GNGTS 2015 - Atti del 34° Convegno Nazionale

holds in particular for the seismic period that roughly started in 1887 in the Firenze zone, but also involves other shorter phases in those zones (Viti et al. , 2015). The fact that the Upper Valdarno and Chiana basins are almost aseismic, while seismic activity in the inner belt mainly occurs within a relatively S-N narrow zone, corresponding to the Elsa-Pesa-Siena-Radicofani basins, might imply that the oroclinal bending of the Chianti- Rapolano-Cetona ridge tends to create a compressional regime at its outer margin (upper Valdarno and Chiana basins) and an extensional/transtensional regime at its inner boundary (Elsa-Pesa-Siena-Radicofani basins). Other considerations about the evidence of belt-parallel compression in the western part of Toscana and northern Lazio are reported by Viti et al. (2015). Velocity field in the Apennine belt by GPS data. The GPS observations obtained from several networks in the study area (involving 450 continuous stations operating over the period 2001-2014) have been considered in order to define the present horizontal velocity field in the central and northern Apennines. The methodology adopted for data processing is described by Cenni et al. (2012, 2013). The resulting velocity field with respect to an Eurasian frame is mapped in Fig. 3. This pattern confirms the major features of the long-term kinematics tentatively deduced by Quaternary deformations� �� ���������� ��� ���� ���� ��� ����� ���� �� ��� �������� ���� ����� ������ ��� , �� ���������� ��� ���� ���� ��� ����� ���� �� ��� �������� ���� ����� ������ in particular the fact that the outer part of the Apennine belt moves faster and with a greater eastward component, with respect to the inner belt. The relatively high density of GPS sites in the central and northernApennines has allowed us to tentatively recognize roughly homogeneous kinematic domains (���� ��� ��� ������� ���������� Fig. 3). The highest velocities (3-5 mm/yr) and a prevailing NE-ward orientation of vectors characterize the outermost belt (east of the blue line in Fig. 3), including the buried thrusts and folds under the Po valley, while the lowest velocities (<1.5 mm/yr), with NW-ward to northward orientation, are observed in the innermost belt (west of the green line in Fig. 3). These two sectors are separated by an axial zone, characterized by intermediate velocity values and average orientation similar to the one that occurs in the outer belt. In the Padanian zone lying north of the buried folds the amplitude of the velocity vectors decreases significantly. Conclusions. It is argued that the complex tectonic pattern presently recognized in the Northern Apennines has mainly been determined by belt-parallel shortening, which has developed at higher rates in the outer sector of the belt (RMU and TE wedges). ���� ����� Most major Fig. 3 – Tentative recognition of the boundaries (green and blue lines) between roughly homogeneous kinematic domains in the central and northern Apennines, based on the space geodetic (GPS) velocity field. Velocity vectors are referred to a fixed Eurasian frame (after Viti et al. , 2015). 138 GNGTS 2015 S essione 1.2