Abstract

Fractures in oriented Middle Devonian shale cores from the Appalachian Plateau province provide evidence for multiple deformation events associated with the Alleghanian orogeny, post-orogenic stress relaxation, and neotectonic (?) stresses. The orientation, distribution, and mineral paragenesis of these fractures are used to establish the timing, regional extent, and stress orientation of each deformation event.

During the Alleghanian orogeny, the Middle Devonian shale section was a regional décollement zone in the central Appalachian Plateau province. The décollement extends from the Appalachian Structural Front to northwestern Pennsylvania and east-central Ohio. It is defined by abundant joints, veins, and slick-ensided fractures localized within multiple zones of organic-rich brown and black shale. These zones are distributed over tens to hundreds of meters of the Middle Devonian shale section.

The shale cores record a continuous counterclockwise rotation of the maximum compressive stress direction in the west-central Appalachians during the Alleghanian orogeny. Three stages are recognized: stage 1—early Alleghanian tectonic jointing and slip along the Middle Devonian shale décollement with shortening directed 340° to 360° stage 2—"main phase" Alleghanian tectonic jointing and detachment that is pre- to syn-folding, with shortening directed 300° to 335° stage 3—late Alleghanian tectonic jointing, detachment, and coal joint formation, that is generally post-folding with shortening directed 270° to 295°. Local Stage 3 shortening directed 255° to 270° resulted in jointing in southwestern Pennsylvania and northern West Virginia.

Discrete post-Alleghanian deformation events include stage 4—post-Alleghanian release jointing associated with regional stress relaxation and a 030°-oriented shale fabric anisotropy; and stage 5—neotectonic (?) jointing related to unloading in the 060°- to 070°-oriented contemporary stress field.

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