Title: A Structural and 40Ar/39Ar Geochronological Re-Evaluation of Low-Angle Normal Faults in Southeastern Idaho
Description: The development of gently inclined faults with large stratigraphic separation has long been enigmatic in the corridor of southeastern Idaho. Recent interpretations have culled examples from across the Basin and Range to suggest that these faults originated at a low dip and represent a regional scale low-angle normal fault system. In contrast, others cite extensive studies from fault mechanics and seismological data that cast doubt on whether these extensional structures could have formed at low inclination in the upper crust. This dissertation reviews the evidence and timing of the proposed Bannock detachment system in the Bannock Range of southeastern Idaho and puts forth a re-evaluation of the styles of extension in the region and a regional framework in which to place them. Chapter 1 re-evaluates gently dipping normal faults in the southern Bannock Range of southeastern Idaho that have previously been interpreted as evidence for a regional detachment system originating and slipping at a low inclination.

Previous work was based on geometrical relations between faults and bedding in lacustrine sediments of the upper Miocene to lower Pliocene Salt Lake Formation. The detachment argument was underpinned by three locations on the Oxford Mountain at which Salt Lake Formation was inferred to have been cut by low-angle normal faults. These locations have been re-evaluated. Two of the locations were found to preserve bedding-to-fault geometries that are well explained by offset from a fault of moderately dipping inclination. The third example is re-interpreted as an unconformable contact, not a fault, an observation that by itself precludes the existence of a detachment at that location. Chapter 2 presents a test of tephronchronology by the 40Ar/39Ar isotopic method. This study compares ages obtained by the geochronologic method of tephrochronology to ages obtained by 40Ar/39Ar single grain laser fusion of feldspars. The results of this study suggest certain considerations must be made when employing the method of tephrochronology for chronological work.

Chapter 3 presents a regional synthesis for the tectonics of southeastern Idaho expanding on the new data presented in chapters 1 and 2. 40Ar/39Ar ages obtained from the Salt Lake Formation show evidence that extension in this region was underway > 15 Ma. Bedding-to-fault cutoff angles for the low-angle faults with the largest stratigraphic separations in the region suggest that the now gently inclined normal faults developed with moderate to steep dips, then tilted to lower inclination during continued extension. A splay of the Paris thrust is interpreted to account for both geometric relations between Paleozoic age rocks and the Neoproterozoic Pocatello Formation, as well as an unconformable contact between Pocatello Formation and late Miocene to Pliocene lake deposits of the Salt Lake Formation. This dissertation focuses on one example of a detachment system. However, it has implications for low-angle faults in general – particularly in regions like the Basin and Range that have had a protracted deformation history.

The examples we have studied are important because they involve strata as young as Pliocene and they provide strong support for the role of tilting in accounting for the present-day attitude of large-offset normal faults, eliminating the need for the well-known mechanical paradox of low-angle normal fault formation.