AN ANALYSIS OF FRACTURE SYSTEMS NEAR THE JUNCTION OF SALT AND CACHE VALLEYS, EASTERN UTAH

Although the scaling and network properties of fracture systems are well documented, we have significantly less data constraining how these properties change during the progressive evolution of mechanically interacting fracture systems comprised of multiple fracture sets. The objective of this study was to determine how the characteristics of individual fracture sets contribute to the overall scaling behavior of an aggregate system of fractures that may have developed during different deformation episodes. We studied a 2 km² section of the Moab member of the Jurassic Curtis Formation, at the junction of Salt and Cache Valleys, near Arches National Park, Utah. The area is nearly 100% exposed, and contains an aggregate fracture network that consists of three easily recognized fracture subsets, each with a unique orientation. We used ArcGIS to digitize fractures of each subset in 25 cm/pixel resolution aerial photos, and then analyzed the length, abundance, and spatial distribution of each subset, as well as the aggregate system of fractures. Over 12,000 fractures were digitized at two observation scales. Although the spatial distribution of fracture abundance of the aggregate system is fairly homogeneous, fracture subsets were inhomogeneous and correlated to nearby faults. Each of the individual fracture subsets had a length distribution showing negative exponential behavior, whereas the aggregate system was best fit by a power-law distribution. These results suggest that while the aggregate system is scale invariant and lacking a characteristic length scale, the subsets each contain a characteristic length scale. Interestingly, the characteristic length scale systematically decreases for each new fracture subset. This may reflect the progressive decrease in size of fracture-bounded rock blocks during development of the aggregate fracture set, and implies that length scales may be used to interpret the relative timing of mechanically interacting fracture sets.