Abstract

The passive to convergent margin transition along western Laurentia drove early development of the North American Cordillera and culminated with the Late Devonian–Mississippian Antler orogeny and emplacement of the Roberts Mountain allochthon in the western United States. New detrital zircon studies in the Pioneer Mountains, east-central Idaho, were conducted to investigate the stratigraphic evidence of this transition and test models for mid-Paleozoic tectonics and paleogeography. Ordovician to Lower Devonian passive margin strata of the Roberts Mountain allochthon and adjacent North American parautochthon contain ca. 1850, 1920, 2080, and 2700 Ma detrital zircons that indicate provenance from the Peace River Arch region of northwestern Laurentia. These detrital zircons are much older than the depositional ages of their host rocks and probably record long-term sediment recycling processes along the Cordilleran margin. Upper Devonian strata, including Frasnian turbidites of the Roberts Mountain allochthon, document the incursion of 450–430 Ma and 1650–930 Ma detrital zircons from an unknown source to the west. Detrital zircon Hf isotope results suggest that the western source was an early Paleozoic arc built on Proterozoic crust, with the Eastern Klamath, Northern Sierra, and Quesnellia terranes as likely candidates. Lower Mississippian syntectonic strata filled a rapidly subsiding, releasing bend basin that was associated with sinistral-oblique plate convergence and reworking of lower Paleozoic rocks in east-central Idaho. The available detrital zircon and stratigraphic data are most consistent with noncollisional models for the Antler orogeny, including scenarios that feature the north to south, time-transgressive juxtaposition of Baltican- and Caledonian-affinity terranes along the Cordilleran margin.

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