RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1004, doi:10.2205/2006ES000191, 2006

Discussion of the Results

[28]  The bimodal distribution of the S p values was caused by two factors: (1) the global first S p mode exists throughout the time interval of 340-200 Ma, irrespective of the distance to the Siberian traps (Figures 8 and 9); (2) the local second, higher S p mode appears only in the time interval of 300-270 Ma (Figure 9) and at relatively small distances from the center of the Siberian traps and disappears away from it (Figures 11 and 12). Consequently, during the time period discussed, the variation magnitude of the geomagnetic field direction was averagely 7o-8o (Figure 6b), this being a global effect characterizing the normal state of the geomagnetic field predominantly of reversed polarity. Moreover the transition from the Kiama reversed polarity hyperchron to the Illawara hyperchron of frequent polarity changes had a poor effect on the average variation magnitude (Figure 6b): it grew 1o-2o larger. This background was overlapped by the anomalous state of the geomagnetic field, which was of local character. Obviously, this was caused by the high disturbance of the "normal" state of the geomagnetic field in the area where the Siberian plume was being generated.

[29]  The large scatter of the S p values in the vicinity of the Siberian trap center suggests the relatively short existence of any large-magnitude variations. It is known that the S p values get into each of the time intervals concerned during the time period of 15-40 million years. The short-time existence of the anomalous magnitudes of the field variations and plume existence is proved by the following fact. Approaching to the centers of modern world magnetic anomalies, we observe the similar pattern of the growing magnitudes of the geomagnetic field direction variations, the lifetime of the world anomalies being shorter than 20 thousand years [Pechersky, 2001]. We can suggests a close relationship between the sources of the world magnetic anomalies and plume formation, this being emphasized by the short existence of the world magnetic anomalies and the flows of most of the Siberian traps. However, apart from the short-term intensive trap formation, there are some long-lived hot spots, such as, the Hawaii, Galapagos, Iceland, and others, the sources of which had been lower-mantle plumes [Courtillot et al., 2003; Ernst and Buchan, 2003]. There is no contradiction here. The impulses causing the generation of the geomagnetic field variations and plumes are very short, yet, the plume chambers produced at the base of the mantle can exist longer than a hundred million years and they are not associated with the core events.

[30]  The S p dependence on the distance to the center of the Siberian traps is obvious in the time interval of 300-270 Ma (Figures 11 and 12). Accordingly, the "retardation" of the magmatism from the exited state of the core, which caused the higher variation of the geomagnetic field was 20-50 million years. These estimates agree with those obtained earlier for the magmatic activities of the modern plumes and for the plumes refer to the Mz-Kz boundary [Pechersky, 2001; Pechersky and Garbuzenko, 2005]. This long interval can be explained by the following two alternatives:

[31]  The first trivial cause is associated with the uncertainty of dating the rocks and paleomagnetic determinations. The highest S p values are referred, as mentioned above, to the time interval of 300-270 Ma, the average time of "retardation" being 35 million years.

[32]  The second nontrivial cause allows us to suggest that this significant event was caused by the repeated "bursts" of the core reactivation which caused the formation of the series of plumes, all of them representing the Siberian superplume. It appears that not all of the rising plumes, which had originated in the time interval of 300-270 Ma in the vicinity of the core-mantle boundary and reached the surface of the Earth, except for the largest and most powerful "burst" which reached the Earth surface in the form of the main stage of the Siberian trap magmatism.

2006ES000191-fig16
Figure 16
[33]  Apart from the S p growth recorded in the time interval of 300-270 Ma, some the other time intervals discussed show some elevated S p values in the areas remote from the center of the Siberian traps (Figures 7, 10, 11, 12, 14, and 15). First, these are randomly scattered sites, possibly, associated with various errors of estimating the precision parameter of the paleomagnetic directions, and, secondly, the compact groups of elevated S p values (Figure 16). Some of these groups, located not far than 30o from the Siberian superplume can be associated with its halo. Yet, this version is not confirmed by the "compact patterns" of these groups of points, these points not covering the center itself. Moreover, the S p values of the Siberian traps proper and of the study objects, located not farther than 30o from the center of traps, show a normal, unimodal distribution (Figure 13). In my opinion, these compact groups denote the regions of the excited states of the Earth core near its boundary with the mantle, the regions of the generation of the global magnetic anomalies and the lower-mantle plumes, the rising of the latter to the Earth surface being "expected" at the time 20-40 million years later than the origin of these compact groups. The fact that the rise of a plume, and, moreover, of the group of plumes (a superplume), is not necessarily vertical, may suggest that the elevated S p clusters, recorded for the time intervals of 300-290 Ma and 285-270 Ma, are the regions of the Siberian superplume generation.


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Citation: Pechersky, D. M. (2006), Geomagnetic field in the vicinity of the Paleozoic-Mesozoic boundary and the Siberian superplume, Russ. J. Earth Sci., 8, ES1004, doi:10.2205/2006ES000191.

Copyright 2006 by the Russian Journal of Earth Sciences

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