THE REVERSAL TEST - AN EXAMINATION OF SECONDARY DIRECTIONS

Paleomagnetic data frequently display bipolar data that are not exactl y antipodal. One cause of this biasing is the addition of another late r (secondary) magnetization that cannot be removed by laboratory demag netization experiments. The magnitude of this hidden secondary magneti zation can be calculated when only two values are known: the non-antip odal angle (measured), and the angle between the Normal direction (mea sured) and secondary direction (measured or assumed). Furthermore, a m inimum magnitude of the secondary magnetization can be calculated with knowledge only of the nonantipodal angle. Applied to three published data sets (sandstone, limestone and diorite intrusion) with non-antipo dal angles of 11 degrees, 28 degrees and 11 degrees respectively; a se condary component is calculated at 25%, 28% and 16% of the characteris tic magnetization remaining after laboratory cleaning. Near surface al teration (weathering) appears to be the source of secondary magnetizat ion in two of these cases. A directionally consistent, biasing effect of a few degrees is made on the mean direction as compared to the conv entional calculation of averaging the nonantipodal directions.