Abstract

New geologic mapping, kinematic analysis, and tephrochronologic age correlations were completed in the southern Portneuf Range of southeast Idaho to characterize the Valley fault, a large-offset normal fault interpreted as the breakaway for the regionally extensive Bannock detachment system. The Valley fault separates ∼11.8–7.57 Ma Salt Lake Formation from underlying Neoproterozoic to Cambrian Brigham Group and Cambrian rocks. Three-point problems indicate that the Valley fault strikes north–northwest (NNW) and dips an average of 9° west–southwest (WSW), in stark contrast to previous work, which interpreted the fault as steeply west-dipping. Footwall strata strike NNW and dip 37 ± 11° east–northeast (ENE). Measured bedding-to-fault cutoff angles are, therefore, ∼46 ± 11°. Hanging-wall strata strike NNW and dip 20 ± 5° ENE adjacent to the fault and steepen progressively down-section to ∼70 ± 10° ENE due to the Valley fault's minor listricity and multiple hanging-wall splays. Beneath the Miocene strata, Cambrian hanging-wall bedrock strike NNW and dip 55 ± 10° ENE.  Adding a 9° Valley fault dip to these bedding dips yields cutoff angles of 29° to 80° for Miocene strata and 64° for hanging-wall bedrock. Top-to-the-west, dip-slip offset across the Valley fault and its hanging-wall splays—using the Miocene unconformity as a marker—is 13.7 ± 1 km. The older, subhorizontal Mine Hollow fault has 1,000 ± 300 m top-to-the-west normal offset.

The initial dip of the Valley fault is interpreted to have been steeper than its present 9° dip. If footwall strata were subhorizontal prior to extension, as indicated by reconstruction of the Miocene unconformity, then the Valley fault had an initial dip of 46 ± 11° WSW based on measured bedding-to-fault angles. Combined with the modern low dip of the Valley fault, these data are interpreted to indicate that the Salt Lake Formation accumulated as growth strata adjacent to the active Valley fault, while the fault as well as footwall and hanging-wall strata tilted northeast through time. The Valley fault's mildly listric shape, the progressive domino-style tilting, and the large amount of total slip suggest that the Valley fault is a Basin-Range normal fault rather than a low-angle breakaway fault for the Bannock detachment system. The preferred model for extension in the region is ∼7 km of uplift and exhumation during two phases of moderate- to high-angle domino-style fault block rotation. First, the Valley fault splays and Mine Hollow fault accommodated 5 km of extension between 11.8 to 9.16 Ma; then the Valley fault accommodated 10 km offset between 9.16 to <7.57 Ma (likely as recent as 5–4 Ma). Our regional cross section and restoration suggest that both the Clifton fault to the west (Bannock detachment) and the Valley fault are two separate faults, each with 13–15 km offset. Reconstruction of these Miocene faults shows that the Mesozoic Paris thrust fault has a preextensional footwall flat length of 30 km, similar to that predicted by other workers.

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