THE ROLE OF PRE-EXISTING STRUCTURES ON THE GEOMETRY OF THE LATE CRETACEOUS POISON CREEK THRUST FAULT OF EAST-CENTRAL IDAHO

This study is focused at the northern margin of the Wyoming salient in the east-central Idaho thrust belt south of Salmon, Idaho. We present new field mapping of the Poison Creek thrust fault, the western most Sevier-correlated thrust. Field results are coupled with thermochronometry of the Poison Creek thrust hanging-wall and detrital zircon analysis of the Ordovician Kinnikinic Quartzite of the Paleozoic passive margin. Within the study area, the Kinnikinic sits with angular unconformity upon Mesoproterozoic Lemhi Group, a relationship that is an expression of the Lemhi Arch, a regional northwest-trending Neoproterozoic and Cambrian paleohigh. Detrital zircon results from the Kinnikinic show two distinct age populations at ~1860 Ma and 2080 Ma. These age-peaks differ from those in known Paleoproterozoic, Mesoproterozoic Belt Supergroup-equivalent, or early Paleozoic rocks in the area. Thus their provenance is a distant one, and not local. The Poison Creek thrust fault accommodated >3 km of stratigraphic offset, placing Mesoproterozoic Lemhi Group atop a thin section of Ordovician Kinnikinic and Saturday Mountain formations, which thicken away from the arch to the southwest. The Poison Creek thrust truncates pre-existing folds. A southeastward-shallowing lateral ramp in the Poison Creek thrust is coincident with several imbricate thrust sheets in the footwall of the main thrust. These observations suggest that the Poison Creek thrust fault truncated a previously deformed section. U-Th/He cooling ages from zircons in the Poison Creek thrust hanging-wall constrain timing of exhumation across the fault to 70 to 58 Ma. Multiple sets of normal faults are also exposed in the mapping area, which cut the contractional structures. These faults largely involve ignimbrites and mafic lava flows of the Eocene Challis Volcanic Group. Overall, results suggest that Mesozoic shortening in this region was contemporaneous with the Absaroka thrust in southeast Idaho. Also, observed out-of-sequence thrusting may have resulted from regional changes in detachment level induced by differences in depositional thickness.