ASYMMETRY IN THE REVERSAL RATE BEFORE AND AFTER THE CRETACEOUS NORMALPOLARITY SUPERCHRON

The geomagnetic reversal chronology shows a decreasing reversal rate, lambda, from about 160 Ma to about 118 Ma, a cessation in the reversal process (the Cretaceous Normal Polarity Superchron) until about 83 Ma , and then an increasing lambda until recent times. Visually, the rate of decrease in lambda leading into the superchron appears lar er than the rate of increase in lambda following the superchron. Statistical tests developed here confirm the visual perception of an asymmetry in the rate of change in lambda. This asymmetry could be caused by a rela tively rapid breakdown of D'', with thermal energy being advected away followed by a slower thickening of the layer through thermal conducti on. However, in the absence of good thermal models as to how different core-mantle boundary conditions affect the geodynamo there are probab ly many apparently viable speculations as to the origin of the observe d rate asymmetry.