Rare, slow but impressive: > 43 ka of rockslide in river canyon incising crystalline rocks of the eastern Bohemian Massif

Abstract

Despite significant progress in understanding the stability of rock slopes, little is known about the time scales of the evolution of slow-moving rockslides. The Ledové sluje rockslide in the Thaya River canyon is a unique and infrequent slope failure developed in crystalline rocks of the Variscan orogen in Central Europe. Fresh topography with trenches, rock walls, slided blocks, scree slopes and crevice-type caves has attracted generations of geologists for more than a century, but questions of mechanism and age of the rockslide have remained unresolved. To address this question, we combined geomorphological research with detailed analysis of the geological structure, electrical resistivity profiling and terrestrial cosmogenic nuclide dating (TCN). Our data show that failure developed above the river undercut bank along a planar sliding surface predisposed by NW-oriented gently dipping metamorphic foliation intersected by steep fractures and faults. Although TCN dating does not allow determination of the entire life span of the rockslide, its scarp predisposed by NE- to ENE-striking fault was largely exposed in the Last Glacial during marine isotope stages (MIS) 3 and 2 between ~ 43 and 23 ka, suggesting slow gradual or multievent movement of the rockslide slope rather than a single catastrophic event. We conclude that, although very rare in Central Europe, rockslides in the crystalline rocks of the Palaeozoic orogens may leave a much longer topographic footprint than in the adjacent Alpine mountain belts.