Devonian paleomagnetism of the North Tien Shan: Implications for the middle-Late Paleozoic paleogeography of Eurasia

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Abstract

The Ural–Mongol belt (UMB), between Siberia, Baltica and Tarim, is widely recognized as the locus of Asia's main growth during the Paleozoic, but its evolution remains highly controversial, as illustrated by the disparate paleogeographic models published in the last decade. One of the largest tectonic units of the UMB is the Kokchetav–North Tien Shan Domain (KNTD) that stretches from Tarim in the south nearly to the West Siberian Basin. The KNTD comprises several Precambrian microcontinents and numerous remnants of Early Paleozoic island arcs, marginal basins and accretionary complexes. In Late Ordovician time, all these structures had amalgamated into a single contiguous domain. Its paleogeographic position is of crucial importance for elucidating the Paleozoic evolution of the UMB in general and of the Urals in particular. The Aral Formation, located in Kyrgyzstan in the southern part of the KNTD, consists of a thick Upper Devonian (Frasnian) basalt–andesite sequence. Paleomagnetic data show a dual-polarity characteristic component (Dec/Inc = 286° / + 56°, α95 = 9°, k = 21, N = 15 sites). The primary origin of this magnetization is confirmed by a positive test on intraformational conglomerates. We combine this result with other Paleozoic data from the KNTD and show its latitudinal motion from the Late Ordovician to the end of the Paleozoic. The observed paleolatitudes are found to agree well with the values extrapolated from Baltica to a common reference point (42.5°N, 73°E) in our sampling area for the entire interval; hence coherent motion of the KNTD and Baltica is strongly indicated for most of the Paleozoic. This finding contradicts most published models of the UMB evolution, where the KNTD is separated from Baltica by a rather wide Ural Ocean containing one or more major plate boundaries. An exception is the model of Şengör and Natal'in [A.M.C. Şengör, B.A. Natal'in, Paleotectonics of Asia: fragments of a synthesis, in: A. Yin and M. Harrison (eds.), The tectonic evolution of Asia, Cambridge University Press, Cambridge (1996) 486–640], in which coherent paleolatitudinal motion of Baltica and the KNTD is hypothesized — the latter as part of the Kipchak Arc. We suggest a parallel hypothesis, which explains coherent motion of the KNTD and Baltica. In particular, we argue that if a basin with oceanic crust ever existed between the KNTD and Baltica, it was a narrow one without (significant) active spreading in Middle to Late Paleozoic time. Notably, the paleogeographic position of Siberia during the Middle Paleozoic and hence, the width of the Khanty–Mansi Ocean between Siberia, on the one hand, and Baltica–KNTD, on the other hand, remains largely unconstrained, because of the paucity of high-quality Silurian, Devonian and Carboniferous paleomagnetic results from Siberia.

Introduction

The Ural–Mongol mobile belt (UMB) stretches for nearly 10,000 km from the Arctic Ocean along the Ural Mountains between Europe and Asia and then onward through Central Asia to almost the Pacific (Fig. 1a). It is one of the largest and most complex mobile belts on the Earth; moreover, its various parts differ considerably from each other in their structural make-up. The Urals, an orogenic belt of more than 2000 km in length (Fig. 1a), display a linear structural pattern, with long narrow sets of folded and imbricated thrusts (e.g., [2]), comparable to the larger-scale aspects of other orogenic belts such as the Rocky Mountains, the Andes, or the Himalayas. The Urals commonly contain Middle Paleozoic island-arc complexes, flysch sequences deposited in marginal seas, and ophiolites as features of relevance to plate-tectonic interpretations.

In contrast to the Urals, the central part of the UMB, that is Kazakhstan, the Altai, and northwestern Mongolia, has a mosaic structure (Fig. 1a). No prevailing structural trend can be observed here. Microcontinents with Precambrian basement are tectonically juxtaposed with Early Paleozoic subduction-related volcanic complexes, accretionary wedges and flysch sequences; short tectonic units often form T- or Y-like junctions. From the end of the Ordovician through the Permian, many strike-slip faults were active and caused the horizontal imbrication of the amalgamated island-arc segments, microcontinents and accretionary wedges. The Late Paleozoic South Tien Shan and Junggar–South Mongol linear fold-thrust belts bound this region to the south.

A number of publications have presented models for the tectonic evolution of the UMB [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], and many of them are very dissimilar. Some authors advocate that the belt was formed by the closure of a Paleoasian Ocean, in which an archipelago of scattered Precambrian microcontinents, oceanic basins and island arc segments existed in the Paleozoic (Fig. 2a). The most important role in the amalgamation of the UMB is ascribed to the diachronous opening and closing of the intervening oceans and, therefore, to diachronous collisions of microcontinents and island arcs. The mosaic structure of the central part of the UMB is assumed to have existed early on and has therefore been called “primary” in this set of models [3], [4], [5], [6], [10], [11]. The basic concepts of such models are similar, but they vary markedly in their details. For instance, some models assume that most microcontinents and island arcs docked to Siberia and formed a composite Siberian–Kazakhstanian continent already in the Ordovician or Silurian [4], [5], [11], whereas in other models several of these units are thought to collide with each other first, thereby forming an independently moving mid-Paleozoic Kazakhstanian continent [6], [10].

A completely different group of models advocates the existence of a continuous volcanic arc system [1], [2], [7], [8], [9]. For instance, Şengör and Natal'in [1] assumed that there was a long continuous Kipchak Arc connecting the Siberian and Baltica cratons in the Early Paleozoic (Fig. 2b). The kinematics of the arc are therefore linked to the motions of Siberia and Baltica. Oceanic crust was subducting westward under the Kipchak arc during most of the Early Paleozoic, and large accretionary wedges were formed. By the Carboniferous, the fragments of the ancient structure had amalgamated into a continent-sized domain, which from that time on can be called the Kazakhstanian continent. The other models of this group differ from that of Şengör and Natal'in [1] in several ways. Yakubchuk et al. [7], [8] assume the existence of two parallel island arcs, while giving a leading role to strike-slip motion and imbrications of island-arc segments. Puchkov [2] and Stampfli and Borel [9] suggest that in the Early Paleozoic the island arc had a rather complicated configuration, but they do not ascribe an important role to strike-slip motions.

The fact that so many dissimilar models can co-exist means that we lack major knowledge about the paleogeography and kinematics of the UMB constituents. Thus, our views on the formation of the Eurasian supercontinent are very preliminary at best. Such a situation is largely due to the scarcity and often poor quality of paleomagnetic data from the region. Were a framework of abundant paleomagnetic results from rocks of different ages and different tectonic units of the UMB available, more stringent constraints on the tectonic evolution of the whole belt could be imposed.

Published pre-Permian paleomagnetic data come mainly from the North Tien Shan, north of the Tarim continental block (Fig. 1b) [12], [13], [14], [15]. In particular, Bazhenov et al. [14] noticed a good fit of paleomagnetic inclination data from the North Tien Shan with the latitudinal motion of Baltica. However, the validity of this conclusion was strongly undermined by the data scarcity from the Early Silurian to the end of the Early Carboniferous. This gap was partly filled by Early Silurian data from South Kazakhstan [15], and in this study we present new paleomagnetic results from Upper Devonian volcanic rocks in the North Tien Shan. With these results added, a temporal sequence of nine paleomagnetic results is available for Late Ordovician to Late Permian time for this area. It allows us to reconstruct the paleolatitudinal movements of this part of Kazakhstan and Kyrgyzstan and to uncover some implications for the tectonic evolution of the UMB.

Section snippets

Regional tectonic setting

One of the major tectonic units of the UMB is the Kokchetav–North Tien Shan domain (KNTD), which stretches from north of Tarim and its marginal South Tien Shan fold belt to the Kokchetav massif in the north (Fig. 1). This domain is located in the central part of Kazakhstan and in the north of Kyrgyzstan, and has a boomerang-like shape, with a nearly N–S trending northern arm and an E–W trending southern one. The KNTD comprises Precambrian microcontinents and Early Paleozoic island-arc volcanic

Geologic description of the study area and sampling

Our study concentrated on the southern slope of the Kyrgyz range, where a thick volcano-sedimentary sequence is exposed (Fig. 3). These volcanic rocks are gently dipping in the west and south but more intense fault-related deformation is observed farther to the east and north. The age of the lower half of the sequence is poorly known; these volcanic rocks may be as old as Ordovician, or even Cambrian (I.L. Zakharov, 1986, pers. comm.; A.P. Bashkirov, 1998, pers. comm.), and are labeled

Methods

The collection was studied in the paleomagnetic laboratory of the Geological Institute of the Russian Academy of Science in Moscow. Cubic specimens of 8-cm3 volume were sawed from hand blocks. One specimen from each hand-sample was stepwise demagnetized in 15–20 increments up to 685 °C in a home-made oven with internal residual fields of approximately 10 nT and measured with a JR-4 spinner magnetometer with a noise level of 0.05 mA/m 1. Demagnetization results were plotted on orthogonal vector

Declinations and rotations

All paleomagnetic data (Table 2) are from the southern part of the boomerang-shaped KNTD (Fig. 1b). This area in the North Tien Shan has suffered several deformation events, the latest of which was in the Late Permian to Early Triassic and involved significant but variable counterclockwise rotations [27]. These rotations sometimes occurred on a local scale and are best interpreted as having been caused by major strike-slip faults crossing the region. Earlier phases of clockwise as well as

Conclusions

Our paleomagnetic study of Upper Devonian andesite and basalt flows from the Aral Formation in the North Tien Shan of Kyrgyzstan yields a characteristic and likely primary magnetization. When combined with other reliable paleomagnetic results from the North Tien Shan, a good fit of the observed paleolatitudes with the reference values for Baltica is obvious. Geologic evidence indicates that a large area in Kazakhstan, the Kokchetav–North Tien Shan Domain (KNTD) was consolidated by Late

Acknowledgments

We thank many people from the Scientific Station of the Russian Academy of Sciences in Bishkek (Kyrgyzstan) for logistic support of the fieldwork, Nina Dvorova for paleomagnetic measurements and Kirill Degtyarev for helpful comments. Thorough and constructive reviews of John Geissman, Stuart Gilder and an anonymous reviewer are gratefully appreciated. This study was supported by the Division of Earth Sciences and the Office of International Science and Engineering's Eastern and Central Europe

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