Résumé
The identification of coseismic and aseismic structures in the core of active faults remains a difficult challenge indispensable for the understanding of fault mechanics. Most of the works focusing on this topic concern long-lived faults where successive slip events have lead to a structurally complex fault zone. In contrast, we present here a micro textural study of a structurally simple "baby" fault, which allows reconstructing its evolution since the undamaged conglomerate host rock. This young (less than 0.7 Ma) strike-slip fault, located in the upper compartment of the Chelungpu fault (Taiwan), has accumulated around 20 m of slip. Geological constraints indicate that the deformation textures formed at depths of less than 1.8 km. The methods of study include high-resolution cathodoluminescence optical and electron microscopy, coupled with image processing. The observed microstructures are interpreted in term of their temporal positioning in the seismic cycle. Our analysis of the fault core reveals several layers of anastomosed fault gouges surrounding lenses of brecciated survivor-pebbles of the original conglomerate. Distinct veins-generations cementing the brecciated lenses are interpreted to be contemporaneous to episodes of slip localization inside the thin gouge layers. Interestingly, only one gouge layer presents an isotropic texture with a homogeneous spatial particle distribution. The other gouge layers are characterized by a foliated texture with an anisotropic spatial particle distribution. We propose that the last coseismic event was located in the fluidized isotropic gouge. During the post and interseismic periods, older isotropic gouges related to past earthquakes were compacted leading to the observed foliated textures. The calcite veins cementing the brecciated survivor-pebbles are attributed to episodic coseismic fluid escape from the pressurized gouge.