Compactive shear bands

High-porosity arkose sandstones of Loiano (LS; Northern Apennines, Italy) are medium to coarse-grained and composed by quartz, feldspar, rock fragments, a few carbonate clasts, and minor accessories. LS are an example of structural-controlled diagenesis, because of the presence of calcite-cement nodules associated to the deformation bands (DBs). A DB consists of a cataclastic and low-porosity tabular zone with a thickness from ≤1 mm up to 2 mm. Slip along single DBs ranges from 4-5 mm up to 2-4 cm. Zone of bands thickness ranges between 0.8 and 60 cm and accommodate offset up to 0.5 m. Lithology (clay content) and rock properties (porosity, sorting, grain-size) are essential factors in determining fault zone characteristics. Different fracture mechanisms characterize quartz (flaking, spalling) and feldspar (intragranular fracturing along cleavage). These different fracture mechanisms control DB’s fabric and grain size distribution. Brittle conditions in the DB core are also demonstrated by bookshelf-like micro-fracturing. DBs are characterized by compaction and cataclasis that result in extensive grain interlocking promoting strain hardening. Porosity reduction produces a corresponding decrease in permeability down to 3 orders of magnitude with respect to the host rock. Cementation increase the cohesion and the reduction of porosity/permeability caused by deformation. Nodules along zone of bands are the most cohesive and weathering-resistant features in the outcrop.