THE GRANGEVILLE, IDAHO 30' X 60' QUADRANGLE: NEW INSIGHTS ACROSS THE ARC-CONTINENT BOUNDARY IN THE CENTRAL NORTH AMERICAN CORDILLERA

Recent mapping of pre-Tertiary basement rocks in the Grangeville 30' X 60' quadrangle provides new insights for the evolution of a part of west-central Idaho that crosses the lithospheric boundary between the Permian to Cretaceous Wallowa arc complex of the Blue Mountains province to the west and Proterozoic and Cretaceous North American cratonic assemblages to the east. Within the Wallowa terrane we now recognize: (1) more extensive exposures of Permian and Triassic basement rocks along the eastern margin of the terrane than originally defined; (2) mostly Triassic basement (rather than Triassic and Permian) in exposures north of Pittsburg Landing; (3) more extensive exposures of rocks we interpret as the Permian Hunsaker Cr. Fm. and the association of several quartz porphyry-hosted Cu-Au mineral deposits with these rocks; and (4) plagioclase-hornblende gneiss and associated calc-silicate rocks along the lithospheric boundary that we now suspect are equivalent to Triassic-Jurassic basin assemblages of the Blue Mountains province.

Three major east-dipping faults occur west of the lithospheric boundary. Southwest of Lucile, the Heavens Gate fault juxtaposes less-deformed Hunsaker Cr. footwall rocks against highly foliated and folded Triassic greenstone and marble to the east. The map pattern suggests that the fault is an early thrust fault with even more significant later normal offset. To the east, we mapped the Rapid River thrust along the eastern contact of the Lucile Slate, a deformed phyllite likely equivalent to the Squaw Creek Schist; most kinematic indicators show top-west transport. Farther east, the Slate Creek thrust places an amphibolite-facies Permian(?) hanging-wall assemblage that forms a structural culmination immediately west of the arc-continent boundary westward over a Jurassic-Triassic(?) greenschist facies footwall assemblage that is folded and overturned to the northwest. Farther north, these 3 faults merge to become a single fault, which in turn merges with, or is truncated by, the NE-striking Klopton Creek-Hammer Creek-Mount Idaho shear zone, a reverse-right lateral structure that offsets the lithospheric boundary east of Grangeville. Later Neogene N-NE-striking normal faults reactivated many of the Mesozoic structures described above.