Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex, western California

The Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of similar to 200-300 degrees C and similar to 8 kbar at subduction-zone depths approaching 30km. Petrologically similar Franciscan metaclastic-rich map units - Yolla Bolly terrane-like rocks, here designated the YB' unit - crop out in the central and southern California Coast Ranges. Recently published detrital zircon UPb SIMS and LA-ICPMS data for 19 YB' metagreywackes indicate maximum ages of formation as follows: similar to 110-115Ma (8) in the NE California Coast Ranges; similar to 95-107Ma (7) in the San Francisco Bay area + Diablo Range; and similar to 85-92Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded YB' strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30 degrees angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the similar to 30 million year interval, similar to 115-85Ma, the locus of YB' accretion, underflow, and tectonic regurgitation evidently moved SE along an similar to 1000 km stretch of the accretionary margin of western California.