INVESTIGATION OF ROCK AVALANCHES IN HIGH MOUNTAINS

Catastrophic rock avalanches (sturzstroms) have been identified as major landforms in high mountains, yet little is known about their distribution, failure mechanisms and frequency. Yet, these large and mega-large rockslides/avalanches are one of the most hazardous of natural phenomena in high mountains releasing as much energy as volcanic eruptions. They are a major mechanism for overall erosion of the mountains and by damming valleys are responsible for subsequent catastrophic floods. We report geomorphological and sedimentological characteristics of rock avalanches from distinctive geomorphic environments within the arid Trans-Himalaya, Northwest India, Tien Shan of central Asia and the Southern Alps of New Zealand.

Rock avalanches are characterised by having long run out distances relative to source area. They have a plan form dominated by a lobate structure, often of a multiple nature, much influenced by the topography of the area surrounding the area of slope failure. They often have a surface dominated by a clast-supported mantle of large angular rock fragments that preserve the original lithostratigraphy of the source area. Besides these surprising morphological features, rock avalanches are characterised by a complex internal structure of isolated boulders supported within a matrix of powder that shows increased shattering or fragmentation with depth. The process of fragmentation contributes an internal dispersive force leading to extraordinary spread and runout. These features must all be taken into consideration in the development of mechanical models of sturzstrom formation and run out.

Rock avalanches are frequently associated with topographic amplification of ground acceleration in high magnitude earthquakes. Dating of such landforms may therefore provide information about high magnitude palaeoseismicity within the high mountains. Although samples from a sturzstrom in northwest India have been submitted for cosmogenic dating, results are not yet available. However, this rock avalanche post-dates recession moraines that have an OSL age estimate of 10.2+2.1 ka and predates the last expansion of valley glaciers since moraines of this stage overlie the rock avalanche debris.