DETRITAL ZIRCON PROVENANCE ANALYSIS OF THE MARIPOSA FORMATION, CALIFORNIA: A RECORD OF THE TECTONIC EVOLUTION OF THE LATE JURASSIC CONTINENTAL MARGIN

The Late Jurassic Period is a critical time of tectonic evolution in the North American Cordillera, with a range of tectonic models proposed to explain the geologic relations in the Klamath-Sierra Nevada segment of the Cordillera. The Late Jurassic depositional age and location of the Mariposa Formation within the Sierra Nevada foothills means that these metasedimentary rocks contain a geologic record ideal for further understanding the tectonic evolution. U-Pb age analysis of detrital zircon from 9 meta-sandstone samples collected along ca. 135 km of Mariposa strike length reveal evidence of shifting provenance from Kimmeridgian to Tithonian time. Calculated MDAs were used as proxies for true depositional ages, given the active arc tectonic setting and abundance of Late Jurassic zircon in all samples. MDAs range from ca. 156 to ca. 147 Ma (Kimmeridgian to Tithonian), indicating at least 9 million years of turbidite deposition in the basin. Samples collected close to the ca. 151-152 Ma Guadalupe Intrusive Complex in the southernmost Mariposa Fm yield MDAs of ca. 152-155 Ma, indicating that development of metamorphic cleavage and intrusion rapidly succeeded deposition, even while deposition continued in other parts of the basin.

All samples contain abundant pre-Mesozoic zircon (37-86% of all grains), with an increasing proportion of Mesozoic arc-derived grains in Tithonian samples relative to Kimmeridgian samples (55% and 41%, respectively). Tithonian samples contain a broader age mode of Middle-Late Jurassic grains with some Early Jurassic grains, while Kimmeridgian samples form a narrower Middle-Late Jurassic age peak with few Triassic grains. Pre-Mesozoic age modes are similar from Kimmeridgian to Tithonian time, but their relative abundance shifts from larger modes of ca. 1800 and 2700 Ma grains in Kimmeridgian samples to larger modes of ca. 1000-1200 and 1400 Ma grains in Tithonian samples. The similarities in both depositional ages and detrital zircon age spectra with those documented for the Galice Fm in the Klamath Mountains suggest that these strata were deposited within an integrated Late Jurassic marginal basin, and their shifting provenance from Kimmeridgian to Tithonian time records the tectonic evolution of the continental margin during changing plate motions of the Late Jurassic Period.