Preexisting fractures and the formation of an iconic American landscape: Tuolumne Meadows, Yosemite National Park, USA

Abstract

Tuolumne Meadows, in Yosemite National Park (USA), is a large sub-alpine meadow in the Sierra Nevada Mountains. Immediately adjacent to Tuolumne Meadows-and underlain by the same bedrock lithology (Cathedral Peak Granodiorite)-are vertical rock faces that provide exceptional opportunities to climbers. While the presence of a broad meadow suggests bedrock erodibility, the vertical rock walls indicate bedrock durability. We propose that the Tuolumne Meadows's landscape is the result of variable glacial erosion due to the presence or absence of preexisting bedrock fractures. The meadows and valleys formed because of concentrated tabular fracture clusters-a distinctive and locally pervasive type of fracturing-that were particularly susceptible to glacial erosion. In contrast, the vertical rock walls consist of sparsely fractured bedrock that was originally bounded by zones of pervasive tabular fracture clusters. Glacial erosion preferentially removed the highly fractured rock, forming prominent ridges in the upland surrounding Tuolumne Meadows. The orientation and spacing of the tabular fracture clusters, relative to ice flow, has exerted a fundamental control on the geomorphology of the area. The erosional variability exhibited by a single lithology indicates that the degree of fracturing can be more important than the host lithology in controlling landscape evolution.

The role of the cryosphere in source-to-sink systems

“…In contrast to fluvial studies, where the importance of bedrock lithology in fluvial incision rates has been demonstrated (e.g., Stock and Montgomery, 1999), the explicit influence of rock type on glacial erosion is still little understood (Alley et al, 2003). Both chemical weathering rates (Anderson, 2005) and fracture intensity may also influence glacial erosion rates (Dühnforth et al, 2010;Becker et al, 2014). The degree to which the presence of subglacial meltwater and sediment at the bed affects both short-term and long-term glacial erosion is also still debated.…”

Glacial landscape evolution by subglacial quarrying: A multiscale computational approach

“…Observations demonstrate that rates of quarrying depend on fracture density and orientation [Dühnforth et al, 2010;Krabbendam and Bradwell, 2011;Hooyer et al, 2012;Becker et al, 2014]. Hooyer et al [2012] compared the orientation of quarried lee surfaces to the direction of basal sliding.…”

Bedrock fracture influences on geomorphic process and form across process domains and scales

“…Becker et al . () show that areas of densely fractured rock in Tuolumne Meadows, USA exhibit low, flat surfaces, in contrast to the more sparsely fractured rock that forms high relief cliff faces and domes. They attribute this contrast to the dominance of glacial quarrying in densely fractured regions versus abrasion in sparsely fractured regions.…”

“…Becker et al . () found that extremely densely fractured zones caused preferential glacial quarrying in Tuolumne Meadows, where topographic highs correspond to areas lacking bands of fractured rock and lows correspond to areas that exhibit these fractured zones (Figure b). This provides direct evidence for Molnar et al .…”

“…Arrows indicate flow direction. References: [1] Pelletier et al, ; [2] Tooth and McCarthy, ; [3] Becker et al, ; [4] Glasser and Ghiglione, ; [5] Rastas and Seppala, ; [6] Anderson, ; [7] Loye et al, ; [8] Butler and Walsh, ; [9] Aich and Gross, ; [10] Velázquez et al, ; [11] Ortega‐Becerril et al ., 2016; [12] Bryan, ; [13] Lamb and Dietrich, . [Colour figure can be viewed at wileyonlinelibrary.com]…”