Clockwise rotation of the western Mojave Desert

Paleomagnetic results from Miocene volcanic rocks in the western Mojave Desert suggest about 25° of clockwise rotation for this region. Twelve sites (110 oriented cores) were collected from the 20 Ma Saddleback Basalt and seven sites (56 oriented cores) from the Red Buttes Quartz Basalt around Boron, California. Structural corrections were made from the attitude of the basalt or the Tropico Group sedimentary rocks in which the sampled basalts are interbedded. Paleomagnetic directions are coincident for the two basalts and show both normal and reversed polarity. After demagnetization to 40 or 60 mT, application of structural corrections, and inversion of reversed sites, the data yield an average direction of I = 51.6° and D = 15.6°, k = 19,α₉₅ = 8.2°. Comparison with the expected direction for Miocene rocks for stable North America shows a clockwise rotation of 23.8° ±11.3° and a flattening of 2.1° ±7.4°. Clockwise rotation of the western Mojave agrees in direction with oroclinal bending of the southern Sierra Nevada due to right-lateral shear along the western margin of North America. Most of this rotation is constrained by other paleomagnetic and structural information to have occurred soon after the sampled basalts were deposited (about 20 Ma) and before about 16 Ma. These clockwise declination anomalies also indicate that any subsequent counterclockwise rotation of the Mojave is small and/or compensated by previous clockwise rotation.