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Alpine inversion in the Bristol Channel Basin includes reverse-reactivated normal faults with hanging wall buttress anticlines. At Lilstock Beach, joint sets in Lower Jurassic limestone beds cluster about the trend of the hinge of the Lilstock buttress anticline. In horizontal and gently north-dipping beds, J 3 joints ( 295±2858 strike) are rare, while other joint sets indicate an anticlockwise sequence of development. In the steeper south-dipping beds, J 3 joints are the most frequent in the vicinity of the reverse-reactivated normal fault responsible for the anticline. The J 3 joints strike parallel to the fold hinge, and their poles tilt to the south when bedding is restored to horizontal. This southward tilt aims at the direction of s 1 for Alpine inversion.Finite-element analysis is used to explain the southward tilt of J 3 joints that propagate under a local s 3 in the direction of s 1 for Alpine inversion. Tilted principal stresses are characteristic of limestone±shale sequences that are sheared during parallel (¯exural-¯ow) folding. Shear tractions on the dipping beds generate a tensile stress in the stiffer limestone beds even when remote principal stresses are compressive. This situation favors the paradoxical opening of joints in the direction of the regional maximum horizontal stress. We conclude that J 3 joints propagated during the Alpine compression caused the growth of the Lilstock buttress anticline. q 2001 Published by Elsevier Science Ltd. IntroductionAn old paradigm is that outer-arc stretching leads to foldrelated jointing (Van Hise, 1896). Outer-arc stretching is associated with tangential longitudinal strain folds that develop when a homogeneous, isotropic layer is buckled (Price and Cosgrove, 1990). The tangential longitudinal strain fold is divided by a neutral surface beyond which the inner arc of the fold is compressed. Joints that initiate in the outer arc of anticlines propagate down to but do not cross the neutral surface. When fold-related jointing penetrates the entire thickness of a bed, the tangential longitudinal strain model is less useful as a mechanism to explain the strain that may have caused jointing. This is also true when jointing is well developed in the limbs of folds or at in¯ection points where strain goes to zero in a tangential longitudinal strain fold. To understand the development of jointing in the limbs of folds, we must look to another model. Flexural¯ow folds have maximum strains at the in¯ection points and zero strain at the hinges (Price and Cosgrove, 1990).We consider the possibility that one or more sets of un®lled joints of the Bristol Channel were contemporaneous with¯exural-¯ow folding during Alpine shortening. Here, we describe ®eld data gathered to test this hypothesis. We presume that the joint geometries interpreted to date from the Alpine inversion re¯ect the superposition of local and remote (i.e. regional) stress conditions. We also use the orientation of systematic joints in carbonate beds to constrain a mechanical model for stress dis...

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