Directional secular variation of the geomagnetic field in Late Devonian volcanics of the Northern Tien Shan

Abstract

We have paleomagnetically studied sixty andesite-basalt lava flows of the Late Devonian (Frasnian) in the Northern Tien Shan. Stepwise thermal demagnetization revealed a high-temperature dualpolarity magnetization in most of the samples. The primary origin of this component was established by the positive reversal and conglomerate tests. We found a clear difference in angular dispersion of this component between the lower and upper portions of the section. In the former, the dispersion is close to the predictions of various models of geomagnetic secular variation, while the scatter in the top part of the sequence is smaller by a factor of four. We conclude that this distinction is most likely due to the geomagnetic field behavior and hypothesize that the magnitude of secular variation could vary severalfold over time intervals of 105−106 years. This is in sharp contrast with other models of secular variation, where this magnitude has been assumed to be rather time-independent (for a given latitude). We discuss probable implications and alternative interpretations of the observed data.