DETRITAL ZIRCON GEOCHRONOLOGY OF TERRANES IN THE SAN JUAN ISLANDS – NORTHWEST CASCADES THRUST SYSTEM, WASHINGTON

The mid-Cretaceous San Juan Islands – northwest Cascades thrust system (NWCS) consists of six or more nappes that are a few kilometers or less thick, range up to 100 kilometers in breadth and are derived from previously accreted Paleozoic and Mesozoic terranes. Detrital zircons in quartzose gneiss of the Yellow Aster Complex are entirely Precambrian, matching zircon age patterns in the miogeocline and outboard Yukon-Tanana, Yreka and Shoo Fly terranes elsewhere in the Cordillera. Protolith of the analyzed rock is quartz arenite; probably of Ordovician age deposited not far outboard of the continent. Amount of possible coastwise displacement is not resolved by zircon ages because latitude discriminate zircon age populations are not apparent in the Ordovician miogeocline from northern B.C. to Mexico. Arc-marginal rocks of the Late Jurassic Constitution Fm., Lummi Fm., Fidalgo Complex and Easton Suite all bear a single or most prominent zircon age peak in the range of 148-155 Ma, and no pre-Mesozoic ages. The zircon patterns suggest that these terranes are mutually related. Zircon ages in these units are similar to Mesozoic zircon age patterns in other Late Jurassic terranes ranging from the Gravina Belt in Alaska to Great Valley Group in California. The age patterns do not constrain displacement, but do support the concept of extensive marginal basins during the Late Jurassic (e.g. Saleeby and Busby-Spera, 1992). Zircons from sandstone of the Bell Pass Mélange yield age populations indicating deposition and mélange formation younger than 119 Ma, and possibly younger than 110 Ma. The Mélange occurs midway in the nappe pile and bears a variety of exotic and unrelated fragments, including the Yellow Aster Complex, the 10x4 km slab of Twin Sisters dunite, Triassic cherts and ocean-island basalts, and Permian blueschist, the entire assemblage of which implies large-scale displacement. A similar youngest zircon age population of 114 Ma was found for the Nooksack Group which is footwall to the NWCS in the Cascades. Thus, the NWCS is younger than 110-114 Ma and formed by tectonism that brought together previously accreted rocks of widely different age and origin. Coast-parallel rather than coast-normal displacement is more easily envisaged for this process. A modern-day analogue could be displacement of the Siletzia or Yakutat terranes.