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In the repeatedly reactivated Gavilgarh–Tan Shear Zone of central India, sheared and foliated pseudotachylyte (Pt–M) veins, with characteristic signatures of viscous deformation, occur as discrete layers within granitic mylonite. Deformation history and age constraints described by earlier workers suggest that semi-brittle reactivation of the shear zone possibly occurred near the frictional-viscous transition at an intermediate depth of the crust that formed these pseudotachylyte layers. Detailed microstructural study and electron back-scattered diffraction (EBSD) analyses of the mylonite and the Pt–M layers were carried out in the present study, which suggest that the mylonite deformed at a high temperature (>500°–600°C) with dominance of Regime 3 Grain Boundary Migration (GBM) recrystallization. Quartz grains in the mylonite deformed dominantly by prism and rhomb slip. In the Pt–M, significant grain size reduction occurred by initial cataclasis and later recrystallization processes. Deformation of quartz grains by a combination of basal and (subordinate) rhomb slip suggests relatively lower temperatures for Pt–M deformation. Grain boundary misorientation distribution (MAD) shows the dominance of low-angle subgrain boundaries in neighbour-pair grains, suggesting the dominance of subgrain rotation (SGR) recrystallization in the Pt–M matrix. Extensive development of chlorite, sericite, and epidote in Pt–M veins suggest hydration during shearing. Prism slip was possibly activated at a lower temperature (<400°C) due to the hydrolytic weakening of quartz. The Pt–M veins developed an interconnected weak layer of biotite, chlorite, and/or epidote during shearing and retrogression that helped in concentrating viscous deformation in these layers. [ABSTRACT FROM AUTHOR]

For projects near the tectonic belt, mylonite of varying metamorphic degrees may be present. The matrix proportion of rock reflects its internal microscopic characteristics, thus it is beneficial for engineering geology to study the effect of the matrix proportion on the mechanical properties and rupture behaviors of rock. Samples of mylonitic granite and granitic protomylonite with varying matrix proportions were obtained from a ductile shear zone for a series of uniaxial compression and acoustic emission (AE) tests. The results showed that with the increase in matrix proportion, the average strength and elastic modulus of the samples increased, and the rock sample with the largest matrix proportion exhibited the maximum peak stress of 244.42 MPa, which was 45.86% greater than the average peak stress of the rock samples with the smallest matrix proportions. For the rock samples with larger matrix proportion, their mechanical parameters exhibited greater dispersion and the large-scale appearance of AE events occurred earlier, showing a relatively gradual failure process. These samples had larger accumulated AE parameter values and greater degree of failure. In contrast, for samples with smaller matrix proportions, the large-scale appearance of AE events occurred close to the peak stress, indicating that the occurrence of damage and fractures was centralized and instantaneous. These samples had lower accumulated AE parameter values and fewer cracks after failure. Additionally, for the rock samples with more matrix proportion, the average variance of the b-value was 1.1, which was lower than that of rock samples with the smallest matrix proportion (the average variance of the b-value was 3.7). Furthermore, it can be predicted that under certain stress, the failure depth around a tunnel is generally smaller when the strength of rock samples with larger matrix proportion is greater.

The Qinling Orogenic Belt (QOB) is a multi‐stage orogenic belt recording subduction and collision processes. The ductile shearing deformation with different properties and ages is developed on the boundary of the different litho‐tectonic units, which can be used to decipher the tectonic evolution of the QOB. Previous studies mainly concentrated on the pre‐Late Mesozoic shearing deformation, while the Late Mesozoic shearing deformation received less attention. It restricts the understanding of the Late Mesozoic tectonic evolution of the QOB. Hence, we conducted detailed field structural analysis and laboratory analysis of the Shagou shear zone (SGSZ) in the middle part of the Shangdan suture zone (SDSZ). The SGSZ, developed in the southern margin of the Baliping pluton, is an S–SSW‐dipping sinistral strike‐slip ductile shear zone with a top‐to‐the‐ESE shear sense. The mylonite protoliths are voluminous Late Triassic granites (200–215 Ma) and a small amount of Neoproterozoic granites (759 Ma) and Early Cretaceous granites (145 Ma). Microstructures and geothermobarometer results suggest that the ductile shearing deformation occurred under high greenschist–low amphibolite‐facies conditions (400–650°C) at middle crustal depths (13–21 km). Zircon U–Pb and amphibole‐biotite 40Ar/39Ar dating results constrain the timing of shearing deformation to 135–145 Ma and the rapid exhumation of the SGSZ to 115–135 Ma. Combining with the regional data, we consider that the sinistral strike‐slip shearing deformation in the SGSZ controls and accommodates the south‐eastward extrusion of the East SQB under the intracontinental compression. Subsequently, the intracontinental extension led to the rapid exhumation of the SGSZ. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between olivine fabric transitions and seismic anisotropy in mantle shear zones, focusing on the Yugu peridotite body in the Korean Peninsula. Olivine, a key mineral in the upper mantle, influences seismic anisotropy through deformation fabrics. The Yugu peridotite body provides insights into these processes within mantle shear zones. Based on microstructures and olivine fabric transitions, this study categorizes peridotites into three groups (PM: proto-mylonite, M: mylonite, and UM: ultra-mylonite) and explores their seismic properties. The findings highlight a direct correlation between olivine fabric strength and seismic anisotropy. Group PM peridotites exhibited higher seismic anisotropy compared to those in Group M and UM peridotites. This study emphasizes that variations in seismic anisotropy within mantle shear zones are primarily driven by the strength of olivine fabric, with additional influences from fabric type and rock composition. The calculated anisotropic layer thickness supports the observation that seismic anisotropy is significantly larger away from the UM peridotites. These insights contribute to understanding of the complex interplay among olivine fabric, seismic anisotropy, and geological processes within mantle shear zones. The implications of this study extend to the improved interpretation of seismic data related to shear localization during peridotite exhumation. [ABSTRACT FROM AUTHOR]

Strain localization processes in the continental crust generate faults and ductile shear zones over a broad range of scales affecting the long-term lithosphere deformation and the mechanical response of faults during the seismic cycle. Seismic anisotropy originated within the continental crust can be applied to deduce the kinematics and structures within orogens and is widely attributed to regionally aligned minerals, e. g., hornblende. However, naturally deformed rocks commonly show various structural layers (e.g., strain localization layers). It is necessary to reveal how both varying amphibole contents and fabrics in the structural layers of strain localization impact seismic property and its interpretations in terms of deformation. We present microstructures, petrofabrics, and calculate seismic properties of deformed amphibolite with the microstructures ranging from mylonite to ultramylonite. The transition from mylonite to ultramylonite is accompanied by a slight decrease of amphibole grain size, a disintegration of amphibole and plagioclase aggregates, and amphibole aspect ratio increase (from 1.68 to 2.23), concomitant with the precipitation of feldspar and/or quartz between amphibole grains. The intensities of amphibole crystallographic preferred orientations (CPOs) show a progressively increasing trend from mylonitic layers to homogeneous ultramylonitic layers, as indicated by the JAm index increasing from 1.9–4.0 for the mylonitic layers and 4.0–4.8 for the transition layer, to 5.1–6.9 for the ultramylonitic layers. The CPO patterns are nearly random for plagioclase and quartz. Polycrystalline amphibole aggregates in the amphibolitic mylonite deform by diffusion, mechanical rotation, and weak dislocation creep, and develop CPOs collectively. The polymineralic matrix (such as quartz and plagioclase) of the mylonite and the ultramylonite deform dominantly by dissolution-precipitation, combined with weak dislocation creep. The mean P and S wave velocities are estimated to be 6.3 and 3.5 km/s, respectively, for three layers of the mylonitic amphibolite. The respective maximum P and S anisotropies are 1.5%–6.4% and 1.8%–4.5% for the mylonite layers of the mylonitic amphibolite, and 6.0%–6.9% and 4.5%–5.0% for the transition layers; but for the ultramylonite layers, these values increase significantly to 8.0%–9.1% and 5.1%–6.0%, respectively. Furthermore, increasing strain (strain localization) generates significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientations of structures in the Hengshan-Wutai-Fuping complex terrains, can generate substantial variations in the orientation and magnitude of seismic anisotropy for the continental crust as measured by the existing North China Geoscience Transect. Thickened amphibolitic layers by extensively folding or thrusting in the middle crust can explain the strong shear wave splitting and the tectonic boundary parallel fast shear wave polarization beneath the Hengshan-Wutai-Fuping complex terrains. Therefore, signals of seismic anisotropy varying with depth in the deforming continent crust need not deduce depth-varying kinematics or/and tectonic decoupling. [ABSTRACT FROM AUTHOR]

Orogen-parallel ductile shear zones are conspicuous structures in the Khondalite Belt, but the timing of shearing remains poorly understood. Here, we present field-based structural and zircon U-Pb geochronological studies on the newly discovered Qianlishan ductile shear zones (QDSZ) in the Khondalite Belt. Our results show that the nearly E-W-trending QDSZ are characterized by steeply S(SW)-dipping mylonitic foliations and mainly display a top-to-N(NE) sense of shearing. Two pre-kinematic intrusions yielded zircon crystallization ages of 2055 ± 17 Ma and 1947 ± 9 Ma, providing the maximum age limit for the QDSZ. Additionally, zircon overgrowth rims from three high-temperature mylonites gave metamorphic ages of 1902 ± 8 Ma, 1902 ± 26 Ma and 1884 ± 12 Ma, interpreted to record the timing of development of the QDSZ. Integrated with previous studies, we propose that the Qianlishan Complex suffered three phases of Orosirian deformation (D1–D3), of which the D3 deformation led to the development of the QDSZ. Deformation events D1, D2 and D3 are considered to have occurred at ca. 1.97–1.93 Ga, 1.93–1.90 Ga and 1.90–1.82 Ga, respectively. These events document that the Khondalite Belt underwent a protracted (>100 Myr) orogenic history in response to the collision between the Yinshan and Ordos blocks. [ABSTRACT FROM AUTHOR]

Jurassic-Cretaceous magmatic rocks are widely distributed in the Jiaodong area, North China. These magmatic rocks are closely associated with gold deposits, but their petrogenesis and geodynamic background are still controversial. The Late Jurassic-Early Cretaceous granitic rocks exposed in the Guocheng area, Jiaodong Peninsula are the focus of this paper. Zircon U-Pb dating, Hf-O isotope and whole rock geochemical analysis are used to determine the petrogenesis of these rocks. Combined with previous data, the geodynamic processes of granite formation in different time intervals are investigated which provide new petrological evidence for understanding the magmatic-mineralization and tectonic background of the Jiaodong area. Samples were collected from the Queshan and Shazibu plutons. The Queshan pluton is a mylonite granite, one sample of which yields a 206U-238Pb zircon age of 156 Ma, while the Shazibu pluton is a quartz monzonite, one sample of which records a 206U-238Pb zircon age of 117 Ma. The two plutons have variable zircon Hf-O isotopic compositions: zircon εHf(t) values of the Queshan pluton are −29.2 to −22.8, and δ18O values are 7.30–7.84 ‰, while the Shazibu pluton has relatively high zircon εHf(t) values (−21.6 to −21.0) and δ18O values of 7.48–8.43 ‰. Whole-rock geochemical data shows that the Queshan pluton has high Sr/Y (60.7–169) and (La/Yb)N (34.6–65.9) values and low Y (3.11–10.3 ppm) and Yb (0.29–0.85 ppm) contents, akin to adakites, whereas granites of the Shazibu pluton are A-type with high Zr+Nb+Ce+Y contents (354–420 ppm) and zircon saturation temperatures (752–795°C). Based on the geochemical and isotopic data, the Late Jurassic Queshan pluton is the product of partial melting of thickened lower crust of the North China Block, while the Early Cretaceous Shazibu pluton formed via fractional crystallization of mafic rocks derived from partial melting of the lower crust of the North China Block. Although both plutons were derived from the lower crust of the North China Block, the nature of the lower crust was modified by involvement of more depleted mantle-derived materials during the Early Cretaceous. From the Late Jurassic to the Early Cretaceous, the rock types (from the adakite to A-type granite) and the nature of the magma source suggest a shift in geodynamic processes. Combined with previous research, the Late Jurassic Queshan pluton is proposed to have formed under low-angle subduction of the ancient Pacific Plate, while the Early Cretaceous Shazibu pluton formed subsequently during Pacific Plate rollback and extension. The Early Cretaceous transformation of tectonomagmatic environments provided favourable conditions for the formation of large-scale gold deposits in the Jiaodong region. [ABSTRACT FROM AUTHOR]

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Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 1017–1019 Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line. [ABSTRACT FROM AUTHOR]

Continental‐scale shear zones in Indochina record Cenozoic tectonic processes in SE Asia. Previous extrusion models link these shear zones to northward indentation of the Indian continent and conflict with distributed Oligocene conjugate strike‐slip pairs, formed by E‐W‐directed compression (present coordinates). This paper presents evidence of Oligocene shearing along the Mogok Metamorphic Belt in Myanmar, the western boundary of the Indochina block. The Kyanigan quarry at northern Mogok Metamorphic Belt exposes paragneiss, marble and quartzite schist with E‐W striking, which would be dragged from N‐S striking foliations of the main Mogok Metamorphic Belt by right‐lateral shearing of the brittle Sagaing Fault. After restoration, right‐lateral shearing 'σ' structures, cored with garnet, in paragneiss in the Kyanigan quarry are consistent with 'σ' and 'δ' structures in the Moulmein granitic mylonite to the south. U–Pb ages of metamorphic zircon of a sheared paragneiss and a leucogranite, cross‐cutting shear foliation in the Kyanigan quarry, and a biotite 39Ar‐40Ar age of mylonite at Moulmein constrain dextral shearing to 33–25.4 Ma, coeval with other 31–24.5 Ma shear zones in Indochina. The eleven NW‐striking left‐lateral shear zones and N‐S to NE‐striking right‐lateral shear zones define Oligocene conjugate fault patterns in Indochina. After restoration of ~80–45° clockwise rotation, these Oligocene conjugate shear zones, coupled with the anticlinal configuration of the Xuelongshan and Doi Inthanon‐Doi Suthep domes, reflect approximate N–S‐directed shortening. These structures to the north with coeval Myanmar‐Sumatra magmatic arc to the south demonstrate that they correspond to back‐arc contraction during northward subduction of the Indian Ocean. Oligocene continental‐scale intra‐continental shearing may be triggered by syn‐subduction compression in SE Asia. [ABSTRACT FROM AUTHOR]

P–T paths were reconstructed using P–T pseudosection modelling from garnets in a finely laminated mylonitic hornblende‐biotite schist with four sub‐cm wide mineralogically distinct layers separated by poly‐crystalline quartz layers from the northern margin of the Western Dharwar Craton (WDC). The almandine‐grossular‐rich garnets, syn/post‐tectonic with respect to the mylonite foliation, exhibit considerable variations in the zoning patterns of divalent cations in the different layers. By contrast, the compositions of the matrix minerals dominated by biotite, chlorite, hornblende and plagioclase are tightly constrained within and across the layers. The varying abundances of major element oxides in the layers reflect variations in the abundances of the minerals, rather than the mineral compositions. P–T conditions estimated using mineral thermo‐barometry in the garnet‐muscovite‐biotite‐plagioclase assemblages are 520–575°C and 6–8 kbar; somewhat higher P–T values (615–665°C, 7–9 kbar) are obtained from hornblende‐garnet‐plagioclase‐quartz assemblages. The P–T paths obtained from the multi‐mineralic layers using P–T pseudosection modelling yield near‐isothermal loading (4–7 kbar) at 500–550°C in all layers. The near‐isothermal loading experienced by the multi‐layered rock suggests rapid crustal thickening involving overthrusting followed by nucleation of steeply dipping transpressional shear zones due to oblique convergence of the WDC with a crustal domain in the north. U‐Th‐total Pb chemical dating of metamorphic monazites indicates the crustal convergence occurred at 2.2–2.1 Ga. The event does not have known equivalents in the neighbouring crustal domains eastwards in India as well as in Madagascar in the west. This suggests the crustal domain may be an exotic block. [ABSTRACT FROM AUTHOR]

The Lancang metamorphic complex is a key component of the Three River Tethys Orogen in the SE Tibetan Plateau, with typical rock assemblages and deformation fabrics recording the subduction of the Paleo-Tethys Ocean and subsequent continental collision. In this study, we present new petrographic and structural observations, together with zircon U-Pb and mica 40Ar/39Ar geochronologic data, to reveal the deformation and kinematics of the Lancang metamorphic complex, thus providing insights into the Permian-Triassic evolution of the eastern Paleo-Tethys domain. The subduction of the Paleo-Tethys Ocean and subsequent collision resulted in a range of structural features, including penetrative regional foliations, thrusts, asymmetric folds at various scales, and ductile deformation fabrics such as asymmetric boudins and porphyroclasts. Zircon U-Pb dating of foliated gabbro and 40Ar/39Ar dating of mica schist and mylonite, which preserve shortening fabrics, suggest that the Paleo-Tethys Ocean was subducted during the Late Carboniferous to Permian-Early Triassic, and the collision of the Baoshan and Simao blocks mainly occurred during ca. 237-230 Ma in the SE Tibetan Plateau. Regional deformation likely shifted from shortening to extension ca. 230 Ma, as reflected by post-collision high-K calc-alkaline magmatism. Our results document regional structural patterns and place timing constraints on the evolution of the Paleo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

The effects of preexisting fabrics on the flow laws and anisotropic deformation of rocks require further study. We conducted triaxial compression experiments on a granitic mylonite parallel to lineation (X), perpendicular to lineation and parallel to foliation (Y), and perpendicular to foliation (Z) under a pressure of 300 MPa, temperatures of 800–1,000°C, and strain rates of ∼2.5 × 10−6–10−4 s−1 using a Paterson gas‐medium apparatus. The low stress exponent (n = 1.9–5.8), high activation energy (Q = 325–802 kJ/mol), and macrostructures (distributed for most samples) and microstructures (such as kinked, folded and elongated biotite, elongated quartz and feldspar, microcracks within quartz and feldspar, and melt wetting and dissolution of quartz and feldspar) suggest that the deformation is dominated by dislocation creep, along with brittle regime at ≤∼850°C and likely diffusion creep at ≥900°C. Dehydration melting of biotite causes more obvious melt wetting of quartz and feldspar boundaries, lower n values at ≥950°C, and the maximum changes in n and Q along the Z‐direction, since the biotite alignment defines the foliation. Under the same conditions, the X‐direction samples consistently display the greatest strengths, which would have been for the Z‐direction samples as reported previously, and most obvious deformation localization, mainly due to the alignment of the elongated quartz and feldspar along this direction. Microcracks always occur in quartz but are tensile when compressed perpendicular to the foliation plane and compressively sheared when shortened parallel to the foliation plane. These tensile microcracks further weaken the rock samples with axes perpendicular to foliation. Plain Language Summary: Geological observations show that foliation (planar fabric) and lineation (linear fabric) are typical characteristics of granitic rocks that experienced ductile deformation in the deep crust. How these fabrics develop during deformation and how they affect strength and strain distributions in the crust have not received sufficient attention, limiting our understanding of rheological property changes during shear zone evolution. In particular, the effects of lineation on rock rheology remain unknown. We performed a series of triaxial compression experiments on granitic mylonite. The results show that sample deformation at 300 MPa, 800°C–1,000°C, and ∼2.5 × 10−6–10−4 s−1 is dominated by dislocation creep, accompanied by brittle deformation at ≤∼850°C for compression parallel to lineation and likely diffusion creep induced by significant dehydration melting of biotite at ≥900°C. Under the same conditions, the samples with axes parallel to the lineation consistently display the greatest strengths, which would have been for the samples with axes perpendicular to the foliation, as reported previously, and the most obvious deformation localization. Interestingly, tensile microcracks within the quartz grains further weaken the samples under compression perpendicular to the foliation. Key Points: Deformation of granitic mylonite is dominated by dislocation creep, along with brittle regime at T ≤ ∼850°C and likely diffusion creep at T ≥ 900°CThe strength (σ) of granitic mylonite is anisotropic, with a maximum parallel to the lineation directionDehydration melting of biotite causes a large change of flow parameters normal to foliation (Z) and tensile microcracks further reduce σ(Z) [ABSTRACT FROM AUTHOR]

The Trans-Baikal parametric borehole drilled out a metamorphic core complex in the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol–Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological, and laboratory–analytical studies made it possible to identify three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (slightly metamorphosed rocks) is represented by a serpentinite mélange separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate as a part of the crystalline basement complex comprises rocks of granite–schist and granite–plagiogneiss subcomplexes. The granite–schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section as layered veins with synkinematic signs. According to the preliminary estimates, the age of biotite granite is about 130 Ma. Schist is characterized by a mylonite structure at intervals of interbedding with biotite granite veins. In accordance with the geophysical survey data on the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors in the seismic RSS–CDP section, which can be traced to a depth of at least 10 km. The granite–plagiogneiss subcomplex is represented by plagiogneiss, leucogranite, and gneissic granite. In the Trans-Baikal parametric borehole, a detailed section of the Borshchovochnyi metamorphic core complex, fully characterized by core samples, was obtained. It should be considered as a reference for studying these structures. The investigation of this section will make it possible later to answer many questions about the formation and evolution of the Central Asian fold belt. [ABSTRACT FROM AUTHOR]

The South Indian Granulite Terrane is traversed by several crustal scale shear zones, however the tectonic significance of the shear zones are poorly understood. The tectonic relevance of the Bavali Shear Zone (BSZ) ‒ in the WNW extremity of the Moyar Shear Zone ‒ at the interface between the Paleoarchean to Neoarchean Western Dharwar Craton (WDC) in the north and the late Neoarchean Nilgiri block in the south is poorly constrained. The most conspicuous feature in the WDC is a set of N-striking gently-plunging upright folds and N-striking dextral shear zones (deformation D4). These D4 structures are superposed on a shallowly-dipping D3 recumbent folds and gently-dipping mylonite fabrics in a suite of anatectic gneisses, lower-grade supracrustal rocks and foliated granitoids. In regional scale, the D3 fold axes curve into the WNW-striking BSZ (D5 deformation), a steep-dipping transpressional shear zone with dextral kinematics. The BSZ is characterized by steeply-plunging stretching lineations sub-parallel to the hinges of reclined folds on the pre-shearing fabrics in the lithologies of the adjacent cratons. Syn-D5 charnockite veins suggest the BSZ formed at T > 850 °C. Existing U–Pb (zircon) dates and monazite chemical dates (this study), indicate that the deformation-metamorphism-magmatism in the WDC and the Nilgiri block occurred between 3400 and 2500 Ma; by contrast the high-T D5 oblique crustal shortening in the BSZ contemporaneous with multiple felsic emplacements was active between 830 and 720 Ma. The BSZ collision orogeny possibly preceded the eventual integration of the Greater India landmass with the Gondwanaland during the early-Palaeozoic. [ABSTRACT FROM AUTHOR]

The Nyakong-Manyi Shear Zone (NMSZ) is a NE-SW elongated corridor found to the northwest of the Foumban-Bankim Shear Zone (FBSZ) along the Central Cameroon Shear Zone. Controversial chronology models has been proposed for the kinematic evolution of the sinistral and dextral shear phases in the Tikar Plain, thus in the FBSZ; early dextral and late sinistral shear phases for some authors and early sinistral and late dextral shear for others. Moreover, the NMSZ kinematic evolution implication on the mylonitization P-T-t path in the area seem to be problematic and the present paper aim is to clear enough those problems; since this shear zone is the main mylonitic corridor that registered the left and right lateral movement in this area. The NMSZ comprises amphibolites, protomylonites, strict sensus mylonites (garnet-kyanite-sillimanite mylonite and garnet-pyroxene mylonite), ultramylonites kyanite-sillimanite and garnet-kyanite-sillimanite gneiss. Field structures testify that the investigated area recorded three deformation phases: (i) the D1 deformation phase which is marked by NW-SE to N-S trending S1 metamorphic foliation with low to moderate dips (15°–45°) that was transposed during the D2 phase, is responsible for a regional metamorphism whose mineral paragenesis is garnet-kyanite-sillimanite; (ii) the early sinistral NNE-SSW to NE-SW shear phase D2 marked by S2 metamorphic and mylonitic foliations; responsible for, L2 stretching mineral lineation, F2 fold axes and B2 boudins structures; (iii) the late dextral NE-SW shear phase D3, characterized by F3 folds, B3 boudins and ductile dextral C3 shear planes. Mineral paragenesis garnet + kyanite + sillimanite and microstructures within gneiss testify that this rock underwent high grade regional metamorphism whose peak conditions are estimated at 11.5–13.5 kbar/850–900 °C. After the peak of metamorphism gneiss was overprinted by high grade pressure mylonitization during the early sinistral and late dextral shear deformations. Microstructural data here indicate a high-grade mylonitization whose P-T conditions are estimated at least at around 10 kbar/750 °C attained during the D2. Shear markers, indicates that the studied area underwent an intense mylonitization at deep crustal deformation level, probably at the ductile-brittle boundary structural level during a major dextral shear deformation. [ABSTRACT FROM AUTHOR]

Panshanlou gold deposit is located in Fuxin, western Liaoning, and is one of the largest mylonite-type gold deposits in China that is currently known to be controlled by ductile shear zones. The mineralization in the Panshanlou gold deposit is found within the ancient Yujianping Group metamorphic mylonite fracture zone. The ore is heavily altered, but has a relatively low grade. Currently, the mine has been in a state of crisis due to years of mining, making it imperative to conduct exploration for replacement resources in order to increase ore reserves and extend the mine’s service life. Panshanlou gold deposit has been studied using deep penetration geochemical exploration methods for mineralization prediction. The main methods employed include analysis of hydrocarbon gases, adsorbed phased mercury, and radon gas in different geological settings. 2 profiles were established, and a total of 73 soil geochemical samples were collected. Each sample was analyzed for adsorbed phased mercury and 7 organic hydrocarbon gas components. Additionally, radon gas measurements were conducted along 36 exploration lines. The research results indicate that methane, ethane, propylene, n-butane, and adsorbed mercury anomalies in the profiles show a good spatial correlation with the ore bodies. Methane and ethane exhibit single-peak anomalies above the ore bodies, while propylene, n-butane, and adsorbed phased mercury show multi-peak anomalies. Lastly, through comprehensive analysis and comparison of the hydrocarbon and mercury anomaly characteristics, 4 prospective mineralization target areas were delineated. [ABSTRACT FROM AUTHOR]

The Great Slave Lake shear zone (GSLsz) is a type example for deeply eroded continental transform boundaries located in the Northwest Territories, Canada. Formed during the oblique convergence of the Archean Rae and Slave cratons, the GSLsz has accommodated up to 700 km of dextral shear. Here we present the results of in situ U‐Pb apatite and titanite geochronology from 11 samples that were collected across the strike of the shear zone. Both geochronometers record a near‐continuous history of ductile shear during crustal cooling and exhumation that spans ca. 1920–1740 Ma. By integrating the geochronological data with structural and metamorphic observations across the structure, we propose a tectonic model for the shear zone that consists of three stages. The first stage (ca. 1920–1880 Ma) is characterized by strain accommodation along two coeval fault strands. During the second stage (ca. 1880–1800 Ma), ductile shear ceases along the northernmost fault strand and the locus of strain migrates southwards toward the hinterland of the Rae cratonic margin. In the third stage (ca. 1800–1740 Ma), ductile strain localizes back along the southern of the two original fault strands, after which the present‐day surface level of the shear zone transitions to brittle shear. Our results highlight both the significance of the lateral migration of the zone of active deformation in major crustal shear zones as well as the localization of strain along existing crustal structures. Key Points: (Re)crystallized apatite and titanite record a near‐continuous history of ductile shear spanning ca. 1920–1740 MaStrain was initially (ca. 1920–1880 Ma) accommodated by two coeval fault strandsA faultward younging in the timing of (re)crystallization is consistent with strain localization during cooling and exhumation [ABSTRACT FROM AUTHOR]

Orogen-parallel ductile shear zones are conspicuous structures in the Khondalite Belt, but the timing of shearing remains poorly understood. Here, we present field-based structural and zircon U-Pb geochronological studies on the newly discovered Qianlishan ductile shear zones (QDSZ) in the Khondalite Belt. Our results show that the nearly E-W-trending QDSZ are characterized by steeply S(SW)-dipping mylonitic foliations and mainly display a top-to-N(NE) sense of shearing. Two pre-kinematic intrusions yielded zircon crystallization ages of 2055 ± 17 Ma and 1947 ± 9 Ma, providing the maximum age limit for the QDSZ. Additionally, zircon overgrowth rims from three high-temperature mylonites gave metamorphic ages of 1902 ± 8 Ma, 1902 ± 26 Ma and 1884 ± 12 Ma, interpreted to record the timing of development of the QDSZ. Integrated with previous studies, we propose that the Qianlishan Complex suffered three phases of Orosirian deformation (D1–D3), of which the D3 deformation led to the development of the QDSZ. Deformation events D1, D2 and D3 are considered to have occurred at ca. 1.97–1.93 Ga, 1.93–1.90 Ga and 1.90–1.82 Ga, respectively. These events document that the Khondalite Belt underwent a protracted (>100 Myr) orogenic history in response to the collision between the Yinshan and Ordos blocks.

The natural radiometric data revealed that eU and eTh-contents of mylonite rocks reach up to 464 and 1283 ppm, respectively. The studied mineralised mylonites are enriched in thorite, ishikawite and zircon as well as other accessories (ferrocolumbite, xenotime, fergusonite, synchysite, cassetrite, apatite, fluorite, galena and barite). Ishikawaite is considered as the almost enriched mineral in uranium contents, whereas UO2 ranges from 31.02 to 50.78 wt%. The higher concentrations of UO2 reached up to 15.88 wt% and ThO2 up to 4.32 wt% related to metamictization of columbite. Some zircon crystals possess higher values of HfO2 reached up to 6.38 wt% could be affected by hydrothermal activity. Both xenotime and fergusonite are enriched in HREE (average ∑HRE2O3 31.19 and 14.1 wt%, respectively) whereas synchysite is main LREE bearing mineral (average ∑LRE2O3 55.78 wt%). [ABSTRACT FROM AUTHOR]

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During the Alpine convergence of the Central Western Carpathians, a set of subparallel NW-SE shear zones formed in the eastern part of the Veporic Unit, near the tectonic contact with the Gemeric Unit. Deformation in low-grade metamorphic conditions was typical of the Alpine movements in the shear zones. Mylonitic rocks of the crystalline complex and their cover formations were studied on a macro and micro scale to characterise the evolution of deformation structures in the shear zones. The differential stress of dynamically-recrystallized quartz, which aggregates in mylonitic rocks, was determined using quartz paleopiezometry. Several types of foliation structures, but mainly penetrative subhorizontal stretching lineation in all observed mylonites, provide evidence of stretching and oriented ductile flow in a NW-SE direction. Shear sense indicators show the subhorizontal movement of the shear zone hanging walls to the SE. Dynamically-recrystallized quartz aggregates in the mylonitic rocks show different values of grain sizes and differential stresses, depending on the structure in which they occur. The lowest differential stresses were found in the quartz aggregates parallel to the S planes of the mylonites. Higher values are related to the C shear bands. The character of the structural setting indicates the formation of the shear zones in the orogen-parallel extension conditions. [ABSTRACT FROM AUTHOR]

A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level. However, the deformation evolution of strain localization at a small-scale remains unclear. This study investigated <10 cm wide shear zones developing in undeformed granodiorites exposed at the boundary of the continental-scale Gaoligong strike-slip shear zone. The small-scale ductile shear zones exhibit a typical transition from protomylonite, mylonite to extremely deformed ultramylonite, and decreasing mineral size from coarse-grained aggregates to extremely fine-grained mixed phases. Shearing sense indicators such as hornblende and feldspar porphyroclasts in the shear zone are the more significantly low-strain zone of mylonite. The microstructure and EBSD results revealed that the small-scale shear zone experienced ductile deformation under medium-high temperature conditions. Quartz aggregates suggested a consistent temperature with an irregular feature, exhibiting a dominated high-temperature prism slip system. Additionally, coarse-grained aggregates in the mylonite of the shear zone were deformed predominantly by dislocation creep, while ultra-plastic flow by viscous grain boundary sliding was an essential deformation process in the extremely fine-grained (∼50 µ m) mixed-phases in the ultramylonite. Microstructural-derived strain rates calculated from quartz paleopiezometry were on the order of 10−15 to 10−13 s−1 from low-strain mylonite to high strained ultramylonite. The localization and strain rate-limited process was fluid-assisted precipitation presenting transitions of compositions as hydrous retrogression of hornblende to mica during increasing deformation and exhumation. Furthermore, the potential occurrence of the small-scale shear zone was initiated at a middle-deep crust seated crustal condition dominated by the temperature-controlled formation and rheological weakening. [ABSTRACT FROM AUTHOR]

Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 1017–1019 Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line.

To improve the geological knowledge of the poorly studied and scarcely accessible Island of Montecristo (Tuscan Archipelago), the Authors performed 1:5000 geological survey, meso- and microstructural study and petrographic analyses on the Messinian pluton, on the associated dyke/vein network and on the remnants of the hornfels cover occurring along the western and southern coasts. The results of this study allow: a) to define a complex system of dykes and hydrothermal veins crossing the pluton and its hornfels. b) to directly correlate the Montecristo hornfels rocks (here grouped in the “Cala S. Maria-Cala dei Ladri Unit”) with the contact metamorphic aureole of the Monte Capanne pluton of the Elba Island (i.e., the Punta Polveraia-Fetovaia Unit). The Cala S. Maria-Cala dei Ladri Unit is mainly composed of alternating metapelites and metasandstones (whose protolith is likely the Quartzarenitic Member of the Palombini Shales and/or the Val Lavagna Shales of Early-Late Cretaceous age) and of minor metaophiolitic rocks; metaserpentinites were also mapped for the first time. c) to recognize low-medium grade hornfels in the remnants of the contact metamorphic aureole and the ubiquitous presence of basal cataclastic horizons, testifying a role of “roof pendant” plates for the Cala S. Maria-Cala dei Ladri Unit outcrops. d) to reconstruct a complex structural evolution for the emplacement of the pluton and for its contact metamorphic aureole. This evolution started with ductile events, i.e., formation of mylonite in the granitoids and of a pervasive S1 schistosity in the hornfels, which was likely related to flattening processes occurred during the balooning of the magmatic body, followed by ductile D2 folding with the detachment of the recrystallized Ligurian covers due to the rise of the partially cooled pluton, and then by a series of jointing/faulting events, whose lineaments were used as pathways for the intrusion of the different dykes and hydrothermal veins; finally, a NW-SE striking transtensive sinistral shear zone (Cala Maestra-Cala Corfù Fracture Zone) with an about ENE-WSW directed σ1 originated in the south/south-western part of the island. This shear zone represents one of the few witness of regional transcurrent structures in the Tuscan Archipelago and was likely active also during the emplacement of the plutonic body. [ABSTRACT FROM AUTHOR]

Oligocene and early Miocene displacement on the Catalina–San Pedro detachment fault and its northern correlatives uncovered mylonitic fabrics that form the greater Catalina metamorphic core complex in southeastern Arizona, USA. Gently to moderately dipping mylonitic foliations in the complex are strongly lineated, with a lineation-azimuth average of 064–244° and dominantly top-southwest shear sense over the entire 115-km-long mylonite belt. Reconstruction of detachment fault displacement based on a variety of features indicates 40–60 km of displacement, with greater displacement in more southern areas. Widespread 26–28 Ma volcanism during early extensional basin genesis was followed by 24–26 Ma granitoid magmatism. Cooling of footwall mylonites continued until 22–24 Ma, as indicated by 40Ar/39Ar mica dates. Lower temperature thermochronometers suggest that footwall exhumation was still underway at ca. 20 Ma. Tectonic reconstruction places a variety of unmetamorphosed supracrustal units in the Tucson and Silver Bell Mountains above equivalent units that were metamorphosed and penetratively deformed in the Tortolita and Santa Catalina Mountains. This restored juxtaposition is interpreted as a consequence of older Laramide thrust burial of the deformed units, with northeast-directed thrusting occurring along the Wildhorse Mountain thrust in the Rincon Mountains and related but largely concealed thrusts to the northwest. Effective extensional exhumation of lower plate rocks resulted from a general lack of internal extension of the upper plate wedge. This is attributed to a stable sliding regime during the entire period of extension, with metamorphic core complex inflation by deep crustal flow leading to maintenance of wedge surface slope and detachment fault dip that favored stable sliding rather than internal wedge extension. [ABSTRACT FROM AUTHOR]

• EBSD kinematic vorticity method for consistent analyses across diverse shear zones. • Kinematic vorticity analyses for metamorphic core complexes show significant pure shear strain. • Diapirically emplaced pluton margins show similar pure shear deformation. • Results consistent with shear zone attenuation and compilation of other studies. • Numerical simulations support buoyant diapirism drives pure shear strain. Kinematic analysis of ductile shear zones is an important method to interpret the dynamic evolution of many tectonic and magmatic processes on Earth, such as orogeny, rifting, and plutonism. Despite decades of study, kinematic vorticity analysis is an underutilized tool to interpret the dynamic drivers of shear zones. Determination of the kinematic vorticity number (W k) quantifies the relative contributions of pure- versus simple-shear strain in shear zones. Here we systematically investigated W k from mylonitic shear zones associated with metamorphic core complexes (MCCs) developed across the central and southern North American Cordillera using electron backscatter diffraction (EBSD) data that allows consistent and reproducible results from a large number of recrystallized grains. We investigated samples from four distinct MCC systems and the structural aureole of a Cordilleran pluton to investigate their kinematics. We find that most MCC samples display pure-to-general-shear strain (average 70 % pure shear), consistent with significant bulk shear-zone shortening (>80 % shortening) observed in many of the MCC systems. Such strain patterns are remarkably similar to deformation observed around plutons that were forcefully emplaced as diapirs. To further validate and investigate these results, we constructed the W k field of these geologic processes using numerical simulations to highlight that pure shear kinematics are more common with diapirism and coupled wallrock deformation compared with discrete detachment-involved normal fault systems. These observations support that buoyant doming can be a viable end-member process to form the investigated MCCs. Our results also suggest that pure shear deformation may be a diagnostic strain characteristic for diapir-like crustal processes, and therefore W k analyses could be used to test similar processes like dome-and-keel models for Early Earth. [ABSTRACT FROM AUTHOR]

• Raised beach is preserved at 3.3–5.3 m amsl in the upper Gulf of Thailand. • C14 and OSL age-dating suggest formation between 3.5–4.0 ka BP. • Provides evidence for Late Holocene local relative sea level (RSL) position. • Extends understanding of geographical variations in RSL across Southeast Asia. Few constructional features of coastal geomorphology have been investigated at the northernmost extremity of the Gulf of Thailand (GoT), with a view to establishing the position (height) of local relative sea level (RSL) during the marine regression following the regional mid-Holocene highstand (MHH) that occurred at approximately 6.5 ka BP. Here, the work investigates a 2 m thick exposure of marine gravels on the coast of Ko Khang Khao islet in the eastern Bay of Bangkok. At an elevation of 3.3–5.3 m above modern sea level, the sequence is interpreted to represent a Holocene raised beach. The unlithified sediments comprise rounded quartz and mylonite pebbles and cobbles, oriented predominantly NE–SW, supported by fossiliferous sands that are rich in marine shells, coral fragments and occasional terrestrial gastropods. The juxtaposition of the marine and non-marine gastropoda of contemporaneous ages makes a compelling story for a coastal storm deposit, thrown up either by a winter monsoon storm, or by a palaeotyphoon that managed to penetrate the upper Gulf. Overlapping results of C14 and OSL age-dating of shell material and mineral sands suggest the raised (storm) beach formed between 3.5 and 4.0 ka BP, i.e. ∼ 2.5–3.0 ka after the MHH peak, at a height of ∼ 1.3–3.3 m above the local RSL position at that time (according to glacial isostatic adjustment modelling). Given the otherwise paucity of data from the upper GoT, the Ko Khang Khao raised beach provides new information that expands our current understanding of geographical variations in RSL across Southeast Asia during the Late Holocene. [ABSTRACT FROM AUTHOR]

A detailed structural study showed that the Mesoproterozoic Karadjilga granite–gneiss massif of the Kyrgyz North Tianshan represents a sheet intrusion, concordant to the bedding of the host Mesoproterozoic metasedimentary strata. The massif has a complex internal structure with strain gradients from unfoliated granite to mylonite. The boundaries of the mylonite, gneiss, and unfoliated granite zones, as well as the foliation in gneisses, are oriented subparallel to the contacts of the massif and are deformed into a single syncline together with the host sedimentary strata. The obtained U–Th–Pb (SHRIMP) age of unfoliated granite (1121 ± 13 Ma) is close to the previously obtained age estimates for granite–gneisses (~1090–1130 Ma), and indicates that, regardless of the degree of deformation, all the rocks of the massif belong to an unified igneous complex. [ABSTRACT FROM AUTHOR]

The present paper is written to bring to light a dichotomy that is noted in a naturally deformed rock, where the sense of shear inferred from quartz shape preferred orientation (SPO) is opposite to that deciphered from the crystallographic preferred orientation (CPO). Microstructural and quartz CPO data (obtained through SEM-EBSD analysis) from a quartz vein hosted in sinistrally sheared mylonitized granite of Gadwal region (Dharwar Craton) are presented. The vein thin section is prepared parallel to the kinematic reference frame (XZ section of the strain ellipsoid) using field-based information, which allows determination of sense of shear using microstructures. Three domains viz. Domain-I, II and III are identified in the rock thin section with grain size being 300–180 µm, 70–35 µm, and <20 µm, respectively. Of these, quartz grains of Domain-II show a well-defined quartz oblique grain shape fabric, which is indicative of dextral sense of shear. However, quartz CPO from the same Domain-II, as well as Domains-I and III, consistently shows a sinistral sense of shear; the latter complements the kinematics noted from the host rock mylonite. The dichotomy noted from Domain-II indicates that there maybe pitfalls in direct interpretation of kinematics based exclusively on quartz SPO analysis. Hence, caution must be exercised while extrapolating microscale kinematics based on quartz SPO to the regional scale. [ABSTRACT FROM AUTHOR]

The present study focuses on the distribution of natural radionuclides and the determination of radioelement favorability in the Nugrus mylonite rock. The radionuclide contents were measured with a NaI(Tl) detector. The concentrations of U, Th, Ra (in terms of eU), and K in the mylonite studied range from 27 to 247 ppm, from 96 to 377 ppm, from 11 to 166 ppm, and from 1.14 to 3.12%, with the averages of 100 ppm, 201 ppm, 55 ppm, and 2.1%, respectively. The interelement relationships indicate that the radioelement distribution is not only magmatic but also hydrothermal. Variations of migration parameters for uranium indicate fluctuations in physicochemical conditions that lead to U accumulation and leaching processes, which are confirmed by increments and decrements in uranium concentrations. The examined samples demonstrate disequilibrium, with eU/Ra (eU) ranging from 1.32 to 3.26, with an average of 1.95, which suggests uranium addition. Uranium, thorium, and potassium favorability indices of the investigated mylonite rocks indicate moderate favorability of these rocks for uranium and thorium and high favorability for potassium. The F-parameter illustrates the presence of different stages of alterations. Mineralogical investigation indicates the presence of such radioactive minerals as thorite and Uranothorite, of Nb–Ta minerals (columbite and ishikawite), and of zircon and xenotime. [ABSTRACT FROM AUTHOR]

Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands. A long-standing controversy surrounds low-angle nature of observed detachment faults within metamorphic core complexes. Here, the authors show that post-orogenic collapse of mountain belts can create a low-angle detachment, resolving the controversy. [ABSTRACT FROM AUTHOR]

The Heilongjiang Complex records the evolution history of the Mudanjiang Ocean, which is of great significance to constrain the amalgamation process of the Jiamusi and Songnen‐Zhangguangcai Range massifs. We carried out petrological, chronological, geochemical, mineral chemical and electron backscatter diffraction (EBSD) fabric analyses on the Heilongjiang Complex in the Yilan area. The latest zircon U–Pb dating results show that the protolith ages of the amphibolite are 261.3 ± 3.0 Ma and 261.8 ± 3.3 Ma, while that of the granodioritic mylonite is 207.8 ± 2.2 Ma. The amphibolites and granodioritic mylonite are enriched in large‐ion lithophile elements (e.g., Rb, Ba, and Sr) and light rare earth elements, with depletion in high‐field‐ strength elements (e.g., Nb, Ta, Zr, and Hf), indicating that the formation of the protoliths of the amphibolites and granodioritic mylonite is related to the subduction of the Mudanjiang oceanic plate under the Songnen‐Zhangguangcai Range Massif. The amphibolites and granodioritic mylonite both experienced two periods of metamorphic and deformation events. The metamorphic degree of the early period of metamorphism is low amphibolite facies, which records a clockwise P–T path from early increased temperature and pressure to a late isothermal depressurization. The P–T path reveals that this period of metamorphism is associated with the collision between two massifs. The later period of metamorphism reaches low greenschist facies, accompanied by deformation, which may be related to the rapid exhumation of the Heilongjiang Complex. This study provides a new perspective for exploring the collision and collage process of the Jiamusi and Songnen‐Zhangguangcai Range massifs. [ABSTRACT FROM AUTHOR]

Strongly deformed footwall rocks exposed in metamorphic core complexes (MCC) of the North American Cordillera were exhumed via ductile attenuation, mylonitic shearing, and detachment faulting. Whether these structures accommodated diapiric upwelling or regional extension via low‐angle normal faulting is debated. The Ruby Mountains‐East Humboldt Range MCC, northeast Nevada, records top‐west normal‐sense exhumation of deformed Proterozoic‐Paleozoic stratigraphy and older basement. We conducted 1:24,000‐scale mapping of the southwestern East Humboldt Range, with integrated structural, geochemical, and geochronological analyses to characterize the geometry and kinematics of extension and exhumation of the mylonitized footwall. Bedrock stratigraphy is pervasively intruded by Cretaceous, Eocene, and Oligocene intrusions, but observations of a coherent stratigraphic section show >80% vertical attenuation of Neoproterozoic to Ordovician rocks. These rocks are penetratively sheared with top‐west kinematics. The shear zone thus experienced combined pure‐ and simple‐shear (i.e., general shear) strain. We argue that this shear zone was syn‐/post‐kinematic with respect to Oligocene plutonism because: (a) mylonitic shearing spatially corresponds with preceding Oligocene intrusions; (b) thermochronology reveals that the shear zone experienced substantial cooling and exhumation after Oligocene plutonism; and (c) the mylonites are crosscut by undated, but likely late Oligocene, leucogranite. We propose that Eocene mantle‐derived magmatism and thermal incubation led to Oligocene diapiric upwelling of the middle crust, with ductile stretching focused on the flanks of this upwarp. Regional Basin and Range extension initiated later in the middle Miocene. Therefore, the development of the East Humboldt Range shear zone was not driven by regional extension and coupled detachment faulting. Plain Language Summary: How the Earth's continental crust stretches and deforms is important for understanding how ancient mountain ranges collapsed in the past and how long these mountains can remain thick and elevated before they undergo the same fate. Regions known as metamorphic core complexes provide important windows into these processes, as they consist of strongly deformed and stretched rocks that were previously deeply buried. These rocks deformed at temperatures that were high enough for the rocks to slowly flow like a liquid. In this study, we mapped, dated, and examined rocks from the East Humboldt Range, northeast Nevada, to investigate exactly how these deeply buried rocks flowed during their migration toward the Earth's surface. Previous models argued that this geology resulted from extreme horizontal extension of the Great Basin over the past 30 million years. However, we interpret that these rocks only flowed locally due to doming of hot, lower crust rocks rising upward, over (geologically) brief timescales of several million years, about 29 million years ago. Therefore, the present‐day extension in the Great Basin is not directly related to this older flowing process. Our interpretation emphasizes that hot crust can flow vertically, like hot wax convecting in a lava lamp. Key Points: The lower plate of East Humboldt Range metamorphic core complex, North American Cordillera, experienced significant attenuationOligocene general shear zone decoupled from Miocene detachment faulting and regional extensionMiocene‐present extension and range tilting resulted in current exposure and geometry of this metamorphic core complex [ABSTRACT FROM AUTHOR]

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The Mahanadi Shear Zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB) separates the southern segment of EGMB with dominant NE–SW structural trend from its northern segment with dominant E–W trend. The MSZ is a high-temperature ductile shear zone in which mylonite–ultramylonite occurs intimately associated with pseudotachylite. The association of two components of contrasting origins (ductile and brittle deformations) is possibly attributed to contemporaneous evolution or multiple stages of evolution. Therefore, field and petrographic studies including clast-size analysis of three samples from different localities across the MSZ have been carried out. Field observations suggest that pseudotachylite veins often cut across the mylonitic fabric. In addition, there is variation in orientation of gneissic and mylonitic fabrics in the clasts within the pseudotachylite zones. The pseudotachylites occasionally exhibit crude flow structures and rims around clasts. These features clearly indicate that the mylonitic fabric in the MSZ predates brittle deformation, and the pseudotachylites have not suffered any subsequent deformation after their formation. Clast-size analysis in all three cases follows the Power Law and demonstrates linear distribution. Lower value for fractal dimension (D = 0.51–0.69) and involvement of large range of size distribution of clasts in the studied pseudotachylite samples are attributed to brittle deformation at high strain rate, independent of the ductile deformation event. The present study suggests evolution of the MSZ in two stages, i.e., early development of mylonite–ultramylonite by ductile deformation at deeper level followed by pseudotachylite development at later stage by brittle deformation at a shallower level, thereby overprinting of brittle deformation over the ductile deformation. Research highlights: The Manuscript describes the coexistence of pseudotachylite, mylonite and ultramylonites in the MSZ. Clast size analysis of pseudotachylite suggests a single brittle deformation event. We have highlighted the deformation evolution of the MSZ in the EGMB. [ABSTRACT FROM AUTHOR]

In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographic preferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four different methods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared: the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), the time-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD). Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars and micas in mylonitic granitoid rocks, have been then considered to solve structural and geological questions related to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying the different techniques are discussed, and the advantages as well as limitations of each method are highlighted. Importantly, our findings suggest that patterns obtained by means of different techniques are quite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) by shape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near to the Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns related to dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likely active as documented by pole figures of positive and negative rhombs (TOF), which reveal also potential mechanical Dauphiné twinning. Results showed that the most complete CPO characterization on deformed rocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well as various mineral phases may be investigated). However, this use is restricted by the fact that (a) there are very few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is a bulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control of the user covering a good amount of investigated grains. CIP and US are not expensive techniques with respect the other kind of investigations and even if they might be considered obsolete and/or time-consuming, they have the advantage to provide a fine and grain by grain “first round” inspection on the investigated rock fabric.

The impact of fluid-rock interaction during deformation is difficult to characterize at the regional scale because of the lack of continuous outcrops to provide textural evidence and meaningful spatial comparisons. Herein we introduce the use of bench top μXRF data as a cost-effective means to quantify chemical and deformation gradients within representative specimens from across the strain gradient associated with the Pocologan Kennebecasis shear zone (PKSZ) in the Canadian Appalachians. Within the Pocologan Harbour granitoid belt, the main foliation is defined by three distinct mineral assemblages characterized by: (A) zoisite, no muscovite; (B) zoisite and muscovite; (C) muscovite and potassium feldspar. We first test whether these mineral assemblages are indicative of primary lithological variation or hydrothermal alteration using major and trace element geochemistry. Then, we test if these mineral assemblages impact the fabric anisotropy using quantitative image analysis of thin section phase maps derived using a benchtop μXRF. We found that whole rock compositions of specimens with an assemblage characterized by potassium feldspar and muscovite show significant element mobility compared to the least altered specimens and that the thin section phase map anisotropy best defines the deformation gradient of the PKSZ regardless of the metasomatic assemblage. This deformation gradient is mimicked by the shape anisotropy of quartz, plagioclase, biotite and muscovite aggregates. Within the most altered specimens, sericite alteration of plagioclase increased the mineral aggregate anisotropy through strain localization in shear bands while the crystallization of potassium feldspar at the expense of plagioclase of muscovite, plagioclase and quartz decreased the mineral aggregate anisotropy which indicates an overall strengthening of the shear zone. Moreover, the combination of grain-size reduction mechanisms and syn -deformation metasomatism in the high strain portion of the shear zone triggered a change of deformation mechanism from plastic deformation to diffusion creep. • Regional-scale mylonite in the Canadian Appalachians • Mylonite deformation involved multiple mineral reactions related to potassic alteration. • Syn-deformation potassic alteration of granitoid lithologies produces competing rheological softening and strengthening. [ABSTRACT FROM AUTHOR]

• Zircon U–Pb and Hf data document 1.9–1.4 Ga westward growth of the Fennoscandian margin. • Sveconorwegian Province is anomalously young and juvenile relative to surrounding crust. • The new data suggest at least 500 km of eastward movement during Sveconorwegian Orogeny. The late Mesoproterozoic Sveconorwegian Orogeny in SW Fennoscandia is characterized by tectonically bound units that record different metamorphic, magmatic, and deformation histories, interpreted to indicate separation by some unknown distance prior to orogeny. New zircon U–Pb and Lu–Hf isotope data from a 1200 km-long NE–SW transect including Archean to 1450 Ma rocks constrain the likely age and isotopic architecture of western Fennoscandia prior to the late Mesoproterozoic Sveconorwegian Orogeny. Zircon age and Hf-isotope patterns indicate that the units comprising the Sveconorwegian Province are both younger and isotopically more juvenile than the surrounding autochthonous Fennoscandian crust, and thus most likely derived from west of the present-day Norwegian coastline. The Mylonite Zone defines a major tectonic structure separating allochthonous Sveconorwegian units in its hanging wall from autochthonous Fennoscandian crust in its footwall. New and compiled metamorphic age data demonstrate that the Mylonite Zone can be traced westward through the Western Gneiss Region, aligning with Nordfjord in western Norway, where it was reused during Caledonian deformation. The proposed westward continuation of the Mylonite Zone accommodated several hundred kilometers of sinistral strike-slip movement. Eastward translation of crust probably took place sometime between 1020 and 990 Ma, coinciding with a magmatic lull, followed by a shift to more evolved isotopic compositions in the hanging wall (Telemark) and high-pressure eclogite-facies metamorphism in the footwall (Eastern Segment) to the Mylonite Zone. Following this relatively short period of compression, the entire orogen and its foreland underwent extension lasting until at least 930 Ma. The nature and fate of the ca. 500 km of crust originally separating the autochthonous and allochthonous units remain elusive. There is no evidence of arc magmatism related to Benioff-style subduction of oceanic crust, and thus we propose an amagmatic Ampferer-style subduction comprising spontaneous subduction of thinned continental crust, as proposed for the Western Alps. Subduction of continental crust and associated radioactive heat-producing elements could also account for the anomalously high temperatures in the lithospheric mantle under the Sveconorwegian Province, which cannot easily be accounted for by other mechanisms. The Sveconorwegian Province may be an anomalous feature in an otherwise larger-scale orogen, the nature of which remains obscure. [ABSTRACT FROM AUTHOR]

The spectrum of slip behavior in crustal faults generates various rock types that can inform the mechanics of earthquake genesis. However, a single fault exposure may contain evidence of slip at various depths and temperatures due to progressive fault rock formation and overprinting during exhumation. Here, we unravel the spatiotemporal evolution of mechanical transitions along the Boundary Canyon detachment, a low-angle normal fault northeast of Death Valley, USA. Field, microstructural, and geochemical characterizations of fault rocks are compared to existing laboratory experiments and combined with a thermo-kinematic model of fault evolution. Together, these constrain the depths of mechanical transitions along the fault and reveal the evolution of earthquake nucleation zone thickness. Fault exposures from different initial paleodepths passed through the mechanical transitions during footwall exhumation, resulting in overprinting of mylonite by cataclasite and ubiquitous late formation of foliated, illite-rich gouge within the uppermost crust. We present evidence of coseismic low-angle normal fault slip (e.g., injection veins of cataclasite, laminar and grain-inertial fluidization). Coseismic slip likely nucleated at strength contrasts within the fault zone (i.e., contacts between quartzite breccia and calc-mylonite; quartz ribbons and mylonite matrix; breccia and clay gouge) at ≈ 5–9.5 km depth. Observations including mutually overprinting cataclasite/ultramylonite and exposures of pulverized gouge support that dynamic rupture propagated down-dip through the brittle-ductile transition zone (≈10–11 km depth) and up-dip through velocity-strengthening fault patches (≈0–5 km depth). Rapid fault exhumation increased the geotherm, leading to upward advection of the brittle-ductile transition and shallowing/thinning of the earthquake nucleation zone. This process may explain the rarity of large magnitude earthquakes on low-angle normal faults. [Display omitted] • Field, micro-structural, and geochemical analysis of fault rocks. • Fault rocks reconstructed to paleo-depth and temperature. • Reconstructed mechanical transitions and 2D static stress models. • Coseismic low-angle, normal-sense slip and dynamic rupture. • Thinning and shallowing of earthquake nucleation zone with exhumation. [ABSTRACT FROM AUTHOR]

In the present study, we unveil the real significance of mylonitic reworking of the polymetamorphic crystalline basement in the Serre Massif of Calabria (Southern Italy). We use a multidisciplinary approach to comprehend the structural, microstructural and petrologic changes that occurred along a, so far, not much considered shear zone affecting the Variscan lower crustal rocks. It was never before studied in detail, although some late Cretaceous ages were reported for these mylonites, suggesting that this shear zone is of prime importance. Our observations reveal now that the formation of the new structural fabric within the shear zone was accompanied by changes in mineral assemblages, in a dominant compressive tectonic regime. During this tectono-metamorphic event, high-P mylonitic mineral assemblages were stabilized, consisting of chloritoid, kyanite, staurolite, garnet and paragonite, whereas plagioclase became unstable. Average peak P–T conditions of 1.26–1.1 GPa and 572–626 °C were obtained using THERMOCALC software. These data question (i) that the Serre Massif represents an undisturbed continuous section of the Variscan crust, as generally suggested in the literature, and (ii) highlight the role of (eo-)Alpine high-P tectonics in the Serre Massif, recorded within mylonite zones, where the Variscan basement was completely rejuvenated. [ABSTRACT FROM AUTHOR]

The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of ~ 70 °C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of ~ 730 °C, 6.6 kbar in the migmatitic gneisses and ~ 500 °C, 4.2 kbar in the serpentinites and Tambo schists 2–4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the ~ 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2–1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45–65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U–Pb dating of zircons shows that partial melting and diking occurred at 29.0–31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North–South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex. [ABSTRACT FROM AUTHOR]

We present a detailed petrological, structural and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu District, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modelling give estimated peak P–T conditions (M1a) of 650–790°C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710°C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710°C to 450°C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590°C to 450°C after low‐P/T‐type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c‐axis fabric opening‐angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 and 66–64 Ma, respectively, with a high cooling rate of ~34°C/Ma using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥10 km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high‐P/T‐type metamorphic rocks led to further cooling between hangingwall and footwall rocks and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high‐P/T‐type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high‐P/T‐type metamorphic rocks. [ABSTRACT FROM AUTHOR]

Abu Rusheid area is a part of the Arabo-Nubian basement exposures and is located in the southern Eastern Desert of Egypt, 95 km southwest of Marsa Alam City on the Red Sea coast. The Precambrian basement rocks include (a) ophiolitic metagabbros, (b) ophiolitic mélange, (c) mylonitic rocks, and (d) granites which were later intruded by lamprophyre dikes, pegmatite, and quartz veins. Geochemical studies revealed that both the mica schist and mylonite are mainly arkoses to greywacke in composition and confined to the passive margin tectonic setting. Chondrite-normalized REE patterns illustrate enrichment in HREEs in mica schist relative to LREEs, and there is a balance between them in mylonite. The minerals that have been identified in both mica schist and mylonite are soddyite, uranophane, kasolite, metazeunerite, thorite, uranothorite, and zinnwaldite. These mineralizations are related to shear zones and mainly coincide with the Najd fault (left lateral strike-slip) shear system. Field spectrometric measurements revealed that the average eU is 219 ppm and eTh is 451 ppm along trenches, whereas along channel samples the average eU is 64 ppm and eTh is 271 ppm. [ABSTRACT FROM AUTHOR]

At the Tick Hill gold deposit, δ18Oquartz data for the mineralised lithologies and surrounding rocks are similar and fall within a narrow range of 10.5–13.7‰ V-SMOW. The highly mineralised quartzo-feldspathic mylonite has quartz δ18O (δ18Oquartz) values of 11.3–13.6‰, which are similar to values for the surrounding rocks both locally and regionally, i.e. δ18Oquartz by itself does not provide a useful exploration tool. The δ18Oquartz values from the Tick Hill area most likely reflect the late Isan hydrothermal overprint at 1525–1520 Ma. The origin of the altering fluids is unclear, as the δ18Oquartz values overlap with reported δ18O values calculated for both metamorphic and igneous fluids. When combining the δ18Oquartz results with δ18Ocalcite results available from the literature, a temperature of 350–550 °C was calculated, which is consistent with observed alteration assemblages associated with gold mineralisation. The δ18Oquartz values (10.5–13.7‰ V-SMOW) of Au-rich quartz–feldspar mylonite are indistinguishable from the altered host rocks both local and regional. The narrow range of δ18Oquartz values for the rock units in the Tick Hill area and Mary Kathleen Domain most likely reflect a regional fluid overprint. The post-mineralised quartz–calcite veins yield higher δ18Oquartz values (14.1–17‰), possibly reflecting (partial) re-equilibration of minerals locally formed in late cataclastic fault rocks. [ABSTRACT FROM AUTHOR]

The Saih Hatat Dome forms a tectonic window under the overlying allochthonous Samail ophiolite and nappes composed of sediments from the Neotethyan Hawasina Basin. In this window, high- to ultra-high pressure/low-temperature eclogites and blueschists of the so-called lower plate are tectonically overlain by metamorphic rocks, forming the so-called upper plate. In 2018, the footwall of the discontinuity became exposed in a 650-m-long and up to 37-m-deep road cut which gave an insight to structural elements in these highly sheared blueschists. Kinematic indicators point to a top-to-north simple shear movement along low-angle discrete shear zones of decimeter to meter thickness, alternating with low-strain zones that are occupied by folded competent rocks. Fold axes are aligned parallel to the mineral stretching lineations and were formed by simple shear-related shortening of a steep limb of a pre-existing N-S oriented syncline. The structural inventory of the outcrop can be interpreted as the result of the Late Cretaceous exhumation of the NE corner of the Arabian Plate. [ABSTRACT FROM AUTHOR]

This article integrates field, powder X-ray diffraction and microstructural data to constrain deformation mechanisms in and the rheology of granite-derived fault rocks exposed along the SE side of the crustal-scale, strike-slip Kellyland fault zone. Deformation in this area of the Kellyland fault zone localized during cooling and is marked by (1) foliated granite, (2) a ∼50 m wide band of pulverized foliated granite, (3) a ∼2.8 m wide breccia zone hosting coeval shear zones, and (4) a >100 m wide ultramylonite zone. The earliest fabric in the foliated granite is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling deformation mechanism. Seismogenic deformation initiated when recorded flow stresses reached 96–104 MPa at temperatures of 400–450 °C and is marked by coeval pulverization and formation of breccia. Interseismic viscous creep at similar flow stresses is recorded by mutual cross-cutting relationships between breccia-hosted shear zones, brittle fractures and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the >100 m wide ultramylonite zone, and seismogenic deformation abated as the ultramylonite formed. The rheology of ultramylonites was governed by grain-size-sensitive creep at 112–124 MPa flow stresses. Hence, from the onset of seismogenesis, the Kellyland fault zone was likely a constant-stress system wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state grain-size-sensitive creep in ultramylonites derived from brittle fault rocks. Flow stresses recorded by these rocks also imply that the whole zone was relatively weak if the brittle–viscous transition and uppermost viscous zone are the strongest part of the crust. [ABSTRACT FROM AUTHOR]