Subsurface and surface data in the San Sepolcro, Casentino and Mugello basins (Northern Apennines of Italy): new constrains on the active tectonics of the area

The seismicity of the axial zone of the northern Apennines indicates that the present-day deformation is characterised by active extension oriented SW-NE occuring within the upper 12-15 km of the crust at extending rates of about 2.5 mm/yr. In the area comprised between Norcia to the SE and Pontremoli to the NW, several moderate magnitude events were recorded both in recent and historical times. The southern area (Norcia-S.Sepolcro) was struck by destructive earthquakes in recent times (Norcia 1979 , Gubbio 1984 , Umbria-Marche 1997-98), while the northern part is apparently less active with low present-day seismic activity but with several historical moderate magnitudes earthquakes. From a structural point of view, the moderate magnitude earthquakes of the southern area occur on SW-dipping normal faults which are antithetic to a low-angle normal fault interpreted as the regional detachment of the area (AltoTiberina fault) which recent microseismic surveys have shown to release microseismicity. In the northern Pontremoli area, an analogous low angle East-dipping detachment has been described on the basis of seismic reflection data and some authors have suggested the presence of such a low-angle detachment also in the area comprised between San Sepolcro to the South and Pontremoli to the North, bordering the Quaternary basins of Casentino, Mugello, Lunigiana and Garfagnana (Etrurian fault system). In this work we present seismic data in the San Sepolcro, Casentino and Mugello basins integrated with deep-wells, surface geological data, aerial photo interpretation and SRTM data analysis. The San Sepolcro basin is about 22 km long and its age is older than lower Pleistocene which is the age of the eldest otcropping deposits. Seismic data in the San Sepolcro area were calibrated with the Pieve S.Stefano well, and with the Crop03 seismic profile. The Pieve S.Stefano well shows at two extensional contacts: i) an shallowest contact is at about 1300 m of depth and superimposes the tuscan flysch (Cervarola) with the lower part of the Umbria-Marche carbonates (Calcare Massiccio) with an elision of about 1000 m. The deepest extensional contact (about 3000 m deep) can be supposed within the Evaporites which show a very small thickness (about 500m) with respect to their "common" thickness (about 1500) as known from other drillings of the region and seismic data. On the basis of our interpretation the San Sepolcro basin infillment is at least 1s (twt), corresponding to about 1000-1200 m and is bordered to the West by at two splays of the Altotiberina fault to the west bordering the Anghiari ridge. Towards the East the basin is borderd by a SW-dipping normal fault antithetic to the Altotiberina fault. The Casentino basin is 15 km long depression infilled with fluvio-lacustrine deposits of late Pliocene-early Pleistocene age and with a minimum thickness of about 60 m. On the basin flanks, five orders of Quaternary fluvial terraces have been recognised by previous authors. The seismic data in the Casentino basin allow us to trace two NE-dipping reflectors the westernmost of which corresponds to a steep topographic signature which we interpret as the surface expression of a NE-dipping normal fault. The seismic signal in correspondance with the fault is also characterised by a set SW-dipping reflectors which may indicate the fault growth. This fault is few km West of the basin border and should be a western splay of a basin-bounding fault. Its topographic signature and the basin shape and morphology let us suppose that this fault system is different in terms of offsets and geometry from the fault system which borders the San Sepolcro basin, the extensional structure being not as outstanding as the San Sepolcro one. Also the existance of a low-angle detachment below the extensional fault system is still under investigation. The Mugello basin is a 25 km long structure infilled with up to 600 m of late Pliocene - early Pleistocene fluvio-lacustrine deposits. Though the tectonic history of the basin is debated, the Quaternary deformation of the basin is SW-NE trending extension