Fault-induced saddle dolomitization during the Late Triassic rifting of Pangea in the southern Adria region (Southern Italy)

Shallow crustal fault zones, particularly those within carbonate successions, are deformation zones that influence subsurface fluid migration, localization, and interaction. These zones, characterized by fault-controlled fracturing and brecciation, can act as important conduits for fluid flow in the Earth's crust. The interaction between fluids and the fractured carbonates rock matrix can induce significant changes in the mineralogical and mechanical properties of fault zones. Therefore, studying fluid-rock interactions in such environments is crucial not only for understanding the natural processes governing subsurface fluid dynamics, deformation, and metamorphism but also for addressing significant challenges in energy and mineral exploration as well as in underground engineering.

This study presents, for the first time in the southern Apennines (southern Italy), evidence of fault-driven hydrothermal dolomitization during the late Triassic rifting event in the western Adria plate. We investigated the fault-controlled saddle dolomite formation in Norian dolomites exposed in the western sector of the Matese Massif. The study focused on dolomite breccias associated with N-S and NNW-SSE striking normal faults. These structures include layers of mature cataclasites made of clasts with angular boundaries within a highly porous matrix, crossed by veins, mosaic and chaotic breccias. The breccias are composed of angular clasts of host rock dolomite, formed by early marine replacive dolomitization of shallow-water carbonates, surrounded by coarse saddle dolomite cement. The saddle dolomite cement is characterized by two distinct phases. The first phase (SD1) is yellow, inclusion-rich, and forms a rim around the clasts, while the second phase (SD2) is euhedral, exhibiting well-defined zoning with a transition from cloudy to limpid crystals. The saddle dolomite cement texture, coupled with decreasing δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values, suggests that it precipitated at temperatures of 100-120°C from fluids that likely interacted with magmatic sources.

U-Pb dating of the dolomite cement provides late Triassic crystallization ages of approximately 206 ± 13 Ma and 217.0 ± 6.6 Ma. Additionally, the ferroan dolomite cement contains quartz and hydrothermal minerals, including fluorite and apatite, in minor quantities. These findings suggest that the brecciation and hydrothermal saddle dolomite precipitation were linked to normal fault activity during the breakup of Pangea, contributing to the separation of the SW sector of Eurasia from the western margin of the Adria Plate.