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 10Be 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 five sites in Tenaya Canyon in Yosemite National Park, California, USA. Most sites yield 10Be concentrations that can be best explained as reflecting 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 specific tectonic history of the Sierra Nevada, governs the pace of glacial erosion in Yosemite. © 2010 Geological Society of America.