Permian palaeomagnetism of East Kazakhstan and the amalgamation of Eurasia

Most of Kazakhstan belongs to the Ural–Mongol belt, the tectonic evolution of which is poorly understood as demonstrated by disparate tectonic models suggested thus far. We undertook a palaeomagnetic study of Upper Permian basalts and andesites from two localities in east Kazakhstan in order to evaluate the final stages of the evolution of this belt and Eurasian amalgamation. Thermal demagnetization revealed a single pre-tilting characteristic component of ubiquitously reversed polarity from all samples. The mean declination of this remanence from one locality agrees rather well with the Permian European palaeomeridian, whereas that from the other is clockwise rotated by 28°± 8° . The overall mean inclination of −49°± 4° differs by 9.7°± 4.2° from the reference inclination calculated, for our localities, from the Eurasian mean pole for the 245–260 Ma interval and is in agreement with 260–275 Ma data. We account for the observed pattern by either a slightly erroneous rock age (lithologies are somewhat older than indicated by geological data) or non-dipole (octopole) components of the geomagnetic field. Because significant relative motion of the study area with respect to Eurasia is not demonstrated, we conclude that welding of Kazakhstan, Europe and Siberia was essentially completed by Mid-Permian time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73262/1/j.1365-246X.2003.01879.x.pd

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

Late Permian palaeomagnetic data east and west of the Urals

We studied Upper Permian redbeds from two areas, one between the Urals and the Volga River in the southeastern part of Baltica and the other in north Kazakhstan within the Ural-Mongol belt, which are about 900 km apart; a limited collection of Lower-Middle Triassic volcanics from north Kazakhstan was also studied. A high-temperature component that shows rectilinear decay to the origin was isolated from most samples of all three collections. For the Late Permian of north Kazakhstan, the area-mean direction of this component is D = 224.3°, I =−56.8°, k = 161, Α 95 = 2.7°, N = 18 sites, palaeopole at 53.4°N, 161.3°E; the fold test is positive. The Triassic result ( D = 55.9°, I =+69.1°, k = 208, Α 95 = 4.2°, N = 7 sites, pole at 57.0°N, 134.1°E) is confirmed by a positive reversal test. The corresponding palaeomagnetic poles from north Kazakhstan show good agreement with the APWP for Baltica, thus indicating no substantial motion between the two areas that are separated by the Urals. Our new mean Late Permian direction for SE Baltica ( D = 42.2°, I = 39.2°, k = 94, Α 95 = 3.5°, N = 17 sites; palaeopole at 45.6°N, 170.2°E) is confirmed as near-primary by a positive tilt test and the presence of dual-polarity directions. The corresponding pole also falls on the APWP of Baltica, but is far-sided with respect to the coeval reference poles, as the observed mean inclination is shallower than expected by 13°± 4°. In principle, lower-than-expected inclinations may be attributed to one or more of the following causes: relative tectonic displacements, quadrupole and octupole terms in the geomagnetic field, higher-order harmonics (incl. secular variation) of the same field, random scatter, non-removed overprints, or inclination error during remanence acquisition and/or diagenetic compaction. Our analysis shows that most mechanisms from the above list cannot explain the observed pattern, leaving as the most likely option that it must be accounted for by inclination shallowing. Comparison with selected coeval results from eastern Baltica (all within Russia) shows that all of them are biased in the same way. This implies that they cannot be used for analysis of geomagnetic field characteristics, such as non-dipole contributions, without a more adequate knowledge of the required correction for inclination shallowing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71899/1/j.1365-246X.2008.03727.x.pd

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

Unraveling the early-middle Paleozoic paleogeography of Kazakhstan on the basis of Ordovician and Devonian paleomagnetic results

It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~. 2000. km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north-central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle-Late Ordovician paleolatitudes and came to the conclusion that a nearly E-W trending active margin of the Kazakhstania landmass had existed at low (~. 10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet-Akbastau, and with the Aktau-Junggar microcontinent by the Ordovician-Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~ 45°. This changed the orientation of the Kazakhstania to NW-SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt