ABSTRACT The rifted Precambrian margin of western Laurentia is hypothesized to have consisted of a series of ~330°-oriented rift segments and ~060°-oriented transform segments. One difficulty with this idea is that the 87 Sr/ 86 Sr i = 0.706 isopleth, which is inferred to coincide with the trace of this rifted margin, is oriented approximately N-S along the western edge of the Idaho batholith and E-W in northern Idaho; the transition between the N-S– and E-W–oriented segments occurs near Orofino, Idaho. We present new paleomagnetic and geochronologic evidence that indicates that the area around Orofino, Idaho, has rotated ~30° clockwise since ca. 85 Ma. Consequently, we interpret the current N-S–oriented margin as originally oriented ~330°, consistent with a Precambrian rift segment, and the E-W margin as originally oriented ~060°, consistent with a transform segment. Independent geochemical and seismic evidence corroborates this interpretation of rotation of Blue Mountains terranes and adjacent Laurentian block. Left-lateral motion along the Lewis and Clark zone during Late Cretaceous–Paleogene time likely accommodated this rotation. The clockwise rotation partially explains the presence of the Columbia embayment, as Laurentian lithosphere was located further west. Restoration of the rotation results in a reconstructed Neoproterozoic margin with a distinct promontory and embayment, and it constrains the rifting direction as SW oriented. The rigid Precambrian rift-transform corner created a transpressional syntaxis during middle Cretaceous deformation associated with the western Idaho and Ahsahka shear zones. During the late Miocene to present, the Precambrian rift-transform corner has acted as a fulcrum, with the Blue Mountains terranes as the lever arm. This motion also explains the paired fan-shaped contractional deformation of the Yakima fold-and-thrust belt and fan-shaped extensional deformation in the Hells Canyon extensional province.

ABSTRACT 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.