Yu. S. Bretshtein and A. V. Klimova
Institute of Tectonics and Geophysics, Far East Division, Russian Academy of Sciences
The area of the mullion articulation of the differently oriented Mongolia-Okhotsk (MO) and Sikhote-Alin (SA) foldbelts, as well as the collision zones between the Siberian and North China cratons, is known for the wide development of geologically different structural facies zones (terrains), which are combined by some geologists into the Amur Plate [Zonenshain et al., 1990]. They produce an intricate mosaic of geologic blocks, the tectonic positions of which cannot be interpreted without using the views of terrains and their close associations in the past with the genetically similar geologic objects proved in the neighboring territories of China, Korea, and Mongolia. The "independence'' of these structural features or their belonging to the China and/or to the Siberian cratons is a subject of many discussions. It appears that this region is a key object for the study and interpretation of the tectonic history of the interaction of these geoblocks at the junction of the Siberian, Pacific, and North China plates.
In spite of the general growth of interest in the study of the geologic history of the region in terms of plate tectonics, paleomagnetic studies were carried out there, until recently, in a sporadic manner. Almost until the middle of the 1990s the single publications dealing with the rocks older than Meso-Cenozoic (Permian-Mesozoic) with references to paleomagnetic data were mainly reviews for the Pacific region as a whole [Ustritskii and Khramov, 1987]. The authors of some papers reported results that did not satisfy the criteria of estimating the reliability of paleomagnetic data collections (the low quality of laboratory magnetic cleaning, the lack of the component analysis of the results of NRM demagnetization, and of the evidence for the use of various tests for detecting NRM ancient and primary components) [Zakharov and Sokarev, 1991]. The rocks that can be ranked at the present time as relatively well studied are only the Mesozoic and Cenozoic rocks of the Primorie and Priamurie regions [Bretshtein, 1988, 1991; Klimova and Bretshtein, 1993; Otofuji Yo-ichiro et al., 1995]. It was only recently the paleomagnetic studies were carried out in some areas of SE Russia, which provided the first paleomagnetic data for the Paleozoic and Mesozoic terrigenous and carbonate rocks of the Primorie, Priamurie, and East Baikal regions [Bazhenov et al., 1999; Bretshtein et al., 1999; Bretshtein et al., 2003; Kurilenko et al., 2001].
It should be noted that some negative circumstances complicated significantly the correlation and interpretation of paleomagnetic data, especially because of the wide development of remagnetized rocks, and also because of the insufficiently exact dating of the rock sequences, in many cases with an accuracy of a series, and most commonly, to a period. The isotopic ages of many igneous rocks were interpreted ambiguously because of a great data scatter. The fauna-based dating of sedimentary rocks admitted, in some cases, the wide age range of their accumulation, during which numerous tectonic events might have occurred, contributing to the remagnetization of the rocks. Many fossil-bearing and magnetically stable rocks could not be used because they belonged to the unstratified matrix in the form of blocks or olistostromes. The substantial amounts of rocks turned out to be almost nonmagnetic (their magnetization was measured at the sensitivity limit of the modern instruments). For this reason, about half of the collected and processed samples were discarded.
Finally, still urgent is the problem of the comparability of paleomagnetic data, namely, of the paleopole positions for the foldbelt terrains of the Far East Region of Russia, as well as for the Siberian and North China plates. As will be demonstrated below, the Paleozoic paleomagnetic data available for the latter are based on the scarce and often fairly contradictory data obtained for the North China and Korea (with the accuracy of the age gradation comparable with that used in our study). The age "sliding'' to some or other side is possible for both regions.
In spite of the sufficiently large volume of laboratory magnetic cleanings and the use of modern data processing techniques, the above-mentioned circumstances predetermined the preliminary character of many of the resulting data. The paleomagnetic data should permanently added, analyzed, periodically revised, and constantly revised.
The laboratory measurements and experimental studies were carried out using modern cryogenic magnetometers, rock generators (spinner magnetometers), and various demagnetizing devices in Russian and foreign laboratories: at the Institute of Tectonics and Geophysics, Far East Division, Russian Academy of Sciences (RAS), at the Geological Institute (RAS), at the Institute of Physics of the Earth (RAS), at the Institute of Geology and Geophysics, Siberian RAS Division, at Far-East Geological Survey, at the "Borok'' Geophysical Observatory of the Institute of Physics of the Earth (RAS), at the Institute of Geophysics, Ukrainian Academy of Sciences, at the Institute of Tectonics, Californian University, and at the Montpellier-2 University, France. These studies included all modern experimental methods, including stepwise thermal demagnetization at temperatures up to 690o C and AF demagnetization up to 100 mT. NRM carriers were investigated to determine various magnetic mineral characteristics, this allowing the estimation of the composition, structural state, and size of magnetic grains and their quantitative relationships in the samples of the study rocks. These characteristics were the coercive force Hc, the value and temperature control of saturation magnetization Js, of remanent saturation magnetization I rs and of magnetic susceptibility c, and the variation of isothermal remanent magnetization as a function of the permanent field of its creation J r(H).
The paleomagnetic data were processed using modern statistical (analytical and graphical) methods using a component analysis [Kirschvink, 1980; Zijderveld, 1967], remagnetization circle remagnetization [Halls, 1978; Khramov et al., 1982], including the separation of a sector for estimating the mean direction [McFadden and McElhinny, 1988], various fold test modifications [McFadden, 1990; McFadden and Jones, 1981; Shipunov, 1995a; Shipunov and Muraviev, 2000; Watson and Enkin, 1993], a reversal test [McFadden and McElhinny, 1990; et al.], conglomerate and burning tests, as well as various methods for isolating pre-, syn-, and post-folding NRM components [Shipunov, 1995b; Shipunov and Bretshtein, 1999]. The computer data processing was performed using the Enkin, McFadden, and Shipunov programs.
The anisotropy of magnetic susceptibility (AMS) and of normal isothermal remanent magnetization (ASIRM) were studied for the purpose of estimating the potential effect of magnetic anisotropy on the distribution of the vectors of the characteristic ChRM component of natural remanent magnetization. Our analysis of the distribution of the rock magnetization vectors, as well as of the results of studying AMS using pilot objects, revealed an obvious correlation (coincidence or similarity) of the dips and strikes of the bedding planes and/or of the secondary schistosity, and also of the AMS and ASIRM planes of the sedimentary rocks with the average direction of the vector for the high-temperature ChRN component of NRM, often irrespective of the age, lithology, dips and strikes, and the tectonic conditions of the rock accumulation. This provided a basis for assuming, by analogy with other investigators [Johns et al., 1997; Tan and Kodama, 2002], that the gently sloping ChRM vectors and the respective low paleolatitude values might have been caused not by the sediment deposition in the area of low paleolatidudes but by the lower inclination in the course of sediment packing and/or by the metamorphism during the subsequent folding, combined with the remagnetization of the rocks.
The regional remagnetization of the rocks and the "unexpectedly'' gentle NRM inclinations (especially in the case of the Late Paleozoic and Mesozoic rocks) were reported earlier for many other regions of Southeast Asia [Dobson and Heller, 1992; Kent et al., 1987; Otofuji Yo-ichiro et al., 1989; Wang and Van der Voo, 1993; Yang et al., 1989], yet, as a rule, they were treated as artifacts, without any detailed analysis of the mechanism that had caused them. On the whole this showed a fairly significant trend: compared to the rigid blocks of the platforms, the fold zones often show the flattening of the ChRM directions for the rocks varying from Cambrian to Triassic in age. Many of the Late Paleozoic and Early Mesozoic rocks of the foldbelts show the substantial "aging'' of the paleopole position, compared to those of the platforms, or are distinguished by the "inadequate'' directions of their characteristic NRM components, often with the predominant contribution of their synfolding and postfolding components and/or of their sums. It appears that this phenomenon is characteristic of all foldbelts of the world and can be a consequence of the postsedimentation packing of the sediments and/or of dislocation metamorphism in the course of orogeny during accretion-collision folding.
Figure 7 shows the "evolution'' of the views of various authors on the potential APWP trend for the North China Platform, based on the results obtained during the last 20 years. This figure also shows the results of our determinations for the Mongolia-Okhotsk (MO) and Sikhote-Alin (SA) superterranes, only part of them being published recently.
On the whole, the trends of the apparent drifting of the paleopole for the MO and SA terranes of the foldbelts and for the North China Platform were found to be almost parallel. The comparison of the various potential versions of these paths (controlled by the choice of the magnetization polarity of the rocks in Paleozoic time) showed their minimum "extension'' when the SE-NW trend was chosen for the Cambrian-Carboniferous interval of time (see Figure 7 (plate 6) for the details). In accordance with the principle of APWP minimization, we preferred direct polarity for the Lower-Middle Paleozoic using the SSW direction of the primary ChRM direction. In this version the MO and SA superterrains, as well as the North China Platform (NCP), are assumed to have been located at the nearly equatorial and subtropic latitudes of the Northern Hemisphere and, probably, in the northern periphery of East Gondwana. With the nearly paleolatitudinal position of the Mongolia-Okhotsk and Sikhote-Alin terranes, many of the independently drifting blocks, as follows from some differences in the position of the paleopole, might have experienced some local differentiated (often differently directed) rotations throughout the Paleozoic. Yet, on the whole, the general drifting trend of these geoblocks from the lower (nearly equatorial) to their present-day positions is fairly demonstrative.
The comparison of the most reliable paleomagnetic data, obtained for the foldbelt terrains of the Far-East territory of Russia, with the most correct results obtained for the North China Platform proved the proximity of their paleopole positions during the respective time interval. These results were obtained for some geographically close local rock sequences in the suture (fold) zones of the North China platform [Gao et al., 1983; He et al., 1988; Huang et al., 1999, 2001; Li et al., 1985; Lin et al., 1985; Ma et al., 1993; Meng and Coe, 1992; Wu et al., 1993; Yang et al., 2002; Zhao et al., 1992, 1993, 1996; Zhenyu et al., 1999; Zhu and He, 1987]. However, these data are often different and contradictory in terms of the paleopole position [e.g., Huang et al., 2000; Zhao et al., 1993]., this being not always caused by the preferred choice of some or other polarity (this being problematic for the Early-Middle Paleozoic). The coincidence of these paleopoles, recorded for the Silurian-Devonian rocks, with the faunally and statistically proved positions of the North China Platform in the Permian-Carboniferous and Mesozoic periods of time [Enkin et al., 1992; Gilder and Courtillot, 1997; Pruner, 1987; Yang et al., 1991, 1992] provides, in our opinion, a good basis for believing that these rocks were remagnetized during the time of the Hercynian-Cimmerian orogenesis. This applies to many of the rock sequences we investigated in the Mongolia-Okhotsk and Sikhote-Alin foldbelts, where the large-scale Late Paleozoic-Mesozoic remagnetization had taken place, and also to the rocks reported in [Kravchinsky et al., 2002]. The latter authors described the positions of the Devonian paleopole determined using the rocks of several rock sequences determined using the rocks of several geologic sections, exposed by the Oldoi River in the Amur R. area, the magnetization of which is believed by these authors to be a primary one. The differences between the resulting positions of the Silurian-Devonian paleopole and the results of other researchers, for instance [Zhao et al., 1993], are explained by Kravchinsky et al. [ by the potential rotations of the blocks around a vertical axis in the sampling site, see a light-color, striated arc-shaped band (see plate 4 in Figure 7.) No geological explanation was found for this rotation by 90o-120o even in terms of the fault tectonics analysis. This interpretations seems to be insufficiently substantiated also in the paleomagnetic aspect: the results of the fold test were not quite correct because they were based on the recording negative polarity and on the use of some formal numerical parameters which were not quite strict in terms of the McElhinny [McElhinny, 1964] and Watson and Enkin [Watson and Enkin, 1993] tests. Neither were they subjected to the "powerful'', mathematically and geophysically substantiated tests offered by [McFadden, 1990; Shipunov, 1995a; Shipunov and Bretshtein, 1999; Shipunov and Muraviev, 2000]. Hence, the detailed age gradation of the Devonian poles is unjustified. Our paleomagnetic study of the same Devonian rocks using the geological rock sequences of the Oldoi R. and Urusha R. basins, which exceed the sample collections of the geologists mentioned above both in space and number, confirmed the paleomagnetic trends obtained previously (Figures 7, where the Silurian and Devonian rocks are shown in different colors), yet they are insufficient to grade the data, as offered without proof in [Kravchinsky et al., 2002]. Yet, the more important fact is that these poles coincide wholly with the areas which include the paleopoles obtained for the Late Paleozoic and Mesozoic rocks. The same applies to the Silurian and Devonian paleopole positions reported in [He et al., 1988; Huang et al., 2000; Li et al., 1985; Meng and Coe, 1992; Yang et al., 2002; Zhenyu et al., 1999; Zhu and He, 1987], which "coincide'' well with the younger Mesozoic poles and differ from the poles obtained in our study and reported in [He et al., 1988; Zhao et al., 1993]. Moreover, the location of the paleopole reported in [Zhao et al., 1993] and the data obtained by Huang et al. [ belong to the rocks of the same age found in the areas spaced a few dozens of kilometers apart.
It follows that the similarity of the positions of some paleopoles of the Middle Paleozoic objects with those of the Mesozoic ones suggests a simpler obvious explanation of this fact by Mesozoic remagnetization during the accretion-collision folding. On the other hand, the polarity version assumed by us for the case of the Cambrian rocks in the Khingan-Bureya terrane gives the anomalous position of the Cambrian paleopole compared to the other terranes of the Sea of Okhotsk coast and the East Baikal region which coincides with the position of the pole for Devonian time. It remains to infer potential remagnetization during the Devonian or the erroneous dating of geological time, which is quite probable for the region concerned because of its poor knowledge.
Worthy of note is the poor situation with getting high-quality, unambiguous pole locations for the Late Triassic and Jurassic rocks, showing a significant scatter for some rock sequences. In any case, the positions of the Jurassic paleopole for some objects, for instance, for the tuffaceous-sedimentary and volcanic rocks of the Uda volcanic belt, are closer to the Siberian poles compared to those of North China. This might have been caused by the fact that the volcanic rocks of the Uda Arc belonged by that time to the stable continental margin (pericratonic trough) of the Siberian plate.
A great difference between the Middle and Late Paleozoic latitudes of the Siberian and North China plates [Khramov et al., 1982; Pecherskii and Didenko, 1995] controlled the "intermediate'' positions of many Mongolia-Okhotsk and Sikhote-Alin terrains in the Paleozoic Paleoasian Ocean. The scatter of the paleolatitude values for the end of the Paleozoic to the beginning of the Mesozoic is a remarkable fact which calls for additional studies. Figure 8 shows merely the paleolatitudes averaged for the whole region (for all terranes where the rocks of the respective ages were investigated), which have been assigned conventionally to the middles of the respective geologic periods. Any more detailed analysis of the latitudes at the present-day knowledge would be speculative and not sufficiently substantiated.
All of these data testify to the fact that we still have a problem both in terms of getting high-quality paleomagnetic inwestigations and their objective interpretation for the whole region of the Asian-Pacific continental margin.
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