GNGTS 2019 - Atti del 38° Convegno Nazionale

138 GNGTS 2019 S essione 1.2 Moho temperatures (>850°C) in theAlps andApennines chains, where the Moho reaches greater depths (>40 km). Such hot Moho underneath the Apennines is consistent with previous studies on the attenuation of Pn and Sn phases (i.e. Mele et al., 1996). Zones of anomalous Moho temperatures perfectly correlate with the well-known areas of recent volcanism (i.e. Tuscany). According to our results, the strongest thermal anomaly lies under the Southern Tyrrhenian Sea. Here the shallowMoho (10–15 kmdepth) exceeds 800°C, resulting in a peak geothermal gradient above 50°C km −1 . The moderate crustal thickness and high heat flow support the hypothesis of a relevant crustal thinning and extensive spreading (Cataldi et al. 1995), followed by basaltic volcanism in the abyssal plain in addition to the calcalkaline, subduction-related volcanism in the nearbyAeolianArc. As an alternative approach for obtaining crustal temperature, we exploit the remarkable seismic signature of the α - β transition of quartz (Diaferia and Cammarano, 2015) that is demonstrated to raise the V P / V S of the bulk rock by around 8%. In our trans- dimensional, joint-inversion of Rayleigh phase velocities and RF, such increase has been detected at nine stations at intra-crustal depths. This allows the estimates of the geothermal gradient, since the temperature of the α - β transition of quartz is known. The comparisons of the two methods shows coherent results (Fig. 2), suggesting that these can be used as viable methods for retrieving the crustal thermal structure, even in active areas. Conclusion. The thermal characterization of the Italian crust is of key importance for a better understanding of its current geological setting and evolution. However, it is challenging due to the scarcity of heat flow data, their uncertainty, and the strong underlying assumptions. We attempted two alternative approaches using thermodynamics and a combination of two distinct seismic data sets (RFs and Rayleigh phase velocities), jointly inverted in a Bayesian framework to retrieve the V S and V P / V S at depth. Our inversion has imaged strong and well-constrained intra- crustal discontinuities that are used interpreted as evidences of the α to β quartz transition, allowing the estimate of the local geothermal gradients. The resulting gradients are in agreement with the known geological contest and comparable to those obtained through thermodynamics. References Cataldi, R., Mongelli, F., Squarci, P., Taffi, L., Zito, G. & Calore, C.; 1995: Geothermal ranking of Italian territory , Geothermics, 1(24), 115–129. Connolly, J.A.D.; 2009: The geodynamic equation of state: what and how . Geochem. Geophys. Geosyst., 10(10). Diaferia, G., & Cammarano, F.; 2017: Seismic signature of the continental crust: What thermodynamics says. An example from the Italian peninsula . Tectonics, 36, 3192–3208. Mele, G., Rovelli, A., Seber, D. & Barazangi, M.; 1996: Lateral variations of Pn propagation in Italy: evidence for a high attenuation zone beneath the Apennines , Geophys. Res. Lett., 23(7), 709–712. Molinari, I., Verbeke, J., Boschi, L., Kissling, E. & Morelli, A.; 2015: Italian and Alpine three-dimensional crustal structure imaged by ambient-noise surface-wave dispersion , Geochem. Geophys. Geosyst., 16(12), 4405-4421. Rutter, E. H.; 1986: On the nomenclature of mode of failure transitions in rocks . Tectonophysics, 122(3-4), 381–387. Fig. 2 - Comparison between the geothermal gradients estimated from the α -β quartz transition and those obtained through thermodynamic modeling.