Loading paths to joint propagation during a tectonic cycle: an example from the Appalachian Plateau, U.S.A.

Based on the timing of joint propagation during the history of burial, lithification, deformation and denudation of clastic rocks within sedimentary basins, four types of joints may be distinguished: tectonic, hydraulic, unloading and release. Tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures, whereas unloading and release joints form near the surface in response to thermal-elastic contraction accompanying erosion and uplift. Tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case. The orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric. Examples of some of these joints are found within the Devonian Catskill Delta of the Appalachian Plateau, New York. During the Alleghanian Orogeny tectonic joints (cross-fold joints) formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross-cutting relationships of folds, cleavage and joints. This sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers. Plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length. The discontinuous rupturing is compatible with models for natural hydraulic fracturing. Release joints (strike joints) post-date the Alleghanian Orogeny as indicated by abutting relationships within the deeper parts of the Devonian clastic section. Unloading joints are orthogonal to the contemporary tectonic stress field.