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The Western Sierras Pampeanas region in the San Juan Province is characterized by thickskinned deformation with approximately N-S trending ranges of average heights of m and a high frequency occurrence of seismic activity. Its location to the east of the mainly thinskinned tectonics of the Argentine Precordillera fold-and-thrust belt suggests that at 30°S, deformation is concentrated in a narrow zone involving these two morphostructural units. In this paper, we present new apatite (U-Th)/He results (AHe) across the northern part of the Sierra de Valle Fértil (around 30ºS) and analyze them in a framework of thermochronologic available datasets. We found Pliocene AHe results for Carboniferous and Triassic strata in the northern Sierra de Valle Fértil consistent with the hypothesis of recent cooling and inferred erosional denudation concentrated along the northern end of this mountain range. Our analysis shows that this northern region may have evolved under different conditions than the central part of the Sierra de Valle Fértil. Previous studies have observed AHe ages consistent with Permian through Cretaceous cooling, indicating the middle part of the Sierra de Valle Fértil remained near surface before the Pampean slab subduction flattening process. Those studies also obtained ~ 5 My cooling ages in the southern part of the Sierra de Valle Fértil, which are similar to our results in the northern end of the range. Taken together, these results suggest a pattern of young deformation in the northern and southern low elevation ends of the Sierra de Valle Fértil consistent with regions of high seismic activity, and Quaternary active faulting along the western-bounding thrust fault of the Sierra de Valle Fértil.

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Research for evaluation of geologic hazards involving earthquakes and volcanic eruptions in southern Argentina seems to have historically received little attention. Nevertheless, the relatively small work done indicates a Neogene tectonic architecture in the area with capability of generating potential hazardous earthquakes in a growing population region. Seismicity and some morphotectonic evidences of Quaternary activity of the Magallanes-Fagnano left-lateral fault system in the transform boundary between South America and Scotia plates, are analysed in this paper. This fault system is considered to be an important seismogenic source, responsible for large earthquakes that have occurred in southern Argentina. Some examples from the South and Austral Andean Volcanic Zones are also examined in order to show recent volcanic activity which also generated crustal seismicity. Preliminary hazard estimation clearly shows the presence of both potentially active volcanic centres in southern Patagonia that may also trigger seismicity and the high probability for large crustal earthquake generation. Figure 2. (a) Google Earth image of the central section of the Magallanes-Fagnano (MF) Fault. Lainez and Irigoyen rivers cross the fault scarp. Negro Lake is an example of a pull-apart basin produced by an 'en-échelon' strike-slip motion; (b) Google Earth image of the area east of Fagnano Lake showing the location of scarps and the narrow valley of Turbio River at its mouth. This figure is available in colour online at wileyonlinelibrary.com/journal/gj. L. PERUCCA ET AL.Figure 3. (a) Aerial oblique view to the fault trace (photo taken by Schiavini); (b) Google Earth image of the central portion of MF fault showing wind gaps and truncated meanders; (c) truncated alluvial plain and meander and aligned forest. This figure is available in colour online at wileyonlinelibrary.com/journal/gj. L. PERUCCA ET AL.

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The area studied includes a little-known portion on the Dry Andes of the San Juan Frontal Cordillera, Argentina, where the hydrological significance of glaciers and rock glaciers was earlier never studied. The surveyed sector includes Cerro El Potro (5,870 m ASL) and nearby mountain chains (28°S). The predominant landforms in these areas were shaped in a periglacial environment superimposed on an earlier glacial landscape. These regions comprise abundant rock glaciers, a noteworthy rock glacier zone in the world, of which little is known in South America. This work employs geomorphological mapping to analyze the distribution of active rock glaciers in relation to altitude, aspect and slope using optical remote sensing techniques with GIS. Statistical estimation techniques were used based on a Digital Elevation Model (DEM) and aerial photos and Spot images interpretation. The specific density of rock glaciers' estimation in the surveyed area (Argentine border) is 1.56% with corresponds to 38 rock glaciers with an area of 5.86 km 2 and 0.12 km 3 of water equivalent. Furthermore, the analytical results show that elevations [4,270 m ASL, a southeastfacing aspect, and slope between 2°and 40°favor the existence of rock glaciers, Finally, a comparison with glacier water equivalent, which covers an area of *16 km 2 and 0.9 km 3 of water equivalent, shows that glaciers are the main stores of water at 28°S (Cerro El Potro Glacier). However, the importance of rock glaciers as water reserves in this portion of Argentina should not be underestimated.