Fringe cracks: Key structures for the interpretation of the progressive Alleghanian deformation of the Appalachian plateau

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.