Physical-transport properties variations within carbonate-bearing fault zones: insights from the Monte Maggio Fault (Central Italy)

Physical properties of fault zones vary with time and space and in particular permeability variations are strictly related to fault zone processes. Results from previous laboratory studies, conducted on sedimentary fault rocks outcropping in Central Italy, show that permeability ranges from 10-16 m2 to 10-20 m2, (Agosta et al., 2007) in response to the intrinsic petrophysical properties of the material, state of stress and fault rock microstructures. Here we investigate the physical properties of carbonate samples collected along the Monte Maggio normal Fault (MMF) that is a regional structure (length ~10 km and displacement ~500 m) located within the active system of the Apennines. In particular we have studied an exceptionally exposed outcrop of the fault within the Calcare Massiccio formation that has been “exhumed” by new roadworks (Fig. 1A). Large cores (100 mm in diameter and up to 20 cm long) drilled perpendicular to the fault plane (Fig. 1B,C) have been used to: 1) characterize the damage zone adjacent to the fault plane and 2) to obtain smaller cores, 38 mm in diameter both parallel and perpendicular to the fault plane, for rock deformation experiments. The MMF shows two types of damage zone (Fig. 1C,D): 1) a cemented and indurated cataclasite (CC), that extends up the 20 cm from the fault plane and 2) a porous cataclasite (PC), that is located adjacent the CC. We performed laboratory measurements of Vp, Vs, and permeability at effective confining pressures up to 100 MPa in order to simulate crustal conditions, at the HP-HT Laboratory of experimental Volcanology and Geophysics (INGV, Rome1). From ambient pressure to 100 MPa, P-wave velocity ranges from 4,9 km/s to 5,9 km/s for PC samples, whereas it is constant at 5,9 km/s for CC samples. Vs show the same behaviour resulting in a constant Vp/Vs ratio of 1,5 and 1,6 for PC and CC respectively. Permeability of CC samples is about 10-19 m2 and it is pressure independent; in contrast, it is higher and pressure dependent for PC samples starting from 10-17 m2 at ambient pressure to 10-18 m2 at 100 MPa of confining pressure. Permeability variations are intimately related to fracture density as well as P-wave velocity. To test the applicability of laboratory data to in-situ condition we have compared laboratory and deep borehole data by assuming 25 MPa = 1 kmof lithostatic load. We selected four boreholes drilled in central Italy that have encountered the Calcare Massiccio (CM) formation: Pieve Santo Stefano 1, Tavullia 1, Villa Degna 1, and Daniel 1. Borehole velocities show significant values in the range of 5,7 – 6,9 km/s and this variation is not depth dependent. The most frequent in-situ values of Vp ~6.3 km/s is ~5% higher than the average velocity registered for both PC and CC samples at 100 MPa in the laboratory. A detailed microstructural analysis conducted by using image analysis on large cores of the MMF damage zone (Fig. 1D) will help in clarifying the relationship between crack density and geometry and elastic wave velocities and permeability.