MIDDLE CRETACEOUS [CA. 105–90 MA] SYNOROGENIC MAGMATISM AND RELATIONS TO SUPERCONTINENTAL RIFTING IN THE MCCALL REGION OF WEST-CENTRAL IDAHO

Changes in lithospheric composition and strength associated with rifting of the Rodinian supercontinent served in localizing late Mesozoic tectonic activity in west-central Idaho. Latest Jurassic to middle Cretaceous magmatism represented by the Hazard Creek complex [HCC] and geochemically similar calcic intrusions [e.g., Cornucopia stock of northeast Oregon] is explained by accretion-related crustal thickening in the Blue Mountains province. Westward-directed thrusting in the Wallowa and Olds Ferry terranes [ca. 150–105 Ma] led to partial melting of metabasaltic source rocks at depths sufficient for garnet stability. Synorogenic magmatism overlapped in time with subduction zone reorganization, which involved an oceanward jump and polarity reversal. Ensuing tectonism along the accretionary margin is attributed to right-oblique plate convergence, northward terrane translation, and clockwise rotation of the Blue Mountains block [Gray et al., 2022]. Transpressional deformation ca. 105–90 Ma was focused along the collisional boundary, where granitic magmas of the southern Idaho batholith were caught between the impinging terrane block and continental margin backstop, i.e., Precambrian rift architecture. LS-tectonite fabric observed in the McCall area—e.g., Little Goose Creek drainage/Fischer Creek Saddle—records the latter part of a progressive deformation history, in the absence of a hit-and-run collider [Tikoff et al., 2022]. In light of the 'new paradigm' developing for the North American Cordillera [Nevadan orogeny of Moores, 1970], a speculative model is proposed. Assumptions: [1] HCC was emplaced into the deforming Olds Ferry terrane, [2] Olds Ferry terrane represents a continent-fringing island-arc, and [3] fringing-arc was constructed on oceanic crust above a westward-dipping subduction zone.

Gray, K., Foster, D., Johnson, K., and Isakson, V., 2022, Rotational tectonics of the Oregon–Idaho–Montana Cordillera: Tectonophysics, v. 833, 229293.

Moores, E., 1970, Ultramafics and orogeny, with models of the US Cordillera and the Tethys: Nature, 228, 837-842.

Tikoff, B., Housen, B., Maxson, J., Nelson, E., Trevino, S., and Shipley, T., 2022, Hit-and-run model for Cretaceous–Paleogene tectonism along the western margin of Laurentia. Geological Society of America Memoir, 220.