, De Paola, N.
, Nielsen, S.B.
, Holdsworth, R.E.
& Bowen, L.
(2018). A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults. Geology 46
Author(s) from Durham
Highly reflective, continuous smooth surfaces, known as “mirror-like surfaces” (MSs),
have been observed in experimental carbonate-hosted faults, which were sheared at both
seismic and aseismic velocities. MSs produced during high-velocity friction experiments
(>0.1 m s–1) are typically interpreted to be frictional principal slip surfaces, where weakening
mechanisms are activated by shear heating. We re-examined this model by performing
friction experiments in a rotary shear apparatus on calcite gouge, at seismic
velocities up to v = 1.4 m s–1 and normal stress σn = 25 MPa, to analyze the evolution of
microstructures as displacement increases. After the onset of dynamic weakening, when
the friction coefficients are low (µ << 0.6), sheared gouges consistently develop a welldefined,
porosity-free principal slip zone (PSZ) of constant finite thickness (a few tens of
micrometers) composed of nanometric material, which displays polygonal grain shapes.
MSs occur at both boundaries of the PSZ, where they mark a sharp contrast in grain size
with the sintered, much coarser material on either side of the PSZ. Our observations suggest
that, with the onset of dynamic weakening, MSs partition the deformation by separating
strong, sintered wall rocks from a central weak, actively deforming viscous PSZ.
Therefore, the MSs do not correspond to frictional slip surfaces in the classical sense, but
constitute sharp rheological boundaries, while, in the PSZ, shear is enhanced by thermal
and grain-size–dependent mechanisms.