Gravitational Slope Deformation vs. Normal Faults: different hazards, similar microstructures?

Carbonate-built rocks of the Central Italian Apennines are cut by fault zones consisting of sharp slip surfaces bounding up to few cm thick principal slip zones (PSZs) surrounded by up to hundreds of meter-thick damage zones. Recent paleo-seismological, geological and geomorphological observations pointed out that the PSZs may accommodate either large landslides (Deep-Seated Gravitational Slope Deformation: DGSD, Galadini, 2006; Moro et al., 2012; Gori et al., 2014, Saroli et al., 2017) and seismic or aseismic crustal scale fault deformation (Normal Faults: NF). Clearly, the distinction between DGSDs and NFs structures based on field and microstructural observations and the individuation of the processes forming the PSZs is of outstanding relevance in geological hazard studies. Currently, most of the sharp slip surfaces exposed in the Italian Central Apennines are mapped as active normal faults, even if the presence of peculiar geomorphological structures (trenches, double crest scarps, counterscarps) would suggest a gravitational origin for some of them. Depending on the geological process (DGSD vs. NF), the slip surfaces and associated slipping zones reach different depths along dip (100-1000 m for DGSD, 10-12 km for NF), and are formed and active over a different range of temperatures (0-30 °C for DGSD vs. 0-250°C for NF), pressures (< 20 MPa for DGSD, 0 to 250 MPa for NF) and slip rates (usually < 10-3 m/s for DGSD, up to ~1 m/s for NF). Such large differences in loading conditions should result in the formation of distinctive secondary fault/fracture networks in the damage zones, possibly recognizable at the outcrop scale or in the microstructures of the slipping zones. Here we discuss the fault/fracture structural networks and the microstructures associated to both DGSDs and NFs by conducting (1) extensive field structural-geology surveys and (2) high-resolution microstructural studies on the PSZs and associated wall rocks.