear waves (SKS, SKKS, PKS and sSKS). The splitting measurements indicate partly complex, laterally varying multilayer anisotropy for individual areas. Consistent measurements at permanent and temporary recording stations over several years and for seismic events of specific source regions allow us to robustly constrain dipping anisotropic structures by adding systematic forward modelling. Although the data coverage is partly limited to only few source regions, our findings support concepts of continental growth due to individual episodes of (paleo-) subduction, each affecting a plunging of the anisotropic fast axis direction due to collisional deformation. Along the northern Scandes the fast axis direction (ϕ) is parallel to the mountain range (NE-SW), whereas an NNW-SSE trend dominates across the southern Scandes. In the south, across the Sorgenfrei–Tornquist Zone, a NW-SE trend of ϕ dominates which is parallel to this suture zone. The Oslo Graben is characterized by an NNE-SSW trend of ϕ. In northern Norway and Sweden (mainly Paleoproterozoic lithosphere), a dipping anisotropy with ϕ towards NE prevails. This stands in contrast to the Archean domain in the NE of our study region where ϕ is consistently oriented NNE-SSW. In the Finnish part of the Svecofennian domain, a complex two-layer anisotropy pattern is found which may be due to lateral variations around the seismic stations and which requires a higher data density than ours for a unique model building. Based on these findings our study demonstrates the importance of long recording periods (in the best case > 10 yr) to obtain a sufficient data coverage at seismic stations, especially to perform meaningful structural modelling based on shear-wave splitting observations.

Shear Wave Splitting and Mantle Flow Beneath Alaska

AbstractShear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific‐Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte‐Fairweather transform margin are parallel to the faults and are likely caused by the strike‐slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along‐strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America‐Pacific relative motion.

Preliminary analysis of the shear-wave splitting observations from the Qiaojia seismic array*

A total of 351 shear-wave splitting results at 25 stations were obtained from the seismic data recorded in period of January, 2013 to December, 2016, by a broadband seismic array deployed in the northern segment of Xiaojiang Fault Zone (n-XJFZ). Meanwhile, the stress field of the n-XJFZ was determined by inverting 140 focal mechanism solutions of the small earthquakes within the study area which were recorded in the same period. This determination confirmed a compressive stress in NW-SE orientation and an extensional stress in the NE-SW orientation, with little difference from those released by previous studies. The shear-wave splitting results show a spatial complexity in polarization orientation, different from one site to another. However, the polarization orientations integrated for the subareas suggest that the fault trends seemingly played important roles. All the subareas bear two dominant orientations, N10°E and N90°E, both of which are different from the azimuths of the principal compressive stress, due to the fault distribution. The time delay averaged over the entire region is 4.56 ms/km, close to that of the upper boundary of the generally accepted interval worldwide but larger than those in most of the investigated regions in the Chinese mainland, which probably implies an alignment of more micro-cracks in the n-XJFZ. Interestingly, the 2014 MS6.5 Ludian earthquake was found to have caused a variation in the time delays of the slow shear waves within the study area though its epicenter was outside. This earthquake resulted in an evident drop of the time delays remaining for 4 months, however, lifted a bit the time-delay level with respect to that prior to the earthquake.

Flow plane orientation in the upper mantle under the Western/Central United States from SKS shear-wave splitting observations

SUMMARY The causes of seismic anisotropy are still under debate. In particular, it is important to understand the extent to which seismic anisotropy is due to more recent geodynamic activities in the asthenosphere, or to frozen-in deformation in the lithosphere. We show that these two endmember cases can in principle be distinguished using shear-wave splitting observations from SKS waves. This is illustrated by the simple example of pure olivine with horizontal a-axis, and differing orientations of the other two axes, namely, vertical b and vertical c. The azimuthal dependence of shear-wave splitting measurements is described by two parameters, which can provide additional information about subsurface deformation. In particular, the oscillation parameter d1 constrains the orientation of foliation. We demonstrate that shear-wave splitting in the Western and Central United States indeed shows the predicted azimuthal dependence, related to a mainly subhorizontally oriented flow plane of deformation in the upper mantle. This has important implications for asthenospheric flow.

Lateral and Vertical Heterogeneity in the Lithospheric Mantle at the Northern Margin of the Pannonian Basin Reconstructed From Peridotite Xenolith Microstructures

AbstractThis study analyzes the microstructures and deformational characteristics of spinel peridotite xenoliths from the Nógrád‐Gömör Volcanic Field (NGVF), located on the northern margin of a young extensional basin presently affected by compression. The xenoliths show a wide range of microstructures, bearing the imprints of heterogeneous deformation and variable degrees of subsequent annealing. Olivine crystal preferred orientations (CPOs) have dominantly [010]‐fiber and orthorhombic patterns. Orthopyroxene CPOs indicate coeval deformation with olivine. Olivine J indices correlate positively with equilibration temperatures, suggesting that the CPO strength increases with depth. In contrast, the intensity of intragranular deformation in olivine varies as a function of the sampling locality. We interpret the microstructures and CPO patterns as recording deformation by dislocation creep in a transpressional regime, which is consistent with recent tectonic evolution in the Carpathian‐Pannonian region due to the convergence between the Adria microplate and the European platform. Postkinematic annealing is probably linked to percolation of metasomatism by mafic melts through the upper mantle of the NGVF prior to the eruption of the host alkali basalt. Elevated equilibration temperatures in xenoliths from the central part of the volcanic field are interpreted to be associated with the last metasomatic event, which only shortly preceded the ascent of the host magma. Despite well‐developed olivine CPOs in the xenoliths, which imply a strong seismic anisotropy, the lithospheric mantle alone cannot account for the shear wave splitting delay times measured in the NGVF, indicating that deformation in both the lithosphere and the asthenosphere contributes to the observed shear wave splitting.

Evaluating geodynamic models for sub-slab anisotropy: Effects of olivine fabric type

We examine spatially varying patterns of sub-slab anisotropy derived from geodynamic models of subduction beneath Central America and Tonga. Invoking a variety of anisotropic fabrics, we compare the predicted sub-slab anisotropy of these models against source-side, shear-wave splitting observations using realistic ray paths. We find that in both regions fabric type has a strong impact on predicted shear-wave splitting. In Tonga, where three-dimensional (3D) return flow dominates, E-type olivine lattice-preferred orientation (LPO) fabric predicts a sub-slab mantle anisotropy that best matches observations. In Central America, where entrained flow dominates, anisotropy from C-type LPO fabric yields the best fit. This highlights the importance of fabric type when interrogating geodynamic models because different regions may be characterized by different LPO fabrics. A primary controller of fabric type is water content. Taken at face value, these results then suggest the sub-slab mantle beneath Tonga is less well hydrated than that beneath Central America.

Complicated seismic anisotropy beneath south-central Mongolia and its geodynamic implications

Two years of high-quality broadband seismic data from 69 temporary stations deployed in south-central Mongolia provide an opportunity to study the anisotropy-forming mechanisms in this area. The majority of shear wave splitting observations determined from the analysis of teleseismic SKS phase are characterized by NW–SE trending fast directions with large splitting delay times (greater than 2.0 s at six stations), which is inferred to be generated by active asthenospheric flow. The variation of the fast direction may be associated with deflection of asthenosphere around the deep Siberian cratonic keel at the base of the lithosphere. Several of the NE–SW trending fast directions with relatively small delay times observed in the Gobi Desert are parallel to the strike of the main faults and sutures, which may represent lithospheric deformation. In addition, it is inferred that small-scale hot mantle upwelling is responsible for generating a cluster of null measurements observed on the south of the Hentiy Mountain.

Vertical coherence of deformation in lithosphere in the NE margin of the Tibetan plateau using GPS and shear-wave splitting data

We present 817 new SKS/SKKS shear-wave splitting observations to infer the mantle deformation field in the NE margin of the Tibetan plateau. The obvious pattern of dense splitting measurements shows that the fast wave polarization directions (φ) are oriented along a dominant NW-SE in the northeastern Tibetan plateau, Alxa block, and the western margin of the Ordos block and rotate to near E-W in the Qinling orogen, Weihe graben and Ordos block. Based on a continuous surface deformation field inferred from GPS observations and faults slip data, we model the predicted fast axis orientations (φc) at each location of averaged splitting observation. Exclusion of 32 averaged observations with larger misfits within the interior of the Ordos block, comparing splitting observations (φ) with predictions (φc) produces an average misfit of 11° indicating vertical coherent of lithospheric deformation in the NE margin of the Tibetan plateau. However, in the Qinling orogen, the thin lithosphere (<100km) could not account for the observed delay times (δt, ~1.2s), and the measured fast directions are parallel to the absolute plate motion (APM) direction, we infer that both the eastward asthenospheric flow and lithospheric left-lateral shear deformation contribute to the observed anisotropy. Within the interior of the Ordos block, where has thick lithospheric root (>300km) and weak anisotropy (δt, ~0.6s), the average misfit increases to 63°, and vertical coherence is no longer present, which suggests the rigid and stable Ordos block with little deformation still remains the lithospheric root of North China Craton.

Gaps, tears and seismic anisotropy around the subducting slabs of the Antilles

Seismic anisotropy in and beneath the subducting slabs of the Antilles is investigated using observations of shear-wave splitting. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to map anisotropy in the crust, the mantle wedge and the slab/sub-slab mantle. To date this is the most comprehensive study of anisotropy in this region, involving 52 stations from 8 seismic networks. Local event delay times (0.21±0.12s) do not increase with depth, indicating a crustal origin in anisotropy and an isotropic mantle wedge. Teleseismic delay times are much larger (1.34±0.47s), with fast shear-wave polarisations that are predominantly parallel to trend of the arc. These observations can be interpreted three ways: (1) the presence of pre-existing anisotropy in the subducting slab; (2) anisotropy due to sub-slab mantle flow around the eastern margin of the nearly stationary Caribbean plate; (3) some combination of both mechanisms. However, there are two notable variations in the trench-parallel pattern of anisotropy — trench-perpendicular alignment is observed in narrow regions east of Puerto Rico and south of Martinique. These observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab. Furthermore, the pattern of anisotropy south of Martinique, near Saint Lucia is consistent with a previously proposed location for the boundary between the North and South American plates.