FAULT DAMAGE ZONE DEVELOPMENT ON THE SEVIER NORMAL FAULT SYSTEM, UTAH

Understanding the development and geometry of fault damage zones is important because these fracture networks control subsurface fluid flow and influence landscape evolution. We examine the dimensions of and fracture intensity within damage zones in the Jurassic Navajo Sandstone associated with the Orderville Salient of the Sevier normal fault system in southern Utah. The Orderville Salient is a zone of complex fault linkage where well-exposed damage zones formed in a variety of structural settings. We surveyed damage zone fracture networks in the hanging wall and footwall of isolated fault segments as well as in fault blocks between segments. We gathered data via ground-based scanline surveys as well as using Structure from Motion (SfM) software to generate 3D models of the ground surface from imagery captured by unmanned aerial vehicle (UAV) flights. We used both scanline and 3D model data to analyze fracture orientation, spacing and intensity. In our comparisons of footwall damage zone widths within the Navajo Sandstone, we found widths ranging from ~34m to 44m across a variety of displacements, suggesting that the width of the damage zone remains relatively constant as displacement increases. Our data show that there is asymmetry in the dominant fracture orientation across the fault – with the dominant fracture set striking ESE in the hanging wall and SSW in the footwall – as well as asymmetry in damage zone width, with the hanging wall damage zone being ~2.5 times wider than in the footwall. We compare our findings of cross-fault asymmetry to strain modeling of the Sevier Fault to understand the causes of these asymmetries. We also find topographic expression of inner (~5m wide) and outer (~40m wide) subzones of the damage zone. This work has implications for fields including groundwater, geothermal energy, and oil and gas production, as fracture intensity and orientation are a control on the movement of subsurface fluids. Our work can also be applied to understanding the impacts of fault dynamics on landscape evolution.