Paleomagnetism of Vendian rocks in the southwest of the Siberian Platform

Analysis of Paleomagnetic Trends

High-Temperature Components of the Aisa, Chistyakova and Moshakova Formations

Figure 10

[52] The high-temperature (HT) component identified in the rock samples of the Aisa Formation shows in the stereogram two almost antipodal clusters (Figure 10 and Table 1), which nevertheless show significantly different mean values (with the 180^o reversal of one of them), namely, g / g_cr = 23.4^o/14.9^o. This fact might have been indicative of the different ages of the direct and reverse polarity magnetization.

[53] However, taking into account the high noise of the signal, as well as the high overlapping of the magnetic component spectra, clearly seen in the analyses of the Zijderveld diagrams, we believe that differences in the mean directions are associated with these circumstances.

[54] Our fold test, performed both at the level of sites and rock samples, gave an indefinite result for the high-temperature component.

[55] The time of the HT component formation, identified in our study of the rocks of the Aisa Formation, can be reconstructed by comparing it with the high-temperature magnetization component of the rocks of the Chistyakova and Moshakova formations, which are comparable in age with the rocks discussed.

Figure 11

[56] This component also shows a bipolar distribution (Figure 11 and Table 2), yet, in contrast to the Aisa high-temperature component, passed a reversal test ( g / g_cr = 1.8^o/19.9^o). The fold test, carried out in its different modifications, proved its prefolding age (Table 2). All of these data suggest that the HT component of the Chistyakova and Moshakova rocks was formed during or soon after the deposition of these rocks.

[57] The paleomagnetic pole calculated for the high-temperature component of the Chistyakova and Moshakova formations (Table 3) does not differ statistically from the respective pole of the Aisa Formation ( g / g_cr = 6.9^o/8.8^o). On the one hand, this supports the above conclusion, on the other hand, this allows us to rank the high-temperature magnetization component, identified in the Aisa Formation, as a primary one. It is important to note that the comparison of the respective poles, computed proceeding from the trends obtained in the modern coordinates, revealed that the angular distance between them grows and their difference becomes significant ( g / g_cr = 15.5^o/13.1^o). The latter means that the fold test, performed at the regional scale also gives the positive result.

[58] The fold test performed for the combined data sample including the paleomagnetic poles, calculated for the sites of the Aisa, Moshakova, and Chistyakova formations also suggests the pre-folding age of the magnetization.

[59] To sum up, the data obtained in this study prove with a high degree of confidence that the high-temperature components of the rocks of the Aisa, Moshakova, and Chistyakova formations originated during or soon after the deposition of these rocks.

Medium-Temperature Magnetization Component of the Aisa and Ust-Tagul Formations

[60] The distribution of the vectors of the medium-temperature (MT) magnetization component in the rocks samples of the Aisa and Ust-Tagul formations is shown in Figure 10. In the stereograms, these vectors produce distinct clusters with the crowding notably greater in the ancient (stratigraphic) system of the coordinates, compared to the modern geographic system (Table 4). Both of the fold tests used in our study, namely, the Enkin test and the Shipunov NFT test, positively suggest the pre-folding age of the formation of the medium-temperature magnetization components of the Aisa and Ust-Tagul rocks. The comparison of these rocks showed that the trends of their MT components did not differ significantly in terms of their statistics ( g / g_cr = 5.9^o/6.2^o [McFadden and McElhinny, 1990]). This proves their almost simultaneous origin, and allows us to discuss them simultaneously. Our simultaneous analysis of the rocks of both formations suggested the pre-folding age of the medium-temperature component.

[61] The age of the MT component could be determined proceeding from the following considerations: first, it cannot be older than the Nemakit-Daldynian (the age of the Ust-Tagul Formation), secondly, the position of its respective paleomagnetic pole is clearly different from the positions of all Siberian paleomagnetic poles, beginning from the Amga one (Middle Cambrian onset). Therefore, the medium-temperature component of the magnetization of the rocks of the Aisa and Ust-Tagul suites took place either at the very end at the Vendian, or during the Early Cambrian. Therefore, since the computed pole resides in the direct vicinity of the Toyonian paleomagnetic pole [Gallet et al., 2003], we have all reasons to believe that the component discussed can be dated Toyonian.

[62] It is of interest to compare our results with the data reported by Gurevich [1981], who studied the paleomagnetism of the Aisa and Ust-Tagul rocks about 25 years ago. Having studied more than 300 rock samples, E. L. Gurevich got very similar paleomagnetic poles for the rocks of these formations, namely, Plat= - 53^o, Plong=116^o, A95=3^o for the Aisa rocks and Plat= - 56^o, Plong=110^o, A95=3^o for the Ust-Tagul rocks. On the basis of these data he derived a conclusion concerning the absence of any significant latitudinal motions for the southern part of the Siberian Platform during the Late Precambrian.

[63] Our data, based on the significantly more detailed and intensive cleaning, confirmed the existence of the magnetization component identified by E. L. Gurevich. However, there are good grounds to believe that the trend reported by him agrees with our medium-temperature component (see Table 3), which seems to be a metachronous one, yet originated not much later than the time of the deposition of the rocks that show this component.

Figure 12

Medium-Temperature Component of the Mota and Irkutsk Formations in the Urik R. Rock Sequence

[64] The practically horizontal bedding of the rocks in this sequence does not allow one to date confidently the origin of the medium-temperature magnetization component, relative to the folding age (Figure 12 and Table 4). Yet, since the time of the dislocation of the rocks in the Urik R. Sequence is unknown, even the results of the fold test could not allow us to tie the age of the medium-temperature component formation to the time scale. At the same time, we know that the paleomagnetic pole calculated using this component (both in the geographic and stratigraphic systems of the coordinates) resides close to the pole of the medium-temperature component, identified as a result of our study in the rocks of the closely spaced region, in the Biryusa R. and Tagul R. rock sequences. The formal test reported by McFadden and McElhinny [1990] confirms that the statistical difference of these poles is insignificant. This justifies our conclusion that the medium-temperature component of the Mota and Irkutsk formations in the Urik R. section originated roughly at the same time, when the medium-temperature component originated in the rocks of the Biryusa-Tagul objects of study.

Medium-Temperature Component in the Outcrops of the Moshakova, Chistyakova, and Ostrovnaya Formations in the Area of the Greben Rock, Yenisey Ridge

Figure 13

[65] The distribution of the vectors of the medium-temperature magnetization component in the outcrops of the rocks of the Moshakova, Chistyakova (Gr) and Ostrovnaya (Ostr) formations in the area of the Greben Cliff is presented in Table 5 and in Figure 13. The fold test in the Enkin modification, performed in our study at the level of sites, suggested the prefolding age of the MT component. However, the procedure of the proportional, 60-percent straightening of the folds in the respective curve showed the distinct crowding maximum, which could record the synfolding age of this component formation. To verify this hypothesis, we used the Watson-Enkin test [Watson and Enkin, 1993], the result of which showed, with a 95-percent probability, that this component originated at the time when the deformation of the study rocks amounted to 38.0-82.8% of their present-day deformation, which suggests the magnetization to be a synfolding process. The NFT test performed by Shipunov [1995] confirms this result. The calculated paleomagnetic pole resided in the area between the Tommotian [Pisarevsky et al., 1997] and Toionian [Gallet et al., 2003] poles of the Siberian Platform, which allows us to suggest the Early Cambrian age of the medium-temperature component concerned.

Intermediate-Temperature Magnetization Component of the Aisa Formation

[66] Unfortunately, in the course of this study we failed to obtain a sufficient amount of information to date the intermediate component identified in two Biryusa outcrops of the Aisa Formation (see Figure 10 and Table 1). Moreover, the character of the Zijderveld diagrams, from which this component was obtained, does not allow us to be sure that we succeeded to get this component in its "pure" form and that its mean value was free from the effects of other magnetization components. We believe it possible to admit that the intermediate component is the "antipode" of the high-temperature component and that it originated at the stage of the early diagenesis in the course of the same process during which the high-temperature component was formed, or somewhat later, when the geomagnetic field had a different polarity. This component will not be discussed in this paper.

High-Temperature Magnetization Component of the Redkolesnaya Formation

[67] The distribution of the trends of the high-temperature (HT) magnetization component in the rocks of this formation exposed by the Irkineeva River and in those of the Greben Rock is shown in Figure 11 and in Table 6. The HT component identified in the outcrops of the river shows two polarities and produces two almost antipodal groups of the vectors. The reversal test gave a positive result ( g / g_cr = 11.2^o/13.9^o), indicating that during the vector reversal of one of these groups, the difference between the average groups is statistically insignificant. The presence of the antipodal groups of the vectors and the positive result of the reversal test can be classified, on the one hand, as the argument in favor of the primary character of the magnetization identified, and on the other hand, as an indication of the "purity" of its identification, that is, the absence of any significant admixture of other magnetization components in the component identified in the Irkineeva River outcrops.

[68] Unfortunately, because of some difficulties of separating the magnetization components described above, this statement cannot be applied to the rocks studied in the Greben Rock area. The fact that the angular distance between the average trends of the direct and reversed polarity, calculated using the trends identified using the outcrops of the Greben Rock, is higher than its critical value ( g / g_cr = 13.0^o/8.0^o) confirms this apprehension. Nevertheless, even in this case the deviation from the antipode pattern is not high suggesting that during the averaging of the vectors of the resulting trend of the HT (high-temperature) component would be slightly displaced relative to its true value.

[69] Taken separately, the data obtained for the outcrops of the Greben Rock do not allow one to perform a fold test (obviously, because of the low variations in the dips and strikes of the rocks). However, the fold test performed using the Irkineeva rock samples and also the whole collection of the Redkolesnaya rock samples proved the pre-folding age of the high-temperature component.

[70] The pre-folding magnetization, the presence of the antipodal groups of the vectors, the difference of the calculated paleomagnetic pole (Table 3) from all of the known younger poles of the Siberian Platform, and the similarity of the average HT directions obtained for the rock sequences, remote from one another, prove that the high-temperature component identified in the Redkolesnaya rocks originated during or soon after the deposition of the rocks.

High-Temperature Magnetization Components of the Ust-Tagul, Mota, Irkutsk, and Ostrovnaya Rock Formations

[71] The vector distribution of the high-temperature components of the rocks of the Ust-Tagul, Mota, Irkutsk, and Ostrovnaya formations are very similar (Figure 12). Proceeding from the close ages of these formations, this similarity can hardly be a random one, obviously reflecting the similar magnetization history of the respective rocks. In order to enhance the systematic constituent of these data distributions, all of the trends were recalculated to the geographical coordinates of the Ostrovnaya rock formation in the vicinity of the Gremyachiy Creek. The resulting total distribution of the high-temperature component vectors is shown in Figure 12. Presented in this Figure is the stereogram of the density distribution of the respective axes. The analysis of these stereograms revealed two obvious clusters corresponding to the two magnetization components, HT1 and HT2. The former, more distinct cluster consists of the vectors with northern and northeastern declinations and moderate positive inclinations (HT1 component). The latter cluster (HT2 component), which is less pronounced, yet, obviously existing one, includes the axes (trends) of NE (and SW) declinations and low (positive and negative) inclinations. All of the clusters are located not far from one another and include an overlapping region. Nevertheless, the distribution of their vectors (axes) suggests that the clusters can be confidently separated. The natural boundary for this separation is the distinct "saddle" clearly manifested between the clusters in the northeast of the stereogram for the density distribution of the axes. It is obvious that this separation is somewhat conventional, because it does not allow one to discard the vectors of the second cluster from the first cluster, which could get into it as a result of the statistical scatter, and vice versa. The consequence of this must be some displacement of the calculated average trend of the first cluster toward the second one, and vice versa. Therefore our average data might have been displaced slightly relative to the true values. However, we hope that this systematic displacement could not be higher than the errors common for paleomagnetic studies.

[72] The average trends of the clusters (components) identified in this study are listed in Table 7. The fold test suggested the pre-folding age of the formation of these components.

The Stable Magnetization Components of the Kliminskaya and Alesha Formations in the Taseeva R. Rock Sequence

Kliminskaya Formation.

[73] The high-temperature magnetization component of NNE declination and low inclination is close to the trend of the HT2 component which we identified in the rocks of the Ust-Tagul, Ostrovnaya, Mota, and Irkutsk formations. However, since this component was recorded only in a few out of the almost fifty samples of the rocks of the Kliminskaya Formation, we will not discuss it here and mention it just for the sake of our complete description.

Figure 14

[74] The medium-temperature magnetization component (see Table 8 and Figure 14) is of undoubted post-folding age, as follows from our Enkin and NFT tests.

Alesha Formation.

[75] The vectors corresponding to the most stable high-temperature magnetization components of these rocks show a fairly complex distribution (Figure 14 and Table 8). However, the analysis of the distribution density of the axes, along which the vectors discussed were directed, the stereogram showed at least two clusters: a large SE-NW one and a smaller, yet, fairly distinct NE one with its center in the region of low inclinations.

[76] The last cluster corresponded to the magnetization component (here referred to as the C component), which showed a bipolar distribution and was ranked as a pre-folding one according to the Enkin and NFT tests.

[77] The C component was ranked as that of the "C" class [McFadden and McElhinny, 1990] of the reversal test ( g / g_cr = 17.7^o/18.9^o) which can be taken, along with the positive fold test, as an argument in favor of its primary origin.

[78] The first cluster, which was examined carefully in the geographical system of coordinates, can be divided into two subclasses (see Figure 14), corresponding to two different magnetization components, namely, the A component (distinguished by its steep inclinations) and the B component. These subclusters and the vectors corresponding to them can be divided conventionally along the "saddle" located in the stereogram between their centers. The fold test performed after their division for the respective vector series showed, in both cases, a negative result, suggesting that the A and B components originated after the folding time. The A component coincided exactly with the trend of the modern magnetic field, which suggested its young age. The B component was found to be close in terms of its trend to the high-temperature component of the Redkolesnaya Formation which seems to be a primary one was formed at the end of the Vendian. At the same time the direct interpretation of the B component as the result of the remagnetization during the Redkolesnaya time is complicated by some difficulties associated with the presence in this region of the dislocated Early and even Late Paleozoic rocks (including the Permian ones). Ranking this fact as the indication that the folding of these rocks continued up to the end of the Paleozoic, then, proceeding from the fact that the direction of the B component is highly discordant to the Late Paleozoic-Cenozoic segment of the Siberian APWP curve, we face the problem that we cannot explain the observed trend in remagnetization terms.

[79] A similar problem arises during the interpretation of the medium-temperature component recorded by us in the Kliminskaya Formation. The pole corresponding to it rests in the vicinity of the Toyonian Pole of the Siberian Platform, suggesting some Early Cambrian remagnetization. On the other hand, if the rock deformation ended only toward the Permian, then, formally speaking, the age of this rock component must be later than Permian, which again leads to the contradiction between the observed trend and the trend corresponding to the APWP segment.

[80] The solution of this contradiction problem might be achieved by admitting the fact that the region of our study might have been poorly subjected to Post Early Cambrian deformation. This can be proved indirectly by the poor deformation of the Middle-Upper Cambrian rocks of the Verkholenskaya (Evenki) Formation [Nalivkin, 1968]. These rocks occur as a gently dipping synform, resting unconformably on the relatively steeply dipping rocks of the Taseeva and Kliminskaya rock series and the horizontal bedding of the Carboniferous rocks in the vicinity of the study area [Nalivkin, 1968]. This contradiction can also be explained by the assumption that the observed rock components occur as an undifferentiated mixture of magnetization components of different ages.

[81] Proceeding from the fact that the data available are definitely insufficient for solving this problem, we do not offer any conclusions concerning the formation time of the medium-temperature component of the Kliminskaya Formation and of the B component of the Alesha Formation. As for the C component of the Alesha Formation, we also restrain from confirming its primary character because the C component has been recorded only in one outcrop. Further studies must confirm or refute its primary character and the very fact of its real existence.

Estimation of the Reliability of the Results Obtained

[82] In the course of this study we derived several paleomagnetic determinations. Some of them were discarded as not sufficiently well founded (the data available for the Alesha and Kliminskaya formations from the Taseeva area). The remaining ones will be used here for the further interpretation.

[83] The data used here (Table 3) were obtained for the high-temperature magnetization components of the Aisa, Moshakova, and upper Chistyakova formations of the Taseeva Series. Proceeding from the fact that these sedimentary rock sequences are close in terms of their age [Kochnev, 2002; Sovetov, 2002a, 2002b, 2002c], and their paleomagnetic poles are not different statistically, we will discuss their common paleomagnetic pole obtained from the averaging of the data obtained for all of the informative outcrops (sites) of these rock sequences. The next important result we used in our study was the paleomagnetic pole calculated using the high-temperature component of the Redkolesnaya Formation. The poles corresponding to the high-temperature components, HT1 and HT2, of the Ust-Tagul, Ostrovnaya, Mota, and Irkutsk formations, and to the medium-temperature components of the outcrops from the Greben Rock area and from the valleys of the Biryusa, Tagul, and Urik rivers will be included into the discussion of the data obtained in this study.

[84] Before proceeding to the interpretation of these results, we will attempt to estimate their reliability. Several formal schemes were offered in the practice of paleomagnetology for this purpose [Li and Powell, 1993; Pecherskiy and Didenko, 1995; Van der Voo, 1993], which may differ in some details, yet, rely on similar criteria. Each of these schemes has its own merits and drawbacks. For this reason we decided to use in this study the most popular of them, namely, the Van der Voo scheme [Van der Voo, 1993], in which, irrespective of the fact whether a given paleomagnetic determination does or does not agree with the successively considered criteria, it gets its Qv estimate in terms of a seven-mark scale. The higher this estimate, the more reliable is the respective paleomagnetic determination. It should be noted that the procedure used to estimate the reliability of the paleomagnetic poles obtained (as expected during testing) using the primary magnetization, that is, the magnetization which was formed during the deposition of the study rocks or soon after it. In our case, these are the Aisa-Taseeva AT pole, the RDK pole obtained for the rocks of the Redkolesnaya Formation, and also the HT1 and HT2 poles calculated using the respective components of the Ust-Tagul, Ostrovnaya, Mota, and Irkutsk formations. In the text that follows we attempt to estimate the reliability of our results using the R. Van der Voo method (see Table 9).

[85] 1. The ages of the study rocks were determined sufficiently well. The modern dating of the deposition of the rocks of the Aisa Formation and of the formations of the Taseeva Series are based not on the direct geochronological data, nor on the biostratigraphic data, but on the basis of their geological history and inter-regional correlations. Moreover, the problem of dating these rocks Ediacarian (Vendian) or Late Baikalian (Late Riphean) is still a matter of hot discussion. Although the dating of the rocks discussed Late Baikalian or Ediacarian is not significant for the further discussion of the pre-Phanerozoic trend of the APWP of the Siberian Platform, our estimate of the former criterion is zero for the "AT" pole.

[86] As regards the RDK, HT1, and HT2 poles, the rocks for which they were obtained are treated by all researchers as Vendian. Moreover, the fauna findings and the chemostratigraphic data [Chechel, 1976; Khomentovskiy et al., 1998] confirm that these rocks deposited from the Late Ediacarian to the beginning of the Tommotian. This allows us to offer an estimate of 1 for the RDK, HT1, and HT2 poles in terms of the first criterion.

[87] 2. This result is based on more than 25 samples. The crowding of the vectors was higher than 10, the confidence angle was less than 16^o. All of the estimates, except the estimate obtained for the HT2 pole, based merely on 11 samples, satisfied this criterion.

[88] 3. Our detailed laboratory measurements made using detailed magnetic cleaning and component analysis yielded the values satisfying this criterion.

[89] 4. The reliability of our paleomagnetic determinations was confirmed by the positive results of the field tests. All of the results reported in this paper were based on the positive results of fold and reversal tests.

[90] 5(a). All of the geological objects examined in this study were located in the areas the tectonic positions of which (the belonging to some or other craton or tectonic block) were defined exactly.

[91] The study rocks outcrop in the regions which are ranked by all of their researchers as the parts of the Siberian Platform. However, it should be taken into account that all of the study rocks are folded and belong to the regions which had experienced notable tectonic deformations, which often show significant faults some of them being of unknown kinematics. These circumstances admonished us from the mechanical application of the results obtained in this study to the whole of the Siberian Platform. For this reason, while choosing the strategy of studying the paleomagnetism of of the Vendian rocks in the southwest of the Siberian Platform, we preferred to study the largest number of the remote rock sequences representing different regions. The results obtained justified this approach. For each time interval, we obtained data for the rock sequences (RDK pole) from different, far-spaced regions (AT, HT1, and HT2 poles), which correlated well with one another, proving the absence of any notable rotations of the study objects relative to one another and relative to the Siberian Platform. This allowed us to refer our results to the whole of the Siberian Platform. The fact that our study areas were spaced far from one another ensured the absence of errors associated with the potential disregard of local tectonics.

[92] 5 (b). The good structural control provided the presence of reliable field data, necessary for the reconstruction of the initial (predeformation) positions of the study objects. The study rocks were perfectly layered. This allowed us to measure their strikes and dips, necessary to determine the trends of the magnetization component identified in the ancient (predeformation) system of the coordinates. The absence of any rotations around the vertical axes, which might have distorted significantly the directions of the ancient magnetization components in the course of their transformation to the stratigraphic coordinate system is proved, as has been mentioned above, by the similarity of the paleomagnetic trends obtained for remote objects. In the cases where we doubted the reliability of the structural control and where the old trends were determined only in some local areas (Taseeva rock sequence), we discarded the data available.

[93] 6. In the cases where the study objects showed the vectors of the direct and reversed polarity with the statistical difference of 180^o, the result obtained satisfied this criterion for the RDK and AT poles. The HT2 component was found to be bipolar, yet, the small number of samples precluded any definite conclusion of the antipodal types of the respective direct and reversed polarity. The HT1 pole was obtained mostly for the monopolar component (only 6 samples, out of 39, showed direct magnetization). The McFadden and McElhinny tests performed for the samples of direct and reversed polarity gave a negative result ( g / g_cr = 14.5^o/12.5^o), probably associated with the inadequate removal of the overlapping components. Thus the HT1 pole did not satisfy the criterion 6 mentioned above.

[94] 7. The absence of similarity between the position of the paleomagnetic pole derived in this study and the positions of the younger poles compared with the Phanerozoic segment of the Siberian APWP [Smethurst et al., 1998] prove that this condition is observed in all of the poles obtained.

[95] To sum up, as follows from the Van der Voo model, the paleomagnetic poles derived show a high-grade reliability with the Qv estimates ranging from 5 (HT2 pole) to 7 (RDK pole).

Figure 15

Interpretation of the Results

[96] Before discussing the results of our interpretation, it should be noted that the territories of the study areas show the ubiquitous metachronous magnetization components which originated, judging by the positions of the respective paleomagnetic poles (GR, BT, UR), at the end of the Vendian to the beginning of the Cambrian (Figure 15). The observed remagnetization is local and seems to be associated with some large tectono-thermal event, or with several events which acourred at the southwestern margin of the Siberian Platform at the end of the Vendian to the Early Cambrian. With all poles calculated from their metachronous components fitting within the same area of the Indian Ocean south of Australia, there is a notable, statistically significant difference between the Urik and Biryusa-Tagul poles, on the one hand, and the Lower Angara pole, on the other, which proves that remagnetization took place in these areas during some close though not simultaneous periods of time.

[97] Since the Angara (GR) pole is located in the area of the potential location of the Tommotian pole of the Siberian Platform [Pisarevsky et al., 1997], while the Urik (UR) pole and, especially, the Biryusa-Tagul (BT) pole tend to closer to the Toyonian pole [Gallet et al., 2003], it can be inferred that the remagnetization front (and, possibly, the deformation front) moved from the northwest to the southeast (in modern coordinates). However, in connection with the potential existence of a small Late Vendian-Early Cambrian APWP loop (see Figure 15), the Urik and Biryusa-Tagul poles can be correlated also with the B1 Nemakit-Daldynian pole [Shatsillo et al., 2005]. In this case, it can be expected that the displacement of the remagnetization front was directly opposite. Be it as it may, the results of our study provide a basis for carrying out detailed paleomagnetic studies aimed to study the history of tectonic movements in the southwest of the Siberian Platform.

[98] Since the metachronous magnetization of the rocks outcropping in the Greben Rock area has a synfolding age, its trend can be used to date the folding in this part of the Lower Angara area. The position of the GR pole at the base of the Phanerozoic trend of the Siberian APWP suggests that the folding, or at least one of its stages, occurred at the beginning of the Cambrian, obviously, during the Tommotian-Atdabanian.

[99] While discussing the regional remagnetization of the rocks in the southwest of the Siberian Platform, it is worth mentioning the independent data obtained recently by Metelkin et al. [2005] for the sedimentary rocks of the Karagas Series and for the basic rocks of the Nersa Complex (Late Riphean) in the rock sequences exposed by the Biryusa R., slightly south of the rocks investigated in our study. In the same area, we also identified a metachronous magnetization component, the paleomagnetic pole of which falls onto the Cambrian segment of the APWP of Siberia (see Pole BKN in Figure 15). Unfortunately, for some objective reasons we failed to date the formation time of the metachronous BKN magnetization, relative the deformation time. Metelkin et al. [2005] offered the potential synfolding age of this component.

[100] The results we obtained in this study for the high-temperature rocks of the Ust-Tugul, Ostrovnaya, Mota, and Irkutsk formations contribute substantially to the discussion of the hypothesis offered for the anomalous behavior of the geomagnetic field at the Vendian-Cambrian boundary and for the reality of the Early Cambrian episode of the true polar wander - IITPW [Kirschvink et al., 1997; Pavlov et al., 2004]. The vector distribution of the high-temperature magnetization components in these rocks resemble amazingly the similar pattern we obtained earlier for the boundary Vendian-Cambrian layers in the Kharaulakh, Baikal, and East Sayan areas [Pavlov et al., 2004; Shatsillo et al., 2005].

[101] Similar to the transitional Vendian-Cambrian rocks in the East Sayan and Baikal regions (South Siberian Platform), as well as in the Kharaulakh area in the northeast of the Siberian Platform, the Vendian-Cambrian rocks studied in the same structural sequence showed two characteristic magnetization components, often replacing each other. One of these components has a reverse polarity and is similar to the so-called "Khramov" directions of the Lower Cambrian, the other showing the alternative "Kirschvink" trend. It is important to note that the respective pair of poles (B1-HT1 and B2-HT2) for the rocks having the same Nemakit-Daldynian age in the Baikal-Central Sayan region generally show a good agreement (Figure 15). Some difference between the HT1 and B1 poles can be explained by the statistic scatter, or by the imperfect quality of the procedure used for the separation of the vector distributions into clusters. In any case, the data obtained in this study confirm confidently the validation of our previous conclusion that the significant number of the Vendian-Cambrian transitional layer of the Siberian Platform show two stable high-temperature components of magnetization, each of which can be regarded as a primary one, which had been formed not later than the Early Cambrian.

[102] Proceeding from this conclusion, earlier, we offered the supposition that some anomalous conditions existed at the Vendian-Cambrian boundary for the generation of the geomagnetic field, when two quasistable states might have existed interchangeably, namely, a dipole field with a dipole oriented averagely along the axis of the Earth rotation and an anomalous dipole field with the substantial deviation of the dipole axis from the Earth rotation axis, or a nondipole field [Pavlov et al., 2004; Shatsillo et al., 2005].

[103] The potential alternative explanation of this supposition follows from the fact that the observed distributions were formed as the result of the superposition of the secondary metachronous magnetization of the Middle-Late Cambrian age over the Kirschvink trends. The assumption of the primary origin of the Kirschvink trends suggests the abnormally high-velocity drifting of the paleomagnetic pole at the end of the Early Cambrian, being one of the corner-stones of the Inertial Interchange True Polar Wander (IITPW).

[104] This explanation seemed to be probable in the case of the Early Cambrian rocks, the "normal (Khramov)" trend of which suggested a pole, close to the Middle and Late Cambrian poles of the Siberian Platform. In this paper, like in the case of the previous one [Shatsillo et al., 2005], we studied older rocks whose normal trend (HT1, B1) differs from all of the known Phanerozoic poles.

[105] However, it should be noted that our discovery of the regional Late Vendian-Early Cambrian remagnetization complicated the interpretation of the observed vector distributions in terms of the anomalous field hypothesis. Since this remagnetization does exist, we can admit that at least some of the two-cluster vector distributions in the Late Vendian rocks can be interpreted as the superposition of the metachronous "Khramov" signal over the primary "Kirschvink" one. However, taking into account the Vendian age of the rocks discussed, the potential Late Vendian or Early Cambrian age of their remagnetization (showing the "Khramov" trend), and the fact that the "Kirschvink" trends were discovered in the Tommotian and Atdabanian rocks [Kirschvink and Rozanov, 1984], we have to admit some short period of time which witnessed the 40 to 50-degree jumps of the paleomagnetic pole from the "Kirschvink" positions to the "Khramov" ones and back, which seems to be scarcely probable.

[106] The important, if not decisive, argument against the IITPW hypothesis is the paleomagnetic determination obtained in the course of this study for the rocks of the Redkolesnaya Formation, which had deposited in the Late Ediacarian to the Early Nemakit-Daldynian. The high-quality pole obtained for the rocks of this formation lies at a distance of more than 40^o from the Tommotian pole, located by J. L. Kirschvink (pole KirCorr in Figure 15) which, in turn, is shifted from the Middle Cambrian pole of the Siberian Platform [Gallet et al., 2003] by a distance of about 67^o. When we used the Kirschvink pole, corrected for the relative rotations of the Aldan and Anabar blocks [Pavlov and Petrov, 1997] (Pole KirCorr in Figure 15), the respective values were 40^o and 49^o. The large distance between the Middle Cambrian pole of the Siberian Platform and the Tommotian-Atdabanian pole obtained by J. L. Kirschvink was explained by him by the IITPW episode. In this case a question arises concerning the explanation of a great distance between the Redkolesnaya pole of fairly similar age and the Tommotian pole obtained by J. L. Kirschvink. Should we admit one more IITPW episode? In our opinion this explanation seems to be highly artificial and improbable. In the case of admitting our hypothesis of the anomalous character of the Vendian-Early Cambrian geomagnetic field instead of the large-scale reciprocating displacement of the pole to the east, following from accepting the Kirshvink pole as the only one possible, we get the monotonous movement of the pole in the eastern direction for a distance which is two-times smaller than the one offered by the Kirschvink version (Figure 15).

[107] To conclude, the data obtained in our study can be regarded as contradicting the IITPW hypothesis and supporting the hypothesis of the anomalous geomagnetic field.

[108] In any case, it is not sufficiently substantiated to use the Kirschvink Tommotian dating, nor our datings, of the respective B2 and HT2 poles performed in this study and earlier [Shatsillo et al., 2005] for plotting the Siberian curve of the apparent pole migration.

Figure 16

[109] The paleomagnetic datings performed in the work of this project allow us to approach the solution of the long-lived and exceptionally acute problem of the Late Riphean-Vendian trend of the Siberian APWP. The essence of this problem is illustrated schematically in Figure 16 and consists of the fact in spite of having a number of fairly good paleomagnetic determinations available for the Middle and the beginning of the Late Riphean of the Siberian Platform, we cannot use all of them, because we do not know the polarities of the Siberian paleomagnetic trends of that time. Accordingly, we do not know whether we deal with the northern or southern poles and cannot be sure in which hemisphere the Siberian Platform was located during the Riphean time. It is obvious that under these conditions the use of the Siberian Precambrian paleomagnetic data for paleotectonic reconstructions and solving other important problems of geology and geophysics is fairly problematic. The importance of this problem can be illustrated by the fact that the confirmation or negation of the possibility of Siberia to be a part of the Rhodinia supercontinent and the very existence of this supercontinent depend critically on the choice of the polarity of the Siberian Precambrian trends [Pavlov et al., 2002].

[110] The uncertainty of the polarity choice for the Siberian Precambrian trends is associated with the total or nearly total absence of reliable paleomagnetic data for the Siberian Platform in the time interval of 950-520 million years. Our data allow us to make an important step of filling this gap.

[111] Up to recently, the Pacific trend of the paleomagnetic poles was believed to be the most preferable one for the Late Riphean to Vendian time (Figure 16a). This choice was made proceeding from the principle of minimizing the movements. However, since the time of getting reliable poles of Siberia for the beginning of the Late Riphean, it seemed to be strange that during the time period measuring hundreds of million years the paleomagnetic pole was displaced merely by 50-60^o, whereas this displacement amounted to more than 130^o during the time period slightly longer than the Phanerozoic time.

[112] The data available for the other continents [Torsvik et al., 1998] also suggested the significantly larger velocities for the movements of the respective paleomagnetic poles in this time interval.

[113] The new paleomagnetic poles of the Siberian Platform, corresponding to the Ediacarian and Late Ediakarian to Nemakit-Daldynian time turned out to be located at the moderate latitudes of the central part of the Indian Ocean. This means that the previous views of the Pacific trend of the Late Riphean and Vendian poles were wrong. The new data reported in this paper suggest that from the Aisa to the Redkolesnaya time (from the end of the Ediacarian to the beginning of the Nemakit-Daldynian) the paleomagnetic pole of the Siberian Platform moved rapidly southward from the central areas of the Indian Ocean. The Siberian Platform itself, whose first southwestern and later south-south western edge were turned to the north, was also located in the southern hemisphere and moved from the tropic latitudes (5-20^o S) to moderate ones (30-50^o S). The data obtained in this study suggest that at that time the Siberian Platform resided at the most southern point of its motion for the last 600 million years. Up to the end of the Vendian and to the beginning of the Tommotian, the Siberian Platform remained at the same latitudes, continuing to rotate clockwise relative to the meridian network. At the beginning of the Tommotian time, facing northward was its southwestern part. That time witnessed the beginning of the slow drifting of the Siberian Platform to the north, which lasted hundreds of million years, with some periods of slowing down and acceleration. In the middle of the Mesozoic the modern northern regions of the Siberian Platform crossed the northern, pole-bordering regions of the planet and continued the movement, initiated about 550 million years ago, now in the southern direction. Our analysis of the migration of the paleomagnetic pole of the Siberian Platform suggests that the end of the Vendian and, possibly, the very beginning of the Cambrian witnessed some very important tectonic rearrangement which caused a cardinal change in the movement of the Siberian Platform and, possibly, of the entire planetary assemblage of the tectonic plates. It appears that the coincidence of the time of the regional remagnetization of the rocks and the change in the drifting pattern of the Siberian Platform was not accidental.

[114] It can be supposed that by the middle-end of the Vendian the moving Siberian Platform encountered some serious obstacle which first slowed down and later stopped its migration to the south. It is possible that the traces of this event are imprinted in the Vendian accretion-collision rock complexes described in the Taimyr folded region [Vernikovskii, 1996].