GNGTS 2016 - Atti del 35° Convegno Nazionale

64 GNGTS 2016 S essione A matrice VIRTUAL OUTCROP AND 3D STRUCTURAL ANALYSIS OF MONTE VETTORE EXTENSIONAL ACTIVE FAULTS M. Menichetti, D. Piacentini, M. De Donatis, M. Roccheggiani, A Tamburini, E. Tirincanti Dipartimento di Scienze Pure e Applicate, Università di Urbino, (PU), Italy A detailed field mapping of spatial geometries of the surface coseismic ruptures of active faults is the basis for the identification of seismogenic structures. Field mapping of fracture distributions, fault offset, and links between geometries along the fault strands are the essential tools for extrapolating and constraining the depth of the fault plane from a kinematic point of view. These geometries can be expressed as orientations derived from strike and dip values collected during a structural survey, in which the kinematic fault analysis are integrated with geophysical data, allowing to constrain any seismogenetic model. Integration of geological and seismological data remains one of the main objectives when identifying active faults and assessing their potential hazard. While large data sets of instrumental seismological data are easy to gather, especially with modern digital seismic stations, field geological data remain very costly in terms of human and economic resources, especially in remote areas. However, evolving technologies have allowed remotely sensed data to be used to obtain a lot of equivalent information (Menichetti et al ., 2016). Earthquakes that produce surface ruptures are not common in central Italy and to this day, there is a lack of consensus on the interpretation of the ground cracks related to the latest 1997 Colfiorito and 2009 L’Aquila earthquakes (Boncio et al ., 2004; Chiaraluce et al ., 2005; Bigi et al ., 2012; Mildon et al ., 2016). An earthquake of Mw=6 struck central Italy between the town of Amatrice (Rieti Province) andArquata del Tronto (Ascoli Piceno Province) onAugust 24, 2016 rupturing an approximately 20 km long SW-dipping normal fault (INGV, 2016). The aftershock area, 50 km long and 15 km wide, experienced major aftershocks and more than ten thousands minor events (INGV, 2016) distributed throughout the morphologically complex landscape of the Sibillini mountain chain and the Laga massif. The region is characterized by stepped mountains alternating with basins, formed during an active extensional tectonic phase that overprinted older compressional structures (Menichetti, 1991; Lavecchia et al ., 1994). The Sibillini mountain chain is a NE verging foreland fold-and-thrust belt, the southernmost part of the northern Apennines that developed during the late Miocene. The entire region has been experiencing extension since the late Pliocene, expressed by a set of extensional faults that crosscut compressional structures forming several intramountain basins. The thrust related anticlines are constituted by Mesozoic and Cenozoic limestone and marls overlaying a few thousand meters thick Mio-Pliocene siliciclastic sequence with an offset of few kilometers (Menichetti, 1982; Lavecchia, 1985; Pierantoni et al ., 2013). The thrust structures show an arcuate shape traced by different stepped segments rotated from NW-SE in the northern sector to N-S in the center and to NE-SW in the southernmost part. The hanging-wall fold structures are characterized by an axial trend parallel to the orientation of the thrust planes, while the footwall structures maintain a NW-SE axial direction. The kinematics of the thrust indicates a complex dextral transpressive component towards ENE that probably reactivates pre-existing Mesozoic structures. The arc apex, where the largest shortening (up to 35%) occurs, coincides with the Mt. Vettore area. The thrust shear zones in the massive limestone are characterized by cataclastic gauge and breccia, while the marly and marly-limestone lithologies contain well- developed S/C-tectonites associated with pervasive foliations. Calcite shear veins indicate a slip vector between N60° to N80° that forms an angle of many degrees with respect to the main trend of the folded structures. Calcite veins are well distributed in the rock mass indicating that the fluid played a role in thrust emplacement (Menichetti, 1982). Both hanging and footwall structures of the Sibillini thrust are affected by pervasive