OF COMPLEX INTERACTIONS OF LITHOLOGIC, STRUCTURAL AND TOPOGRAPHIC BOUNDARY CONDITIONS, CLIMATE CHANGE AND ACTIVE TECTONICS ROCK AVALANCHING IN THE NW ARGENTINE ANDES AS A RESULT

In NW Argentina rock avalanching occurs in two geomorphic settings: A) narrow valleys draining large basins and B) mountain fronts bordered by wide piedmont areas. In the narrow valley environment, the deposits are relatively young. Landslide events concentrate during humid climate periods, and they occur with recurrence intervals of a few ka. In contrast, the deposits in piedmont settings are significantly older, while rock avalanches occur with recurrence intervals of several tens ka, and do not show any direct relation with climate change. Common to the regional distribution in both settings is the influence of lithology, structural control, as well as tectonic activity. Rock avalanches have occurred only in granites, low-grade metamorphic rocks and coarse clastic sedimentary rocks. These lithologies are competent enough to form steep slopes and provide planar structures that dip towards the valley. Because all rock-avalanches originated in the hanging wall of reverse faults with important Neogene displacement causing mountain-front oversteepening, it is inferred that most collapses were tectonically conditioned and/or triggered. However, only at a few sites detailed sedimentologic studies of related sediments show unequivocally that strong seismic activity triggered landsliding. Geological evidence and comparison with empirical data suggest that these earthquakes have been either crustal and of magnitude > M 7 or very shallow and of a magnitude > M 5.5.