Noise across Olduvai Subchron: Paleomagnetic study of a Pliocene lava succession from Javakheti Highland (Georgia, Lesser Caucasus)

Abstract

Well-exposed lava sequences with available geochronological information may provide invaluable information on the fine characteristics of the Earth's magnetic field variation. A paleomagnetic study has been performed on twenty Pleistocene age, sub-horizontal lava flows in the Javakheti Highland (Lesser Caucasus). The sequence is formed by calc-alkaline basaltic lavas and is divided by a thin sedimentary layer in its upper part. We obtained two new radiometric ages: an age of 1.93±0.09 Ma for the flows below the sedimentary layer and 1.78±0.11 Ma for the upper part of the sequence. Thermomagnetic susceptibility vs. temperature curves indicate that remanence is carried by ferrimagnetic minerals like magnetite, titanomagnetite or titanomaghemite. Reliable paleomagnetic results could be obtained from all 20 analyzed flows, yielding the following reverse-polarity mean direction for the whole sequence: inclination I =-61.5°and declination D =200.1° (N = 20; α95 =3.1°; k=114). This direction does not agree with the expected one. The discrepancy can be explained by an apparent clockwise 17.1 ± 6.1° vertical-axis rotation. Alternatively, such moderate deviation may be due to some unstable geomagnetic regime or undersampled paleosecular variation. Isotope ages obtained in the present study indicate that the Toloshi sequence was emplaced during the normal-polarity Olduvai subchron, but palaeomagentic results indicate a reverse-polarity magnetization. At the same time, the analysis of virtual geomagnetic pole (VGP) scatter indicates that the Toloshi sequence was probably formed during a short time, insufficient to average paleosecular variation. These results suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode.