RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1002, doi:10.2205/2005ES000185, 2006
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Figure 2 |
[30] Worthy of mention are the data obtained by Gurevich et al. [2004] and by Heunemann et al. [2003] for trap-type effusive rocks in the area of Norilsk and in the north of the Putorana Plateau, respectively.
[31] In the Norilsk region (Talnakh, Listvyanka, and Kaerkan areas) samples were collected from lava flows and small intrusions at 35 sites. The characteristic magnetization components showed both direct and reversed polarity and were ranked to be substantially antipodal ones, with some virtual poles being fairly widely scattered (the clustering factor of 6.5). Heunemann et al. [2003] suggest that the trap rock sequence records a transition from direct to reversed polarity, the stable field being recorded in some stratigraphically lower rocks. We believe that the data available for these 35 sites should be discarded from the calculation of the Siberian magnetic pole.
[32] The 60 lava flows studied in the Abagalakh rock sequence (the northern part of the Putorana Plateau and the valleys of the Abagalakh and Ikon rivers). Heunemann et al. [2003] believe that the lower 16 lava flows recorded the latest period of the reversed to normal polarity transition. The magnetization of the remaining 44 lava flows reflect the trend of the stable (unreversed) geomagnetic field and, hence, can be used to calculate the magnetic pole.
[33] Pavlov et al. [2001] studied several lava flows and small intrusions at seven sites west of Norilsk City. Their characteristic magnetization showed both direct and reversed polarity, The reversal test gave a positive result. The respective paleomagnetic pole, shown in Table 2, was found to be somewhat different from the pole reported by Pavlov et al. [2001]. This was caused by the fact that during the revision of initial data some samples with noise were discarded, and the closely spaced sites were combined.
[34] The results of the paleomagnetic studies of traps and of some rocks remagnetized by them, outcropping in the valleys of the right tributaries the Podkamennaya Tunguska River, known as the Bolshaya Nirunda and Stolbovaya rivers, and also in the valley of the Kotui River (Maimecha-Kotui area), were published in 2003 by Veselovsky et al. [2003].
[35] Studied in the Stolbovaya R. Valley were four sites from a large intrusion in the river mouth and three sites in three outcrops of remagnetized Ordovician rocks. Depending on the choice of a method for calculating the mean values, namely, breaking the outcrop of remagnetized rocks into sites (version 1), or considering each of them as one site (version 2); calculating the mean values at the site level (version 1) or at the level of objects, where the object is one remagnetized outcrop, one igneous rock body, etc., (version 2), the respective paleomagnetic poles had somewhat different coordinates (see Table 2).
[36] In the Bolshaya Nirunda R. Valley we studied a large igneous rock body and some remagnetized rocks in three outcrops of Ordovician sedimentary rocks. Similar to the Stolbovaya R. objects of study, the mean trends of the Bolshaya Nirunda R. objects of study could be calculated using two methods, one corresponding to Version 1 (see above) and used by (M. L. Bazhenov et al., in press, 2005) the other corresponding to Version 2 used by Veselovsky et al. [2003] to their data.
[37] Five sites from 5 lava flows were studied in the Kotui R. Valley. The recorded characteristic magnetization showed both direct and reversed polarity.
[38] Kravchinsky et al. [2002] studied several trap lava flows in the Alakit-Markha area of the Vilyui region, in the vicinity of the Sytikan, Aikhal, and Jubilean kimberlite pipes. The data reported by these authors are not discussed here because these pipes are located at a significant distance from the Permian-Triassic trap rocks, and the association of their magnetization with the trap emplacement seems to be insufficiently obvious.
[39] Apart from the data that were published earlier, in this paper we also use the data obtained for the trap rock bodies and the sediments remagnetized by them from the Kulyumbe and Moyero river valleys (M. L. Bazhenov et al., in press, 2005). In the Kulyumbe area samples were collected from 6 lava flows, 7 sills, and 13 outcrops of sedimentary rocks, which appeared to be wholly remagnetized by the traps. In the Moyero R. Valley, results were obtained for 11 intrusions and 11 outcrops of sedimentary rocks, also remagnetized by the traps.
[40] In the case of the Moyero R. area, the data obtained for the sedimentary rocks showed extremely high clustering (K=1327 for the case of 50-percent rectification, K=793 in the geographical system of coordinates, and K=805 in the stratigraphic coordinates) and a significant difference of their mean values from the respective value calculated for the igneous rocks. For this reason, the results obtained for the remagnetized rocks were discarded from the calculation of the mean value for the region as a whole (version 2). This decision was made proceeding from the following two alternative hypotheses. One of the inferred the extremely rapid magnetization of the rocks, during which the secular variations had been averaged. On the contrary, the other hypothesis inferred some fairly long-lasting remagnetization which had been associated with some unknown remagnetization event.
[41] The other method of calculation (version 1) inferred, like in the case of the objects from the Bolshaya Nirunda and Stolbovaya River valleys, the breaking of the remagnetized rock outcrops into sites and the calculation of the average value for the region using the sites where samples were collected both of sedimentary and igneous rocks.
[42] Because of the high clustering of the trends obtained for the sills and remagnetized sedimentary rocks in the lower reaches of the Kulyumbe River, a view was advanced that the former could be interpreted as the single-event apotheses (offshoots) of a large igneous rock body emplaced in the close vicinity, while the latter were remagnetized during the intrusion of these apotheses. Proceeding from this assumption it was suggested to interpret all of the objects studied in the lower reaches of the Kulyumbe River (except for the KV7 sill having a different polarity (see Table 1 in M. L. Bazhenov et al., in press, 2005), this table being available also at the address of http://paleomag.ifz.ru/bazhenov-tab.html) as the products of some short-time event, assigning all of them the same weight, like in the case of the lava flows in the upper reaches of the Kulyumbe River and of the KV7 sill. The average trend calculated using the Devonian remagnetized red rocks, based on the samples collected in different places of the same outcrop, were also recommended to be taken into account, each of them having its own singular weight. This procedure of computing the mean values is also included in the rules recommended for version 2. Like in all other regions, in the case of the Kulyumbe area, this version implies that each isolated outcrop of remagnetized sedimentary rocks can be treated as one site irrespective of the number the samples available.
[43] The paleomagnetic poles calculated using the above procedures for the study areas are listed in Table 2. The location of the regions of the Siberian Platform, whose poles are used in this paper, is shown in Figure 2.
[44] In spite of the fact that the poles were calculated using different methods (version 1 and version 2) the resulting average poles, namely NSP2 (M. L. Bazhenov et al., in press, 2005) and VP (this paper) are located at a distance of merely 2.4o from each other. This distance is notably smaller than the critical angle ( g c=5.7o [McFadden and McElhinny, 1990], which makes it statistically insignificant.
Citation: 2006), New paleomagnetic data for the Permian-Triassic Trap rocks of Siberia and the problem of a non-dipole geomagnetic field at the Paleozoic-Mesozoic boundary, Russ. J. Earth Sci., 8, ES1002, doi:10.2205/2005ES000185.
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