Thermal History of Rocks in Southern San Joaquin Valley, California: Evidence from Fission-Track Analysis (1)

Abstract

The theory of the fission-track method and its application to sedimentary basin analysis is illustrated by a case study in the southern San Joaquin Valley, California. Fission tracks provide a powerful tool for studying the thermal history of sedimentary basins because the two minerals most commonly used in fission-track studies, apatite and zircon, occur as detrital constituents in many sedimentary rocks, and their annealing temperatures span the main temperature range for oil generation. Fission tracks also provide information on the sedimentation record and provenance of rocks in a basin. We have used fission-track analysis to study the thermal and depositional history of the subsurface Tertiary sedimentary rocks on both sides of the active White Wolf reverse fault in the southern San Joaquin Valley. The distinctly different thermal histories of the rocks in the two structural blocks are clearly reflected in the apatite fission-track data, which suggest that rocks in the rapidly subsiding basin northwest of the fault have been near their present temperature for only about 1 m.y. compared with about 10 m.y. for rocks southeast of the fault. These estimates of heating time agree with previous estimates for these rocks. Zircon fission-track data indicate that the Tertiary sediments were derived from parent rocks of more than one age. However, from at least the Eocene to late Miocene or Pliocene, the major sediment source was rocks related to the youngest Sierra Nevada Mesozoic intrusive complexes, which are presently exposed east and south of the southern San Joaquin Valley.