Joint development normal to regional compression during flexural-flow folding: The Lilstock buttress anticline, Somerset, England

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, J₃ joints (295-285° strike) are rare, while other joint sets indicate an anticlockwise sequence of development. In the steeper south-dipping beds, J₃ joints are the most frequent in the vicinity of the reverse-reactivated normal fault responsible for the anticline. The J₃ 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 σ₁ for Alpine inversion. Finite-element analysis is used to explain the southward tilt of J₃ joints that propagate under a local σ₃ in the direction of σ₁ for Alpine inversion. Tilted principal stresses are characteristic of lomestone-shale sequences that are sheared during parallel (flexural-flow) folding. Shear tractions on the dipping beds generate a tensile stress in the stiffer limestone beds even when remote principal stresses are compressice. This situation favors the paradoxical opening of joints in the direction of the regional maximum horizontal stress. We conclude that j₃ joints propagated during the Alpine compression caused the growth of the Lilstock buttress anticline.