Ductile Shear Zones Research Articles

Peridotites with back-arc basin affinity exposed at the southwestern tip of the Mariana forearc

Peridotites at water depths of 3430 to 5999 m have been discovered using the submersible Shinkai6500 (dives 6K-1397 and 6K-1398) on the southwestern slope of the 139°E Ridge (11°12′N, 139°15′E), a small ridge at the southwesternmost tip of the Mariana forearc near the junction with the Yap Trench and Parece Vela Basin. The peridotites studied consist of 17 residual harzburgites and one dunite and show various textures with respect to their depths. Peridotites with coarse-grained (> 1 mm) textures were sampled from the shallowest part (3705–4042 m) of the dive area, and peridotites with fine-grained (< 0.5 mm) textures were sampled deeper (5996 m). Olivine crystal-fabrics vary with grain size, with (010)[100] A-type patterns for the coarse-grained peridotites, {0kl}[100] D-type patterns for the fine-grained peridotites, and various indistinct patterns in samples of variable grain sizes. Fine-grained peridotites with D-type olivine crystal-fabrics could result from deformation under relatively higher flow stresses, suggesting that a ductile shear zone in the lithospheric mantle could occur in the deepest part of 139°E Ridge. Spinel Cr# range from relatively low (0.36) to moderately high (up to 0.57), and correlate with Ti contents (0.07–0.45 wt.%). The trace element patterns of clinopyroxene similarly exhibit steepening slopes from the middle to the light REEs regardless of textural variations. These mineralogical and geochemical features would result from melt-rock interactions under conditions of relatively shallow lithospheric mantle, which are much more comparable with the Parece Vela Basin peridotites than the Mariana forearc peridotites. Consequently, the Parece Vela Basin mantle is more likely exposed on the inner slope of the westernmost Mariana Trench, presumably due to the collision of the Caroline Ridge.

Branches of the Karakoram fault in Eastern Pamir

ABSTRACT The tectonic evolution of the Karakoram fault in the Pamir Plateau is controversial because the timing and kinematics of several branches of the fault system are unclear. Here we conduct field observations on the East Pamir fault, the North Muskol shear zone, and the Xindi-Waqia fault in the eastern Pamir, and obtain zircon U-Pb and biotite 40Ar/39Ar ages from these ductile shear zones. All three faults/shear zones are hosted by granitic mylonites. The East Pamir fault is a northwest-striking, normal-dextral shear zone that cuts the Rushan-Pshart suture. Zircon U-Pb dating yields a mean age of 201.5 ± 0.5 Ma for the mylonites in this shear zone. Biotite 40Ar/39Ar analysis yields two weighted mean plateau ages, 10.65 ± 0.13 Ma, and 11.04 ± 0.15 Ma. An undeformed granitic vein that cuts the ductile shear zone gives a mean zircon U-Pb age of 9.6 ± 0.3 Ma. We interpret that the granitic protolith crystallized at 201.5 Ma, and the East Pamir fault underwent rapid cooling and exhumation at ca. 11 Ma and ceased by 9.6 Ma. The North Muskol shear zone is an east-striking extensional shear zone along the northern boundary of the Shatput-Muztaghata dome. Two mylonitic samples show mean 206Pb/238U ages of 17.9 ± 0.2 Ma and 17.6 ± 0.3 Ma, which are interpreted as the timing of crystallization of the granitic protolith. The Xindi-Waqia fault is a northwest-striking ductile shear zone and brittle fault that bounds the Muztaghata dome. We obtain two zircon U-Pb ages (231.8 ± 1.1 Ma, 235.9 ± 0.9 Ma) and two biotite 40Ar/39Ar plateau ages (between 6.60 ± 0.10 Ma and 6.87 ± 0.10 Ma) from the mylonites in this fault, and interpret that the granitic rocks were emplaced in the Late Triassic and exhumed in the late Miocene. The newly obtained U-Pb ages, combined with the previously published data, indicate that Rushan-Pshart suture in Pamir marks the closure of an ocean basin within the Palaeo-Tethys Ocean, and cannot be correlated with the Bangong-Nujiang suture in Tibet. We establish a systematic correlation in which the two Palaeo-Tethyan sutures (Tanymas-Jinshajiang and Rushan-Pshart) in Pamir represent the western extensions of the Jinshajiang and Longmu Co-Shuanghu sutures in the Tibetan Plateau, respectively, and the Central Pamir is the western extension of the Tianshuihai-North Qiangtang terrane. This indicates that the dextral slip along the Karakoram fault system in east-central Pamir is 140 ± 10 km, and the mean slip rate since the onset of the Karakoram fault at ca. 18 Ma is 7.8 ± 0.5 mm/yr. Furthermore, our analyses suggest that the North Muskol shear zone was active at ca. 18 Ma before the arrival of the Karakoram fault system then. The East Pamir fault must have been active at ca. 11 Ma, but ceased by ca. 9.6 Ma, and the slip along the Karakoram fault system migrated northeastward and primarily accommodated by the Tashkurgan fault soon after 9.6 Ma. The Xindi-Waqia fault is the southern extension of the ductile shear zone of the Kongur Shan extensional system, as they are highly similar in geometry, kinematics, and timing. Overall, as the Karakoram fault system propagated northward in Pamir during the Cenozoic, crustal deformation migrated northeastward among the Karakoram fault branches from Ladakh to the Kashgar-Yecheng transfer system, and a complex evolution path can be portrayed.

Structures and chronology of the Yabrai shear zone in the Alxa, NW China: Constraints on the late Paleozoic shear system in central segment of the Central Asian Orogenic Belt

Large-scale strike-slip faults often develop in the collisional and accretionary orogenic belts around the world. The Central Asian Orogenic Belt (CAOB) contains numerous strike-slip faults and provides opportunities to decipher the relative movements among different components. Here, we report new data from the ENE-trending Yabrai dextral ductile shear zone in the Alxa region (NW China) of the southernmost CAOB. We performed field mapping, structural analysis, and muscovite 40Ar/39Ar dating to constrain the geometries, kinematics and timing of the shear zone. The Yabrai shear zone crops out for more than 150 km, and the kinematic indicators from the outcrops, microstructures, and quartz c-axis fabrics suggest a consistent dextral shear motion. The microstructures and EBSD results indicate intermediate-to low-temperature deformation (∼300–530 °C). Muscovite 40Ar/39Ar dating from mylonitic samples yield ages of 254–251 Ma, which imply that the shear zone was active in the Late Permian. The displacement of Yabrai shear zone is estimated to be ca. 30 km. An integration of spatiotemporal patterns of shear zones and regional aeromagnetic data suggests that a giant “S–C”-type shear system occurred in the Alxa region in the Late Permian. Based on this work and previous studies, a crustal-scale strike-slip system of late Paleozoic is developed between the CAOB and the southern cratons. It formed in a transcurrent regime after the closure of the Paleo-Asian Ocean, which was a response to the deformation of the whole CAOB in the end of late Paleozoic.

Structural Anatomy of the Intraterrane Shear Zones in the Archean Bundelkhand Craton, North-Central India and Its Possible Linkage to Supercontinent Assembly: Insights from Field- and AMS-Based Kinematic Analysis

Abstract Delineation and characterization of the intraterrane shear zones (ITSZs) are very significant in establishing suitable models to understand the tectonic evolution of continental blocks and often provide crucial information on supercontinent assembly. The granitoid rocks of the Bundelkhand Craton (BC) host mesoscale ductile shear zones of varying width and direction and typically represent decoupling zones (high and low strain), characteristic of ITSZs. The detailed study on the structural anatomy and nucleation of the ductile shear zones in the BC suggests the presence of four distinct sets of ITSZs along ~N-S, ~NE-SW, ~NW-SE, and ~ E-W directions. The nucleation of the ITSZs in the granitoid rocks is possibly a result of reactivation of the precursor fractures due to the rheological transformation induced by a fluid phase, which percolated through the fractures. Also, the thickness of the ultramylonite zone in the ITSZs is attributed to the width and style of the brittle fracturing. Here, we also construe a relative chronology of the development of fabric elements related to the ITSZs in BC with the help of cross-cutting relationships as observed in the field and further corroborated it to the available chronological data. This suggests, ~E-W trending ITSZs along the Bundelkhand tectonic zone (BTZ) and Raksa shear zone (RSZ) as the oldest followed by the evolution of ~N-S, ~NE-SW, and ~ NW-SE trending ITSZs. The kinematic analysis of these ITSZs, with the help of angular relationship between the measured mylonitic foliation and determined magnetic foliation data (by AMS analysis), suggests that fabric development in the ITSZs is dominantly governed by general shear except the oldest ~E-W trending ITSZs, which shows pure shear dominated progressive deformation. Also, with the help of field evidences and magnetic fabric analysis, we conclude that transpression was involved over a protracted period for the evolution of the ITSZs in the BC. Finally, we argue that the BTZ served as the site for the localization of the ~E-W trending ITSZs and represents the zone of amalgamation between North BC and South BC during the formation of Kenorland.

A likely geological record of deep tremor and slow slip events from a subducted continental broken formation

Fluids in subduction zones play a key role in controlling seismic activity, drastically affecting the rheology of rocks, triggering mineral reactions, and lowering the effective stress. Fluctuating pore pressure is one important parameter for the switch between brittle and ductile deformation, thus impacting seismogenesis. Episodic tremor and slow slip events (ETS) have been proposed as a common feature of the geophysical signature of subduction zones. Their geological record, however, remains scanty. Only the detailed and further characterization of exhumed fossil geological settings can help fill this knowledge gap. Here we propose that fluctuating pore pressure linked to metamorphic dehydration reactions steered cyclic and ETS-related brittle and ductile deformation of continental crustal rocks in the subduction channel of the Apennines. Dilational shear veins and ductile mylonitic shear zones formed broadly coevally at minimum 1 GPa and 350 °C, corresponding to ~ 30–40 km depth in the subduction zone. We identify carpholite in Ca-poor metasediments as an important carrier of H2O to depths > 40 km in cold subduction zones. Our results suggest that the described (micro)structures and mineralogical changes can be ascribed to deep ETS and provide a useful reference for the interpretation of similar tectonic settings worldwide.

Relations between inherited basement fabric and fault nucleation in a continental setting: The Rio do Peixe Basin, NE Brazil

The control of normal fault growth by preexisting basement structures is still a worldwide matter of debate. The objectives of the present study are (i) to investigate where rift faults nucleate in continental-scale ductile shear zones and how these zones influence fault architecture; (ii) how subvertical basement fabric controls listric fault geometry and evolution; (iii) how long reactivation of faults lasts; and (iv) which are the implications of fault reactivation for the oil industry. We contribute to these issues by defining the 3-D relationship between preexisting ductile strike-slip shear zones, the geometry and evolution of subsequent rift-related normal faults using airborne magnetic, 2-D and 3-D seismic reflection, and structural data from the Rio do Peixe Basin, a Cretaceous intracontinental basin in northeastern Brazil. Our results indicate that Precambrian mylonitic foliation and faults share a common strike at the surface, but not a common dip, mostly because the Precambrian shear zones are planar and the reactivated faults are listric (curved). The combined magnetic anomaly and seismic reflection help understand the 3-D geometrical relation between inherited structures and Cretaceous brittle reactivation, which is not obvious from the field data. As inferred from subsurface information, the mylonitic foliation (sub-vertical dip) and shear zone boundary (ca. 60° dip, parallel to brittle faulting) are not parallel. The basin-bounding faults reactivated along ductile shear zones that separate major terranes with mechanical contrast. We conclude that the map view spatial coincidence of Precambrian ductile shear zones and main brittle faults can be deceiving. Mylonitic foliation and faults are parallel along strike but not along dip, indicating that the geometry of rift faults is not controlled by the mylonitic foliation, but by the mechanical contrast between tectonometamorphic units separated by ductile shear zones.

Deformation, thermochronology and tectonic significance of the crustal-scale Cubatão Shear Zone, Ribeira Belt, Brazil

Crustal-scale shear zones are key structures in orogenic belts linked to the assembly of West Gondwana. The Cubatão Shear Zone (CSZ) represents a significant segment of a crustal-scale transcurrent shear zone system that cuts Paleoproterozoic to Neoproterozoic units in the Ribeira Belt (SE Brazil). Despite its tectonic importance, the relationship between transcurrent deformation and orogenesis is still controversial. This work aims to understand this relationship by integrating the transcurrent deformation history of the CSZ with regional orogenic processes through structural analyses, phase-equilibria modelling, and zircon and apatite U-Pb-REE analyses. The results indicate that the CSZ developed within a transpressional setting with a dominant subhorizontal stretching lineation at medium-temperature conditions of 460–520 °C and 4.5–9.5 kbar, consistent with quartz recrystallisation microstructures indicative of subgrain rotation recrystallisation. Igneous-type apatite from the mylonites of the CSZ record ages of ca. 610–570 Ma, which are interpreted as reset ages due to ductile deformation at medium-temperature conditions. Zircon U-Pb geochronology indicates that mylonites are sourced from the Juquiá Granite and the Atuba Complex. The CSZ activity is coeval with the main period of activity of other ductile shear zones from the Ribeira Belt, as indicated by available geochronological data. This age interval coincides with the main period of other regional shear zones activity, voluminous granitic magmatism, regional metamorphism and convergence of the cratons surrounding the Ribeira Belt, indicating active participation of the transcurrent shear zones in the orogenic processes that built up West Gondwana.

Shear zones in the southern Eyre Peninsula, South Australia: Quartz c-axis fabrics in granulite facies mylonitic orthogneisses and relationship to mafic dykes

The south eastern portion of the Eyre Peninsula, South Australia, is dominated by a suite of granites and orthogneisses deformed in steep-dipping shear zones during the Kimban Orogeny (1730–1690 Ma). The largest is the transpressional Kalinjala Shear Zone with shortening strains of ≥90% and shear strains of γ = ≥20, with other near-parallel secondary shear zones displaying similar kinematics. These ductile shear zones are dominated by mylonitic orthogneisses, with shearing commonly nucleating on, or adjacent to, precursor mafic dykes. The mafic dykes are highly sheared, folded and boudinaged with a newly formed matrix assemblage (orthopyroxene + clinopyroxene + garnet + magnetite + quartz + K-feldspar) that indicates the peak P/T metamorphic conditions of 0.7–1.0 GPa and 750–850 °C. Partial melting, developed as leucosomes, is associated with this metamorphism. Microstructural relationships between quartz crystallographic preferred orientation (CPO) and shape preferred orientations (SPO) analyses of these polymineralic rocks are best interpreted in terms of a granulite facies isothermal temperature regime of ~750 °C. The quartz c-axis fabrics, obtained using an optical fabric analyser, reveal a range of CPOs from weak girdles normal to the shear plane, to maxima parallel to the Y-strain axis. There is an overall symmetric orthorhombic pattern, with c-axis fabrics oriented with respect to the XY plane of flattening. There is a minor component of non-coaxial deformation in some shear zones. These fabrics indicate the increased activation of predominantly the prism-<a > slip system in quartz within the shear zones, which is characteristic for these high-temperature and high-strain deformation conditions.

Mesozoic intracontinental ductile shearing along the Paleozoic Shangdan suture in the Qinling Orogen: Constraints from deformation fabrics and geochronology

AbstractThe Qinling Orogen, located between the North and South China Blocks, records subduction-collisional orogeny along the Paleozoic Shangdan and Triassic Mianlue sutures, and Mesozoic intracontinental orogenesis, which all played an important role in building the present tectonic framework and topography. The strike-slip Shagou ductile shear zone that overprints the Paleozoic Shangdan suture between the North and South Qinling Belts is crucial for understanding the Mesozoic intracontinental deformation in the Qinling Orogen. The microstructures, asymmetrical fabrics, and kinematic vorticities (0.81–0.95) suggest sinistral simple shear. The quartz c-axis patterns from felsic mylonites exhibit Y-maximum fabrics, indicating the activity of prism &amp;lt;a&amp;gt; slip, while those from amphibole-rich mylonites display both Y-maximum and Z-maximum fabrics showing the combined activity of prism &amp;lt;a&amp;gt; and basal &amp;lt;a&amp;gt; slip systems. In the mafic mylonites, the plagioclase fabrics were induced by combined (010)[100] and (001)[100] slip systems, while the amphibole fabrics were probably related to anisotropic growth or passive rotation of rigid clasts. Equilibrium P–T conditions of 4.28–6.12 kbar and 646–727 °C estimated from geothermobarometry suggest that the main deformation occurred under amphibolite facies conditions at middle–lower crustal depths. Zircon U-Pb ages constrain their protolith rocks to have crystallized at 816 ± 25 Ma and 726–718 Ma, while the intense mylonitization and sinistral shearing occurred at ca. 200–187 Ma. A U–Pb zircon age of 132 ± 3 Ma from a granitic dike cutting the foliation and an amphibole 40Ar/39Ar plateau age of 119.0 ± 0.9 Ma from mylonites together suggest that the Shagou shear zone evolved through decompression and exhumation stages in the time period of 132–119 Ma.

Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric, and geochronology of the Taibai shear zone, Qinling Orogen, China

Abstract The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons that formed after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations indicate that the Taibai shear zone deformed in response to pure shear-dominated (54%–65%) transpression and top-to-NW shear sense as a result of NE–SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism &amp;lt;a&amp;gt; slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase–hornblende thermometer—constrain the main deformation temperatures to a range of 400–650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal &amp;lt;a&amp;gt; slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U–Pb ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during Early Cretaceous time.

Structural elements and evolution of the Yangmuchuan dome: Constraints on the tectonics of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton

The nature of assembly of the Columbia supercontinent is well-characterized by the orogenic belts that were formed during amalgamation of continental blocks. The Jiao-Liao-Ji Belt (JLJB) in the eastern North China Craton possesses complex deformation structures that exemplify the Paleoproterozoic orogeny. A series of granitic gneiss domes in the southern JLJB characterizes the structural deformation in the Paleoproterozoic orogenic belts.Located in the middle segment of the JLJB, the Yangmuchuan dome is composed of three major elements, i.e., the Liaoji granites in the core, and sheared metamorphic rocks in the mantle and outer shell. Bedding-parallel ductile shear zones and weakly deformed metamorphic rocks between them are well-preserved in the latter two elements. Structural analysis reveals that the Yangmuchuan dome mainly experienced two deformation stages (D1 and D2). The D1 deformation is characterized by pure shear-dominated deformation structures with occurrence of penetrative foliations, symmetrical boudinages, mineral lineations and rootless folds in all the rocks from the mantle and outer shell. Dominated by simple shearing, the D2 deformation is the key stage contributing to the formation of the dome structure in the area. The D2 deformation is manifested as occurrence of A-type folds, S-C fabrics, mylonitic foliation S2, mineral stretching lineation L2 in granites, and in layered rocks in the mantle and outer shell. They were superposed on and transformed, along high strain zones, D1 deformation structures. Magnetic fabrics of the Liaoji granites from the core show a generally consistent orientation with the foliations and stretching lineations in the sheared granites. LA-ICP-MS zircon U-Pb dating on syn-tectonic leucogranitic dykes indicates that the peak deformation (D2) of the Yangmuchuan dome occurred from >1887 to 1808 Ma. Geometrical and kinematic analysis from the structural, microstructural and magnetic fabric data reveal that formation of the Yangmuchuang gneiss dome resulted from dominant subhorizontal flow and subsidiary vertical flow in the middle and lower crust as the consequence of orogenic activity along the JLJB during the assembly of the Columbia supercontinent.

Strain Partitioning along Terrane Bounding and Intraterrane Shear Zones: Constraints from a Long-Lived Transpressional System in West Gondwana (Ribeira Belt, Brazil)

Abstract Shear zones are zones of localized high strain accommodating differential motion in the lithosphere and impacting the crustal rheology and deformational history of orogenic belts. Although terrane bounding shear zones are widely studied, intraterrane shear zones and their tectonic significance, especially in association with supercontinent assembly, is a largely unexplored topic. The Ribeira Belt (SE Brazil), a Neoproterozoic-Cambrian orogenic belt from West Gondwana, is dissected by a crustal-scale NE-trending transcurrent shear zone system that juxtaposes composite terranes. Despite its extensive coverage and complexity, this shear zone system remains poorly investigated. In this paper, we explore the thermal and deformational regimes, and timing of ductile shearing using a multiscale approach combining structural analysis derived from remote sensing and field-based structural data, microstructures, quantitative structural analysis, and multimineral U–Pb geochronology (zircon, titanite, monazite, and xenotime). Our data, combined with previously published data, indicate a transitional northeastward increase in metamorphic conditions from lower greenschist to granulite facies conditions (from 250–300 to 750–800°C), reflecting the different crustal levels that are exposed. Vorticity and finite strain data indicate a complex strain regime with varied contributions of pure and simple shear and oblate-shape ellipsoids in strike-slip shear zones and prolate-shaped ellipsoids in dip-slip reverse shear zones. The strain set suggests that all shear zones were developed under subsimple shear deformational regimes involving thrusting and folding followed by wrench tectonics. The pure shear component of deformation was accommodated in folded domains between shear zones. Geochronological data suggest intermittent ductile shear zone activations from ca. 900–830 to 530 Ma, partially coeval with at least two major episodes of terrane accretion at 850–760 Ma and 610–585 Ma. The spatial and temporal record of shear zones within the Ribeira Belt indicates that some relate to assembly of the belt and represent either terrane bounding structures (e.g., Itapirapuã shear zone) or intraterrane structures (e.g., Ribeira, Figueira, and Agudos Grandes shear zones), whereas others are terrane bounding, postcollisional shear zones (e.g., Taxaquara shear zone) reactivated in an intracontinental setting (560–535 Ma).

Correlation between gravitational and magnetic anomalies and crustal susceptibility in the Three Gorges area, China

The Huangling Anticline, which formed during the Jinning movement, is the structural core of the Yangtze River Three Gorges area. In this study, we investigated the geological structure of the Huangling Anticline through correlation analysis among magnetic, Bouguer gravity, and crustal susceptibility anomalies of the Three Gorges and adjacent areas. Aeromagnetic anomalies, which were processed by reducing to the pole, reveal four main aeromagnetic anomaly regions. Three-dimensional (3D) magnetic susceptibility constrained inversion was used to obtain the susceptibility distribution from 0 to 30 km depth. We focused on the spatial distribution of four major rock masses: Sandouping, Huanglingmiao, Dalaoling, and Xiaofeng. The low susceptibility of the Huanglingmiao rock mass is tentatively explained by the combined effect of weathering and stripping of the upper body and a tectonic ductile shear zone at the bottom.

Formation of the Hengshanli granitic gneiss dome in the Paleoproterozoic Jiao-Liao-Ji Belt, North China Craton

A group of granitic gneiss domes that may provide constraints on the tectonic evolution of early Precambrian orogenic belts are well preserved in the LiaoJi segment (or LJOB-Liaoji orogenic belt) of the Jiao-Liao-Ji Orogenic Belt (JLJOB), eastern North China Craton. To deeply understand the genesis of the domes and how the crustal mass flowed in collisional orogenic belt, we carried out structural analysis of the Hengshanli dome in the western part of the LJOB. Detailed analyses of deformation structures, microstructures and fabrics, and geochronology of syntectonic granitic dykes demonstrate that three episodes of structural deformation contributed to the formation of the dome structure. During D1, a pure shear-dominated deformation, penetrative foliations (S1), symmetrical lenses or boudinages, intrafolial folds (F1) and mineral lineations (L1) developed in the interlayered rocks of viscosity contrasts. The D2, the WNW-ESE orientated sub-horizontal simple shear-dominated deformation, is characterized by asymmetrical A-type folds (F2), stretching lineations (L2) , mylonitic foliations (S2), localized ductile shear zones, axial planar foliations, rotated porphyroblasts, L>S tectonites and S-C fabrics that indicate high-strain simple shear deformation. A D3 brittle deformation is evidenced by E-W directed open folds (F3) by flexural folding mechanism. LA-ICP-MS U-Pb dating of zircons from the syntectonic granitic dykes reveals that the main deformation event occurred from ca. 1946 to 1844 Ma. The dating also reveals that the Liaoji granites at the core were emplaced at ca. 2147 Ma and experienced anatexis at ca. 1864 Ma and 1846 Ma. It is suggested that the mechanism of doming is a combination of middle and lower crustal sub-horizontal flow, diapirism and folding superposition. Convergent flow of remobilized magma at depth and sub-horizontal simple shearing of metasedimentary rocks triggered middle and lower crustal flow in association with diapirism during the doming. The granitic gneiss domes are the products of dominant lateral and subsidiary vertical crustal mass flow under E-W directed compressional regime in the LJOB. We suggest that the orogenic belt experienced peak deformation at ca. 1946-1844 Ma, accompanied with syntectonic metamorphism and magmatism at nearly the same time.

Textures of gold-bearing sulfides and gold precipitation mechanism, Wangu gold deposit, Jiangnan Orogen

万古金矿床位于江南造山带中部，赋存于新元古界冷家溪群浅变质岩系中，受NNE-NE向长沙-平江断裂带和近EW向九岭-清水韧性剪切带联合控制，金资源量约85t。其主要矿石类型为毒砂-黄铁绢英岩型和石英-硫化物脉型，其次为构造角砾岩型。毒砂和黄铁矿为该矿床主要的载金矿物，分布广泛。金成矿作用可分为四个阶段，I为乳白色石英-绢云母-白钨矿阶段；Ⅱ为烟灰色石英-绢云母-毒砂-黄铁矿-金阶段；Ⅲ为烟灰色石英-绢云母-黄铁矿-毒砂-多金属硫化物-金阶段；Ⅳ为乳白色石英-方解石阶段。其中，Ⅱ、Ⅲ为成矿主阶段。根据成矿主阶段毒砂电子探针分析结果，Ⅱ阶段毒砂中As的含量在42.19%~44.84%之间，均值为43.42%（n=56），Ⅲ阶段毒砂中As的含量在40.08%~43.36%之间，均值为42.08%（n=19）。通过毒砂温度计相图估算出Ⅱ、Ⅲ阶段的形成温度和硫逸度分别为364±21℃、319±22℃和10^-9.7~10^-7、10^-11.5~10^-8.6。电子探针数据揭示的载金毒砂和黄铁矿中不可见金含量分别为0.01%~0.66%和0.01%~0.11%。黄铁矿Au-As元素投点分布于金溶解度曲线两侧，说明其内金主要以纳米级颗粒和固溶体金或晶格金的形式赋存；其中Ⅱ阶段黄铁矿纳米级金颗粒占比为73.33%，多于Ⅲ阶段黄铁矿（67.80%）。以上数据说明在水岩反应过程中，围岩中的含铁矿物与成矿流体中的H₂S发生反应，生成毒砂和黄铁矿。伴随着强烈的水岩反应，成矿温度和硫逸度降低，成矿Ⅱ阶段至Ⅲ阶段主要载金硫化物由毒砂转变为黄铁矿，强烈的硫化作用导致金-硫络合物失稳并释放金，金以置换的方式进入硫化物晶格或以显微-超显微金颗粒的形式沉淀，形成含金硫化物；即硫化作用是导致万古矿床不可见金沉淀的主导机制。;The Wangu gold deposit, with a proven gold resource of~85t, is located in the middle section of the Jiangnan Orogen and occurs in metamorphic rock series of the Neoproterozoic Lengjiaxi Group. It formed under the control of the NNE-NE-trending Changsha-Pingjiang fault zone and the EW-trending Jiuling-Qingshui ductile shear zone. The main ore types of the deposit include arsenopyrite-, pyrite-, sericite-, and quartz-altered slate and quartz-sulfide veins, followed by slate breccia within hydrothermal quartz. Arsenopyrite and pyrite are the main gold bearing minerals in the deposit, which are widely distributed in the deposit. Gold mineralization can be divided into four stages:I, the milky quartz-muscovite-scheelite; Ⅱ, the smoky gray quartz-muscovite-arsenopyrite-pyrite-gold; Ⅲ, the smoky gray quartz-muscovite-pyrite-arsenopyrite-polymetallic sulfide-gold; and Ⅳ, the milky quartz-calcite. Among them, the Ⅱ and Ⅲ are the main mineralization stages. Based on the results of Electron Probe Micro Analysis (EPMA) of arsenopyrite from the main mineralization stages, the contents of As in arsenopyrite at stage Ⅱ range from 42.19% to 44.84%, with an average of 43.42% (n=56). The contents of As in arsenopyrite at stage Ⅲ range from 40.08% to 43.36%, with an average of 42.08% (n=19). According to the phase diagram of arsenopyrite thermometer, the formation temperature of Apy-1 in the stage Ⅱ is estimated to be 364±21℃ with a sulfur fugacity varying within 10^-9.7~10^-7. The formation temperature of Apy-2 in the stage Ⅲ is 319±22℃, and its sulfur fugacity is 10^-11.5~10^-8.6. The contents of invisible gold in gold-bearing arsenopyrite and pyrite were 0.01%~0.66% and 0.01%~0.11%, respectively, revealed by EPMA data. The Au-As data of pyrite are plotted on both sides of the gold solubility curve, indicating that gold in pyrite mainly exists in the form of nano-scale particles and solid solution or lattice gold. The proportion of nanometer gold particles in Py-1 pyrite is 73.33%, more than that in Py-2 (67.80%). The above data indicate that in the process of water-rock reaction, the iron-bearing minerals in surrounding rocks react with H₂S in ore-forming fluid to form arsenopyrite and pyrite. Accompanied by a strong water-rock reaction, the main gold-bearing sulfides changed from arsenopyrite to pyrite during mineralization from stage Ⅱ to Ⅲ with the decrease of mineralization temperature and sulfur fugacity, i.e., strong sulfurization results in instability of the gold-sulfur complex and the release of gold. As a result, gold is deposited into the sulfide lattice by substitution or in the form of microscopic-ultra microscopic gold particles to form gold-bearing sulfide. Therefore, sulfidation is the main mechanism of the precipitation of invisible gold in sulfides at the Wangu gold deposit.

Kinematics and structural evolution of the Anziling dome-and-keel architecture in east China: Evidence of Neoarchean vertical tectonism in the North China Craton

The debate on the role of vertical versus horizontal tectonism in Archean cratons is intimately linked to the initiation of plate tectonics. The dome-and-keel architecture has been considered as a consequence of vertical tectonism. Although such a structural pattern is documented in some Mesoarchean and older cratons, such as the Kaapvaal and Pilbara cratons, whether it also occurs in Neoarchean cratons is poorly constrained. Determining the kinematics, structural evolution, and the timing of these structures is crucial in understanding how the tectonic behavior operated during the evolution of the early Earth. The North China Craton, especially its eastern part, is a Neoarchean continental block and preserves typical greenstone-granite rock assemblages. Detailed structural mapping reveals that the Anziling area (east China) is characterized by a typical dome structure without significant reworking by later deformation. The dome is in tectonic contact with a supracrustal rock assemblage that is now the dip-slip Shuangshanzi ductile shear zone. In the supracrustal rocks, compositional layers are folded into upright isoclinal folds. Meanwhile, along the shear zone, foliation varies from NNW to SW with sub-vertical dip. Mineral stretching lineations indicate a sinistral shear sense with a slightly oblique-slip component in the north, but show NWW-directed and SW-directed steep dip-slip shear in the west and south, respectively. Kinematic indicators imply that the granitic dome formed through a vertically upward movement accompanied by an uneven clockwise rotation. The supracrustal rocks sank downwards to form the regional keel structure. Structural data suggests that the Anziling area is a typical dome-and-keel structure. U-Pb zircon dating on pre-, syn-, and post-tectonic dykes indicate that the dome-and-keel structure formed at 2530−2500 Ma, and was intimately related to the emplacement of tonalite-trondhjemite-granodiorite granitoids. New data from this study suggest that until the late Neoarchean, the vertical tectonism was still a dominant tectonic regime that was operating in the eastern North China Craton.

Kinematic vorticity, finite strain, and deformation thermometry analyses of the exhumed mylonites in the Samen ductile shear zone (Sanandaj-Sirjan Metamorphic Belt, Iran)

Quantitative microstructural investigations involving finite strain and vorticity analyses of quartz-rich mylonites provide significant information regarding the kinematic characteristics of the Samen shear zone (SSZ) within the Sanandaj–Sirjan Metamorphic Belt, Zagros Mountain of Iran. A dominant top-to-the-SE sense shear in the study area is recognized by several kinematic indicators. The results of the finite strain analyses indicate approximately plane strain deformation conditions (Rxz = 2.46–3.53). The mylonitic granitoid rocks record general-shear deformation (Wm = 0.37–0.94) associated with a spatial increase in simple-shear component (23%–78%) from W to E. In the study area, kinematic analyses indicate spatial variations of finite strain, deformation temperature, and the kinematic vorticity numbers resulted in partitioned progressive deformation. These spatial variations reveal an increase of all values towards the Samen thrust sheet. Ductile SSZ developed under greenschist-amphibolite facies metamorphic conditions (464–619 °C), as indicated by the presence of quartz, feldspar, biotite, muscovite, hornblende mineral assemblages, opening angles (OAs) of quartz c-axis fabrics and recrystallization regimes. The dynamic and kinematic characteristics of the study area were generally controlled by ongoing oblique convergence of the Afro-Arabian plate and the Iranian plateau with estimation of a horizontal shortening of ∼20–38% measured perpendicular to the boundaries of the SSZ.

Kinematics and Geochronology of Late Paleozoic–Early Mesozoic Ductile Deformation in the Alxa Block, NW China: New Constraints on the Evolution of the Central Asian Orogenic Belt

Abstract Strike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.

Interactions between large-scale strike-slip intersecting faults: Implication from the Tan–Lu and Xiangfan–Guangji fault zones in eastern China

ABSTRACT The large-scale strike-slip Tan–Lu Fault Zone (TLFZ) and Xiangfan–Guangji Fault Zone (XGFZ) both terminate in the southeastern corner of the Dabie Orogen at an angle of almost 90°, and this corner therefore provides a very good natural laboratory for understanding the mechanism by which large-scale strike-slip faults terminate. We present new geochronological and structural data for the southeastern tip of the XGFZ and the southwestern tip of the TLFZ. The NW–SE-striking XGFZ records ductile shearing in its northwestern segment, characterized by discrete dextral shear zones that formed at temperatures of 350–400°C, as indicated by quartz c-axes fabrics and microstructures. In the southeastern segment of the XGFZ, WNW–ESE-trending thrusts are displayed. The NE–SW-striking TLFZ is characterized by discrete NE–SW-trending sinistral ductile shear zones in the Qianshan–Tongcheng segment, brittle left-lateral strike-slip faults in the Taihu–Qianshan segment, and thrusts to the south of Taihu. The trends of these thrusts change progressively southward from NE–SW to ENE–WSW and E–W. New zircon U–Pb dating results and previous cooling biotite 40Ar/39Ar ages constrain the timing of shearing in the XGFZ to 112–102 Ma (late Early Cretaceous), which is the same as the age of faulting in the TLFZ (110–102 Ma). The large-scale strike-slip TLFZ and XGFZ both terminate at their tips as thrusts. We suggest that interactions between the two faults led to the kinematic changing from strike-slip to thrusting, with this playing an important role in controlling the termination of these two large-scale intersecting strike-slip faults. The dextral shearing of the NW–SE-trending XGFZ, the sinistral shearing of the NE–SW-trending TLFZ, and the nearly E–W-trending thrust faults all indicate continental-scale N–S compression in eastern China during the late Early Cretaceous. This compression resulted from rapid NNW-ward oblique subduction of the Paleo-Pacific Plate.

Paleoproterozoic polyphase deformation in the Helanshan Complex: Structural and geochronological constraints on the tectonic evolution of the Khondalite Belt, North China Craton

The Khondalite Belt is widely regarded as a Paleoproterozoic collisional orogen between the Yinshan and Ordos Blocks in the Western Block of the North China Craton. However, the deformation history of the Khondalite Belt is poorly understood, and many aspects of the tectonic processes involved in the collision remain enigmatic. In this paper, we carried out detailed field-based structural analyses integrated with geochronological studies in the Helanshan Complex, one of the multiply deformed high-grade gneiss terrains in the Khondalite Belt. Three major phases of deformation (D1-D3) were identified in the Helanshan Complex. D1 mainly produced S(SW)-verging recumbent to overturned intrafolial isoclinal folds F1, originally sub-horizontal penetrative transposition foliations S1 and SSW-plunging mineral lineations L1. D1 deformational fabrics occurred coevally with the granulite-facies peak metamorphism. D2 dominantly generated steeply SE(E)-plunging asymmetrical upright open folds F2 that successively refolded the previous D1 structures. D3 gave rise to a series of NE- to E-trending ductile shear zones, featured by top-to-the-(N)NW oblique thrusting with a component of sinistral strike-slip movement. LA-ICP-MS U-Pb zircon dating results of seven deformation-related samples reveal that D1 occurred at some time from 1954 Ma to 1911 Ma, most likely at 1954–1949 Ma. D2 successively happened in the period of 1911–1870 Ma, and D3 had broadly taken place at ∼ 1870 Ma. The D1-D3 deformation sequences recorded tectonic processes including the initial top-to-the-SSW thrusting and crustal thickening, subsequently followed by exhumation of the khondalites in the Paleoproterozoic. Combined with previous studies, these new structural and geochronological data indicate that the Khondalite Belt underwent a long-term (>100 Myr) orogenic history and developed polyphase deformation, high-grade metamorphism as well as magmatism in response to the NNE-SSW-directed oblique collision between the Yinshan and Ordos Blocks at ∼ 1.95 Ga.