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Abstract

The late Mesozoic accretionary boundary in west-central Idaho has played a critical role in tectonic models proposed for the northwestern U.S. Cordillera. From west-to-east, major elements include the Permian to Jurassic Wallowa island-arc terrane, a poorly understood transition zone consisting of the Riggins Group assemblage and deformation belt along the west side of the island arc-continent boundary, Late Jurassic to Cretaceous arc-continent boundary, and Precambrian North American margin intruded by the Cretaceous–Paleogene Idaho batholith. We focus on the transition zone in the area between White Bird and Riggins, Idaho, which includes a contractional belt in variously deformed and metamorphosed rocks of island-arc affinity. We propose that the rocks of the entire transition zone, including those originally defined as the Riggins Group, are likely of Wallowa terrane origin and/or related basinal assemblages. Ultramafic rocks in the transition zone are possibly related to a Jurassic or Cretaceous basinal assemblage that includes the Squaw Creek Schist of the Riggins Group. Our recent work addresses the kinematic history of structures in the contractional belt. The belt was reactivated in the Neogene to accommodate mostly brittle normal faulting that strongly influenced preservation of the Miocene Columbia River Basalt Group at this location along the eastern margin of the flood basalt province. This field guide provides a road log for examining the geology between Moscow and New Meadows, Idaho, along U.S. Highway 95.

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