DETRITAL ZIRCON ANALYSIS OF THE GALICE AND MARIPOSA FORMATIONS, CALIFORNIA AND OREGON: A RECORD OF THE LATE JURASSIC CORDILLERAN MARGIN (Invited Presentation)

The Late Jurassic tectonic evolution of the North American continental margin is critical for understanding subsequent terrane accretion and displacement. Upper Jurassic strata of the Galice Formation in the western Klamath Mountains and Mariposa Formation in the western Sierra Nevada foothills are the youngest rocks deformed during Late Jurassic Nevadan deformation in Oregon and California. Large detrital zircon age and Ɛ_Hf datasets from the Galice and Mariposa Formations support the longstanding correlation between Galice and Mariposa strata and characterize sediment provenance through time and space within each basin. Our data require that both basins received abundant continentally derived zircon and proximal volcanic arc detritus throughout turbidite deposition from ca. 159 Ma to 148 Ma. However, subtle differences between the pre-Mesozoic age distributions and Mesozoic zircon Ɛ_Hf values compiled for each basin reveal nuances in provenance that can be directly related to the basin’s location relative to sediment sources. We use mixture-modeling to assess the relative contributions of various pre-Mesozoic source regions in the Klamath-Sierran arc and retroarc region to the Galice and Mariposa pre-Mesozoic detrital zircon age signatures. The relative latitudinal position of the Galice and Mariposa basins with respect to their source regions can account for the differences in source contributions to each basin, and our results are consistent with <200 km of post-Jurassic dextral displacement within the Sierra Nevada magmatic arc. These geochemical and age-based provenance results, combined with depositional age constraints for each basin, are most consistent with models for the Late Jurassic Nevadan orogeny that call on changing plate kinematics resulting in periods of transtension and transpression along the margin, rather than westward subduction of the North American plate beneath an island archipelago or double subduction of the Mezcalero plate.