Fluid overpressure along an Oligocene out-of-sequence thrust in the Shimanto Belt,SW Japan |
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| Institution: | 1. UMR CNRS 6249, Université de Franche-Comté, 16 route de Gray, 25030 Besançon, France;2. UMR8538, Laboratoire de Géologie de l’École Normale Supérieure, 75005 Paris, France;3. Laboratoire de Génie Civil et géo-Environnement (LGCgE/EA4515), building SN5, University Lille 1, 59655 Villeneuve d’Ascq, France;4. UFR Sciences de la Terre, Cité Scientifique, 59655 Villeneuve d’Ascq, France;1. Civil Engineering Department, Yancheng Institute of Technology, Yancheng, China;2. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China;3. School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 2, China;1. Department of Applied Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan;2. Tono Geoscience Center, Sector of Decommissioning and Radioactive Waste Management, Japan Atomic Energy Agency, Gifu 509-5102, Japan;3. Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Kochi 783-8502, Japan |
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| Abstract: | Out-of-sequence thrusts (OSTs) exposed in ancient accretionary prisms are considered as fossil analogs of present-day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity. In modern as well as in ancient accretionary prisms, first-order megasplay faults or OSTs truncate or merge with faults of lesser importance called second-order OSTs. Structural analysis of the Makinokuchi fault, a branch of an Oligocene to lower Miocene second-order OST in the Tertiary Shimanto Belt of central Kyushu, SW Japan, brings information about the conditions of deformation at the time of thrusting. The studied exposure shows that the fault footwall and, to a much lesser extent, the fault hanging-wall, consist of quartz-cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component. The footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias. The continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault. Fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions (245–285 °C and 5–8 km depth). These similarities suggest that the fluids responsible for syn-tectonic hydraulic brecciation were collected from the footwall through the conduits. The fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone. Our analysis shows that fluid overpressures can play a key role in the growth and activity of second-order OSTs in accretionary prisms and suggests that fluids collected along second-order OSTs or splay faults may flow upward along first-order OSTs or megasplay faults. |
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| Keywords: | Accretionay prism Out-of-sequence thrust fault Fault breccia Fluid Hydraulic fracturing Japan Oligocene |
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