Lateglacial rock slope failures in north-west Ireland : age, causes and implications

Abstract

Nine postglacial quartzite rock slope failures (RSFs) in north-west Ireland were dated using cosmogenic 10Be. Weighted mean RSF ages range from 17.7 ± 0.9 to 12.5 ± 0.7 ka or 16.6 ± 0.7 to 11.7 ± 0.5 ka, depending on assumed 10Be production rate. All dated RSFs occurred within ∼5000 years following ice-sheet deglaciation at ∼17.4ka (∼16.3 ka) and all but two occurred within 2000 years after deglaciation. The timing of RSFs rules out glacial ‘debuttressing’, permafrost degradation and enhanced deglacial cleft-water pressures as triggers of failure in most cases. We infer that paraglacial stress release and associated fracture propagation were critical in reducing rock masses to critical stability, although earthquakes caused by Lateglacial glacio-isostatic rebound and/or release of stored tectonic stresses may have triggered failure in some or all cases. In conjunction with data from related studies, our results imply that most undated RSFs outside the limit of Younger Dryas glaciation in the British Isles are of Lateglacial age, and that numerous Lateglacial RSFs occurred inside these limits, with subsequent removal of debris by glaciers. They support the view that paraglacial RSF activity in tectonically stable intraplate terrains was concentrated within a few millennia following deglaciation.