Joints and décollement zones in Middle Devonian shales: Evidence for multiple deformation events in the central Appalachian Plateau
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 decollement zone in the central Appalachian Plateau province. The decollement 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. 68 refs., 10 figs., 3 tabs.