We present the results of geological and palaeomagnetic mapping of several lava profiles in Southwestern Iceland, covering a transition from reverse to normal polarity about 2 M.y. ago. The transition was first described by SIGURGEIRSSON (1957). We confirm that the virtual-pole path of this transition is not confined in longitude. We also present and discuss instances of complex pole paths in older strata in Iceland, and we provide new evidence for the general reliability of palaeomagnetic directions from lava flows.

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The paleomagnetism of the loess unit L₈ in Xifeng section of the Chinese Loess Plateau has been studied to determine the behavior of the Matuyama-Brunhes transitional field. Three primary results have been obtained. (1) The M-B transition is located in the low part of L₈ and the time duration of this transition defined by directional changes is estimated to be about 3, 600 yrs, whereas the intensity changes last about 8, 000 yrs, both of the estimations being based on the SPECMAP δ¹⁸O time scale (IMBRIE et al., 1984). (2) The morphology of the M-B transition field is characterized not by a single reversal course, but at least by three reversals and one aborted reversal course. The VGP paths obtained are not consistent with a non-dipole axisymmetric field. (3) The geomagnetic fields during this transition seem to have the configurations that dipole field component is at least comparable to the non-dipole ones in magnitude, which does not support the commonly-accepted viewpoint.

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A detailed record of the Matuyama-Jaramillo (reversed-normal) transition has been obtained from a loess section near the city of Lanzhou, in the Gansu province of China. At the position in the section where the reversal occurs, the accumulation rate has been estimated at 28 cm per thousand years. A continuous section of 6 m was collected for investigation using both thermal and alternating field demagnetisation. Results indicate that the reversal in direction had a duration of approximately 5000 years while the field strength was much reduced for a period of at least 9000 years. Secular variation in both direction and strength of the field is important throughout the sequence, being particularly strong while the field strength was low. As a consequence, the stability of the field leading up to the reversal in direction is poor, with a number of apparent short duration “reversals” due to a relative enhancement of the non-dipole field. Fourier analysis of the record shows an important periodicity close to 750 years in the field strength and periodicities in the horizontal and vertical field components which can be correlated with more recent secular variation. During the reversal, the VGP path is restricted in longitude, initially following the coast of Africa until it reaches the equator. It then swings west to the Caribbean before moving up through North America to complete the reversal. The second part of this path is within one of the preferred longitude bands recently highlighted by LAJ et al. (1991).

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Here we present a detailed record of the Matuyama/Brunhes (M/B) polarity transition recorded in a loess section at Xifeng (36°N, 108°E), in the central part of the Chinese loess plateau. The transition zone, which occurs in loess unit L8, has a thickness of 3.8 m and was sampled in full. At this point in the section the loess accumulation rate has been estimated at 10 cm/ka, and with the size of sub-sample used provides a potential time resolution of 240 years. Magnetite has been identified as the main magnetic mineral with a small contribution from maghaemite or cation deficient magnetite. The remanence of all samples was investigated using thermal demagnetisation from room temperature up to 650°C. The results indicate a duration of 32, 000 years for the decay and recovery of the main dipole intensity, and 20, 000 years for the period of major directional change. During the transition the field intensity falls to an average of 30% of its value before and after the transition, although there are strong variations about this weak value which accompany a sequence of 4 reversed/normal/reversed fluctuations of the local field vector. These fluctuations immediately preceed the final reversed-normal transition, after which the field intensity recovers to its pre-transition value. Calculated VGP paths of most stages are continuous and confined to preferred longitude bands which include America, the Atlantic Ocean or Africa. Field behaviour during the transition period suggests that non-dipolar terms are dominant while the dipole field is weak, with the dipole decay and recovery associated with the increased influence of low order zonal terms of the geomagnetic field.

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The Olduvai termination record recovered from remanent magnetization of the Red Loam has demonstrated the possibility of recording and preserving the geomagnetic polarity transition in eolian deposits and displayed some characteristics different from those of the Olduvai termination records available. The paleofield direction undergoes three phases of change during the polarity transition. In the first phase, only the declination changes, resulting in the VGP's moving along a meridional band, bounded by 30°E-150°W and 50°E-130°W, in high northern latitudes. The following phase is characterized by swift change in both declination and inclination. The VGPs move to the far hemisphere, migrate southerly, and come back to the near hemisphere at moderate southern latitude. A smooth approaching to the expected reverse polarity-like direction constitutes the third phase. The estimated durations of these three phases are 8900, 5300, and 6600 years, respectively. The VGP path is longitudinally confined and strongly near- and west-sided.

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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) × 10²² Am² (from 20 out of 25 sites), and for the Rajmahal Traps 5.5 ± 1.9 × 10²² 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 × 10²² Am² (n = 37). From this it appears that the Mesozoic dipole low had ended before the onset of the Cretaceous Quiet Zone.

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