Brittle deformation in phyllosilicate-rich mylonites: implications for failure modes, mechanical anisotropy, and fault weakness

Phyllosilicates, even in relatively small quantities, dramatically influence the mechanical behavior of rocks. In laboratory triaxial tests on foliated rocks, for content in phyllosilicates greater than 20-25%, a relevant mechanical anisotropy appears, as the internal friction coefficient (tangential stress/normal stress at failure) varies between 0.3 and 0.7 with orientation of the sample with respect to the maximum compressive stress. This reflects different fracture modes: when the foliation is favorably oriented, fractures develop along it and the rocks are weak, whilst when fractures cut the foliation at a high angle, rocks are stronger. This kind of mechanical anisotropy is one possible explanation for the relative and absolute fault weakness shown by non-Andersonian misoriented faults (i.e. faults with an orientation, with respect to the regional stress field, not fulfilling Anderson’s theory of faulting). Examples of misoriented faults are low angle normal faults (LANFs), high angle reverse faults and strike slip faults developed at a high angle with the most compressive regional stress axis. In this thesis I have considered two field examples of misoriented faults represented by the Simplon Line Fault Zone (SFZ), in the Swiss Alps, and a zone of (ultra)cataclastic bands in the Grandes Rousses Massif of the French Alps (GRM). These structures have been characterized from the regional scale (paleostress), to the meso-scale (fault zone architecture), and micro-scale (optical microscope, SEM and micro-CT). Moreover, I have characterized the petrophysical properties and mechanical anisotropy of the GRM rocks with density, porosity, uniaxial (UCS) and triaxial (TXT) lab tests performed at the Environmental Science Centre of the British Geological Survey in Keyworth (Nottingham,UK). Both fault zones have been involved in Alpine brittle deformations, which acted on rocks characterized by a pre-existing schistosity. The SFZ is a LANF developed in mylonitic phyllosilicate-rich paragneiss and orthogneiss, characterized by the alternation of anastomosing phyllosilicate films and quartz and feldspar lithons/layers. The schistosity is misoriented for brittle reactivation (high angle with respect to σ1), and its weakness is demonstrated by its brittle activation with the progressive development of cataclasites, which nucleate in the phyllosilicate levels. Brittle deformation in the GRM micaschists, around the glacial plain of the Saint Sorlin Lakes, shows two different failure modes, depending on the schistosity orientation. For a σ1/foliation ẞ angle of ca. 80°, we observe the development of Andersonian conjugated fractures, with a stair-stepping failure path controlled by either [001] mica planes or brittle fractures crosscutting the more competent quartz and feldspar layers. With a ẞ angle of ca. 72° we observe no Andersonian fractures and the complete brittle activation of the schistosity, along which penetrative ultracataclastic seams are developed. The mechanical characterization of GRM’s samples with UCS and TXT highlights the relationships between elastic moduli, peak strength, and failure modes, as a function of the ẞ angle. Peak strength follows a continuously varying anisotropy model, since we observe a progressive and continuous strength decrease from ẞ = 0° to ẞ = 45°, and a similarly smooth and continuous increase up to ẞ = 90°. If we consider the different failure modes, we can see a progressive transition from an Andersonian behavior (ẞ = 0° or 90°), a hybrid failure mode dominated by stair-stepping fractures, and a failure mode dominated by slip along phyllosilicate films (20° < ẞ < 70°). Field and lab results evidence the primary role of phyllosilicate rich foliation in nucleation and development of fracturing and faulting.