STRUCTURAL AND LITHOLOGIC CONTROLS ON FRACTURE DEVELOPMENT WITHIN A SEGMENTED NORMAL FAULT ZONE, SOUTHERN UTAH

Fractures form within a segmented fault system in response to both regional and evolving local stresses as segments propagate and interact. By documenting the variability of fracture intensities and geometries proximal to faults, we can better predict how fault segments in the subsurface might propagate and affect local stresses and resulting fracture networks. We focus on the segmented Sevier Fault zone, which lies within the transition between the Basin and Range Province and the Colorado Plateau. The fault zone is well exposed adjacent to the Orderville geometric bend (OGB), where three major fault segments interact near Orderville, UT. The Jurassic Navajo sandstone, which consists of a lower red oxidized zone and an upper bleached white zone, provides most fracture data, thus reducing variability in fracture characteristics caused by lithology. Using Google Earth satellite imagery at a standard eye elevation and ArcGIS analysis, we traced more than 1000 fractures and analyzed them by structural position.

Our analysis revealed changes in fracture network characteristics across the OGB, including: 1) steeply-dipping fracture sets in transition zones display strikes subparallel to the NNE-striking fault segments; 2) fracture sets in some segment footwalls display strikes that deviate ~30 degrees from fault strike; and 3) several locations in segment footwalls display fractures that bend as they approach fault segments. The abrupt changes in fracture set orientations across the OGB may be related to the “open” or “closed” nature of fault segments, which affects the local stress fields by either permitting or preventing stress-field communication across fault segments. We also establish that fracture intensity increases up-section from the lower oxidized zone into the bleached zone, suggesting that changes in cementation may impact elastic moduli and therefore fracture intensity. Our results suggest that as a segmented fault system evolves, fracture network characteristics in transition zones between segments may be more predictable than in footwalls on the margins of the system. Additionally, because fracture networks strongly affect fluid flow in subsurface systems, we can use these results to better predict elevated hydraulic conductivity and flow-rate anisotropy in structurally similar systems.