The role of flexure in joint development during drape folding and oroclinal bending

According to Anderson's relationship between stress state and fault systems, the axis of the least principal stress (sigma3) during thrust faulting in foreland environments should be vertical. Such a stress state favors the propagation of horizontal joints. Despite the Anderson paradigm for stress during thrusting in forelands, vertical joints suggest that sigma 3 was horizontal during some stages of foreland development. Vertical joints are a manifestation of bedding flexure that generates a local stress state favoring a horizontal sigma3. This thesis studies flexure-induced joint development in a drape fold of the Laramide Orogeny at Circle Ridge, Wyoming, and joint development about an oroclinal bend of the Alleghanian Orogeny at the Allegheny front, Pennsylvania.; Circle Ridge is a basement-cored, doubly-plunging anticline of Laramide age located in the Wind River Basin, Wyoming. Outcrop fracture patterns were compared with early fracture development predicted by models of the elastic stretch during basement-cored folding of elastic sequences comparable to the stratigraphic section found at Circle Ridge. A fold amplitude of only 15 m is required to generate a fracture distribution observed at Circle Ridge for joints parallel to the fold axis, provided interbed slip takes place in the basal Cambrian shales and the Niobrara Formation. Later during folding, interbed slip took place in the Gypsum Springs Formation. Basement involvement in the deformation will not change the fracture pattern described for joints parallel to the fold axis. Stretching parallel to the fold axis direction is a mechanism that may explain joints perpendicular to the fold axis.; The Allegheny front is a major step in the Appalachian detachment and defines the western limit of the Valley and ridge province. It is involved in an oroclinal bend concave to the southeast. Valley and Ridge tectonics in the Allegheny front involves two compressional stages, with drag rotation of cross-fold joints in the southwest side of the salient during the first stage, or Reading Prong stage, and drag rotation of cross-fold joints in the northeast side during the second stage, or Blue Ridge stage. Pre-fold cross-fold joints propagated in the Reading Prong stage when tectonic transport was in a 325° direction. Major Alleghanian thrusting was during the Blue Ridge stage, when tectonic transport was in a 290° direction, a more westerly direction than the previously rotated cross-fold joints.