Surface microscopic examination of quartz grains from rock avalanche basal facies

The Wenchuan earthquake, which occurred on 12 May 2008, triggered many rock avalanches that presented abundant fractured surface textures. The surface textures of the rock avalanche clasts bear abundant characteristic signatures that are significant for understanding the mechanical behavior of rock avalanche dynamic fragmentation. To study this mechanism of rock avalanche basal facies further, two large rock avalanches were investigated using microscopic surface texture examination of quartz grains sampled from their basal facies with a scanning electron microscope. After the morphological and statistical analyses of the surface textures on the quartz grains, the following features are noted: (i) surface textures on the quartz grains are all fresh, with obvious irregular shapes and very sharp edges; (ii) the display of brittle fractures, including conchoidal fractures, step fractures, concavities, and V depressions or marks, is very common; (iii) cracks or fissures are also visible on the quartz grains. The display of these features reveals that a brittle fracturing process occurred during the formation of these surface textures on the quartz grains, indicating the existence of a high-stress history suffered by the particles in the basal facies. Hence, by the mechanical analysis of the formation of these surface textures, the authors propose that both the overburden pressure and the self-excited vibration energized by the undulated slip surface may play dominant roles in the occurrence of the particle dynamic fragmentation in the basal facies.