GNGTS 2018 - 37° Convegno Nazionale

196 GNGTS 2018 S essione 1.2 COMBINED GRAVITY AND MAGNETIC ANOMALIES MODELING OF CENTRAL ITALY P. Mancinelli 1 , M. Porreca 1 , C. Pauselli 1 , G. Minelli 1 , M.R. Barchi 1 , F. Speranza 2 1 Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Italy - Member of CRUST (Centro inteRUniversitario per l’analisi SismoTettonica tridimensionale con applicazioni territoriali) 2 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Introduction. In complex geodynamic areas characterized by active tectonics, the combined gravity and magnetic modeling can contribute to validate subsurface geological models, defining the geometries and the thickness of the geological formations involved into the seismicity. Moreover, when the modeling is constrained to well documented geological observations both at the surface and at depth (borehole and seismic reflection profiles), its interpretation is certainly more straightforward. In these cases, relationships between the mechanical properties of the crust and the distribution of the seismicity can be also addressed. The Umbria-Marche Apennines (e.g. Barchi et al. , 2001) represent a typical fold and thrust belt (Fig. 1) mainly formed during the Late Miocene and successively dissected by extensional tectonics, still active, as demonstrated by the 2016-2017 seismic sequence (e.g. Tinti et al. , 2016; Chiaraluce et al. , 2017). Methods. The subsurface geology of the investigated sector of the Apennines has been widely studied in the last three decades (e.g. Bally et al. , 1986; Lavecchia et al. , 1994; Barchi and Mirabella, 2009; Barchi, 2010; Bigi et al. , 2011 and references therein). These authors propose contrasting structural styles, characterized by either thin-skinned or thick-skinned tectonics involving the sedimentary cover – consisting of Miocene-to-Quaternary turbidites, Meso-Cenozoic pelagic calcareous sequence and Triassic evaporites – and the deeper basement respectively. In this work, we report the results obtained by the combined gravity and magnetic modeling of the region affected by the 2016-2017 seismic sequence, at the border between the Northern and the Central Apennines (Fig. 1). The magnetic data used in this work (Fig. 2a) are derived from the aeromagnetic map obtained by integrating previous datasets (Caratori Tontini et al. , 2004 and references therein). Across the study area, the magnitude of the anomaly is comprised between -15 and 50 nT, with a general eastward increasing trend. The Bouguer anomaly map (Fig. 2b) was calculated from ~50,000 original data points provided by the Italian oil company (eni) across Central Italy, using a reduction density of 2670 kg m -3 . The Bouguer gravity anomaly shows an eastward-decreasing trend from maximum values of ~10 mGal (10 -5 m sec -2 ) to minimum values of -70 mGal toward the eastern sector of the study area. Moreover, the modelling was constrained to the recent geologic model from Porreca et al. (2018) and to all the petrophysical properties (density and magnetic susceptibility) of the bodies involved in the modelling available from the literature. Results and conclusions. In the grav-mag modeling of the section 2 (Fig. 3), where the geometries of the top of the basement and of the overlying Meso-Cenozoic units were constrained to the reference geological model, a coherent fit both at long and short wavelengths, is obtained between the two models. The resulting best-fitting geometries for sections 1 and 3 are in agreement with the structural model of section 2. In fact, both magnetic and gravity calculated anomalies produced along these sections, fit the observed anomalies producing errors with magnitude similar to those obtained on the geologically-constrained section 2. In our opinion, these results further validate the initial geological model proposed by Porreca et al. (2018), which is consistently supported across the entire study area. The spatial distribution of the basement is coherent across the entire area, with the top of the