Search

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.

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).

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.

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.

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.