НОВЫЕ ПАЛЕОМАГНИТНЫЕ ДАННЫЕ ПО СИЛУРИЙСКИМ И ДЕВОНСКИМ ВУЛКАНИТАМ ЧИНГИЗСКОЙ ОСТРОВНОЙ ДУГИ КАЗАХСТАНА И ИХ ВКЛАД В ПРЕДСТАВЛЕНИЯ О ТЕКТОНИЧЕСКОЙ ЭВОЛЮЦИИ УРАЛО-МОНГОЛЬСКОГО ПОЯСА

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is the key to the formation of the Eurasian supercontinent during Paleozoic time, but the views on this complicated process remain very disparate and sometimes controversial. Three volcanic formations of the Middle Silurian, LowertoMiddle Devonian and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A singlepolarity characteristic component in midSilurian andesites yields a positive intraformational conglomerate test, whereas dualpolarity prefolding components are isolated from the two Devonian collections. These new data were evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Late Carboniferous time. A comparison of the Chingiz paleolatitudes with those of Siberia shows similarities between the northward motion and rotational history of the Chingiz unit and those of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.Тектоническая эволюция Урало-Монгольского подвижного пояса (УМП) многие десятилетия является предметом исследования огромного количества авторов. Однако, несмотря на все усилия, тектонические реконструкции разных авторов различаются самым радикальным образом, а во многом являются взаимоисключающими. Один из способов прояснить ситуацию – получить последовательности разновозрастных палеомагнитных определений и на их основе оценить кинематику ключевых структур УМП. При палеомагнитных исследованиях среднепалеозойских вулканитов Чингизской палеоостровной дуги на северо-востоке Казахстана в андезитах середины силура была выделена первичная компонента намагниченности, что подтверждается положительным тестом галек для внутриформационного конгломерата. В двух среднедевонских объектах также была выделена первичная намагниченность, для которой тест складки и тест обращения положительны. Объединив все имеющиеся данные по этому региону, мы получили последовательность палеомагнитных определений в интервале с позднего кембрия до поздней перми, что позволило уверенно определить, в каком полушарии находилась Чингизская палеодуга. Сделан вывод, что эта структура устойчиво смещалась к северу и пересекла экватор в силуре. Имеющиеся данные так же уверенно указывают на вторичную природу изгиба вулканических поясов Казахстана, имеющих подковообразные очертания. Сравнение этих данных с кривой кажущейся миграции полюса Сибирской платформы позволяет говорить о том, что большую часть палеозоя Чингизская палеодуга двигалась согласованно с Сибирской платформой, что накладывает жесткие ограничения на эволюцию УМП

PALEOMAGNETISM OF SILURIAN AND DEVONIAN VOLCANICS FROM THE CHINGIZ ISLAND ARC, KAZAKHSTAN, AND ITS BEARING ON TECTONIC EVOLUTION OF THE URAL-MONGOL BELT

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is the key to the formation of the Eurasian supercontinent during Paleozoic time, but the views on this complicated process remain very disparate and sometimes controversial. Three volcanic formations of the Middle Silurian, LowertoMiddle Devonian and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A singlepolarity characteristic component in midSilurian andesites yields a positive intraformational conglomerate test, whereas dualpolarity prefolding components are isolated from the two Devonian collections. These new data were evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Late Carboniferous time. A comparison of the Chingiz paleolatitudes with those of Siberia shows similarities between the northward motion and rotational history of the Chingiz unit and those of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt

Neotethys and the India–Asia collision: insights from a palaeomagnetic study of the Dazhuqu ophiolite, southern Tibet

Since 1999, independently derived geophysical and geological models have been published arguing for an intra-oceanic subduction system along essentially the entire width of the India–Eurasia collision belt. This idea conflicts with earlier proposals, where in the eastern part of the convergence zone Neotethyan mid-ocean ridge-generated lithosphere directly north of the Indian craton was consumed beneath Eurasia in Tibet in an Andean-type plate configuration. New palaeomagnetic data are reported from three Barremian–Aptian (∼ 120 Ma) sequences of chert, siliceous mudstones and volcaniclastic rocks. These rocks form the uppermost part of the Dazhuqu supra-subduction zone ophiolite terrane, which crop out along substantial portions of the India–Eurasia (= Yarlung Tsangpo) suture zone in southern Tibet. The declination data provide little regionally-useful tectonic information; they are dominated by local rotations, presumably related to the Dazhuqu terrane's initial obduction onto the India plate in the Palaeocene and subsequent movement(s) as India later collided and indented into Eurasia. The inclination data are, however, useful because they yield consistent sub-equatorial formation sites (2.7 °S ± 3.0°, 7.9 °N ± 2.7°, 1.4 °N ± 5.7°), which correspond with the location of the Neotethyan intra-oceanic subduction system inferred from the seismic tomographic data. At the time these Dazhuqu terrane rocks formed, the intra-Tethyan subduction zone would have been about 2500 km south of Eurasia