Intensities of the Earth's magnetic field about 60 m.y. ago determined from the Deccan Trap Basalts, India

The Thelliers' method was applied to 28 samples from 14 magnetically stable lavas of the Deccan traps, India. They correspond to a time interval longer than 104 years but shorter than 3 m.y. about 60 m.y. ago. Sixteen samples from nine lavas were successful in experiments and gave estimates of paleointensities of the geomagnetic field. Unsuccessful samples suggest that changes in their blocking temperature spectra were induced by the heating. Those samples with reversed polarity gave paleomoments of the geomagnetic field not much different from present or very recent values (mean, 8.40 × 1025 G cm3). On the other hand, those corresponding to a reversed normal transition period gave substantially smaller values (mean, 2.85 × 1025 G cm3). Paleointensity data obtained so far by the Thelliers' method suggest a binary structure of the geomagnetic field for the Cenozoic and Late Cretaceous; i.e., the strength of the geomagnetic field, except in the Brunhes normal epoch and short transition periods, has been similar regardless of the polarity, normal or reversed. It is suggested that with the use of a precisely controlled magnetic field and magnetically stable samples it should be possible to perform the Thelliers' experiments by single heatings, instead of the conventional paired heatings, to individual temperatures. The modification will greatly reduce the labor in paleointensity studies without losing the merits of the Thelliers' method.