Brittle Shear Zones Research Articles

Deformation partitioning in the mid- to deep sheared crust of the Variscan Fautea-Favone metamorphic complex (southern Corsica, France)

ABSTRACT The Variscan basement in Corsica (France) is exposed as scattered metamorphic complexes preserved in the late Permian batholith in the south-western sector of the island. We present the first tectono-metamorphic map (at 1:10000 scale) of the Variscan high-grade metamorphic complex exposed in one of these complexes in the Fautea-Favone area (south-eastern Corsica). Our geological mapping distinguished the Fautea Migmatitic Complex (FMC), to the south, and the Tarcu-Favone Gneissic Complex (TFC), to the north. Considering the type and distribution of deformation structures, and lithotypes, the TFC and FMC are further divided in three sub-units. The two complexes are separated by a c. 1 km-thick zone where the two complexes are amalgamated in an anastomosed pattern (Transitional Units, TRU). Both complexes are lately affected by ductile to brittle localized shear zones with both dextral and sinistral kinematics, frequently bounding lozenge-shaped low-strain domains.

Geological Characteristics and Preliminary Genesis Exploration of the Sakay Gold Deposit in Vientiane, Laos

The Sakay gold deposit in Vientiane, Laos, is located in the Indochina landmass of the southeastern segment of the Tethys orogenic belt, specifically within the Vientiane-Pakse micro-landmass and the Vientiane-Pakse metallogenic belt. This area is regionally significant for the concentration of minerals such as gold, copper, and tin. The host rocks of the deposit are intermediate volcanic lavas and volcanic tuffs, occurring in near-east-west brittle shear structural fractures through hydrothermal filling and metasomatism. The ore exhibits granular texture, subhedral texture, porphyritic texture, and oriented polycrystalline texture, with structural features such as disseminated, vein-like, and cataclastic breccia. The main ore minerals are pyrite, sphalerite, galena, and chalcopyrite, while the gangue minerals are primarily quartz, calcite, and dolomite. Gold is mainly present as included gold or fissure gold within the crystal lattices and microfractures of minerals such as pyrite and sphalerite. Based on mineral assemblages and generation timing, the mineralization can be divided into three stages: arsenopyrite-pyrite-dolomite-quartz (I), sphalerite-galena-chalcopyrite-calcite (II), and siderite (III), with the latter stages often overlaying the former, showing evident cross-cutting and metasomatic phenomena. The surrounding rocks in the mining area are altered, mainly showing silicification, carbonatization, limonitization, sericitization, and chloritization. Preliminary studies suggest that this deposit is a low-temperature hydrothermal gold deposit within a brittle shear zone.

Long-Lived Seismic Instability of Large Intraplate Brittle Shear Zones Revealed by Distributed Slip Zones and Paleoseismic Frictional Melt, Eastern Botswana

Long-Lived Seismic Instability of Large Intraplate Brittle Shear Zones Revealed by Distributed Slip Zones and Paleoseismic Frictional Melt, Eastern Botswana

Contrasting assemblages of secondary minerals after beryl from the granitic pegmatites Drahonín IV and Věžná I; evidence for high variability of mineralised fluids in the Rožná-Olší ore field area, Czech Republic

Abstract This work presents data for the mineral assemblages, composition and Raman spectroscopy of proximal secondary Be and associated minerals in pseudomorphs after beryl from granitic pegmatites located along the contacts of major regional geological units. The pegmatites differ in their position relative to the ductile to brittle shear zones within the Rožná-Olší ore field (U-deposit), Czech Republic. Extensive dissolution of beryl crystals in the beryl–columbite pegmatites Drahonín IV and Věžná I situated within or close to the shear zones is evident in contrast to minor alteration of beryl in the Dolní Rožínka and Kovářová pegmatites located outside of the shear zones. Near-total replacement of beryl crystals, up to 40 cm in length, from the Drahonín IV pegmatite, located in the Olší shear zone formed the following secondary Be minerals in order of their abundance: bavenite–bohseite > bertrandite ≫ milarite > hydroxylgugiaite. This assemblage is also characterised by the presence of sulfides (pyrite, galena, sphalerite) and zeolites. Such an extensive replacement process required a substantial fluid flow and is very possibly related to the pre-uranium quartz–sulfide and carbonate–sulfide mineralisation events within the Rožná-Olší ore field. Alteration products resulting from breakdown of beryl in the Věžná I pegmatite follow the sequential substages (bertrandite + K-feldspar ± harmotome → epididymite + K-feldspar → hydroxylgugiaite + K-feldspar) and locally show cross-cutting textures. These assemblages were generated by post-magmatic residual fluids (early assemblage bertrandite + K-feldspar) as well as fluids related to a retrograde stage of metamorphism, compositionally contrasting with the host serpentinite, and perhaps also hydrothermal processes associated with the Olší shear zone. The pegmatites Dolní Rožínka and Kovářová, located outside of the shear zones, exhibit only a low degree of alteration and have differing textural and paragenetic development. Highly variable assemblages of secondary minerals after beryl are excellent mineral indicators of hydrothermal overprinting in granitic pegmatites during a variety of subsolidus processes.

Polyphase deformation history and its structural controls on auriferous quartz vein stockwork in the Xiaoqinling district, southern margin of the North China Craton

Polyphase deformation history and its structural controls on auriferous quartz vein stockwork in the Xiaoqinling district, southern margin of the North China Craton

Selection and characterization of the target fault for fluid-induced activation and earthquake rupture experiments

Abstract. Performing stimulation experiments at approximately 1 km depth in the Bedretto Underground Laboratory for Geosciences and Geoenergies necessitates identifying and characterizing the target fault zone for on-fault monitoring of induced fault slip and seismicity, which presents a challenge when attempting to understand seismogenic processes. We discuss the multidisciplinary approach for selecting the target fault zone for experiments planned within the Fault Activation and Earthquake Ruptures (FEAR) project, for which the aim is to induce the fault slip and seismicity for an earthquake magnitude of up to 1.0 while enhancing the monitoring and control of fluid-injection experiments. Structural geological mapping, remote sensing, exploration drilling and borehole logging, ground-penetration radar, and laboratory investigations were employed to identify and characterize the target fault – a ductile–brittle shear zone several meters wide with an intensely fractured volume spanning over 100 m. Its orientation in the in situ stress field favors reactivation in normal to strike-slip regimes. Laboratory tests showed slight velocity strengthening of the fault gouge. The fault's architecture, typical for crystalline environments, poses challenges for fluid flow, necessitating detailed hydraulic and stress characterization before each of the FEAR experiments. This multidisciplinary approach was crucial for managing rock volume heterogeneity and understanding implications for the dense monitoring network. Successfully identifying the fault sets the stage for seismic activation experiments commencing in spring 2024.

The Intraplate Stress Field of West Africa

Abstract West Africa continues to host a growing number of low and intermediate‐magnitude earthquakes (M2‐5) along its passive margins, and its continental interior. Earthquake activity in these regions raises the need to comprehend the causes and the tectonic controls of the seismicity. Unfortunately, such studies are rare. Here, we apply single‐station inversion techniques to constrain fourteen focal mechanisms, computed after compiling a set of high‐quality waveforms. We describe the connection between seismicity, the contemporary stress field, anthropogenic activity and Holocene fault scarps in the region. Our results indicate transpressive stresses acting on the inherited brittle structures in the passive margins. We also observe a compressive regime in the intracontinental failed rifts. We attribute the seismicity to the reactivation of “weak” faults in the Neoproterozoic and Mesozoic failed rifts, the passive transform structures, and the intracratonic Precambrian brittle shear zones.

Deformation and Frictional Failure of Granular Media in 3D Analog and Numerical Experiments

Frictional sliding along grain boundaries in brittle shear zones can result in the fragmentation of individual grains, which ultimately can impact slip dynamics. During deformation at small scales, stick–slip motion can occur between grains when existing force chains break due to grain rearrangement or failure, resulting in frictional sliding of granular material. The rearrangement of the grains leads to dilation of the granular package, reducing the shear stress and subsequently leading to slip. Here, we conduct physical experiments employing HydroOrbs, an elasto-plastic material, to investigate grain comminution in granular media under simple shear conditions. Our findings demonstrate that the degree of grain comminution is dependent on both the normal force and the size of the grains. Using the experimental setup, we benchmark Discrete Element Method (DEM) numerical models, which are capable of simulating the movement, rotation, and fracturing of elasto-plastic grains subjected to simple shear. The DEM models successfully replicate both grain comminution patterns and horizontal force fluctuations observed in our physical experiments. They show that increasing normal forces correlate with higher horizontal forces and more fractured grains. The ability of our DEM models to accurately reproduce experimental results opens up new avenues for investigating various parameter spaces that may not be accessible through traditional laboratory experiments, for example, in assessing how internal friction or cohesion affect deformation in granular systems.

Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit, North China: Evidence from geochronology and noble gas isotopes

Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit, North China: Evidence from geochronology and noble gas isotopes

Hydrothermally altered shear zones: A new reservoir play for the expansion of deep geothermal exploration in crystalline settings

Our study investigates the genesis of granitic reservoirs formed within a brittle shear zone in central Finland, evaluating their feasibility as deep geothermal targets. We employ a suite of laboratory-based experiments and cutting-edge analytical mineral techniques to determine the petrophysical and thermal properties of targeted granitic rocks, elucidating their formation processes. The most favorable reservoir properties were observed in granites affected by cataclasis and mineral dissolution, leading to a notable secondary porosity of ∼20 %. Reservoir quality is largely controlled by the pore network morphology. Alongside fractures, interconnected moldic, sieve, and interparticle pores contribute to substantial permeability of ∼5 × 10−14 m2 (50mD), even under high confining pressures of 50MPa (∼2km). A new geothermal play is here presented: brittle shear zones that have undergone high-temperature (∼200–300 °C) hydrothermal alteration. These findings will significantly enhance our capacity to identify and exploit prolific permeable zones in deep crystalline settings globally, thereby making a substantial contribution to our ongoing transition to cleaner energy resources.

Lava – substrate interaction: Constraints on flow emplacement and basal sintering, Lebuj rhyolitic flow, Tokaj Mountains, Carpathian-Pannonian region

The surface extrusion of rhyolite lava is linked with a complex edifice morphology and texturally diverse internal structure. The rarely exposed basal flow/dome zones provide a textural record of the mechanical and thermal stress between the overlying lava and existing topography. The Lebuj flow (Tokaj Mountains, Hungary) developed in a Miocene caldera setting, where the erosion exposed its basal zone, including various textures due to the lava substrate interaction. The flow margin is contacted with underlying volcaniclastics along a steeply inclined (50–75°) plane. The lithology describes a complex ductile-brittle transition in the flow and lithification process in the substrate. The relict obsidian grains (marekanite, 0.1% H2O) in the perlite (2–3% H2O) are proof of an incomplete hydration process below the glass transition temperature. The dynamic loading of the lava caused irregular fragmentation developing a brittle basal shear zone with lens-like glass deformation. The substrate has suffered syn-emplacement lithification and the primary glass structure is completely re-crystallized. FTIR and Raman measurements identified low-temperature phyllosilicate minerals and SiO2 polymorphs, which caused additional porosity loss and densification. Using cooling time-temperature-porosity information given by this reconstruction, we suggest parallel-acting processes at elevated (groundmass crystallization-devitrification) and lower temperature (hydration-secondary mineralization) range. These define a relative timescale for the textural development in the lithologically heterogeneous contact zone.

On the Scale‐Dependence of Fault Surface Roughness

Abstract Defining roughness as the ratio of height to length, the standard approach to characterize amplitudes of single fault, joint and fracture surfaces is to measure average height as a function of profile length. Empirically, this roughness depends strongly on scale. The ratio is approximately 0.01 at a few mm but 10× smaller at a few tens of meters. Surfaces are rougher at small scales. However, these conclusions are metric‐dependent. If instead height is averaged over wavelength, roughness is nearly Brown spatial noise, having almost scale‐independent apparent surface height to wavelength ratio. The small deviation from scale‐independence is of the opposite sense than found using the standard metric; surfaces are slightly rougher at long wavelengths. Some natural surfaces may be Brownian within the measurement uncertainties. These contradictions are curiosities of surfaces that have Hurst exponents between 0.5 and 1, as natural fault surfaces do. The wavelength‐based analysis of roughness and how it changes with scale are straight‐forward; a normalized Fourier transform approximately preserves amplitude and its scale dependence in the wavelength domain. Among the conclusions from reconsideration of scale dependence are that the scale dependence is weak and much smaller than that of other fault and shear zone properties. Background and aftershock seismicity, jogs and step‐overs indicate strong localization (smoothing) with slip and scale. The lack of strong scale dependence to surface roughness suggests it is not the dominant control on brittle shear zone evolution.

Geology of the Copper Mineralization in Proterozoic Ajabgarh Meta-Sediments of, Dokan-Dariba Belt, Sikar District Rajasthan Northwestern India

The Khetri copper belt is a well-known metallotect in northern part of Delhi fold belt in Rajasthan. On the eastern margin of the Khetri sub basin of North Delhi basin separated by a basement high, another sub basin Alwar-Ajabgarh sub basin exposes that a thick sequence of Ajabgarh group of rocks overlies a thick arenaceous sequence of Alwar group of Delhi Super Group of rocks. The Ajabgarh meta sediments here in the Neem Ka thana area are characterized by presence of Bornite dominated copper mineralization with silver association and minor presence of Pb. The mineralization has been described by various workers as strata-bound, hypogene and IOCG. But these inferences are based on part information and the inference drawn is sectorial in nature. The current study includes a holistic study based on exploration over a period of more than two decades and the data generated suggest thereof, that this syngenetic sulphide mineralization associated with the sedimentation of marl and carbonate rocks. Subsequently it has been relocated during 2nd deformation accompanied by epigenetic component of mineralization depicted in terms of vein filled coarse grained aggregates of bornite and chalcopyrite disposed across the general disposition of litho-package. The EPMA and fluid inclusion data generated from the area indicate association of typical hydrothermal environment minerals like, Perkrite, Wittchenite, Aguilarite, Molybdnite etc. The mineralizing fluids have been trapped between the temperature ranges of 130°C to 375°C with average being 250°C to 300°C. The fluid salinity also varies from near pure hot water to moderately saline fluid indicative of multi-episodic mineralization of syngenetic nature coupled with epigenetic component. The ore textures indicate 500°C temperature range; certain intergrowths of minerals like specular hematite and bornite suggest the occurrence of hypogene environment induced due to emplacement of granite/pegmatite on the eastern and southern margins of the belt. The strata bound nature suggests the euxogenic environment facilitated by carbonate facies of rocks. The parallelism of sulphide with the rock fabric was attained during first deformation and the epigenetic component coupled with the hypogene assemblage was deposited during the D2 deformation in the brittle ductile shear zones and limb shears, between the temperature ranges of 130°C to 570°C as deduced from petrochemical data. Hence a comprehensive model is suggested here on evolution of process of mineralization in the Neem Ka Thana belt.

Drowned in granite - retrieving the tectono-metamorphic history of the Janub metamorphic complex, the northernmost part of the Arabian-Nubian Shield

Drowned in granite - retrieving the tectono-metamorphic history of the Janub metamorphic complex, the northernmost part of the Arabian-Nubian Shield

Episodic alteration within a gold-bearing Archean shear zone revealed by in situ biotite Rb–Sr dating

Episodic alteration within a gold-bearing Archean shear zone revealed by in situ biotite Rb–Sr dating

Origin of the Woxi orogenic Au-Sb-W deposit in the west Jiangnan Orogen of South China: Constraints from apatite and wolframite U-Pb dating and pyrite in-situ S-Pb isotopic signatures

Origin of the Woxi orogenic Au-Sb-W deposit in the west Jiangnan Orogen of South China: Constraints from apatite and wolframite U-Pb dating and pyrite in-situ S-Pb isotopic signatures

Fault zone fluid pathways in the carbonate Irecê basin, NE Brazil

Abstract We applied field structural data and isotope geochemical (δ13C, δ18O and 87Sr/86Sr) analyses to understand the relationship among calcite veins, fault damage zones and carbonate host rocks in a thrust fault damage zone in the Achado quarry, Irecê Basin in the São Francisco Craton, NE Brazil. Our results reveal three hydrological packages with different rheological behaviours in a stratified carbonate succession. The upper package includes the Achado fault damage zone that is characterised by interlayered dolomitised grainstones and mudstones. These rocks display high positive δ13C values (10‰–13‰), negative δ18O values (mean −6.34‰) and radiogenic (87Sr/86Sr) isotope values (0.70885–0.71519). A second package is marked by a cataclastic brittle shear zone lateral parallel to dolograinstones bedding. These rocks show low to positive δ13C values (−3.41‰ to +8.85‰), more positive δ18O values (mean −3.73‰) and radiogenic (87Sr/86Sr) isotope values (0.71039–0.71373). The lower package is characterised by well‐preserved pristine limestone succession that shows δ13C values ranging between −0.46‰ and +3.17‰, mean δ18O = −5.41‰ and less radiogenic 87Sr/86Sr values (0.70762–0.70818). In contrast to the upper and intermediate packages, rocks from the lower one exhibit very low permeability and behaved as a seal for fluid migration. Fluid flow occurred several times during basin evolution, for example along syn‐rift fault damage zones, bedding‐parallel carbonate breccia, thrust faults, cataclastic shear zones, synorogenic conjugate shear fractures or joints and opening mode I fracture‐fill calcite veins. These fractures allowed pervasive fluid flow in the porous intermediate cataclastic shear zone where fluids flowed and formed veins, as diffuse fluid flow in randomly oriented fracture swarms, or channelised fluid flow in aligned fracture corridors. They record significant centimetre‐scale to km‐scale hydrological behaviour within carbonate layers. Most carbonates that are associated with veins, fault damage zones and hydraulic breccia were formed by fluids of the same origin with low δ13C (−6.0 to −2.0‰) and δ18O (−6.0 to −8.5‰) values, and more radiogenic 87Sr/86Sr values compared to the carbonate host rocks.

Mineralization and Structural Controls of the AB-Bid Carbonate-Hosted Pb-Zn (±Cu) Deposit, Tabas-Posht e Badam Metallogenic Belt, Iran

The Ab-Bid deposit, located in the Tabas-Posht e Badam metallogenic belt (TPMB) in Central Iran, is the largest Pb-Zn (±Cu) deposit in the Behadad-Kuhbanan mining district. Sulfide mineralization in the Ab-Bid deposit formed in Middle Triassic carbonate rocks and contains galena and sphalerite with minor pyrite, chalcopyrite, chalcocite, and barite. Silicification and dolomitization are the main wall-rock alteration styles. Structural and textural observations indicate that the mineralization occurs as fault fills with coarse-textured, brecciated, and replacement sulfides deposited in a bookshelf structure. The Ab-Bid ore minerals precipitated from high temperature (≈180–200 °C) basinal brines within the dolomitized and silicified carbonates. The sulfur isotope values of ore sulfides suggest a predominant thermochemical sulfate reduction (TSR) process, and the sulfur source was probably Triassic-Jurassic seawater sulfate. Given the current evidence, mineralization at Ab-Bid resulted from focusing of heated, over-pressurized brines of modified basinal origin into an active fault system. The association of the sulfide mineralization with intensely altered wall rock represents a typical example of such features in the Mississippi Valley-type (MVT) metallogenic domain of the TPMB. According to the structural data, the critical ore control is a bookshelf structure having mineralized dextral strike-slip faults in the northern part of the Ab-Bid reverse fault, which seems to be part of a sinistral brittle shear zone. Structural relationships also indicate that the strata-bound, fault-controlled Ab-Bid deposit was formed after the Middle Jurassic, and its formation may be related to compressive and deformation stages of the Mid-Cimmerian in the Middle Jurassic to Laramide orogenic cycle in the Late Cretaceous-Tertiary.

Post-Cretaceous brittle tectonics in the Tunas Alkaline Complex, Paraná, Brazil

We present a study of the brittle tectonics of Tunas Alkaline Complex focusing on the kinematic and geometric analysis of faults and brittle shear zones deforming the complex. The data acquired and respective interpretations sought to contribute to the post-Cretaceous tectonic structural models of southern Brazil. The ~83 Ma-old alkaline complex is made up of syenites, alkali syenites, plutonic and volcanic breccias, as well as trachyte, microsyenite, and bostonite dykes. Morphostructural analysis using remote sensing combined with right dihedral kinematic analysis of fault-slip data and relations of intersections among structures allowed us to characterize four Cenozoic paleostress fields and their respective tectonic pulses: (I) Eocene-Oligocene NW-SE extension responsible for normal oblique-slip faults striking N60-70E; (II) Oligocene-Miocene NE-SW compression associated with tectonic accommodation of the South American Plate at circa 27-26 Ma, and responsible for WNW-ESE-trending sinistral and NE-SW-trending dextral strike-slip faults; (III) Plio-Pleistocene N-S to NNW-SSE compression related to dextral reactivation of NW-SE-striking faults, sinistral reactivation of NE-SW-trending faults, and generation of NNW-SSE-trending strike-slip faults; and (IV) Pleistocene-Holocene NW-SE to WNW-ESE compression that caused dextral reactivation of ENE-WSW-trending faults and sinistral strike-slip reactivation of NNW-SSE-trending faults.

Assessment of open spaces related to Riedel-shears dip effect in brittle shear zones

Assessment of open spaces related to Riedel-shears dip effect in brittle shear zones