THE EVOLUTION OF SUBSIDIARY FRACTURE NETWORKS IN SEGMENTED NORMAL FAULT SYSTEMS

Normal faults commonly form segmented, en echelon patterns with segments that propagate and overlap as extension increases. The local stress field changes within and adjacent to the fault transfer zone, so subsidiary fracture orientations that form between and adjacent to the fault segments may change over time as the stress state evolves. Because fractures commonly increase permeability, the results of our work inform our understanding of fluid flow anisotropy and connectivity at segment boundaries, impacting our access to natural resources.

The segmented Sevier fault zone lies within the transition from the Basin and Range province to the stable Colorado Plateau. The well-studied fault zone strikes north-northeast, dips west, and has excellent outcrop exposure near Orderville, in Southern Utah. We focused on two major, right-stepping fault segments that overlap by approximately 6 km, with strain within the transfer zone also accommodated by two minor faults. In the field, we documented fracture orientation and spacing in the Navajo and Temple Cap sandstones, and we annotated field photos to reveal relationships between joint sets.

Our analyses of fracture data reveal a dominant, steeply-dipping, NNE-striking joint system subparallel to the Sevier fault segments across much of the transfer zone. However, we document an abrupt 25 degree clockwise rotation in the strike of the dominant joint set from west to east across one of the minor normal faults within the transfer zone, and to the east of the easternmost major segment, joint orientations are much less systematic. Fracture intensities vary significantly both laterally and vertically within the transfer zone, and to the south of the primary study area, where strain is accommodated by just one fault, we observed tightly-spaced (<0.5m) small-offset (<0.5m) normal faults within 10 m of the fault. Our results suggest that joints formed within transfer zones are strongly systematic with highly variable intensities both horizontally and vertically. These results also show that the systematic nature of fracturing dies out quickly outside of the transfer zone. We suggest that the total volume of rock impacted by systematic fracturing within a transfer zone far exceeds that in locations where only a single dominant fault accommodates strain.