Sinistral Shear Research Articles

A suture related accretionary wedge in the Gondwana assembly: Insights from serpentinites in the Hoggar shield, Algeria

The assembly of West Gondwana supercontinent involved several complex processes that led to the formation of the Hoggar shield during the Panafrican-Brasiliano event. We present the first petro-geochemical, geochronological and field data of serpentinite lenses exposed along the In-Tedeini domain, within deformed talc-schist and magmatic-sedimentary formations. The serpentinites preserved the main characteristics of their parent peridotites (Al/Si: 0.004–0.03; Mg/Si: 1.14–1.62; Al2O3: 0.15–1.37 wt%; Mg#: 85.3–94; Ti: 9.34–120 ppm; Nb: 0.007–0.46 ppm), attesting to a highly depleted mantle wedge protolith involved in a subduction zone. This is in agreement with the high Cr# (0.55–0.6), low to moderate Mg# (0.36–0.65) and low TiO2 contents (<0.1 wt%) of their constitutive Cr-spinels. Geochemical modelling suggest that both the North and South In-Tedeini serpentinite units have experienced intense and similar fluid-induced dynamic melting episodes. The evolution of these two units has diverged, with the Southern In-Tedeini unit being refertilized by a small volume of island-arc basaltic melts generated in the mantle wedge. Serpentinization of these rocks probably occurred under static conditions at high temperature corresponding essentially to amphibolite-facies conditions. Field relations suggest that the exhumation of the massive serpentinites occurred along major sinistral shear zones steeply dipping eastward, assisted by talc-schists that highly localized transpressive deformation. First U-Pb zircon ages obtained from a metasomatic chloritite in North In-Tedeini serpentinites; they may have recorded the age (770 ± 5 Ma) of the subduction related Panafrican island arcs and the emplacement (631 ± 10 Ma) of In-Tedeini serpentinites within the crust; or they may rather correspond to the serpentinization events endured by the rocks. All together, the reported results support the presence of a major suture zone, oriented NNW-SSE. This suture is outlined by mantle serpentinite lenses exhumed in a collisional accretionary wedge, which connects the western and the central Hoggar, following a Panafrican east-dipping subduction. This tectonic system would have contributed to the closure of the Goiás-Pharusian ocean.

The Borborema Strike-Slip Shear Zone System (NE Brazil): Large-Scale Intracontinental Strain Localization in a Heterogeneous Plate

Abstract Large-scale strike-slip faults are fundamental tectonic elements of the continental lithosphere. They constitute plate boundaries (continental transforms), separate terranes with contrasting geological histories within accretionary orogens, or accommodate heterogeneous deformation in intracontinental settings. In ancient orogens, where deeper levels of the crust are exposed, these faults are expressed as shear zones materialized by up to tens of km-wide mylonitic belts. The Borborema shear zone system in northeastern Brazil is one of the largest and best-exposed intracontinental strike-slip shear zone systems in the world, cropping out over 250,000 km2. Here, we review its main geophysical, structural, petrologic, and geochronologic characteristics and discuss the factors controlling its development. This complex continental scale shear zone system is composed of a set of NE- to NNE-trending dextral shear zones from which there are two major E-trending dextral shear zones with horse-tail terminations into the transpressional belt branch, as well as several smaller E-trending dextral and NE-trending dextral and sinistral shear zones. The major shear zones are marked by extensive linear or curvilinear magnetic gradients, implying their continuation at depth. The major shear zones are materialized by migmatite to amphibolite-facies mylonites, but the entire system shows evidence of late deformation at lower temperatures. The system developed during the late stages of the Neoproterozoic Brasiliano (Pan-African) orogeny (mainly from 590 to 560 Ma), postdating by more than 20 Ma the main stage of contractional deformation. Localization of strike-slip shearing in this intraplate setting was controlled by rheological contrasts between blocks with distinct Paleoproterozoic histories, the presence of preorogenic Neoproterozoic rifts, the craton geometry, and zones of enhanced magmatic activity, highlighting the importance of rheological heterogeneity in controlling shear zone nucleation and evolution.

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.

Microstructure and Crystallographic Preferred Orientations of an Azimuthally Oriented Ice Core from a Lateral Shear Margin: Priestley Glacier, Antarctica

A 58 m long azimuthally oriented ice core has been collected from the floating lateral sinistral shear margin of the lower Priestley Glacier, Terra Nova Bay, Antarctica. The crystallographic preferred orientations (CPO) and microstructures are described in order to correlate the geometry of anisotropy with constrained large-scale kinematics. Cryogenic Electron Backscatter Diffraction analysis shows a very strong fabric (c-axis primary eigenvalue ∼0.9) with c-axes aligned horizontally sub-perpendicular to flow, rotating nearly 40° clockwise (looking down) to the pole to shear throughout the core. The c-axis maximum is sub-perpendicular to vertical layers, with the pole to layering always clockwise of the c-axes. Priestley ice microstructures are defined by largely sub-polygonal grains and constant mean grain sizes with depth. Grain long axis shape preferred orientations (SPO) are almost always 1–20° clockwise of the c-axis maximum. A minor proportion of “oddly” oriented grains that are distinct from the main c-axis maximum, are present in some samples. These have horizontal c-axes rotated clockwise from the primary c-axis maximum and may define a weaker secondary maximum up to 30° clockwise of the primary maximum. Intragranular misorientations are measured along the core, and although the statistics are weak, this could suggest recrystallization by subgrain rotation to occur. These microstructures suggest subgrain rotation (SGR) and recrystallization by grain boundary migration recrystallization (GBM) are active in the Priestley Glacier shear margin. Vorticity analysis based on intragranular distortion indicates a vertical axis of rotation in the shear margin. The variability in c-axis maximum orientation with depth indicates the structural heterogeneity of the Priestley Glacier shear margin occurs at the meter to tens of meters scale. We suggest that CPO rotations could relate to rigid rotation of blocks of ice within the glacial shear margin. Rotation either post-dates CPO and SPO development or is occurring faster than CPO evolution can respond to a change in kinematics.

Two-stage strike-slip faulting of the Altyn Tagh Fault revealed by magnetic fabrics in the Qaidam Basin

The timing and mechanism of the changes in the stress/strain field direction on the northeastern Tibetan Plateau are controversial, which limits our understanding of the tectonic evolution of the northeastern Tibetan Plateau and the Altyn Tagh Fault (ATF). Here, we conducted a detailed anisotropy of low-field magnetic susceptibility analysis of successive fluvial-lacustrine sediments deposited since the early Oligocene at Eboliang in the middle-western Qaidam Basin. Magnetic fabrics of the upper part (the Qigequan Formation) are predominantly considered to be primary sedimentary magnetic fabrics, while those of the lower-middle parts (the Shangganchaigou to Shizigou Formations) are mainly pre-folding, layer-parallel shortening induced embryonic tectonic fabrics. The magnetic lineation revealed ~50° successive clockwise rotations of the compressional strain orientations from nearly N-S to NE-SW in the middle-western Qaidam Basin during ~15–7 Ma. The nearly N-S compressional event is consistent with the generally N-S-directed India-Asia collision and thus was attributed to its far-field effects, whereas the post-mid-Miocene transition of ~50° clockwise rotations of the compressional strain orientations is consistent with the tectonic deformation in the basin interior. This transition lagged far behind that in the northern Qaidam marginal thrust belt (NQMTB), which began in the early Oligocene. The laterally diachronous transition in compressional strain orientation and deformations in the middle-western and northern Qaidam Basin were most likely due to the two-stage, sinistral strike-slip faulting of the ATF, with confined sinistral shear occurring along the ATF and partially transferred into the NQMTB during the early Oligocene to middle Miocene, while distributed sinistral shear occurred on the northeastern Tibetan Plateau beginning in the middle Miocene. A bidirectional propagation of compressional strain variation and tectonic deformation from the northern Qaidam Basin since the early Oligocene is proposed to reflect the tectonic evolution of the northeastern Tibetan Plateau.

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.

Crustal deformation across the western Altyn Tagh fault (86° E) from GPS and InSAR

SUMMARYWe combine Global Positioning System (GPS) velocity field with Interferometric Synthetic Aperture Radar (InSAR) results to study the interseismic deformation across the western Altyn Tagh fault (ATF) at longitude 86° E. GPS and InSAR data are consistent after correcting for the contribution from vertical deformation in the InSAR line-of-sight map. InSAR and GPS data identify an area of ∼2 mm a−1 sinistral shear and ∼6 mm a−1 of NS shortening located ∼150 km south of the ATF near the Manyi fault system. Excluding the data located in that area, Bayesian inversion of a 2-D profile across the ATF indicate a locking depth of 14.8 ± 3.5 km and a slip rate of 8.0 ± 0.4 mm a−1, lying at the lower range of previously published estimates. In addition, we find no significant offset between the fault at depth and the surface fault trace and no asymmetry of the interseismic profile that implicitly reveal lateral variations of the elastic strength across the ATF. Detailed analysis of InSAR profile across the fault show no distinguishable surface creep along the western ATF. Our study highlights how different data sets, data selection and model assumption might impact results on the ATF slip rate, locking depth and rheological contrast across the fault.

Reactivated shear zones: A case study in a tectonic superposition zone between the Southern Brasília and Ribeira orogens, southeastern Brazil

New data are presented related to the evolution of major shear zones in the Neoproterozoic Southern Brasília and Ribeira orogens, considering their attitude, kinematics and metamorphic conditions. The integration and analysis of geological maps, aerogeophysical and new geochronological U-Pb data, allowed to unravel the kinematics and reactivation of these shear zones. The first major event related to the collision of the São Francisco and Paranapanema paleocontinents is represented by the Alterosa Suture Zone (ASZ), interpreted as the suture of the Southern Brasília Orogen, which is a low to medium angle shear zone with mylonitic fabric indicative of granulite facies conditions. Geochronological analyzes in zircon from this zone provided three age peaks of 630 Ma, 610 Ma and 600 Ma associated with the collision of this orogen. A second regional event related to the Ribeira Orogen was responsible for the formation of steep NE-SW shear zones of low to high metamorphic grade and the local reactivation of the ASZ. Metamorphic zircon from one of these shear zones yielded an age of 574 ± 2 Ma, with minor rim growth and/or lead loss at 553 ± 2 Ma and 524 ± 4 Ma. The age of 574 ± 2 Ma fits well in the peak activity period of the Ribeira Orogen (600-560 Ma). The ages of 553 ± 2 Ma and 524 ± 4 Ma probably reflect reactivation. Several of these shear zones show evidence of original sinistral and later dextral kinematics. The paper also presents a new structure resulting from this dextral reactivation of former sinistral shear zones, which is named here “arrow tip structure”.

Structural evolution of the Kanggur tectonic belt: implications for the latest carboniferous to Permian orogeny in the Eastern Tianshan, NW China

ABSTRACT The E-W Kanggur tectonic belt, a major tectonic belt in the Eastern Tianshan (ETS) of the Central Asian Orogenic Belt (CAOB), records the amalgamation history between the ETS and the Tarim Craton. Two generations of structural deformation (D1 and D2) are recognized by field observations in the Jingerquan, Huangshan-Jingxia, Yamansu North Mountain, and Nanbeidagou-Kanggurtag areas. Early deformation is exhibited by vertical ‘A’ type (asymmetrical and symmetrical) S0 folds, lenticular symmetrical boudins, S-C fabric, and nearly vertical S1 foliation, demonstrating an ~N-S compressional setting. Late deformation is characterized by ~E-W-trending dextral shearing and local S1 folding. Transpressional deformation explains the kinematic compatibility of these fold and shear structures. The new U-Pb zircon and Ar/Ar age data in this study provide constraints on the timing of deformation, suggesting that D1 occurred in the late Carboniferous-early Permian (300–286 Ma) and that dextral shearing (D2) likely occurred in the middle-late Permian (262–233 Ma). The ~N-S contraction was driven by orogenic thickening during the convergence of the North Tianshan (NTS) island arc system, the Central Tianshan (CTS), and the Tarim Craton. Dextral transpression is related to the oblique convergence between the Eastern Tianshan (ETS) and Tarim Craton. On a larger scale, dextral kinematics along the Main Tianshan Shear Zone in the Eastern and Western Tianshan, together with sinistral shearing in northeastern Kazakhstan and the Chinese Altai, accommodated the eastward escape of central orogenic materials, which were possibly affected by the convergence of the Siberian, Baltic, and Tarim Cratons.

Geology and geochronology of the Tokuzbay gold deposit in the Chinese Altai: A case study of collision-related orogenic gold deposits in Central Asian Orogenic Belt

Orogenic gold deposits are one of the most important gold deposits in the world. However, it is controversial that orogenic gold deposits are only generated at subduction-accretion or both in collisional setting. The Altai Orogen experienced a complex subduction accretion–collision history, including magmatism and metamorphism, and forming giant gold deposits. The Tokuzbay gold deposit consists of quartz vein ores and disseminated ores hosted in dikes. The ore bodies are controlled by secondary shear zone of the NW-, NNW-trending Ma’erkakuli Fault and occurred in meta-sediments, i.e., meta-sandstone and phyllite of Devonian Altai Formation, and diorite dikes and granite porphyritic dikes. Wall rock alteration is represented by silicification, pyritization, and muscovite alteration. The ores are restricted by ductile and brittle deformation in the shear zone. Large amounts of native gold and auriferous tellurium were precipitated during a transition from ductile to brittle regime. These characteristics indicate that the Tokuzbay gold deposit is an orogenic gold deposit in the Altai Orogen. The zircon LA-ICP-MS U-Pb ages of the ore-bearing granite porphyritic dikes and diorite dikes are 390 ± 2.5 Ma (MSWD = 0.87) and 378 ± 4 Ma (MSWD = 0.21), respectively, indicating that the magmatic rocks were emplaced during subduction of the Paleo Asian Ocean beneath Altai Orogen. Muscovite samples associated with gold from the auriferous quartz vein and disseminated ores hosted in diorite dikes returned Ar-Ar ages of 292.7 ± 1 Ma (MSWD = 1.3) and 293.9 ± 1.2 Ma (MSWD = 0.7), indicating that gold mineralization in quartz vein is synchronized with disseminated ores. The gold mineralization is restricted to the Early Permian and overlaps with the collision between Altai Orogen and Junggar Plate. The ore-hosting magmatic rocks related to subduction are more than 100 million years earlier than the ore-forming events. Combined with the regional tectonic evolution and the shear characteristics of the gold district, it is well proved that sinistral shearing trigged by the collision of Junggar Plate and Altai Orogen contributed to gold mineralization of the Tokuzbay deposit. Considering the Altai gold deposits in Kazakhstan, it is revealed that Permian collision between Junggar Plate and Altai Orogen triggered a large number of orogenic gold mineralization.

Polyphase gold mineralization at the Yaou deposit, French Guiana

Abstract The Yaou deposit, located in French Guiana within the Guiana Shield, is one of the most promising gold deposits of the regional Paleoproterozoic greenstone belt. It displays numerous quartz monzodiorite bodies aligned along a sinistral shear zone where a five-deformation phases model is established at the camp scale. The ductile D 1/2YA phase is responsible for the main penetrative foliation while the D 3YA phase is related to shearing. An intrusive event is identified as being pre to syn-D 3YA . The following phase D 4YA represents a brittle quartz-carbonate veining set hosted preferentially within intrusive bodies and along the shear zone. A local D 5YA brecciation event crosscuts the D 4YA veins. Among this deformation history, two auriferous events (D 3YA and D 4YA ) control the overall grade of the Yaou gold deposit. More specifically, most of the Au grade is associated with the main economic D 4YA veining event, where the gold is visible and linked to Py 4 within an ankerite/hematite rich alteration halo. At the microscopic scale, results of in situ analyses using LA-ICP-MS on pyrite show that metasediment-hosted Py 0 is a primary source of submicroscopic gold having a low contribution to the total endowment. Py 3 shows some gold content due to possible remobilization of Au D0YA . Gold in Py 4 is found as submicroscopic gold, as micro-inclusions and as infilling fractures in association with elements such as Te, Ag and Bi. Most contribution to the Au grade is from micro-inclusions and, to a lesser extent, from free and submicroscopic gold. The ore shoot locations are lithologically controlled for Au D0YA (metasedimentary unit-hosted), structurally controlled (shear zone-hosted) for Au D3YA and rheologically controlled for the Au D4YA (intrusion-hosted). The deposit is clearly polyphase both at the macroscopic and the microscopic scales, invisible gold is associated with As whereas visible gold is observed as inclusions in pyrite with high contents of Ag, Te and Bi. We define an early low-grade enrichment of Au D0YA to Au D3YA followed by a later high-grade event, Au D4YA supporting polyphase mineralization processes. This study confirms that orogenic gold deposits can be formed by remobilization and/or new gold inputs during multiple deformation, veining and hydrothermal events.

A Lower Devonian age for the Corvock Granite and its significance for the structural history of South Mayo and the Laurentian margin of western Ireland

The Silurian Croagh Patrick succession, which crops out just south of a fundamental Caledonian structural zone near Clew Bay, western Ireland, is a series of psammites and pelites with a strong penetrative cleavage. These rocks are intruded by the Corvock granite. A suite of minor intrusions associated with the granite contains the regional cleavage whereas the Corvock granite is undeformed. New U–Pb dates are 413 +7 / –4 Ma for a strongly cleaved sill and 410 ± 4 Ma for the main granite, and closely constrain the age of crystallization of the granite and coeval cleavage formation as Lower Devonian (Lochkovian or Pragian), implying syn- to late-kinematic granite emplacement. These data are consistent with evidence for strong sinistral shear shown by the Ox Mountains granodiorite just to the northeast dated at 412.3 ± 0.8 Ma. This Devonian cleavage is superimposed on Ordovician rocks of the South Mayo Trough. The localisation of the strong deformation is interpreted as being due to its position at a restraining bend during regional sinistral motion on a segment of the Fair Head – Clew Bay Line to the north. Contemporaneous deformation in the syn-kinematic Donegal batholith suggests a transfer of sinistral motion to this intra-Grampian structure rather than simple along-strike linkage to the Highland Boundary Fault in Scotland. Our new data indicate diachronous deformation during the late Silurian and early Devonian history of the Irish and Scottish Caledonides and also support previous interpretations of diachronous deformation between these areas and the Appalachian orogens.

Tectonic record and gold mineralization in the central part of the Neoproterozoic Keraf suture, Gabgaba district, NE Sudan

The central Keraf suture recorded complex lithospheric-scale deformation associated with the formation of the Neoproterozoic Arabian-Nubian shield and hosts the WG03, Central Zone and UTM gold deposits, Gabgaba district, NE Sudan. This study aims at (i) unravelling the tectonic history recorded by the central Keraf suture and (ii) interpreting the spatial and genetic relationships between structural setting and gold mineralization. To this end, we coupled field petrographic observations and structural measurements made along a 50 km-long geological cross-section through the Gabgaba district with macroscopic and microscopic description of mineralized structures.The structural setting of the central Keraf suture is dominated by progressive deformation, from ductile D1a Keraf thrusting to ductile-brittle D1b sinistral shearing in a prolonged northwest-orientated convergence regime. A first mineralization event, leading to a minor gold endowment, relates to magmatic accretion pre-dating D1a Keraf suturing. The second gold episode is characterized by ore veining as a response to progressive late collisional D1b Keraf strike-slip shearing and may be classified as orogenic gold. Most of the gold budget of the Gabgaba district formed at this stage.In summary, the central Keraf suture exemplifies that the Arabian-Nubian shield contains significant polyphase gold mineralization along lithospheric-scale structures. These structures may record a complex tectonic evolution from magmatic to collision, with a strong structural control on gold endowment.

Deformation, Magmatism, and Sulfide Mineralization in the Archean Golden Mile Fault Zone, Kalgoorlie Gold Camp, Western Australia

AbstractThe Golden Mile fault zone is a key controlling structure to the estimated 75 Moz gold endowment of the Kalgoorlie gold camp in the Yilgarn craton of Western Australia. The earliest structures in the fault are F1 folds that developed during D1 recumbent-fold and thrust deformation (&amp;lt;2685 ± 4 Ma). These F1 folds are overprinted by a pervasive NW- to NNW-striking S2 cleavage related to sinistral shearing beginning with 2680 ± 3 Ma D2a sinistral strike-slip and culminating with ca. 2660 Ma D2c sinistral-reverse movement. The majority of deformation in the fault zone correlates to ca. 2675 Ma D2b deformation, which is characterized by sinistral-normal kinematic indicators. Late, ca. 2650–2640 Ma D3 dextral-reverse kinematic indicators overprint the earlier D2 structures. Pyrrhotite-chalcopyrite-pyrite-sphalerite-galena assemblages were emplaced throughout the D2 event within NE-trending D2a tensile fractures, NW- to NNW-striking D2b normal faults and associated breccias, and NW- to NNW-striking D2c low-angle veins, with the latter D2b and D2c structures correlating to the Fimiston and Oroya mineralization types, respectively. All D2a-, D2b-, and D2c-related sulfides in the Golden Mile fault zone show similarly restricted δ34S (~1.0–4.5‰) and elevated Δ33S (~2.0–3.0‰) values that reflect strong local sulfur contribution from shales of the Lower Black Flag Group and host-rock buffering of hydrothermal fluids related to the Fimiston and Oroya mineralization events. This host-rock buffering decreased fluid fO2, favoring the development of pyrrhotite-pyrite stable sulfide assemblages and causing respective decreases and increases in fluid Au-Te and Pb-Bi-Sb concentrations. At the camp scale, the Golden Mile fault zone exerted a primary control on the distribution of porphyry dikes and gold deposits; however, magma and hydrothermal fluid circulation was favored in adjacent, higher-order structural sites due to the fault zone’s incompetent rheology and tendency for ductile deformation and diffuse fluid flow. Other Archean examples such as Au deposits of the Larder Lake-Cadillac deformation zone in the Superior craton illustrate that this type of diffuse fluid flow in large-scale crustal fault zones can result in disseminated economic mineralization. However, this study highlights that host-rock effects on fluid chemistry in large-scale crustal fault zones exercises a strong control on a fluid’s propensity to form ore. The results of this study emphasize that both the rheology and chemistry of rocks within and adjacent to large-scale deformation zones act as important controls on the formation of gold ore in Archean terranes.

Kinematic analysis of the Dschang granitic pluton (West-Cameroon): Implications to the Pan–African deformation of the Central African fold belt in Cameroon during the post-collisional history of western Gondwana

The Pan-African (578–563 Ma, U-Pb zircon age) Dschang granitic pluton belongs to the edge of the central domain of the Central African Fold Belt in Cameroon. It is made up of biotite granites that intruded pre-Pan-African biotite-hornblende orthogneiss basement. A new detailed structural study testifies that this area recorded four deformation phases (D1-D4). The D1-related structures described in the basement rocks are folded. They were progressively transposed into the main S2 foliation during D2 deformation which is synchronous with the emplacement of E-W granitic dykes. The D2 strikes NNW-SSE to NW-SE. It is characterized by main S2 metamorphic foliation (mean pole of the whole data at 269°/66°), NW–SE low to moderate plunging lineations (mean lines of the whole data at 159°/15°), F2 folds with NW plunging axes, C2 sinistral E-W ductile shear indicators (marked by injection of granite dykes into the biotite-hornblende orthogneiss). The D3 phase is marked in the biotite-hornblende orthogneiss by C3 dextral shear planes, followed by dextral rotated boudins (ß3). In the granites, it is characterized by S3 magmatic foliation (mean pole of the whole data at 323°/61°) due to the reorientation of platy megacrystals of K-feldspar, F3 folds profile with moderate plunging axes (mean lines of the whole data at 145°/28°) and sinistral vergence. The D4 phase is usually ductile. It is characterized by minor faults with C4 sinistral shear planes which crosscut the S3 magmatic foliation. The D2 phase is synchronous with the early emplacement of the Dschang granites at 578 Ma whereas D3 marks its end at 563 Ma. Mesoscopic (phenocrysts alignment, elongated enclaves, schlierens) and microscopic features (quartz, plagioclase and alkali feldspar deformation microstructures) are consistent with a continuum of deformation from magmatic to submagmatic states in the Dschang granites, the later state being more common. We conclude that the history of the Dschang area began with the emplacement of biotite-hornblende orthogneiss before 578 Ma; it was followed by the emplacement of granite that started towards the end of the D2 phase at 578 Ma (probably aided by the E-W sinistral shear fractures) and ended during the D3 phase at 563 Ma. This is consistent with the emplacement of the Dschang granitic pluton during the transition period between the sinistral left lateral wrench movement and the dextral right lateral wrench movement of the history of western Gondwana. However, the scarcity of typical structures of a synkinematic intrusion or a post-magmatic ductile shearing (mylonites, S/C-structures) in the study granite suggests its emplacement into a low strain crustal domain within an anastomosing pattern of shear zones.

Remote sensing and structural studies of the Central Asir Shear Zone, Western Arabian Shield: Implications for the late Neoproterozoic E-W Gondwana assembly

The Central Asir Shear Zone (CASZ) is a N- to NNE-oriented inhomogeneous strain zone parallel to the Nabitah shear belt in southern Saudi Arabia. It is ~200 km long and 20 km wide, and traverses the Asir terrane. The deformed terrains in the vicinity of the CASZ have experienced a prolonged history involving three successive phases of deformation: arc-collision, dextral transpression and sinistral strike-slip faulting. Remote-sensing data analysis and field investigation provide insights into the geological and structural features along the CASZ. Image transformation and enhancement reveal faults and folds, and discriminate between granitoids, metavolcanics, gabbros and younger (Ablah Group) sediments. The obtained evidence points to polyphase deformation of the area, characterized by narrow zones of high strain, folding and shearing enclosing slices of less deformed rocks. Lineament analysis shows predominantly NNE, NNW, N and NE trends of folds and shear zones, and NW, NE and E fracture trends. The CASZ has a >100 m.y. long deformation history, including a change from dextral to sinistral shearing. Both shearing senses are found on the older N-S and younger NW-SE fault systems. N-S oriented shearing was accompanied by intrusion of granitoids and localized hydrothermal alteration along N to NNE trending shear zones. Strain analysis shows low strain in the Ablah basin sediments, consistent with Ablah deposition after the tectonic climax in the region.

Two key switches in regional stress field during multi-stage deformation in the Carboniferous–Triassic southernmost Altaids (Beishan, NW China): Response to orocline-related roll-back processes

AbstractThe orogenic architecture of the Altaids of Central Asia was created by multiple large-scale slab roll-back and oroclinal bending. However, no regional structural deformation related to roll-back processes has been described. In this paper, we report a structural study of the Beishan orogenic collage in the southernmost Altaids, which is located in the southern wing of the Tuva-Mongol Orocline. Our new field mapping and structural analysis integrated with an electron backscatter diffraction study, paleontology, U-Pb dating, 39Ar-40Ar dating, together with published isotopic ages enables us to construct a detailed deformation-time sequence: During D1 times many thrusts were propagated northwards. In D2 there was ductile sinistral shearing at 336–326 Ma. In D3 times there was top-to-W/WNW ductile thrusting at 303–289 Ma. Two phases of folding were defined as D4 and D5. Three stages of extensional events (E1–E3) separately occurred during D1–D5. Two switches of the regional stress field were identified in the Carboniferous to Early Permian (D1-E1-D2-D3-E2) and Late Permian to Early Triassic (D4-E3-D5). These two switches in the stress field were associated with formation of bimodal volcanic rocks, and an extensional interarc basin with deposition of Permian-Triassic sediments, which can be related to two stages of roll-back of the subduction zone on the Paleo-Asian oceanic margin. We demonstrate for the first time that two key stress field switches were responses to the formation of the Tuva-Mongol Orocline.

Development and Interplay of Two Orthogonal Fault Systems in the NW Qaidam Basin, Northern Tibetan Plateau

How fold and thrust belts (FTBs) evolve over time and space in a transpressional regime remains poorly understood. Based on high-resolution 3D seismic reflection data and remote sensing images, we herein present a detailed structural analysis of the Cenozoic faults in the NW margin of the Qaidam Basin that is bounded to the north by the left-reverse Altyn Tagh fault system. Two sets of orthogonal, basement-involved faults with contrasting geometries, kinematics, and temporal development are identified. One set consists of generally E-W-striking, N-dipping, reverse faults with a component of sinistral shear. They are parallel or subparallel to the Altyn Tagh fault system, led to southward tilting of the basement, and formed a local unconformity between the middle Miocene Shangyoushashan formation and underlying strata. They developed in an out-of-sequence order, and were mostly active during 43.8–15.3 Ma but in relatively tectonic quiescence with limited weak reactivation since then. The second set is mainly composed of the NNW-striking reverse faults with dextral shear components. They are approximately perpendicular to the Altyn Tagh fault system, and intensively active since ∼15.3 Ma, much later than the initiation of the E-W-striking faults. Together with published results, we ascribe the development of these two sets of orthogonal faults as the transition from transpression to left lateral slip on the central segment of the Altyn Tagh fault system. The two fault sets interplayed with each other in two ways: 1) the older E-W-striking faults were offset by younger NNW-striking faults, and 2) the younger NNW-striking faults curved to link with the preexisting E-W-striking faults. Our findings reveal that transpressional-dominated FTBs evolve in a more complicated way than the contractional-dominated ones, and more site-based case studies are needed to reveal the underlying primary principles.

Magnitude, timing, and rate of slip along the Atacama fault system, northern Chile: implications for Early Cretaceous slip partitioning and plate convergence

Displacement estimates along the Atacama fault system (AFS), a crustal-scale sinistral structure that accommodated oblique convergence in the Mesozoic Coastal Cordillera arc, vary widely due to a lack of piercing points. We map the distribution of plutons and mylonitic deformation along the northern c. 70 km of the El Salado segment and use U–Pb geochronology to establish the slip history of the AFS. Along the eastern branch, mylonitic fabrics associated with the synkinematic c. 134–132 Ma Cerro del Pingo Complex are separated by 34–38 km, and mylonites associated with a synkinematic c. 120–119 Ma tonalite are separated by 20.5–26 km. We interpret leucocratic intrusions to be separated across the western branch by c. 16–20 km, giving a total slip magnitude of c. 54 ± 6 km across the El Salado segment. Kinematic indicators consistently record sinistral shear, and zircon (U–Th)/He data suggest dip-slip motion was insignificant. Displacement occurred between c. 133–110 Ma at a slip rate of c. 2.1–2.6 km Myr –1 . This slip rate is low compared to modern intra-arc strike-slip faults, suggesting (1) the majority of lateral slip was accommodated along the slab interface or distributed through the forearc or (2) plate convergence rates/obliquity were significantly lower than previously modeled. Supplementary material: Full U-Pb, (U-Th)/He, petrographic, and structural data with locations is available at https://doi.org/10.6084/m9.figshare.c.5262177 Thematic collection: This article is part of the Isotopic dating of deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Structural Setting, Paleomagnetism, and Magnetic Fabric of Miocene Plutons in a Transpressional Sinistral Shear Zone, Tonalá, Chiapas, Mexico: Evidence of Shortening During Magma Emplacement

AbstractWe studied the structure, paleomagnetism, and magnetic fabric of Miocene granitoids intruded along the left‐lateral, transpressional, Tonalá shear zone in Chiapas, southern Mexico. Magmatic fabrics delineate patterns of regional folding striking parallel to the shear zone, suggesting syn‐magmatic shortening. Magnetic susceptibility fabrics are well developed, and mean magnetic susceptibility averages 18.6 × 10−3 SI. The magnetic fabric of the plutons is controlled primarily by MD and finer PSD magnetite, which also carries the magnetic remanence. The orientations of macroscopic mineral fabrics and magnetic susceptibility fabrics are consistently parallel in protomylonites, but their coincidence is good to poor in granitoids that preserve magmatic fabrics. Most site mean directions are NW‐NE directed of moderate positive inclination (or antipodal), but on a local scale site, mean directions are very discordant. The overall mean direction, omitting highly discordant sites, is of Decl. = 359.6° and Incl. = +41.9° (k = 14.4, α95 = 8.1°, N = 24 sites); it is nearly concordant with the North America reference direction, and the slight difference may indicate a gentle, north‐side down regional tilt. High directional dispersion is interpreted to reflect relative rotation about local vertical to sub‐vertical axes of plutons, or parts of plutons, along the shear zone, with some rotations interpreted to be caused by progressive deformation within the shear zone. Paleomagnetic and AMS data from the protomylonite rocks are interpreted to have been affected by the distortional strain, which accommodated shape and volume change in the rock. We suggest that the introduction of granitic magma into middle crustal levels critically enhanced the crustal response to shortening within the shear zone, possibly due to melt compressibility.