Evidence of subduction-related fluid circulation along the active Altotiberina low-angle normal fault

Abstract:

Slip along non-Andersonian low angle normal faults (LANFs) is facilitated by fluids along the fault plane, the record of which is preserved in deformation fabrics in exhumed shear zones bounding metamorphic core complexes. However, opportunities to directly sample fluid-related fault fabrics associated with active low-angle detachments are limited; the Altotiberina low-angle fault in central Italy is one of only a few active LANFs, and provides a unique opportunity to investigate relationships between fluids and dynamic fault processes. Although this active fault is not directly accessible, it is structurally connected to the surface via a series of epidetachment normal faults, including the Gubbio fault, which have operated in tandem with the Altotiberina fault since upper Pliocene time.

Determining the source of fluids within fault damage zones is complicated by the superposition of fluid-related fabrics. Implementing clumped isotope (Δ47) techniques offers an opportunity to distinguish between fluid sources based on temperature and stable isotopic composition. We combine the clumped isotope geothermometer with field observations, microstructural analysis, energy-dispersive x-ray (EDS), and cathodoluminescence to investigate the Gubbio fault core. Several distinct phases of calcite reveal incorporation of high temperature fluids that have isotopic compositions incompatible with an upper crustal source. Our results suggest that metamorphic fluids migrated along the Altotiberina fault from depths greater than 10 km. We conclude that the Altotiberina fault is hydraulically connected along structural pathways between the upper and lower crust, and at depth fluid overpressures are sufficient enough to overcome hydrostatic pressures. We also suggest that the source of the fluids driving slip along the Altotiberina fault are derived from the nearby subduction zone, an interpretations that implies an external condition driving LANFing in the Northern Apennines, which is consistent with the Miocene to present evolution of this retreating convergent margin.