PRELIMINARY PALEOMAGNETIC DATA FROM THE EASTERN MONO BASIN, CALIFORNIA AND NEVADA

The Adobe Hills in the eastern Mono Basin, California, are located between two regions with different mechanisms for transferring slip between subparallel strike-slip faults. The geometry and orientation of the prominent sinistral faults in the area are consistent with previous data for a simple shear that links the fault systems and has resulted in clockwise vertical-axis block rotation. This study hypothesizes that early Miocene deformation was located in eastern California near the Sierra Mountain Front prior to stepping east into the central Mina Deflection during the mid-late Pliocene. By using paleomagnetic data to quantify vertical-axis rotation, comparisons of each sections mean directions can then be distinguished. All sampling sites are located in stratigraphically continuous basalt flow sections within separate structural blocks. This sampling scheme allows us to evaluate an absolute magnitude of vertical axis rotation. In addition, we report new 40Ar/39Ar age determinations that yielded mid to late Pliocene dates between 3.17 Ma – 3.63 Ma, with an anomalous age of 31.53 Ma for AH-68. Petrographic analyses reveal that the basalts are primarily composed of a plagioclase matrix with large concentrically zoned augite, olivine, and opaque crystals of maghemite. The basalts from the Adobe Hills have yielded a stable single component magnetization that decays fairly linear to the origin during alternating field demagnetization, although some sites yield a more complex demagnetization behavior involving more than one magnetization component and required a combination of alternating field and thermal demagnetization to isolate the characteristic remanent magnetization. Forty sites from the Adobe Hills basalts indicate that some structural blocks experienced clockwise vertical axis rotation ranging from +15° ± 10° to +50° ± 10° in an area that occur outside of what was previously thought to be unrotated. This study provides the first paleomagnetic data for this area and will help form a better understanding of the processes involved with the transfer of deformation and extent of the areas of impact for future studies concerning slip transfer systems.