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Marginal parts of a plate and subducting slab can play important roles in geodynamics. This is because in areas where a plate interacts with other plates or with the mantle thermal, geochemical, and mechanical interactions are expected. The Philippine Sea (PHS) slab that subducts beneath the Japan arcs has such an edge. To examine the relationship between arc magmatism and the slab edge in the transition zone from Northeast Japan to Central Japan, we investigated isotopic systematics of the regional volcanic rocks, incorporating data from literature and new data for five isotopic ratios of Sr, Nd, and Pb. The new data included major element compositions of 22 samples from the back-arc area, and 5 isotopic ratios for 6 samples selected from Pleistocene to early Quaternary epochs. Consequently, several findings were determined based on the spatial variation of the isotopic ratios and the estimated amount of slab-derived fluid: (1) the amount of fluid derived from the two subducting slabs (i.e., the Pacific slab and the PHS slab) decreases northward from a significantly high value (~ 5 wt% fluid added to the source mantle), away from the seismically determined edge of the PHS slab; (2) the proportion of the PHS component in the total slab-derived fluid also decays northward; and (3) the PHS component spreads to the north beyond the seismically determined edge of the PHS slab. These observations strongly suggest that the existence of an aseismic PHS slab beneath southernmost parts of Northeast Japan delivers the PHS component to the arc magmatism. As was indicated by previous geodynamical studies, subduction of the PHS and PAC slabs may generate suction force towards the corner of mantle wedge, which might account for the large amount of fluid near the seismically determined slab edge as described in (1) above. [ABSTRACT FROM AUTHOR]

The Pacific and Okhotsk plates are converging at a rate of about 9 cm/a, thereby forming the Japan Trench subduction zone. Two large earthquakes occurred in the offshore region of Fukushima on 13 February 2021 (M w 7.1) and 16 March 2022 (M w 7.3). The occurrences of the two earthquakes are close in space and time, thus constituting the Fukushima-Oki earthquake doublet. The joint inversions of the onshore and offshore strong-motion data and the teleseismic P waves reveal that both earthquakes belong to intraslab events, and the unilateral ruptures of the 2021 and 2022 events propagate southwestward and northeastward, respectively. Both events are dominated by thrust motions, and major fault slips are mainly localized away from the rupture initiating area. The geometries and locations of the two seismogenic faults suggest that the Fukushima-Oki doublet arises from the reactivations of two preexisting bending-related faults inside the subducting Pacific plate, which is driven by the downdip compressional stress regime caused by plate unbending. Furthermore, the regional tectonic settings indicate a complicated seismogenic environment for the Fukushima-Oki doublet, in which the dehydration embrittlement and seamount subduction are likely to play significant roles. Additionally, the distinctive rupture patterns of both events are also associated with the variation of fluid content and the small-scale structure of a slab slice enclosed by the two seismogenic faults within the subducting plate, which may control the heterogenous distributions of fault slips and aftershocks. The analysis of Coulomb failure stress changes suggests that the 2021 event and the 2022 foreshock collectively increase the stress loading, which may facilitate the occurrence of the 2022 mainshock, and the seismicity of the seismic belt consisting of the three intraslab sequences following the great 2011 Tohoku-Oki earthquake is also likely to remain active in the future. • Source rupture processes of two Fukushima-Oki earthquakes in northeastern Japan. • Joint inversions of onshore and offshore strong-motion and teleseismic waveforms. • Two unilateral ruptures separately propagating southwestward and northeastward. • Both intraslab events caused by reactivations of preexisting outer-trench faults. • Seismogenesis attributed to heterogenous fluid content and regional tectonics. [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]

Zones of fluid-bearing anomalies occur in volcanic regions, on a plate interface, or in the mantle wedge of a subduction zone. The various causes include slab dehydration, serpentinization or metamorphic reactions in different thermal regimes. To reveal the impact factors that might have contributed to slab melting and arc magmatism behind the Japan Trench, we determined high-resolution three-dimensional (3-D) structures of seismic velocities (Vp, Vs), Vp/Vs ratio and temperatures. The temperatures (T) were calculated from the inverted seismic structures, and two petrological models of the upper mantle: a peridotite assemblage and a pyrolite assemblage, which differ from the results of the thermal simulation. The multi-parameter structures obtained for Vp, Vs, Vp/Vs and T show good consistency in the upper mantle behind the Japan Trench. The temperatures derived from the peridotite assemblage were found to match the mean geotherm and simulated temperature of the upper mantle more closely than those from the pyrolite model. A layer approximately 10 km thick with low-Vp and Vs, high-Vp/Vs, and slightly high-T anomalies was observed on the upper boundary of the subducting Pacific slab. This distinctive anomalous layer is interpreted as partial melting of the oceanic crust due to deep-seated metamorphic reactions whose characteristics are dependent on the source of fluids, mineral composition, and the thermal regime. Such a process enriches the peridotite content of the basalt underneath the island arc in the mantle wedge. Localized zones of significantly low-Vp and low-Vs, high-Vp/Vs and high-T perturbations were imaged in the mantle wedge under the active volcanoes, suggesting partial melting of peridotite-rich mantle material to produce tholeiitic magma. The present study demonstrates that fluids released from slab dehydration, mineral composition and the thermal regime play crucial roles in both arc magmatism, and slab melting in the subduction zone. [Display omitted] • High resolution seismic and thermal structures are imaged in the NE Japan subduction zone. • Slab melting is controlled by the source of fluids and thermal regime behind the Japan Trench. • Fluids and mineral composition play a key role in slab melting and arc magmatism. [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]

• Effects of anisotropic thermal conductivity on the thermal structure are investigated. • Two regions with a high degree of anisotropy are observed in the mantle wedge. • Slab temperature changes only slightly due to the anisotropic thermal conductivity. The effects of anisotropic thermal conductivity due to the crystallographic-preferred orientation (CPO) of olivine in the mantle wedge on the steady-state thermal structure of the Tohoku subduction zone, Northeast Japan, are investigated using geodynamic modeling. Four different olivine crystal fabrics are modeled based on theoretical CPO development in the mantle wedge: random (i.e., isotropic thermal conductivity), A-type, C-type, and E-type fabrics. The thermal conductivity in the mantle wedge is highly anisotropic in the regions just above the descending slab and near the base of the overriding plate. The obtained temperature differences between the anisotropic and isotropic cases are only up to 30 °C within the slab. Therefore, the impact of anisotropic thermal conductivity on the slab temperature may be limited when anisotropy is only considered in the mantle wedge. [ABSTRACT FROM AUTHOR]

Abstract: Oceanic plates subducted at trenches penetrate into the deep mantle, and encounter a structural boundary at a depth of 410km where olivine, the dominant element of mantle rocks, transforms into a higher density form wadsleyite. This transformation may be delayed within the coldest core of subducting plates (slabs) due to kinetic effects, and it has been suggested that metastable olivine may persist deeper than 410km. Using high density seismic array data in Japan, we show the direct image of the structure corresponding to this metastable olivine wedge (MOW) beneath southwest Japan. Numerical simulation of a subducting slab, including the kinetic effect of water (H2O) on the olivine–wadsleyite transformation, indicates that the presence of the imaged MOW requires an insignificant amount of water (less than 100wt. ppm) be present in the slab mantle, thus a deep dry cold slab. We infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab, but no significant amount within the slab itself. We also demonstrate that a numerical simulation including the kinetics of 660-km phase transformation can reconcile the observed deep depression of the 660-km discontinuity with a gentle Clapeyron slope. [Copyright &y& Elsevier]

Abstract: The petrological consequences of deep subhorizontal deflection (“stagnation”) of subducting slabs should affect both apparent seismic velocity structures and slab morphology. We construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high-pressure phase transitions in mantle minerals, focusing upon Japan as our study area. We calculate associated thermo-petrological buoyancy forces and bending moments which (along with other factors such as viscosity variations and rollback dynamics) may contribute to slab deformation. We consider effects of variations in depth of stagnation, post-stagnation dip angle, phase transition sharpness, transition triplication due to multiple intersection of geotherms with phase boundaries, and potential persistence of metastable phases due to kinetic hindrance. We also estimate seismic velocity anomalies, as might be imaged by seismic tomography, and corresponding seismic velocity gradients, as might be imaged by receiver-function analysis. We find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation. Such buoyancy forces vary with the depth at which stagnation occurs, so that slabs may seek an equilibrium slab stagnation depth. Metastable phase bending moment gradients further enhance slab stagnation, but they thermally decay after ∼600–700km of horizontal travel, potentially allowing stagnant slabs to descend into the lower mantle. Stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660-km seismic discontinuity, affecting the seismological visibility of such features. Seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique (travel-time tomography vs. boundary-interaction phases). While seismic tomography appears to yield images of stagnant slabs, discontinuity topography beneath Japan resolved by migrated receiver functions appears to be consistent with slab penetration of the transition zone. However, model slabs which bottom around ∼780–810km and then bend upwards by a few degrees can match both the tomographic and receiver-function images. [Copyright &y& Elsevier]

Abstract: The metaperidotites found in the Happo-O''ne region of central Japan are mostly lherzolites to harzburgites with subordinate dunites. Their protoliths originated as a series of refractory residues by near-fractional melting, 15–25% melting in the spinel field, which is confirmed by a HREE melting model of the apparently primary clinopyroxene. These metaperidotites display U-shaped primitive mantle (PM)-normalized REE patterns (0.02–0.5 times PM) and are highly enriched in LILE (0.2–20 times PM) relative to HFSE (<0.2 times PM), providing evidence for mantle-wedge metasomatism. In-situ analyses confirmed that clinopyroxenes and tremolite are enriched in fluid-mobile elements (B, Sr, Pb, Li, Cs, Ba and Rb; 0.1–100 times PM) coupled with depletion of HFSE (Ta, Hf, Th, Zr, Ti and Nb; <0.7 times PM)+U; these chemical features of clinopyroxenes and tremolite are similar to those of their whole rocks, and reflect slab-fluid metasomatism. We recognized three stages of clinopyroxenes that are different in morphology and REE patterns, but show the same major-element chemistry. Nearly euhedral clinopyroxene (Cpx1) displays a spoon-shaped REE pattern (0.2–3 times PM), reflecting the first stage (stage 1) of metasomatism. Acicular clinopyroxene (Cpx2) after tremolite+olivine shows a U-shaped REE pattern (0.06–1 times PM) (stage 2), whereas fine-grained clinopyroxene (Cpx3) derived from orthopyroxene shows very low concentrations of REE (stage 3). These three successive stages of retrogressive clinopyroxene formation possibly denote a retrogressive chemical change of the involved fluids during multi-stage metasomatism. The fluid of stage 1 is highly enriched in LREE (0.2–2 times PM), MREE, Pb, Sr, Li and Rb relative to the stage-3 fluid that is very low in MREE, Pb, LREE and Sr, but has high Na, Mn, Ba, B and Cs. The stage-2 fluid, which is low in LREE (0.06–1 times PM), Li and Pb, and very low in Cs, Rb and Hf relative to the fluid of stage 1, is considered to be a transitional fluid between stages 1 and 3. The retrogressive change of the fluid composition is possibly equivalent to a transversal change of the slab-derived fluid within the mantle wedge due to a change of metamorphic condition, depth of the subducting slab and an episode of metasomatism during the exhumation of the Happo-O''ne peridotites. [Copyright &y& Elsevier]

Abstract: Hypocenter relocations of earthquakes in the Pacific slab have shown an anomalous deepening of a seismic belt in the upper-plane of the double seismic zone at depths of 80–120km beneath the Hokkaido corner, while it is located at depths of 70–90km in the surrounding Tohoku and eastern Hokkaido areas. Seismic tomographic inversions performed beneath the Hokkaido corner have shown that a low-velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the Pacific slab and makes direct contact with the upper surface of the Pacific slab. These observations suggest that: 1) the low-velocity zone just above the Pacific slab is part of the subducted Kuril forearc sliver. 2) The contact with the subducted, and so relatively cold, sliver materials prevents the mantle wedge from heating the Pacific slab and causes a lower temperature condition in the Pacific slab crust beneath the Hokkaido corner than in the surrounding areas. 3) As a result, a delay of eclogite-forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there. [Copyright &y& Elsevier]

Abstract: We examine the upper mantle and transition zone beneath the western Pacific using multiple ScS reverberations. A low-velocity zone (LVZ) is found above the 410-km discontinuity oceanward of the subducting Honshu slab at an average depth of 356km, with a thickness that ranges from 50 to 75km assuming the LVZ continues to the 410-km discontinuity, which is locally elevated. The low-velocity region is evident in previous tomographic studies, and our results suggest that the anomaly is best explained by a layer of partial melt. The layer may be entrained from above by subduction or produced in situ by the combined effects of water and temperature. A self-consistent model that explains local P-wave velocities (Obayashi, M., Sugioka, H., Yoshimitsu, J., Fukao, Y., 2006. High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity. Earth and Planetary Science Letters 243, 149–158) and our observations calls for a maximum temperature anomaly of ∼150°C and a resulting maximum olivine water content after melting of 0.100wt. %. [Copyright &y& Elsevier]

Abstract: Both the global and regional P wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions. In particular, low-velocity anomalies have been observed beneath the descending high-velocity slabs in a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, make it difficult to substantiate this feature and evaluate the vertical extent of the low-velocity structure. From broadband waveform modeling of triplicated phases near the 660-km discontinuity for three deep events, we constrained both the P and SH wave velocity structures around the base of the upper mantle in northeast Asia. For the two events beneath the Southern Kurile, the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending Pacific slab. Our preferred models consistently suggest normal-to-lower P and significantly low SH velocities above and below the 660-km discontinuity extending to about 760-km depth compared with the global IASP91 model, corroborating previous observations for a slow structure underneath the slab. In contrast, both high P and SH velocity anomalies are shown in our preferred models for the Japan subduction zone region, likely reflecting the structural feature of a slab stagnant above the 660-km discontinuity. The velocity jumps across the 660-km discontinuity were found to be on average 4.5% and 7% for P and S waves under the south Kurile, and 3% and 6% under the Japan subduction zone. The respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under-slab areas. Based on mineral physics data, the depth-averaged ∼1.5% P and ∼2.5% SH velocity differences in the depth range of 560–760km between the two regions could be primarily explained by a 350–450K temperature variation, although the presence of about 0.5–1wt.% water might also contribute to the subtle velocity variations near the base of the transition zone in the Southern Kurile. From our modeling results, we speculate that the slow structure in the Southern Kurile may be correlated to the low-velocity zone observed previously around the 410-km discontinuity under Northern Honshu. If this is the case, both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast Asia. [Copyright &y& Elsevier]

Abstract: We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan. The Kurose xenoliths have less radiogenic Os isotopic ratios (187Os/188Os=0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima. [Copyright &y& Elsevier]

Tungsten is strongly incompatible during magmatic processes and is fluid mobile in subduction zones. Here we show that W isotope fractionation in arc lavas provide a powerful new tool for tracing slab dehydration and melting in subduction zones. Geochemically well characterized, representative arc-lavas from three subduction zones were chosen for this study to evaluate W isotope fractionation under different sub-arc conditions. Arc-lavas from SW Japan are produced by subducting a young, hot slab, and lavas from the volcanic front and rear arc of the Sangihe and Izu arcs are produced during subduction of a cold slab. The heaviest W isotope compositions (δ 184 W ∼ 0.06 ‰) are observed in fluid-rich samples from the volcanic fronts of the Sangihe and Izu arcs. With increasing distance from the volcanic front, more melt-rich samples are characterized by progressively lighter W isotope compositions. Enriched alkali basalts from SW Japan, thought to be the product of mantle melting at a slab tear, and adjacent shoshonites have the lightest W isotope compositions (δ 184 W ∼ 0 ‰). The correlation of W isotope fractionation with various indices of fluid release (e.g., Ce/Pb, Ba/Th) suggests that the heavy W isotope signatures record fluid recycling near the volcanic front due to dehydration of the subducted slab. Upon release of the heavy W, the residual slab preferentially retains isotopically light W, which is released during subsequent melting of drier lithologies in hot subduction zones, such as SW Japan. These data suggest that W isotopes can be used as a tracer of slab dehydration, potentially helping to determine the onset of cold subduction zone magmatism and hence, modern-style plate tectonics. • The W stable isotope composition of bulk earth is equal to chondritic values. • Cold, fluid-rich subduction zones have heavy W stable isotope compositions. • Hot, dry subduction zones have light W stable isotope compositions. • Tungsten stable isotopes have the potential to be used as a tracer for slab dehydration. [ABSTRACT FROM AUTHOR]