Tikoff, B., Kahn, M.J., Gaschnig, R.M., Michels, Z.D., Davenport, K., Hole, J.A., Stanciu, C.A., Fayon, A.K., and Kruckenberg, S.C.
In: Haugerud, R.A., and Kelsey, H.M., eds., From the Puget Lowland to East of the Cascade Range: Geologic Excursions in the Pacific Northwest: Geological Society of America Field Guide 49, p. 229-254, doi:10.1130/2017.0049(09).
Publication year: 2017


The Salmon River suture zone is the boundary between the accreted (Blue Mountain) terranes and cratonic North America in western Idaho. This region was the focus of study by the EarthScope IDOR (IDaho-ORegon) project that integrated structural geology, geochemistry, geochronology, and seismology. This field trip traverses from western Idaho to eastern Oregon, covering the Atlanta lobe of the Idaho batholith, Blue Mountains terranes, and the middle Cretaceous western Idaho shear zone that separates these two domains. The main component of the Atlanta lobe is the Atlanta peraluminous suite, and it intruded from 83 to 65 Ma, was derived from crustal melting, and lacks a regionally consistent fabric. The crust below the Idaho batholith is relatively thick and seismic velocities are consistent with the entire crust being relatively felsic. The western Idaho shear zone overprints the Salmon River suture zone and obscures most evidence for the suturing. It is the present boundary between Blue Mountains terranes and cratonic North America. From studies along this transect, we have determined that the western Idaho shear zone exhibits dextral transpressional deformation, was active from ca. 103 to 90 Ma, and magmatism occurred during deformation; presently exposed levels on this transect record deformation conditions of 730 °C and 4.3 kbars. There is an ~7 km vertical step in the Moho at or slightly (<20 km) east of the current exposure of the western Idaho shear zone, separating thicker crust to the east from thinner crust to the west. Blue Mountains terranes immediately outboard of the western Idaho shear zone likely were located farther south during the middle Cretaceous and underwent strike-slip displacement during western Idaho shear zone deformation. The Olds Ferry terrane—the accreted terrane located immediately west of the western Idaho shear zone—was underplated by mafic magmatism, likely in the Miocene during eruption of the Columbia River basalt group. The field trip will utilize StraboSpot, a recently developed digital data system for structural geology and tectonics, so participants can investigate the relevant data associated with the IDOR EarthScope project.