Ductile Shear Research Articles

Genesis of the Graphite from the Tugeman Graphite Deposit, Xinjiang, China: Evidence for Carbon Isotope Refining by Fluids Associated with the Ductile Shear Zone

The Altun orogenic belt is situated along the northern boundary of the Tibetan Plateau. In this study, we present an analysis of the ore deposit, mineral composition, and carbon isotope signatures of the Tugeman graphite deposit within the Altun orogenic belt. The graphite in the Tugeman graphite deposit occurs within graphite-bearing schists and marble. Graphite enrichment is observed in the ductile shear zone. The carbon isotope values of graphite range between −18.90‰ and −10.03‰ (with an average value of −12.70‰). These values differ significantly from those observed in organic matter and marine carbonates, suggesting the occurrence of a mixing process involving reduced carbon fluid derived from biological organic material during regional metamorphism as well as a potential influx of oxidized carbon fluid from external sources. In addition, the metamorphic temperature of Tugeman graphite calculated from Raman spectroscopy is between 494 °C and 570 °C, which indicates that the disordered material is transformed from greenschist-amphibolite facies metamorphism to moderate-crystalline graphite. Combining the geological and carbon isotope characteristics of the Tugeman graphite deposit, we argue that the Tugeman graphite deposit is a regional metamorphic graphite deposit of biogenic origin, and during the late stage of metamorphism, it underwent interaction with fluids.

Comparison of a copper processed by asymmetric unidirectional rolling (AUR) and cross rolling (ACR): Strain distribution, microstructure, texture, and mechanical properties

In this work, the effect of strain path on the strain distribution, microstructural evolution, crystallographic texture, and mechanical behavior of electrolytic tough-pitch copper was investigated by simulation and experimental procedures. The asymmetric unidirectional and cross rolling processes with a thickness reduction of 90% were performed at room temperature. The asymmetric cross-rolling (ACR) gives much better through-thickness uniformity of shear strain distribution compared to the asymmetric unidirectional-rolling (AUR). The average equivalent strain of AUR and ACR after 90% rolling reduction was 3.99 ± 1.22 and 3.66 ± 0.26, respectively. Both AUR and ACR processes improved the particle distribution in the matrix. However, the particle distribution of the AUR sample was slightly more uniform compared to the ACR sample. The shape of primary grains in the AUR copper was straight while that in the ACR sample was zigzag. The average dislocation density of the AUR sheet was larger than that of the ACR sample (2.41 × 1016 m−2 vs. 2.31 × 1016 m−2). The major recrystallization mechanism in the AUR and ACR samples was discontinuous and continuous dynamic recrystallization, respectively. The main deformation textures (Copper, S, and Brass components, and α and β fibers) were weak in both samples due to the domination of recrystallization and shear textures in the cold-rolled copper. The recrystallization texture components in the upper region were stronger than in the lower region. The uniformity of the microhardness profile of the ACR sample was more than that of the AUR sample. The AUR sample had a larger strength and a smaller ductility compared to the ACR sample. The results showed that the strain path of asymmetric rolling was effective on the strain-hardening behavior of copper during the tensile test. The fractograph of the ACR sheet exhibited ductile dimples. However, the fractograph of the AUR sample revealed many ductile shear dimples and flat surfaces.

Protracted localization of metamorphism and deformation in a heterogeneous lower-crustal shear zone

Deformation of the continental crust is influenced by the pre-existing structural framework, fluid availability, strain rate, and pressure-temperature conditions. We investigate the evolution of a large (hundreds-of-meters wide) heterogeneous shear zone and associated brittle and ductile deformation structures (Lofoten, Norway) using structural analysis, mineral-chemical and microstructural observations, and U-Pb dating. The shear zone developed through a sequence of metamorphic stages: (1) migmatization and granulite-facies metamorphism, (2) eclogite-facies metamorphism, and (3) amphibolite-facies metamorphism. Stage (1) was related to magmatic activity at lower-crustal conditions prior to 1.7 Ga. Stage (2) likely occurred during the early phases of the collision between Baltica and Laurentia (Caledonian collision). Stage (3) was related to shortening during the main phase of the Caledonian collision (∼430–400 Ma) and was accompanied by hydration of the shear zone. We demonstrate how mechanical heterogeneities influence the deformation style from the centimeter to meter scale. Zones with a pre-existing fabric deformed by ductile shearing and folding, whereas homogeneous, dry rocks fractured. Fractures provided precursors for small-scale shear zones. These contrasting deformation styles occurred repeatedly. Consequently, pre-existing structures define the deformation style and serve as conduits for the channelization of fluids over extended periods of time.

The origin and evolution of the ore-forming fluids at the Manondo-Choma gold prospect, Kirk range, southern Malawi

Abstract This study provides an in-depth analysis of fluid inclusions (FIs) and stable isotopes from the Manondo-Choma gold prospect in southern Malawi to understand the ore-forming mechanisms, genesis, and evolution of the hydrothermal fluids responsible for gold mineralization at the Manondo-Choma gold prospect. FIs and microthermometry studies were carried out on mineralized quartz veins from the area. The Manondo-Choma gold prospect is located in the southern Malawi within the Kirk range. Gold is mostly found in quartz veins within metamorphic rocks, such as gneiss and schists, and is structurally controlled by the NE–SW ductile shear zones. Gold mineralization is linked to quartz sulfide veins. Mineralization occurs in the following three stages: early, middle, and late, of which the middle stage is more plorific. The following three FI types were recognized in the quartz veins: pure carbonic, aqueous carbonic (CO2–H2O) and aqueous (H2O–NaCl) inclusions. Gold and associated mineralization were likely precipitated due to the lowering of pressure and fluid immiscibility. Oxygen isotope data indicate that the source of ore-forming fluids at the Manondo-Choma gold prospect was largely metamorphic in origin with minor magmatic input.

The ore-forming process of the Qingshan gold deposit, Qaidam Basin: Evidence from LA-ICP-MS trace element analysis and S-Pb isotopic compositions

The Qingshan gold deposit is located in the Tanjianshan ore collection area, which is in the northern margin of Qaidam Basin and is one of the most important gold and polymetallic metallogenic belts in China. Based on the field investigation and optical microscopic observations, mineralization at the Qingshan deposit can be divided into three stages: (1) quartz-pyrite stage, (2) quartz-sulfide-gold stage, (3) quartz-calcite stage. Pyrite from the stage 1 (Py1), stage 2 (Py2) and stage 3 (Py3) are selected for LA-ICP-MS trace element analyses. Pyrite from the stage 1 (Py1), stage 2 (Py2) are selected for in situ sulfur isotopes analysis. Pyrite from the stage 1 (Py1), stage 2 (Py2) and stage 3 (Py3), galena from the stage 1 (Gn1) and stage 2 (Gn2) are selected for in situ lead isotopes analysis. The result shows that Py1 is relatively rich in Co, Ni, Zn and Ag contents and lacking in Sb contents. Py2 is relatively rich in Cu, As, Sb, Au, Pb and Ag contents and lacking in Co, Ni and Bi contents. Py3 is relatively rich in Co, Ni and Bi contents and lacking in Zn, As, Au and Pb contents. The Au/As ratio shows that invisible gold occurs in the form of solid solution (Au+). The values of Au/Ag and Co/Ni ratio show that the pyrites in the Qingshan deposit are hydrothermal origin. The S isotope values of pyrite range from −4.33 to 4.97‰ (mean: 3.71‰, n = 31). The Pb isotope compositions with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values ranging from 18.002 to 18.791, 15.567 to 15.584, and 38.410 to 38.485, respectively. The results of in situ S-Pb isotopic indicate that sulfur and metals of the Qingshan deposit were mainly derived from the upper crust and orogenic belts. The Qingshan gold deposit is classified as an orogenic gold deposit, and was closely related to a large-scale ductile shear zone, which formed during the process of subduction between the Qaidam block and the Qilian block in the Variscan period. The mixing of fluids destroyed the stability of Au(HS)2- and is most likely the reason for gold precipitation.

Early Paleozoic oblique convergence from subduction to collision: Insights from timing and structural style of the transpressional dextral shear zone in the Qilian orogen, northern Tibet of China

Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE−striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via field-based, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (∼500−550 °C and ∼5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep−dominated deformation. A strain rate of 10−12 s−1 and a differential stress of 25−39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47−0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence.

Long‐Lived (180 Myr) Ductile Flow Within the Great Slave Lake Shear Zone

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

Calibration of ductile and shear damage models for carbon and stainless steel shear studs

AbstractSteel‐concrete composite beams are commonly used in bridge construction. The corrosion of the supporting steel beams and the shear connectors are however known as a major cause of premature deterioration of composite bridges. Stainless steel beams and shear connectors offer a viable alternative solution to the steel corrosion problem in composite bridges located in harsh environmental conditions. This paper presents experimental and numerical investigations of the tensile and shear behaviour of stainless steel welded shear connectors. The results of tensile coupon and double shear tests on stainless steel and carbon steel shear connectors to characterise the tensile and shear response are reported. Numerical modelling is carried out using the ABAQUS finite element software to develop calibrated ductile damage and shear damage fracture models for stainless steel shear connectors. The development and validation of the numerical models together with the calibration of the fracture models are presented and comparisons are made with those of carbon steel shear connectors.

Reactivated fracture-controlled uranium mineralization: An example from NNE-SSW Kamaguttapalle–Kammapalle tract, Kadapa district, Andhra Pradesh, India

The Cuddapah Basin (CB) and its basement can be considered as an appropriate uranium metallogenic province. The basement complex of CB in the Eastern Dharwar Craton (EDC), India, has been the target for structurally controlled uranium exploration since the early 1980s. Most of the brittle-ductile deformation induced reactivated narrow linear fracture zones often exhibit significant surface and sub-surface uranium occurrence. There are about 10 fracture zones associated with uranium mineralization. Amongst them, the Kamaguttapalle–Kammapalle fracture zone (KKF) in the south of the CB is quite interesting from an exploration viewpoint with a suitable geologic setting in space and time frame. Geomorphic, structural and petro-mineralogic exploration guides imply the possibility of economic uranium concentration along the NNE-SSW trend where tectonic reactivation took place over pre-existing ductile shear zone. Progressive exhumation and transition from ductile to brittle deformation regime led to the development of dominant strike slip tectonics with an associated riedel fracture system, which are subsidiary coeval shear fractures used in depicting sense of movement. Intense illitization and hematitization are often observed. Quartz reefs and veins along the NNE-SSW trend cut the preexisting mylonites at places. About 9 stages of fracturing is observed out of which 3 stages are related to uranium mineralization. All the 9 deformations are related to strike slip tectonics which in turn is part of Grenville orogeny. Paragenetic sequence and mode of occurrences of uranium phases deciphers about the importance of low to moderate level epi‑meso type hydrothermal (100°–250 °C) fluids as carrier of U. Reactivated fractures and quartz veination also act as controlling factors of mineralization. Strain analysis points to positive and negative flower structures at different segments of the fracture zone. Based on the litho-structural attributes it is established that maximum compression (σ1) acted along the NNW-SSE direction. Fractured quartz veins provide well defined permeability and trapping facility by restricting transverse fluid movement as a mechanical barrier. The presented model gives an idea on role of strike-slip tectonics in uranium mineralization for narrowing down the target zone for sub-surface exploration.

The deformation and burst behavior of Zircaloy-4 cladding tubes with hydride rim features subject to internal pressure loads

This study is motivated by the desire to conduct a separate effects test replicating specific aspects of an in-pile pulse reactor test on high exposure light water reactor (LWR) fuel rods. The principal purpose of the tests was to determine whether temperature and or hydrogen concentration thresholds exist in assessing the vulnerability of a stress relieved zirconium alloy cladding tubes to low strain ruptures. A secondary objective was to determine whether observations of the cladding stress during deformation could aid in the understanding of the crack formation and propagation in cladding tubes with hydride rim features. The study utilized artificially hydrided Zircaloy-4 cladding tubes and an isothermal, internal pressure test that was controlled with an in-situ strain measurement. The test results show a clear transition in failure vulnerability between room temperature and 150 °C. Below 150 °C, cladding tubes rupture at very low uniform plastic strain—even at relatively modest cladding hydrogen levels. At and above 150 °C, cladding tubes only rupture at low uniform plastic strain if the cladding hydrogen content is greater than 500 ppm and usually experience at least some modest amount of plastic deformation prior to rupture. Post-test metallographs from non-ruptured cladding tubes show numerous primary cracks forming in the hydride rim and arresting at the metal/hydride interface. This observation combined with the decrease in bulk yielding of hydrided cladding tubes when compared to fresh non-hydrided tubes leads to the conclusion that the fracture stress of the hydride rims is related to and bounded by the yield stress of the zirconium metal. Some evidence is presented for a change in crack propagation mechanism from one of void formation on hydride plates and linkage at temperatures below 150 °C to more classical ductile shearing beneath the primary crack tip at temperatures above 150 °C. This apparent change in crack propagation mechanism may help explain the observed temperature and hydrogen concentration thresholds.

Modeling Deep Rooted Thrust Mechanism of Crustal Thickening in Eastern Tibet

AbstractTo test Eastern Tibet crustal thickening modes, we compare 2‐D numerical models of two emblematic end‐member models, with either an obstacle in the low viscosity lower crust or a thrust embedded in the high viscosity one. We show that the obstacle halts the viscous lower crustal flow potentially initiated by the weight of the high Central Tibet, generating a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. On the contrary, including a low viscosity discontinuity in the upper crust, mimicking a shallow steep listric fault as inferred in the region, reproduces a sharper exhumation profile, as constrained from thermo‐kinematic inversions of thermochronological data, and the lack of foreland basin, as observed in the field. Moreover, such fault drives deformation throughout the entire crust, suggesting a deep crustal ductile shear zone limiting the more ductile deformation in the lower crust although no discontinuity is imposed.

Devonian-Carboniferous extension and Eurekan inversion along an inherited WNW-ESE-striking fault system in Billefjorden, Svalbard.

Background: The Billefjorden area in central Spitsbergen hosts thick Lower-lowermost Upper Devonian, late-post-Caledonian collapse deposits presumably deformed during the Late Devonian Svalbardian Orogeny. These rocks are juxtaposed against Proterozoic basement rocks along the Billefjorden Fault Zone and are overlain by uppermost Devonian-early Permian deposits of the Billefjorden Trough, a N-S-trending Carboniferous rift basin bounded by the Billefjorden Fault Zone. Methods: We interpreted seismic reflection (also depth-converted), bathymetric, and exploration well data. Results: The data show abundant Early Devonian, WNW-ESE-striking (oblique-slip) normal faults segmenting the Billefjorden Trough, and a gradual decrease in tectonic activity from the Early Devonian (collapse phase) to early Permian (post-rift phase). Early Devonian-Middle Pennsylvanian WNW-ESE-striking faults were mildly reactivated and overprinted and accommodated strain partitioning and decoupling in the early Cenozoic. This resulted in intense deformation of Lower Devonian sedimentary rocks and in the formation of bedding-parallel décollements, e.g., between the Lower Devonian Wood Bay and the uppermost Pennsylvanian-lowermost Permian Wordiekammen formations. This suggests that intense deformation within Devonian rocks in Dickson Land can be explained by Eurekan deformation alone. Eurekan deformation also resulted in the formation of WNW-ESE- and N-S- to NNE-SSW-trending, kilometer-wide, open folds such as the Petuniabukta Syncline, and in inversion and/or overprinting of Early Devonian to Early Pennsylvanian normal faults by sinistral-reverse Eurekan thrusts. WNW-ESE-striking faults merge at depth with similarly trending and dipping ductile shear zone fabrics in Proterozoic basement rocks, which likely formed during the Timanian Orogeny. Conclusions: A NNE-dipping shear zone, which is part of a large system of Timanian thrusts in the Barents Sea, controlled the formation of WNW-ESE-striking Devonian-Mississippian normal faults and syn-tectonic sedimentary rocks in Billefjorden. Eurekan strain partitioning and decoupling suggest that the Svalbardian Orogeny did not occur in Svalbard.

Structure of the Assynt window, Moine Thrust Zone and relationship of thrusts to alkaline igneous complexes, Caledonian orogeny, NW Scotland

AbstractThe Moine Thrust Zone forms the Caledonian orogenic thrust front where the Moine Supergroup metamorphic rocks have been thrust westward across the Laurentia plate stable foreland, comprising Archean-Proterozoic granulite and amphibolite facies rocks (Lewisian gneisses), with unconformably overlying Mesoproterozoic and Neoproterozoic Torridonian clastic sediments and Cambrian-Ordovician passive margin sedimentary rocks. Four major thrusts beneath the Moine thrust in the Assynt window include the (i) Ben More Thrust, which places the Loch Ailsh syenite intruded into Lewisian basement and Cambrian-Ordovician sedimentary rocks over the Sole thrust sheet, (ii) Glencoul thrust, which places Lewisian basement and folded cover rocks over Cambrian-Ordovician sedimentary rocks, (iii) Borralan thrust, which carries a large alkaline syenite intrusion beneath the Ben More roof thrust and (iv) the Sole thrust sheet, which carries imbricated Cambrian-Ordovician sedimentary rocks and lamprophyre sills over the stable foreland. Three further thrust sheets within the Lewisian basement gneisses are now recognised through restoration of balanced cross-sections, which were responsible for doming of the Assynt window. Although the Moine thrust is mapped as a single line on the map it encompasses, (a) deep ductile shear zone formed of mylonites derived from hangingwall Moine schists, footwall Cambrian quartzites and Ordovician limestones, and basement Lewisian gneisses, (b) roof thrust of the Glencoul and Ben More Thrust sheets and (c) brittle out-of-sequence motion where the Moine schists have been thrust over mylonites, which directly overlie the stable foreland (Knockan Crag).

New Constraints on the Late Oligocene‐Miocene Thermo‐Tectonic Evolution of the Southeastern Tibetan Plateau From Low‐Temperature Thermochronology

AbstractThe southeastern (SE) Tibetan Plateau (Yunnan) is characterized by low‐relief uplands that were deeply incised by large rivers. The thermal history of basement rocks in this region remains poorly investigated, while this data is needed to elucidate the complex relationship between tectonics and climate in shaping the surface. To better understand its thermo‐tectonic evolution, we carried out apatite fission track thermochronology on 31 samples collected from a large area that covers different tectonic units, including a vertical profile in the middle Mekong River valley; additional zircon LA‐ICP‐MS U‐Pb dating was performed on four basement rocks. Our results confirm that a large portion of Mesozoic crystalline rocks constitute the basement of the SE Tibetan Plateau. Inverse thermal history modeling of fission track data reveal extensive late Oligocene to Miocene rapid basement cooling and exhumation episodes from both inside and outside the active zones (i.e., ductile shear zone and river valley). These thermal events were coincident with the activities of large‐scale strike‐slip faults that dominate the structural framework. Combined with the published data, we propose that widespread crustal shortening and thickening took place in the SE Tibetan Plateau during the Oligocene‐Miocene in the context of a compressive tectonic regime. Low‐temperature thermochronological data reveal that both tectonic forcing and climate‐driven erosion have played important roles in exhuming the basement rocks in the region. It is also deduced that the present‐day relatively low‐elevation landscape of the Yunnan area resulted from complex interaction between regional tectonic activity and surficial erosion since the late Oligocene.

Andean-type, bivergent crustal shortening in the Rinkian orogen: New constraints on the tectonic evolution of Laurentia–West Greenland in the Paleoproterozoic

Abstract The “Rinkian belt” of West Greenland is a metamorphic terrain of Paleoproterozoic age comprising: (1) the north Rinkian fold-thrust belt (FTB)—a pro- or fore-arc domain, highly deformed and metamorphosed with widespread anatexis; (2) the Prøven Igneous Complex—a magmatic arc characterized by hypersthene granitic rocks (“charnockites”); (3) the south Rinkian FTB—an inverted back-arc basin; and 4) a continental margin or foreland. Recognition of this tectonic architecture demonstrates that the “Rinkian” is a bona fide orogenic belt—the Rinkian orogen— and not simply the imbricated lower plate of the Nagssugtoqidian orogen. Arc plutons of the Prøven Igneous Complex were emplaced into the Karrat Group at ca. 1.90–1.85 Ga, dividing a back-arc basin into pro- and retro-arc domains. In the former—the north Rinkian FTB—WSW-directed thrusting (deformation events D1-D2) and high-grade metamorphism were taking place by ca. 1.875 Ga and were continuous through ca. 1.850 Ga with a peak temperature at ca. 1.830 Ma accompanied by anatexis in the Karrat Group and lower units of the Prøven Igneous Complex. In the retro-arc domain—the south Rinkian FTB—thrusting to the E (D1) began at ca. 1.870 Ma followed by thrusting to the W (D2) at ca. 1.830–1.820 Ga with displacement focused into a major high-temperature ductile shear zone which carried the Prøven Igneous Complex in the hanging wall of an Andean-type, crustal-scale, “pop-up” structure. High-temperature deformation continued during D3 when the pro-arc, arc, and retro-arc domains were shortened by bivergent detachment folding and thrusting at ca. 1.820–1.810 Ga.

Decoupled Oligocene mylonitic shearing and Miocene detachment faulting in the East Humboldt Range metamorphic core complex, northeast Nevada, USA

Abstract The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension, thus explaining the spatial relationship between structures.

Extreme ductile thinning of Cambrian marbles in the Northern Snake Range metamorphic core complex, Nevada, USA: Implications for extension magnitude and structural evolution

The detailed stratigraphy of rocks within the Northern Snake Range metamorphic core complex provides an exceptional opportunity to investigate the geometry and magnitude of ductile strain during high-magnitude continental extension. In the northern part of the range, Middle-Late Cambrian marbles in the footwall of the Northern Snake Range dècollement (NSRD), which have a stratigraphic thickness of 1107 ± 107 m in adjacent ranges, were thinned during Late Eocene-Late Oligocene ductile extension. From west to east across the range, these rocks have been thinned from 869-935-m-thick (15–21% structural thinning) to 54-88-m-thick (92–95% structural thinning) across a 12 km lineation-parallel distance. Ductile extensional strain was accompanied by the development of pervasive linear-planar fabrics and produced megaboudins of calcareous schist units that are 100-500-m-long, 15-25-m-thick, and separated by as much as 1 km. The magnitude of lineation-parallel ductile extension increases eastward across the range from 24 ± 21% to 1226 ± 256%, and total ductile extension across the range is 12.1 ± 2.2 km (167 ± 31%). Quartz recrystallization microstructures and published calcite-dolomite thermometry indicate deformation temperatures of ∼400–550 °C during initial Late Eocene-Late Oligocene ductile extensional shearing. West to east across the range, rocks in the NSRD footwall experienced a progressively longer ductile extensional strain history and a prolonged residence time at higher temperatures, which was aided by the eastward migration of denudation-related cooling and was likely enhanced by strain heating and/or a pre-extensional eastward dip of footwall rocks. These factors promoted the development of the extreme strain gradient.

Early Neoproterozoic tectonics in the Marwar Crustal Block, NW India, the relevance of the Phulad Shear Zone, and implications for Rodinia reconstruction

Abstract Palaeomagnetic studies in the Malani Igneous Suite (ca. 770–750 Ma) of the Marwar Crustal Block, NW India, provide essential constraints on palaeogeographic reconstructions of the Rodinia Supercontinent. The Malani Igneous Suite is intrusive into megacrystic granite and granitegneissic enclaves of the Marwar Crustal Block. A crustalscale NE–SW ductile transpressional shear zone with a southeasterly dip known as the Phulad Shear Zone (820–810 Ma) separates this Marwar Crustal Block from the rocks farther east. The preshearing tectonic evolution of the Marwar Crustal Block is poorly understood. Three phases of ductile deformations (D1, D2, and D3, with associated fabrics S1, S2, and S3) were identified in the Marwar Crustal Block. The D1 deformation is restricted to enclave gneisses. The megacrystic granite was emplaced syntectonically during D2 deformation. The S2 magmatic foliation (NNW–SSE) in the megacrystic granite is oblique to the Phulad Shear Zone. The D3 deformation in the megacrystic granite is synchronous with the Phulad Shear Zone mylonites. Another porphyritic granite (Phulad granite, ca. 820 Ma) emplaced syntectonically during D3 deformation along and across the Phulad Shear Zone. U-Pb zircon dates in the Marwar Crustal Block document ca. 890 Ma and ca. 860 Ma magmatic dates. U-Pb-Th monazite dates in the Marwar Crustal Block show a strong peak at ca. 820 Ma. By integrating critical field relations, deformational patterns, and geochronology, we argue that the Marwar Crustal Block shows distinct geological evolution prior to its accretion with the remaining parts of India. We propose that the accretion of the Marwar Crustal Block must be younger than ca. 860 Ma and culminated during ca. 820–810 Ma to form the Greater India landmass along the Phulad Shear Zone.

Constraining the evolution of shear zones in the Himalayan mid crust in Central–Western Nepal: implications for the tectonic evolution of the Himalayan metamorphic core

AbstractStructural analysis, petrochronology and metamorphic petrology enable identification and bracketing of the timing of a newly mapped high-temperature ductile shear zone (Jagat Shear Zone (JSZ)) in the Himalayan metamorphic core in Central-Western Nepal. In situ U-Th-Pb monazite petrochronology constrains the timing of top-to-the-S/SW shearing between 28–27 Ma and 17 Ma. Burial and prograde metamorphisms in footwall rocks were linked to thrust-sense movement along the JSZ, while the hanging wall rocks were retrogressed and exhumed. The identification and age of the JSZ (as part of a regional system of shear zones: the High Himalayan Discontinuity (HHD)) coupled with the localization and timing of activity of the Main Central Thrust (MCT) (i) fills a gap in tracing the HHD along orogenic strike, (ii) supports the identification of the position and timing of the long-debated MCT and (iii) helps to place the boundaries of the Himalayan metamorphic core and its internal architecture. Thus, our study is a significant step towards a precise identification of the burial, assembly and exhumation mechanisms of the Himalayan metamorphic core.

Stratigraphic and mineralization age, Sr–Nd isotope constraints on the source of fluids in the Yindonggou Ag–Au(–Pb–Zn) deposit, Qinling Orogen, China

The Yindonggou deposit, located in the northwestern margin of the Wudang Block, is the largest Ag–Au deposit (reserves: 2000 t Ag and 20 t Au) in the area. Quartz vein- and altered rock-type orebodies are hosted in the meta-volcanic rocks of the Wudang Group, and are controlled by the Yindongyan anticline and ductile shear zone along the anticlinal axis. The metamorphosed quartz keratophyre tuff from the Wudang Group yield LA-ICP-MS zircon concordant 206Pb/238U age of 738.30 ± 5.90 Ma, indicating a Neoproterozoic volcanic event. Four stages of mineralized quartz veins were found in the deposit. One hydrothermal sericite sample collected from the stage 2 smoke-gray quartz Ag–Au veins yield a well-defined 40Ar/39Ar isotopic plateau age of 234.65 ± 3.51 Ma, and 39Ar/36Ar-40Ar/36Ar normal isochron age of 220.11 ± 14.32 Ma, suggesting a Middle to Late Triassic (ca. 234 Ma) mineralization at Yindonggou. The difference between diagenetic and mineralization ages indicates that the Yindonggou deposit is not a volcanic eruption-related deposit, but a vein type epigenetic ore deposit. Five wall-rock samples from the Wudang Group of the Yindonggou deposit yield ISr(234 Ma) ratios of 0.714890–0.723704, averaging 0.719938; the (143Nd/144Nd)i values ranging from 0.512018 to 0.512078, averaging 0.512055, with the εNd(234 Ma) values of −6.22 to −5.05. Galena from stage 1 have ISr(234 Ma) values of 0.710801–0.718999, (143Nd/144Nd)i values of 0.511954–0.512234, and εNd(234 Ma) values of −7.47–2.01. Pyrite and sphalerite from stage 2 yield ISr(234 Ma) ratios of 0.704389–0.719884, (143Nd/144Nd)i values of 0.511920–0.512402 and εNd(234 Ma) values of −8.13 to 1.28. Both εNd(234 Ma) and ISr(234 Ma) values of the sulfides indicate that the ore-forming fluids were contaminated by wall rocks from the Wudang Group. The early stage sulfides are representative of the initial ore-forming fluids, which is similar to metamorphosed basic rocks and Mian-Lue ophiolite. This indicates that the deep-sourced initial ore-forming fluids with high εNd(234 Ma) and low ISr(234 Ma) were derived from the metamorphic devolatilization of the subducted oceanic crust. The Yindonggou is an orogenic-type deposit formed in Triassic oceanic subduction-related accretionary orogeny.