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The modern volcanoes of Indonesia, represent the cycling of material and heat along the Southeast (SE) Asian subduction zone. This volcanic activity is well exemplified in the middle section of Java Island, which is found on the Sunda arc portion of the SE Asian convergent system. Central Java's volcanoes inherit their geochemistry from silicate melts that formed due to hydrous melting at a range of depths above their subducting slab: ∼136 to ∼272 km. It has been shown that across-arc variations in magmatic trace elements like boron (B) can shed light on the quantity and type of slab-derived fluid released in a forearc to back-arc sequence. In this study, we present major, trace and radiogenic chemistry data for five volcanoes arranged in a roughly N-S lineament across Central Java. We show that the observed chemical changes across the section of the Sunda arc can be explained by variations in different kinds of source from fluid mobile material being devolatilized across the arc. Our mixing models suggest that the magmas underlying these volcanoes see a decreasing ratio of sediment to altered oceanic crust (AOC) contributions from the volcanic front (Merapi volcano) toward the rear-arc (Muria). We estimate that a maximum ∼1% contribution of sediment-derived fluid is sufficient to explain the chemical systematics of this arc segment. Shallow slab dehydration, mostly from the sediment cover, at low temperature (at <60 km of slab depth) transfers the fluid high in B/Nb, Ba/Nb, 87Sr/86Sr and 207Pb/204Pb to the mantle beneath this region's arc front, inducing flux melting. Farther from the trench, the signature of the subducting oceanic plate increases and peaks at around ∼160–170 km of slab depth, carrying substantial slab liquids with moderate to low B/Nb and Ba/Nb with lower 87Sr/86Sr and 207Pb/204Pb to the mantle beneath middle- and rear-arc region. Beyond 200 km, in the rear-arc, evidence for slab liquid is low, but not non-existent. Unexplained Ba-enrichment in magmas from across the arc may account for the role of subduction serpentinite dehydration in the subduction channel or local crustal assimilation. This study provides a conceptual model for material transfer from the subducting slab in SE Asia. Our findings have important implications for the understanding of geochemical cycling in this region, as well as the impact of these cycles on present and past volcanism. • Slab liquid with different proportion of SED/AOC is evident across Central Java, high at arc front and low at the rear-arc • Shallow slab liquid is high in FME, 87Sr/86Sr, 207Pb/204Pb; deep slab liquid has moderate/low levels of those components • Elevated Ba/Nb across the arc magmas may hint to the importance of subducting serpentinite in subduction channel [ABSTRACT FROM AUTHOR]

Subducted slabs provide the primary driving force for mantle convection and plate tectonics. The strength of the slab directly controls the transfer of forces between it and the lithospheric plates at the surface. However, slab viscosity constrained by surface topography and gravity data is significantly lower than that suggested by mineral physics laboratory experiments. It is unclear whether the free-slip top boundary condition used by these studies affects the inverted slab viscosity. In this study, we constrain the subducting slab viscosity structure by comparing the computed topography and gravity in slab subduction models with a free surface with observations. The free-surface model results support relatively weak slabs (20–120 times more viscous than the upper mantle) at the bending region, consistent with previous studies with a free-slip top boundary. The viscosity of the slab below the bending region barely affects the surface topography and gravity field, and both strong and weak slabs fit the observed topography and gravity field, suggesting that extra independent observations are needed to constrain the deep slab viscosity. We investigate the comprehensive relations between subduction interface viscosity, surface topography and gravity anomaly, and trench motion. Models with trench advance have significantly low topography and gravity above the volcanic arc, contradicting subduction zone observations. Together with present trench motion observations and previous studies, we support the idea that the trench retreats under normal single-slab subduction conditions. • We constrain slab viscosity with topography and gravity anomalies in slab subduction numerical models with a free surface. • Model results support relatively weak slabs (20–120 times more viscous than the upper mantle) at the bending region. • Trench advance requires strong subduction interfaces, which result in significant arc depressions that contradict the observation. [ABSTRACT FROM AUTHOR]

The complex tectonic setting of south-central Alaska is characterized by a transition from normally-dipping subduction of the Pacific plate in the west to near-flat slab subduction of the overthickened Yakutat microplate in the east. Many previous studies have characterized both the isotropic and anisotropic subsurface structure of this region, but only a few studies have characterized anisotropy using receiver functions. Here, we present anisotropy-aware receiver function analysis for two transects of permanent seismic stations in south-central Alaska, one covering a normally-dipping segment of the subduction zone, and one covering the adjacent flat slab segment. Beneath the normally-dipping segment, there is evidence for shearing and possibly serpentinization at the top of the slab in the shallow forearc, and for variation in mantle flow geometry with depth, possibly a result of oblique subduction and/or the adjacent flat slab segment, or an arc magmatism-related process. Additionally, there appears to be significant crustal deformation associated with the volcanic arc. We also identify significant crustal deformation and anisotropy along the flat slab segment, likely a result of the subducting Yakutat microplate, with crustal deformation geometry appearing to vary along the transect. There also appears to be evidence for water-rich conditions at the top of the flat slab, shedding light on the distribution of volatiles in a flat slab setting that lacks an active volcanic arc. • Anisotropy-aware receiver functions for normally-dipping and flat slab segments. • Complex deformation in the mantle wedge above the normally-dipping Pacific plate. • Complex crustal deformation and possibly water-rich mantle above the flat Yakutat slab. [ABSTRACT FROM AUTHOR]

The origin of the high velocity anomalies in cold subduction slabs revealed by seismological tomography remains unknown. Garnet is a major component of the pyrolitic mantle and subduction slabs and crucially affects their physical properties, such as densities and velocities. Based on previous studies, the predicted velocities of garnets with various compositions are much different from the measured values. Here, we measured the velocities of four hot-pressed natural garnet samples, namely, Alm 65.1 Grs 17.5 Prp 8.5 Sps 2.0 And 6.9 , Alm 60.9 Grs 25.9 Prp 8.2 Sps 2.0 And 3.0 , Alm 49.6 Grs 5.3 Prp 37.8 Sps 1.7 And 5.6 , and Grs 89.0 Prp 0.4 Sps 6.3 And 4.3 , at pressures up to 11 GPa using the ultrasonic interference method in conjunction with a Walker-type multi anvil, to further constrain the composition dependence on sound velocities. Our results show that the velocities and the bulk and shear moduli monotonically increase with pressure but display a complicated dependence on composition. Accordingly, we have determined the dependences of P - and S -wave velocities of garnets on pressure and composition and reconstructed the velocity profiles of pyrolite, eclogite, harzburgite, and a garnet layer at depths of 90–300 km in a cold subducted slab. Our model proposes that the presence of a garnet layer with a thickness of 5.9–15.9 km atop the slab could cause velocity contrasts of 4.0–6.0% for V P and 4.1–7.7% for V S between the slab and the overlying pyrolite. The presence of a layer with a high proportion of garnet could contribute to observed high-velocity anomalies in cold subduction zones. • The velocities of four hot-pressed garnet samples up to 11 GPa were obtained. • The relationship between garnet wave velocity and composition was determined. • A garnet layer could cause high velocity anomalies in cold subduction zones. [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]

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. • MTZ discontinuities beneath Java and adjacent areas are imaged using receiver functions with an unprecedented resolution. • The abnormally thin MTZ underneath Lombok and Sumbawa Islandssuggests the presence of significant positive thermal anomaly. • The discontinuity depth undulations in eastern Java are associated with toroidal mantle flow traversing a slab window. [ABSTRACT FROM AUTHOR]

Forward and inverse P- wave velocity models along an E -W 280-km-long wide-angle reflection/refraction profile at 21°N across the northeastern South China Sea margin and Manila subduction zone reveal a 10–12 km thick thinned continental crust (TCC) located west of the Manila transcurrent fault (MTF) and Manila slab. The TCC is divided in two parts separated by the intra-continental velocity difference boundary (VDB). West of the VDB, the TCC is located above a high velocity layer (HVL). East of the VDB, the TCC consists of three layers: an undefined body with a velocity inversion at its base above the upper and lower crusts. The VDB and MTF merge east of the Hengchun peninsula showing that the TCC located east of the VDB decreases in size and disappears to the north. The distribution of earthquakes (EQs) shows that ∼80 km of crust have subducted east of the MTF since the onset of Luzon arc collision with Eurasia at ∼7 Ma. All published tomographies suggest the subduction of oceanic crust (OC). However, due to crustal corrections, up to a maximum width of 40 km of TCC might eventually exist in the upper part of the Manila slab instead of OC. Two end-members with 40 km of TCC or not in the upper part of the Manila slab were restored at ∼7 Ma. Since that time, the north Luzon island moved ∼600 km in the northwest while part of this motion was absorbed near the Hengchun peninsula by shrinking of Eurasia TCC, resulting in a ∼ 20° clockwise rotation of the Manila slab and MTF. Small Luzon arc blocks started to rotate 10°-30° clockwise at ∼7 Ma below northern Taiwan, 3–4 Ma in the Coastal range and 1 Ma in Lutao and Lanyu islands and not south of the Hengchun peninsula. Thus, both the clockwise rotation of the individual Luzon arc blocks and of the Manila slab and MTF might explain the collision process of the Luzon arc with the Eurasia continent. • Thinned continental crust (TCC) west of the Manila transcurrent fault and Manila slab. • Two adjacent stripes separated by an intra-continental velocity difference boundary composed the TTC. • The left-lateral motion guides the driving force controlling the collision of Luzon arc and Eurasian TCC. [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]

The Laramide orogeny (~ 80–50 Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000 km inboard of the Farallon Plate subduction margin. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. Three proposed factors are: (1) a westward (trenchward) increase in North America motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a low-density oceanic plateau. This study uses 2D upper mantle scale numerical models to investigate these factors. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (> 100 Myr) and denser than the mantle. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau that includes an 18-km-thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. The slab suction force has only a minor effect on slab flattening, but the thickness of the Colorado Plateau lithosphere controls the depth of the flat slab. With a continental velocity of 4 cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15 Ma after plateau subduction. The flat slab underthrusts the continent at ~ 200 km depth, eventually extending > 1500 km inboard of the trench. [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 geodynamic evolution of the Western Mediterranean related to the closure of the Ligurian-Tethys ocean is not yet fully resolved. We present a new 3D numerical model of double subduction with opposite polarities fostered by the inherited segmentation of the Ligurian-Tethys margins and rifting system between Iberia and NW Africa. The model is constrained by plate kinematic reconstructions and assumes that both Alboran-Tethys and Algerian-Tethys plate segments are separated by a NW-SE transform zone enabling that subduction polarity changes from SE-dipping in the Alboran-Tethys segment to NW-dipping in the Algerian-Tethys segment. The model starts about late Eocene times at 36.5 Ma and the temporal evolution of the simulation is tied to the geological evolution by comparing the rates of convergence and trench retreat, and the onset and end of opening in the Alboran Basin. Curvature of the Alboran-Tethys slab is imposed by the pinning of its western edge when reaching the end of the transform zone in the adjacent west-Africa continental block. The progressive curvature of the trench explains the observed regional stress reorientation changing from N-S to NW-SE and to E -W in the central and western regions of the Alboran Basin. The increase of the retreat rates from the Alboran-Tethys to the Algerian-Tethys slabs is compatible with the west-to-east transition from continental-to-magmatic-to-oceanic crustal nature and with the massive and partially synchronous calc-alkaline and alkaline magmatism. • We propose a 3D numerical model of opposing subduction simulating the tectonic setting of the Western Mediterranean. • The retreat rates of the slabs are consistent with the duration of opening of the Alboran and Algerian basins. • The progressive curvature of the Alboran-Tethys slab is consistent with the regional stress evolution of the Alboran Basin. [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]

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. • The MTZ discontinuities in Taiwan and its adjacent regions are imaged using receiver functions of unprecedented resolution • The geometry of subducted slab segments in the vicinity of the MTZ is constrained • The break-off Philippine Sea Plate drops into the lower mantle transition zone along with dehydration • The eastward-dipping South China Sea Slab penetrates the d410 [ABSTRACT FROM AUTHOR]

Convergent plate dehydration is of significance in determining the coupling strength and seismicity activity at variable subduction contacts. Characterized by decadal M > 7.5 devastating earthquakes offshore and a lack of a magmatic arc in north-central Peru, the Peruvian megathrust remains enigmatic in terms of the along-strike heterogeneity of the thermal regime, slab dehydration, and seismotectonics. Hydrothermally controlled petrological metamorphism of multiple facies within the subducted oceanic lithosphere and subsequent fluid channeling and migration lead to fault instability and seismic and aseismic slips on the megathrusts. Based on 3D thermal modeling, we estimate the features of the Peruvian subduction hydrothermal regime. Our results show that (1) the shallow seismogenic zone (M > 7.5 earthquakes) along coastal Peru exhibits high slab dehydration rates and gradients and is associated with fault segmentation and the updip migration of fluids controlling the occurrence of great megathrust earthquakes; (2) the low slab dehydration rate (<0.01 wt%/km) in north Peru contributes to the absent volcanism in north Peru, while the high slab dehydration rate (>0.02 wt%/km) beneath south Peru facilitate the recurrence of magmatic volcanism; (3) the subduction-related slip behaviors at a certain depth resulting from heterogeneous variations in the coupled plate interface are determined by slab dehydration and petrological metamorphism in the subducted Nazca plate. The variation in slab dehydration contributes to the seismotectonic segmentation and slip variation associated with changes in the pore fluid pressure on the subducted megathrust. • Dehydration embrittlement resulting from the eclogitization of the MORB layer and harzburgitization of the ultramafic layer influence the occurrence of subduction earthquakes at various depths beneath Peru. • The along-strike fluid content variation contributes to the forearc slab segmentation and seismotectonics which are influenced by the plate topographic features entrained into subduction channels. • Migration of subducted oceanic ridge with a thick oceanic crust leads to the MORB-dominant flattened slab and multi-layered complicated hydrothermal regimes. [ABSTRACT FROM AUTHOR]

Geological observations have revealed a rapid evolution and obvious east–west differential evolution in the North Sulawesi subduction zone. The Celebes Sea plate has inserted itself under the north arm of Sulawesi Islands, which has simultaneously rotated clockwise. The rotation of the north arm of the Sulawesi Islands might be critical in facilitating tectonic processes, such as the slab subduction and rollback of the Celebes Sea plate. For the east–west differences along the North Sulawesi subduction zone, a numerical model with the convergence rate of the plates as the basic variable is established to quantitatively describe the evolution process of the North Sulawesi subduction zone. Our results reproduced the east–west differences of the subducting Celebes Sea plate, showing a shallow–deep–shallow subduction style. We propose that the variable velocity ratio of the overriding plate to the subducting slab could be the principal reason for the differential subduction along the strike of the North Sulawesi subduction zone. We conclude that the residual slab and the rate of the eastern continental plate limit the downward movement of the subducted slabs of the eastern Sulawesi. Furthermore, the reason for the shorter subducted slab at the extreme western Sulawesi is the fact that subduction occurs outside the rotation radius. Moreover, the widespread extension at the western Sulawesi has a limited correlation with the clockwise rotation. • We combined 2D numerical simulation with seismicity to obtain an evolution model of the North Sulawesi subduction zone. • Variable velocity ratio of the overriding plate to the subducting slab induced the East-West subduction difference. • Connection between the subduction of Celebes Sea and the rotation of the north arm of the Sulawesi Islands is discussed. [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]

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 present a series of three-dimensional numerical models investigating possible thermal and mechanical effects of ridge subduction and slab window migration. Ridge subduction is a fundamental process of plate tectonics, and the geologic manifestations in the overriding lithosphere have been investigated in many convergent margin settings. Many of the geologic effects of slab window migration (e.g., anomalous high-T metamorphism in the forearc, non-arc-like magmatism in the volcanic arc) are explained primarily by the introduction of a region of hot, upwelling asthenospheric mantle in the subduction zone environment. Using end-member boundary conditions and idealized geometries, our models address the thermal and corresponding mechanical manifestations of slab window migration, and our results provide a general agreement with observations from interpreted regions of slab window migration. Our models show that protracted heating in the forearc region should result in protracted high-temperature metamorphism, associated rheologic weakening and strain partitioning and changes in the topographic uplift pattern in regions affected by slab window migration. Although these models are idealized, they do provide valuable insight into the geodynamics of slab window environments and provide a valuable starting point for further exploration. [Copyright &y& Elsevier]

Abstract: The detachment of subducted lithosphere (slabs) has been proposed in a number of locations as the cause for observed volcanism and regional tectonic evolution. For example, along-strike progression of uplift in the Adriatic Sea and west-to-east progression of volcanism in the Trans-Mexican Volcanic Belt may be caused by propagating detachment of subducted slabs. An understanding of the dynamics of slab detachment is necessary to relate the timing of detachment to surface observations. We demonstrate using two-dimensional (2D) numerical models that the timing, occurrence, and surface effects resulting from the detachment of subducted lithosphere are strongly dependent upon the stiffness (i.e. stress supported viscously) of the lithosphere and rheology of the upper mantle. In particular, in models with a Newtonian upper mantle where plastic yielding is the only weakening process within the slab, detachment does not occur. Models accounting for non-Newtonian upper mantle deformation demonstrate two modes of detachment: (1) slow and deep detachment of strong slabs (500 MPa or greater maximum yield strength) controlled by heating of the slab, and (2) fast and shallow detachment of weaker slabs (300 MPa) controlled by yielding within the slab interior. For slabs with a maximum yield strength of 500 MPa, the time to detachment increases with slab age (for ages ranging from 40 to 120 My), indicating that detachment time is controlled by the integrated stiffness of the slab, which limits the sinking rate. However, the detachment time for weaker slabs (300 MPa) is independent of age because weaker slabs sink rapidly, and the time to detachment is limited by the rate of flow of the surrounding mantle. [Copyright &y& Elsevier]

Abstract: Many subduction margins that rim the Pacific Ocean contain complex records of Cenozoic slab-window volcanism combined with tectonic disruption of the continental margin. The series of slab windows that opened beneath California and Mexico starting about 28.5 Ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime. The timing and areal extent of the resultant slab-window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin. The initial Pioneer slab window thermally weakened the overlying western Transverse Ranges and California Borderlands region starting about 28.5 Ma. A second thermal pulse occurred in the same region starting about 19 Ma during growth of the Monterey slab window. This additional heating, combined with the capture of a partially subducted Monterey plate fragment by the Cocos plate, initiated the pulling apart and rotation of the adjacent continental margin. Similarly, the capture of Guadalupe and Magdalena plate fragments by the Pacific plate and initiation of the Guadalupe–Magdalena slab window about 12.5 Ma are coeval with Baja California pulling away from the Mexico continental margin, with the break along the Comondú arc, in crust already thermally weakened by about 10 My of volcanism. In coastal California, distributed crustal extension and subsidence accompanied the new transform plate boundary, and continued until the slab windows cooled and plate motion coalesced along a through-going system of strike-slip faults. The transform boundary continues to evolve, and forward modeling predicts an instability with the current configuration as a result of convergence between the Sierra Nevada and Peninsular Ranges batholiths, starting about 2 My in the future. The instability may be resolved by a shift in the locus of transform motion from the San Andreas fault to the eastern California shear zone, or by breaking off another fragment of the Mojave or southern Sierra Nevada crustal blocks and translating it northward. [Copyright &y& Elsevier]

Abstract: We present small-scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions. The oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate. In this configuration, the long-term process of subduction is not steady-state. Slab interaction with the upper mantle-lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes, followed by periods of slab steepening. The overriding plate tectonic regime is influenced by the dynamics of subduction, slab anchoring favouring trench perpendicular shortening. When the slab is anchored, slab flattening further favours shortening, while slab steepening favours extension or smaller shortening rates. Non-steady-state long-term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present-day subduction zones. Experiments suggest that, despite boundary conditions applied on the converging plates do not change, tectonics pulses within the overriding plate may be caused by this non-steady-state behaviour. [Copyright &y& Elsevier]

Abstract: The Schellart''s [Schellart, W.P., 2007, The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle. Tectonophysics, 445, 363–372.] paper uses slab dip and upper plate extension for testing the westward drift. His analysis and discussion are misleading for the study of the net rotation of the lithosphere since the first 125 km of subduction zones are sensitive also to other parameters such upper plate thickness, geometry and obliquity of the subduction zone with respect to the convergence direction. The deeper (>125 km) part cannot easily be compared as well because E- or NE-directed subduction zones have seismic gaps between 270–630 km. Moreover the velocity of subduction hinge cannot be precisely estimated and it does not equal to backarc spreading due to accretionary prism growth and asthenospheric intrusion at the subduction hinge. It is shown here that hinge migration in the upper plate or lower plate reference frames supports a general global polarization of the lithosphere in agreement with the westward drift of the lithosphere. The W-directed subduction zones appear controlled by the slab–mantle interaction with slab retreat imposed by the eastward mantle flow. The opposite E-NE-directed subduction zones seem rather mainly controlled by the convergence rate, plus density, thickness and viscosity of the upper and lower plates. Finally, the geological and geophysical asymmetries recorded along subduction and rift zones as a function of their polarity with respect to the tectonic mainstream are not questioned in the Schellart''s paper, but they rather represent the basic evidence for the westward drift of the lithosphere. [Copyright &y& Elsevier]

Oceanic plate subduction can exert extensional, compressional or both types of deformations simultaneously in overriding plates. In this study, we explore the factors controlling these two competing modes of deformation in response to contrasting stress fields. Two-dimensional computational fluid dynamics models are presented to show that the dip and age of subducting slabs are crucial parameters to modulate the extensional versus compressional deformations. Shallow-dipping (< 30°) and/or young (< 60 Myr) subducting slabs favor compressional tectonics to form an orogenic wedge (accretionary type), whereas steeply dipping (> 45°) and/or old (> 60 Myr) slabs facilitate extensional tectonics, expressed as a back-arc basin (non-accretionary type). We validate the parametric analysis by comparing our model results with the observed modes of overriding plate deformation in natural subduction zones. Unlabelled Image • Numerical simulations are run to understand the mode of plate margin deformation as a function of subducting slab dip and age. • Shallow and/or young slabs develop accretionary type plate margin while steep and/or old slabs exhibit non-accretionary type. • The model results are in agreement with the styles of plate margin deformation in natural subduction zones. [ABSTRACT FROM AUTHOR]

Understanding how long-term subduction dynamics relates to the short-term seismicity and crustal tec tonics is a challenging but crucial topic in seismotectonics. We attempt to address this issue by linking long-term geodynamic evolution with short-term seismogenic deformation in the Northern Apennines. This retreating subduction orogen displays tectonic and seismogenic behaviors on various spatiotemporal scales that also characterize other subduction zones in the Mediterranean area. We use visco-elasto-plastic seismo-thermo-mechanical (STM) modeling with a realistic 2D setup based on available geological and geophysical data. The subduction dynamics and seismicity are coupled in the numerical modeling, and driven only by buoyancy forces, i.e., slab pull. Our results suggest that lower crustal rheology and lithospheric mantle temperature modulate the crustal tectonics of the Northern Apennines, as inferred by previous studies. The observed spatial distribution of upper crustal tectonic regimes and surface displacements requires buoyant, highly ductile material in the subduction channel beneath the internal part of the orogen. This allows protrusion of the asthenosphere in the lower crust and lithospheric delamination associated with slab retreat. The resulting surface velocities and principal stress axes generally agree with present-day observations, suggesting that slab delamination and retreat can explain the dynamics of the orogen. Our simulations successfully reproduce the type and overall distribution of seismicity with thrust faulting events in the external part of the orogen and normal faulting in its internal part. Slab temperatures and lithospheric mantle stiffness affect the cumulative seismic moment release and spatial distribution of upper crustal earthquakes. The properties of deep, sub-crustal material are thus shown to influence upper crustal seismicity in an orogen driven by slab retreat, even though the upper crust is largely decoupled from the lithospheric mantle. Our simulations therefore highlight the effect of deep lower crustal rheologies, self-driven subduction dynamics and mantle properties in controlling shallow deformation and seismicity. • Simulation links crustal tectonics, seismicity with long-term geodynamics driven solely by slab pull. • Lithospheric delamination allowed by a ductile lower crust is needed for a realistic stress pattern. • Deep material properties (e.g., slab temperature, stiffness) significantly affect crustal seismicity. [ABSTRACT FROM AUTHOR]

Vertical edges along subducted slabs have been recognized in the majority of subduction zones. Surprisingly, slab edges evolved into Subduction-Transform-Edge-Propagator (STEP) faults in only a few regions; the conditions under which STEPs form are special. It is relevant to constrain the conditions that facilitate STEP fault initiation because they leave a clear geological footprint in the overriding plate, whereas vertical tears generally do not. We therefore study a candidate region for STEP fault initiation in the western Hellenic Subduction Zone. We investigate the structure and seismicity of the shallow western Hellenic Subduction Zone using a recent full-waveform inversion model which both captures details of crustal and upper-mantle structure, yielding constraints in the depth interval from 10 to 200 km where lithosphere-mantle interactions have tectonic expressions. The western end of the Hellenic Subduction Zone is fragmented near the Kefalonia Transform Fault. We identify a separate Epirus lithospheric fragment that is roughly vertical below the southern Albanides. We also identify a new and major contrast within the lithospheric mantle of the Ionian ocean basin, which aligns with a gradient in free-air anomalies. In the overriding plate, the Kefalonia Transform Fault zone accommodates right lateral strike-slip deformation. We interpret this fault zone as a proto-STEP fault that formed simultaneously with Pliocene fragmentation of the Epirus fragment. Comparing the recent evolution of the NW Hellenic slab edge with currently active STEPs indicates that along-trench variations in convergence velocity are a prerequisite for STEP fault initiation. Such velocity variations may result from subduction of continental crust along part of the trench. Resistance to sea ward tear propagation by a mechanically strong subducting plate may prevent variations in convergence velocity to occur, and thus STEP fault initiation. The amount of time over which a velocity contrast persists will also be relevant for STEP fault initiation. Mechanical coupling between upper- and lower-plate, and the deformability of the upper-plate appear to also play a role in the initiation of the STEP fault once a slab is fragmented. • The western end of the Hellenic slab is fragmented near the Kefalonia Transform Fault (KTF). • The separate Epirus fragment is roughly vertical below the southern Albanides. • We interpret the KTF as a proto-STEP fault that initiated simultaneous with Pliocene fragmentation of the Epirus fragment. [ABSTRACT FROM AUTHOR]

Gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub-slab mantle to flow through the gaps and into the mantle wedge. This through-slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and/or dynamic uplift in the overriding plate, while the altered mantle flow patterns affect the trench evolution. Numerous geodynamic models have investigated the processes that form slab gaps, but few studies have examined the dynamics of slab gap-altered mantle flow, its effects on trench morphology and kinematics, or the controlling parameters on these processes. Here, laboratory subduction models with a pre-cut gap in a subducting silicone plate are used to explore how slab gap size, and slab gap depth influence the surrounding mantle flow field and trench dynamics. Results suggest that both the vertical extent and the depth of the top (trailing edge) of the slab gap are crucial parameters for modulating overall subduction dynamics. They show that a slab gap, which occurs near the surface and initially comprises 30% of the subducting plate width, can extend enough vertically in the slab to produce significant vertical flow through the gap. Changes to the trench geometry and kinematics are also evident in the models, such that double- and triple-arc geometries are formed during subduction of a shallow slab gap. All of these results are consistent with observations of slab gaps and their induced surface expressions, or the lack thereof, in Eastern Anatolia, East Java, Italy, and Argentina. Unlabelled Image • Subduction dynamics can be largely affected by the opening of slab gaps. • Effects on the surface are only observable when the slab gap is located near-surface. • Vertical mantle flow through the slab gap can happen only when the gap has several 100 km vertical extent. [ABSTRACT FROM AUTHOR]

Numerical simulations have indicated that the Large low-Shear Velocity Provinces (LLSVPs) dynamically interact with subducted slabs in D″, and slabs in turn play an important role in influencing the shape and location of the LLSVPs. In this study, we find evidence for the existence of the D" discontinuity near the northeastern margin of the Perm Anomaly — a smaller LLSVP beneath Eurasia, by analyzing the Scd (the lower mantle triplication) phases generated from two deep events occurring in the northwestern Pacific subduction zone and recorded at stations in Europe and Turkey. The D″ discontinuity in the north is characterized with a +2% velocity increase occurring at a height of 300 km above the core-mantle boundary (CMB), whereas the discontinuity in the south is characterized with a +3% velocity increase occurring at a height of 260 km above the CMB. The D″ discontinuity is most likely attributed to a phase transition from perovskite to post-perovskite in conjunction with the presence of relic slab materials (likely related to the subducted Mongol-Okhotsk slab). Our observations– along with previous seismic anisotropic detections– provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the LLSVP. • The D″ discontinuity serves as evidence of the existence of a paleoslab near the northeastern margin of the Perm Anomaly. • The paleoslab is likely attributed to the subducted Mongol-Okhotsk slab. • The impingement of the Mongol-Okhotsk slab may lead to deformation in D″ near the NE margin of the Perm Anomaly. [ABSTRACT FROM AUTHOR]

• Slab breakoff occurs almost synchronously in a narrow range of depths when the passive margin is parallel to the trench. • Slab breakoff may occur over a wide range of depths and times in models with variable geometries for the oceanic to continental transition zone. • Phases of uplift and subsidence result from dynamic and isostatic adjustments associated with the lateral propagation of breakoff at depth. We study slab breakoff of subducting plates with variable orientations of passive margins (ocean-continent transition: OCT) using three-dimensional laboratory experiments. Our results show that the initial obliquity of the OCT with the trench determines the depth and timing of slab breakoff. The reference model involves a straight OCT parallel to the trench. Slab breakoff first starts in the central section of the subduction zone at a depth equivalent to ∼250 km and then propagates rapidly towards the edges of the subduction zone where it develops at slightly higher depths (∼280 km). In the models where passive margin arrives first at the edges of the subduction zone, slab breakoff starts at that location and propagates laterally towards the center of the subduction zone. It results in higher slab breakoff depths at the edges of the subduction zone than at the center (maximum difference of 170 km). In models where the passive margin arrives first in the center of the subduction zone, slab-breakoff starts at that location and propagates towards the edges of the subduction zone over a period that can be as large as 31 Ma and with a large range of depths (maximum difference of 170 km). The rate of lateral propagation of slab breakoff decreases with the increasing initial obliquity of the OCT with the trench. The model with the maximum obliquity shows a correlation between the surface topography and the slab breakoff at depth. The plate surface is uplifted after the oceanic and continental slab segments are mechanically decoupled, and in the final stage of the experiment, when the slab is educted, subsidence develops as the isostatic and dynamic support diminish. [ABSTRACT FROM AUTHOR]

• We present 3D seismic attenuation (Q) model of the Nazca Plate subduction zone. • Our model shows clear high- Q slab and low- Q continental plate structures. • Our model for the Brazilian Shield shows high- Q at 40–80 km depth in the backarc. • Along-arc slab Q variations indicate different slab hydration states. • Relatively high- Q at the mantle wedge corner indicates cold thermal state. Seismic attenuation provides key constraints on understanding the structure and dynamics of subduction-zone system. In this study, we provide the first three-dimensional P and S -wave attenuation models beneath the Nazca-South America subduction zone by inverting t * data from 397 local shallow and intermediate-depth earthquakes. The study area covers the southern part of the Peruvian flat-slab region (11°–15 °S) (where the subducting Nazca Ridge is migrating), and the Central Andean Plateau (∼13°–18 °S) (associated with the subducting Nazca Plate) and the Brazilian mantle lithosphere. We obtain five major features in our attenuation model with seismicity including (1) well-defined along-strike and along-dip Nazca slab structure in terms of low attenuation, (2) high attenuation associated with the passage of Nazca Ridge and present location of the oceanic ridges, (3) high attenuation in the backarc mantle wedge and continental crust, (4) clear image of Brazilian Shield in terms of low attenuation, underthrusting to the west, and (5) high attenuation related to the slab dehydration beneath the volcanic arc at 100 and 200 km depths. In particular, prominent low- Q zones in the continental crust and mantle wedge beneath the active volcanic arc reflect source zones of arc magmatism caused by fluids from the slab dehydration. The observed along-arc differences in slab Q estimates can be attributed to different hydration states between the flat and normal-dip slabs due to the subduction of the Nazca Ridge. [ABSTRACT FROM AUTHOR]