Bedrock fracture control of glacial erosion processes and rates

Abstract

The importance of rock type and tectonic history on rates of glacial erosion, and the relative roles of glacial quarrying and abrasion, are poorly understood. We use concentrations of cosmogenic 10 Be in glacial polish and measurements of bedrock fracture spacing to explore the relationship between erosion rates and rock fracturing at 23 sites along Tuolumne River valley and fi ve sites in Tenaya Canyon in Yosemite National Park, California, USA. Most sites yield 10 Be concentrations that can be best explained as refl ecting solely postglacial nuclide accumulation. Six sites, however, display anomalously high concentrations, implying incomplete removal of the pre-glacial nuclide inventory during the last glaciation; these require that erosion in the last glacial cycle was <2-3 m. These low-erosion sites occur preferentially in massive units of the Cathedral Peak Granodiorite and the El Capitan Granite. Fractures in high-erosion sites are more closely spaced than in low-erosion sites, with spacings that average 1.1 ± 0.03 m and 3.3 ± 0.1 m, respectively. Our data suggest that the distance between fractures in the rock, dictated in part by the original spacing in a particular pluton, and in part by the specifi c tectonic history of the Sierra Nevada, governs the pace of glacial erosion in Yosemite.

Ice flow models and glacial erosion over multiple glacial–interglacial cycles

“…where K is a constant that characterizes the erodibility of the subglacial material (Laitakari et al, 1985;MacGregor et al, 2009;Duhnforth et al, 2010) and l is another constant generally equal to 1 (Table 1). While a few recent studies have used more sophisticated rules for erosion (MacGregor et al, 2009;Iverson, 2012), this is the same rule as used in previous ICE-Cascade and other glacially influenced, landscape evolution models (e.g., Braun et al, 1999;Herman and Braun, 158 R. M. Headley and T. A. Ehlers: Ice flow models and glacial erosion over multiple glacial-interglacial cycles 2008; Kessler et al, 2008;Egholm et al, 2012b;Yanites and Ehlers, 2012).…”

“…In addition, sediment that is eroded then deposited is reincorporated into the model with the same erodibility as the original bedrock. A variety of research related to modeling and field data of different erosional systems has shown that the rock erodibility, whether this varies due to fracturing, rheology, or composition, can have a significant effect on the ultimate form of the landscape (Duhnforth et al, 2010;Ward et al, 2012).…”

Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion

“…Cuesta-type landscapes can be modified by glacial erosion, but the resultant 'hard-bed landform assemblage' has a very different geomorphology, commonly characterized by bedrock megagrooves (e.g. Zumberge, 1954;Krabbendam and Bradwell, 2011;Eyles, 2012 (Mathes, 1930;Dühnforth et al, 2010).…”

Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

“…Equally, numerous studies suggest a strong link between the presence of preexisting mechanical discontinuities (bedding and joints) and plucking (e.g., Gordon, 1981;Rastas and Seppälä, 1981;Rea, 1994;Glasser et al, 1998). Dühnforth et al, (2010) showed that within one rock type (granodiorite) that underwent valley glaciation in Yosemite, areas of close joint spacing experienced faster, deeper glacial erosion than areas of wide joint spacing; and they attributed this to enhanced plucking in areas of close joint spacing. Hardness and joint density are therefore expected to provide strong controls on the mode of erosion (i.e., abrasion vs. plucking).…”