The origin of microcontinents in the Central Asian Orogenic Belt: Constraints from paleomagnetism and geochronology

Abstract

The Central Asian Orogenic Belt (CAOB) is widely recognized as a locus of Asia's main growth during the Neoproterozoic–Paleozoic, but its evolution remains controversial. The views on the most enigmatic, late Neoproterozoic to Cambrian, stages are critically dependent on the origin and subsequent kinematics of numerous microcontinents that comprise the structure of Kazakhstan, Tien Shan, Altai and Mongolia.We report new paleomagnetic data and U–Pb zircon ages from Neoproterozoic volcano-sedimentary rocks from the Lesser Karatau block in central Kazakhstan. The laser ablation U–Pb age of felsic tuff of the Kurgan Fm. is 766±7Ma. Thermal demagnetization revealed that most studied samples retained a dual-polarity pre-tilting component whose primary origin is supported by a conglomerate test. According these paleomagnetic data, the Lesser Karatau microcontinent was located at a paleolatitude of 34.2±5.3°, N or S, at about 770Ma. There is only one additional CAOB microcontinent, the Baydaric microcontinent in central Mongolia, for which reliable paleomagnetic data indicate a paleolatitude of 47±14°, N or S, at about 770–805Ma (Levashova et al., 2010).Several lines of evidence favor the view that the above CAOB microcontinents were originally parts of two larger domains, thus allowing extrapolation of the above paleomagnetic data to much larger territories, the Kazakhstan and Mongol domains that, in turn, might have belonged to major cratonic areas. A comparison of our paleomagnetic data with those from the larger cratonic nuclei provides first-order constraints on the origins of the CAOB microcontinents. We compare the existing tectonostratigraphic correlations between the Neoproterozoic to early Paleozoic sections of the microcontinents with coeval sections on the margins of Tarim, Australia, South China, Siberia, and North China. This combined analysis excludes a southern hemispheric location for the CAOB microcontinents at 750–800Ma. Of the several cratons that were located in the northern hemisphere at that time we favor a hypothesis that the Kazakhstan and Mongol domains had originally belonged either to Tarim or South China.