Constraints on uplift in the Franciscan Subduction Complex from apatite fission track analysis

The Franciscan accretionary prism of California contains large belts of sediments that were subducted to depths of 10 to 30 km, metamorphosed there under high‐pressure, low‐temperature conditions, and later uplifted back to the surface. Reconnaissance apatite fission track data (40 samples) place constraints on the uplift histories of these and related lower‐pressure rocks. Franciscan metagreywackes from a few areas (parts of the Diablo Range, parts of the northern Coast Ranges' Eastern and Central belts) retain imprecise fission track evidence of cooling in the middle or Late Cretaceous(?). Using annealing temperatures of fission tracks in apatite and assuming 9°C/km subduction‐period geothermal gradients (estimated from blueschist mineralogy, paleogeothermal gradients in the Great Valley forearc basin, plate motion reconstructions, and other data), these rocks were uplifted through about the 12 km depth level at that time. Since some of these rocks were probably metamorphosed at depths of 15–25 km, substantial uplift had occurred by the middle or Late Cretaceous(?). Rocks from several areas retain fission track evidence of cooling in the early to middle Tertiary, including the Diablo Range (roughly 32 Ma), San Francisco (about 43 Ma), Chiles Creek in Napa County (about 45 Ma), and parts of the northern Coast Ranges' Eastern and Central belts (early to middle Tertiary). Great Valley forearc basin sandstones in the Clear Lake outlier retain evidence of cooling at roughly 35 Ma. Assuming 8.5°–10.5°C/km gradients, rocks from these areas were buried at least 8–10 km in the early or middle Tertiary and experienced at least a few kilometers of uplift at that time. In the northern Coast Ranges, two areas retain evidence of cooling in the late Tertiary (Coastal belt at roughly 20–10 Ma; Yolla Bolly Mountains in Eastern belt at roughly 20 Ma). Assuming 12°C/km gradients, they were buried at least 7 km in the late Tertiary and were uplifted at least a few kilometers at that time. The data thus suggest in general that uplift was occurring in various areas of the Franciscan forearc more or less continuously thoughout much of the period that subduction was occurring. This progressive uplift is most compatible with models where uplift is driven by synsubduction underplating of subducted sediments at the base of the accretionary prism, because those models predict continuing, long‐term uplift throughout the period of subduction but also allow episodicity in uplift as underplating rates and locations vary. Modern geothermal gradients in most Franciscan outcrop areas are moderately high (about 28°–35°C/km), a consequence of latest Cenozoic termination of subduction and development of the San Andreas transform margin. Franciscan and Great Valley samples show no fission track evidence of latest Cenozoic exposure to high temperatures (>100°C). Therefore they were only shallowly buried during the postsubduction period, and postsubduction uplift has been relatively minor (maximum of about 3.2 km).