Abstract The Wallowa terrane is one of five pre-Cenozoic terranes in the Blue Mountains province of Oregon, Idaho, and Washington. The other four terranes are Baker, Grindstone, Olds Ferry, and Izee. The Wallowa terrane includes plutonic, volcanic, and sedimentary rocks that are as old as Middle Permian and as young as late Early Cretaceous. They evolved during six distinct time segments or phases: (1) a Middle Permian to Early Triassic(?) island-arc phase; (2) a second island-arc phase of Middle and Late Triassic age; (3) a Late Triassic and Early Jurassic phase of carbonate platform growth, subsidence, and siliciclastic sediment deposition; (4) an Early Jurassic subaerial volcanic and sedimentary phase; (5) a Late Jurassic sedimentary phase that formed a thin subaerial and thick marine overlap sequence; and (6) a Late Jurassic and Early Cretaceous phase of plutonism. Rocks in the Wallowa terrane are separated into formally named units. The Permian and Triassic Seven Devils Group encompasses the Middle and Late(?) Permian Windy Ridge and Hunsaker Creek Formations and the Middle and Late Triassic Wild Sheep Creek and Doyle Creek Formations. Some Permian and Triassic plutonic rocks, which crystallized beneath the partly contemporaneous volcanic and sedimentary rocks of the Seven Devils Group, represent magma chambers that fed the volcanic rocks. The Permian and Triassic plutonic rocks form the Cougar Creek and Oxbow “basement complexes,” the Triassic Imnaha plutons, and the more isolated Permian and Triassic plutons, such as those in the Sheep Creek to Marks Creek chain and in the southern Seven Devils Mountains near Cuprum, Idaho. The Seven Devils Group, and its associated plutons, are capped by the Martin Bridge Formation, a Late Triassic platform and reef carbonate unit, with associated shelf and upper-slope facies, and overlying and partly contemporaneous siliciclastic, limestone, and calcareous phyllitic rocks of the Late Triassic and Early Jurassic Hurwal Formation. Younger rocks are a subaerial Early Jurassic volcanic and sedimentary rock unit of the informally named Hammer Creek assemblage, and a Late Jurassic overlap sedimentary unit, the Coon Hollow Formation. Late Jurassic and Early Cretaceous plutons intrude the older rocks. Lava flows of the Miocene Columbia River Basalt Group overlie the pre-Cenozoic rocks. Late Pleistocene and Holocene sedimentation left discontinuous deposits throughout the canyon. Most impressive are deposits left by the Bonneville flood. The latest interpretations for the origin of terranes in the Blue Mountains province show that the Wallowa terrane is the only terrane that, during its Permian and Triassic evolution, had an intra-oceanic (not close to a continental landmass) island-arc origin. On this field trip, we travel through the northern segment of the Wallowa terrane in Hells Canyon of the Snake River, where representative rocks and structures of the Wallowa terrane are well exposed. Thick sections of lava flows of the Columbia River Basalt Group cap the older rocks, and reach river levels in two places.

The 39,000-km 2 Idaho batholith lies 600 km east of the present Pacific coastline and east of the Columbia River basalt plateau. The batholith is Late Cretaceous in age and is emplaced immediately east of the Triassic Seven Devils volcanic arc, an apparently allochthonous terrane which may be part of the recently recognized "Wrangellia terrane" of western Canada and southern Alaska. Locus of a Late Cretaceous subduction zone related to the Idaho batholith is not yet defined but must lie west of the Seven Devils arc. Country rocks of the Idaho batholith are Proterozoic Belt metasediments and pre-Belt basement orthogneisses. Pre-batholithic, sillimanite-zone, regional dyna-mothermal metamorphism, apparently Jurassic or Cretaceous in age, is broadly concentric to the northern half of the batholith, extending for a few to several kilometers beyond the contact. The Idaho batholith is dominantly medium-grained, massive to moderately foliated, muscovite-biotite granite and granodiorite. Gneissic tonalite, rich in biotite and hornblende, forms a 12-16 kilometer-wide western border zone of the batholith, and tonalite or trondhjemite form satellitic plutons for 50-70 kilometers to the west. Granodiorite makes up a 10-20 kilometer-wide border zone against exposed country rocks of most of the batholith and surrounds the voluminous granite of the batholith interior. Such granodiorite has not been documented adjacent to the pre-Belt basement rocks of the northwest-trending Salmon River arch, which divides the Idaho batholith into a northern Bitterroot lobe and a southern Atlanta lobe. Granodiorite may have formed a broad shell over most of the batholith which may have originally extended across the deeply eroded Salmon River arch. The borders of a few large separate intrusions have been partly documented, especially in the interior of the Bitterroot lobe, but for most of the batholith, separate major bodies are as yet unknown. Foliation in the western tonalitic border zone of both lobes dips 50 to 70 degrees eastward under the batholith. To the east in the main body of the batholith, the foliation weakens and gradually arches to nearly horizontal in the interior. Southwestern and northeastern border zones of the deeper northern part of the batholith are marked by large alternating sheets of granitic and high-grade country rocks. Large, tabular or contorted inclusions and nebulous schlieren are abundant towards the interior. The north-trending Bitterroot dome of the northeastern Bitterroot lobe appears to have formed a mushroom-shaped diapir into the country rocks, then rose isostatically in response to eastward unloading of the 6000 km 2 Sapphire tectonic block. The base of the flanks of this part of the batholith has been mapped on the southwest and northeast. Chemically, the Idaho batholith appears related to the volumetrically minor "sodic series" recognized by Tilling in the Boulder batholith to the east. Radiometric ages in both batholiths appear to be similar. The hornblende-bearing tonalites of the western border and more-mafic satellitic plutons to the west, north, and northeast of the Idaho batholith appear to have the "I-type", "magnetite-series" mineralogy of Chappell and White and of Ishihara, respectively. The muscovite-bearing main units of the batholith appear to have "S-type" and probably "ilmenite-series" mineralogy. Sources for magmas forming the tonalitic western border zones of the Idaho batholith were probably mafic-rich rocks of the upper mantle or of subducted oceanic crust or young continental margin volcanic rocks. Magmas forming the granodiorite-granite main body of the batholith wre probably derived by partial melting of Precambrian continental basement rocks. The main body of the Bitterroot Lobe of the batholith appears to have been emplaced at a depth of 15 to 20 km, and much of the magma was probably generated at not much greater depth. Metallic mineral deposits are essentially absent from the deeper Bittrroot lobe. Correlation of magnetite-series granitic rocks and Cu-Mo mineralization and ilmenite-series granitic rocks with Sn-W mineralization can be partly documented for the Atlanta lobe and for plutons east of the batholith, but data are sketchy and, in part, appear contradictory.

ABSTRACT An important element in reconstructions of the Cordilleran margin of North America includes longstanding debate regarding the timing and amount of rotation of the Blue Mountains in eastern Oregon, and the origin of geometric features such as the Columbia Embayment, which was a subject of some of Bill Dickinson’s early research. Suppositions of significant clockwise rotation of the Blue Mountains derived from Dickinson’s work were confirmed in the 1980s by paleomagnetic results from Late Jurassic–Early Cretaceous plutonic rocks, and secondary directions from Permian–Triassic units of the Wallowa–Seven Devils arc that indicate ~60° clockwise rotation of the Blue Mountains. This study reports new paleomagnetic data from additional locations of these Late Jurassic–Early Cretaceous plutonic rocks, as well as Jurassic sedimentary rocks of the Suplee-Izee area. Samples from three sites from the Bald Mountain Batholith, two sites from small intrusive bodies near Ritter, Oregon, and six sites from the Wallowa Batholith have well-defined magnetization components essentially identical to those found by previous workers. The combined mean direction of both sets of data from these Late Jurassic to Early Cretaceous intrusive rocks is D = 30, I = 63, α 95 = 6°. Samples from Jurassic sedimentary rocks in the Suplee-Izee area include four sites of the Lonesome Formation, three sites of andesitic volcanics in the Snowshoe Formation, and three sites from the Trowbridge Formation. The Lonesome and Trowbridge samples all had very well-defined, two component magnetizations. The in-situ mean of the combined Lonesome and Trowbridge Formations is D = 28, I = 63, α 95 = 15°. Upon tilt-correction, the site means of these units scatter and fail the paleomagnetic fold test in spectacular fashion. The similarity between the directions obtained from the remagnetized Jurassic rocks, and from the Late Jurassic to Early Cretaceous plutonic rocks suggests that a widespread remagnetization accompanied emplacement of the intrusives. Similar overprints are found in Permian and Triassic rocks of the Blue Mountains. Directions from 64 sites of these rocks yields a mean of D = 33°, I = 64°, k = 26, α 95 = 3.7°. Comparing the directions with North America reference poles, a clockwise rotation of 60° ± 9° with translation of 1000 ± 500 km is found. Together with data from Cretaceous and Eocene rocks, clockwise rotation of the Blue Mountains has occurred throughout the past ca. 130 Ma, with long-term rotation rates of 0.4 to 1 °/Ma. Approximately 1000 km of northward translation also occurred during some of this time.

ABSTRACT The Salmon River suture zone in west-central Idaho is a steep ocean-continent plate boundary separating Paleozoic-Mesozoic island-arc terranes and the ancestral western Laurentian margin that characterizes much of the central North American Cordillera. In the Riggins region, the most complete record of arc-continent collision and subsequent modification of the accretionary boundary is exposed because of the lower abundance of Cretaceous plutonism as compared to exposures of the boundary regionally along strike, and the deep degree of erosion along the Salmon River Canyon. Using recent mapping, microtectonic analysis, geochronological data, and structural models, this field trip explores the time-transgressive structures currently exposed across the Salmon River suture zone from the eastern foothills of the Seven Devils Mountains into the Salmon River Canyon. The Salmon River suture zone contains a Late Jurassic to Early Cretaceous, west-vergent thrust belt that is overprinted along its eastern extent by the Late Cretaceous, transpressional western Idaho shear zone and Late Cretaceous(?) and Cenozoic ductile-brittle extensional structures. A distinct amalgamation of metavol-canogenic and metasedimentary rocks characterizes the thrust belt and includes the (1) northeastern Wallowa terrane, (2) western Salmon River belt, formally grouped with the Wallowa terrane, and (3) eastern Salmon River belt, known locally as the Riggins Group and Pollock Mountain Amphibolite. The western Idaho shear zone overprints the easternmost rocks and structures associated with the eastern Salmon River belt. It also contains heterogeneous units of tonalite, trondhjemite, and grano-diorite orthogneiss, as well as individual tonalite, granodiorite, and granite plutons that display a gradation and partitioning of deformation and strain internally within the shear zone. East of the magmatic injection zone located along the arc-continent boundary, Laurentian continental metasedimentary rocks and tonalite and granodio-rite plutons occupy the eastern portions of both the shear zone and larger suture zone. Geochronologic data, obtained largely from metaplutonic rocks in the McCall region south of the Riggins region, provide the temporal resolution to constrain current tectonic models proposed for Salmon River suture zone evolution.

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.