EVOLUTION OF FRACTURE NETWORKS WITHIN A NORMAL FAULT TRANSFER ZONE: A CASE STUDY FROM THE SEVIER FAULT SYSTEM, SOUTHERN UTAH

As normal faults develop, they commonly initiate as discontinuous, segmented systems, with an en echelon pattern in map view. As strain accumulates, segments propagate and interact through changes in the local stress field, causing fractures in adjacent rocks to form in a range of orientations. Because fractures impact fluid flow in subsurface systems, we investigated the connection between fault segment interaction and subsidiary structure formation along a transfer zone between segments of the Sevier fault zone, near the town of Orderville, in southern Utah. We focus on a well-documented right step in the west-dipping fault, where spectacular lateral and vertical exposures of the Jurassic Navajo sandstone provide insights about the evolution of similar systems in the subsurface.

We integrated field data with 3D computer modeling to test hypotheses of fracture formation associated with the lateral propagation of two en echelon fault segments. Because previous geologic mapping revealed >6 km of strike-parallel segment overlap in the study area, we documented the fracture network in relation to these fault segments across the overlap zone. Fracture data include fracture orientation, opening width, intensity, and vertical persistence. We also developed 3D computer models to simulate laterally propagating en echelon fault segments, modeling increasing stages of overlap with increasing 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 mean stress.

Our model results reveal how the stress field around the fault transfer zone may have evolved over time and provide support for our interpretations of field data, which suggest that the orientations of subsidiary structures documented in field research are consistent with those expected as fault segments propagate and overlap. However, our field data suggest that while fracture orientations are consistent with formation in this structural context, the intensities, both relative to fault segments and vertically through the exposed Navajo sandstone, are not predictable. We plan to investigate controls on these less predictable variables in future research.