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\begin{document}

\title{The Frictional-Viscous Transition in Experimentally Deformed Granitoid
Fault Gouge}



\author[1]{Weijia Zhan}%
\author[2]{André Niemeijer}%
\author[1]{Alfons Berger}%
\author[1]{Natalia Nevskaya}%
\author[2]{Christopher James Spiers}%
\author[1]{Marco Herwegh}%
\affil[1]{University of Bern}%
\affil[2]{Utrecht University}%


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  \date{June 17, 2024}


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\begin{abstract}
In crustal faults dominated by granitoid gouges, the frictional-viscous
transition marks a significant change in strength constraining the lower
depth limit of the seismogenic zone. Dissolution-precipitation creep
(DPC) may play an important role in initiating this transition,
especially within polymineralic materials. Yet, it remains unclear to
what extent DPC contributes to the weakening of granitoid gouge
materials at the transition. Here we conducted sliding experiments on
wet granitoid gouges to large displacement (15 mm), at an effective
normal stress and pore fluid pressure of 100 MPa, at temperatures of
20-650\selectlanguage{ngerman}°C, and at sliding velocities of 0.1-100 \selectlanguage{greek}μ\selectlanguage{english}m/s, which are relevant
for earthquake nucleation. Gouge shear strengths were generally
\textasciitilde{}75 MPa even at temperatures up to 650\selectlanguage{ngerman}°C and at
velocities \textgreater{} 1 mm/s. At velocities \selectlanguage{english}[?] 1 mm/s, strengths
decreased at temperatures \selectlanguage{english}[?] 450degC, reaching a minimum of 37 MPa at the
highest temperature and lowest velocity condition. Microstructural
observations showed that, as the gouges weakened, the strain localized
into thin, dense, and ultrafine-grained (\selectlanguage{english}[?] 1 \selectlanguage{greek}μ\selectlanguage{english}m) principal slip zones,
where nanopores were located along grain contacts and contained minute
biotite-quartz-feldspar precipitates. Though poorly constrained, the
stress sensitivity exponent n decreased from [?]17 at 20\selectlanguage{ngerman}°C to
\textasciitilde{}2 at 650°C at the lowest velocities. These findings
suggest that high temperature, slow velocity and/or small grain sizes
promote DPC-accommodated granular flow over cataclastic frictional
granular flow, leading to the observed weakening and strain
localization. Field observations together with extrapolation suggest
that DPC-induced weakening occurs at depths of 7-20 km depending on
geothermal gradient.%
\end{abstract}\selectlanguage{ngerman}%



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