Tectonic Evolution of the Condrey Mountain Schist: An Intact Record of Late Jurassic to Early Cretaceous Franciscan Subduction and Underplating

The Klamath Mountains in northern California and southern Oregon are thought to record 200+ m.y. of subduction and terrane accretion, whereas the outboard Franciscan Complex records ocean-continent subduction along the North American margin. Unraveling the Klamath Mountains' Late Jurassic history could help constrain this transition in subduction style. Key is the Mesozoic Condrey Mountain Schist (CMS), comprising, in part, a subduction complex that occupies a structural window through older, overlying central Klamath thrust sheets but with otherwise uncertain relationships to more outboard Klamath or Franciscan terranes. The CMS consists of two units (upper and lower), which could be correlated with (a) other Klamath terranes, (b) the Franciscan, or (c) neither based on regional structures and limited extant age data. Upper CMS protolith and metamorphic dates overlap with other Klamath terranes, but the lower CMS remains enigmatic. We used multiple geochronometers to constrain the timing of lower CMS deposition and metamorphism. Maximum depositional ages (MDAs) derived from detrital zircon geochronology of metasedimentary rocks are 153–135 Ma. Metamorphic ages from white mica K-Ar and Rb-Sr multi-mineral isochrons from intercalated and coherently deformed metamafic lenses are 133–116 Ma. Lower CMS MDAs (<153 Ma) predominantly postdate other Klamath terrane ages, but subduction metamorphism appears to start before the earliest coherent Franciscan underplating (ca. 123 Ma). The lower CMS thus occupies a spatial and temporal position between the Klamath Mountains and Franciscan and preserves a non-retrogressed record of the Franciscan Complex's early history (>123 Ma), otherwise only partially preserved in retrogressed Franciscan high grade blocks.