Structural evolution and permeability of normal fault zones in highly porous carbonate rocks

The structural evolution of cataclastic fault cores from nucleation to growth was studied for the case of normal fault zones affecting high-porosity carbonates in the Hyblean Plateau, Sicily. A comparison was made between faults with increasing displacements affecting a similar lithology at shallow depth conditions. In the first few millimetres to centimetres adjacent to the fault surface, porosity is significantly reduced by a sequence of pore collapse, grain crushing, rotation-enhanced abrasion and calcite precipitation, which is a function of increasing displacement. Consequently, fault planes have strongly reduced permeability even for very small displacements. Adjacent damage zones are characterized by fracture density and connectivity increasing toward the fault plane. Cataclastic rock production and consequent fault-core development initiate as the fault displacement reaches values of 1–5 m. This displacement threshold coincides with a decrease in the widening of the damage zone per unit increase of fault displacement, which relates to a change in the mechanism of deformation accumulation in the fault zone. The change is interpreted to result from strain-softening in the fault zone due to the onset of cataclasis. Permeability data indicate that normal faults in high-porosity carbonates are effective transversal seals even at very small offsets and their combined conduit-barrier hydraulic behaviour is accentuated as displacement increases. These results show some similarities but key differences with respect to fault zone development for the analogous cases of sandstones and low-porosity limestones.