Fossil chemical-physical (dis)equilibria between paleofluids and host rocks and their relationship to the seismic cycle and earthquakes

Understanding the behavior of fluids in seismically active faults and their chemical-physical (dis)equilibrium with the host rock is important to understand the role of fluids upon seismicity and their possible potential for forecasting earthquakes. The small number of case studies where seismic and geochemical data are available and the lack of accessibility to fault zones at seismogenic depth for recent earthquakes limit our understanding of fluid circulation and its relationship to seismicity. The study of fault-fluid relationships in exhumed faults can broaden the number of case histories and improve our understanding of the role of fluids in the seismic cycle in different tectonic settings. Here we use new geochemical and thermal data and a review of published studies from the Apennines fold-and-thrust belt (Italy) to provide a model of fluid circulation during the seismic cycle related to either the local orogenic compressional or post-orogenic extensional tectonics. We also suggest a workflow based upon different methods to identify tectonic-related chemical-physical (isotopic and thermal) (dis)equilibria in fluid-rock systems during the seismic cycle. The proposed workflow involves multiscale structural and isotope geochemical analyses, radiometric dating, and burial-thermal modeling. It is applied to carbonate-hosted faults exhumed from a depth shallower than 4 km (temperature ≤ ~130 °C and pressure ≤ ~130 MPa). We show that in the Apennines, during syn-orogenic shortening, thrusting is mostly assisted by fluid circulation in an effectively closed system where fluid and host rock remain close to chemical and thermal equilibrium. In contrast, post-orogenic normal faulting occurs in association with upward and/or downward open fluid circulation systems leading to chemical-physical disequilibria between the host rock and the circulating fluids. Isotopic and thermal fluid-rock disequilibria are particularly evident during pre- and co-seismic extensional deformation. Mineralizing fluids, whose temperature can vary between 30 °C warmer and 16 °C colder than the host rock, result from the mixing of fluids derived from both the deforming host rock and external sources (meteoric or deep crustal). The proposed workflow offers the potential to track past seismic cycles and provide indications on actual fluid-earthquake relationships including the identification of potential seismic precursors and modes of triggered seismicity that might be different in extensional and compressional tectonic settings.