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Late Cretaceous age of fractures in the Sierra Nevada batholith, California

P. Segall·E.H. McKee·S.J. Martel·B. D. Turrin
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Abstract

Regional sets of steeply dipping joints and faults are common throughout the Sierra Nevada batholith, yet relatively little is known about how or when they formed. Within some east-northeast-striking, left-lateral fault zones in the Mount Abbot quadrangle of the central Sierra Nevada, the host granodiorite is hydrothermally altered to a lower greenschist assemblage that contains muscovite. The muscovite yields a mean K-Ar and 40 Ar/ 39 Ar age of 79 Ma, which provides a minimum age for the faulting. Field relations show that these faults developed from earlier formed, mineralized joints, so these ages also provide a minimum age for the jointing. Published ages of biotite, hornblende, and zircon from the host granodiorite of Lake Edison are 80 Ma (K-Ar), 85 Ma (K-Ar), and 90 Ma (U-Pb), respectively. The geochronology, field relations, and hydrothermal mineral assemblages together suggest that the mineralized joints and faults all formed between 85 and 79 Ma, soon after the host pluton was emplaced.

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P. Segall, E.H. McKee, S.J. Martel, B. D. Turrin. 1990. Late Cretaceous age of fractures in the Sierra Nevada batholith, California. https://doi.org/10.1130/0091-7613(1990)018%3C1248%3Alcaofi%3E2.3.co%3B2

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