Structure and shear-sense indicators of the Mesoproterozoic basement of the North Tianshan microcontinent: Example of granitoid gneisses of the Karadjilga pluton, NW Kyrgyzstan

<p>The Mesoproterozoic Karadjilga pluton is a poorly studied fragment of the North Tianshan microcontinent located in the western Central Asian Orogenic Belt. Metasedimentary rocks surrounding the pluton consist of marbles and mica schists of the Mesoproterozoic Ortotau Group. These rocks constitute a major west-northwest trending syncline with steep to subvertical limbs. The hinge of the fold is well expressed in the west part of the syncline and plunges east with 30-40° angle of plunge. Eastern termination of the syncline is cut by faults. Granitoid gneisses and granites of the Karadjilga pluton crop out in the core of the syncline. The contacts of the pluton are sub-parallel to bedding and schistosity in surrounding rocks. Primary magmatic contacts are locally reworked by reverse faults and thrusts. Our detailed mapping and structural study revealed inhomogeneous deformation of rocks of the Karadjilga pluton. The following rock types are identified: 1) undeformed granite 2) foliated granite 3) granite-gneiss and 4) mylonite. Undeformed granites form <25-30% of total volume of the pluton and are most widespread in the northeast part of the pluton. On some geological maps they are shown as Ordovician or Devonian. However, U-Pb dating of 9 zircon grains by SHRIMP-II (VSEGEI, St. Petersburg, Russia) yielded a 1125±5 Ma concordant age. It agrees with previously reported U-Pb SHRIMP ages for deformed granites and gneisses (Degtyarev et al., 2011; Kröner et al., 2013) and indicates that undeformed granites belongs to the same Mesoproterozoic magmatic complex. Foliated granites and gneisses prevail and constitute up to 60-70% of total volume. They form west-northwest trending zones alternating with mylonites or undeformed granite. Mylonites are subordinate and occur mainly along the contacts of the pluton. Shear zones seem to be approximately parallel to the schistosity of deformed granites, but their geometry needs more study and mapping. Shear-sense indicators were studied in the oriented thin sections and are represented mainly by sigma and delta structures and oblique foliation with rare folds and other indicators. In all but one sample only strike-slip displacement has been identified. In the northern part of the pluton sinistral displacement predominates, whereas dextral displacement prevails in the southern part of the pluton. Shear zones are most widespread on the margins of the Karadjilga pluton, but locally also occur in the central part of the pluton, where they form narrow west-northwest trending zones. According to shear-sense indicators, displacement within the Karadjilga pluton occurred mainly in the approximately west-east direction that strongly differs from the north-south sense of displacement in the Paleozoic thrust and fold belts of Tianshan.</p><p>The study was supported by the RFBR project 20-05-00252.</p>

A reinterpretation of the Snow Lake gold camp, Trans-Hudson Orogen, Canada: The use of cleavages as markers to correlate structures across deformed terranes

The Snow Lake gold camp is located within amphibolite facies volcanic rocks of the ca. 1.88 – 1.87 Ga Flin Flon-Glennie Complex (FFGC) in the Trans-Hudson Orogen, Manitoba. During thrusting and collision with the Archean Sask craton, volcanic rocks were interleaved with turbidites of the ca. 1.855 - 1.84 Ga Burntwood Group and sandstone and conglomerate of the ca. 1.845 - 1.835 Ga Missi Group. The main cleavage in the turbidites was previously attributed to thrusting and used as a marker for correlating structures across the camp. A re-examination of this cleavage suggests that it overprints the thrust faults and formed during later collision between the FFGC and the Archean Superior craton. This has important implications as it further suggests that (1) previously unrecognized, early brittle thrust faults repeat volcanic stratigraphy and may have created the boundary conditions that enabled the formation of ductile thrust faults, fold nappes, and mega sheath folds; (2) shear sense indicators along ductile thrust faults formed during their reactivation as sinistral shear zones rather than during thrusting; and (3) peak metamorphic conditions were caused by thrusting and stacking during collision with the Sask craton but were attained later during collision with the Superior craton due to the time lag between orogenesis and the re-equilibration of regional isotherms. Results from this study may be applicable to other complexly deformed terranes where the dominant regional cleavage differs in expression in mixed volcanic and sedimentary successions and has been used as a marker for correlating structures.

Structural and microtectonic analyses of Barpak of Gorkha district, west-central Nepal

Geological mapping was carried out in the Barpak-Bhachchek area of the Daraudi River valley, Gorkha district, West-Central Nepal for structural analysis. The area comprises rocks of the Higher Himalayan Crystalline and the Lesser Himalayan Sequence. Pelitic and psammitic schist, quartzite, calc-quartzite, dolomitic marble, graphitic schist, gneiss are the main rock types within the Lesser Himalayan Sequence, whereas banded gneiss and quartzite form a significant portion of the Higher Himalayan Crystalline in the study area. The area is affected by poly-phase deformation. Lesser Himalayan Sequence has suffered five deformational phases (DL1-DL2, D3-D5) whereas the Higher Himalayan Crystalline has suffered four deformational events (DH1, D3-D5). The Lesser Himalayan Sequence lying to the northern limb of the Gorkha-Kuncha Anticlinorium is contort into doubly plunging to dome-and-basin-like en echelon type of non-cylindrical folds as Baluwa Dome and Pokharatar Basin (DL2 and D4). The direction of shearing as indicated by shear sense indicators (C' Shear band and Mica fish) is top-to-south coinciding with regional sense of shear related to the MCT propagation. The dynamic recrystallization direction, obtained from rock dominant with phyllosilicate minerals is top-to-south and coincides with mineral lineation and indicate the mineral lineation is contemporary with dynamic recrystallization during the MCT propagation.

Measurement of 3D-Shape Preferred Orientation (SPO) Using Synchrotron μ-CT: Applications for Estimation of Fault Motion Sense in a Fault Gouge

We propose a 3D-shape preferred orientation (SPO) measurement method of rigid grains using synchrotron micro-computational tomography (μ-CT). The method includes oriented sampling, 3D μ-CT imaging, image filtering, ellipsoid fitting, and SPO measurement. After CT imaging, all processes are computerized, and the directions of thousands of rigid grains in 3D-space can be automatically measured. This method is optimized for estimating the orientation of the silt-sized rigid grains in fault gouge, which indicates P-shear direction in a fault system. This allows us to successfully deduce fault motion sense and quantify fault movement. Because this method requires a small amount of sample, it can be applied as an alternative to study fault systems, where the shear sense indicators are not distinct in the outcrop and the fault gouge is poorly developed. We applied the newly developed 3D-SPO method for a fault system in the Yangsan fault, one of the major faults in the southeastern Korean Peninsula, and observed the P-shear direction successfully.

Orthogonal Tectonic and Magmatic Fabrics in a Layered Granite-Gneiss at Remal Dam Site, India: Implications for Fabric Generation and Superposition

<p>The Remal granite-gneiss is situated close to the tectonic boundary between the Singhbhum Craton and the Rengali Province in the state of Odisha, eastern India. This granite-gneiss contains two prominent fabric elements - a sub-horizontal to gently dipping felsic fabric S<sub>ign</sub>, believed to be of igneous origin that predates a sub-vertical gneissosity S<sub>1</sub> which is of tectonic origin. S<sub>ign</sub> layers have a non-uniform, arcuate geometry and grain-size within the layers show systematic variations. S<sub>1</sub> is defined by metre-scale segregations of biotite-poor and biotite-rich domains whose orientations are constant. S<sub>ign</sub> layers are arranged rhythmically in cross-section and either curve into parallelism with or truncate against layers above and below; the entire assembly resembles cross beds developed in sediments. Some of the layers develop trough cross-bedding similar to those seen in mafic intrusions such as the Skaergaard Complex, indicative of slumping of a crystallizing mush along an inclined depositional plane at the time of crystallization. The S<sub>ign</sub> layers are composed of quartz, K-feldspar and plagioclase with abundant graphic intergrowths and myrmekite, and lack any evidence of compaction. Plagioclase grains are often zoned, and dihedral angles between mineral grains is significantly different from the equilibrium value of 120°, testifying to the preservation of the igneous nature of this fabric without significant solid state modification. In contrast, S<sub>1</sub> is sub-parallel to localized mylonite zones within the granite-gneiss composed of chlorite and epidote, indicative of deformation under greenschist facies conditions. The mylonitized zones contain prominent dextral shear sense indicators and is believed to have originated due to the amalgamation of the Rengali Province with the Eastern Ghats Mobile Belt along the east-west trending, sub-vertical Brahmani Shear Zone further to the south. The S<sub>1</sub> gneissosity appears to have developed as a result of this deformation event. EBSD analyses of quartz grains within the granite-gneiss reveal distinct variations in the distribution of <c> axes in different domains. Close to the mylonite zones, deformation of quartz has been dominantly accommodated by basal <a> slip with a dextral shearing overprint while away from these zones and S<sub>1, </sub>the <c> axes are distributed in clusters without any systematic pattern. The persistence of an earlier igneous layering, despite the subsequent development of a gneissosity concomitant with localised mylonitisation, indicates that the later deformation event has not obliterated the earlier formed igneous fabric. The study also demonstrates that development of a gneissosity does not necessarily require deformation operating at moderate to high temperature, and can stabilize even under greenschist facies conditions.</p>

The Vari Unit in the hanging wall of the West Cycladic Detachment System (Agios Georgios, Greece): A small island with a big message

<p>The timing and kinematics of low-angle normal faulting in the Cyclades (Aegean region, Greece) has been a matter of debate, mainly because the detachments arch over the islands and only remnants of the fault systems and small klippen of hanging-wall rocks are preserved. The small (4.3 km<sup>2</sup>) island of Agios Georgios, 20 km south of the Attica Peninsula, lies structurally above the West Cycladic Detachment System (WCDS). The island consists of metavolcanic greenschists and grey pelitic schists defined by an Ms+Chl+Ep/Czo+Ab±blue Amp assemblage. Zoned blue amphiboles occur both within albite porphyroclasts and the matrix in greenschists, and in pelitic schists. A locally strongly deformed granitoid dominates the east end of the island, which also contains zoned amphiboles (partly relict magmatic?), actinolite often replacing hornblende and two generations of white mica. The granitoid yields a zircon U-Pb date of 248.2 ± 1.0 Ma (MSWD: 0.83) with Variscan inheritance. Dynamically recrystallized feldspar in both the granitoid and host rocks suggests upper greenschist to amphibolite facies conditions. The white mica in the host rock forms bundles that define an anastomosing foliation, and although strongly deformed in many parts, stretching lineations and shear-sense indicators show variable orientations. New <sup>40</sup>Ar/<sup>39</sup>Ar geochronology on the mica yields c. 60-47 Ma dates, and new zircon (U-Th)/He dates indicate cooling <200°C at c. 21 Ma. The new geochronology and lack of a strong top-to-SW directed deformation, diagnostic of the WCDS, indicates that the island lies well above the detachment system.</p><p>Thirty km to the NNE, along a section perpendicular to the strike of the WCDS, lies the small (18.5 km<sup>2</sup>) island of Makronisos, where the WCDS and the footwall are exposed. The island consists of greenschists and pelitic/graphitic schists with quartzites and blue-grey mylonitic marbles, and metabasites containing blue amphiboles. The structurally highest level consists of white to pale grey/reddish ultramylonites up to 40 m thick lying on the central ridge of the island and along the NE and NW coasts. Strongly clustered stretching lineations and macro-/microscopic shear-criteria indicate a top-to-SSW shear sense. Deformation mechanisms in quartz (LT-bulging) and calcite (recrystallization) indicate deformation at ~300°C. Albite porphyroclasts may preserve an older foliation and an earlier, higher grade metamorphism. This record is consistent with cooling during top-to-SSW deformation. New <sup>40</sup>Ar/<sup>39</sup>Ar analyses on white mica yield ages of 22-15 Ma, markedly younger than the ages from Ag. Georgios. Published zircon (U-Th)/He dates from the Western Cyclades footwall record cooling at 14-9 Ma. The relict status of the HP-mineral assemblages suggests Makronisos is part of the Lower Cycladic Blueschist Nappe (i.e. below the Trans Cycladic Thrust) and hence the ultramylonites are interpreted as the footwall of the early ductile high strain zone forming the WCDS. The older dates and distinctly different structural styles on Ag. Georgios indicate that as the hanging wall to the WCDS it is comparable to the Vari Unit on Syros, likely an extensional allochthon of the Pelagonian block.</p>

Strain Pattern Analysis of Mylonites From Sitampundi-Kanjamalai Shear Zone, Thiruchengode, South India

This study aims to investigate the petrography and strain pattern of mylonites from parts of N-S trending Sitampundi-Kanjamalai Shear Zone (SKSZ) around Thiruchengode. The petrographic study indicates the presence of recrystallized quartz, K-feldspar, plagioclase, biotite and some hornblende. The kinematic analysis of Mylonites was done with the help of shear sense indicators such as recrystallized type quartz (quartz ribbon) around the cluster of feldspar, S-C fabric shows dextral shear sense and some sinisterly shear sense in some parts of SASZ which can be considered as a product of partitioning of both strain and vorticity between domains. These all indicates the simple shear extension along E-W direction and the mylonitic foliation shows the pure shear compression along N-S direction. Further the study of bulk strain analysis by Flinn plot method using L and T section of mylonite shows k<1 which lies in the field of flattening zone of finite strain. The kinematic vorticity number is calculated by Rxz/β method which gives the value of 0.36 indicating the general shear. The rigid grain graph shows that the pure shear component is more ­­­­dominant than the simple shear component. The analysis leads to the conclusion that the mylonite has experienced a high temperature shearing of above 700°cat deep crustal level.

Kinematic analysis of rock flow and deformation temperature of the Sirjan thrust sheet, Zagros Orogen, Iran

Microstructural, finite strain and vorticity analyses of quartz-rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the Sirjan thrust sheet exposed in a structural window within the Sanandaj–Sirjan High Pressure – Low Temperature (HP–LT) metamorphic belt that forms part of the hinterland of the Zagros orogenic belt of Iran. A dominant top-to-the-SW sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins, rotated porphyroclasts, mica fish and S/C fabrics. Quantitative analyses reveal approximately plane strain deformation conditions with Rxz values ranging from 2.5 to 4.3 and increasing towards the Sirjan thrust. Opening angles of quartz c-axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 ± 50°C in the hanging wall rocks. Oblique grain shape and quartz c-axis fabrics were used to estimate the degree of non-coaxiality during deformation. The obtained vorticity profile indicates a down-section increase in kinematic vorticity number (Wm) from 0.6 to 0.89. This range of vorticity numbers confirms contributions of both simple (41–68 %) and pure shear (32–59 %) deformation components. The structural characteristics of the study area ultimately were controlled by oblique motion of the Afro-Arabian plate relative to the Iranian plate.

Geology of the orogenic Cheminis gold deposit along the Larder Lake – Cadillac deformation zone, Ontario

The Larder Lake – Cadillac deformation zone (LLCDZ) is one of two major, auriferous, deformation zones in the southern Abitibi subprovince of the Archean Superior Province. It hosts the Cheminis and the giant Kerr Addison – Chesterville deposits within a strongly deformed band of Fe-rich tholeiitic basalt and komatiite of the Larder Lake Group (ca. 2705 Ma). The latter is bounded on both sides by younger, less deformed, Timiskaming turbidites (2674–2670 Ma). The earliest deformation features are F1 folds affecting the Timiskaming rocks, which formed either during D1 extensional faulting or during early D2 north–south shortening related to the opening and closure, respectively, of the Timiskaming basin. Continued shortening during D2 imbricated the older volcanic rocks and turbidites and produced regional F2 folds with an axial planar S2 cleavage. D2 deformation was partitioned into the weaker band of volcanic rocks, producing the strong S2 foliation, L2 stretching lineation, and south-side-up shear sense indicators, which characterize the LLCDZ. Gold is present in quartz–carbonate veins in deformed fuchsitic komatiites (carbonate ore) and turbiditic sandstone (sandstone-hosted ore), and in association with disseminated pyrite in altered Fe-rich tholeiitic basalts (flow ore). All host rocks underwent strong mass gains in CO2, S, K2O, Ba, As, and W, during sericitization, carbonatization, and sulphidation of the host rocks, suggesting that they interacted with the same hydrothermal fluids. Textural relationships between alteration minerals and S2 cleavage indicate that mineralization is syn-cleavage. Thus, gold was deposited as hydrothermal fluids migrated upward along the LLCDZ during contractional, D2 south-side-up shearing. The gold zones were subsequently modified during D3 reactivation of the LLCDZ as a dextral transcurrent fault zone.