Dynamic disintegration processes accompanying transport of the Holocene Flims sturzstrom (Swiss Alps)

A sturzstrom is a large-scale landslide that evolves during transport into a rapid granular flow (rock avalanche) by a dynamic disintegration process. Numerous theoretical hypotheses have been proposed to explain the excessive mobility of these catastrophic mass movements, but the role of disintegration processes remains poorly understood. Exceptional outcrop conditions at the giant (12 km(3)) Flims sturzstrom (Swiss Alps) permit the analysis of disintegration processes that result from shearing during transport. In confined zones of the rock mass, shearing is concentrated along preferred surfaces (originating as bedding planes), which evolve into shear band layers. Differential velocities of slabs separated by these shear layers induce oblique fractures in the slabs by accommodation. Contacts between grains are maintained in the shear layers. In unconfined zones near the top and the lateral margins shearing generates dilatancy. The rock mass is affected by anisotropic dispersion without mixing, and disintegrated material is preferentially produced to the top due to inflation. The rock slide then evolves to a granular flow. The consequence of these processes is the formation of two distinct depositional facies in the sturzstrom. The structured facies exhibits a stratified aspect close to the base, reflecting confined slab-on-slab shearing motion. Towards the top and distally, it evolves into a chaotic facies with some isolated pockets of the structured facies. Reduction of confining forces on the moving sturzstrom allows transformation of the structured facies into the chaotic facies. This characteristic transition, observable in many sturzstrom deposits, seems to be necessary for the rock avalanche process to occur, and, offers insights into the long-runout characteristics of these catastrophic mass movements. (C) 2004 Elsevier B.V. All rights reserved.