GNGTS 2015 - Atti del 34° Convegno Nazionale

has involved the L’Aquila fault system. This decoupling and the consequent acceleration of the Gran Sasso wedge, has induced shear stress in the RMU units, causing the formation of a major fault system (Norcia-Colfiorito-Gualdo Tadino-Gubbio), which has been the source of major earthquakes. The roughly belt-parallel compression exerted by the RMU wedge causes shortening in the Padanian side of the Toscana-EmiliaApennines. One possible effect of this process is recognized in the Sillaro front (�� Fig. 1d), where the �� Umbria-Marche turbiditic units, outcropping in the Romagna Apennines, underthrust the Emilia Apennines, still covered by the Ligurian units. Another major consequence of belt-parallel shortening in the Emilian Apennines may be the roughly NNWward lateral escape of the Piacenza wedge (�� Fig. 1d). The decoupling between this wedge and the Ligurian Apennines is accommodated by sinistral transpressional motion at the Bedonia-Varzi fault system and by roughly N-S divergent motion at the Villarvernia-Varzi (VV) fault, both characterized by significant seismic activity �� (Viti et al. , 2015 and references therein)� �� . The kinematics proposed for the Piacenza wedge is compatible with the configuration of the Emilia folds, presumably formed by the roughly northward displacement of this indenter. The decoupling of the Piacenza wedge from the eastern portion of the Emilia Apennines could be accommodated by transpressional deformation �� at the Enza-Taro fault system, evidenced by various tectonic features (Viti et al. , 2015)� �� . It is worth noting that the Enza-Taro fault may be the surface projection of a discontinuity (Giudicarie) that affects the Adria domain �� (Fig. 1d). Belt-parallel shortening in the inner side of the northern Apennines. �� Considering the available evidence (Viti et al. , 2015 and references therein), we argue �� that the late Quaternary tectonic activity in this sector of the northern Apennines has been driven by belt-parallel compression, induced by the push of the inner sectors of the southern and central Apennines (Fig. 1). �� The effects of the proposed kinematic/dynamic context have probably been emphasized by the peculiar mechanical properties of the crust in the Roman magmatic body, that, being pervaded by magmatic intrusions, is presumably characterized by higher rigidity with respect to the surrounding orogenic structures and can thus more efficiently transmit the roughly northward push of the central and southern Apennines. In particular, we argue that since the upper Pleistocene theAlbano-Chianti-Rapolano-Cetona ridge has undergone significant belt-parallel shortening, which was mostly accommodated by uplift and some lateral shifts �� (Fig. 1c)� �� . In particular, the sinistral lateral displacement and uplift of the Rapolano ridge along the Ambra fault system, may have generated the relief now located between the Upper Valdarno and Chiana basins. The compressional interaction between the Chianti ridge and the Northern RMU wedge may have enhanced the relief that now separates the Pistoia-Firenze basin from the Upper Valdarno basin. The above hypothesis is consistent with the following observed features (Fig. 2): - Uplifted Pleistocene deposits have been recognized in the segments of the above ridge and in the adjacent basins, such as the Elsa-Pesa, Siena, Radicofani and Upper Valdarno� �� . �� This phenomenon has also been recognized at the easternmost margin of the Firenze basin. - �� During the late Pleistocene, the Arno river has undergone a drastic deviation, turning from a roughly southward path to the present one. This event could be an effect of the dextral displacement and uplift that the Rapolano ridge (Fig. 2) has undergone to accommodate the supposed longitudinal shortening. This hypothesis is also compatible with the location of the 1558 earthquake (M 5.8) that probably activated the sinistral Rapale fault in the Ambra valley. - The Cetona and Rapolano ridges are dissected by SW-NE sinistral transversal fault systems, such as the Sentino and Sarteano ones, where pull-apart mechanisms are recognized�. - Some Quaternary NE-SW trending sinistral transtensional fault systems are recognized in the basins located along the inner side of the Albano-Chianti, Rapolano and Cetona ridges, i.e. the Elsa-Pesa and Siena-Radicofani. For instance, the Poggibonsi lineament separates the Elsa-Pesa from the Siena basin. More to the south, the sinistral Arbia fault could represent the westward prosecution, through the Siena Basin, of the Ambra fault. 136 GNGTS 2015 S essione 1.2