Paleomagnetism of Upper Jurassic basalts from Transbaikalia: new data on the time of closure of the Mongol-Okhotsk Ocean and Mesozoic intraplate tectonics of Central Asia

The Upper Jurassic basalts (150-160 Ma) described as the Ichetui Formation over the territory of the Tugnui, Margintui, and Maly Khamar-Daban volcanic structures have been studied paleomagnetically. It is shown that natural remanent magnetization still contains a component which may reflect the geomagnetic field direction at the beginning of the Late Jurassic. This is supported by reversal and conglomerate tests. Calculation of mean paleopole gives: Plat = 63.6, Plong = 166.8, alpha 95 = 8.5 degrees. These values well coincide with the data for the Badin Formation from Mogzon depression, which lies east of the study area and approximately dates from the Kimmeridgian-Oxfordian interval of the Late Jurassic. At the same time, those poles statistically differ from the European and Southeast Asian poles of the same age. The available paleomagnetic data suggest that at the beginning of the Late Jurassic the Mongol-Okhotsk Ocean was probably still open. Since the early Late Jurassic the continental blocks of Southeastern Asia and Siberian part of the Eurasian plate had been approaching, with the Siberian domain rotating clockwise. Analysis of the total of data shows that sinistral strike-slip deformations were present not only in southern Siberia but also between the Siberian and European Platforms. Thus, the deformations of the Central Asian crust in the early Late Jurassic reflect the intraplate strike-slip motions coeval with the closure of the Mongol-Okhotsk Ocean and are governed by the clockwise rotation of the Siberian part of the Eurasian plate relative to its European part. (c) 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.