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Classification of fault breccias and related fault rocks
Geological Magazine
Abstract
Despite extensive research on fault rocks, and on their commercial importance,
there is no non-genetic classification of fault breccias that can easily be
applied in the field. The present criterion for recognizing fault breccia as
having no ‘primary cohesion’ is often difficult to assess.
Instead we propose that fault breccia should be defined, as with sedimentary
breccia, primarily by grain size: with at least 30% of its volume comprising
clasts at least 2 mm in diameter. To subdivide fault breccias, we advocate the
use of textural terms borrowed from the cave-collapse literature –
crackle, mosaic and chaotic breccia – with bounds at 75% and 60%
clast content. A secondary breccia discriminant, more difficult to apply in the
field, is the ratio of cement to matrix between the clasts. Clast-size issues
concerning fault gouge, cataclasite and mylonite are also discussed.
... Fieldwork was performed at the outcrop scale, aimed at investigating individual contacts between lithologies and tectonic structures, including folds, thrusts and faults. Faults recognized in the field have been characterized in terms of geometry, distribution of damage, type and composition of fault rocks and kinematics (Woodcock and Mort, 2008). We carried out UAV-based Digital Outcrop Model on the largest outcrop of faulted rocks (lat. ...
... Limestone and dolostone characterized by the presence of angular fragments with jigsaw texture and calcite-filled dilational patches and vein arrays (Figs. 6B and G) support the interpretation that the brecciated rock consist of dilation chaotic breccia (Woodcock et al., 2007;Woodcock and Mort, 2008), associated with a net volume increase during formation (Tarasewicz et al., 2005). The main slip domains show a single, large (up to 3 m) gouge locally surrounded by the huge cataclasites and brecciated damage zone. ...
... The main slip domains show a single, large (up to 3 m) gouge locally surrounded by the huge cataclasites and brecciated damage zone. Differently, the quartzite involved in the fault zone alternate m-sized brecciated bodies (chaotic breccia; Woodcock and Mort, 2008) with phyllosilicate-rich gouges. A further different texture, characterized by cohesive fine grained cataclasite and absence of gouge, is preserved by the marble at the contact with the quartzite, consisting of thin proto-cataclasite. ...
We report the first description of a ~ 15 km long NE-SW-striking transtensive fault network crosscutting the metamorphic units of the Ligurian Alps. The main fault zone, hereby named Horse Head Fault Zone, is up to 250 m thick, involves quarzite, metarhyolite, marble and alternation of dolostone and limestone and minor pelite. Relatively narrow (~ 1–3 m-thick) fault cores are characterized by gouge and cataclasites, surrounded by brecciated damage zones as thick as tens to hundreds of meters. Damage zones show widespread evidence for dilation in the form of dilation breccia, large calcite crystals and aggregates, and centimeter- to meter-thick veins. Moreover, the fault zone contains a multitude of polished slip surfaces with multiple sets of slickensides and slickenfibers. Oblique to strike-slip kinematics dominates over the large part of the fault mirrors and both overprint and are overprinted by down-dip slip surfaces. The fault network includes dominant NE-SW right-lateral faults with a minor normal component and NW–SE left-lateral steep faults with a negligible reverse component, consistent with a km-size dextral NE-SW-striking Riedel shear zone, in turn representing an antithetic R’ of the regional sinistral shear zone constituted by the Ligurian Alps after the nappe stacking. The Horse Head Fault Zone accommodated km-scale displacement before the Early Miocene, as it is sealed by the sedimentary deposits of the Finale Ligure Basin, thus predating the Corsica-Sardinia drifting. Results of this work constraint the bending of the Ligurian Alps as part of the Western Alpine arc as accomplished through two consecutive, late Oligocene and Early Miocene, stages driven by the combination of Adria rotation and the rollback of the Apennine subduction.
... As a result, structural breccia and fault surfaces from pre-Quaternary activity have often been preserved. For instance, in the northern fault zone, early compressive foliation, structural lenses, and breccia can be observed alongside loosely consolidated fault rocks, cataclasites, and localized thin layers of fault gouge [28] (Woodcock and Mort, 2008). In the southern fault zone, early structural lenses and compressive foliation are well-developed, with brecciated rock bands showing preliminary cementation. ...
... As a result, structural breccia and fault surfaces from pre-Quaternary activity have often been preserved. For instance, in the northern fault zone, early compressive foliation, structural lenses, and breccia can be observed alongside loosely consolidated fault rocks, cataclasites, and localized thin layers of fault gouge [28] (Woodcock and Mort, 2008). In the southern fault zone, early structural lenses and compressive foliation are well-developed, with brecciated rock bands showing preliminary cementation. ...
The northern segment of the Tanlu fault zone, which encompasses the Dunhua–Mishan and Yilan–Yitong fault zones, plays a critical role in the tectonic framework of Northeast China. This study focuses on the Hunhe fault, part of the Liaoning segment of the Dunhua–Mishan fault zone, which exhibits concealed characteristics and an NE–NEE orientation. We employ remote sensing and field investigations to accurately delineate the Hunhe fault’s location, scale, and tectonic activity. The findings indicate that the Hunhe fault displays significant spatial variability in tectonic activity. Some segments show evidence of late Quaternary activity, contradicting prior research that classified the Hunhe fault as an active fault during the MIS (Marine Isotope Stages) 20-103MIS 20-103- MIS6-19MIS6-19 period and assessed its seismic potential differently. Recent field investigations suggest considerable spatial variability in tectonic activity, indicating segmental characteristics. In this study, the Hunhe fault is divided into segments based on five aspects: the fault structure and movement characteristics of the fault; transverse faults and obstruction structures; geological and geomorphological characteristics; seismic features; and fault activity. The detailed segments are as follows: the Shenyang segment, the Fushun segment, the Zhangdang-Nan Zamu segment, and the Nan Zamu to Ying Emeng East section. These findings aim to enhance the understanding of the seismic hazard potential associated with the Hunhe fault, highlighting the need for ongoing research to address its complexities and implications for regional seismic risk assessment.
... • Tectonic breccias are clastic deposits consisting of more than 30% of large (>2 mm) particles (Woodcock and Mort, 2008). They are further classified into three sub-groups based on clast arrangements: 1. Crackle breccias are characterized by thin seams of cement or matrix that separate clasts without any apparent displacement. ...
... • Cataclasites are fine-grained fault rocks with less than 30% of large clasts (>2 mm) (Woodcock and Mort, 2008). The classification proposed by Billi (2010) categorizes cataclastic rock fabrics based on particle size distribution (PSD) (Storti et al., 2003) and clast-matrix ratio, utilizing both optical and scanning electron microscopy to assess the level of maturity. ...
This study presents, for the first time in the southern Apennines (Southern Italy), evidence of fault-driven hydrothermal dolomitization during the late Triassic rifting of the western Adria Plate, through examination of fault-controlled saddle dolomite formation in Norian (Upper Triassic) dolomites, exposed in the western sector of the Matese Massif. This investigation focuses on dolomite breccias associated with N-S and NNW-SSE striking normal faults. These structures include layers of mature cataclasites made of clasts with angular boundaries within a highly porous matrix, crossed by veins, mosaic and chaotic breccias. The breccias consist of angular clasts of host rock dolomite, derived from the early marine replacive dolomitization of shallow-water carbonates, surrounded by coarse (ca. 500 μm) saddle dolomite cement. The saddle dolomite cement is characterized by two distinct phases. The first phase (SD1) is yellow in color and inclusion-rich, forming a rim around the clasts. The second phase (SD2) is euhedral and exhibits well-defined zoning, with a transition from cloudy to limpid crystals. The saddle dolomite cement texture and the decreasing δ18O and 87Sr/86Sr values suggest a precipitation temperature of about 100-120°C from a fluid that might have interacted with a magmatic source. The U-Pb dating of the dolomite cement indicates a late Triassic crystallization age of approximately 206 ± 13 Ma and 217.0 ± 6.6 Ma. Furthermore, within the ferroan dolomite cement, quartz and hydrothermal minerals, notably fluorite and apatite, occur in minor quantities. These findings suggest that the brecciation and hydrothermal saddle dolomite precipitation were related to the activity of normal faults during Pangea breakup, resulting in the separation between the SW sector of Eurasia and the western margin of the Adria Plate.
These data provide the first evidence of Triassic syn-tectonic saddle dolomites in this region of the Apennine belt and highlight the important role of U-Pb dating of fracture-filling dolomite cements in unraveling the tectonic evolution of structurally complex areas.
... These samples were categorised into four mesostructural groups: high-angle faults, shear zones, thrusts, and veined rocks. As displayed in Fig. 2a, samples were further classified into macrostructural types on the basis of a revised nomenclature of deformed mineralised rocks, modified after Woodcock and Mort (2008), which for the first time also takes into account veins and brittle-ductile structures. ...
... a) Nomenclature of deformed mineralised rocks (modified afterWoodcock and Mort, 2008). b) Microfabric occurrence of mineralised rocks as constrained in this study. ...
Carbonate deformation through the brittle-ductile transition (BDT) remains incompletely documented in the field. We therefore investigate the exhumation of the SW-Helvetic nappe stack using a new multi-method approach that integrates optical observations with a revised nomenclature, thermochronology, stable isotopes, and clumped isotope thermometry, aiming to constrain the time-temperature history of BDT deformation processes in carbonates. Single grain (U–Th)/He zircon and apatite fission track ages establish new burial/exhumation trajectories from different nappes, allowing us to infer the thermal history of the Rawil Depression. This prominent doubly-plunging hinge zone between the Mont Blanc and Aar Crystalline Massifs underwent post-nappe faulting, now constrained to the Tortonian-Early Pliocene, reflected in differential exhumation rates of ∼0.2 km/Myr between the most depressed area and its easternmost side. Calculating and modelling the rock-buffered clumped isotope temperatures (ranging from ∼250 °C to 55 °C), we indirectly date the BDT processes, exemplified by the Rezli Fault. On this structure, possible shear heating during mylonitisation is indirectly dated at 18–15.5 Ma. Progressive embrittlement started around 11–9 Ma at temperatures of about 150–110 °C. Lower clumped isotope temperatures correspond to recent brittle faulting between 9 and 5 Ma. Our results on the regional evolution can be applied to analogous seismogenic carbonate-rich crustal sections.
... In these cases, the cause of the cohesiveness or incohesiveness of fault rocks should be clarified before their classification. To avoid this problem, alternative classifications that exclude cohesion have been proposed for classifying fault rocks (Killick, 2003;Woodcock and Mort, 2008). For example, Woodcock and Mort (2008) suggested a revised classification that uses clast size as the primary criterion. ...
... To avoid this problem, alternative classifications that exclude cohesion have been proposed for classifying fault rocks (Killick, 2003;Woodcock and Mort, 2008). For example, Woodcock and Mort (2008) suggested a revised classification that uses clast size as the primary criterion. In their scheme, "incohesive fault rocks" in the classification of Sibson (1977) are termed "fault gouge", and breccia is classified based solely on clast size. ...
... Our work informs and may help revise classic models that depict fault deformation and fault-related rocks as a function of depth and posit that shallow fault-related rocks in the upper ∼2-3 km are typically incohesive (Figure 1; Scholz, 1988Scholz, , 2019Sibson, 1977;Woodcock and Mort, 2008). However, the composition and structure of active faults may consist of cohesive, altered, and highly sheared rocks (Boulton et al., 2017) and thin fault cores within damage zones comprised of complexly deformed and altered rocks (Chester and Logan, 1986;Chester et al., 1993;Mitchell and Faulkner, 2009). ...
... The presence of well-indurated fault-related rocks at such shallow depths is inconsistent with fault-zone models that postulate that the upper several kilometers of faults consist of unconsolidated to poorly consolidated gouge or breccia, clay gouge, or pulverized rocks (Figure 1; c.f., Woodcock and Mort 2008;Scholz 2019). Cohesive, mineralized, and 'cemented' cataclasite and intact fault gouge developed in the upper 2-3 km of this major fault zone. ...
Quantifying shallow fault zone structure and characteristics is critical for accurately modeling the complex mechanical behavior of earthquakes as energy moves within faults from depth. We examine macro- to microstructures, mineralogy, and properties from drill core analyses of fault-related rocks in the steeply plunging ALT-B2 geotechnical borehole (total depth of 493 m) across the San Gabriel Fault zone, California. We use macroscopic drill core and outcrop-sample analyses, core-based damage estimates, optical microscopy, and X-ray diffraction mineralogic analyses to determine the fault zone structure, deformation mechanisms, and alteration patterns of exhumed deformed rocks formed in a section of the fault that slipped 5-12 million years ago, with evidence for some Quaternary slip. The fault consists of two principal slip zones composed of cohesive cataclasite, ultracataclasite, and intact clay-rich, highly foliated gouge within upper and lower damage zones 60 m and 50 m thick. The upper 6.5 m thick principal slip zone separates Mendenhall Gneiss and Josephine Granodiorite, and a lower 11 m thick principal slip is enclosed within the Josephine Granodiorite. Microstructures record overprinted brittle fractures, cohesive cataclasites, veins, sheared clay-rich rocks, and folded foliated and carbonate-rich horizons in the damage zones. Carbonate veins are common in the lower fault zone, and alteration and mineralization assemblages consist of clays, epidote, calcite, zeolites, and chloritic minerals. These data show that shallow portions of the fault experienced fluid-rock interactions that led to alteration, mineralization, and brittle and semi-brittle deformation that led to the formation of damage zones and narrow principal slip zones that are continuous down-dip and along strike.
... The Acquasanta Breccia is a well cemented tectonic breccia, which can be defined as a protocataclasite, namely a "non-foliated crackle-to-mosaic breccia" sensu Woodcock and Mort (2008). It is very compact and composed of heterometric serpentinite clasts, up to about half a meter across, usually angular or subangular in shape, and cemented by abundant fine-grained, dark green/blackish or red-brownish material (Figs. 4 and 5). ...
... ing and microfaulting. This mechanism induced a homogeneously distributed crushing that produced a cataclasite or a protocataclasite, namely a "non-foliated crackle-to-mosaic breccia" sensu Woodcock and Mort (2008). This structure was achieved by frictional processes that induced grain-size reduction along clast boundaries, which developed a finegrained matrix in the interspaces between the grains. ...
This paper sheds light on syn-exhumation, pervasive cataclastic processes occurred on competent serpentinites of a metaophiolitic sub-unit of the Voltri Massif. The reported data highlight the importance of the compositional and structural heterogeneities within the ophiolite-bearing exhumation channels, since the observed pervasive cataclastic processes affected only the more competent rock bodies of the metaophiolitic unit, giving origin to a serpentinite breccia, named as Acquasanta Breccia. This breccia is cemented by random-oriented chrysotile fibers that give rise to a very uncommon microstructure, as highlighted by optical microscopy, Micro-Raman spectroscopy, SEM-EDS and TEM investigations. The Acquasanta Breccia underwent a sequence of syn-exhumation tectonic events, accompanied by the onset of different generations and types of chrysotile, which occur both in veins and in the rock matrix. The breccia records the intermediate and late stages of the exhumation of the Voltri meta-ophiolites, which occurred after the development of the retrograde greenschist-facies foliation and before the late shallower faulting events.
... The similarity in the lithologies present in these breccias together with the superimposed, i.e. multiple brecciation of the same rocks hampers the interpretation of the processes responsible for the formation of the breccias (François, 1973;Cailteux and Kampunzu, 1995;Wendorff, 2000;Cailteux et al., 2018;Jackson et al., 2003;Mambwe, 2017;Selley et al., 2018). Regarding the economic importance and resource potential of the breccias and their relation to the geodynamic evolution of the Katanga basin, as well as the tectonic evolution after the Lufilian orogeny, the aim of this review is to: (1) discuss the diverse genetic models and terminologies used for the breccias occurring in the Katanga Supergroup; (2) propose a uniform classification by using the terminology of Morrow (1982) coupled to the mechanisms of brecciation defined by Jébrak (1997), and in accordance with the more recent classification of Woodcock and Mort (2008); (3) place the breccias in the geodynamic context according to their lithostratigraphical position in both the DRC (KCB) and Zambia (ZCB); and (4) highlight the resource potential. The data used here are from published papers by the authors and others, and from new field work in the CACB. ...
... Open fault zones which formed during transpressional or transtensional tectonism created the accommodation space for the formation of chaotic fault breccia (cf. Woodcock and Mort, 2008;Woodcock et al., 2014). Such fault breccia can be subjected to subsequent hydrofracturing and in situ fragmentation and friction caused by the tectonic activity combined with fluidization (see Murphy, 1984;Jébrak, 1997;Dewey et al., 1998;Branquet et al., 1999;Quintana et al. 2006;Woodcock et al., 2006). ...
The Katanga Supergroup, host of several world class Neoproterozoic sediment-hosted stratiform copper and cobalt deposits, is part of the Central Africa Copperbelt (CACB). The origin and classification of the breccia, which is one of the most characteristic features of the Katanga Supergroup, has been a matter of discussion since the 19th century. Different nomenclatures and various classifications have been used in the mining operations across the CACB. The difficulty to identify genetically related types of breccia creates some problems in regional correlations and comparisons. In addition, their origin is most relevant for the metallogenesis of the world-class copper and cobalt sediment-hosted deposits. This paper classifies the breccia according to their characteristics, reviews the mechanisms of brecciation, and thus addresses their origin, and highlights their potential in metal resources. The different types of breccias found in CACB include sedimentary breccia (gravity flow, sedimentary breccia stricto sensu), dissolution and collapse breccia, hyaloclastite breccia, hydraulic breccia and tectonic breccia (thrust-breccia and fault breccia). The spatial and chronological distribution of these breccias in the CACB follows the geodynamic history of the area starting from early Neoproterozoic rifting up to the Lufilian orogeny and post-Lufilian tectonism.
... The fault rock close to the Cama fault surface is a chaotic to mosaic breccia (Woodcock and Mort, 2008) composed of incipient angular clasts (1-10 mm in size) cut by numerous fractures oriented 50 • -90 • to the slip surface (very rough due to karst processes) and forming conjugated pairs (Fig. 9a). Most fractures are filled with secondary sparite, composed of blocky and almost euhedral calcite grains (Bons et al., 2012), with straight and indented (white in color arrows in Fig. 9d) boundaries ( Fig. 9c and d). ...
... The fault scarp is discontinuous and crops out only in the middle and southern sectors, with maximum height of ~2 m. The fault core is almost absent, except close to the south-eastern tip, where it consists of mosaic fault breccias (Woodcock and Mort, 2008) and the damage zone is < 40-m-thick and not affected by secondary faults (Fig. 6). Extensional fractures are usually spaced >10 cm apart, except in few areas close to the master fault, where they are spaced <3 cm apart and conjugated themselves (Fig. 6) with orientation consistent with the Pleistocene extensional phase. ...
... Despite decades of research into the nature, origin, and behavior of faults, the associated terminology and classification are still not universally agreed upon (Woodcock & Mort, 2008). Depending upon the formation process, depth of occurrence, geological activities, and hydrogeological and anthropogenic activities, faults show wide variations in their behaviour. ...
The presence of hidden or concealed faults in and around mine openings and excavations has always been a unique challenge to mine management. Typical to Indian mining conditions, the limited availability of geotechnical and structural data during exploration results in a vast data gap and uncertainty in mine planning. To define the structural complexity of the mine and adequacy of
geotechnical data gathering, preliminary data from 200 coal blocks spread across different parts of India data were collated to determine the fault density, and an attempt was made to correlate its impact of mining operation and likelihood of fault induced operational and safety risks. Out of 200 coal block data assessed, it was observed that 109 coal blocks (55%) had no information about the faults in the borehole database gathered, indicating that either the deposits are free from major structural features such as faults or the borehole density adopted is not adequate to capture the existing structural complexities. To validate the challenges of
near fault zones, data from 09 large operational opencast mines complied, and it was observed that mines with higher fault density showed indications of several issues ranging from slope instabilities, water seepage, migration of coal fire and gases, ore dilution along with hindrance to mine operations such as the deployment of a surface miner, highwall mining, re-organization of mine excavation schemes and sterilization of resources, among others. An analysis of “Fault Density versus Level of Exploration” showed a clear correlation with an R2 value of 0.53, with a mean fault density value of 0.2/sq.km and 1.40/sq.km for regionally explored and fully explored coal blocks, respectively. Fault density was very low in regional exploration compared to the detailed one, but operational mines even showed a higher number of faults, indicating significant gaps in early-stage structural database gathering.
Keywords: Fault and Shear zones, degree of complexity, fault density, coal mines
... This breccia consists of angular to rounded limestone clasts with axes of 1-7 mm (in the two-dimensional view provided by thin sections) surrounded by matrix/cement in which clasts are < 0.1 mm in length. The fault breccia is defined as a protocataclasite, according to the classification of Woodcock and Mort (2008). The composition of the protocataclasite displays large spatial variations, with some portions containing abundant clasts (Fig. 6c), whereas others are dominated by fine matrix (Fig. 6d). ...
Reconstructions of palaeoseismicity are useful for understanding and mitigating seismic hazard risks. We apply cosmogenic ³⁶Cl exposure-age dating and measurements of rare-earth elements and yttrium (REE-Y) concentrations to the palaeoseismic history of the Sparta Fault, Greece. Bayesian-inference Markov chain Monte Carlo (MCMC) modelling of ³⁶Cl concentrations along a 7.2 m long vertical profile on the Sparta Fault scarp at Anogia indicate an increase in the average slip rate of the scarp from 0.8–0.9 mm yr⁻¹ 6.5–7.7 kyr ago to 1.1–1.2 mm yr⁻¹ up to the devastating 464 BCE earthquake. The average exhumation of the entire scarp up to the present day is 0.7–0.8 mm yr⁻¹. Modelling does not indicate additional exhumation of the Sparta Fault after 464 BCE. The Sparta Fault scarp is composed of fault breccia, containing quartz and clay-lined pores, in addition to host-rock-derived clasts of calcite and microcrystalline calcite cement. The impurities control the distribution of REE-Y in the fault scarp surface and contribute spatial variation to ³⁶Cl concentrations, which precludes the identification of individual earthquakes that have exhumed the Sparta Fault scarp from either of these data sets. REE-Y may illustrate processes that localize slip to a discrete fault plane in the Earth's near-surface, but their potential use in palaeoseismicity would benefit from further evaluation.
... These gouges are produced through the intense pulverization of the host rock, induced by heat and frictional forces. Typically, fault gouges are composed of fine-grained material that is incohesive and clasts no larger than 2 mm, which do not exceed 30% of the total material (Woodcock and Mort, 2008). Although the fault gouge zone can vary greatly in thicknesses from a few mm to several tens of km (Scholz, 1987;Sibson, 2003), the primary deformation zone located along the principal slip surface (Engelder, 1974), with narrow zone ranging from < 1 mm to several cm in thickness (Faulkner et al., 2003;Sibson, 2003;Billi, 2005;Smith et al., 2011;Woo and Han, 2019). ...
The fault gouge, a crushed fault rock resulting from seismic slip, exhibits variations in crystallinity and particle size contingent on the magnitude of deformation. Therefore, the fabrication of fault gouge analogues with carefully controlled properties such as crystallinity, grain size, and mineral composition is necessary to systematically study their role in a range of fault slips. While the single mineral fault gouge analogues have been reproduced using a milling method, the fabrication methods for multi-mineralic fault gouge analogues with varying particle size and crystallinity are rarely reported, yet. In this study, we propose two methodologies for fabricating multi-mineralic fault gouges controlling the degree of amorphization and the particle size using a high-energy ball mill: mixing-grinding (MG) and grinding-mixing (GM) types. The MG type was designed to simulate the fault gouge that the same grinding energy is applied to all the constituent minerals, while the GM type can be controlled the degree of deformation of each mineral according to the research objective. To investigate the effects of these methodologies on the reduction in crystallinity and particle size, we compared the characteristics of MG type_6h and GM type_6h samples, both ground for 6 h. Consequently, despite undergoing the same grinding duration, the GM type_6h sample exhibited more significant reduction in crystallinity and more heterogeneous particle size. Additionally, we fabricated the GM type_Xc50 sample, where the crystallinity of all constituent minerals was reduced to < 50%. For this, each mineral was ground for an optimized duration that reduced its crystallinity to < 50%, after which the samples were mixed. Consequently, the GM type_Xc50 sample demonstrated the greatest reduction in crystallinity and the most uniform particle size distribution. To fabricate the generation process of natural fault gouge, it is appropriate to use the MG type to apply the same energy to constituent mineral, resulting the deformation reflecting its hardness. The GM type is recommended for use in fabricating the characteristics of the fault gouge that has undergone intense deformation at the slip surface, which allows for controlling the uniform crystallinity and particle size reduction of each constituent mineral. Our results suggest that the diverse nature of naturally occurring fault gouges can be fabricated in laboratory settings by adjusting the grinding conditions. This study offers effective methods for fabricating fault gouge analogue for frictional experiments for fault slip and potential applications in simulating geochemical reactions in fault zones.
... Regarding the brittle deformation, only those structures with mappable-scale extension and/or an important displacement considering the map scale must be reported in the final version of the map together with the associated kinematics, if recognisable. The types of fault rocks must be classified considering the classification based on the clast/matrix ratio (Sibson, 1977;Passchier & Trouw, 2005;Woodcock & Mort, 2008;Fossen, 2016), reporting in the legend in the description of fault rocks the recognised kinematic indicators. ...
... Dilational jogs are intersected by fractures and veins ( Fig. 2a-g), often showing stockwork textures. Crackle to chaotic hydrothermal breccias (Woodcock and Mort, 2008) occur in dilational jogs (Fig. 2g). Fault tips are characterized by horsetail veins branching from the principal fault ( Fig. 3a and b). ...
... The samples were collected in the same study area along different fault zones, characterized by different kinematics (normal vs strike-slip), ages, and dimensions. The chosen hand specimens derive form the following structural domains (Woodcock and Mort, 2008) : ...
Aiming at assessing the porosity and permeability properties, we present the results of microstructural and laboratory measurements, including density, porosity, VP, VS, and electrical resistivity. These measurements were performed in dry and in saturated conditions on 54 blocks of Mesozoic carbonate host rocks and fault breccias collected in Greece. The host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias from the Senonian and Vigla formations. These rocks exhibits average density values, low porosity values, and medium-to-high P- and S-wave velocities. Fault breccias originate from high-angle extensional and strike-slip fault zones, displaying a wider range of density, porosity values up to 5–10 times higher than host rock, along with ultrasonic velocities. Regardless of lithology, the carbonate host rocks might include vugs due to selective dissolution. Conversely, the fault breccia samples feature microfractures. Slight textural anisotropy is documented in the carbonate host rocks, while a higher degree of anisotropy characterizes the fault breccias. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa, revealing that transport properties along microcracks in fault breccias can significantly increase with increasing depth. To assess rock permeability and porosity-permeability relations, three different protocols were employed. Two of them were based on the Effective Medium Theory, where permeability was computed by inverting ultrasonic measurements, assuming an array of penny-shaped cracks embedded in an impermeable host matrix. The aspect ratio and crack width were obtained by the seismic measurements, modeling either by assuming all cracks as isolated or unconnected or all cracks connected into the network. The application of these two protocols showed a systematic variation of permeability with porosity. In contrast, the results of the third protocol, based on the digital image analysis outcomes only, did not exhibit systematic variation. This behavior was interpreted as a result of the not-selective dissolution of the outcropping carbonates causing a wide range of measured fracture aperture values. This study found that carbonate host rocks lacked a clear poro-perm trend due to the presence of stiff, sub-rounded pores and small vugs. On the contrary, fault breccia exhibited a linear increase in permeability with porosity due to a connected pore network including microfractures.
... Hydrothermal breccias are occasionally associated with the veins (Fig. 9e). These breccias exhibit chaotic to crackle textures (sensu Woodcock and Mort, 2008), with subangular to subrounded host rock clasts embedded in a carbonate cement primarily consisting of calcite, with minor dolomite (Fig. 9e). The host rock fragments are generally not in contact with one another (Fig. 9e). ...
This study examines the impact of polyphase tectonics on the development of structurally controlled hydrothermal fluid pathways in carbonate geothermal reservoirs. Our case study focuses on hydrothermal carbonate veins and vein-filled faults in Devonian carbonates from the North Rhine-Westphalia region in western Germany, currently being explored as a potential low-enthalpy geothermal reservoir at depths of 4–5 km. These veins, which can be up to 20 m thick, are subvertical and strike NNW, N, or ESE. They exhibit pinch-and-swell undulating geometries, faulted vein-wall contacts, and occasional fillings of hydrothermal breccias. Vein-filled normal faults show hybrid shear-dilatant openings, with fault tips characterized by horsetail vein terminations. The textures, orientations, and age of the veins suggest their formation at low confining pressures and at depths < 2 km during the Post-Variscan East-West extension. Orthogonal North- and East-striking strike slip faults, inherited from the Variscan orogeny, were likely reactivated during post-Variscan extension, with a significant dilatant component that formed the observed veins. In the Ruhr Basin, the dilation tendency analysis indicates that NNW-striking veins or joints are optimally oriented for re-opening under the current strike-slip stress regime, characterized by NNW-trending maximum horizontal stress. The intersections of fractures may currently create moderately SSE-plunging linear zones of enhanced fluid flow. The main uncertainty regards the presence of similar structures at geothermal reservoir depths of ∼4–5 km in the Middle Devonian carbonates underneath the Ruhr Basin. As the study veins are likely to have formed at depths <2 km, the existence of analogous dilatant conduits at greater depths remains speculative. Eventually, we propose that zones characterized by open discontinuities and channelized fluid flow in carbonate geothermal reservoir in strike-slip tectonic settings can be: (a) dilational jogs between overlapping strike-slip faults, (b) bends along faults, and (c) strike-slip fault terminations with horsetail extensional structures. These zones can also be prone to enhanced karstification.
... The Mez211002 sample comes from the inner brecciated part of the eclogitic metabasite lens. Polished saw-cut sections in Fig. 4a and b show that the breccia is a mosaic breccia following the terminology of Woodcock and Mort (2008). Clasts are not in contact but are separated by an omphacite matrix and show minor rotations of less than 10°. ...
The study of rock chemistry is a milestone in understanding fluid-rock interactions and fluid migration in subduction zones. When combined with thermodynamic models, it can provide direct insight into fluid composition, metasomatic reactions, and pressure-temperature (P-T) conditions, as well as their role in rock deformation. Here, a shear zone-located in the Mont Avic area of the Zermatt-Saas zone (Western Alps)-is analyzed. This shear zone consists of several blocks of different lithotypes, including a Ca-rich metasomatite block embedded in a serpentinite mylonitic matrix, and structurally underlies a coherent eclogitic mafic unit. This work aims to estimate the pressure-temperature conditions of the Ca-rich metasomatism and the amount of fluid involved. The brecciation exhibits mosaic breccia textures with clasts comprising ∼ 80 vol % of garnet, together with omphacite, epidote, titanite, rutile, and apatite hosted in an omphacite matrix. Quantitative chemical mapping of the garnet reveals primary garnet cores with embayment and lobate edges with a chemical composition similar to unaltered reference eclogite garnet. These primary garnet cores are overlain by Ca-rich metasomatic garnet rims with oscillatory chemical zoning. The oscillatory chemical zoning, together with the morphology of the primary garnet cores, suggests repeated influxes of external Ca-rich fluid that destabilized the primary garnet cores and promoted the growth of Ca-rich rims. Mass balance calculations between precursor metabasite and Ca-metasomatite indicate multiple fluid sources involving dehydrated serpentinite, calcic metasediments, and metabasites with time-integrated fluid fluxes calculated between 11.5×10 3 and 5.5×10 4 m 3 fluid m −2 rock , consistent with channelized fluid flow in an open system. Thermodynamic modeling of garnet from unbrecciated and non-metasomatized metabasites-from the Savoney eclogitic mafic unit-indicates peak metamorphic conditions of 2.5 ± 0.1 GPa and 535 ± 40°C, consistent with regional estimates. Pressure-temperature conditions of metaso-matism were constrained using P-X and T-X phase modeling (where X represents changes in bulk CaO and Na 2 O composition) between 2.6-2.2 GPa and 570-500°C, showing that Ca-rich fluid percolation occurred close to the metamorphic peak (i.e., prograde to the peak or early exhumation path).
... The primary structural features are poorly foliated serpentinite fragments, which exhibit angular morphologies. These fragments develop crackle or mosaic-type breccia (in the sense of Woodcock & Mort, 2008) fabric and domino-type dismembering (Figure 4a). The inter-clast space within the breccia is filled with olivine-rich veins. ...
Serpentinites are major carriers of volatiles in deep subduction zones, releasing most fluids in the 500–650°C range. Despite fundamental implications for mass transfer and intermediate‐depth seismicity, the mechanical role of these fluids is unclear. To characterize the mechanical role of fluids at (ultra)high‐pressure conditions, we perform a petro‐structural analysis on olivine‐rich veins from the Western Alps meta‐ophiolite. Some veins formed through dilational and mixed dilational‐shear fracturing without significant shear‐related deformation. However, field and microstructural observations indicate transient shearing and dilational fracturing at high pore fluid pressures. These include: (a) foliated sheared veins; (b) newly formed olivine and Ti‐clinohumite within mineral lineations coating sheared veins and shear bands; (c) Olivine and Ti‐clinohumite mineral fibers sealing porphyroclasts; (d) mutual crosscutting relationships among dilational and shear features. Dilational veins prevail in low‐strain areas, while sheared veins and shear bands dominate within high‐strain zones toward the ultramafic sliver boundaries. These strain variations underscore the role of local stress regimes during serpentinite dehydration. Consequently, areas experiencing stronger shear stresses around large‐scale blocks or mechanical weakening during fluid circulation are prone to draining overpressurized fluids. These interface‐parallel and fracture‐controlled pathways thus facilitate fluid escape from the dehydrating downgoing slab. Transient events of dilational fracturing and brittle‐ductile shearing, along with strain localization in highly comminuted olivine‐bearing sheared veins, may have resulted from strain rate bursts potentially related to (sub)seismic deformation. These observations are in line with geophysical data indicating high pore fluid pressures within the intermediate‐depth seismicity region.
... The igneous character of the protoliths was determined following the classification guidelines provided by Le Maitre et al. (2002). Additionally, six cataclasites from the PFS (three samples) and CFS (three samples) were classified according to the criteria outlined byWoodcock and Mort (2008) and examined to identify deformational textures and structures followingPasschier and Trouw (2005) andTrouw et al. (2010). The fault rocks were described in terms of microstructural domains which may have occurred during the same deformation event. ...
Plutonic rocks typically have negligible matrix porosity and permeability. However, fractures and mineral alterations create storage space and flow pathways that turn plutonic rocks into fluid reservoirs. Despite significant hydrocarbon discoveries, naturally fractured reservoirs in plutonic rocks have been poorly studied. In most Colombian basins, the crystalline basement has undergone multiple deformational events and is thrust over the Cretaceous to Cenozoic source and reservoir rocks of the conventional petroleum system. This structural configuration is ideal for the migration of oil into a fractured basement. A multiscale fracture analysis, including field, petrographical and petrophysical techniques was conducted on the Permian and Jurassic plutonic basement of Upper Magdalena Basin in order to understand the controls on brittle deformation, the development of fracture networks and their potential to form hydrocarbon reservoirs. The results indicate that protolith textures and structures, including magmatic and mylonitic foliation, favours fracturing. Dykes exhibit higher fracture density (7–48 fractures/m), porosity (mean = 0.4%) and permeability (mean = 125,818.75 mD) than the host rock (2–25 fractures/m; 0.23%; 12,066.09 mD). Intersection zones from regional faults, are characterized by the highest fracture and lineament intensity. Our results suggest that dyke swarms and interacting damage zones can significantly enhance the reservoir quality of plutonic rocks by providing storage in fractures and fluid pathways to the host rock.
... 7a-e and 11). The stockwork veins tend to concentrate in a narrow host rock volume of approximately several meters in width, making the wall rock into crackle-chaotic breccia ( Fig. 11a and b; Woodcock and Mort, 2008). The extension veins are syntaxial and exhibit diverse degrees of sealing (perfectly sealed, partially sealed, or unsealed) and show a blocky to elongate blocky texture ( Fig. 11b and c). ...
This study explores the coupled processes of structural inversion and fluid migration around the northeastern part of the Mesozoic Chungnam Basin in the southwestern Korean Peninsula, focusing on quartz vein systems genetically linked to an orogenic gold deposit. Our results show that the inversion structures reflect the coexistence of strike-slip simple shear and pure shear components caused by NW–SE crustal shortening, leading to a transpressional deformation model. The quartz veins, filling the high-angle faults with a reverse slip component and adjacent hydrofractured wall rocks, represent transient fluid flow related to seismic faulting and fault valving during the inversion. New K–Ar ages of illite polytypes in fault gouges, determined using the Illite-age-analysis (IAA) method, indicate Late Jurassic–Early Cretaceous fault reactivations. Notably, ca. 160 Ma 2M1 illite age from a fault zone filled with an orogenic gold–bearing quartz vein indicates that the inversion and auriferous hydrothermal fluid flow started with a magmatic quiescence in the southern Korean Peninsula likely owing to the flat subduction of the Paleo-Pacific Plate. Our findings suggest that the orogenic gold–bearing vein system was likely sourced from the subcrustal metamorphic fluid in the flat subduction zone of the Mesozoic East Asian Continental margin.
... Microstructural and microchemical constraints of dilation breccia formation X-Ray Microscopy and petrographic analysis highlight a dilation breccia structure, with internally weakly foliated omphacitite fragments ranging in size from a few microns to several centimetres (Figs. 2 The omphacitite breccia can be classified as crackle and mosaic breccia 43 . Locally, the breccia shows also jadeitite veins oriented subparallel to the foliation visible in the clasts. ...
Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H2 and light hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH4-H2-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H2-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH4-H2-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO2-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.
... Hence, we summarize those aspects here before focusing on the petrological evolution below. The damage zone of the small-displacement fault zone in Í Botni (IBO: location in Figure 1d, detail in Figure 2) consists of a range of variably altered cataclastic breccias (Figure 2b) in contact with the fault core, grading back to undeformed host rock through a zone of chaotic, mosaic, and crackle breccias (fault rock classification from Woodcock & Mort, 2008) (Figures 2b and 2c). In the large-displacement Gøtugjógv (GOT: location in Figure 1c, detail in Figure 3) and Selatrað (SEL: location in Figure 1c, detail in Figure 4) fault zones, the damage zone is characterized by high concentrations of meter-to decameter-scale fractures and secondary faults. ...
Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip‐related deformation on upper‐crustal faults in basaltic rocks is closely associated with fluid‐rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt‐derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault‐rock petrology is determined by optical petrography and automated phase identification based on micrometer‐scale chemical maps from scanning electron microscope X‐ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X‐ray fluorescence. The fault rocks show intense fluid‐mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along‐fault percolation of carbonated meteoric water, increasing fluid‐rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure‐solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid‐mediated alteration by CO2 and water, while other species are redistributed within the fault zones.
... We employed these rock block to perform experimental analyses aimed at measuring the values of porosity, density, ultrasonic velocities and electrical resistivity at room pressure and temperature. In the field, the hand specimens were collected because representative of the following carbonate lithologies (Woodcock and Mort, 2008): 115 -Host Rocks exposed Away from major Fault Zones (HR-AFZ); ...
Aiming at assessing the porosity and permeability properties, we present the results of microstructural and laboratory measurements of density, porosity, VP, VS, and electrical resistivity performed in dry and in saturated conditions on 54 blocks of Mesozoic carbonate host rocks and fault breccias. Host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias pertaining to the Senonian and Vigla formations. These rocks show average density values, low values of porosity, and medium-to-high P- and S-wave velocities. Fault breccias derive from high-angle extensional and strike-slip fault zones, and are characterized by a wider range of density, porosity values up to 5–10 times higher than host rock, and low ultrasonic velocities. Independently on lithology, the carbonate host rocks might include vugs due to selective dissolution. Differently, the fault breccia samples include microfractures. A slight textural anisotropy is documented in the carbonate host rocks, whereas a higher degree of anisotropy characterizes the fault breccias. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa, showing that transport properties along microcracks in fault breccias can significantly increase with increasing depth. In order to assess rock permeability and porosity-permeability relations, three different protocols are employed. Two of them are based on the Effective Medium Theory, so that permeability is computed by inverting ultrasonic measurements and assuming an array of penny-shaped cracks embedded in an impermeable host matrix. Accordingly, the aspect ratio and crack width are obtained by the seismic measurements. Two end terms have been modelled by assuming all cracks isolated, and unconnected or all cracks connected into the network. Application of these two protocols shows a systematic variation of permeability with porosity, whereas the results of the third one, based on the digital image analysis outcomes, do not exhibit systematic variation. We interpret this behavior as due to the not-selective dissolution of the outcropping carbonates causing a wide range of measured fracture aperture values.
... A classi cação morfológica de brechas não considerou a origem genética como proposto por Woodcock & Mort (2008;Figura 1). Nesta proposta considera a proporção de clastos grandes (>2mm), clastos pequenos (0,1-2 mm) e matriz (>0,1mm), além da proporção clastopreenchimento, arredondamento e no grau de rotação dos clastos. ...
Brechas cimentadas (BCs) subverticais truncam camadas de dolomito da Formação Serra do Quilombo de idade Ediacarana, porção superior do Grupo Araras da Bacia Araras-Alto Paraguai (BAAP), sul do Cráton Amazônico. A investigação estrutural destas ocorrências exposta principalmente em frentes de lavra de minas permitiu identi car três morfologias principais de brechas: "crackle", "mosaico" e "caótico", além de ocorrer como preenchimento de veios de falha. O estudo revelou que as BCs exibem forte controle estrutural, exibindo um zoneamento desde ocorrências em veios até o padrão caótico próximo às falhas tectônicas principais. O contato basal com os calcários da Formação Guia facilitou o processo de brechação ligado a propagação de fraturas e falhas, além de posteriormente funcionar como um caminho para o uido sobre pressão. Os resultados obtidos fornecem novos dados sobre o evento hidráulico-termal nos dolomitos ediacaranos da BAAP que podem ser relacionados as fases evolutivas da tectônica regional que afetou esta parte do sul do Cráton Amazônico. PALAVRAS-CHAVE: Neoproterozoico, Bacia Araras-Alto Paraguai, Sul do Craton Amazônico, Brecha hidráulica, hidrotermalização. INTRODUÇÃO A origem de brechas cimentadas (BC) que cortam verticalmente as camadas de dolomito da Formação Serra do Quilombo, Ediacarano é alvo de debates dentro do quadro evolutivo da Bacia Araras-Alto Paraguai (BAAP) do sul do Cráton Amazônico. Trabalhos prévios têm con-siderado como deposicional ou preenchimento de falhas tectônicas com in uência hidrotermal (Nogueira & Riccomini, 2006; Milhomem et al., 2013). A análise geométrica-estrutural reve-lou que estas brechas estão relacionadas à um evento tectono-termal que afetou a BAAP. Estes litotipos foram observados na frente de lavra da mina de calcário da Emal-Camil na região de Cáceres, Estado do Mato Grosso, e fornecem indicações importantes sobre a distribuição e migração de uídos durante os eventos de deformação transtensional que inverteu moderada-mente a BAAP (Santos et al. 2020). O estudo destas rochas permitiu detalhar a tipologia e os processos de formação fornecendo subsídios para posicioná-las no espaço e no tempo da evo-lução geológica do Sul do Cráton Amazônico.
... The carbonates rock masses (limestones and dolomites) found in several hundred meters thick zones along the study area (i.e., the western coastal Lefkada island, Greece) are highly tectonized, almost disintegrated, and at places powdered (Rondoyanni, et al., 2012) forming limestone/dolomite fault breccias. Based on the observed fragment sizes, the limestone/dolomite fault breccias can be classified given the Woodcock and Mort (2008) classification system. The majority of the observed breccias are crackle breccias. ...
Fault zones comprise parent carbonate rocks that are altered due to fault deformations and are transformed into fault rocks. They typically exhibit substantial heterogeneity since they comprise intact, unaltered stiff rock fragments which are surrounded by a mainly soft, weak matrix. In the present study, the major types of fault rocks detected in carbonate fault zones are documented and a methodology to derive their mechanical properties is introduced. These types include the parent/host rock, welded fault breccias, unwelded fault breccias and matrix material. Carbonate fault rocks from western Lefkada island, Greece, were studied in the laboratory to establish a relationship between their Uniaxial Compressive Strength and their textural features, i.e., the geometrical aspects of their fragments and matrix (size, shape, distribution). The results indicate that the Uniaxial Compressive Strength of the welded/unwelded breccias is a function of the corresponding strength of their components (fragments and matrix) and the matrix ratio (the percentage of matrix in a given volume). Due to the weak nature of the matrix, the Uniaxial Compressive Strength of the matrix cannot be readily measured. Therefore, suggestions for the determination of its Uniaxial Compressive Strength based on field assessment are proposed.
... 20 Ma identical to that of unaltered schists that has been interpreted as the cooling age of the basement exhumation and the 40 Ar-39 Ar age of sericite in the sulfide ore is disturbed by the Zn-Pb mineralization thermal event (Gilg et al., 2006); the volcanic rock clasts in the ore-hosting breccias have undergone chloritization, but this alteration does not occur in the marble and schist clasts and thus is less likely associated with the Zn-Pb mineralization. The large vertical extent of the breccia pipe containing clasts of exotic volcanic rocks derived from the footwall sequence indicates that the breccias are unlikely to have been generated by carbonate dissolution-collapse processes or tectonic or hydraulic faulting (Sangster, 1988;Jébrak, 1997;Loucks et al., 2004;Woodcock and Mort, 2008). However, upward transportation of rock clasts is common in a diapiric zone (e.g., Warren, 2016;Stern et al., 2011;Thomas et al., 2015;Lawton and Amato, 2017). ...
The Angouran deposit (19.3 Mt at 23.4% Zn and 4% Pb) is the second-largest Zn-Pb deposit in Iran. The deposit is hosted in a Neoproterozoic−Cambrian marble-schist sequence within a breccia pipe in a domal structure, with sulfide mineralization under low-temperature hydrothermal conditions (<200 ºC). The features of the ore-hosting breccias are similar to known halokinetic diapir breccias in the world but evaporite minerals are subtle. The common types of breccia clasts in the Angouran breccia pipe include a matrix-supported angular clast (float breccia) with highly variable sizes and orientations and exotic volcanic clasts. The volcanic clasts were derived from the underlying Miocene volcanic rocks, evidenced by the consistent petrography and zircon U-Pb ages dated at 20−19 Ma. Abundant smithsonite pseudomorphs after anhydrite and anhydrite inclusions within sphalerite and pre-ore marcasite in the breccia matrix indicate that the breccia pipe contains abundant anhydrite prior to the Zn-Pb mineralization. The enrichment of evaporite is also supported by the occurrence of considerable double-terminated quartz crystals that contain spherical and tabular carbonate inclusions and anomalously high Li, Na, and K concentrations, relatively high B concentration, and high δ18O values (up to 28.3‰). These observations suggest the Angouran deposit formed in a former halokinetic diapir breccia pipe. The halokinetic diapirism was possibly triggered by thrust loading of the marble-schist sequence over the Miocene evaporite beds during the Arabia-Eurasia continental collision. Halokinetic structures elsewhere in the Angouran region warrant this consideration. Most of the evaporite minerals in the breccia pipe were dissolved and replaced before and/or during subsequent Zn-Pb sulfide and smithsonite mineralization events. This study provides a good example for the identification of vanished evaporites, halokinetic structure, and associated Mississippi Valley-type mineralization.
... Mechanisms to explain shallow slip deficits and fault-zone plasticity include frictional slip on weak fault-related rocks, pore-fluid pressure weakening (Marone and Saffer, 2007), or bulk plastic yielding (Roten et al., 2017) due to the presence of lower cohesive and frictional strength rocks in fault damage zones (Chester et al., 1993;Isaacs et al., 2008;Jeppson et al., 2010;Bradbury et al., 2011;Jeppson and Tobin, 2015). Most fault zone classification schemes and interpretations (Sibson, 1977;Woodcock and Mort, 2008) depict the uppermost parts of fault zones as consisting of low or non-cohesive fault zones comprised of 'incohesive breccias'. Most of these models fail to account for the nature and extent of a range of fault-related rocks observed in the rock record, especially foliated fault-related rocks that form in the upper crust (c. ...
We examine fault-related rocks from the upper parts of the active San Andreas and ancient San Gabriel Faults, southern California, to determine the nature and origin of micro-scale composition and geochemistry of fault-related rocks. These data constrain the nature and extent of fluid-rock interactions and processes by which slip is accommodated on shallow portions of these faults. The steeply dipping San Gabriel Fault (SGF) was sampled in a steeply inclined borehole to a depth of 400 m, and the San Andreas Fault (SAF) was sampled by seven northeast-plunging boreholes to a depth of 250 m. Fault damage zones 100m+ wide exhibit narrow fault core zones within broad damage zones. Petrographic, mineralogic, whole-rock geochemical analyses and synchrotron-based X-ray fluorescence mapping of whole drill core and thin sections reveal evidence for repeated syntectonic hydrothermal alteration, Fe-Mn rich mineralization, shearing, and brecciation that resulted in the formation of foliated cataclasites and clay and chlorite-rich shear zones in fractured network fault zones. Mineralization and alteration include clay and chlorite development, carbonate and zeolite mineralization, and the mobility of trace and transition elements in the deformed rocks. Textural evidence for repeated shearing, alteration, vein formation, brittle deformation fracture, fault slip, pressure solution, and re-lithification of faulted rocks suggests that hydrothermal alteration occurred during deformation at shallow levels. The rock assemblages likely represent significantly weakened rocks that have the potential to slip at low shear stresses, experience creep, distribute seismic energy within and near the fault, and show that hydrothermal conditions in faults may exist at very shallow levels of active faults.
... In order to document the internal structure as well as mineralogical and chemical properties of the fault rocks at the Cunielongba exposure, we collected a series of samples from NE to SW on the outcrop along the river. The fault core at the Cunielongba exposure consists of light yellow breccia, yellow breccia, black fault gouge, and gray breccia, classified by their appearance, color, and fragmented grain sizes (e.g., Woodcock & Mort, 2008), and the damage zones containing the light yellow and gray sandstone were distinguished by the increasing density of fractures as well as by Cheng et al., 2011;Wen et al., 2008), and focal mechanisms (earthquake.usgs.gov and Papadimitriou et al., 2004). ...
While the Xianshuihe fault displays continuous creeping behavior, it is also the most seismically active fault in the eastern Tibetan Plateau, and its earthquake mechanisms remain unclear. Here, we aim at the stick‐slip portion of the creeping Qianning segment of the Xianshuihe fault to determine the characteristics of fault rocks and how fluids at depth influence fault behavior. Field survey, optical and scanning electron microscope observations, X‐ray diffraction and fluorescence analyses, as well as carbon and oxygen isotope analyses were performed on the collected samples. The fault core consists of 3–5 cm‐thick black fault gouge and ∼2.5 m‐thick breccia, surrounded by ∼12 m damage zone. In contrast to fault breccia (1–8 cm in diameter), the black fault gouge, which represents the principal slip zone of repeated seismic events, contains angular quartz particles (∼10 μm on average) and clays dominated by illite. The fluid‐rock interactions altering silica minerals into illite, and the thermal decomposition of carbonate minerals, passively increase the relative content of quartz and feldspar (total 63%–73%) in the fault gouge. The deeply sourced CO2 (from mantle and metamorphic degassing) within the hydrothermal fluids causes carbonate precipitation in breccias (21%–53%), composed of calcite, dolomite, and aragonite. These fluid‐assisted reactions lead to more abundant strong mineral phases (quartz, feldspar, and carbonates, 64%–87%) than weak clays (12%–36%) within the fault core, and locally strengthen the fault, which inhibits slow release of stress at shallow depth and promotes seismic rupture of the fault.
... We also consider an over-consolidated material for which both the density and strength of the material are supposed to have increased with time under compaction with high normal stress (Morrow & Byerlee, 1989). This can originate from cementation due to rock dissolution or partial melting (Di Toro et al., 2009;Fondriest et al., 2020;Rodrigues et al., 2021;Walderhaug, 1994), from local comminution (i.e., wear particle formation) or from the introduction of other sediments within the fault gouge (Lee & Kim, 2005;Woodcock & Mort, 2008). In Casas et al. (2022), we simulated a cemented granular fault gouge (dry contact) with 2-D Discrete Element Modeling (DEM), establishing a link between the initial amount of porosity and cementation within the gouge and the type of rheological behavior observed. ...
Fault zones are usually composed of a granular gouge, coming from the wear material of previous slips, which contributes to friction stability. When considering a mature enough fault zone that has already been sheared, different types of infill materials can be observed, from mineral cementation to matrix particles that can fill the remaining pore spaces between clasts and change the rheological and frictional behaviors of the gouge. We aim to understand and reproduce the influence of grain‐scale characteristics on slip mechanisms and gouge rheology (Riedel bands) by employing the discrete element method. A 2D‐direct shear model is considered with a dense assembly of small polygonal cells of matrix particles. A variation of gouge characteristics such as interparticle friction, gouge shear modulus or the number of particles within the gouge thickness leads to different Riedel shear band formation and orientation that has been identified as an indicator of a change in slip stability. Interpreting results with slip weakening theory, our simulated gouge materials with high interparticle friction or a high bulk shear modulus, increase the possible occurrence of dynamic slip instabilities (small nucleation length and high breakdown energy). They may give rise to faster earthquake ruptures.
Mineral deposits are made up of admixtures of ore minerals and gangue from which metal is extracted. Concentration of such ore minerals may vary from very small mineral occurrence to economically exploitable large resource. The size, level of concentration, shape, extension, geographic location, the feasibility of mining and beneficiation, logistics, infrastructures, and social, economic, and political stability of the country, etc., are some of the important decision-making factors, to develop a potential ore deposit for exploitation. Ore minerals have different primary and secondary textures, which helps in identifying the suitable beneficiation processes to recover the metal (s) from them. Deposits of such ore minerals are formed by means of various geological processes, such as magmatic, hydrothermal, metamorphic, and sedimentary. There are several criteria adopted by various geoscientists time to time based on which ore deposits are classified. Based on mechanical, chemical, and biological process of their concentration, they are categorized as Eluvial, Colluvial, Fluvial or Alluvial, Aeolian or aeolian, ultramafic, Mafic, Felsic, Porphyritic, Skarn, Volcanogenic, Sedimentary Exhalative, Mississippi Valley Type, metamorphic, residual, Supergene, and Sedimentary deposits. Based on their genesis, they are classified as syngenetic and epigenetic deposits, while based on conditions of origin, they are grouped as epithermal, mesothermal, and hypothermal Deposits. Metallic mineral deposits are categorized based on nature of metal, such as Ferrous, non-ferrous, Nobel or PGM, Rare Earth, and Radioactive deposits, while non-metallic mineral deposits are classified based on commodity and their applications, like used for manufacturing of fertilizers, cements, refractories, ceramics, glass, abrasives, cosmetics, gemstones, organic or fossil fuels, such as coal, natural gas, and petroleum, and miscellaneous Industrial mineral deposits.
A new general and genetic classification is proposed that can be applied to sedimentary deposits and rocks (sedimentites), in six main groups depending on the processes that originate them, this classification seeks the standardization and systematization of the criteria and the most used terms, and proposes some new terms: (1) detrital sedimentites formed by fragmentation of rocks “in situ”; (2) clastic sedimentites formed by erosion, transport and sedimentation of solid materials; (3) biogenic sedimentites formed by biological productivity and activity “in situ”; (4) chemical sedimentites formed by crystal precipitation from solutions “in situ” within the basin; (5) mixed sedimentites, those formed by mixed sedimentary processes, e.g., biogenic accumulation and clastic sedimentation, among others; and (6) diagenetic modification-transformation sedimentites, entities formed “in situ” by compaction- reorganization, alteration-transformation, dissolution-collapse, precipitation-cementation, crushing and grinding, or deformation-remobilization processes. Additionally, a new compositional-mineralogical classification of the sedimentites in eleven groups are proposed: silicate; carbonate; phosphate; iron, manganese, aluminum and titanium oxides and hydroxides; sulfate and salts; organic rich; nitrate; sulphide; borate; native rich (metals and the non-metals); and mixed. This systematization offers the advantage of allowing a better and easier sequencing of the classification of the sedimentites and facilitates the improvement in the teaching and learning processes. This systematization offers the advantage of allowing a better and easier sequencing of the classification of sedimentary deposits and rocks and facilitates the improvement in the teaching and learning processes.
The Calcare Cavernoso, a well-known and controversial lithostratigraphic unit of the Northern Apennines,
represents a thick layer with a predominantly cataclastic brecciated texture, that is located at the base of the
Tuscan Nappe and that plays an important role in the evolution of the Northern Apennines. Due to its primary
lithological features, this lithostratigraphic unit favored the development of regional tectonic detachments during
the compressional and extensional phases that affected the orogenic chain. It consists of slightly fractured to
fully cataclastic limestone and dolomitic limestone originated from Upper Triassic dolostones, limestones and
gypsum/anhydrites as a result of the interaction of tectonic and diagenetic-autoclastic processes. On the basis
of stratigraphic-structural field relationships and textural characteristics of two significant areas in southern
Tuscany, it can be pointed out how the Calcare Cavernoso there exhibits the features of a tectonic large-scale
breccia in which diagenetic and autoclastic processes were certainly important, but are subordinate to, and
superimposed by the cataclastic process. The recognition of different breccia types, examined through textural
and image analyses, combined with field mapping has made it possible to discriminate between breccias of
different origin (tectonics vs. sedimentary) and to understand their origin and role in the stratigraphic-structural
framework of the Northern Apennines. This interpretation, which emphasises a main tectonic role for the Calcare
Cavernoso of southern Tuscany, allows us to characterise it as a tectonic chaotic unit or lithostratigraphic complex
with features of a broken-and-transformed formation, mainly derived from the disruption of Upper Triassic primary
evaporite-dolomitic strata at the base of the Tuscan Nappe. This new interpretation of the Calcare Cavernoso
requires a different approach for geological mapping in terms of formal lithostratigrapic units and also in terms of
geomechanical parameters.
Understanding the behavior of fluids in seismically active faults and their chemical-physical (dis)equilibrium
with the host rock is important to understand the role of fluids upon seismicity and their possible potential for
forecasting earthquakes. The small number of case studies where seismic and geochemical data are available and
the lack of accessibility to fault zones at seismogenic depth for recent earthquakes limit our understanding of
fluid circulation and its relationship to seismicity. The study of fault-fluid relationships in exhumed faults can
broaden the number of case histories and improve our understanding of the role of fluids in the seismic cycle in
different tectonic settings. Here we use new geochemical and thermal data and a review of published studies
from the Apennines fold-and-thrust belt (Italy) to provide a model of fluid circulation during the seismic cycle
related to either the local orogenic compressional or post-orogenic extensional tectonics. We also suggest a
workflow based upon different methods to identify tectonic-related chemical-physical (isotopic and thermal)
(dis)equilibria in fluid-rock systems during the seismic cycle. The proposed workflow involves multiscale
structural and isotope geochemical analyses, radiometric dating, and burial-thermal modeling. It is applied to
carbonate-hosted faults exhumed from a depth shallower than 4 km (temperature ≤~130 ◦C and pressure ≤
~130 MPa). We show that in the Apennines, during syn-orogenic shortening, thrusting is mostly assisted by fluid
circulation in an effectively closed system where fluid and host rock remain close to chemical and thermal
equilibrium. In contrast, post-orogenic normal faulting occurs in association with upward and/or downward
open fluid circulation systems leading to chemical-physical disequilibria between the host rock and the circulating
fluids. Isotopic and thermal fluid-rock disequilibria are particularly evident during pre- and co-seismic
extensional deformation. Mineralizing fluids, whose temperature can vary between 30 ◦C warmer and 16 ◦C
colder than the host rock, result from the mixing of fluids derived from both the deforming host rock and external
sources (meteoric or deep crustal). The proposed workflow offers the potential to track past seismic cycles and
provide indications on actual fluid-earthquake relationships including the identification of potential seismic
precursors and modes of triggered seismicity that might be different in extensional and compressional tectonic
settings.
A conceptual state of the art review of the research on fault zone shear resistance is presented. Recent works on the subject are analyzed in the context of the approaches formulated in the authors’ presentations at the Sixth Conference “Triggering Effects in Geosystems”. The analysis of the results obtained in the last two or three decades by different research teams shows that the frictional properties of the slip zone gouge play a determining role in the rupture initiation and propagation patterns. The refinement of the methods for processing the parameters of weak seismicity, aimed at estimating the “slowness” of microearthquakes confined to a fault zone, may lead to new approaches in fault zone monitoring to derive indirect information on the material composition of a fault slip zone and, thus, on its seismogenic potential. At present, such methods may be useful in solving the problems of reducing the damage caused by man-made earthquakes.
Unraveling the age and kinematics of low temperature deformation events is crucial in understanding the late‐stage evolution of orogens. However, accurate age constraints can often be challenging to obtain due to unideal outcrop conditions, large sedimentary hiatuses or the lack of well‐defined thermal events. In this study, we show on the example of the Nekézseny Thrust, a poorly exposed late orogenic thrust in the southern Western Carpathians, that a combined approach of structural analysis and multi‐method thermochronology can provide the necessary temporal, kinematic and thermal constraints for a detailed reconstruction of the deformation history. While structural mapping revealed that the Late Cretaceous Uppony Gosau Basin in the footwall of the Nekézseny Thrust underwent a significant post‐Campanian and pre‐Miocene shortening, K/Ar dating of fault gouge samples from the main fault zone constrained the primary thrusting event to the Maastrichtian. Based on the acquired apatite fission‐track and (U‐Th)/He ages, subsequent heating of the Upper Cretaceous sediments due to tectonic burial was limited to 75–100°C, followed by deformation‐related and gradual cooling between the Eocene and Early Miocene. Considering the reconstructed deformation history, as well as the large‐scale tectonic affinity of the displaced units in its footwall and hanging wall, the Nekézseny Thrust is a far‐traveled (ca. 600 km) segment of the Late Cretaceous Alps‐Dinarides contact zone, whose development was linked to the switch from lower plate to upper plate position with respect to the Sava Zone and Alpine Tethys sutures, respectively.
Davis, G.H., Bos Orent, E., Clinkscales, C., Ferroni, F.R., Gehrels, G.E., Guns, K.A., George, S.W.M., Hanagan, C.E., Hughes, A.N., Iriondo, A., Jepson, G., Kelty, C., Krantz, R.W., Levenstein, B.M., Lingrey, S.H., Miggins, D.P., Moore, T., Portnoy, S.E., Reeher, L.J., and Wang, J.W., in press, 2023, Structural analysis and chronologic constrains on progressive deformation within the Rincon Mountains, Arizona: Implications for development of metamorphic core complexes: Geological Society of America Memoir 222, p. 1-125. https://doi.org/10.1130/2023.1222(01)
The goal of this study was to accurately evaluate the lateral sealing ability of a fault in siliciclastic stratum based on previous analysis of the lateral sealing of faults by a large number of scholars in the published literature and physical simulation experiments. Content of the clay mineral phase and the diagenetic degree of fault rock were investigated as the main factors to evaluate the lateral sealing of faults. Based on this theory, the configuration relationship between the clay content and burial depth of fault rock (SGR&H) threshold evaluation method for the lateral sealing of faults was established. Then, we applied these results to evaluate the lateral sealing ability of faults in the Beixi, Beier, Wuerxun, and Surennuoer areas in the Hailar Basin, China. The variation in SGR boundary values with burial depth between the lateral opening and moderate sealing area, as well as between the moderate and strong sealing area of the faults, are obtained. Compared with the previous methods, the SGR&H threshold method transforms the static SGR value of a formation or even a region into a dynamic SGR value that changes with the burial depth, which can fully characterize the differences in the conditions required for sealing faults with different internal structures at different depths. In determining the lateral sealing ability of faults by comparing the evaluation results, we discovered the following. (1) In the same area, the sealing thresholds of faults within different layers are different because the deep strata are subjected to greater pressures and longer loading times, so these faults are more likely to seal laterally. (2) In the same layer, the sealing thresholds of faults in different areas are also different. The higher the thickness ratio between the sandstone and the formation (RSF), the smaller the entry pressure of the fault rock when it has reached a critical seal state, so the SGR&H thresholds are relatively small. Compared to the previous methods, the SGR&H threshold method in this article reduces the exploration risk of faults with relatively low diagenetic degree in shallow strata, and also increases the exploration potential of faults with relatively high diagenetic degree in deep strata. The evaluation results are more consistent with the actual underground situation.
Metamorphic fluids and deformation zones are fundamental carriers of carbon from the deep Earth to shallower reservoirs. It is well established that water weakens rocks and contributes to trigger seismicity. Conversely, the potential effects of carbon-rich aqueous fluids on deep-seated rocks remain little studied. Some of these fluids may be reduced and transport energy sources such as natural H 2 and CH 4 . Mechanisms and pathways capable to transport these deep energy sources towards shallower reservoirs, where they may be used by microbial life in the subsurface biosphere, remain unidentified. Here we present direct geological evidence of seismic failure of mechanically strong rock types due to the accumulation of CH 4 -H 2 -rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H 2 -rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modelling predicts that the production and accumulation of carbon-rich fluids can produce overpressure much more easily than C-poor aqueous fluids, and that CH 4 -H 2 -rich fluids are more favourable to produce overpressure than CO 2 -rich ones. This study provides tangible evidence for the migration of deep Earth energy sources along tectonic discontinuities and suggests causal relationships with brittle failure of hard rock types that may trigger seismicity at forearc depths.
The Etam Shear Zone is a N to NNE trending strike-slip zone, and it displays a sinistral shearing mylonitic (ductile) zone and a brittle zone. Field observations and microstructural studies reveal that the ductile zone experienced strain partition. Primary minerals (quartz, K-feldspar, Plagioclase, and biotite) in wall rock (granodiorite) are gradually reduced towards the intensively deformed shear zone center. Quartz recrystallization also increases in the same direction. The structural evolution reveals the following: (1) a first phase of deformation D 1 underlined by S 1 foliation and mylonitic schistosity (Sm). This phase is oriented N10°E to N22°E and is associated with L 1 lineation (NNE-SSW, average plunge of 10° NE), F 1 fold (fold axis~ S60E), and asymmetric β 1 boudins of domino's type; (2) a second phase D 2 that develop F 2 folding, C 2 shearing planes (N60E to N75E and dipping strongly W to NW), and a late D 2 brittle event. Microstructures such as recrystallization, sub-grain boundary, myrmekites, and undulose extinction characterize the sub magmatic and post magmatic deformation. Kinematic markers (rotated mineral, shearing planes, and fold vergence) reveal an earlier dextral shearing movement in biotite gneiss followed by sinistral shear sense recorded in mylonites. The mica-chlorite-epidote assemblage defines a greenschist facies in the Etam Shear Zone. Metasomatic reactions (Plagioclase + K-feldspar + water = Quartz + Albite + Epidote + Chlorite) during pervasive alteration of some minerals have probably released silica during deformation that was carried by hydrothermal fluids and precipitated in pull-apart fracture to form hydrothermal syn-kinematic quartz veins.
Fluids influence fault zone strength and the occurrence of earthquakes, slow slip events, and aseismic slip. We introduce an earthquake sequence model with fault zone fluid transport, accounting for elastic, viscous, and plastic porosity evolution, with permeability having a power‐law dependence on porosity. Fluids, sourced at a constant rate below the seismogenic zone, ascend along the fault. While the modeling is done for a vertical strike‐slip fault with 2D antiplane shear deformation, the general behavior and processes are anticipated to apply also to subduction zones. The model produces large earthquakes in the seismogenic zone, whose recurrence interval is controlled in part by compaction‐driven pressurization and weakening. The model also produces a complex sequence of slow slip events (SSEs) beneath the seismogenic zone. The SSEs are initiated by compaction‐driven pressurization and weakening and stalled by dilatant suctions. Modeled SSE sequences include long‐term events lasting from a few months to years and very rapid short‐term events lasting for only a few days; slip is ∼1–10 cm. Despite ∼1–10 MPa pore pressure changes, porosity and permeability changes are small and hence fluid flux is relatively constant except in the immediate vicinity of slip fronts. This contrasts with alternative fault valving models that feature much larger changes in permeability from the evolution of pore connectivity. Our model demonstrates the important role that compaction and dilatancy have on fluid pressure and fault slip, with possible relevance to slow slip events in subduction zones and elsewhere.
La generación de espacios dilatantes en las rocas hospedantes es fundamental para facilitar la circulación y acumulación de fluidos mineralizantes con potencial económico. En depósitos diseminados, los cambios volumétricos y morfológicos en rocas con cuarzo y feldespato, debido a sus coeficientes de dilatación (contracción) térmica diferencial relativa, determinan dilatancia haciéndolos porosos y permeables a los fluidos. Las constantes de dilatación térmica contrastantes para el cuarzo y el feldespato en procesos termocrecientes y termodecrecientes se revisan mediante el modelado volumétrico del efecto de una variación significativa de la transición de cuarzo α−β alrededor de 575 °C. Este fenómeno podría variar desde 0.4 % para un granitoide con bajo contenido de cuarzo hasta 1.2 % durante el enfriamiento de un boxwork porfírico típico. En el caso de cuerpos vetiformes, el fallamiento produce dilatación por flexión transtensiva con aumento de su influencia cuando muestran formas irregulares, escalonadas o flexuradas. De acuerdo con la Ley de Anderson, las porciones de estas fallas serán dilatantes cuando el ángulo con el esfuerzo principal compresivo subparalelo (γ = ángulo entre σ1 y la superficie de falla) es pequeño. El conocimiento morfoestructural y cinemático de los mecanismos generadores de dilatancia controlante de las mineralizaciones constituye una valiosa y práctica metodología de trabajo, que contribuye al pronóstico de localización de los mejores sectores de mineralización en volumen y calidad durante las actividades de prospección y exploración. Ambos mecanismos físicos que generan la dilatancia en las rocas huésped son factores de suma importancia en la definición económica de las mineralizaciones que las ocupan.
The generation of dilating spaces in the host rocks is essential to facilitate the circulation and accumulation of mineralizing fluids with economic potential. In disseminated deposits, the volumetric and morphological changes in rocks with quartz and feldspar, due to their coefficients of relative differential thermal expansion (contraction), determine dilatancy, making them porous and permeable to fluids. The contrasting thermal expansion constants for quartz and feldspar in thermo-increasing and thermo-decreasing processes are reviewed by volumetric modeling of the effect of a significant variation of the quartz transition α−β around 575 °C. This phenomenon could vary from 0.4 % to 1.2 % for granite with different quartz content during the cooling of a typical porphyry boxwork. In the case of vein bodies, faulting produces dilation by transtensive bending with an increase in thickness when they show irregular, stepped and/or bended shapes. According to Anderson's Law, portions of these faults will be dilatant when the principal stresses are subparallel (γ = the angle between σ1 and the fault surface). The morphostructural and kinematic knowledge of the mechanisms that generate the controlling dilatancy of mineralization constitutes a valuable and practical work methodology that contributes to the location forecast of the best mineralization sectors in terms of volume and quality during prospecting and exploration activities. Both described physical mechanisms that generate dilation in the host rocks are factors of great importance in the economic definition of the mineralizations control.
Physical factors likely to affect the genesis of the various fault rocks-frictional properties, temperature, effective stress normal to the fault and differential stress-are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge-breccia- cataclasite series-pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10-15 km.
The results are presented from an integrated sedimentologic, structural, stratigraphic, and diagenetic study of syndepositional
faults and fractures that cut the Permian Capitan reef and equivalent platform strata within three outcrop windows in Slaughter Canyon, Guadalupe Mountains, New Mexico. The studied faults are dip slip, have displacement of up to 30 m parallel the shelf margin, and are vertically and laterally segmented. The faults dip steeply shelfward and basinward, and grew incrementally during deposition of the Yates and Tansill Formations. Steep polycyclic paleocavern systems developed along faults and fractures. These can have cumulative vertical relief of more than 270 m, extend at least 1.2 km along strike, and are typically less than 10 m wide but can attain a width of 90 m. The paleocaverns can extend more than 110 m below the top of the Capitan reef. The paleocaverns are filled mainly with sediments deposited during Capitan progradation and aggradation. Seven Permian lithologies are distinguished: (1) limestones and limestone breccias, (2) microspar-lithified breccias, (3) carbonate-rich breccias, (4) reworked and remnant breccias, (5) beige dolomitic siltstone–sandstone and associated breccias, (6) pink dolomitic siltstone–sandstone and associated breccias, and (7) spar-cemented breccias. The fills vary vertically and along strike within the paleocaverns but have a clear organization and stratigraphy. The integration of the stratigraphy of the paleocaverns with the structural and sequence stratigraphic framework developed in back-reef strata provides evidence for incremental fault growth and multiple episodes of dissolution, brecciation, collapse, deposition, cementation, and dolomitization within the paleocaverns during Capitan times. Solution-modified syndepositional faults extend at least 33 km along strike from Slaughter Canyon and are considered to be an integral component of the Capitan platform. The fault-zone paleocaverns contain a unique internal record of events and processes that have no counterpart in the shelf succession. Their study provides new insights into the internal heterogeneity and diagenesis of the Capitan platform. The results have important implications for the Capitan platform, and more generally for the heterogeneity of syndepositional fracture-controlled karst systems formed in carbonate platforms with steep unstable margins, subject to compaction-induced tilting and/ or developed in active tectonic settings.
Geologic mapping, interpretation, and field checking of recent aeromagnetic data suggest the existence of a closely associated series of faults and splays extending from Alabama to Virginia, herein termed the Eastern Piedmont fault system. Characteristic magnetic anomalies were found to be associated with known faults and were used to trace them through covered intervals. The fault system extends northeastward from the Goat Rock fault of Alabama and west-central Georgia, crossing the lower Piedmont of South Carolina, passes beneath a segment of the Coastal Plains in the Carolinas, and then flanks the Raleigh belt in North Carolina and continues into Virginia. From east-central Georgia to Virginia, cataclastic rocks along the faults of the system are bounded to the northwest and southeast by rocks of the Carolina slate belt, forming perhaps the most extensive fault system in eastern North America. Its movement history is similar to that of the Brevard fault: an early ductile mylonitic phase, followed by periods of brittle deformation. We interpret the fault system to have been initiated during collapse of the late Precambrian early Paleozoic Carolina slate belt island arc. The Paleozoic continental suture probably lies farther east, buried beneath the Coastal Plain.
Paleocave systems form an important class of carbonate reservoirs that are products of near-surface karst processes and later burial compaction and diagenesis. Features and origins of fractures, breccias, and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems. Information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale, pore networks, and spatial complexities. Spatial complexities in paleocave reservoirs result from near-surface and burial processes. Near-surface processes include dissolutional excavation, clastic sedimentation, chemical precipitation, and localized fracturing, brecciation, and collapse of cave walls and ceilings. Burial processes begin as cave systems subside into the subsurface. Remaining cave passages commonly collapse and early-formed breccia clasts are rebrecciated. Differential compaction of strata around and over collapsed passages produces fractures, crackle breccias, and mosaic breccias. Near-surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity. Hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed-paleocave systems. The coalescing of passages in a cave system into larger, connected porosity zones results from a combination of multiple, cave-forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured. This combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across, commonly forming large exploration targets. Final size, pore-network types, and spatial complexities of coalesced collapsed-paleocave systems are products of their evolution from near-surface development through burial into the deeper subsurface. The coalesced collapsed-paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved.
Many traditional terms for fault-related rocks have undergone recent dynamic metamorphism under high-pressure discussions by various groups of specialists. A generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic, engineering, and legal purposes. Such a framework is proposed here, focusing on a rate-of-strain versus rate-of-recovery diagram and relating this framework to the products of brittle and ductile deformation along faults.
Surface-rupture patterns and aftershock distributions accompanying moderate to large shallow earthquakes reveal a residual brittle infrastructure for established crustal fault zones, the complexity of which is likely to be largely scale-invariant. In relation to such an infrastructure, continued displacement along a particular master fault may involve three dominant mechanical processes of rock brecciation: (a)attrition brecciation, from progressive frictional wear along principal slip surfaces during both seismic and aseismic sliding, (b)distributed crush brecciation, involving microfracturing over broad regions when slip on the principal slip surfaces is impeded by antidilational jogs or other obstructions, and (c)implosion brecciation, associated with the sudden creation of void space and fluid-pressure differentials at dilational fault jogs during earthquake rupture propagation. These last, high-dilation breccias are particularly favorable sites for hydrothermal mineral deposition, forming transitory low-pressure channels for the rapid passage of hydrothermal fluids. Long-lived fault zones often contain an intermingling of breccias derived from all three processes.
Breccias are among the most widely distributed rock textures found in hydrothermal vein-type deposits. Previous studies have mainly been interested in developing qualitative descriptive approaches, leading to a confusing profusion of terms. Brecciation originates in numerous ways, resulting in highly complex classification systems and frequent misinterpretations of facies. Field observations are difficult to reconcile with physical theories of fragmentation, partly due to the fact that few satisfactory quantitative tools have been developed. A review of the main brecciation processes occurring in hydrothermal vein-type deposits allows for the discrimination between chemical and physical mechanisms, including tectonic comminution, wear abrasion, two types of fluid-assisted brecciation (hydraulic and critical), volume expansion or reduction, impact and collapse. Each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry, including fragment morphology, size distribution of the fragments, fabric, and dilation ratio. The first two parameters are especially important because: (1) the morphology of the fragments allows chemical and physical (mechanical) breccias to be distinguished, and (2) the particle size distribution (PSD) is a function of the energy input during breccia formation. The slope of the cumulative PSD (fractal dimension) ranges from high values for high-energy brecciation processes, to low values for low energy processes indicated by an isometric distribution. The evolution of a vein system can be divided into three stages: propagation, wear and dilation. These stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity. These two thresholds also mark the transition between different types of brecciation. Mineralization occurs during all three stages and may display different textures due to pressure variations. The use of quantitative parameters in fault-related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment, including structural setting and crustal level, as well as fluid–rock interactions. Recognition of the different breccia types could also be important during the early stages of mineral exploration.
The aim of this article is to extract from the existing literature a consistent nomenclature that can be used in the description of coherent fault rocks. The nomenclature is dealt with in this paper. Typical microstructures illustrating each is presented in a later paper (White et al ., 1982). It will be shown that a simple set of nomenclature can be extracted from the literature, so long as genetic connotations are kept to a minimum. The sequence, with increasing shear strain is country rock–protomylonite–blastomylonite–mylonite–ultramylonite if the rock has a well developed foliation; country rock–protocataclasite–cataclasite–ultracataclasite if it is without a foliation.
It is emphasized that a mylonite is basically a fine-grained schist that has formed within fault zones. It is the association with faulting that distinguishes a mylonite from a fine grain schist.
Provides a review of coarse fragmental rocks of all origins, with an emphasis on breccias and related metallic ore deposits. Contents include sections dealing with breccias; rudrocks; subaerial and shallow subsurficial non-volcanic environments; marine carbonates, siliciclastics and evaporites; volcanic environments and associations; hot spring systems; intrusive granitic, intrusive mafic-ultramafic, and alkaline-carbonatite associations; tectonic-metamorphic associations; Terrestrial meteorite impacts, problematica, and anthropogenic coarse fragmentites; Lunar and planetary coarse fragmentites. Also contains a general index, an index of coarse fragmentite names, locality index and metals index. -A.W.Hall
Understanding ore textures is fundamental to unraveling the genesis of an ore deposit, which in turn allows exploration and mining geologists to build their conceptual models of the deposits they encounter and leads to more successful exploration and exploitation. This book is specifically designed for the field geologist working without the benefits of sophisticated chemical, mineralogical or petrological support. It covers the basic building blocks of textural recognition beginning with infill (direct precipitation from hydrothermal fluids into 'cavities'), alteration (the results of hydrothermal fluid reactions with wall rocks) and overprinting (the normal complexity caused by successive introduction of hydrothermal fluids usually accompanied or preceded by renewed fracturing) and ends with a detailed examination of textures associated with tectonic and intrusive breccias. © Springer-Verlag Berlin Heidelberg 2009. All rights are reserved.
Many traditional terms for fault-related rocks have undergone recent dynamic metamorphism under high-pressure discussions by various groups of specialists. A generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic, engineering, and legal purposes. Such a framework is proposed here, focusing on a rate-of-strain versus rate-of-recovery diagram and relating this framework to the products of brittle and ductile deformation along faults.-Authors
Breccias are commonly regarded as having random fabrics, but clast alignment can cause significant fabric anisotropy. Orientation and shape are two complementary aspects of clast anisotropy. Standard circular statistics can be used to characterize orientation anisotropy, such as the direction and length of the mean resultant vector (with standard error and confidence intervals), and the concentration parameter, kappa. The Rayleigh test can be applied to establish statistical significance. Shape anisotropy can be characterized by the aspect ratio. However, none of these quantities fully describe anisotropy. The axial ratio of the finite shape matrix, a concept borrowed from strain analysis, accounts for both anisotropy components. A review of brecciation mechanisms suggests that anisotropy may be imparted by: 1) preferred fracturing directions 2) fabrics in rocks prior to brecciation 3) laminar flow during transport 4) preferred orientation during deposition. Anisotropy may be reduced by turbulent flow during transport. The orientation anisotropy and the final shape matrix are the most sensitive discriminants among breccias from the Proterozoic Mount Isa inlier, where breccias are prominent hosts in several ore deposits. These parameters reveal fabrics that can not be discerned without measurement. Breccias that have considerable clast transport distances have low anisotropies and no preferred orientations, consistent with random fabrics that were formed by fluidization. Such a fluidized breccia hosts the Cu-Au deposit at Ernest Henry mine, and regional examples contain infill sulphides. In situ breccias, on the other hand, have orientation and total anisotropies that are inherited from the fragmentation process, and regional examples are not mineralized. Clast roughness in the Mount Isa breccias varies very little, but particle size distribution, clast/(matrix+infill) ratios, and circularity, are additional useful parameters to characterize breccias and understand their genesis.
We propose a classification scheme that unifies terminology for all
primary volcaniclastic deposits, assigns initial depositional mechanism
as the basis for classifying them, and refines the grain-size classes
used to pigeonhole samples. By primary
volcaniclastic deposits and rocks, we mean the entire range of
fragmental products deposited directly by explosive or effusive
eruption. This definition thus focuses on the primary transport and
deposition of particles, rather than those processes by which the
particles form or the nature of the fluid in which they are carried. We
favor this approach for all primary volcaniclastic deposits because they
typically contain assemblages of clasts formed by different processes
and/or at different times that are subsequently brought together during
eruption.
Faults are important to the economic geologist in that they can either control the formation of some classes of ore deposits or modify the position and geometry of ore bodies. Fault rock classification is an important part of understanding the structural geology of an area. In this paper practical modifications are suggested to the most commonly used classifications of fault rock, to make it more applicable for the economic geologist. The system recommended is based in the first instance on the classification according to cohesiveness of the rock. The second defining attribute is whether the matrix is foliated or has a random fabric, providing the subdivision of cohesive fault rocks into the mylonitic and cataclastic series. The third attribute used to categorize fault rocks is the proportion of matrix to clasts, and differs in detail from those previously published. The terminology relating to crush breccias used by earlier systems is considered confusing and superfluous and has been eliminated. Although of lesser importance to the economic geologist, the terminology evolving for mylonites and mylonitc gneisses is included and is based on internal fabrics and textures. Application of this simplified classification to geological mapping in underground exploration development and logging of faults in core will enhance the geologist's ability to interpret the structure of ore deposits.
The oblique-reverse Dent Fault, northwest England, throws Carboniferous limestone units in the footwall against mudstone-dominated lower Paleozoic rocks in the hanging wall. The fault zone cuts the kilometer-wide steep limb of a precursory forced monocline. However, individual fault strands comprise centimeter-scale cataclasite cores fringed in the footwall carbonates by damage zones, some meters to tens of meters wide, composed of random-fabric dilation breccias. Breccia texture and microstructure, revealed by stained thin sections and peels, imply rapid coseismic fragmentation and then interseismic resealing by void-filling cements. The cements varied in composition through time from calcite to dolomite and then to ferroan calcite. Pervasive dolomitization of the protolith is common in the breccia zones. A key observation is that each volume of dilation breccia shows only limited refracture. This tendency to single-phase brecciation suggests that cementation caused reseal-hardening of breccia with respect to intact protolith. Breccia thickness and refracture are greatest at jogs in the Dent Fault, but breccia distribution suggests that damage also accumulated in fault walls and at propagating fault tips. Dilation breccias are a common but poorly documented product of brittle deformation of limestone. Their reseal histories can provide valuable general clues to fault zone evolution.
A geometric classification of fault breccia borrowed from the cave-collapse literature has been suggested as an alternative to available genetic classifications. Here, image analysis is used to explore geometric discrimination between the visually assigned classes of crackle breccia, mosaic breccia and chaotic breccia, using samples from the well-understood Dent Fault, northwest England. Clast sphericity and surface roughness show some correlation with the breccia classes, but particle size distributions and their fractal dimension show none. A more useful parameter is the percentage of sample area occupied by clasts. Crackle breccia has >75% clasts, mosaic breccia 60–75% clasts, and chaotic breccia has
A body of chaotic breccia along the reverse-oblique Dent Fault zone is ascribed to hanging-wall collapse into persistent voids created by geometric mismatch of fault walls, although some implosion into transient voids is a possibility The breccia comprises a 20 in wide body of hanging-wall lithologies, with a chaotic clast-supported fabric that contrasts with the fitted-fabric breccias typical of the Dent Fault damage zone. The breccia body has crude bedding defined by clast shape and size contrasts. The void fill is cut by Variscan fault strands, which, together with its ferroan calcite and barite cement, prove its late Carboniferous rather than recent age. It is shown that ally fault void, transient or persistent, had a smaller aperture than the final width of the breccia body, and no more than 5 m; a span that can be supported to depths of 2 or 3 km. However, cement zonation in the breccia fill suggests that the void opened in multiple increments, each of In aperture compatible with the maximum displacement in any one event along the Dent Fault. The Dent Fault example highlights the possible general importance of fault-void collapse but also the problems in distinguishing it from implosion processes.
Two breccias, the Doughboy and Cathedral Rock breccias, hosted by the Willyama Supergroup in the Olary Domain, northeastern South Australia, were mapped and described in this study. Mapping of the breccias has demonstrated that they are controlled by the regional deformational history of the area and they are intimately related to episodes of hydrothermal fluid flow and alteration. The clast morphology, particle size distribution, dilation ratio, structural position and contact relationships with the host rocks allow the degree of maturity and the nature of the fluid rock interactions for each breccia to be determined. Breccia clast size distribution and morphology were quantified using fractal analysis techniques. Textural analysis revealed significant differences in particle size distribution and clast morphology for the different breccias. The results of the fractal analysis indicate that the Doughboy breccia is the result of multiphase tectonic and fluid-assisted failure on a fault plane, whereas the Cathedral Rock breccia is the result of fluid-assisted failure during fold-related faulting. Due to the high fluid pressure constraints on reverse fault initiation, large fluid pressure differentials are generated during the opening of dilatant sites. It is the opening of these dilatant spaces which provides the room to accommodate and the mechanism for brecciation. Reverse faulting created dilatant spaces that, due to the large pressure differential between the wall rock and the dilational space, initiated failure of the wall rock. Whether this was a single event such as at Cathedral Rock or occurred during multiple events such as at Doughboy can be deduced via the fractal characteristics. Collectively, these observations shed light on the processes that occur during regional deformational events associated with multiple generations of hydrothermal activity. The relationship between fluid pressure and brecciation may explain why they are the sites where the most intense effects of hydrothermal activity in the Olary Domain occur.
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//journals.cambridge.org Downloaded: 11 Dec 2014 IP address: 169.230.243.252 RAPID COMMUNICATION 437 Figure 2. Some grain-size criteria relevant to fault rocks. breccia–gouge transition, though his table 1 implies a larger limit. Qualitatively, Higgins characterized fault breccias as having at least 30 % by volume of fragments 'visible to the naked eye'. Wise et al. (1984) suggested clast-size limits for the protomylonite–mylonite transition at 0.5 mm and for the mylonite–ultramylonite transition at 0.1 mm (Fig. 2).
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