Mechanical and chemical processes in the fault zone crosscutting slope carbonates of Apulian carbonate platform and their effect on the fault permeability (Gargano Promontory, Italy)

Fault zones typically consist of a highly fractured and permeable rock mass (damage zone) surrounding a fault core with low permeability. The fluid flow property of such a typical fault zone is determined by a combination/interaction of chemical and mechanical processes. Generally, mechanical processes, for example mode-I fracturing and the subsequent shearing of these pre-existing fractures with formation of secondary tails dilational structures, may significantly increase the amount of fracture porosity in the damage zone. However, mainly in carbonates rocks, chemical processes such as dissolution and reprecipitation of minerals, can have the opposite effect on porosity and, hence, permeability by cementing and sealing the fractured rock masses. In this work, we examine a normal fault crosscutting a slope succession of the Apulian carbonate platform cropping out in the Gargano Promontory of southern Italy (Lower Cretaceous Casa Varfone Formation). First, a mesoscale structural analysis of the fault zone was carried out in the field. Then, a microstructural characterization of representative samples collected from the fault core (cataclastic carbonates) and damage zone (fractured carbonates) was performed in order to describe the chemical and mechanical processes that took place during and after the faulting. Additionally, laboratory experiments of the same representative samples allowed us to assess their main petrophysical properties (porosity and permeability). The results are consistent with this fault juxtaposing reefal and slope carbonates of the footwall against basinal limestone of the hanging wall. The fault core is mainly composed of fine-grained carbonate cataclasites either cohesive or not. The footwall damage zone is made up of fractured reefal breccias, whereas the hanging wall one of fractured fragmented to pulverized micritic carbonates. Microstructural analyses focused primarily on rock texture (matrix and clasts amount and composition, cement composition and type). A special attention has been paid to the relative timing of formation of the main elements (pores, cement, stylolites, fractures and veins) in order to distinguish the different stages of dissolution, fracturing, fragmentation and cementation. The results of this study will provide new insights on the aforementioned competing mechanical and chemical processes during progressive deformation. Furthermore, this knowledge may shed lights on spatial and temporal fault permeability in the Apulian carbonates of southern Italy.