--> ABSTRACT: Thermal History of the Los Angeles Basin: Evidence from Fission-Track Analysis, by Nancy D. Naeser, Thane H. McCulloh, Kevin D. Crowley, Chris M. Reaves; #91003 (1990).
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ABSTRACT: Thermal History of the Los Angeles Basin: Evidence from Fission-Track Analysis

Previous HitNancyTop D. Naeser, Thane H. McCulloh, Kevin D. Crowley, Chris M. Reaves

Fission-track studies in two areas of the Los Angeles basin (the deep central syncline and the Santa Fe Springs oil field) are helping clarify the thermal history of this young, prolific petroliferous basin. In both areas, the thermal histories are constrained by exceptionally well-documented burial histories. Temperature gradients (about 27°C/km in the central syncline and 38°C/km at Santa Fe Springs) are thoroughly established. Very short effective heating times are suggested in both areas (about 1.4 m.y. for deep strata at Santa Fe Springs and even shorter in the central syncline) when geotherms are combined with burial-history curves.

Preliminary fission-track analysis of detrital apatite separated from core samples of sandstones of Pliocene age from exploratory holes in the central syncline indicates that tracks are totally annealed where temperatures exceed about 140°C (4500 m deep). Using commonly accepted kinetic models for apatite annealing, this temperature indicates an effective heating time of approximately 1.0 m.y.

In contrast, fission-track analysis of apatites from Pliocene to Miocene sandstones from a well on the northwest edge of the Santa Fe Springs oil field shows fission tracks are preserved in apatites at exceptionally high temperatures; track loss is not complete until temperatures reach 167-178°C (3795-4090 m deep). Average composition of the apatites, determined by electron microprobe, is F1.78Cl0.01(OH)0.21 The very low chlorine content of the apatites indicates that they are not unusually resistant to annealing. The exceptionally high annealing temperatures indicate that either the present high geothermal gradient at Santa Fe Springs oil field is a product of extremely youthful changes in thermal regime or that commonly accepted kinetic annealin models for apatite are inappropriate.

Study is proceeding to resolve this conflict. Additional samples from key areas in the Los Angeles basin are being analyzed, and refined thermal modeling is underway.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990