Abstract Recent mapping, U-Pb zircon geochronology, trace-element geochemistry, and tracer isotope geochemistry of plutonic and volcanic rocks in the Wallowa and Olds Ferry terranes of the Blue Mountains Province yield new insights into their tectonic evolution and pre-accretion history. Igneous rocks of the Wallowa arc terrane formed in two magmatic episodes of contrasting duration and geochemical characteristics. Magmatism in the first episode lasted for at least 20 Ma (ca. 268–248 Ma), spanning the Middle Permian to the Early Triassic and was of generally calc-alkaline affinity. Rock units associated with this episode include the Hunsaker Creek and Windy Ridge formations of the Wallowa terrane, as well as potentially equivalent tonalite and diorite plutonic rocks in the Cougar Creek Complex and related basement exposures, which show midcrustal levels of the terrane. The second episode of magmatism in the Wallowa arc was remarkably brief (U-Pb zircon dates range from 229.43 ± 0.08 Ma to 229.13 ± 0.45 Ma) and dominated by mafic to intermediate compositions of tholeiitic affinity. Rock units associated with the second episode may include the Wild Sheep Creek and Doyle Creek formations, as well as ubiquitous dikes and plutons in the Cougar Creek Complex and similar basement exposures. After 229 Ma, the Wallowa arc apparently became dormant. The record of igneous activity in the Olds Ferry arc contrasts with that of the Wallowa in its age range and the continuity of calc-alkaline magmatism. Radiometric ages and stratigraphic field relationships allow the magmatic history of the Olds Ferry terrane to be divided into at least three cycles separated by brief hiatuses and collectively spanning the late Middle Triassic through the Early Jurassic (ca. 237– 187 Ma). Rock units related to these episodes are divided by unconformities, and they include the Brownlee pluton, lower Huntington Formation, and upper Huntington Formation. Magmatic activity in the Olds Ferry arc may have persisted until at least 174 Ma, based on the presence of volcanic ash horizons in the lower portion of the overlying Weatherby Formation of the Izee basin. All cycles of Olds Ferry magmatism display generally calc-alkaline affinity. The contrasting magmatic histories of the Wallowa and Olds Ferry arc terranes provide the basis for at least two conclusions. First, these arcs formed as separate tectonic entities, rather than as a single composite arc. Second, progressive closure of the ocean basin between the arcs in the Late Triassic and Early Jurassic was related to continued subduction beneath the Olds Ferry arc, but the Wallowa arc was apparently dormant during much of that interval.

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.