ANALYZING DEFORMATION WITHIN A NORMAL FAULT TRANSFER ZONE USING SFM 3D MODELING

Within segmented normal fault systems, strain and stress vary between overlapping fault segments. We examine the distribution and orientation of fractures in a major transfer zone of the Sevier fault, south-central Utah, where canyons eroded into the Jurassic Navajo sandstone provide excellent exposure. We focus on the easternmost fault within a transfer zone, where strain is accommodated by multiple faults, and we compare fracture characteristics to a locality where one fault accommodates strain. To gather data from inaccessible canyon walls, we used unmanned-aerial-vehicle images combined with SfM (structure from motion) technology to build sub-decimeter-resolution 3D models.

We found that where strain is accommodated by a single fault segment (~880 m throw), fracture intensity is localized and high (>1.5 m^-1) within the footwall. Intensity decreases with distance from the fault plane, abruptly terminating ~20 m from the fault. In contrast, the fractured footwall of the transfer zone fault displays lower values near the fault (<0.50m^-1) without an observed decrease in intensity until fracturing terminates ~180 m from the fault.

We used SfM modeling to document the vertical fracture network in the footwall of the transfer zone fault. Fracture intensity changed, usually abruptly, from base to top of the ~200-m exposure, with values of 0.20, 0.57, 0.20, 0.41, and 0.46 m^-1. Abrupt changes in fracture intensity appear to coincide with the resistance to erosion or changes in outcrop coloration, but in one case, the vertical change in intensity is accommodated by fracture branching.

We suggest that because strain in the transfer zone is accommodated by multiple faults, fracturing intensity is lower but more widely distributed than at the single fault locality. We posit that vertical variations in fracture intensity were mechanically controlled by a combination of lithologic variations, post-lithification fluid alteration (oxidized vs. bleached sandstone), and primary structures (i.e., cross-bedding and cross-bed set boundaries). Our results imply that lateral fluid flow at the single-fault locality would be more localized but at higher flow rates relative to the transfer zone. In addition, differences in vertical fracture intensity indicate that lateral flow would vary within the stratigraphic section.