Cenozoic tectonic rotation of the Mojave Desert, California as indicated by paleomagnetic studies

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Thesis (Ph. D.)--University of Massachusetts at Amherst




Strain distribution across broad plate boundaries is inadequately understood. California's Mojave Desert has been adjacent to Tertiary convergent and transform plate margins and its general geology is fairly well known, making it an excellent location for crustal motion studies.^ Models for the Cenozoic history of the Mojave Desert predict crustal motions, but paleomagnetism is the primary method for quantifying such motions. The Barstow, California area has good exposure of appropriate rocks ranging from Oligocene-earliest Miocene to Pliocene in age. Standard paleomagnetic sampling and analysis of these volcanic units yielded primary paleomagnetic directions, which spatially and temporally constrain crustal motions in the Barstow area. Observed magnetic directions were compared with expected directions for cratonic North America allowing determination of crustal rotations and translations.^ Paleomagnetic directions from the Lane Mountain Quartz Latites and Jackhammer Formation suggest 55$\sp\circ$ to 75$\sp\circ$ of clockwise rotation of the Barstow area in earliest Miocene time which may be related to oroclinal bending of the southern Sierra Nevada batholith. These units may also record a geomagnetic reversal.^ Twenty-three degrees of counterclockwise rotation of the Pickhandle Formation coincides with early Miocene northeast-southwest extension and detachment faulting in the Mojave Desert. Other parts of the desert experienced variable senses and amounts of rotation concurrently. Drag along transfer zones or detachment surface geometry appear to have caused upper plate rotations in the extended terranes, while the lower plates remained unrotated. These results suggest that extension in the Mojave Desert is related to similarly oriented Miocene extension in adjacent areas.^ This study, like most paleomagnetic work in the western and central Mojave Desert, shows no evidence for post-18 Ma rotation, suggesting that post-10 Ma right-lateral faulting there has produced relatively little crustal deformation. The exception to this generalization is the northeast corner of the Mojave Desert which underwent about 28$\sp\circ$ of post-18 Ma clockwise rotation. This enigmatic and structurally different northeastern area has apparently experienced atypical post-middle Miocene activity. Paleomagnetic flattening data and structural constraints indicate that post-Oligocene north-south translation of crustal blocks in the Mojave Desert has been insignificant.