USING DISCORDANT ZIRCON TO RE-EVALUATE THE NORTHERN EL PASO TERRANE: LATE PALEOZOIC TECTONIC EVOLUTION OF SOUTHWESTERN LAURENTIA, EXTENSION OF THE PERMIAN ARC, AND A MIDDLE JURASSIC HYDROTHERMAL EVENT WITHIN THE SOUTHEASTERN SIERRA NEVADA ARC

Quantitative modeling of strongly discordant, detrital zircon U-Pb isotopic data from metasedimentary pendants of the northern El Paso terrane yields upper and lower intercept information that constrains protolith identities as well as the timing of lead loss. Previous work in the Kern Plateau identified metasedimentary rocks with detrital zircon populations sourced near the Peace River Arch (Canada), but structural observations seemingly contradicted proposed correlations to the Roberts Mountain allochthon. Modeling of four, strongly discordant detrital zircon samples reveals Laurentian passive margin strata within the Kennedy Meadows and Bald Mountain pendants. These new samples are combined with published data to identify segments of the Roberts Mountain thrust in the Kern Plateau that were offset more than 350 km by late Paleozoic sinistral translation along the truncated margin of southwestern Laurentia. Published speculations regarding the presence of Devonian to earliest Permian strata deposited locally in the Kern Plateau are consistent with our field observations and detrital zircon data.

Late Paleozoic translation of the truncated Roberts Mountain allochthon likely occurred along the braided Kern Plateau shear zone, which engulfs the Kern Plateau pendants. New single-crystal U-Pb-Hf data from plutons intruding the shear zone are virtually identical to published data from the El Paso Mountains, demonstrating that the Sierra Nevada-Mojave arc initiated in the late Early Permian (ca. 274 Ma) along the entire length of the El Paso terrane and likely continued to be active into the Middle Triassic (ca. 240 Ma). Previously implicated arc activity in the Kern Plateau during the Late Triassic, however, is not corroborated by single-crystal U-Pb analysis. Published structural evidence indicating reactivation of the late Paleozoic Kern Plateau shear zone is reinterpreted as indicating oblique compression during arc initiation followed by Middle Jurassic extension in the southeastern Sierra Nevada arc, which facilitated intense hydrothermal activity and zircon lead loss.