Tettonica pleistocenica tra il Monte Soratte e i Monti Cornicolani (Lazio)

Abstract

Systematic analyses of the tectonic structures of the Pliocene-Pleistocene units of the Tiber lower valley, between Mount Soratte and Cornicolani Mts. in central Italy, show a complex neotectonic framework. Mount Soratte is a horst, which developed on the culmination of a northeastern vergent thrust sequence. Northeastwards and southwards it is bordered by N50°-70°W striking normal faults, with reconstructed minimum compressive stress axis oriented N20°-40°E. On the Mt Soratte southwestern slope normal faults reutilize the ramps of thrust planes. This tectonic system was active from Lower Pliocene up to early Middle Pleistocene, and brought about the formation of NW-SE oriented basins which would be filled by a thick sedimentary sequence. On the south of Mt. Soratte, en échelon N-S striking discrete tectonic elements affected both sedimentary and volcanic deposits. Slickensides along fault planes show a sequence of right-lateral strike-slip events superimposed on a dip-slip extensional event. The maximum compressive stress axis of this strike-slip tectonic system is oriented N20°-N40°E. These discrete tectonic elements are 3 km long and slightly overlap each other with average separation of 4 km; the right-stepping en-échelon array forms a 15 km long shear zone. A well defined N20° right-lateral strike-slip fault ("Sabina fault", Alfonsi et al., 1991), active from post-Upper Miocene times, represents the continuation of this shear zone in the Apennines. On the SE of Mt. Soratte, in the Acque Albule area (Tivoli), a 30 km long dextral strike-slip shear zone affects the Upper Pleistocene deposits. This shear zone contains N-S striking strike-slip segments and N20° oblique divergent splays; their interaction brought forth well-defined releasing features (Acque Albule pull-apart basin). Strike-slip tectonic structures are there still active after 360 ka, and their connection with the Colli Albani volcanic evolution is still a matter of discussion. In both the areas (Acque Albule and Mt. Soratte), the strike-slip structures are marked by both the rising up of mineralized fluids and by travertine deposition. The evolution of the shear zones has been reconstructed on the basis of the geometric pattern of such structures. The comparison with sand-box models shows that the Mt. Soratte shear zone is less evolved then the Acque Albule one, in terms of horizontal displacement along the buried master fault. This conclusion, which is confirmed by the kinematiccharacters of tectonic elements, indicates that there is a transition from a strike-slip shear zone (Acque Albule) to a transtensional one (Mt. Soratte). Different geodynamic hypotheses could be proposed in order to explain the relation between strikeslip and extensional tectonics. Strike-slip elements might develop as transfer faults of areas undergoing different extension rates; strike-slip tectonics might be generated by an inversion of stress axes in Upper Pleistocene, the two shear zones, strictly connected with the "Sabina fault", have discontinuosly been active since post-Upper Miocene times: their regional elongation and their possible connection with volcanic features suggest that the strike-slip tectonic pattern is linked to the complex geodynamic evolution of the Latium Tyrrhenian margin.