Lateral Variation in Rock Magnetic Properties and Natural Remanence in a 15 my old Lava Flow, Sheep Creek Range, North Central Nevada
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A paleomagnetic and rock magnetic survey of two vertical profiles through a basaltic lava flow in the Sheep Creek Range of Northern Nevada showed that magnetic properties of the lava vary laterally. Previous studies of this flow have indicated that it erupted 15.6 million years ago, during a transitional to normal state of the geomagnetic field when the field may have been changing as rapidly as a few degrees per day. As the flow cooled from 630 to 300?C over several months, the ferromagnetic minerals may have recorded the direction of the earth?s magnetic field as it changed producing an inhomogeneous remanent magnetizatios in the flow. This interpretation is highly controversial and more work is needed to confirm. In this study samples were taken at 20 cm intervals from the bottom of the flow to the top for both profiles and their remanent magnetizations were measured. Thermal demagnetization steps ranging from 150?C to 680?C show the variation of the field direction as the flow cooled. In Profile A, samples more than 0.5 meters above the flow base reveal a direction like that of the overlying flow while the samples from Profile B have a multicomponent magnetization with directions intermediate between those of flows both above and below it. Rock magnetic measurements reveal that Profile B is less completely oxidized than Profile A, perhaps indicating that Profile B cooled more quickly even though the flow is more than twenty-five percent thicker there.