Seasonally intermittent water flow through deep fractures in an Alpine Rock Ridge: Gemsstock, Central Swiss Alps

- Geological structure of a rock ridge is obtained with seismic refraction tomography.
- Rock temperature is measured in a horizontal borehole intersecting steep joints.
- Summer precipitation flows into rock joints, inducing short-term thermal anomalies.
- Snow melt water does not flow into rock joints in spring due to a basal ice layer.
- There is no hydrostatic forcing due to a lack of permanent ice plugs in rock joints.

Geological investigations and seismic refraction tomography reveal a series of 70° steep, parallel and continuous fractures at 2950 m asl within the Gemsstock rock ridge (Central Swiss Alps), at the lower fringe of alpine permafrost. Temperature measurements in a 40 m horizontal borehole through the base of the ridge show that whilst conductive heat transfer dominates within the rock mass, brief negative and positive temperature anomalies are registered in summer. These have very small amplitudes and coincide with summer rainfall events lasting longer than 12 h. In contrast, a complete lack of anomalous thermal signals during spring snowmelt suggests that runoff does not penetrate the open joints, despite high snow water equivalents of around 400 mm. This is attributed to the development of an approximately 20 cm thick, continuous and impermeable basal ice layer which forms at the interface between the snow cover and the cold rock on the shady North facing rock wall during snowmelt. Spring snowmelt water therefore does not affect rock temperatures in the centre of the rock mass, despite the presence of deep open joints. The mechanical impact of snowmelt infiltration on rock wall stability at depth is thus assumed to be negligible at this site.