Mountain landslides and moraines in the Chilean Andes

Abstract

Due to intense erosion in the Andean valleys, the terminal and marginal moraines of the last Pleistocene glaciation were preserved only under favorable morphological conditions. In the case of many barriers and valley steps in the Chilean Andes that were interpreted so far as moraines, they are not glacial deposits, but debris bodies of powerful prehistoric mountain landslides.

Against a glacial genesis and in favor of a mountain landslide interpretation of these debris bodies, we have their extraordinarily large thickness compared to the moraines of the neighboring valleys, the size of the individual blocks, their decompaction without loss of coherence, the absence of rounding of the material, as well as local reasons (especially the petrographic composition). An investigation of the distribution of prehistoric mountain landslides has also a practical significance. According to experience, mass movements occur with particular frequency precisely where mountain landslides had previously originated. In the largely uninhabited Chilean Andean valleys, the dangers of mountain landslides are not as great as in densely populated Andean mountain ranges, especially in Peru. Nevertheless, a potential hazard occurs as a consequence of mountain landslides, especially due to the damming and subsequent rupture of lakes and as a result of which the lower sectors of the valleys and even the Chilean intermediate depression may be flooded. Special attention should be paid to the region of the Laja River, where the Laja Lagoon is dammed by fills from the young Antuco volcano, which is located in the middle of the valley. Rupture lake sediments deposited here in the recent geological past are indicative that slow or rapid lake ruptures may also be repeated in the future.