Contribution of deep-seated bedrock landslides to erosion of a glaciated basin in southern Alaska

Landslides represent a key component of catchment-scale denudation, though their relative contribution to the erosion of glaciated basins is not well known. Bedrock landslide contribution was investigated on the surface of one of eleven glaciers on a glaciated ridge in the Chugach-St Elias Range of southern Alaska, where the debris from four major landslides is easily distinguished from moraines and other supraglacial material. A series of aerial and satellite photos from 1972 to 2000 and field observations in 2001 and 2002 indicate that three of four landslides have fallen onto the surface of the glacier since about 1978. The landslides, which originated from the steeply dipping (60-70 degrees) bedrock walls, were deposited onto the glacier in the ablation zone and are currently being transported downstream supraglacially. Individual glacial valleys with topographic relief of similar to 400 m are cut into high-grade metamorphic rock characterized by a steep north-dipping foliation and fractured by numerous large joints. Measurements of landslide area and average thickness obtained from high-resolution survey data indicate a total landslide volume of similar to 2(.)3 x 105 m(3). This volume suggests a basin-averaged erosion rate from landslides of 0(.)48 mm a(-1). An overall basin-scale erosion rate of 0(.)7 to 1(.)7 mm a(-1) can be inferred, but depends on the percentage of the total-basin sediment yield contributed by supraglacial sources. A mean rockwall retreat rate of 6(.)7 mm a(-1) is calculated and is considerably higher than published rates, which range from 0(.)04 to 4(.)0 rum a(-1). Controls on landslide generation include seismicity, freeze-thaw processes, topography, rock strength, and debuttressing. It is likely all of these factors contribute to failure, although the primary controls for the landslides in this study are thought to be rock strength and topography. The absence of landslides on ten of the eleven glaciers on this ridge is attributed to landslide magnitude-frequency relationships and short temporal scale of this study. Large-volume bedrock landslides (> 100 000 m(3)) may have low frequency, occurring less than once in a 55-year time frame. Copyright (c) 2005 John Wiley & Sons, Ltd.