USING 3D GIS TO REVEAL EXTENSIONAL FAULT AND MEGABRECCIA GEOMETRY, WEST SIDE OF THE SHEEP RANGE, NEVADA

The west side of the Sheep Range in southern Nevada exposes a terrain about 5 km wide and 28 km long between the Quaternary alluvium and main structural block of the range. In the south adjacent to the Las Vegas Valley Shear Zone this consists of a series of very large, mostly coherent blocks of Paleozoic bedrock, which grade along strike into a 1500 m thick Tertiary basin with isolated, discrete megabreccia slides. Three dimensional GIS allows reconstruction of the geometry of the individual fault and megabreccia blocks. This work can be done in a platform like Google Earth, which requires no downloading of digital data and limited user training, and can be done anywhere in the world. Scanned maps, including many from the USGS National Geologic Data Base, can be overlaid. Users must understand map scale issues, the resolution of the digital elevation models, and issues with image seams, clouds, and color contrast in the Google Earth imagery. Moving the analysis partly or entirely to the desktop GIS allows higher resolution topographic data, and the choice of imagery. For the Sheep Range, USGS currently has 4 different sets of orthoimagery at 1 m or better pixel resolution. In the megabreccia regions with extreme slopes, this allows several looks with potentially different shadows. The Sheep Range does not yet have high resolution 3DEP coverage, but a rapidly increasing portion of the US has 1-3 m resolution topography, including parts of the Tin Mountain landslide. Previous solutions have automated the computation of three point problem, but the GIS can automatically compute a planar trend surface along each line segment, and a series of three point problems. This can highlight changes along strike, errors in contact tracing or digitization, and limitations of the input digital data. Digital investigation allows better visualization of this large area with rugged topography and limited road access, and shows multiple generations of sliding, different fault and block geometries, and the changes in structures along strike of the mountain front.