Statistical Model of Secular Variation and Excursions and Drilling to the Moho

Almost all models for Earth's magnetic field secular variation derived from paleomagnetic results disregard results that give low latitude Virtual Geomagnetic Poles (VGPs). These results are regarded as "unusual" or "not part of the regular secular variation process". These low latitude VGPs are regarded as being produced at a time when the field is undergoing an excursion or a reversal and that therefore they should not be included in a secular variation study. However, it is almost impossible to determine logically the latitude of VGP below which results should be dismissed from study of the secular variation. And for some studies, it is important to include all results to arrive at a valid conclusion. This is the case with the paleomagnetic evidence that will be collected during the drilling through the oceanic crust to the Mohorovicic discontinuity, marking the top of the mantle. Accordingly, the results from the data set originally used by McElhinny and McFadden (1997) updated by addition of new results was expanded to include all the results giving low latitude VGPs that were left out of the updated data set. These expanded results were subjected to statistical analysis based on different latitude bands of the observation sites. This showed that for all latitude bands, the Fisher distribution gave a very bad fit to the data. However the data from every observational latitude band could be fitted by a Fisher distribution for about 90% of the data, plus a uniform distribution in latitude for the other 10% of the data. In addition, the Fisher distribution angular standard deviation showed a linear increase with observation latitude, rising from 11.5º at 10º latitude to 20º at 66º latitude. These results can be used to define a simple model which allows us to estimate the scatter to be found at any latitude, and in particular to determine the scatter in inclination that would be obtained from unoriented samples of basalt recovered during the Integrated Ocean Drilling Program (IODP). Since the magnetization of the lower crust is believed to contribute significantly to sea floor spreading magnetic anomalies it is important to be able to distinguish between normally and reversely magnetized rocks in the lower crust in order to estimate their contribution to the sea floor spreading magnetic anomalies. In the absence of magnetic orientation this can only be done by studying their magnetic inclination. For very low latitudes, there is only a small likelihood of being able to use inclination to determine polarity, whereas at higher latitudes, the probability of being correct rises. In order to achieve an accuracy of 95% the paleolatitude of the chosen hole has to be at an absolute latitude greater than 19º. Improvement of this result by moving to even higher latitudes is slow. 97% accuracy is only achieved at an absolute latitude greater than 28º. The current plan calls for a major hole to be drilled on a site that was formed at the equator, thus rendering it impossible to tell anything about polarity unless the samples can be oriented.