Page 13 - Tondietall2015

Italian Journal of Geosciences Accepted manuscript

Figure 9 shows a scatter plot of the mean permeability values measured in the host rock,
damage zone and fault core of the different faults as a function of the porosity measured by image
analysis (2D data set).

The interpolation functions of both rocks show have similar slope, they are just translated
vertically due to the two orders of magnitude difference in permeability.

The undeformed Favignana grainstones are characterized by a mean permeability value of
approximately 70 Darcy. Within the damage zones (Zones II and III) this rocks recorded the
precipitation of calcite cement and a non-destructive compactive granular flow and pore collapse.
These processes reduced the pore amount and connectivity resulting in a mean permeability of
about 20 Darcy. Within fault cores, extensive pressure solution occurred and the deformation
evolved from particulate flow to compactant cataclastic flow, in agreement to TONDI (2007). The
progressive dissolution increased the ratio of clay minerals with respect to the carbonate particles.
This local variation in the bulk rock composition, together with the tendency of platy clay minerals
to align themselves parallel to the fault slip direction as a consequence of the shear (EICHHUBL et
alii, 2005), comported a decrease of the permeability down to a value of 0.5 Darcy (mean value).
These observations are consistent with the permeability and porosity reduction measured by
CRAWFORD et alii (2002) that carried out systematic permeability measurements on synthetic fault
gouges with increasing clay content. Their results show a decrease of the gouge permeability up to
three orders of magnitudes simply by varying the percentage of clay from 0 to 100.

The Orfento host rock has a mean permeability value of about 0.5 Darcy, this value is equal to
the permeability measured in the Favignana fault cores. As a consequence of the non-destructive
compactant granular flow, the mean permeability of the damage zones (Zone II) was decreased to
about 0.15 Darcy. Moreover, the composition of the fault cores (Zone I) is rich in residual insoluble
material and silt-to-clay-sized particles, in agreement with TONDI et alii, (2012). This composition
is the result of extensive pressure solution associated to cataclasis. The resulting mean permeability
of these zones is about 0.01 Darcy.