MULTI-METHOD DETRITAL THERMOCHRONOLOGY OF THE GREAT VALLEY GROUP, CALIFORNIA

To illustrate the power of simultaneously using several age-dating methods on detrital samples, we here present the preliminary results of a study that applies single-grain U/Pb (zircon), ⁴⁰Ar/³⁹Ar (micas and K-feldspar), AFT, ZFT, and (U-Th)/He (apatite) dating, along with vitrinite reflectance measurements on a suite of samples of the Late Cretaceous Great Valley Group (GVG), near Coalinga (San Joaquin Valley, California). Published paleocurrent data and U/Pb ages of detrital zircons show that these sediments have a provenance in the Sierra Nevada. Conventional petrographic provenance studies, along with Sr and Nd isotope studies, and the evolution of the U/Pb grain-age histograms tell an internally consistent story of increasing geochemical and geomorphic maturity of the Cretaceous Sierra Nevada magmatic arc with time.

AFT dating of the same sediments tells a complementary story. Four of five GVG samples have AFT ages that range from 71-84Ma. They are at most partially annealed and represent a minimum constraint on the exhumation age of the Sierra Nevadan sediment source, which closely follows the crystallization age as measured by the U/Pb SHRIMP data. The structurally deepest AFT sample has an apparent age of 13Ma. It is very proximal to the fault contact of the GVG with the New Idria serpentinite body, which forms the core of the San Joaquin Ridge anticline. Coleman (1996) argued that the New Idria body represents subducted oceanic crust that was stranded at depth, serpentinized, and then rose rapidly during the Middle Miocene from depths on the order of 20 km. We interpret the 13 Ma fission track age to record thermal resetting of the fission track clock during tectonic juxtapostion of the basal GVG against the hot, rapidly rising serpentinite body. This is a very unusual mechanism for resetting the fission track system and would require very rapid rise of the serpentine body to transfer sufficient heat. The rising New Idria body deposited on the order of 15 km3 of clastic serpentinite in the Great Valley basin (Big Blue Formation). An apatite fission track sample from these deposits yields a mixture of middle Miocene and Cretaceous single grain ages and confirms that substantial portions of the body rose very rapidly to shed detritus with only a very short gap between the fission track and depositional age.