Understanding the distribution and geometry of faults and fractures is critical for predicting both subsurface permeability architecture and the integrity of geological fluid barriers, particularly in rocks with low primary porosity and permeability. While fracture patterns in relatively competent, weathering-resistant (therefore often well-exposed) rocks are generally well studied in outcrop, the role of mechanically weak layers in defining fracture patterns is frequently overlooked or under-represented. Here we show that rock composition, specifically clay and silicate minerals versus carbonate content, exerts a strong control on fault and fracture propagation and bed-containment within a mechanically layered, Cretaceous carbonate sequence at Canyon Lake Gorge, Texas. We find that relatively incompetent, clay-rich layers limit fault and fracture propagation, and cause bed-containment of fractures in more competent beds. In our results, no clear relationships exist between mechanical layer thickness and fracture abundance. These results are important for understanding the relative importance of composition versus bed thickness on fracture abundance in the subsurface, and for predicting fracture-controlled fluid flow pathways, seals and fracture connectivity across beds with variable compositions, thicknesses and competences.

The Viggiano Mt. platform carbonates form a layered succession cross-cut by a dense array of pressure solution seams, and five sets of fractures and veins, which together form a sub-seismic structural network associated with polyphasic tectonic evolution. To assess the influence exerted by depositional and diagenetic heterogeneities on fracture geometry, distribution and multiscale properties, we present the results of stratigraphic, petrographic, mineralogical and mesoscale structural analyses conducted at the Viggiano Mountain, southern Italy. Based on rock textures and fossil associations, we documented that the Sinemurian–Pleinsbachian carbonates were deposited in a low-energy open lagoon, the Toarcian carbonates in a ramp setting rimmed by sand shoals, and the Cenomanian carbonates in a medium- to high-energy, lagoonal–tidal setting. Fracture-density (P20) and intensity (P21) values computed after circular scanline measurements show similar trends in both Sinemurian–Pleinsbachian and Toarcian carbonates, consistent with the bed and bed-package heterogeneities acting as efficient mechanical interfaces during incipient faulting. On the other hand, P20 and P21 do not show very similar variations throughout the Cenomanian carbonates due to pronounced bed amalgamation. Throughout the study area, the aforementioned parameters do not vary in proportion to the bed thickness, and show higher values within the coarse-grained carbonate beds. This conclusion is confirmed by results of linear scanline measurements, which focus on the P10 properties of the most common diffuse fracture set. The original results reported in this work are consistent with burial-related, physical–chemical compaction and cementation processes affecting the fracture stratigraphy of the Mesozoic platform carbonates.

Bedding-perpendicular joints striking parallel (longitudinal) and perpendicular (transverse) to both the axis of the hosting anticline and the trend of the foredeep-belt system are widely recognized in fold-and-thrust belts. Their occurrence has been commonly attributed to folding-related processes, such as syn-folding outer-arc extension, although they can also be consistent with a pre-folding foredeep-related fracturing stage. Here we report the pre-folding fracture pattern affecting the Pietrasecca Anticline, in the central Apennines (Italy), resolved by a detailed field structural analysis. Field observations, scan-lines and interpretation of virtual outcrops were used to study the intensity, distribution and the orientations of fracture pattern along the anticline. The fracture pattern of the Pietrasecca Anticline consists of longitudinal and transverse joints, oriented approximately perpendicular to bedding, and of a pre-folding longitudinal pressure-solution cleavage set, which is oblique to bedding regardless of the bedding dip. Cross-cutting relationships show that joints predated the development of the pressure-solution cleavage. Furthermore, joint intensity does not relate to the structural position along the anticline. Taken together, these observations suggest that jointing occurred in a foredeep environment before the Pietrasecca Anticline growth. Our work further demonstrates that joints striking parallel and orthogonal to the main fold axis do not necessarily represent syn-folding deformation structures.

The appraisal and development of fractured reservoirs are challenging tasks because of the combined variations in reservoir quality and natural fracture distribution. Fracture models are built routinely to support both appraisal and development decisions for such reservoirs. One of the key objectives of these models is to generate three-dimensional (3D) fracture properties for dynamic flow simulations. In this paper, we illustrate the discrepancy between the scale of observation required to build a thorough geological understanding of the subsurface and the simplification imposed by modelling. We use a case study carried out in the context of an exploration campaign of a Cretaceous carbonate reservoir in Egypt. After describing the regional geological setting of the area, we share a series of detailed observations on open-hole logs, cores, thin-sections and borehole images. These observations are focused on reservoir architecture, fracture typology and fracture connectivity. Observations are then integrated with the regional geological context to build a conceptual fracture model and to characterize the uncertainty affecting the essential parameters of this model. The conceptual model, combined with 3D seismic data, is used to define a fracture modelling strategy. This strategy includes a drastic simplification of the conceptual model to generate 3D discrete fracture network scenarios that are calibrated using pressure communication data from the exploration wells. Another extreme simplification is then necessary to populate 3D simulation grids. In the case study presented, the key tuning parameters to obtain a dynamic match are the grid cell orientation and the width of the modelled fault damage zone.

The Niagara Escarpment is a geological feature located in southern Ontario, Canada, and the northeastern United States, comprising highly fractured sandstone, shale and carbonates deposited during the Ordovician and Silurian periods. Differential erosion of the strata has generated a steep cliff face which bisects the city of Hamilton, Ontario. Geological fractures are widespread in the escarpment and result in the formation of unstable blocks of rock subject to erosion through rockfall. This presents structural stability issues of concern due to the proximity of the escarpment to urban infrastructure. We quantify and analyse fracture networks in the escarpment using a combined field- and numerical-modelling-based approach. The location, orientation and aperture of fractures were documented from local outcrops using scanline and area survey methods. Clusters of poles describing the orientation of geological discontinuities were identified in spherical projections, defining three sets: (1) a sub-vertical stratabound set striking N–S, (2) a sub-vertical stratabound set striking E–W, and (3) a set parallel to horizontal sedimentary bedding planes which we infer controlled sub-vertical fracture geometry. Discrete fracture network modelling of fracture sets highlights their high degree of connectivity, and contribution to local geohazards, and quantifies their role in controlling fluid flow through escarpment strata, which is dependent on fracture aperture. Additionally, bedding planes have the potential to act as free surfaces, facilitating stress conditions in which approximately cuboid blocks are produced, and increasing the risk of rockfalls. We conclude that fractures present a first-order control on the fluid-flow properties and stability of the Niagara Escarpment.

Hydrofractures, or hydraulic fractures, are fractures where a significantly elevated fluid pressure played a role in their formation. Natural hydrofractures are abundant in rocks and are often preserved as magmatic dykes or sills, and mineral-filled fractures or mineral veins. However, we focus on the formation and evolution of non-igneous hydrofractures. Here we review the basic theory of the role of fluid pressure in rock failure, showing that both Terzaghi’s and Biot’s theories can be reconciled if the appropriate boundary conditions are considered. We next discuss the propagation of hydrofractures after initial failure, where networks of hydrofractures may form or hydrofractures may ascend through the crust as mobile hydrofractures. As fractures can form as a result of both tectonic stresses and an elevated fluid pressure, we address the question of how to ascertain whether a fracture is a hydrofracture. We argue that extensional or dilational fractures that formed below c. 2–3 km depth are, under normal circumstances, hydrofractures, but at shallower depth they may, but must not be hydrofractures. Since veins and breccias are often the products of hydrofractures that are left in the geological record, we discuss these and critically assess which vein structures can, and which do not necessarily, indicate hydrofracturing. Hydrofracturing can suddenly and locally change the permeability in a rock by providing new fluid pathways. This can lead to highly dynamic self-organization of crustal-scale fluid flow.

Fractures that form when fluid pressure ruptures the rock are referred to as fluid-driven fractures or hydrofractures. These include most dykes, inclined sheets and sills, but also many mineral veins and joints, as well as human-made hydraulic fractures. While considerable field and theoretical work has focused on the geometry and arrest of hydrofractures, how they select their propagation paths, particularly in layered and faulted rocks, has received less attention. Here I propose that of all the possible paths that a given hydrofracture may follow, it selects the path of least (minimum) action as determined by Hamilton’s principle. This means that the selected path is that along which the energy transformed (released) multiplied by the time taken for the propagation is a minimum. Hydrofractures advance their tips/fronts in steps, with a time lag between the fracture front and the fluid front. In the present framework, each step is then controlled by Hamilton’s principle. The results suggest that when the hosting rock body is regarded as homogeneous, isotropic and non-fractured, hydrofracture paths are everywhere perpendicular to the trajectories of the minimum compressive (maximum tensile) principal stress σ3 and follow the trajectories of the maximum principal compressive stress σ1. When applied to layered and faulted rock body, the results indicate that hydrofracture paths may follow existing faults for a while, depending primarily on (1) the dip of the fault (steep faults are the most likely to be used by vertically propagating hydrofractures), and (2) the tensile strength across the fault compared with the tensile strength of the host rock along a path following the direction of σ 1. The results suggest that hydrofractures may use faults as parts of their paths primarily if the fault is steeply dipping and with close to zero tensile strength.

We consider the implications of adding a cap to the yield surface for elastic–plastic and elastic–visco-plastic solids with coupling between deformation, fluid flow and mineral reactions. For a suitable combination of (low) permeability and strain rate, opening-mode veins can form in compression. Such behaviour is enhanced by dissolution and by simultaneous mineral reactions with negative ΔV. These are the opening-mode equivalents of compaction bands in rocks with high permeability. Stylolite parallel veins are considered as forming in this way; such veins are commonly the laminated or ribbon-quartz veins associated with intense gold mineralization. Axial plane veins and melt segregations are in this class also. The addition of a yield surface cap limits the permissible stress states portrayed by a failure-mode diagram and has implications for breccia formation. Failure discontinuities that form at the cap require a decrease in fluid pressure to form as opposed to extension joints and veins that require an increase in fluid pressure; discontinuities that form at the cap are in orientations that are commonly interpreted as reactivated early discontinuities. The switching between high fluid pressure and low fluid pressure, which we call mode-switching, arises from competition between mineral dissolution and deposition. This is an alternative to the fault-valve mechanism and does not require fault reactivation or failure at a ‘seal’ linked to seismicity or fault reactivation. The capped yield surface concept provides a unifying self-consistent approach for vein/breccia formation and for the kinematics of brittle and visco-plastic rocks.

Calcite veins are common in organic-rich mudrocks, but their genesis and ability to transmit fluids are debated. A combined microstructural and isotopic investigation of an array of calcite veins recovered in core from the Marcellus Formation reveals that the veins grew via a combination of continuous fibrous growth and punctuated fracture-opening increments. Continuous opening is the result of pressure-solution creep and involves no mechanical fracturing, but rather the growth of a pressure fringe around a pre-existing, sealed fracture. In contrast, incremental opening is accomplished by overpressured, mineral-saturated fluid, which repeatedly ruptures the rock at the cement / host-rock interface. Punctuated growth increments occurred repeatedly throughout an otherwise protracted, continuous growth history, indicating that the present structures preserve hybrid deformation conditions between brittle, fluid-assisted cracking and plastic strain. Stable isotopic signatures match those of a regional opening-mode fracture set that formed in response to catagenetic fluid overpressures amid a tectonically imposed (Alleghanian) stress field. It is concluded that calcite veins form as opening-mode hydraulic fractures and are susceptible to increments of brittle reactivation, even while inelastic growth processes widen and fill the veins with fibrous cement.

In this study, we present a new granular rock-analogue material (GRAM) with a dynamic scaling suitable for the simulation of fault and fracture processes in analogue experiments. Dynamically scaled experiments allow the direct comparison of geometrical, kinematical and mechanical processes between model and nature. The geometrical scaling factor defines the model resolution, which depends on the density and cohesive strength ratios of model material and natural rocks. Granular materials such as quartz sands are ideal for the simulation of upper crustal deformation processes as a result of similar nonlinear deformation behaviour of granular flow and brittle rock deformation. We compared the geometrical scaling factor of common analogue materials applied in tectonic models, and identified a gap in model resolution corresponding to the outcrop and structural scale (1–100 m). The proposed GRAM is composed of quartz sand and hemihydrate powder and is suitable to form cohesive aggregates capable of deforming by tensile and shear failure under variable stress conditions. Based on dynamical shear tests, GRAM is characterized by a similar stress–strain curve as dry quartz sand, has a cohesive strength of 7.88 kPa and an average density of 1.36 g cm−3. The derived geometrical scaling factor is 1 cm in model = 10.65 m in nature. For a large-scale test, GRAM material was applied in strike-slip analogue experiments. Early results demonstrate the potential of GRAM to simulate fault and fracture processes, and their interaction in fault zones and damage zones during different stages of fault evolution in dynamically scaled analogue experiments.

The Niagara Escarpment is a fractured Palaeozoic sedimentary cuesta, subject to year-round weathering in a temperate climate. We examined the temperature of the rock surface and fractures at three in situ sites with varying aspect and lithology, as well as the surface and interior of three control blocks maintained in outdoor conditions between December 2020 and March 2021. The objectives were to examine the interplay between freeze–thaw and thermal weathering in the winter months and to identify potential factors influencing these processes. Both diurnal-scale and prolonged freeze–thaw cycles differing in spatial and temporal extent were identified, coincident with periods of high moisture. We frequently observed rapid temperature changes (>1 °C min−1) at sites with strong insolation, which implies that the temperature regime is suitable for thermal shock and fatigue to occur. Site-specific factors, such as the aspect of the escarpment face and lithology, impact the mechanism and extent of weathering. Southeast-facing sites with high insolation are dominated by diurnal-scale freeze–thaw; west- and east-facing sites with lower insolation experience a more prominent prolonged freeze–thaw cycle. Across all sites there is a gradient between surface and fracture temperature that follows diurnal trends in air temperature and insolation. Variability in the surface-fracture gradient may enhance weathering processes by shifting the orientation and magnitude of stress, and by changing the spatial distribution of freezing and thawing. Our research indicates that site-specific factors and pre-existing fractures moderate the influence of air temperature and insolation on thermal gradients, and ultimately the weathering regime.

Deformation in intraplate domains is usually considered as a consequence of tectonic events at plate boundaries. Nevertheless, the occurrence of intraplate earthquakes such as the recent Le Teil event in the south of France along the Cévennes Fault System (CFS), on 11 November 2019, Mw = 4.9, questions whether this far-field deformation only occurs during tectonic pulses at plate boundaries, or if it corresponds to low-intensity but regional continuous deformation through time. To address this question, we have coupled U–Pb geochronology of fault-related calcites with structural analysis along a major fault system (the CFS) in the South-East Basin, France. We evidence (1) an Albian activity of the CFS and (2) a continuous compressional activity of the CFS and satellite structures during the whole Eocene and probably during the Late Cretaceous – Palaeocene, including periods (e.g. Lutetian) usually considered as phases of tectonic quiescence. We thus demonstrate that the tectonic reactivation of this intraplate fault system is not restricted to periods of high rates of deformation at plate boundaries.

Jurassic carbonate strata in the eastern Paris Basin exhibit several generations of faults, tension gashes and stylolites. Although their relative chronology can sometimes be determined according to cross-cutting relationships, the duration of major deformation phases and their influence on fluid flow and carbonate cementation are still uncertain. This contribution aims to clarify the timing of brittle deformation and associated calcite cementation. Tension gashes filled by calcite in Jurassic carbonates were sampled in outcrops and boreholes and dated through U–Pb geochronology. Almost all the sampled fractures were cemented during the Cenozoic period. Continuous deformation spread from c. 50 to 30 Ma. Tension gashes oriented N10° to N20° dated at 48–43 Ma show the main Pyrenean contractional stage. A second set of calcites were dated at c. 35–33 Ma and document a Late Eocene – Oligocene extension. A transition from the compressional to the extensional regime is expressed by tension gashes dated between 43 and 35 Ma. Finally, tension gashes oriented N150° to N175°, dated between 32 and 18 Ma, may result from the propagation of the horizontal stress generated by the Alpine orogen or by late Pyrenean deformation. Clumped isotope thermometry on five samples revealed both low crystallization temperatures (from 27 to 53 °C) and the meteoric origin of calcite-precipitating fluids. Our research therefore documents a continuous fracturing from Ypresian to Rupelian times, and less expressed brittle deformation during the Miocene period.

Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

A combined structural and geochronological study was carried out to identify the tectonic evolution of the northern Verkhoyansk Fold-and-Thrust Belt, formed on the east margin of the Siberian Craton during late Mesozoic collision. Fault and fold geometries and kinematics were used for palaeostress reconstruction along the Danil and Neleger rivers cross-cutting the central and western parts of the Kharaulakh segment of the northern Verkhoyansk. Three different stress fields (thrust, normal and strike-slip faulting) were identified after separation from heterogeneous fault-slip data. Thrust and normal faulting stress fields were found in both areas, whereas a strike-slip faulting stress field was only found in Neoproterozoic rocks of the Neleger River area. U–Pb laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) dating of calcite slickenside samples reveals the following succession of major deformation events across the northern Verkhoyansk: (i) The oldest tectonic event corresponding to the strike-slip faulting stress field with NE–SW-trending compression axis is Early Permian (Cisuralian, 284 ± 7 Ma) and likely represents a far-field response to the late Palaeozoic collision of the Kara terrane with the northern margin of the Siberian Craton. (ii) A slickenfibrous calcite age of 125 ± 4 Ma is attributed to the Early Cretaceous compression event, when the fold-and-thrust structure was formed. (iii) U–Pb slickenfibre calcite ages of 76–60 Ma estimate the age of a Late Cretaceous – Palaeocene compression event, when thrusts were reactivated. Slickensides related to both (ii) and (iii) compressional tectonic events formed by similar stress fields with W–E-trending compression axes. (iv) From the Palaeocene onwards, extensional tectonics with approximately W–E extension predominated.

Orogenic forelands host interactions between deformation and static or migrating fluids. Given their accessibility and dimensions, these areas are not only historic landmarks for structural geology, but they are also areas of prime interest for georesource exploration and geological storage, and loci of potential geohazards. Geochemical techniques applied on cements filling tectonic structures and associated trapped fluids can constrain the temperature, pressure, origin and pathways of fluids during deformation and allow the characterization of the past fluid system. In this review focused on calcite cements, we first present and critically discuss the most used geochemical techniques to appraise specific parameters of the fluid system. Second, we summarize the outcomes of selected case studies where the past fluid system was reconstructed with consideration of tectonics, either at the scale of the individual fold/thrust or at the scale of the fold-and-thrust belt. At first order, the past fluid system evolves in a similar way with respect to the considered stage of deformation, being rather closed to external fluids when deformation is bounded to mesoscale structure development, and opening to vertical flow when thrust and folds develop. In a more detailed view, it seems that the past fluid system evolves and distributes under the influence of the structural style, of the geometry of the major faults and of the lithology of the sedimentary succession. Through this review, we illustrate the concept of geochemistry-assisted structural geology through case studies where the geochemistry of calcite veins constrained subsurface geometries and structural developments in orogenic forelands.

Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events.

Fluids are commonly invoked as the primary cause for weakening of detachment shear zones. However, fluid-related mechanisms such as pressure-solution, reaction-enhanced ductility, reaction softening and precipitation of phyllosilicates are not fully understood. Fluid-facilitated reaction and mass transport cause rheological weakening and strain localization, eventually leading to departure from failure laws derived in laboratory experiments. This study focuses on the Miocene Raft River detachment shear zone in northwestern Utah. The shear zone is localized in the Proterozoic Elba Quartzite, which unconformably overlies the Archaean basement, and consists of an alternating sequence of quartzite and muscovite-quartzite schist. In this study, we characterize fluid-related microstructures to constrain conditions that promoted brittle failure in a plastically deforming shear zone. Thin-section analyses reveal the presence of healed microcracks, transgranular fluid inclusion planes and grain boundary fluid inclusion clusters. Healed microcracks occur in three sets, one sub-perpendicular to the mylonitic foliation, and a set of two conjugate microcracks oriented at ∼40–60° to the mylonitic foliation. Healed microfractures are filled with quartz, which has a distinct fabric, suggesting that microcracks healed while the shear zone was still at conditions favourable for quartz crystal plasticity. Transgranular fluid inclusion planes also occur in three sets, similar in orientation to the healed microfractures. Fluid inclusions commonly decorate grain and subgrain boundaries as inter- and intragranular clusters. Our results document ductile overprint of brittle microstructures, suggesting that, during exhumation, the Raft River detachment shear zone crossed the brittle–ductile transition repeatedly, providing pathways for fluids to permeate through this shear zone.

Interactions of hydrothermal and deformation processes in active fault zones are important in light of seismic and geothermal activities. This study investigates different tectonites of the Grimselpass breccia fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). We combine microstructural and geochemical investigations to decipher the interaction of grain-size reduction and associated dilation (volume gain) during frictional deformation with geochemical processes such as mass gain via precipitation, grain growth and healing. Three types of tectonites were investigated in the fault zone: (1) cockade-bearing tectonites, (2) cataclasites and (3) fault gouges. They differ in terms of microstructure of clasts, mass and volume gain/loss, as well as type and degree of cementation. Clast size distributions show decreasing median values from low-strain cataclasites to high-strain cataclasites. Mass/volume gains are up to 20 %, with samples of smallest clast sizes often showing the highest mass gain. In the case of cockade-bearing tectonites large volume and mass gain occur. In particular, the precipitation of SiO2-rich cements is inferred to (i) reduce permeability and (ii) induce a gain in strength of the respective fault rocks. Location, timing and degree of healing directly control the fault architecture by deactivating fault strands from the circulation of hydrothermal fluids. In this case, new rupturing events are necessary to reactivate locked fault strands. Such feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, representing a process chain for the long-term preservation of orogenic hydrothermal systems.