Subnormal geothermal gradients in the Great Valley Forearc Basin, California, during Franciscan Subduction: A fission track study

Abstract

Outboard parts of forearcs commonly have subnormal geothermal gradients because relatively cold lithosphere subducts beneath them. The strata of the 10‐ to 15‐km‐thick Late Jurassic‐Tertiary Great Valley forearc basin accumulated in the forearc of the Franciscan subduction zone of coastal California. In this study, paleogeothermal gradients in this basin were reconstructed to constrain the tectonic history of the convergent plate margin.Backstripping was used to reconstruct the burial histories of outcropping basin strata, and apatite fission track analysis was used to determine the maximum burial temperatures experienced by 64 rock samples. Plots of maximum burial temperatures versus maximum burial depths indicate earliest Tertiary (time of deepest burial) forearc basin geothermal gradients of about 9°C/km. Backstripping shows that strata from the lower parts of two sections were deeply buried (>6.5 km) for the period 90–65 Ma without experiencing high temperatures (≥105°C), indicating that gradients were at least moderately subnormal throughout that period. The data permit gradients no greater than about 15°C/km at 90 Ma, decreasing to no greater than about 9°C/km at 65 Ma. Lower strata in another section were probably deeply buried without experiencing high temperatures from 130 or 120 Ma until the latest Cretaceous or early Tertiary, indicating that subnormal gradients probably existed throughout that period. K‐Ar ages of Franciscan Complex blueschists indicate subnormal gradients in the California forearc in the Late Jurassic and Early Cretaceous, fission track data from the Sierra Nevada indicate subnormal gradients in the early and middle Cenozoic, and modern heat flow patterns indicate subnormal gradients in the late Cenozoic. Together, these indicators suggest that thermal gradients in the forearc were continuously subnormal from the initiation of Franciscan subduction in the Late Jurassic until the termination of subduction in the late Cenozoic.Gradients in the range that would permit blueschist facies metamorphism in the Franciscan Complex were thus long‐lived. The broad range of K‐Ar ages noted for Franciscan blueschists need not reflect multiple, discrete metamorphic events, but are also consistent with a continuous, extended period of high P/T conditions. Blueschists need not have been uplifted soon after metamorphism to avoid overprinting by normal P/T conditions, but rather may have remained at depth for extended periods.Preliminary modeling indicates that an interruption in subduction of longer than 5–20 m.y. should cause detectable heating of forearc basin strata, while a much shorter interruption should heat accretionary wedge rocks. Since there is no evidence of such heating, it appears that convergence at the Franciscan trench was essentially continuous. If allochthonous terranes traveled northward parallel to the California margin during the Late Jurassic to late Cenozoic period of Franciscan subduction, they apparently did so during oblique subduction, not during extended interludes of pure transform motion.