Abstract

Late Tertiary extension produced a north-northwest-trending half-graben in the southern Portneuf Range north of Preston, Idaho. Conglomerates and tuffs of the Miocene-Pliocene Salt Lake Formation that fill the half-graben dip steeply toward the fault that bounds the basin on the east, here called the Valley Fault. Mapped facies relationships and reversed stratigraphy in the clasts derived from upper Proterozoic and lower Paleozoic rocks in the surrounding hills show that the basin filled as it formed. Mapping in rocks unconformably underlying the Salt Lake Formation show that the half-graben is superimposed on an open north-northwest-trending anticline. This anticline is inferred to be above a ramp in the Paris thrust or a higher decollement within the Paris thrust plate. Steep dips in the basin fill show that the hanging-wall block of the Valley Vault has been rotated as much as 90°; this indicates that the fault is listric. A simple geometric reconstruction indicates that the fault flattens to horizontal at a depth of about 4,000 meters below the present surface; the Proterozoic stratigraphy, as known from adjacent ranges, indicates that the Kelley Canyon Formation, a thick shale, would be present at this same depth. This shale is inferred to be in the upper plate of the Paris thrust; hence, it lies on the west limb of the anticline. It would provide a westward-dipping plane of weakness that would contain the Valley Fault. Westward slip of the upper block exceeds 6 kilometers.

The Miocene-Pliocene age of the basin fill in the southern Portneuf Range suggests that Basin and Range extension began no later than Miocene in this part of southeastern Idaho; seismicity in the region indicates that it continues. The inferred depth of faulting implies a thin-skinned mode of deformation.