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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 array The Tarim lower crust is underthrusting to ∼75 km depth beneath the Tibetan Plateau The Altyn Tagh range was uplifted rapidly since late Miocene through the thickening of the upper crust

The slab breakoff has been applied to many collisional orogen around the world to explain postcollisional magmatism and metamorphism. For Phanerozoic orogens, multiple lines of evidence indicate that ultrahigh-pressure metamorphic rocks are exhumed from depths of 100–400 km, implying deeper slab breakoff depths. However, the slab breakoff depth in Archean orogens is less certain, and the factors governing the depth of slab breakoff remain poorly constrained. In this study, we use a 2D numerical model to identify the key parameters controlling slab breakoff and to quantitatively evaluate the depth of slab breakoff. The results indicate that a high degree of eclogization of oceanic crust, fast convergence rates, and low radiogenic heat production of continental crust generally promote slab breakoff at greater depths. Otherwise, the shallow slab breakoff mode is favored. For Archean collisional orogens, it is more likely for shallow breakoff to occur, due to the higher radiogenic heat production and mantle potential temperature, potentially explaining the rare occurrence of ultra-high pressure metamorphic rocks in the Archean. We also quantified the effects of other factors, including the density of subcontinental lithospheric mantle and age of oceanic lithosphere, which play secondary roles in determining slab breakoff depth.

Metamorphic transformations in subduction lithosphere are triggered by pressure and temperature changes occurring under stress. This anisotropic stress field can in turn be locally altered by the transformation pattern, as reactions induce significant changes of the material properties of the rocks. The granulite to eclogite transformation constitutes a striking example of a pressure‐driven transformation potentially able to generate significant volume forces due to densification and known to be associated with transient weakening. However, the feedback mechanisms between pressure variations and the evolution of the physical properties of rocks during eclogitization remain poorly constrained. Formalizing these interactions is thus required to understand how eclogitization initiates and propagates under stress. In this study, mechanical numerical models are used to explore the evolution of eclogitization in a matrix‐inclusion system subjected to shear boundary conditions, where pressure variations control the physical properties of the materials. Our results show that the initiation of the transformation is controlled by both the strength of the protolith and by the degree of overstepping of the transformation. Eclogite structures then systematically propagate in the direction normal to the principal shortening direction. In contrast, other parameters such as the density variations involved in the transformation, the initial difference in strength between the protolith and the inclusion, and the shape and orientation of the inclusion do not play a major role on the transformation initiation itself but enhance or inhibit its propagation.

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 H 2 O in the form of OH groups. H 2 O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H 2 O, pyroxene from ~ 50 to ~ 310 µg/g H 2 O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H 2 O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H 2 O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H 2 O 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 H 2 O increase from ~ 450 to ~ 2500 µg/g H 2 O 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.

Eclogitization reactions of plagioclase-bearing rocks in water-limited environments are often incomplete. In such domains, metamorphism and strain localization interact, yielding complex strain-transformation patterns in the field. This is the case in the granulite facies anorthosites of Holsnøy, where partial eclogitization proceeds along digitations in the least strained domains. These areas are among the rare examples of preserved early eclogitization textures. The aim of this study is to assess the mechanical alteration induced by plagioclase breakdown reactions, through a detailed petrological study of the very first increments of the eclogitization process, expected to be preserved at the tip of a digitation in an apparently undeformed granulite block. We show that (i) the zoisite-forming reaction within plagioclase grains occurs unbalanced and initiates mechanical twinning, and subgrains individualization, and (ii) kyanite-analcime association at plagioclase grain boundaries gives evidence of partial melting, which might have affected the rock near the peak P – T conditions in response to early water infiltration. This transient partial melting stage results in the re-crystallization of μm-scale plagioclase grains. Intra-grain and grain boundary transformations therefore induce an effective grain size reduction. It constitutes an alteration of the overall aggregate properties of the burying continental rocks, prior to any significant deformation, at potentially low differential stress. Partially transformed granulites are eventually weaker than initial granulites and are prone to later strain localization.

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 rocks The transformation of hydrated rocks into HP granulites along a collision gradient reproduces the slab tomographic velocity increase at 30 km The often‐supposed eclogitization produces velocities that are significantly higher than the crustal top of the European Alpine slab

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.

2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.

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.

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.