Thermal History by Fission-Track Dating, Tejon Oil Field Area, California

Data that have been obtained from deep drill holes in several areas where the approximate duration of heating events is known suggest that fission tracks in apatite and zircon are totally annealed when the grains are heated at temperatures of 135 to 105°C and 200 to 175°C, respectively, over periods of 10⁶ to 10⁸ years. Annealing can pose problems for determining the primary ages of samples, but if offers a powerful method for studies of their thermal and tectonic history. This paper explores the application of this method to determining the thermal history of sedimentary basins using detrital grains separated from drill-hole samples.

Detrital apatite and zircon have been separated from Eocene to Miocene sandstone recovered from deep drill holes in the southern San Joaquin Valley of California (Tejon oil field area). The Tejon area is divided by the seismically active White Wolf fault. Fission-track data show that apatite in the downthrown block immediately northwest of the fault is totally annealed at a maximum paleotemperature (reconstructed from laumontite geothermometry) of 135 to 140°C, suggesting heating of 10⁶ years’ duration. The higher paleotemperature (> 150°C) indicated for total annealing of apatite in the upthrown block shows that these samples could have been at the suggested maximum paleotemperatures for no more than 10⁵ years. The relative short duration of heating over the whole Tejon area is supported by the lack of annealing in zircon, even at paleotemperatures as high as 179°C. These conclusions are consistent with the thermal history suggested by laumontite crystalhzation viewed with stratigraphic and structural evidence.