FAULT-TIP DAMAGE ZONE DEVELOPMENT DURING NORMAL FAULT PROPAGATION WITHIN THE SEVIER NORMAL FAULT ZONE, SOUTHERN UTAH

As normal faults propagate, the rock around the fault plane becomes affected by strain and creates an envelope of intense fracturing known as a damage zone. To determine the relationship between damage zone width and fault displacement, we focus on the Sevier fault zone in southwestern Utah, where there is very good exposure of damage zones in a variety of structural settings. We document damage zone fracture networks in the hanging wall and footwall in close proximity to a fault tip in order to determine if damage zones grow proportionally to the length and displacement of a fault, or if the width increases quickly before the majority of the displacement occurs.

Data was gathered via ground-based scanline surveys in the field, documenting fracture orientation and position. Video of inaccessible outcrops using unmanned aerial vehicle (UAV) flights and Structure from Motion (SfM) modeling software was used to generate high resolution, georeferenced 3D models of the outcrops. We use this data to analyze fracture orientation, spacing, and intensity.

The data reveals that fault damage zone development is asymmetric within the Navajo Sandstone, with a wider damage zone in the hanging wall relative to the footwall of the same lithology. The data also reveals a higher intensity of fracturing within the hanging wall relative to the footwall, as well as a much higher intensity relative to fracturing associated with the transfer zone between this segment and a nearby synthetic fault segment. The fractures within the damage zone of the hanging wall and the transfer zone display greater clustering than the fractures within the footwall damage zone. However, the fracture intensity of the tip zone is very similar to the fracture intensity of an area nearby that displays much greater displacement, suggesting that the majority of damage zone width and fracturing occurs early on in fault propagation.

Our study suggests that faults that do not display much displacement also have considerable fracturing, and can be investigated for further research. Because fracture networks increase permeability in rock and help control subsurface flow, investigations like this have implications for groundwater flow rates, hydrocarbon migration, ore mineralization, and geothermal energy production potential.