Types Of Shear Zones Research Articles

Ar-Ar chronology of the brittle deformation age for the ore-controlling shear zones in the Jiaodong Peninsula and it's constrains on gold mineralization

胶东地区是典型的剪切带型金矿集中区，精确厘定该地区控矿剪切带的活动时代，可为探讨剪切带与金矿成因关系提供关键的时限约束，并且对矿床成矿模式的建立具有至关重要的意义。大量矿床地质证据显示胶东地区控矿剪切带中脆性变形活动对金矿体的形成具有直接影响，但其脆性变形时代尚不十分清楚。据此，本文在详细研究金矿体赋存特征的基础上，系统选取胶东焦家、玲珑、邓格庄、乳山这四个金矿区控矿剪切带断层泥中白云母进行⁴⁰Ar-³⁹Ar年代学研究。定年结果显示，胶东地区焦家剪切带、招平剪切带以及牟乳剪切带的脆性变形时代分别为110.3±1.5Ma、122.8±1.7Ma、119.6±1.2Ma~115.8±1.4Ma。其中焦家剪切带的脆性变形时代明显晚于招平和牟乳剪切带，可能代表了焦家蚀变岩型矿化形成后易遭受后期的构造叠加。综合胶东各金矿区控矿剪切带变形时代、岩体侵位时代、成矿时代及剪切带活动特征和矿体产出特征，本文认为在多期岩体侵位以及控矿剪切带递进变形过程中，剪切带韧性变形区中由高压流体作用产生的同期脆性破裂可形成脉型矿化，如乳山金矿；而在脆-韧性和脆性变形区中发生的大规模脆性变形可导致脉型和蚀变岩型矿化的形成，如玲珑、邓格庄和焦家金矿。但随着剪切带的递进变形和隆升剥蚀，后期多期次的脆性构造变形叠加，可导致多种矿化类型出现在同一构造部位，如焦家金矿中石英脉型和蚀变岩型矿化。;Jiaodong Peninsula is a typical concentration area for shear zone type gold deposits. Accurate determination of the deformation age of ore-controlling shear zones in this area can provide a key time limit for exploring the relationship between shear zones and the genesis of gold deposits, and is of vital significance to the establishment of metallogenic mechanism. A large number of geological evidences show that the formation of gold orebodies is directly controlled by brittle deformation activities of ore-controlling shear zones in Jiaodong Peninsula, but this deformation age is still unclear. Based on detailed study of the occurrence characteristics of orebodies, we systematically selected muscovite samples from fault gouge in Jiaojia, Linglong, Denggezhuang and Rushan gold mining areas for the Ar-Ar chronology study. ⁴⁰Ar/³⁹Ar dating results show that the brittle deformation ages of Jiaojia, Zhaoping and Muru shear zones in Jiaodong Peninsula are 110.3±1.5Ma, 122.8±1.7Ma and 119.6±1.2Ma~115.8±1.4Ma, respectively. Among them, the brittle deformation age of Jiaojia shear zone is obviously later than those of Zhaopin and Muru shear zones, which may represent that the altered rock type mineralization in Jiaojia deposit is susceptible to structural superposition in the later period. Comprehensively comparing shear zone deformation ages, pluton emplacement ages, metallogenic ages, shear zone activities and orebody occurrence characteristics in Jiaodong gold deposits, it is suggested that: (1) The contemporaneous brittle fractures caused by high-pressure fluids in ductile deformation region of shear zones can lead to the formation of vein type mineralization, e.g. Rushan gold deposits; (2) Large-scale brittle deformation in brittle-ductile and brittle region can lead to the formation of vein type and altered rock type mineralization, e.g. Linglong, Denggezhuang and Jiaojia gold deposits. Both stages take place during the process of multi-stage emplacement and progressive deformation of ore-controlling shear zones. However, with the progressive deformation and uplift denudation of shear zones, the superposition of brittle deformation in the late stage may lead to the occurrence of multiple mineralization types in the same structural site, e.g. quartz vein type and altered rock type mineralization in Jiaojia gold deposit.

Hydromechanical processes and their influence on the stimulation effected volume: observations from a decameter-scale hydraulic stimulation project

Abstract. Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment fractures associated with one out of two shear zone types were hydraulically reactivated. The two shear zone types differ in terms of tectonic genesis and architecture. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in 11 boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to 3 orders in magnitude. With one exception, jacking pressures were unchanged by the stimulations. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone architectures govern the seismo-hydromechanical response. The elevated fracture fluid pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15–20 m from the injection point is characterized by variable extensional and compressive strain produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which strain magnitudes decrease with increasing distance. These compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. Our hydromechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud and can be subdivided into a primary stimulated and secondary effected zone.

Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20m×20m×20m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

Preliminary Study on the Characteristics of Ductile Shear Zone Type Gold Deposit

The ductile shear zone type gold deposit is a very important type of gold deposit. For the study of metallogenic characteristics of ductile shear zone, the development is extremely rapid, and there are already quite mature research methods and relatively complete research contents. This paper mainly focuses on the structural characteristics, fabric characteristics of the shear zone, the relationship between the deformation of the shear zone and mineralization, the relationship between gold deposits and shear zones, the geochemical characteristics of the shear zone gold deposits, the metallogenic model and the future. The research direction has been made a simple narrative.

New Insight into the Genetic Mechanism of Shear Zone Type Gold Deposits from Muping-Rushan Metallogenic Belt (Jiaodong Peninsula of Eastern China)

Most gold deposits are genetically controlled by shear zones, which are called shear zone type gold deposits (SZTGD). A better understanding of kinematics of shear zones and its constraint on the ore-forming process is critical to reveal the genetic mechanism of the SZTGD and favorable to mineral exploration. By conducting detailed structural analysis including field and microscopic observations and electron backscatter diffraction (EBSD) and fractal dimension analysis in the Muping-Rushan shear zone (MR) as well as several gold deposits, the kinematic characteristics of the MR are well recognized and the metallogenic process of the SZTGD are discussed. The main conclusions are as follows: (1) petrology, geometry, kinematics, macro- and micro-structures imply that the MR has experienced a progressive shearing history exhumed via middle crust to subsurface level under the NW-SE extensional regime from late Jurassic to early Cretaceous; (2) in the MR, gold may precipitate both in the brittle fractures at middle crust level and brittle deformation part at shallow crust level during the stress-chemical process and (3) comparison of gold deposits between the MR and other areas show that the SZTGD has a uniform metallogenic mechanism, which is from (multi-stage) pluton emplacement, hydrothermal fluid action, shearing action, brittle fracturing, sudden reduction of fluid pressure, flash vaporization to (gold) mineralization.

Metallogenic Regularity and Prospecting Predictions of Gold Deposits in China

In this paper, based on the analysis of 1572 small and above-scale gold deposits in China, the gold deposits are divided into four types for prediction purposes: sedimentary, volcanic rock, magmatic hydrothermal and shear zone types. The spatial and temporal distribution of gold deposits in China is non–uniform, as 60% of the deposits are distributed in the land block and Mesozoic deposits account for 71.5% of the national total. It is found that intrusive and sub-volcanic rocks are the main ore–forming geological bodies. Here, the method for accurately identifying the spatial positions of concealed rocks is summarized. Through the geological profile study of 450 representative gold deposits, this paper suggests that there are mainly two kinds of gold deposits in China. One is characterized as an association of Au and Ag, with formation temperatures of approximately 250°C; it has very low concentrations of As and Sb. The fineness of the gold is generally below 900. Unequal proportions of K–feldsparization and albitization formed in the early stage. The other is an association of Au, As and Sb with a formation temperature below 250°C; the fineness of the gold is generally higher than 900; it has low Ag and high As and Sb concentrations. Alterations were mainly in the form of silicon with some secondary quartz formed in the early mineralization stage. Taking into account the elemental geochemical characteristics, the mechanism of gold migration and precipitation in the transient transition process from fluid to solid minerals in the ore-forming structure is discussed. The discussion provides a reference for future gold prospecting.

Lattice-preferred orientation development in experimental and natural fine-grained dolomite shear zones

We compare microstructures and lattice preferred orientations (LPO) in shear zones formed in two experimentally deformed and two naturally deformed dolomites. Experimental samples were deformed at 900 °C and laboratory strain rates, and naturally deformed dolomites were collected from the Pioneer Landing thrust and Town Knobs thrust of the Southern Appalachians, deformed at ~300 °C and ~240 °C, respectively, at tectonic strain rates. Coarse-grained host dolomite, in all samples, exhibits undulatory extinction, subgrains, recrystallized grains, and, in the naturally deformed dolomites, fractures. Two types of fine-grained shear zones are characterized in experimentally deformed dolomites: 1) a fine-grained zone of recrystallized dolomite generated during axial shortening of coarse-grained dolomite that initially deformed by dislocation creep, with strain localization initiated by grain-size sensitive deformation processes and 2) a synthetic, fine-grained dolomite layer placed between coarse-grained dolomite shear pistons. Microstructures and lattice preferred orientations of the experimentally sheared dolomite reflect the concomitant operation of dislocation creep, diffusion creep, and grain boundary sliding. Microstructures, including subgrains, recrystallized grains, and four-grain junctions, and lattice preferred orientations that consist of weak c-axis maxima in the natural shear zones are similar to the experimental shear zones, however, a slightly stronger LPO, more common subgrains and the lack of four-grain junctions indicate that dislocation creep dominates within dolomites deformed at lower temperatures (240 °C).

Quantifying the Partition Between Seismic and Aseismic Deformation Along Creeping and Locked Sections of the North Anatolian Fault, Turkey

Shallow aseismic creep is a key deformation component along plate boundaries that contributes to the energy budget during the seismic cycle. Several major active continental faults show spatial alternation of creeping and locked sections. The present study focuses on the evaluation of the aseismic part of the total displacement along the North Anatolian Fault in Turkey. Detailed microstructural analyses of finite strain were performed using various methods, based on change of length or angle, on six representative samples collected over 32 outcrops along locked and creeping sections of the fault. Chemical analyses were used to map mineral composition of fault rocks and to calculate relative volume changes associated with creep. The relationship between finite strain and volume change allowed quantifying the evolution of the penetrative pressure solution cleavage mechanism of creep. In volcanic and analogous creeping rocks, finite strain measurements revealed two spatial scales of strain that correspond to the alternation of two types of shear zones, with cleavages either oblique or sub-parallel to the fault displacement. Using geodetic and geologic data, cumulative aseismic displacement was calculated in the range 9–49% of the total 80-km displacement in the creping sections and was negligible in locked sections. The large uncertainty in the kilometer-width creeping sections was related to the difficulty of quantifying the high strain values associated with high shear displacement and for which measurement uncertainties are large. A promising way to improve such quantification would be to develop reliable statistical analysis of cleavage orientation in the field.

Characteristics of Ductile Shear Zone and Its Ore-controlling Effect in Dongtian Polymetallic Mining Area

The genesis of the Red Pacific polymetallic deposit in Yanbian area has long been regarded as a volcanic hydrothermal deposit or a volcanic jet deposit. In the field investigation and comprehensive study, authors found that the so-called 'metamorphic volcanic rocks and volcanic clastic rocks' is actually a set of highly deformed terrigenous clastic rocks. The main rock types are metamorphic conglomerate, sericite schist, sericite to form a structural deformation belt having a width of 50-400 m. The deformed conglomerate rock is the conglomerate of the valley flag, and the latter two are the former siltstone, the silty mudstone and the tectonic deformation zone are basically the same as those of the strata and ore body. The red and Taiping polymetallic ore bodies are produced in the Miaopun Formation in the deformation zone. The ore and the surrounding rock also have certain deformation structures, such as pleated structure, shear page and so on. Therefore, the cause of this deposit is ductile shear zone type, layer structure ore control.

Structural Analysis: Folds Classification of metasedimentary rock in the Peninsular Malaysia

Understanding shear zone characteristics of deformation are a crucial part in the oil and gas industry as it might increase the knowledge of the fracture characteristics and lead to the prediction of the location of fracture zones or fracture swarms. This zone might give high influence on reservoir performance. There are four general types of shear zones which are brittle, ductile, semibrittle and brittle-ductile transition zones. The objective of this study is to study and observe the structural geometry of the shear zones and its implication as there is a lack of understanding, especially in the subsurface area because of the limitation of seismic resolution. A field study was conducted on the metasedimentary rocks (shear zone) which are exposed along the coastal part of the Peninsular Malaysia as this type of rock resembles the types of rock in the subsurface. The analysis in this area shows three main types of rock which are non-foliated metaquartzite and foliated rock which can be divided into slate and phyllite. Two different fold classification can be determined in this study. Layer 1 with phyllite as the main type of rock can be classified in class 1C and layer 2 with slate as the main type of rock can be classified in class 1A. This study will benefit in predicting the characteristics of the fracture and fracture zones.

Microstructures of shear zones from selected domains of the Western Tatra Mountains

The paper is focused on the meso- and microstructural characteristics of selected shearing zones in the Western Tatra Mts. The domains of crystalline rocks studied (Długi Upłaz Ridge, Rakoń Mt., Zabraty Ridge and Zabrať Pass, Wołowiec Mt., Trzydniowiański Mt., Czubik Mt. and Jarząbczy Mt.) show evidences of heterogeneous shearing with development of shear zones. Four types of shear zones were distinguished: (1) ductile shear zones in gneisses, (2) brittle-ductile complex shear zones, (3) gneisses with clear later brittle deformation; (4) leucogranites, produced by anatexis with later brittle deformation. The development of these shear zones is characterised by occurrences of various types of fault rocks: cataclasites, S-C cataclasites and mylonites. The different shearing-related rocks and structures are interpreted as an effect of protolith diversity and strain partitioning. Shape-preferred orientation is an important structural feature of all the shearing-related rocks. Kinematic analyses revealed generally a southward sense of shearing documented by structures related to brittle conditions and northwards sense of shearing recognised in ductily sheared crystalline rocks. The time relationships between different shear zone types are discussed, leading to the conclusions that the 1 st type of shear zone is pure Variscan, the 2 nd type is Variscan with Alpine brittle deformation, and the 3 rd and 4 th types are mainly Alpine brittle deformations of Variscan syntectonic anatectic leucogranites.

Recycling and utilisation of industrial solid waste: an explorative study on gold deposit tailings of ductile shear zone type in China.

Tailings are solid waste arising from mineral processing. This type of waste can cause severe damage to the environment during stockpiling as a result of the leaching of something harmful into the ecosystem. Gold deposit of ductile shear zone type is an important type of gold deposit, and the recycling of its tailings has been challenging researchers for a long time. In this article, the characteristics of this type of tailings were systematically studied by using modern technical means. Considering the characteristics of the tailings, clay was selected to make up for the shortcomings of the tailings and improve their performance. Water and raw materials were mixed to produce green bodies, which are subsequently sintered into ceramic bodies at 980 °C~1020 °C (sintering temperature). The results showed that some new kinds of mineral phases, such as mullite, anorthite and orthoclase, appear in ceramic bodies. Furthermore, the ceramic bodies have a surface hardness of 5 to 6 (Mohs scale), and their water absorption and modulus of rupture can meet some technical requirements of ceramic materials described in ISO 13006-2012 and GB 5001-1985. These gold mine tailings can be made into ceramic tiles, domestic ceramic bodies, and other kinds of ceramic bodies for commercial and industrial purposes after further improvements.

Geological and geochemical implications of the genesis of the Qolqoleh orogenic gold mineralisation, Kurdistan Province (Iran)

Abstract The Qolqoleh gold deposit is located in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), within the NE-SW trending Qolqoleh shear zone. Oligocene granitoids, Cretaceous meta-limestones, schists and metavolcanics are the main lithological units. Chondrite-normalised REE patterns of the ore-hosting metavolcanics indicate REE enrichment relative to hanging wall (chlorite-sericite schist) and footwall (meta-limestone) rocks. The pattern also reflects an enrichment in LREE relative to HREE. It seems that the LREE enrichment is related to the circulation of SO4 2- and CO2-bearing fluids and regional metamorphism in the Qolqoleh shear zone. Both positive and negative Eu anomalies are observed in shear-zone metavolcanics. These anomalies are related to the degree of plagioclase alteration during gold mineralisation and hydrothermal alteration. In progressing from a metavolcanic protomylonite to an ultramylonite, significant changes occurred in the major/trace element and REE concentration. Utilising an Al-Fe-Ti isocon for the ore-hosting metavolcanics shows that Sc, Y, K, U, P, and M-HREE (except Eu) are relatively unchanged; S, As, Ag, Au, Ca, LOI, Rb and LREE are enriched, and Sr, Ba, Eu, Cr, Co and Ni decrease with an increasing degree of deformation. Based on geochemical features and comparison with other well-known shear zones in the world, the study area is best classified as an Isovolume-Gain (IVG) type shear zone and orogenic type gold mineralisation. Based on the number of phases observed at room temperature and their microthermometric behaviour, three fluid inclusion types have been recognised in quartz-sulphide and quartz-calcite veins: Type I monophase aqueous inclusions, Type II two-phase liquid-vapour (L-V) inclusions which are subdivided into two groups based on the homogenisation temperature (Th): a) L-V inclusions with Th from 205 to 255°C and melting temperature of last ice (Tm) from -3 to -9°C. b) L-V inclusions with higher Th from 335 to 385°C and Tm from -11 to -16°C. Type III three-phase carbonic-liquid inclusions (liquid water-liquid CO2-vapour CO2) with Th of 345-385°C. The mean values of the density of ore-forming fluids, pressure and depth of mineralisation have been calculated to be 0.79-0.96 gr/cm3, 2 kbar and 7 km, respectively. The δ18Owater and δD values of the gold-bearing quartz-sulphide veins vary from 7.2‰ to 8‰ and -40.24‰ to -35.28‰, respectively, which are indicative of an isotopically heavy crustal fluid and likely little involvement of meteoric fluid. The δ18Owater values of the quartz-calcite veins have a range of -5.31‰ to -3.35‰, and the δD values of -95.65‰ to -75.31‰, which are clearly lower than those of early-stage quartz-sulphide-gold veins, and are close to the meteoric water line. Based on comparisons of the D-O isotopic systematics, the Qolqoleh ore-mineralising fluids originated from metamorphic devolatilisation of Cretaceous volcano-sedimentary piles. Devolatilisation of these units occurred either synchronously with, or postdates, the development of penetrative (ductile) structures such as shear zones and during overprinting brittle deformation

Numerical investigation of the interplay between wall geometry and friction in granular fault gouge

AbstractThe influence of surface roughness is central in understanding the behavior of various types of shear zones including faults, landslides, and deformation in glacial till. All of these systems contain a non‐planar rough wall, which interacts with either a gouge zone or another wall. We use the 3‐D discrete element method (DEM) to investigate both the effect of boundary roughness and friction. Granular non‐cohesive gouge is sandwiched between rough walls with large grooves, or smooth walls composed of spherical particles that can be adjusted to control roughness. Roughness and gouge properties are scaled to laboratory friction experiments. We vary friction between the particles and the wall and monitor shear strength, height, coordination number, distribution, and orientation of particle forces, localization, and porosity distribution in the shear zone. We find that, on the first‐order, strength is controlled by particle‐particle friction and mechanical coupling of the fault zone wall to the gouge. Rough boundaries (RMS roughness > grain radius) force more shear within the gouge zone, dilating the layer and sliding more grains, which leads to large stress necessary to shear the layer. When large amplitude roughness is removed, and roughness is at the grain‐scale, the coupling, and thus the strength, is controlled by both wall and particle friction as well as fine‐scale boundary roughness. These differences are reflected in profiles of shear within the gouge zone and offset at the boundary in smooth models. From our simulations, we quantify how and why rough natural faults will have a higher overall strength.

FEM analysis of deformation localization mechanisms in a 3-D fractured medium under rotating compressive stress orientations

Stress distributions and deformation patterns in a medium with a pre-existing fracture set are analyzed as a function of the remote compressive stress orientation (σH) using finite element models with increasingly complex fracture configurations. Slip along the fractures causes deformation localization at the tips as wing cracks or shear zones. The deformation intensity is proportional to the amount of slip, attaining a peak value for α=45° (α: angle between the fracture strike and σH) and slip is linearly proportional with fracture length. Wing cracks develop for high deformation intensities for 30°<α<60°, whereas primary plastic shear zones develop for low deformation intensities. Additionally, two types of secondary shear zones develop for α<30° and α>60°, with constant angles of 135° and −60° with σH, respectively.Mechanical interaction between fractures in a fracture zone, quantified as change in slip compared to an isolated fracture, decreases with increasing fracture separation. Fracture underlap elongates the fracture length and therefore increases the amount of slip, while fracture overlap exhibits the opposite effect. Fracture slip decreases with an increasing amount of directly adjacent fractures. Mechanical interaction becomes negligible for fracture configurations with spacing-to-length and spacing-to-overlap ratios exceeding 0.5 and that in this case fractures are decoupled.Independent of the pre-existing fracture configuration, the development of a secondary systematic fracture set driven by a remote stress rotation is dominated by σH; development of wing cracks or shear zones is restricted to the fracture tips. Blocks with tapered geometries are present in models with a variable fracture strike, where the maximum principal stress (σ1, applying the geological convention that compressive stresses are positive) trajectories consistently deviate from σH; the presence of two systematic σ1 trajectory orientations suggests that two types of secondary features could develop in one re-activation phase.

Low-temperature deformation of limestone, Isola Palmaria, northern Apennine, Italy – The role of primary textures, precursory veins and intracrystalline deformation in localization

Fine-grained limestones of the Tuscan nappe from Isola Palmaria (northern Apennine, Italy) contain sporadically developed, bedding-parallel shear zones, some of which exhibit well-developed calcite mylonites. The presence of shear zones that formed at temperatures less than 250 °C allows the study of limestone deformation mechanisms active under low-temperature, upper-crustal conditions. Two materially distinct types of shear zones developed during the syn-nappe stage of deformation and have been characterized by integrated field, strain, microstructural (optical microscopy, cathodoluminescence, transmission electron microscopy) and EBSD fabric studies. Type A shear zones occur within decametre-thick packages accommodating heterogeneously distributed strain in very fine-grained limestone (micrite). Type B shear zones are localized within calcite veins that develop into recrystallized calc-mylonites. Both types form at the contact with dolomite, although this does not play an important role in strain localization. In both cases, low-temperature ductility occurs by extensive dislocation glide sustained by athermal network-accommodated recovery. Although, the end-state microstructures of both shear zone types cannot be differentiated optically, differences do exist in the crystallographic preferred orientations (CPO) and the grain boundary structure of deformed micrite and vein calcite. Deformed micrite grain boundaries are rich in voids consistent with extensive fluid flux that can enhance material transport and independent grain displacements, while suppressing CPO development. The crystallographically defined grain boundaries in recrystallized vein calcite suggest that lower grain boundary diffusivity enables more typical dislocation-mediated CPO formation.

Softening the lower crust: Modes of syn-transport transposition around and adjacent to a deep crustal granulite nappe, Parry Sound domain, Grenville Province, Ontario, Canada

[1] The Parry Sound domain is a granulite nappe-stack transported cratonward during reactivation of the ductile lower and middle crust in the late convergence of the Mesoproterozoic Grenville orogeny. Field observations suggest the following with respect to the ductile sheath: (1) Formation of a carapace of transposed amphibolite facies gneiss derived from and enveloping the western extremity of the Parry Sound domain and separating it from high-strain gneiss of adjacent allochthons. This ductile sheath formed dynamically around the moving granulite nappe through the development of systems of progressively linked shear zones. (2) Transposition initiated by hydration (amphibolization) of granulite facies gneiss by introduction of fluid along cracks accompanying pegmatite emplacement. Shear zones nucleated along pegmatite margins and subsequently linked and rotated. The source of the pegmatites was most likely subjacent migmatitic and pegmatite-rich units or units over which Parry Sound domain was transported. Comparison of gneisses of the ductile sheath with high-strain layered gneiss of adjacent allochthons show the mode of transposition of penetratively layered gneiss depended on whether or not the gneiss protoliths were amphibolite or granulite facies tectonites before initiation of transposition, resulting in, e.g., folding before shearing, no folding before shearing, respectively. Meter-scale truncation along high-strain gradients at the margins of both types of transposition-related shear zones observed within and marginal to Parry Sound domain mimic features at kilometer scales, implying that apparent truncation by transposition originating in a manner similar to the ductile sheath may be a common feature of deep crustal ductile reworking.

Subglacial deformation and water-pressure cycles as a key for understanding ice stream dynamics: evidence from the Late Ordovician succession of the Djado Basin (Niger)

Subglacial deformation is crucial to reconstructing glacier dynamics. Sediments associated with the Late Ordovician ice sheet in the Djado Basin, Niger, exhibit detailed structures of the subglacial shear zone. Three main types of subglacial shear zones (SSZ) are discriminated. The lowermost SSZ, developed on sandstones, displays Riedel macrostructures and cataclastic microstructures. These resulted from brittle deformation associated with strong glacier/bed coupling and low pore-water pressure. Where they developed on a clay-rich bed, the overlying SSZ display S–C to S–C′ fabrics, sheath folds, and dewatering structures. These features indicate high ductile shear strain and water overpressure. On fine-grained sand beds, the SSZ exhibit homogenized sand units with sand stringers, interpreted as fluidized sliding beds. The succession of subglacial deformation processes depends on fluid-pressure behavior in relation to subglacial sediment permeability. Fluid overpressure allows subglacial sediment shear strength and ice/bed coupling to be lowered, leading to ice streaming.

Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

In the Neves area, the pre-Alpine intrusive protolith of the Zentralgneise unit (Tauern window, Eastern Alps) is well preserved in a kilometric-scale low-strain domain without pervasive Alpine deformation. It is compositionally heterogeneous, consisting predominantly of granodiorites, with lesser leucocratic granites, and different generations of lamprophyres and aplites. The intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes. Incipient Alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities, resulting in the development of several different types of shear zones. Fractures without alteration haloes initially accommodate slip entirely on the fracture itself. With increasing deformation, a foliation is progressively developed in the adjacent host rock, eventually producing a single heterogeneous “ductile” shear zone with the typical sigmoidal foliation pattern. Strong layers (aplite dykes and bleached alteration haloes developed to either side of precursor fractures) localize shear on their boundaries to produce characteristic paired shear zones. Shearing is more evenly distributed within weak layers (lamprophyres and quartz veins), with a marked discontinuity in shear strain against the adjacent, little deformed granodiorite. Shear zone development was accompanied by the formation of new fractures and quartz-rich veins in the host rock, which in turn also localized shear. Magmatic contacts, fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations. The kinematics of the overprinting (strike-slip) shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis σ 1 (here oriented ca. 345°). Discontinuities of almost all orientations show shear reactivation, even in the case of very low resolved shear stress, indicating an overall viscous response of the system without a specific yield stress. The geometry and kinematics of the shear zone network suggest that the overall deformation in low-strain domains was close to coaxial. During deformation along the shear zone network, compatibility was maintained by fracturing (with the development of new quartz veins) and by distributed ductile deformation of the host rock, especially within contractional domains of the shear network and at shear zone intersections. Deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities. Shear zone development in intact rock is always preceded by fracturing, which localizes subsequent shear reactivation.

Shear zones in porous sand: Insights from ring-shear experiments and naturally deformed sandstones

We have used thin section and particle size analyses to relate stress–strain relationships in ring-shear experiments with burial depth at the time of faulting in naturally deformed samples. We show that the burial depth (level of stress) and the amount of shear displacement at the time of deformation are important factors influencing the type of grain breakage and also the type of shear zone that forms. Further, petrographic image analyses with porosity estimations show systematic change related to progressive development of the shear zones and the development of two end-member types of shear zones: (a) Shear zones with diffuse boundaries formed at low levels of stress, and (b) Shear zones with sharp boundaries formed at higher levels of stress. We consider the mechanism of deformation at shallow depth/low level of stress to be dominated by reorganization, rolling and flaking of grains. This mechanism causes rough surfaces of the grains at the margins and within the shear zones. At greater depths or at higher levels of stress, the predominant mode of fracturing transgresses from flaking to grain splitting, resulting in lower porosity values and greater particle size reduction. Furthermore, this transition results in a slow increase in the power law dimension from 1.4 to 1.6 with respect to the increased displacement.