New contributions to the Early Pliocene geomagnetic field strength: Case study of southern Caucasus volcanics

Abstract

We carried out a Thellier paleointensity study on a ~ 3.8 My Pliocene lava flow succession from Georgia (southern Caucasus). Previous paleomagnetic studies on this succession revealed that eight consecutive lava flows record a reverse polarity direction at the base of the section followed by a thick normal polarity zone of eighteen consecutive flows. 27 samples from 9 flows from both polarity zones were preselected for paleointensity experiments because of their low magnetic viscosity index, stable remanent magnetization, reversible thermomagnetic curves and minor fraction of grains with a multidomain magnetic structure. Altogether, 13 samples from 6 different cooling units yielded reliable paleointensity estimates with flow-mean virtual dipole moments rang- ing from 5.8 to 7.6*1022 Am2. Our results, although not numerous, are of high technical quality and comparable to other paleointensity data recently obtained on younger lava flows. The NRM fractions used for paleointensity determination range from 28 to 65% and the quality factors varies between 4.7 and 19.4, being normally greater than 5. The mean virtual dipole moment (VDM) obtained in this study is slightly lower than the present day geomagnetic field strength but it is in accordance with the mean early Pliocene worldwide VDM. The results are also similar to those recently reported for the late Miocene (8-10 My), which may indicate that geomagnetic field strength was stable and relatively high from about 10 to 4 My. More data are needed to better understand the transition mode between Mesozoic low and the present high geomagnetic field.