Rapid Variation of the Geomagnetic Field in Reversal: Evidence from the Sheep Creek Range, North-Central Nevada

A paleomagnetic study of middle-Miocene basalts from the Sheep Creek Range of north-central Nevada was conducted to investigate prior evidence of extremely rapid change in geomagnetic field direction during a polarity reversal, and early results indicate that the event in question might not have been a reversal at all. In a single lava flow, magnetic directions varied from transitional at the margins to normal polarity in the center. To improve on past research, we drilled core samples in two evenly spaced vertical profiles, to gain a picture of magnetic field behavior at even intervals of time as the lava flow cooled. We also collected samples at four other basalt localities in the region in order to gather more information about the transition period, and to get samples from the earliest sections to cool, in order to characterize with certainty whether the event was a reversal or not. Early measurements have shown that alternating-field demagnetization is effective in removing VRM and lightning-strike overprinting in the Sheep Creek Range basalts. Preliminary results indicate that transitional magnetic directions are present in the lowest existing Miocene basalts; consequently it is impossible to determine the polarity of the geomagnetic field immediately before the transitional period began. The event that is recorded in the remanent magnetizations of the basalts could be either a reverse-to-normal polarity reversal, or an excursion of a normal field. Regardless, further research on the basalt samples collected in this study, especially the vertical profiles, will improve our understanding of geomagnetic field behavior during periods of significant fluctuation in field strength and magnetic pole location