Correlation of the Miocene Peach Spring Tuff with the geomagnetic polarity time scale and new constraints on tectonic rotations in the Mojave Desert, California

Abstract

We report new paleomagnetic results and 40 Ar/ 39 Ar ages from the Peach Spring Tuff (PST), a key marker bed that occurs in the desert region between Barstow, California, and Peach Springs, Arizona. The 40 Ar/ 39 Ar ages were determined using individual hand-picked sanidine crystals from ash-flow specimens used in previous paleomagnetic studies at eight sites correlated by mineralogy, stratigraphic position, and magnetic inclination. Site-mean ages, which range from 18.43 Ma to 18.78 Ma with analytical precision (1 s.d.) typically 0.04 Ma, were obtained from areas near Fort Rock, AZ; McCullough Mts, NV; Cima Dome, Parker Dam, Danby, Ludlow, Kane Walsh, and Stoddard Wash, CA. The regional mean age determination is 18.71 ± 0.13 Ma, after the data were selected for sanidine crystals that yielded greater than 90% radiogenic argon (N=40). This age determination is compatible with previous 40 Ar/ 39 Ar dating of the PST after taking various neutron-flux monitor calibrations into account. We report paleomagnetic results from eight new sites that bear on reconstructions of the Miocene basins associated with the Hector Formation, Barstow Formation, and similar fine-grained sedimentary deposits in the Barstow region. Key findings of the new paleomagnetic study pertain to age control of the Hector Formation and clockwise rotation of the Northeast Mojave Domain. Our study of a rhyolitic ash flow at Baxter Wash, northern Cady Mountains, confirms the correlation of the PST within the Hector Formation and prompts reinterpretation of the previously determined magnetostratigraphy. Our model correlates the PST to the normal-polarity zone just below the C6-C5E boundary (18.748 Ma) of the astronomically tuned Geomagnetic Polarity Time Scale. After emplacement of the Peach Spring Tuff at Alvord Mountain and the Cady Mountains, the southern part of the Northeast Mojave Domain (between Cady and Coyote Lake faults) underwent clockwise rotation of 30°–55°. Clockwise rotations increase with distance northward from the Cady fault and may reflect Late Miocene and younger accommodation of right-lateral motion across the Eastern California Shear Zone. The new results also expand the area known to be affected by the Peach Springs eruption, and confirm that a pink ash-flow tuff surrounding Daggett Ridge near Barstow is part of the PST.