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 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.

The Upper Triassic (Carnian–Norian) Martin Bridge Formation of northeastern Oregon, southeastern Washington, and western Idaho is characterized by rapidly shifting depositional processes within a tropical volcanic island arc setting. Martin Bridge sequences in the Hells Canyon and northern Wallowa Mountains document shallow-water peritidal evaporitic sediments that are succeeded by deeper and predominantly subtidal deposits. This indicates drowning of the carbonate platform and a transition to deeper-water turbiditic sedimentation before a late Triassic transition into the overlying mid-Norian to Jurassic Hurwal Formation. At the type locality in the southern Wallowa Mountains, dysaerobic shales, carbonate debris sheets, and turbiditic sediments indicate distal slope and basinal environments while other facies at other sites in the Wallowa Mountains and Hells Canyon areas indicate reef and shallow-water platform settings. In this paper we formally recognize the name Martin Bridge Formation and reinstate the type locality in the southern Wallowa Mountains as the principal unit strato-type. An additional reference section is given at Hurricane Creek in the northern Wallowa Mountains. The Martin Bridge is formally divided into four members: the Eagle Creek and Summit Point Members are introduced and formally proposed herein and the BC Creek and Scotch Creek Members also are elevated to formal status. A partial reconstruction of the Wallowa terrane during deposition of the Martin Bridge Formation suggests a north-south (or northeast-southwest) trending platform margin facing a forearc basin situated to the east (or southeast). The lithofacies and paleontological characteristics of the Martin Bridge can be put into the framework of a depositional and a tectonic model to help better explain many of the stratigraphic and paleontologic problems previously encountered. We believe that the Wallowa terrane provides one of the best and most complete examples yet known for shallow-water carbonate depositional patterns in an oceanic island arc setting.