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

Vertical joints in Devonian elastic sedimentary rocks of the Finger Lakes a rea of New York State are ornamented with arrays of fringe cracks that reve al the complex deformational history of the Appalachian plateau detachment sheet during the Alleghanian orogeny. Three types of fringe cracks were map ped: gradual twist hackles, abrupt twist hackles, and kinks. Gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an ove rall vertical direction within the bed hosting the parent crack and are fou nd in all elastic lithologies of the detachment sheet. Abrupt twist hackles propagate as planar features in thick shale beds above or below the siltst one beds hosting parent joints. Kinks propagate horizontally as planar surf aces 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 or ientation and magnitude of the remote stress held, internal fluid pressure, and the elastic properties of the bed. The twist angle of gradual twist ha ckles is larger in coarser elastic beds, indicating that stress and interna l pressure are more important parameters than elastic properties in control ling 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 s hale beds is used to estimate the maximum horizontal stress difference in t he shale beds as S-H - S-h approximate to 2.5 MPa when S-H - S-h approximat e to 12 MPa in sandstone beds. The twist angle of the fringe cracks and the abutting relationships of pare nt joints give an indication of the overall change in stress held orientati on within the detachment sheet during Alleghanian tectonics. These parent j oints indicate a regional clockwise stress rotation of Alleghanian age conc ordant with the twist angle of fringe cracks throughout the western part of the study area. A counterclockwise twist angle in the eastern portion indi cates a local stress attributed to drag where no salt,vas available to deta ch the eastern edge of the plateau sheet. The clock,vise change in stress o rientation is consistent with the rotation in stress orientation found in t he 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.