Evolving fluid pathways in a shallow mega-thrust shear zone (Northern Apennines, Italy)

Megathrust shear zones are weak interplate faults that accommodate deformation under low effective stress in a fluid-rich environment. The evolution of stress and fluids during megathrusts activity can be reconstructed from syn- and post-tectonic mineral veins in exhumed settings.

The Sestola–Vidiciatico Unit (SVU) in the Northern Apennines is a fossil analogue of a shallow subduction megathrust mélange. It represents the plate boundary shear zone (200–400 m thick) between the Ligurian prism and the underthrusted Adria microplate active during the early–middle Miocene with peak temperatures of ~170 °C. The SVU is composed of kilometre-scale slices of marls, shales and sandstones derived from the Ligurian prism and its sedimentary cover, thrust along a basal décollement onto younger Adria-derived foredeep turbidites.

This study focuses on a well-exposed outcrop along a south-dipping thrust ramp of the basal décollement of the SVU. The footwall consists of sandstones and siltstones of Langhian age, overthrust by a slice of Aquitanian marls, and by an upper slice of Priabonian – Bartonian claystone. We performed structural mapping, microstructural and geochemical analyses (O-C stable isotopes, trace and major element geochemistry), and fluid inclusion studies on calcites filling shear and extensional veins and cementing tectonic breccias.

Marls and claystones within the SVU are bounded by sharp thrust surfaces decorated by multiple generations of shear veins. In the vicinity of the main thrusts, marls and claystone are crosscut by pervasive shear fractures, bounding flattened and elongated lithons defining a foliation at low angle to the thrusts. Deformation in the footwall includes oblique cleavage, bedding-parallel shear planes, and a conjugated set of NNE-SSW left-lateral and N-S trending right-lateral subvertical transtensional faults showing mutually crosscutting relationships with the basal thrust of the SVU. Calcite shear veins mark thrust surfaces, whereas transtensional faults in the footwall are marked by shear and extension veins, as well as calcite-cemented breccias. Two different calcite phases have been observed: an early-stage calcite, rich in host-rock inclusions and a later inclusion-free calcite.

Geochemical and thermometric results point to two groups of distinct mineralizing fluids circulating through the fracture network: (1) diluted seawater precipitating early-stage calcites at low temperatures (< 50 °C up to 70 °C); (2) an external low-salinity fluid precipitating later-stage calcites at higher temperatures (~80-100° C).

Our data suggest a transition from low temperature and low salinity fluids, possibly from mixing of seawater and fluids released from clay dehydration during progressive burial of the SVU, to the ingression of moderately hot fluids (up to 120 °C) external from the system. This indicates that the onset of fluid circulation by faulting is modulated by the embrittlement and seismic ruptures in subduction zones, favoured by a low-stressed environment.