Examination of sedimentary basins in east-central Idaho and analysis of previous work in western Montana indicate that late middle Eocene to Oligocene extension north of the eastern Snake River Plain was confined to a north- to north-northwest-trending rift zone, 100 ±25 km wide. In east-central Idaho, six remnants of half grabens preserve up to 2.6 km of Eocene to Oligocene boulder to pebble conglomerate, sandstone, and intercalated tuff. The Paleogene basin-bounding normal faults strike north to north-northwest, dip west, and accommodated up to 10 km of dip slip. Clasts at the base of each basin-fill sequence were derived almost exclusively from underlying middle Eocene volcanic rocks, whereas higher in the section, the detritus was derived increasingly from pre-Tertiary formations exposed in the footwall of the basin-bounding normal faults. The uppermost units of the basin-fill deposits typically lack volcanic clasts. Pebble-count data indicate that the basin-fill deposits were produced by unroofing of Proterozoic to Tertiary rocks in the uplifted footwall east of each basin. Slip on the basin-bounding normal faults created substantial relief and spawned gravity-emplaced slide blocks and megabreccias that interfinger with thick alluvial-fan deposits in the basins. In contrast, previous studies of the upper Eocene to lower Miocene Renova Formation in southwestern Montana suggest that relatively low relief and tectonic quiescence characterized the region east of the proposed rift zone. I speculate that a river flowing from the rift zone into a basin east of the rift shoulder may have carried fine-grained sediment into the Renova basin.
Throughout east-central Idaho, initial slip on the north- to north-northwest-striking normal faults was closely associated with rhyolitic volcanism during the waning phases of Challis magmatism ca. 47-45 Ma. 40Ar/30Ar ages, however, show that sedimentation and extension outlasted volcanism by more than 15 m.y. and were largely amagmatic. Gravitational collapse of previously thickened crust probably drove this episode of extension within and west of the Sevier fold and thrust belt. I interpret the rift zone, which can be traced into western Montana and southern British Columbia, as a northward continuation of a west-directed belt of late Paleogene extension previously documented in the Great Basin.