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Loss of slab-derived volatile and fluid-mobile elements (FME) through the forearc may represent a large, unaccounted for flux out of convergent margins. To address volatile recycling through the forearc and potential changes in fluid source from the progressively dehydrating subducting slab, we collected water samples from 44 different cold and thermal (>40 °C) springs on a transect across the outer forearc (∼20–40 km to the subducting slab), forearc (∼40–80 km to the slab), and arc (∼80–120 km to the slab) in Costa Rica. We focused on the heavy halogens (Cl, Br, I), as well as B and Li, given their high fluid mobility and thus limited potential for modification by subsequent fluid-rock interaction. Chlorine, boron, and lithium stable isotope ratios show dramatic ranges from −1.7 to +1.0 ‰ (n = 43), −12.0 to +30.9 ‰ (n = 35), and −2.4 to + 27.5 ‰ (n = 38), respectively, with distinct changes across the transect consistent with a sedimentary pore fluid component in the outer forearc, fluids sourced from dehydration of the slab (sediment and altered oceanic crust) in the forearc, and minimal slab input near the arc. Variations in elemental ratios (Br/Cl, I/Cl, B/Cl, Li/B) across the transect are also largely consistent with evolving FME sources during subduction. Boron isotope ratios in some higher-temperature fluids are modified due to phase separation, whereas, lithium and chlorine isotope ratios are minimally modified. Springs located in the outer forearc along a major fault (Falla Morote) deviate from the consistent across-arc trend and record almost the full range of observed isotopic values. This extreme range in values along the fault likely represents a combination of adsorption/desorption effects, weathering, and mixing with crustal fluids. Mass balance calculations show that up to ∼ 70 % of Cl, ∼50 % of Br, ∼30 % of B, ∼15 % of I, and minimal Li may be recycled through the outer forearc and forearc springs of the Costa Rican margin, therefore fluid emission via springs in global forearc regions are a potentially significant output for FME at convergent margins. [ABSTRACT FROM AUTHOR]

Sound velocities and densities of Al‐bearing superhydrous phase B (Al‐bearing SuB) were investigated up to 24 GPa and 1300 K by synchrotron X‐rays combined with ultrasonic interferometry techniques in a multi‐anvil apparatus. We found that Al + H incorporation decreases the adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. The presence of hydrous pyrolite with ∼10 wt.% Al‐bearing SuB in cold subducting slabs could explain up to ∼2% high velocity anomalies at the bottom of the mantle transition zone (500–660 km) while it turns into ∼7.7% low velocity anomalies below 660 km. Al‐bearing SuB is reportedly stable at mantle temperatures, where it could account for ∼12.2% low velocity anomalies beneath subduction zones in the uppermost lower mantle. Plain Language Summary: Among dense hydrous magnesium silicates, Superhydrous phaseB (SuB) is believed to hold large amount of water and transport it to the deep mantle. Recent studies showed that Al can largely increase the thermal stability region of SuB, suggesting this phase may also be an important carrier of water in the mantle transition zone (MTZ) and uppermost lower mantle (ULM). Here we investigated the sound velocities of Al‐bearing SuB up to 24 GPa and 1300 K using ultrasonic interferometry combined with synchrotron X‐ray techniques in a multi‐anvil apparatus. We found that Al + H incorporation decreases adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. Based on our elasticity data, we calculated velocity of a hydrated pyrolite composition, containing up to 10% of Al‐SuB along temperature profiles corresponding to cold slab and hot mantle at the depths of the MTZ and the ULM. We found that the presence of hydrous pyrolite may explain high velocity anomalies up to 660 km depth at MTZ while low velocity anomalies appear below 660 km along cold slab geotherm. Nevertheless, low velocity anomalies are generated in the whole depth range along a normal geotherm. Key Points: We constrained the Al + H incorporation on sound velocities of superhydrous phase B (SuB) up to 24 GPa and 1300 KAl + H incorporation which promoted by increased temperature significantly affect velocities of SuBAccumulation of Al‐bearing SuB can explain velocity contrast with various magnitude owing to the change of Al + H content with temperature [ABSTRACT FROM AUTHOR]

Ultrahigh-temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900°C. The duration and heat source of UHT metamorphism are critical constraints on the tectonic evolution of orogenic systems. Here, we report the discovery of a sapphirine-bearing granulite from the east-central Himalaya which preserves UHT evidence. The reconstructed pressure-temperature-time path indicates that the temperature increased by almost 200°C within ~2 Ma which is consistent with rapid asthenospheric heat input. Numerical simulations illustrate potential mechanisms for such heating: juxtaposition of the deep crustal accretionary system with the upwelling asthenospheric mantle through newly developed apertures generated by slab break-off and/or associated vertical tearing of down-going Indian lithosphere. Spatial-temporal consistencies among the UHT metamorphic phases, postcollisional magmatism, geophysical constraints, and crustal deformation indicate that slab break-off or tearing controls broad swaths of Himalayan tectonics. The consequent upwelling asthenosphere may have been a significant heat source for the Miocene Himalaya and for similar ancient collisional orogenic systems. [ABSTRACT FROM AUTHOR]

In this study, we have determined the single‐crystal elasticity of clinohumite [Mg8.85Ti0.19Si3.93O16(OH1.11F0.89)] using Brillouin measurement up to 21 GPa at 300 K and 1 bar at 750 K, respectively. The elasticity of clinohumite was determined to be KS0 = 126.2(3) GPa, G0 = 79.7(2) GPa with pressure derivatives KS′ = 4.2(1), G′ = 1.3(1), pressure derivatives ∂KS/∂T = −0.024(1) GPa/K, and ∂G/∂T = −0.011(1) GPa/K). We comprehensively examined the effects of varying H2O, fluorine content and thermal states, on the velocity and density structures of the subducted harzburgite layer. Assuming a typical H2O content of 2 wt.% within harzburgite, our modeling has shown that hydrous harzburgite with clinohumite as the decomposition product of serpentine along a hot slab geotherm even has the VP and VS 0.4–0.8(6)% greater than it dry counterpart at 250–380 km depth. Yet in the top transition zone, the addition of H2O and F can effectively lower the sound velocities and density. The F‐bearing hydrous harzburgite has the VP and VS 1.1(5)–1.3(3)% lower than its dry counterpart, and only 0.6(5)% and 2.3(5)% greater than the pyrolitic mantle. Along cold slab geotherm, phase A will replace clinohumite as the dominant hydrous phase in the harzburgite, the VP and VS are 4.8(5)–5.3(3)% and 5.9(5)–6.0(3)% greater than the pyrolitic mantle in the upper mantle. In the top transition zone, the difference is approximately 3% in VP and 5% in VS. Our results provide crucial experimental evidence for future assessments of the seismic signals of subducted slabs with different hydrous minerals and thermal states. Plain Language Summary: Subducted slab plays a crucial role in connecting the Earth's surface to the interior. Transportation of H2O and other important volatiles to the Earth's mantle via subducted slab can greatly influence a series of physical properties of mantle minerals. Clinohumite is one of the most important H2O and fluorine (F) carriers in the subducted slab. Here, we determined the single‐crystal elasticity of F‐bearing clinohumite at high pressures and temperatures. The obtained elasticity data were applied to model the velocity profiles of the hydrous harzburgite. Our results show that the addition of H2O and F can enhance the velocity difference between harzburgite and pyrolitic mantle at 250–380 km depth along the hot slab geotherm, and VP and VS of harzburgite with 2 wt.% H2O and clinohumite as the H2O‐bearing phase are 0.4–0.8(6)% greater than those of dry harzburgite. In the transition zone depth, addition of H2O and Fe lowers the sound velocities of harzburgite. Hydrous harzburgite has a lower VP and VS than its corresponding dry phase. These new velocity constraints can help us better understand the circulation of H2O and F within Earth's interior and identify the detected seismic observations of subducted slabs with different compositions and thermal states. Key Points: Single‐crystal elasticity of F‐bearing clinohumite was determined to 21 GPa and 750 K and shows strong dependence with F contentThe obtained results were used to model the velocity profiles of the hydrous harzburgite layer considering the thermal state and compositionH2O and F in clinohumite have significant influence on the velocity profile of subduction slab [ABSTRACT FROM AUTHOR]

The far limit of a plate subduction process and its related far‐field dynamic process are fundamental topics for plate tectonics. The Great Khingan Range (GKAR) in the western flank of NE China is currently under the far‐field influence of the Pacific plate subduction. Benefiting from the newly deployed seismic arrays in the northern GKAR, we take full advantage of seismic data from both temporary and permanent networks and employ an improved scheme of the H‐κ stacking method by introducing surface wave dispersion to obtain the integrated maps of Moho depth and crustal bulk Vp/Vs ratio in this region. Our results show that the Moho depth has a giant step near the North–South Gravity Lineament (NSGL). Meanwhile, the distribution of bulk Vp/Vs ratio presents a north–south difference roughly by 50°N, where the south subregion consists of a collage of high and low Vp/Vs ratio; by contrast, the north subregion is characterized by unified high values. The east‐west structural discrepancies across the NSGL from the Earth's surface to the mantle transition zone indicate the difference in the strength of modifications between the near and far‐fields from the Pacific plate subduction. The complicated distribution of the Vp/Vs ratio in the south subregion supports that secondary small‐scale upwellings underneath the Cenozoic volcanic groups dominate the tectonic reworking in this area. The lack of Cenozoic volcanism and a more straightforward distribution of the Vp/Vs ratio in the north subregion might allude to a tectonically inactive part of NE China. Plain Language Summary: We use an improved H‐κ stacking method to construct an integrated map of Moho depth and crustal bulk Vp/Vs ratio across the Great Khingan Range (GKAR). Our results show a significant step in the Moho depth near the North–South Gravity Lineament, which probably marks the boundary between near‐ and far‐field locations from the subduction. We also identify a prominent discrepancy in the crustal bulk Vp/Vs ratio between the south and north subregions of the GKAR. The south subregion is characterized by a collage of high and low Vp/Vs ratio, while the north subregion has a much more straightforward pattern. The localized high Vp/Vs ratio in the south subregion coincides with the Cenozoic volcanism, which tells us the far‐filed effects of the subduction are dominated by a secondary small‐scale mantle upwelling. In contrast, the unified high Vp/Vs ratio and no sign of Cenozoic magmatism in the south subregion may indicate a tectonically inactive region in NE China. The unique mechanism of the far‐field effects and the particular location of the north subregion of the GKAR can explain how the north–south difference in crustal property is formed. Key Points: A new integrated map of Moho depth and bulk Vp/Vs ratio across the Great Khingan Range (GKAR) is built with an improved H‐κ stacking methodThe far‐filed effects of the stagnant Pacific slab on the GKAR are most likely achieved by a small‐scale secondary mantle upwellingThe north subregion of the GKAR is tectonically inactive, which may have retained its crustal state since the Paleozoic Era [ABSTRACT FROM AUTHOR]

Recent observations beneath Kanto, Japan have shown that seismic activity and seismic attenuation within the overlying continental plate change with time due to drainage caused by slow‐slip events (SSEs) along the upper boundary of the Philippine Sea plate. However, associated changes in rock properties have not been investigated. In this study, we estimate frequency‐dependent shear‐wave anisotropy to provide a detailed insight into the structural change associated with drainage. We perform shear‐wave splitting analysis in frequency ranges of 1–4, 2–6, and 4–8 Hz for 306 earthquakes that occur during September 2009–August 2021 and recorded at the Metropolitan Seismic Observation network. Obtained time differences between fast and slow S waves (delay time) range from almost zero to 0.16–0.18 s, exhibiting spatio‐temporal variation and frequency dependence. The fast S‐wave polarization directions are generally consistent with the direction of the maximum horizontal compressional axis in the study region, which suggests that the observed anisotropy is probably caused by the NE–SW‐oriented fractures developed under the regional stress field. The temporal variation in delay times is correlated with SSEs activity with a lag time of 0.0–0.1 year. Furthermore, comparisons between observed frequency‐dependent delay times and numerical calculation of fracture‐induced anisotropy suggest that the average fracture radius is almost constant (0.30–0.35 m) over time but fracture density temporally varies from 0.025 to 0.035. We infer that the fracture density is probably enhanced by opening of the NE–SW‐oriented fractures during the upward migration of fluids that are expelled from the plate interface. Plain Language Summary: Measurement of shear‐wave polarization anisotropy characterizes the intensity and orientation of fractures within the rocks beneath the Earth's surface. Furthermore, its frequency dependence provides important information for determining the fracture size and density. This study reveals that the strength of anisotropy shows temporal variations associated with the inferred periodic drainage from the subducting slab. We discuss a possible mechanism of the structural change focusing on the frequency dependence of the observed anisotropy and propose that the opening of fractures occurs immediately above the subducting slab following the periodic drainage. The obtained results will provide important constraints on fluid‐rock interaction above the subducting plate interface. Key Points: Shear‐wave splitting analysis in Kanto, Japan suggests that seismic anisotropy is orientated sub‐parallel to the regional stressAnisotropy above the plate interface is enhanced with a lag of 0.0–0.1 years from the occurrence of slow‐slip eventsThe observed frequency‐dependent anisotropy suggests that fractures open responding to drainage from plate interface [ABSTRACT FROM AUTHOR]

Anatolia is the global archetype of tectonic escape, as witnessed by the devastating 2023 Kahramanmaraş Earthquake sequence, and the 2020 Samos Earthquake, which show different kinematics related to the framework of the escape tectonics. Global Positioning System (GPS) motions of the wedge-shaped plate differ regionally from northwestwards to southwestwards (from east to west). Anatolia was extruded westward from the Arabian-Eurasian collision along the North and East Anatolian fault systems, rotating counterclockwise into the oceanic free-faces of the Mediterranean and Aegean, with dramatic extension of western Anatolia in traditional interpretations. However, which is the dominant mechanism for this change in kinematics, extrusion related to the Arabia/Eurasia collision or rollback of the African slab beneath western Anatolia is still unclear. To assess the dominant driving mechanisms across Anatolia, we analyze recent GPS velocity datasets, and decomposed them into N-S and E-W components, revealing that westward motion is essentially constant across the whole plate and consistent with the slip rates of the North and East Anatolia fault zones, while southward components increase dramatically in the transition area between central and western Anatolia, where a slab tear is suggested. This phenomenon is related to different tectonic driving mechanisms. The Arabia-Eurasia collision drives the Anatolian Plate uniformly westwards while western Anatolia is progressively more affected by the southward retreating African subducting slab west of the Aegean/Cypriot slab tear, which significantly increases the southward component of the velocity field and causes the apparent curve of the whole modern velocity field. The 2020 and 2023 earthquake focal mechanisms also confirm that the northward colliding Arabian Plate forced Anatolia to the west, and the retreating African slab is pulling the upper plate of western Anatolian apart in extension. We propose that the Anatolian Plate is moving westwards as one plate with an additional component of extension in its west caused by the local driving mechanism, slab rollback (with the boundary above the slab tear around Isparta), rather than separate microplates or a near-pole spin of the entire Anatolian Plate, and the collision-related extrusion is the dominant mechanism of tectonic escape. [ABSTRACT FROM AUTHOR]

The origin of volcanism along the Kuril–Kamchatka Island Arc (KKIA) was analyzed. Geophysical observations show variations in the slab parameters. Different widths of the volcanic belt in the northern and southern parts of the KKIA are caused by changes in the slab dip angle. The rift system of the Bussol Strait may be generated by significant changes in the slab velocity in the central segment of the KKIA. We proposed that the back-arc basin plays a predominant role in the formation of various parameters of the slab and manifestations of volcanism along the KKIA. [ABSTRACT FROM AUTHOR]

Aqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high‐pressure experiments and natural‐case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high‐pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in‐situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile‐brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite‐blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite‐bearing eclogites imply interaction with an externally‐derived fluid, whereas rare phengite‐free, kyanite‐eclogites only record internally‐derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non‐random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point‐like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak‐pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile‐brittle deformation of eclogite in a subduction setting. [ABSTRACT FROM AUTHOR]

In the Dom Feliciano Belt, Brazil, the Pinheiro Machado Complex (PMC) includes diorites, tonalites, granodiorites, syenogranites and granites, whose evolution is related to several magmatic pulses and complex petrogenetic processes. Two magmatic stages were identified (early and late), resulting in different rock subgroups. The geochemical data showed that the early magmatism was chemically affected by partial melting. Geochemical modelling results suggest that fractional crystallization processes with assimilation of around 40% from the crustal basement and the decoupling of assimilated magma are crucial for the PMC rocks' genesis. Geochemical data also show that during the early magmatism, the subsequent process of early diorite anatexis developed by heating and continuous activity of the underlying magma chamber possibly occurred at a melting rate of 5%–10%. The hybrid rocks have contributions from the mixing process related to early terms, showing geochemical correlations in the major element curves, for the early diorite and syenogranitic melt members, at 60%–50% and 50%–40%, respectively. Slab failure tectonic context is related to the multi‐intrusive events dynamics recorded in the studied rocks. Recharge and melting events of the recently formed crust due to the constant heating of new pulses of deep slab melting would explain the magmatic interactions observed in the Complex. The results demonstrate that the studied rocks crystallized in an open system, including mixing processes to form hybrid rocks, physical disaggregation and assimilation of early intrusions, truncation, dragging and erosion of early mushes by younger pulses. [ABSTRACT FROM AUTHOR]

Increasing evidence from seismic methods shows that anisotropy within subduction zones should consist of multiple layers. To test this, we calculate and model shear wave splitting across the Alaska-Aleutians Subduction Zone (AASZ), where previous studies have argued for separate layers of anisotropy in the subslab, slab and mantle wedge. We present an updated teleseismic splitting catalogue along the span of the AASZ, which has many broad-band seismometers recently upgraded to three components. Splitting observations are sparse in the Western Aleutians, and fast directions are oriented generally trench parallel. There are significantly more splitting measurements further east along the AASZ. We identify six regions in the Central and Eastern Aleutians, Alaskan Peninsula and Cook Inlet with a high density of splits suitable for multilayered anisotropy analyses. These regions were tested for multilayer anisotropy, and for five of the six regions we favour multiple layers over a single layer of anisotropy. We find that the optimal setup for our models is one with a dipping middle layer oriented parallel to palaeospreading. A prominent feature of our modelling is that fast directions above and below the dipping layer are generally oriented parallel to the strike of the slab. Additionally, we lay out a framework for robust and statistically reliable multilayer shear wave splitting modelling. [ABSTRACT FROM AUTHOR]

The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local- S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local- S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local- S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local- S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike. [ABSTRACT FROM AUTHOR]

Subducted slabs have been detected in the lower mantle for almost 30 years, yet the presence of foundered cratonic segments in the lower mantle is still unclear and inadequately investigated. We present the first P-wave radial anisotropy tomography of southern Africa (our model SA-RAnis2024), which reveals a contrasting feature of preserved northwest and modified southeast Kalahari cratonic root. Segments from the modified cratonic lithosphere are inferred to have dropped into the shallow lower mantle where seismic evidence of isolated high-velocity anomalies are observed. We detect such a high-velocity anomaly under the southwest margin of the Kalahari craton, which possibly detached from the southeast Zimbabwe craton at ca. 60 Ma based on plate reconstructions. Foundered segments can be partially brought back up to shallow depths, and contribute to the geochemical heterogeneity of younger lithosphere, through large-scale mantle convection. [ABSTRACT FROM AUTHOR]

Unique intraplate volcano eruptions and westward volcano migration since the Oligocene have been observed in northeast China, where an overriding continental zone is tectonically controlled by the subduction of the northwestern Pacific plate and the opening of Japan Sea. Interestingly, these intraplate magmatic events occur around a subsiding basin (the Songliao Basin), but no volcanic activity has been observed within the Songliao Basin. The geodynamic mechanism responsible for these volcanoes remains unclear. To address the geodynamic process beneath northeast China, we conducted numerical models constrained by data from regional reconstruction and seismic and volcanic studies. The vertical velocity field of mantle convection and lithospheric partial melting structures derived from our models show that mantle upwelling and melting centers migrate from east to west in northeast China with the westward propagation of the sub-horizontal slab in the transition zone, leading to the observed volcano migration. Also, with the subduction retreat of the northwestern region of the Pacific plate and the opening of the Japan Sea, significant lithospheric thickness differences developed between the Changbaishan-Mudanjiang region and the Songliao Basin, leading to lithospheric unstable dripping. This dripping structure prevents the partial melting of the lithosphere but facilitates the subsidence of the Songliao Basin. Moreover, the lithospheric dripping model successfully predicts upper mantle structures consistent with the proposed tomography model, the observed Moho depth, and surface topography variations. Thus, lithospheric dripping induced by lithospheric thickness differences and the retreating subduction of the Pacific slab provides a robust mechanism for the unique geodynamic processes in northeast China. [ABSTRACT FROM AUTHOR]

The subcontinental lithospheric mantle (SCLM) is now suggested by many workers to play a role in the formation of orogenic gold deposits in active continental margins, given that the gold and volatiles may be introduced into the SCLM during plate subduction. The giant Cretaceous Jiaodong gold province within the North China block occurs in a convergent margin setting where there is no terrane accretion, thus eliminating from consideration the commonly accepted metal and fluid source reservoir for orogenic gold--the metamorphosed upper crust of an active orogenic belt. Thus, the auriferous fluids were released below the high-grade Archean cratonic rocks from either enriched SCLM or directly from the subducted oceanic slab. Mercury (Hg) isotopes allow fingerprinting of isotopically distinct reservoirs, discriminating between the two possibilities. We present Hg isotope data for a set of pyrite grains, native gold, and lamprophyres from six main gold districts in the Jiaodong gold province. These samples dominantly yielded near-zero Δ199Hg (0‰ ± 0.1‰), within uncertainty of the mantle Δ199Hg value. The results provide novel evidence that the SCLM predominantly controlled the Hg budget of the Jiaodong gold deposits, implying that the SCLM acted as a buffer for the crustal mass-independent fractionation Hg signatures and a storage reservoir for gold and volatiles. Consequently, during slab subduction, gold and volatiles were recycled and stored within the SCLM and were later released from the mantle during uplift and relaxation of compressional tectonics. [ABSTRACT FROM AUTHOR]

The Bayingobi basin is located in the middle of Central Asia Orogenic Belt, at the intersection of Paleo‐Asian Ocean and Tethys Ocean, as well as the junction of multiple tectonic plates. This unique tectonic setting underpins the basin's intricate history of tectonic activity. To unravel the multifaceted tectono‐thermal evolution within the southwestern region of the basin and to elucidate the implications of sandstone‐hosted uranium mineralization, granitic and clastic rock samples were collected from the Zongnai Mts. uplift and Yingejing depression, and apatite fission track (AFT) dating and thermal history simulation analysis were performed. AFT dating findings reveal that the apparent ages of all samples fall within the range of 244 Ma to 112 Ma. In particular, the bedrock of the Zongnai Mts. and Jurassic detrital apatite fission tracks have undergone complete annealing, capturing the uplift‐cooling age. Meanwhile, the AFT ages of Cretaceous detrital rocks are either equivalent to or notably exceed the age of sedimentary strata, signifying the cooling age of the provenance. A comprehensive examination of AFT ages and palaeocurrent direction analyses suggests that the Cretaceous source in the Tamusu area predominantly originated from the central and southern sectors of the Zongnai Mts. uplift. However, at a certain juncture during the Late Early Cretaceous, the Cretaceous provenance expanded to include the northern part of the Zongnai Mts. uplift. Based on the results of thermal history simulations and previous studies, it is considered that the Tamusu area has undergone four distinct tectonic uplift events since the Late Paleozoic. The first is the Late Permian to Early Triassic (260–240 Ma), which is associated with the closure of the Paleo‐Asian Ocean and the accretionary orogeny within the Alxa region. The second uplift event took place in the Early Jurassic (190–175 Ma) and corresponded to intraplate orogeny following the closure of the Paleo‐Asian Ocean. The third uplift event is the Late Jurassic to Early Cretaceous (160–120 Ma), which is linked to the East Asia's position as the convergence center of multiple tectonic plates during this period. The fourth uplift event is linked to the Late Early Cretaceous (112–100 Ma), driven either by the westward subduction of the eastern Pacific plate or the mantle upwelling resulting from the Bangong–Nujiang oceanic lithosphere subduction and slab break‐off. The primary stress orientation for the first three tectonic uplift phases approximated a nearly SN direction, while the fourth stage featured a principal stress direction of NW. The fourth tectonic uplift event of the Late Early Cretaceous and basaltic eruption thermal event during this period likely exerted a significant influence on the formation of the Tamusu sandstone‐hosted uranium deposit. [ABSTRACT FROM AUTHOR]

It is well established that Cretaceous magmatism in the South China Block (SCB) is related to the Paleo‐Pacific subduction. However, the starting time and the associated deep crust‐mantle processes are still debatable. Mafic dike swarms carry important information on the deep earth (including mantle) geodynamics and geochemical evolution. In the Jiangnan Orogen (South China), there is no information on whether the Mesozoic magmatic activities in this region are also directly related to the Pacific subduction or not. In this study, we present detailed zircon U‐Pb geochronological, whole‐rock element and Sr‐Nd isotope data for Early Cretaceous Tuanshanbei dolerite dikes, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA‐ICP‐MS zircon U‐Pb dating suggests that this dolerite erupted in the Early Cretaceous (∼145 Ma). All samples have alkaline geochemical affinities with K2O + Na2O = 3.11–4.04 wt%, K2O/Na2O = 0.50–0.72, and Mg# = 62.24–65.13. They are enriched in LILE but depleted in HFSE with higher initial 87Sr/86Sr ratio (0.706896–0.714743) and lower εNd(t) (–2.61 to –1.67). They have high Nb/U, Nb/La, La/Sm and Rb/Sr, and low La/Nb, La/Ta, Ce/Pb, Ba/Rb, Tb/Yb and Gd/Yb ratios. Such geochemical signatures suggest that the fractional crystallization is obvious but crustal contamination play a negligible role during magmatic evolution. Tuanshanbei dolerite were most likely derived from low‐degree (2%–5%) partial melting of a phlogopite‐bearing mantle material consisted of ∼85% spinel peridotite and ∼15% garnet peridotite previously metasomatized by asthenosphere‐derived fluids/melts with minor subduction‐derived fluids/melts. Slab‐rollback generally lead to the upwelling of the hot asthenosphere. The upwelling of asthenosphere consuming the lithospheric mantle by thermo‐mechanical‐chemical erosion. The lithospheric mantle may have partially melted due to the heating by the upwelling asthenosphere and lithospheric extension. It is inferred that the Tuanshanbei dolerite might be associated with the initial slab rollback and corresponding lithospheric extension occurred potentially at ca. 145 Ma. [ABSTRACT FROM AUTHOR]

The westernmost Nankai Trough, southwest Japan, exhibits a rapid along-strike reduction in plate coupling in the proximity to the subducting Kyushu-Palau ridge. Yet how and to what extent the ridge subduction impacts physical properties at the megathrust have not been investigated. Here we present high-resolution seismic P-wave velocity models along the forearc wedge in the western Nankai Trough derived from full-waveform inversion analyses of seismic refraction data. The velocity models show that where the plate coupling is weak and the plate boundary presumably hosts slow earthquakes, the upper plate exhibits lower seismic velocities indicating higher degree of fracturing over a ~ 100 km length along trough. Intriguingly, the extent of the upper-plate low-velocity features is significantly larger than the surficial width of the Kyushu-Palau ridge, and this low-velocity zone is underthrust by the slab with increased crustal thickness by 2–4 km. Seismic reflection images consistently reveal that the thicker slab crust has appreciable basement roughness extending ~ 60 km from the eastern margin of the Kyushu-Palau ridge beneath the western Shikoku basin. We suggest that such a thicker and rugged slab crust, together with the main body of the Kyushu-Palau ridge, can cause significant fracture zones in the overriding plate, decrease the interplate coupling and produce preferable conditions for shallow slow earthquakes to occur when subducted. The results may also provide structural constraints on the western limit of future megathrust earthquakes in the Nankai Trough. [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]

Numerical models of subduction commonly use diffusion and dislocation creep laws from laboratory deformation experiments to determine the rheology of the lithosphere. The specific implementation of these laws varies from study to study, and the impacts of this variation on model behavior have not been thoroughly explored. We run simplified 2D numerical models of free subduction in SULEC, with viscoplastic slabs following (1) a diffusion creep law, (2) a dislocation creep law, and (3) both simultaneously, as well as several variations of model 3 with reduced resistance to bending. We compare the results of these models to a model with a constant-viscosity slab to determine the impact of the implementation of different lithospheric flow laws on subduction dynamics. In creep-governed models, higher subduction velocity causes a longer effective slab length, increasing slab pull and asthenospheric drag, which, in turn, affect subduction velocity. Numerical and analogue models implementing constant-viscosity slabs lack this feedback but still capture morphological patterns observed in more complex models. Dislocation creep is the primary deformation mechanism throughout the subducting lithosphere in our models. However, both diffusion creep and dislocation creep predict very high viscosities in the cold core of the slab. At the trench, the effective viscosity is lowered by plastic failure, rendering effective slab thickness the primary control on bending resistance and subduction velocity. However, at depth, plastic failure is not active, and the viscosity cap is reached in significant portions of the slab. The resulting high slab stiffness causes the subducting plate to curl under itself at the mantle transition zone, affecting patterns in subduction velocity, slab dip, and trench migration over time. Peierls creep and localized grain size reduction likely limit the stress and viscosity in the cores of real slabs. Numerical models implementing only power-law creep and neglecting Peierls creep are likely to overestimate the stiffness of subducting lithosphere, which may impact model results in a variety of respects. [ABSTRACT FROM AUTHOR]

Ahi Tupua, the central section of the Taupō Volcanic Zone in the central North Island of New Zealand, encompasses Taupō and Ōkataina calderas and has been the most frequently active and productive silicic magma system worldwide during the Quaternary. The entire Taupō Volcanic Zone is underlain by the Hikurangi Subduction Zone where a Large Igneous Province, the Hikurangi Plateau, is being subducted, but Ahi Tupua exhibits much higher rates of magma output than either the northern or the southern sections. Trace element signatures of Ahi Tupua eruptive products suggest that both decompression melting and flux melting play a role in this rifted arc setting, with fluid flux signatures being more prevalent in regions of active caldera volcanism. Intermediate‐depth (50–300 km) earthquakes provide a means of studying faulting and fluid flow processes within and around the subducting slab beneath Ahi Tupua. Using data from the national seismic network and a 13‐station temporary network ("ECLIPSE"), we repick and relocate 397 intermediate‐depth earthquakes of magnitudes M2.5+ that occurred in a 29‐month interval, and compute focal mechanisms for a subset of 47 earthquakes. We observe some seismicity that may be within the mantle wedge but most of the relocated earthquakes occur within the crust and uppermost mantle of the subducted slab and exhibit a weak transition from predominantly normal‐faulting in the slab crust to predominantly reverse‐faulting in the slab mantle. No double seismic zone is observed beneath Ahi Tupua. These observations are consistent with dehydration of the slab and flow of slab‐derived fluids along existing faults into the mantle wedge, that drives flux melting and accounts for the distinctive geochemical signatures and voluminous output of Ahi Tupua calderas. Plain Language Summary: Ahi Tupua is a volcanic region in the North Island of New Zealand that is the most active silicic volcanic system on Earth. It includes the active caldera volcanoes Taupō and Ōkataina. This volcanic region lies above the Hikurangi Subduction Zone, and studies of past eruptions suggest that Ahi Tupua's high rate of activity is driven by fluids released deep in the subduction zone. In this study, we use seismic data from across the entire North Island to investigate earthquakes occurring at depths of 50–300 km within the Hikurangi Subduction Zone. We show that these earthquakes suggest that the subducting oceanic crust is releasing a substantial amount of water into the overlying mantle, causing a large amount of melting and, ultimately, the distinctive volcanism of Ahi Tupua. Key Points: We relocate 397 intermediate‐depth earthquakes beneath Ahi Tupua, central Taupō Volcanic Zone, and compute 47 focal mechanismsMost seismicity occurs within the slab crust and mantle, with subordinate mantle wedge and plate interface faultingHypocenters and focal mechanisms are consistent with slab‐derived fluid fluxing melting of the mantle wedge beneath Ahi Tupua [ABSTRACT FROM AUTHOR]

Subduction of carbonate‐bearing oceanic plates into Earth's interior recycles carbon from the surface to the deep mantle. The subducted carbonates can significantly affect mantle properties and dynamics. Magnesite is recognized as one of the major potential carbon hosts in the deep mantle because of its stability up to deep lower mantle conditions. However, despite many previous studies on the equation of state and elastic properties of magnesite, large discrepancies still exist for its elastic moduli and their pressure and temperature derivatives. Here we report in situ density and elastic wave velocity measurements on a natural magnesite at simultaneous high pressure‐temperature conditions up to ∼8 GPa–1073 K in a multi‐anvil apparatus using ultrasonic and synchrotron X‐ray techniques. Global fitting of the data set to finite strain equations yields KS0 = 114.0 ± 1.2 GPa, KS′ ${K}_{S}^{\prime }$ = 4.9 ± 0.3, ∂KS∂TP ${\left(\frac{\partial {K}_{S}}{\partial T}\right)}_{P}$ = (−0.019 ± 0.002) GPa/K, G0 = 68.6 ± 0.4 GPa, G′ = 1.6 ± 0.1, and ∂G∂TP ${\left(\frac{\partial G}{\partial T}\right)}_{P}$ = (−0.018 ± 0.001) GPa/K. Compared to other major upper mantle and transition zone minerals, magnesite has intermediate VP, the lowest VS, and the lowest density. Thus, magnesite possesses higher VP/VS ratio than other mantle minerals in normal mantle regions, whereas this feature is less pronounced in subduction zone environments. Modeling of the velocity profiles of carbonated lithologies along different geotherms suggests that moderately‐enriched magnesite domains are unlikely to be detected seismically in the Earth's mantle. Plain Language Summary: Carbon can be transported into Earth's interior by subducting slabs in the form of carbonates. Magnesite (MgCO3) is recognized as one of the most important oxidized carbon hosts inside the Earth. Accurate knowledge of the thermoelastic properties of magnesite at Earth's mantle conditions is thus needed in order to evaluate its seismic detectability and effect on mantle seismic structure and dynamics. Despite many previous efforts in determining the properties of magnesite, large discrepancies still exist for its elastic moduli and their pressure and temperature derivatives. In this study, we determined both the density and elastic wave velocity of magnesite at high pressure and temperature conditions up to ∼8 GPa–1073 K using in situ ultrasonic measurements combined with synchrotron X‐ray techniques in a multi‐anvil press. Our high‐quality data tightly constrained the thermoelastic parameters of magnesite, which were then used to model the velocity and density of magnesite and magnesite‐bearing rocks in the Earth's mantle along different geotherms. Our results show that despite the fact that magnesite has the lowest VS compared to other major mantle minerals, moderately‐enriched magnesite domain is difficult to detect in the Earth's mantle based on its seismic velocity. Key Points: In situ density and elastic wave velocity measurements on magnesite at simultaneous high P‐TThe thermoelastic properties of magnesite are tightly constrained by high‐quality P‐ρ‐T‐VP‐VS data setModerately‐enriched magnesite domains are difficult to detect seismically in the Earth's mantle [ABSTRACT FROM AUTHOR]

The North Sulawesi subduction zone is characterized by southward subduction of the Celebes Sea slab to a depth of ∼250 km, mainly overlying the Sangihe slab that subducts west from the Molucca Sea and penetrates the mantle transition zone. The palaeo‐subducted Sula slab dips northward and partially underlies both the Sangihe and Celebes Sea slabs. Adjacent subduction zones with horizontal overlapping subducting slabs in the upper mantle have unclear dynamic interactions. An extensive strike‐slip fault forms the western boundary of the active North Sulawesi subduction zone, providing an ideal setting to study mantle flow between overlapping slabs. We use local S‐wave and teleseismic S and SK(K)S waveform splitting analysis to measure seismic anisotropy in the northern Sulawesi region. Our observations reveal typical mantle wedge corner flow within the Sangihe subduction system. In the Gulf of Tomini, the observed trench‐oblique fast‐axis orientations above the Celebes Sea slab are likely a consequence of the interaction between two subducting slabs. The southernmost measurement with an E–W‐trending fast direction in the mantle wedge might be related to the subduction of the Sula slab. Furthermore, fault‐parallel fast‐axis orientations of anisotropy near the southern segment of the Palu‐Koro fault are attributed to large‐scale shearing across this lithospheric‐scale strike‐slip fault system. Overall, our observations suggest that the strain caused by lithospheric and asthenospheric deformation is mainly confined within the microplate, displaying a restricted flow pattern and localized effects due to the size of the plate boundaries, such as the Palu‐Koro fault. Plain Language Summary: North Sulawesi is bordered by the Makassar Strait, Celebes Sea, Molucca Sea, and Gulf of Tomini and hosts the Palu‐Koro fault. The region hosts two active subduction zones: the smaller, southward dipping North Sulawesi subduction zone and the larger westward dipping Sangihe subducting slab. Another fossilized submerged slab –Sula slab lies beneath both the Celebes Sea and Sangihe slabs to the south. Thus, it is an ideal area to investigate how the interaction between neighboring subduction zones can affect the upper‐mantle deformation and whether a strike‐slip fault could contribute to the mantle dynamics. We use shear‐wave splitting analysis to measure seismic anisotropy to reveal the lithospheric deformation and upper mantle flow patterns of this interactive area. Our results show that the Celebes Sea slab controls mantle wedge dynamics in the North Sulawesi subduction zone, and the interaction between the Celebes Sea and Sangihe slabs occurs where two slabs entirely overlap. The strike‐slip fault at the western boundary only appears to have a local influence on the regional strain field in western Sulawesi. Key Points: We employ shear‐wave splitting analysis to characterize the seismic anisotropy to infer upper‐mantle deformation in north SulawesiWe find evidence for slab‐slab dynamic interactions and constrain their influencing spatial rangeThe Palu‐Koro fault marks the west limit of lithospheric deformation imposed by the complex subduction system [ABSTRACT FROM AUTHOR]

The SS precursors have been extensively utilized in mapping the mantle transition zone (MTZ). However, their applications are often challenged by weak phases that arise from small impedance contrasts of the mantle discontinuities, noise contamination, and localized thermal/compositional heterogeneities. We develop a new data processing workflow for more reliable MTZ imaging by adopting the recently proposed robust damped rank‐reduction (RDRR) method from exploration seismology. This method exploits the signal coherency in the four‐dimensional data and allows simultaneously attenuating noise and interpolating missing traces. We utilize synthetic data sets generated with realistic earth structures and MTZ topography to evaluate the capability of the proposed workflow. Our test results show that the RDRR method can well capture the topographic variation of mantle discontinuities, improving the SNR of the SS precursor data by an order of magnitude. The application to SS precursors from the western Pacific successfully removes contaminating noises, mitigates imaging artifacts of small‐scale anomalies, and improves the lateral coherency of the MTZ structure, revealing a clear first‐order structural transition. Compared with earlier global and regional models, our model reveals more structural details including a localized thin MTZ near the Changbai volcano in East Asia. This observation, in conjunction with the reported low‐velocity structure in the MTZ in earlier tomographic studies, may support the presence of deep mantle up‐welling through a slab gap. In summary, our work enables resolving MTZ structures with high fidelity and highlights the importance of advanced array methods in improving SS precursor imaging. Plain Language Summary: Underside S wave reflections (SS precursors) have placed important observational constraints on the thermal and chemical processes in the Earth's mantle transition zone. Despite continuing attempts at enhancing the acquisition design, network irregularity still produces incomplete and noisy SS precursor data, which poses significant challenges to the conventional processing method. Traditional processing schemes stack SS precursors using large (e.g., 10°) bins to provide a sufficient amount of traces to improve the data quality and the detection of mantle interfaces. However, when the SS precursor data are incomplete and contaminated by strong noises, the stacking process becomes less adequate in recovering useful signals. Therefore, we introduce a novel SS precursor imaging workflow based on a recently developed multi‐dimensional data processing method from exploration seismology. The proposed processing scheme takes advantage of signal coherency in a higher dimensional space, thus allowing us to simultaneously reduce noise and interpolate missing traces. We demonstrate the effectiveness of our method using synthetic and real SS precursors collected from the western Pacific subduction zone. A comprehensive analysis of the imaging results shows that the proposed workflow improves the reliability of the MTZ structures compared with the conventional stacking method. Key Points: We adopt the robust damped rank‐reduction (RDRR) method into the conventional SS precursor workflow to improve the MTZ imagingRDRR method allows simultaneously attenuating the noise and interpolating the missing traces in the four‐dimensional dataApplication of our method to SS precursor data from western Pacific resolves MTZ structures at improved resolution [ABSTRACT FROM AUTHOR]

Tonga is a convergent plate boundary between the Pacific and Australian plates and is the fastest and the most seismically active deep subduction system in the world. We focused on southern Tonga (south of latitude 22°S) and the mantle transition zone (depths of 410–670 km), where seismic activity forms two subparallel bands of events in the east and west. We performed stress analysis by inverting focal mechanisms of earthquakes available in the Global Centroid Moment Tensor catalog and revealed two distinct stress regimes in the slab. While the stress orientation in the eastern slab segment conforms to the down‐dip compressional stress along the entire slab, the stress orientation in the western slab segment is different, having the maximum compression in the vertical direction. This suggests that the western segment can represent a stagnant slab with flattening and bending, as proposed by modeling studies. Its connection with the younger actively subducting slab is supported by the horizontal westward shift at 520 km depth. The stress analysis also indicates substantially different fault orientations in both segments. In the actively dipping slab, the majority of activated faults are predominantly sub‐horizontal. However, they are significantly inclined from vertical in the stagnant slab segment. A higher scatter in fault orientations in the stagnant slab suggests deformation, fragmentation and rheological complexity resulting from bending and flattening. Plain Language Summary: Tonga is a part of Tonga‐Kermadec, the 2,550‐km long subduction system in SW Pacific. It represents a convergent plate boundary between the Pacific and Australian plates. It is the fastest converging and the most seismically active deep subduction in the world. In the mantle transition zone, especially at depths greater than 500 km, the geometry of the slab becomes complex, forming separated slab segments. We performed stress analysis in the mantle transition zone (depths 410–670 km) with a focus on southern Tonga (south of latitude 22°S) where seismicity forms two subparallel bands of events in the east and west. We inverted publicly available data, focal mechanisms of earthquakes from the Global Centroid Moment Tensor catalog, and revealed two distinct stress regimes. While the stress orientation in the eastern segment matches the down‐dip compression of the subducting slab, the stress orientation of the western segment is different with the maximum compression in the vertical direction. This suggests that the western segment represents a stagnant slab with flattening and bending. Such findings are supported by the horizontal westward shift at 520 km depth and by substantially different fault orientations in both segments. Key Points: We performed stress analysis in southern Tonga mantle transition zone from focal mechanisms of earthquakes listed in Global Centroid Moment Tensor catalogWe revealed two distinct stress regimes: down‐dip compression matching subducting slab in the east and vertical compression in the westWestern segment suggests a stagnant slab with flattening and bending supported by horizontal shift and different fault orientations [ABSTRACT FROM AUTHOR]

It was recognized that two magmatic belts in the Lhasa‐Tengchong terrane formed due to the Mesozoic‐Cenozoic Tethyan evolution. Still, their spatiotemporal variations of magmatic flare‐ups/lulls are rarely discussed. Here we use the new U‐Pb and Lu‐Hf isotopic data of captured zircons and a comprehensive data set to show that the flare‐up of northern magmatic belt has peak ages of 110 Ma in central and northern Lhasa and 120 Ma in eastern Tengchong, possibly related to the tectonic transition from Meso‐ and Neo‐Tethyan double subduction to Neo‐Tethyan single subduction. For the southern magmatic belt, the flare‐ups at 100–85 Ma and 65–45 Ma in eastern southern Lhasa indicate obvious juvenile crustal growth, while flare‐ups at 75–45 Ma in western southern Lhasa and Tengchong record ancient crustal reworking. Such flare‐up variations in the southern magmatic belt possibly resulted from asynchronous changes in the Neo‐Tethyan slab dip. Plain Language Summary: The Tethyan slab subduction and following collision of India and Asia produced two magmatic belts in the Lhasa‐Tengchong terrane. How did the magmatic flare‐ups or high‐flux episodes and sources spatiotemporally vary? We combine the new U‐Pb and Lu‐Hf isotopic data of captured zircons (xenocrystic zircons caught by volcanics from country rocks) and a comprehensive data set to show that the peak ages of early Cretaceous flare‐up of the northern magmatic belt are ca.10 Ma younger in central and northern Lhasa than in eastern Tengchong, possibly related to the diachronous closure of the Meso‐Tethys and flat subduction of the Neo‐Tethys. The southern magmatic belt in Tengchong and western southern Lhasa exhibit 75–45 Ma flare‐ups featured by the reworking of ancient crust, different from the eastern southern Lhasa with the flare‐ups at 100–85 Ma and 65–45 Ma which are dominated by the juvenile crustal growth. Such flare‐up variations may be related to changes of the Neo‐Tethyan slab dip. Key Points: The Neo‐Tethyan arc has asynchronous magmatic flare‐ups and diverse magmatic sourcesThe spatiotemporal changes of slab dip triggered the asynchronous flare‐ups of the Neo‐Tethyan arc [ABSTRACT FROM AUTHOR]

The detachment (i.e., break‐off) of down‐going subducting oceanic slabs is a major geodynamic event with far‐reaching consequences, one of which is the reduction of the slab pull force acting on the trailing plate. We investigate the motion of the Sinai Microplate where a recent (∼1 Myr ago) slab break‐off occurred along its sole converging plate boundary (Cyprian Arc) with the overriding Anatolia Microplate. Based on new bathymetric mapping, high‐resolution seismic reflection imaging, geodetic and earthquake data, we show that Sinai is actively moving in a northwest direction with respect to Nubia. Our results indicate that despite the recent slab break‐off, Sinai has and is still being pulled (or pushed) toward the overriding Anatolia Microplate. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the upper mantle flow induced by the Afar Plume. Plain Language Summary: Subduction of the oceanic lithosphere into the mantle exerts the prime force that drives plate tectonic motions. The termination of subduction due to the tearing of the down‐going oceanic slab is, therefore, an important geodynamic phenomenon with far‐reaching kinematic consequences. Here we show that despite the recent tearing of the leading oceanic slab of the Sinai Microplate, convergence motion between Sinai and Anatolia Microplates continued without substantial changes. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the motion of the upper mantle flow induced by the Afar Plume. Key Points: Within the eastern Mediterranean, the Sinai Microplate is moving in a northwest direction with respect to the Nubian PlateThe Sinai Microplate is still converging with the overriding Anatolia Plate, despite the recent break‐off of the leading subducting slab [ABSTRACT FROM AUTHOR]

This study integrates geophysical‐geochemical data to investigate the thermochemical structure of the lithosphere and sublithospheric mantle, along the Southern Tyrrhenian Basin, Apennines, Adriatic Sea, Dinarides, and Carpathians‐Balkanides. We present the lithospheric structure of the Adria microplate and the two opposing mantle slabs along its NE and SW margins. The modeling shows the presence of two asthenospheric mantle wedges aligning with the Apenninic and Dinaric continental mantle slab rollback, along with cold (−200°C) sublithospheric anomalies beneath Adria's NE and SW margins. In the northern Adria region, the lithosphere undergoes synchronous thinning in the Tyrrhenian domain and thickening toward the forefront of the northern Apennines. This is associated with the northeastward rollback of the SW Adriatic slab, leading to subsequent delamination of the continental mantle. In the southern Adria region, the complex deep structure results from the variably oriented lithospheric slabs, and nearly 90‐degree shift of the tectonic grain between the southern Apennines and the Calabrian Arc. At the SW Adria margin, beneath the northern Apennines, the thermal sublithospheric anomaly is attached to the shallower lithosphere, while a slab gap is modeled in the southern Apennines. One possibility is that the gap is due to a recent horizontal slab tear. Along the NE margin of Adria, the thermal anomaly penetrates to depths of about 200 km in the northern Dinarides and 280 km in the southern Dinarides, shallower than the SW Adria anomaly, which extends to at least 400 km depth. Plain Language Summary: We integrate geological and geophysical data (e.g., elevation, gravity, geoid, seismic tomography) to investigate the density and temperature distribution of the crust and lithospheric mantle down to 400 km, along a profile that extends from the Southern Tyrrhenian Basin and Apennines in Italy to the Southern Dinarides and Carpathians‐Balkanides in southern Europe. Taking advantage of our previous research in this region, we provide an integrated view of the lithospheric structure and the uppermost mantle of the Tyrrhenian Basin, Apennines, Adriatic Sea and Dinarides. Our results show notable variations in both the crust and the base of the lithosphere. These variations are observed to be shallower beneath the basins and deeper beneath the mountain belts, particularly in areas with elevated topography. Additionally, our observations highlight the presence of two cold and dense zones situated in the distal margins of the Adria microplate, specifically beneath the Apennines and Dinarides. The thermal anomalies indicate the presence of two oppositely subducting slabs with different geometries and depth of penetration. Their current location, depth and shape have largely controlled the geodynamic evolution of the study region in the last 30 My. Key Points: We unveil the thermochemical structure of the lithosphere and uppermost mantle of the Adria Microplate and its marginsResults reveal mantle wedges aligning with Apenninic and Dinaric slabs, and cold sublithospheric anomalies beneath NE and SW Adria marginsIn N Apennines the slab is attached while a gap is observed in S Apennines. At Adria's NE margin, the slab penetrates deeper southward [ABSTRACT FROM AUTHOR]

The major- and trace-element compositions of amphiboles in andesite from Quaternary Yufu Volcano, northeastern Kyushu, Japan were analysed to investigate the generation processes of andesitic magma from Yufu Volcano. The amphiboles in andesite from Yufu volcano can be divided into two groups based on major-element composition: pargasite and magnesio-hornblende. To estimate temperature, pressure, and major- and trace-element compositions of melts in equilibrium with amphiboles, we used the recently proposed methods that can calculate temperature, pressure, major element compositions, and partition coefficients of trace-element between amphibole and melt using only the major-element compositions of amphibole. The estimated temperature, pressure, and major-element composition of melt in equilibrium with the amphibole phenocrysts indicate that each group crystallised under different conditions. These differences suggest that two magma chambers at different depths existed beneath Yufu Volcano and that the andesitic magma of Yufu Volcano was formed by mixing of the two magmas. The trace-element compositions of melts in equilibrium with the pargasite and magnesio-hornblende, estimated by applying the partition coefficients calculated from major-element compositions of amphibole to trace-element compositions of amphiboles, indicate magma derived from slab melt and the partial melting of crustal material, respectively. Because magma is a mixture of minerals and melt, we estimate the chemical compositional ranges of the two end-member magmas on the Y versus SiO2 diagram from the mixing relationship between amphibole and estimated melt, as well as phenocrysts of plagioclase, clinopyroxene, and orthopyroxene. The overlap of the estimated compositional range with the trend of whole-rock composition represents the chemical compositions of the end-members of magma mixing, yielding estimates of the mafic (SiO2 ≈ 45 wt %) and felsic (SiO2 ≈ 68 wt %) end-member magmas. Furthermore, we estimate the concentrations of other elements in the end-member magmas by substituting the estimated SiO2 concentrations of the magmas into linear regression equations between the whole-rock contents of other elements and SiO2. The trace-element compositions of the mafic and felsic end-member magmas, as estimated in this study, have similar features to those of gabbroids and Cretaceous granitic rocks, respectively, that are presumed to lie beneath Yufu Volcano. These similarities could be explained by the possibility that the compositions of the end-member magmas were influenced by basement rocks. [ABSTRACT FROM AUTHOR]

The stability and breakdown of mineral phases in subducting slabs control the cycling of trace elements through subduction zones. Stability of key minerals and the partitioning of trace elements between these minerals and liquid phases of interests have been charted by natural sample analysis and experimental constraints. However, systematic study from arc front to far back arc has rarely shown that the expected geochemical variations of the slab liquid are actually recorded by natural samples. Complexities arise by uncertainties on the nature of the slab component (melts, fluids and supercritical liquids), source heterogeneities and transport processes. Using data from olivine-hosted melt inclusions sampled along and across the NE Japan and southern Kurile arcs, we demonstrate that experimentally and thermodynamically constrained phase stabilities in subducted materials indeed control the trace element signatures as predicted by these models and experiments. The main reactions that can be traced across arc are progressive breakdown of light rare earth element-rich accessory phases (e.g. allanite), enhanced dehydration of the lithospheric mantle (serpentine breakdown) and changes in the nature of the slab component. This work elucidates subduction zone elemental cycling in a well-characterized petrogenetic setting and provides important constraints on the interpretation of trace element ratios in arc magmas in terms of the prograde metamorphic reactions within the subducting slab. [ABSTRACT FROM AUTHOR]

Seismic anisotropy is key to constrain mantle flow, but it is challenging to image and interpret it. Existing large-scale tomography models of seismic anisotropy typically show large discrepancies, which can lead to completely distinct geodynamical interpretations. To better quantify the robustness of anisotropy tomography, we create a 2-D ridge-to-slab geodynamic model and compute the associated fabrics. Using the resulting 21 elastic constants, we compute seismic full waveforms, which are inverted for isotropic and radially anisotropic structure. We test the effects of different data coverage and levels of regularization on the resulting images and on their geodynamical interpretation. Within the context of our specific imposed conditions and source–receiver configuration, the retrieved isotropic images exhibit substantial artificial slab thickening and loss of the slab's high-velocity signature below ∼100 km depth. Our results also show that the first-order features of radial anisotropy are well retrieved despite strong azimuthal anisotropy (up to 2.7 per cent) in the input model. On the other hand, regularization and data coverage strongly control the detailed characteristics of the retrieved anisotropy, notably the depth–age dependency of anisotropy, leading to an artificial flat depth–age trend shown in existing anisotropy tomography models. Greater data coverage and additional complementary data types are needed to improve the resolution of (an)isotropic tomography models. [ABSTRACT FROM AUTHOR]

Sulfur degassed at volcanic arcs calls for dissolved sulfate ions (S6+) released by subduction-zone fluids, oxidizing (in association with carbon) the subarc mantle, but sulfur speciation in subduction fluids at subarc depths remains unclear. We apply electrolytic fluid thermodynamics to model the dissolution behavior of pyrite in metacarbonate sediments as a function of P, T and rock redox state up to 4.3 gigapascals and 730°C. At subarc depth and the redox conditions of the fayalite-magnetite-quartz oxygen buffer, pyrite dissolution releases oxidized sulfur in fluids by disproportionation into sulfate, bisulfite, and sulfide species. These findings indicate that oxidized, sulfur-rich carbon-oxygen-hydrogen-sulfur (COHS) fluids form within subducting slabs at depths greater than 100 kilometers independent from slab redox state and that sulfur can be more effective than the concomitantly dissolved carbon at oxidizing the mantle wedge, especially when carbonates are stable. [ABSTRACT FROM AUTHOR]

The 180° curvature of the Banda arc at the eastern end of the Java‐Banda subduction zone reflects complicated geodynamic processes. A detailed investigation of its anisotropic structure would reveal its subduction dynamics, further resolving the controversial issue on how the highly arcuate Banda arc formed. We apply tilting‐axis anisotropic tomography to obtain a high‐resolution 3‐D anisotropic model beneath the Java‐Banda region. Our results show significant differences between Java and Banda in the pattern of anisotropy in both the subducting slab and its surrounding mantle, which reflect two distinctly different deformation modes in the two domains. Our results support the single‐slab subduction model for the Banda region. In addition, trench‐normal and upright fast‐velocity‐planes appear in the deep upper mantle, which may indicate material migrations in the big mantle wedge. Fast‐velocity‐planes in the shallow mantle exhibit a toroidal distribution, reflecting past counter‐clockwise rotation and asthenospheric material extrusion. Plain Language Summary: How the great curvature of the Banda arc formed remains in debate. Previous studies have proposed a single slab model and a double slab model for this subduction zone. Seismic anisotropy in the subducting slab and its surrounding mantle could provide clues on which model is correct. In this work, we perform the first tomographic inversion for tilting‐axis anisotropy to reveal the 3‐D anisotropic structure of the Java‐Banda subduction zone. The anisotropic features in the subducted slab and its surrounding mantle revealed by our anisotropic tomography support the single‐slab subduction model for the formation of the Banda arc. Key Points: Significant anisotropic differences exist between Java and Banda in both the mantle and slabSlab subduction is the main cause of seismic anisotropy in the Java‐Banda arcOur results support the single slab model for the Banda subduction zone [ABSTRACT FROM AUTHOR]

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

Complex zoning in crystals including repeated resorption and overgrowth is characteristic for arc magmas and occurs in response to closed-system changes in magmatic P–T– f O2 conditions and open system processes such as magma mixing and degassing or regassing. However, over which time frame do such changes occur? Do zoning patterns record changes occurring during the polybaric ascent of magmas that carry crystals or glomerocrysts sourced from variably mushy magma reservoirs, or alternatively indicate the uptake of antecrysts that experienced long periods of cold storage in plutonic precursors? A priori , these scenarios are endmember models, with the former transcrustal magmatic systems, where the crystals record changing conditions during magma ascent or changing interstitial melt compositions, traditionally preferred over the latter, which we here term transcrustal plutonic systems. In subsolidus plutonic systems, aphyric parental melts would acquire their entirely antecrystic crystal cargo during ascent from plutonic protoliths, and only crystal rims may be related to the host magma. We discuss the evidence for dominantly plutonic antecrystic cargo in some continental arc magmas, identified by considering mineral phase proportions, hydration of crystal rims that indicate hydrothermally altered cargo picked up by fresh melts, and uranium isotope disequilibria between crystals and matrix. We then turn to two-pyroxene thermobarometry and review the evidence for plutonic antecryst dominance revealed by this method in southwest Japan and the southern Taupo Volcanic Zone. We provide additional two-pyroxene data from the Andes, the Cascades, and the Tatun Volcano Group in northern Taiwan, corroborating that the uptake of crystals by aphyric to scarcely phyric melts is prevalent in continental arc magmatic systems. Thus, in many cases transcrustal plutonic systems seem to dominate, implying that a significant proportion of parental melts of continental arc magmas are variably enriched in silica, too hot to carry crystals, and typically too hot and not hydrous enough to be generated by differentiation in frequently postulated lower crustal hot zones, as we will demonstrate here. Our data indicate that in continental subduction zones, the mantle wedge is the source of a diversity of melt compositions (low- to high-silica), irrespective of the age and temperature of the subducting slab. Before discussing some of the implications of the prevalence of non-canonical transcrustal plutonic systems for the thermal structure of the crust, magma ascent processes, volcano monitoring, economic geology, as well as the evolution of continental crustal growth and recycling through deep time, we critically evaluate this novel perspective in terms of published data that might favour more traditional supersolidus transcrustal magmatic systems. This contribution provides the community with the opportunity to consider significantly colder crustal environments than typically accepted, and outlines avenues of future research. [ABSTRACT FROM AUTHOR]

Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ∼2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields-generated by westward migration of the Tonga Trench and associated slab rollback-have deflected the conduit northward over the past 2-3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries. [ABSTRACT FROM AUTHOR]

Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8-7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ∼0.02‰. Approximately 6-9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab-an application that can be applied to arc magmas worldwide. [ABSTRACT FROM AUTHOR]

Knowledge of the state of stress in subducting slabs is essential for understanding their mechanical behavior and the physical processes that generate earthquakes. Here, we develop a framework which uses a high-resolution focal mechanism catalog to determine the change in the position of the neutral plane before and after the M9 Tohoku-oki earthquake to determine that the deviatoric stress within the slab at intermediate depths must be very low (~1 MPa). We show that by combining the static stress calculated from coseismic slip distributions with the stress orientations before and after the mainshock, we can determine the full deviatoric stress tensor within the subducting slab at intermediate depths. These results preclude earthquake source mechanisms that require large background driving stresses, favoring a mechanically weak subducting slab, thus providing quantitative constraints on the physical processes that generate intermediate-depth earthquakes. [ABSTRACT FROM AUTHOR]

Recent observations revealed the existence of ice slabs and aquifers on the Greenland ice sheet (GrIS). Both affect the ice sheet's hydrology: ice slabs facilitate runoff and aquifers modulate drainage to the bed. However, their climatic drivers and history remain unclear, as most observations cover only two decades. Here, we present a model simulation of the evolution of GrIS ice slabs and aquifers (1980–2020), evaluated using radar measurements. The results show that accumulation, melt and rain rates are good predictors for the spatial distribution of ice slabs and aquifers. Both features were already present in the late 1980s, and their extent remained relatively constant until the beginning of this century, after which increased melt led to their expansion. We show that almost any transect from the coast to the ice‐sheet interior will cross either an ice slab region, or an aquifer, or both. Plain Language Summary: The Greenland ice sheet is covered by a layer of old, porous snow called firn, which has the capacity to prevent surface melt runoff. The firn can however, form thick ice slabs or store large amounts of water underground. These change the fate of surface melt by respectively enhancing runoff and draining water to the ice sheet's bed. While observations have shown where these phenomena occur, it is still unknown what the exact atmospheric conditions are that lead to their formation. Here, we tackle this problem using computer simulations. Our results show that the amount of snowfall, melt and rain determine whether ice slabs or aquifers can form. The results also suggest that ice slabs and aquifers are more abundant than previously assumed, and that their observed expansion started only in the early 2000s, and they will continue to expand if the climate gets warmer and melt and rain become more abundant. Key Points: Accumulation, melt and rain are good predictors of ice slab and firn aquifer locationsThe expansion of modeled ice slabs and aquifers on the Greenland ice sheet started in the early 2000sWe show that, in between the ablation zone and the accumulation zone, there are always ice slabs and/or aquifers present [ABSTRACT FROM AUTHOR]

Analysis of the upper mantle plumes spatial distribution in the inner part of the Sunda arc shows a number of plume bodies interrupting the stagnant slab framed from the south by the sinking slab of the Sunda arc. Possible mechanisms resulting in this structure of the mantle are (i) W-E-trending toroidal mantle flow through a gap in a flat slab and (ii) rollback capable of forming a gap in a flat slab and launching upper mantle plumes in it without deep (>1000 km) roots. The space above the top of the slab consists of local hot mantle bodies, which are considered secondary plumes and often form local rift segments. The 3D visualization of velocity variations δVp in the Tibet and Central Asia region demonstrates structural patterns similar to those in the Sunda arc area. One can observe a region of subhorizontal slab fragments and a gap, which is recorded by plume anomalies of deep and secondary origin. The motion vectors of the rock mass along the fault planes of the Sunda arc, detected from seismic events, are directed outward from its center of curvature, in which secondary upper mantle plumes are concentrated. This indicates the development of thrust processes at the arc front unassociated with the subducting plate. The arc thrusting is accompanied by a small number of displacements along antithetic thrust faults. The displacement azimuths along the Himalayas are fan-shaped, oriented toward Hindustan. This shows that the main indicator of tectonic activity (seismic events) has a direction of rock mass displacement to the south from the extensional back-arc basin within Tibet with the development of thrust deformations during movements along the detachment planes. Two directions of seismic movements are distinguished in the Himalayan arc, as well as in the Sunda arc. The first direction follows the Indian Plate subduction model. The second direction includes variable azimuth rock mass movements along the thrust planes onto the Indian Plate. [ABSTRACT FROM AUTHOR]

We present a new tectonic plate reconstruction that suggests substantial revisions to events associated with development of the Peruvian flat slab and resolves several long-standing issues regarding the subduction of bathymetric highs in the region. The Tuamotu Plateau is widely considered to be the product of Easter Plume magmatism, and plate reconstructions suggest it formed following initial plume ascent at ca. 55 Ma. The Nazca Ridge is also linked to the Easter Plume and is an obvious candidate to be the spreading ridge conjugate to the Tuamotu Plateau. Models for the paired evolution of the two ridges, however, generally stop at ca. 33 Ma because of the inability of plate reconstructions to associate the two ridges across a spreading center prior to this time. In addition, seafloor magnetic data demonstrate that the Tuamotu Plateau developed at a complexly shaped and evolving mid-oceanic ridge that precluded development of a simple mirror image conjugate of the type commonly employed in Nazca Ridge reconstructions. Seafloor isochrons also suggest that a ridge jump separated the Tuamotu Plateau from its conjugate at ca. 42 Ma. Global plate models offer an alternative approach to assessing conjugate development, by showing how a hypothetical conjugate to the Tuamotu Plateau is built up over time. Using such a model, we found that the conjugate that developed during the main stage of Tuamotu growth (55 Ma to 42 Ma) cannot be the Nazca Ridge, which appears to have initiated at ca. 42 Ma, when the Easter Plume diverted volcanism southward. We named the newly recognized conjugate the Enigma Ridge. Importantly, subduction of this ridge starting from ca. 17 Ma on the north Peruvian trench can account for the missing slab buoyancy previously attributed to the hypothesized, but controversial, Inca Plateau. The Enigma Ridge must still be providing far more buoyancy over a much greater area than the Nazca Ridge, which only began to subduct rather recently. [ABSTRACT FROM AUTHOR]

The distribution of different stress states in the subducting slab indicated by centroid moment tensor solutions for intra-slab earthquakes can help constrain the rheological properties of the slabs. A comparison of slabs in the western and eastern Pacific realms shows contrasting patterns in the stress states down to depths of ~ 350 km. The majority of slabs in the western Pacific show a pair of down-dip compression (DC) and down-dip tension (DT) domains in the upper and lower parts of the slab reflecting the effects of the slab unbending during progressive subduction. In contrast, slabs in the eastern Pacific show predominantly in-plane DT stress irrespective of slab geometry. Two-dimensional numerical simulations assuming constant slab thickness and viscosity indicate that the development of slabs with in-plane DT stress at depths of 100–300 km requires the slabs to be thin and have a low viscosity (1023 Pa s). Weak slabs bend easily and tend to fold when they encounter increased resistance to downward movement at the 660-km boundary. The associated DC stresses are not transmitted up the slab so negative buoyancy of the slab and DT stress dominates at intermediate depths for this type of slab. Most experimentally derived rheological parameters predict a high viscosity (> 1024 Pa s) for such slabs. However, two-dimensional numerical simulations using temperature- and pressure-dependent viscosity show that a relatively low activation energy (~ 110 kJ/mol) for diffusion creep is a possible explanation for the observed distribution of stresses in the slabs. Such low activation energies are compatible with recent experimental work on diffusion creep of polyphase mantle materials in which a low effective activation energy for creep results from a slow grain growth due to pinning effect of the secondary phase. The simulations provide a mechanical explanation for the observed dominantly DT stress state at 100–300 km depths for young slabs and paired DT and DC stress states at the same depth range for old slabs. [ABSTRACT FROM AUTHOR]

The distribution of different stress states in the subducting slab indicated by centroid moment tensor solutions for intra-slab earthquakes can help constrain the rheological properties of the slabs. A comparison of slabs in the western and eastern Pacific realms shows contrasting patterns in the stress states down to depths of ~ 350 km. The majority of slabs in the western Pacific show a pair of down-dip compression (DC) and down-dip tension (DT) domains in the upper and lower parts of the slab reflecting the effects of the slab unbending during progressive subduction. In contrast, slabs in the eastern Pacific show predominantly in-plane DT stress irrespective of slab geometry. Two-dimensional numerical simulations assuming constant slab thickness and viscosity indicate that the development of slabs with in-plane DT stress at depths of 100–300 km requires the slabs to be thin and have a low viscosity (1023 Pa s). Weak slabs bend easily and tend to fold when they encounter increased resistance to downward movement at the 660-km boundary. The associated DC stresses are not transmitted up the slab so negative buoyancy of the slab and DT stress dominates at intermediate depths for this type of slab. Most experimentally derived rheological parameters predict a high viscosity (> 1024 Pa s) for such slabs. However, two-dimensional numerical simulations using temperature- and pressure-dependent viscosity show that a relatively low activation energy (~ 110 kJ/mol) for diffusion creep is a possible explanation for the observed distribution of stresses in the slabs. Such low activation energies are compatible with recent experimental work on diffusion creep of polyphase mantle materials in which a low effective activation energy for creep results from a slow grain growth due to pinning effect of the secondary phase. The simulations provide a mechanical explanation for the observed dominantly DT stress state at 100–300 km depths for young slabs and paired DT and DC stress states at the same depth range for old slabs. [ABSTRACT FROM AUTHOR]

Cretaceous through Eocene plutonic rocks in northeastern Washington, USA, document a 60 m.y. history of crustal thickening and subsequent collapse and extension in response to two terrane-accretion events. Rocks emplaced 113-53 Ma have increasing La/Yb ratios reflecting orogenic plateau development after arrival of the Insular terrane by 100 Ma. Plutons emplaced 52-45 Ma (the Challis episode) document collapse of this plateau and define a SW-younging age progression attributed to breakoff and rollback of the Farallon slab following accretion of the Siletzia terrane at ca. 50 Ma. All of the rocks have chemical traits of arc magmas, likely inherited from their lower-crustal sources, but low B/Be ratios and the lack of evidence for amphibole fractionation indicate the Eocene magmas formed under drier conditions than are typical of active subduction settings. These magmas also originated at greater depth (eclogitic vs. gabbroic source) and were emplaced more shallowly than the earlier ones. All rocks have overlapping Sr-Nd and O isotopic data, indicating significant contributions from older continental crust, and depleted mantle Nd model ages become older toward the east, defining three regions that correspond with previously inferred lowercrustal domains. Farallon slab rollback also drove extension (core complex formation, dike swarms) and crustal uplift, which, along with voluminous magmatism, define the Challis episode. This tectonic model is further supported by seismic tomography, which has identified remnants of a detached slab in the upper mantle beneath the region. [ABSTRACT FROM AUTHOR]

The distribution of Oligo--Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90°E and 84°E) and flattened subducted lithosphere (both 86°E and 81°E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 °C) were recorded in the Oligo--Miocene ultrapotassic-potassic rocks at 90°E and 84°E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86°E and 81°E. Besides, the melting depth of the Oligo--Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81°E and 90°E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction. [ABSTRACT FROM AUTHOR]

Recycling of Fe-rich materials through subduction may affect the element budgets and redox properties of the mantle, thus in- fluencing the differentiation trends and mineralization types of mantle-derived magmas. However, the effects of different recycled Ferich materials on the mantle are dependent on their lithologies, which are still poorly constrained. Stable Fe isotopes can act as useful tracers for distinguishing among different recycled Fe-rich lithologies, and their imprints may be documented in mantle-derived magmas. This study focuses on the Fe isotopes of ferrobasalts and the associated dacites and magnetite ores in the Chagangnuoer Fe deposit of the Awulale Arc, Central Asia, to identify recycled Fe-rich materials and explore the Fe enrichment mechanism in continental arcs. Our results indicate that the ferrobasalts and dacites possess the lowest known δ56Fe, -0.40‰ ± 0.04‰ (2SE), among their counterparts worldwide. The low δ56Fe signatures are considered to originate from a hybridized mantle source, which may have been modified by recycled ferrocarbonates. The recycled ferrocarbonates may have melted during the decompressional heating stage of the slab subduction of the South Tianshan Ocean, coupled with asthenospheric upwelling under "wet" mantle conditions. The addition of ferrocarbonate melts to the mantle might have decreased the oxygen fugacity of the mantle wedge to below the fayalite--magnetite--quartz buffer, accounting for the Fe enrichment in arc tholeiites and large-scale Fe mineralization along the Awulale. Notably, our study reveals a novel carbonate recycling pathway in the cold subduction zones, where ferrocarbonates were subducted into the mantle and then recycled by the upwelling asthenosphere to mix with the fluid-metasomatized mantle through mantle convection. [ABSTRACT FROM AUTHOR]

The late Paleozoic tectonic evolution of the Lhasa terrane and opening of the Sumdo Paleo-Tethys Ocean remain controversial due to limited data and the ambiguous tectonic significance of Paleozoic magmatism in the southern Lhasa subterrane. Here, we explore the petrogenesis of meta-basalt from the Tangjia area and meta-gabbro from the Sumdo area to provide new constraints on the evolution of the Lhasa terrane. The geochemical characteristics of meta-gabbro are similar to those of oceanic island basalt and have positive whole-rock εNd(t) values (+0.08 to +1.03) and zircon εHf(t) values (+3.4 to +4.9), which suggests that they originated from asthenospheric mantle with minor contributions from lithospheric mantle in an intracontinental rift setting. The meta-basalt yields positive whole-rock εNd(t) values (+6.47 to +6.60) and zircon εHf(t) values (+14.2 to +15.2) and has enriched mid-oceanic-ridge--like geochemical characteristics; we suggest that the meta-basalt was sourced from the depleted mantle and interacted with enriched components in an initial oceanic basin setting. Zircon U--Pb geochronology of meta-basalt and metagabbro yielded crystallization ages of ca. 338 Ma and ca. 345 Ma, respectively. We suggest that the Sumdo Paleo-Tethys Ocean opened in the early Carboniferous. Negative buoyancy of Paleo-Tethys oceanic slab during subduction toward Eurasia drove the opening of the Sumdo Paleo-Tethys Ocean. Given that the Sumdo Paleo-Tethys Ocean was a limited oceanic basin, the Lhasa terrane was not completely separated from the northern margin of Gondwana before the Permian. [ABSTRACT FROM AUTHOR]

Arc magmatism along convergent plate boundaries has been a major contributor to continental crustal growth. During the late Paleoproterozoic--Mesoproterozoic assembly of the supercontinent Nuna, following arc-backarc-continent collisions, the Makkovik Orogen, along the southern margin of the Archean North Atlantic Craton, experienced a pulse of continental arc magmatism at ca. 1800 Ma. Lithogeochemistry indicates compositions typical of ferroan, alkali to alkali-calcic, A-type granites. The granitic rocks have εNd(t) values varying from -5.3 to +2.0 and model ages [T(DM)] from 2750 to 1880 Ma. The very high εNd(t) and young model ages imply a substantial juvenile mantle component in their formation. Oxygen fugacity (ƒO2) ratios, estimated by the proxy ratio of FeOT/(FeOT + MgO), range from 0.58 to 0.99, indicating the parental magmas formed in both reducing and oxidizing conditions. The granites also preserve an alkali-calcic alteration that together with inferred ƒO2 makes them more prospective for a variety of intrusion-related styles of mineralization including iron-oxide-copper-gold and uranium. Based on the petrology, lithogeochemistry, and isotopic compositions, the ca. 1800 Ma magmatic event is interpreted to have been developed post-collisionally following the docking of the Cape Harrison Arc/micro-continent with the North Atlantic Craton during the assembly of the supercontinent Nuna. This collision was followed by slab rollback (possibly breakoff), extensional collapse, and mantle upwelling resulting in the generation of the abundant ca. 1800 Ma felsic magmatism. The timing of the ca. 1818-1799 Ma plutonism in the Makkovik Orogen is coeval with significant felsic plutonism preserved in the Julianehåb Igneous Complex (Ketilidian Orogen) of southern Greenland. [ABSTRACT FROM AUTHOR]

Differentiating between advancing and retreating subduction zones and delineating the timing and processes for switching between these end-member types are critical factors in understanding orogenic evolution. In this study, we used temporal composition variations and spatiotemporal distribution of igneous rocks to constrain the types and directions of the subduction zone beneath the Lhasa terrane in southern Tibet during the mid-Mesozoic. Comprehensive geochronological and geochemical data from mid- Mesozoic magmatic rocks in five areas of the central Lhasa terrane document two compositionally distinct magmatic suites: older (ca. 172-150 Ma) high-Mg diorite-granodiorite and younger (ca. 150-130 Ma) low-Mg granodiorite-syenogranite. The high-Mg rocks with enriched isotopes were likely generated by the mixing of Lhasa terrane basement- derived magmas with mantle-derived magmas that incorporated extensive contributions from subducted sediments. The low- Mg rocks with relatively depleted isotopes are interpreted as derived from a mixed lowercrust source (ancient basement and juvenile crust) with minor mantle contributions followed by crystal fractionation. Both high-Mg and low-Mg rocks show arc signatures and formed the 1200 km Shiquan-Zhegu-Menba arc. A combination of the Mesozoic geological and geochemical data suggests that both the Neo-Tethyan and Bangong Tethyan slabs were likely subducting beneath the Lhasa terrane, creating two opposing subduction zones. The change from high-Mg to low- Mg magmatism within the Shiquan-Zhegu- Menba arc likely records a tectonic switch from advancing to retreating subduction of the south-dipping Bangong Tethyan slab. Our results provide a good example for identification of subduction polarities and types of ancient orogenic belts through spatiotemporal distributions and changes in geochemical compositions of igneous rocks. [ABSTRACT FROM AUTHOR]