NEW CONSTRAINTS ON TECTONIC ROTATIONS IN THE MOJAVE DESERT, CALIFORNIA: PALEOMAGNETISM AND 40AR/39AR DATING OF THE MIOCENE PEACH SPRINGS TUFF

We report new paleomagnetic results and ⁴⁰Ar/³⁹Ar ages from a key marker bed in the region between Barstow, California and Peach Springs, Arizona. The single-crystal ⁴⁰Ar/³⁹Ar ages were determined from ash-flow specimens used in previous paleomagnetic studies at sites correlated to the Miocene Peach Springs Tuff (PST). Eight site-mean ages, which range from 18.43 Ma to 18.78 Ma, were obtained from areas near Fort Rock, AZ; McCullough Mts, NV; Cima, Parker Dam, Danby, Ludlow, Kane Wash, and Stoddard Wash, CA. The regional mean age determination is 18.71 ± 0.13 Ma (1 sd of the mean), after the data from all sites were selected for sanidine crystals that yielded greater than 90% radiogenic argon (N = 40). This age is compatible with previous ⁴⁰Ar/³⁹Ar dating of the PST (18.5 ± 0.2, 18.42 ± 0.07 Ma), taking various neutron-flux monitor calibrations into account. We report paleomagnetic results from 8 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 region. The results extend and confirm correlation of tuffs near Barstow (Daggett Ridge and Harvard Hill) with the PST. Other key findings pertain to age control of the Hector Formation and clockwise rotation of the Northeast Mojave Domain. Our paleomagnetic study of a rhyolitic ash flow at Baxter Wash, northern Cady Mountains, confirms the correlation of the PST in the Hector Formation and prompts reinterpretation of the previously determined magnetostratigraphy. Our model correlates the PST to a normal-polarity zone just below the C6-C5E boundary (18.75 Ma) of the astronomically tuned Geomagnetic Polarity Time Scale. After emplacement of the Peach Springs Tuff at Alvord Mountain and the Cady Mountains, the southern part of the Northeast Mojave Domain (between Cady and Coyote Lake faults) rotated clockwise 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.