TECTONIC EVOLUTION OF THE CONDREY MOUNTAIN SCHIST: AN INTACT RECORD OF LATE JURASSIC TO EARLY CRETACEOUS FRANCISCAN SUBDUCTION AND UNDERPLATING

The Klamath Mountain accreted terranes in northern California and southern Oregon and the outboard Franciscan accretionary complex together record 300+ m.y. of subduction along the western North American margin. The Condrey Mountain Schist (CMS) subduction complex is the structurally lowest unit in the Klamaths and is exposed in a window through the other Klamath terranes, which were assembled prior to 153 Ma. Previous studies genetically linked the CMS to other Klamath terranes on the basis of regional relationships. Published CMS emplacement ages, however, range from 155 to <130 Ma and temporally overlap with both Klamath terrane emplacement and the poorly constrained early Franciscan history (ca. 168-123 Ma). To resolve whether the CMS is correlative with other Klamath terranes or instead reflects early Franciscan subduction, we collected samples across 10 km of CMS structural thickness and employed a variety of geochronometers to better constrain the timing of deposition, subduction, and metamorphism.

The CMS consists of a greenschist/epidote-amphibolite facies upper unit (upper CMS) and epidote-blueschist facies lower unit (lower CMS; 350-450°C, 0.8-1.1 GPa). The lower CMS is dominated by metasedimentary rocks with intercalated metamafic and metaultramafic lenses that underwent coherent deformation and underplating. Maximum depositional ages (MDAs) derived from detrital zircon U-Pb geochronology on metasedimentary rocks range from 153 Ma to 135 Ma. Zircon rims yielded ages of 145-125 Ma, while K-Ar on white mica and Rb-Sr mineral isochrons on intercalated epidote blueschists average 129 ± 8 Ma. Lower CMS MDAs constrain the sedimentary protolith depositional ages to <153 Ma. Based on detrital zircon rim, K-Ar white mica, and Rb-Sr ages, subduction and metamorphism (<145 Ma) only slightly postdated deposition.

The transition from Klamath-style terrane accretion to Franciscan-style subduction appears to only be preserved in high grade blocks and isolated slabs in the Franciscan. Late Jurassic to Early Cretaceous MDAs and metamorphic age constraints from the lower CMS post-date final assembly of the other Klamath terranes. This precludes correlation with these other terranes and suggests that the CMS is a coeval and non-retrogressed, coherently underplated record of the early Franciscan margin.