Abstract
The spatial separation of macroscopic rheological behaviours has led to
independent conceptual treatments of frictional failure, often referred
to as brittle, and viscous deformation. Detailed microstructural
investigations of naturally deformed carbonate rocks indicate that both,
frictional failure, and viscous mechanisms might operate during seismic
deformation of carbonates. Here, we investigate the deformation
mechanisms that were active in two carbonate fault zones in Greece by
performing detailed slip-system analyses on data from automated
crystal-orientation mapping transmission electron microscopy and
electron backscatter diffraction. We combine the slip system analyses
with interpretations of nanostructures and predictions from deformation
mechanism maps for calcite. The nanometric grains at the principal slip
surface should deform by diffusion creep but the activation of the
(0001)<-12-10> slip system is evidence for a
contribution of crystal plasticity. A similar crystallographic preferred
orientation appears in the cataclastic parts of the fault rocks despite
exhibiting a larger grain size and a different fractal dimension,
compared to the principal slip surface. The cataclastic region exhibits
microstructures consistent with activation of the
(0001)<-12-10> and
{10-14}<-2021> slip systems.
Post-deformational, static recrystallisation and annealing produces an
equilibrium microstructure with triple junctions and equant grain size.
We propose that repeated introduction of plastic strain and
recrystallisation reduces the grain size and offers a mechanism to form
a cohesive nanogranular material. This formation mechanism leads to a
grain-boundary strengthening effect resulting in slip delocalisation
which is observed over six orders of magnitude (μm–m) and is expressed
by multiple faults planes, suggesting cyclic repetition of deformation
and annealing.