STRUCTURE, INTRUSIVE HISTORY, AND LOCATION OF THE EASTERN MARGIN OF THE WALLOWA TERRANE, WESTERN IDAHO

Geologic mapping and zircon U-Pb dating (LA-ICPMS) in a NW-SE transect near Lucile, Idaho has better defined the eastern edge of the Wallowa terrane and places it directly against continental North America. A series of NE-striking, moderately SE-dipping faults separate five rock assemblages between the Snake River and the initial ⁸⁷Sr/⁸⁶Sr 0.704/0.706 line (basement boundary with continental North America). The northern continuation of the Heavens Gate fault passes just west of Lucile and separates lowermost Wallowa terrane (Permian trondhjemite and Permian Hunsaker Creek Fm) in the west from intensely deformed Triassic-Jurassic(?) volcaniclastic rocks, limestone, phyllite, and argillite in the east. A strongly mylonitized Permian quartz porphyry sill immediately west of the fault yielded an age of 260 ± 9 Ma. Kinematic indicators within mylonites along the fault indicate early tops NW (thrust) movement, followed by tops SE (normal) displacement. A small tonalite stock in the Triassic-Jurassic(?) section yielded a 115.1 ± 2.7 Ma U-Pb zircon age. A deformed dike along strike to the south gave an age of 113.0 ± 2.6 Ma on rims and Permian and Jurassic ages on cores; the core ages are similar to the Wallowa basement to the west. The next fault to the east is the Rapid River thrust, which appears to carry an overturned section of the same Triassic-Jurassic(?) strata present in the hanging wall of the Heavens Gate fault. Farther east is the Slate Creek fault, which carries likely Permian Hunsaker Creek and intrusive Permian basement at higher metamorphic grade northwestward. Intrusive rocks increase in abundance farther to the east, and Permian and Triassic intrusive rocks continue to within 1.5 km of the 0.704/0.706 line along Slate Creek; eastward the plutons are Cretaceous. A Triassic tonalite, dated at 233.1 ± 5.6 Ma, is 3 km west of the 0.704/0.706 line. Country rock here consists of plagioclase-hornblende gneiss and associated calc-silicate rocks interpreted as a regionally extensive fine-grained basinal assemblage, possibly equivalent to the Triassic-Jurassic Hurwal Fm and the correlative(?) Squaw Creek Schist and Lucile Slate. These results imply that the Wallowa terrane is juxtaposed directly against continental North America at this latitude, without intervening terranes such as the Olds Ferry arc.