GEOLOGIC SIGNIFICANCE OF CRETACEOUS TECTONOMETAMORPHIC UNITS IN THE FRANCISCAN COMPLEX, WESTERN CALIFORNIA

The Franciscan Yolla Bolly terrane of the northeastern California Coast Ranges consists mainly of quartzose metagraywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals including ubiquitous lawsonite ± pumpellyite ± aragonite ± glaucophane and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under P-T conditions of ~200-300°C and ~8 kbar at subduction-zone depths approaching~30 km. Petrologically similar Franciscan metaclastic-rich map units (i.e., Yolla Bolly terrane-like rocks, here designated the “YB” unit) crop out in the central and southern California Coast Ranges. Recently published detrital zircon U-Pb SIMS data for 19 “YB” metagraywackes indicate maximum ages of formation as follows: ~110-115 Ma (8) in the NE California Coast Ranges; ~95-107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85-92 Ma (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 mappable entities. Based on the current and likely Cretaceous 30° angular divergence between NS-paleomagnetic striping of the Farallon oceanic plate and the NNW-trending California convergent margin, I suggest that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust + its largely clastic sedimentary overburdon may have resulted in the progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagraywacke section. During the ~30 Myr interval ~115-85 Ma, the locus of “YB” accretion, underflow, and regurgitation evidently moved southeastward along a ~1000 km stretch of the accretionary margin of western California.