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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.

Abstract The Middle Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth, covering over 210,000 km 2 of Oregon, Washington, and Idaho and having a volume of 210,000 km 3 . A well-established regional stratigraphic framework built upon seven formations, and using physical and compositional characteristics of the flows, has allowed the areal extent and volume of the individual flows and groups of flows to be calculated and correlated with their respective dikes and vents. CRBG flows can be subdivided into either compound flows or sheet flows, and are marked by a set of well-defined physical features that originated during their emplacement and solidification. This field trip focuses on the Lewiston Basin, in southeastern Washington, western Idaho, and northeastern Oregon, which contains the Chief Joseph dike swarm, where classic features of both flows and dikes can be easily observed, as well as tectonic features typical of those found elsewhere in the flood basalt province.

The Late Triassic (Carnian-Norian) time interval is well represented by strata in the Wallowa and Baker terranes and in the Izee-Suplee area of central Oregon. Strata of the Olds Ferry terrane, by contrast, have received little attention beyond field mapping and reconnaissance-level efforts. This paper describes the sedimentology of Carnian-Norian deposits in the Olds Ferry terrane and describes a model for arc-flanking mixed carbonate-volcaniclastic sedimentation. A distinctive thin-bedded limestone unit in the lower Huntington Formation is underlain and overlain by volcanic and shallow intrusive rocks. Ammonite and bivalve age assignments indicate that the limestone unit spans the late Carnian to early Norian; portions of the section may, more specifically, represent the welleri or dilleri ammonite zones. Four lithofacies—Graded Skeletal Packstone and Mudstone, Tuffaceous Peloidal Grainstone, Skeletal Peloidal Packstone, and Lapilli-Tuff Breccia—record episodic sedimentation influenced by earthquakes, storms, and volcanic eruptions on an oxygen-poor carbonate slope apron. The limestone unit represents an echinoderm- and mollusk-dominated heterozoan faunal assemblage characterized by abundant delivery of unlithified carbonate sediment to deeper water.

Maps of the distribution of Middle Proterozoic through Cretaceous rocks beneath the Tertiary unconformities in eastern Idaho effectively remove differential uplift associated with Basin and Range tectonism and reveal preextensional structural relief associated with folds and thrusts in the western part of the Idaho-Wyoming-Montana thrust belt. North of the Snake River Plain, the paleogeologic map shows that regionally extensive Middle Proterozoic to Triassic strata and the Cretaceous Idaho batholith were variably uplifted and exposed prior to formation of the Eocene Challis volcanic field. In the Beaverhead Mountains, Ordovician and Triassic strata are juxtaposed along the Hawley Creek thrust. In the northern Lemhi and Lost River ranges, west- and south-dipping homoclines beneath the Eocene unconformity are interpreted to reflect folding above frontal and lateral ramps in the footwall of the Hawley Creek thrust system. In the southern Lemhi and Lost River ranges, the White Knob Mountains, and eastern Pioneer Mountains, Mississippian to Permian strata underlie most of the unconformity. Stratigraphic offset is evident along the exposed Pioneer and Copper Basin thrust faults, but along the exposed Glide Mountain thrust as well as the concealed White Knob, Grouse, and Lost River-Arco Hills thrusts uplift was insufficient to juxtapose rocks of different systems. To the west, the Idaho batholith and Pennsylvanian-Permian rocks are juxtaposed beneath the Eocene unconformity. Several kilometers of overburden covered both the batholith and Permian rocks in Cretaceous time and were regionally eroded prior to extrusion of Eocene volcanic rocks. South of the Snake River Plain and west of the exposed traces of the Paris and Putnam thrust faults, Late Proterozoic to Permian strata unconformably underlie Miocene sedimentary and volcanic rocks. The Paris thrust separates Late Proterozoic through Mississippian strata in its hanging wall from Pennsylvanian through Triassic rocks in its footwall, and the Putnam thrust separates Late Proterozoic to Ordovician strata from Pennsylvanian to Triassic strata. Beneath the unconformity, stratigraphic displacement appears to diminish northwestward along the Paris thrust and southeastward along the Putnam thrust, supporting the interpretation that displacement is progressively transferred from one thrust to the other. Lower Paleozoic strata generally underlie the unconformity between the emergent thrust system and the Arbon and Malad valleys. West of these valleys, Pennsylvanian-Permian strata everywhere underlie the unconformity. A simple flat-ramp-flat-thrust fault geometry of the Paris-Putnam thrust system may explain the outcrop pattern, with hanging-wall and footwall flats beneath the regions of low structural relief, separated by a footwall ramp located beneath the modern Malad and Arbon valleys. Paleogeologic maps show that the Paris-Putnam thrust sheet was probably not an important source of quartzose clasts in the Cretaceous-Tertiary Harebell and Pinyon formations of northwest Wyoming.

Rocks in the eastern Klamath Mountains were affected by polyphase tectonism. The older Callahan tectonic event, of Late Ordovician–Early Silurian age, produced high-pressure and epizonal metamorphism and is expressed in the Yreka-Callahan nappes by intrafolial folds. The main tectonic event occurred during Late Devonian and Early Mississippian time and is interpreted here to be coeval with the Antler orogeny, defined in the Roberts Mountain allochthon to the east in Nevada. It resulted in the amalgamation of lower Paleozoic Trinity ophiolite and the pre-Devonian to Early Devonian intra-oceanic island arc with flysch-type sediments deposited in a continent margin (Yreka-Callahan assemblage), and a metamorphic sequence of amphibolite, marble, and quartz-mica-schist of the Central Metamorphic Belt. The magmatic evolution of Paleozoic and lower Mesozoic rocks in the eastern Klamath Mountains is characterized by the predominance of convergent plate boundary magmatism. Nevertheless, lower Paleozoic pillow basalts associated with coralline limestone occur as olistoliths in the Yreka-Callahan assemblage. They display either T mid-ocean ridge basalt (MORB) affinities with flat rare earth element (REE) patterns or mild alkalic affinities marked by higher contents in TiO 2 and HFS elements and LREE-enriched patterns. The Ashgillian calc-alkaline differentiated suite, characterized by LREE-enriched patterns and low ɛ Nd (T) + 3.5, was erupted through continental crust in an Andean margin setting. The pre-Devonian to Early Devonian volcanic rocks that show tholeiitic affinities with very depleted to somewhat enriched LREE patterns and high ∊ Nd (T) values represent the remnants of an intra-oceanic immature island arc. After the Late Devonian(?)–Early Mississippian tectonic event, island-arc magmatism resumed with the eruption of depleted low-K tholeiitic basalt and andesite, characterized by depleted LREE patterns during Late Devonian(?) or Carboniferous-Permian(?) time (Gregg Ranch and Copper Peak sections). A major pulse in volcanism occurred during Middle to Late Permian and is now represented by the voluminous sequence of volcanogenic debris and flows of the Nosoni, Dekkas, and Bully Hill Formations. This volcanism, formed of predominantly low-K tholeiitic basalt and andesite, characterized by flat REE patterns and high ɛ Nd (T) + 4 to 8, with minor calc-alkaline, alkaline (enriched LREE patterns), and boninitic (Mg-Cr-rich) basalts, was erupted through a thickened crust. At the end of the Permian island-arc volcanism, uplift and faulting occurred in the arc. The magmatic affinities of the middle to late Paleozoic volcanic rocks in the eastern Klamath Mountains suggest an evolution through time from an immature to a mature island-arc setting. The Permian-Triassic history of the Blue Mountains (Seven Devils Group; Huntington Formation) presents many similarities with that of the eastern Klamath terrane. In both areas, evidence for Permian volcanism is similar with flat REE patterns and high ɛ Nd (T) + 6.7 to 8.4. Triassic volcanic rocks display low-K tholeiitic affinities (flat REE patterns) with minor calc-alkaline flows in the uppermost levels of the volcanic pile. Apparently, compressive structures related to the Sonoma orogeny are absent in both areas. Thus the eastern Klamath and Blue Mountains could have formed part of the same arc in Permian-Triassic time.