Quartz C-axis Fabrics Research Articles

Inferring the Orientations of the Foliation and Lineation Defined by Quartz c-Axis Fabrics: Methods and Applications

Inferring the Orientations of the Foliation and Lineation Defined by Quartz c-Axis Fabrics: Methods and Applications

Transpression in the Eastern Jiangnan Orogen and its implications for ductile deformation process and regional Tectonics of the South China block

The transpressive deformation in the eastern Jiangnan Orogen during the Early Paleozoic is the structural response to the oblique convergence of the Yangtze-Cathaysia blocks, and this relationship is critical for understanding the tectonics of South China. The Jiangwan–Huangshan shear zone (JHSZ) and Baiji–Sanyang shear zone (BSSZ) exhibit NEE–SSW-trending oblique dextral and sinistral shearing, respectively, and their evolution resulted in local E–W dextral strike-slip shearing. Most kinematic vorticity values range from 0.4 to 0.81, which implies that the JHSZ and BSSZ deformed under the contribution of both pure-shear and simple-shear components, suggesting complicated deformation histories for those orogenic shear zones. The quartz and feldspar deformation mechanisms, opening angles and lattice preferred orientation (LPO) patterns of the quartz c-axis fabrics suggest shear deformation temperatures between 400 and 550 °C in the eastern Jiangnan Orogen. Locally deformation occurred under high-temperature (>600 °C) conditions. Combined with previous data and regional geology, these findings indicate that the shearing deformation in the eastern Jiangnan Orogen accommodated the oblique convergence between the Yangtze and Cathaysia blocks during the Early Paleozoic, which was possibly caused by the final assembly of Gondwana.

PT conditions of deformation of the Gol-e-Gohar shear Zone, SW Iran: Insights from analysis of quartz c-axis fabrics, recrystallization mechanisms and syndeformational fluid inclusions

An investigation into the correlation between microstructural characteristics and deformation temperatures of quartz in the middle crust is of utmost importance in yielding substantial knowledge of the nature of deformation zones. Mylonitic rocks from the Gol-e-Gohar Shear Zone were investigated to infer innovative insights into the ductile deformation pressure–temperature (PT) conditions of the Sanandaj–Sirjan metamorphic rocks within the Zagros, southwest Iran. This approach used a multi-method strategy including quartz c-axis fabrics, recrystallization mechanisms and fluid inclusions (Fls) data. Geothermobarometric analysis based on these data highlight deformational PT conditions ranging from ∼400 to ∼500 °C and P < 5 kbar that correspond to ductile deformation with greenschist to lower-amphibolite conditions at depth of ∼ 12–19 km. Measured opening angles of quartz c-axis fabric and dominance of basal < a > slip system indicate crystal plastic deformation occurred in T > 400 °C. Microstructures highlighting the occurrence of Grain Boundary Migration (GBM) confirm temperature of 500–600 °C. The deduced P–T–salinity Fls history of deformation-related fluid inclusions is highlighted by Fls (B-type) with H2O-CO2–rich metamorphic fluids. These fluids were trapped during ductile deformation in temperatures ranging from ∼ 355 to ∼ 535 °C and pressure of P < 5 kbar. The petrofabric analyses show that the mylonitic rock in the Gol-e-Gohar shear zone have undergone metamorphism at the amphibolite facies condition, and then followed by a retrograde greenschist facies metamorphism. These results emphasize the utility of microstructural and fluid inclusion techniques to constrain the history of mylonitic rocks exhumation in the deformation zones.

Early Paleozoic ductile deformation of the South China Block: the polyphase shear zones in the eastern Jiangnan belt

Three parallel east–west-trending shear zones are components of the polyphase shear zones in the eastern Jiangnan belt. The meso- and microstructural characteristics and the estimates of deformation temperatures, vorticities and geochronology of these shear zones provide significant information regarding the tectonic evolution of the South China Block (SCB) during the early Paleozoic. Most shear sense indicators (e.g. asymmetric folds, S-C fabrics and rotating porphyroclasts) reveal that the kinematics were characterized by east–west dextral shear. Finite strain measurements indicate an oblate strain ellipsoid shape with S-type and SL-type tectonites. The rotated rigid porphyroclasts and Mohr circle methods yield vorticity ( Wk ) values of 0.58–0.80 and 0.66–0.89, respectively, indicating that the flow was generally shear-dominated with almost equal proportions of simple and pure shear. Both mineral deformation thermometers and quartz c -axis fabrics indicate that the deformation occurred at medium temperature conditions of 400–550°C. Combined with our new age and regional geological data, the east–west-trending shear zones might represent an increment of deformation during the last period of the early Paleozoic orogeny in the SCB.

Buoyancy-driven exhumation deformation: Evidence from the Sulu orogen, eastern China

Whether deformation of exhumed crust in a collisional orogen is driven by buoyancy or tectonic stress remains uncertain. The Sulu orogen in eastern China contains slowly subducted and exhumed high-pressure (HP)−ultrahigh-pressure (UHP) terranes, which provide a good opportunity to understand whether the exhumation deformation was driven by buoyancy. We used field and microscopic observations as well as quartz c-axis fabrics to determine the deformation kinematics, temperatures, and evolution of the exhumed crustal slices. Deformed and undeformed dikes were dated by using the zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry method to constrain the timing of deformation. Our data demonstrate that each crustal slice was involved in pervasive top-to-the-NW (hinterland) or top-to-the-SE (foreland) ductile deformation during Late Triassic exhumation, and that the base of each slice records deformation that was superimposed during the subsequent exhumation of the underlying slice. The crustal exhumation of the southern Sulu orogen is consistent with the multistage ductile extrusion model. The kinematics of exhumation deformation within each crustal slice conform to an upward asymmetric flow. The flow velocity and corresponding shear sense were affected by temperature variations in each HP slice, whereas they were sensitive to migmatization within the UHP slices. The southern Sulu orogen examples show that crustal flow deformation during exhumation was driven by buoyancy and controlled by viscosity. Unlike the consistent kinematics of tectonics-driven deformation, the kinematics of buoyancy-driven deformation are characterized by variations in the senses of shear within a single crustal slice, and this can therefore be used to distinguish the two types of deformation.

Timing of the Kaiyuan-Jiapigou shear zone in the northern margin of the North China Craton: Implications for closure of the Mongol-Okhotsk Ocean

The ductile Kaiyuan–Jiapigou Shear Zone (KJSZ) is located within the eastern part of the northern margin of the North China Craton (NCC). The foliation within the shear zone generally dips at 55° towards N20° and contains a lineation that plunges at 40° towards N30°. Field and thin section observations and quartz c-axis fabrics of the KJSZ indicate that this zone records top-to-the-SSW shearing associated with a NNE–SSW regional compression event. Muscovite, biotite, and hornblende 40Ar/39Ar step-heating ages constrain the timing of shearing and the tectonothermal history of the KJSZ. Quartz c-axis fabrics and mineral deformation features indicate the deformation within the KJSZ occurred at temperatures of 400°C–450°C, which are lower than the Ar–Ar closure temperature of hornblende but higher than the closure temperatures of muscovite and biotite. Samples from the KJSZ yield 40Ar/39Ar step-heating ages from 164.6 ± 0.5 to 165.2 ± 0.6 Ma for hornblende samples and from 161.4 ± 0.4 to 163.1 ± 0.4 Ma for mica samples. Combining these ages with the temperature of deformation within the KJSZ indicates that this shear zone most likely formed between 165.2 ± 0.6 and 161.4 ± 0.4 Ma. Linking these new data with the tectonics and geochronology of the eastern part of the northern margin of the NCC suggests that the deformation within this region was most likely controlled by a nearly N–S compressional event associated with the far field effect of the Mongol–Okhotsk Ocean closure during the Middle–Late Jurassic.

Correlation among the Ailaoshan–Song Ma–Song Chay orogenic belts and implications for the evolution of the eastern Paleo-Tethys Ocean

The timing and mechanism of the combination between the South China Block (SCB) and the Indochina Block (IB) are controversial. Three ophiolitic mélange zones (Ailaoshan, Song Ma, and Song Chay) have been proposed as suture zones within this collisional orogen. However, the relationships among the three corresponding tectonic belts are unclear. In this study, we present detailed structural data for the three tectonic belts. The bulk architectures of the Ailaoshan, Song Ma, and Song Chay belts correlate well with one another. This similarity is also revealed by our new zircon UPb geochronological results from the Song Ma and Song Chay ophiolites. The regional deformation age is constrained to between 250 and 240 Ma by our new muscovite 40Ar/39Ar ages, and the medium-low temperature conditions are revealed by the quartz c-axis fabric. Considering the transformation effect of the Cenozoic large-scale sinistral strike-slip of the RRF and DBF, the Early Mesozoic Ailaoshan, Song Ma, and Song Chay suture zones should represent different segments of the same belt. Based on this hypothesis, we compiled the ages of the magmatism in this region, which allows us to propose an evolutional model as follows: i) ∼380–310 Ma continental rifting and subsequent Ailaoshan–Song Ma–Song Chay ocean spreading as a branch of the Paleo-Tethys, ii) ∼310–250 Ma oceanic subduction coeval with continental-arc magmatism, iii) ∼250–240 Ma continental collision, iv) ∼240–220 Ma post-collisional extension.

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.

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.

Tectonic transformation from orogen-perpendicular to orogen-parallel extension in the North Himalayan Gneiss Domes: Evidence from a structural, kinematic, and geochronological investigation of the Ramba gneiss dome

The North Himalayan Gneiss Domes consist of a series of isolated domes cored by leucogranite and/or gneiss body. The Ramba dome core comprises gneiss and two-mica granite, mantled mainly by schist sequences, and the degree of deformation decreases outward from pervasive mylonitization to weak mylonitization. Pre-doming deformation is characterized by the formation of regional-scale top-to-the-north shearing (D1) in the Tethyan Himalayan Sequence, locally preserved strong folding (F1), and intrusion of 46.3 Ma-40.6 Ma leucogranite. The D2 deformation consists of map-scale ductile shearing fabrics and top-to-the-north detachment along the South Tibetan Detachment System (STDS) in the middle and lower units of the dome. Intrusion of 26.1–21.0 Ma syn-kinematic leucogranite veins in the schist units occurred during the D2 north-directed extensional episode. The D3 deformation was characterized by top-to-the-east shear during 12.0–10.2 Ma, concurrent with the initiation of activity in the N–S-trending Yadong–Gulu rift. The latest deformation (D4) involved the ascent of two-mica leucogranite through diapirism at about 8.17 Ma. Microstructural analyses, as well as quartz c-axis fabrics measured by EBSD (Electron backscatter diffraction), indicate deformation temperature ranges of D2 extensional shearing from 500 to 600 °C in the middle unit of the Ramba dome to >600 °C in the lower unit. These new data imply that this type of doming in the Ramba region represents a classic type of transformation from orogen-perpendicular to orogen-parallel extensional deformation during Miocene India–Asia collision.

Quartz C-Axis Fabric Characterization of the Strike-Slip Ductile Deformation within the Three Pagodas Shear Zone, Western Thailand

Quartz C-Axis Fabric Characterization of the Strike-Slip Ductile Deformation within the Three Pagodas Shear Zone, Western Thailand

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.

Structural and rheological features of the northeastern Sanyang shear zone in the eastern Jiangnan orogen: Indications for early Paleozoic orogeny in the South China Block

The N55°E−trending Sanyang shear zone is located in the eastern Jiangnan orogen, and the northeastern junction between the Cathaysia and Yangtze blocks is the key to understanding the evolution of the South China Block (SCB) for early Paleozoic crustal reworking. Most shear-sense indicators (e.g., asymmetric folds, porphyroclasts, S–C fabrics, and mica fish) indicate that the Sanyang shear zone is characterized mainly by oblique sinistral shear and locally by dextral shear. Mineral deformation behavior, quartz C-axis fabrics, and the Kruhl thermometer demonstrate that shear deformation developed under high greenschist-to low amphibolite facies conditions with temperatures ranging from 450 to 550 °C. Differential stresses deduced from recrystallized quartz grain sizes were approximately 22.4–46.5 MPa, and strain rates were on the order of 10−9 to 10−10 S−1. Three dimensional (3D) strain ellipsoid measurements and the Flinn diagram of the Sanyang shear zone indicate that all samples yield oblate and flattened strain ellipsoids, which imply a contractional regime. The kinematic vorticity (Wk) 0.716–0.999, estimated by using 3D strain ellipsoid-based methods, indicates a high simple shear contribution in the shear zone. All structural, strain, and vorticity analyses suggest that the Sanyang shear zone experienced general shear with transpression. The numerical estimates allow kinematic modeling of the Sanyang shear zone, which provides important information about the structural framework for the SCB. The development of the Sanyang shear zone was a response to the oblique convergence of the Yangtze and Cathaysia blocks during the early Paleozoic orogeny.

The East Variscan Shear Zone: a structural and geochronological review for improving paleogeographic reconstruction of the southern Variscides

The Alpine orogenesis partially overprinted the Southern European Variscan Belt complicating paleogeographic reconstructions. A main debate concerns the extension and the tectonic significance of the regional-scale East Variscan Shear Zone (EVSZ). This work reviews the data obtained thanks to a systematic and multidisciplinary study of the Ferriere-Mollières shear zone in the Argentera Massif, the Emosson-Bèrard shear zone in the Aiguilles Rouge Massif and the Cavalaire Fault in the Maures Massif. A pure shear-dominated transpression with a a variable component of simple shear is recognized in the studied shear zones. Syn-kinematic paragenesis, microstructures and quartz c-axis fabrics revealed syn- shear decreasing temperature starting from amphibolite-facies. Petrochronology constrained the timing of deformation. Ages range between ~340-330 Ma and ~320 Ma. Older ages are interpreted as the beginning of the activity of the EVSZ along its older branches (Ferriere-Mollières shear zone) while ages of ~320 Ma, obtained in all the shear zones, demonstrate that they were active at the same time. The data revealed a similar tectono-metamorphic evolution of the shear zones and highlights that transpressional deformation affected the European Variscan Belt after the metamorphic peak linked to the initial collisional stage. The review constrains the extension, the kinematics and the timing the EVSZ.

Formation of the Cenozoic Ailao Shan mid-crustal tectonic discontinuity: Role of Oligo-Miocene stratified sub-horizontal middle to lower crustal flow in the southeastern Tibetan Plateau

Structural, kinematic and geothermometry analyses are applied to unravel the structural evolution of the Ailao Shan Tectonic belt (ALTB) with a specific goal of understanding how crustal masses in southeastern Tibetan Plateau flowed during the Indian-Eurasian continental collision. A mid-crustal tectonic discontinuity that is characterized by a narrow ultramylonite belt separates the highly sheared high-grade metamorphic unit (HMU) from the low-grade metamorphic unit (LMU) along the ALTB. It is shown that rocks from the HMU and LMU possess deformation associations of identical structural geometries and kinematics, but at contrasting metamorphic conditions. Calculated mean kinematic vorticity numbers indicate that the ultramylonite belt was affected by dominant simple shearing, while the HMU and LMU on both sides of the belt were deformed by general shearing dominated by pure shearing. Geothermometry analyses on sheared two-mica schists from the HMU and quartz veins from the LMU reveal different deformation conditions, i.e., 610–777 °C, 0.4–0.6 GPa and 320–391 °C, respectively. There is also a sharp decrease in deformation temperature inferred from deformation microstructures and quartz c-axis fabrics, from 550–650 °C for the HMU to 350–400 °C for the LMU. These results suggest that there is an apparent temperature break of ca. 220 °C over a total structural distance of approximately 1 km between these two units at the present outcrop level. Our results show that the HMU and LMU are kinematically linked while rheologically decoupled, and were originally deformed by sub-horizontal shearing at different crustal levels in the same strain field. The ultramylonite belt between them potentially inherited a preexisting basement/cover contact along the ALTB and brought the two units into contact. We suggest that progressive stratified flow of middle to lower crustal masses during the extrusion of the Sundaland block at Oligocene to Early Miocene was responsible for the concurring high- and low-temperature fabrics at different crustal levels. Contemporaneous doming during the progressive crustal flow resulted in exhumation of the lower crustal rocks. During the process, the ultramylonite belt occurred as a tectonic discontinuity (TDC) that leads to the loss of a thick pile (ca. ∼6.3–7.8 km) of crustal masses in the middle crust.

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.

Orogen-parallel mid-lower crustal ductile flow during the late Triassic Qinling orogeny: structural geology and geochronology

ABSTRACT The North and South China Block collision in Triassic resulted in the Qinling orogeny with massif batholith intrusion. In this study, we investigated the deformation structures of the Foping domal zone, the Ningshan bedding-parallel shear zone, and the Yangtian-Tongchewan strike-slip shear zone in East Qinling, aiming at unravelling orogen-parallel mid-lower crustal ductile flow during this late-stage orogeny. Field measurements of foliated felsic migmatites in the Foping domal zone show a subhorizontal stretching lineation associated with top-to-east shearing, and the quartz c-axis fabrics of rock samples from leucosome display characteristic prism <c> slip system (>650°C), consistent with granulite-facies metamorphic condition. The ductile fabrics observed along the W-E extending Yangtian-Tongchewan mylonitic zone along the southern Foping domal zone indicate dextral strike-slip shearing. The Ningshan bedding-parallel shear zone was mapped in the Palaeozoic sedimentary strata along the southern rim of the Foping domal zone and it exhibits subhorizontal stretching lineation. Zircon U-Pb dating results of migmatite from the Foping domal zone, mylonite from the Yangtian-Tongchewan shear zone and syntectonic granitic dyke from the Ningshan bedding-parallel shear zone, together with mica 40Ar/39Ar dating results, allow to constrain the timing of coeval development of these structures in Late Triassic at 220–208 Ma, broadly consistent with the pluton intrusion. The overall pattern is interpreted to result from a local indentation of the Hannan Massif with a salient boundary geometry, which possibly controlled the lateral extrusion of the orogenic wedge, orogen-parallel crustal ductile flow as well as the Late Triassic plutonic emplacements in East Qinling.

Tectonics, geochronology, and mineralization of the Shuidegou gold deposit in the Mianlue belt, western Qinling Mountains, China

The gold mineralization in the Mianlue belt, western Qinling Mountains, has been a hotspot in geoscience research in recent years. In particular, important breakthroughs have been achieved in terms of prospecting in the medium-sized Shuidegou gold deposit, located in the middle of the Mianlue belt. Here, we present the results of systematic tectonic and chronological analyses conducted on the Shuidegou gold deposit. The reconstructed zircon U–Pb chronologies of the ore-bearing altered rocks and of the ore-bearing vein demonstrated that the Shuidegou gold deposit formed at ~183 Ma and was further enriched at ~ 150 Ma. The macro- and micro-structures identified in deformed geologic bodies and quartz c-axis fabrics in the mining area suggest a division of the deformation sequence into four phases, corresponding to: (1) a Mid–Late Triassic collision and convergence of middle–deep deformation structures during the main orogenic period (D1); (2) a Late Triassic–Early Jurassic collision and convergence during the late-stage thrust-nappe tectonic deformation (D2); (3) a Mid–Late Mesozoic shallow strike-slip shear structure stage during intracontinental orogeny (D3); (4) post-Cretaceous, shallow brittle thrust extrusion strike-slip structure (D4). Based on the deformation and zircon U–Pb chronological features of the Shuidegou gold deposit, we believe that the main metallogenic age (~183 Ma) corresponds to the thrust-shear deformation stage (D2) and that it is genetically connected with magmatism. Notably, the late-stage further enrichment(~150 Ma) occurred in the sinistral strike-slip stage. Ore-bearing fluids may have been restricted by the far-field effect of the Pacific plate subduction along the eastern edge of East Asia. Based on the macro- and micro-deformation features, the chronological data relative to the key geologic bodies, and the regional geologic background, we subdivided the mineralizing process of the Shuidegou gold deposit is composed into three stages: (1) the formation of an ore-guiding channel, (2) the deposition and mineralization of ore-forming elements, and (3) a later further enrichment. The study of ore-controlling structure showed that the development of the Shuidegou gold deposit was mainly controlled by WNW- (E–W) and NNE-trending structures during the two metallogenic periods (~183 Ma and ~ 150 Ma, respectively). Concerning this aspect, the Shuidegou gold deposit contrasts with some typical gold deposits in the same region.

Late Triassic contractional tectonics in the overriding plate of the Sulu orogenic belt, Eastern China

It is well known that the Dabie-Sulu orogenic belt resulted from the northward continental subduction of the South China Block (SCB) beneath the North China Block (NCB) during the Triassic. Correlating the deformation events recorded in the subducted and overriding plates is essential to understand its tectonic evolution, but related deformation is poorly documented in the overriding plate. In this contribution, we present detailed structural analysis, anisotropy of magnetic susceptibility (AMS), and geochronological data in the Linsishan unit of the Jiaobei massif, the overriding plate of the Sulu orogenic belt. The Linsishan unit preserves a significant ductile deformation characterized by NNW-, NE-, or SE-dipping foliations with NE–SW trending mineral lineations. Coupling mesoscopic and magnetic fabrics augment the NE–SW linear structures. The NE-dipping monoclinal structures with top-to-SW kinematics could be interpreted as a SW-directed thrusting, the deformation temperature of which is 350–450 °C according to the quartz c-axis fabrics. The timing of deformation is constrained to be 223–221 Ma based on 40Ar/39Ar dating of the deformed rocks and linked with a syn-collisional stage in the Sulu orogenic belt. Combining these new results with previous works, we propose a three-stage evolution model for the Sulu orogenic belt, namely (1) subducted SCB experienced ultra-high pressure (UHP) metamorphism before 230 Ma; (2) SW-directed thrusting was developed in the overriding plate, coeval with the syn-collisional exhumation of UHP rocks during 230–220 Ma; (3) post-collisional migmatization, magmatism, extension, and UHP rocks exhumation took place at 220–205 Ma.

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

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