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Article Dans Une Revue Tectonophysics Année : 2018

Fault imprint in clay units: Magnetic fabric, p-wave velocity, structural and mineralogical signatures

Résumé

We investigated the evolution of the structure, physical properties and fluid flow properties across a fault zone within the Toarcian shale formation from the Tournemire Underground Research Laboratory (France). With this purpose, an integrated approach was used based on the analysis of petrofabric (anisotropy of magnetic susceptibility, AMS, and of p-wave velocity, APV) and the mineralogy of two cores intercepting the F2 fault, a strike-slip fault with a small vertical offset. The fracture patterns indicated that the fault registered a multiphase tectonic evolution. Two strike-slip regimes, with approximate NE-SW and NW-SE compression, and a normal regime with a WNW-ESE direction of extension were identified. Most of the N-S fault planes collected in TF1 core, have a shallow dip with values between 5° to 45°. We further evidenced a close relationship between the fault architecture and petrophysical properties. A preferential NW-SE orientation of the K1 axes was observed that is in agreement with the extensional direction of the dextral movement of the fault. Within the fault core, an important decrease in anisotropy was observed and indicates clay platelets rotations related to tectonic deformation. A weaker anisotropic change was observed in ASM1. This difference may be related to the local variability of the fault zone and suggests the need to study the characteristics of a fault zone both across its different architectural zones and along the main fracture plan. Finally, we identified evidence of minerals alteration related to fault reactivation and fluid circulation. Fluid circulation enabled the transformation of siderite in magnetite, a neo-formation of kaolinite and a precipitation of calcite were at vicinity of the damage zone/fault core boundary. In TF1, while the major mineralogical changes occur in the damage zone. In ASM1, a higher illite-smectite mixed layers and calcite content was observed in the eastern side of the fault.
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Dates et versions

hal-02006500 , version 1 (04-02-2019)

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Eva Moreno, Catherine Homberg, Johann Schnyder, Alain Person, Christian David, et al.. Fault imprint in clay units: Magnetic fabric, p-wave velocity, structural and mineralogical signatures. Tectonophysics, 2018, 745, pp.264-277. ⟨10.1016/j.tecto.2018.07.017⟩. ⟨hal-02006500⟩
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