Recognizing the seismic cycle along ancient faults: CO2-induced fluidization of breccias in the footwall of a sealing low-angle normal fault

The Zuccale low-angle normal fault exposed on the island of Elba, Italy, is a crustal-scale structure which contains a strongly foliated fault core. In the immediate footwall of the Zuccale fault, cohesive fault-related breccias which were initially deformed by typical frictional deformation mechanisms experienced fluidization over areas of at least 10−2–10−3 km2.Three internal variants of fluidized breccia are recognized, with each related to a separate fluidization event. Characteristics of the fluidized breccias include: (1) a highly irregular ‘intrusive’ boundary with the overlying fault core; (2) no grain-scale evidence for typical frictional deformation mechanisms; (3) an association with carbonate cements indicating that fluids contained CO2; and (4) a clast-preferred orientation suggesting that fluids were moving vertically and spreading laterally as they encountered the foliated fault core.Our observations suggest that the fluidized breccias are representative of the interseismic period along the Zuccale low-angle fault, and developed across small fault patches during build-ups in fluid overpressure. Attainment of a critical fluid overpressure triggered embrittlement and the formation of low-angle slip surfaces and sub-vertical tensile veins within the overlying fault core, which may account for the presence and the dimensions (10−1–10−3 km2) of rupture surfaces which produce microseismicity along active low-angle normal faults in central Italy.