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The Molucca Sea region is an active arc-arc collision zone whose structure and mantle dynamics are essential for reconstructing the tectonic evolution of Southeast Asia. The key to understanding the tectonic processes of the region is to clarify the morphology of the subducted slabs. However, geometric details of the subducted Molucca Sea slab and other slab segments under this region remain controversial due to the lack of high-resolution mantle tomography. In this study, we jointly invert a large number of local and teleseismic travel-time data recorded at 99 local seismic stations to determine a high-resolution model of P-wave mantle tomography beneath the Molucca Sea region. Our tomography reveals two aseismic slab segments in the deep upper mantle and the mantle transition zone, which may reflect the earlier subducted Molucca Sea slab. The total length of the Molucca Sea slab is estimated to be ∼1800 to 1900 km, being ∼300 km longer than previous estimates. • The Molucca Sea slab has experienced break-offs. • Aseismic slab fragments broken off from the early subducted Molucca Sea slab are revealed in the mantle transition zone. • The maximum length of the Molucca Sea slab is ∼1800 to 1900 km. [ABSTRACT FROM AUTHOR]

Across the entire mantle we interpret 94 positive seismic wave-speed anomalies as subducted lithosphere and associate these slabs with their geological record. We document this as the Atlas of the Underworld , also accessible online at www.atlas-of-the-underworld.org , a compilation comprising subduction systems active in the past ~ 300 Myr. Deeper slabs are correlated to older geological records, assuming no relative horizontal motions between adjacent slabs following break-off, using knowledge of global plate circuits, but without assuming a mantle reference frame. The longest actively subducting slabs identified reach the depth of ~ 2500 km and some slabs have impinged on Large Low Shear Velocity Provinces in the deepest mantle. Anomously fast sinking of some slabs occurs in regions affected by long-term plume rising. We conclude that slab remnants eventually sink from the upper mantle to the core-mantle boundary. The range in subduction-age versus – depth in the lower mantle is largely inherited from the upper mantle history of subduction. We find a significant depth variation in average sinking speed of slabs. At the top of the lower mantle average slab sinking speeds are between 10 and 40 mm/yr, followed by a deceleration to 10–15 mm/yr down to depths around 1600–1700 km. In this interval, in situ time-stationary sinking rates suggest deceleration from 20 to 30 mm/yr to 4–8 mm/yr, increasing to 12–15 mm/yr below 2000 km. This corroborates the existence of a slab deceleration zone but we do not observe long-term (> 60 My) slab stagnation, excluding long-term stagnation due to compositional effects. Conversion of slab sinking profiles to viscosity profiles shows the general trend that mantle viscosity increases in the slab deceleration zone below which viscosity slowly decreases in the deep mantle. This is at variance with most published viscosity profiles that are derived from different observations, but agrees qualitatively with recent viscosity profiles suggested from material experiments. [ABSTRACT FROM AUTHOR]

A study of teleseismic P-wave tomography was carried out along the Central Orogenic Belt of China (COBC) consisting of the Qinling, Tongbai, Hong'an, Dabie and Sulu orogens that were created by northward subduction of the South China Block beneath the North China Block to subarc depths in the Paleozoic to Mesozoic. The results show the presence of high velocity anomaly underneath the COBC and its adjacent cratonic regions but the absence of high velocity anomaly in the lower lithosphere beneath the COBC. The high velocity anomaly is deep underneath the COBC, possibly representing the relic of the subducted Paleo-Tethyan oceanic slab predating the continental deep subduction of the South China Block. The high velocity anomaly is also deep underneath the southeastern part of the North China Block, possibly being a relic of the subducted Paleo-Pacific slab. However, no relic of the deeply subducted continental slab is seismically detected in the lower lithosphere beneath the Dabie and Sulu orogens, though the occurrence of ultrahigh-pressure metamorphic rocks in the two orogens has clearly demonstrated continental deep subduction to subarc depths in the Triassic. Instead there is only the relatively flat and uniform Moho boundary beneath the COBC region, indicating that the subduction-thickened continental lithosphere along the COBC has lost its root. It is inferred that the orogenic root beneath the COBC region has been foundered into the asthenospheric mantle at the postcollisional stage. The foundering would have taken place in the Early Cretaceous as indicated by the extensive occurrence of magmatic and migmatitic rocks along the COBC. As a consequence, the seismic tomography cannot detect the deeply subducted continental slab remnant from the Triassic period. It is possible that the tomographic visibility of continental subduction in the lower lithosphere is limited to the past 100 Myr. [ABSTRACT FROM AUTHOR]

We investigate the 3-D P and S wave velocity structure of the East Japan forearc using a large number of high-quality arrival-time data of local earthquakes recorded by a permanent seafloor seismic network (S-net) and the land-based Kiban seismic network. Geometries of the subducting slab boundary and the Conrad and Moho discontinuities under the forearc are refined by referring to results of previous studies including active-source seismic soundings and seismicity located by the S-net. The obtained P and S wave velocity images are quite similar to each other, suggesting that they reflect robust features of the 3-D seismic structure. The subducting Pacific slab exhibits high velocities, whereas significant low-velocity (low-V) anomalies are revealed in the overlying Okhotsk plate beneath the forearc, which may reflect accretionary materials containing abundant fluids. The subducting oceanic crust atop the Pacific slab is revealed clearly as a thin low-V layer extending from the trench axis down to ~100 km depth under the volcanic front. Our results of earthquake relocation using the 3-D velocity model indicate that the double seismic zone in the subducting Pacific slab occurs from the trench axis down to ~180 km depth. The occurrence and focal mechanisms of the double seismic zone are caused by slab deformation during its subduction, such as slab bending or unbending, as well as other processes including hydration, dehydration, compositional variations, and phase changes in the subducting slab. [Display omitted] • Seafloor S-net and Kiban network data are used to study the 3-D forearc structure. • The Conrad, Moho and Pacific slab geometries in the East Japan forearc are refined. • P and S wave velocity images are quite similar, reflecting robust structural features. • The subducting oceanic crust is revealed down to ~100 km depth under the volcanic front. • The intraslab double seismic zone occurs from the trench axis down to ~180 km depth. [ABSTRACT FROM AUTHOR]

We present a high-resolution 3-D model of P wave tomography of the crust and mantle down to 1600 km depth under Southeast Asia (SE Asia), which is determined by inverting more than 1.3 million P wave arrival time data of local, regional and teleseismic events recorded at 1241 seismic stations deployed in SE Asia and mainland China. Strong and weak smoothing inversions are conducted to reveal different characteristic scales of velocity anomalies in the mantle. Both seismic and aseismic portions of the subducted plates in different parts of the circular subduction system in SE Asia are clearly revealed as high-velocity bodies in our tomographic images. The intraplate volcanoes in Borneo are underlain by significant low-velocity anomalies in the upper mantle, whereas a clear high-velocity anomaly is visible in the mantle transition zone and the lower mantle. We deem that hot and wet upwelling flows in the big mantle wedge above the stagnant slab in the mantle transition zone may have caused the intraplate volcanism in Borneo. A large low-velocity anomaly consisting of a flat head and a twisted tail extending to the bottom of our model is imaged beneath the southeastern Asian basalt province. The low-velocity anomaly possibly represents a hot zone whose shape may have been modified by mantle wind associated with a plate rotation. The hot zone has a lower-mantle origin and probably contains some small mantle upwellings inside, with the Hainan plume being a major trunk. • Aseismic portions of the subducted slabs beneath SE Asia are revealed. • Hot upwellings in the big mantle wedge caused intraplate volcanism in Borneo. • The Hainan plume has a complex geometry due to mantle wind. [ABSTRACT FROM AUTHOR]

The motivation for this work was to model the crustal thickness, and the geometry of the Mohorovičić (Moho) discontinuity, the crust-upper mantle boundary around the Japanese archipelago. The western Pacific Ocean is one of the most eminent subduction zones in the world. The subduction zone complexity is due to alteration within the geometry of the stagnant slab. We compiled a three-dimensional density distribution model for subduction plates around the Japan Islands, and estimated the depth to Moho using the constrained non-linear inversion of gravity data derived from the latest Gravity field and Ocean Circulation Explorer, or GOCE, model known as GOCO06s. The study had three steps, starting with the data corrections followed by the inversion, and ending with the modeling and comparing of the estimated results with previous models and studies. In this study, we used available receiver function results, seismic tomography models, global crustal thickness, and geological and tectonic information as constraints. The model estimated the crustal thickness, surface geometry of the Moho, and geometry of the stagnant slab in the Kuril Trench, Japan Trench, Nankai Trough, and Izu-Bonin Trough. In gravity inversion, we used 34 km and 600 kg m−3 as the Moho depth reference and density contrast, respectively. Results showed that the crustal thickness ranged from 14 to 43 km with a − 0.78 mean misfit for Moho depths derived from seismic P-wave analysis. The calculated gravity data had a 0.16 mGal misfit with the observed data. The normalized analytical signal helped to sharply delineate the different plates and their edges. Three cross-sections were conducted to model the stagnant slab along the subduction zone of the Eastern Eurasian plate, western Pacific plate, Okhotsk plate, and Philippine Sea plate. The eastern area of the entire region (Pacific plate) had a low crust thickness and high density compared to the western area (Eurasian plate), which had a high crust thickness and low density. • 3D gravity inversion of the Northwestern Pacific Plate at the Japan Islands. • Estimating the Moho depths around the Japan Islands. • 3D density modeling of the Stagnant Slabs and Subduction plates. [ABSTRACT FROM AUTHOR]

Although it is well understood that mantle flow in a subduction setting can develop seismic anisotropy, many tomographic studies still assume an isotropic mantle. In this study, we explore how unaccounted-for seismic anisotropy contributes to P-wave anomalies, focusing on apparent sub-slab low velocities. We use data from the westernmost Mediterranean, where the Alboran slab has subducted beneath the Gibraltar Arc. Our isotropic P-wave velocity model shows a low-velocity anomaly at depths of 75–225 km to the west of the high-velocity-anomaly that represents the Alboran slab. We explore whether this low-velocity anomaly could be an anisotropy-induced artifact as a significant shear-wave-splitting (SWS) along the arc is observed. We consider five different hypothetical anisotropy models representing different mantle configurations. The anisotropy models are based on 1) the SWS observations, 2) the depth distribution of the low-velocity anomaly and 3) geodynamic modeling of the development of entrained and toroidal mantle flow in generic subduction zones. Given the underdetermined nature of an anisotropic inversion, we include the anisotropy models as a priori constraints on the tomography. The tomographic results including anisotropy models have smaller travel time residuals and isotropic anomalies in the region of interest while fitting the SWS observation better than the isotropic model. We propose that unaccounted-for seismic anisotropy can produce artifacts in isotropic tomography of real datasets, and this is at least partially the case for the low-velocity anomaly below the Alboran slab. Further, we show that not taking this into account can lead to erroneous conclusions about mantle temperature and the presence of melt. From the insights that our anisotropy models provide, we also suggest that toroidal mantle flow is currently dominant below and around the Alboran slab. • Seismic tomography considering seismic anisotropy in the westernmost Mediterranean. • Anisotropic models improve fit to travel time and SWS observations. • Best-fit model includes only anisotropy from toroidal flow. • Results show isotropic models overestimate the magnitude of sub-slab low-velocities. • Inclusion of anisotropy affects interpretations of temperature and melt content. [ABSTRACT FROM AUTHOR]

Abstract: We determine a 3-D P-wave velocity structure of the mantle down to 700 km depth under the Kamchatka peninsula using 678 P-wave arrival times collected from digital seismograms of 75 teleseismic events recorded by 15 portable seismic stations and 1 permanent station in Kamchatka. The subducting Pacific slab is imaged clearly that is visible in the upper mantle and extends below the 660-km discontinuity under southern Kamchatka, while it shortens toward the north and terminates near the Aleutian–Kamchatka junction. Low-velocity anomalies are visible beneath northern Kamchatka and under the junction, which are interpreted as asthenospheric flow. A gap model without remnant slab fragment is proposed to interpret the main feature of high-V anomalies. Combining our tomographic results with other geological and geophysical evidences, we consider that the slab loss may be induced by the friction with surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow and the presence of Meiji seamounts. As a result, the slab loss and the subducted Meiji seamounts have jointly caused the Pacific plate to subduct under Kamchatka with a lower dip angle near the junction, which made the Sheveluch and Klyuchevskoy volcanoes shift westward. [Copyright &y& Elsevier]