PALEOMAGNETISM OF PALEOGENE BASALTS FROM THE TIEN SHAN, KYRGYZSTAN: RIGID EURASIA AND DIPOLE GEOMAGNETIC FIELD

Abstract

Several conflicting explanations, invoking persistent non-dipole fields or tectonic motion on various scales, were suggested to account for anomalously low paleomagnetic inclinations in Cenozoic rocks of Central Asia. In order to better understand this problem we undertook a paleomagnetic study of Paleogene basalts from a part of the Tien Shan close to the China–Kyrgyzstan border. Stepwise thermal demagnetization showed that only a single-component remanence of reversed polarity is present in these rocks above 200–260°C. The overall mean direction of this remanence (D=194.6°, I=−54.0°, α95=3.8°, n=18 sites) agrees well with the Eurasian reference directions for the Paleogene which is in sharp contrast with most earlier published Cenozoic inclinations from Central Asia which are 20–30° shallower than the expected values. Good agreement of the reference inclinations and those from basalts rules out models which invoke non-dipole fields and/or large-scale tectonic motions for explanation of the Cenozoic inclination anomaly. Instead, a mechanism related to natural remanent magnetization acquisition in redbeds is strongly indicated. The above conclusion also implies that the differences between the reference data and measured inclinations in Cretaceous redbeds from Central Asia are also of non-tectonic origin and a revision of our views on evolution of this region is required.