DECIPHERING THE TECTONIC EVOLUTION OF THE LATE JURASSIC CONTINENTAL MARGIN FROM DETRITAL ZIRCON PROVENANCE ANALYSIS OF THE GALICE AND MARIPOSA FORMATIONS, OREGON AND CALIFORNIA

The location of Late Jurassic deposition of the Galice (Klamath Mtns) and Mariposa (Sierra Nevada foothills) strata relative to the North American continental margin remains a subject of debate. Detrital zircon U-Pb age and Hf isotopic data from 20 metasandstone samples yield maximum depositional ages (MDAs) of ca. 160-151 Ma for the Galice Fm and ca. 160-148 Ma for the Mariposa Fm. These data indicate synchronous deposition that continued through initial Nevadan contractional deformation ca. 155 Ma. Compiled age spectra from the Galice and Mariposa Fms contain 50% and 56% pre-Mesozoic zircon, respectively, and individual samples with the oldest MDAs contain 75% pre-Mesozoic zircon. Mesozoic zircon grains have ε_Hf values from +15 to -20, with low values indicating incorporation of continental material into the magmas from which these zircon crystallized. The abundant pre-Mesozoic zircon and non-radiogenic Mesozoic zircon suggest deposition adjacent to or within the North American continental margin.

The Oxfordian through Tithonian MDAs (ICS timescale 2022/v10) permit assessment of the evolution of sediment sources. Pre-Mesozoic age spectra from the Galice Fm show minor changes in the abundance of different age modes between Oxfordian and Kimmeridgian compilations, and these age modes match sources in the older Klamath terranes and the Sierra Nevada pre-batholithic framework. The Kimmeridgian-Tithonian compilation from the Mariposa Fm contains a nearly identical age spectra as the Kimmeridgian Galice compilation. In contrast, the Oxfordian compilation from the Mariposa Fm is distinct from the other compilations, and is most similar to sources in the NV-UT backarc basin and passive margin. These results suggest that both the Galice and Mariposa Fms were linked to the continental margin by Oxfordian time (ca. 160 Ma), and that they became part of an integrated basin system with shared provenance through Kimmeridgian-Tithonian time, during the Nevadan orogeny.