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[Display omitted] • Ediacaran magmatism and sedimentation occurred in the Bayankhongor Zone and the northeastern edge of the Baidrag Block. • Magmatic and sedimentary units formed in a back-arc basin (not belonging to ophiolite) • Multiple crustal blocks and oceans closing by subduction in peri -Siberian realm. Geochronological and geochemical investigation of the magmatic and sedimentary rocks from the south-eastern tip of the Bayankhongor Zone (central Mongolia) constrains the Neoproterozoic evolution of the northern margin of the Baidrag Block. There, ultramafic to felsic igneous rocks of the Khan-Uul Massif intruded the volcano-sedimentary sequence of the Ulziit Gol Unit. U–Pb zircon ages show that both the plutonic and volcanic rocks were coeval products of the same Ediacaran (598–564 Ma) magmatism. Most of the samples have low contents of high field strength elements typical of arc-related magmas; however, some mafic rock samples display N-MORB-like chemistry. Magmatic rocks in the Khan-Uul Massif and Ulziit Gol Unit yielded exclusively positive initial ε Hf in zircon values (+4.9 to +13.8), implying derivation from depleted-mantle sources. Furthermore, heterogeneity of the whole-rock initial ε Nd values (–4.0 to +2.1) documents that fractional crystallization was accompanied by variable crustal contamination. The detrital zircon age patterns of the sandstone and tuffites in the Ulziit Gol Unit indicate that the sequence was filled dominantly by Ediacaran magmatic detritus derived from the Khan-Uul Massif and its volcanic equivalents, while the older cratonic material from the Baidrag basement represented only a subordinate component. Combined magmatic and sedimentary records imply that both the Khan-Uul Massif and the Ulziit Gol Unit may have formed in the same back-arc basin environment. This challenges the previous view that the entire Bayankhongor Zone was ophiolitic in nature. Distribution of coeval Ediacaran supra-subduction systems on the scale of the Mongolian Collage indicates the closure of multiple oceanic basins rimming the Siberian Continent. Following Rodinia break-up, this peri -Siberian realm was presumably formed by sub-parallel continental ribbons and oceanic basins. It is proposed that the amalgamation of these blocks and closure of intervening oceans reflected the Ediacaran advancing mode of the Palaeo-Pacific subduction. [ABSTRACT FROM AUTHOR]

Tectonic mélanges have diverse origins, variable lithologies, complex structures, and significant differences in engineering characteristics; they are the fundamental source of crucial geological safety risks and cause large-scale geohazards. Based on an analysis of the basic characteristics of 7 tectonic mélange belts in the eastern Tibetan Plateau, this article focuses on the Jinsha River tectonic mélange belt. This study employed remote sensing image interpretation and geological surveys, and the basic characteristics of 87 landslides with volumes greater than 1 × 106 m3 that are distributed in the mélange belt along the upstream Jinsha River were identified. According to an engineering geological theory analysis and typical cases investigation, a comprehensive classification method was established for the slide-controlling modes of mélange high slopes, including 3 categories and 7 subcategories, revealing the main controlling factors and structural features for deformation and failure. In this study, the distribution of high-susceptibility zones and high-risk zones for large-scale landslides along the Sichuan‒Tibet section upstream of the Jinsha River was delineated. The results provide a basic theory and insights into the early identification and risk prevention of potential large-scale landslides and disaster prevention and mitigation of major engineering projects in the alpine gorge area of tectonic mélange belts. [ABSTRACT FROM AUTHOR]

The frictional velocity dependence and healing behavior of subduction fault zones play key roles in the nucleation of stick‐slip instabilities at convergent margins. Diagenetic to low‐grade metamorphic processes such as pressure solution are proposed to be responsible for the change in frictional properties of fault materials along plate interfaces; pressure solution also likely contributes to the acceleration of healing according to previous studies. Here, we report friction studies for temperatures of 20–100°C and normal stresses from 20 to 125 MPa on samples collected from ancient subduction fault zones, the Lower Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two mélanges correspond to the updip and downdip limits of the seismogenic zone and include deformation features that indicate lower and higher degrees of pressure solution. Our data show that the Lower Mugi mélange exhibits velocity‐weakening to velocity‐neutral frictional behavior under low normal stress and that the Makimine mélange sample shows velocity‐strengthening behavior under high normal stress. We suggest that mineralogical changes due to diagenesis and metamorphism influence fault slip behavior. We measure frictional healing in slide‐hold‐slide experiments for the Lower Mugi mélange sample and document the role of pressure solution in fault healing. Our results show that frictional healing increases at higher temperatures. The microstructures related to pressure solution found in the post‐experimental gouges support the idea that the enhanced healing is related to pressure solution. Plain Language Summary: Investigating the mechanical properties of rock materials in plate boundary faults is crucial for the understanding of subduction zone earthquakes. The nucleation of earthquakes along the plate boundary occurs in a zone where friction drops during slip and fault strength rebuilds after an earthquake. The mechanical properties of fault materials in this zone are likely influenced by local diffusion of mobile elements during the interseismic period. Here, we perform friction experiments using two samples of ancient plate boundary faults that were near the updip and downdip limits of the zone, respectively. The first set of experiments reveals a variation in frictional response to slip under an experimental setup analogous to subduction zone conditions. Variation in the magnitude of interseismic pressure solution strain preserved in the samples is likely the cause of varying responses that lead to the nucleation and cessation of earthquakes. We conduct another series of experiments under varying hydrothermal conditions that reveal a faster restrengthening of the fault material at elevated temperatures. This is likely related to the temperature dependence of pressure solution, which is found to be an operative deformation mechanism based on the examination of post‐experimental materials. Key Points: Mélanges bounding the upper and lower limits of the seismogenic zone show changes in frictional velocity‐dependence with normal stressMicrostructures of the sheared samples suggest that temperature dependence of healing occurs via diffusive mass transferThermally‐activated deformation mechanisms can modulate frictional velocity‐dependence and healing behavior of plate boundary faults [ABSTRACT FROM AUTHOR]

Tectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions. Plain Language Summary: Temperature influences the processes responsible for triggering earthquakes at subduction zones. This inference is supported by the observed concentration of seismic activities within a specific temperature range along the plate interface. To investigate the topic, we examine a distinctive rock type known as tectonic mélanges, which has undergone subduction in the past and has since been exposed. These mélanges contain mineral veins formed through the movement of materials; a factor that has shown correlation with the occurrence of earthquakes at subduction zones. To determine the temperatures at which the mechanism operates, leading to the formation of the veins, we employ two methodologies: quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry. Our findings indicate that, regardless of the geographical locations or the types of veins, they formed within a temperature range of 100–250°C. This consistency implies the existence of a consistent depth range within subduction zones, where fluids are predominantly present, facilitating material movement and consequently, the development of these mineral veins. Key Points: Quartz‐calcite veins in ancient accretionary prisms record temperatures of 100–250°C, implying a fluid‐rich zone along plate interfacesFluid isotopic compositions suggest rock‐buffered fluids as major vein‐forming fluids derived from the dehydration of host rocksReduced vein formation, along with clay hydration at 250–350°C, may cause a shift in hydrogeology and mechanics of subduction fault zones [ABSTRACT FROM AUTHOR]

Abstract Tectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions.

The Wunuer tectonic melange is composed of rock blocks and matrixes with different properties, belonging to sandy tectonic melange. The blocks can be sorted into local blocks and exotic blocks, which are various in size, composition and shape, disorderly distributed in the melange. The matrix is foliated argillaceous rock or phyllite slate, with intense shear deformation and disordered schistosity, distributed around the blocks. The comprehensive study shows that the Wunuer tectonic melange, obviously different from the slump deposit of sedimentary origin, is a typical sandy tectonic melange, serving as an important part of Xinlin-Xiguitu suture zone. [ABSTRACT FROM AUTHOR]

Mélange is characterized by the block-in-matrix fabric in which rigid blocks of different sizes, lithologies, and ages are distributed in a ductile matrix. Though mélange is a significant component of orogenic belts, in the Ukrainian Carpathians, mélanges of tectonic origin have not been reported. We have described the mélange zones widespread in the Pieniny Klippen Belt, Marmarosh Klippen Zone, and the inner nappes of the Outer Ukrainian Carpathians, as well as the typical deformation structures developed within them. The mélange matrix is characterized by a scaly fabric formed by cleavage surfaces and somewhere arranged in S-C structures. Lenticular clasts within the matrix show their long axis aligned to the tectonic foliation. Rotation of the boudins occurs against the shear direction following the formation of the S-C structures. The rigid clasts somewhere demonstrate sigma-type rotation structures. Some blocks within the mélange are highly fractured up to tectonic breccias. The study of tectonic slicken-sides and other deformation structures within the Pieniny belt demonstrates the presence of regular stress fields correlated with the geodynamics of the Carpatho-Pannonian region. The main stress field indicates the SW-NE regional compression related to the formation of Carpathian nappes and S-N — trending dextral strike-slip faults. Our study in the Priborzhava quarries records the Oash right-lateral strike-slip fault zone. In the Pieniny Klippen Belt some oblique normal faults of the Carpathian direction are related to the Transcarpathian Depression formation. The study showed that the mélange zones in the inner part of the Ukrainian Carpathians were formed largely due to strike-slip movements. In contrast, in the outer part of the Carpathian orogen, they formed mainly due to thrusting.

Abstract The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

Based on detailed field investigations and structural mapping in the Petit-Saint Bernard Pass area (French-Italian border), the Versoyen Complex is newly defined as an imbrication of four structural units. The first one is composed of the remnant of the Valaisan ocean floor ("basalt" and "black schist" formation) and the other three units -grouped under the name of "schistes à blocs"-contain blocks with different origin: oceanic, continental or a mix of both, embedded in a matrix of grey micaschists. Kinematic analysis highlights the compressional style of this complex with a well-developed stack of tectonic slivers. Rocks in the tectonic mélange include Paleozoic green gneiss and grey-green micaschists blocks, together with Upper Paleozoic granitoid and layered gabbro, which display a typical orogenic calc-alkaline signature, comparable to that of coeval plutonic rocks of the Briançonnais continental basement attributed to a late Variscan back-arc environment. A new isotopic dating on a calc-alkaline layered gabbro block gave a U/Pb zircon age of 310 ± 4 Ma, in the range of other dated plutonic rocks such as the Punta Rossa and Aiguille du Clapet megablocks. These Paleozoic continental basement rocks of the Versoyen Complex were initially rift allochthons, but are now outcropping as a tectonic mélange, with imbricated blocks and slivers within an Early Cretaceous grey micaschists matrix. Radiolarians sampled by Beltrando et al. (2012) within the grey micaschists have been attributed to the Late Jurassic - Early Cretaceous. Additional stratigraphic and geodynamic constraints allow us to restrict the deposition of the Versoyen sediments from the Aptian to Cenomanian times. The Versoyen Complex is unconformably overlain by the Valaisan Trilogy (Aroley-Marmontains-Saint Christophe) of Cretaceous age. The Turonian Aroley strata deposited on the Versoyen "schistes à blocs" units and the absence of any Aroley limestones within the underlying tectonic mélange testify a clear stratigraphic unconformity, mapped regionally, between the Versoyen Complex and Valaisan Trilogy. Both are affected by a high-pressure metamorphism related to the Alpine collision around 40 Ma and trace of former metamorphism are nowhere observed suggesting that the former mélange did not result in a significant tectonic thickening. We interpret the Versoyen Complex, as a pre-Alpine Cretaceous subduction-related accretionary prism, formed during the closure of the Valaisan Ocean. Valaisan rifting initiated during the Late Jurassic, as a consequence of an eastwards propagation of the North-Atlantic rifting towards the Alpine region. This propagation followed pre-existing Variscan structures along the Zone Houillère, explaining the present marked differences between Variscan basements on each side of the Valaisan suture (External Crystalline Massifs and Briançonnais basement). Due to the southward subduction of the Valaisan Ocean, this passive margin was later on deformed through compressional shearing tectonics, developing a "schistes à blocs" fabric and imbricating the former rift allochthons. During the Tertiary Alpine collision, Versoyen and Valaisan Trilogy units were deeply underthrusted, and then exhumed under the Subbriançonnais units - including the Petit-Saint Bernard and Arguerey Liassic calcschists, which underthrusted in turn the Briançonnais Front. [ABSTRACT FROM AUTHOR]

Yeşilöz Makaslama Zonu (YMZ), Alanya bölgesinde yer alan Alanya Nap Kompleksi’nin, Mahmutlar alt napı içerisinde belirlenmiş büyük bir makaslama zonudur. Nap, yerleşimi sırasında eksenel metamorfik folyasyon ile ilksel transpoze kıvrımlar ve düşey açık kıvrımlar tarzında deformasyon yapılarının gelişimine neden olmuştur. KB doğrultulu bu zon, 2 km genişliğe ve 10 km’nin üzerinde haritalanabilir uzunluğa sahiptir. Makaslama zonu, napa ait metamorfik kayaçlardan, düşey folyasyona sahip milonitleşmiş fillitler içerisinde yer alan merceksi mermer ve kuvarsit blokları ile temsil edilmektedir. Bu kırılgan kayaç kütleleri, nap içerindeki yapılara benzer şekilde kesilmiş ve parçalanmış düşey kıvrımlar ve naptaki metamorfik folyasyonun kalıntılarını içeren fillitik makaslama lensleri sergilemektedir. Makaslama zonu içerisindeki kinematik belirteçler, sağ yanal doğrultu atımlı bir deformasyona işaret etmektedirler. Mahmutlar napının tektonik yerleşimi, altındaki napın üst kesimlerinde hafif dereceli bir metamorfizmaya neden olurken, derinlere kadar uzanan makaslama zonunun gelişimiyle de etkilemiştir. Nap, makaslama zonunun oluşumu sırasında ve sonrasında daha fazla translasyonal harekete uğramamıştır. Bu durum büyük olasılıkla, sünümlü doğrultu atımlı hareketin, Alanya Nap Kompleksi içerisinde sonra etkin olan daha genç napların yerleşimi ile gelişen kalıntı kısalmanın karşılanması ile ilgili olmalıdır.

Discontinuous ophiolites within the Indus–Tsangpo suture zone (ITSZ) in southern Tibet represent remnants of Neo‐Tethyan oceanic lithosphere, which are considered to have formed during the late Jurassic to early Cretaceous. We report field observations, petrological and geochronological data for some mafic intrusions in a tectonic mélange in the southern ophiolitic belt of the western ITSZ. Zircon from a gabbro yields an age of 243 ± 1 Ma with εHf(t) values of +7.9 to +13.2. Rare earth element (REE) pattern of this rock resembles that of normal mid‐ocean ridge basalt. We suggest that such gabbro and surrounding mafic–ultramafic rocks represent the remnants of middle Triassic oceanic lithosphere in the ITSZ. Our findings indicate that the Neo‐Tethys Ocean between India and the Lhasa terrane existed from at least the middle Triassic, contrasting with the commonly held view that the Neo‐Tethys Ocean opened along the northern edge of Gondwana during the late Triassic–early Jurassic. [ABSTRACT FROM AUTHOR]

Ophicalcites exposed on the island of Tinos, Greece, occur as ellipsoidal bodies within greenschist-facies phyllites of the Upper Cycladic Unit. Close to their outcrops, blocks of serpentinites, metabasic rocks and metasediments were identified, implying a tectonically dismembered ophiolitic sequence in the study area. The ophicalcites comprise brecciated serpentinites cemented by calcite. Based on textural, mineralogical and deformation features, five ophicalcite varieties were discriminated, reflecting calcite precipitation, sedimentary features and increasing brecciation. Serpentinitic fragments comprise antigorite, while Cr-spinel, magnetite, talc and chlorite are accessory minerals. Carbonate veins consist of calcite and minor dolomite, talc, chlorite, and rarely epidote. Bulk rock chemical compositions and Cr-spinel mineral composition point towards a supra-subduction environment. Carbon and oxygen isotope ratios of calcite imply precipitation from mixed marine and hydrothermal fluids, followed by isotope exchange due to late, greenschist-facies overprint. The Tinos ophicalcites record intraoceanic exhumation of the ultramafics at the seafloor, where faulting and serpentinization caused an extensive network of fractures, healed by carbonates. Such intraoceanic deformation can be attributed either to obduction tectonics expressed by thrusting of oceanic piles, or to transpressional(?) transform faults, or more probably to slip along detachment fault of an oceanic core complex. [ABSTRACT FROM AUTHOR]

The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision. [ABSTRACT FROM AUTHOR]

The Horné Belice Group of the Považský Inovec Mts. represents a tectonic mélange containing numerous native and exotic olistoliths that accumulated on the outer Tatric active margin during the stacking of the Western Carpathian Middle Group of Nappes. Thin-section samples of the pelagic limestones from 3 different olistoliths located on the NE slopes of the Humienec elevation point in the Rázová Formation were studied and analysed. Two limestone samples represent foraminifera biomicrite (foraminifera wackestone). The sample HR1 contained Globotruncana cf. linneiana (d' Orbigny), Globotruncana cf. arca (Cushman), Globotruncana ventricosa White, Globotruncana sp., Globotruncanita stuartiformis (Dalbiez), Globotruncanita subspinosa (Pessagno), Falsomarginotruncana cf. desioi (Gandolfi), Muricohedbergella sp. and Heterohelix sp. The sample PI4 contained Globotruncana ventricosa White, Globotruncana sp., Globotruncanita cf. subspinosa (Pessagno), and most commonly form Globotruncanita calcarata (Cushman), Globotruncanella havanensis (Voorwijk), Muricohedbergella cf. monmouthensis (Olsson) and Heterohelix sp. Microfauna from both samples points to late Campanian age of the pelagic limestones. New results confirm diachronity of Rázová and Hranty formations (at least during Campanian) and tectonic activity leading to formation of the olistoliths after Campanian. [ABSTRACT FROM AUTHOR]

Amphibolites in the Shuixiakou area of the southern Dunhuang Orogenic Belt, southernmost Central Asian Orogenic Belt (CAOB), occur as lenses within hornblende-biotite-plagioclase gneiss or pelitic schist, exhibiting block-in-matrix feature of tectonic mélange. Three generations of metamorphic mineral assemblages (M1, M2, and M3) have been recognized in the garnet-bearing amphibolite lenses. The metamorphic prograde assemblage (M1) is documented with inclusion trails (hornblende + plagioclase + quartz) within garnet porphyroblasts, and are estimated to be formed under 610–690 °C and 6.5–10.2 kbar. The metamorphic peak assemblage (M2) consists of garnet + hornblende + clinopyroxene + plagioclase + quartz in the matrix and records metamorphic peak P-T conditions of 720–750 °C and 13.4–14.7 kbar. The retrograde assemblage (M3) is represented by the symplectic assemblage (hornblende + plagioclase + quartz ± biotite ± magnetite) rimming the garnet porphyroblast, formed in the decompression stage under P-T conditions of 630–730 °C and 3.8–7.2 kbar. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. SIMS dating of metamorphic zircons from the amphibolites confirm that the high-pressure metamorphism (M2) occurred at ca. 438–398 Ma.

The article proposes a model of collisional and accretionary events of the Neoproterozoic at the western margin of the Siberian Craton based on the results of geological, petrological, and geochronological studies of Precambrian complexes of the Yenisei Ridge and comprehensive geophysical data. Mineralogical and petrological evidence of paleosubduction is presented, represented by relicts of glaucophane-bearing high-pressure mineral assemblages in metabasites (630‒620 Ma) within the tectonic mélange of the suture zone and by metamorphosed ophiolites and island arc complexes of the Isakovka and Predivinsk terranes (700–600 Ma) of the Sayan–Yenisei accretionary belt. The proposed geodynamic evolutionary model of the Yenisei paleosubduction zone includes reconstructions for time intervals of 740‒700, 640‒600, 580‒540 Ma, during which a suture zone was formed as a result of collision of the Kas–Turukhansk microcontinent with the Siberian Craton. Tectonized fragments of the suture partially outcrop on the surface on the right bank of the Yenisei River. The bulk of the deformed and metamorphosed oceanic crust is buried beneath the Ediacaran and Phanerozoic sedimentary cover in the eastern part of the Kas–Turukhansk microcontinent, which is traceable by geophysical data. The history of the region's geological development in the Late Meso‒Neoproterozoic correlates with synchronous endogenic events along the Arctic margin of the Nuna and Rodinia paleocontinents, which confirms the spatial proximity of the Siberian and North Atlantic cratons (Laurentia and Baltica) in a wide time range. [ABSTRACT FROM AUTHOR]

The Chañaral tectonic mélange (northern Chile) is a local unit within the late Paleozoic accretionary complex formed at the southwestern margin of Gondwana. The structural characteristics of the studied mélange were mostly developed during a first deformation phase (D1) and include a block-in-matrix fabric, lineations (L1) and foliations (S1), tight to intrafoliar folds, S-C and S-C-C′ composite planar fabrics, and a conspicuous spatial separation of domains with predominantly linear and linear-planar fabrics. Folding during a second stage (D2) modified the orientation of the previous fabrics and structures. The eastern boundary of the Chañaral mélange is N-S to NNW-SSE oriented, moderately dipping to the east. Its western boundary is not exposed. The plane showing the maximum structural asymmetry (the vorticity normal section) is ENE-WSW directed, and sub-vertical. Kinematic criteria consistently reveal top-to-the-WSW displacement. A kinematic model of triclinic transpression with inclined extrusion has been applied to evaluate the D1 structural features of the Chañaral mélange. The pitch of the simple shear direction on the deformation plane ranged from 60°N to 90°. The pitch of the estimated extrusion direction was of 30°-40°S. The coaxial component was clearly constrictional (logarithmic K value of 2 to 5). The vorticity number has not been constrained by the model, but its spatial variation can explain the domainal distribution of the fabrics in the mélange. The simulated particle paths show the predominance of material displacement parallel to the margin, with low to moderate down-dip displacements, which is in accordance with the low-pressure metamorphic assemblages found in the mélange. The convergence direction between the blocks separated by the mélange unit was N50°-60°E. Kinematic blocking of the Chañaral mélange, probably related to the accretion of an oceanic volcanic domain, allowed the D2 folding of the previous structures, a process that, at least initially, proceeded without a change in the convergence direction. Unlabelled Image • The late Paleozoic Chañaral tectonic mélange (northern Chile) was part of a subduction channel. • Flow kinematics evidence oblique extrusion and limited downdip particle displacements. • Structural analysis of tectonic mélanges should consider 3D models for oblique tectonic settings. [ABSTRACT FROM AUTHOR]

Amphibolites in the Shuixiakou area of the southern Dunhuang Orogenic Belt, southernmost Central Asian Orogenic Belt (CAOB), occur as lenses within hornblende-biotite-plagioclase gneiss or pelitic schist, exhibiting block-in-matrix feature of tectonic mélange. Three generations of metamorphic mineral assemblages (M1, M2, and M3) have been recognized in the garnet-bearing amphibolite lenses. The metamorphic prograde assemblage (M1) is documented with inclusion trails (hornblende + plagioclase + quartz) within garnet porphyroblasts, and are estimated to be formed under 610-690 °C and 6.5-10.2 kbar. The metamorphic peak assemblage (M2) consists of garnet + hornblende + clinopyroxene + plagioclase + quartz in the matrix and records metamorphic peak P-T conditions of 720-750 °C and 13.4-14.7 kbar. The retrograde assemblage (M3) is represented by the symplectic assemblage (hornblende + plagioclase + quartz ± biotite ± magnetite) rimming the garnet porphyroblast, formed in the decompression stage under P-T conditions of 630-730 °C and 3.8-7.2 kbar. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. SIMS dating of metamorphic zircons from the amphibolites confirm that the high-pressure metamorphism (M2) occurred at ca. 438-398 Ma. [ABSTRACT FROM AUTHOR]

The geological, geochemical, and isotope-geochronological evidence of the events at the final stage of the Neoproterozoic history of the Yenisei Range is considered (beginning from the formation of fragments of the oceanic crust in the region and their accretion to the Siberian Craton until the postaccretionary stage of crustal tension and onset of the Caledonian orogeny). Based on an analysis of new data on the petrogeochemical composition, age, and geodynamic nature of the formation of contrasting rocks in the composition of tectonic mélange of the Near-Yenisei (Prieniseiskaya) regional shear zone, we have found the chronological sequence of events that marks the early stages of the Paleoasian Ocean evolution in the zone of its junction with the Siberian Craton. These events are documented by the continental marginal, ophiolitic, and island-arc geological complexes, each of which has different geochemical features. The most ancient structures are represented by fragments of oceanic crust and island arcs from the Isakovka terrane (700-620 Ma). The age of glaucophane-schist metamorphic units that formed in the paleosubduction zone corresponds to the time interval of 640-620 Ma. The formation of high-pressure tectonites in the suture zone, about 600 Ma in age, marks the finishing stage of accretion of the Isakovka block to the western margin of the Siberian Craton. The final events in the early history of the Asian Paleoocean were related to the formation of Late Vendian riftogenic amygdaloidal basalts (572 ± 6.5 Ma) and intrusion of postcollisional leucogranites of the Osinovka massif (550-540 Ma), which intruded earlier fragments of the oceanic crust in the Isakovka terrane. These data allow us to refine the Late Precambrian stratigraphic scheme in the northwestern Trans-Angarian part of the Yenisei Range and the evolutionary features of the Sayan-Yenisei accretionary belt. The revealed Late Neoproterozoic landmarks of the evolution of the Isakovka terrane are attributed to the terminal phase of the breakup of Rodinia, separation of the Siberian Craton, and opening of the Paleoasian Ocean. [ABSTRACT FROM AUTHOR]

Northwestern Ilocos Norte in Luzon, Philippines, exposes cherts, peridotite and a variety of metamorphic rocks including chlorite schist, quartzo-feldspathic schist, muscovite schist and actinolite schist. These rocks are incorporated within a tectonic mélange, the Dos Hermanos Mélange, which is thrust onto the turbidite succession of the Eocene Bangui Formation and capped by the Upper Miocene Pasuquin Limestone. The radiolarian assemblages constrain the stratigraphic range of the cherts to the uppermost Jurassic to Lower Cretaceous. Stratigraphically important species include Eucyrtidiellum pyramis ( Aita), Hiscocapsa acuta ( Hull), Protunuma japonicus ( Matsuoka & Yao), Archeodictyomitra montisserei ( Squinabol), Hiscocapsa asseni ( Tan), Cryptamphorella conara ( Foreman) and Pseudodictyomitra carpatica ( Lozyniak). The radiolarian biostratigraphic data provide evidence for the existence of a Mesozoic basinal source from which the cherts and associated rocks were derived. Crucial to determining the origin of these rocks is their distribution and resemblance with known mélange outcrops in Central Philippines. The mélange in the northwestern Ilocos region bears similarities in terms of age and composition with those noted in the western part of the Central Philippines, particularly in the islands of Romblon, Mindoro and Panay. The existence of tectonic mélanges in the Central Philippines has been attributed to the Early to Middle Miocene arc-continent collision. This event involved the Philippine Mobile Belt and the Palawan Microcontinental Block, a terrane that drifted from the southeastern margin of mainland Asia following the opening of the South China Sea. Such arc-continent collision event could also well explain the existence of a tectonic mélange in northwestern Luzon. [ABSTRACT FROM AUTHOR]

A rapid reduction in sediment porosity from 60 to 70 % at seafloor to less than 10 % at several kilometers depth can play an important role in deformation and seismicity in the shallow portion of subduction zones. We conducted deformation experiments on rocks from an ancient accretionary complex, the Shimanto Belt, across the Nobeoka Thrust to understand the deformation behaviors of rocks along plate boundary faults at seismogenic depth. Our experimental results for phyllites in the hanging wall and shale-tuff mélanges in the footwall of the Nobeoka Thrust indicate that the Shimanto Belt rocks fail brittlely accompanied by a stress drop at effective pressures < 80 MPa, whereas they exhibit strain hardening at higher effective pressures. The transition from brittle to ductile behavior in the shale-tuff mélanges lies on the same trend in effective stress-porosity space as that for clay-rich and tuffaceous sediments subducting into the modern Nankai subduction zone. Both the absolute yield strength and the effective pressure at the brittle-ductile transition for the phyllosilicate-rich materials are much lower than for sandstones. These results suggest that as the clay-rich or tuffaceous sediments subduct and their porosities are reduced, their deformation behavior gradually transitions from ductile to brittle and their yield strength increases. Our results also suggest that samples of the ancient Shimanto accretionary prism can serve as an analog for underthrust rocks at seismogenic depth in the modern Nankai Trough. [ABSTRACT FROM AUTHOR]

This paper presents a geological map and interpretative cross-sections, which illustrate the structure of a major fault zone of the Alps, that is, the Ligurian segment of the Penninic Basal Contact (PBC), which led to the emplacement of the orogenic wedge onto the European crust. Chaotic deposits, whose origin is debated, characterize the footwall of this complex thrust zone. The lower part of the sequence containing chaotic deposits consists of low deformed paraconglomerates and exotic megablocks embedded with turbidites. Conversely, the highly deformed upper part of the sequence englobes fragments of substratum-derived succession and is bounded by thrust planes. The nature of these chaotic deposits suggests an origin by gravitational processes related to the unstable front of the advancing wedge associated with offscraping of tectonic slices during the thrusting of allochtonous nappes along the PBC thrust zone. [ABSTRACT FROM AUTHOR]

Himalayan fold belt has full of geological surprises, 'mélange' is one of them which create difficulties during tunneling. Such mélange completely went unnoticed during surface mapping and geotechnical investigation preceding the construction of the Udhampur railway tunnel (URT). During the construction, the mélange zone has encountered across the tunnel, which occurs along the Tanhal thrust (equivalent to MBT) that separates the Murree Group and the Shiwalik Group. The mélange was characterized by a chaotic, heterogeneous geological mixture of stronger blocks (scale independence) and weaker sheared fine-grained matrix, often termed as 'block-in-matrix rocks' or bimrocks, which enforced mixed face tunneling. The heterogeneity in a tectonic mélange led to stress concentrations in the rock blocks, and there were relatively high deformations within the matrix also. Release of stress from the blocks due to excavation, with unfavorable joint and thrust orientations enforced brittle failure of the blocks (face and crown collapses) while matrix deformation (time dependent) caused convergence of primary support later. Additionally, the clay minerals with high swelling potential within the matrix swelled and created pressure on the primary support. Due to the geomechanical heterogeneity in mélange, homogenizing the rock-mass by the commonly used quantitative systems might have lead to an inappropriate design and construction. The adopted New Austrian Tunneling Method (NATM) proved to be an useful tool for tunneling. [ABSTRACT FROM AUTHOR]

The lithological nature of major interplate boundaries is estimated by a field analysis of a well preserved exhumed subduction channel in the Caucasus Karabakh region. From this field example the subduction channel is a narrow geological object of about 500 m width formed at approximate depth of 10 km along an Andean-type subduction zone. It is comprised by an upper ‘sedimentary’ channel formed by an upper section of detrital and volcanic rocks thrusted on top of pelagic sediments scrapped off the oceanic floor. This sedimentary mélange is thrusted on top of an intensely deformed tectonic mélange. The tectonic mélange comprises blocks of basalt from the oceanic floor and a focussed deformation zone 50–100 m in width. This zone is mainly formed by mud-supported conglomerates exhibiting a chlorite + carbonate matrix with blocks of basalt, cross-cut by numerous chlorite-carbonate-epidote-albite veins. It overlies an undeformed ocean floor section. Superposed chlorite- and calcite-bearing veins in the mélange evidence high fluid:rock ratios of 0.3–2.3, with varied δ 18 O and δ 13 C isotopic ratios (+17 < δ 18 O < +25‰; −7 < δ 13 C < +4‰), which agrees with fluid mixing between pelagic sediments and a hydrothermal component at temperatures ranging from 120 to 400 °C, and thus mixing between deep and shallow reservoirs along the subduction interface. These data show that the several fluid reservoirs situated along the interplate boundary could have been connected by high-magnitude co-seismic displacements along the subduction zone. These subduction channel features are confronted to other similar fossil examples and current settings, such as the Andes accretionary prism to propose a reconstructed geometry of the interplate contact zone from the surface to the base of the crust. [ABSTRACT FROM AUTHOR]

The fluid pressure ratio and effective friction coefficient of shear veins in an exhumed accretionary complex in the Cretaceous Shimanto Belt, southwest Japan, were constrained by fluid inclusion and stress inversion analyses. The shear veins are considered to be developed along the decollement. Temperature and pressure conditions for shear veins were found to be in the range of 175–225°C and 143–215 MPa, respectively. Fluid pressure ranged between the lines of 70–100% lithostatic pressure, when the geothermal gradient was assumed on the basis of the slab age at the trench. The effective friction coefficient of shear veins was estimated to be 0.1–0.2 by stress inversion analysis, which corresponds to a fluid pressure ratio of 0.67–0.83. Both pressure–temperature and friction coefficient results indicate that the fluid pressure ratio is higher than about 0.7. In this paper, we propose the procedure to constrain paleo-fluid pressure ratio from natural materials. Wedge states were examined using the weak decollement friction coefficient, with shallow and steep taper angles indicating that the wedge state is stable or compressively critical, and is controlled by the fluid pressure ratio within the wedge. [ABSTRACT FROM AUTHOR]

In the central-northern margin of the North China Craton, the tectonic boundary between the Jining and Huai'an Complexes is well exposed in the Gushan area. In this boundary zone, exotic blocks of orthopyroxene-bearing monzonitic gneiss, garnet-bearing mafic granulite, marble, and calc-silicate were identified in the meta-psammitic and meta-pelitic matrix. The likely nature of the rock units is a tectonic mélange, which is a key to understand the tectonic evolution of the craton. In this paper, geochronological and geochemical studies have been carried out on representative lithologies from the Gushan area to discuss their petrogenesis and geodynamic settings. The opx-bearing monzonitic gneisses (U–Pb zircon emplacement age of 2.17 Ga) are high-K calc-alkaline to shoshonitic I-type rocks with trace-element magmatic arc signature. The trace elements, whole rock Sm–Nd and zircon Lu–Hf isotopes suggest that they were possibly derived from partial melting of Neoarchean lower crust (garnet amphibolite) in a continental arc. The protoliths of the garnet-bearing mafic granulites are inferred to have formed between 2.37 Ga and 1.95 Ga, possibly at 2.2 Ga. They show N-MORB geochemical affinity and were most likely produced by partial melting of spinel lherzolite in a mid-ocean ridge setting. Detrital zircons from one metapsammite sample (garnet-bearing quartzofeldspathic gneiss) yield nearly concordant 207 Pb/ 206 Pb ages ranging from 2.1 Ga to 2.0 Ga. In situ U–Pb and Hf isotopic data of zircons from the metapsammite sample support that the metasedimentary protoliths were probably derived from a mid-Paleoproterozoic continental arc. Three metamorphic age groups (1.95–1.93 Ga, 1.86–1.83 Ga, and 1.80 Ga) are revealed by the opx-bearing monzonitic gneisses, garnet-bearing mafic granulites, garnet-bearing quartzofeldspathic gneiss, and sillimanite–garnet–K-feldspar gneiss. The predominant group (1.95–1.93 Ga) is considered to record the age of near-peak granulite facies metamorphism. The 1.86–1.83 Ga group may result from decompression or the thermal imprint of the collision between the Eastern and Western Blocks. The 1.80 Ga group is consistent with the age (1.81 Ga) of the ductile deformation of the garnet–K-feldspar mylonite, and is interpreted as the timing of a relatively rapid tectonic extension. Thus, the Paleoproterozoic Gushan mélange formed and evolved in a subduction–collision–exhumation process, and finally situated in a likely suture zone between the Eastern and Western blocks or sub-cratons. [ABSTRACT FROM AUTHOR]

The Dorud–Azna region of the Sanandaj–Sirjan metamorphic zone (SSMZ) in NW Iran is part of the Zagros orogenic belt and exposes an amphibolite-grade metamorphic succession of mixed continental (e.g., the granitic Galeh–Doz orthogneiss) and oceanic or rift (e.g., various amphibolites) origin, which is intruded by the Darijune gabbro. Laser-ablation ICP–MS U–Pb zircon ages of 608 ± 18 Ma and 588 ± 41 Ma of the granitic Galeh–Doz orthogneiss indicate a Panafrican basement similar to that of the Yazd block of Central Iran. We therefore interpret this part of the SSMZ to represent a previously subducted part of the Panafrican basement of the Iranian microcontinent. Based on geochemistry, amphibolites and metagabbros from the Amphibolite unit representially potentially the cover on the Panafrican Galeh–Doz orthogneiss are interpreted to have their origin in a likely Upper Paleozoic to Mesozoic rift or E-MORB ophiolite succession. One of the metagabbros, here termed Dare-Hedavand metagabbro yield a 206 Pb/ 238 U mean age of 314.6 ± 3.7 Ma. Talc-bearing greenschists to epidote amphibolites of the Triassic June complex formed in a similar, but younger environment. Furthermore, the metamorphic complex is intruded by the Darijune gabbro, which yields a mean ICP–MS U–Pb zircon age of 170.2 ± 3.1 Ma. The final stage of likely Jurassic greenschist facies-grade metamorphism postdates the gabbro intrusion. We interpret the Darijune gabbro to have resulted from the initial subduction of Neotethyan oceanic lithosphere in a continental arc setting. The new data combined with data from literature indicate that the SSMZ represents a tectonic element of a mixed continental and oceanic origin, likely accreted to Central Iran during a Jurassic tectonic process. The new data also constrains thick-skinned nappe stacking within the Sanandaj–Sirjan zone. [ABSTRACT FROM AUTHOR]

Abstract: The Tam Ky-Phuoc Son Shear Zone (TKPSSZ) is one of the most pronounced crustal structures in central Vietnam and juxtaposes high-grade rocks of the Kontum Massif against lower grade rocks of the Truong Son Fold Belt. An internal zone comprises highly strained rocks, including ultramafic–mafic tectonites, bounded by mylonitic zones that straddle tens to hundreds of kilometers. An external zone comprises mainly high-grade metamorphic complexes (southern flank), and weakly metamorphosed siliciclastic and carbonate rocks with subordinate greenstone (northern flank). Detailed structural analysis reveals that the shear zone is a multi-deformed terrane: D1 produced a regional high grade schistosity and mylonitization; D2 generated regional northwest–southeast trending fold arrays and brittle–ductile shear zones; D3 locally produced northeast-trending folds; brittle faulting occurred during D4–D5. U–Pb dating of zircon and monazite suggests that D1 involved metamorphism and felsic magmatism at ca. 430Ma, recording part of a regional collisional orogeny. Monazite and titanite growth at ca. 250–240Ma in basement rocks is synchronous with widespread syn- to post-D2 magmatism between ca. 260 and 245Ma suggesting a second major collisional event during the Indosinian Orogeny. D3 may have occurred as part of (or soon after) this Permo-Triassic event. Deformation during post-collisional stages, perhaps in response to extrusion/wedging and oroclinal rotation of terranes, led to post-D3 structures (D4–D5). The E–W trending TKPSSZ is here shown to be a continuation of the N–S trending Po Ko Shear Zone (PKSZ). Most of the significant lode gold occurrences in central Vietnam occur along this TKPSSZ–PKSZ structure and are associated with, and controlled by, D1 ductile to ductile–brittle high-strain zones. Mineralized later-stage structures support remobilization and reconcentration of ore during subsequent events at ca. 240Ma (Re–Os molybdenite age). The TKPSSZ–PKSZ D1 structure likely represents part of a paleosuture zone, marking the closure of an ancient ocean basin through terrane assembly in the Early Paleozoic. [Copyright &y& Elsevier]

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of peridotite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust- and mantle-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism. [ABSTRACT FROM AUTHOR]

Laterally extensive belts of mélange characterize Phanerozoic convergent plate margins, but are rare in Archean terranes. We document a late Archean mélange in the Zanhuang Massif of the North China Craton (NCC). The Zanhuang mélange separates a passive margin to foreland basin sequence developed on the western edge of the Eastern Block of the NCC from an arc terrane consisting of trondhjemitic, tonalitic and granodioritic (TTG) gneisses in the Central Orogenic Belt (COB) of the NCC. The mélange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles and quartzites mixed with exotic tectonic blocks of ultramafic and metagabbroic rocks, metabasalts that locally include relict pillow structures, and TTG gneisses. All units in the mélange have been intruded by mafic dikes that were subsequently deformed, and are now preserved as garnet–amphibolite boudins. We interpret the mélange to mark the suture zone between the Eastern Block and the arc terrane in the COB. Field relationships and geochemistry suggest that the exotic ultramafic–metagabbroic–metabasaltic blocks are possible slivers of an intra-oceanic arc or fore-arc ophiolite incorporated into the mélange during the arc–continent collision process. A circa 2.5Ga granitic pluton intrudes the mélange and undeformed circa 2.5Ga pegmatites cut the mélange. Tectonic models for the evolution of the COB are varied, but include models that favor collision at 2.5Ga, 2.1Ga, and 1.8Ga. This work shows clearly, from field structural relationships and geochronology, that the first collision must have occurred prior to 2.5Ga, consistent with late Archean suturing of the western margin of the Eastern Block with an arc terrane (Fuping terrane) during an arc–continent collision. The presence of an Archean mélange with exotic blocks in a suture zone between an Archean arc and continental margin is clear evidence for the operation of plate tectonics at circa 2.5Ga. [ABSTRACT FROM AUTHOR]

Abstract: For the reconstruction of Alpine tectonics of the Eastern Alps, the evaporitic Permian to Lower Triassic Haselgebirge Formation plays a key role in (1) the origin of Haselgebirge bearing nappes, (2) the inclusion of magmatic and metamorphic rocks revealing tectonic processes not preserved in other units, and (3) the debated mode of emplacement of the nappes, namely gravity-driven or tectonic. Within the Moosegg quarry of the central Northern Calcareous Alps gypsum/anhydrite bodies are tectonically mixed with lenses of sedimentary rocks and decimeter- to meter-sized tectonic clasts of plutonic and subvolcanic rocks and rare metamorphics. We examined various types of (1) widespread biotite–diorite, meta-syenite, (2) meta-dolerite and rare ultramafic rocks (serpentinite, pyroxenite) as well as (3) rare metamorphic banded meta-psammitic schists and meta-doleritic blueschists. The apparent 40Ar/39Ar biotite ages from three biotite–diorite, meta-dolerite and meta-doleritic blueschist samples with variable composition and fabrics range from 248 to 270Ma (e.g., 251.2±1.1Ma) indicating a Permian age of cooling after magma crystallisation or metamorphism. The chemical composition of biotite–diorite and meta-syenite indicates an alkaline trend interpreted to represent a rift-related magmatic suite. These, as well as Permian to Jurassic sedimentary rocks, were incorporated during Cretaceous nappe emplacement forming the sulphatic Haselgebirge mélange. The scattered 40Ar/39Ar white mica ages of a meta-doleritic blueschist (of N-MORB origin) and banded meta-psammitic schist are ca. 349 and 378Ma, respectively, proving the Variscan age of pressure-dominated metamorphism. These ages are similar to detrital white mica ages reported from the underlying Rossfeld Formations, indicating a close source–sink relationship. According to our new data, the Haselgebirge bearing nappe was transported over the Lower Cretaceous Rossfeld Formations, which include many clasts derived from the Haselgebirge Formation and its exotic blocks deposited in front of the incoming nappe comprising the Haselgebirge Formation. [Copyright &y& Elsevier]

Abstract: Exhumed high pressure–low temperature metamorphic mélanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction. Rigid blocks of widely varying size are embedded in a weak matrix, which takes up the deformation and controls the rheology of the composite. The microfabrics of a highly deformed jadeite-blueschist from the Franciscan Complex, California, are investigated to help understand deformation mechanisms at depth. The specimen shows a transposed foliation with dismembered fold hinges and boudinage structures. Several generations of open fractures have been sealed to become veins at high-pressure metamorphic conditions. The shape of these veins, frequently restricted to specific layers, indicates distributed host rock deformation during and after sealing. Small cracks in jadeite and lawsonite are healed, with tiny quartz inclusions aligned along the former fracture surface. Large jadeite porphyroblasts show strain caps and strain shadows. Open fractures are sealed by quartz and new jadeite epitactically grown on the broken host. Monophase glaucophane aggregates consist of undeformed needles with a diameter between 0.1 and 2μm, grown after formation of isoclinal folds. Only quartz microfabrics indicate some stage of crystal-plastic deformation, followed by annealing and grain growth. Aragonite in the latest vein generation shows retrogression to calcite along its rims. The entire deformation happened under HP-LT metamorphic conditions in the stability field of jadeite and quartz, at temperatures between 300 and 450°C and pressures exceeding 1–1.4GPa. The microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism, accompanied by brittle failure and vein formation at quasi-lithostatic pore fluid pressure. This indicates low flow strength and, combined with high strain rates expected for localized deformation between the plates, a very low viscosity of material in the interplate shear zone at a depth>30–45km. [Copyright &y& Elsevier]

Abstract: Fabric analyses based on the anisotropy of magnetic susceptibility (AMS) provide a criterion with which to distinguish tectonic from sedimentary mélanges in the Tertiary Shimanto accretionary complex, Yakushima Island, SW Japan. Sedimentary mélanges are characterized by fabric overprinting associated with compaction and diagenesis. The magnetic ellipsoids for such mélanges represent a cleavage/bedding-parallel magnetic foliation, similar to that obtained for coherent sedimentary layers in nearby rocks. The tectonic mélanges and tectonically formed duplex structure are characterized by grain rotation, whereby the grain short-axis is rotated about axes oriented parallel to the intersection between S- and C-planes and the long axis of pencil cleavage, perpendicular to the shear direction. The AMS data obtained for tectonic mélanges and duplex structure show a less intense magnetic foliation compared with the sedimentary mélanges, with K max (the maximum magnetic susceptibility) axes oriented parallel to the intersection between S- and C-planes and the long axis of pencil cleavage, and K min (the minimum magnetic susceptibility) axes showing a girdle distribution about K max . These contrasting paths of fabric development may reflect different stages of mélange formation during the overall history of sediment deposition, burial, accretion, and uplift. Sedimentary mélanges are likely to be strongly compacted, given that they formed soon after sedimentation. In contrast, tectonic mélanges and duplex structure contain a shear-induced fabric because they develop at a relatively late stage, once the sediments have been sufficiently compacted for such a fabric to develop, possibly at levels near the maximum burial depth. [Copyright &y& Elsevier]

Abstract: Serpentinites can enhance the exhumation of HP-rocks by lowering the bulk density of subducted material and by establishing a low-viscosity channel between the lower and upper plates; this mechanism has been proposed for a 100m-scale tectonic mélange outcropping inside the Voltri Massif (Ligurian–Piemontese Units, Western Alps, Italy). We want to test if the mechanism of the subduction channel is feasible at a larger scale, i.e. for the km-scale Voltri Massif, that consists of metaophiolites recording blueschist to eclogite facies peak conditions. We identified a study area in the eastern part of the massif made of highly sheared serpentinite which hosts lenses of metagabbros; a strong strain partitioning between host-rock and the lenses and between core and rims of the lenses occurs. We coupled P–T pseudosections, garnet–omphacite geothermometry and petrographic observations and we obtained clockwise P–T paths with almost isothermal decompression trajectory. The analyzed blueschist lens recrystallized at peak conditions of 10–15kbar and 450–500°C; the eclogitic ones reached peak metamorphic conditions ranging from about 21kbar and 450–490°C to 22–28kbar and 460–500°C; the studied bodies thus recorded different metamorphic peaks acquired at slightly different T conditions. Comparing our results with literature data, a large scatter in the metamorphic peak conditions is displayed, with peak temperatures up to 650°C and pressures ranging from 18 to 25kbar. (1) The presence of poorly deformed lenses (from m- to km-scale) of heterogeneous lithologies inside highly sheared serpentinite and metasediments country-rocks, (2) the strong strain partitioning and (3) the heterogeneous metamorphic peak conditions of the different lenses allow us to suggest that the Voltri Massif matches the requirements of a tectonic mélange; in particular the eastern sector may thus represent a “fossil” serpentinite-subduction channel, comparable to the small-scale one already described in the western sector of the Voltri Massif. [Copyright &y& Elsevier]

Abstract: In this paper we present U–Pb age data of detrital and magmatic zircons from sedimentary and volcanic rocks in the Zhangguangcailing Group (ZG), NE China, to constrain the tectonic evolution of the region as a whole. The ZG consists of slightly metamorphosed volcanic and sedimentary rocks, and from bottom to top is divided into the Zhenggou, Hongguang, and Xinxing suites. Zircons from three schists in the Zhenggou suite give 12 main groups of age populations (from 226 to 861Ma), whereas zircons from a basaltic andesite give a Late Triassic age (211±2Ma). In the Hongguang suite, zircons from two schists and a greywacke give 6 main groups, 5 main groups, and 3 main groups of age populations (from 262 to 916Ma), respectively, whereas zircons from a gabbro–diorite and two volcanic rocks give ages of 259, 219, and 317Ma, respectively. Zircons from a schist in the Xinxing suite give 12 main groups of age populations (from 284 to 1837Ma). These results, together with the field relationships of the rocks, indicate (1) that the sedimentary and volcanic rocks of the ZG have diverse ages (from Early Paleozoic to Early Mesozoic), (2) that the rocks do not lie in a logical stratigraphic sequence, and (3) that the rocks represent a tectonic mélange. Based on the youngest age (211Ma) for volcanic rocks within the ZG, and the age of deformation (174–184Ma) of the Heilongjiang Complex, we propose that the tectonic mélange formed during the Late Triassic–Early Jurassic. Furthermore, the occurrence of detrital zircons with ages of 0.8–0.9, 1.8, and 2.4–2.5Ga implies the existence of Neoproterozoic magmatism and the remnants of a Precambrian basement in the Songnen–Zhangguangcai Range Massif, NE China. [Copyright &y& Elsevier]

Mud volcanoes at convergent margins are important pathways through which clay minerals and fluids, collectively known as mud breccia, originating from deep within forearcs, are ejected at the surface, opening an important window to shallow level processes in subduction zones. Although the mud breccia is the only detachable part of a subducting slab at shallow depths, its origin and implications for the chemistry of crustal material getting recycled into the mantle remain largely unknown. To understand the chemical transformation of slabs within forearcs and the chemistry of recycled components in accretionary subduction zones, we carried out a detailed mineralogical, geochemical and Sr-Nd-Pb isotopic study of matrix of the mud breccia ejected at the mud volcanoes of the Andaman accretionary prism, located at the Indian Plate-Burma Plate convergent margin. Our current and an earlier study on these mud volcanoes reveal that the clay-quartz rich mud matrix, and accompanying water from mineral dehydration and thermogenic hydrocarbons are extracted from depths more than 6 km in the forearc, from tectonic mélanges located at the base of the accretionary prism, in the décollement zone. The mud breccia ascends rapidly through the heavily deformed accretionary wedge and gets emplaced into the overlying soil horizon, incorporating rock clasts from the geological formations of the wedge and minor organic matter from the soil. Major-Trace element contents and isotopic ratios of mud matrix indicate that they contain materials derived from both the altered oceanic crust and terrigenous sediment of the slab, with the former contributing the most (> 80%). All data point to the scenario that only a small fraction of the terrigenous sediment of the slab gets recycled into the mantle at the Andaman subduction zone, which in turn gets reflected in the relatively pristine isotopic compositions of the arc lavas erupted at the Barren Island volcano. [ABSTRACT FROM AUTHOR]

The Kanayama unit of the Mesozoic Mino-Tamba accretionary complex in eastern southwest Japan is a thick mélange unit composed mainly of abundant muddy matrix and variously sized clasts of chert, hemipelagic siliceous mudstone, and sandstone. Two episodes of deformation formed the mélange fabrics: the first involved fragmentation of sandstone layers in response to mud injections; the second involved layer-parallel, noncoaxial shear that resulted in the mixing of pelagic to terrigenous clasts and the formation of S-C-like asymmetric fabrics with scaly foliation. The S-C fabrics exhibit a predominantly sinistral shear sense in plan view. After fold and tilt corrections, the restored slip motion inferred from the S-C fabrics indicates a bulk top-to-the-south shear. Combining the unit's terminal fossil age and radiometric age with a regional plate reconstruction model, these data suggest that the Kanayama mélange formed along a décollement during oblique subduction of the Izanagi plate in the Early Cretaceous. [ABSTRACT FROM AUTHOR]

Quartz, carbonates and other minerals as e.g., realgar are present in veins and caverns in sedimentary rocks in the Western Carpathians. In the Polish segment, they have been characterized from the mineralogical, petrologic, and geochemical points of view, as well as fluid inclusions. Their characters are discussed from perspective of a description of particular types of chaotic complexes—the tectonic mélange zones, distinguished in the Western Carpathian area over the last two decades. The mélange zones are considered to be geochemical systems open to fluid flow, a site for mineral crystallization and/or migration zones of hydrocarbons and mineralized waters. In this context the tectonic mélange in the Jabłonki/Rabe vicinity (SE Poland, the Bieszczady region) in comparison to that of the Mszana Dolna tectonic window area are proposed as the examples. The trapping conditions of fluids (brine and methane) in the minerals in the mélange zones appear to have been 180–205 °C and ~550–570 bars, and 220 °C and 500 bars for calcite and quartz, respectively. The general trend of the increase in temperatures and pressures from west towards east and south-east in the mélange zones points to an increase in the degree of exhumation of different parts of the Carpathians. [ABSTRACT FROM AUTHOR]

Numerous studies exist on exhumed tectonic mélanges along subduction channels whereas, in accretionary wedge interiors, deformation mechanisms and related fluid circulation in tectonic mélanges are still underexplored. We combine structural and microstructural observations with geochemical (stable and clumped isotopes and isotope composition of noble gases in fluid inclusions of calcite veins) and U-Pb geochronological data to define deformation mechanisms and syn-tectonic fluid circulation within the Mt. Massico intra-wedge tectonic mélange, located in the inner part of the central-southern Apennines accretionary wedge, Italy. This mélange developed by shear deformation at the base of a clastic succession. Deformation was characterized by disruption of the primary bedding, mixing, and deformation of relicts of competent olistoliths and strata within a weak matrix of deformed clayey and marly interbeds. Recurrent cycles of mutually overprinting fracturing/veining and pressure-solution processes generated a block-in-matrix texture. The geochemical signatures of syntectonic calcite veins suggest calcite precipitation in a closed system from warm (108°-147 °C) paleofluids, with δ 18O vlaues between +9‰ and 14‰, such as trapped pore waters after extensive 18O exchange with the local limestone host rock and/or derived by clay dehydration processes at T > 120 °C. The 3He/4He ratios in fluid inclusions are lower than 0.1 Ra, indicating that He was exclusively sourced from the crust. We conclude that: (1) intraformational rheological contrasts, inherited trapped fluids, and low-permeability barriers such as marly-shaly matrix, can promote the generation of intra-wedge tectonic mélanges and the development of transient fluid overpressure; (2) clay-rich tectonic mélanges, developed along intra-wedge décollement layers, may generate low-permeability barriers hindering the fluid redistribution within accretionary wedges. • Intra-wedge tectonic mélange generate by intraformational rheological contrasts. • Veining and pressure-solution processes generated block-in-matrix texture. • Calcite veins precipitated from warm (108–147 °C) trapped formation waters. • Clay-rich stylolites can favour the development of fluid overpressure. • Intra-wedge tectonic mélanges may generate efficient barriers to fluid flow. [ABSTRACT FROM AUTHOR]