The effect of clay content on the dilatancy and frictional properties of
fault gouge.
Abstract
Mature fault cores are comprised of extremely fine, low permeability,
clay-bearing gouges. Saturated granular fault materials are known to
dilate in response to increases in sliding velocity, resulting in
significant pore pressure drops that can suppress instability. Up to
now, dilatancy has been measured only in clay-poor gouges. Clay minerals
have low frictional strengths and, in previous experiments, even small
proportions of clay minerals were shown to affect the frictional
properties of a fault. It is important, therefore, to document in detail
the impact of the proportion of clay on the frictional behaviour and
dilatancy of fault rocks. In this work, a suite of triaxial deformation
experiments elucidated the frictional behaviour of saturated, synthetic
quartz-clay (kaolinite) fault gouges at effective normal stresses of 60
MPa, 25 MPa and 10 MPa. Upon a 10-fold velocity increase, gouges of all
clay-quartz contents displayed measurable dilatancy with clay-poor
samples yielding comparable changes to previous studies. Peak dilation
did not occur in the pure quartz gouges, but rather in gouges containing
10 to 20 wt% clay. The clay content of the simulated gouges was found
to control the gouge frictional strength and the stability of slip. A
transition occurred at ~40 wt% clay from strong,
unstably sliding quartz-dominated gouges to weak but stably sliding
clay-dominated gouges. These results indicate that in a low
permeability, clay-rich fault zone, the increases in pore volume could
generate pore-fluid pressure transients, contributing to the arrest of
earthquake nucleation or potentially the promotion of sustained slow
slip.