CORRELATION, PROVENANCE, AND ACCRETIONARY HISTORY OF FRANCISCAN AND RELATED UNITS IN NORTHERN CALIFORNIA AND SOUTHERN OREGON

Northern California and southern Oregon contain a number of fault-bounded units of the Franciscan assemblage which have been proposed as correlative to more extensively studied units to the south. Collectively, these units represent the oldest known clastic material in the Franciscan assemblage and thus record the onset of accretion along the Mesozoic plate edge. This study analyzed ~2500 detrital zircon grains collected from 10 samples of Franciscan and nearby related units, with the aim of better characterizing proposed correlations and provenance and constraining the timing of the onset of accretion. Peak metamorphic temperatures were also determined for higher-grade Franciscan units, based on laser Raman analysis of carbonaceous material.

Maximum depositional ages (MDAs) of the Colebrooke Schist, South Fork Mountain Schist, and Redwood Schist, all proposed as subunits of the Pickett Peak terrane, range from 131 – 142.5 Ma. All three units contain significant numbers of Sierra Nevadan-aged grains as well as older grains, including peaks at ~415 Ma, 600 Ma, 1000 – 1200 Ma, 1450 Ma, and 1650 Ma + older grains. This suite of older grains matches the expected distributions of zircon ages for grains sourced from Jurassic sandstones of the Colorado Plateau and, in combination with the Sierra Nevadan grains, is interpreted as evidence for a similar North American provenance for these units. With the exception of a single anomalously low temperature from the Colebrooke Schist, all three units show peak metamorphic temperatures which range from 342 – 389°C. The Days Creek formation of the Snow Camp terrane has a MDA of 132 Ma while a sample from some of the northernmost outcrops mapped as Franciscan Central Belt, at Patrick’s Point, yielded a MDA of 137 Ma, older than previously reported MDAs of ~90 Ma from Central Belt exposures further south. Both the Days Creek and the Patrick’s Point samples show a provenance which matches the Pickett Peak samples, and they are similarly interpreted. New maximum depositional ages presented here, integrated with existing 40Ar/39Ar cooling ages, corroborate earlier work suggesting a profound Early Cretaceous shift from non-accretion to accretion beneath the edge of the North American plate.