Michael J. Valentine

Cenozoic crustal rotations in the Mojave Desert from paleomagnetic studies around Barstow, California

Paleomagnetic data from the Mojave Desert around Barstow, California suggest 55° to 75° of clockwise rotation of this area in earliest Miocene time, followed by 23° of counterclockwise rotation in the Early Miocene, and no rotation after 18 Ma. Earliest Miocene clockwise rotation of the Barstow area is suggested by data from nine sites in the Oligocene-Miocene Lane Mountain Quartz Latite and Jackhammer Formation and may be related to oroclinal bending of the southern Sierra Nevada batholith. Data from 13 sites in the Early Miocene Pickhandle Formation indicate counterclockwise rotation, which coincides with Early Miocene E-NE-W-SW extension and detachment faulting in the Mojave Desert. Other parts of the Mojave Desert experienced variable senses and amounts of rotation at this time which may be related to drag along transfer zones within the upper plate, while the lower plate was not rotated. These results suggest that Miocene extension in the Mojave Desert is related to similarly oriented extension in the Colorado River extensional corridor. Early Miocene to Pliocene rocks of the Andesite and Dacite of the Calico Mountains, Barstow Formation, Andesite of Murphys Well, and Black Mountain Basalt show no evidence for significant rotation of the Barstow area after 18 Ma. Most other paleomagnetic data from elsewhere in the Mojave are consistent with a lack of significant post-18 Ma rotations, suggesting that post-10 Ma right-lateral faulting there has produced relatively little crustal rotation. The exception to this generalization is the northeast corner of the Mojave Desert which underwent substantial post-18 Ma clockwise rotation. Paleomagnetic flattening data also indicate that post-Oligocene N-S translation of crustal blocks in the Mojave Desert has been insignificant.