Moho carbonation at an ocean-continent transition

Carbonation of mantle rocks during mantle exhumation is reported in present-day oceanic settings, both at mid-ocean ridges and ocean-continent transitions (OCTs). However, the hydrothermal conditions of carbonation (i.e., fluid sources, thermal regimes) during mantle exhumation remain poorly constrained. We focus on an exceptionally well-preserved fossil OCT where mantle rocks have been exhumed and carbonated along a detachment fault from underneath the continent to the seafloor along a tectonic Moho. Stable isotope (oxygen and carbon) analyses on calcite indicate that carbonation resulted from the mixing between serpentinization-derived fluids at ~175 °C and seawater. Strontium isotope compositions suggest interactions between seawater and the continental crust prior to carbonation. This shows that carbonation along the tectonic Moho occurs below the continental crust and prior to mantle exhumation at the seafloor during continental breakup.

Structural deformation-metamorphism and exhumation processes of Yuanmou metamorphic complex, Yunnan, China

<p>Within orogenic zone and continental extensional area, it often developed metamorphic complex or metamorphic gneiss dome that widely exposed continental mid-lower crustal rocks, which is an ideal place to study exhumation processes of deep-seated metamorphic complex and rheology. The Yuanmou metamorphic complex is located in the south-central part of the "Kangdian Axis" in the western margin of Qiangtang Block and Yangtze Block, which is a part of the anticline of the Sichuan-Yunnan platform. Many research works mainly focus on the discussion of intrusion ages, aeromagnetic anomalies, and polymetallic deposits. However, the exhumation process and mechanism of the Yuanmou metamorphic complex are rarely discussed and still unclear. This study, based on detailed field geological observations, optical microscopy (OM), cathodoluminescence (CL), electron backscatter diffraction (EBSD) and electron probe (EMPA) were performed to illustrate the geological structure features, deformation-metamorphic evolution process and its tectonic significance of Yuanmou metamorphic complex during the exhumation process. All these analysis results indicate that the Yuanmou metamorphic complex generally exhibits a dome structure with deep metamorphic rocks and deformed rocks of varying degrees widely developed. Mylonitic gneiss and granitic intrusions are located in the footwall of the Yuanmou, which have suffered high-temperature shearing. The mylonitic fabrics and mineral stretching lineations in the deformed rock are strongly developed, forming typical S-L or L-shaped structural features. The high-temperature ductile deformation-metamorphism environment is high amphibolite facies, that is, the temperature range is between 620 ~ 690 ℃ and the pressure is between 0.8 ~ 0.95 Gpa. In the deformed rocks closed to the detachment fault, some of the mylonite fabric features are retained, but most of them have experienced a strongly overprinted retrogression metamorphism and deformation. At the top of the detachment fault zone, it is mainly composed of cataclasites and fault gouge. The comprehensive macro- and microstructural characteristics, geometry, kinematics, and mineral (amphibole, quartz and calcite) EBSD textures indicate that the Yuanmou metamorphic complex has undergone a progressive exhumation process during regional extension, obvious high-temperature plastic deformation-metamorphism in the early stage, and superimposed of low-temperature plastic-brittle and brittle deformation in the subsequent stage, which is also accompanied by strong fluid activities during the exhumation process.</p>

Permian rifting and detachment faults and their role in Alpine collisional tectonics

<p>The Permian was a time of strong crustal extension in the area of the later-formed Alpine orogen. This involved extensional detachment faulting and the formation of metamorphic core complexes. We describe (1) an area in the Southern Alps (Valsassina, Orobic chain) where a metamorphic core complex and detachment fault have been preserved and only moderately overprinted by Alpine collisional shortening, and (2) an area in the Austroalpine (Schneeberg) where Alpine deformation and metamorphism are intense but a Permian low-angle normal fault is reconstructed from the present-day tectonometamorphic setting. In the Southern Alps case, the Grassi Detachment Fault represents a low-angle detachment capping a metamorphic core complex in the footwall which was affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, c. 289 Ma and c. 287 Ma, the older of which was syn-tectonic with respect to the extensional mylonites (Pohl, Froitzheim, et al., 2018, Tectonics). Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S and the sense of transport top-South. In this area, there is no evidence of Permian strike-slip faulting but only of extension. In the Schneeberg area of the Austroalpine, a unit of Early Paleozoic metasediments with only Eoalpine (Cretaceous) garnet, the Schneeberg Complex, overlies units with two-phased (Variscan plus Eoalpine) garnet both to the North (Ötztal Complex) and to the South (Texel Complex). The basal contact of the Schneeberg Complex was active as a north-directed thrust during the Eoalpine orogeny. It reactivated a pre-existing, post-Variscan but pre-Mesozoic, i.e. Permian low-angle normal fault. This normal fault had emplaced the Schneeberg Complex with only low Variscan metamorphism (no Variscan garnet) on an amphibolite-facies metamorphic Variscan basement. The original normal fault dipped south or southeast, like the Grassi detachment in the Southern Alps. As the most deeply subducted units of the Eoalpine orogen (e.g. Koralpe, Saualpe, Pohorje) are also the ones showing the strongest Permian rift-related magmatism, we hypothesize that the Eoalpine subduction was localized in a deep Permian rift system within continental crust.</p>

Fluid circulations associated with the necking of the crust: the example of the Mont-Blanc detachment fault

<p><strong> </strong>The external crystalline massifs of the Alps, which include the Mont-Blanc massif, are found in between the external and internal parts of the orogen. The external parts correspond to the proximal domain of the Alpine Tethys (Helvetic domain), whereas the internal part corresponds to the former distal domain of the margin (Penninic domain). Therefore, the Mont-Blanc massif is a key place for understanding the proximal-distal transition during Jurassic rifting of the Alpine Tethys. </p><p>Despite numerous seismic observations at modern passive margins, the tectono-sedimentary and fluid evolution recorded in these domains called necking zone remain poorly understood. Many questions remain concerning the thermal evolution, the origin and composition of the fluids, their link to large-scale hydrothermal systems, and the impact of element transfer on the diagenesis of syn-rift sediments.</p><p> </p><p>Here we focus on the Col du Bonhomme (southern Mont-Blanc massif near Bourg St-Maurice, France), where late Triassic / early Jurassic to late Jurassic sediments preserve pre-Alpine contacts between the sediment and the basement.  The syn-rift sedimentary tract is composed of Sinemurian to Pliensbachian sandstones called “Grès Singuliers”, lying unconformably above the pre-rift and over an exhumed fault plane corresponding to the top basement.</p><p>Characterization of the faults and overlying sediments requires a multi-scale and multi-disciplinary approach combining field observation, petrography, sedimentology, structural geology, and geochemistry. The protolith of the fault rocks is a Variscan migmatitic gneiss. The damaged zone consists of cataclasites and the core zone is made of black gouge. The gouge is overlaid conformably by Liassic sandstones that contain reworked clasts of cataclasite. The observations that the top basement fault is cut by a Pliensbachian high-angle normal fault and Triassic clasts occur in the gouge enables to date this fault as Early Jurassic. </p><p>At the micro scale, the basement shows hydratation leading to chloritization of biotite and sericitisation of feldspaths (orthoclase and plagioclase). A strong hydration-assisted deformation with increase of deformation toward the fault core leads to the formation of cataclasites. They are composed of quartz, sericite with small remnants of orthoclase, chlorites with secondary pyrites and rutiles. The fault core is a black gouge with grain size comminuition and mineral neoformation.</p><p>Evidence for fluid flow is observed in the fault leading to the hydrothermal alteration of the basement (sericitisation of feldspath and corrosion of quartz)  and the formation of syn-gouge quartz and quartz-adularia veins in the black gouge (datation using the Rb-Sr an adularia and U-Pb on calcite method is in progress) . </p><p>Based on our observations we interpret the fault observed at Col du Bonhomme as a Jurassic exhumation fault associated with the necking of the European crust during Jurassic rifting. This preliminary work shows that the fault acted as an important pathway for crustal fluids with important transfer of silica and at least K, Fe and Ti.  The Col du Bonhomme area gives an opportunity to study fluid circulation and basement alteration along a rift-related detachment fault in the necking domain and therefore to understand fluid-mediated element mobility during rifting.</p><p><strong>Keywords :</strong> Detachment fault, Mont-Blanc massif, Fluid circulation , Alpine Tethys, Necking zone</p>

Exhumation of Metamorphic Core Complexes through Progressive Subhorizontal Shearing, Doming, and Detachment Faulting: Insights from the Cretaceous Liaonan Metamorphic Core Complex, Eastern North China Craton

The Early Cretaceous Liaonan metamorphic core complex (MCC), eastern North China craton, provides a field setting to evaluate progressive middle-upper crustal subhorizontal shearing, doming, and detachment faulting. The MCC is bounded by a western Jinzhou detachment fault zone (JDFZ) and a southern Dongjiagou shear zone (DSZ) that were primarily suggested to be two segments of the master detachment fault zone. Integrated structural, microstructural, quartz c-axis fabrics, and fluid inclusion analysis and zircon U-Pb dating on mylonites and syn-kinematic granites along the DSZ and JDFZ reveal that the DSZ possesses deformation characteristics that are obviously different from those along the JDFZ. The DSZ is composed of a Lower Unit of sheared Archean gneisses and an Upper Unit of sheared Neoproterozoic metasedimentary rocks, between which there is an obvious tectonic discontinuity contact (TDC). Rocks from below and above the TDC possess structures and fabrics with consistent geometries and kinematics with those along the JDFZ. A metamorphic break exists between the two units that were sheared at contrasting deformation conditions. Dating of zircons from syn-kinematic granitic dikes from DSZ yields an age of ca. 134 Ma, which is similar to the ages of early shearing along the JDFZ. It is concluded that the Jinzhou and Dongjiagou faults formed parts of a detachment faulting with top-to-the WNW kinematics. Exhumation of the Liaonan MCC shearing initiation along both the JDFZ and DSZ at an early stage (ca. 133~134 Ma), subsequent progressive shearing, and doming during slow cooling and exhumation before ca. 120 Ma, followed by fast cooling and rapid exhumation of the MCC by detachment faulting along the JDFZ until ca. 107 Ma.

Supplemental Material: Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge

Additional information and figures describing the acquisition and processing of the micro-earthquake data set, including links to data repository and 3-D visualization of the earthquake catalog.<br>