Joint spacing in the Caples Lake granodiorite of the Sierra Nevada Batholith in Eldorado National Forest, California: A comparative analysis of joint sets and data resolution

Joints are the most common deformation structure in the Earth’s upper crust and exert a significant influence on structural stability, landscape morphology, and fluid flow . Therefore, a greater understanding of fracture parameters (e.g., length, aperture, etc.) allows us to more accurately predict their presence, persistence, and prevalence, in the subsurface . We study the fracture spacing of two sub-orthogonal joint sets—66 NE-246 SW and 330 NW-150 SE—in the Caples Lake granodiorite of the Sierra Nevada Batholith, California. Specifically, we investigate 1) their spacing distributions with a keen interest in power-law (fractal) spacing, 2) distribution comparisons between master and cross joints, and 3) the usability of Google Earth datasets in joint spacing analyses. Spacing was calculated from position data obtained in the field and on Google Earth along one-dimensional traverses orthogonal to the mean joint strike of a set , with a target sample size of 100 for a stable fractal dimension. We tested fractal behavior through log- log cumulative frequency vs. spacing plots , determined the spacing distribution with the Chi- squared (χ2) goodness-of-fit test, and compared distributions with the Kolmogorov-Smirnov statistic and the Coefficient of Variation . All four datasets exhibit non-fractal behavior and can instead be better described by lognormal or gamma distributions. This may be the result of sampling biases such as truncation or censoring , which can be possibly overcome with greater sample sizes and extending our lower limit of measurement an order of magnitude into the millimeters. Master and cross joints have slightly different distributions as expected from joints of different age; however, this relationship is still unclear and should be further explored with a greater sample size and less opportunistic sampling scheme that encourages shorter traverses further upslope on an outcrop. Google Earth datasets were significantly inadequate for joint spacing analyses. As expected, they routinely underestimate smaller spacings and as a result generate larger, artificial spacings, and distributions of shifted form and position. Within fracture analysis, Google Earth should remain a tool for field site reconnaissance and mapping only