JOINTING WITHIN THE OUTER ARC OF A PERIPHERAL BULGE AT THE ONSET OF THE ALLEGHANIAN OROGENY

NW-trending joints are pervasive throughout the Upper Devonian shale succession of western New York while ENE-trending joints are pervasive but more common in black shale. The fact that joints of both sets terminate against higher modulus carbonate concretions indicates that they formed as natural hydraulic fractures produced when organic-rich rocks, including the Rhinestreet and Dunkirk shales, were buried to the oil window (McConaughy and Engelder, 1999). Both NW and ENE joints formed within a regional stress field related to the Alleghanian clockwise rotational transpressive collision of Gondwana against Laurentia. North-trending (NS) joints that predate the NW joints and are most densely formed at the contacts of gray shale and overlying black shale units complicate this tectonic scenario. These joints appear to have originated in the higher modulus carbonate by elastic contraction. Specifically, a tensile Earth stress causing a uniform level of extensional strain over the entire Upper Devonian shale succession was enhanced in the stiffer carbonate. Propagation of these early joints required an axis of tensile stress oriented ~EW, consistent with passage of an inferred westward-migrating peripheral bulge in advance of oblique plate convergence in New England. Upper Devonian shale hosting the NS joints crops out in that area of the Appalachian Basin where models show that tensile stress caused by uplift and lithospheric flexure related to passage of the peripheral bulge, relative to burial-induced compressive stress, is optimum. Eventually, subsidence of the peripheral bulge carried the shale succession into the oil window during the Alleghanian Orogeny resulting in the propagation of regionally pervasive fluid driven joints.