OBSERVING FRACTURE PATTERNS AT 3 SCALES IN THE SIERRA NEVADA BATHOLITH, MOKELUMNE WILDERNESS, CALIFORNIA

The Sierra Nevada Batholith stretches almost 400 miles along the length of California and is well exposed within the Sierra Nevada Mountains. Our study area is located ≈ 20 miles southwest of Lake Tahoe in Mokelumne Wilderness, encompassing ≈ 72 mi², and was investigated in Google Earth, in field, and with drone imagery. In Google Earth, primary domains were determined on the basis of mega-lineaments. These mega-lineaments can be traced for a kilometer or more, and are identified by linear bands of vegetation. There were 3 distinct areas that possessed high density of these mega-lineaments, and individual sites for more detailed study were chosen to investigate congruency between mega-lineaments and intervening areas. Outcrop images from 22 sites (≈ 1,000 m across), were taken in Google Earth and traced in Adobe Illustrator. Orientations of traced joints were computed and the two dominant sets displayed were trending NE-SW and NW-SE. A consistent orthogonal set was visible with locally clear truncations, which were analyzed using histograms. There was not significant consistency in which direction was truncating versus truncated. Subsequent field work indicated this was in part due to exfoliation slab break-up that reactivated earlier structures. Field work consisted of 2 weeks of data collection from 6 more accessible sites. While the same preferred orientations seen in Google Earth were seen in the field, field data displayed more variable joint orientations. On the tops and upper slopes of ridges, an abundance of exfoliation fractures had modern joints mirroring older fabrics. Valley floors are thought to exhibit pre-exfoliation joint patterns that formed at depth. Field truncation data yielded consistent results that represented a clear age relationship between ENE-WSW and NNW-SSE, with the former displaying an older joint set. Drone imagery was taken at 5 sites and analysis is in progress. Initial results indicate the same truncation relationships as the field data. We believe that there may be a Google Earth ‘filter’ that favors the earlier formed and overall longer joints that developed at depth and possess shadowing, vegetation, and weathering features that delineate the individual fractures, at the expense of shorter and more irregular joints related to exfoliation and exfoliation slab break-up.