The complex geology of Italy is marked by two orogens, the Alps to the north and the Apennines along the peninsula and Sicily. The Alps are due to the thrusting toward the west and northwest of the Adriatic plate over the European plate, while the Apennines have been generated by the subduction of the Adriatic plate toward the west. This area has been object of multidisciplinary studies and many aspects are still debated. This study aims to define geophysical models of the lithosphere-asthenosphere system in Italy and surrounding areas by using the non-linear inversion of surface-waves dispersion curves and the forward inversion of Bouguer anomalies. Applying the non-linear inversion method, known as Hedgehog, (Valyus et al., 1969; Valyus 1972; Knopoff, 1972), starting from the surface-wave dispersion curves, structural models (shear-wave velocity vs. depth) have been calculated in all the Italian region and surrounding areas. The dispersion curves database (Pontevivo and Panza, 2002, Pontevivo, 2003, Raykova, 2004), determined by the two stations method (Panza, 1976) or by FTAN (Frequency Time Analysis) to extract the fundamental mode of Rayleigh waves (Levshin et al., 1972; 1992), has been used to realize tomography maps (Ditmar and Yanovskaya, 1987 and Yanovskaya and Ditmar, 1990). The analyzed dispersion curves have periods in the range T=7s - T=150s which is appropriate to explore the Vs velocity structures down to about 350 km deep. A grid has been imposed on the tomographic maps, with dimensions comparable with the resolving power of the data, in our case 1° x 1°. The tomographic values on the knots have been averaged to define the local dispersion curves. For each cell the uppermost crustal structure (the upper 6-10 km) has been fixed according to independent data from literature, the deeper structure, below the inverted layers, has been fixed according to already published data (Du et al., 1998). Because of the well-known non-uniqueness of the inverse problem, a set of models fits the observational data, with an equivalent level of reliability. To choose one representative model for each cell, the LSO (Local Smoothing Optimization) method, which consists in finding, for each cell, the representative solution so that the lateral velocity gradient between neighbouring cells is minimized, has been developed. Using this method a 1D velocity model has been chosen for each cell. It is used as constrain to perform the forward inversion of Bouguer anomaly in Italy. The gravimetric observations have been provided by DIIAR of Politecnico of Milano. From these data we construct 20 anomaly profiles along the Tyrrhenian and Adriatic coastlines, perpendicular to the Apenninic chain and the Alps and perpendicular to the direction of the subduction in the Southern Tyrrhenian subduction zone. The seismic and gravimetric data analysis put in evidence: a continental crust present along the Alpine and Apennine chain and along Dinarides, where it has been observed a possible lithospheric doubling consistent with the presence of the Adria plate subduction; an oceanic crust in the southern Tyrrhenian basin, where the presence of a partially molten mantle (mantle wedge) has been observed below the Moho, in correspondence of the huge volcanic bodies in the basin. The presence of mantle wedge, a possible deep magma source, has been detected also in correspondence of Eolie islands, Vesuvio, Phlegraean Fields, Stromboli and the recent volcanoes of Tuscany, Latium and Sardinia. A possible lithospheric doubling or presence of a shallow layer of consolidated magma, consistent with gravimetric models, has been observed in correspondence of Ischia. Well marked is the subduction of the Adriatic-Ionian lithosphere, the lid present below the Moho along the eastern Adriatic coast, dips moving through the Italian coasts. Here, the presence of a soft mantle over the lid is consistent with the concept of mantle wedge lying on a subducting plate. Similar structures can be seen also along the Apenninic subduction. Along the arc from Puglia to the eastern Calabrian coasts it has been observed a relatively slow layer within the lithosphere consistent with the presence of serpentinized peridotite, which could results from the Jurassic extensional phase.
Lithosphere-asthenosphere System in Italy and Surrounding Areas: Optimized Non-liner Inversion of Surface-wave Dispersion Curves and Modelling of Gravity Bouguer Anomalies
Biancamaria Farina
2007-01-01
Abstract
The complex geology of Italy is marked by two orogens, the Alps to the north and the Apennines along the peninsula and Sicily. The Alps are due to the thrusting toward the west and northwest of the Adriatic plate over the European plate, while the Apennines have been generated by the subduction of the Adriatic plate toward the west. This area has been object of multidisciplinary studies and many aspects are still debated. This study aims to define geophysical models of the lithosphere-asthenosphere system in Italy and surrounding areas by using the non-linear inversion of surface-waves dispersion curves and the forward inversion of Bouguer anomalies. Applying the non-linear inversion method, known as Hedgehog, (Valyus et al., 1969; Valyus 1972; Knopoff, 1972), starting from the surface-wave dispersion curves, structural models (shear-wave velocity vs. depth) have been calculated in all the Italian region and surrounding areas. The dispersion curves database (Pontevivo and Panza, 2002, Pontevivo, 2003, Raykova, 2004), determined by the two stations method (Panza, 1976) or by FTAN (Frequency Time Analysis) to extract the fundamental mode of Rayleigh waves (Levshin et al., 1972; 1992), has been used to realize tomography maps (Ditmar and Yanovskaya, 1987 and Yanovskaya and Ditmar, 1990). The analyzed dispersion curves have periods in the range T=7s - T=150s which is appropriate to explore the Vs velocity structures down to about 350 km deep. A grid has been imposed on the tomographic maps, with dimensions comparable with the resolving power of the data, in our case 1° x 1°. The tomographic values on the knots have been averaged to define the local dispersion curves. For each cell the uppermost crustal structure (the upper 6-10 km) has been fixed according to independent data from literature, the deeper structure, below the inverted layers, has been fixed according to already published data (Du et al., 1998). Because of the well-known non-uniqueness of the inverse problem, a set of models fits the observational data, with an equivalent level of reliability. To choose one representative model for each cell, the LSO (Local Smoothing Optimization) method, which consists in finding, for each cell, the representative solution so that the lateral velocity gradient between neighbouring cells is minimized, has been developed. Using this method a 1D velocity model has been chosen for each cell. It is used as constrain to perform the forward inversion of Bouguer anomaly in Italy. The gravimetric observations have been provided by DIIAR of Politecnico of Milano. From these data we construct 20 anomaly profiles along the Tyrrhenian and Adriatic coastlines, perpendicular to the Apenninic chain and the Alps and perpendicular to the direction of the subduction in the Southern Tyrrhenian subduction zone. The seismic and gravimetric data analysis put in evidence: a continental crust present along the Alpine and Apennine chain and along Dinarides, where it has been observed a possible lithospheric doubling consistent with the presence of the Adria plate subduction; an oceanic crust in the southern Tyrrhenian basin, where the presence of a partially molten mantle (mantle wedge) has been observed below the Moho, in correspondence of the huge volcanic bodies in the basin. The presence of mantle wedge, a possible deep magma source, has been detected also in correspondence of Eolie islands, Vesuvio, Phlegraean Fields, Stromboli and the recent volcanoes of Tuscany, Latium and Sardinia. A possible lithospheric doubling or presence of a shallow layer of consolidated magma, consistent with gravimetric models, has been observed in correspondence of Ischia. Well marked is the subduction of the Adriatic-Ionian lithosphere, the lid present below the Moho along the eastern Adriatic coast, dips moving through the Italian coasts. Here, the presence of a soft mantle over the lid is consistent with the concept of mantle wedge lying on a subducting plate. Similar structures can be seen also along the Apenninic subduction. Along the arc from Puglia to the eastern Calabrian coasts it has been observed a relatively slow layer within the lithosphere consistent with the presence of serpentinized peridotite, which could results from the Jurassic extensional phase.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
