On the nucleation of non-Andersonian faults along phyllosilicate-rich mylonite belts

Abstract

The weakness of fault zones is generally explained by invoking an elevated fluid pressure or the presence of extremely weak minerals in a continuous fault gouge horizon. This allows for faults to slip under an unfavourable normal to shear stress ratio, in contrast to E. M. Anderson’s theory of faulting. However, these mechanisms do not explain why faults should nucleate in such an orientation as to make them misoriented and non-Andersonian. Here we present a weakening mechanism, involving the mechanical anisotropy of phyllosilicate-bearing mylonite belts, which is likely to influence the nucleation of faults in addition to their subsequent activity. Considering three natural examples from the Alps (the Simplon, Brenner and Sprechenstein- Mules fault zones) and a review of laboratory tests on anisotropic rocks, we apply anisotropic slip tendency analysis and show that misoriented weak faults can nucleate along a sub-planar phyllosilicate-rich mylonitic foliation, constituting a large-scale mechanical anisotropy belt and preventing the development of Andersonian optimally oriented faults

Structural and temporal evolution of a reactivated brittle–ductile fault – Part I: Fault architecture, strain localization mechanisms and deformation history

“…We suggest that high differential stresses caused by the contrast in effective viscosity between quartz veins and the surrounding phyllonite promoted crystalplastic deformation by sub-grain rotation, similar to that described by Trepmann and Stöckhert (2009). Bons (1988) showed that chlorite can deform by dislocation creep at temperatures as low as c. 250 • C. Crystal plastic deformation in chlorite and other phyllosilicates is attained by very easy glide along the (001) slip planes, whereas dislocation climb and the activation of hard slip systems is facilitated by kinking and diffusion (Bons, 1988). We suggest that crystal plastic creep became an effective deformation mechanism in the KF as temperatures exceeded c. 250 • C in the UpP.…”

“…The ability of phyllonitic fault rocks to deform by stable creeping has been attributed elsewhere to either increased fluid pressures (Rice, 1992), frictional grain sliding Tesei et al, 2012), pressure solution (Gratier et al, 2011), dislocation creep (Bons, 1988) or a combination of these mechanisms (e.g. Bos and Spiers, 2002).…”

Stress, faulting, fracturing and seismicity: the legacy of Ernest Masson Anderson

“…Such assemblages are likely to display anisotropy in their frictional behaviour. Bistacchi et al (2012) extend the slip tendency analysis of Morris et al (1996) to rocks with directional variations in frictional resistance to sliding. Improved laboratory characterization of rock mechanical properties is the basis for several recent approaches to brittle fracture in the crust, representing another departure from Andersonian orthodoxy.…”

“…These models could be extended to analyse in more detail the situation reported in Tingay et al (2012), exploring the parameter space for controlling stress rotations above and below a detachment within the sediment package. Bistacchi et al (2012) develop a novel model of low-angle detachment instability due to anisotropic coefficients of sliding friction. Many low-angle detachments contain zones of strongly aligned fabrics defining an intense structural anisotropy, with these local fabrics oriented parallel to the detachment.…”

Geology of the Brenner Pass-Fortezza transect, Italian Eastern Alps

“…The regional-scale exhumation of the Tauern window associated to the extensional activity of the Brenner Fault Zone (Behrmann, 1988;Bistacchi, Massironi, Menegon, Bolognesi, & Donghi, 2012;Massironi, Bistacchi, & Menegon, 2011;Selverstone, 1988) resulted in cooling of the Penninic units and transition from ductile to brittle deformations. This is evidenced by a penetrative west-dipping, top-down SC fabric developed in the ridge between Vipiteno and Spina del Lupo, corresponding to the footwall of the Brenner Line, which is hidden by Quaternary deposits in the Isarco valley (small outcrops around Vipiteno and the Brenner Pass).…”