2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 205-13
Presentation Time: 12:00 PM


PUTNAM, Roger L., Department of Earth Science, Columbia College, Sonora, CA 95370, STOCK, Greg M., National Park Service, Yosemite National Park, El Portal, CA 95318, MATASCI, Battista, University of Lausanne, Center for Research on Terrestrial Environnement, Lausanne, 1000-1018, Switzerland and GLAZNER, Allen F., Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, roger.putnam.3@gmail.com

El Capitan in Yosemite Valley, California is a 1 km-tall x 1.5 km-long cliff that displays the interior of the Sierra Nevada Batholith. This near-perfect exposure provides an excellent opportunity to study the emplacement and subsequent weathering of granitic rocks. Detailed mapping of the spatial extents of the different rocks used to be hindered by the challenge of performing field mapping on a 3,000 foot-tall cliff. However, recent advances in high-resolution photography and terrestrial LiDAR scanning technology allowed us to make the first comprehensive geologic map of El Capitan at decimeter scale. The timing and emplacement dynamics of the 7 rock types exposed on the face were evaluated using this unique, vertical geologic map.

Three dimensional mapping and remote sensing allowed us to test hypotheses about bedrock controls on the shape of El Capitan. Terrestrial LiDAR scans of El Capitan were color coded by slope-aspect in Coltop3D software by Terranum. Slope-aspect data, coupled with detailed understanding of the bedrock geology, allowed us to assess the fracture characteristics of the dominant rock types on El Capitan. Mafic rocks (e.g. the diorite of North America and the tonalite of the Gray Bands) have greater fracture density and tend to form smaller fractures than felsic rocks (e.g. the El Capitan and Taft Granites.) Since weaker rocks cannot accumulate large amounts of stress, they will often fracture easily; suggesting that the mafic rocks of El Capitan have lower rock-mass strength than their more silicic counterparts. Furthermore, the mafic rocks on El Capitan are often positioned at localized slope breaks. These results suggest that the distribution of granitic rock types on El Capitan may have a first-order control on its shape. This raises the possibility that subtle differences in the composition of granitic rocks affect their weathering characteristics and contribute to the geomorphic evolution of granitic landscapes.

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