GNGTS 2017 - 36° Convegno Nazionale

116 GNGTS 2017 S essione 1.2 Only in two cases the rivers could flow along their original course, in correspondence of the two (already mentioned) NNE-SSW valleys crossing the fold (blue river segments in Figs. 1D and 2A). It is worth noting that these valleys are located exactly in correspondence of the abrupt change in the attitude of the folded strata (e.g. Fig. 2A). We integrated field observation with the interpretation of seismic reflection sections, with the purpose of reconstructing the general geometry and evolution of the TF. As shown in Fig. 3, the backlimb of the TF is affected by three main backthrusts (faults B1, B2 and B3 in Fig. 3). In regard to the fold’s forelimb, at its foot, a main north-dipping thrust has propagated very close to the surface, based on the results of seismic sections (Fig. 3A). Such fault (F1 in Fig. 3) belongs to a fan of reverse low-angle thrusts (comprising F2 and F3 in Fig. 3), as shown in the interpreted seismic section Nr. 3 (see Fig. 2A for location) and in the 3D model (Fig. 3B-E). Lower faults (F2 and F3) cut through a tight syncline and then gradually fade out southward; on the contrary, the shallower fault (F1) seems to crop out at the surface. Through field observation we noticed that, in correspondence of the possible emersion of F1, there is morphological evidence of an almost NW-striking morphological scarp, as high as 7.5 m, measured in the field by tape and GPS profiles (e.g. Fig. 2E-F). This scarp is 13-km-long (dashed line in Fig. 2A) and may represent the surface fault rupture trace of an active main reverse fault, here named Tsaishi fault, or may represent a fold scarp linked with a very shallow reverse fault. Furthermore, the area immediately north of the inferred scarp is affected by a number of small water streams characterized by entrenching due to intense erosion (e.g. Fig. 2D); the gully in Fig. 2D is 2 meters lower than the surrounding topography. Once the creeks have flowed through the scarp, they suddenly changed their morphology to a Fig. 3 - A) Interpreted time-migrated seismic reflection sections number 3 (located in Fig. 2A). B-E) 3D reconstruction of the TF, based on seismic reflection sections and field data. Abbreviations: F1, F2 and F3: south-vergent thrust faults; B1, B2 and B3: northvergent backthrusts; UD and LD: upper and lower detachments; T: Tertiary; Cr: Cretaceous; J: Jurassic. Geographic coordinates are shown in UTMWGS84 projection.