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Levashova N.M.,Russian Academy of Sciences | Meert J.G.,74 Williamson Hall | Gibsher A.S.,Russian Academy of Sciences | Grice W.C.,74 Williamson Hall | Bazhenov M.L.,Russian Academy of Sciences
Precambrian Research

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 ± 7. Ma. 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 770. Ma. 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 770-805. Ma (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 750-800. Ma. 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. © 2010 Elsevier B.V. Source

Meert J.G.,74 Williamson Hall | Gibsher A.S.,Russian Academy of Sciences | Levashova N.M.,Russian Academy of Sciences | Grice W.C.,74 Williamson Hall | And 2 more authors.
Gondwana Research

The Cambrian explosion, c. 530-515. Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635-542. Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~. 750-635. Ma Neoproterozoic glacial episodes by 20-30. million years. In this paper, we present evidence for the oldest discovery of the 'Ediacara' discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. "Marinoan") glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata. © 2010 International Association for Gondwana Research. Source

Levashova N.M.,Russian Academy of Sciences | Kalugin V.M.,Russian Academy of Sciences | Gibsher A.S.,Russian Academy of Sciences | Yff J.,74 Williamson Hall | And 3 more authors.

Existing views on the tectonic evolution of the Central Asian orogenic belt (CAOB) are highly controversial and the Neoproterozoic to Cambrian stages of this evolution remain the most enigmatic. However, the views on the Paleozoic evolution of the CAOB crucially depend on these early stages, as different choices of the starting point lead to very dissimilar Paleozoic reconstructions. In this context numerous microcontinents with the Precambrian basement that are included in the mosaic structure of Kazakhstan, Tien Shan, Altai and Mongolia are of particular interest. We undertook a paleomagnetic, geochemical and geochronological study of the Neoproterozoic volcanics from one of these units - the Baydaric microcontinent in Central Mongolia. According to U-Pb (laser ablation) dating the age of the studied Dzabkhan Volcanics is about 770-805Ma. Thermal demagnetization revealed that most of the studied samples retained a pre-tilting component, whose primary origin is supported by a conglomerate test. These new data, together with available geological information allow us to conclude that about 770-800Ma ago the Baydaric domain was located at a latitude of 47±12 +16° N and belonged to one of the following plates: India, South China, Tarim or Australia. © 2010 Elsevier B.V. Source

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