Geodynamics of the western part of the Mongolia–Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia
Introduction
The Mongolia–Okhotsk belt has been identified long ago (Nagibina, 1963; Yanshin, 1974) as a zone of middle Paleozoic–early Mesozoic folded marine sequences intruded by granites. The belt extends from the Khangay mountains in Central Mongolia as far as the Uda Gulf in the Okhotsk Sea (Fig. 1). The middle Paleozoic–late Mesozoic magmatic provinces formed in the continental conditions around the belt are related to its evolution (Nagibina, 1963; Yanshin, 1974).
Until recently the geodynamics of the Mongolia–Okhotsk belt have remained insufficiently analysed. Shortage of paleomagnetic evidence for stratified sequences, scarcity of age determinations for igneous rocks, and poorly constrained geometry of major faults led to ambiguous interpretations of the main geological events. Many geologists dated the belt as late Paleozoic and associated Mesozoic and even Permian tectonics and magmatism with intraplate phenomena (either rifting or `doming') that would be mere reactivation of pre-existing features (Nagibina, 1963; Komarov, 1972; Khrenov, 1981; Gordienko, 1987; Kovalenko and Yarmoliuk, 1990).
According to other views, Siberia and Mongolia had been separated by an enormous gulf of the Paleopacific, called the Mongolia–Okhotsk ocean (Zonenshain et al., 1990), since the Early Permian when the western part of Mongolia joined Siberia in the region of Khangay. Closure of the Mongolia–Okhotsk ocean, mainly as a result of rotation of Siberia with respect to Mongolia and the associated collision of the two continental blocks, produced the Mongolia–Okhotsk belt (Zonenshain et al., 1990; Zorin et al., 1995). This oceanic gulf either had been closing gradually (scissors-like) from Triassic to Late Jurassic (Zonenshain et al., 1990), or shut in a single episode at the Early/Middle Jurassic boundary (Zorin et al., 1995Zorin et al., 1998a).
Şengör et al. (1993) suggested that the Mongolia–Okhotsk oceanic gulf (they call it the Khangay–Khentey ocean) had existed rather since the Vendian–Cambrian than the earliest Permian. The author of this study used to date the ocean as Silurian in age (Zorin et al., 1993). However, such reconstructions are inconsistent with paleomagnetic (Zhao et al., 1990; Pruner, 1992) and paleobiogeographic data (Nie et al., 1990) which place Mongolia far from Siberia at least until the Permian. Şengör et al. (1993) attribute the greatest part of the Mongolia–Okhotsk belt to the Altaids which are a system of fold belts formed by accretion at the continental margin of Siberia and along island arcs from the Vendian to the early Mesozoic. They suggest that the western (greatest) part of the Mongolia–Okhotsk ocean closed in the Triassic as a result of strike-slip motions whereby no distinct suture formed, and date the closure of its eastern part as late Mesozoic (Şengör and Natal'in, 1996). To my view, they unreasonably broaden the concept of the Mongolia–Okhotsk belt by including the Paleozoic folded sequences of South Mongolia (Şengör et al., 1993, p. 299, fig. 2) which are commonly distinguished as a separate belt related to the evolution of the northern branch of the Paleotethys (Zonenshain et al., 1990).
As yet there is no unanimity about the position of the suture between Siberia and Mongolia which is usually called the Mongolia–Okhotsk suture. Before first attempts to explain the geological structure of this region in terms of plate tectonics were undertaken, a Mongolia–Okhotsk lineament had been held to exist as a northwestern boundary of the Mongolia–Okhotsk fold belt (Nagibina, 1963; Solonenko, 1977; Khrenov, 1988). In a pilot work on plate tectonics of Russia and the bordering countries Zonenshain et al. (1990)associated the lineament with the suture between Siberia and Mongolia. At the same time, a more detailed paleogeodynamic analysis allowed the inference that the suture coincides with the northwestern boundary of the fold belt only in the northeastern and central parts of the latter. In its western part, at the Russia–Mongolia border, the suture passes into the southeastern boundary of the belt (Zorin et al., 1994, Zorin et al., 1995).
As mentioned above, some authors believe that in the Mesozoic, and even as early as the Permian, the territory of the Trans-Baikal region (southeastern East Siberia) and Mongolia was broadly involved in continental rifting (Gordienko, 1987; Khrenov, 1988; Kovalenko and Yarmoliuk, 1990). Only recently an attempt has been made to attribute Permian, Triassic and Early Jurassic sub-alkaline and alkaline magmatism to the back of the Andean-type active continental margin of Siberia (Zorin et al., 1995, Zorin et al., 1998a) and to consider Early Cretaceous rifting as a separate post-collisional episode in the history of the Mongolia–Okhotsk belt (Zorin et al., 1997).
More constraints on the crustal structure and geodynamics of the western part of the Mongolia–Okhotsk belt have been obtained during the last decade due to transects performed as part of the Global Geoscience Transects Project of the International Lithosphere Program. Four transects (Fig. 1) have been compiled on the basis of interpretation of geological and geophysical data (Zorin et al., 1993, Zorin et al., 1994, Zorin et al., 1995, Zorin et al., 1998a). The transects traverse the Precambrian southern Siberian platform, the Late Precambrian and early Paleozoic Baikal fold area, the western part of the Mongolia–Okhotsk belt, and the Mongolia–China provinces subjected to folding in early Paleozoic, middle Paleozoic and late Paleozoic–early Mesozoic time. Each of the four cited papers considers the crustal structure of all transected geological provinces and the stages of geodynamic evolution of East Siberia and Mongolia since the Late Precambrian to the late Cenozoic. Data on the Mongolia–Okhotsk fold belt are thus scattered throughout separate works neither of which contains full description of its geodynamic history. At the same time, new data have been lately obtained on the position of paleomagnetic poles for some localities of the belt (Kuzmin and Kravchinsky, 1996) and on absolute ages of granitoids in its surroundings (Yarmoliuk et al., 1997). The present paper is an attempt to summarise the transect results related uniquely to geodynamics of the Mongolia–Okhotsk fold belt and to put them into the context of other recent data.
Section snippets
Devonian–Carboniferous active margin of Siberia and the Onon island arc
Evolution of the Mongolia–Okhotsk fold belt is associated first of all with the Mongolia/Siberia collision and the processes in the vicinity of the Mongolia–Okhotsk ocean which existed as an enormous gulf of the Paleopacific between the two continental blocks. The gulf arose as late as the Early Permian (see below). However, Devonian and Carboniferous complexes play a significant role in the structure of the Mongolia–Okhotsk fold belt and should be considered in this exposition as well.
Sedimentary and igneous rocks
The first, Early Permian, episode of the Siberia/Mongolia collision in the region of Khangay is evidenced by the presence of continental molasse that covers intricately folded turbidites. The molasse is composed of conglomerates and coarse-grained sandstones, intercalated with felsic volcanics (Filippova, 1969). Voluminous intrusion of calc-alkaline granites and granodiorites in the Khangay zone likewise occurred in the Early Permian (Fig. 6). In the Late Permian, felsic intrusive magmatism was
Early Cretaceous rifting
In the Early Cretaceous, collisional calc-alkaline and sub-alkaline magmatism within the western part of the Mongolia–Okhotsk belt gave way to eruption of alkaline basalts and small volumes of alkaline rhyolites. Concurrently, rather narrow elongated basins (15–25 km wide and over 200 km long) came into existence in the Khilok and South Khentey zones and in the periphery of the Onon zone (Fig. 11). The basins accumulated 1.5–2.0 km thick continental clastic deposits (sandstones, siltstones, and
Discussion
Two peculiarities of the Mongolia–Okhotsk belt and its adjacent areas have attracted the attention of many investigators during the last 30 years. These peculiarities are the broad distribution of so-called `intraplate' granitoid magmatism and the relatively small areas of the late Paleozoic and early Mesozoic marine sedimentary series (Nagibina, 1963; Komarov, 1972; Khrenov, 1981; Yarmoliuk et al., 1997).
These specific features of the Mongolia–Okhotsk belt can be explained in frames of the
Conclusion
This review is a synthesis of new data on the Mongolia–Okhotsk belt obtained during work on the Russia–Mongolia transects and data scattered throughout separate publications (Zorin et al., 1993, Zorin et al., 1994, Zorin et al., 1995, Zorin et al., 1997, Zorin et al., 1998a). This synthesis has allowed a clearer formulation of some of the conclusions drawn in those articles. The transect work and the present study have yielded important results that are new as compared to earlier published
Acknowledgements
I dedicate this paper to the memory of the late Lev P. Zonenshain, who was a pioneer in explaining the evolution of the Mongolia–Okhotsk fold belt in terms of plate tectonics. I thank my colleagues V.G. Belichenko, E.Kh. Turutanov, E.V. Sklyarov, A.M. Mazukabzov, V.M. Kozhevnikov, V.V. Mordvinova, I.B. Filippova, A.B. Dergunov, S.V. Ruzhentsev, O. Tomurtogoo, N. Arvisbaatar, and P. Khosbayar for fruitful collaboration in the Global Geoscience Transect Project. I appreciate the help of A.I.
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