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Determining subduction‐related processes is crucial for understanding lithological heterogeneity, as substantial quantities of slabs are recycled into the mantle. Molybdenum isotopes are valuable for distinguishing sources materials due to the significant isotope differences between the crust and the mantle. In this study, we systematically investigate Mo isotopes in a suite of well‐characterized continental basalts from Keluo and Halaha‐Chaoer, located in Northeast China. The δ98Mo values of Keluo range from −0.41 to −0.23‰, with an average of −0.34‰, while Halaha‐Chaoer samples ranged from −0.18 to −0.12‰, with an average of −0.15‰. The δ98Mo values of Keluo basalts are lighter than the mean value of fresh oceanic basalts (−0.21‰), whereas those of Halaha‐Chaoer basalts are similar to oceanic basalts. Combined with other geochemical indications (LOI, Ce/Pb, La/Yb and so on), the Mo isotopic variations cannot be attributed to chemical weathering, continental crust contamination or magmatic processes. Instead, the δ98Mo variations in this study are explained by the incorporation of different oceanic crustal materials into the magma sources. Correlations of δ98Mo with Ba/Th, Th/U, 143Nd/144Nd indicated that both sediment and altered oceanic crust have significantly influenced these variations. This study demonstrates the potential of Mo isotopes to distinguish different types of recycled oceanic crust materials. [ABSTRACT FROM AUTHOR]

Subduction infancy corresponds to the first few million years of the start of subduction following heat transfer from the incipient mantle wedge towards the slab‐top, as witnessed by metamorphic soles which represent slivers of oceanic crust metamorphosed up to granulite facies conditions welded beneath obducted ophiolites. In this study, integrated petrological, geochemical, mineralogical, geochronological, and thermodynamic studies were carried out on samples from the Yushugou high‐temperature metamorphic ophiolitic complex (YHTM) in the South Tianshan Accretionary Complex (STAC), where a massive exposure of coherent granulite accompanied by a thick peridotite body is preserved. Bulk‐rock compositions and Sr–Nd–Hf isotopes demonstrate the petrogenesis of meta‐basalts with oceanic island basalt (OIB)‐like and mid‐ocean ridge basalt (MORB)‐like affinities, with little infiltration by subduction‐derived melts and/or fluids (e.g., no negative Nb–Ta anomalies). Thermodynamic modelling and U–Pb chronology reveal that the YHTM meta‐basalts experienced granulite facies metamorphism of ~840–940°C and ~0.92–1.02 GPa at c. 392 Ma and then possibly reheating and zircon alteration in the Carboniferous. In addition, detrital zircons in sedimentary host rocks of the YHTM show limited Precambrian records and offer maximum depositional ages of c. 410–400 Ma together with the oldest Palaeozoic cluster around c. 470–450 Ma. It is suggested that the YHTM granulites could be of Ordovician–Silurian protolith and such an age pattern significantly deviates from those of adjacent terranes (the Central Tianshan, STAC, and North Tarim Craton). Combined with the compilation of pressure–temperature–time estimates of the YHTM and ages of regional ophiolites, arc intermediate‐mafic rocks, A‐type granites, and deformation, a model of induced, temporarily northward, intra‐oceanic subduction initiation is proposed, which probably occurred along the previously existing weak zone close to a seamount or oceanic plateau in the earliest middle Devonian during the northward subduction of the South Tianshan Ocean (STO). Anomalously high geothermal gradients could be triggered by asthenosphere upwellings, further facilitating the formation of OIB‐type metamorphic soles. The YHTM, which represents the remnant of metamorphic soles and associated ophiolites, was finally emplaced to the north margin of the STAC during the relatively long‐term (c. 160 million years) accretion and continuous subduction of the STO before its closure. This finding also presents a new natural example of OIB‐type metamorphic soles as a snapshot of fossil intra‐oceanic subduction infancy during the complex evolutionary history of the STO. [ABSTRACT FROM AUTHOR]

The petrogenesis and geodynamic setting of the Mesozoic magmatic rocks in the Erguna Block, NE China remains controversial, especially the relationship between magmatism and the subduction history of the Mongol–Okhotsk oceanic plate. Here we present data for the Early Jurassic–Early Cretaceous adakite-like magmatic rocks from Chaoman Farm in the northeastern part of the Erguna Block. Zircon U-Pb dating reveals that the syenogranites crystallized at around 190–180 Ma, while the monzonites, quartz diorite porphyries, and quartz monzonite porphyries were emplaced at around 147–143 Ma. The syenogranites, monzonites, quartz diorite porphyries, and quartz monzonite porphyries are adakite-like rocks. The syenogranites and quartz monzonite porphyries were produced by the partial melting of a thickened ancient mafic lower continental crust and a thickened juvenile lower crust, respectively. Meanwhile, the monzonites and quartz diorite porphyries were formed as a result of partial melting of the oceanic crust. In conclusion, the occurrence of these Early Jurassic magmatic rocks was closely linked to the process of southward subduction of the Mongol–Okhotsk oceanic plate. On the contrary, the Late Jurassic to early Early Cretaceous magmatism (147–143 Ma) occurred in an extensional environment, and was probably triggered by upwelling of the asthenosphere. [ABSTRACT FROM AUTHOR]

We apply the stable potassium isotope system (41K/39K) to well-studied Mariana arc lavas, in which inter-island geochemical variability has been interpreted to reflect near constant addition of an aqueous fluid flux, that dominantly samples the subducting mafic oceanic crust and variable amounts of sediment melt addition to the sub-arc mantle wedge. The nature of the sediment component in the Mariana arc lavas remains enigmatic, with a mixture of melts from all subducted sediment lithologies and the altered-mafic oceanic crust seeming likely, but some interpretations point towards a component from melting of the volcaniclastic sediment alone. We present K isotopic data on a set of well-characterised Mariana arc lavas and sediment samples (from Ocean Drilling Program site 801). Our data show that the majority of Mariana arc lavas are isotopically heavy (ẟ 41K), by up to ∼0.2 ‰, relative to mid-ocean ridge basalt (MORB), but most of the subducting sediment samples are slightly isotopically lighter than, or indistinguishable from MORB. The volumetrically important volcaniclastic sedimentary unit however is significantly isotopically lighter than MORB, by ∼0.8 ‰, which reflects marine diagenetic processes. Thus, volcaniclastic material significantly influences the bulk sediment K isotope composition. We show that ẟ 41K compositions of the Mariana arc lavas can be reproduced by the addition of an aqueous fluid with isotopically heavy K (relative to MORB) and an additional fraction of an incompatible-element-enriched isotopically light melt component. Modelling of K isotopes together with K/La and radiogenic Nd indicate that the melt component is best explained by a mix of melts from bulk sediment and altered-mafic oceanic crust. Our results show that distinctive K isotopic variations in subduction zone inputs and K isotopic fractionation during dehydration reactions makes K a useful tracer of subduction zone processes. [ABSTRACT FROM AUTHOR]

Rhombic structures have been observed in the Qom-Zefreh-Nayin-Dehsheir-Baft region, specifically along the direction of the dextral faults, which have caused significant changes in strike length. This study investigates the geological features and fault interactions in the region through the examination of aerial images, fault-lithology correlations, petrology, crustal thickness, and seismic studies. The analysis of aerial photos and geological correlations revealed the presence of ophiolites and pluto-volcanics associated with faults and rhombic structures. By conducting field geology and combining various geological studies, a pull-apart basin was identified in the area, contributing to the formation of three rhombic structures. This basin played a crucial role in the genesis of the region's ophiolites and pluto-volcanics. The research suggests that the initial tensional stress leading to the pull-apart basin was caused by the right step of a dextral fault within the Urumieh-Dokhtar magmatic arc. This fault formation occurred due to the oblique Arabian subduction towards the Iranian plateau. During the Zagros orogeny, the stretched area persisted, leading to the formation of oceanic crust in this location. The subduction angle changes from subduction to super-subduction, resulting in the classification of the region into two types: C and E genes. Different types of magma, including alkaline, subalkaline, shoshonite, calcalkaline, and adakitic, were identified in this region. The study highlights the significance of tholeiitic arcs, abyssal features, crust thickness, and seismicity in understanding oblique diagonal subduction models and tensional pull-apart basins, which are crucial in the transition from subduction to super-subduction. This research offers valuable insights into the geological complexities of the region and opens up opportunities for further exploration of similar models. [ABSTRACT FROM AUTHOR]

Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. It remains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact (MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to the accumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, with some of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here, we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subduction initiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMB temperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements in LLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications for understanding the diverse tectonic regimes of rocky planets. Plain Language Summary: Plate tectonics remains unique to Earth, but when and how it started is debated. Earth's oldest minerals indicate a clement surface by 4.3 Ga, resembling the modern Earth with its granitic crust and oceans. Granite is most easily explained as originating from subduction. However, the mechanisms for subduction initiation, especially so soon after the Moon‐forming giant impact, remain elusive. Earlier studies indicate that the core‐mantle boundary (CMB) temperature is increased due to accumulation of the impactor's core during the impact. Our recent work further shows that the lower half of Earth's mantle remains mostly solid after this impact and that parts of the impactor's mantle might have survived as the two seismically‐observed large low‐shear velocity provinces (LLSVPs). In this study, we perform whole‐mantle convection models to illustrate that strong mantle plumes can arise, weaken the lithosphere, and eventually initiate subduction ∼200 Myr after the giant impact. Our systematic computations reveal that the hot CMB temperature after the impact is the primary factor determining whether there is early subduction initiation, with enrichment of heat‐producing elements in LLSVPs as another potential contributor. Our model ties the Moon's formation to incipient subduction, providing insights for understanding the diverse tectonic regimes of rocky planets. Key Points: The mantle thermochemical structure left by the Moon‐forming impact triggers strong plumes that may have initiated the first subductionThe strong mantle plumes may rise from a heated core or from large low‐shear velocity provinces enriched in heat‐producing elementsEarly giant impact events may have a profound influence on the diverse tectonic regimes of rocky planets [ABSTRACT FROM AUTHOR]

Potassium (K) isotopic compositions for a suite of crustal rocks (plutonic unit, sheeted dike complex, and volcanic sequence) in the Troodos ophiolite, Cyprus, were determined to investigate fluid-transport processes during subduction inception. The gabbros and plagiogranites in the plutonic unit have restricted K isotopic compositions (0.01–0.20 ‰ and −0.17 to 0.02 ‰, respectively), whereas diabases in the sheeted dike complex have variable δ41K values from −0.24 to 1.13 ‰. The δ41K values of the volcanic sequence range from −0.26 to −0.07 ‰ in the lower pillow lavas, and from −0.71 to 0.34 ‰ in the upper pillow lavas. These δ41K values are clearly distinct from those of subducting sediments and altered oceanic crust, ruling out direct crustal input as the cause of the unusually heavy K isotopic compositions. Rather, the high K/Th, K/U, and Sr/Th ratios of the investigated samples, together with binary mixing modelling, reveal that the anomalously heavy K isotopic compositions in Troodos rocks are attributed to the influx of fluids during subduction inception. Notably, the δ41K values in the crustal rocks of the Troodos ophiolite are more variable and extend to higher values than those of arc-front and rear-arc lavas of the Izu arc and arc lavas of Martinique in the Lesser Antilles, indicating continuous K isotope fractionation during slab dehydration. Thus, the heavy K isotopic compositions of the Troodos ophiolite reflect K isotope fractionation during dehydration of a shallow slab within a newly formed subduction system, where heavy K isotopes preferentially enter into the aqueous fluids. Later, progressive breakdown of K-bearing phases at deeper levels results in isotopically lighter hydrous melts, which can be delivered to the arc. [ABSTRACT FROM AUTHOR]

Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE-striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via fieldbased, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (~500-550 °C and ~5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep-dominated deformation. A strain rate of 10-12 s-1 and a differential stress of 25-39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47-0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence. [ABSTRACT FROM AUTHOR]

An eclogite from the Early Palaeozoic Fleur‐de‐Lys Supergroup in Newfoundland was studied because of its biotite porphyroblasts, which very rarely occur in this rock type. Thermodynamic modelling suggests that eclogitic biotite in common metabasite (former basalt–gabbro) is limited to (1) bulk‐rock compositions, which are relatively rich in Fe2+ and K and poor in Fe3+, and (2) the low‐pressure range of the eclogite facies. The latter reason is supported by the determination of the pressure–temperature (P–T) path of the Newfoundland eclogite. Chemical zonation of garnet, presence of phengite with Si contents of ~3.4 per formula unit, Zr contents in rutile and petrographic observations resulted in a P–T trajectory starting at medium‐pressure conditions. Nearly isothermal burial led to a peak pressure of 18–19 kbar at ~575°C, followed by exhumation and slight heating. Deformation occurred at or close to the peak pressure. Subsequent introduction of hydrous fluids caused the formation of porphyroblasts of biotite and Ca–amphibole in the pressure range of 12–17 kbar at peak temperatures of 625–640°C. Retrogression led to very fine‐grained symplectites around omphacite and phengite and marginal replacement of biotite porphyroblasts by plagioclase and titanite. Geodynamic scenarios invoking either a flat subduction of oceanic crust followed by continent–continent collision or intracontinental subduction along a transpressional fault system might best explain the formation of eclogite with biotite porphyroblasts in general. For the Newfoundland eclogite, the latter scenario is preferred. [ABSTRACT FROM AUTHOR]

Orogenic gold systems are arguably the most variable mineral system globally in terms of an extreme range of depositional depths, corresponding P–T conditions and wallrock alteration assemblages, structural controls and styles, and element associations. This diversity has ignited controversy on genetic models for the two decades since orogenic gold became a widely accepted term. From the diverse genetic models proposed, the two groups of fluid-source models that meet most genetic constraints are the following: (1) deposition from crustal fluids via metamorphic devolatilization at the amphibolite-greenschist transition, or potentially even deeper under specific tectonic conditions, and (2) deposition from sub-crustal fluids either by direct devolatilization of subducted oceanic crust and overlying sediment wedge or of previously metasomatized and fertilized mantle lithosphere. Both models normally postulate gold deposition within a geodynamic system that evolves from extension through compression into syn-gold transpression. Crustal metamorphic models normally invoke subduction-driven geodynamic systems that involve advection of crustal metamorphic fluids up crustal-scale faults. In contrast, sub-crustal devolatilization models involve subduction-related processes as both geodynamic drivers and gold sources with fault-controlled fluid conduits extending to below the Moho. The overall lack of orogenic gold and other subduction-related mineral systems during the unique Boring Billion (1.8–0.8 Ga) period provides an important constraint on this genetic debate. Boring Billion orogens had varying geodynamic drivers, asthenosphere upwelling, and low-P metamorphic terranes with crustal-scale faults, all parameters consistent with formation of orogenic gold systems, during subduction-independent accordion-type tectonics. The absence of orogenic gold during the Boring Billion provides critical evidence against the crustal metamorphic model and furthers the sub-crustal model which requires subduction as both the geodynamic driver and auriferous fluid source. [ABSTRACT FROM AUTHOR]

In the regime of plate tectonics, the subduction of an oceanic plate generally terminates with the collision and accretion of continental terranes. Then, a new subduction zone may form in the neighboring oceanic plates, which is defined as the terrane collision‐induced subduction initiation (SI). Based on the analyses of the western Pacific subduction system in the Cenozoic, three types of collision‐induced SI have been observed: subduction polarity reversal, subduction transference and far‐field subduction. However, the dynamics and controlling factors of SI mode selection after terrane collision are not clear. In this study, a multi‐terrane collision model has been conducted with variable rheological strength of continental terranes and different convergence velocities. The model results indicate that the relative strength of the terranes controls the SI mode selection, with the new subduction zone tending to form beneath weaker terranes. In addition, the higher convergence velocity can facilitate the collision‐induced SI. An analytical study of force balance has been further conducted, which provides a mechanical explanation for the numerical prediction of a weak overriding terrane as a favorable SI site. The numerical models and force balance analyses are further compared with the natural cases in the western Pacific subduction system. This indicates that subduction polarity reversal is the most favorable mode after terrane collision in the western Pacific, possibly due to the weakness of overriding plate during the preceding subduction‐induced fluid/melt activity. This comprehensive study provides systematic constraints for the dynamics of collision‐induced subduction jump, especially for the western Pacific subduction zones in the Cenozoic. Plain Language Summary: The Earth's rocky surface is composed of both oceanic and continental crusts. The oceanic plate is denser and can sink into the Earth's interior as a subducting slab. A specific subduction process terminates when the whole oceanic plate is consumed and the neighboring continental blocks collide together. Then, the continued convergence may lead to the formation of a new subduction zone in the neighboring oceanic plate; however, the localization of new subduction zone varies. In the western Pacific subduction system in the Cenozoic, a new subduction zone may form in three different positions of the overriding and subducting plates. The mode selection of the new subduction zone formation plays an important role in Earth's plate tectonic evolution; however, its mechanism is not clear. In this study, systematic numerical models and force balance analyses have been conducted, which indicate that the relative strength of the continental blocks controls the position of new subduction zone formation, which favors the formation beneath weaker blocks. This prediction is further confirmed by the natural cases in the western Pacific system. This comprehensive study improves our understanding of the formation of new subduction zones as well as the more general subduction dynamics and plate tectonics. Key Points: Terrane collision may lead to three different modes of subduction jump: polarity reversal, transference, and far‐field subductionThe collision‐induced subduction initiation prefers to occur beneath the rheologically weak overriding terraneSubduction polarity reversal is the dominant mode in the western Pacific due to the widely distributed rheologically weak island arc [ABSTRACT FROM AUTHOR]

The Pampean flat slab in central Chile and Argentina is characterized by the inland migration and subsequent cessation of arc volcanism since the mid‐Miocene. Slab flattening also affects the distribution and number of intermediate‐depth earthquakes and the evolution of the overlying continental thermal structure. In this study, we combine thermal‐mechanical models with petrological models to examine temporal changes in pressure, temperature, and composition during flat‐slab subduction and estimate water carrying capacity, predicted melt distributions and predicted changes in melt composition. Model results indicate that the present‐day flattened Nazca plate carries water to ∼700 km inland from the trench and could cause flux melting if the material above the slab remains fertile. Observed slab seismicity matches areas where hydrated materials have ∼>3 wt% H2O in the oceanic crust and mantle lithosphere. Seismicity increases as slab water carrying capacity decreases (slab dehydration). As P‐T conditions and compositions of the rock trapped above the slab change during slab flattening, flux melting switches from a peridotite‐dominated early phase to a combined mid‐ocean ridge basalt/eclogite and peridotite melting at ∼8 Ma. The results provide broad consistency with known earthquake distributions, seismic velocities, and observed temporal and spatial changes in volcanic patterns above the Pampean flat slab and point toward the role of melt depletion in the decrease and ultimate cessation of arc volcanism in this region. Key Points: We estimate the water carrying capacities, melt distributions, and composition during the Pampean flat‐slab subductionThe predicted hydrated areas match the observed slab seismicity in the Pampean regionFlux melting above the flat slab is predicted to migrate inland, providing constraints on the causes of the spatiotemporal changes in magmatism [ABSTRACT FROM AUTHOR]

The structural response to compression of the synthetic high-pressure hydroxide perovskite MgSi(OH)6, the so-called "3.65 Å phase," has been determined to 8.4 GPa at room temperature using single-crystal XRD in the diamond-anvil cell. Two very similar structures have been determined in space groups P21 and P21/n, for which differences in oxygen donor-acceptor distances indicate that the non-centrosymmetric structure is likely the correct one. This structure has six nonequivalent H sites, of which two are fully occupied and four are half-occupied. Half-occupied sites are associated with a well-defined crankshaft of hydrogen-bonded donor-acceptor oxygens extending parallel to c. Half occupancy of these sites arises from the averaging of two orientations of the crankshaft H atoms (|| ±c) in equal proportions. The P21 and P21/n structures are compared. It is shown that the former is likely the correct space group, which is also consistent with recent spectroscopic studies that recognize six nonequivalent O-H. The structure of MgSi(OH)6 at pressures up to 8.4 GPa was refined in both space groups to see how divergent the two models are. There is a very close correspondence between the responses of the two structures implying that, at least to 8.4 GPa, non-centrosymmetry does not affect compressional behavior. The very different compressional behavior of MgO6 and SiO6 octahedra observed in this study suggests that structural phase transformations or discontinuities likely occur in MgSi(OH)6 above 9 GPa. [ABSTRACT FROM AUTHOR]

The paradigm of plate tectonics holds that ocean plates are rigid during drift and only experience tectonic deformation at subduction zones, but new findings from the Pacific challenge this idea. Geological and geophysical evidence from the Ontong Java, Shatsky, Hess, and Manihiki oceanic plateaux indicates that extensional deformation during plate drift is a widespread phenomenon across the Pacific plate. These anomalously thick oceanic plateaux are weaker regions of the ocean lithosphere and more prone to tectonic deformation. Numerical geodynamic models demonstrate that a slab pull force from distant subduction plate boundaries can be effectively transmitted to oceanic plateaux through strong ocean lithosphere and cause substantial extension during plate drift. Our findings reveal that a wide expanse of the Pacific has experienced syn‐drift plate tectonics linked to pull from the western Pacific subduction factory. Plain Language Summary: New findings from the Pacific Ocean challenge the conventional understanding of plate tectonics. It was previously believed that oceanic plates remained rigid during plate drift and only experienced deformation at subduction zones. However, geological and geophysical evidence from the Ontong Java, Shatsky, Hess, and Manihiki oceanic plateaux suggests that extensional deformation is a common occurrence during plate drift. These plateaux, which are weaker regions of the ocean lithosphere, are more susceptible to tectonic deformation. Through numerical geodynamic models, we have demonstrated that the slab pull force exerted by distant subduction plate boundaries can effectively cause substantial extension in these oceanic plateaux. This study reveals a significant presence of intra ocean plate deformation associated with the Western Pacific subduction factory. Key Points: Seismic and petrologic data indicate that oceanic plateaux on the Pacific Plate are undergoing extensional deformation during plate driftNumerical modeling finds a slab pull force may be causing the extension as the force is transmitted far away from the subduction zoneOceanic plates can experience substantial tectonic deformation during their drift to subduction [ABSTRACT FROM AUTHOR]

Recent research has shown that shallow tremors tend to occur in spatial association with heterogeneous conditions, which are formed by seamounts or ridges on the subducted oceanic crust. In the southern Kuril Trench, shallow tremors have been observed on the western side, although no apparent heterogeneity in the subducted oceanic crust has been identified. To investigate structures spatially associated with these shallow tremors, we processed reflection seismic data acquired along seven profiles. We examined structures ranging from the seafloor to the top of the oceanic crust, within a depth of up to 20 km. Our results reveal that undulations observed at the top of the subducted oceanic crust are related to the distribution and number of shallow tremors. The undulations would play a crucial role in the distribution of shallow tremors. Plain Language Summary: Shallow tremors, often occurring near subducted seamounts and ridges underground, are small earthquakes that take place at shallower depths of a plate boundary compared to megathrust earthquakes. While the mechanisms behind these two types of earthquakes are not yet fully comprehended, there is an observed correlation, particularly in terms of their locations. At the southern Kuril Trench, shallow tremors have only been detected on the western side, where there aren't any subducted seamounts or ridges. Seismic surveys carried out in the southern Kuril Trench have provided detailed images of the subsurface structures. These images indicate that shallow tremors are more likely to happen in areas where the subducted oceanic crust has pronounced undulations. It's worth noting that only these undulations on the oceanic crust—and not other significant structures—have been linked to the occurrence of shallow tremors. Key Points: Seismic reflection surveys in the Kuril Trench image undulations in the subducted oceanic crust, as well as strong overlying reflectorsAmplitudes and wavelengths of undulations correlate with shallow tremor occurrencesUndulations and the distribution of strong reflections on the plate boundaries shown in previous studies also generally correspond [ABSTRACT FROM AUTHOR]

Metasomatism of the sub-continental lithospheric mantle (SCLM) can result in enrichment in volatile elements introduced via subduction of seawater-altered oceanic lithosphere. Subduction-related metasomatism can introduce chlorine isotope heterogeneity to the SCLM inherited from the subducted source, which can be used to characterize the source of metasomatism. This study presents Cl isotope compositions (δ37Cl) of variably metasomatized continental lithospheric mantle xenoliths from the Colorado Plateau and southern Rio Grande Rift (southwestern United States). Fertile mantle xenoliths that represent asthenosphere that has recently been emplaced at the base of the North American lithosphere have δ37Cl values of 0.0 ‰ to +0.1 ‰, suggesting that the convecting mantle has an average δ37Cl value similar to seawater and chondrites. In contrast, metasomatized xenoliths span a range of δ37Cl values from +0.1 ‰ to +1.9 ‰, extending above the average convecting mantle value. The range in δ37Cl values can be explained by mixing between a subduction-related fluid with δ37Cl values between +1.0 ‰ and +1.9 ‰ and a shallow crustal fluid with a δ37Cl value less than +0.3 ‰. The most likely source of the high δ37Cl subduction-related signature is the lower crust in the subducted Farallon plate. Lower crustal Cl likely entered the SCLM either through direct dehydration of lawsonite ± phengite in the deeply subducted Farallon plate or from a mixed serpentinite-oceanic crust source in the subducted Farallon. Regardless of the mechanism, this study provides evidence for subduction of crustal Cl into the mantle, which can introduce isotopic heterogeneity into the SCLM and has implications for the volatile cycling in the HIMU mantle and SCLM. [ABSTRACT FROM AUTHOR]

In the most internal zone of the Variscan Orogen, a stack of ophiolites and (ultra)high-P belts hold evidence for c. 60 Myr (c. 400–340 Ma) of almost continuous metamorphism and accretion onto the continental margin of Gondwana. Subduction of the peri-Gondwanan lithosphere led to three successive (ultra)high-P metamorphic episodes prior to the continental collision between Gondwana and Laurussia that shaped Pangea. A first high-P, high-T metamorphic belt developed during subduction of a peri-Gondwanan magmatic arc in the Early Devonian. Related to this event is the ensuing opening of intra-Gondwana Middle Devonian oceanic basins, which rapidly closed leading to ophiolite accretion. The second high-P metamorphic episode is related to the formation of a serpentinitic mélange after the sequential subduction of oceanic and transitional crust during Middle Devonian time. Tectonic blocks of subducted crustal material recording high-P, low-T metamorphism were detached from the slab and incorporated into a low-viscosity channel developed between the downgoing slab and the mantle wedge. As convergence progressed, a Late Devonian high-P, low- to intermediate-T metamorphic belt developed due to subduction of the most external margin of Gondwana. Subsequently, the scenario switched to continental collision. In this context, Early Carboniferous intracontinental subduction of the Gondwanan foreland below the accretionary wedge triggered ultra-high-P metamorphism and ensuing exhumation via trans-lithospheric diapirism and doming up to the base of the accretionary wedge. [ABSTRACT FROM AUTHOR]

Contrasting views exist in regard of the evolution of metamorphic rocks in the southeastern Pohorje Mountains (Mts), located in the southeastern Eastern Alps. Major debated points are whether micaschists have experienced ultrahigh‐pressure metamorphism in the Late Cretaceous (Eo‐Alpine) and whether they were continuously exhumed or experienced a multiple subduction–exhumation process from that time on. Therefore, we studied micaschist sample 18Slo39 with two generations of garnet and phengitic muscovite from this area. Our detailed study of this rock included petrographic observations, chemical analyses of minerals with the electron microprobe, pseudosection modelling, conventional geothermometry, and monazite in‐situ U‐Th‐Pb dating using laser‐ablation inductively coupled plasma (ICP) mass spectrometry. The following results were obtained: The studied micaschist was subject to a peak pressure of 1.31 ± 0.14 GPa at 603 ± 26°C in Eo‐Alpine times: 90.62 ± 2.78 (2σ) Ma (Stage I). Contact metamorphism at pressure–temperature conditions of 0.66 ± 0.10 GPa and 577 ± 23°C was induced by the intrusion of the Pohorje pluton (Stage III). We determined an early Miocene age of 18.33 ± 0.43 (2σ) Ma for this intrusion. Based on this study and the previously reported data for a micaschist (16Slo12) taken in the vicinity of sample 18Slo39, a geodynamic model is proposed for the region of the Pohorje Mts considering Eo‐Alpine subduction of oceanic crust and European continental crust, of which the micaschist was part of. Another high‐pressure event in the Eocene (Stage II) was the result of intracontinental subduction because of transpression by the Periadriatic fault system that separates the Eastern Alps from the Southern Alps. This type of subduction gave rise to magma generation and ascent to form the Pohorje pluton, which caused contact metamorphism in its vicinity. [ABSTRACT FROM AUTHOR]

Marine multichannel and wide‐angle seismic data constrain the distribution of seamounts, sediment cover sequence and crustal structure along a 460 km margin‐parallel transect of the Hikurangi Plateau. Seismic reflection data reveals five seamount up‐to 4.5 km high and 35–75 km wide, with heterogeneous internal velocity structure. Sediment cover decreases south‐to‐north from ∼4.5 km to ∼1–2 km. The Hikurangi Plateau crust (VP 5.5–7.5 km/s) is 11 ± 1 km thick in the south, but thins by 3–4 km further north (∼7–8 km). Gravity models constructed along two seismic lines show the reduction in crustal thickness persists further east, coinciding with a bathymetric scarp. Gravity data suggest the transition in crustal thickness may reflect spatial variability in deformation and lithospheric extension associated with plateau breakup. Variability in the thickness of subducting crust may contribute to differences in megathrust geometry, upper‐plate stress state and high‐rates of contraction and uplift along the southern Hikurangi margin. Plain Language Summary: The thickness of crust arriving at subduction zones exerts a major influence on the configuration, structure, and distribution of stresses at plate boundaries. A region of thickened oceanic crust, the Hikurangi Plateau, is subducting beneath the North Island of New Zealand. To understand the structure of this Plateau, we analyze seismic data along a 460 km long transect, which reveals five large seamounts, and a 3–4 km reduction crustal thickness between the southern and northern areas of the Plateau. Gravity data show this difference is persistent across the plateau and reveal structures suggesting the crust may have thinned when the Hikurangi plateau separated from a second plateau (Manihiki), which is thought to have formed at the same time. We propose that the subduction of thicker crustal along the southern Hikurangi margin may contribute to the shallow angle of the subducting plate, and high rates of uplift and deformation in the overthrusting plate. Key Points: Crustal thickness of the Hikurangi Plateau reduces by ∼30% between the southern (10–11 km) and northern (7–8 km) PlateauCoincident contrasts in plateau fabric may attribute the thickness contrast to rifting from the Manihiki PlateauThickness contrasts may impact the geometry of the subducting plate and rates of upper‐plate compression and uplift along the Hikurangi margin [ABSTRACT FROM AUTHOR]

Understanding deep nitrogen (N) cycling better requires investigating the delivery of N to subduction systems via various lithologies. Input to subduction zones through mafic rocks is more voluminous and massive as compared to sedimentary rocks which calls for a thorough investigation of the behavior of N in metabasic rocks. Here we estimate the delivery of N to subduction zones at forearc depths by investigating the geochemistry of amphibolites and epidote-blueschists from the Central Qiangtang Metamorphic Belt in Tibet where the metabasic rocks likely represent the transition from oceanic to continental subduction. The rocks contain 21–147 ppm N with δ15N values from +1.8 ‰ to +10.0 ‰, and 147 ppm N is the highest that has been reported in a metabasic rock thus far. Given the N abundances for most of the rocks are much higher than those of altered oceanic crust (i.e. basalts, sheeted dikes and gabbros; 6.0 ± 4.7 ppm), the N is likely neither magmatic nor was introduced in the rocks during hydrothermal alteration prior to subduction. This is confirmed by the K 2 O/Th versus Ba/Th, Th/U versus Th and Ba/Rb versus K 2 O plots where these rocks align with the trend of metamorphic fluid alteration rather than seafloor hydrothermal alteration. A two-step process led to N acquisition in the metabasic rocks. In the first step, the metabasic rocks acquired their N from metasediment-derived fluids during metamorphism in the subduction channel. In the second step, some of the metabasic rocks underwent N loss and concomitant enrichment in 15N due to devolatilization within the subduction channel. We modeled the N fluxes at forearc depths in 55 modern-day subduction zones via metasedimentary and metabasic rocks assuming their minimum, median and maximum N concentrations to assess their relative importance in delivery of N to subduction zones. We find that metabasic rocks supply comparable fluxes of N to metasedimentary rocks at forearc depths, even though metasedimentary rocks have at least an order of magnitude higher N abundance than metabasic rocks. The importance of metabasic rocks in N delivery reinforces the need to investigate the behavior of N in them from more locations globally to improve our understanding of deep N cycling. [ABSTRACT FROM AUTHOR]

Fluid–rock interaction within the altered oceanic crust and across the slab–mantle boundary during subduction facilitates element transfer, but the dynamics of fluid transport and fluid–rock exchange during upward fluid migration are still unclear. A study of metamorphic fluid–rock interaction within a section of subducted oceanic crust was carried out on eclogites and metasediments of the ultra-high-pressure Lago di Cignana Unit (NW Italian Alps). The P–T modeling of a quartzschist shows that garnet grew during the prograde and sporadically during the retrograde path and that phengite mainly records the peak to retrograde conditions. Microscale geochemical analysis of garnets has revealed a systematic evolution of oxygen isotopic composition with garnet major element zonation, with extreme within-sample core–rim variations in δ18O between 18 and 4‰ providing evidence for external fluid influx. Garnet in eclogites and calcschists, as well as garnet cores in quartz-rich lithologies, shows normal compositional zoning, as expected for prograde garnet growth, and a relatively constant oxygen isotopic composition. The outer garnet growth zones within a few metasediments show reverse or discontinuous zoning and progressively lower δ18O. Despite major element zoning, the isotopic composition of mica is homogeneous across chemical zoning in one eclogite and one quartzschist, but shows 6‰ variability in another quartzschist. In the underlying Zermatt–Saas serpentinites, antigorite from nine serpentinite samples shows some variation in δ18O, with average δ18O values for individual samples ranging from 1 to 6‰. These results provide evidence for two main stages of external fluid infiltration: (i) fluids from the dehydration of mafic lithologies entered the sequence at peak conditions around 3 GPa, as indicated by the oxygen composition of intermediate zones of mica and garnet, and (ii) low δ18O fluids from serpentinites infiltrated parts of the sedimentary package during exhumation prior to 1.5 GPa, as recorded by the 4‰ garnet outer rims. Samples recording external fluid infiltration are concentrated in the lower part of the sequence, indicating channelized fluid flow, suggesting focused fluid infiltration due to permeability contrasts between metasedimentary and eclogitic lithologies. Channelized fluid flow in the ultra-high-pressure metasediments of Lago di Cignana has not resulted in systematic decarbonation of the metasediments. [ABSTRACT FROM AUTHOR]

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization. [ABSTRACT FROM AUTHOR]

This study aims to explain the nonuniform earthquake pattern along the Cascadia Subduction Zone. In particular, we investigate the relationship between the tectonic features of the subducting oceanic Juan de Fuca slab and the onshore seismicity pattern. We have integrated multiple geophysical data sets toward three general objectives. The first study intends to study variations in physical properties along three 2‐dimensional models through regions of different seismicities that combine public gravity, magnetic, and seismic data sets. These models reveal multiple zones of decreased crustal density that we interpret as regions of weaker oceanic crust. The second objective is to delineate major tectonic features by performing spatial analysis of potential fields. The overall methodology comprises gravity and magnetic data filtering, followed by lineaments mapping and cross‐referencing interpretation with available seismic reflection data. This process allows delineating zones of crustal weakness by extrapolating outside our three 2‐D models. We also map multiple seamounts that appear to cluster along identified zones of weaker crust. Third, we investigate the relationship between the mapped tectonic elements, namely the zones of weak crust with accompanying seamounts, and the observed seismicity trends within the subducted slab. The alignment between those suggests that mapped weak crust zones and associated seamounts may have an influence on the overall subduction process. As more of these structures are heading toward the Washington portion of the margin than to the Oregon portion, more earthquakes are observed in the north than in the south. Plain Language Summary: This study examines why earthquakes in the Cascadia Subduction Zone are not the same everywhere. We focused on the area where the Juan de Fuca plate, a piece of oceanic lithosphere, is being pushed underneath the North American continent. We used information from various geophysical data sets to create 2‐D models that show the different physical properties of crustal rocks in regions with different densities of earthquakes. We found that there are areas with weaker crust, and that seamounts clustered there. We also found that there are weaker crust zones and seamounts heading toward Washington than Oregon, which may explain why there are more earthquakes in the north than in the south along this margin. Key Points: Multiple geophysical data sets were used to create three 2‐D models revealing weaker oceanic crust zonesMapped new linear features from disturbances in sea‐floor magnetic stripes and seamounts from gravity dataTectonic features on the Juan de Fuca plate spatially correlate with earthquakes within the subducting slab [ABSTRACT FROM AUTHOR]

We map the characteristic signature of the subducting Juan de Fuca and Gorda plates along the entire Cascadia forearc from northern Vancouver Island, Canada, to Cape Mendocino in northern California, USA, using teleseismic receiver functions. The subducting oceanic crustal complex, possibly including subcreted material, is parameterized by three horizons capable of generating mode‐converted waves: a negative velocity contrast at the top of a low velocity zone underlain by two horizons representing positive contrasts. The amplitude of the conversions varies likely due to differences in composition and/or fluid content. We analyzed the slab signature for 298 long‐running land seismic stations, estimated the depth of the three interfaces through inverse modeling and fitted regularized spline surfaces through the station control points to construct a margin‐wide, double‐layered slab model. Crystalline terranes that act as the static backstop appear to form the major structural barrier that controls the slab morphology. Where the backstop recedes landward beneath the Olympic Peninsula and Cape Mendocino, the slab subducts sub‐horizontally, while the seaward‐protruding and thickened Siletz terrane beneath central Oregon causes steepening of the slab. A tight bend in slab morphology south of the Olympic Peninsula coincides with the location of recurring large intermediate depth earthquakes. The top‐to‐Moho thickness of the slab generally exceeds the thickness of the oceanic crust by 2–12 km, suggesting thickening of the slab or underplating of slab material to the overriding North American plate. Plain Language Summary: The tectonic Juan de Fuca plate, that underlies the easternmost North Pacific Ocean off‐shore Vancouver Island, Washington, Oregon, and northern California, is being pushed beneath the North American continent by plate tectonics. On its way deep into the Earth, the plate deforms. In this study, we analyze seismograms of distant earthquakes which were recorded within the study area. Through signal and data processing, we decipher information about the location, orientation, and properties of the down‐going oceanic plate beneath the continent. The data show that the plate protrudes shallowly dipping under the continent beneath the Olympic Peninsula (Washington) and Cape Mendocino (California), while it dips down more steeply under central Oregon and Vancouver Island (British Columbia). This configuration suggests that Siletzia, an old and rigid basalt plateau that forms the central part of the study area, controls the shape of the down‐going plate. Furthermore, the oceanic plate appears to significantly thicken at depth, which may indicate that parts of it accumulate at the base of the continent. These results are important to better understand the subduction process and may help to infer the location of the deeper extent of rupture in a future potential strong earthquake. Key Points: We model Cascadia subduction stratigraphy as three dipping horizonsSlab morphology is controlled by crystalline terrane backstopsA near‐ubiquitous ∼2–10 km thick ultralow velocity zone in the tremor zone correlates with the E‐layer [ABSTRACT FROM AUTHOR]

Serpentinization is among the most important, and ubiquitous, geological processes in crustal–upper mantle conditions (<6 GPa, <600°C), altering the rheology of rocks and producing H2 that can sustain life. While observations are available to quantify serpentinization in terrestrial and mid‐ocean ridge environments, measurements within subduction zone environments are far more sparse. To overcome this difficulty, we design a methodology to quantify and offer a first‐order estimate of the magnitude of "slab‐serpentinization" that has occurred over the last 5 Ma within the world's subduction zones by coupling four discrete tectonic and geophysical datasets—(a) raster grids of relic abyssal peridotite (peridotite exhumed from slow spreading mid‐ocean ridges but unaffected by pre‐subduction serpentinization) within ocean basins, (b) slab geometry, (c) thermal profiles and a (d) plate‐tectonic model. Averaged per year, our results suggest that 4.2–24 • 107 kg of H2 per annum could be generated from "slab‐serpentinization" within a subduction zone. Our estimate is 3–4 orders of magnitude lower than what is thought to be produced at mid‐ocean ridges, and 1–2 orders of magnitude lower than what could occur through serpentinization at trench flexure and when including possible mantle wedge serpentinization. Higher hydrogen production is correlated most strongly with the spreading history of ocean basins, underlaying the importance of the tectonic history of a slab prior to subduction. Plain Language Summary: The fate of most ocean crust formed at mid‐ocean ridges is to eventually subduct and be recycled into the mantle. Subduction zones therefore represent a key link between the rocks we see at the surface of the Earth, both in oceans and continents, and the underlaying mantle. However, subduction zones are impossible to observe directly and therefore difficult to fully understand the processes that shape them. Here, we designed a framework that coupled a series of discrete data sets to model how the composition of each subducting slab across the globe differs in order to provide an accurate estimate of "slab‐serpentinization." Serpentinization is the process that converts mantle rocks to serpentinite through exposure to water. A by‐product of this process is the formation of hydrogen gas. Using our framework, we estimated bulk fluxes of serpentinization in subducting slabs, and the corresponding flux of hydrogen. Key Points: First‐order estimate of the gross flux of "slab‐serpentinization" and the resulting (possible) hydrogen production (4.2–24 • 107 kg of H2 per annum)Seafloor spreading history and ocean basin evolution (prior to subduction) impart the strongest control on slab‐serpentinization possibility [ABSTRACT FROM AUTHOR]

Torn and bent slabs are usually associated with flat‐slab subduction where the descending plate develops a horizontal geometry beneath the overlying continent. How such slab dynamics modify the surrounding mantle flow and the overriding plate remains enigmatic. Here, we conduct three‐dimensional subduction numerical experiments to investigate the flat slab to steep‐angle slab transition region and examine the impact of slab geometry changes on mantle flows. The results show that the along‐strike change due to flattening a segment of slab induces oblique flow toward the mantle corner at the transition region where flat slab bends to steep‐subducting slab. Slab tears can occur due to the buoyancy contrast between an oceanic ridge and the surrounding dense oceanic crust and/or presence of weak zones. A vertical tear at the side edge of a flat slab causes toroidal flow around the steep‐angle slab where sub‐slab mantle floods rapidly through the tear. Flow through the tear spreads to all directions including upwelling toward the continental base that may trigger slab and partial melting and thus affect arc magmatism. A horizontal tear that occurs ahead of the flat portion where slab resumes its steep‐angle results in enhanced plate‐motion parallel flow. Notably, the tear‐induced flow behaves as a rapid injection through the slab breach with the peak velocity up to an order of magnitude higher than plate motion, lasting for 1–2 million years after initially tearing. This rapid pulse flow might be recorded in surface tectonics as distinct transient events of topography or metamorphism. Key Points: Along trench variations in slab geometry due to bending and tearing during flat‐slab development induces distinct mantle flowVertical tear generates toroidal flow along steep‐subducting plate and horizontal tear induces plate‐motion parallel flowTear induced flow is transient and the peak velocity is up to an order of magnitude higher than plate motion [ABSTRACT FROM AUTHOR]

Devastating megathrust earthquakes and slow earthquakes both occur along subducting plate interfaces. These interplate seismic activities are strongly dependent on the nature of the plate interface, such as the shape of the plate interface and the materials and physical conditions along the plate interface. The oceanic plate, which is the input to the subduction zone, is the first order control on the nature of the plate interface. To reveal the nature of the subduction inputs to the northeastern Japan arc, we have conducted large-scale controlled-source seismic surveys of the northwestern part of the oceanic Pacific plate. The obtained seismic data have revealed (1) oceanic plate structural evolution caused by plate bending prior to subduction, suggesting the promotion of the oceanic plate hydration; (2) spatial variation of the oceanic plate structure, such as variations in the thickness of sediment and crust; (3) that the spatial variations are caused by both ancient plate formation processes and more recent volcanic activities; and (4) that spatial variations of the nature of the subduction inputs show good correlation with the along-strike variations in the seismic structure and seismic activities after subduction, including the coseismic slip distribution of the 2011 Tohoku earthquakes and the structural differences between the northern and the southern Japan Trench. These observations indicate that the incoming oceanic plate structure is much more spatially variable than previously thought and also imply that the spatial variation of the subduction inputs is a key controlling factor of the spatial variation of various processes in subduction zones, including interplate seismic activities and evolution of the forearc structure. [ABSTRACT FROM AUTHOR]

The article offers information on the role of fluid overpressures and hydrogeology in the occurrence of slow slip along subduction zones. Through seismic imaging and ocean drilling, the study reveals a previously unknown fluid reservoir within the volcanic upper crust of the subducting Hikurangi Plateau.

To ascertain the Early-to-Middle Jurassic tectonic setting in the central Great Xing'an Range, this study investigated the Early and Middle Jurassic granitoids exposed in the Chaihe area in the central Great Xing'an Range based on isotopic chronology and petrogeochemistry. The results of this study show that the Early and Middle Jurassic granitoids have emplacement ages of 179–172 Ma. Moreover, the Early and Middle Jurassic granitoids are high-K calc-alkaline unfractionated I-type granitoids and high-K calc-alkaline fractionated I-type granitoids, respectively. The magma sources of the Early and Middle Jurassic granitoids both originated from the partial melting of newly accreted lower crustal basaltic rocks. Meanwhile, the Middle Jurassic magma sources were mixed with mantle-derived materials or ocean-floor sediments formed by the dehydration and metasomatism of subducted slabs. The Early and Middle Jurassic granitoids in the study area were formed in the subduction environment of the oceanic crust, in which the Mongol-Okhotsk oceanic plate was subducted southward beneath the Eerguna and Xing'an blocks. Moreover, the Siberian plate began to collide and converge with northeast China during the Middle Jurassic. [ABSTRACT FROM AUTHOR]

Subduction initiation is a pivotal process in plate tectonics. Models of subduction initiation include the collapse of passive margins, oceanic transform faults, inversion of oceanic core complexes, and ridge failure but have ignored the potential effects of continental crust relicts within the oceanic crust. In this paper, we explore the role of microcontinents on subduction initiation through two-dimensional thermo-mechanical numerical modeling. We consider three scenarios with variable ages of oceanic crust surrounding the microcon-tinent and parametrically examine the microcontinent characteristics (size, crustal thickness, thermal gradient, and rheology), oceanic plate age, and convergence rates. Results suggest that moderate-size (≥300 km) microcontinents can nucleate subduction initiation at the junction between continental and oceanic plates. A large part of the microcontinent would be dragged into the subduction zone, and the subsequent asthenosphere upwellings would incorporate part of the microcontinent. Our numerical models add a new hypothetical scenario for subduction initiation, especially for those places where a young and buoyant plate subducts beneath an older and denser oceanic plate. Moreover, they can explain the origin of exotic crust materials and ultrahigh-pressure minerals in supra-subduction zone ophiolites. [ABSTRACT FROM AUTHOR]

[Display omitted] • Eclogitization kinetics of oceanic crust layers controlled Precambrian subduction. • Discrete eclogitization induced flat subduction in Precambrian. • Flat subduction builds short-lived overthickened layered mantle. Flat (shallow) subduction is mainly proposed as an Archean-Paleoproterozoic process, contributing to the growth of continental lithosphere. However, numerical models of Precambrian subduction commonly show steep subduction. Here we investigate the effects of lithology-dependent eclogitization (minor for gabbroic lower crust, strong for basaltic upper crust) of oceanic crust during subduction. We performed 2D petrological-thermomechanical modeling of subduction at potential mantle temperatures up to 250 °C warmer than present, using a numerical approach that accounts for buoyancy effects from both mantle depletion and eclogitization. The modeling reveals that increases in mantle potential temperature and the related thickness of oceanic crust and depleted mantle may induce transition to the flat subduction regime at ΔT ≥ 150 °C but only when a delayed eclogitization of gabbroic compared to basaltic crust is taken into account. Otherwise, subduction operates in the steep slab regime. Flat Precambrian subduction models show episodic bimodal magmatism within arcs located > 400 km from the trench, caused by short-lived (1–4 Myr) subduction transients consisting of three consecutive stages: (1) twisting downward of the eclogitized slab portion, (2) subvertical descent and break-off of this twisted portion, (3) rising of the remnant non-eclogitised slab tip and continuation of flat subduction. Limited formation of TTG-series granitoids is caused by partial melting of the subducting basaltic layer but becomes less pronounced at ΔT = 250 °C. Flat subduction produces an overthickened, layered keel-like continental mantle root, with an intermediate layer of subducted oceanic crust. However, construction of long-lived mantle keels by this mechanism might be limited by peeling off of the gradually eclogitizing oceanic crust and the underlaying mantle when plate convergence ends. At elevated mantle temperatures flat subduction produces voluminous hydration of the mantle wedge, forming a subcrustal serpentinite mélange layer that becomes a potential source of fluids in the subsequent evolution of the overlying continental crust. [ABSTRACT FROM AUTHOR]

The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘ C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µ g g -1 H 2 O) and grossular-rich garnet (200–1800 µ g g -1 H 2 O) contain at least 1 order of magnitude more H 2 O than almandine-rich garnet (< 120 µ g g -1 H 2 O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D [m 2 s -1 ]) = - 24.5 at 540 ∘ C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H 2 O to be calculated. The result shows that ∼ 3360 kg H 2 O km -1 (section of oceanic crust) yr -1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean. [ABSTRACT FROM AUTHOR]

High Ba-Sr granitic rocks are widespread in Phanerozoic orogenic systems, and their petrogenesis is important for revealing the evolutionary process of the Proto-Tethys Ocean in the North Qilian orogenic belt. This paper presents a combination of zircon U-Pb age, whole-rock major and trace element concentrations, and Sr-Nd-Hf isotopic data for Caowa high Ba-Sr dioritic intrusion from the eastern part of the North Qilian orogenic belt, aiming to decipher its petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating yield an emplacement age of 450 ± 2 Ma for the Caowa intrusion, indicating a magmatic activity of the Late Ordovician. The Caowa quartz diorites contain moderate contents of SiO2, MgO, Mg#, and resultant high concentrations of Na2O + K2O, Fe2O3T, and Al2O3, displaying calc-alkaline and metaluminous characteristics. The studied samples have relatively elevated Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (e.g., Ba, Th, U) and depletions in HFSEs (e.g., Nb, Ta, Ti), resembling those of typical high Ba-Sr granitoids in subduction zones. Together with enriched Sr-Nd isotopic composition [(87Sr/86Sr)i = 0.7082–0.7086, εNd(t) = −5.1 to −4.9], and the wide ranges of zircon εHf(t) values (−13.2 to +8.5), it suggests that these high Ba-Sr quartz diorites were derived from a mixture magma source between the ancient crust materials and the enriched lithospheric mantle metasomatized by fluid released from subducted oceanic crust or sediment. Taking into account the ophiolites, high pressure metamorphic rocks, and arc magmatic rocks in the region, we infer that due to the influence of the northward subduction of the Qilian Proto-Tethys Ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician and resulted in extensive metasomatism of lithospheric mantle by fluids derived from oceanic crust or sediments, and the Caowa high Ba-Sr quartz diorites were generated in the process of crust–mantle interaction during the Late Ordovician. [ABSTRACT FROM AUTHOR]

An analysis of two decades (2003–2022) of seismicity recorded by the Spanish and Portuguese seismic networks along the West Iberian passive margin has resulted in a better understanding of the distribution of moderate seismic activity in this intraplate submarine area. The study provides a precise trend of specific alignments inferred from the density maps of seismicity, giving an accurate depiction of event distribution along two wide stripes that extend 700 km long through the ocean floor in a WNW-ESE direction. These bands are parallel to the Africa-Eurasia plate boundary but are distinctly separated from its related seismicity by approximately 300 km and 700 km, respectively. This is a sufficient distance to be considered as intraplate activity. When trying to relate this seismicity to structural and geophysical features, a conclusive picture doesn't emerge. The earthquakes occur indiscriminately across thinned continental, hyperextended, and exhumed mantle rift domains. They fade out in the proximity of undisputed oceanic crust, but some events extend beyond. The hypocentral depths signal a considerable amount of events nucleating in the upper mantle. The focal mechanisms, although scarce, are predominantly strike-slip. Considering these observations, hypothesis ranging from subduction initiation, development of strained corridors or local structures of the margin, have been discussed in order to explain this relatively anomalous seismicity. However, some of them do not portray convincing arguments, while others are too unspecific. None of them are flawless, suggesting that several factors may be at play. Despite being one of the most probed passive margins in the world, the present geodynamical status of the West Iberian Margin manifested in its modern seismicity remains unknown. Interpreting this data within a global tectonic plate framework, together with the potential addition of seafloor seismometers, may provide the key to understanding this activity along one of the most archetypical margins of the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

The Qushenla Formation volcanic rocks are widely exposed in the northern margin of the Bangong-Nujiang suture zone (BNSZ). Research on these rocks is of great significance for understanding the tectonic evolution of the Bangong-Nujiang Tethys Ocean (BNTO). In this study, a systematic geological survey was conducted on the Qushenla Formation volcanic rocks that are widely exposed in the Nawucuo area, in the northern margin of the western segment along the BNSZ. The whole-rock geochemistry, zircon U-Pb dating, and in situ zircon Lu-Hf isotopes were carried out in this study, aiming to constrain the formation age, rock genesis, magma source and tectonic setting of the volcanic rocks. The zircon U-Pb dating shows that the Qushenla Formation volcanic rocks in the western BNSZ erupted during the period of 120–108 Ma, i.e., Early Cretaceous. The Qushenla Formation volcanic rocks are a suite of intermediate-basic volcanic and pyroclastic rocks belonging to the medium-K calc-alkaline series. They are relatively enriched in light rare earth elements (LREEs) and incompatible elements such as Rb, K, La, Th, Sm, and Hf, whereas depleted in heavy REEs (HREEs) and high field strength elements (HFSEs) such as Nb, P, Zr, and Ti. The in situ zircon εHf(t) values of the volcanic rocks range from 8.95 to 12.57, with an average of 10.40. The Mg#, Th/La and Th/Ce values are between those of the mantle-derived magma and the continental crust. The formation of the Qushenla Formation volcanic rocks can be explained through the following process: (1) As the nospheric materials that upwelled during the rollback of the subducting Tethys Ocean slab induced the large-scale partial melting of the mantle wedge and the formation of the initial basaltic magma. (2) These mantle-derived magmas ascended and induced the partial melting of the lower crust to generate peraluminous melts. (3) The mixing of the peraluminous melts and mantle-derived melts generated the initial magma with homogeneous Sr-Nd isotope compositions. (4) Last, the eruption of the magma produced the widespread Qushenla Formation volcanic rocks at the surface. When combining this information with the regional geological background, it is believed that the Qushenla Formation volcanic rocks, the Meiriqicuo Formation volcanic rocks and the Late Jurassic–Early Cretaceous intrusive rocks together constitute the tectonic magmatic arc of the active continental margin on the southern margin of Qiangtang, which was formed in the tectonic setting of the northward subduction of the Bangong-Nujiang oceanic crust beneath the Qiangtang Block. [ABSTRACT FROM AUTHOR]

Large-scale Mesozoic granitoids are exposed in the Greater Khingan Mountains. Their relationship with the Mongolia–Okhotsk and the Paleo-Pacific Ocean is still under discussion and a matter of debate. In this study, field observations were made and a total of 18 granitoids exposed in the vicinity of the Heihe–Baishilazi area in the northern part of the Greater Khingan Mountains were sampled for petrological, geochronological, and geochemical research. In addition, to complement this study, 90 granitic samples from the Xinghua, Dajinshan, Yili, Chabaqi, and Sankuanggou areas in the Greater Khingan Mountains were compiled in order to reveal rock assemblages, magma sources, and then inquire into the tectonic background. Zircon LA–ICP–MS U–Pb dating indicates that two samples from the Heihe area were formed in the Early Jurassic period (194.2 ± 1.4 Ma and 183.1 ± 1.3 Ma), and the εHf(t) values and TDM2 of the zircons were mainly +5.8 to +10.7 and 528 Ma to 834 Ma, respectively, with a large variation range. The intrusive rocks from the Greater Khingan Mountains (108 in total) belonging to the T1T2G1G2 assemblage contained tonalites (T1), trondhjemites (T2), granodiorites (G1), and granites (G2). These granitoids are presented as subalkaline series in a plot of total alkali versus SiO2 (TAS diagram), medium-K calc-alkaline and high-K calc-alkaline series on SiO2 versus K2O diagram, with metaluminous to peraluminous characteristics on an A/CNK versus A/NK diagram. These are shown as a MA (magnesium andesite) series and LMA (lower (or non) magnesium andesite) series on a SiO2 versus MgO diagram, which can be further divided into the higher-pressure TTG subtype of the MA (corresponding to high-SiO2 adakite (HSA)) series and the lower-pressure TTG subtype of LMA (corresponding to typical calc-alkaline suprasubduction zone rocks). In addition, granitoids were enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depleted in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs), corroborating a suprasubduction zone environment. Regional correlations as well as geochemical characteristics indicate that the rocks from the Greater Khingan Mountains formed in a subduction zone environment during the Early Jurassic; primary magma had presumably originated from the melting of young and hot oceanic crust under eclogite to amphibolite facies conditions. According to the spatial variation in rock assemblages (T1T2G1 to G1G2 and G2), we speculate that the northeastern Heihe, Baishilazi, and Xinghua areas as well as the westward Dajinshan area were adjacent to the ocean and formed an outer subduction zone, whereas the southwestward Sankuanggou, Yili, and Chabaqi areas were adjacent to the continent, forming an inner subduction zone. The distribution sites of the inner and outer subduction zones indicate southward and southwestward ocean subduction. Therefore, we propose a direct connection with southward subduction of the Mongolia–Okhotsk Ocean. [ABSTRACT FROM AUTHOR]

Due to the complexity of 2D magnetic anomaly maps north of 18°S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi‐method approach to reconstruct the formation of the basin at ∼16°S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km‐long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response. Plain Language Summary: The opening of back‐arc basins is often described as analogy to mid‐ocean ridge spreading, where the only difference is the force driving the extension. However, the northern Lau Basin is a prime example for the shortcomings of this analogy since its crust preserves an image of its complex tectonic history. The complexity results from the short‐lived nature of zones of active rifting and spreading in the northern Lau Basin, which is very different from the temporally and spatially steady nature of spreading centers at mid‐ocean ridges. The analysis of different methods (wide angle seismic data using ocean bottom seismometers, multi‐channel seismic, magnetic, gravity, and geochemical data) has led us to conclude that the Lau Basin's crust at 15°30–17°20′S was formed by a dynamically changing, both in regard of magma composition and position, extensional system that consists of the Fonualei Rift and Spreading Center and the Mangatolu Triple Junction. Nevertheless, the crustal zonation, formed by the varying subduction influence during its formation, is still preserved and affects the stress response of the crust and thus the present‐day tectonic behavior. Key Points: Oceanic crust in the north‐eastern Lau Basin formed at the now reorganized FRSC‐MTJ systemThe position and the opening mechanisms of back‐arc basin spreading center's change more dynamically at mid‐ocean ridgesDifferent opening mechanisms at the southern Mangatolu Triple Junction and northern Fonualei Rift Spreading Center despite their proximity [ABSTRACT FROM AUTHOR]

Determining subducted processes is very important for understanding lithological heterogeneity because substantial quantities of slabs are recycled into the mantle. Molybdenum isotopes have been used to distinguish sources materials because of the significant Mo isotopic differences between the crust and the mantle. Here, we undertake a systematic Mo isotopic investigation on a suite of well-characterized continental basalts from Keluo and HalahaChaoer, located in Northeast China. δ98Mo of Keluo varied from -0.41 to -0.23‰ with an average of -0.34‰, and samples from Halaha-Chaoer ranged in δ98Mo from -0.18 to -0.12‰ with an average of -0.15‰. The former is lighter than the mean value of fresh oceanic basalts (-0.21‰), while the latter is similar to oceanic basalts. Combined with other geochemical indications (LOI, Ce/Pb, La/Yb and so on), the Mo isotopic variations cannot be interrupted by chemical weathering, continental crust contamination, or magmatic processes. The δ98Mo variation in this study is explained by adding various oceanic crustal materials in magma sources. The relationships of δ98Mo with Ba/Th, Th/U, 143Nd/ 144Nd for the samples indicated that sediment and altered oceanic crust have contributed to the significant variations in these samples. This study indicates the great potential of Mo isotopes to distinguish different types of recycled oceanic crust materials in the mantle. [ABSTRACT FROM AUTHOR]

Determining subducted processes is very important for understanding lithological heterogeneity because Substantial quantities of slabs are recycled into the mantle. Molybdenum isotope have been used to distinguish the addition of crustal materials since the significant Mo isotopic differences of reservoirs. Here we undertake a systematic Mo isotopic investigation on a suite of well-characterized continental basalts from Keluo and Halaha-Chaoer, located at Northeast China. The δ98Mo varied from -0.41 to -0.23 ‰ with an average of -0.34 ‰ for Keluo samples and from -0.18 to -0.12 ‰ with an average of -0.15 ‰. The former is lighter than fresh oceanic basalts (-0.21 ‰) while the latter is similar to oceanic basalts. The Mo isotopic variations, combine with other geochemical indications (LOI, Ce/Pb, La/Yb and so on), cannot be interrupted to chemical weathering, continental crust contamination or partial melting, but are explained by the addition of various oceanic crustal materials in magma sources. The relationships between δ98Mo and Ba/Th, Th/U, 143Nd/144Nd for the samples suggest sediment, fresh and altered oceanic crust have contributed to the large variations in these samples. The study here indicates the great potential of Mo isotopes to distinguish different types of recycled oceanic crust materials in the mantle. [ABSTRACT FROM AUTHOR]

Because of the steep subduction of a highly concave slab, researchers have characterized megathrusts under the Marianas as among the coldest and curviest plate coupling interfaces in various circum-Pacific subduction zones. Seismic tomography indicates that the heterogeneous underlying plate varies markedly in its subduction angle, velocity, and flexure along the strike and dip, while their effects on the thermal structure and intraslab earthquake occurrence remain enigmatic. By incorporating the 3-D MORVEL velocity and state-of-the-art slab geometry into thermomechanical modeling, we estimated the 3-D subduction thermal state and hydrothermal regime below the Marianas. We find that (1) the concave slab geometry and the complexity of the intraslab velocity variation in the Marianas are associated with a heterogeneous along-strike thermal regime and a cold mantle wedge beneath the central Marianas; (2) amphibolitization and eclogitization of subducted oceanic crust cause variations in fluid pressure and fluid release from the subduction interface, which may influence the distribution of interface seismicity in the Mariana system; (3) the concentration of active hydrothermal vents in the trench > 8 km deep is accompanied by a large temperature gradient and subsequent remarkable slab dehydration in the southern Marianas; and (4) slab dehydration (> 0.02 wt%/km) from 30 to 80 km indicates notable fluid release and potential fluid migration in subduction channels, which may correspond to the large water flux at depth beneath the Marianas. [ABSTRACT FROM AUTHOR]

The 290–280 Ma mafic Liuyuan Complex, embedded in the terranes constituting the southern edge of the Central Asian Orogenic Belt, has been interpreted to be either an ophiolite or an intracontinental large igneous province. Our detailed mapping shows that the Liuyuan Complex preserves an almost complete oceanic crustal section, with basal troctolite cumulates, varitextured gabbro affected by intense sub‐seafloor hydrothermal circulation, a newly discovered, and laterally‐continuous sheeted dyke complex, capped by basaltic pillows. This architecture is inconsistent with a continental rift‐layered intrusion model for the Liuyuan Complex, which we interpret as oceanic crust formed by seafloor spreading. The trace element ratios (Th/Yb, V/Ti, La/Sm, and La/Nb) of the lavas is consistent with a back‐arc basin environment. We suggest that the slab subducting to the north and forming the Ganquan arc system rolled back around 292 Ma, opening a back‐arc basin (the Liuyuan Complex). This oceanic basin started to close from ca. 281 Ma, with north‐dipping subduction under the accreted terranes constituting the southern active margin of Composite Siberia. The Liuyuan Complex was eventually obducted southwards over the Ganquan arc as part of the upper plate during the final closure of the back‐arc basin. [ABSTRACT FROM AUTHOR]

The carbon cycle is an important process that regulates Earth's evolution. We compare two typical periods, in the Paleoproterozoic and Neoproterozoic, in which many geological events occurred. It remains an open question when modern plate tectonics started on Earth and how it has influenced the carbon cycle through time. In the Paleoproterozoic, intense weathering in a highly CO2 and CH4 rich atmosphere caused more nutritional elements to be carried into the ocean. Terrestrial input boosted high biological productivity, deposition of sediments and the formation of an altered oceanic crust, which may have promoted an increase in the oxygen content. Sediment lubrication and a decrease in mantle potential temperature made cold and deep subduction possible, which carried more carbon into the deep mantle. Carbon can be stored in the mantle as diamond and carbonated mantle rocks, being released by arc and mid‐ocean ridge outgassing at widely different times. From the Paleoproterozoic through the Neoproterozoic to the Phanerozoic, the carbon cycle has promoted the evolution of a habitable Earth. [ABSTRACT FROM AUTHOR]

A nearly balanced carbon budget between subduction input and degassing output has likely controlled the long‐term surface environment and habitability of Earth throughout geological history. However, the ongoing extensive hydration and carbonation of the mantle in trench outer‐rise regions may affect the global carbon budget. In this study, we show that the carbon content of the lithospheric mantle can be inferred from geophysical data and thermodynamic modeling. Based on the seismic velocity anomaly in trench outer‐rise regions, we estimated that the total carbon flux due to mantle carbonation is 7–31 Mt C/year, with possible fluid‐to‐rock mass ratios of 250–1000. These values are similar to the carbon uptake by altered oceanic crust, indicating that mantle carbonation has a significant effect on the subduction carbon budget. Although there are large uncertainties on the estimates of the subduction and degassing carbon fluxes, secular cooling of the mantle leads to the development of outer‐rise faults associated with bending of the oceanic lithosphere and increased mantle carbonation, which may disrupt the self‐regulating system of the global carbon cycle on geological timescales. [ABSTRACT FROM AUTHOR]

The tectonic properties of the Nanping‐Ninghua tectonic belt (NNTB) play a crucial role in understanding the tectonic evolution of the Wuyi orogenic belt, which is related to the Neoproterozoic‐Paleozoic tectonic evolution of the Cathaysian Block. In this study, We present comprehensive petrological, geochronological, and geochemical data on the metavolcanic rocks from the Louqian Formation in Changting, Fujian Province. Our results show that volcanism in Changting occurred between 740.4 and 723.5 Ma, based on zircon U–Pb data for the metavolcanic rocks. The metavolcanic rocks have SiO2 contents ranging from 68.22 to 71.56 wt %, high Al2O3 and Na2O + K2O contents (13.30–15.19 wt % and 7.56–8.25 wt %, respectively), and low CaO, MgO, and FeOT contents (0.09–1.25 wt %, 0.72–0.96 wt %, and 1.57–3.16 wt %, respectively). These samples are enriched in large‐ion lithophile elements (LILEs) such as Ba and Rb, and depleted in high‐field‐strength elements (HFSEs) including Nb, Ta, and Ti, with significant negative Eu anomalies, indicating an affinity for arc igneous rocks. Based on previous geological studies, we suggest that the NNTB formed in an active continental margin environment during the Nanhua Period. The subduction of oceanic crust led to the formation of the volcanic belt along the Nanping‐Ninghua continental margin, which was active until 696 Ma. During the Neoproterozoic, the North and South Wuyi terranes did not merge to form a single Wuyi terrane, and the Cathaysia Block was composed of several terranes. [ABSTRACT FROM AUTHOR]

Whether or not oceanic crust basalts are affected by plate subduction is a hot topic of debate. The South China Sea is one of the largest marginal basins in the western Pacific Ocean and has been surrounded by subduction of the Pacific plate and Indian plate, yet, to date, no study has clearly shown evidence of subduction in the geochemistry of volcanism in the basin due to a lack of sampling of igneous crust basalts on the seafloor. The International Ocean Discovery Program Expedition 349 cored seafloor basalts near the fossil spreading ridges of the eastern (Site U1431) and southwestern (Site U1433 and U1434) subbasins in the South China Sea. The recovered basalt samples indicated a pyroxenite-bearing peridotite mantle source. Here, we report Mg isotopic data from 14 of these oceanic crust basalt samples. The δ26Mg values of most basalts from the three drill holes were higher (up to -0.10‰) than that of the average mantle (-0.25‰). The lack of correlations of δ26Mg with geochemical indices of magmatic processes (e.g., MgO, CaO/Al2O3, La/Sm, Nb/Zr) suggests that crystal fractionation and partial melting had insignificant effects on the Mg isotopic compositions of the South China Sea basalts. Thus, the variations in Mg isotopes were inherited from their mantle sources. Considering the highly varied Ce/Pb ratios and elevated 87Sr/86Sr values but mantle-like 143Nd/144Nd values, we propose that the varied δ26Mg values were likely caused by metasomatism of subduction-released fluids. The coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-δ26Mg values of the South China Sea basalts resulted from mixing among pyroxenite-bearing peridotite mantle, the nearby Hainan plume materials, and subducting serpentinite-released fluids. Therefore, these Mg isotopes suggest that the mantle source of the South China Sea basalts was influenced by subducted materials, providing further evidence of the initial expansion, formation, and evolution of the South China Sea during plate subduction. [ABSTRACT FROM AUTHOR]

In the Mesozoic, the age of Paleo‐Pacific plate that flatly subducted beneath the South China was ∼80 Ma, resulting in a cold and dense oceanic slab. The transition process from flat to steep subduction of this old slab and factors that affect it are still unclear. Using 2‐D thermomechanical numerical models, we investigate the dependence of the transition mode on the size of TOC (length and thickness) and the subduction rate. The results indicate that under low subduction rates (e.g., 1 cm/yr), even a 100‐km long eclogitized TOC can induce slab delamination beneath continental lithosphere. The maximum subduction rate for slab delamination to occur increases with an increasing length or thickness of TOC. However, when the subduction rate is over a certain critical value (e.g., ≥4 cm/yr), oceanic slab rolls back. The presence of the TOC delays the initial rollback time, as compared to cases with no TOC. In addition, with a TOC of small size (e.g., 100‐km length and 20‐km thickness) under a certain range of subduction rate, an 80‐Ma old slab can maintain a long‐lasting flat subduction. The comparisons between the numerical results and global plate motion models indicate that the northwestward motion of South China Block since ca. 195 ± 5 Ma induced the delamination of oceanic slab. The arrival of the TOC in the Paleo‐Pacific Plate to the continental arc should be later than the previously estimated ca. 250 Ma. Determination of the TOC size requires three‐dimensional simulations and more detailed geological records. Key Points: The maximum subduction rate for slab delamination to occur increases with an increasing length or thickness of the thickened oceanic crustThe arrival time of the thickened oceanic crust to the continental arc of the South China Block is expected to be later than ca. 250 MaThe size of the thickened oceanic crust can be estimated from the arrival time of the thickened oceanic crust and the subduction rate [ABSTRACT FROM AUTHOR]

Late Miocene to Late Pleistocene alkaline lavas in the northernmost part of the Antarctic Peninsula and its off-lying islands are the latest stage of magmatic activity that took place in response to lithospheric extension in the back-arc region of the South Shetland subduction system. The alkaline magmatism occurred much later than the main pulse of Cretaceous arc magmatism and generated basaltic extrusive rocks during several sub-aqueous/sub-glacial and sub-aerial eruption periods. The suite consists primarily of alkali olivine basalts with oceanic island basalt (OIB)-like trace element signatures, characterized by elevated highly to less incompatible element ratios compared to MORB. The samples have higher 87Sr/86Sr (0.70301–0.70365), and lower 143Nd/144Nd (0.51283–0.51294) and 176Hf/177Hf (0.28291–0.28298) than depleted MORB mantle. Their lead isotope ratios vary within a limited range with 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb ratios of 18.797–18.953, 15.577–15.634, and 38.414–38.701, respectively. Sr, Nd, Hf and Pb isotope systematics suggest involvement of diverse source materials in the genesis of the alkaline magmas. Evaluation of radiogenic isotope and trace element data indicates that the source of the alkaline melts had a complex petrogenetic history, reflecting the effects of mantle hybridization along the slab mantle interface through interaction of mantle wedge peridotites with volatile-bearing, siliceous melts produced by melting of subducted sediments and basaltic oceanic crust. Hf-Nd isotope and trace element projections further demonstrate that the metasomatizing melt was likely generated by eclogite partial melting at sub-arc to post-arc depths, in equilibrium with a garnet-bearing residue and involved breakdown of high field strength elements (HFSE) retaining phases. Consumption of metasomatic amphibole during partial melting of hybridized peridotite at the wet solidus appears to have had a significant effect on the final melt compositions with high HFSE, Na and H2O contents. [ABSTRACT FROM AUTHOR]

SUMMARY: Oceanic plateaus (or aseismic ridges) can be either subducted into the deep mantle, or accreted onto the overriding plate. Furthermore, some oceanic plateaus can change subduction mode from steep to flat-slab subduction. What factors control the fate of oceanic plateaus during subduction remain enigmatic. Here, we investigate the controls on these modes and their respective geological effects using 2-D thermomechanical simulations. We systematically examine the characteristics of an oceanic plateau (including crustal thickness and length), plateau-trench distance, convergence rate and eclogitization of the oceanic crust. Our models confirm that the size of the plateau and eclogitization are the main factors controlling the subduction characteristics. For the eclogite models, a relatively thin oceanic plateau (≤20 km thick) undergoes steep subduction, a moderate-scale plateau (25–30 km thick) favours flat-slab subduction and large-scale plateaus (≥35 km thick) are more susceptible to collide and accrete to the overriding upper plate. Eclogitization significantly reduces the formation chance and duration of flat-slab subduction. The switch from flat-slab to steep subduction occurs rapidly (<5 Ma), and the steepening occurs twice as fast as the flattening. The plateau-trench distance determines the location of the break-off, and shallow break-off (<300 km) of the frontal slab will significantly change the subduction pattern. Either fast convergence rates (≥8 cm yr−1) or overthrusting of the overriding plate promotes the formation of flat-slab subduction. The mode changed from flat-slab to steep subduction explains the landward migration of magmatism followed by a trenchward migration in Eastern China since the Mesozoic. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt is bounded on the north by the Siberian Craton and on the south by the North China Craton and the Tarim Craton. It is one of the largest Phanerozoic accretionary orogenic belts on Earth. Since the early Paleozoic, its eastern part has experienced the compound orogenesis and mineralization of three major tectonic systems: the closure of the Paleo-Asian Ocean, the closure of the Mongolian–Okhotsk Ocean, and the subduction of the Paleo-Pacific Plate. From Zhangwu County in the south to East-Ujimqin Banner in the north, a 500 km magnetotelluric profile adjacent to Northeast China has been studied. With 100 sites of magnetotelluric data processing and analysis, we apply a two dimensional inversion in TE and TM modes and obtain a resistivity model up to a 100 km depth. We have discovered two high-resistivity anomalies with opposite dip directions in the upper mantle on both sides of the Solonker Suture Zone, which provide an evidence of the bi-directional subduction pattern of the oceanic crust and the position of the closure of the Paleo-Asian Ocean. In addition, the whole study area presents an approximate basin-range coupling relationship. In the northern part of the study area, the low-resistivity anomalies below it have an apparent north-dipping characteristic, which may be related to the asthenosphere upwelling from west to east. In addition, they may be related to the upwelling of mantle materials, and provide sources of ore-forming material for the Baiyinnuoer mining area through post-collision extension. In the central part of the study area, there are several large-scale high-resistivity anomalies below the Baolidao Belt. The different dip directions reveal the experiences of several subductions and collisions. In the southern part of the study area, the Bainaimiao Belt is located between the southern margin of the Songliao Basin and the northern margin of North China Craton. The main resistivity anomalies below are all south-dipping. [ABSTRACT FROM AUTHOR]