Strain hardening as a mechanism for slip nucleation and arrest in phyllosilicate-rich rocks

The processes governing the nucleation and arrest of a rupture during slow slips remain speculative. The importance of understanding slip arrest mechanisms lies in the potential for slow slips to trigger destructive earthquakes and in the fact that not all slow slips lead to the nucleation of regular earthquakes.

At seismogenic depths (<30 Km, 100°-400°C), phyllosilicate-rich rocks (i.e., claystones, metasediments, serpentinites) are widespread lithologies that are also frictionally weak (µ≪0.6). The presence of these rock favors slip nucleation at weak fault patches which may or may not develop into fast unstable slip.

We performed hydrothermal friction experiments at the temperatures and pressures relevant to the seismogenic zone to understand the mechanism(s) behind slip nucleation and arrest. We tested experimental gouges of phyllite (Rio Marina Fm.) and meta-sandstone (Verrucano Fm.) from a natural shear zone exposed at the Elba Island (Italy). Experiments were performed at a shearing velocity of 10 µm/s over a wide range of effective normal stresses (20 to 150 MPa), high temperature (350 °C) conditions and to high strains (displacements up to 40 mm) using two hydrothermal Rotary shear machines hosted in Padova University (Italy) and Utrecht University (Netherlands).

            Experimental results show that the phyllite sheared at low effective normal stresses (20-50 MPa), show a low friction coefficient of µ ~ 0.3 and a strain weakening behavior. With increasing normal stress (up to 150 MPa) we observe an initial low friction (0.35) that evolves with a strain hardening trend up to µ ~ 0.7-0.9. Conversely, experiments on the meta-sandstone show generally higher friction (0.6-0.7) even at small strains at all normal stress conditions.

Frictional weakness is due to the phyllosilicates’ ability to develop efficient foliations that accommodate the deformation. At low effective normal stresses (up to 60 MPa), we observe the development of a through-going phyllosilicate network within the phyllite gouge resulting in the observed low friction and strain weakening evolution. Conversely, at high normal stress, a through-going weak phyllosilicate network cannot develop because of the presence of frictionally strong high stress asperities, from which phyllosilicates have been extruded. The observed strain hardening and high friction trend results from comminution of intervening quartz and feldspar grains that we correlate with the occurrence of ultracataclasites in the microstructures. The experimental results on the meta-sandstone confirm this hypothesis, showing a friction and a microstructure similar to the “hardened” phyllite gouges. EDS maps of chemical elements in the phyllite gouge sheared under high normal stress confirm the absence of an interconnected network of phyllosilicates. In natural shear zones, at seismogenic depth, we may observe slip nucleation in weak phyllosilicate rich rocks (µ ~ 0.3). However, the fault patch may quickly strengthen if the propagating slip is fast enough to disrupt the foliation, which would decelerate slip. Our study provides a new mechanism by which slow slip events may nucleate and spontaneously arrest, potentially halting the growth of rupture into regular earthquakes.