GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 63 GRAVITATIONAL SLOPE DEFORMATION VS. NORMAL FAULTS: DIFFERENT HAZARDS, SIMILAR MICROSTRUCTURES? L. Del Rio 1 , M. Fondriest 1 , M. Moro 2 , M. Saroli 3 , S. Gori 2 , E. Falcucci 2 , E. Spagnuolo 2 , G. Di Toro 1,2 1 Dipartimento di Geoscienze, Università degli Studi di Padova, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Roma, Italy 3 Università degli Studi di Cassino, Italy Introduction. 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. 0-80°C for NF), pressures (< 20 MPa for DGSD, 0 to 100 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. Geological setting and case studies. The rocks outcropping in the selected area of the Central Apennines (Fig. 1a) consist of Jurassic-Cretaceous shallow-water to basin limestones and dolostones plus Plio-Pleistocene cemented conglomerates and Quaternary fluvio-lacustrine deposits. Because of the different mechanical properties (Young modulus, friction coefficient, fracture toughness, cohesion, etc.) of these rocks, we selected scarps of faults and DGSD cutting similar rocks to reduce the role of rock composition and fabric in controlling the fracture pattern and the microstructures of the slipping zones. Though paleo-seismological and geomorphological studies suggest that some structural features are mainly associated to DGSDs while other to seismic faulting, some scarps of active seismogenic faults were thought to accommodate also DGSD (Moro et al. , 2012; Gori et al. , 2014). Because of the continuum between the NF and DGSD fault/fracture structural networks, we selected the following case studies (underlined: field surveys and microstructural investigations completed; numbers refer to the boxes in Fig. 1a): 1. Valle Force Fault (VFf) , anthitetic normal-fault bordering a small intra-mountain basin. This ~1 km in length normal fault cuts shallow-water limestones and Pleistocene carbonate breccias. 2. Campo Felice Fault (CFf) (length ~15 km, slip rate ~0.8-1.3 mm/yr) vs. Monte Ocre fault-DGSD ( Of) . The latter is geometrically linked to the Campo Felice Fault and was reactivated as DGSD. Similar host rocks: shallow-water wackestone carbonates. 3. Rocca Preturo Fault (RPf) (length~10 km, slip rate ~0.23-0.34mm/yr) vs. Sant’Erasmo (SAE) DGSD . The latter is a small antithetic fault at the footwall of the Rocca Preturo Fault and was reactivated as DGSD. Similar host rocks: rudstones with intercalated marls. 4. Alto di Cacchia (AC) DGSD . This ~500 m long scarp is considered a pure DSGD (end