Ductile Fault Research Articles

METAMORPHISM AND DEFORMATION OF GOLD-BEARING NEOPROTEROZOIC WONAKA SCHIST BELT, NORTHWEST-NIGERIA.

The role of metamorphism and deformation is indispensable in the occurrences of gold mineralization worldwide. In this work, deformation and metamorphic conditions for gold-bearing Neoproterozoic Wonaka Schist Belt; located around Kutcheri town of Tsafe Local Government of Zamfara State, was investigated. This is achieved using metamorphic litho-minerals obtained from ternary plots via X-Ray fluorescence (XRF) geochemical data, and directly using minerals phases from X-Ray Diffraction (XRD) technique. Index minerals identified from petrographic analysis previously suggest low to medium-grade metamorphism (M1). XRD analysis indicates quartz, albite, oligoclase, microcline, chlorite, and biotite, suggesting greenschist to lower amphibolite facies (M2). Sillimanite, andalusite, kyanite, staurolite, chlorite, biotite, and garnet were identified from the ternary plots using XRF major oxides, indicating upper amphibolite to granulite facies metamorphism (M3). This is typical of prograde metamorphism, granulite facie metamorphic grade is indicated. Na2O/Al2O3 versus K2O/Al2O3 for petrogenetic character suggests shale provenance, while the trace elements spider diagram indicates Wonaka litho-units as co-genetic compositionally, as high concentrations of V and Cr linked the petrogenetic affinity to mafic sources. Three circles of deformations are indicated; ductile deformation (D1) of the paleosome Schist producing foliations and lineation, brittle type (D2) in mid Pan-African and was accompanied by several fractures and felsic intrusions. Late Pan-African (D3) involves the folding of banded orthogneisses, the development of boudinage as well as intense shearing (ductile fault). Geospatial analysis of the fractures suggests that they represent regional Pan-African sutures cross-cutting Nigeria into the Atlantic and up to South American plate. The research therefore concludes that Au-fluid emanating through this regional event, utilizes D2 as channel ways and loci. D3 with M3 engulfed the entire structures repositioning the geometry to its present disposition.

Role of Crack Interaction on Shear Localization in Porous Granular Rocks Deformed in the Brittle and Ductile Fields

AbstractCrack interactions leading to shear localization were quantified using microstructural analysis for brittle faults and high‐temperature ductile faults formed during experiments on quartz sandstone. In both faulting regimes, the nucleation of macroscopic faults results from the interactions of microfractures at two length scales in ensemble. Brittle faults nucleate when the longest mesoscale shear fractures and transgranular tensile cracks critically interact. In contrast, ductile faults nucleate when the longest mesoscale shear fractures and multi‐grain scale intergranular shear cracks critically interact. For both faulting regimes, we conclude the interaction and coalescence of the longest mesoscale shear fractures is the fundamental process responsible for fault nucleation. Hence, mesoscale shear fractures, which accommodate the majority of axial strain prior to shear localization in both faulting regimes, also serve as the nucleus of macroscopic faults. Locally, the growth of the mesoscale shear fractures is promoted by the interaction and coalescence of the multi‐grain scale cracks in both faulting regimes. We hypothesize that attainment of a critical microstructure for shear localization (i.e., local clustering of the longest microfractures) requires a characteristic amount of plastic axial strain, which depends on deformation conditions. In brittle faulting, distributed microfracturing is confined within limited regions of the rock volume, which expedites crack clustering and fault nucleation at low characteristic strains. In ductile faulting, distributed microfracturing occurs more uniformly throughout the rock volume, delaying shear localization to high characteristic strains. Accurate prediction of shear localization requires models that describe crack interactions of the largest flaws that account for crack clustering.

Deformation‐induced graphitization and muscovite recrystallization in a ductile fault zone

AbstractA suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation‐induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X‐ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution‐precipitation processes. Thermal conditions were constrained to be near isothermal using calcite‐dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1‐full width‐at‐half‐maximum (FWHM), G‐FWHM, Raman band separation (RBS), and a lesser‐known vibrational mode B2g‐FWHM, showed robust linear trends across the same sampling transect. However, the G‐FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain‐driven reduction in graphite crystallite size (decrease in G‐FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an R2 value of 0.92 predicts the graphite D1‐FWHM values from the XRD‐derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.

Modes and geometry of crustal-scale detachment folding in hot orogens—Insights from physical modeling

Detachment folding can be defined as the displacement and buckling of a competent layer above a rheologically weak horizon during tectonic shortening, frequently addressed in the context of salt tectonics. More recent studies have focused on influence of detachments in large-scale lithospheric deformation where the detachment horizon is represented by partially molten lower crust.This study addresses the geometric, kinematic and dynamic behavior of crustal-scale detachment folds using heated paraffin wax as an analogue for partially molten lower crust. Various thermal and shortening rate scenarios were tested in order to characterize deformation patterns within hot orogens in general, and to find an appropriate range of temperature gradients and shortening rates for the detachment folding regime. Five different regimes of lower crustal deformation were identified: 1. Homogeneous thickening or bulging, 2. Short-wavelength folding, 3. Development of diapir-shape folds or ductile faults, 4. Detachment folding and 5. Formation of lower crustal finger-like protrusions.Models are compared to various natural prototypes worldwide, in particular a series of metamorphic domes in the Central Asian Orogenic Belt (CAOB). Detailed analysis of the kinematic-dynamic evolution of the detachment folding scenario revealed an asymmetrical evolution of the folds associated with rotation of the limbs, as well as flexural flow of the lower weak mushy crust around the molten core. Pressure gradients in the fold cores saturated by melt controlled the sequential injections and outflows of partially/molten material between folds’ cores and the melt source layer at the base of the system. This resulted in accumulation of melt in the foreland of the accretionary zone, with higher melt absorption potential for newly developed folds. These observations may have significant implications for the development of pseudo-symmetrical metamorphic domes in the CAOB.

Bridging the Gap between Long–Term Orogenic Evolution (>10 Ma Scale) and Geomorphological Processes That Shape the Western Alps: Insights from Combined Dating Approaches

Constraining the relative roles of erosion and tectonics in the evolution of mountain belts is a challenging scientific goal. In this review article on the Western Alps, we show how it becomes possible to “bridge the gap” between the long–term (>Ma) orogenic evolution controlled by tectonics and exhumation processes and the recent geomorphological evolution that is accessible on an annual–decadal basis. Advances in mineral dating that have grown in relation to deformation in the ductile and brittle crustal fields have allowed us to constrain the evolution of deformation through time and depth. A drastic change from early collision, dominated by rapid underthrusting of the European plate, to a more stagnant syn–collisional tectonic context is documented since about 26–20 Ma by syn–kinematic phengites and vein–hosted monazites along the Alpine arc. The overall dextral kinematic context is accompanied by local extensional domains in the Simplon and High Durance Valley. Activation of the Simplon ductile fault is documented from 20 Ma, whereas the High Durance extensional system commenced after 10 Ma. The application of cosmogenic nuclide dating of incised river gorges demonstrates that the erosion pattern of the Western Alps follows a different evolution within the valleys dominated by upstream glacial erosion than in peripheral watersheds devoid of glaciers. The very low peripheral incision is found to be similar to the vertical GPS signal, suggesting equilibrium of tectonic uplift and incision, whereas the glacial–dominated valleys exhibit significantly increased and transient river incision during interglacials and a constant ongoing tectonic regime.

A single extensional, diffuse, ductile fault zone in the Goriganga section, western Himalaya: Part of the upper South Tibetan Detachment System?

The Trans-Himadri Detachment Fault (T-HDF) of the Kumaon region of western Himalaya, a north-easterly steeply dipping discrete brittle/semi-brittle fault, had previously been regarded as a strand of the South Tibetan Detachment System (STDS). Yet, elsewhere in the Himalaya, the STDS is defined as a diffuse, ductile low-angle normal-sense shear zone. This aspect of the STDS remained conjectural and was even refuted by various workers in the Goriganga valley in the Kumaon. We have systematically documented the STDS as a low-angle, northerly dipping, ductile, diffuse shear zone, with a true thickness of >5 km in this transect. In the present scenario, the shear zone is located only within the psammo-pelitic basal part of the Tethyan Sedimentary Sequence (TSS), and bypassed the rock units of the Greater Himalayan Sequence (GHS). Multiple grain-statistical parameters of mylonitized micaceous quartzites point to an anastomosing architecture which displays a single precursory damage zone and a wider core for the entire STDS shear zone. The 40Ar*/39Ar geochronology and oxygen isotope analysis of the micas in mylonites demonstrated that the STDS evolved in weak lithologies, between c.14–11 Ma, at ∼300–350 °C ambient temperature, at or immediately below the brittle-plastic transition depth in the quartzo-feldspathic continental crust.Consideration of similar range of optimum ages of cessation of the extensional activity and already published timings of the end of brittle motion of the STDS in adjacent regions suggests that this wide diffuse shear zone is a deeper counterpart of the brittle upper STDS. This is in sharp contrast to the exposures of the detachment elsewhere in the Himalaya, where diffuse zone demonstrably constitutes the lower branch of this detachment. This finding has potential implications on overall understanding of the deformation related to the STDS and late Miocene geodynamic evolution of the GHS in this region.

Seafloor overthrusting causes ductile fault deformation and fault sealing along the Northern Hikurangi Margin

IODP Site U1518, drilled during IODP Expeditions 372 and 375, penetrated a large-offset (∼6 km) thrust, the Pāpaku fault, rising from a megathrust that hosts recurring slow slip events along the Hikurangi margin. Although drilling intersected the fault zone at only ∼300 m below the seafloor within porous silty mudstone, it exhibits intense tectonic ductile deformation, including finely banded mudstones contorted into decimeter-scale folds; elongate mudstone clasts with grain tail complexes; stacked and truncated silt beds in distorted mudstones; and soft sediment injections. Locally, these ductile features are overprinted by brittle deformation, including normal faults, fracture arrays, and breccias. The more consolidated hanging wall is dominated by brittle structures, whereas the footwall exhibits ductile and brittle deformation that decreases in intensity with depth. The intense tectonic ductile deformation and asymmetric distribution of structures across the fault zone at Site U1518 can be explained by seafloor overthrusting. The emplacement of the hanging wall upon the footwall flat overrode high-porosity, undeformed, and previously unburied sediments, localizing shear deformation within these weak sediments. In contrast, the overconsolidated hanging wall preferentially experienced brittle deformation during folding and displacement. Interstitial pore water geochemical profiles at Site U1518 show a repetition of near-seafloor diagenetic sequences below the fault, consistent with overthrusting of previously unburied strata. The preserved diagenetic profiles in the footwall suggest that overthrusting occurred within the last 50-100 kyr, and indicate little along- or across-fault fluid flow at the location of Site U1518. Thus the Pāpaku fault appears to define a low-permeability seal that restricts footwall consolidation, maintaining locally high pore fluid pressures and low fault strength. If similar low permeability structures occur elsewhere along the margin, they could support regionally high pore pressure conditions favorable to the occurrence of SSEs on the Hikurangi megathrust fault.

Geological study of a Mewoulou-Nkolbisson ductile strike-slip fault segment (Western Yaoundé, Cameroon): evidence of hazards related to structural landforms

Geological study of a Mewoulou-Nkolbisson ductile strike-slip fault segment (Western Yaoundé, Cameroon): evidence of hazards related to structural landforms

Isotope Geochemistry of the Shenshuitan Gold Deposit within the Wulonggou Gold Field in the Eastern Kunlun Orogen, Northwest China: Implications for Metallogeny

The Shenshuitan gold deposit is located within the Eastern Kunlun Orogen in northwest China. The gold mineralization here occurs primarily within the ductile fault XI. The sulfide mineral assemblage is dominated by pyrite and arsenopyrite, with minor pyrrhotite, chalcopyrite, galena, and sphalerite. Host rocks predominantly consist of Ordovician silicic slate and late Silurian granites, and their alterations include silicification and sericitization. The measured δD and δ18O values of quartz and sericite range from −113.9‰ to −93.1‰ and from 4.6‰ to 12.0‰, respectively. Bulk and in situ δ34S values of sulfides range from −7.3‰ to +9.6‰ and from −3.92‰ to 11.04‰, respectively. Lead isotope compositions of sulfides show 206Pb/204Pb ratios from 18.071 to 19.341, 207Pb/204Pb ratios from 15.530 to 15.67, and 208Pb/204Pb ratios from 37.908 to 38.702. Collectively, the isotope (H, O, S, and Pb) geochemistry suggests that the ore-forming fluids were of a metamorphic origin mixed with meteoric water and that the sulfur and lead were sourced from a mixture of host rocks and original ore-forming metamorphic fluids. Lastly, this deposit can be classified as an orogenic gold deposit associated with the final collision between the Bayan Har–Songpanganzi Terrane and the Eastern Kunlun Orogen during the Later Triassic.

Late Carboniferous paleoelevation of the Variscan Belt of Western Europe

We present the first stable isotope paleoaltimetry estimates for the hinterland of the eroded Variscan Belt of Western Europe based on the hydrogen isotope ratios of muscovite from syntectonic leucogranites that have been emplaced at ∼315 Ma. We focus on the Limousin region (Western Massif Central, France) where peraluminous granites are spatially associated with strike-slip and detachment shear zones that developed as a consequence of Late Carboniferous syn- to post-orogenic extension. In this region, we show that the north-east corner of the Millevaches massif (located at the junction between brittle and ductile fault systems) represented a pathway for Earth surface-derived fluids that penetrated the crust and reached the ductile segment of the low-angle Felletin detachment zone. Using microstructural, thermometry, hydrogen isotope and 40Ar/39Ar geochronological data, we show that these Variscan meteoric fluids interacted with hydrous silicates during high temperature deformation between ∼318 and 310 Ma. For paleoaltimetry purposes, we reference our hydrogen isotope record (δD) of ancient meteoric fluids from mylonitic rocks to ∼295 Myr-old records retrieved from freshwater shark remains preserved in the Bourbon l'Archambault basin that developed in the external zones of the orogen. A ∼76‰ difference in δDmeteoric water values between the Millevaches massif (δDmeteoric water value = −96 ± 8‰) and the Bourbon l'Archambault foreland basin (δDwater value = −20 ± 6‰) is consistent with paleoaltimetry estimates of 3.4 ± 0.7 km based on a modern lapse rate of ∼-22‰/km for δDwater values. The rather large difference in δD values between the foreland basin and the continental interior suggests that the hinterland of the Variscan belt of western Europe was high enough to act as a barrier to moisture transport from the south-south-east and induce an orographic rain shadow to the north.

Permeability Testing of Drill Core from Basement Rocks in the Fault-Hosted Gryphon U Deposit (Eastern Athabasca Basin, Canada): Insights into Fluid–Rock Interactions Related to Deposit Formation and Redistribution

Fluid percolation conditions in fault zones are often inferred in geologic studies, but the porosity and permeability of rocks are rarely tested during mineral exploration drilling. Routine permeability and porosity tests on drill core, with adequate detection limits and accuracy, can provide new insights into present- and paleo-fluid flow processes and deposit formation in sedimentary basins and permeable fault zones (e.g. uranium deposits, sub-sea hydrothermal ore deposits). In this study, geological and hydrogeological observations were combined with results of over a thousand permeability tests at spot-on unconfined drill core, extracted along transects through the Gryphon U deposit and fault zone, using a pressure-decay N2 gas probe with adequate seal to the rock surface. The new seal method allowed a wide range of permeability determinations under field conditions (over 5 × 10–20 to 10–12 m2). The penetrative macropore networks were observed directly from gas discharge patterns on drill core surfaces. For the tested crystalline basement rocks below the Athabasca Basin, we inferred that the matrix permeability and porosity distribution appear to be preserved since the late Paleoproterozoic/Mesoproterozoic, under the cover of a sedimentary basin. The permeability values at the present time, and the patterns of rock alteration and deformation, offer insights to paleo-hydrogeological conditions. The gneissic and pegmatitic host rocks have relatively low permeability at present time, except fractured (discrete flow channels) and/or intensely altered to a more porous rock. The alternating zones of silicification and desilicification appear to pre-date the intense argillic alteration, and it may have been a result of earlier hydrothermal activity. The Gryphon U deposit occurs in elongated narrow “lenses” along several fault strands of a fault zone (below the unconformity surface and within basement rocks). U-ore is a cementing mineral in fault-related fractures and fault rocks, but remnant flow channels in fractures in U-ore are still preserved. The most permeable and porous fault rocks are coarse gouges (~ 40% porosity, ~ 10–15 m2 permeability) with approximately three orders of magnitude higher rock matrix permeability than the gneissic host rocks. Such fault rocks locally contain carbonaceous matter, and similar fault rocks may have been preferentially mineralized by U to form the U ore lenses, but the role of brittle-ductile and ductile fault and shear zone rocks in U mineralization is not yet clear at Gryphon. Our review of U–Pb uraninite ages at Gryphon suggests a protracted faulting and U-mineralization history. In response to numerous episodic events of brittle deformation and hydrothermal alteration, the permeability and porosity of the host rocks and U-ores have evolved, as observed by multiple cross-cutting reaction fronts. We inferred from the test results and the alteration halo at the U deposit, affecting a wide part of the fault zone, that the permeability and porosity increased by the downward ingress of U-rich acidic basinal brine that percolated into the basement-hosted (rooted) shear/fault zone and nearby crystalline basement rocks. However, the younger prominent U roll/redox-fronts caused local-scale reduction in porosity and permeability by hematite cementation.

Deep Ductile Shear Zone Facilitates Near‐Orthogonal Strike‐Slip Faulting in a Thin Brittle Lithosphere

AbstractSome active fault systems comprise near‐orthogonal conjugate strike‐slip faults, as highlighted by the 2019 Ridgecrest and the 2012 Indian Ocean earthquake sequences. In conventional Mohr‐Coulomb failure theory, orthogonal faulting requires a zero frictional coefficient (pressure‐insensitive), which is unlikely in the brittle lithosphere. The simulations developed here show that near‐orthogonal faults can form in the brittle layer by inheriting the geometry of orthogonal shear zones nucleated in the deep ductile (pressure‐insensitive) layer. In particular, if the brittle layer is sufficiently thinner than the ductile fault root, near‐orthogonal faulting is preserved at the surface. The preservation is further facilitated by a depth‐dependent strength in the brittle layer. Conversely, faults nucleated within the brittle layer are unlikely to form at orthogonal angles. Our model thus offers a possible explanation for orthogonal strike‐slip faulting and reveals the significant interactions between the structure of faults in the brittle upper lithosphere and their deep ductile roots.

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.

Deformation fabrics and strain localization mechanisms in graphitic carbon-bearing rocks from the Ailaoshan-Red River strike-slip fault zone

Graphitic carbon-bearing rocks generally occur in low-to high-grade metamorphic units. In many brittle faults, graphitic carbon is often associated with gouge or low-grade metamorphic rocks whereas in ductile faults, graphitic carbon commonly occurs in marble, schist or gneiss. Carbonaceous material gradually transforms from an amorphous into an ordered crystalline structure by increasing thermal metamorphism. The degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hangingwall unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperatures determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium-grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower-grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip-localized micro-shear zones and laterally continuous or discontinuous slip-planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back-scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism<a> slip systems, which indicate increasing deformation temperatures. Comparison of quartz and graphite LPOs indicates that graphite LPOs show higher temperature conditions. According to the graphitic slip-planes, micro-shear zones with grain-size reduction and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine-grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.

Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic, geochronologic, and structural analysis of the Diana Complex in the southern Carthage-Colton mylonite zone

Abstract Crustal-scale shear zones can be highly important but complicated orogenic structures, therefore they must be studied in detail along their entire length. The Carthage-Colton mylonite zone (CCMZ) is one such shear zone in the northwestern Adirondacks of northern New York State (USA), part of the Mesoproterozoic Grenville province. The southern CCMZ is contained within the Diana Complex, and geochemistry and U-Pb zircon geochronology demonstrate that the Diana Complex is expansive and collectively crystallized at 1164.3 ± 6.2 Ma. Major ductile structures within the CCMZ and Diana Complex include a northwest-dipping penetrative regional mylonitic foliation with north-trending lineation that bisects a conjugate set of mesoscale ductile shear zones. These ductile structures formed from the same 1060–1050 Ma pure shear transitioning to a top-to-the-SSE shearing event at ∼700 °C. Other important structures include a ductile fault and breccia zones. The ductile fault formed immediately following the major ductile structures, while the breccia zones may have formed at ca. 945 Ma in greenschist facies conditions. Two models can explain the studied structures and other regional observations. Model 1 postulates that the CCMZ is an Ottawan orogeny (1090–1035 Ma) thrust, which was later reactivated locally as a tectonic collapse structure. Model 2, the preferred model, postulates that the CCMZ initially formed as a subhorizontal mid-crustal mylonite zone during collapse of the Ottawan orogen. With continued collapse, a metamorphic core complex formed and the CCMZ was rotated into is current orientation and overprinted with other structures.

Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of the Late Silurian Shidonggou granite from the Wulonggou area in the Eastern Kunlun Orogen, Northwest China

ABSTRACTThe Wulonggou area in the Eastern Kunlun Orogen (EKO) in Northwest China is characterized by extensive granitic magmatism, ductile faulting, and orogenic gold mineralizations. The Shidonggou granite is located in the central part of the Wulonggou area. This study investigated the major as well as trace-element compositions, zircon U–Pb dates, and zircon Hf isotopic compositions of the Shidonggou granite. Three Shidonggou granite samples yielded an average U–Pb zircon age of 416 Ma (Late Silurian). The Late Silurian Shidonggou granite is peraluminous, with high alkali contents, high Ga/Al ratios, high (K2O + Na2O)/CaO ratios, and high Fe2O3T/MgO ratios, suggesting an A-type granite. The Shidonggou granite samples have zircon εHf(t) values ranging from −7.1 to +4.4. The Hf isotopic data suggest that the Late Silurian granite was derived from the partial melting of Palaeo- to Mesoproterozoic juvenile mantle-derived mafic lower crust. Detailed geochronological and geochemical data suggest that the Late Silurian granite was emplaced in a post-collisional environment following the closure of the Proto-Tethys Ocean. Combining data of other A-type granitic rocks with ages of Late Early Silurian to Middle Devonian, such post-collisional setting related to the Proto-Tethys Ocean commenced at least as early as ~430 Ma (Late Early Silurian), and sustained up to ~389 Ma (Middle Devonian) in the EKO.

Earthquake cycle simulations with rate-and-state friction and power-law viscoelasticity

We simulate earthquake cycles with rate-and-state fault friction and off-fault power-law viscoelasticity for the classic 2D antiplane shear problem of a vertical, strike-slip plate boundary fault. We investigate the interaction between fault slip and bulk viscous flow with experimentally-based flow laws for quartz-diorite and olivine for the crust and mantle, respectively. Simulations using three linear geotherms (dT/dz=20, 25, and 30K/km) produce different deformation styles at depth, ranging from significant interseismic fault creep to purely bulk viscous flow. However, they have almost identical earthquake recurrence interval, nucleation depth, and down-dip coseismic slip limit. Despite these similarities, variations in the predicted surface deformation might permit discrimination of the deformation mechanism using geodetic observations. Additionally, in the 25 and 30K/km simulations, the crust drags the mantle; the 20K/km simulation also predicts this, except within 10km of the fault where the reverse occurs. However, basal tractions play a minor role in the overall force balance of the lithosphere, at least for the flow laws used in our study. Therefore, the depth-integrated stress on the fault is balanced primarily by shear stress on vertical, fault-parallel planes. Because strain rates are higher directly below the fault than far from it, stresses are also higher. Thus, the upper crust far from the fault bears a substantial part of the tectonic load, resulting in unrealistically high stresses. In the real Earth, this might lead to distributed plastic deformation or formation of subparallel faults. Alternatively, fault pore pressures in excess of hydrostatic and/or weakening mechanisms such as grain size reduction and thermo-mechanical coupling could lower the strength of the ductile fault root in the lower crust and, concomitantly, off-fault upper crustal stresses.

Detrital zircon ages in Korean mid-Paleozoic meta-sandstones (Imjingang Belt and Taean Formation): Constraints on tectonic and depositional setting, source regions and possible affinity with Chinese terranes

Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Th–Pb isotopic data of detrital zircons from mature, quartz-rich meta-sandstones are used to constrain possible tectonic affinities and source regions of the rhythmically layered and graded-bedded series in the Yeoncheon Complex (Imjingang Belt) and the correlative Taean Formation. These metamorphic marine turbidite sequences presently occur along the Paleoproterozoic (1.93–1.83Ga) Gyeonggi Massif, central Korea’s main high-grade metamorphic gneiss terrane. Yet, detrital zircons yielded highly similar multimodal age spectra with peaks that do not match the age repartition in these basement rocks, as late (1.9–1.8Ga) and earliest (∼2.5Ga) Paleoproterozoic detrital modes are subordinate but, in contrast, Paleozoic (440–425Ma) and Neoproterozoic (980–920Ma) spikes are prominent, yet the basement essentially lacks lithologies with such ages. The youngest concordant zircon ages in each sample are: 378, 394 and 423Ma. The maturity of the meta-sandstones and the general roundness of zircons of magmatic signature, irrespective of their age, suggest that sediments underwent considerable transport from source to sink, and possibly important weathering and recycling, which may have filtered out irradiation-weakened metamorphic zircon grains. In combination with these isotopic data, presence of a low-angle ductile fault contact between the Yeoncheon Complex and the Taean Formation and the underlying mylonitized Precambrian basement implies that they are in tectonic contact and do not have a stratigraphic relationship, as often assumed. Consequently, in all likelihood, both meta-sedimentary formations: (1) are at least of early Late Devonian age, (2) received much of their detritus from distant (reworked) Silurian–Devonian and Early Neoproterozoic magmatic sources, not present in the Gyeonggi Massif, (3) and not from Paleoproterozoic crystalline rocks of this massif, or other Korean Precambrian basement terranes, and (4) should be viewed as independent tectonic units that had sources not exposed in Korea.A thorough literature review reveals that the Yeoncheon Complex and the Taean Formation were potentially sourced from the Liuling, Nanwan and Foziling groups in the Qinling–Dabie Belt, which all show very similar detrital zircon age spectra. These immature middle–late Devonian sandstones were deposited in a pro-foreland basin formed as a result of the aborted subduction of the South Qinling Terrane below the North Qinling Terrane, which was uplifted and eroded during post-collision isostatic rebound. The submarine fans where the mature distal turbiditic Yeoncheon and Taean sandstones were deposited may have constituted the eastern terminal part of a routing system originating in the uplifted and eroded middle Paleozoic Qinling Belt and adjacent part of the foreland basin.

Fault-Related Controls on Upward Hydrothermal Flow: An Integrated Geological Study of the Têt Fault System, Eastern Pyrénées (France)

The way faults control upward fluid flow in nonmagmatic hydrothermal systems in extensional context is still unclear. In the Eastern Pyrénées, an alignment of twenty-nine hot springs (29°C to 73°C), along the normal Têt fault, offers the opportunity to study this process. Using an integrated multiscale geological approach including mapping, remote sensing, and macro- and microscopic analyses of fault zones, we show that emergence is always located in crystalline rocks at gneiss-metasediments contacts, mostly in the Têt fault footwall. The hot springs distribution is related to high topographic reliefs, which are associated with fault throw and segmentation. In more detail, emergence localizes either (1) in brittle fault damage zones at the intersection between the Têt fault and subsidiary faults or (2) in ductile faults where dissolution cavities are observed along foliations, allowing juxtaposition of metasediments. Using these observations and 2D simple numerical simulation, we propose a hydrogeological model of upward hydrothermal flow. Meteoric fluids, infiltrated at high elevation in the fault footwall relief, get warmer at depth because of the geothermal gradient. Topography-related hydraulic gradient and buoyancy forces cause hot fluid rise along permeability anisotropies associated with lithological juxtapositions, fracture, and fault zone compositions.

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone, New Hampshire, USA

AbstractWe describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na‐poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric‐forming Na‐rich muscovite is also progressively replaced by fabric‐forming Na‐poor muscovite. The mineralogy of the new phyllonitic fault‐rock produced is dominated by Na‐poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric‐forming Na‐rich muscovite is selectively replaced at high‐strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high‐grade porphyroblasts by weaker Na‐poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na‐rich muscovite‐defined foliation destroys not only the metastable high‐grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.