Fault-healing and tribochemical processes in granodiorite under hydrothermal conditions

Understanding the mechanical and geochemical processes of fault rock development is a key clue into the understanding of fault healing rates. Fault healing rate β – the change of the static friction coefficient (Δμ) with log time (β = μ₀ + Δμ/log(1+t_hold /t_cutoff)) – is a significant parameter in the seismic cycle, controlling the storage of the elastic strain energy in the fault wall rocks and allowing earthquakes to repeatedly occur in pre-existing faults.

Fault healing is investigated with slide-hold-slide (SHS) experiments aimed at reproducing the seismic cycle. However, most of these experiments have been conducted under room conditions, while natural earthquakes nucleate at temperatures T > 150°C and in presence of pressurized fluids. Under these conditions, fluid-rock interaction (reaction kinetics, pressure-solution transfer, sub-critical crack growth, etc.) may impact severely on β and on the magnitude of Δμ.

In this study, motivated by the evidence of intense fluid-rock interaction in exhumed seismogenic faults hosted in the continental crust (Gomila et al., 2021, G³), we performed SHS experiments in a rotary shear apparatus equipped with a dedicated hydrothermal vessel. The goal is to investigate (1) the tribochemical processes and healing behavior of gouge-bearing faults made of granodiorite and, (2) explore how the mechanical properties and healing rates evolve with fault maturity (e.g., fault displacement, duration of fluid-rock interaction).

For the simulated gouge samples (grain size < 75 µm), three set of experiment of SHS were conducted, the first with run-in duration of 500s, whereas the 2^nd and 3^rd with 5000s, and geochemically contrasted against a non-sheared sample. The fluid (deionized water) saturated gouges were kept under an effective normal stress (σ_n^eff) of 10 MPa, a fixed temperature T of 300°C and a constant pore fluid pressure P_f of 25 MPa, and they were slid for ca. 15 mm and 60 mm at a slip rate of 10 µm/s. Hold periods between slip events ranged from 3s to 10000s (1^st and 2^nd experiments) and from 3s to 300000s (3^rd), to investigate the dependence of β and the underlying tribochemical processes with both cumulative slip and duration of the experiment.

Under these hydrothermal conditions, Δμ first increased with holding time (β value of ca. 2.0x10^-2 , independently of run-in duration) and then decreased (β = -3.6x10^-2, β = -3.0x10^-2 and β = -2.6x10^-2, for the 1^st, 2^nd and 3^rd experiment, respectively). Bulk XRF analyses on sheared samples show an enrichment of TiO₂, MgO and P₂O₅, while a loss of MnO and CaO oxides with respect to the non-sheared sample. Detailed SEM-EDS analyses show a main mineral loss of biotite and quartz within the main slip zone.

This suggest that under hydrothermal conditions, total shear displacement and duration of the fluid-rock interaction enhance mineral reactions that promote negative healing rates (β < 0) in faults during the seismic cycle. This would imply that during the life-span of an evolving fault, as it matures, it would be possible to (1) lower the fault yield strength due to and increasing fluid-rock interaction, henceforth (2) increase the recurrence but decrease the intensity of the seismic activity.