Fluid-rock chemical-physical equilibrium/disequilibrium inferred from tectonic carbonates in the Apennines (Italy): an advanced approach to track seismic cycles and earthquakes

Abstract

Geofluids play an important role to seismic faulting both at hypocentral depth during earthquake nucleation and at shallower crustal levels during seismic rupture propagation. Pre- to co-seismic anomalies of crustal fluid circulation have thus far been identified by hydrogeochemical and seismological (Vp/Vs) monitoring and are generally interpreted as potential precursors, triggers, and/or facilitators of large magnitude earthquakes. Structural and geochemical studies on syn-tectonic mineralizations have highlighted different patterns of fluid ingress, circulation and fluid-rock interaction during the seismic cycle of thrusts and normal faults. Understanding fault rock-fluid relationships in exhumed faults is useful for revealing the role of fluids also in still ongoing seismic cycles from different tectonic settings. We present a review of published studies and original data on the chemical-physical characteristics of syn-tectonic mineralizations formed by fluids that assisted fault-related deformation in the Apennines (Italy). We use our data to build a general model of fluid circulation during thrusting and normal faulting during the seismic cycle, and define a multi-technique workflow to identify tectonic-related physical/chemical equilibria/disequilibria in fluid-rock systems. The proposed workflow relies upon multiscale structural analysis, stable C, O, and clumped isotope analysis, radiometric dating and burial-thermal modeling. The chosen study area is the Central Apennines fold-and-thrust belt, where post-orogenic extensional deformation, characterized by numerous Mw ≥ 6.5 earthquakes, currently overprints Neogene-Quaternary folds and thrusts. We show that thrusting mostly develops in closed fluid systems where fluid and host rock are close to chemical equilibrium. Subsequent normal faulting is characterized by a dominant upward and/or downward open fluid circulation system with an overall range of δ18O of paleofluids from -10‰ to +13.5‰ (V-SMOW). Isotopic and thermal fluid-rock disequilibria are particularly evident during pre-/ and co-seismic extensional deformation with mineralizations that are up to 30° C warmer and 16° C colder than the host rock and are associated with the ingress of exotic (meteoric or deep crustal) fluids.