GNGTS 2017 - 36° Convegno Nazionale

114 GNGTS 2017 S essione 1.2 observations; ii) field geological-structural data; iii) instrumental and historical seismicity; and iv) seismic reflection sections. The studied fold is located in the Western Republic of Georgia, at the south-western tip of the Rioni Basin (RB) uplifted area, at the foothill of Greater Caucasus (Fig 1C). This area is characterized by ongoing mountain building processes, due to collision between the Arabian and Eurasian plates (Fig. 1B) since Tertiary times, as testified by GPS data and by diffuse compressional seismicity, with magnitudes as great as 7 (Tsereteli et al. , 2016); in particular, the M S 6.0, 1614 A.D. Tsaishi earthquake occurred just south of the anticline (Fig. 1C, Varazanashvili et al. , 2011). In accordance with the westward ongoing propagation of the closure of the Transcaucasian depression, the convergence rate between the Greater and the Lesser Caucasus ranges from 4 mm/yr in the RB to the west to 14 mm/yr in the Kura foreland to the east (Reilinger et al ., 2006). The RB and Kura basins (KB) formed as foreland basins during Oligocene-early Miocene times and were then involved into the orogenic fold and thrust belts (Alania et al. , 2016 and references therein). Field and geophysical data. Our field survey has been intended firstly to reconstructing the shallow structure of the TF and to find evidence of recent and still active uplifting, as well as folding and faulting processes. By way of DEM- and field-based geomorphological observations, we observed that the TF crops out for a total length of 29.4 km, taking into account that its NW part seems to be truncated (Fig. 1C and 2A). We also observed that the fold axis trends NW-SE in the western segment, whereas it trends E-W to WNW-ESE in the eastern segment (Fig. 2A). In the field, we collected a total of 106 measurements of strata attitude along the western segment, and 30 measurements along the eastern segment (Fig. 2A). Regarding the 10-km- long western TF, the northern flank (backlimb) shows dip directions between N00–138° and an average value (Av.) of N45°; dip angles range 5–60° with an Av. of 28°. The southern flank (forelimb) shows dip directions ranging N10–300° with anAv. of N193°; dip angles are between 5° and 89°, the Av. is 46° (Figs. 2). At two sites we observed the presence of overturned strata. This segment can be classified as an asymmetric anticline with a clear vergence to the SW and a NE-dipping axial surface (Schmidt’s stereogram in Fig. 2). In correspondence of the eastern TF, we observed strata attitude changes: dip directions mostly range N2–345° (N70° as Av.) along the northern flank (backlimb), and N119–211° (N179° as Av.) along the southern flank (forelimb) (Fig. 2A). Dip angles are between 5° and 60° (24° as Av.), and 8-60° (22° as Av.) respectively (Fig. 2A). At a general level, the TF can be described as an asymmetric anticline with a clear vergence to the SW and an axial surface dipping to the NE, as shown in the Schmidt’s stereogram in Fig. 2A; the field-reconstructed hinge line trends N147° along the NW part, and N125° along the SE part. More to the E, near the Khobi river valley, the hinge line trend changes to about E-W, whereas eastward there is another slight clockwise rotation of strata. In summary, for the eastern part of the TF, from west to east, the hinge line trends N87°, N94° and N110° (Fig. 2A). Regarding the time evolution of the TF, based on our field observation, the folding process most likely started at the onset of the middle Miocene (e.g. Fig 2B); however, previous authors suggest the possibility of earlier local uplift in the Oligocene. The folding process is presently active, giving rise to a south-vergent anticline, as shown by upwarped late Quaternary river deposits identified in the field. For example, near the NW termination of the TF, there is a series of river terraces of Holocene age, elongated in a NNE-SSW direction (Tibaldi et al. , 2017a). The surfaces of these terraces show a very shallow dip southward, consistent with the direction of river flow. However, approaching the TF, south of the town of Zugdidi, the dip of the uppermost (i.e. the locally older) terrace surface is northward; some outcrops also show the presence of beds of unconsolidated river conglomerates that dip 7–8° to the north (Fig. 2C), suggesting a very recent uphill tilting of the river deposits. In addition, by observing the plain of the RB near the TF, we noticed that rivers show clear diversions as they approach the TF (red river segments in Fig. 1D), whose growth represented an obstacle to their southward flow.