Geophysical models of the Lesachriegel and Gradenbach deep-seated mass-movements (Schober range, Austria)

Two examples (Lesachriegel and Gradenbach) of deep-seated gravitational creep of slopes or saggings located in the crystalline rocks of the Schober mountain range, Austria, have been examined. The retreat of the Alpine glaciers after the last ice age, at 15,000 years BP is considered to have initiated these movements. Both mass-movements have had phases of accelerated movement during the last 50 years. The sagging masses and the escarpment areas have been delineated using existing geological and geomorphological information, as well as field observations and digital terrain models. Models of the basal planes have been derived from seismic investigations and boreholes. The areas and volumes of the creeping rock masses are 1.6 km2 and 0.06 km3 for Lesachriegel and 1.7 km2 and 0.12 km3 for Gradenbach. The pre-failure topography gives an idea of the slope topography before creep or sagging processes started. The surfaces have been modeled taking morphological and kinematic plausibility and the dilation of the rock masses into account. Several geometric and physical parameters (e.g. displacement of centre of gravity, initial and average sliding angle, porosity, potential energy release) have been derived from structural models. Total displacements of the centres of gravity and within flow channels have been calculated by applying a special form of the condition of continuity (mass conservation). From these results, Lesachriegel has been classified as a mature sagging and Gradenbach as an early stage sagging. At Lesachriegel, present creep rates and total displacements conform to the idea that glacial retreat after the ice age initiated creep processes. At Gradenbach, the data indicate a significant acceleration in recent times. It is assumed that either the development of deep gravitational creep after glacial retreat took a very long time (> 10,000 years) or the creep process stopped at an early stage and was reactivated more recently. Our present knowledge of the history of the Gradenbach mass-movement is not sufficient to identify a particular factor responsible for a reactivation of the creep process.