CONDITIONS OF INITIATION OF THE FRANCISCAN SUBDUCTION COMPLEX FROM LU-HF AGES ON HIGH-GRADE BLOCKS

Lu-Hf growth ages on garnet amphibolite, eclogite, and garnet-glaucophane schist blocks from the Franciscan complex in the central and northern Coast Ranges of California give highly precise ages that allow us to place new constraints on the early thermal history of the Franciscan subduction zone. The oldest age is 168.7±0.8 on plagioclase-bearing garnet amphibolite from Panoche Pass, metamorphosed at around 700°C. We interpret this as a remnant of a high-temperature metamorphic zone developed beneath the hot hanging wall of the newly initiated subduction zone, which suggests that initiation was virtually coeval with the formation of the tectonically overlying Coast Range Ophiolite. Garnet growth ages from other blocks correlate well with the maximum temperature of metamorphism, suggesting that accretion continued during cooling at an average rate of about 15°C/m.y. over a period of more than 20 m.y.

The thermal history requires high initial geothermal gradients within both the footwall and the hanging wall of the subduction zone and a relatively slow subduction rate of the order of 10 km/Ma during the initial stages of Franciscan subduction. Such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre. The data also suggest slow exhumation rates and significant residence time at depth of the earliest Franciscan rocks.

Apatite and zircon fission-track analyses from Franciscan greywacke adjacent to these blocks indicate that their final exhumation took place in late Cretaceous to Eocene time. Exhumation from ca. 40 km depth therefore took between 70 and 120 m.y. If exhumation was a result of corner flow circulation in the wedge, this comparatively slow rate (~0.5 mm/yr) suggests that the bulk effective viscosity of the wedge is likely to have been at least 6 E-20 Pa.sec., four to five orders of magnitude greater than implied by flow-melange models for Franciscan exhumation.