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The Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O > 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (>300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km. [ABSTRACT FROM AUTHOR]

Pre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions. [ABSTRACT FROM AUTHOR]

Linking the timing of allanite growth to metamorphic conditions in metapelites is particularly challenging because of the large variety of allanite textures and chemical compositions. This study focuses on five garnet–biotite metapelites retrogressed to different extents, from the internal domain of the Longmen Shan (eastern Tibet) from which few petrochronological data documenting the metamorphic ages are available. Microstructural observations were combined with whole-rock compositions, detailed mineral characterization, phase equilibria modelling, and in situ U–Th/Pb allanite dating to gain insights into the allanite reactivity relative to other rock-forming minerals. All samples experienced similar peak temperatures of 560–600°C. Allanite is the main rare earth element (REE)-bearing accessory mineral but it exhibits different textures, such as epidote rims with distinct REE contents as well as late inclusion-like dissolution features. Garnet is, along with allanite, critical to reconstruct the REE budget of these rocks. In the two samples where allanite is observed as inclusions in garnet, garnet shows no textural zoning and a low Y content (<90 ppm). This is attributed to early Y fractionation in allanite and epidote rims with no further equilibration with garnet. This allanite is therefore pre-garnet (T  < 520°C). In the sample where allanite is observed only in the matrix, both garnet and allanite–epidote rims exhibit more complex textural and compositional zoning, with a higher Y content in garnet. The Y incorporation in garnet is attributed to a series of reactions involving allanite, interpreted as syn- to post-garnet growth (T  > 520°C). This relative chronology is confirmed by in situ U–Th/Pb allanite dating: pre-garnet allanite shows ages of c. 200 Ma, whereas syn- to post-garnet allanite has ages of c. 180 Ma. The timing of allanite appearance strongly correlates with the biotite-in reaction predicted by the models and observed in microstructures. In the two samples collected close to the major Wenchuan Shear Zone—deformed up to mylonitization and retrogressed under greenschist-facies conditions (3–4 ± 1 kbar, 350–400°C)—allanite still preserves the peak metamorphic ages (c. 200 and c. 180 Ma). Allanite and its epidote rims, however, are fragmented and partly replaced by a retrograde assemblage of Qz + Pb-depleted (<100 ppm) monazite, providing an age <90–100 Ma. Our results allow different metamorphic stages for the studied transect in the central Longmen Shan to be refined: a prograde path at c. 200 Ma, a thermal relaxation and exhumation from c. 180 Ma, and a late greenschist overprint. This study also shows that when allanite is the principal phase in a mineral assemblage datable by U–Th/Pb, detailed observations of allanite textures may be highly informative in constraining the timing of its growth relative to other major mineral phases. [ABSTRACT FROM AUTHOR]

Abstract: Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim 40Ar/39Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology. [ABSTRACT FROM AUTHOR]

The deep structure and deformation mechanisms of the Longmen Shan thrust belt (Sichuan, China), at the eastern border of the Tibetan plateau, were largely debated after the devastating Mw 7.9 Wenchuan earthquake (2008). Recent geophysical studies and field investigations have been focused on the active motion of the major Beichuan fault, which ruptured during the earthquake. However, the total exhumation across the fault still remains unclear. In the hanging wall of the Beichuan fault, the South China block is exhumed in the Pengguan massif. Close to the Beichuan fault, the rocks of the Pengguan massif underwent greenschist facies metamorphism associated with brittle-ductile deformation. No metamorphism is observed in the footwall of the fault. In this study, we characterize and date the metamorphic history recorded in the hanging wall of the Beichuan fault in order to constrain the depth and timing of exhumation of the rocks of the Pengguan massif along the fault. A high-resolution petrological approach involving chemical analyses and X-ray maps was used to analyze the micrometric metamorphic minerals. The P-T conditions of the greenschist facies metamorphic event were estimated by an inverse multi-equilibrium thermodynamic approach. The results, 280 ± 30 °C and 7 ± 1 kbar, suggest that the rocks of the Pengguan massif were exhumed from ca. 20 km depth. Our results underline the importance of the thrusting component in the long-term behavior of the Beichuan fault and provide a minimal depth at which the fault is rooted. In situ laser ablation 40 Ar/ 39 Ar dating of metamorphic white mica revealed that the greenschist overprint occurred at 135–140 Ma. The Pengguan massif was therefore partially thrusted along the Beichuan fault during the Lower Cretaceous, long before the Eocene-Miocene exhumation phase which is well-constrained by low-temperature thermochronology. Our results provide the first independent depth information for the exhumation history of the Pengguan massif and reveal a previously undocumented Lower Cretaceous tectonic event that marks the onset of the thick-skinned deformation in the external domain of the Longmen Shan (East of the Wenchuan Fault). [ABSTRACT FROM AUTHOR]

Pressure-temperature-deformation (P-T-ε) paths for metamorphic rocks that experienced a complex deformation history commonly rely on the link between successive metamorphic assemblages and the different microstructures preserved at the sample scale. However, with changing P-T conditions, metamorphic minerals in a specific microstructure can chemically re-equilibrate. The direct link between deformation phases and mineral compositions for thermobarometry purposes can therefore be distorted. This study focuses on a series of garnet-biotite metapelites from the Longmen Shan (Sichuan, China) that preserve muscovite of different chemistry in distinct microstructures. To quantify the degree of re-equilibration of muscovite, a microstructural study was coupled with high-resolution chemical mapping. The chemical evolution of muscovite was modeled along a P-T loop previously constrained with phase equilibria calculations, semi-empirical and empirical thermobarometry. Results show that metapelites experienced a three-stage metamorphic history: (1) heating and burial up to 11 ± 2 kbar, 530 ± 20 °C, (2) minor decompression and heating up to 6.5 ± 1 kbar, 575 ± 10 °C and (3) decompression and cooling down to 4 ± 1 kbar, 380–450 °C. Muscovite partially re-equilibrated by pseudomorphic replacement during the three metamorphic stages, although it is mainly observed in prograde microstructures preceding the pressure peak. The main factors controlling the re-equilibration are the intensity of the deformation and the fluid availability during metamorphism. The P-T conditions of metamorphic assemblages thus reflect pulses of fluids release that enhanced mineral resorption and local replacement. Such micro-chemical behavior is probably relatively common in metapelites and questions the reliability of the P-T predictions based on relict phase chemistry, apparently preserved in microstructures that might have been affected by later re-equilibration. [ABSTRACT FROM AUTHOR]

This study explores the relative importance of fluid availability, temperature and mineral assemblage variations on the development of macro and microstructures in greenschist-facies shear zones of two granitoids of the Axial Zone (central Pyrenees) emplaced at the end of Variscan orogeny, at similar structural levels. The investigated shear zones formed during alpine compression at comparable temperature. In the Bielsa granitoid, deformation was distributed in a dense network of shear zones. Extensive mineralogical transformations resulted in variations in major, minor and rare earth element contents pointing to pre-kinematic hydrothermal alteration (at Permian-Triassic) at 270–350 °C and further alteration in fluid-abundant conditions during Alpine shearing. Quartz was fractured, exhibits weak plastic deformation and was weakly reworked, mainly by dissolution-reprecipitation. Strain was accommodated in the mica-rich matrix (40–50% vol. of mica). Mica significantly grew during pre-kinematic hydrothermal alteration. In the Maladeta granite, deformation was localized in sparse shear zones. Hydrothermal alteration occurred at similar to slightly higher temperature (280–380 °C). Elemental variation related to alteration was weaker, pointing to fluid-deficient conditions before and during Alpine compression. Limited fluid resulted in lower mica content (20–28% vol.) than in Bielsa. Quartz exhibits stronger intragrain plastic deformation, dynamic recrystallization by subgrain rotation and reworking by dissolution-reprecipitation and nucleation. Strain was accommodated by both quartz aggregates and mica-rich matrix. The transition from dominant brittle behavior of quartz to dynamic recrystallization by subgrain rotation occurred in a narrow range of 350–380 °C, well below 450 °C as generally described. The lower temperature and less hydrated conditions in Maladeta are in apparent contradiction with stronger quartz recrystallization and strain localization than in Bielsa. By promoting phyllosilicate growth and phase mixing before shearing, the metamorphic pre-conditioning of the granitic protolith appears to be the main factor controlling strain distribution at sample scales and differences in tectonic styles between the two massifs. • Deformation is localized only in granite showing weak pre-kinematic alteration. • Direct transition from fracturing to subgrain rotation in quartz occurs at T < 370 °C. • Pre-shearing fluid-rock interaction promotes phyllosilicate growth and phase mixing. • Pre-conditioning of protolith controls the (micro)structural style in low-T shear zones. [ABSTRACT FROM AUTHOR]

Mutations in the transcription factors encoded by PHOX2B or LBX1 correlate with congenital central hypoventilation disorders. These conditions are typically characterized by pronounced hypoventilation, central apnea, and diminished chemoreflexes, particularly to abnormally high levels of arterial PCO2. The dysfunctional neurons causing these respiratory disorders are largely unknown. Here, we show that distinct, and previously undescribed, sets of medullary neurons coexpressing both transcription factors (dB2 neurons) account for specific respiratory functions and phenotypes seen in congenital hypoventilation. By combining intersectional chemogenetics, intersectional labeling, lineage tracing, and conditional mutagenesis, we uncovered subgroups of dB2 neurons with key functions in (i) respiratory tidal volumes, (ii) the hypercarbic reflex, (iii) neonatal respiratory stability, and (iv) neonatal survival. These data provide functional evidence for the critical role of distinct medullary dB2 neurons in neonatal respiratory physiology. In summary, our work identifies distinct subgroups of dB2 neurons regulating breathing homeostasis, dysfunction of which causes respiratory phenotypes associated with congenital hypoventilation. [ABSTRACT FROM AUTHOR]

The Pyrenean mountain belt experienced a polyphased history from the Variscan to the Alpine orogeny. Deciphering the different phases of metamorphism and deformation in the present structure of the belt is difficult, and particularly challenging in the Axial zone (central Pyrenees), where peak metamorphic conditions remained below 450 °C. This is however crucial to understand the evolution of the Pyrenean belt after pluton emplacement at c. 310-320 Ma. In this study* we characterize the age and conditions of different stages of greenschist-facies metamorphism in the Bielsa Variscan basement massif (below the Gavarnie unit, in the southern Axial Zone) and its Permo-Triassic sedimentary cover. At its northern and southern borders, the massif is extensively metasomatized throughout a vertical section of at least 2000 m depth. Similar alteration features have been observed eastward in the Pyrenees and are therefore probably significant at the scale of the central Axial zone. Field observations near Lake of Urdiceto, in the northern part of the massif, show that the sedimentary cover and its basement are tightly and harmonically folded. Close to the lithological contacts between the basement and the sedimentary cover, mylonitic bands extend ~E-W. Further away from the contact, zones of brittle deformation alternate with undeformed granite and mylonites. In all samples, at microscopic scale, the magmatic mineral assemblage is replaced by greenschist-facies metamorphic minerals, including chlorite, white mica, rutile and titanite. Chlorite-white mica thermobarometry and LA-ICP-MS in-situ U-Pb rutile and titanite dating indicate that the alteration occurred at ~300-350°C, ~2 kbar and 310-280 Ma. In samples with higher strain, secondary chlorite, white mica, monazite and rutile overprint earlier metasomatic events. P-T-t conditions for this event are estimated at ~200-250°C, 3-4 kbar and c. 40 Ma. Mylonitic bands preserve both metamorphic stages. Sediments exhibit two generations of veins (extensional and transtensional) filled with two generations of chlorite and rutile partially reset at Triassic and Eocene times. These results suggest that (1) the sedimentary cover and the basement experienced a similar history since sediment deposition, (2) the Bielsa massif was heterogeneously altered and deformed prior to the Pyrenean orogeny (late-Variscan) without recording the Cretaceous rifting phase preserved in the eastern Pyrenees, (3) Alpine metamorphism is preserved and localized in the most deformed areas of the massif. The spatial distribution of the deformation and metamorphism have been likely controlled by the first late-Variscan alteration, in Bielsa as in the other massifs of the Axial zone: the crust must have been significantly weakened prior to the Pyrenean shortening.*This study is part of the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS). [ABSTRACT FROM AUTHOR]

This study uses low-T petro-chronology to establish a link between the rheological effects of low-T fluid-rock interactions and fluid flow. Newly-formed mineral assemblages during fluid-rock interactions are controls of fluid paths and deformation mechanisms during the evolution of orogens, where polyphased deformations are commonplace. In the cold and external domains of mountain belts, these interactions are therefore difficult to distinguish and characterize because they occur at low-T and low-P conditions. For this reason, in the central Pyrenees (Axial zone), the metamorphism associated to the Variscan deformation is hardly distinguishable from the Alpine one. In this study*, we have characterized the different phases of low-grade metamorphism recorded in the Variscan granitic Bielsa massif, underthrusted below the Gavarnie unit during the Cenozoic orogenic evolution. In the studied area (Lake of Urdiceto), undeformed granite alternates with fractured and mylonitic rocks. In all samples, regardless the strain, the original magmatic mineral assemblage of biotite ± amphibole, plagioclase, K-feldspar, quartz and ilmenite is altered to a Fe-rich chlorite (chlorite1) ± white mica, ± prenhite. The titanium content of magmatic biotite and amphibole is incorporated in secondary Ti-bearing phases (rutile1 or titanite). The replacement reactions are pseudomorphic and fluid proceeds through microcraks. In 'discretely' deformed samples, flakes of chlorite and titanite are folded and cross-cut by fractures filled with Fe-poor chlorite (chlorite2), rutile2 ± monazite. In mylonites, rutile2 is the principal accessory mineral. Crystallization temperatures estimated with the multi-equilibrium approach for chlorite1 range between 300 and 350°C, while they range between 200 and 250°C for chlorite2. In-situ U/Pb LA-ICP-MS dating of rutile1, titanite and monazite yields ages of c. 310-280 Ma, while rutile2 and younger monazite domains show ages of c. 40-50 Ma. We conclude that the Bielsa granitic basement was softened and altered at low-T conditions at the late-Variscan, allowing a diffuse deformation. Low-T metamorphism and deformation was then localized during the Pyrenean phase as attested by the occurrence of the youngest rutile2 and monazite in the only mylonites. Furthermore, the occurrence of titanite over rutile is observed to be mainly dependent on the CO2 activity of the fluid phase. Rutile occurs along mylonitic bands, where a localization of the fluid flow accompanying the Pyrenean metamorphism is speculated. The composition of secondary chlorite also varies from rutile-bearing to titanite-bearing samples. Hence, the spatial occurrence of accessory minerals and the chemical variations of chlorite2 allow the composition and circulation paths of the paleofluids to be mapped. *This study is part of the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS). [ABSTRACT FROM AUTHOR]

The Longmen Shan, eastern and steepest margin of the Tibetan plateau, is often seen as the archetype example of an orogenic system built by crustal channel flow extrusion since the Miocene. This model is controversial as other studies propose an accretionary prism mechanism. A key difference between these models resides in the kinematics proposed for the Wenchuan - Maoxian (WM) fault zone, a major tectonic structure of the Longmen Shan. We constrain the Cenozoic kinematics of the WM fault zone by combining structural observations, fault gouge K/Ar dating and 40Ar/39Ar dating of syn -kinematic white mica. Normal / right lateral ductile deformation occurred at 28.0 ± 0.9 Ma while top-to-the-east reverse deformation at 15.4 ± 0.2 Ma. K/Ar ages of authigenic illite from two fault gouges show that brittle right-lateral / reverse deformation was active at 6.9 ± 2.9 Ma. These ages are consistent with the relative vertical motions across the fault zone deduced from thermochronology. Three deformation phases can be identified: right-lateral / normal in the Middle Oligocene (∼30–25 Ma), reverse in the middle Miocene (25–15 Ma), and right-lateral / reverse since the upper Miocene (since ∼6 Ma). The WM fault zone never experienced pure normal motion, and only shortening since the Oligocene, in contrast to predictions of lower crustal channel-flow extrusion models. These results are in favor of a crustal accretionary prism model for the Cenozoic building of the eastern Tibetan plateau. [Display omitted] • First account of three Cenozoic tectonic phases along the Wenchuan - Maoxian fault. • Direct deformation dating results coherent with thermochronology. • No pure normal motion on the fault in Cenozoic, no normal component since 25 Ma. • Fault Kinematics dismiss the crustal channel-flow extrusion model in Longmen Shan. [ABSTRACT FROM AUTHOR]