Transition Zone Lower Mantle Research Articles

Crystallographic Preferred Orientation of Phase D at High Pressure and Temperature: Implications for Seismic Anisotropy in the Mid‐Mantle

AbstractSeismic anisotropy has been widely observed in the lower mantle transition zone and the uppermost lower mantle near several subducting slabs, which is typically attributed to the crystallographic preferred orientation (CPO) of constituent minerals. In this study, well‐controlled uniaxial and simple shear deformation experiments were conducted on Mg‐endmember phase D and Al‐bearing phase D aggregates at 20 GPa and 500–1,000°C. Strong (0001) fabrics were observed in the uniaxially compressed samples. The CPO from all experiments, including extension and simple shear, indicated that the most active slip systems in phase D were <110>(0001) and <100>(0001), with a dominance of <110>(0001). The calculated seismic velocity suggests that the deformed phase D generates positive radial seismic anisotropy (VSH > VSV) in both subhorizontal shearing and subvertical compression, causing significant shear‐wave splitting time. Our results suggest that the observed seismic anisotropy in the mid‐mantle in several cold subduction zones can be well explained by the presence of deformed phase D.

The structure of the mantle transition zone and its implications for mantle upwelling beneath the central Tibetan plateau from triplicated P-waveforms modeling

The Tibetan plateau is considered as an ideal place for studying the continental collisions, tectonic movements and regional geodynamics. Although multiple episodes of igneous activities in Tibetan plateau have been suggested derived from mantle upwelling, their origins and corresponding geodynamic models remain vigorously debate. This study presents a high-resolution upper mantle velocity model of P-wave and its lateral variation beneath the central Tibetan plateau based on triplicated waveform modeling. The results show that, compared with IASP91 model, the 660-km discontinuity is deepened approximately 17–28 km from the Qiangtang terrane to the Beishan block. The lower mantle transition zone (MTZ) is characterized by high P-wave velocity anomalies (HVA) in range of 0.3%–0.9%. We infer that they are mainly related to the hydrous materials with low temperature in the lower MTZ. Besides, the depression of the 410-km discontinuity is about 6–15 km. A remarkable low velocity layer (LVL) is detected atop the 410-km discontinuity, exhibiting the thickness of 31–40 km and low P-wave velocity anomaly of −2.5% to −1.6%. This is against completely temperature-dominated origin of the LVL, and suggests that the material composition has also contributes. Additionally, we propose a new geodynamic model to demonstrate the asthenospheric upwelling beneath the Qiangtang and Songpan-Ganzi terranes, which suggests that the passively ascending water-enriched materials from the lower MTZ and their dehydration are responsible for the observed LVL atop the 410-km discontinuity, as well as the origin of the Cenozoic magmatic intrusion and volcanic eruption of the northern Tibet volcano in Pleistocene.

The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations

AbstractAlaska is a tectonically active region with a long history of subduction and terrane accretion, but knowledge of its deep seismic structure is limited by a relatively sparse station distribution. By combining data from the EarthScope Transportable Array and other regional seismic networks, we obtain a high‐resolution state‐wide map of the Moho and upper‐mantle discontinuities beneath Alaska using teleseismic SH‐wave reverberations. Crustal thickness is generally correlated with elevation and the deepest Moho is in the region with basal accretion of the subducted Yakutat plate, consistent with its higher density due to a more mafic composition. The crustal thickness in the Brooks Range agrees with the prediction based on Airy isostasy and the weak free‐air gravity anomaly, suggesting that this region probably does not have significant density anomalies. We also resolve the 410, 520, and 660 discontinuities in most regions, with a thickened mantle transition zone (MTZ) and a normal depth difference between the 520 and 660 discontinuities (d660‐d520) under central Alaska, indicating the presence of the subducted Pacific slab in the upper MTZ. A near‐normal MTZ and a significantly smaller d660‐d520 are resolved under southeastern Alaska, suggesting potential mantle upwelling in the lower MTZ. Beneath the Alaska Peninsula, the thinned MTZ implies that the Pacific slab may not have reached the MTZ in this region, which is also consistent with recent tomography models. Overall, the results demonstrate a bent or segmented Pacific slab with varying depths under central Alaska and the Alaska Peninsula.

Cold and Wet Mantle Transition Zone Beneath the Mediterranean Sea Revealed by the Electrical Image

A three-dimensional electrical conductivity model of the mantle beneath North Africa and Southern Europe is obtained by geomagnetic depth sounding. C-responses are estimated from geomagnetic data observed at observatories in and around the region and converted to the electrical structure of the mantle transition zone. The limited-memory quasi-Newton method is chosen to minimize the nonlinear objective function of inversion, while the forward modeling relies on a staggered-grid finite difference method in the spherical coordinate system. The data misfit term of the inversion function is measured using the L1-norm in order to suppress the response instability caused by the significant noise contained in the observed data. In order to adequately correct the ocean effect in observatories near the coast, a fixed shell comprised of ocean and land is incorporated in inversion. A banded zone with reduced conductivity is present in the three-dimensional model, primarily seen in the lower mantle transition zone and lower mantle beneath the Mediterranean Sea. Combining laboratory-measured conductivity models, we propose that subducted slabs causing reduced temperature and a water reservoir in the mantle transition zone should be responsible for the observed electrical model.

Global water distribution in the mantle transition zone from a seismic isotropic velocity model and mineral physics modeling

Although the discoveries of hydrous ringwoodite inclusions and ice-VII inclusions in natural diamonds suggest a hydrous mantle transition zone (MTZ), water content and distribution in the MTZ remain unclear. Here combining a global P- and S-wave isotropic velocity tomography and mineral physics modeling, we image the water distribution in the MTZ. Our results indicate that the MTZ is a main water reservoir inside the Earth, and the total water content of the MTZ is about 0.64–1 seawater. The upper MTZ (410–520 km) and the lower MTZ (520–660 km) contain 0.3–0.5 wt% and 0.15–0.2 wt% water, respectively, implying water contents of the MTZ decrease with increasing depths. The most hydrous regions are mainly located near subduction zones, where the upper MTZ and the lower MTZ can contain water up to 0.5–1 wt% and 0.2–0.5 wt%, respectively, indicating water is transported into the MTZ by hydrous slabs. In addition, old subducted slabs in the western Pacific subduction zone are more hydrous than young subducted slabs in the eastern Pacific subduction zone. We also propose a water circulation model which integrates our results of the water content and distribution in the MTZ.

Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

AbstractSeismic anisotropy has been widely observed near the subducting slabs in the lower mantle transition zone (MTZ) and is often interpreted by the lattice preferred orientation (LPO) of constituent minerals. Akimotoite is one of the dominant minerals near the cold subducting slabs. Therefore, we conducted the well‐controlled uniaxial and shear deformation experiments on the MgSiO3 akimotoite aggregates at 21–23 GPa and 900–1300°C by using the D111‐type Kawai‐type multianvil apparatus. We observed strong LPOs and the most dominant slip system of akimotoite is suggested to be (0001). The elastic wave velocities of deformed samples were calculated to be strong azimuthal and polarization anisotropy with the velocities of horizontally polarized shear waves greater than that of vertically polarized shear waves for the horizontal mantle shearing. Our results provide important implications for the origin of observed seismic anisotropies and the mantle flow directions in the lower MTZ.

Mapping the mantle transition zone beneath the Indian Ocean geoid low from Ps receiver functions

The most pronounced high-amplitude (−40 to −106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 ± 13.6 to 459.7 ± 2.9 km and 643.1 ± 7.4 to 710.2 ± 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive ~800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from −0.89 to −3.52 (%) at d410, and at dd60 range from −1.13 to −7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.

Ab initio thermal expansion and thermoelastic properties of ringwoodite (<i>γ</i>-Mg<sub>2</sub>SiO<sub>4</sub>) at mantle transition zone conditions

Abstract. Thermal convection in the Earth's mantle is driven by lateral variations in temperature and density, which are substantially controlled by the local volume thermal expansion of the constituent mineral phases. Ringwoodite is a major component of the lower mantle transition zone, but its thermal expansivity and thermoelastic properties are still affected by large uncertainties. Ambient thermal expansion coefficient (αV0​​​​​​​), for instance, can vary as much as 100 % according to different experimental investigations available from the literature. In this work, we perform ab initio density functional theory calculations of vibrational properties of spinel-structured Mg2SiO4 ringwoodite in order to provide reliable thermophysical data up to mantle transition zone conditions. Temperature- and pressure-dependent thermal expansivity has been obtained by phonon dispersion calculations in the framework of quasi-harmonic approximation (QHA) up to 25 GPa and 2000 K. Theoretical analysis of vibrational spectra reveals that accurate prediction of IR and silent modes, along with their relative mode Grüneisen parameters, is crucial to define thermal expansivity. A six-parameter analytical function is able to reproduce ab initio values fairly well in the whole investigated P–T range, i.e., αV(P,T)=(1.6033×10-5+8.839×10-9T+11.586×10-3T-1-6.055T-2+804.31T-3) ×exp⁡(-2.52×10-2P), with temperature in kelvin and pressure in gigapascal. Ab initio static and isothermal bulk moduli have been derived for ringwoodite along with their P, T and cross derivatives, i.e., K0 = 184.3 GPa, KT,300 K = 176.6 GPa, K0′ = 4.13, KT,300K′ = 4.16, ∂KT∂TP = −0.0233 GPa K−1 and ∂2KT∂P∂T0=1.0×10-4 K−1. Computed thermal expansivity and thermoelastic properties support the evidence that QHA performs remarkably well for Mg2SiO4 ringwoodite up to mantle transition zone temperatures. Since volume thermal expansion of ringwoodite is strongly pressure-dependent and its pressure dependence becomes more marked with the increasing temperature, internally consistent assessments and empirical extrapolation of thermoelastic data to deep mantle conditions should be taken with care to avoid inaccurate or spurious predictions in phase equilibrium and mantle convection numerical modeling.

Mantle transition zone discontinuities beneath Taiwan and its adjacent areas: Implications for slab subductions

How deep the two opposite dipping subduction zones in the vicinity of Taiwan, northwestward-dipping Philippine Sea Plate (PSP) and eastward-dipping South China Sea Slab (SCS), penetrated down into mantle, are hotly on debate and insufficiently imaged by seismic tomographic inversions, either due to inadequate quantities of seismic data or limited vertical resolution of the techniques used by previous investigations. Here we employ an unprecedented number of 55,847 high-quality P-to-S radial receiver functions recorded by 304 broadband seismic stations located in SE China and adjacent areas to image the topography of the mantle transition zone (MTZ) discontinuities (d410 and d660) to delineate the configurations of the subducted slab segments and analyze the impacts of thermal and water content anomalies on the MTZ. The close-to-normal wavespeed-corrected d410 and a ~ 10 km depression of the corrected d660 to the east of northern Taiwan are revealed, indicating that the PSP breaks off above the d410 and the delaminated PSP drops into the lower MTZ along with slab dehydration. To the east of southern Taiwan, where the SCS begins to subduct eastward, the uplift of the apparent d410 is nearly twice as much as that of the d660. The positive wavespeed anomaly above the d410 and low temperature in the vicinity of the d410 are enough to contribute to the uplift of MTZ discontinuities, suggesting that the SCS just reaches the d410, rather than penetrating the entire MTZ.

Velocity Anomalies Around the Mantle Transition Zone Beneath the Qiangtang Terrane, Central Tibetan Plateau From Triplicated P Waveforms

AbstractThe Qiangtang terrane exhibits many unique features compared with surrounding areas in the Tibetan plateau. It has been largely remodeled by the northward downwelling of the Indian lithosphere and the vertical upwelling of the Eurasian asthenosphere. Here, we investigate the P‐wave velocity around the mantle transition zone (MTZ) beneath the Qiangtang terrane using high‐frequency triplicated waveforms recorded by the China Digital Seismic Network. We found a low‐velocity layer (LVL) atop the MTZ and a high‐velocity anomaly (HVA) in the lower part of the MTZ. They are characterized by thickness of 36 km with velocity drop of −2.0% and 155 km with velocity increment of 1.2%, respectively. The 410 and 660 km discontinuities are depressed by different depths of 12 and 30 km, suggesting a thickened MTZ. In combination with previous multi‐disciplinary observations, we infer that the LVL is a hydrous partial melt affected by dehydration and high temperature, and the HVA is due to water and low temperature conditions. The downwelling cold continental lithosphere may have accumulated in the lower MTZ and induced the hydrous hot MTZ materials to upwelling until the shallow depths of the upper mantle beneath the Qiangtang terrane.

Elastic properties of Fe-bearing Akimotoite at mantle conditions: Implications for composition and temperature in lower mantle transition zone

The pyrolite model, which can reproduce the upper-mantle seismic velocity and density profiles, was suggested to have significantly lower velocities and density than seismic models in the lower mantle transition zone (MTZ). This argument has been taken as mineral-physics evidence for a compositionally distinct lower MTZ. However, previous studies only estimated the pyrolite velocities and density along a one-dimension (1D) geotherm and never considered the effect of lateral temperature heterogeneity. Because the majorite-perovskite-akimotoite triple point is close to the normal mantle geotherm in the lower MTZ, the lateral low-temperature anomaly can result in the presence of a significant fraction of akimotoite in pyrolitic lower MTZ. In this study, we reported the elastic properties of Fe-bearing akimotoite based on first-principles calculations. Combining with literature data, we found that the seismic velocities and density of the pyrolite model can match well those in the lower MTZ when the lateral temperature heterogeneity is modeled by a Gaussian distribution with a standard deviation of ∼100 K and an average temperature of dozens of K higher than the triple point of MgSiO3. We suggest that a harzburgite-rich lower MTZ is not required and the whole mantle convection is expected to be more favorable globally.

Elasticity of hydrous ringwoodite at mantle conditions: Implication for water distribution in the lowermost mantle transition zone

The mantle transition zone (MTZ) is potentially a geochemical water reservoir because of the high H2O solubility in its dominant minerals, wadsleyite and ringwoodite. Whether the MTZ is wet or dry fundamentally impacts our understanding of the deep-water distribution, geochemical recycling, and the pattern of mantle convection. However, the water content in the MTZ inferred from previous studies remains disputed. Seismic observations such as velocity anomalies were used to evaluate the water content in the MTZ, but the hydration effect on the velocities of MTZ minerals under appropriate pressure (P) and temperature (T) conditions is poorly constrained. Here we investigated the elastic properties and velocities of hydrous ringwoodite at high P-T conditions using first-principles calculations. Our results show that the hydration effects on elastic moduli and velocities of ringwoodite are significantly reduced by pressure but strongly enhanced by temperature. The incorporation of 1.0 wt% water into ringwoodite decreases the compressional and shear velocities of the pyrolitic mantle by −1.0% and −1.4% at the conditions of MTZ, respectively. Using results from seismic tomography and together with the topography of the 660-km discontinuity, we evaluate the global distribution of water in the lower MTZ. We find that about 80% of the MTZ can be explained by varying water content and temperature, however, the remaining 20% requires the presence of high-velocity heterogeneities such as harzburgite. Our models suggest an average water concentration of ∼0.2 wt% in the lower MTZ, with an interregional variation from 0 to 0.9 wt%. Together with our previous work, we conclude that the water concentration in the MTZ likely decreases with depth globally and the whole MTZ contains the equivalent of about one ocean mass of water.

Evidence of Bermuda Hot and Wet Upwelling From Novel Three‐Dimensional Global Mantle Electrical Conductivity Image

AbstractA model of electrical conductivity in the mid–mantle transition zone was obtained with improved constraints. An L1‐norm regularization inversion algorithm is proposed here that reduces the influence of noisy data on three‐dimensional geomagnetic depth sounding inversion from C‐responses. Here the regularization is implemented by using an L1‐norm to measure the predicted data error, which is normalized by the C‐response covariance, but an L2‐norm is used to measure the regularization term associated with model parameters. The limited‐memory quasi‐Newton method (L‐BFGS) is used to invert for the three‐dimensional electrical conductivity model. The model is discretized by curved rectangular prisms in spherical coordinates. Sensitivity tests show that for good‐quality data contaminated by Gaussian noise, L1 inversion, which could perform as well as L2 inversion, can adequately recover the main features of the electrical conductivity structure within the region of data coverage. When data errors are drawn from an exponential distribution, L1 inversion obtains relatively reliable reconstruction of the electrical structure, even when the noise level is comparable to that of actual C‐responses. C‐responses from 129 low‐latitude and midlatitude geomagnetic observatories are inverted using L1‐norm minimization of the data error. The resulting model reveals an electrically conductive feature in the lower mantle transition zone and upper lower mantle that is broadly coincident with that found in previous studies. The reduced influence of data with large variances on L1‐norm misfits, along with inclusion of responses estimated from more observatories, makes L1 inversion more clearly identify these deep conductive features while identifying previously obscured anoconductive zones. A feature of particular interest is the high electrical conductivity anomaly beneath the Bermuda‐Sargasso Sea region in the mid–mantle transition zone and the uppermost lower mantle. Rock physics analysis indicates that the anomaly is most possibly caused by the wet upwelling material with excessive ~650 K higher temperature, suggesting a narrow tail with a broad head.

Electrical conductivity in the mantle transition zone beneath Eastern China derived from L1-Norm C-responses

SUMMARY Constraining the distribution of water in different regions of the mantle remains one of the significant challenges to comprehend the global deep water cycle. Geomagnetic depth soundings can provide such constraint through the electrical conductivity structure. Hence, this study aims to propose a regularization technique that can estimate previously unavailable C-response. In the method, the objective function comprised an L1-norm measured data prediction error and a spectral smoothness constraint term. We used the data error of C-response to weight the predicted error. The L-BFGS method was introduced to determine the minimum point of the objective function, and the regularization parameter decreased adaptively during inversion. Thus, the geomagnetic data processed yielded high-quality C-responses in 31 stations in Eastern China. In addition, we obtained 1-D electrical conductivity profiles in the mantle transition zone (MTZ) beneath Eastern China from C-responses using the L-BFGS method. Compared with the global 1-D model, the conductivity–depth profiles revealed that the MTZ beneath Eastern China is more conductive in the east but more resistive in the west. The conversion of these conductivities to water content based on the mineral physics suggested that the MTZ beneath Eastern China is characterized by a high water concentration, approximately 0.2 and 1 wt per cent in the upper and lower MTZ, respectively. Owing to the inclusion of more stations, the water-rich region could be constrained roughly to the east of the North–South Gravity Lineament (NSGL). Further considering seismic images in the same area, this water content distribution pattern suggested that the front of the stagnant Pacific Plate in the lower MTZ might have reached the NSGL. However, the dehydration reactions in the stagnant slab were more active in the eastern part. Perhaps, some of these fluids migrated into the upper MTZ and could be the source of the trapped water found in the xenoliths from the deep upper mantle beneath Eastern China.

Mantle Transition Zone Structure Beneath Northeast Asia From 2‐D Triplicated Waveform Modeling: Implication for a Segmented Stagnant Slab

AbstractThe structure of the mantle transition zone (MTZ) in subduction zones is essential for understanding subduction dynamics in the deep mantle and its surface responses. We constructed the P (Vp) and SH velocity (Vs) structure images of the MTZ beneath Northeast Asia based on two‐dimensional (2‐D) triplicated waveform modeling. In the upper MTZ, a normal Vp but 2.5% low Vs layer compared with IASP91 are required by the triplication data. In the lower MTZ, our results show a relatively higher‐velocity layer (+2% Vp and −0.5% Vs compared to IASP91) with a thickness of ~140 km and length of ~1,200 km atop the 660‐km discontinuity. Taking this anomaly as the stagnant slab and considering the plate convergence rate of 7–10 cm/year in the western Pacific region during the late Cenozoic, we deduced that the stagnant slab has a subduction age of less than 30 Ma. This suggests that the observed stagnancy of the slab in the MTZ beneath Northeast Asia may have occurred no earlier than the Early Oligocene. From the constraints derived individually on Vp and Vs structures, high Vp/Vs ratios are obtained for the entire MTZ beneath Northeast Asia, which may imply a water‐rich and/or carbonated environment. Within the overall higher‐velocity stagnant slab, a low‐velocity anomaly was further detected, with a width of ~150 km, Vp and Vs reductions of 1% and 3% relative to IASP91. Such a gap may have provided a passage for hot deep mantle materials to penetrate through the thick slab and feed the Changbaishan volcano.

Garnet‐controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

AbstractDeep mantle plumes and associated increased geotherms are expected to cause an upward deflection of the lower–upper mantle boundary and an overall thinning of the mantle transition zone between about 410 and 660 km depth. We use subsequent forward modelling of mineral assemblages, seismic velocities, and receiver functions to explain the common paucity of such observations in receiver function data. In the lower mantle transition zone, large horizontal differences in seismic velocities may result from temperature‐dependent assemblage variations. At this depth, primitive mantle compositions are dominated by majoritic garnet at high temperatures. Associated seismic velocities are expected to be much lower than for ringwoodite‐rich assemblages at undisturbed thermal conditions. Neglecting this ultralow‐velocity zone at upwelling sites can cause a miscalculation of the lower–upper mantle boundary on the order of 20 km.

Creep strength of ringwoodite measured at pressure–temperature conditions of the lower part of the mantle transition zone using a deformation–DIA apparatus

Creep strength of ringwoodite is important for understanding complicated patterns of the mantle convection in and around the mantle transition zone. To determine the creep strength of ringwoodite, we expanded pressure–temperature conditions of in situ stress–strain measurements in a deformation–DIA apparatus combined with synchrotron X-ray to those of the lower part of the mantle transition zone. The expansion of the pressure–temperature conditions was made by shrinking anvil truncation to 2.0 mm and the development of a cell assembly for in situ deformation experiments up to 1700 K. Utilizing the developed technique, creep–strength measurements on polycrystalline ringwoodite were performed at 16.9–18.0 GPa and 1300–1700 K during axial deformation with strain rates of 1.48–3.59×10−5 s−1 to strains of 13.2–24.9%. Based on mechanical and microstructural observations, we infer that ringwoodite deformed by exponential dislocation creep through the Peierls mechanism at 1300–1400 K and power-law dislocation creep at 1500–1700 K. The creep strength of ringwoodite is apparently lower than that of bridgmanite, wadsleyite and olivine. The present result implies the possibility that the lower mantle transition zone is a low-viscosity layer. Further creep–strength data of these minerals are necessary to be determined above 13.5 GPa and high temperatures to determine viscosity structure in and around the lower mantle transition zone at strain rates relevant to the mantle convection.

P wave tomography and anisotropy beneath Southeast Asia: Insight into mantle dynamics

AbstractSoutheast Asia is surrounded by subduction zones resulting from the interactions of several lithospheric plates. Its evolution has been also influenced by active tectonics due to the Indo‐Asian collision in the Cenozoic. In this study, we use a large number of arrival‐time data of local and regional earthquakes to determine 3‐D P wave tomography and azimuthal anisotropy in the mantle beneath SE Asia. High‐velocity (high‐V) anomalies representing the subducting slabs are clearly visible in the upper mantle and the mantle transition zone (MTZ). Low‐velocity (low‐V) zones with trench‐normal anisotropy are revealed in the uppermost mantle, which indicate back‐arc spreading or secondary mantle‐wedge flow induced by the slab subduction. In contrast, trench‐parallel anisotropy dominates in the deep upper mantle and reflects structures either in the subducting slab or in the upper mantle surrounding the slab. The trench‐parallel anisotropy is also significant in the lower MTZ, which may contribute to shear wave splitting observations. A low‐V body extending down to the lower mantle is visible under the Hainan volcano far away from the plate boundaries, suggesting that Hainan is a hot spot fed by a lower‐mantle plume. The low‐V body under Hainan is connected with low‐V zones in the upper mantle under SE Tibet and Vietnam. Our P wave anisotropy results reflect significant mantle flow existing in the asthenosphere from SE Tibet to Hainan and further southwestward to Vietnam. The present study, especially the 3‐D P wave anisotropy results, provides important new insight into mantle dynamics in SE Asia.

Mantle transition zone discontinuities beneath the contiguous United States

AbstractUsing over 310,000 high‐quality radial receiver functions recorded by the USArray and other seismic stations in the contiguous United States, the depths of the 410 km and 660 km discontinuities (d410 and d660) are mapped in over 1,000 consecutive overlapping circles with a radius of 1°. The average mantle transition zone (MTZ) thickness for both the western and central/eastern U.S. is within 3 km from the global average of 250 km, suggesting an overall normal MTZ temperature beneath both areas. The Pacific Coast Ranges and the southern Basin and Range Province are underlain by a depressed d410, indicating higher‐than‐normal temperature in the upper MTZ. The proposed Yellowstone and Raton hot spots are not associated with clear undulations of the MTZ discontinuities, but d410 beneath another proposed hot spot, Bermuda, is depressed significantly and d660 has a normal depth. Low‐temperature regions are found in the upper MTZ associated with the subducted Juan de Fuca slab beneath the northern Rocky Mountains and in two circular areas beneath the northern Basin and Range Province and the southern Colorado Plateau. Part of the Great Plains is characterized by a depressed d660. This observation, when combined with results from seismic tomography, suggests the existence of a cold region in the lower MTZ, probably associated with subducted Farallon slab segments.