Prevailing Stress Field Research Articles

Unconventional Reservoir Characterization and Formation Evaluation: A Case Study of a Tight Sandstone Reservoir in West Africa

Unconventional reservoirs, including gas shales and tight gas sands, have gained prominence in the energy sector due to technological advancements and escalating energy demands. The oil industry is eagerly refining techniques to decipher these reservoirs, aiming to reduce data collection costs and uncertainties in reserve estimations. Characteristically, tight reservoirs exhibit low matrix porosity and ultra-low permeability, necessitating artificial stimulation for enhanced production. The efficacy of the stimulation hinges on the organic material distribution, the rock’s mechanical attributes, and the prevailing stress field. Comprehensive petrophysical analysis, integrating standard and specialized logs, core analyses, and dynamic data, is pivotal for a nuanced understanding of these reservoirs. This ensures a reduction in prediction uncertainties, with parameters like shale volume, porosity, and permeability being vital. This article delves into an intricate petrophysical evaluation of the Nene field, a West African unconventional reservoir. It underscores the geological intricacies of the field, the pivotal role of data acquisition, and introduces avant-garde methodologies for depth matching, rock typing, and the estimation of permeability. This research highlights the significance of unconventional reservoir exploration in today’s energy milieu, offering a granular understanding of the Nene field’s geological challenges and proffering a blueprint for analogous future endeavours in unconventional reservoirs.

Influence of fluid pressure changes on the reactivation potential of pre-existing fractures: a case study in the Archaean metavolcanics of the Chitradurga region, India

AbstractThe metavolcanics of Chitradurga region host numerous shallow crustal veins and fractures and faults of multiple orientations. Several high and low Pf cycles have been recorded in the region, leading to the reactivation of most of the pre-existing fractures for high Pf and selective reactivation of some well-oriented fractures under low Pf conditions. The pre-existing anisotropy (magnetic fabric) in the metavolcanics acted as the most prominent planar fabric for fracture propagation and vein emplacement under both conditions, thereby attaining maximum vein thickness. In this study, we emphasize the reactivation propensity of these pre-existing fracture planes under conditions of fluid pressure variation, related to the high and low Pf cycles. Multiple cycles of fluid-induced fracture reactivation make it difficult to quantify the maximum/minimum fluid pressure magnitudes. However, in this study we use the most appropriate fluid pressure magnitudes mathematically feasible for a shallow crustal depth of ∼2.4 km. We determine the changes in the reactivation potential with states of stress for the respective fracture orientations under both high and low Pf conditions. Dependence of fluid pressure variation on the opening angle of the fractures is also monitored. Finally, we comment on the failure mode and deformation behaviour of the fractures within the prevailing stress field inducing volumetric changes at the time of deformation. We find that deformation behaviour is directly related to the dip of the fracture planes.

In Situ Stress State of the Ruhr Region (Germany) and Its Implications for Permeability Anisotropy

In this study, we carried out reactivation potential analysis of discontinuities revealed from four exploration boreholes penetrating heavily faulted and folded Upper Carboniferous rock strata of the Ruhr region. We performed this study based on the notion that slip is controlled by the ratio of shear to effective normal stresses acting on a pre-existing plane of weakness in the prevailing stress field configuration. The results of this analysis were supported by indicators of localized fluid flow, both on micro- and macro-scales, which confirm relationship between secondary permeability and in situ stress state in the Ruhr region. Findings from this study, in conjunction with results of destructive laboratory testing, indicate that the steep NW–SE- and NNE–SSW-striking planar discontinuities are likely to be either close to the critical state or critically stressed in the in situ stress configuration in the Ruhr region. These planar structures, as evidenced by indicators of localized permeability, are the main fluid pathways in the studied region. The NE–SW-striking discontinuities, on the other hand, are most likely to be closed and hydraulically inactive in the prevailing stress state. Based on results gained from this study, implications for utilization of deep geothermal energy in the region were discussed.

A new model for the optimal structural context for giant porphyry copper deposit formation

AbstractPorphyry-type deposits are the main global source of copper and molybdenum. An improved understanding of the most favorable structural settings for the emplacement of these deposits is necessary for successful exploration, particularly considering that most future discoveries will be made under cover based on conceptual target generation. A common view is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip fault systems that favor local extension within a regional compressive to transpressive tectonic regime. However, the role of such a structural context in magma storage and evolution in the upper crust remains unclear. In this work, we propose a new model based on the integration of structural data and the geometry of magmatic-hydrothermal systems from the main Andean porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We suggest that the magma differentiation and volatile accumulation required for the formation of a porphyry deposit is best achieved when the fault system controlling magma ascent is strongly misoriented for reactivation with respect to the prevailing stress field. When magmas and fluids are channeled by faults favorably oriented for extension (approximately normal to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during the late stages of the evolution of porphyry systems and in the epithermal environment. This new model has direct implications for conceptual mineral exploration.

Structural settings of gold deposits within the Reefton goldfield, western New Zealand

ABSTRACT The Reefton goldfield includes the large Globe Progress and Blackwater deposits as well as ∼30 smaller deposits which are localised along a network of late syn- to post-metamorphic faults and shear zones that extend 25+ km in a northerly direction. The Globe Progress deposit is located in a unique structural site where this district-scale zone of mineralised faults and shear zones splits into two strands. Lense-shaped zones of mineralised quartz veins <100 m long which form the smaller deposits plunge parallel to intersections between their host faults and the S2 cleavage or bedding. Veining likely developed in minor dilational jogs where the faults refract across cleavage or bedding. Some dilational jogs formed during dextral strike-slip and transpressional-reverse movement along the host faults in response to a NE-SW oriented σ1. Faults that host the smaller deposits are located on east-dipping fold limbs, whereas the larger, tabular rather than rod-shaped, Birthday Reef at Blackwater is the only deposit and associated fault located on a WNW-dipping fold limb. This unusual structural setting was presumably more conducive to dilational opening in the prevailing stress field. None of the deposits within the goldfield are located in major fold hinges.

Sand injectites: from source to emplacement – an example from the Miocene age Frimmersdorf Seam, Garzweiler Open-cast Mine, Lower Rhine Embayment

Abstract Sand injectites are reported from a Miocene-age lignite seam in the Lower Rhine Embayment, which is exploited in the Garzweiler open-cast mine, Germany. Owing to the ongoing mining of the seam, the sand injectite morphology and extent were documented over a 3 year period. The combination of fieldwork, geophysical and drilling well data, as well as a 3D reconstruction of the host unit, reveal that the sand injectites within the Frimmersdorf Seam (= host unit) are highly variable, and include sills, dykes, reticulate (i.e. an interconnected network of thin sand veins) and irregular-shaped sand bodies. The injectites are connected to the underlying Frimmersdorf Sand or the overlying Neurath Sand (= parent sand units), or can be distributed throughout the seam. The differentiation of sand injectites from syn-depositional sand bodies is based on the orientation (discordant/concordant) and the presence of primary sedimentary structures in the latter. The prevailing stress field across the Lower Rhine Basin, and the coeval nature of fault activity, controlled the formation of sand injectites in the region. While such structures have been noted for over 30 years, this study is the first to provide a detailed analysis of these economically important features, including their classification, extent and morphology.

Oblique reactivation of lithosphere-scale lineaments controls rift physiography – the upper-crustal expression of the Sorgenfrei–Tornquist Zone, offshore southern Norway

Abstract. Pre-existing structures within sub-crustal lithosphere may localise stresses during subsequent tectonic events, resulting in complex fault systems at upper-crustal levels. As these sub-crustal structures are difficult to resolve at great depths, the evolution of kinematically and perhaps geometrically linked upper-crustal fault populations can offer insights into their deformation history, including when and how they reactivate and accommodate stresses during later tectonic events. In this study, we use borehole-constrained 2-D and 3-D seismic reflection data to investigate the structural development of the Farsund Basin, offshore southern Norway. We use throw–length (T-x) analysis and fault displacement backstripping techniques to determine the geometric and kinematic evolution of N–S- and E–W-striking upper-crustal fault populations during the multiphase evolution of the Farsund Basin. N–S-striking faults were active during the Triassic, prior to a period of sinistral strike-slip activity along E–W-striking faults during the Early Jurassic, which represented a hitherto undocumented phase of activity in this area. These E–W-striking upper-crustal faults are later obliquely reactivated under a dextral stress regime during the Early Cretaceous, with new faults also propagating away from pre-existing ones, representing a switch to a predominantly dextral sense of motion. The E–W faults within the Farsund Basin are interpreted to extend through the crust to the Moho and link with the Sorgenfrei–Tornquist Zone, a lithosphere-scale lineament, identified within the sub-crustal lithosphere, that extends &gt; 1000 km across central Europe. Based on this geometric linkage, we infer that the E–W-striking faults represent the upper-crustal component of the Sorgenfrei–Tornquist Zone and that the Sorgenfrei–Tornquist Zone represents a long-lived lithosphere-scale lineament that is periodically reactivated throughout its protracted geological history. The upper-crustal component of the lineament is reactivated in a range of tectonic styles, including both sinistral and dextral strike-slip motions, with the geometry and kinematics of these faults often inconsistent with what may otherwise be inferred from regional tectonics alone. Understanding these different styles of reactivation not only allows us to better understand the influence of sub-crustal lithospheric structure on rifting but also offers insights into the prevailing stress field during regional tectonic events.

On the Geomorphology and Tectonic Position of Ciletuh-Jampang Area, West Java, Indonesia

The Ciletuh-Jampang area is located in the western tip of the Southern Mountain of West Java, Indonesia. The area is presently well publicized in an effort to bring the unique geological characteristics exposed in this particular place to become the Global Geopark Network (GGN) of UNESCO. Ciletuh area exhibits a rare mega-aphitheater morphology. Stratigraphically West Java composed of clastic sediments of continental and volcanic island arc origin with the interruption of sub-aerial volcanic rocks and the coastal reefs banks developed along the coasts both in the north and the south. The age of the rock ranges between Eocene to Recent. The analysis of regional structure shows that a set of deep seated faults was recognized dominating the area in NNW-SSE and NNE-SSW directions. The released faulting occurred in E-W direction. The low angle shallow upthrustings facing north of skin structure in E-W direction dominate the upper part of the rock sequence. The structural analysis revealed the continuous prevailing stress field in SSW-NNE direction since Eocene time. The advancing landslides are readily observable in the rim of the amphitheater of Ciletuh. This phenomenon seems to demonstrate the initial stage (primarumpf) of the geomorphologic cycle taking into account the flat surface of Jampang area in the surroundings. Further investigations revealed the geomorphologic evidences of the formation of plateau, namely the erosion surface, old terraces, mountain tables, old beach ridges and the characteristics of drainage pattern. Two remnants of main plateau were recognized in Ciletuh-Jampang area, namely Bentang Plateau and Jampang Plateau. Further investigations show that at least five locations of plateau were identified in western part of Java Island, namely Dieng Plateau (+ 2,000 m), Pengalengan Plateau (+1,300 m), Bentang Plateau (+ 1,000 m), Jongrangan Plateau (+ 850 m) and Jampang Plateau (+ 700 m). A rough estimation reveals the annual rate of lifting in Java Island since Pliocene time ranges most likely between 5.0 and 5.4cm.

Seismicity in the Vicinity of the Tristan Da Cunha Hot Spot: Particular Plate Tectonics and Mantle Plume Presence

AbstractEarthquake locations along the southern Mid‐Atlantic Ridge have large uncertainties due to the sparse distribution of permanent seismological stations in and around the South Atlantic Ocean. Most of the earthquakes are associated with plate tectonic processes related to the formation of new oceanic lithosphere, as they are located close to the ridge axis or in the immediate vicinity of transform faults. A local seismological network of ocean‐bottom seismometers and land stations on and around the archipelago of Tristan da Cunha allowed for the first time a local earthquake survey for 1 year. We relate intraplate seismicity within the African oceanic plate segment north of the island partly to extensional stresses induced by a bordering large transform fault and to the existence of the Tristan mantle plume. The temporal propagation of earthquakes within the segment reflects the prevailing stress field. The strong extensional stresses in addition with the plume weaken the lithosphere and might hint at an incipient ridge jump. An apparently aseismic zone coincides with the proposed location of the Tristan conduit in the upper mantle southwest of the islands. The margins of this zone describe the transition between the ductile and the surrounding brittle regime. Moreover, we observe seismicity close to the islands of Tristan da Cunha and nearby seamounts, which we relate to ongoing tectono‐magmatic activity.

Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field

In this study, we examine the fracture network of the Tolhuaca geothermal system located in the Southern Andean volcanic zone that may have acted as a pathway for migration and ascent of deep-seated fluids under the far/local stress field conditions of the area. We collected the orientation, slip-data and mineralogical content of faults and veins recovered on a ca. 1000 m deep borehole (Tol-1) located in the NW-flank of the Tolhuaca volcano. Tol-1 is a non-oriented, vertical borehole that recovered relatively young (<1 Ma) basaltic/andesitic volcanic rocks with subordinate pyroclastic/volcanoclastic interbedded units of Pleistocene age. Here, we examined and measured the inclination, geometry, texture, mineralogy, and relative sense of displacement of veins and faults. To determine the actual azimuthal orientation of fault and veins we reoriented 66 segments (89 standard mini-cores) of Tol-1 using stable Characteristic remanent magnetization component (ChRM) obtained by thermal demagnetization methodology. Paleo-declination of ChRM vectors was used to re-orient the borehole pieces, as well as fault and veins, to a common anchor orientation value consistent with the Geocentric Axial Dipole approximation (GAD). Inversion of RM-corrected fault-slip data reveals a local tensional stress field with a vertically oriented σ1 axis (083/74) and a subhorizontal, NS-trending σ3 axis (184/03). Within the topmost 400 m of the borehole, faults and veins are randomly oriented, whereas below 400 m depth, faults and veins show preferential NE-to EW-strikes and steep (>50°) dips. The EW-striking veins are compatible with the calculated local stress field whereas NE-striking veins are compatible with the regional stress field, the morphological elongation of volcanic centers, alignments of flank vents and dikes orientation. Our results demonstrate that the paleomagnetic methodology proved to be reliable and it is useful to re-orient vertical boreholes such as Tol-1. Furthermore, our data show that the bulk transpressional regional stress field has local variations to a tensional stress field within the NE-striking fault zone belonging to the Liquiñe-Ofqui Fault System, favoring the activation of both NW- and NE-striking pre-existent discontinuities, especially the latter which are favorably oriented to open under the prevailing stress field. The vertical σ1 and NS-trending subhorizontal σ3 calculated in the TGS promote the activation of EW-striking extensional veins and both NE and NW-striking hybrid faults, constituting a complex fluid pathway geometry of at least one kilometer depth.

The 8 June 2008 Mw 6.4 Earthquake in Northwest Peloponnese, Western Greece: A Case of Fault Reactivation in an Overpressured Lower Crust?

Abstract On 8 June 2008 a strong ( M w 6.4) earthquake occurred in the area of northwest Peloponnese, western Greece. The event originated in the lower crust and was caused by the rupture of a previously unknown fault probably inherited from past tectonic phases. In this study we perform a stress inversion of all available focal mechanisms in this area in order to obtain an estimate of the regional stress field. It is shown that the maximum principal stress axis has an azimuth of N273°E and forms an angle of 63° with the fault’s strike, which is well-constrained by seismological observations. This implies that the fault was severely misoriented with respect to the prevailing stress field assuming friction coefficients in the range 0.65–0.85. Calculation of pore-fluid factors for a variety of input parameters seems to confirm the presence of elevated fluid pressure near the hypocenter because they reach superhydrostatic to lithostatic values (0.80–1.0). The source of these fluids is probably of deep origin and may have to do with upper mantle degassing. This suggestion is supported by the presence of mantle helium in spring waters close to the epicenter and by low Pn velocities consistent with partially molten mantle beneath northwest Peloponnese.

Software for slip-tendency analysis in 3D: A plug-in for Coulomb

Slip-tendency analysis is a valuable tool in fault reactivation evaluation and seismic hazard assessment as it provides a means of quantifying the slip potential on mapped or suspected faults in a known or inferred stress field. We developed an interactive graphic tool to perform slip-tendency analysis. The application is written in MATLAB in the form of plug-ins for COULOMB, a graphic-rich deformation and stress change open-source software. In addition to identifying the faults most prone to reactivation, we compute and plot synthetic focal mechanisms from the direction and sense of likely slip. This allows compatibility between focal mechanisms and geological structures to be verified. Both individual faults and fault networks can be considered in three dimensions. The potential for slip depends on the prevailing stress field, the fault surface orientation and the coefficient of friction. These parameters are given interactively in a Windows environment. The application thus offers an easy-to-use graphical interface with the possibility of fast parameter modification and 3D visualization of the results.

Rock Stress, Rock Stress Measurements, and the Integrated Stress Determination Method (ISDM)

The primary objectives of this work are to (1) improve the understanding of the prevailing stress distribution at the Aspo Hard Rock Laboratory (HRL) in SE Sweden by employing an integrated stress determination approach, and in order to accomplish this (2) extend the existing stress integration methodology denominated integrated stress determination method (ISDM; Cornet in Comprehensive Rock Engineering vol 3, Pergamon Press, Oxford, pp 413–432, 1993a). The new developments of the ISDM involve a 12-parameter representation of the regional stress field in the rock mass (i.e., the full stress tensor and its variation with depth) that is applicable to hydraulic stress data (sleeve fracturing, hydraulic fracturing, and hydraulic tests on pre-existing fractures), overcoring data (CSIR- and CSIRO-type of devices), and to combinations of hydraulic and overcoring stress data. For the latter case, the elastic parameters of the overcoring technique may be solved in situ by allowing the hydraulic stress data to constrain them. As a result, the problem then involves 14 model parameters. Results from the study show that the ISDM effectively improves the precision of the prevailing stress field determination and that it is especially powerful for identification of consistencies/inconsistencies in an existing data set. Indeed, this is the very basic premise and goal of stress integration; combine all available data to achieve as complete a characterization of the mechanical stress model as possible, and not to identify a solution that fits only loosely the maximum amount of stress data.

Buoyancy‐driven deformation and contemporary tectonic stress in the lithosphere beneath Central Italy

AbstractWe studied the continental deformation and modelled the contemporary flow and stress distribution in the lithosphere beneath Central Italy. We made use of a revisited crust and uppermost mantle Earth structure that supports delamination processes. The model behaviour is primarily determined by the thick high density lithospheric root to the east and the low‐viscosity shallow mantle wedge to the west. The rate of the modeled crustal motion is in agreement with GPS data and the pattern of lithospheric flow explains the heat flux, the regional geology and provides a new background for the genesis and age of the recent Tuscan magmatism. The modelled stress in the lithosphere is spatially correlated with the prevailing stress field and the gravitational potential energy patterns and shows that buoyancy forces, solely, can explain the coexisting regional contraction and extension and the unusual sub‐crustal seismicity.

Stress and crustal anisotropy in Marlborough, New Zealand: evidence for low fault strength and structure-controlled anisotropy

The major strike-slip faults in the greater Marlborough region, central New Zealand, are of both scientific and societal interest as they accommodate relative plate motion in the upper plate of an oblique subduction zone and pose a high seismic risk to central New Zealand. Studies in California suggest that some plate-bounding strike-slip faults are frictionally weak and that crustal anisotropy is controlled by the ambient stress. Whether these observations are more generally applicable to major strike-slip faults has yet to be determined. We have used inversions of focal mechanism and first motion data to calculate the principal stress directions in Marlborough and related them to the geometry of the major faults. The average angle between the axis of maximum horizontal compressive stress (SHmax) and the average strike of the major faults is 60°; this is substantially higher than the ∼30° expected for reactivation of a vertical strike-slip fault given Byerlee friction and hydrostatic fluid pressure. This geometry can be explained, however, by the faults having a moderately low friction coefficient (∼0.35), a moderately high fluid pressure (∼0.7 × lithostatic) or some combination of the two. This observation substantiates the hypothesis that the San Andreas fault is not unique in being frictionally weak. We have also conducted shear-wave splitting analysis on local S phases to determine the directions of crustal anisotropy and investigated their orientations with respect to the geological fabric and the principal stress directions. The anisotropy determined using shallow earthquakes reveals that the fast direction is 65 ± 50° and is generally aligned with the NE-CSW-striking faults, and we therefore conclude that crustal anisotropy in Marlborough is controlled more by the geological structures than by the prevailing stress field.

Structural controls on hydrothermal flow in a segmented rift system, Taupo Volcanic Zone, New Zealand

AbstractVigorous hydrothermal convection transfers 10 times the average continental heat flow through the central Taupo Volcanic Zone (TVZ), a region of active extension (approximately 8 mm year−1) and productive rhyolitic volcanism. Over 20 high‐temperature (&gt;250°C) geothermal fields occur within Quaternary pyroclastic basins, with convective circulation to depths of 7–8 km presumably extending through basement rocks. Parallel‐striking normal faults, fractures and dikes dissect the convective regime, interacting with fluids to either enhance or restrict flow according to the relative permeability of structure and host rock. In the basement, high bulk permeability is maintained by focussed flow through faults and associated fractures well oriented for reactivation in the prevailing stress field. In contrast, distributed flow through fault‐bounded compartments prevails within Quaternary basins, masking any signal of deeper structural control. Exceptions occur where more competent rocks are exposed at the surface. As in narrow magmatic rifts elsewhere, the extensional fabric is partitioned into discrete rift segments linked along strike by accommodation zones. Eighty per cent of TVZ geothermal fields correlate spatially with rift architecture, with 60% located in accommodation zones. We suggest that segmented rift fabrics generate bulk permeability anisotropy that is to some extent predictable, with rift segments characterized by enhanced axial flow, and accommodation zones characterized by locally enhanced vertical permeability that is tectonically maintained. This provides a plausible explanation for the common occurrence of geothermal fields within accommodation zones and their notable absence within densely faulted rift segments. Maintenance of structural permeability in zones of active hydrothermal precipitation necessarily requires repeated brittle failure. Geothermal plumes therefore exploit tectonically maintained permeability within accommodation zones, with rift segments functioning mostly as drawdown regions. The influence of rift architecture on flow paths has important implications for geothermal extraction and epithermal mineral exploration within the TVZ and other structurally segmented hydrothermal systems, both active and extinct.

In situ stress measurement and its application for hydro-electric projects—an Indian experience in the Himalayas

Any attempt to design engineering structures in rock mass requires knowledge of the prevailing in-situ stress field. It is always advisable to measure it, in whatever best way possible. The prior knowledge of the prevailing stress field helps in the alignment and the design of support system for the underground cavern, estimation of grouting pressure in the dam foundation, design of steel liners for Head Race Tunnel, Penstock, etc. There are various methods of determination of in-situ stresses in rock mass. However it has been experienced that the hydrofracturing technique is the most convenient to conduct and interpret the test results. This paper briefly describes the various uses of stress measurements, details of hydraulic fracturing test system, test procedure adopted and the concept of hydrofracturing in arriving at the in-situ stresses of the rock mass. The results for the above applications conducted at four mega hydroelectric projects in India and neighbouring countries of Bhutan and Nepal located in the Himalayas are presented.

A simplified model of stress-slip relationships: application to the Frøy field, northern North Sea

We apply an analytical method, Sortan, to determine fault slip as induced by far-field stress. The Sortan method is based on the Wallace–Bott hypothesis. The method is applied to all major faults of the Frøy oil field located at the eastern margin of the southern Viking Graben, North Sea. The purpose was to quantify the most likely senses of motion experienced by the faults during the Late Jurassic–Early Cretaceous rifting event.Because strike-slip displacement is generally difficult to determine based on classical exploration methods, particular attention was paid to the simulated strike-slip component along the major faults. Depth-converted structural maps were used to determine fault attitudes. The presence at the northern edge of the field of a major syn-rift dextral strike-slip fault, the North Frøy Fault, allowed us to constrain the most likely characteristics of the prevailing stress field during rifting. According to different possible stress states, different models were run. The results from the Sortan modelling indicate that the strike-slip component of motion was in general minor on the major faults within the oil field. The inferred stress state is found consistent with rifting involving fault-related displacement strongly dependent on σ 3 orientation. This stress state prevailed at least during the latest stages of rifting (Latest Jurassic–Early Cretaceous) of the northern North Sea. This case study shows that where paleostress inversion methods cannot be applied in the absence of direct observation, the Sortan method has the potential to constrain the slip behaviour of the brittle structure. It also enables one to rapidly check the overall mechanical consistency of tectonic interpretations.

Fluid chemistry of veining associated with an ancient microearthquake swarm, Benmore Dam, New Zealand

Steeply dipping strata in the vicinity of Benmore Dam, Otago, New Zealand, are complexly deformed metasedimentary rocks of the Torlesse Supergroup (Middle Triassic age). Over an exposed area ∼100 m wide × 25 m high, these strata are disrupted by a fault-fracture mesh comprising conjugate Coulomb shears interlinked by extensional and extensional-shear fractures, all formed in a common stress field and hosting quartz + prehnite ± epidote ± calcite veining. The combined effect of these structures is shortening perpendicular to beddings and subvertical extension so that in their present attitude, they correspond to a set of conjugate thrust faults with associated extension fractures. On the evidence of incremental vein textures, the development of this distributed fault-fracture mesh is interpreted as resulting from a fluid-driven microearthquake swarm, which postdated regional low-grade metamorphism. Mechanical considerations suggest that the migrating hydrothermal fluids were significantly overpressured, possibly to approximately lithostatic values, if the mesh structure developed in its present attitude. Fluid-inclusion microthermometric studies show that Benmore vein quartz contains two-phase aqueous inclusions with salinities between 1.4 and 2.9 wt% NaCl equivalent and homogenization temperatures ( T h ) between 189 and 217 °C. The assemblage quartz + prehnite + epidote suggests trapping temperatures ( T t ) of ∼280 °C, requiring the addition of an ∼70 °C correction to T h values. Late calcite contains inclusions with noticeably lower salinity (0.0–0.9 wt% NaCl) and T h values (129–175 °C). Studies on quartz + pumpellyite ± calcite veins from nearby Lake Aviemore show similar fluid-inclusion salinity and T h values. Fluid-inclusion gas analyses show all the vein samples to be dominated by H 2 O (99.3–99.9 mol%) with few other gases apparent, including CH 4 (≤0.5%), N 2 (≤0.1%), CO 2 (≤0.1%), and C 2 -C 4 hydrocarbons. Cation and anion analyses, when combined with the gas data, show that NaCl dominates the fluid-inclusion salinities. Oxygen isotope results, when combined with calculated T t values, indicate that the water responsible for the deposition of Benmore and Aviemore quartz had δ 18 O compositions of 9.4‰ and 4.8‰, respectively. Calcite δ 13 C values between−25.3‰ and−38.0‰ are indicative of oxidation of CH 4 to CO 2 as a result of hydrothermal fluids interacting with organic- rich sediments. Fluid-inclusion \({\delta}D_{H_{2}O}\) values for Benmore range between −73‰ and −89‰ compared to−109‰ for the one Aviemore sample. This research has demonstrated that (1) water of meteoric origin, probably from subantarctic latitudes, penetrated to ≥6 km depth and underwent an oxygen isotope shift before depositing the Benmore-Aviemore veins; (2) the migrating hydrothermal fluids were likely overpressured well above hydrostatic to near lithostatic values if the mesh structure was active in its present orientation; and, (3) fluid migration was coupled to distributed brittle failure in the prevailing stress field, “self-generating” a permeable fault-fracture mesh.

A Brittle Failure Mode Plot Defining Conditions for High-Flux Flow

Stress and fluid-pressure conditions for the initial formation within intact rock of faults (shear fractures), extension fractures, and extensional shears, and for the reshear of existing faults, may all be represented on a generic failure plot of differential stress (σ1– σ3) versus least effective compressive stress, σ3' = (σ3– Pf), scaled to nominal tensile strength, T (~ half the cohesive strength). Plots of this kind may be used to define maximum sustainable overpressure in different tectonic environments and the structural conditions under which the flow of large fluid volumes may occur through fault-fracture meshes containing gaping extension and extensional shear fractures. They may also be adapted to different tectonic regimes and correlated to depth for particular fluid-pressure conditions. High-flux flow through distributed fault-fracture meshes requires the tensile overpressure condition, σ3' σ3), which can only be achieved in the absence of throughgoing cohesionless faults that are well oriented for frictional reactivation in the prevailing stress field. High-flux flow through distributed fault-fracture meshes, intrinsically a transient, pulsing phenomenon, may therefore occur as follows: (1) in effectively intact low-pemeability crust devoid of throughgoing favorably oriented faults, (2) where existing faults have become severely misoriented in the prevailing stress field, and (3) where existing faults have regained cohesive strength through hydrothermal cementation.