Structural diagenesis of carbonate fault rocks exhumed from shallow crustal depths: An example from the central-southern Apennines, Italy

This contribution focuses on field and laboratory analyses of carbonate fault cores pertaining to high-angle extensional fault zones currently exposed in the central and southern Apennines, Italy. The fault zones studied crosscut Mesozoic platform-related carbonate rocks, strike ca. NW-SE, and dip steeply SW. They formed during the Plio-Quaternary downfaulting of the Apennine fold-and-thrust belt and were exhumed from shallow crustal levels (<1.5 km). The carbonate fault cores include grain-supported, matrix-supported, and cement-supported fault rocks, fluidized layers of ultracataclasites with injection veins, and main slip surfaces. Our results of microstructural, petrographic, and cathodoluminescence analyses highlight the contrasting diagenetic evolution of calcite- and dolomite-rich fault rocks. Physical compaction was common within the dolomite-rich fault rocks, whereas chemical compaction coupled with dissolution characterized the calcite-rich fault rocks. Furthermore, multiple generations of calcite cements are documented in the fault zones. The first generation consists of a microcrystalline calcite cement, which developed around survivor grains and lined intergranular pores. The second generation is made up of light-luminescent, fibrous calcite crystals, which precipitated within open fractures and around survivor grains. The third generation consists of an euhedral calcite cement that surrounded survivor grains and infilled both open fractures and intergranular pores.