Antitaxial calcite veins in shales associated with normal fault systems: evidence from the central Southern Alps (N Italy)

Antitaxial fibrous veins are commonly found in low permeability strata in sedimentary basins worldwide. Many studies refer to those veins as “beef” since they are bedding-parallel veins of fibrous material (Cobbold et al., 2013). Recently, Delogkos et al. (2022) suggested in their work that extension is accompanied by bed-parallel slip (BPS), especially in rift systems with low-moderate basin extensions. This study aims to provide more clues on the formation mechanism of bed-parallel antitaxial calcite veins associated to a normal fault system, since they have significant impacts on the mechanical properties and fluid flow behaviour of the host shale. This work concentrates on the late Norian succession of the Seriana Valley area (central Southern Alps, N Italy), characterized by the Riva di Solto Shale. Here, the unit is in the footwall of the Amora Fault System, a N-S trending rift-related normal faults system that formed during the Early Jurassic as a consequence of the opening of the Alpine Tethys. It consists of black, thin laminated organic rich shales, marls and limestones arranged in asymmetric cycles. At the mesoscale, E-dipping normal faults synthetic to the Amora Fault System can be observed offsetting the shale layers. These are characterized by the widespread presence of calcite veins, that developed parallel to the bedding or on low-angles N-dipping planes, which often show successive folding and thrusting. The microstructural analysis of 8 sampled veins allowed their classification as antitaxial calcite veins, as described by Bons et al. (2012), and revealed the presence of three antitaxial growth stages in most of the samples, confirmed by the cathodoluminescence analysis. The O-C stable-isotopes analyses shows that the antitaxial calcite veins precipitated from a fluid with δ13C bufferd from the host-rock, and δ18O shifting towards more negative values in the latest growth stage, possibly due to change in the temperature of the fluids. The U-Pb dating gave an Early Jurassic age, confirming that this system is contemporaneous and related to the Amora Normal Fault System. According to the structural and geochemical analyses, we suggest that the antitaxial calcite veins in the Riva di Solto Shale formed during the Early Jurassic by hydraulic fracturing in a mechanically anisotropic rock with a tensile failure normal to the maximum principal stress (σ1 ). Fluid overpressure enabled the development of temporary fluid-flow pathways that were subsequently sealed to form mineral vein networks with a “crack-seal slip cycle” repeated several times.