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Tectonic Controls on Mudrock Geochemisry, Mesozoic Rocks of Eastern Oregon and Western Idaho, U.S.A.: Implications for Cordilleran Tectonics

¹ Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, U.S.A.; tlamaski{at}uoregon.edu
² Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, U.S.A.
³ School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164-6376, U.S.A.

Tectonic models for the Mesozoic evolution of accreted island arcs and subduction complexes in the Blue Mountain Province (BMP) of eastern Oregon and western Idaho vary widely. The BMP is situated between coeval accreted terranes of southern British Columbia and the western U.S., and is critical for interpreting Mesozoic paleogeography of the U.S. Cordillera. In this study, we interpret the Triassic–Jurassic evolution of the BMP using geochemistry as a tool to evaluate the provenance of mudrocks in fine-grained turbidites. Temporal and spatial variations in mudrock geochemistry indicate that during Late Triassic to Early Jurassic time the Wallowa terrane was an intra-oceanic island arc and the Olds Ferry terrane was a pericratonic (continental-fringing) island arc. Subsequently, a regionally extensive Middle to Late Jurassic marine basin received input from continental sources and sediment may also have been derived from coeval terranes to the south. Comparison of provenance data from the BMP with similar data from accreted terranes to the north and south suggests that Triassic–Jurassic basins of the western U.S. share a long-term provenance history within a complex system of continent-fringing and intra-oceanic island arcs. In contrast, it appears that the Quesnel terrane of southern British Columbia had a distinct provenance with a more direct link to Laurentia during Triassic time. Early to Middle Jurassic sedimentary rocks from southern British Columbia and the western U.S. show a high degree of geochemical provenance similarity. This may indicate reorganization of formerly discrete tectonic regions into a nascent, integrated western Laurentian margin prior to complete development of a Cretaceous Andean-type margin. The results of this study demonstrate the utility of mudrock geochemistry in tectonic analyses and show that major- and trace-element geochemistry of mudrocks is useful for developing a comprehensive understanding of regional tectonic evolution.