Paleomagnetism of Carboniferous sediments in the Hexi corridor: its origin and tectonic implications

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Abstract

We report paleomagnetic results for Carboniferous rocks from the eastern part of the Hexi Corridor, which we have demonstrated to be the western extension of the North China block (NCB). The characteristic high-temperature remanence component (B-component) exhibits predominantly reverse polarity, and is generally separable by thermal demagnetization temperature steps between 350 and 580°C. This B-component can also be isolated by alternating fields (AFs) cleaning at a peak value of 20–30 mT using a hybrid thermal–AF demagnetization method. The B-component shows a positive fold test at the 95% confidence level, indicating that it was acquired before Late Triassic to Early Jurassic folding. We interpret the B-component to be of primary origin acquired during the Carboniferous. The corresponding pole positions are at 14.0°E, 10.5°N, α95=6.2° for the Early-Middle Carboniferous (late Visean to Namurian) and at 10.2°E, 33.3°N, α95=16.7° for the Late Carboniferous (Stephanian). The preliminary Late Carboniferous pole is in agreement with that for the NCB, supporting earlier claims that the Hexi Corridor has been the western extension of the NCB since the Ordovician. On the other hand, these two Carboniferous paleopoles are distinct from the coeval ones for Inner Mongolia, Outer Mongolia, South China, Tarim and stable Europe at greater than 95% confidence, suggesting that these continental blocks could not have been connected in their present configuration during Carboniferous time. The significant difference in paleolatitude between the NCB and Australian part of Gondwanaland according to the two new Hexi Corridor poles indicates that they had drifted apart by the late Visean.

Introduction

The Phanerozoic apparent polar wander (APW) paths for three major continental blocks of China (North China, South China and Tarim) have been recently revised based upon a significant amount of new paleomagnetic results [1], [2], [3], [4]. Earlier published APW paths for the three major continental blocks of China [5], [6], [7], [8], [9] were revised on the bases of newly available paleomagnetic poles for the Early Paleozoic resulting in better understanding of the paleogeography and kinematic history during this time [1], [2], [10], [11], [12], [13]. Paleogeography and tectonics during the mid-Paleozoic, however, have remained unresolved problems for the three major continental blocks of China owing to the serious lack of paleomagnetic data for the Middle and Late Paleozoic. For the North China block (NCB) this lack is due in part to the weak magnetic remanence of Upper Carboniferous to Middle Permian sedimentary rocks, which are generally made up of light colored coarse-grained sandstones, and also to a major stratigraphic hiatus from the Upper Ordovician to the Lower Carboniferous [14].

The relationship between the Hexi Corridor terrane and stable NCB, defined as the eastern part of the NCB beyond the Helanshan fold belt (HFB; Fig. 1a), has been constrained recently by a paleomagnetic study in the eastern part of the Hexi Corridor, Ningxia of North China [15]. This study demonstrated that a paleomagnetic pole for Early-Middle Ordovician limestone rocks from the eastern part of the Hexi Corridor is in good agreement with NCB poles, implying that the Hexi Corridor had been part of the NCB since the Ordovician in its present configuration. This finding paleomagnetically favors that some post-Ordovician Paleozoic lithologies missing on the stable NCB are present in the Hexi Corridor and forms the basis for consideration of post-Ordovician Paleozoic problems of the NCB using Silurian, Devonian and Carboniferous results from the eastern part of the Hexi Corridor terrane, extensively we sampled recently in several sections.

The Miboshan area southwest of Ningxia, where an Early-Middle Ordovician paleomagnetic pole had been obtained [15], is just about 10 km from our Silurian and Devonian sampling sections, the Xiaoyuchuan section, and our Silurian and Devonian results from three separated sampling sections (Zhaohuajing, Shixiagou and Xiaoyuchuan) show good consistency, indicating that no relative rotation took place between them [13]. Thus, our results from the eastern part of the Hexi Corridor for the Ordovician, Silurian and Devonian, may be considered free of local rotation effects and to be representative for the region and for the NCB. With the aim of better understanding the paleogeography and tectonics of the NCB during the Carboniferous we present paleomagnetic results from the Xiaoyuchuan section of the eastern part of the Hexi Corridor, southwest Ningxia, North China.

We compare the results with the two major continental blocks of China and adjacent areas during the Carboniferous, and present a new paleogeographic reconstruction.

Section snippets

Geological setting and paleomagnetic sampling

The Hexi Corridor area is located in the southern margin of the Alashan/Hexi Corridor terrane, to the west of the Ordos Basin across the HFB, and at the juncture of two major Chinese blocks, North China and Tarim (Fig. 1a). Geological evidence and paleomagnetic results strongly suggest that the Alashan/Hexi Corridor terrane has formed the western extension of the NCB since Ordovician time [14], [15], [16].

In the eastern part of the Hexi Corridor, the slightly metamorphosed pre-Cambrian basement

Method

All specimens were subjected to either progressive thermal or hybrid thermal–alternating field (AF) demagnetization. The hybrid thermal–AF technique (i.e. AF after thermal demagnetization) was used for specimens with a hard viscous/secondary remanent magnetization that was not appreciably removed by AF treatment alone and showing unstable behavior during thermal demagnetization associated with peak temperatures above 300°C. Thermal demagnetization was carried out with a Natsuhara TSD-1

Origin of the components of magnetization

The fold test indicates that the B-component was acquired before folding whereas the A-component was recorded after folding with the relevant folding to be of Early to Middle Jurassic age. The corresponding in-situ paleomagnetic pole of the A-component, at 224.3°E, 67.8°N, with semi-axis along and perpendicular to the great circle of ellipse dp/dm=7.7/12.6, is not statistically distinguishable from those of the Middle-Late Jurassic or Early Cretaceous for the NCB [24], [25] at the 95%

Conclusions

  • 1.

    Late Carboniferous paleomagnetic pole from the eastern part of the Hexi Corridor, being not significantly different from earlier, provisional findings for the stable NCB, reinforce the claim that the Hexi Corridor terrane had been the western extension of the NCB since Ordovician time.

  • 2.

    Two Carboniferous paleomagnetic poles from the eastern part of the Hexi Corridor are distinct with the coeval ones for Inner Mongolia, Outer Mongolia, South China, Tarim and stable Europe, at greater than 95%

Acknowledgements

We are very grateful to the Geological Bureau of Ningxia Hui Autonomous Region and Lixin Bai for organizing the field trip and to Jianjun Lu for his participation and help in the field. We wish to thank C. Klootwijk and P. Schmidt for their careful reviews and comments. We also acknowledge K. Hoffman for providing constructive review of the manuscript. This study was supported by the NSF of China (No. 49874014) and the CAS (KZCX1-07), and partially supported by the Ministry of Personnel of

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