Joint development normal to regional compression during flexural-flow folding: the Lilstock buttress anticline,Somerset, England |
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| Institution: | 1. Microbiology Department, Faculty of Science, Ain shams University, Egypt;2. Egyptian Petroleum Research Institute (EPRI), Cairo, Egypt;3. Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt;4. Department of Industrial Biotechnology, Genetic Engineering and Biotechnology Research Institute, University of Sadat City (USC), Sadat City, Egypt;5. School of Chemical Engineering, University of Adelaide, Adelaide, Australia;1. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China;2. Centre for Exploration Targeting, The University of Western Australia, Crawley, WA 6009, Australia;1. Instituto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro, Asa Norte, CEP 70.910-900, Brasília - DF, Brazil;2. Observatório Sismológico, Universidade de Brasília, Campus Universitário Darcy Ribeiro, Asa Norte, CEP 70.910-900, Brasília - DF, Brazil;1. Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen;2. Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia |
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| Abstract: | 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 limestone–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 compressive. 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. |
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