TESTING MODELS OF EN ECHELON NORMAL FAULT EVOLUTION USING 3D COMPUTER MODELING

As normal fault systems develop, they often form a segmented, en echelon pattern. As the system evolves, fault segments interact with each other through changes in the local stress field. These changes may cause subsidiary structures such as joints to form in a range of orientations, and these structures impact permeability and fluid flow connectivity. By investigating the connection between fault segment interaction and subsidiary structure formation, we gain a better understanding of similar systems in the subsurface.

We studied the Sevier fault zone near Orderville, Utah, to document subsidiary structures related to a right-step in the fault zone. The Sevier fault is one of the easternmost Basin and Range fault systems at the transition into the relatively stable Colorado Plateau to the east. We chose this location because it has excellent outcrop exposure, with a classic example of a breached relay ramp between the overlapping (>6000 m overlap) fault segments. To investigate how the evolution of the Sevier fault segments affected the stress field and resulting subsidiary structures, we modeled the system using the Fault Response Module within the Move2016 suite, 3D computer modeling software. We created 3D models of an en echelon fault pair at different stages of overlap and with different magnitudes of normal dip-slip fault displacement. For each model, we evaluated spatial variations in vertical displacement, strain dilation, stress in the East-West and North-South directions, and coulomb stress state. Our model results reveal how the stress field around the fault transfer zone may have evolved over time creating the subsidiary structures we documented in field research. By analyzing these models, we found that changes in the stress field and the resulting joint pattern strongly depend on the magnitude of fault overlap. 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.