Mantle Transition Zone Thickness Research Articles

Unveiling the Distinct Structure of the Upper Mantle Beneath the Canary and Madeira Hotspots, as Depicted by the 660, 410, and X Discontinuities

AbstractThe Canary and Madeira Islands are two distinct hotspots in the Central‐East Atlantic that are close to each other. Their volcanism is generally attributed to underlying mantle plumes, but the detailed structure of these plumes is still not well understood. The thermal and compositional structure of the plume introduces complexities in the phase transitions of the mantle, which impact the depth and magnitude of seismic discontinuities. We use 1,268 high‐quality receiver functions from stations located at the two hotspots to detect P‐to‐s converted phases through a common‐conversion point stacking approach and conduct a detailed analysis of mantle seismic discontinuities. The results show that both hotspots are characterized by a thin mantle transition zone (MTZ), with sharp 410 and 660 discontinuities at depths of 428–421 km and 647–664 km, beneath the Canaries and Madeira respectively. The results indicate that the Canary plume crosses the MTZ, whereas the Madeira plume mainly influences the upper portion of the MTZ. Furthermore, we find reliable detections of a sharp X discontinuity beneath the Canaries at 287 km. Its presence suggests the accumulation of silica‐rich recycled eclogite at these depths. We also use the amplitudes of P410s and PXs to derive velocity jumps at corresponding discontinuities. Based on these measurements, we estimate that the basalt proportion is 60%–80%, with accumulation being more significant in the Canaries than in Madeira. The MTZ thickness, the presence of the X discontinuity, and the high basalt proportion provide compelling evidence for a deep‐rooted thermochemical plume beneath the study area.

A Tilted Broad Plume Underneath the Greenland Cratonic Keels

AbstractTo advance our comprehension of the complex geological history and mantle dynamics in the North Atlantic region, we employ all available broadband seismic data recorded in Greenland to reveal an abnormal mantle transition zone (MTZ) structure. Central and eastern Greenland exhibits depressed 410 and 660 km discontinuities (d410 and d660, respectively) bordering the MTZ, indicative of a substantial thermal anomaly associated with an underlying plume, surpassing the 1,800°C threshold for post‐garnet phase transitions at the d660. Variations in MTZ thickness across Greenland stem from differing temperature anomalies at the d410 and d660, possibly linked to a tilted plume within the MTZ. These findings corroborate geodynamic models, elucidating the interaction between post‐garnet phase transitions and upwelling plumes. The results shed light on the origin of the enigmatic Icelandic hotspot track and its influence on the thermal and lithospheric structures beneath Greenland.

Mantle Wavespeed and Discontinuity Structure Below East Africa: Implications for Cenozoic Hotspot Tectonism and the Development of the Turkana Depression

AbstractEthiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co‐located, previously unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub‐lithospheric slow wavespeeds imply that Turkana's present‐day low elevation is explained best by Mesozoic and Cenozoic‐age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.

Seismic image of the mantle transition zone beneath northeastern China: evidence for stagnant Pacific subducting slab, lithospheric delamination and mantle upwelling

SUMMARY We provide a comprehensive image of the mantle transition zone (MTZ) beneath northeastern China by performing Variable Bin Radius Stacking of receiver functions. A massive seismic data set consisting of over 133 000 receiver functions recorded by 1208 broad-band stations is processed. Our results reveal fine-scale topography on the 410- and 660-km discontinuities defining the upper and lower bounds of the MTZ, lateral variations in the MTZ thickness and slab interfaces within the MTZ. In particular, unambiguous images of the slab interfaces provide direct evidence for the presence of the stagnant Pacific subducting slab below the eastern portion of the study area. A widespread deepening of the 410-km discontinuity is consistent with a hot and wet low-velocity upper mantle resulting from dehydration of the stagnant slab. Prominent depressions are evident in the depth to the 660-km discontinuity, with a thickened MTZ associated with the cold stagnating slab. Localized uplifts of the 660-km discontinuity are possibly caused by partial melt under the slab. These features attest to the influence of the Pacific plate on the MTZ. Additionally, a pronounced upwarp on the 660-km interface with a thin MTZ agrees with a previously hypothesized mantle upwelling through a slab window, possibly triggered by the sinking of the stagnant slab. Moreover, the western part of the study region is characterized by alternating ups and downs of the 410-km interface, while the topography of the 660-km discontinuity is relatively flat. We propose the western region is dominated by foundering of delaminated lithospheric blocks that induced upward mantle return flows upon entrance into the MTZ.

Topographic variations of mantle transition zone discontinuities in the Java subduction zone and adjacent regions from receiver function analyses

An unprecedented number of broadband seismic data from 165 stations were utilized to constrain the topographic variations in seismic discontinuities at the top and bottom of the mantle transition zone (MTZ) in the Java subduction zone. After moveout corrections under a nonplane wavefront assumption and time-to-depth conversions based on the IASP91 Earth model, the depth undulations of two discontinuities that border the MTZ (d410 and d660) were obtained by stacking P receiver functions that were grouped into circular bins with a radius of 1°. The apparent depths exhibit systematically spatial variations that range from 385 to 427 km for the d410 and 608 to 700 km for the d660, which result in a MTZ thickness of 217–309 km. The mean values are 406 ± 12, 665 ± 19, and 258 ± 18 km, suggesting thicker than normal MTZ by an average of 8 km. The major factor that accounts for the systematic depth variations across the study area originates from the subducted slab that perturbs the entirety of the MTZ, which induces a negative thermal anomaly, high velocity zone, and abnormal water presence concentrated in the lower MTZ of western Java. In Lombok and Sumbawa Islands, after the thermal and velocity heterogeneities from the subducted slab were considered, the remnant d660 uplift is found due to a significant thermal anomaly with a magnitude of up to +200 K near the d660. In eastern Java and adjacent regions, advective thermal upwelling associated with toroidal mantle flow that traversed a slab window in central Java induces a low velocity anomaly with a magnitude that is greater than the high velocity anomaly caused by the subducted slab, which apparently depressed the d410 and d660 by the same amount.

Joint inversion of PP and SS precursor waveforms and Rayleigh wave phase velocities for global mantle transition zone structure

SUMMARYWe have compiled a new data set of global PP and SS precursor waveforms that we jointly invert in combination with fundamental-mode and higher-order Rayleigh-wave phase velocities for upper mantle and mantle transition zone (MTZ) structure. We observe clear S410S, S520S, S660S and P410P precursor arrivals, but not P660P, because of interfering phases. Traveltimes and amplitudes of precursor phases reflect a complex interplay of data and modelling factors, implying that MTZ structure is best resolved through direct inversion of waveforms. To model waveforms as accurately as possible, we account for effects arising from data processing, shallow structure, incoherent stacking, attenuation and source effects, among others. As part of the inversion, we consider two independent model parametrizations to obtain quantitative insights into the seismic and thermochemical constitution of the MTZ. These include a ‘classical’ seismic parametrization based on a layered seismic velocity structure and a thermodynamic parametrization, where seismic profiles are self-consistently built from mineral physics data. The results show lateral variations in thermal, compositional and discontinuity structure that partly correlate with tectonic setting. The mantle beneath continents and subduction zones is found to be colder in comparison to oceans and hotspots as reflected in MTZ thickness. In terms of composition, we find that subduction zones are enriched in basalt. Mid-MTZ structure shows a trend from simple sub-ocean single- to complex circum-Pacific subduction-zone-related dual-discontinuity structure—the possible signature of oceanic crustal transport to the MTZ. Statistical analysis indicates that a mechanically mixed mantle matches seismic data better than an equilibrated mantle across ∼2/3 of the globe. Finally, while a large part of the seismic data can be matched by an iso-chemical and adiabatic mantle, complexities within the MTZ are not entirely captured by this assumption.

Evidence for melt leakage from the Hawaiian plume above the mantle transition zone

Dehydration reactions at the top of the mantle transition zone (MTZ) can stabilize partial melt in a seismic low-velocity layer (LVL), but the seismic effects of temperature, melt and volatile content are difficult to distinguish. We invert P-to-S receiver function phases converted at the top and bottom of a LVL above the MTZ beneath Hawaii. To separate the thermal and melting related seismic anomalies, we carry out over 10 million rock physics inversions. These inversions account for variations arising from the Clapeyron slope of phase transition, bulk solid composition, dihedral angle, and mantle potential temperature. We use two independent seismic constraints to evaluate the temperature and shear wave speed within the LVL. The thermal anomalies reveal the presence of a hot and seismically slow plume stem surrounded by a “halo” of cold and fast mantle material. In contrast to this temperature distribution, the plume stem contains less than 0.5vol% melt, while the surrounding LVL—within the coverage area—contains up to 1.7vol% melt, indicating possible lateral transport of the melt. When compared to the melting temperatures of mantle rocks, the temperature within the LVL, calculated from seismic observations of MTZ thickness, suggests that the observed small degrees of melting are sustained by the presence of volatiles such as CO2 and H2O. We estimate the Hawaiian plume loses up to 1.9Mt/yr H2O and 10.7Mt/yr CO2 to the LVL, providing a crucial missing flux for global volatile cycles.

Topography of the 410 and 660 km Discontinuities Beneath the Cenozoic Okavango Rift Zone and Adjacent Precambrian Provinces

AbstractBy taking advantage of the recent availability of a broadband seismic data set from Networks NR and BX covering the entire country of Botswana, we conduct a systematic receiver function investigation of the topography of the 410 and 660 km discontinuities beneath the incipient Okavango rift zone (ORZ) in northern Botswana and its adjacent Archean‐Proterozoic tectonic provinces in southern Africa. Similar to a previous mantle transition zone (MTZ) discontinuity study using data from a 1‐D profile traversing the ORZ, a normal MTZ thickness is observed in most parts of the study area. This is inconsistent with the existence of widespread positive thermal anomalies in the MTZ and further implies that active thermal upwelling from the lower mantle plays an insignificant role in the initiation of continental rifting. The results also suggest that cold temperature presumably associated with thick cratonic keels has indiscernible influence on the thermal structure of the MTZ. The expanded data set reveals several isolated areas of slight (~10 km or smaller) MTZ thinning. The largest of such areas has a NE‐SW elongated shape and is mostly caused by relative deepening of the 410 km discontinuity rather than shallowing of the 660 km discontinuity. These characteristics are different from those expected for a typical mantle plume. We speculate that the thinner‐than‐normal MTZ may be induced by minor thermal upwelling associated with late Mesozoic‐early Cenozoic lithospheric delamination, a recently proposed mechanism that might be responsible for the high elevation of southern Africa.

Slab Dehydration and Mantle Upwelling in the Vicinity of the Sumatra Subduction Zone: Evidence from Receiver Function Imaging of Mantle Transition Zone Discontinuities

AbstractA total of 10,586 P‐to‐S radial receiver functions recorded by 64 broadband seismic stations were utilized to image the 410 and 660 km discontinuities (d410 and d660, respectively) bordering the mantle transition zone (MTZ) beneath the Sumatra Island, the Malay Peninsula, and the western margin of the South China Sea. The d410 and d660 were imaged by stacking receiver functions in successive circular bins with a radius of 1°, after moveout corrections based on the 1‐D IASP91 Earth model. The resulting apparent depths of the discontinuities exhibit significant and spatially systematic variations. The apparent depths of the d410 and d660 range from 382 to 459 km and 637 to 700 km with an average of 406 ± 13 and 670 ± 12 km, respectively, while the corresponding values for the MTZ thickness are 217 to 295 km and 261 ± 13 km. Underneath southern Sumatra and adjacent regions, the MTZ is characterized by an uplifted d410 and a depressed d660. While the former is probably caused by the low temperature anomaly, the latter is most likely related to a combination of the low temperature anomaly and dehydration associated with the subducted Australian Plate that has reached at least the d660. In contrast, an abnormally thin MTZ is imaged to the southwest of the Toba Caldera. This observation, when combined with results from previous seismic tomography studies, can be explained by advective thermal upwelling through a slab window.

Mantle Structure in the Central Mediterranean Region From P and S Receiver Functions

AbstractWe investigate the upper mantle discontinuities in the central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S wave velocity profiles (down to 300‐km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, lithosphere‐asthenosphere boundary, and an underlying low‐velocity layer between ~60‐ and ~200‐km depth. The low‐velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the lithosphere‐asthenosphere boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of ~14 km, as inferred from a reduced P660s‐P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S‐to‐P conversion points) two strong signals from nonstandard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.

Mantle Transition Zone Thickness Beneath the Middle East: Evidence for Segmented Tethyan Slabs, Delaminated Lithosphere, and Lower Mantle Upwelling

AbstractWe use a very large seismic data set to provide a comprehensive image of the mantle transition zone (MTZ) beneath the Middle East. We utilized the technique of Common Conversion Point stacking of P wave receiver functions to investigate the topography on the 410‐ and 660‐km discontinuities defining the upper and lower boundaries of the MTZ. Our results show significant topography on the 410‐ and 660‐km discontinuities and corresponding variations in the MTZ thickness. The MTZ topography is broadly consistent with the results of seismic tomography studies, implying the presence of both cold thermal anomalies imparted by detached Tethyan slabs and lithospheric segments and hot thermal anomalies induced by upwelling of lower mantle material. The MTZ topography in the northern Middle East is dominated by the presence of patches of cold material that are intermittently separated by regions of hot to normal MTZ. Our results suggest that instead of a continuous slab, the Tethyan slab in the Middle East is strongly segmented along the strike of the subduction boundary. Furthermore, we find evidence for a significant gap in subduction extending from the eastern edge of the Cyprean arc to NW Iran. The southern Middle East is dominated by the processes related to the mantle upwelling beneath the Afar depression. Our results imply that buoyant lower mantle material enters the MTZ beneath the Afar depression and then spreads laterally to the northeast beneath the western Arabia, flowing within the MTZ and in the upper mantle.

Mapping Mantle Transition Zone Discontinuities Beneath the Central Pacific With Array Processing of SS Precursors

AbstractWe image mantle transition zone (MTZ) discontinuities beneath the Central Pacific using ~120,000 broadband SS waveforms. With a wave packet‐based array processing technique (curvelet transform), we improve the signal‐to‐noise ratio of SS precursors and remove interfering phases, so that precursors can be identified and measured over a larger distance range. Removal of interfering phases reveals possible phase shifts in the underside reflection at the 660, that is, S660S, which if ignored could lead to biased discontinuity depth estimates. The combination of data quantity and improved quality allows improved imaging and uncertainty estimation. Time to depth conversions after corrections for bathymetry, crustal thickness, and tomographically inferred mantle heterogeneity show that the mean depths of 410 and 660 beneath the Central Pacific are 420 ± 3 km and 659 ± 4 km, respectively. The mean MTZ thickness (239 ± 2 km) is close to global estimates and suggests an adiabatic mantle temperature of ~1,400°C for the Central Pacific. Depth variations of the 410 and 660 appear to be relatively small, with peak‐to‐peak amplitudes of the order of 10–15 km. The 410 and 660 are weakly anticorrelated, and MTZ is thinner beneath Hawaii and to the north and east of the hotspot and thicker southwest of it. The relatively small discontinuity topography argues against the presence of large‐scale (more than 5° wide) thermal anomalies with excess temperatures over 200 K across the transition zone. The data used cannot exclude stronger thermal anomalies that are of more limited lateral extent or that are not continuous across the MTZ.

Mantle transition zone discontinuities beneath the Indochina Peninsula: Implications for slab subduction and mantle upwelling

AbstractWhile the northward indentation of the Indian into Eurasian plates has been intensively investigated, its oblique subduction beneath the Indochina Peninsula (ICP) and the role it played on mantle structure and dynamics remain enigmatic. In this first regional‐scale receiver function study of the mantle transition zone (MTZ) discontinuities beneath the ICP and its surrounding areas, we stack ∼12,000 receiver functions recorded at 33 stations using a non–plane wave common‐conversion‐point stacking technique. Systematic spatial variations of MTZ thickness with departures between −21 and +24 km from the globally averaged value are revealed, providing independent evidence for the presence of slab segments in the MTZ beneath the central and a slab window beneath the western ICP. The results also support the existence of broad mantle upwelling adjacent to the eastern edge of the slab segments, which might be responsible for the widespread Cenozoic volcanisms and pervasively observed upper mantle low velocities in the area.

Mantle transition zone beneath a normal seafloor in the northwestern Pacific: Electrical conductivity, seismic thickness, and water content

We conducted a joint electromagnetic (EM) and seismic experiment to reveal the mantle structure beneath a normal seafloor at 130–145 Ma in the northwestern Pacific, where the seafloor is relatively flat and the underlying mantle is expected to be normal (free from tectonic perturbations). In the experiment, we deployed state-of-the-art instruments in two arrays from 2010–2015. Here, we report the result of analyses of the EM and seismic data for investigating the mantle transition zone (MTZ) structure. The EM data analysis revealed that an electrical conductivity structure below both arrays was approximated by an average 1-D model of the north Pacific, and showed a possible downward increase in conductivity at the top of the MTZ. From the P-wave receiver function analysis, perturbations in the MTZ thickness from a global average were estimated to be +20 km and +2 km below the northern and southern arrays, respectively, from which temperature profiles in the MTZ below these two arrays were then estimated. We jointly interpreted the profiles of electrical conductivity and thus estimated temperature, with reference to the experimental values of the effects of water on the electrical conductivities of MTZ minerals (wadsleyite and ringwoodite) from mineral physics. The upper bound of the water content below the northern array was determined to be 0.4 wt.% or 0.04 wt.%, depending on different results of mineral physics, and that below the southern array was determined to be slightly smaller. The lower bound of the water content was not constrained by our data. Our results indicate that the MTZ beneath the normal seafloor in the northwestern Pacific is drier than subduction zones, and may be a water-poor region in a plum-pudding mantle model.

Crustal structure and mantle transition zone thickness beneath a hydrothermal vent at the ultra-slow spreading Southwest Indian Ridge (49°39′E): a supplementary study based on passive seismic receiver functions

As a supplementary study, we used passive seismic data recorded by one ocean bottom seismometer (OBS) station (49°41.8′E) close to a hydrothermal vent (49°39′E) at the Southwest Indian Ridge to invert the crustal structure and mantle transition zone (MTZ) thickness by P-to-S receiver functions to investigate previous active seismic tomographic crustal models and determine the influence of the deep mantle thermal anomaly on seafloor hydrothermal venting at an ultra-slow spreading ridge. The new passive seismic S-wave model shows that the crust has a low velocity layer (2.6 km/s) from 4.0 to 6.0 km below the sea floor, which is interpreted as partial melting. We suggest that the Moho discontinuity at ~9.0 km is the bottom of a layer (2–3 km thick); the Moho (at depth of ~6–7 km), defined by active seismic P-wave models, is interpreted as a serpentinized front. The velocity spectrum stacking plot made from passive seismic data shows that the 410 discontinuity is depressed by ~15 km, the 660 discontinuity is elevated by ~18 km, and a positive thermal anomaly between 182 and 237 K is inferred.

Passive rifting of thick lithosphere in the southern East African Rift: Evidence from mantle transition zone discontinuity topography

AbstractTo investigate the mechanisms for the initiation and early‐stage evolution of the nonvolcanic southernmost segments of the East African Rift System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa rift zones over a 2 year period from mid‐2012 to mid‐2014. Stacking of over 1900 high‐quality receiver functions provides the first regional‐scale image of the 410 and 660 km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the rift zones. When a 1‐D standard Earth model is used for time‐depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area. In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper mantle high‐velocity anomalies. These findings suggest that it is unlikely for a low‐velocity province to reside within the upper mantle or MTZ beneath the nonvolcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi rift zone, an observation that is consistent with strain localization models and fault polarity and geometry observations. We postulate that the Malawi rift is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early‐stage development of rift zones in southern Africa.

Structure of the mantle transition zone beneath the North China Craton

Detailed information on deep structural features can improve current understanding of lithospheric thinning and destruction of the North China Craton. In this study, we analyzed receiver functions (RFs) from teleseismic P waveforms recorded by 246 seismic stations beneath the North China Craton (NCC) to determine the topography of the mantle transition zone (MTZ). The results reveal specific features in the 410 and 660km discontinuities and outlined MTZ thickness beneath the study area. The MTZ structure shows two anomalously thickened regions beneath the NCC (up to 30–40km thicker than the global average), which are mainly concentrated in the eastern and western sections of the eastern NCC (ENCC). The thickened regions are discontinuous and separated by a distance of about 400km. Double 660km discontinuities also appear in the southeastern ENCC. Combining these results with previous findings, we propose a model in which MTZ thickening is caused by discrete remnants of the subducted western Pacific slab. Discontinuities resulting from the stagnant slabs may retain evidence of changes in the rate of Pacific plate subduction as well as the phase transition of the subducted slabs within the MTZ.

The mantle transition zone beneath West Antarctica: Seismic evidence for hydration and thermal upwellings

AbstractAlthough prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high‐quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 and ∼0.48 Hz, respectively. Single‐station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210 ± 15 km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225 ± 25 km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic.

Lateral Variations of the Mantle Transition Zone Structure beneath Eastern China

Abstract The mantle transition zone (MTZ) structure beneath eastern China is studied by using P ‐wave receiver functions. The 410 km discontinuity is relatively flat whereas the depth of the 660 km discontinuity varies significantly under the study area. This indicates that the 660 is more influenced by laterally varying temperature. The 520 km discontinuity is continuously observed at ∼520 km depth under a limited area. Within the South China Block, the MTZ thickness under the Yangtze Craton is larger than normal whereas that under the Cathaysia Block is nearly normal. This indicates colder temperature affecting the MTZ beneath the Yangtze Craton. Combined with seismic tomography, our result suggests that the stagnant ancient slab beneath eastern China is mainly trapped around the base of the MTZ beneath the Yangtze Craton in the study area.

Seismic signature of the mantle transition zone beneath eastern Tibet and Sichuan Basin

The strong interaction between the eastward flow escaping from Tibet and the rigid Sichuan Basin resulted in the rise of the Longmenshan. However, the detailed dynamics in the mantle remains controversial. In this study, the structure of the mantle transition zone (MTZ) beneath eastern Tibet and Sichuan Basin is investigated using 5080 receiver functions from 51 broadband stations. The depth of the 410km discontinuity is close to the global average, except for the Longmenshan where the 410 and 660-km discontinuities are found to be depressed by up to 10–25km and 5–10km, respectively. The observed simultaneous depressions of the 410 and 660-km discontinuities distributed along the LMS, together with proofs from tomography and regional tectonics, suggest that asthenospheric flow sinks into the MTZ, resulting in a high velocity zone, as well as variation in the MTZ thickness. The depressions are not from the traditional Clapeyron slopes or temperature variation. Also, the depression of the 410km discontinuity and the dehydration of wadsleyite are syngenetic, both of which originate from the dry mantle flow traveling across the old 410km interface.