How does Brittle Deformation of Phyllosilicate-rich Mylonites Work? Implications for Fault Weakness

Abstract:

One mechanism explaining the nucleation and propagation of weak faults with non-Andersonian attitude is the mechanical anisotropy of phyllosilicate-rich mylonitic rocks. We characterized the mechanical anisotropy and (micro-)failure modes of phyllosilicate-rich mylonites from the Grandes Rousses Massif (Helvetic-Dauphinois Domain, French Alps), deformed under brittle conditions after exhumation from metamorphic conditions. We performed uniaxial (UCS) and triaxial (TXT) tests varying the σ1/schistosity angle at varying confining pressures.

Fractures obtained in the lab and observed in nature are stair-stepped at millimeter-scale, and are composed of low-angle segments developed along phyllosilicate layers and high-angle segments cutting the quartz-feldspar layers with an Andersonian (shear fracture) or tensional joint orientation.

UCS at 90° show high strength and failure mode characterized by both low-angle segments along schistosity and high-angle ones cutting quartz-feldspar layers. UCS at 0° show lower strength and axial splitting along schistosity.

TXT cover a complete range of inclinations, from 0° to 90°, at confining pressures of 60MPa and 120MPa. Maximum strength is achieved at 0°, strength and minimum strength, attained at 45°, is around 50% with a significant strength anisotropy. The “fault zone” develops mainly along schistosity for tests at 20-70°, whilst Andersonian shear fractures are observed at 0-20° and 70-90°.

In the field we defined the paleo-stress field and measured the σ1/schistosity angle (the schistosity is undulated). Noteworthy, also in the field a transition from a failure mode dominated by slip along schistosity to an Andersonian behavior was observed at 70°.

Combining laboratory and field data, we conclude that mechanical anisotropy of foliated rocks is relevant in a wide range of σ1/schistosity angles and, due to failure modes dominated by along-schistosity slip, influences the angle to σ1 at which new macroscopic fault zones nucleate, which is very different from predictions of Anderson’s theory.