A study of the time-averaged paleomagnetic field for the last 195 million years

Abstract

During the last decade a great deal of paleomagnetic results have accumulated, and their accuracy and reliability have been increased significantly by improved demagnetisation techniques. In the present thesis all available paleomagnetic data have been analysed for the purposes of providing more understanding of characteristics of the geomagnetic field through geological time. Also, physical implications for geodynamo theory have been sought! As preparatory procedures, data selection, the separation of data according to polarity, and correction for plate tectonics effects have been carried out. With such selected (and corrected) paleomagnetic data, spherical harmonic analyses and statistical analyses of the paleosecular variatioll have been carried out for the time-averaged paleomagnetic field from Recent back to 195 Myr ago. The results of these analyses have been compared with the reversal frequency characteristics of the paleomagnetic field. Thefollowing conclusions have been drawn from the analyses: (1) During time intervals for which the reversal frequency characteristic is stable, the normal and reversed polarity fields can be taken to be the same in terms of time-averaged Gauss coefficients. However, during time intervals for which the reversal frequency characteristic is unstable, the two polarity fields· can be taken to be different from each other. (2) The zonal quadrupole and octapole field components reverse together with the axial dipole component when the field reverses its polarity. (3) The zonal octapole component has been dominant (up to a maximum strength of order G3=0.16) relative to the quadrupole component (maximum strength of order G2=0.11) through time, regardless of polarity. ( 4) The variations with time of the time-averaged Gauss coefficients for the normal polarity field have similar patterns to the variations with time of those for the reversed polarity field. (5) When the reversal frequency is low, the paleosecular variation is relatively small and is found to be mainly due to changes in the nondipole field. However, when the reversal frequency is high, the paleosecular variation is relatively large, and is found to be mainly due to dipole wobble. (6) The variations with time of the paleosecular variation characteristics for the normal polarity field have similar patterns to the variations with time of those for the reversed polarity field. These results provide some important implications which any satisfactory geodynamo model should meet.