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The in-situ stress state of the eastern margin of the Sichuan Basin, where there is a complicated fault zone, is poorly understood. This study offers vital insights into the in-situ stress state based on information from 17 wellbores in the Wufeng-Longmaxi Shale near the eastern boundary fault (i.e., the Qiyueshan Fault) of the Sichuan Basin, China. Drilling-induced tensile fractures and borehole breakouts revealed a decrease in the angle between the SHmax (maximum horizontal stress) orientation and fault strike when the wellbores are close to the damage zone of the Qiyueshan Fault. This stress rotation may be caused by the lateral contrast in the Poisson's ratio between the country rock and damage zone of the Qiyueshan Fault. A high angle between the SHmax orientation and fault strike also appeared in the individual fault blocks of the Qiyueshan Fault as a result of local stress concentration. The higher magnitudes of horizontal stresses indicate stronger lateral compression in the Qiyueshan Fault Zone. Owing to the lateral compression of a “sandwich” consisting of a high-E (Young's modulus) roof and low-E shale intervals, natural fractures mainly appeared in the low-E shale intervals when the wellbore was close to the fault damage zone. The low fracture intensity in the low-E shale intervals of the wellbores located at the boundaries of the stress concentration blocks may be associated with an extensional stepover process.

Stress memory in the Wufeng-Longmaxi Shale located in the vicinity of the boundary fault of the Sichuan Basin was extracted by Kaiser effect analysis; subsequently, shale-gas retention capacity as determined by stress memory was assessed with a simulation. Due to gas generation and geothermal heating, the Kaiser effect rarely reflects stress states before the maximum burial depth. The strong deformation within and close to the fault damage zone negated the stress memory from the early exhumation episode; instead, the stress state of the recent exhumation episode was recorded. The maximum effective horizontal stress (σH-eff) at 120 Ma was ∼85 MPa at ∼6000 m, with a maximum sustainable pressure of ∼135 MPa. The porothermoelastic mechanism caused a decrease of the pressure magnitude but an increase of the pore-fluid factor during exhumation. After the Wufeng-Longmaxi Shale was uplifted from ∼6000 m to ∼3500 m, the pore-fluid factor almost reached 1 and horizontal hydrofracturing caused shale-gas leakage. The σH-eff at 20 Ma was ∼73 MPa at ∼3500 m and it had a gradient of 38 MPa/km. Reverse faulting at 20 Ma induced a remarkable pressure drop, which indicates shale-gas leakage at that time. Subsequently, a transition from reverse to strike-slip faulting further induced fluid pressure reduction and shale-gas leakage.

The Flat Creek Member of the Utica Formation along Canajoharie Creek south of Canajoharie, NY, is populated with unmineralized pancake joints, a term for horizontal, bed-parallel partings. Each pancake joint propagated outward from a central initiation point with radial growth in excess of 0.5 m in some instances. Propagation arrests with out-of-plane growth displaying either a tilt characteristic of an arrest line or bifurcation into a complex set of cylindrical splays or hackles. A rough surface morphology relative to vertical fractures (i.e., joints and veins) in the Flat Creek coupled with bifurcation at arrest are the manifestation of an accelerating crack tip reaching critical growth at the shear-wave velocity of the shale. The lack of mineralization and containment between vertical fractures are consistent with propagation during exhumation. One driving mechanism for a late propagation event could be axial splitting which places pancake joints in a class of brittle fracture that includes sheeting joints. The high-speed propagation is a manifestation of the rapid release of fracture energy characteristic of sheeting joints. However, we also consider a second driving mechanism, natural hydraulic fracturing. In the Appalachian Basin, high pore pressure is maintained in gas shale at depths less than 50% of the original overburden. If abnormal pore pressure is maintained for 300 my or more, it is present when exhumation carries the shale upward through the crossover depth where the least stress becomes vertical. If pore pressure exceeds the vertical stress, then pore pressure will lift overburden. A possible mechanism for maintaining pore pressure during exhumation to shallow depths is a poroelastic phenomenon known as Skemption's behavior. We posit that pancake joints differ from common sheeting joints because of the additional contribution from abnormal pore pressure as a driving mechanism that allows for deeper propagation depth.

In order to assess the nature of the Appalachian Basin Quiet Zone (ABQZ), a region of low seismic activity, a minimum horizontal stress profile from sonic logs was calibrated using hydraulic fracture stress measurements in a well from McKean County, PA. The calibrated minimum horizontal stress profile that best fits with the hydraulic fracture stress data was established using an extended Eaton's model that treats rock as a transverse isotropic material with a vertical symmetry axis (TIV). To achieve a best fit, several assumptions are introduced: (1) a constant tectonically-induced elastic strain of eHmax = 3.3 × 10−4, (2) an anisotropic velocity model ANNIE approximation for shales, and (3) a bimodal pore pressure distribution with abnormally high pore pressure in the Marcellus gas shale. Other calibration methods assume an isotropic medium, a constant tectonic strain gradient corresponding to zero stress at ground surface, or constraints from sidewall core data to estimate stiffness constants. By testing four calibration methods, we find that stress profiles are affected by multiple factors, including the ratio of maximum tectonically-induced elastic strain over minimum tectonic strain, pore pressure in the Marcellus, Biot's coefficients, and elastic properties. Sensitivity analysis was conducted to analyze the influence of these parameters. Our stress profile is most influenced by pore pressure and tectonically-induced elastic strain while other factors had much smaller effects. Tectonically-included strain in the ABQZ is insufficient to achieve the critical stress necessary to induce frictional slip on favorably oriented faults in the sedimentary cover which presumably speaks to the lack of earthquakes in the basement of the ABQZ. However, induced earthquakes from deep disposal wells suggest that basement at the edge of the ABQZ may be decoupled from the sedimentary cover, in which case the cover has relaxed through a time-dependent deformation mechanism such as pressure solution creep.

On January 1, 2009, a concrete slab covering a water-pump vault of a water well 400 m north of a Marcellus gas well in Dimock, Pennsylvania, USA was reported to have split into three pieces while being overturned. It was suggested that the cycling on of a water pump sparked the deflagration of a methane-air mixture causing the slab to overturn. Here, the conditions necessary to generate an explosion consistent with evidence, mainly a split and overturned concrete slab unmarked by soot or other evidence of a flame, are analyzed. Using more than one approach, calculations show that the maximum pressure to lift the concrete slab was roughly 0.3 bar. Considering among others the flammable range of methane, the explosion pressure as a function of equivalence ratio, the presence of methane gradients inside the vault, the absence of soot and possible ignition sources, the analysis did not yield a well-defined, credible gas explosion scenario to explain the observed damage, although the possibility cannot be ruled out with absolute certainty.

Both bedding-parallel slickensides and cleavage duplexes are forms of mesoscopic-scale detachment faulting populating black (Marcellus and Geneseo/Burket) and intervening gray (Mahantango) shales of the Middle Devonian, a section known for abnormal pore pressure below the Appalachian Plateau. The abundance and the orientation of slickensides and cleavage duplexes in the more organic-rich black shale relative to gray shale suggests that maturation-related abnormal pore pressure facilitates detachment, a mesoscopic manifestation of the Hubbert–Rubey pore pressure model for overthrust faulting. The former are discrete slip surfaces whereas the latter consists of nested, anastomosing slip surfaces, either cutting through bedding or on disrupted bedding surfaces stacked as mesoscopic versions of thrust duplexes. Cleavage duplexes are between a few cm and over 1 m thick with their hanging walls commonly transported toward the Appalachian foreland, regardless of local limb dip. Cleavage duplexes are most common near the stratigraphic maximum flooding surface, the organic-rich section most prone to develop maturation-related pore pressure in the Middle Devonian gas shales. Bedding-parallel slickensides are somewhat more evenly distributed in the black shale but also found in overlying gray shale. In both black and gray shales, slickensides are more abundant on the limbs of folds, an indication of pore-pressure-related flexural-slip folding. On the macroscopic scale, the Pine Mountain Block of the Southern Appalachian Mountains was enabled by a basal detachment cutting along the Upper Devonian Chattanooga black shale which has a thermal maturity sufficient for the generation of abnormal pore pressure. The Pine Mountain block is a large-scale overthrust showing little evidence of collapse of the hinterland side, a credible example of a pore-pressure-aided overthrust fault block of the type envisioned by the Hubbert–Rubey model.

More than 2 × 10 4 m 3 of water containing additives is commonly injected into a typical horizontal well in gas shale to open fractures and allow gas recovery. Less than half of this treatment water is recovered as flowback or later production brine, and in many cases recovery is

The oldest joint set in Devonian rocks of western New York state has an atypical NS strike and predates regionally more abundant NW-striking and ENE-striking joints driven by hydrocarbon-related fluid decompression. The NS joints originated in higher modulus diagenetic carbonate and were driven initially by a different mechanism, either joint-normal stretching and/or thermoelastic contraction. The origin of these joints in higher modulus carbonate concretions indicates the presence of a tensile stress produced by uniform regional extensional strain. Upper Devonian shale hosting the NS joints crops out in that area of the Appalachian Basin where a Morrowan erosional unconformity marks the region of maximum upward lithospheric flexure of an early Alleghanian forebulge. The NS strike of these early joints points to a forebulge stretching axis oriented approximately east-west and associated in time and space with crustal loading that drove both the Northfieldian Orogeny and the underplating of the Bronson Hill Anticlinorium in New England. Ultimately, subsidence of the Morrowan forebulge buried the Upper Devonian shale succession to the oil window during the latter part of the Alleghanian tectonic cycle resulting in the propagation of fluid driven NW- and ENE-trending joints in black shale.

Linear elastic fracture mechanics predicts that joint orientation is controlled by the stress field in which the joints propagate. Thus joint sets are effective proxies for stress trajectories during joint growth. Complexity in joint orientation indicates stress trajectory variability, a phenomenon quantified using an eigenvalue method that measures dispersion of joint normal vectors (i.e. poles) around the mean vector. Ratios between the eigenvalues of a joint orientation tensor give the clustering strength (ζ) and the shape factors (γ) of the distribution. A joint set that forms in a relatively isotropic rock subject to a rectilinear stress field should exhibit strong clustering and small random orientation variation that can be described by the Fisher statistical model. However, most joint orientation distributions in bedded rocks have non-random variation, greater in strike than in dip. This relative stability of the vertical stress orientation is strongest when joints are bounded by bedding interfaces, reflecting the tendency for deflection in the local stress field arising from the growth of side cracks, joint segments and adjacent joints in joint zones. Even when joint growth across bedding interfaces indicates negligible strength anisotropy, joint orientation distributions reflect less joint–joint interaction during vertical growth than during horizontal growth. As strike variation grows due to the presence of a non-rectilinear stress field, the orientation data better fit a Kent statistical model. Joint sets formed during fold development or in rocks with irregular bedding boundaries are more weakly clustered with Fisher-like orientation distributions. Orientation distributions for joint sets formed throughout a stress rotation have Kent-like shapes that indicate the magnitude of stress trajectory variation and clustering strength that depends on the joint density at each increment of the stress rotation.

Transected joints (i.e. systematic joints that strike at an angle to the present fold axis trend) occur on the flanks of Split Mountain, a Laramide anticline near the eastern end of the Uinta Mountains, Utah. The common orientation on both flanks for these WNW-striking joints is inconsistent with joints driven by a syn-folding stretch normal to the direction of highest curvature. A smaller dispersion of the poles to these transected joints occurs when they are rotated with bedding to their ‘pre-fold’ orientation. This dispersion of poles is inconsistent with a post-fold genesis in a regional stress field but permits the possibility that these WNW joints propagated as a systematic set prior to Laramide folding. A pre-fold interpretation is substantiated by a regional WNW-striking joint set within Cretaceous and older rocks in the surrounding Piceance, Uinta, and southeastern Sand Wash basins. During tilting accompanying the upfolding of Split Mountain, most joints of this WNW-striking regional set remain locked without slipping under a shear stress. Fracture toughness and frictional strength are two rock properties that serve to lock a joint until a critical resolved shear stress is achieved. A gravity load caused down-dip slip on some joints that were tilted to a dip of about 62°. This suggests that a local principal stress remained roughly vertical during bedding rotation. Assuming fracture strength and friction prevented slip on most joints during tilting, the ratio of least horizontal, S h , to vertical stress, S v , at the critical tilt angle was approximately 0.55.

Alpine inversion in the Bristol Channel Basin includes reverse-reactivated normal faults with hanging wall buttress anticlines. At Lilstock Beach, joint sets in Lower Jurassic limestone beds cluster about the trend of the hinge of the Lilstock buttress anticline. In horizontal and gently north-dipping beds, J3 joints ( 295‐2858 strike) are rare, while other joint sets indicate an anticlockwise sequence of development. In the steeper south-dipping beds, J3 joints are the most frequent in the vicinity of the reverse-reactivated normal fault responsible for the anticline. The J3 joints strike parallel to the fold hinge, and their poles tilt to the south when bedding is restored to horizontal. This southward tilt aims at the direction of s1 for Alpine inversion. Finite-element analysis is used to explain the southward tilt of J3 joints that propagate under a local s3 in the direction of s1 for Alpine inversion. Tilted principal stresses are characteristic of limestone‐shale sequences that are sheared during parallel (flexural-flow) folding. Shear tractions on the dipping beds generate a tensile stress in the stiffer limestone beds even when remote principal stresses are compressive. This situation favors the paradoxical opening of joints in the direction of the regional maximum horizontal stress. We conclude that J3 joints propagated during the Alpine compression caused the growth of the Lilstock buttress anticline. q 2001 Published by Elsevier Science Ltd.

Two general types of sedimentary structures serve as initiation points (IP) for cross-fold joints in siltstone beds of the Ithaca Formation, Appalachian Plateau, New York: planar bed boundaries and geometrically complex features such as bedforms, trace fossils, and soft-sediment deformation structures. The relative abundance of each IP type varies with the stratigraphic position of the jointed body. When they are mechanically isolated by thicker shales, siltstone bodies have a higher percentage of IP associated with complex features. This association supports the hypothesis that sedimentary structures serve to concentrate stress, thus becoming preferred sites for primary joint initiation. Secondary joint initiation occurs at planar bed boundaries through interaction with pre-existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints. The propagation path of joints passing through planar bed boundaries of composite joints can be traced backward to primary IP at complex sedimentary structures. Modeling suggests that when beds are mechanically coupled, the modest joint-tip stress concentration across a shale layer (e.g.

Four trends of joint sets (WNW‐ESE, NW‐SE, NNW‐SSE and NE‐SW) are found in upper Turonian carbonate rocks within the Neqarot syncline of south-central Israel. The two most predominant sets strike parallel to the trend of maximum compressive stress directions (SH) associated with the plate-related Syrian Arc stress field (SAS; WNW‐ESE) active during the Cretaceous to present and the perturbed regional stress field (NNW‐SSE) related to stress accumulation on the Dead Sea Transform during the Miocene to the present. Eighty-two percent of the beds in this study contain joints parallel with the latter trend, whereas 42% contain joints parallel to the former trend. All beds with layer thickness to spacing ratio (FSR) . 1.5 have NNW‐SSE joint sets compatible with the Dead Sea Transform stress field (DSS), whereas all joints sets that are not compatible with the DSS stress field fall beneath this value for FSR. Considering lithology, joints in five of six chalky limestone beds and all marly limestone beds are compatible with the DSS, whereas joints compatible with the SAS do not develop in these marly and chalky limestone beds. In the study area, the joint sets lack a consistent formation sequence where more than one set is found in a single bed. We use these observations to conclude that all studied joints are Miocene or younger, that the regional stress field from the Miocene to the present fluctuated, between DSS and SAS states, and that the higher FSRs correspond to a greater amount of joint-normal strain in response to the DSS. q 2001 Elsevier Science Ltd. All rights reserved.

Structural geometries and kinematics based on two sets of joints, pinnate joints and fault striations, reveal that some mesoscale faults at Split Mountain, Utah, originated as joints. Unlike many other types of faults, displacements (D) across faulted joints do not scale with lengths (L) and therefore do not adhere to published fault scaling laws. Rather, fault size corresponds initially to original joint length, which in turn is controlled by bed thickness for bed-confined joints. Although faulted joints will grow in length with increasing slip, the total change in length is negligible compared to the original length, leading to an independence of D from L during early stages of joint reactivation. Therefore, attempts to predict fault length, gouge thickness, or hydrologic properties based solely upon D‐Lscaling laws could yield misleading results for faulted joints. Pinnate joints, distinguishable from wing cracks, developed within the dilational quadrants along faulted joints and help to constrain the kinematics of joint reactivation. q 2001 Elsevier Science Ltd. All rights reserved.

The interaction between propagating joints and embedded concretions in a Devonian black shale near Seneca Lake, NY, permits identification of the loading configurations responsible for two joint sets of different ages striking at nearly the same orientation. The earlier set consists of systematic joints cut by later Alleghanian joints of the Appalachian Plateau. The later set consists of non-systematic curving cross joints that abut these same Alleghanian joints. Field evidence shows that concretions functioned as stiff inclusions in a compliant black shale. As a consequence of this elastic contrast, local perturbations in the remote stress field persisted around the concretions during burial, tectonic deformation, and exhumation. These stress perturbations influenced joint propagation paths of both joint sets. Our conclusions about loading configurations are based on finite-element modeling of the effect of the local stress perturbation on concretion-modified joint propagation. Modeling shows that the local stress perturbation from a thermoelastic loading was responsible for deflecting cross joints away from concretions in a curved propagation path near the concretion. This load configuration also led to arrest of cross joints before they penetrated the shale–concretion interface. At greater distances from the concretion, the propagation path of cross joints was controlled by the contemporary tectonic stress field. The interface between concretions and the surrounding shale was strongly bonded, as indicated by the crossing of the interface by some of the systematic ENE joints. Higher compressive stress levels within the concretions relative to the shale suppressed joint development in the concretion, causing the arrest of those joints once they had driven across the interface and a short distance into the concretion. Numerical modeling shows that interface penetration by the systematic ENE joints is consistent with a fluid load, the same loading configuration postulated for the subsequent Alleghanian joints. The traces of the systematic ENE joints align on opposite sides of concretions, rather than curving toward the concretion as predicted by two-dimensional models of the fluid load. Co-planar traces are indicative of large, planar joints propagating in-plane around the concretion, making it energetically inefficient for the crack front to curve as it enters the local stress perturbation near the concretion. A fluid load for the systematic ENE joints came from high pore pressure during the pre-Alleghanian stages of burial of the Devonian Catskill delta complex.

One model for the development of hybrid shear fractures is transitional–tensile fracture propagation, a process described as the in-plane propagation of a crack subject to a shear traction while held open by a tensile normal stress. Presumably, such propagation leads to a brittle structure that is the hybrid of a joint and a shear fracture. Crack–seal veins with oblique fibers are possible candidates. While these veins clearly show shear offset, this is not conclusive evidence that a shear traction was present at the time of initial crack propagation. Many recent structural geology textbooks use a parabolic Coulomb–Mohr failure envelope to explain the mechanics of transitional–tensile fracturing. However, the laboratory experiments cited as demonstrating transitional–tensile behavior fail to produce the fracture orientation predicted by a parabolic failure envelope. Additional attempts at verification include field examples of conjugate joint sets with small acute angles, but these conjugate joints form neither simultaneously nor in the stress field required by the transitional–tensile model. Finally, linear elastic fracture mechanics provides strong theoretical grounds for rejecting the notion that individual cracks propagate in their own plane when subject to a shear traction. These observations suggest that transitional–tensile fracture propagation is unlikely to occur in homogeneous, isotropic rock, and that it is not explained by a parabolic Coulomb–Mohr failure envelope as several recent structural geology textbooks have suggested.

In rocks without systematic mechanical discontinuities (e.g., granite), joint spacing follows an approximately log-normal frequency distribution (i.e., the distribution has a kurtosis near zero). Joint spacing in rocks with systematic mechanical boundaries differs from the spacing in isotropic rocks, exhibiting consistently positive values of kurtosis (i.e., the distribution is more clustered around the mode than a perfectly log-normal distribution). We attribute this difference in joint-spacing distribution to mechanical boundaries such as bed partings in sedimentary rocks that constrain joint height and control joint spacing. Existing systematic joints can also act as mechanical boundaries during the development of later ‘cross’ joints. Two parallel mechanical boundaries determine the mechanical-layer thickness that influences joint spacing. In this paper, we investigate the effect of sampling geometry and mechanical discontinuities on joint-spacing statistics. In many situations, neither pavement surfaces nor properly oriented boreholes are available for measuring the spacing of cross joints that develop between existing systematic joints. When joint-spacing data come from scanlines that are oblique to a systematic joint set (e.g., crossing many systematic joints on a sub-vertical outcrop face), we consider whether the median spacing of the systematic joint set is statistically equivalent to the mechanical-layer thickness thought to control cross-joint spacing between individual pairs of systematic joints. The basis of our analysis is the fracture spacing index (FSI), which is the slope of a line fitted to a plot of mechanical-layer thicknesses vs. median joint spacing. We collected joint-spacing data along oblique scanlines from a large outcrop of the Devonian Brallier Formation, a distal turbidite sequence, near Huntingdon, PA. Our analysis indicates that the spacing of cross joints correlates better with a mechanical-layer thickness defined by the median systematic (‘strike’) joint spacing (r 2 D 0:78, FSID 1.02) than with a mechanical-layer thickness defined by the stratigraphic bed thickness (r 2 D 0:69, FSID 0.97). This is consistent with the conclusions of previous workers. We also note that the spacing data from the cross joints exhibit a higher (i.e., more positive) value of kurtosis than the data from the earlier strike joints (1.66 vs. 0.98). This is consistent with the idea that mechanical discontinuities alter joint spacing in a systematic manner. In this case, the cross joints may have been influenced by two sets of mechanical boundaries (bedding and existing joints), whereas the earlier strike joints were constrained by only one set (bedding). © 1998 Elsevier Science B.V. All rights reserved.

Using the Griffith energy-balance concept to model joint propagation in the brittle crust, two laboratory loading configurations serve as appropriate analogs for in situ conditions: the dead-weight load and the fixed-grips load. The distinction between these loading configurations is based largely on whether or not a loaded boundary moves as a joint grows. During displacement of a loaded boundary, the energy necessary for joint propagation comes from work by the dead weight (i.e., a remote stress). When the loaded boundary remains stationary, as if held by rigid grips, the energy for joint propagation develops upon release of elastic strain energy within the rock mass. These two generic loading configurations serve as models for four common natural loading configurations: a joint-normal load; a thermoelastic load; a fluid load; and an axial load. Each loading configuration triggers a different joint-driving mechanism, each of which is the release of energy through elastic strain and/or work. The four mechanisms for energy release are joint-normal stretching, elastic contraction, poroelastic contraction under either a constant fluid drive or fluid decompression, and axial shortening, respectively. Geological circumstances favoring each of the joint-driving mechanisms are as follows. The release of work under joint-normal stretching occurs whenever layer-parallel extension keeps pace with slow or subcritical joint propagation. Under fixed grips, a substantial crack-normal tensile stress can accumulate by thermoelastic contraction until joint propagation is driven by the release of elastic strain energy. Within the Earth the rate of joint propagation dictates which of these two driving mechanisms operates, with faster propagation driven by release of strain energy. Like a dead-weight load acting to separate the joint walls, pore fluid exerts a traction on the interior of some joints. Joint propagation under fluid loading may be driven by a release of elastic strain energy and/or work by the pore fluid during poroelastic contraction. Again, propagation velocity dictates the driving mechanism, with critical to supercritical joint propagation accompanying fluid decompression and subcritical crack propagation taking place under a constant fluid drive. During axial shortening a jointing process called axial splitting may occur following wing crack growth from an initial crack tilted relative to the maximum compressive stress. Here the joint-driving mechanism may be either the work of a remote stress (i.e., a dead weight) or the release of elastic strain energy (i.e., fixed grips).

Plane strain fracture toughness (KIc) values are determined for the modified ring (MR) test through numerical simulation of crack growth to highlight the sensitivity of MR KIc values on applied displacement or force boundary conditions, slip conditions at the specimen-platen interface, and the Poisson ratio (v) of the test material. Numerical calculation of fracture toughness in the MR test is traditionally conducted assuming a uniform force along the specimen loading surfaces and no slip between the specimen and the loading platens. Under these conditions KIc increases by 30–40% as v decreases from 0.4 to 0.1. When slip is allowed at the specimen-platen interface under a uniform force, KIc values are independent of v, and for any given v, are 5–25% less than those determined under a no-slip boundary condition. Under a uniform displacement of the specimen loading surfaces, KIc is essentially independent of v, regardless of specimen-platen interaction. Moreover, although KIc values determined under unifor displacement and no-slip boundary conditions are always higher than those determined under uniform displacement and slip-allowed boundary conditions, the average difference in KIc for these two methods is less than 5% for the two specimen geometries examined. This suggests that under uniform displacement conditions, KIc is essentially independent of specimen-platen interaction. Because KIc values determined from MR testing are strongly dependent on the modeling procedure, future reports of KIc determined from this test should be accompanied by detailed reports of the modeling procedure. Until further testing reveals the most accurate simulation technique, we advocate use of a uniform displacement formulation for KIc determination from MR testing because results from this method are insensitive to most modeling parameters. Numerical results from models conducted under uniform force, no-slip boundary conditions should be viewed as an upper bound to KIc.

Extensional strain accommodated by brittle deformation was measured in adjacent mudstone and dolostone units at Arroyo Burro beach, California. The dolostone failed in effective tension whereas the mudstone failed in shear, both in response to the same extensional tectonic event. In the mudstone unit initial bed-length and fault-displacement methods document extensional strains of 6.6% ± 0.3% and 6.0% ± 0.3%, respectively. Upon adjusting the displacement estimates according to theoretical fault displacement population analysis, the corrected strain becomes 9.7% ± 0.3% for the mudstone. Measurements of vein apertures in the dolostone document extensional strain that varies according to scale, with outcrop vein scanlines indicating a strain of 3.4% ± 0.1%, and thin-section scanlines yielding a strain of 5.8% ± 0.2. Applying theoretical fault displacement population analysis to vein apertures in dolostone shows that small veins below the detection limit of outcrop surveys contribute significantly to fracture-related strain within the dolostone. This difference in extensional strain may arise because veins measured in outcrop extend across the entire bed height and thus are controlled by the dolostone mechanical layer thickness, whereas microscopic veins measured in thin-section terminate without regard to a bounding layer. The corrected dolostone strain becomes 10.2% ± 1.0%, matching the revised strain calculated in the adjacent mudstone unit and indicating uniform displacement boundary conditions for the two markedly different lithologies. The ~10% extensional strain at Arroyo Burro indicates significant strike-parallel (NW-SE) extension accommodated by brittle failure during development of the western Transverse Ranges fold and thrust belt.

Bedding-perpendicular joints confined to individual beds in interbedded sedimentary rocks commonly exhibit spacings which are proportional to the thickness of the jointed bed, and which vary according to lithology or structural position. The mechanical explanation for this relationship is well understood when the joints are driven by far-field crack-normal tensile stresses, but poorly understood for cracks driven by elevated fluid pressures, where the crack-driving stress is the difference between the crack-normal compression and the fluid pressure in the crack. Through a series of finite-element numerical models, we investigate how various parameters influence the driving-stress distribution around pressurized cracks in layered media, and thereby identify factors influencing the spacing of fluid-driven joints. For the situation we modeled, we observe that: (1) crack-driving stress is reduced in the vicinity of pressurized joints, and that the extent of the stress reduction depends on the contrast in elastic properties between the layers; and (2) crack-driving stress distribution depends on the ambient pore pressure during jointing. These results indicate the spacing of fluid-driven joints should depend on lithology and pore pressure.

The magnetic anisotropy of Devonian black shale samples was measured from two cores drilled in the Appalachian Plateau. The mineralogy of the shales is predominantly clay, with small quantities of quartz and minor amounts of opaques and chlorite. Magnetite is the predominant ferromagnetic mineral present in the samples. The magnetic fabric was measured at both room temperature and liquid-nitrogen temperature and is dominated by a well-defined bedding (vertical) compaction and a lesser defined magnetic lineation. Measurements of the anisotropy of magnetic susceptibility (AMS) at liquid-nitrogen temperature, which enhances the paramagnetic contribution in the rock, showed a strong increase in both the bulk susceptibility and susceptibility differences. This increase suggests that the AMS is controlled by the paramagnetic minerals, particularly the clays and chlorite. Strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry. Good correlations have been found (1) between the orientation of the magnetic lineation and the long axes of the chlorite crystals, and (2) between the degree of magnetic foliation and the amount of vertical compaction. The magnetic lineation also agrees well with the direction of seismic anisotropy over the Plateau. The anisotropy of the anhysteretic remanence, which expresses the anisotropy due to the ferromagnetic component in the rocks, shows a weaker correlation with the amount of vertical compaction. A weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post-dates the main Alleghanian orogeny. The magnetic anisotropy of the Devonian shales mirrors the compaction and tectonic fabric on the Appalachian Plateau.

Concentrated slip zones bound subsidiary shears at three scales within the Cerro Brass Fault, which cuts Cambrian dolomites of the N&any Anticlinorium, Pennsylvania. On the outcrop scale thrust faults along both Cerro Brass fault zone boundaries acted as concentrated slip zones bounding subsidiary shears developed along original bedding planes. The outcrop scale subsidiary shears are zones of concentrated slip bounding hand- specimen-scale subsidiary shears. In turn, the hand-specimen-scale subsidiary shears are concentrated slip zones bounding even smaller-scale subsidiary shears. Subsidiary shears in Cerro Brass fault zone are analogous to subsidiary shears (i.e. RI and R2) commonly found in laboratory gouge-friction experiments. The orientations of subsidiary shears at the outcrop and hand-specimen scales define a 'Riedel within Riedel' geometry in which the original bedding played the role of R, shears at the outcrop scale and, at the same time, operated as boundary faults at a smaller scale. The presence of subsidiary shears on more than one scale suggests that the Coulomb failure theory is not sufficient to explain their origin

This work presents the results of structural and fluid inclusion analyses of brittle-ductile shear zones (bdsz) cutting Lower Paleozoic carbonates in the Appalachian fold and thrust belt. Strike parallel extension of the beds riding up the ramp of Yellow Spring thrust resulted in development of conjugate bdsz by progressively non-coaxial deformation. During the shearing, syntectonic extension veins were formed by successive episodes of mode-I cracking and sealing by the precipitation of quartz, saddle dolomite and calcite in paragenetic order. Geobarometry on co-existing methane-rich and water-rich fluid inclusions reveals the trapping of fluids at large range of fluid pressures. The values of maximum and minimum fluid pressures correspond to the events of crack propagation and complete sealing respectively. Contrary to ductile shear zones in high-grade metamorphic rocks, this study in unmetamorphosed rocks shows that bdsz evolve by influx of locally derived methane bearing brines of connate or evolved formation water origin at pressures of 80–40 MPa, temperatures of 155–165°C and a depth of 6 km. It is shown that the salinity and composition of the mineralizing fluids change from one crack-seal episode to another, in spite of a broad consistency in the pressure-temperature conditions.

On the Appalachian Plateau of New York and Pennsylvania plume patterns are present on surfaces of all cross-fold joints that cut siltstones, but they are not common on either cross-fold joints or strike joints cutting shales. Near Watkins Glen, New York, joints cutting siltstones display two types of plumose patterns; each particular type correlates with a specific cross-fold joint set. The more easterly striking cross-fold joints (345° i.e., N15°W), cutting thin siltstone beds embedded in thicker shale formations, have straight plume patterns with axes parallel to bedding. More westerly striking cross-fold joints (335°), cutting thick siltstone beds, have curved plume patterns with axes that either curve or show fan-like rhythmic patterns that alternately increase and decrease in intensity. Joints cutting only shales exhibit no distinct surface morphology other than long arcuate arrest lines. The fan-like rhythmic patterns of plumes suggest that these joints formed by a cyclic process (perhaps related to pore pressure variations) rather than by one massive rupture.

Based on the timing of joint propagation during the history of burial, lithification, deformation and denudation of clastic rocks within sedimentary basins, four types of joints may be distinguished: tectonic, hydraulic, unloading and release. Tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures, whereas unloading and release joints form near the surface in response to thermal-elastic contraction accompanying erosion and uplift. Tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case. The orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric. Examples of some of these joints are found within the Devonian Catskill Delta of the Appalachian Plateau, New York. During the Alleghanian Orogeny tectonic joints (cross-fold joints) formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross-cutting relationships of folds, cleavage and joints. This sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers. Plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length. The discontinuous rupturing is compatible with models for natural hydraulic fracturing. Release joints (strike joints) post-date the Alleghanian Orogeny as indicated by abutting relationships within the deeper parts of the Devonian clastic section. Unloading joints are orthogonal to the contemporary tectonic stress field.

A pole-figure goniometer was used to measure the preferred orientation of the basal planes of chlorite in 87 samples from the Upper Devonian clastic rocks of the Appalachian Plateau, New York. Interpretation of the preferred orientation according to the theory of March yields an estimate of how much an element of the sediment has changed shape since deposition; this deformation can be interpreted according to three distinct models for the strain history. All begin with compaction, followed by one of three types of tectonic strain: (1) a plane strain, that conserves bedding-plane area by compensating for horizontal shortening with horizontal elongation at right angles; (2) one that uniaxially shortens a horizontal line normal to the fold axes without compensating elongation: and (3) a plane strain again, which conserves the area in the vertical plane containing a shortened horizontal direction by stretching it vertically. For lack of suitable names, we will simply call the three total strains type (b), (u) and (v) from the initials of the distinguishing features of their tectonic increment. The horizontal strain components estimated from preferred orientation are compared at a number of localities with those implicit in the principal horizontal diameters of the elliptical outlines of deformed crinoid columnals found on bedding planes. Expressed for constant bedding-plane area, or type (b) total strain, the strain in the direction of greatest elongation measured by preferred orientation was found to range from no recorded strain to 0.19, with a median of 0.04. As a rule, however, the more intensive strains were recorded in shales with a fine-grained phyllosilicate content exceeding 85% by volume, in which the mean was 0.07. Deformed crinoid columnals in the same set of samples have elongations parallel to their long axes that range from 0.04 to 0.10, with a median and average of 0.07. The discrepancy between the two sets of strain measures, at least in the coarser and more permeable rock types, may either be real and caused by easier flushing of dissolved ions or may be caused by imperfect strain recording in coarse matrix materials.

Work of the past decade concerning tectonic cleavage has focused on nine topics: (1) appropriate terminology for cleavage classification, (2) processes involved in cleavage formation, (3) controls on cleavage distribution and morphology, (4) depth dependence of cleavage formation, (5) controls and causes of cleavage initiation, (6) nature of lithologic changes during cleavage development, (7) relation of cleavage to strain, (8) relation of cleavage to stress and (9) chronology of cleavage development. This paper reviews these issues as they apply to the formation of disjunctive cleavage in sedimentary rocks. Discussion is limited to cleavage that formed at shallow crustal conditions in which rock-water interaction, rather than plastic flow, recrystallization, or grain growth, is the dominant deformation process.

In situ and laboratory measurements of ultrasonic velocity have been compared to test for the effect of strain relaxation on rock properties. For granites, the larger changes in compressional-wave velocity (Vp) correlate with higher in situ stresses, the maximum Vp being parallel to the directory of maximum compressive stress. For sedimentary rocks, the relation between changes in Vp and in-situ stress is dependent on lithology. In sandstone, the direction associated with the maximum decrease in Vp accompanying strain relaxation is parallel to the maximum compressive stress. Limestone is virtually isotropic with little decrease in Vp on strain relaxation.

Since hydraulic fractures exhibit a strong tendency to propagate in a plane normal to the least principal stress, the preferred plane of propagation under thrust stress regime ( S h > S v ) conditions is horizontal. This can lead to complications in applying the hydrofracture stress measurement technique as horizontal fractures do not directly sample the horizontal stress field. Recent experiences in conducting measurements in thrust regimes have highlighted two problems that might be encountered. The first is the possibility of horizontal fracture initiation at the wellbore. The second relates to the seemingly common problem of deciding whether ISIPs which lie close to the estimated overburden reflect least horizontal stress levels (which happen to coincide with Sv) or vertical stress levels (resulting from fracture rotation to a horizontal plane in response to S h > S v ). If the latter is true, then the ISIPs represent only lower bounds to the true values of S h . We present two datasets which have bearing on the two problems noted above. In the first we review measurements conducted in a vertical borehole penetrating granite in which the vast majority of induced fractures were horizontal at the wellbore. Evidence suggests that these fractures were not a result of packer-induced stresses or incipient natural fractures but rather were a consequence of both high horizontal stress levels and fluid infiltration of the wellbore wall during the 15 sec of pump time required to attain breakdown. The absolute magnitudes of the horizontal stresses is not determined. However, through consideration of the elastic stress distribution about a vertical borehole it is possible, in principle, to estimate the horizontal stress difference S H −S h from the horizontal fracture initiation pressure. In the second dataset we present measurements conducted in three boreholes penetrating a sandstone/shale sequence in which the induced fractures were determined to be vertical at the wellbore. However, by modelling the anticipated effects of topography it is clear that the ISIPs above a certain stratigraphic horizon consistently reflect the vertical stress, and not S h . No evidence of dual shut-in pressures, which might provide a measure of the magnitude of the least horizontal stress in these beds, was observed. Neither did the form of the post-shut in pressure decline for these beds display any characteristics that might have served to distinguish them as horizontal fracture controlled. This example shows that the presence of a vertical fracture trace at the wellbore cannot be taken as proof that the ISIP reflects S h .

The dissolution of crinoid columnals during tectonic deformation of the Appalachian Plateau was enhanced by stress-induced changes of chemical potential of calcite in solution at the surface of the crinoid columnal. Pressure solution on the outside surface occurred within areas of highest normal stress developed at grain to grain contacts whereas free-face dissolution on the inside bore (the axial canal) of the columnal occurred where parts of the crystal lattice next to the surface were subject to the highest strain, as indicated by the presence of mechanical twins. For free-face dissolution, the chemical potential of the solute is affected more by the larger strain energy associated with mechanical twinning than strain energy from elastic strain prior to mechanical twinning of the crystal lattice. These observations suggest that free-face dissolution as well as pressure solution may contribute significantly to dissolution during rock deformation by diffusive mass transfer.

The permeability of whole and jointed Barre granite was measured at pressures up to 2 kbars. Jointed samples were actually split cylinders joined by surfaces with controlled surface roughness. Samples with induced tension fractures were also measured. The permeability of the whole rock ranged from about 10−6 to 10−7 darcies. The permeability of the jointed rock ranged from about 8 × 10−5 darcies at low pressure down to that of the whole rock at high pressures. Permeability was not a simple function of the difference between external confining pressure (Pc) and internal fluid pressure (Pf). Changes in permeability were found to be proportional to (b dPf − a dPc) where b/a

Residual strain, a self-equilibrating recoverable strain that remains in rocks even after external forces and moments are removed, is found NNW of the folded Appalachian plateau in the Devonian Onondaga limestone and the Silurian Lockport dolomite and Grimsby sandstone of western New York. This residual strain is manifest upon overcoring by a NNW directed maximum expansion of the limestone and sandstone and a random maximum expansion of the dolomite. Strains, recorded with strain gauge rosettes bonded to outcrops, are as high as 200 μϵ (microstrain). Double overcoring of the sandstone and dolomite relieves smaller strains of the same orientation as the initial overcore. X-ray analysis of the Grimsby sandstone shows that the elastic residual strain locked in quartz grains is characterized by a NE principal extension of 60 μϵ and a 10–30 μϵ NW principal compression oriented 30° counterclockwise from the NNW compression indicated by overcoring. Sonic velocity tests on samples in the lab indicate that Grimsby sandstone is anisotropic with a NNW maximum P-wave velocity of 4.05 km/sec. This anisotropy correlates with the residual strain in Grimsby sandstone. Mechanical twinning of calcite within both the Onondaga limestone and Grimsby sandstone indicates that the rock contains a permanent compressive strain of less than 2% in the NNW direction. The permanent strain becomes progressively smaller in a series of samples from Syracuse to Buffalo, New York. The development of solution cleavage in Onondaga limestone also indicates a NNW compression. No evidence of permanent strain was found in the dolomite. The NNW compression of the limestone and sandstone is normal to the fold axes of the Appalachian foreland fold and thrust belt. This geometric relationship indicates that the residual strain well beyond the outermost Appalachian folds was caused by the same tectonic stresses responsible for folding, the Appalachians during the late Paleozoic. Strain within the Appalachian plateau below the Silurian salt horizon suggests either the presence of a second decollement in, perhaps, Ordovician shales or a general NNW shortening of the crust under the Appalachian plateau.

In situ measurements of both instantaneous and time-dependent relaxation of the Algerie granite at a quarry near East Otis, Massachusetts, indicate that the orientation of instantaneous maximum expansion is 30° different from the orientation of the time-dependent maximum expansion. Based on comparison of these measurements with the regional contemporary tectonic stress and with measurements of petrofabrics of the sampled unit, it appears that the instantaneous relaxation orientation (ENE) is controlled by contemporary tectonic stress, whereas the time-dependent relaxation orientation is controlled by healed microcracks in the granite. The curves for time-dependent relaxation best fit a power-law function of time with the exponent varying between 0.211 and 0.872.

Davis, D.M. and Engelder, T., 1985. The role of salt in fold-and-thrust belts. In: N.L. Carter and S. Uycda (Editors), Collision Tectonics: Deformation of Continental Lithosphere. Tectonophysics, 119: 67-88. The style of deformation in thin-skinned fold-and-thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks. Evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported. Fold-and-thrust belts that form atop a salt layer, such as the Appalachian Plateau, the Franklin Mountains in northwestern Canada, and the Jura of the Alps, among others, share several readily observable characteristics. As predicted by a simple mechanical model for fold-and-thrust belts, a detachment in salt permits a thrust belt to have an extremely narrow cross-sectional taper. In addition, predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt. Other common attributes of salt-basal thin-skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin.

Tectonic cleavage has developed in Lower Devonian limestones of the Hudson Valley Fold-Thrust Belt in New York State. Morphology and distribution of cleavage in these limestones is controlled by the amount of clay-quartz matrix present and by strain. Only limestones with greater than 10% clay-quartz matrix developed widespread cleavage. X-ray diffraction analyses indicate that the clay matrix was altered during deformation; the width of the 001 illite peak in cleaved lime wackestone is less than that in uncleaved lime wackestone from the same stratigraphic level. A minimum of 10% clay is necessary to provide interconnectivity between sites of dissolution and the free-fluid system. Preliminary analyses of bulk chemistry indicate that calcite is removed from the local rock system during cleavage development, supporting proposals that circulation of fluid through the rock plays a major role in the development of cleavage. Textural evidence suggests that both pressure solution and free-face dissolution contribute to the removal of ions at grain boundaries. Cross-cutting relations indicate that cleavage was initiated early during the development of the fold-thrust belt, but continued to develop during the late stages of folding.