Fluid conduits in carbonate‐hosted seismogenic normal faults of central Italy

We studied the structures and stable isotope geochemistry of carbonate fault rocks in four normal faults of central Italy. The faults juxtapose Meso‐Cenozoic carbonates of the footwalls against continental basins of the hanging walls. Footwall rocks exposed along fault scarps have been exhumed from depths of ∼1 km. The fault rocks are systematically arranged in each fault and can be separated into five distinct domains. Farthest from the main fault contact are undeformed host rock and fractured host rock (domain 1). Progressively closer to the fault contact, in the core of the fault, are gouge (domain 2), cataclasite (domain 3), and cement‐dominated horizons with planar slip surfaces (domain 4). Thin horizons of brecciated hanging wall sediments (domain 5) are adjacent, and locally accreted, to the footwall. Structural and stable isotope data are consistent with compartmentalization of fluid in the faults during exhumation. The data are most consistent with these fluids being predominantly evolved meteoric water rather than fluids from the mantle, crustal magmas, and/or devolatilizing carbonate rocks. Meteoric water infiltrated domains 4 and 5 of the faults, either from hanging wall sediments or directly at the land surface. The gouge and cataclasites of domains 2 and 3 were impermeable barriers to movement of meteoric water into domain 1 and undamaged host rocks of the footwall. The results of this study do not support models of earthquake nucleation and rupture that envision large volumes of deep‐seated fluids passing upward through shallow portions of these seismogenic faults.