Page 9 - Tondietall2015

Italian Journal of Geosciences Accepted manuscript

TONDI (2007) documented that single faults are made of three distinct adjacent tabular zones
(Zones I, II and III) that are recognizable in thin section (fig. 5). The Zone I represents the fault
core, which is characterized by a well-developed, continuous zone of grain size and porosity
reduction. In thin section the Zone I appears as made of comminuted grains and red-brown
insoluble residual material (mostly iron oxides), with associated slip surfaces. Porosity is less than
1%. Zones II and III form the fault damage zone. Zone II, which bounds on both sides Zone I, is a
compacted grain zone (without grain size reduction) with micro-stylolites localized at the grain
contacts; generally, in terms of thickness it is one half of Zone I. Porosity of zone II ranges between
1% and 3% (TONDI, 2007). The Zone III represents a thicker volume, surrounding both Zones I and
II, in which porosity reduction is due to the extensive precipitation of calcite cements. The residual
porosity of this zone ranges between 3% and 7%.

The micro-mechanisms responsible for the formation of compactive shear band-based faults in
these rocks has been described by TONDI, 2007. First particulate flow, involving grain translation,
rotation associated to pore collapse takes place, and then the so-formed compactive shear bands are
overprinted by pressure solution processes, which facilitates the compaction (Zone II). In this Zone,
intergranular pressure solution destroys the pore space leaving a rock characterized by sutured
grains with no intergranular space. Cementation does not occur in this zone. At incremental stages
of deformation the intergranular pressure solution determines partial dissolution of the grains and
grain size reduction. A large amount of residual material is left behind by this process (Zone I). The
intergranular pressure solution is also responsible of the dissolved CaCO3 that precipitates in
adjacent pores of Zone III as calcite cement reducing porosity.

MAJELLA SITE
At the Madonna della Mazza quarry, the Orfento carbonate grainstones are crosscut by
extensional and strike-slip faults (and compactive shear bands): i) coeval, conjugate normal faults
striking NNW-SSE and WNW-ESE, respectively, which are both characterized by dip angles