ONSET OF SUBDUCTION ACCRETION IN THE FRANCISCAN COMPLEX VARIES ALONG STRIKE: NEW U-Pb DETRITAL ZIRCON INSIGHTS FROM THE SKAGGS SPRINGS AND GOAT MOUNTAIN SCHISTS

The Franciscan Complex of California represents a Jurassic to Cenozoic accretionary wedge formed during subduction of the Pacific Plate beneath the North American Plate. The complex is marked by an eastward increase in metamorphic grade and depositional age with significant internal metamorphic and age breaks. The Skaggs Springs and Goat Mountain schists represent relatively high-grade outliers and are of uncertain age and origin compared to adjacent material. U-Pb detrital zircon isotopic analyses were conducted using laser ablation inductively coupled plasma mass spectrometry to clarify the depositional age of clastic material at each location. Results from three samples (878 new analyses) of the Skaggs Springs Schist reveal a maximum depositional age of 120 - 125 Ma, approximately 20 - 25 Ma younger than previously reported. The younger depositional age is at odds with existing Ar-Ar geochronology, which yielded white mica ages of 132 Ma, interpreted to reflect the timing of metamorphism of the unit. New U-Pb detrital zircon data are consistent with interpretations that the Skaggs Springs Schist represents a structurally high thrust sheet situated in an anomalously outboard position. The Goat Mountain samples yield more complex results. Two samples (492 new analyses) yielded maximum depositional ages of ca. 97 Ma. A third sample (295 new analyses) returned a significantly older maximum depositional age of 144 Ma, one of the oldest maximum depositional ages extracted from the Franciscan. 144 Ma is in the age range of the basal forearc basin strata, suggesting that accretion of clastic units forms the earliest stages of trench-forearc clastic sedimentation, pushing the date of the onset of accretion in the Franciscan Complex from ca. 120 Ma to ca. 144 Ma. These findings suggest that multiple Franciscan subunits, unrecognized on existing geologic maps, are exposed at Goat Mountain. New detrital zircon data, in the context of existing detrital zircon data from clastic rocks of the Franciscan Complex, suggest that subduction accretion occurred in Early Cretaceous time in Northern California and in the Late Cretaceous in southern California and Baja California. We explore possible mechanisms explaining along-strike variations in accretion age of Franciscan clastic rocks.