SUBDUCTION INITIATION ALONG THE CALIFORNIA PLATE MARGIN: TIMING AND THERMAL EVOLUTION OF THE FRANCISCAN COMPLEX

Subduction of the Farallon plate began along central western North America in the Late Jurassic. Timing relations for subduction initiation are most evident in California from the classic triad of the Franciscan accretionary prism, Great Valley forearc basin above the Coast Range Ophiolite (CRO), and the batholithic roots of the Sierran volcanic arc. The CRO, upturned and variably exposed along about 1000 km of the margin length (taking into account offset along San Andreas related faults), formed between 169-163 Ma (U-Pb zircon ages). The oldest blueschist facies metamorphics in the Franciscan, the mafic "high-grade" gar-epi blueschist/eclogite, mostly occur as blocks scattered in shale-matrix melange. Ar-isotopic ages indicate recrystallization between ~155 to 140 Ma. Ar-ages for law ± arag coherent blueschist metasediments are as old as ~125 Ma (South Fork Mountain Schist) and as young as ~90 Ma.

The simplest explanation for regional geologic relationships is that Franciscan subduction began at about 155 Ma beneath young, thin lithosphere capped by the CRO. Enough convergence (200 km?) occurred to generate some magmatism in the Sierra (e.g., Guadalupe pluton), but there was little sediment atop the subducting plate and accretion was negligible. From about 145 to 125 Ma, convergence was slow (1 cm/yr?) as evident from the scarcity of magmatics in the Sierra, but continuous as evident from the retrograde blueschist facies overprint widely recognized in the high-T blueschist/eclogite blocks (plate interface gradients ~15°C/km). The Knoxville Fm. accumulated in the Great Valley forearc basin, while the trench created by the early phase of fast (5 cm/yr?) subduction became filled causing clastic strata to prograde far seaward. At about 125 Ma, subduction sped up greatly as evident from the voluminous plutonism in the Sierra Nevada and global plate motion reconstructions. Underplating of the early Cretaceous trench-filling strata formed the coherent terranes of the Eastern Belt. Sediment reaching the trench axis was then quickly transported into the subduction channel shear zone. Eventually the combination of a sufficient supply of trench sediment, fast subduction speed (10± cm/yr), and plate dip caused deeply subducted mud-rich sediments to upwell forming the melange of the Central Belt.