APPLICATION OF CLUMPED ISOTOPES TO INVESTIGATE THE ROLE AND SOURCE OF FLUIDS IN LOW-ANGLE NORMAL FAULTING

The Altotiberina normal fault (ATF) is an active low-angle detachment located in central Italy, and provides a unique opportunity to investigate both fluid involvement in active low-angle slip and the relationships between the low-angle detachment and the associated epidetachment faults. Elevated borehole fluid pressures and texture evidence from the seismically active epidetachment faults suggests that fluids play an important role in the aseismic creep of the ATF as also recorded by surface geodetic measurements. Previous studies concluded that fluids associated with hydraulic fracturing along the epidetachment faults were meteoric in origin. However, published stable isotope ratios were inconclusive, and deformation fabrics in fault zone calcite support the presence of elevated fluid temperatures, which is more consistent with deep fluid circulation or a deep fluid source.

Clumped isotope (Δ₄₇) analysis, a powerful paleoclimate and paleoaltimetry tool, has been utilized less frequently in tectonic studies. This technique provides high precision temperature estimates in conjunction with stable isotopic values useful in distinguishing fluid source. Using the Δ₄₇ method, we demonstrate that the seismically active epidetachment faults are hydraulically connected to the low-angle ATF. We conclude that deeply-sourced overpressurized fluids that permeate the ATF hanging-wall fault system are likely derived from the adjacent active subduction margin, and have played a continuous role throughout the Quaternary evolution of the active ATF system. Our conclusions suggest that subduction derived fluids have possibly driven the tectonic style of hinterland extension across the Italian Northern Apennines since the onset of extension.