USING 3D COMPUTER MODELS TO TEST EN ECHELON NORMAL FAULT EVOLUTION

We studied the en echelon Sevier normal fault zone near Orderville, Utah, to develop a predictive model of subsidiary structure formation during the propagation of overlapping fault segments. The Sevier fault, one of the easternmost Basin and Range fault systems, is at the transition into the relatively stable Colorado Plateau. We chose a study site along the system where outcrop exposure is excellent, with a classic example of a breached relay ramp between overlapping fault segments. Previous research reveals that as normal fault segments propagate both laterally and vertically, they interact with each other through changes in the local stress field. These changes cause subsidiary structures such as joints and shear fractures to form in a range of orientations adjacent to and between the faults, which impacts permeability and fluid flow connectivity. By investigating the connection between fault segment interaction and subsidiary structure formation, we can gain a better understanding of similar systems in the subsurface.

We modeled the system using the 3D Fault Response Module within the Move2016 suite to investigate how two propagating fault segments affected the stress field and resulting subsidiary structures. We created models at different stages of lateral overlap and with different magnitudes of normal dip-slip fault displacement. For each model, we evaluated spatial variations in vertical displacement, strain volume dilation, stress in the East-West direction, and the mean stress state. Our model results reveal that changes in the local stress field and the resulting joint pattern strongly depends upon the magnitude of fault overlap. These results are consistent with our documentation of joints and fractures in field exposures. We also show that the magnitude of segment overlap impacts fracture dilation within the fault transfer zone, which has implications for the development of permeability in similar fault systems. We then developed a more complex 3D fault system to more accurately represent the real-world fault system observed near Orderville. Although our initial results are not conclusive, we plan to refine our modeled system to more completely build a transferable model that can be applied to similar, less well-exposed fault systems elsewhere.