Subduction-related accretionary complexes are fundamental building blocks in orogenic belts and are considered to be a hallmark of more recent plate-tectonic processes (Phanerozoic). Active accretionary wedges can reflect a long-lived history that may span 50 or more millions of years, whereas ancient accretionary complexes can even reflect a longer history that may involve collisional deformation events as well as offscraping, underthrusting, and underplating. Accretionary complexes commonly include a mixture of rock types that could not have formed in a single tectonic setting. The architecture of active subduction-related accretionary wedges has been studied by marine geophysical surveys that have yielded complex structural histories involving imbrication of oceanic lithosphere, fore-arc extension and/or contraction, delineation of supra-subduction thrust belts and/or broad zones of mélange, and tectonic wedging. Despite the significance of these marine geophysical studies in developing a broad understanding of the development of accretionary wedges in a variety of tectonic settings, many aspects of the deformational history of long-lived accretionary complexes are still poorly understood and require the direct examination of exposed, on-land, examples to explore these processes.
The composite Baker terrane of northeastern Oregon, provides a natural laboratory to study the nature of structural development and crustal growth in a well-exposed, ancient accretionary complex. In this project, the investigators will evaluate a tectonic model in which the Baker terrane is a composite supra-subduction accretionary complex that developed marginal to North America and was the principal focus of lithospheric deformation related to a collisional orogeny in the Blue Mountains province. The proposed study makes specific predictions about the crustal structure of the Blue Mountains that will be tested through detailed geologic and structural mapping, radiolarian biostratigraphy, microstructural studies of fault-related rocks, geochemical and Nd and Sr isotope studies, detrital zircon studies, and U-Pb radiometric dating. In particular, the model proposed is that the northern margin of the Baker terrane is a fundamental lithospheric boundary that separates the far-traveled Wallowa island-arc terrane from the Baker accretionary complex terrane.
This research will involve the training of graduate students at the University of Wyoming. Undergraduate students will also be employed as geologic field assistants. In addition, samples and data collected from this research will be used extensively in the classroom. For example, samples will be used in undergraduate Earth systems science, petrology, and structural geology classes, and in a graduate course in microstructural analysis. Finally, the Baker terrane is a major component of the Blue Mountains province, and a thorough knowledge of these accreted rocks (i.e., the structural basement in regard to the widespread younger rocks of the region) is essential to the success of proposed geophysical surveys through the Blue Mountains province (e.g., the GeoSwath initiative within the EarthScope project).
