The role of permafrost in large rock slope failures in the early Holocene - An analysis of the Köfels rock slide (3.1 billion m³)

Some of the largest rock slope failures in the European Alps today occurred with a significant lag time of several thousand years after glaciers retreated from the Last Glacial Maximum. Amongst other processes, the inert permafrost evolution can partly explain the delayed failure timing. Here, we challenge this hypothesis using the Köfels rock slide (Austria, 3.1 km³ failure volume, 9527–9498 cal BP) by retracing thermal conditions and reassessing the failure mechanics of one of the most prominent crystalline rock slides in the Alps. Our coupled simulations of ice-permafrost temperatures suggest that permafrost was maintained at the upper slope parts, mainly above glacier surface elevations, throughout the past 40 ka. Yet in the period before the rock slope failure, permafrost was preserved only at the highest elevation. It was lost within the hanging valley behind the failed mountain flank. This likely led to hydrogeological recoupling, enhancing groundwater flow towards stability-relevant zones. In addition, we deploy new rock mechanical tests to assess the rock bridge shear strength of the local orthogneiss and run mechanical simulations in order to decipher and better discuss the complex promoting factors of the Köfels rock slide (currently in process). Large permafrost rock slope failures are not only attributed to the loss of cohesive rock and ice material strength by warming temperatures, rather than to the interwoven hydro-thermo-mechanical system adaptation resulting from permafrost degradation.