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The left‐lateral Altyn Tagh Fault (ATF) system is the northern boundary of the Tibetan Plateau resulted from the India–Eurasia continental collision. How intracontinental deformation across the central ATF responds to the distal collision remains elusive, primarily due to unclear crustal structure. We obtained detailed crustal structure across the central ATF using receiver functions recorded by ∼NW–SE oriented linear dense array. The images reveal the Tarim lower crust is underthrusting beneath the Tibetan Plateau and reaches to a maximum depth of ∼75 km and undergoing partial eclogitization. The two south‐dipping interfaces imaged beneath the Altyn Tagh Range (ATR) represent the thrusting Northern Altyn Fault and its branch fault. Oblique convergent forces extruded upper crustal materials along the thrust faults, creating the pop‐up structure of ATR, supported by low Vp/Vs ratios. Our balanced cross‐section for the Moho suggests intracontinental deformation in the ATR has accelerated since the late Miocene. Plain Language Summary: The Altyn Tagh Fault (ATF), serving as the northern boundary of the Tibetan Plateau, demarcates the Tarim Basin from the Qaidam Basin. Understanding how intracontinental deformation across the boundary region would better inform the uplift and expansion of the plateau. This study reveals the fine crustal structure by analyzing seismic data from a ∼NW–SE oriented linear dense array across the central ATF. Combined with fault slip rates, we propose that the Tarim lower crust is underthrusting beneath the Tibetan Plateau, leading to the extrusion of upper crustal materials and the rapid uplift of the Altyn Tagh Range since the late Miocene, which provides insight into the lateral growth of the plateau. Key Points: Detailed crustal structure beneath the central Altyn Tagh Fault was imaged by receiver functions of a dense 2‐D seismic arrayThe Tarim lower crust is underthrusting to ∼75 km depth beneath the Tibetan PlateauThe Altyn Tagh range was uplifted rapidly since late Miocene through the thickening of the upper crust [ABSTRACT FROM AUTHOR]

The Sierra Nevada de Santa Marta (SNSM) in northwestern Colombia is one of the world's highest coastal mountains, with an elevation above 5 km. Gravity measurements show that the SNSM has a positive Bouguer anomaly (>+130 mGal), indicating that the mountain lacks a crustal root. In this work, we test the hypothesis that these observations can be explained by gravitational removal of the dense lower lithosphere. We use 2D numerical models to examine the dynamics of lithosphere removal and its effect on surface elevation and gravity anomaly. The models consist of continental lithosphere that includes a pre‐thickened crustal region, representing the SNSM. In our preferred model, the dense mantle lithosphere and crustal root are gravitationally unstable and undergo removal as local drips within ∼10 Ma from the onset of foundering. This creates an area of thinned crust (∼38 km) underlain by a buoyant sublithospheric mantle where melting and low seismic velocities are predicted. Subsequent non‐isostatic forces maintain a topography of 3.3 km with a 2D Bouguer gravity anomaly of +103 mGal. Parameter tests show that a strong lower‐crustal rheology provides greater support for the high topography and that a weak mantle lithosphere rheology produces faster removal. The models demonstrate that local lithosphere dynamics can explain the first‐order observations in the SNSM. We propose that lithosphere removal could have occurred recently (∼2 Ma), explaining the localized low seismic velocity zone below the SNSM, or at 56–40 Ma, inducing anomalous short‐lived Eocene magmatism. Plain Language Summary: Most high mountain regions are formed during continental deformation near tectonic plate boundaries, creating an area of thick low‐density crust. The Sierra Nevada de Santa Marta (SNSM) is an unusual mountain in northwestern Colombia. It is isolated from the main Andes mountain belt, but it has an elevation above 5 km and geophysical measurements indicate that the crust is unexpectedly thin. Here, we test the idea that the SNSM once had a thick crust, but the lower part of this layer was anomalously dense and sank into the deep mantle. We use 2D computer simulations to investigate this phenomenon and its effect on the Earth's surface. The models start with a ∼125 km wide area with a thicker crust and a deflection in the underlying rigid mantle lithosphere. Because the mantle lithosphere is cooler and denser than the mantle below, it sinks as a drip. This process induces an increase in the density of the lower crust such that it also sinks as a drip. The models show that the removal holds a high elevation region with a locally thin crust that is underlain by a low seismic wave velocity zone and melting, consistent with SNSM observations. Key Points: Mantle lithosphere and crustal root are gravitationally removed as local drips, generating a thinned crust within 10 MaA removal event can explain a localized and positive Bouguer gravity anomaly in the elevated Sierra Nevada de Santa Marta, ColombiaNon‐isostatic forces have a significant role in the support of high topography during removal [ABSTRACT FROM AUTHOR]

Abstract The left‐lateral Altyn Tagh Fault (ATF) system is the northern boundary of the Tibetan Plateau resulted from the India–Eurasia continental collision. How intracontinental deformation across the central ATF responds to the distal collision remains elusive, primarily due to unclear crustal structure. We obtained detailed crustal structure across the central ATF using receiver functions recorded by ∼NW–SE oriented linear dense array. The images reveal the Tarim lower crust is underthrusting beneath the Tibetan Plateau and reaches to a maximum depth of ∼75 km and undergoing partial eclogitization. The two south‐dipping interfaces imaged beneath the Altyn Tagh Range (ATR) represent the thrusting Northern Altyn Fault and its branch fault. Oblique convergent forces extruded upper crustal materials along the thrust faults, creating the pop‐up structure of ATR, supported by low Vp/Vs ratios. Our balanced cross‐section for the Moho suggests intracontinental deformation in the ATR has accelerated since the late Miocene.

Recent geophysical campaigns in the Alps produce images with seismic property variations along the slab of sufficiently fine resolution to be interpreted as rock transformations. Since the reacting European lower crust is presumed responsible for the variations of velocities at the top of the Alpine slab, we sampled local analogs of the lower crustal lithologies in the field and modeled the evolution of equilibrium seismic properties during burial, along possible pressure‐temperature paths for the crustal portion of the slab. The results are then compared to the range of the S‐wave velocities obtained from the S‐wave velocity tomography model along the CIFALPS transect. The velocity increase from 25 to 45 km within the slab, in the tomographic model is best reproduced by the transformation of specific lithologies in the high‐pressure granulite facies along a collisional gradient (30°C/km). Although the crust is certainly not completely homogeneous, the best candidates for the rocks that make up the top of the Alpine dip crustal panel are a kinzigite from Monte San Petrone, a gneiss from the Insubric line, and blueschist mylonite from Canavese. While they may not represent the entirety of the crust, they are sufficient to explain the tomographic velocity of the Alpine slab. A lateral lithological contrast inherited from the Variscan orogeny is not required. Eclogitization, suggested as the first‐order transformation in convergence zones, could be a second‐order transformation in collisional wedges. These results also imply a partially re‐equilibrated thermal gradient, consistent with the Alpine thermal state data at depth. Plain Language Summary: Tomography, that is, imaging of deep geological structures based on seismic wave travel to time anomalies, is now so sensitive that it allows us to see changes in the properties of rocks buried at depth beneath mountain belts. In the Alps, the European plate is imaged down to 80 km and shows a sharp velocity increase close to 30 km depth. By calculating the bulk seismic wave velocities on exhumed analogs sampled throughout the Western Alps, the present study proposes to interpret the velocity jump as the consequence of the transformation of the European lower crust from amphibolite to granulite (the high‐temperature metamorphic rocks produced during a collision) rather than the usually admitted transformation to eclogite (the higher pressure metamorphic rocks produced during subduction). This has implications regarding the present‐day thermal structure of the Alps: the Western Alps are not a frozen subduction zone but a collision zone exposing subduction‐related rocks and structures at the surface only. Key Points: The lower crustal top of the European Alpine slab is mostly composed of felsic to intermediate rocksThe transformation of hydrated rocks into HP granulites along a collision gradient reproduces the slab tomographic velocity increase at 30 kmThe often‐supposed eclogitization produces velocities that are significantly higher than the crustal top of the European Alpine slab [ABSTRACT FROM AUTHOR]

Abstract Recent geophysical campaigns in the Alps produce images with seismic property variations along the slab of sufficiently fine resolution to be interpreted as rock transformations. Since the reacting European lower crust is presumed responsible for the variations of velocities at the top of the Alpine slab, we sampled local analogs of the lower crustal lithologies in the field and modeled the evolution of equilibrium seismic properties during burial, along possible pressure‐temperature paths for the crustal portion of the slab. The results are then compared to the range of the S‐wave velocities obtained from the S‐wave velocity tomography model along the CIFALPS transect. The velocity increase from 25 to 45 km within the slab, in the tomographic model is best reproduced by the transformation of specific lithologies in the high‐pressure granulite facies along a collisional gradient (30°C/km). Although the crust is certainly not completely homogeneous, the best candidates for the rocks that make up the top of the Alpine dip crustal panel are a kinzigite from Monte San Petrone, a gneiss from the Insubric line, and blueschist mylonite from Canavese. While they may not represent the entirety of the crust, they are sufficient to explain the tomographic velocity of the Alpine slab. A lateral lithological contrast inherited from the Variscan orogeny is not required. Eclogitization, suggested as the first‐order transformation in convergence zones, could be a second‐order transformation in collisional wedges. These results also imply a partially re‐equilibrated thermal gradient, consistent with the Alpine thermal state data at depth.

Geotribometamorphism means a process of destructive-constructive changes of the rocks in the seismotectonic zones in the Earth's crust. Due to the friction between rock blocks and layers, kinetic energy is generated, which deforms and destroys the rocks until mylonitization and melting in an environment of high temperature and pressure. Subsequently, the disintegrated material recrystallized into new high-pressure rocks such as eclogites, garnet lherzolites, phengite and kyanite schists and calciphyres. These rocks mark paleoseismic zones and events that occurred during the Precambrian and Phanerozoic. [ABSTRACT FROM AUTHOR]

The incipient development of diagnostic high‐pressure assemblages—the 'eclogitization'—of granitoids, such as plagioclase breakdown and small‐scale formation of garnet and phengite does not require exogenous hydration because unlike dry protoliths like basalt/gabbro or granulite, granitoids s.l. contain crystallographically bound H2O in biotite. During high‐pressure overprint, partial biotite breakdown causes a localized increase in the chemical potential of H2O (μH2O). Transport of H2O into nearby plagioclase induces the formation of diagnostic eclogite facies assemblages of jadeite–zoisite–K‐feldspar–quartz ± kyanite ± phengite that pervasively replace former cm‐sized plagioclase without requiring the participation of free H2O. Depending on pressure–temperature evolution, similar textures may involve albite instead of jadeite, consistent with the general absence of Na‐clinopyroxene in high‐pressure metagranitoids and kindred gneisses. Plagioclase breakdown may also occur due to simple burial because compression leads to an increase of μH2O, without requiring additional influx of H2O at the texture scale. However, the addition of biotite‐derived H2O into plagioclase sites likely increases reaction rates. In parallel, ∼100‐μm‐thick complementary coronae involving garnet | phengite–quartz develop at former biotite–plagioclase/K‐feldspar interfaces due to the coupled diffusion of FeO–MgO–H2O from biotite towards feldspars and minor CaO in the opposite direction. The reaction textures likely create structural weaknesses and preferential fluid pathways that facilitate further hydration and/or deformation along the prograde path, thereby obliterating the textures. If exogenous H2O is introduced, it is accommodated in phengite growing at the expense of igneous K‐feldspar and possibly in epidote‐group minerals. Upon decompression, such hydrated rocks would dehydrate, thereby favouring fluid‐assisted retrogression and loss of diagnostic eclogite facies assemblages at lower pressure. Whereas the prograde reaction textures are only preserved at closed‐system conditions and in the absence of deformation, they are suggested to commonly form during orogenic metamorphism of granitoids and quartzofeldspathic gneisses that dominate the continental crust in high‐pressure terranes such as the Western Italian Alps and the Western Gneiss Region (Norway). [ABSTRACT FROM AUTHOR]

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H2O in the form of OH groups. H2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H2O, pyroxene from ~ 50 to ~ 310 µg/g H2O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H2O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H2O increase from ~ 450 to ~ 2500 µg/g H2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform. [ABSTRACT FROM AUTHOR]

Botswana is a country with relatively low seismic activity that experienced an unexpected Mw 6.5 earthquake on 3 April 2017. Using data from the first countrywide seismic network, we established a Botswana earthquake catalog for the period January 2014 to February 2018. Two areas of elevated seismic activity were detected. The first one is the Okavango Rift Zone in northern Botswana, an area that is known to be active. The other one is associated with the 2017 mainshock and its aftershocks in central Botswana; it follows the Paleoproterozoic suture between the Limpopo Belt and the Kaapvaal Craton. Double‐difference relocation of these events revealed a reactivated fault system with a northwesterly strike with the aftershocks occurring at shallower depths than the mainshock at 29 km. The focal mechanisms of the mainshock and selected aftershocks are of normal faulting type with strikes similar to the orientation of the fault system. The unidirectional rupture of the mainshock in the lower crust combined with the upward migration of the aftershocks along the Moiyabana Fault and a thin low velocity anomaly the uppermost mantle are consistent with the events being produced by eclogitization of a dry metastable granulite facies rock by fluid intrusion from the mantle in an extensional stress regime. The Okavango Rift Zone is generally interpreted as the terminus of the southwestern branch of the East African Rift System. We suggest that the recent earthquakes in central Botswana may be considered as the southern continuation of this branch. Plain Language Summary: Earthquakes do not happen very often in Botswana, but an unexpected magnitude 6.5 earthquake occurred on 3 April 2017. We investigated earthquake occurrence in Botswana for the period January 2014 to February 2018 using data from the first countrywide seismic network. We found that the magnitude 6.5 earthquake in central Botswana was located deep in the crust and that the aftershocks were all at shallower depths. The main‐ and aftershocks were located along the boundary between two cratonic blocks that were welded together 2.6 Gyr ago. The earthquakes show that this fault zone is now reactivated in a stress regime that leads to rifting. It is likely that the earthquakes are caused by fluid infiltration from the mantle into the crust, altering the rock of the ancient fault zone. It is generally thought that the seismically active Okavango Rift Zone in northern Botswana is the (southern) end of the southwestern branch of the (continental scale) East African Rift Zone. We suggest that this branch has progressed southwards through central Botswana. Key Points: New Botswana earthquake catalog indicates rifting along Okavango Rift Zone and Limpopo BeltLower crustal earthquakes triggered by mantle fluids and eclogitization of dry metastable granulite rockSouthwestern branch of East African Rift System may continue through central Botswana [ABSTRACT FROM AUTHOR]

Abstract To analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data-processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1 s and two-dimensional for periods greater than 10 s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80–100 km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.

Metamorphic transformation rates are classically determined on decimetre‐scale field samples and from laboratory experiments at smaller scales. Here we present a geophysical approach based on field data and joint geophysical–petrological modelling to quantify the average rate of metamorphic transformations at the 10–100‐kilometre and million‐year scales. The model simulates the eclogitization of Indian lower crust as it penetrates beneath southern Tibet. Metamorphic transformation of the lower crust is tracked by its densification, the effect of which is then compared to observed gravity anomalies. From the modelling we find that the Indian lower crust's overall densification requires a partially hydrated initial composition. Moreover, the modelled evolution of this densification compared to what is predicted by pressure–temperature–density grids is consistent with delayed, far‐from‐equilibrium metamorphism. The Indian lower crust descends underneath the Himalaya until beneath southern Tibet in a thermodynamically metastable state until the first dehydration reactions are reached. This observation is used to determine the average rate of metastable rock transformation to an eclogite facies assemblage, constrained at between ~6 × 10−9 and 5 × 10−7 g/cm2/year, and reaction affinity at 0.8–1.6 kJ/mol oxygen. Compared to field and laboratory data, this range of results matches the effective rates typically associated with regional metamorphism. This fit and correlation across the scales legitimates the use of transformation rates determined at small scales in large‐scale geodynamic studies. [ABSTRACT FROM AUTHOR]

Using data from 186 stations belonging to the USArray Transportable Array, a three‐dimensional shear wave velocity model for the southeastern United States is constructed for the top 180 km by a joint inversion of receiver functions and Rayleigh wave phase velocity dispersion computed from ambient noise and teleseismic earthquake data. The resulting shear wave velocity model and the crustal thickness and Vp/Vs (κ) measurements show a clear spatial correspondence with major surficial geological features. The distinct low velocities observed in the depth range of 0–25 km beneath the eastern Gulf Coastal Plain reflect the thick layer of unconsolidated or poorly consolidated sediments atop the crystalline crust. The low κ (1.70–1.74) and slow lowermost crustal velocities observed beneath the eastern Southern Appalachian Mountains (including the Carolina Terrane and Inner Piedmont) relative to the adjacent Blue Ridge Mountains and Valley and Ridge can be interpreted by lower crustal delamination followed by relamination. The Osceola intrusive complex in the central Suwannee Terrane has similar crustal characteristics as the eastern Southern Appalachian Mountains and thus can similarly be attributed to crustal delamination/relamination processes. The Grenville Province and adjacent areas possess relatively high κ values which can be attributed to mafic intrusion associated with crustal extension in a recently recognized segments of the eastern arm of the Proterozoic Midcontinent Rift. Plain Language Summary: The southeastern United States accommodates some of the most diverse geological features in the world, as a result of several episodes of continental rifting, formation of ocean basins, and continental collision over the past several billion years. Imaging the interior structure in the area provides key information needed for better understanding how the Earth has operated and evolved over its history. In this study we use ground vibrations generated by distant earthquakes and ambient noise to image the structure in the top 180 km beneath the Southern Appalachian Mountains and the bordering coastal plains. The observations suggest that the eastern part of the Southern Appalachian Mountains, including the Carolina Terrane and the Inner Piedmont, may have lost its original lower part of the crust which has been replaced by a layer of more felsic (i.e., rich in silicate minerals) materials derived from the mantle. The measurements also support a recently proposed model that the Midcontinent Rift could have extended more southward than previously thought, and provide some intriguing preliminary observations for additional investigations, such as the low mantle velocities and their possible linkage with an area of thinner than normal lithosphere in northern Florida. Key Points: Crustal and mantle structure for the top 180 km beneath southeastern U.S. was imaged using receiver functions and Rayleigh wave dispersionResults support lower crustal delamination/relamination in the Carolina Terrane and Inner Piedmont of the southern Appalachian MountainsHigh crustal Vp/Vs values observed in the Grenville Province suggest mafic intrusion associated with the Proterozoic Midcontinent rift [ABSTRACT FROM AUTHOR]

The partially eclogitized crustal rocks on Holsnøy in the Bergen Arcs, Norway, indicate that eclogitization is caused by the interplay of brittle and ductile deformation promoted by fluid infiltration and fluid‐rock interaction. Eclogitization generated an interconnected network of millimeter‐to‐kilometer‐wide hydrous eclogite‐facies shear zones, which presumably caused transient weakening of the mechanically strong lower crust. To decipher the development of those networks, we combine detailed lithological and structural mapping of two key outcrops with numerical modeling. Both outcrops are largely composed of preserved granulite with minor eclogite‐facies shear zones, thus representing the beginning phases of eclogitization and ductile deformation. We suggest that deformation promoted fluid‐rock interaction and eclogitization, which gradually consumed the granulite until fluid‐induced reactions were no longer significant. The shear zones widen during progressive deformation. To identify the key parameters that impact shear zone widening, we generated scale‐independent numerical models, which focus on different processes affecting the shear zone evolution: (i) rotation of the shear zones caused by finite deformation, (ii) mechanical weakening due to a limited amount of available fluid, and (iii) weakening and further hydration of the shear zones as a result of continuous and unlimited fluid supply. A continuous diffusion‐type fluid infiltration, with an effective diffusion coefficient around D=10−16m2s, coupled with deformation is prone to develop structures similar to the ones mapped in field. Our results suggest that the shear zones formed under a continuous fluid supply, causing shear zone widening, rather than localization, during progressive deformation. Key Points: Continuous fluid supply causes shear zone wideningShear zones widen during strain accumulation [ABSTRACT FROM AUTHOR]

To analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data-processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1 s and two-dimensional for periods greater than 10 s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80–100 km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range. [ABSTRACT FROM AUTHOR]

The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (

Growing evidence supports that the widely developed Mesozoic folds and magmatism in the South China Block are attributed to the flat‐slab subduction, followed by slab steepening, of the Paleo‐Pacific Plate (PPP). However, the dynamics of the transition from flat‐ to steep‐slab subduction, as well as the relationship between tectonic processes and surface deformation and magmatic events, are poorly constrained. Using a 2‐D thermomechanical numerical model, we systematically investigate the effects of slab rollback or delamination following the eclogitization of a subducted oceanic plateau on the surficial topography, structural deformation, and magmatism. Our results indicate that the subduction of an oceanic plateau drives flat‐slab subduction, which further leads to the development of a wide thrust fault zone, an uplifted and magmatically active foreland, and the foreland basin subsidence. This model is further complicated by the transition from flat‐ to steep‐slab subduction through slab rollback or delamination which is controlled by the subduction rate and the timing of oceanic plateau eclogitization. The comparisons between our numerical results and the geological data from South China reveal that the slab delamination model reproduces the regional evolution of the Mesozoic South China. Based on our models, we suggest that the eclogitization during flat‐slab subduction is not instantaneous, which is instead at least 18 Myr after the oceanic plateau reaches the ideal conditions of phase change. Key Points: Eclogitization of oceanic plateau starts at 18 Myr after initial flat‐slab subductionSlab rollback versus delamination are controlled by subduction rate and timing of eclogitizationMesozoic South China was controlled by flat‐slab subduction and later slab delamination [ABSTRACT FROM AUTHOR]

Abstract We examine spatio-geometric patterns in the density of seismicity within the Cascadia forearc to gain insight into controls on seismogenesis. Tremor epicenters exhibit the most regular distribution defining a 40–80 km wide band extending along almost the entire convergent margin from the southern terminus of Gorda plate subduction in northern California to the central Explorer plate below northern Vancouver Island. Based on prior characterization of constituent low-frequency earthquakes, the up- and down-dip limits of tremor are assumed to represent slab isodepth contours at nominal values of 28 and 45 km, between which fluids at near-lithostatic overpressures are trapped within the subducting slab. Epicenters of earthquakes within the North American crust are anticorrelated with those of tremor, and concentrated in Washington and northern California where they are sandwiched between tremor and the Cascade volcanic arc. Seismicity within the subducting plate possesses the most limited epicentral distribution. Seismicity is confined to shallow depths off Vancouver Island and in northern California, and projects to greater depths beneath Washington and southern British Columbia. Comparison of seismicity patterns with long-wavelength slab geometry and thermo-petrologic constraints suggests that seismicity occurrence in Cascadia is governed by an interplay between slab strain, metamorphic dehydration within the subducting oceanic plate, and a plate boundary seal that controls where fluids enter the overriding plate. The inferred fluid evolution model harbors interesting implications for mantle wedge hydrology, forearc crustal composition and volcanism in Cascadia and other warm subduction zones. Highlights • Seismicity density in Cascadia is analysed in context of slab strain, metamorphic reactions and plate boundary permeability • Tremor serves as proxy for plate boundary seal and supplies constraints on slab depth • Crustal seismicity concentrates where fluids leave subducting plate in regions of high slab curvature • Fluid-evolution model holds implications for mantle wedge hydrology, forearc crustal composition and volcanic extrusion rates [ABSTRACT FROM AUTHOR]

The origin of intermediate-depth seismicity has been debated for decades. A substantial fraction of these events occurs within the upper plane of Wadati–Benioff double seismic zones believed to represent subducting oceanic crust. We deformed natural lawsonite-rich blueschist samples under eclogite-facies conditions ( 1.5 < P < 3.5 GPa ; 583 K < T < 1121 K ), using a D-DIA apparatus installed at a synchrotron beamline continuously monitoring stress, strain, phase changes, and acoustic emissions (AEs). Two distinct paths were investigated: i) heating during deformation at pressures >2.5 GPa to maximum temperatures ranging from 762 to 1073 K, during which lawsonite and glaucophane became gradually unstable while entering the stability field of lawsonite–eclogite and the breakdown reaction of lawsonite was only crossed in case of the highest final temperature; ii) heating while deforming at a pressure <2 GPa to a maximum temperature of 1121 K associated with crossing the breakdown reaction of lawsonite and successively entering the stability fields of epidote–blueschist and eclogite–amphibolite but not of lawsonite–eclogite. Upon entering the Lws-Ecl stability field samples exhibited brittle failure, accompanied by the radiation of AEs. In-situ X-ray diffraction and microstructural analysis demonstrate that fractures are topologically related to the formation of omphacite. Amorphous material was detected along the fractures by transmission-electron microscopy without evidence for free-water. Since the newly formed omphacite crystals are small compared to the initial grains, we interpret the observed mechanical instability as a transformation-induced runaway under stress triggered during the transition from lawsonite–blueschist to lawsonite–eclogite. In contrast, we find no microstructural evidence that the breakdown of lawsonite, and hence the liberation of water leads to the fracturing in samples that experienced the highest quench temperatures of 1073 and 1121 K, although some AEs were detected during an experiment performed at 1.5 GPa. Our experimental results challenge the concept of “dehydration embrittlement”, which ascribes the genesis of intermediate-depth earthquakes to the breakdown of hydrous phases in the subducting oceanic plate. Instead we suggest that grain-size reduction (transformational faulting) during the transition from lawsonite–blueschist to lawsonite–eclogite leads to brittle failure of the deviatorically loaded samples. [ABSTRACT FROM AUTHOR]

Permian-aged metagabbros from the eclogite type-locality in the eastern European Alps were partially to completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro-to-eclogite transition, allowing the details of the eclogitization process to be investigated. Original, anorthite-rich igneous plagioclase is pervasively replaced by fine-grained intergrowths of clinozoisite, kyanite and Na-rich plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100-200 μm thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet. Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds. Given that the igneous plagioclase is pervasively replaced by clinozoisite, kyanite and Na-rich plagioclase, whereas kyanite-diopside symplectites are confined to narrow rim zones, we suggest that the development of these textures was controlled by the (im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of anhydrous plagioclase indicates that H2O diffusivity occurred on a mm-scale. By contrast, the size of the anhydrous diopside-kyanite and diopside-garnet symplectites indicate that Fe-Mg-Ca-Na diffusivity was limited to a 10s of μm scale. Chemical potential relations calculated in the idealized NCASH chemical system show that the clinozoisite-kyanite-albite intergrowths formed due to an increase of μH2O to plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid conditions indicated by this texture span from virtually dry conditions ( [ABSTRACT FROM AUTHOR]

We present the results of a comparative study of the geochemical changes of amphibolites and gneisses from the Belomorian mobile belt in response to plagiomigmatization, high-pressure metamorphism, two-feldspar migmatization, and secondary amphibolization during Paleoproterozoic (Svecofennian) tectonometamorphic activation. It is established that most of the Paleoproterozoic metamorphic processes are nonisochemical for major and trace elements, which is possibly caused by the interaction of protolithic rocks with metamorphic fluids. The finds of eclogites within the Belomorian mobile belt are spatially and genetically related to the large fields of apoamphibolite and apogneiss plagiomigmatites. Apoamphibolite eclogites, Grt-Aug eclogite-like rocks, apoamphibolite and apogneiss plagiomigmatites were formed by a single process initiated by the influence of an alkaline fluid on the amphibolite-gneiss complex. This was accompanied by the depletion of the rocks in HREE, enrichment in LREE, disappearance of the negative and formation of the positive europium anomaly. The formation of later two-feldspar migmatites was related to the reworking of the gneiss-migmatite-amphibolite complex by more acid fluids, which led to the depletion of microclinized rocks in LREE. Secondary amphibolization and epidotization of the basites and metabasites did not affect their REE distribution pattern. [ABSTRACT FROM AUTHOR]

Data on the localization of eclogitized mafic bodies in steeply and superposed gently dipping ductile shear zones in the area of Gridino village are considered. The process of eclogitization in steeply dipping ductile shear zones, which affect basites of rootless bodies (fragments of tectonic mélange) and dikes, was determined by deformations. Such a relation between them is most evident in narrow ductile shear zones that cross Early Proterozoic (age of ca. 2.4 Ga) gabbronorite dikes. Gabbronorites involved in deformations of ductile shear zones experienced eclogitization, reflected in the total recrystallization of magmatic minerals with the formation of equilibrium high-pressure mineral parageneses, ordered metamorphic textures, and linear strain patterns. Synchronous metamorphic transformations in gabbronorites, which avoided plastic deformations and retained massive structures (in wall rocks of shear zones) resulted in the formation of reaction coronas at the boundaries of magmatic mineral grains. Younger Fe gabbro dikes, localized in steeply dipping ductile shear zones, which cross gabbronorite dikes, are totally eclogitized. This determines the temporal lag between the formation of gabbonorite dikes and eclogitization in steeply dipping ductile shear zones and indicates its Svecofennian age. As in steeply dipping ductile shear zones, eclogitized basites constituting rootless bodies in their gently dipping counterparts are characterized by ordered metamorphic textures indicating the deformation nature of eclogitization. Gently dipping ductile shear zones are never crossed by Early Proterozoic basite dikes, although they contain boudins of them, which implies the younger (Svecofennian) age of these zones. The absence of eclogitization features in flat ferruginous basite bodies, which take part in the structure of gently dipping ductile shear zones and demonstrate orientation of metamorphic structures similar to that of gneisses, indicates that bodies of eclogitized basites were involved into these zones after eclogitization. An Archean age of these seems unlikely, since rootless bodies of eclogitized basites are never crossed by Proterozoic basite dikes in the study area. The textural data imply the deformation nature and Svecofennian age of eclogitized basites of the area of Gridino village. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau is formed by continuing convergence between Indian and Asian plates since ∼50 Ma, involving more than 1400 km of crustal shortening. New seismic data from western Tibet (the TW-80 experiment at 80°E) reveal segmentation of lower crustal structure by the major sutures, contradicting the idea of a mobile lower crust that flows laterally in response to stress variations. Significant changes in crustal structure and Moho depth occur at the mapped major tectonic boundaries, suggesting that zones of localized shear on sub-vertical planes extend through the crust and into the upper mantle. Converted waves originating at the Moho and at a shallower discontinuity are interpreted to define a partially eclogitized layer that extends 200 km north of the Indus–Yarlung Suture Zone, beneath the entire Lhasa block at depths of between 50 and 70 km. This layer is thinner and shallower to the north of the Shiquanhe Fault which separates the northern Lhasa block from the southern part, and the degree of eclogitization is interpreted to increase northward. The segmentation of the Tibetan crust is compatible with a shortening deformation rather than shear on horizontal planes. Unless the Indian-plate mantle lithosphere plunges steeply into the mantle beneath the Indus–Yarlung suture, leaving Indian-plate crust accreted to the southern margin of Tibet, then it too must have experienced a similar shortening deformation. [ABSTRACT FROM AUTHOR]

Western Hubei Province is at the southern end of the 3000-km-long north-south-oriented Xing'anling–Taihangshan–Wulingshan topographic step in China, which separates high-rising plateaus and mountain ranges in the west from low-elevation plains in the east. We calculated teleseismic P receiver functions of 32 permanent broadband seismic stations in Hubei Province and estimated crustal thicknesses under them using the H - κ method. We also obtained detailed crustal structural images along three profiles using the CCP stacking method. The results show an east-west crustal thickness increase in the study area from 30–35 km to 45–50 km in less than 20 km of horizontal distance, most likely in a step-wise fashion. The thin crust beneath the Nanxiang and Jianghan basins in eastern Hubei extends into the interior of the Wuling Uplift and the Huangling Massif in western Hubei. The lack of mirror symmetry between the Moho and surface topography suggests that part of the mountain ranges in western Hubei is either compensated by non-Airy-type isostasy models or is not in isostatic equilibrium but supported by the strength of the lithosphere. The brittle or localized ductile deformation in the lower crust/uppermost mantle as indicated by the abrupt Moho steps seems to be decoupled with brittle deformation in the upper crust. The CCP images also reveal an apparent double Moho beneath the Wudang Mts. which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern China in the Late Mesozoic. The Moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle. [ABSTRACT FROM AUTHOR]

New data are obtained on the major-component and minor- and trace-element composition of metamorphosed gabbronorites and Fe-Ti gabbro in the Krasnaya Guba area, Belomorian Mobile Belt. These two magmatic complexes, which are spatially combined but are of different age and were produced in different geodynamic environments, were simultaneously metamorphosed in the Paleoproterozoic to parameters transitional between the amphibolite and eclogite facies. Eclogite metamorphism was associated with perceivable changes in the concentrations of alkalis and alkali earth elements, and the metamorphic transformation of the plagioclase eclogite (which can be metamorphosed gabbronorite and/or gabbro) into amphibolite and biotite amphibolite was associated with changes in the concentrations of practically all trace elements. This implies that the metamorphic processes were not isochemical and were accompanied by significant removal and introduction of incompatible elements. However, some trace-element ratios (such as Ti/Y, Ti/Zr, Zr/Y, La/Sm, and Nb/Th) remained unmodified in the course of metamorphism and remained the same as in the pristine gabbronorite or gabbro and likely can be employed as indicators of the composition of the probable protolith and the conditions under which it was produced. [ABSTRACT FROM AUTHOR]

Abstract: Consolidated crust in the North Barents basin with sediments 16–18km thick is attenuated approximately by two times. The normal faults in the basin basement ensure only 10-15% stretching, which caused the deposition of 2–3km sediments during the early evolution of the basin. The overlying 16km of sediments have accumulated since the Late Devonian. Judging by the undisturbed reflectors to a depth of 8s, crustal subsidence was not accompanied by any significant stretching throughout that time. Dramatic subsidence under such conditions required considerable contraction of lithospheric rocks. The contraction was mainly due to high-grade metamorphism in mafic rocks in the lower crust. The metamorphism was favored by increasing pressure and temperature in the lower crust with the accumulation of a thick layer of sediments. According to gravity data, the Moho in the basin is underlain by large masses of high-velocity eclogites, which are denser than mantle peridotites. The same is typical of some other ultradeep basins: North Caspian, South Caspian, North Chukchi, and Gulf of Mexico basins. From Late Devonian to Late Jurassic, several episodes of rapid crustal subsidence took place in the North Barents basin, which is typical of large petroleum basins. The subsidence was due to metamorphism in the lower crust, when it was infiltrated by mantle-source fluids in several episodes. The metamorphic contraction in the lower crust gave rise to deep-water basins with sediments with a high content of unoxidized organic matter. Along with numerous structural and nonstructural traps in the cover of the North Barents basin, this is strong evidence that the North Barents basin is a large hydrocarbon basin. [Copyright &y& Elsevier]

Abstract: Studies on lower crustal and mantle xenoliths as well as geophysical data provide important information on the cratonic lithosphere. While geothermobarometric calculations of a majority of mantle xenoliths are in agreement with the typically low surface heat flow values of a craton (~40mW/m2), P–T estimates for lower crustal xenoliths deviate significantly from the cratonic geotherms. Independent from the individual cratonic history, the temperatures are ~200–300°C higher than what is expected at the base of the lower crust (~500–600°C at ~1.3–1.6GPa). Possible explanations may be a lack of equilibration to the cratonic geotherm or a relatively recent localized heat input. The presence of granulitic rocks under eclogite-facies conditions which are expected to prevail in the lower cratonic crust has consequences for the interpretation of geophysical rock properties. A mafic granulite which has been preserved under eclogite-facies conditions has densities and P-wave velocities similar to a felsic composition equilibrated to eclogite-facies conditions. Furthermore, phase diagrams calculated from xenolith bulk compositions demonstrate that eclogitization at relatively high temperatures as required for delamination of continental crust can only be triggered at significantly higher pressures than lithostatic at the base of the lower crust. As long as P–T conditions and the rock composition entail the assemblage to be granulitic, the addition of fluid at temperatures above 800°C will not result in eclogitization, but rather in melt generation. This can also lead to an increase in density of up to 3%, however, this is strongly dependent on the amount of water saturation. [Copyright &y& Elsevier]