Possible Recording of Rapidly Changing Transitional Geomagnetic Field Within a Single Lava Flow, Sheep Creek Range, North Central Nevada

New results demonstrate strong lateral inhomogeneity in the paleomagnetic and rock magnetic properties of a 15.6 my old lava flow from the Sheep Creek Range (north central Nevada) that was erupted during a geomagnetic polarity transition. The 4 to 5 m thick flow lies stratigraphically between two flows with remanence directions (D=349, I=-48 and D=166, I=-49) that differ by 83 degrees. We collected from the flow two profiles of oriented samples at approximately 20 cm vertical spacing. Detailed thermal demagnetization experiments show that samples through nearly the entire flow thickness at profile 1 contain a pattern of multiple remanence directions consistent with the hypothesis that that the flow cooled and acquired its magnetization as the transitional geomagnetic field underwent substantial and rapid directional change. Curiously, only those samples within 0.5 of the base of profile 2, just a few meters horiztonally from profile 1, exhibit the multicomponent remanence. Samples higher in the flow at profile 2 all have a single component remanence with same direction as the overlying flow. This pattern may have resulted from exceptionally deep thermal remagnetization by the overlying flow. Rock magnetic properties also differ between the two profiles. For example, the strength of an isothermal remanent magnetization acquired in a field of 80 mT decreases monotonically by a factor of 3 from flow base to flow top in profile 1, but is constant (and comparable to that at the top of profile 1) through the upper 2.5 m of profile 2. A plausible interpretation of these and other rock magnetic differences is that the flow at profile 1 was cooled so rapidly (or somehow insulated so effectively from reheating from above) that oxidation of the magnetic minerals was much less complete than at profile 2. Work in progress, including examination of polished thin sections, will clarify the origin of the lateral inhomogeneity and help us understand whether this lava flow has recorded very rapid geomagnetic change.