UPLIFT OF COHERENT BLUESCHIST TERRANES IN THE EASTERN BELT OF THE FRANCISCAN COMPLEX OF NORTHERN CALIFORNIA

Petrologic and microstructural data was obtained from nearly 500 samples collected along 11 transects across six map unit contacts in the Eastern Belt of the Franciscan Complex of northern California. The primary objective was to identify structural and petrologic constraints on the sense and magnitude of fault offset along these contacts. The metamorphic mineral assemblage in metagraywacke across the 25 km-wide study area is qtz + ab + chl + mus + law ± pump and in metapelites it is qtz + ab + chl + mus ± law. Rare jadeite is present in the west and paragonite is widespread in the easternmost unit, the South Fork Mountain Schist. Qtz, cal, and arag veins are common in all units except the easternmost area where only qtz veins were found. Phase equilibria, illite and chlorite crystallinity, the 2M1/2M1+1Md polytype ratio, and a west-to-east change in the strain style of detrital quartz grains from dominantly pressure solution to undulatory extinction to recrystallization all indicate maximum temperatures were ~200 to 250°C in the west and up to ~350°C in the east. Phengitic mus, law, and arag, indicate metamorphic pressure across the area were ~6 kb. The petrologic data are compatible with burial of as little as ~5 kb in the South Fork Mountain Schist and as much as ~7 kb in the west where jd is present in several samples. No significant petrologic or microstructural contrasts were detected for samples collected from units (Chicago Rock Melange, Hammerhorn Ridge Metagraywacke or Valentine Spring Formation) that were supposedly separated by post-metamorphic thrust faults of regional extent. The vertical component of post-metamorphic fault offset at the location of the contacts was small (< few km), if any. The results of this study are consistent with a tectonic model for the evolution of the Franciscan accretionary prism in which the overriding forearc block (capped by the Coast Range Ophiolite) acted as a source of heat during the first few tens of m.y. of convergence. As subduction continued, temperature/depth gradients became <10°C/km. Slow synsubduction uplift of the Eastern Belt was driven by deeper-seated underplating and trenchward-directed gravitational spreading, which caused thinning and uplift of the early-formed coherent blueschist terrane and overlying cover rocks at the edge of the overriding North American plate.