Provenance and thermal history of the Franciscan accretionary complex: Constraints from zircon fission track thermochronology

Abstract

The Franciscan accretionary complex of California contains large belts of sediments that were subducted to depths of ≈10 to 30 km, metamorphosed under high‐pressure and low‐temperature conditions, and later returned to the surface. Reconnaissance zircon fission track (FT) data (21 samples) place constraints on the sedimentary provenance, thermal history, and/or exhumation history of various Franciscan units. In a transect across the northern Coast Ranges through Mendocino Pass, metagraywacke samples from the Valantine Springs Formation of the Franciscan Eastern belt experienced maximum temperatures (Tmax) ranging from ≈250° to >≈310°C. Zircon FT and previously reported apatite FT data indicate that parts of the western Eastern and eastern Central belts cooled at about 100–70 Ma from ≈300°C to ≈100°C. Considering the low paleogeothermal gradient of roughly 9°C/km during Franciscan subduction, these rocks were apparently uplifted and exhumed from ≈30 to ≈10 km depth at this time. One sample from the Eyler Mountain sequence in the northwestern Diablo Range similarly shows Tmax ≥ ≈310°C and major cooling at ≈100–70 Ma. This uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the Franciscan wedge accompanied by extensional thinning at shallower levels, as a result of massive sediment influx into the Franciscan trench from the Sierra Nevada magmatic arc. In the northern Coast Ranges, the western part of the Central belt and the Coastal belt experienced lower Tmax, probably <200°C, and retain source area (detrital) zircon FT ages. Burnt Hills terrane samples in the Diablo Range also experienced Tmax <≈230°C and retain detrital ages. The detrital single‐grain age patterns from these areas are compatible with those expected from the Sierra Nevada magmatic arc and its country rocks. This is consistent with derivation of Franciscan sediments from arc terranes in California and provides no particular suggestion that these parts of the Franciscan are highly allochthonous. Paleogene Coastal belt sandstones also yield detrital ages compatible with derivation from the Sierra Nevada but also include early Tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the Idaho Batholith.