Mesozoic subduction along the western margin of North America in California jumped west in mid-Jurassic time to form the Franciscan accretionary wedge outboard of the Coast Range Ophiolite and the Great Valley forearc basin. Based on Mesozoic plate motion analysis for the Pacific region, overall rates of subduction are likely to have been on the order of 50 km/m.y. New and previously published Lu–Hf garnet growth ages on high-grade blocks in the Franciscan, however, suggest that early Franciscan metamorphism took place in a slowly cooling environment, characteristic of slow subduction rates. In order to assess rates of early Franciscan subduction, 2-D thermal modeling was carried out based on the following assumptions: (1) Subduction was initiated at 170 Ma, based on the age of the oldest tectonic block, and on the age of the Coast Range Ophiolite, believed to have formed in a forearc setting behind the newly initiated subduction zone. (2) At the start of subduction the hanging wall mantle wedge was at asthenospheric temperatures, consistent with initiation of subduction at the ophiolite spreading center. (3) The earliest subducted lithosphere was 10 m.y. old at the start of subduction. (4) Tectonic blocks were metamorphosed at an average pressure of 1.4 GPa, corresponding to a depth of 43 km beneath mantle rocks (assuming ρ = 3.3 g cm−3).
Subduction rates were investigated between 1 and 300 km/m.y., and the best fit between the ages and estimated metamorphic temperatures of the blocks was found to be 10 ± 5 km/m.y. This is five times slower than the ~50 km/m.y. determined from plate motion analysis for that time, and suggests that some reappraisal of the configuration and velocities of the oceanic plates may be needed.