ROTATIONAL TECTONICS OF THE OREGON–IDAHO–MONTANA CORDILLERA

Paleomagnetic data compiled along a line extending from northeastern Oregon through north-central Idaho into southwestern Montana indicate adjacent crustal blocks experienced vertical axis clockwise rotation over late Mesozoic–early Cenozoic time [Elston et al., 2002; Housen & Dorsey, 2005; Lewis et al., 2014]. From southwest to northeast, rotated blocks include accreted island arc terranes of the Blue Mountains province [Oregon], calc-alkaline intrusive bodies emplaced into the arc–continent boundary [Idaho], and passive margin metasedimentary assemblages of the Rocky Mountain foreland [Montana]. Tectonic reconstructions of the Blue Mountains call for ≤66° rotation as a single large block [>50,000 km2], with counterclockwise restoration forming an elongate arc system aligned with Klamath Mountains terranes to the south [California]. East- to northeast-trending terrane belts are tucked into the Columbia Embayment [Dickinson and Thayer, 1978], which corresponds to a right-angle bend in the arc–continent boundary [western Idaho suture zone]. Results from metaluminous plutons sampled immediately north/cratonward of the boundary support ~30° rotation; restoration is consistent with the orientation of inferred transform faults associated with break-up of Rodinia. East of the Idaho batholith [Bitterroot lobe], Mesoproterozoic strata of the Rocky Mountain foreland [Belt basin] show transport far east of their depositional sites. Rotation of major thrust slabs [~20°; Lewis-Eldorado-Hoadley, Sapphire, Lombard] was coeval with latest Cretaceous–Paleocene movement along the Lewis and Clark line [Sears & Hendrix, 2004]. In our view, right-oblique plate convergence/accretion of the Blue Mountains block drove the rotation of thrust slabs and development of associated salients [e.g. Helena] in the Sevier belt. Alternatively, we consider a hit-and-run tectonic model [Maxson & Tikoff, 1996] whereby outboard arc terranes collided with and caused the rotation of previously accreted terranes of the Blue Mountains. In either case, transpressional terranes in the Cordilleran hinterland [e.g. Wrangellian composite: Oregon/Idaho–British Columbia–Alaska; Gray et al., 2019] played a dynamic role in transmitting rotational strains across the accretionary boundary into the continental interior.