Fault-healing and tribochemical processes in granodiorite under hydrothermal conditions
Authors: Gomila Rodrigo , Feng Wei , Di Toro Giulio 
Published: February 2023

Understanding the mechanical and geochemical processes of fault rock development is a key clue into the understanding of fault healing rates. Fault healing rate &#946;&#160;&#8211; the change of the static friction coefficient (&#916;&#956;) with log time (&#946; = &#956;0 + &#916;&#956;/log(1+thold /tcutoff)) &#8211; 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&#176;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 &#946;&#160;and on the magnitude of &#916;&#956;.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 &#181;m), three set of experiment of SHS were conducted, the first with run-in duration of 500s, whereas the 2nd and 3rd with 5000s, and geochemically contrasted against a non-sheared sample. The fluid (deionized water) saturated gouges were kept under an effective normal stress (&#963;neff) of 10 MPa, a fixed temperature T of 300&#176;C and a constant pore fluid pressure Pf of 25 MPa, and they were slid for ca. 15 mm and 60 mm at a slip rate of 10 &#181;m/s. Hold periods between slip events ranged from 3s to 10000s (1st and 2nd experiments) and from 3s to 300000s (3rd), to investigate the dependence of&#160;&#946;&#160;and the underlying tribochemical processes with both cumulative slip and duration of the experiment.Under these hydrothermal conditions, &#916;&#956; first increased with holding time (&#946; value of ca. 2.0x10-2 , independently of run-in duration) and then decreased (&#946; = -3.6x10-2, &#946; = -3.0x10-2 and &#946; = -2.6x10-2, for the 1st, 2nd and 3rd 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 (&#946;&#160;< 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.