Weakness and mechanical anisotropy of phyllosilicate-rich cataclasites developed after mylonites of a low-angle normal fault (Simplon Line,Western Alps) |
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| Institution: | 1. Departamento de Química (Módulo 13), Universidad Autónoma de Madrid, 28049 Madrid, Spain;2. Departamento de Química Orgánica (Módulo 1), Universidad Autónoma de Madrid, 28049 Madrid, Spain;3. División Académica de Ciencias Básicas, Universidad Juárez Autónoma de Tabasco, Mexico;4. Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain;1. School of Environmental and Life Sciences, University of Newcastle, Newcastle NSW, Australia;2. School of Earth, Atmosphere & Environment, Monash University, Clayton VIC, Australia;1. Western Australian Argon Isotope Facility, Department of Applied Geology and JdL-CMS, Curtin University, GPO Box U1987, Perth, WA 6845, Australia;2. Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia;3. School of Chemistry and Biochemistry, The University of Western Australia, Crawley, WA 6009, Australia;4. Department of Earth Sciences, University of Western Ontario, London, ON N6A 3K7, Canada;1. Institute of Modern Optics, Department of Physics, Harbin Institute of Technology, Harbin 150001, China;2. Key Laboratory of Micro-Optics and Photonics Technology of Heilongjiang Province, Harbin 150001, China;1. State Key Laboratory of Geological Processes and Mineral Resources and Institute of Earth Sciences, China University of Geosciences, Beijing 100083, PR China;2. CSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA 6102, Australia;3. School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;4. Department of Applied Geology, Curtin University, Bentley, WA 6845, Australia;5. Department of Geosciences, National Taiwan University, Taipei 106, Taiwan;6. Geological Survey of Norway-NGU, 7491 Trondheim, Norway;7. Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology-NTNU, 7491 Trondheim, Norway |
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| Abstract: | The Simplon Fault Zone is a late-collisional low-angle normal fault (LANF) of the Western Alps. The hanging wall shows evidence of brittle deformation only, while the footwall is characterized by a c. 1 km-thick shear zone (the Simplon Fault Zone), which continuously evolved, during exhumation and cooling, from amphibolite facies conditions to brittle-cataclastic deformations. Due to progressive localization of the active section of the shear zone, the thermal-rheological evolution of the footwall resulted in a layered structure, with higher temperature mylonites preserved at the periphery of the shear zone, and cataclasites occurring at the core (indicated as the Simplon Line). In order to investigate the weakness of the Simplon Line, we studied the evolution of brittle/cataclastic fault rocks, from nucleation to the most mature ones. Cataclasites are superposed on greenschist facies mylonites, and their nucleation can be studied at the periphery of the brittle fault zone. This is characterized by fractures, micro-faults and foliated ultracataclasite seams that develop along the mylonitic SCC′ fabric, exploiting the weak phases mainly represented by muscovite and chlorite. Approaching the fault core, both the thickness and frequency of cataclasite horizons increase, and, as their thickness increases, they become less and less foliated. The fault core itself is represented by a thicker non-foliated cataclasite horizon. No Andersonian faults or fractures can be found in the footwall damage zone and core zone, whilst they are present in the hanging wall and in the footwall further from the fault. Applying a stress model based on slip tendency, we have been able to calculate that the friction coefficient of the Simplon Line cataclasites was <0.25, hence this fault zone is absolutely weak. In contrast with other fault zones, the weakening effect of fluids was of secondary importance, since they accessed the fault zone only after an interconnected fracture network developed exploiting the cataclasite network. |
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| Keywords: | Simplon Line phyllosilicates Low Angle Normal Fault weak fault mechanical anisotropy Slip Tendency |
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