Fault zone evolution and fluid circulation within active extensional faults in carbonate rocks

Structural and geochemical methods applied to the seismically-active extensional Tre Monti Fault (central Apennines, Italy) were used to develop a conceptual evolutionary model of seismic faulting with fluid involvement for shallow (<= 3 km depth) extensional faults in carbonate rocks. The relative chronology of these structures was reconstructed through cross-cutting relationships and cathodoluminescence analyses. C- and O-isotope data from different generations of fault-related mineralizations show a shift from marine- to meteoric-derived fluid circulation during exhumation from 3 to <=1 km depths and concurrent fluid cooling from ~68 to <35 °C. Between ~3 km and ~1 km depths, impermeable barriers within the sedimentary sequence created a semi-closed hydrological system, where marine-derived fluids circulated within the fault zone at temperatures between 60° and 75°C without any mixing with meteoric-derived fluids. During fault zone exhumation at depths <= 1 km and temperatures <35 °C, the hydrological circulation became open and meteoric-derived fluids progressively infiltrated and circulated within the fault zone. The presence of low-permeability clayey layers in the sedimentary sequence contributed to control the type of fluids infiltrating into the fault zone. These results can foster the comprehension of fault-related fluid circulation within seismogenic faults at shallow depths in carbonate rocks of other fold-thrust belts involved in post-collisional seismogenic extensional tectonics.