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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]

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 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]

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]

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 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]

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]

Slab breakoff can explain the postcollisional igneous and metamorphic events in many collisional orogens and triggers the transition from initial continental collision to deep subduction. However, the relationship between the deep-seated geometry of slab breakoff and shallow tectono-magmatic events remains unclear. In this study, we report new geochronological, geochemical, and Hf isotope data for granitic rocks from the Qiangtang block. Three granitic samples yielded zircon U-Pb ages of 226-214 Ma and mostly gave negative zircon εHf(t) values of -20.1 to +0.57. These granitoids are K-rich, calc-alkaline, magnesian, and metaluminous to weakly peraluminous. We propose that the granites were generated by partial melting of compositionally heterogeneous lower crust in the Southern Qiangtang block. Our new data, along with previously published data for igneous and metamorphic rocks in the Qiangtang block, suggest that the Longmu Co--Shuanghu suture zone represents a remnant of the closure of the Paleo-Tethys Ocean, and that the Late Triassic tectono-magmatic evolution was controlled by slab breakoff after continental collision. The contrasting magmatic and metamorphic ages show that the two stages of tectono-magmatic activity in the Late Triassic were due to a change in the subduction angle of the continental lithosphere from steep (225-215 Ma) to shallow (215-205 Ma) during slab breakoff. Furthermore, we infer that slab breakoff of the oceanic lithosphere caused extensive crustal melting and reworking and initiated far-field subduction. Our results provide new insights into the slab breakoff processes that control the geometry of subducting continental lithosphere and related postcollisional magmatic and metamorphic events in collision zones. [ABSTRACT FROM AUTHOR]

The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction. [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 simple 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 in parallel. We compare the results of these models to a model with a constant-viscosity slab to determine the relative importance of each creep mechanism in subducting lithosphere and the impact of the implementation of different lithospheric flow laws on subduction dynamics. We find that dislocation creep dominates diffusion creep throughout subducting lithosphere with moderate (5 mm) grain size in the upper mantle. However, both diffusion and dislocation creep predict very high viscosities in the cold core 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. Our models also demonstrate a feedback between effective slab length and subduction velocity. Analogue and numerical models with constant-viscosity slabs lack this feedback, but still capture the qualitative patterns observed in more complex models. [ABSTRACT FROM AUTHOR]

To a large extent, the thermal structure of a subduction zone determines where seismicity occurs through controls on the transition from brittle to ductile deformation and the depth of dehydration reactions. Thermal models of subduction zones can help understand the distribution of seismicity by accurately modelling the thermal structure of the subduction zone. Here, we assess a common simplification in thermal models of subduction zones, i.e. constant values for the thermal parameters. We use temperature-dependent parameterisations, constrained by lab data, for the thermal conductivity, heat capacity, and density to systematically test their effect on the resulting thermal structure of the slab. To isolate this effect, we use the well-defined, thoroughly studied, and highly simplified model setup of the subduction community benchmark by in a 2D finite-element code. To ensure a self-consistent and realistic initial temperature profile for the slab, we implement a 1D plate model for cooling of the oceanic lithosphere with an age of 50 Myr instead of the previously used half-space model. Our results show that using temperature-dependent thermal parameters in thermal models of subduction zones affects the thermal structure of the slab with changes on the order of tens of degrees and hence tens of kilometres. More specifically, using temperature-dependent thermal parameters results in a slightly cooler slab with e.g. the 600 ∘ C isotherm reaching almost 30 km deeper. From this, we infer that these models would predict a larger estimated seismogenic zone and a larger depth at which dehydration reactions responsible for intermediate-depth seismicity occur. We therefore recommend that thermo(-mechanical) models of subduction zones take temperature-dependent thermal parameters into account, especially when inferences of seismicity are made. [ABSTRACT FROM AUTHOR]

Passive seismic studies have suggested that slab tearing occurs beneath North Sumatra, albeit the slab morphology is somewhat poorly constrained. The lack of resolution on the slab means that the presence of a tear has not been validated, and that the main factors responsible for slab tearing are hard to discern. We use a large seismic data set to determine a new 3‐D high‐resolution tomographic model of the crust and upper mantle beneath North Sumatra, which reveals a well‐defined low‐velocity channel that cuts across the subducting slab through a cavity. This feature cannot be reconciled with a tear in the slab, as previously suggested. Our relocated earthquakes exhibit a protruding cluster of seismicity along the subducted Investigator Fracture Zone, indicating that the high reliefs of the Investigator Fracture Zone possibly enhance the coupling between the subducted slab and the overriding plate. Shear stress resulting from the differential coupling in the slab may have created an area of mechanical weakness in correspondence of the fossil‐spreading ridges, which eventually led to the formation of a slab window. This window allows the exchange of mantle material from the subslab region up to the Toba volcano. We propose that the previously damaged parts (e.g., fracture zones and spreading centers) of the oceanic plate could help promote the growth of a slab window. Plain Language Summary: Seafloor fabrics are damaged traces of the oceanic plate, which may break again during subduction, leading to tearing, slab window or full slab detachment. Slab breaking is a pivotal process in the tectonic evolution of the lithosphere and underlying mantle, and can lead to complexities in slab deformation and mantle dynamics, new intraplate magmatism, and rapid uplift. Slab tearing in the subducted Indo‐Australian oceanic lithosphere beneath North Sumatra has been suggested by studies of seismicity and 3‐D seismic tomography. However, the crucial contributing factor to the initiation of slab breaking is a matter of debate. Our high‐resolution tomographic results reveal the fine crustal and upper‐mantle structure under North Sumatra, showing a very low P‐wave velocity hole cutting through the subducting slab. According to the location of seafloor fabric structures (e.g., fracture zones and fossil ridges in the Wharton basin) and the corresponding low‐velocity hole in our tomographic images, we suggest that the subducted seafloor fabrics are responsible for the slab window of the subducting Indo‐Australian plate beneath North Sumatra. We propose that the weak compartments of fossil‐spreading ridge and the obstructed Investigator Fracture Zone are keys for the onset of slab breaking. Key Points: A 3‐D P‐wave tomography model beneath North Sumatra is obtained using local and teleseismic data recorded by a dense seismic networkA low‐velocity channel is revealed in the subducting slab beneath the Toba calderaSlab window beneath Toba may be related to pre‐existing seafloor fabric structures in the oblique subduction system [ABSTRACT FROM AUTHOR]

Dehydration of subducting oceanic lithosphere (slab) induces Li‐isotope fractionation between the fluid and the slab, suggested by the δ7Li variation (∼10‰) in exhumed subduction complexes. Given that arc magmas represent melt of the supraslab mantle, a large δ7Li variation is anticipated for arc volcanic rocks. However, the δ7Li values in these rocks are mostly homogeneous within the range of mid‐ocean ridge basalts (+1.6 to +5.6‰). The lack of a subduction‐related δ7Li signature has been explained by (a) homogenization by mixing of different magma sources, (b) loss of Li from the slab via dehydration, or (c) homogenization by diffusive exchange of slab‐derived Li and the mantle. The Chugoku district in SW Japan is an ideal place to study the process responsible for Li‐isotope variation in arc magmas, since the Chugoku volcanic rocks show large δ7Li variation (−1.9 to +7.4‰). High δ7Li values (+6.3 to +7.4‰) are found in some high‐Sr andesites and dacites (adakites) whereas low δ7Li values (−1.0 to −0.1‰) are found in high‐Mg andesites. The parental magmas of these rocks have been sourced from subducted oceanic crust and sediments, respectively, with various extents of the interaction with wedge mantle. The limited extents of Li isotope modification are indicated by the similarity of the δ7Li values of these rocks and their supposed sources. The models for a slab dehydration and a diffusive exchange between slab‐derived melt and mantle demonstrate that the δ7Li signatures of the sources can be preserved in the adakites if they ascent rapidly in mantle. Plain Language Summary: Many of Earth's volcanoes occur near the trenches, where one plate subducts beneath another. These volcanoes are formed as a result of the melting of mantle that contains materials of subducted plates and the transportation of melts through the mantle. A quantitative understanding of these important processes has been hampered by the lack of geochemical tools to examine consecutive chemical reactions beneath the volcanoes. Here we conducted a lithium isotope investigation of the volcanic rocks from the Chugoku district in Southwest Japan. This district hosts the volcanoes of andesites and dacites which have been derived from the melting of materials in the subducted plate. We found that these volcanic rocks preserve Li‐isotope compositions of the original materials when they melted. This in turn suggests that these magmas ascent rapidly (<300,000 years) in the mantle. Key Points: Volcanic rocks in the Chugoku district in Southwest Japan show large δ7Li variation (−1.9‰ to +7.4‰)Li isotopes fractionate to the limited extent during arc magma productionSlab‐derived δ7Li features can be preserved if slab‐derived fluids/melts have distinct δ7Li values from mantle and ascend rapidly [ABSTRACT FROM AUTHOR]

We present new zircon U-Pb ages and Hf isotope compositions as well as whole-rock major- and traceelement geochemical and Sr-Nd isotopic data for silicic plutonic and volcanic rocks from the Duolong area of central Tibet. Combined with existing data, our new data indicate that these plutonic and volcanic rocks were formed in two stages ca. 120 Ma and ca. 110 Ma, respectively, in a postcollisional extensional setting that was triggered by slab breakoff. The similar geochemical compositions of granitoids and rhyolites, combined with their close spatial and temporal relationships, suggest that they were both derived from juvenile crustal material within a single magmatic system. We propose that the two inferred crustal melting events in the Duolong area were caused by two episodes of deep mantle activity triggered by the transition of the plate subduction angle from steep to shallow in response to the ascent of buoyant continental lithosphere during slab breakoff. Furthermore, rapid surface uplift during the late Early Cretaceous caused by slab breakoff made an important contribution to the formation of the proto– Tibetan Plateau. This study provides new insights into postcollisional tectonomagmatism and plateau uplift in central Tibet triggered by slab breakoff. We propose more generally that tectonic uplift during postcollisional processes (i.e., slab breakoff and lithospheric delamination) is a major contributor to plateau uplift in collision zones. [ABSTRACT FROM AUTHOR]

Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ18O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds. [ABSTRACT FROM AUTHOR]

Episodes of slow and fast plate subduction, slab rollback, and backarc opening are widely documented; e.g., in the central Mediterranean region. Pervasive damage by fluids is emerging as a possible weakening mechanism that could lead to slab segmentation and breakoff. We show that low-velocity anomalies within the Ionian slab present in along-dip seismic tomography profiles are traces of past damaging events generated by water penetration into the oceanic lithosphere when it was at the trench. We propose that the stepwise evolution of the central Mediterranean over the past 21 m.y. has been governed by strength changes of the Ionian slab induced by damage. More generally, we speculate that the cyclicity of such a process in space and time in the different circum-Mediterranean subduction events may have led to the present shape of the Mediterranean basin. [ABSTRACT FROM AUTHOR]

The timing and origins of the Early Cenozoic magmatic flare-up in southern Tibet remain controversial, mainly because of the lack of available data for the rare, small-volume, ocean island basalt (OIB)-like mafic rocks that can shed insights into this geological event. We have confirmed that Early Cenozoic OIB-like basalts in the Zhongba area, southern Tibet, were closely related to this magmatic flare-up, which occurred in response to deep mantle geodynamics. In this study, we present detailed geochronological and geochemical data for the Zhongba basalts. These Eocene (ca. 52 Ma) OIB-like basalts have negative K, Zr, Hf, and Ti anomalies, low Fe/Mn and high Fe/Zn ratios, and uniform and enriched Sr–Nd–Hf and HIMU-like Pb isotopic compositions (87Sr/86Sr(i) = 0.7052–0.7053; εNd(t) = − 1.7 to − 1.3; εHf(t) = − 1.1 to − 0.3; 206Pb/204Pb = 19.83–19.87), indicative of derivation from carbonated peridotite. Based on the regional tectonic evolution, we propose that the carbonated peridotite source of the Zhongba basalts was generated by the reaction between depleted mid-ocean ridge basalt-like mantle peridotite and melts from subducted Neo-Tethys carbonate-bearing oceanic crust. This carbonated peridotite was possibly preserved at the thermal boundary between the lithosphere and asthenosphere and underwent partial melting to produce the Zhongba basalts in response to slab break-off of the Neo-Tethys oceanic lithosphere at ca. 52 Ma. The slab break-off triggered the magmatic flare-up and other geological processes (e.g., a decrease in the velocity of the Indian Plate) in southern Tibet during the Eocene. [ABSTRACT FROM AUTHOR]

Spatiotemporal constraints for Late Cretaceous tectonism across the Colorado Plateau and southern Rocky Mountains (northern Arizona-New Mexico, USA) are interpreted in regards to Laramide orogenic mechanisms. Onset of Laramide arch development is estimated from cooling recorded in representative thermochronologic samples in a three-step process of initial forward models, secondary HeFTy inverse models with informed constraint boxes, and a custom script to statistically estimate timing of rapid cooling from inverse model results. Onset of Laramide basin development is interpreted from increased rates of tectonic subsidence. Onset estimates are compared to published estimates for Laramide timing, and together suggest tectonism commenced ca. 90 Ma in northwestern Arizona and progressed eastward with later onset in northcentral New Mexico by ca. 75-70 Ma. The interpreted sweep of onset progressed at a rate of ~50 km/m.y. and was approximately half the 100-150 km/m.y. rate estimated for Late Cretaceous Farallon-North America convergence during the same timeframe. Previous suggestions that the Laramide tectonic front progressed at a rate similar to convergence via basal traction are not supported by our results. We thereby suggest that (1) a plate margin end load established far field compression and that (2) sequential Laramide-style strain was facilitated by progressive weakening of North American lithosphere from the dehydrating Farallon flat slab. Results are compared to models of sweeping tectonism and magmatism in other parts of the Laramide foreland. Discussions of the utility of the custom script and the potential for stratigraphic constraints to represent only minimum onset estimates are also presented. [ABSTRACT FROM AUTHOR]

The northwestern corner of South America presents a highly complex geodynamical setting including the convergence between South America, Nazca, and Caribbean plates; which also entails heterogeneity and complexity in the lithospheric structure. Here, we present depth‐migrated common‐conversion point (CCP) teleseismic receiver function profiles carried out using seismological data between 2011 and 2020 from the Colombian Geological Service, combined with some stations of observatories and international networks. These profiles provide better constraints on the lateral interpretation of the main lithospheric discontinuities in this region. We provide an updated crustal thickness map displaying some differences from previous studies, especially in the northernmost part of the cordilleran system. Our results also show some intracrustal features such as the regional detachment surface beneath the Eastern Cordillera and the presence of deep melt reservoirs under the Central Cordillera and the Paipa‐Iza complex in the Eastern Cordillera. We also discuss the lithospheric thickness and the potential existence of a highly hydrated and serpentinized mantle wedge under the forearc over the slab segment south of the Caldas tear. Furthermore, the CCP profiles further support the subduction system segmentation under northwestern South America, with at least five slab segments, two of them overlapping between ∼5.5 and ∼8°N, three belonging to the Nazca plate, and two having a Caribbean origin. The Bucaramanga nest appears to be linked to the interaction between the two northmost slab segments, while the Cauca cluster might be related to a slab dehydration process of a highly fractured zone. Plain Language Summary: The Colombian tectonic configuration is highly complex with the convergence of three major tectonic plates. Despite extensive recent studies in this region, some of the important tectonic features are still under discussion. In this study, we present cross‐section images of the lithospheric and upper mantle structure beneath Colombia, carrying out an analysis of teleseismic wave conversions corresponding to data collected between 2011 and 2020 by the Colombian Geological Service, volcanological and seismological observatories, and international stations. The results show that the crust is thinner than estimated in previous studies in the northernmost part of the Colombian cordillera system, and provide some material on intracrustal features and lithospheric thickness. Moreover, our images support other studies on the subduction system in this area, showing the presence of five slab segments on the Pacific side subducting beneath Colombia, including an overlapping segment between ∼5.5 and ∼8°N. The two densest seismic clusters, the Bucaramanga nest and the Cauca cluster, seem to be linked to the interaction of two plates at depth and the dehydration of a fractured slab zone, respectively. Key Points: Crustal thickness beneath northwestern South America with some significant changes from previous studiesWe observe five different slab segments under northwestern South AmericaRegional detachment surface under the Eastern Cordillera ranging between 10 and 30 km depth [ABSTRACT FROM AUTHOR]

The formation of the Central Andes dates back to ∼50 Ma, but its most pronounced episode, including the growth of the Altiplano‐Puna Plateau and pulsatile tectonic shortening phases, occurred within the last 25 Ma. The reason for this evolution remains unexplained. Using geodynamic numerical modeling we infer that the primary cause of the pulses of tectonic shortening and growth of the Central Andes is the changing geometry of the subducted Nazca plate, and particularly the steepening of the mid‐mantle slab segment which results in a slowing down of the trench retreat and subsequent increase in shortening of the advancing South America plate. This steepening first happens after the end of the flat slab episode at ∼25 Ma, and later during the buckling and stagnation of the slab in the mantle transition zone. Processes that mechanically weaken the lithosphere of the South America plate, as suggested in previous studies, enhance the intensity of the shortening events. These processes include delamination of the mantle lithosphere and weakening of foreland sediments. Our new modeling results are consistent with the timing and amplitude of the deformation from geological data in the Central Andes at the Altiplano latitude. Plain Language Summary: The Central Andes is a subduction‐type orogeny that formed as a result of the interaction between the Nazca oceanic plate and the South American continental plate over the last 50 million years. Growth of the Andes is primarily the result of crustal shortening. Nevertheless, "geological" data compiled from previous studies have shown that phases of drastic pulsatile shortening occur at 15 and 5 Ma. In this study, we used high‐resolution 2D numerical geodynamic simulations to investigate the link between oceanic and continental plate dynamics and their interaction. We find that when the oceanic plate steepens in the mantle transition zone, the trench retreat is hindered. Coupled with the weakening of the continental plate through the slab flattening and subsequent delamination of the lithospheric mantle, this leads to pulsatile shortening phases of a magnitude equivalent to that suggested by the data. Key Points: The steepening of the slab due to slab buckling hinders the trench retreating and explains the main pulsatile phases of the deformation during the last 25 MaThe absolute motion of the overriding plate controls the regime of subduction dynamicsFlat slab and eclogitization are required to weaken and then shorten the overriding plate when the slab steepens and the trench is hindered [ABSTRACT FROM AUTHOR]

Subducted remnant slabs play an important role in the geodynamics and evolution of the Earth, but their behaviour in the mantle is not well understood. Terrestrial heat flow and thermal lithosphere structure provide crucial constraints on lithospheric dynamics. Based on whole‐well steady‐state temperature logs and conductivity measurements of core samples from a scientific borehole, we obtained a new high‐quality terrestrial heat flow values for East Asia. The heat flow was calculated at 60.6 ± 16.0 mW/m2 with a high crust/mantle heat flow ratio of 1.41, suggesting a hot‐crust‐cold‐mantle lithospheric structure. The thermal lithospheric thickness of eastern Fujian is estimated to be 152–166 km, indicating a thick and cold lithosphere. We demonstrate that the cooling of the lithosphere is caused by a remnant palaeo‐slab of the Palaeo‐Pacific plate subducted beneath SE China. The remnant slab blocked heat convection into the bottom of the lithosphere of the Eurasian Plate. [ABSTRACT FROM AUTHOR]

Voluminous moderate- to high-magnesian [Mg# = molar Mg/(Mg + Fe2+) = 44–64] andesitic and dacitic rocks with high silica (mostly 61–66 wt%) adakitic affinity (Y = 13–22, Yb = 1.3–2.1, Sr/Y = 18–44, La/Yb = 10–25) and common mafic magmatic enclaves (MMEs) are first reported in the Keçiboyduran stratovolcano (KSV) from the Cappadocia volcanic province (CVP), Central Anatolia, Turkey. We present comprehensive whole-rock geochemistry and Sr–Nd–Pb isotope data, mineral chemical compositions and 40Ar–39Ar ages for KSV samples. Based on the volcanostratigraphy and 40Ar–39Ar dating results, two successive eruption ages of 2.2–1.6 Ma (stage I: amphibole-rich) and 1.6–1.2 Ma (stage II: pyroxene-rich) were established for the KSV, corresponding to the Gelasian and Calabrian stages of Early Pleistocene, respectively. Textural and geochemical evidence indicates that the KSV magnesian andesites–dacites are products of a hybrid magma formed by mixing between mantle-derived mafic and crust-derived felsic magmas with further fractionation and minor contamination during magma storage and ascent. Our new data, combined with previous geological and geophysical results suggest that parental magnesian mafic melts of the KSV rocks originated from a heterogenous mantle source generated through the metasomatism of mantle wedge material by subducted sediment-derived melts, and then partially melted through asthenospheric upwelling in response to slab break-off. The mafic magma underplated the overlying lower crust, resulting in its partial melting to generate crustal felsic magma. Both magmas mixed at lower crustal levels creating MME-rich hybrid magmas. Subsequently, the hybrid magmas were emplaced at different depths of the crust (c. 4–11 and 11–15 km for the stage I and II, respectively), where they crystallized at moderate temperatures (c. 1180–840 °C) and under relatively high oxygen fugacity (LogƒO2 = − 11.4 to − 9.2), water-rich (H2Omelt = 5.6–3.6 wt%) and polybaric (~ 1.2 to 5.1 kbars) conditions, and underwent fractionation of primarily amphibole ± pyroxene causing adakitic affinity. We propose a new petrogenetic model for the early Quaternary magnesian/adakitic andesites/dacites of the CVP in a post-subduction tectonic setting. Our results provide robust evidence for slab break-off of the eastern Cyprus oceanic lithosphere and put further constraints on the tectonic evolution of the eastern Mediterranean collision zone during the Early Quaternary. [ABSTRACT FROM AUTHOR]

The stratigraphic development of foreland basins has mainly been related to surface loading in the adjacent orogens, whereas the control of slab loads on these basins has received much less attention. This has also been the case for interpreting the relationships between the Oligocene to Micoene evolution of the European Alps and the North Alpine foreland basin or Molasse basin. In this trough, periods of rapid subsidence have generally been considered as a response to the growth of the Alpine topography, and thus to the construction of larger surface loads. However, such views conflict with observations where the surface growth in the Alps has been partly decoupled from the subsidence history in the basin. In addition, surface loads alone are not capable of explaining the contrasts in the stratigraphic development particularly between its central and eastern portions. Here, we present an alternative view on the evolution of the Molasse basin. We focus on the time interval between c. 30 and 15 Ma and relate the basin-scale development of this trough to the subduction processes, and thus to the development of slab loads beneath the European Alps. At 30 Ma, the western and central portions of this basin experienced a change from deep marine underfilled (Flysch stage) to overfilled terrestrial conditions (Molasse stage). During this time, however, a deep marine Flysch-type environment prevailed in the eastern part of the basin. This was also the final sedimentary sink as sediment was routed along the topographic axis from the western/central to the eastern part of this trough. We interpret the change from basin underfill to overfill in the western and central basin as a response to oceanic lithosphere slab-breakoff beneath the Central and Western Alps. This is considered to have resulted in a growth of the Alpine topography in these portions of the Alps, an increase in surface erosion and an augmentation in sediment supply to the basin, and thus in the observed change from basin underfill to overfill. In the eastern part of the basin, however, underfilled Flysch-type conditions prevailed until 20 Ma, and subsidence rates were higher than in the western and central parts. We interpret that high subsidence rates in the eastern Molasse occurred in response to slab loads beneath the Eastern Alps, where the subducted oceanic slab remained attached to the European plate and downwarped the plate in the East. Accordingly, in the central and western parts, the growth of the Alpine topography, the increase in sediment flux and the change from basin underfill to overfill most likely reflect the response to slab delamination beneath the Central Alps. In contrast, in the eastern part, the possibly subdued topography in the Eastern Alps, the low sediment flux and the maintenance of a deep marine Flysch-type basin records a situation where the oceanic slab was still attached to the European plate. The situation changed at 20 Ma, when the eastern part of the basin chronicled a change from deep marine (underfilled) to shallow marine and then terrestrial (overfilled conditions). During the same time, subsidence rates in the eastern basin decreased, deformation at the Alpine front came to a halt and sediment supply to the basin increased possibly in response to a growth of the topography in the Eastern Alps. This was also the time when the sediment routing in the basin axis changed from an east-directed sediment dispersal prior to 20 Ma, to a west-oriented sediment transport thereafter and thus to the opposite direction. We relate these changes to the occurrence of oceanic slab breakoff beneath the Eastern Alps, which most likely resulted in a rebound of the plate, a growth of the topography in the Eastern Alps and a larger sediment flux to the eastern portion of the basin. Beneath the Central and Western Alps, however, the continental lithosphere slab remained attached to the European plate, thereby resulting in a continued downwarping of the plate in its central and western portions. This plate downwarping beneath the central and western Molasse together with the rebound of the foreland plate in the East possibly explains the inversion of the drainage direction. We thus propose that slab loads beneath the Alps were presumably the most important drivers for the development of the Molasse basin at the basin scale. [ABSTRACT FROM AUTHOR]

Flat-slab subduction has affected parts of North America, South America, and Asia over the past 250 m.y. In these areas, reconstructions show that the subducting plate became subhorizontal below the continent for ~5 to >30 m.y., followed by foundering of the slab and resumption of steep-angle subduction. Using two-dimensional numerical models, we examine the factors that control the development and removal of a flat slab. Models are based on the Late Cretaceous to Oligocene Farallon flat slab below the southwestern United States. We find that the primary control on subduction geometry is the oceanic plate density structure. Subduction of a buoyant oceanic plateau creates a flat-slab segment that moves inboard at approximately the rate of continental trenchward motion (4-5 cm/yr). Steepening is initiated with eclogitization of the oceanic plateau crust. Once the plateau density exceeds that of the mantle, the slab undergoes rollback through progressive trenchward-directed detachment from the continent at a rate of 2-10 cm/yr. Rollback is enhanced by: (1) weakening of the overlying continental mantle lithosphere, inferred to result from slab-derived hydrous fluids, and (2) a slowdown in plate velocities; the rate and amount of oceanic eclogitization are second-order effects. Conversely, rollback is hindered by a strong oceanic plate and interactions between the slab and high-viscosity lower mantle. For the ~2000-km-long Farallon slab, the Conjugate Shatskey Rise plateau must have remained buoyant for 20-30 m.y. after subduction. This was followed by rapid rollback caused by both plateau eclogitization and continental weakening, leaving an area of thinned and hydrated continental lithosphere. [ABSTRACT FROM AUTHOR]

• Zircon and titanite in xenoliths document Eocene metasomatism of the Wyoming craton lower crust. • Metasomatism by hydrous and carbonated fluids linked to subduction of the Shatsky conjugate and Farallon slab beneath the northern margin of the craton. • Metasomatism, slab foundering, and heating triggered partial loss of the Wyoming craton lithosphere. New zircon and titanite U-Pb and trace-element data from minette- and kimberlite-hosted lower crustal xenoliths from near the northern margin of the Wyoming craton document multiple periods of metasomatism from 62 to 50 Ma. Metasomatism was either coincident or preceded eruption of the host magmas by up to 14 Myr (eruption dated at 52 Ma and 48 Ma based on new zircon U-Th/He data). Thus, fluid/melt interactions were not solely related to magmatic entrainment and eruption. Accessory mineral trace element data reveal metasomatism by fluids from multiple sources, including water and carbonatite potentially sourced from the subducted Farallon slab. These metasomatic episodes are interpreted to reflect pan-lithospheric hydration events that weakened the deep cratonic lithosphere and facilitated partial removal following rollback and foundering of the Farallon plate. The new data corroborate previous suggestions that subduction-related metasomatism may be a key requisite for the destruction of otherwise stable cratons. [ABSTRACT FROM AUTHOR]

The geodynamic evolution of the Himalayan–Tibetan orogen during the transition from Neo-Tethyan oceanic to Indian continental subduction remains unclear. Therefore, we conducted a new geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study of post-collisional diabase dikes in the central segment of the southern Lhasa subterrane. The diabase dike samples yield zircon U–Pb ages of 51–49 Myr, consistent with the timing of the Gangdese magmatic flare-up (ca. 53–50 Myr). In addition, the diabase dikes exhibit a geochemical intraplate affinity and depleted Sr–Nd–Pb isotopic compositions. Their variable Nb contents allow, the dikes to be divided into low-Nb (<8 ppm) and high-Nb (>9 ppm) types, which represent the melting products of slab fluid-modified Gangdese lithospheric mantle and a mixed mantle source comprising Gangdese lithospheric and deep asthenospheric mantle, respectively. In addition, the wide variation in εHf(t) values (−10.0 to +12.3) of zircons from these diabase dikes is consistent with the Gangdese zircon Hf isotopic 'pull-down' at ca. 50 Ma. Integrating our new data with published results suggests that the zircon Hf isotopic 'pull-down' was a direct consequence of the involvement of subducted Indian continental components in the formation of the Gangdese continental lithosphere. The combined data further suggest that upwelling of the asthenosphere, triggered by breakoff of the Neo-Tethyan oceanic slab, resulted in extensive mantle and crustal melting in the Lhasa terrane and the Tethyan Himalaya in southern Tibet. • The diabase dikes in the Pengcuolin–Dazhuqu region were emplaced at ca. 50 Ma. • Indian continental components were involved in the Gangdese lithosphere at ca. 50 Ma. • The breakoff of the Neo-Tethyan slab is believed to have occurred at ca. 53–50 Ma. [ABSTRACT FROM AUTHOR]

The origin of the Chon Aike silicic large igneous province (SLIP) is a matter of intense debate, with contrasting hypotheses that range from intraplate settings linked to mantle plume impingement to subduction-related protracted arc magmatism in an active margin. In this study, we propose a new model for the origin of this SLIP based on a multidisciplinary dataset from Patagonia, a comprehensive literature review of southwestern Gondwana, and the results of 2-D thermochemical modeling. We demonstrate that the partial melting of subducted rocks during a massive slab break-off and the subsequent piecemeal sinking of a previously flattened oceanic lithosphere beneath southwestern Gondwana best reconciles most of the data from this magmatic province. Geophysical, geochronological, geochemical, and isotopic data from Chon Aike SLIP, combined with the understanding of the tectonic regime, ore deposits, Jurassic geological events in southwestern Gondwana and numerical modeling results, support an origin primarily linked to the partial melting of partially eclogitized metabasaltic and metasedimentary rocks, enhanced by a warm ambient mantle associated with supercontinent thermal insulation and the thermal effects of the Karoo mantle plume impingement. The demise of the flat slab through large-scale slab break-off would have led to the partial melting of a mixture largely composed of these extensively underplated components and mantle batches. These melts would have variably interacted with mantle and continental rocks and melts, resulting in the formation of most of the Chon Aike SLIP. The re-establishment of the magmatic arc after the initial stages of slab break-off seems to have only affected the northwestern part of this SLIP. [ABSTRACT FROM AUTHOR]

Based on recent results of AlpArray, we propose a new model of Alpine collision that involves subduction and detachment of thick (∼ 180 km) European lithosphere. Our approach combines teleseismic P-wave tomography and existing local earthquake tomography (LET), allowing us to image the Alpine slabs and their connections with the overlying orogenic lithosphere at an unprecedented resolution. The images call into question the conventional notion that downward-moving lithosphere and slabs comprise only seismically fast lithosphere. We propose that the European lithosphere is heterogeneous, locally containing layered positive and negative Vp anomalies of up to 5 %–6 %. We attribute this layered heterogeneity to seismic anisotropy and/or compositional differences inherited from the Variscan and pre-Variscan orogenic cycles rather than to thermal anomalies. The lithosphere–asthenosphere boundary (LAB) of the European Plate therefore lies below the conventionally defined seismological LAB. In contrast, the lithosphere of the Adriatic Plate is thinner and has a lower boundary approximately at the base of strong positive Vp anomalies at 100–120 km. Horizontal and vertical tomographic slices reveal that beneath the central and western Alps, the European slab dips steeply to the south and southeast and is only locally still attached to the Alpine lithosphere. However, in the eastern Alps and Carpathians, this slab is completely detached from the orogenic crust and dips steeply to the north to northeast. This along-strike change in attachment coincides with an abrupt decrease in Moho depth below the Tauern Window, the Moho being underlain by a pronounced negative Vp anomaly that reaches eastward into the Pannonian Basin area. This negative Vp anomaly is interpreted as representing hot upwelling asthenosphere that heated the overlying crust, allowing it to accommodate Neogene orogen-parallel lateral extrusion and thinning of the ALCAPA tectonic unit (upper plate crustal edifice of Alps and Carpathians) to the east. A European origin of the northward-dipping, detached slab segment beneath the eastern Alps is likely since its down-dip length matches estimated Tertiary shortening in the eastern Alps accommodated by originally south-dipping subduction of European lithosphere. A slab anomaly beneath the Dinarides is of Adriatic origin and dips to the northeast. There is no evidence that this slab dips beneath the Alps. The slab anomaly beneath the Northern Apennines, also of Adriatic origin, hangs subvertically and is detached from the Apenninic orogenic crust and foreland. Except for its northernmost segment where it locally overlies the southern end of the European slab of the Alps, this slab is clearly separated from the latter by a broad zone of low Vp velocities located south of the Alpine slab beneath the Po Basin. Considered as a whole, the slabs of the Alpine chain are interpreted as highly attenuated, largely detached sheets of continental margin and Alpine Tethyan oceanic lithosphere that locally reach down to a slab graveyard in the mantle transition zone (MTZ). [ABSTRACT FROM AUTHOR]

The Pamir plateau protrudes ∼300 km between the Tajik‐ and Tarim‐basin lithosphere of Central Asia. Whether its salient location and shape are caused by forceful indentation of a promontory of Indian mantle lithosphere is debated. We present a new local‐seismicity and focal‐mechanism catalog, and a P‐wave velocity model of the eastern part of the collision system. The data suggest a south‐dipping Asian slab that overturns in its easternmost segment. The largest principal stress at depth acts normal on the slab and is orientated parallel to the plate convergence direction. In front (south) of the Asian slab, a volume of mantle with elevated velocities and lined by weak seismicity constitutes the postulated Indian mantle indenter. We propose that the indenter delaminates and overturns the Asian slab, underthrusts the Tarim lithosphere along a compressive transform boundary, and controls the location and shape of the Pamir plateau. Plain Language Summary: The Pamir plateau stands out between the Tajik basin to the west and the Tarim basin to the east. Its location and shape are either caused by a part of the Indian continent that protrudes below Pamir's crust, or thinned lithosphere of a former Asian basin existed in the place of the Pamir and subducted during the collision of India with Asia. Our new seismological data show that the Asian slab—a displaced part or slice of the Tarim‐Tajik‐basin lithosphere—is overturned beneath the eastern Pamir. A zone of high seismic velocities, indicative of a relatively cold and rigid mantle lithosphere, occurs in front (south) of the Asian slab. A seismically active zone with low seismic velocities is squeezed between this structure and the Tarim lithosphere. Together, these observations trace the northern and eastern margin of the Indian mantle indenter that predefines the shape of the Pamir plateau. Key Points: New local earthquake catalog and seismic P‐wave velocity model for the eastern PamirElevated velocities outline the northern and eastern margin of the Indian mantle indenter beneath the Pamir plateauThe indenter overturns the eastern end of the Asian slab and terminates along a transform margin against the Tarim block [ABSTRACT FROM AUTHOR]

Distributions of slabs within Earth's mantle are increasingly used to reconstruct past subduction zones, based on first‐order assumptions that slabs sink vertically after slab break‐off, and thus delineate paleo‐trench locations. Non‐vertical slab motions, which occur prior to break‐off, represent a potentially significant source of error for slab‐based plate reconstructions, but are poorly understood. We constrain lateral migration of the Indian slab and overlying India‐Asia collision zone by comparing tomographically imaged mantle structure with plate‐kinematic constraints. Following coupling of the Indian and Australian plates at the onset of collision, ∼1,000 km lateral migration of the Indian slab was driven by vertical subduction of the Australian slab. The sinking behaviors of individual slabs do not evolve in isolation, but instead influence, or are influenced by, other slabs in the same plate network. Hence, lateral slab migrations may be determined by interpreting the sinking behavior of slabs collectively, and with respect to plate kinematics. Plain Language Summary: To understand the links between plate tectonics and mantle processes, researchers must determine how tectonic plates have moved with respect the evolving mantle through geological time. To overcome this problem, recent studies use the locations of subducted slabs in the deep mantle to reconstruct plate motions, based on the hypothesis that slabs sink vertically through the mantle, and therefore mark the surface locations of past subduction zones. Here, we test slab sinking hypotheses, and their use in plate reconstruction modeling, by investigating the sinking kinematics of the subducting Indian and Australian slabs during the India‐Asia collision. Our analysis indicates that since onset of collision at ∼45–40 Ma, the Indian slab migrated laterally, ∼1,000 km northwards through the mantle, driven by subduction of the neighboring Australian slab. We arrive at this new interpretation because we interpret Indian and Australian slab kinematics collectively, and with respect to India‐Australia plate motions. Our study shows that the sinking behavior of one slab can influence that of another slab in the same network. Slab‐based plate reconstructions should therefore interpret slabs of the same network collectively, and with respect to plate motions, in order to constrain non‐vertical slab motions and avoid potentially significant plate reconstruction errors. Key Points: Subduction of Australian oceanic lithosphere drove northward motion of coupled India‐Australia plate since onset of collision at 45–40 MaBuoyant Indian continent stalled subduction of Indian slab whilst Australian slab subduction drove motion of coupled India‐Australia plateApproximately 1,000 km north lateral migration of Indian slab occurred to maintain compatibility with plate kinematics of coupled India‐Australia plate [ABSTRACT FROM AUTHOR]

Hot mantle plumes and ancient cold slabs have been observed beneath modern midocean ridges, but their specific and detailed effects on mid-ocean ridge crustal accretion are poorly understood. The oceanic lithosphere beneath the Southeast Indian Ocean displays unique morphological, geophysical, and geochemical characteristics, which may reflect the influence of both mantle anomalies and upwelling plumes on seafloor spreading. In this study, we combined gravity-derived oceanic crustal thickness with plate tectonic reconstructions to investigate patterns of asymmetry in thickness of crust accreted at the Southeast Indian Ridge over the last 50 m.y. Our results reveal several distinct features: (1) small-scale, short-lived asymmetries in the thickness of crustal accretion of up to 0.75 km are alternatively distributed on the southern and northern flanks of the 90°-120°E Southeast Indian Ridge segment. These can be explained by variations in mantle depletion or mantle temperature. (2) Two large-scale, long-lived (duration of ~50 m.y.) asymmetries in crustal accretion of >2.5 km are observed around the Kerguelen Plateau and Balleny Islands, which we attribute to excess crust from the off-axis Kerguelen and Balleny mantle plumes. (3) Two large-scale, long-lived (duration of ~50 m.y.) asymmetries in crustal accretion of 0.75-2.5 km are observed on the northern flank of the westernmost (70-80°E) Southeast Indian Ridge and the southern flank of the eastern (120°- 140°E) Southeast Indian Ridge segment, respectively. We attribute these to asymmetry in mantle temperature of up to 20-53 °C. We suggest these asymmetric temperatures across the Southeast Indian Ridge are associated with the foundered lithospheric fragments of the Indian Craton triggered by the African Large Low-Shear-Velocity Province during the breakup of Gondwanaland and an intraplate subducted slab of the Paleo- Tethys Ocean, respectively. The remnant craton fragments and subducted oceanic slab may have moved north in concert with the northward-migrating Southeast Indian Ridge beginning at 50 m.y. ago [ABSTRACT FROM AUTHOR]

We propose that subducting slabs may cause lithospheric removal by directing mantle flow along the craton margin. This process could carve and shape the cratons, leading to conditions that impact the overall (in)stability of the lithosphere. We use three-dimensional geodynamic models to investigate how subduction-driven directed flow interacts with cratonic lithosphere of differing shape, concluding that the margin shape controls both channelization of flow around the craton as well as the potential for destruction. While the simulations show that all craton shapes aid in channelization, the cratons with straight vertical margins are the most resistant to deformation, and the cratons with gradually thickening margins are less resistant to deformation. The dependence on shape could contribute to the progressive removal of cratonic lithosphere along its margin in a runaway process until a more stable vertical margin shape evolves. [ABSTRACT FROM AUTHOR]

Sulphur is of particular importance in the subduction zone system, as it is thought to be intimately related to the arc redox state and metallogenesis above the slab. Knowing the sulphur isotope compositions of the subducted slab, and identifying the δ34S variations between the pre-subduction oceanic lithosphere and its high-pressure (HP) equivalents is of great importance for understanding the sulphur behaviour in subduction zones and the long-term sulphur recycling of the Earth. In this study, we undertook secondary ion mass spectroscopy (SIMS) in-situ δ34S analyses of sulphides from metasediments, mafic blueschists/eclogites, and ultramafic slab serpentinites from the Southwestern Tianshan HP–UHP metamorphic belt, which represents one of the deepest fossil subduction zones on Earth. By integrating the in-situ δ34S compositions of sulphides studied here and those reported from worldwide HP–UHP metamorphic terranes, we found that prograde to peak metamorphic sulphides of metasediments, metabasites/metagabbros, and slab HP serpentinites have δ34S values ranging from –33 to –6‰, –4 to +4‰, and +2 to +18‰, respectively. However, retrograde metasomatism of HP rocks during exhumation may alter the δ34S compositions of sulphides. The integrated dataset proposes that sulphides in subducting slab sequences retain their protolith's δ34S signatures during HP metamorphism. The δ34S inheritance indicates that no large isotope fractionation is expected to occur in subduction zones. It further suggests that evaluations of slab subduction effects on the δ34S signature of arc magmatism and the deep mantle should consider pre-subduction processes such as ridge geodynamic setting and hydrothermal alteration. [ABSTRACT FROM AUTHOR]

The Messinian Salinity Crisis (5.97–5.33 Ma) was caused by the closure of the Atlantic‐Mediterranean gateways that cut through the Gibraltar orogenic system. The geodynamic drivers underlying gateway closure and re‐opening are still debated. Here, we interrogate the gateway successions to find the imprints of surface deformation, infer the timing and nature of associated geodynamic drivers, and test such inferences against numerical simulations of slab dynamics. We find that since the latest Miocene, a tectonic framework was established in the gateway region dominated simultaneously by (a) relative plate convergence, (b) slab tearing under the eastern Betic Cordillera and (c) mantle resistance against north‐northeastward dragging of the Gibraltar slab by the African plate's absolute motion. We propose that mantle‐resisted slab dragging and slab tearing operated in concert closing the gateways that caused the Messinian Salinity Crisis, whereas sinking of heavy oceanic lithosphere located between buoyant continental plates re‐opened the Strait of Gibraltar at 5.33 Ma. [ABSTRACT FROM AUTHOR]

The high Sr/Y geochemical feature of granitoids can be attributed to various mechanisms, and elucidating genesis of high Sr/Y granitoids provides insights into the material recycling and magmatic processes at depth. In southeastern Central Asian Orogenic Belt (CAOB), many Middle Permian granitoids exhibit high Sr/Y ratios, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Palaeo-Asian ocean. Here we present new zircon U-Pb ages, Lu-Hf isotopes and whole-rock geochemical data for the Middle Permian high Sr/Y monzogranites from central Inner Mongolia, southeastern CAOB. LA-ICP-MS zircon U-Pb data shows that these high Sr/Y rocks were emplaced during 273–261 Ma. They are calc-alkaline, sodium-rich and metaluminous to weakly peraluminous, with enriched large-ion lithophile elements (Rb, Th, K and Pb) and depleted high field strength elements (Nb, Ta, P and Ti), suggesting a mafic lower crustal source rather than evolved potassic crustal materials. Their relatively low (Gd/Yb)N (1.1–2.0), (Dy/Yb)N (1.0–1.3), Nb/Ta (7.9–10.9) ratios and flat heavy rare earth element patterns are characteristics of derivation from a relatively shallow depth with amphibolite as dominant residue. They also have highly variable εHf(t) values (−8.2 to +10.0) and TDMC (1814 to 649 Ma), similar to those of the Early Palaeozoic high Sr/Y intrusions along the Bainaimiao arc belt. Combined with data from literatures, we suggest that the high Sr/Y monzogranites in this study were probably generated by reworking of the newly underplated juvenile high Sr/Y lower crust of the Bainaimiao arc belt. Moreover, taking into account the regional investigations, the sublinear distributed Middle Permian magmatic rocks in the southeastern CAOB were likely associated with the incipient slab break-off of the Palaeo-Asian oceanic lithosphere following initial collision between the North China craton and the South Mongolia terranes. [ABSTRACT FROM AUTHOR]

A total of 431 well‐defined and 632 null shear–wave splitting measurements obtained from 115 broadband seismic stations located in the eastern Himalayan syntaxis and adjacent areas is largely inconsistent with predicted fast orientations by absolute plate motion models. Spatial coherency analysis of the splitting parameters suggests that the observed azimuthal anisotropy is mostly located in the upper asthenosphere or the transitional layer between the lithosphere and asthenosphere, and the disagreement between the fast orientations and regional tectonic fabrics suggests an insignificant lithospheric contribution to the observed anisotropy. The observations may be attributed to flow systems that are driven by the westward rollback of the Indian slab beneath the Indo‐Burma block and are modulated by a previously revealed gap between the northward and eastward subducting slabs of the Indian plate. Key Points: Seismic anisotropy may be explained by a rollback of the Indian slab and modulation by a slab gapThe rotational pattern of fast orientations in SE Tibetan Plateau may be attributed to flow in the mantle wedge escaped from the slab gapN‐S anisotropy in NE India and Burma block and E‐W anisotropy in Indochina may reflect sub‐slab trench parallel flow from slab rollback [ABSTRACT FROM AUTHOR]

[Display omitted] • The Gaga diorite with adakitic features was generated from Neo-Tethyan oceanic slab. • Mafic enclave was formed by mixing of melts from lithosphere mantle and oceanic slab. • The genetic transition of Eocene granitoids implies the process of slab breakoff. The India-Asia collision holds significant implications for understanding continental collision dynamics. It is essentially controlled by various dynamic processes, including oceanic slab subduction, slab rollback, and slab breakoff, all of which are accompanied by extensive magmatic activities in convergent margins. However, the magmatic consequences of the Neo-Tethyan slab breakoff during the India-Asian collision remain ambiguous. In the middle segment of the southern Lhasa terrane, two episodes of magmatism (∼55 Ma and ∼ 50 Ma) have been preserved. In this study, we present whole-rock major and trace elements, zircon U-Pb ages, and whole-rock Sr-Nd-Hf isotopes of the ∼ 55 Ma Gaga pluton and conduct a comprehensive comparison with the two episodes of magmatic mixing events. The Gaga host diorites were formed by the partial melting of the Neo-Tethyan oceanic slab and underwent amphibole fractional crystallization. The Gaga gabbroic enclaves, which are associated with magma mixing, originated from lithosphere mantle metasomatized by subduction-related fluids. Notably, the Gaga pluton exhibits more prominent arc-type characteristics of Nb-Ta and Zr-Hf negative anomalies than the ∼ 50 Ma intrusive rocks in adjacent areas. The genetic transition of Eocene magmatic rocks indicates that the ∼ 55 Ma Gaga pluton formed during the initial stage of slab breakoff, and the ∼ 50 Ma magmatic pluton was produced during the late stage of slab breakoff. [ABSTRACT FROM AUTHOR]

Abstract Shear-wave splitting provides insight into geodynamic processes such as lithospheric deformation and upper mantle flow. This study presents shear wave splitting parameters determined from XKS (SKS, SKKS and PKS) and Pms phases (from receiver functions) recorded by the 2D seismic array, SANDWICH, deployed in central Tibet from November 2013 to April 2016. The XKS splitting measurements show a generally strong anisotropy with an average of 1.3 s, that even includes 17 stations with delay times no less than 2.0 s. Interestingly, significantly weak splitting is also observed between Nam Tso and Siling Tso near 90°E. Spatial coherence analysis of the splitting parameters indicates an upper asthenospheric source as the primary cause of seismic anisotropy in the study region. The regionally dominant ENE-WSW oriented upper-mantle anisotropy can emerge from the corner flow in the overlying mantle wedge induced by the subduction of the Indian lithospheric slab. The crustal anisotropy obtained through Pms splitting analysis might reflect lattice preferred orientation of anisotropic minerals formed by the ENE oriented middle-lower crustal flow, with a minor contribution to the strong anisotropy. The weak splitting observations in the east are likely caused by the nearly vertical α -axis of olivine induced by upwelling asthenosphere at a slab tear in the Indian lithosphere. Graphical abstract Highlights • A 2D seismic array SANDWICH was operated in central Tibet across BNS. • Splitting parameters of XKS and Pms waves were measured. • The strong anisotropy overall mainly comes from corner flow induced by Indian lithosphere. • The crust anisotropy caused by crustal flow contributes to the strong anisotropy. • The weak anisotropy near 90°E south of BNS is related to asthenosphere upwelling. [ABSTRACT FROM AUTHOR]

Variations of redox conditions (i.e., oxygen fugacity, f O 2) accompanying slab dehydration in subduction zones are subject to ongoing controversies, especially since the interplay between redox sensitive elements during prograde metamorphism remains complex and, likely, variable at the slab scale. Here we investigate f O 2 variations during serpentinite dehydration and their feedback on the stability of sulfur and carbon compounds by studying the eclogitic Monviso meta-ophiolite (Western Alps, Italy). Despite a complex metamorphic history, the Monviso massif has preserved a complete section of oceanic lithosphere, from seafloor metasediments, meta-ophicarbonates and metabasites to deep-seated metagabbros and metaserpentinites. By bringing new estimates in the northern massif, we show that these lithologies have recorded a homogeneous pressure and temperature (P-T) climax, at 520–570 °C and 2.6–2.7 GPa, on the whole meta-ophiolite. Despite this homogeneous P-T record, serpentinite forming minerals imply strong variations in f O 2 according to their position in the slab, from high f O 2 conditions (~ FMQ +2) in the deep-seated lithologies made of heazlewoodite-magnetite-olivine assemblages to low f O 2 (~ FMQ −4) in the paleoseafloor lithologies made of pentlandite-awaruite-olivine. This redox gradient is opposed to what is expected at mid-ocean ridges and is therefore likely set up during serpentinite dehydration. Such variations of f O 2 conditions also influenced carbon distribution and redox state within the lithosphere. In particular, reducing conditions associated with brucite breakdown in paleoseafloor serpentinites promote the formation of disordered carbonaceous matter over inorganic carbonates. Newly-formed disordered carbonaceous matter could subsequently be recycled in the deep mantle, with the potential to play a major role on the deep carbon cycle. • Heterogeneous oxygen fugacity recorded in serpentinites of the Monviso meta-ophiolite. • Subducting slab redox gradient set up during serpentinite dehydration at eclogite facies P-T conditions. • Carbonaceous matter precipitation enhanced by carbonate reduction in the slab. [ABSTRACT FROM AUTHOR]

The propagation of slab break-off (slab tearing) is usually attributed to laterally variable plate convergence systems with a spatial transition between simultaneous oceanic subduction and continental collision. To study the process of slab tearing in a non-collisional geodynamic context, here we use a 3D thermo-mechanical numerical approach to model the oblique subduction of a homogeneous oceanic plate. We investigate the effects of the following parameters: (1) subduction obliquity angle, (2) age of oceanic slab, and (3) partitioning of boundary velocities (i.e., the ratio between the subduction component and the advance of the overriding plate in the total convergence). In our simulations, the retreat of the subduction zone leads to a thinning of the fore-arc and back-arc lithosphere, which are decoupled from the subducting slab by the rise of the hot asthenosphere from the underlying mantle wedge. As a consequence of the initial obliquity of the active plate margin, slab roll-back velocities are subject to progressive along-trench variations. Consistent with the gradual rotation of the trench, the front of the decoupling between the overriding and downgoing plates (together with predicted magmatic activity and topographic uplift) migrates in a horizontal direction. In the experiments with low angles of subduction obliquity (< 15°), relatively old subducting plates (> 50 Ma), and in the absence of the subduction component in the overall shortening, slab detachment either develops simultaneously along the entire length of the subduction zone or does not occur at all. In contrast, with higher subduction obliquity (≥ 15°), younger slabs (≤ 50 Ma) and in the presence of a boundary push on the oceanic side, the initial slab break-off is followed by the gradual growth of the "tear" window in the direction opposite to the migration path of the previously established plates decoupling. The sharp contrast in trench retreat rates between subduction zone segments affected and unaffected by slab detachment results in the arcuate shape of the trench. Furthermore, the direction of slab tearing may change from horizontal to vertical, eventually leading to the formation of a transform fault on the subducting plate. Our results show striking similarities with several features – such as trench curvature, subduction zone segmentation, magmatic production, lithospheric stress/deformation fields, and associated topographic changes – observed in many subduction zones (e.g., Marianas, New Hebrides, Mexico, Calabrian). • The propagation of slab break-off is modelled in non-collisional settings. • Initial subduction obliquity leads to along-trench variations in slab retreat rates. • The young age of the subducting plate and high subduction obliquity favour slab tearing. • Originally horizontal slab tearing can turn into a vertical transform fault. • Slab tearing favours strong trench curvature as observed in many subduction zones. [ABSTRACT FROM AUTHOR]

Water transported by deep subduction to the mantle transition zone (MTZ) that is eventually released and migrates upwards is invoked as a likely cause for hydroweakening and cratonic lithosphere destruction. The destruction of the North China Craton (NCC) during the Mesozoic has been proposed to be related to hydroweakening. However, the source of water related to large-scale craton destruction in the NCC is poorly constrained. Some suggest that the water was mainly released from a flat-lying (or stagnating) slab in the MTZ, whereas others posit that most water was released from a previously existing strongly hydrous MTZ then perturbed by the stagnating subduction in the MTZ layer. In this study, we use numerical modeling to evaluate the water carrying ability of flat-lying slabs in the MTZ with different slab ages and water contents to simulate its maximum value and discuss its potential role on large-scale hydroweakening and craton destruction. Our results reveal that a single flat-lying slab in the MTZ cannot provide enough water for large-scale cratonic lithosphere hydroweakening and thinning. Water estimates invoked for craton destruction as experienced by the NCC can only be the result of long-term piling of multiple slabs in the MTZ or penetrating deeper into the lower mantle. [ABSTRACT FROM AUTHOR]

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

The oceanic plateau is proposed to be produced by the plume head. Seeking the missing plume head associated with seamount chains is an important task to evaluate the plume theory. While the subduction initiation and process of the oceanic plateau is debatable, recent seismic studies suggest that the Hawaiian-Emperor oceanic plateau has been subducted into the lower mantle. The sinking rate is higher than previously estimated and even higher than that of the normal slab. It is intriguing, as the phase transitions would cause a density barrier for the basaltic materials from ∼660-km to ∼720-km depths, and slow down the subduction of the slabs, especially for the oceanic plateau with a thickened basaltic crust. Here, we perform geodynamic numerical experiments to investigate the foundering process of flat stagnant slabs from the transition zone into the lower mantle. We compare models of both the normal slab and slabs bearing oceanic plateau and suggest that a basaltic underplate layer beneath the peridotite mantle lithosphere in the oceanic plateau can lead to a higher foundering rate. The underplated oceanic plateau can reach ∼1000-km depth in ∼50 Myrs. These models provide a new perspective in understanding the fate of subducted oceanic plateau, suggesting that missing plume heads might be found in the lower mantle. • Provides a new perspective in seeking subducted plume head in the mantle. • Presents various foundering processes of flat stagnant slabs from the transition zone into the lower mantle. • Underplated oceanic plateau can reach ∼1000-km depth in ∼50 Myrs, agreeing with recent seismic imaging studies. [ABSTRACT FROM AUTHOR]

A large volume of continental lithosphere should have been emplaced beneath the Tibetan Plateau during the convergence between the Indian and Asian plates. However, geophysical studies have imaged an obviously thickened crust and a normal to slightly thickened mantle lithosphere beneath the plateau. Most of the additional mantle lithosphere is proposed to have detached and descended into the deep mantle to the mantle transition zone (MTZ). Recent studies suggest that the Indian lithospheric slab has torn into sections with different subduction angles. However, the locations of these slab sections, especially the high-angle one, require further structural constraints. Here, we used newly collected data from the broadband seismic network SANDWICH in central Tibet to study the thickness of the MTZ using the receiver function method. Our results show that there are two MTZ regions that are approximately 15 km thicker than the average one. One region is beneath the Lhasa terrane centered at 90°E, 31°N, and the other one is beneath the Qiangtang terrane centered at 88°E, 33°N. Because the upper and lower boundaries of the MTZ are both rock phase transition discontinuities that are sensitive to temperature. We interpret the thickened MTZ beneath the plateau as evidence of the presence of cold Indian slab and mantle lithosphere. The high-angle Indian lithospheric slab between the Yadong-Gulu Rift and the Pumqu-Xianza Rift is subducting deep into the MTZ, producing a decrease in temperature and thickening the MTZ beneath Lhasa terrane. In contrast, the Qiangtang anomaly corresponds to a piece of delaminated thickened Tibetan lithosphere. Based on the extensive distributions of Eocene to Oligocene potassic rocks and Mid-Miocene to Quaternary potassic rocks, we suggest that the thickened mantle lithosphere beneath the northern Qiangtang terrane and Songpan-Ganzi terrane detached in a piecewise manner, resulting in subsequent plateau uplift and recent potassic volcanism. [ABSTRACT FROM AUTHOR]

We use numerical models to investigate the dynamics of two interacting slabs with parallel trenches. Cases considered are: a single slab reference, outward dipping slabs (out-dip), inward dipping slabs (in-dip) and slabs dipping in the same direction (same-dip). Where trenches converge over time (same-dip and out-dip systems), large positive dynamic pressures in the asthenosphere are generated beneath the middle plate and large trench-normal extensional forces are transmitted through the middle plate. This results in slabs that dip away from the middle plate at depth, independent of trench geometry. The single slab, the front slab in the same-dip case and both out-dip slabs undergo trench retreat and exhibit stable subduction. However, slabs within the other double subduction systems tend to completely overturn at the base of the upper mantle, and exhibit either trench advance (rear slab in same-dip), or near-stationary trenches (in-dip). For all slabs, the net slab-normal dynamic pressure at 330 km depth is nearly equal to the slab-normal force induced by slab buoyancy. For double subduction, the net outward force on the slabs due to dynamic pressure from the asthenosphere is effectively counterbalanced by the net extensional force transmitted through the middle plate. Thus, dynamic pressure at depth, interplate coupling and lithospheric stresses are closely linked and their effects cannot be isolated. Our results provide insights into both the temporal evolution of double slab systems on Earth and, more generally, how the various components of subduction systems, from mantle flow/pressure to interplate coupling, are dynamically linked. [ABSTRACT FROM AUTHOR]

Adakitic and potassic magmatic rocks are widespread in the Himalaya and the Tibetan Plateau, and their petrogenesis might provide potential insights into the deep crustal processes. However, the detailed spatiotemporal distributions of these rocks and their controlling mechanisms remain ambiguous. Here we report geochemical data, zircon U-Pb ages and Hf isotopic compositions of east-west (E-W) directed Qunrang monzonite porphyry dikes located in the Tethyan Himalaya. These rocks are acid and potassic with low Mg, low abundances of compatible elements, low Y and HREE, but with high Al, high Sr (822–1386 ppm) and Ba (2027–6091 ppm), high Th (62–84 ppm), U (9–16 ppm) and Th/U ratios (4–7), and thus high Sr/Y and (La/Yb) N ratios. The above geochemical features, combined with their negative whole-rock ε Hf (t) values (−3.9 to −4) and negative to small positive zircon ε Hf (t) values (−7.2 to +0.5), suggest that they were derived from partial melting of a thickened lower continental crust consisting mainly of garnet-amphibolite which were triggered and hybridized by underplating enriched mantle-derived shoshonitic melts. Zircon U-Pb ages of the Qunrang monzonite porphyry yielded a weighted mean age of 10.02 ± 0.33 Ma. Based on these observations, synthesized with the spatiotemporal distribution of the Oligocene-Miocene adakitic and potassic rocks in the southern Tibet, we propose that lateral detachment and longitudinal tearing processes might be potentially accounted for the eastward and southward younging trend of these magmatic rocks, whereas slab rollback and hinge advance might result in the unique E-W directed Qunrang adakite-like potassic dikes and the northward reversal younging trend. [ABSTRACT FROM AUTHOR]

The Cimmerian blocks are ribbons-like micro-continents that detached from the northern margin of Gondwana during the closure of the Paleotethys ocean at the end of the Permian. While it has been hypothesized that slab pull forces induced by subduction of the Paleotethys beneath Eurasia could have driven extension in the lower plate after the Paleotethys mid-oceanic ridge subduction, so far, numerical modeling studies have concluded that these forces alone are not sufficient to drive continental breakup. However, the previous modeling studies neglect olivine phase transitions in the mantle and likely underestimated the slab pull. Here, we present numerical simulations that consider the evolution of the density of the slab associated with metamorphic reactions and analyze carefully when, where, and how the slab pull can lead to the formation of micro-continents. Our results show that the marked increase in slab density related to olivine phase transitions at 410 km discontinuity, in addition to gravitational potential energy, can cause continental rifting of the lower plate. Nonetheless, continental breakup is only achieved in absence of horizontal shear flow at the base of the continental lithosphere. In these successful experiments, the deformation localizes 100-200 km on shore due to bending stress pointing out that this process probably always leads to the formation of small continental ribbons. Our results confirm that the tensional force induced by the Paleotethys subduction dynamics may have caused the detachment of the Cimmerian blocks. Since continental breakup of the lower plate is promoted in the absence of basal shear below Gondwana's lithosphere (with respect to the underlying mantle), we argue that the formation of the Cimmerian blocks provides information on the relative motion between the tectonic plates and the underlying mantle. Our simulations demonstrate that there is a significant time-lag between ridge subduction and continental breakup (i.e. the time needed for the slab to reach the 410 km discontinuity), which is rarely accounted by paleogeographic reconstructions. • Consider realistic density increase slab pull contribution to plate force budget. • Slab pull can trigger continental breakup in the lower plate after ridge subduction. • Models confirm a delay between Cimmerian blocks break-up and ridge subduction. • Instance of lower plate breakup may provide information on relative plate motion. • There is predictable a time-lag between ridge subduction and continental breakup. [ABSTRACT FROM AUTHOR]

The whole rock compositions of the blocks and the surrounding matrix of the Dalrymple Amphibolite are investigated in this study to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite. Similarities in their rare earth and trace element patterns indicate the genetic relationship between the mafic lavas of the Palawan Ophiolite and the metamafic blocks of the Dalrymple Amphibolite. This confirms that the metamafic blocks represent the basalt to gabbro section of the oceanic lithosphere of the subducting slab. The matrix surrounding the blocks exhibit highly variable phase assemblages. In order to determine its petrogenesis, we distinguished groups of components/elements which behave similarly (Group 1 TiO 2 , Al 2 O 3 , Zr, Th and the light rare earth elements; Group 2 Cr, Ni and MgO) based on statistical (correlation coefficient) analyses. These groups indicate mixing of metasedimentary (Group 1) and metamafic (Group 2) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO 2 and Al 2 O 3) in the matrix samples were fitted against a modelled concentration by changing the end-member and their relative proportions. The end-members and mixing ratio with the highest regression value (r2) was selected to obtain the modelled composition of the matrix. The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO 2 , Al 2 O 3 , Cr, Nd, Zr, and Hf are immobile. This assumption is supported by the prevalence of kyanite, ilmenite and zircon in the matrix mineral assemblage. This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. In particular, the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones. Mass balance calculations further revealed that an early fluid infiltration event likely occurred following the mixing process. This preferentially leached out elements which are either fluid-mobile (e.g. CaO and SiO 2) or are not incorporated into the growing minerals in the matrix. The strong control of mineral assemblage of the matrix in its chemistry is exhibited by a number of samples which showed variable degrees of losses and gains in elements traditionally interpreted to be fluid immobile (e.g. heavy rare earth elements and Y). A later hydration event linked to retrograde metamorphic stage imprinted gains of K 2 O, Rb, and Ba in the matrix samples with the growth of replacement minerals (e.g. muscovite on kyanite). This later fluid infiltration event possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction. • Mélange sequence formed on the slab-mantle wedge interface of an incipient arc. • Metamafic blocks derived from the basalt-gabbro section of the subducting slab. • Petrogenesis of the matrix determined through statistics and isocon analyses. • Mixing and fluid infiltration affected the matrix of the Dalrymple Amphibolite. • Distinct fluid generations leached out some elements while adding others. [ABSTRACT FROM AUTHOR]