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To investigate continental dynamics underneath the south‐central Tibetan plateau, which composes the Himalayan, Lhasa, and Qiangtang blocks, we have conducted comprehensive examinations of seismic azimuthal anisotropy in the crust using receiver functions (RFs) and crustal and mantle anisotropy using teleseismic shear wave splitting (SWS) analysis. In the Qiangtang block, the observed predominantly E‐W fast orientations from RF and SWS analyses with similar magnitude are interpreted as resulting from eastward crustal flow with minor contributions from the mantle. In the Lhasa block, the crustal anisotropy is approximately N‐S oriented, which is parallel to the strike of rift basins and southward crustal flow. Anisotropy revealed by SWS demonstrates a rotation from E‐W in the north to NE‐SW in the south, which can be interpreted as reflecting mantle flow field induced by the northward movement of the subducting Indian plate. The addition of PKS and SKKS measurements and extension of epicentral distance range to 171.8° for SWS analysis revealed dominantly strong E‐W oriented anisotropy in most parts of the Himalayan block, where most previous studies reported pervasively null measurements. The absence of azimuthal anisotropy is observed in two regions in the Himalayan block which is attributable to mantle upwelling through a previously identified slab window. A two‐layered anisotropy structure with different fast orientations for the upper and lower layers can be constrained in the southern Qiangtang and the vicinity of the Main Boundary Thrust. Plain Language Summary: In our study exploring the underground dynamics beneath the south‐central Tibetan plateau, an area encompassing the Himalayan, Lhasa, and Qiangtang blocks, we utilized advanced seismic techniques to uncover the structure and dynamics of the earth's crust and mantle. In the Qiangtang region, our findings suggest an eastward crustal movement with minimal mantle influence. In the Lhasa block, the observed crustal anisotropy aligns with rift basins with a reduced magnitude, indicating a smaller‐scale southward flow. In the same area, we also observed a transition in mantle flow patterns from east‐west in the north to northeast‐southwest in the south, attributed to the northward movement of the Indian plate. Contrary to previous studies that mostly found no anisotropy in the Himalayan block, our research detected strong anisotropy with east‐west orientations. In two areas of the Himalayan block, the absence of anisotropy likely results from molten rock rising through a gap in the Indian plate. Moreover, we discovered a two‐layered structure of rock alignment near the Main Boundary Thrust and the Qiangtang block. Key Points: Significant anisotropy is revealed in the Himalayan block, where previous studies suggest pervasively null measurementsThe crust significantly impacts observed anisotropy in teleseismic shear wave splitting, potentially leading to double‐layered anisotropyAsthenospheric upwelling through a slab window beneath the Himalayan block accounts for the pervasive null measurements [ABSTRACT FROM AUTHOR]

The Banda arc‐continent collision zone signifies one of the most seismically active and tectonically intricate zones. The high convergence rate across the region, coupled with the exceptionally arcuate arc and subducted slab, makes it an ideal locale for investigating interactions between plate (slab) kinematics and plastic flow in the asthenosphere, which can be diagnosed by seismic anisotropy from shear wave splitting analyses. In total, 206 pairs of splitting measurements using teleseismic SKS, SKKS, and PKS, along with 43 pairs using local S phases, are obtained by utilizing broadband seismic data from five permanent seismic stations. To reduce the ambiguity in determining the origin of anisotropy leading to the teleseismic splittings, which lack vertical resolution, crustal anisotropy is constrained according to the sinusoidal moveout of converted S phases at the Moho using receiver functions. A layered anisotropic structure based on joint analyses of the anisotropy measurements characterizing different depth layers suggests the presence of trench‐parallel flow both in the mantle wedge and the sub‐slab region. The northeastward motion of the slab, entrained by the fast‐moving Australian Plate, deflects asthenospheric materials. The modulation results in trench‐parallel plastic mantle flows and leads to the steepening of the southern portion of the asymmetric spoon‐shaped Banda slab. In the shallower part of the sub‐slab region, the northeastward Australian Plate motion produces simple shear in the transitional layer between the rigid lithosphere and the viscous asthenosphere. The shear deformation induces seismic anisotropy with resulting fast orientations in accordance with the plate motion direction. Plain Language Summary: The Banda region represents a nascent arc‐continent collision with the Australian continental lithosphere. The volcanic arc and non‐volcanic arc‐islands display a unique horseshoe shape. The subducted slab is anomalously bent 180°, leading to a spoon‐shaped morphology with a steeper southern portion. The role of the kinematics of the Australian Plate and this bent slab in shaping the asthenosphere is not clear. This study uses seismic anisotropy measured using data recorded over 14 years by five broadband seismic stations on the non‐volcanic arc‐islands to explore this intriguing problem. Results suggest that the northeastward movement of the Australian Plate makes asthenospheric materials flow along the depth contour of the slab, steepening the Banda slab's southern part. In the sub‐slab region, the upper portion follows the movement direction of the Australian Plate, while the deeper part is influenced more by slab deflection, directing asthenospheric flow. Key Points: Mantle flow field associated with the spoon‐shaped Banda slab is investigated using seismic azimuthal anisotropy measurementsObservations can be explained by trench‐parallel flow both in the mantle wedge and the sub‐slab regionNortheastward movement of the slab relative to the asthenosphere induces the two flow systems and leads to steepening of the southern slab [ABSTRACT FROM AUTHOR]

To delineate the mantle flow fields in the vicinity of a previously proposed slab window and the possible roles that they may play in the formation of potassic volcanism in Central Java, we conduct shear wave splitting analyses using both local S and teleseismic XKS waves (including SKS, SKKS, and PKS) recorded by 121 onshore stations and two ocean bottom seismometers (OBSs). The XKS fast orientations from the OBSs are trench normal and in accord with previous subslab anisotropy measurements. In the eastern part of Central Java, the XKS and local S fast orientations from the onshore stations are mostly trench‐parallel; in contrast, in the western part of Central Java, the XKS fast orientations are trench‐normal while the local S measurements are spatially varying. The observations can be attributed to four flow systems including (a) subslab trench normal mantle flow in areas away from the trench which is entrained by the Australian Plate, (b) trench normal flow that goes into the mantle wedge from the subslab area through a slab window beneath the western part of the study area, (c) trench‐parallel subslab flow near the trench beneath the eastern part of the study area which is driven by slab‐rollback, and (d) dominantly trench‐parallel flow system in the mantle wedge reflecting the horizontal component of the escaped flow system through the slab window. We propose that the vertical component of the escaped flow is responsible for the formation of the potassic volcanoes in the adjacent oceanic area. Plain Language Summary: It has been well established that the vast majority of the world's volcanoes not associated with spreading centers are located along volcanic arcs, ultimately originating from dehydration of water‐bearing minerals brought down by subducting oceanic slabs. Some volcanoes occur in areas away from the volcanic arcs and are usually geochemically different from arc volcanoes. The intraplate volcanoes in Central Java are potassic (K)‐rich and have been hypothesized to be caused by a supply of volcanic material in the deeper asthenosphere compared to the arc volcanoes. The most frequently cited mechanism is mantle upwelling through a slab window. The window was imaged by some of the seismic tomography studies, but its location is debated due to diminishing resolution at greater depth and limited station coverage. More importantly, whether there is indeed a flow system traversing the slab window remains unrevealed. Here we conducted seismic azimuthal anisotropy investigation using splitting analyses of two kinds of shear waves that are originated at different depths. Our results indicate that mantle flow from the subslab enters the mantle wedge by traversing a slab window, and suggest that the vertical component of this flow system accounts for the formation of the potassic volcanoes in the area. Key Points: Azimuthal anisotropy beneath Central Java is delineated using teleseismic and local S wave splitting with unprecedented spatial resolutionMost results are consistent with subslab entrained flow traversing a slab window into the mantle wedge and flows parallel to trenchThe vertical component of the escaped flow through the slab window accounts for producing the potassic volcanoes in the back‐arc region [ABSTRACT FROM AUTHOR]

A total of 431 well‐defined and 632 null shear–wave splitting measurements obtained from 115 broadband seismic stations located in the eastern Himalayan syntaxis and adjacent areas is largely inconsistent with predicted fast orientations by absolute plate motion models. Spatial coherency analysis of the splitting parameters suggests that the observed azimuthal anisotropy is mostly located in the upper asthenosphere or the transitional layer between the lithosphere and asthenosphere, and the disagreement between the fast orientations and regional tectonic fabrics suggests an insignificant lithospheric contribution to the observed anisotropy. The observations may be attributed to flow systems that are driven by the westward rollback of the Indian slab beneath the Indo‐Burma block and are modulated by a previously revealed gap between the northward and eastward subducting slabs of the Indian plate. Key Points: Seismic anisotropy may be explained by a rollback of the Indian slab and modulation by a slab gapThe rotational pattern of fast orientations in SE Tibetan Plateau may be attributed to flow in the mantle wedge escaped from the slab gapN‐S anisotropy in NE India and Burma block and E‐W anisotropy in Indochina may reflect sub‐slab trench parallel flow from slab rollback [ABSTRACT FROM AUTHOR]

Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ~200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behindthe-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America. [ABSTRACT FROM AUTHOR]

The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction. [ABSTRACT FROM AUTHOR]

• Tibetan 410 and 660 km discontinuities are imaged using receiver functions • Central Tibet's d660 is deepened by a slab-related thermal anomaly and dehydration • MTZ thickens near Tengchong volcanic field due to slab dehydration • MTZ thins in western Tian Shan, suggesting active thermal upwelling To investigate the three-dimensional spatial distribution of subducted oceanic slab segments and their consequential effect on the thermal and hydrous composition of the mantle transition zone (MTZ) beneath the entire Tibetan Plateau, we conducted a comprehensive analysis of approximately 655,000 P-to-s receiver functions (RFs), obtained from 735 broadband seismic stations. These RFs were utilized to delineate the 410 km (d410) and 660 km (d660) seismic discontinuities, which represent the upper and lower boundaries of the MTZ, respectively. The RFs were grouped within circular bins with a radius of 1 degree and stacked to image the discontinuities. The mean apparent depths derived based on the 1-D IASP91 Earth model for the d410 and d660 across the entire study area are 412.2±8.3 km and 668.9±8.5 km respectively, and the MTZ thickness is 256.5±6.9 km. The observed apparent depths underwent subsequent correction utilizing multiple velocity models. Several areas in central Tibet exhibit a normal d410 and an anomalously deep d660, which can be attributed to the combined effect of the negative thermal anomaly and dehydration associated with subducted slab segments that have penetrated at least to the d660 depth. The anomalous thickening of the MTZ beneath the southeastern Tibetan Plateau surrounding the Tengchong volcanic field can be explained by the dehydration of the subducted Indian Slab. Significant thinning of the MTZ associated with the deepening of the d410 beneath the western Tian Shan may indicate active thermal upwelling originating from the MTZ. [ABSTRACT FROM AUTHOR]

The 410 and 660 km discontinuities (d 410 and d 660) bordering the mantle transition zone (MTZ) beneath NE Asia, including NE China, Eastern Mongolia, and southern Siberia, are imaged in successive circular bins with a radius of 1 degree by stacking a total of 274,413 P -to- s radial receiver functions recorded by 799 broadband seismic stations. After moveout correction based on the 1-D IASP91 Earth model, the resulting apparent depths of the discontinuities exhibit significant and spatially systematic variations. Three approximately N-S elongated narrow zones with significantly thickened MTZ are observed, which may be associated with the thermal effect and dehydration of subducted slabs. The major volcanoes in NE China are underlain by a d 660 that is apparently depressed by ∼19 km, which can be interpreted by the presence of an anomalously high water concentration in the lower MTZ released from the stagnated slabs. Low wavespeed anomalies above the d 410 west of the Datong volcanic fields are underlain by an MTZ with normal thickness, and are attributable by dehydration of the leading portion of the stagnant Pacific slab in the MTZ that is revealed in an N-S oriented narrow zone east of this area. The lateral shift of the upper mantle low wavespeed zone and the area with thickened MTZ may suggest a westward drift of the upper mantle relative to the subducted slab. An abnormally thin MTZ is observed beneath the Hangay Dome in central Mongolia, suggesting the possible existence of thermal upwelling from the lower mantle through the MTZ. • The 410 and 660 discontinuities in NE Asia are imaged with unprecedented resolution. • N-S oriented narrow zones with thick than normal MTZ indicate cold slab and hydrous MTZ. • Cenozoic volcanism is underlain by MTZ thickening and is caused by slab dehydration. • Thin MTZ beneath the Hangay Dome suggests thermal upwelling from the lower mantle. [ABSTRACT FROM AUTHOR]

• Mantle transition zone (MTZ) study of Borneo and the southern South China Sea (SCS). • Significant MTZ thickening is revealed to favor the existence of the Proto-SCS. • Post-subducted materials develop additional 350 and 730 km discontinuities. • Slab dehydration may contribute to post-spreading volcanism of the southern SCS. The South China Sea (SCS) is a hydrocarbon-rich major marginal sea in the western Pacific Ocean. One of the proposed driving mechanisms for its formation is the slab-pull of a Mesozoic ocean basin known as the Proto-South China Sea (PSCS), which subducted beneath northern Borneo between the Eocene and the early Miocene. However, the present-day depth extent of the slabs and even the existence of the PSCS remain highly debated. By stacking P-to-S conversions from the 410 km and 660 km discontinuities bordering the mantle transition zone (MTZ), we demonstrate significant MTZ thickening beneath the southern margin of the SCS, which, when combined with previously-revealed high-velocity anomalies in the MTZ, provides evidence for the existence of the PSCS. Chemically and rheologically heterogeneous materials segregated from the slab segments may have generated layered structures as evidenced by the discontinuities observed at the depths of ∼350 km and ∼730 km. Dehydration melting from the slab segments may be a viable mechanism for the extensive post-spreading magmatism in the southern SCS. [ABSTRACT FROM AUTHOR]