Paläogeographische Entwicklung Kasachstans im Paläozoikum
Zusammenfassung der Projektergebnisse
Paleozoic Eurasia is made up of several continental blocks (Baltica, Siberia, Kazakhstan, Turan and Tarim), which were welded together during Carboniferous and Permian times. Within this agglomerate, Kazakhstan occupies a central position, and recognizing the tectonic evolution of this area is of great importance for understanding the geological history of entire Eurasia. Yet, the evolution of Kazakhstan is still not unambiguously understood and the main questions to be answered are related to (a) the identification of individual terranes, now integrated into Kazakhstan, (b) the reconstruction of the drift histories of these terranes prior to amalgamation an the drift history of the Kazakhstan continent after amalgamation, © the definition of the origin of the curvature of the Paleozoic fold belts, which form a giant tight loop. Addressing these problems a paleomagnetic study has been conducted in South Kazakhstan. The rocks exposed here, range in age from the early Ordovician to the Carboniferous. In total, 16 localities (187 sites, 1100 samples) with different ages and lithologies were investigated. Prefolding components of magnetization have been isolated in Lower Ordovician (the Lower Arenigian), Silurian, Lower to Middle Devonian, and Lower Carboniferous formations. The pre-folding component of magnetization from the Lower Arenigian red-beds defines the location of one of the major Lower Paleozoic microcontinents of Kazakhstan when all Kazakh terranes were still separated from each other. This is the first and only case in Kazakhstan where paleomagnetic data were obtained for one of the original terranes before amalgamation. The pre-folding components of magnetization isolated in the Silurian and Lower Devonian rocks in the South Chu-Yili mountains show a northerly declination and positive inclination and indicate a northerly paleolatitude of the area of around 12° if a normal polarity is assumed. The most striking result obtained in this study is probably the direction of the pre-folding component of magnetization identified in rocks of early Devonian age from the Koktas formation which is significantly different from both the reference directions for Baltica and Siberia. The resulting paleolatitude of 64° exceeds any expected value for the Paleozoic. In the Kendyktas ridge, a pre-folding component of magnetization was isolated in the Upper Devonian – and Lower Carboniferous red beds implies a paleolatitude of 22° N. Based on the data from this study and the data published during the last decade it becomes obvious that since the Middle Ordovician, North and South Kazakhstan show no significant difference in latitudinal positions. The majority of the paleomagnetic data indicate that in the Palaeozoic, both South and North Kazakhstan were situated slightly further to the North than it would be expected if Kazakhstan was a part of Baltica, or slightly further to the South than it would be expected if Kazakhstan was a part of Siberia. Since the Ordovician up to the Permian, Kazakhstan moved from southern latitudes into northern latitudes with drift rates close to those of Baltica and Siberia. The paleomagnetic rotations observed for South Kazakhstan cannot be reconciled with tectonic models such as the one for the evolution of the Kipchak arc. This model assumes, that since the Early Devonian southern Kazakhstan had experienced clockwise rotation of about 90° relative to Baltica and about 30° clockwise rotation with respect to Siberia.
Projektbezogene Publikationen (Auswahl)
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Paleomagnetism of Palaeozoic rocks from South Kazakhstan: Preliminary results and interpretation, in EGS-AGUEUG Joint Assembly, Nice, 2003
Alexyutin, M.V., Bachtadse V., Alexeiev, D.V., Nikitina, O.I.
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Paleomagnetism of Silurian and Devonian rocks from Central and Northern Kazakhstan - comparison with South Kazakhstan directions. in AUG Meeting, San Francisco, 2004
Alexyutin, M.V., Bachtadse V., Alexeiev, D.V.
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Paleomagnetism of the Devonian and Carboniferous rocks from Chu-Yili mountains, Southern Kazakhstan, in EUG Meeting, Nice, 2004
Alexyutin, M.V., Bachtadse V., Alexeiev, D.V.
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"Paleomagnetism of the Ordovician and Silurian rocks from the Chu-Yili and Kendyktas mountains, South Kazakhstan." Geophysical Journal International, 162 (2), 321-331, 2005
Alexyutin, M. V., Bachtadse V., Alexeiev, D.V., Nikitina, O.I.
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Paleozoic Geography and Paleomagnetism of Kazakhstan, Dissertation, Ludwig-Maximilians-Universität München, 89.pp., 2005
Alexyutin, M.V.
