Hot fluid pumping along shallow-level collisional thrusts: The Monte Rentella Shear Zone, Umbria Apennine, Italy

The characteristics of a shallow-level shear zone that is representative of the deformation in the external sectors of the Northern Apennine fold-and-thrust belt are described. The characterization involved an integrated approach using microstructural analysis of deformation fabrics, vitrinite reflectance measurements, XRD analysis on clay minerals and carbon and oxygen stable isotopes analyses. This data set provides the evidence that the thrust was active at very shallow depths (ca. less than 3 km), with maximum paleotemperatures ranging from 60° to 100–110 °C. The regime during fault activity evolved through cycles of compaction and dilation linked to transient build up of fluid overpressure and injection. The alternating cycles of fluids supply generated a fault-fracture mesh with a complex network of blocky and striped veins that formed at temperatures ranging from 150° to 200 °C, not compatible with the conditions in the host rocks. This evidence implies that the shear zone was flooded by hot fluids coming upward from diagenetic and low-grade metamorphic dehydration of clay minerals active at deeper structural levels. The fluids were thus highly channelled and focused where deformation also focused, producing a local pronounced isotopic difference between fluids and host rock.

► Multidisciplinary analysis of fault–fluid interactions in a collisional thrust of N Apennines.
► Fault-fracture mesh with associated cyclic injection of overpressured fluids and striped veins.
► Vitrinite reflectance and XRD on clay minerals data indicate very shallow thrust activity (<3 km).
► C and O stable isotopes analyses suggest fluid in disequilibrium with host rock at T of 150–200 °C.
► Thrust flooded by hot fluids coming from low-grade metamorphic reactions at deeper structural levels.