Architecture, Microfabric and Formation Conditions of the Basal Contact Zone of the Flims Rock Avalanche (Switzerland)

Many theories about processes and conditions of rock avalanches lack field evidence, due to difficulties in monitoring such events and the rarity of accessible locations to study corresponding structures in bedrock outcrops. This study provides a detailed investigation of the basal contact zone (including the rupture/sliding surface) of the Flims rock avalanche at two sites (from the proximal and distal release area) in terms of architecture, microfabric, and formation conditions. In addition, we compare our findings with shallow seismotectonic fault zones and derive indications for processes that have occurred before, during, and after the failure of the Flims rock avalanche.

 

Field observations document the wide natural variability of the basal contact zone architecture within the rock avalanche source area. The studied contact zone was formed at about 500 m depth as a stepped or undulating structure, few centimeters to several meters thick. It consists of chaotic breccia and locally features an up to 10 cm thick mesocataclasite, granular fault injections, and striated pavements indicating highly localized shear deformation. The pavements represent the main rupture/sliding plane of the rock avalanche and occur either as a sharp boundary to the intact bedrock or as parallel planes within mesocataclasite. In the proximal area of the source zone, a gradual increase of grain comminution towards the rock avalanche basal rupture/sliding surface suggests that most deformation and movement within the rock avalanche was concentrated in this narrow zone. In the more distal area, the deformation and movement were distributed on both the basal rupture surface and internal shear zones.

 

Microstructural investigations of the contact zone reveal deformations older than the mesocataclasite and pavement, including mylonites and calcite veins related to the previous tectonic history, and an old healed breccia, possibly formed during pre-failure damage in this zone. The architecture of the rock adjacent to the rupture/sliding surface observed in this study shows similarities to observations from shallow seismotectonic fault zones and high-strain and high-speed shear experiments. The analogies help to understand processes that led to the formation of the rupture plane and its increased mobility: Observations of cataclasite at the basal rupture zone suggest that the movement of the rock mass first was slow (< 0.4 m/s) and crushed the rock near the basal rupture surface by constrained comminution, inducing a granular flow. An acceleration of the slip rate to over 1 m/s led to dynamic weakening and the development of a distinct rupture/sliding surface. With the formation of a thin rupture surface, several coupled processes (grain boundary sliding, frictional heating, and thermal decomposition) might have caused a further decrease of the frictional resistance on this plane, resulting in increased mobility of the rock avalanche in the source area. Evidence for these processes is given by the occurrence of rounded nano-grain structures on the pavement of the basal rupture surface, which are possible remains of thermal decarbonation. This decarbonation implies a very local temperature rise due to frictional heating (> 720 °C), less than 10 µm away from the rupture surface.