JOINT SPACING ANALYSES IN GRANITES OF THE SIERRA NEVADA BATHOLITH NEAR SILVER LAKE, CALIFORNIA

Mathematical characterization of joints continues to be important for many reasons due to their influence on structural stability and fluid flow, and their sheer prevalence in the upper crust. Among parameters of note, joint spacing—including fractal behavior—has attracted significant attention. The Sierra Nevada mountains of eastern California, comprised mostly of early-mid Mesozoic granites, provide recently glaciated and extensive exposures that aid quantitative characterization. Two dominant, sub-orthogonal joint sets were commonly sub-parallel to earlier structures, including micro-veins with bleached margins, thin and distributed shear zones, dikes, pegmatites, and a weak magmatic foliation. Unloading and surface processes appear to have reactivated joints in these earlier directions. Joints measured include these later phases and earlier ones formed at depth.

Preliminary spacing analyses were performed using Google Earth. Joint positions were recorded along a continuous traverse sub-perpendicular to a dominant, steeply dipping set. Ten more discontinuous traverses were completed in the field with a measuring tape at four primary sites.

Spacing distributions were largely log-normal while some were best approximated by a negative exponential function (x = spacing, y = frequency). Log-log plots (x = log spacing, y = log frequency) exhibit no linear trends and thus do not show fractal behavior. Rather, they are characterized by a concave down curve with two distinct, seemingly linear tails. Semi-log spacing plots (log spacing or log frequency) though show strong, consistent linear trends. Google Earth data roughly mirrors these results but with subtle differences likely due to a much higher sample size (N = 169) than the maximum obtained in the field (N = 95). However, a direct comparison between Google Earth and field data is not yet possible as different sites (though near each other) were used. Log-normal spacing distributions have been found by other researchers in other granites, vein systems, and layered beds. How different jointing episodes and sample size contribute to the overall and log-log distributions remains to be worked out but is of key interest.