Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

2a). We mapped it in detail between the river located 800 mwest of the border to the east, and the FagnanoLake under which the rupture continues westward. In the western sector of the mapped segment, the relief rises steeply to the east, the fault intersecting a hill ~30 meters above lake level. In this sector, the northern block is at a higher elevation compared to the southern block of the fault, with a scarp reaching up to 20 meters in height. Conversely, along the gentle slope of the eastern side of the hill, the southern block is higher with a scarpwhose elevation varies between 2 and 5meters. The hill cut by the fault consists inbasal till, suggesting that the vertical offset recorded in these scarps corresponds toanaccumulated long-termverticaloffset, since theendof the lastglacialperiod. The fact that the apparent vertical displacement along the scarp depends on topography is compatible with a left-lateral kinematics of the fault. We excavated a trench and sampled tilted trees in this fault segment. The detailedmorphology of this fault segment, and the description of the place where we excavated the trench is described below. The second and third segments, located NW of the previously mentioned segment, were identified in areas near the lake. In both cases, the ruptures show positive reliefs in the northern block, with post-glacial scarps of ~1 m in height, and a straight trend with an azimuth of N95ºE. However, in both cases, it is difficult to observe their continuity towards the east. The fourth segment, extends onshore on approximately 500 meters with an E-W strike (Figure 2a). This segment is marked by a scarp with a higher northern block, with a post-glacial scarp height increasing approaching the lake. Close to the lake, the northern block is also deformed by gravity movements affecting glacial deposits. The last segment we identified is located in the western part of the study area, approximately 4 km west of Estancia Lago Fagnano. The N95ºE rupture extends for over 1 km on the southern slopes of Mount Hope, and develops in the recent cover, partially covering the basement rocks consisting of Upper Jurassic volcaniclastics rocks from the Tobifera Formation (Klepeis, 1994). Although the slope of the hill increases towards the north, it is interesting to note that, in some parts along this rupture, the southern block exhibits positive topography with respect to the northern block that may, again, result from left-lateral strike-slip motion. Locally, we observed open cracks parallel to the fault in basement rocks accommodating the gravitational collapse of part of the southern block towards the valley. In addition to the previously located ruptures, one relevant testimony to mention is that following the 1949 earthquake, it was reported that forest in the delta of the Betbeder River, south of Lake Fagnano, was affected by subsidence (Figure 2a). This report is another clue suggesting that the rupture from the 1949 earthquake extended, at least, to the western edge of Lake Fagnano, and accommodated the subsidence of the southern block. We conducted detailed mapping of the geometry and dimensions of fault-related structures observed in the easternmost part of the study area. From west to east (see Figure 2b): (zone a) In this area, a pop-up structure, measuring approximately 60 m in length, 30 m in width, raises the surface by 2.5-3 m relative to the base of the scarp. Within the pop-up, a fewmeters-wide fracture (sag-pond structure) with a N095°E strike lowers the surface. Because the pop-up elevates this area compared to its surroundings, it prevents the sag- pond from being filled with water, and we excavated a trench that crosses this depression. The trench was excavated in an area where the rupture occurs in semi- consolidated basal till. The dimensions of the excavated trench are 4 meters in length, with a maximum depth of 1.5meters below the surface. On both walls, we observe at least two faults which are filled with softer sediment constituting the recent soil, which have been sampled for 14 C dating. Detailed analysis of the trench is still ongoing, alongside awaiting geochronological results from 14 C dating and dendrochronological analyses being conducted on samples of tilted trees extracted from the same area. (zone b) About 90 m east from the trench, we identified an oblique Riedel-type fracture, extending approximately 15 m with a N75ºE strike (20-25º with respect to the fault strike). Subsequently, we observed another 50 m-long and ~25 m-wide pop- up structure, and reaching up to 2 m in height. In this pop-up too, a deep crack develops, corresponding to a sag-pond about 2 meters-wide and up to 40 m in length, which is filled with water due to its depth (Figure 3b). (zone c)This section is characterized by smaller pop-upstructures (10-20 meters in length and width and ~1-1.5 meters in height), some of them associated withRiedel fractures that do not exceed 15 meters inlength, with a N75-80ºE strike (15-25º with respect tothe fault strike). (zone d) In the easternmost part ofthe map, the rupture shows fewer structures. Weidentified an oblique fracture, approximately 20 m inlength with an azimuth of N115ºE, interrupting the main rupture. Moving eastward, a small-sized pop-up (10 m in length and 5 m in width) not exceeding 1 m in height. Finally, towards the east, the main rupture is linear with a constant azimuth of N100ºE.