GNGTS 2014 SESSIONE 1.1 THE SOUTHERN MATESE ACTIVE FAULT SYSTEM: NEW GEOCHEMICAL AND GEOMORPHOLOGICAL EVIDENCE 1 2 3,4 4 2 2 2 A. Ascione , S. Bigi , G. Ciotoli , A. Corradetti1, G. Etiope , L. Ruggiero , P. Sacco , C. Tartarello , S. Tavani1, E. Valente5 1 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università Federico II, Napoli, Italy 2 Dipartimento di Scienze della Terra, Università La Sapienza, Roma, Italy 3 Istituto di Geologia Ambientale e Geoingegneria – CNR-IGAG, Roma, Italy 4 Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Roma, Italy 5 Dipartimento di Scienze Umanistiche, Sociali e della Formazione, Università del Molise, Campobasso, Italy Introduction. Assessment and mitigation of seismic hazard rely on the identification and geometric characterization of active structures. Crucial, to this task, is the definition of the relationships between shallow and deep-seated active structures, through a comprehensive approach combining surface and subsurface geology with seismological information. Indeed the detailed reconstruction of the active tectonics frame at the surface may provide a reliable picture of seismically active structures (e.g., Amoroso et al., 2014), also in those areas featuring complex structural setting, e.g., a strong rheological contrast between deep and shallow structural levels [e.g., the southern Apennines: Ascione et al. (2013)]. The spatial distribution of surface deformation may be effectively detected by a variety of geomorphological indicators, and temporally constrained by the stratigraphical record. However, the topographical signature of faulting depends not only on fault offset and behaviour (in terms of slip per event and recurrence time; e.g., Slemmons and Depolo, 1986), but also on both the resistance to erosion of the faulted bedrock, and the sensitivity of the geomorphological scenario (e.g., Ascione et al., 2013). ����������������������������������������������������������In areas characterised by highly erodible bedrock types or alluvial deposits, soil-gas measurement has received much attention as an elective method for tracing active hidden faults (e.g., Baubron et al., 2002; Fu et al., 2005; Ciotoli et al., 2007, 2014; Al-Hilal and Al-Ali, 2010; Walia et al., 2010), and monitoring seismic activity (e.g., Toutain and Baubron, 1999; Yang et al., 2005; Kumar et al., 2009; Walia et al., 2012). �����������������The stress/strain changes related to seismic activity may control fluid circulation at depth forcing crustalfl uid to migrate upwards, especially along faults (King, 1986; Ciotoli et al., 2007). Fluid circulation at depth may alter the geochemical characteristics of the fault core (Annunziatellis et al., 2008), which in turn can influence soil gas concentrations at the surface (e.g., Baubron et al., 2002; 11 001-260 volume 1 11 24-10-2014 16:51:54 GNGTS 2014 SESSIONE 1.1 Fig. 1 – Historical earthquakes and seismogenic structures of the Matese area: a) epicenters of historical earthquakes [squares are proportional to the rupture surface produced by the events; from Boschi et al., (1997)] and epicenters of instrumental seismicity between 1983 and 2000 from INGV seismic database (from Milano et al., 2005); the dashed white frame indicates location of the area in b; b) seismogenic sources (from DISS Working Group, 2010), and epicentre (star) and focal mechanism (inset) of the December 29, 2013 earthquake (data from http://cnt.rm.ingv. it/tdmt.html); the white frame indicates location of the Colle Sponeta area. 12 001-260 volume 1 12 24-10-2014 16:51:56 GNGTS 2014 SESSIONE 1.1 Lombardi and Voltattorni, 2010). In fact in seismically active zones anomalous concentrations of various minor and trace gas species (i.e., CO2, Rn, He, H2, CH4) are a widely observed process (e.g., Ciotoli et al., 2007, 2014; Voltattorni et al., 2012). The degassing process, occurring as advective migration of free gas phase in fractured rock volumes at depth, indicates that active faults are characterized by high permeability, and thus act as preferential conduits in the crust (Ciotoli et al., 1998, 2007, 2013, 2014; Toutain and Baubron, 1999; Annunziatellis et al., 2008). An integrated approach, based on the search for geomorphological and geochemical indicators of faults showing evidence of late Quaternary activity at the surface is used in our study, which is being carried out in the framework of the INGV-DPC 2014-15 Progetto S1, and aims at providing new data on the active tectonics of the area including the southern Matese ridge and adjoining valleys (Fig. 1). The Matese ridge falls in the epicentral area of strong historical earthquakes, the most destructive, with X-XI MCS intensities and Mw from 6.7 to ≥ 7, occurred in 1349, 1456, 1688 and 1805 (e.g., Gasperini et al., 1999; Galli and Galadini, 2003; Di Bucci et al., 2005; Fracassi and Valensise, 2007; Locati et al., 2011; Rovida et al., 2011; Fig. 1a). Such strong earthquakes affected the area with coseismic ruptures and a large number of secondary geological effects, including landslides, sinkhole collapses and changes of discharge rate and chemical properties of major springs (e.g., Porfidoet al. 2007; Di Bucci et al., 2005, 2006; Fracassi and Valensise, 2007; Galli and Naso, 2009; Del Prete et al., 2010). The seismogenic sources of the 1805, 1688 and 1349 earthquakes according to the DISS Working Group (2010; and references therein) are shown in Fig. 1b. In the last decades, the area has been affected by low-magnitude background seismicity characterised by both sparse earthquakes and seismic sequences (Milano et al., 2005; Chiarabba et al., 2011; Fig. 1a). From the end of 2013 to the early 2014, the area has been struck by a further seismic sequence. The epicentre of the ML=4.9 main event, which has occurred on 29th December and has been characterised by a normal faulting mechanism, has been localised in the SE part of the Matese ridge (http:// cnt.rm.ingv.it/tdmt.html; Fig. 1b), i.e. between those which are acknowledged as the main seismogenic structures, and in area in which no fault strand showing evidence of recent activity had been mapped to date (e.g., Cinque et al., 2000). This points to the need of focussing the investigation both within and around the Matese ridge in order to better define the overall surface deformation scenario. In this paper we present new, preliminary data from the southern Matese ridge area, with a particular focus on those providing evidence of active tectonics in the Colle Sponeta area (Fig. 1b). Geological setting. The study area is located in the Campania-Molise sector of the southern Apennines. The southern Apennines is a NE-directed fold and thrust belt formed in Miocene to Quaternary times, with thrusting coexisting with back-arc extension in the southern Tyrrhenian basin since the late Miocene (e.g. Cinque et al., 1993). Back-arc extension caused formation, since the Early Pleistocene, of large peri-Tyrrhenian grabens (e.g., Caiazzo et al., 2006) in some of which volcanism developed since the Middle Pleistocene (e.g., Radicati Di Brozolo et al., 1988). Thrusting ceased in the early Middle Pleistocene, and a new tectonic regime with NE- SW oriented maximum extension was established in the chain (e.g., Cello et al., 1982; Cinque et al., 1993; Montone et al., 1999; Patacca et al., 2008). The structures related to this regime include dominantly extensional faults that postdate and dissect the thrust belt (e.g., Cello et al., 1982; Ascione et al., 2013). Based on fault-plane solutions, normal faults also control seismogenesis in the mountain belt, which is affected by low to moderate events punctuated by strong earthquakes, mostly following the chain axis and originating on NW-SE trending faults (e.g., DISS Working Group, 2010). In the study area, tectonic units of the fold and thrust belt are composed of Mesozoic- Tertiary successions covered by Neogene foredeep basin deposits. These consist, from the top, of carbonate successions (Apennine Platform, outcropping in the Matese ridge), pelagic basin 13 001-260 volume 1 13 24-10-2014 16:51:56 GNGTS 2014 SESSIONE 1.1 successions (Molise-Sannio Basin, outcropping to the N and E of the ridge), and the buried Apulian platform carbonates (Mazzoli et al., 2000). The thrust pile is dissected by NW-SE extensional structures and E-W trending high-angle faults generally showing left-lateral activity overprinted by either dip-slip or oblique right-lateral motion associated with reactivation during the Middle Pleistocene to Present- tectonic regime (Mazzoli et al., 2000, and references therein). The area including the southern Matese ridge and adjoining valleys is affected by several extensional faults showing geomorphological-stratigraphical evidence of activity during the late Quaternary. These include a dense net of minor (few km long) faults, mostly with NW-SE, E-W and N-S trends, and some major (tens of km long) extensional fault zones with overall WNW-ESE trends (Cinque et al., 2000). The main one of the latter structures bounds towards the NE the middle Volturno river basin (location in Fig. 1b), in which the top of the carbonates is lowered below the sea level (Corniello and Russo, 1990). A further main structure is the ≈ 20 km long Pozzilli–Capriati (Brancaccio et al., 1997), or Aquae Iuliae fault (Galli and Naso, 2009). This fault zone recorded repeated surface ruptures during the late Holocene, with the last one being associated with the Mw ≈ 6.7, 1349 earthquake (Galli and Naso, 2009). To the seismic shaking of this event has been associated the formation of karst collapse sinkholes in the Telese area (Del Prete et al., 2010). Indeed, the Matese ridge southern boundary is characterised by several karst collapse sinkholes, particularly clustered around Telese and Pratella (location in Fig. 1b). Such phenomena are interpreted as the response to intense dissolution associated with the rise of deep-seated fluids along active faults (Santo et al., 2011).