RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1004, doi:10.2205/2006ES000191, 2006
Variations in the Geomagnetic Field Direction
[7] The total magnitude of the geomagnetic field
direction variations was determined using the
standard angular deviation
S=81/ K1/2,
where
K is the precision parameter in the
sphere statistics
[Khramov et al., 1982].
Using the same method, we determined
S values for the whole of
the Neogaea
[Pechersky, 1998]
and for the vicinity of the Mz-Kz boundary
[Pechersky and Garbuzenko, 2005].
[8] In contrast to the other geomagnetic field characteristics,
briefly reported above, the abundant data
available for the magnitude of the field
direction variations, both in time and space, are
sufficient to analyze not only the global
behavior of this field characteristic, but also
some local features, relative, in particular, to
the center of the Siberian traps.
[9] In order to describe the
S behavior in the vicinity of
the Paleozoic-Mesozoic (P/T) boundary,
I used the Paleomagnetic Database (GPMDB-2005),
and chose the paleomagnetic data, ranging roughly
from 200 Ma to 340 Ma in age. This choice was
based on the following reasons: (a) the basic
impulse of the Siberian trap activity can be
dated 251 Ma, some less intensive magmatic
activity was recorded later, during the Triassic
[Gurevich et al., 2004;
Ivanov et al., 2005].
In order to reconstruct a
more complete pattern of the geomagnetic field
variation, I used the time interval after the end
of the Siberian trap magmatism, namely, 250 Ma to
200 Ma; (b) earlier, using the examples of the
modern igneous activity of the plumes
[Pechersky, 2001],
and of the plumes in
the vicinity of the Mz-Kz boundary
[Pechersky and Garbuzenko, 2005],
we found that closer to the plume epicenter the
S value and its scatter increase notably, this
growth being especially significant during the
period of ~20-50 million years before the
beginning of the high magmatic activity of the
plume at the Earth surface. In this connection I
used the time interval overlapping the potential
time of the plume rise, namely, 270-300 Ma. For
comparison, I added the time interval of
310-340 Ma, preceding the beginning of the
Siberian Plume formation.
[10] The next step was to sort out the data chosen from
the paleomagnetic data base into four categories:
[11] (1) the unreliable paleomagnetic data
obtained for not more than ten samples, the
thermal demagnetization was not higher than
200o C, the AF demagnetization was not higher
than 15 mT, the precision parameter
K<7,
the confidence angle
a95>25, the
coordinates of the paleopole were different
greatly from the average pole for the given time
for the continent in question, the paleomagnetic
determinations being considered as the results of
the remagnetization of the older rocks. The
determinations of this kind were discarded;
[12] (2) the low reliability of the paleomagnetic
determinations with the number of the samples not
more than 20, the cleaning temperature lower than
400o C, the AF cleaning being lower than
30 mT. The index of the paleomagnetic reliability
was 0.1 for this group of samples. This index was
used as a weight value in calculating the average
S values;
[13] (3) the intermediate reliability of the paleomagnetic
determinations with the number of the
samples higher than 20, the thermal cleaning was
performed at temperatures not lower than 500o
C, the AF cleaning was not less than 50 mT. There
were examples of positive geophysical tests
(fold, pebble, reversal or baking tests). The
index of paleomagnetic reliability was 0.5. This
index was used as the weight in calculating the
average
S values.
[14] (4) the high reliability of paleomagnetic determinations
with the number of samples larger than
20, the obligatory complete thermal
demagnetization and AF demagnetization with the
component analysis, the identification of the
characteristic NRM component, and the positive
fold, pebble, baking, and reversal tests. The
index of paleomagnetic reliability, as high as
1.0, was used as the weight in calculating the
average
S values.
[15] It should be emphasized that the geophysical tests of
paleomagnetic reliability, such as the fold,
pebble, reversal, and baking tests, are important
to prove the identification of the primary NRM
component, yet, they cannot be used to prove that
the
S value corresponds to the variation
magnitude of the geomagnetic field direction. As
will be demonstrated below, the scatter of the
S values is very large, primarily, because of
some technical and methodological reasons, such
as not complete cleaning, measurement errors,
magnetic biasing in the course cleaning, the
uncertainty of the age interval during the
calculation of the paleomagnetic direction, and
the like. For this reason we can be sure only of
the average and modal
S values.
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Figure 2
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[16] Based on the types of the rocks we classified our
determinations into four groups: sedimentary,
redbeds, volcanic, and intrusive rocks. Most of
our data belong to the sedimentary rocks. For
this reason, the results of this study are
combined into groups in terms of their ages and
paleomagnetic reliability. A separate group
combines the data for Australia, which are
characterized, in the age interval discussed, by
the elevated
S values (Figure 2), which are
unrelated to the Siberian plume.
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Figure 3
|
[17] The paleomagnetic determinations, which remained
after our data sorting were classified into the
following age groups: 340-315, 310-290,
285-270, 265-245, and 240-200 Ma. Each of
these intervals was characterized by the
respective maps of paleotectonic reconstructions
and APW paths for all continents, mainly after
[Torsvik and Van der Voo, 2002]
with the additions for China and other regions after
[Bretstein and Klimova, 2005;
Didenko et al., 1994;
McElhinny and McFadden, 2000;
Pechersky and Didenko, 1995;
Scotese and McKerrow, 1990;
Smethurst et al., 1998].
The paleomagnetic data points available were plotted
on these maps, and after that were determined
their ancient coordinates. There are two models
of paleotectonic reconstructions: the GAD model,
where the geomagnetic field is assumed to be a
dipole one and the G3 model which takes into
account a nondipole, octupolar component
[Torsvik and Van der Voo, 2002].
Without dwelling on the substantiation of these models,
I determined the ancient coordinates of the
paleomagnetic observation sites for both models.
The coordinates of the Siberian traps center,
reconstructed for the time of 250 Ma, were found
to be 57o N, 30o E, using the GAD model,
and 60o N, 45o E, using the G3 model. In
both cases we determined the distances, along the
great-circle arc, from each of the paleomagnetic
determination site to the center of the Siberian
traps. I assumed that during the igneous
activity of the Siberian plume the position of
its epicenter had not changed notably. In terms
of clarifying the dependence of the total field
direction variation on the distance of the
observation site from the trap center, the
difference between these two models was
insignificant (Figure 3): it was usually not
higher than 10o. It was only in the case of
the distances to the center of the Siberian
traps amounting to more than
80o, it was
sometimes as large as
20o and more, in the
region where the association of the Siberian
plume and the field direction variation was
hardly probable. Therefore it is enough to
consider only one model, in this paper we will
dwell on the G3 model.
[18] In determining the coordinates of the paleomagnetic
observation site, we used two factors: the
paleolatitude, calculated from the paleomagnetic
inclination of the given site, and the position
of the site in the map of the "suitable" age
reconstruction. The fact is that in many cases
the Database offers a fairly broad range of rock
and paleomagnetic determinations ages. For this
reason, I selected a map where the position of
the point in question corresponded best of all to
its paleolatitude determined using the
paleomagnetic inclination. Also, we took into
account the agreement of the polarity of the
given paleomagnetic determination with the
time-scale of geomagnetic polarity
[Gradstein et al., 2004],
within a five-million year averaging.
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Figure 4
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[19] The
S value is known to vary with the latitude:
it diminishes roughly by two times slowly from
the equator to the pole. This is valid for the
intervals of the steady-state geomagnetic field
throughout the Neogaea
[Pechersky, 1996].
According to the dependence of
the
S values with the latitude, all of the
S determinations were reduced to one paleolatitude,
namely, to the latitude of the pole ( S p ). Figure 4a clearly shows the
S dependence
on the paleolatitude of the observation site,
which vanishes after its reduction to the
latitude of the pole (Figure 4b).
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Figure 5
|
[20] Depending on the model used (GAD or G3),
the paleolatitudes of the observation points were
found to be somewhat different and, hence, the
S p values were found to be different,
too. This difference is illustrated in Figure 5,
which shows that in most cases the difference
between the
S p values of both models is
not higher than 1-2o.
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Figure 6
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|
Figure 7
|
|
Figure 8
|
[21] All of our paleomagnetic determinations were
distributed, with some or other accuracy, over five-million
year intervals (Figure 6a). For each of the
five-million year interval I calculated its
average value and performed a smoothing operation
for the time intervals of 10 million years, the
smoothing step being 5 million years (Figure 6b).
The resulting pattern (Figure 6b) shows the
behavior of the variation direction magnitude of
the "normal" geomagnetic field, where the
scatters of different origin are smoothed off.
This Figure shows that the direction of the
"normal" field varies insignificantly, even
without smoothing from
6o
to
10o, the
changes being minimal in the Kiama hyperchron
during the stable state of the geomagnetic field,
and more notable in the time interval of
320-300 Ma, which preceded the Kiama hyperchron
(Figures 1, 6b). The P/T boundary is not expressed in
the
S p behavior, being restricted to the
region of some
S p value growth which
began at the time of 260 Ma (Figure 6b). Against
a background of the "normal" field there is some
tendency toward the
S p growth in the
direction closer to the center of the Siberian
traps by a distance less than
40o
(Figure 7). This trend was also recorded in the
summarized
S p distribution histogram (Figure 8).
This Figure clearly shows the main distribution
with the
8o mode, against the background of
which clearly shows up the second group of the
higher
S p values (10-13o) with a
mode of
11o, related mainly to the sites
located closer than
40o to the center of the
Siberian traps.
[22] Discussed below in more detail is the behavior
of the variation of the geomagnetic field direction
during the main time intervals relative to the
P/T boundary and to the center of the Siberian
trap, which is believed to have been the
epicenter of the plume. It should be emphasized
that the distance from the plume epicenter to the
region of its origin at the core-mantle boundary
was about 3000 km, and its rising path being
often inclined
[Ernst and Buchan, 2003].
Combined with the insufficient
accuracy of the paleomagnetic direction
determinations and reconstructions, this fact
might have caused the significant scatter of the
S p values. It is pertinent to remind
that each of these time intervals embraces 15 to
40 million years. Consequently, with the above
mentioned uncertainty of the distance to the
source of the plume and the long period of time
compared to the time of the unstable state at the
core-mantle boundary and, hence, the unstable
state of the geomagnetic field, the conditions
closer to the plume epicenter must be reflected,
first, in the growth of the variation magnitude,
that is, in the
S p value and, secondly,
in the growth of the
S p value scatter.
Moreover, the
S p distribution relative
to the Siberian plume epicenter could be
disturbed notably by the other plumes of
different ages, which might have been active
during the time period discussed. Apart from the
reasons mentioned above, one should not forget
technical errors, the inaccuracies of the
measurements, the "purity" of identifying the
primary NRM components (the presence of stable
secondary components), and the like. All of them
augmenting the scatter of the
S p unit
values, the mean and modal values being more
objective quantitative characteristics.
|
Figure 9
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Figure 10
|
[23] (A) 340-315 Ma. The distribution of the
S p is characterized by the predominance of low values
ranging from
4o to
8o with a distinct
mode of
8o (Figure 9e), corresponding to the
"normal" field in this time interval (Figure 6b).
The distribution of the
S p values is not
controlled by the distance to the center of the
Siberian traps (Figure 10).
|
Figure 11
|
[24] (B) 310-290 Ma. The
S p values show a bimodal
distribution (Figure 9d). The first
7o mode
corresponds to the "normal" field in this time
interval (Figure 6b). The second
10o mode
depend on the distance to the center of the
Siberian traps, which is indicated clearly by
the average
S p values (Figure 11). The
largest
S p values were found for the
time of 290-300 Ma.
|
Figure 12
|
[25] (C) 285-270 Ma. The
S p values show a bimodal
distribution, including one distinct mode of
8o and a less distinct mode of
11o (Figure 9c).
The first mode corresponds to the "normal"
field, the second mode being associated with the
S p growth closer to the center of the
Siberian traps, as seen from the behavior of the
average
S p values (Figure 12). The
highest
S p values were recorded for the
time interval of 270-280 Ma.
|
Figure 13
|
|
Figure 14
|
[26] (D) 265-245 Ma. The
S p values showed
a unimodal distribution with a mode of
8o, which
corresponds to the "normal" field, the mode
associated with the Siberian Plume is absent
(Figure 9b). This was confirmed by the histograms
of the unit
S p data selection obtained
for the data points residing not far than
30o
from the center of the Siberian traps
(Figure 13a) and by the selection of the Siberian
traps alone (Figure 13b). Some
S p raise
at the distance of less than
20o
(Figure 14)
is proved by a mere couple of reliable data
points and averagely does not exceed the "normal"
field (Figure 6b).
|
Figure 15
|
[27] (E) 240-200 Ma. This time interval is characterized
by the
S p unimodal distribution (Figure 9a) with
a mode of
8o,
which corresponds to the
"normal" field, its somewhat unstable
distribution being obviously associated with the
variation of the "normal" field at that time
(Figure 6b). As could be expected, the
S p does not show any connection with the
distance to the Siberian traps (Figure 15).

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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