HR: 15:10h
AN: G43D-07    [Abstracts]
TI: Use of Geodetic Laser Scanning to Evaluate the Curvature of Bedrock Surfaces in an Investigation of Sheeting Joint Formation
AU: * Martel, S J
EM: smartel@hawaii.edu
AF: Department of Geology & Geophysics, University of Hawaii 1680 East-West Road, Honolulu, HI 96822, United States
AU: Mitchell, K
EM: kellyjm@hawaii.edu
AF: Department of Geology & Geophysics, University of Hawaii 1680 East-West Road, Honolulu, HI 96822, United States
AB: We are using aerial and tripod-mounted geodetic laser scanning (GLS) data, together with photography and large-scale geologic mapping, to investigate the formation of sheeting joints in Yosemite National Park. Sheeting joints are opening-mode fractures that form subparallel to the topography, and over broad areas in Yosemite they define the bedrock surface. Rock slabs bounded by sheeting joints superficially resemble the layers of an onion. Our hypothesis is that sheeting joints form where a tensile stress normal to the topographic surface exists in the shallow subsurface. This condition is met where k₂ P₂₂ + k₃ P₃₃ > γ cosβ, where k₂ and k₃ are the principal curvatures of the bedrock surface, P₂₂ and P₃₃ are the corresponding normal stresses parallel to the principal stresses, γ is the unit weight of the rock, and β is the slope angle. Sheeting joints are predicted where at least one of the principal curvatures is sufficiently convex (negative) and the corresponding normal stress is sufficiently compressive (negative). We use aerial GLS data with a vertical resolution of ~10 cm and a point spacing of ~1 m to measure the slope and curvature of the bedrock surface at the scale of a ridge or valley. We use tripod-mounted GLS data with a point spacing of ~5 cm, large-scale geologic mapping, and photographs to detect steps between consecutive sheeting joints, with the step height giving the sheet joint spacing. Outcrops hosting sheeting joints have a stair-step appearance with a distinctive curvature signature: high convex curvature at the top of a step, and high concave curvature at the step bottom. Steps between sheeting joints with a spacing of less than a meter or so are difficult to detect using the aerial GLS data. Apparently the interpolation of aerial data onto a grid, necessary for our curvature codes, and the smoothing of gridded data to filter out trees compromises the value of the aerial GLS data in detecting the step edges, even though the vertical resolution of the GLS data should be quite adequate for measuring the step height. A curvature code that does not require gridded data could help detect step edges. The steps can be detected readily with the tripod-mounted GLS data, large-scale geologic mapping, and photographs, which show shadows cast by the steps. Our analyses to date with all the data sets supports our hypothesis for sheet joint formation.
DE: 1209 Tectonic deformation (6924)
DE: 1294 Instruments and techniques
DE: 1295 Integrations of techniques
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
SC: Geodesy [G]
MN: 2007 Fall Meeting