EarthRef.org Reference Database (ERR) -- Sherwood et al. 1993

We present the detailed results of our palaeointensity experiments on mid-Cretaceous basaltic lavas from the Rajmahal Traps in northeastern India and several formations in Israel. The aim of this study was to provide the first estimates of the geomagnetic field strength during the long period of normal polarity, the Cretaceous Quiet Zone. Using rock magnetic techniques-strong-field thermomagnetic behaviour, low-field low temperature susceptibility variation, and hysteresis properties, supported by optical and electron microscopy we have determined the magnetic mineralogy of the 48 sample sites. Deuteric oxidation is largely absent in both Israeli and Indian lavas, resulting in low Curie temperatures. Some degree of low temperature oxidation is always found in the Indian lavas, but is not common in the Israeli rocks. Stepwise thermal and alternating field demagnetisation show that many Israeli rocks have strong very soft overprints which may be lightning strikes. Although Cretaceous field directions are recorded in both Israeli and Indian lavas, and indicate that most of the lavas erupted during a period of normal polarity. the quality of the palaeomagnetic data is much higher in the Indian lavas with lower within site scatter. For each site, two samples were measured using the modified Thellier palaeointensity technique, and two using the Shaw technique with Rolph-Shaw correction. The magnetic mineralogy of the rocks are not conducive to obtaining good quality palaeointensity data. The within site scatter was often very high, particularly in the case of the Israeli lavas, where the Thellier technique often failed because secondary components could not be removed and the Shaw method failed because the rocks demagnetised completely at low alternating fields. This has led to one or two of the palaeointensity results being rejected from most sites, and a few Israeli sites being rejected altogether. The mean virtual dipole moment (VDM) from the Israeli lavas is 4.7 +/-2.2 (1 SD) x 10(22) Am2 (from 20 out of 25 sites), and for the Rajmahal Traps 5.5 +/- 1.9 x 10(22) Am2 (from 23 out of 23 sites). Both mean VDMs are lower than our previously published preliminary estimates based only on Thellier data. From radiometric ages it would appear that the igneous activity in the Ramon and the Rajmahal areas is almost contemporaneous; hence the mean VDMs from these two formations should give a similar estimate of the mean dipole moment of the Earth at the beginning of the Cretaceous Quiet Zone. Combining the intensity data from these two areas, the estimated mean VDM for this time is 5.4 +/- 1.9 x 10(22) Am2 (n = 37). From this it appears that the Mesozoic dipole low had ended before the onset of the Cretaceous Quiet Zone.