Appalachian Stress Study: 3. Regional scale stress variations and their relation to structure and contemporary tectonics

Abstract

Using commercial hydrofracture data to supplement published stress data, we map regional variations in least horizontal stress magnitude (Sh) and orientation within the Paleozoic sediments of the Appalachian Plateau. Systematic trends revealed by the data are examined for correlations with structure (the study area includes a prominent salt detachment) and tectonic history. The dominant stress component present at all depths (including basement) throughout the region is ENE oriented compression and is most likely of tectonic origin. No deviations from this rule are found that are statistically significant. Near‐surface (<600 m) stresses are consistently greater than the overburden with “strike‐slip” regime conditions prevailing below. However, where the Salina salt beds are present, “thrust” regime conditions extend deeper, reaching the salt in most places where data are available (limited to where the salt is shallower than 1.5 km). At the salt, a discontinuity in the trend of Sh with depth marks the abrupt transition to the underlying strike‐slip regime characterized by least stress ratios (i.e., Rh = Sh/Sv) in the range 0.5–0.7 and extending to basement. The comparatively high stress found above the salt may reflect a difference between the strain histories of the overlying and underlying sections facilitated by mechanical decoupling by the salt and perhaps includes a component of stress remnant from NW oriented Alleghanian shortening of the upper section. The lower (subsalt) stress regime extends beyond the eastern salt margin and displays remarkable uniformity over distances of hundreds of kilometers. This is particularly evident across Ohio, where an Rh value of 0.6±0.05 appears valid down to basement. North of Ohio, values increase markedly, whereas to the south of the salt pinch‐out, in West Virginia and eastern Kentucky, values as low as 0.35 are found in Devonian shales at 1–1.5 km depth, although values deeper in the section may be higher. The northward increasing trend can be reasonably well modeled as the effect of persisting lithospheric flexure due to the Pleistocene ice sheet. However, flexure was much greater in the past, and as Devonian shales in south Ohio/West Virginia are currently only marginally stable, a modest increase over the present flexure would be expected to cause shear failure. This suggests that the very low stress in the shales of south Ohio/West Virginia is of different origin and more recent than the Pleistocene.

Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis

“…Extensive salt detachments often constitute stress discontinuity horizons (Davis & Engelder 1985): for example, Evans (1989) found a discontinuity in horizontal stress magnitude across the salinian salt of the Appalachian basin, and röckel and Lempp (2003) report a discontinuity in horizontal stress magnitude and direction at the Zechstein salt in the North German basin. It is thus of interest to compare Fig.…”

Stress field constraints on intraplate seismicity in eastern North America

“…In southeastern Canada and the northeastern United States the tangential stress due to rebound is subparallel to the regional ENE SHmax orientation, while the radial rebound stress is subparallel to S hmin. In a two-dimensional analysis of reboundrelated flexural stresses by Clark [1982], the ice sheet load was approximated as a right circular cylinder of paraboidal The rebound flexural stresses have been interpreted as sufficient to influence the stress field in the upper 1-2 km [Clark, 1982;Evans, 1989]. The potential perturbation at earthquake depths (typically 5-10 km in this region, Table 1) can be evaluated drawing on the analysis of frictional analysis presented above.…”

Determination of In-Situ Stress and Geomechanical Properties from Borehole Deformation

“…The basin wide stress study indicated σ h /σ ν value of up to 0.7 in the corresponding depth at this well location [26]. With the vertical stress of 54 MPa from Table 1, the upper bound minimum horizontal stress will be 37.8 MPa.…”

“…Table 3 shows the comparison of the PR-GA-ANN based inverse analysis results of geomechanical properties and horizontal in-situ stresses with the field observation or reports of the area for the well MIP 3H. The basin wide stress study indicated σh/σν value of up to 0.7 in the corresponding depth at this well location [26]. With the vertical stress of 54 MPa from Table 1, the upper bound minimum horizontal stress will be 37.8 MPa.…”

“…Evans also stated in the Appalachian Stress Study Report that the magnitude of σH varies from high in the northern part of the basin to low values in the south; the stress state in the Devonian shale, of which Middlesex formation is a part, is either strike slip or normal fault regime due to the pinchout of the underlying salt [26]. The location of the studied well is around the pinch-out area [27].…”