VELOCITY STRUCTURE OF THE VULSINIAN VOLCANIC COMPLEX (LATIUM, ITALY) FROM SEISMIC REFRACTION DATA AND 3-DIMENSIONAL INVERSION OF TRAVEL-TIMES

We have interpreted three reversed seismic refraction profiles and three related fans in the Monti Vulsini Volcanic Complex, modelling travel times and amplitudes calculated using the asymptotic ray theory. The interpretation of the refraction lines revealed complex structure in the shallow crust (0-7 km), characterized by strong lateral heterogeneities. The three-layer seismic model of the volcanic area is characterized by relatively high P wave velocities, generally ranging from 4.0-4.5 km/s in the uppermost layer to 6.7-7.1 km/s at a depth of about 7 km. The upper layer (0.5-2.7 km thick) corresponds to the volcanic cover and the upper part of the flysch sequence. The depth to the top of the middle layer, corresponding to the lower part of the flysch unit and to the Meso-Cenozoic carbonate sequence, is very irregular and is strongly controlled by the tectonic evolution of the area. The thickness of this layer ranges between 2.2 and 5.0 km. The third layer extends beneath the whole Vulsinian region and is characterized by high P velocity (6.7-7.1 km/s) at relatively shallow depth (5-7 km). We interpret the high velocity observed in the third layer as being caused by mafic intrusive rocks, such as gabbros, or by high-grade metamorphic rocks, such as schists, granulites, or metatuffs. The results of the refraction modelling have been compared with a three-dimensional (3-D) P velocity model calculated by inverting travel time residuals from explosions and local earthquakes. Generally, close agreement is observed between refraction and inversion models. The fan profiles, as well as three unreversed refraction profiles and 3-D inversion, showed a velocity decrease, as low as 5%, within the deepest layer in the central part of the volcanic complex. The low-velocity zone in the central region could be related either to high temperatures and/or partial melt in the intrusive (or metamorphic) body, or to the deepening of the top of the third layer.