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More than 1 million wells may have been completed using hydraulic fracturing techniques in the USA alone; however, there have been few case studies exploring the impacts on water resources due to methane migration. This study evaluated the results of three investigations initiated by the US Environmental Protection Agency, that were subsequently described in published studies at Dimock in Pennsylvania, Parker-Hood County in Texas, and Pavillion in Wyoming, as well as another study completed at Sugar Run in northeast Pennsylvania. In addition, earlier investigations at Shaws Corner in Pennsylvania, Jackson County in West Virginia, Garfield County in Colorado, and Bainbridge in Ohio are summarized. The most common cause of incidents was the presence of uncemented sections of production casings in wells that allowed gas migration from intermediate depths to shallow freshwater aquifers. In three cases, an inadequate depth of the primary top of cement (TOC) also contributed to impacts. Sources of methane were best identified through analyses of isotopes on samples from production casings, annular spaces, and water wells. In Dimock, some isotope signatures changed with time, after the completion of remedial actions. In Parker-Hood County, where impacts were not related to gas well activity, noble gas analyses were also needed to determine the source of gas. At Pavillion, where maximum methane concentrations in water wells were

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.

Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO42− concentrations dropped. These observations are attributed to iron and SO42− reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO42− that could distinguish newly leaked methane from older methane sources in aquifers.

Within the Appalachian plateau detachment sheet of southwestern Pennsylvania tectonic thickening is a central element to the growth of anticlines in a Silurian-Devonian section containing a three-tiered mechanical stratigraphy (that is, a basal detachment zone, a lower imbrication zone, and an upper wedge zone). The detachment zone is predominantly within disturbed shale of the Silurian Vernon Formation which sits above a disturbed surface on the Lockport dolomite. Large-scale anticline growth is, in part, a consequence of small-scale blind imbrication of the more competent mechanical layers in the lower portion of the detachment sheet (the imbrication zone). Here, salt within the Silurian Syracuse Formation hosts secondary detachment responsible for imbrication and the development of triangle zones in the core of the anticlines. Some fold amplification is also accomplished by extensive, smaller-scale thrust wedging and concomitant tectonic thickening of the less competent Devonian section in the upper portion of the detachment sheet (the wedge zone). These spatially periodic anticlines are separated by horizontally bedded synclines characterized by the absence of thrust wedging and fault imbrication across all three zones. Tectonic thickening continued behind the foreland propagation of the detachment-tip line within the Vernon shale as indicated by a systematic hinterland increase in the cross-sectional width and amplitude of the anticlines. On the basis of seismic images, each detachment sheet anticline is situated above prominent, periodically-spaced, pre-Alleghanian structures in the footwall of the detachment sheet. These footwall structures arise from a combination of thrust imbrication at the depth of the Ordovician Trenton Group and high-angle, basement-involved faulting at the depth of the Cambrian Gatesburg Formation. Some of the high-angle fault displacement is a consequence of Alleghanian inversion on faults associated with the extensional Rome Trough and other basement structures developed during the Late Proterozoic rifting of the eastern margin of Laurentia. Evidence for tectonic inversion is present in seismic reflection images that show buttress anticlines in the Cambro-Ordovician carbonate section. The superposition of detachment-sheet anticlines above footwall structures strongly suggests that the foreland transport was disrupted by topographic bumps on the base of the detachment zone. Bending of the detachment sheet over these irregularities may have promoted the spatially periodic collapse and concomitant tectonic thickening in the over-riding sheet as it was pushed laterally toward the foreland. A strain hardening of the detachment zone part of the detachment sheet at the topographic irregularities permitted the growth of double-sided tapered wedges (that is, each anticline). The overall structural profile of the detachment sheet is consistent with the growth of a Coulomb wedge whose low basal friction is interrupted by patches of strain hardening.

Vertical joints in Devonian clastic sedimentary rocks of the Finger Lakes area of New York State are ornamented with arrays of fringe cracks that reveal the complex deformational history of the Appalachian plateau detachment sheet during the Alleghanian orogeny. Three types of fringe cracks were mapped: gradual twist hackles, abrupt twist hackles, and kinks. Gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet. Abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints. Kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds. The breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field, internal fluid pressure, and the elastic properties of the bed. The twist angle of gradual twist hackles is larger in coarser clastic beds, indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown. Assuming that the vertical stress axis (S v ) equals 78 MPa at 3 km burial depth, the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as S H ‐ S h ∪ 2.5 MPa when S H ‐ S h ∪ 12 MPa in sandstone beds. The twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during Alleghanian tectonics. These parent joints indicate a regional clockwise stress rotation of Alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area. A counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet. The clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the Pennsylvania Valley and Ridge, but is opposite to the sense of rotation in the southwestern portion of the detachment sheet (western Pennsylvania and West Virginia). The two regional rotation domains are separated by the Juniata culmination.

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.

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

The modified ring test is used to determine the fracture toughness of synthetic, granular, fresh-water ice average density 0.891 Mg m−3 and firn (average density 0.605 Mg m 3) from depths between 26 and 27.2 m in the E core of the Greenland Ice Sheet Project II. Average fracture toughness is 145.7kPa m2 for the manufactured ice and 108.6kPam½ for the firn. Comparison between the ice and firn suggests that ice-fracture toughness decreases with decreasing density (i.e. increasing porosity), suggesting lateral and vertical variations in the near-surface fracture resistance of glaciers and ice sheets may be related to firn densification. The modified ring test has many advantages over conventional, notch-based specimens in that complications which arise in notched specimens due to crack-length, loading-rate, notch-acuity and specimen-size effects are irrelevant for a modified ring-specimen geometry.

The debate surrounding the safety of shale gas development in the Appalachian Basin has generated increased awareness of drinking water quality in rural communities. Concerns include the potential for migration of stray gas, metal-rich formation brines, and hydraulic fracturing and/or flowback fluids to drinking water aquifers. A critical question common to these environmental risks is the hydraulic connectivity between the shale gas formations and the overlying shallow drinking water aquifers. We present geochemical evidence from northeastern Pennsylvania showing that pathways, unrelated to recent drilling activities, exist in some locations between deep underlying formations and shallow drinking water aquifers. Integration of chemical data (Br, Cl, Na, Ba, Sr, and Li) and isotopic ratios ((87)Sr/(86)Sr, (2)H/H, (18)O/(16)O, and (228)Ra/(226)Ra) from this and previous studies in 426 shallow groundwater samples and 83 northern Appalachian brine samples suggest that mixing relationships between shallow ground water and a deep formation brine causes groundwater salinization in some locations. The strong geochemical fingerprint in the salinized (Cl20 mg/L) groundwater sampled from the Alluvium, Catskill, and Lock Haven aquifers suggests possible migration of Marcellus brine through naturally occurring pathways. The occurrences of saline water do not correlate with the location of shale-gas wells and are consistent with reported data before rapid shale-gas development in the region; however, the presence of these fluids suggests conductive pathways and specific geostructural and/or hydrodynamic regimes in northeastern Pennsylvania that are at increased risk for contamination of shallow drinking water resources, particularly by fugitive gases, because of natural hydraulic connections to deeper formations.

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.

A 1-km profile of in situ stress and geophysical log data was acquired in the Wilkins well to study the relationship between rock properties and in situ stress contrasts. The Wilkins well penetrates Devonian clastic rocks on the Appalachian Plateau near the town of South Canisteo, New York. Open hole hydraulic fracture stress measurements were made in stratigraphic sequences where geophysical logs indicated significant bed-to-bed variations in elastic and lithologic properties. Analysis of stress magnitudes and interval-averaged geophysical data shows that principal horizontal stress magnitudes correlate directly with elastic stiffness and inversely with clay content. A similar relation is found for older Paleozoic strata penetrated by a well at Auburn, New York. Correlations between stress magnitude and geophysical properties observed in the Wilkins and Auburn wells provide strong evidence that bed to bed stress variations arise from a uniform ENE-WSW directed strain acting on beds of different Young's modulus rather than from variations in rock shear strength. Because of their high Young's modulus, sandstones, siltstones, and limestones in the northern Appalachian Basin are likely to be stronger barriers to hydraulic fracture propagation than shales. Porosity logs in the Wilkins well show that the large decrease in horizontal stress found at the base of the Rhine street Formation occurs where shales are less compacted. The correlation with undercompaction is consistent with a paleo-overpressure drainage mechanism as the cause for the stress decrease.

In a search for proof that some joints on the Appalachian Plateau are neotectonic, orientation data were collected on more than 1800 fractures in exposures of the Lockport Dolomite in western New York and southern Ontario. Of greatest interest was a late-forming ENE joint set present in the central and western portions of the field area. These joints increase in abundance with proximity to the Niagara Escarpment. The shape of the east–west trending Niagara Escarpment is defined by a series of asymmetric reentrants with a strong ENE linear trend, reflecting the bedrock joint pattern. The joints appear to have formed near the Earth's surface in response to low tensile stresses developed in bedrock adjacent to the retreating escarpment. Since the current shape and position of the escarpment were controlled by glacial activity during the last 3 m.y., it is possible that the ENE joints and reentrants are neotectonic features.

Bedding-parallel (BPV), strike (SV), and cross-fold (CFV) veins represent a sequence of polyphase fracturing during the development of a fault-bend fold in lower Paleozoic carbonate beds in the Appalachian Valley and Ridge province, Pennsylvania. Brecciation and layer-parallel shearing played important roles in the development of the earliest vein set (BPV) which propagated prior to folding. Fluid inclusions in calcite BPV are highly saline brines (23.4 wt% NaCI) trapped at conditions close to lithostatic (P ≤ 180 MPa, T ≤ 267 °C). While passing through the lower kink plane of a fault-bend fold, strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces. Stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp. Renewed extension upon passing through a second kink plane led to propagation of antitaxial SV along the stylolitized joints by the crack-seal process. Slightly less saline fluids (22.4 wt% NaCI) were trapped in SV at fluid pressures ≤144 MPa and temperatures ≤217 °C. With the concurrent formation of lateral ramps, the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial CFV. Due to further mixing of fresh waters, the salinity of fluids forming CFV decreased (20.5 wt% NaCl) with trapping conditions at P ≤ 116 MPa and T ≤ 179 °C. The fluid evolution path from BPV to CFV through SV shows a modest decrease in salinity with a sharp decrease in possible trapping pressures.

When dividing Pennsylvania counties into five groups, Casey et al. reported an upward trend in radon concentration between 2004 and 2012 in all groups. NBC News subsequently warned, “Rising levels of toxic gas found in homes near fracking sites” (NBC News 2015). This headline is misleading. The county groups of most interest—high, low, and no Marcellus activity—show a difference in adjusted geometric mean indoor radon concentration of < 0.8 pCi/L during any given year. Because the counties with no drilling activity had higher indoor radon concentrations than those with low drilling activity, it is unreasonable to attach a physical meaning to the difference of < 0.4 pCi/L between high- and no-activity counties at any time during Marcellus development. This result (i.e., < 0.4 pCi/L) would not be expected if ambient radon gas was escaping in significant volume from Marcellus drill pads. While it is fair to separate counties with high and low activity, both groups are part of the same geological province, the Appalachian Plateau. High- and low-activity counties are interspersed in a patchwork such that, prior to any Marcellus activity, the two data sets should have shown similar but not necessarily identical trends. Figure 4 of Casey et al. shows that with the exception of 1995, 1996, and 2000, they do exhibit the same up–down trends in radon, with predicted indoor radon concentrations in high-activity counties offset upward by a fraction of a pCi/L long before the arrival of drilling in Pennsylvania. The same trends carry through the years of Marcellus drilling. The no-activity counties are part of a different geological province, possibly giving rise to a different radon trend (Rodgers 1971), but even data from the no-activity counties demonstrate a similar up–down trend that carries through from before the arrival of Marcellus activity. It is even more difficult to make the case that radon trends correlate with hydraulic fracturing, or fracking, if one considers the true arrival date of significant high-volume fracking in all but Washington County, Pennsylvania. Arguably, major drilling activities were not under way until the second half of 2008, and significant production of Marcellus gas was not under way until 2009 (PA DEP 2015). The authors state explicitly that “[o]nly unconventional wells (horizontal wells, hydraulic fracturing) were included” in their study. There was only one horizontal Marcellus well drilled in Pennsylvania in 2006 and only five by the end of 2007, all in Washington County (PA DEP 2015). Yet Figure 3A of Casey et al. indicates as many as 320 horizontal wells were drilled by the end of 2007, an obvious error in their paper. Figure 4 indicates radon concentration was trending upward in 2004 in all regions, long before fracking hit Pennsylvania. Prior to 2004, there is a clear down–up–down trend in all five study regions so that an up-trend after 2003 just mirrors a similar five-year cycle starting about 1994. Even implying a link between Marcellus activity and radon as much as 12.5 miles from the nearest drill pad unreasonably stretches any message found in the data. The only way significant amounts of natural gas–related radon will enter homes at distances outside well-pad setbacks is through use of gas for heating or cooking. The cities on gas service should be most affected; in my experience, most rural homes are not connected with natural gas service. With heat turned off in summertime, the most common entry point for radon is, to my knowledge, through gas stoves on the first floor, which consume a small fraction of the natural gas burned in basement furnaces during the winter heating season. Yet, statistics from the Pennsylvania Department of Environmental Protection show that summertime radon concentrations are lower in both basements and first floors (Robert Lewis, personal communication, 14 May 2015), the latter with open windows being the logical entry point for direct diffusion from nearby pads. Throughout the year, first-floor radon concentration consistently remains at about half the basement concentration (Robert Lewis, personal communication, 14 May 2015). This is the behavior expected if radon enters from soil and migrates to the first floor from the basement by the stack effect. Because up–down trends in radon were present in Pennsylvania prior to the arrival of Marcellus drilling, another explanation is necessary. Something as simple as soil moisture could account for the variation of radon with time. Trends that are similar back to 1989 suggest that the arrival of drilling activity is just a confounding factor making it more difficult to identify the real cause of the decadal-scale up–down trend in radon throughout Pennsylvania. In sum, Casey et al. raised an alarm without justification. The literature shows that such alarms stress communities near drilling activity (Ferrar et al. 2013). Thus, it behooves health educators to be circumspect before placing statements in the peer-reviewed literature that can be manipulated by the media to cause public fear and concomitant stress-related health symptoms.

[1] At Zion National Park (ZNP) the landscape is a consequence of differential weathering of the Navajo Sandstone where closely spaced vertical joints constitute joint zones that erode to form regularly spaced (half kilometer) slot canyons striking 351°. Between these joint zones is a set of isolated joints striking 339°. Fracture interaction and horsetail/wing crack development indicate that the 339° striking joint set is younger than the 351° striking joint zones, despite the lateral extent of stress reduction shadows in the vicinity of the large-scale joint zones. In addition to an older, less pervasive, ∼020° joint set, this sequence of jointing records a counterclockwise rotation of the regional extension directed from WNW to WSW in the Navajo Sandstone at ZNP. ZNP is located at the western margin of the Colorado Plateau, ∼100 km east of the major normal faults of the northeastern central Basin and Range subprovince. Extension within the eastern central Basin and Range initiated during the Miocene and exhibited a WSW extension direction [Anderson, 1971; Wernicke et al., 1988; Snow and Wernicke, 2000]. The correlation between nearby Basin and Range extension and the extension direction for the 351° tending joint zones of ZNP is so close that the jointing at ZNP is interpreted as evidence for modest, yet pervasive Basin and Range extension in the western margin of the Colorado Plateau.

Hydrostatic compression tests were used to measure the variation in mechanical properties of Devonian core cut through an ancient pressure transition zone in the Appalachian Plateau of western New York. The properties of this core are strongly influenced by low aspect ratio microcracks developed during stress relief. In the siltstones, linear compressibility decreases with increasing confining pressure to a minimum value between 14 and 16 X 10-6 MPa-1 in all directions. In the shales, linear compressibility parallel to bedding is approximately 10 X 10-6 MPa-1 for all confining pressures, but normal to bedding it decreases with increasing confining pressure throughout the range of test pressures (140 MPa), reaching minimum values between 25 and 40 X 10-6 MPa-1 at high pressure. The high linear compressibility normal to bedding reflects compression and closure of a dense population of bedding-parallel microcracks. Microcrack porosity is lowest in the core 50 m above the ancient transition zone and is higher further above and below the zone. In conclusion, rocks within the ancient transition zone have lower intrinsic compressibilities than overlying rocks (8-12 X 10-6 MPa-1 versus 9-18 X 10-6 MPa-1). Such a marked change in elastic properties near the transition zone may serve as a seismic reflector.

This paper describes the effect of bedding-plane anisotropy on the fracture toughness, Klc, and the relationship between fracture toughness and fracture surface topography of the Ithaca Formation, a Devonian-aged siltstone of the Appalachian Plateau from Watkins Glen, New York. The Ithaca siltstone is the focus of a number of studies to decipher the tectonics of the Appalachian Plateau. Our fracture toughness measurements used the chevron-notched short bar and the indentation/strength test methods. The chevron-notched short-bar test was conducted using three different sample diameters (109, 98, and 12.7 mm). The KIcresults in MP✓am are: 2.67 ± 0.07 and 1.93 ± 0.18 at 109 mm, 2.41 ± 0.13 and 1.82 ± 0.06 at 98 mm, and 1.74 ± 0.11 and 1.35 ± 0.12 at 12.7 mm, for the bedding-perpendicular and bedding-parallel fracture orientations, respectively. The indentation/strength test was conducted for cracks in the bedding-perpendicular orientation only, for which an average toughness of 1.26 ± 0.06 MPa✓m was observed. These results suggest that KIc is not a material constant, and therefore little value is placed on fracture toughness measurements using a single sample size. Although the fracture surfaces appear macroscopically to be similar, the surface topographies were quantitatively distinct in their fractal dimension D. The magnitude of D specifies the degree of correlation between points on the fracture surface, and increases as the fracture surface topography becomes more jagged. The average fractal dimensions of the bedding-perpendicular and bedding-parallel fractures are 1.37 and 1.32, respectively. This study found a good correlation between fracture toughness and fractal dimension, with toughness increasing for an increasing fractal dimension.

In many sedimentary basins of the world the minimum hori- zontal stress, S h , is greater in overpressured zones than in normally pressured zones at equivalent depths. A common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress, S v and S h within certain bounds, and the difference is smaller under lower effective stress (i.e., higher pore pressure, P p ). However, in the overpressured parts of the central North Sea graben, United Kingdom, and the Sable subbasin of the Scotian Shelf, Canada, conventional friction envelopes underestimate the magnitude of S h . These data instead indicate that S h increases at a rate proportional to but less than the rate of increase of P p , a condition consistent with a P p -induced deformation of the rock called poroelastic behavior. This paper argues that, whereas friction may govern S h in normally pressured basins, poroelastic behavior is responsible for the unusually high S h in the overpres- sured parts of these same basins. Data on the P p and S h gradients from these basins suggest that Δ S h /Δ P p ∼ 0.7.

Cross joints are late-formed nonsystematic fractures that extend across intervals between systematic joints. Traces of such cross joints are seen on bedding-plane surfaces of Devonian Catskill clastic sedimentary rocks of the Appalachian Plateau of western New York State, where the maximum horizontal principal stress (S H ) is oriented ∼N65°E, as indicated by in situ stress measurements. Between pairs of closely spaced systematic joints, traces of cross joints are commonly planar and orthogonal to the preexisting joints. However, in the mid-region between some widely spaced systematic joints in western New York, cross joints strike parallel to the S H of the present lithospheric stress field, but then curve to abut the preexisting joints at right angles. A curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints, and the perpendicular termination indicates that the preexisting joints were open. Depending on their age, the strike of the mid-region of curving cross joints denotes the orientation of either the neotectonic stress field or its Tertiary predecessor in the North American lithosphere.

On the Appalachian Plateau of New York, ENE striking joints parallel the mid-continent contemporary stress field and do not correlate with other structures. Joints within Ohio, Indiana, Michigan, and West Virginia also parallel the mid-continent maximum horizontal compression as indicated by hydraulic fracture measurements. In northern Indiana, where the contemporary stress field deviates significantly from the orientation of the mid-continent stress, the attitude of the joints deviates accordingly. In addition to joints, structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid-continent stress field. From this I conclude that these joints are mode I cracks whose orientation corresponds to the contemporary tectonic stress field. In itself, the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds.

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.

We made 29 in situ doorstopper strain relaxation measurements distributed among eight sites spaced on a 35-km transect running from the foothills of the San Gabriel Mountains, across the San Andreas fault, into the western Mojave desert southeast of Palmdale, California. This was a pilot study to test if such measurements can detect the regional stress field and any modification of it in a tectonically complex area. Strain was measured by overcoring strain gauge rosettes which had been bonded to the flattened bottom of a shallow borehole. Measurements of stress were repeatable at each site. We found NNE trending maximum compressive stress (σ1) at our sites farthest from the fault. This orientation is parallel to the σ1 inferred from fault plane solutions of major southern California earthquakes and compares favorably with deep hydrofracture stress measurements made near our sites. Nearer the San Andreas fault, the orientation of σ1 was approximately east-west north of the fault, and northwest-southeast to northsouth south of the fault. The repeatability of measurements at a site and the favorable comparison of our measurements with Tullis' [1977] near-surface stress measurements suggest that we reliably determined the stress field orientation present at a site. It was not possible, using the available data, to distinguish the stress caused by residual or topographic effects from tectonically applied stress or to account for modification of the stress field by decoupling across fractures.

To compare the orientation and development of jointing with the orientation and magnitude of finite strain recorded in the Upper Devonian rocks of the Appalachian plateau, New York, we mapped systematic joint sets on an area of 20,000 km2. In this area, Wedel [1932] mapped folds with limb dips of less than a degree and axes that change strike by 30° from 090° in the east to 060° in the west. We observed two different cross-strike joint sets that maintain their approximate cross-strike position from east to west. Yet, in detail the angle between the sets is 18°±2° in the east and 30°±4° in the west. In many outcrops one joint set parallels the direction of maximum compressive strain (eƒ) as recorded by deformed fossils, whereas the other joint set never parallels eƒ. Rare calcite-filled joints are oriented parallel to the direction of eƒ. In addition, the calcite-filled joints both cut and are cut by solution cleavage. These observations suggest that the joint set paralleling eƒ formed during the deformation event represented by the deformed fossils. The joint set that does not parallel eƒ somehow reflects a deformational event other than that producing fossil distortion, as suggested by strain relaxation experiments. In an outcrop of the Machias formation, where the direction of eƒ is 15° from the trace of a cross-strike joint set, the tensor average from 14 subsurface strain relaxation tests shows in situ maximum comprehensive strain (eoc = 10−4) parallel to cross-strike joints but not the direction of eƒ. Strain relaxation is coaxial with a fabric anisotropy indicated by sonic velocity tests. Our idea is that the orientations of cross-strike joints parallel to eoc were controlled by the same rock property that causes strain relaxation on overcoring, whereas the orientations of the cross-strike joints parallel to eƒ were controlled by the stress field causing the fossil distortion. Placing these structural data in a regional context allowed us to construct a dynamic and kinematic model for the structural evolution of the New York plateau. The model indicates variation in both boundry conditions and material behavior through a series of four distinct deformational events, which begin prior to lithification and end in the Recent. Thus our analysis suggests that the structural features we have used represent a set of highly sensitive tools for investigating the deformational history of the Appalachian foreland.

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.

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.

The strata of the northern Umbrian Apennine fold belt are cut by an array of mesoscopic faults that generally display strike- or oblique-slip offset. The majority of these faults have traces less than a few metres long and represent displacements of < 10 cm. Fault surfaces are associated with stylolites and are coated with elongate calcite fibers, suggesting that movement occurred by the mechanism of pressure-solution slip. Crosscutting relationships indicate that faulting occurred before, during, and perhaps after regional folding. The slip on the faults permitted translations of mesoscopic blocks with respect to one another, thereby accommodating regional strain. There is a great range among fault attitudes, but two clusters forming a conjugate set with about a 90° dihedral angle stand out. The mean trend of left-lateral faults of this pair is N72°E, whereas the mean trend of right-lateral faults is N16°W. The bisector between these two fault clusters is about 15° away from the normal to the regional fold axes. This unusual orientation pattern of mesoscopic faults of the study area may indicate that the mechanics of initiating faults in rocks undergoing pressure-solution deformation is different from that in rocks undergoing purely brittle deformation. Alternatively, the fault pattern may indicate that the faults represent slip on pre-existing fractures. If this latter situation is true, the geometry of the fault array may merely reflect the geometry of the pre-existing joint array.

Strain relaxation stress measurements were made in the Mojave Desert southeast of Palmdale, California, at two sites during the summers of 1979 and 1980, using the U.S. Bureau of Mines technique to depths of about 30 m. The field data and finite element modeling studies demonstrate that thermally induced stress dominates the results obtained in the upper 6 m. At depths greater than 6 m the average orientation for the horizontal maximum compressive stress at these sites is N22°W±9° at 2 km south of the San Andreas fault and N13°W±2° at 20 km north of the fault. These azimuths compare favorably with the average of N21°W determined with nearby hydrofracture stress measurements (Zoback et al, 1980). Savage et al. (1981) also found a NNW orientation for the maximum shortening from a geodetic network with a 15-km aperature in the Palmdale area. The fact that essentially the same orientation is recovered by three different techniques which sample to different depths and over different areal extents argues strongly for a contemporary tectonic origin for the stress. Finite element models of the San Andreas fault in southern California develop a stress field similar to that observed regionally (∼15°E for the maximum compressive stress) away from the fault when displacements corresponding to relative motion between lithospheric plates are applied on the boundaries of the models. Near the fault, however, the model principal stresses are rotated counterclockwise similar to those measured near Palmdale, demonstrating the influence of the faults on the principal stress orientations.

This study presents a classification/characterization of Appalachian faults. Characterization factors include timing of movement relative to folding, metamorphism, and plutonism; tectonic position in the orogen; relations to existing anisotropies in the rock masses; involvement of particular rock units and their ages, as well as the standard Andersonian distinctions. Categories include faults with demonstrable Cenozoic activity, wildflysch-associated thrusts, foreland bedding-plane thrusts, premetamorphic to synmetamorphic thrusts in medium- to high-grade terranes, postmetamorphic thrusts in medium- to high- grade terranes, thrusts rooted in Precambrian basement, reverse faults, strike-slip faults, normal (block) faults, compound faults, structural lineaments, faults associated with local centers of disturbance, and geomorphic (nontectonic) faults.