Multimethod detrital thermochronology of the Great Valley Group near New Idria, California

Abstract

The simultaneous use of several thermochronological methods on replicate sedimentary rock samples can reveal their pre- and postdepositional history. Single grain U/Pb dating of zircon, zircon and apatite fi ssion track dating, and vitrinite refl ectance measurements were performed on Cretaceous through Miocene sedimentary rocks of the Great Valley Group and the Temblor Formation near Coalinga and New Idria, California. The data show that the Sierra Nevada was exhumed and cooled at ~0.5–1 km/m.y. or ~20 °C/m.y. during the Cretaceous. After deposition in the Great Valley forearc basin, Sierra Nevada erosional products were buried at great depth under low thermal gradients. At ca. 12–14 Ma, northward progression of the Mendocino triple junction triggered folding on the eastern fl anks of the California Coast Ranges and rapid exhumation of the New Idria serpentinite diapir. This middle Miocene event caused the deposition of spectacular deposits of sedimentary serpentinite (Big Blue Formation). The rapid rise of the hot serpentinite body created a thermal pulse that may have provided the enigmatic heat source for oil fi elds in the shallow Vallecitos syncline, a few kilometers north of New Idria.