Australia: The Land Where Time Began
Siberian Traps – Volcanic Pulses as inferred from Permo-Triassic Geomagnetic Secular Variations
Pavlov et al. say there is not much evidence to suggest the eruption of the Siberian Traps was a gradual process, rather, it is believed the emplacement occurred in a series of brief, voluminous pulses of volcanism, as has been demonstrated for the Deccan Traps (Chenet et al., 2008, 2009) India and for the Karoo Traps (Moulin et al., 2011, 2012), South Africa.
A palaeomagnetic study of several trap volcanic sections from the Norilsk and Maymecha-Kotuy area was carried out with the aim of finding evidence of such pulses during the formation of the Siberian Traps, and to constrain eruptive activity. Palaeomagnetic directions published previously (Heunemann et al., 2004) from the Norilsk area were used to complement the study.
A total of 79 flows from the Kotuy region (Medvejia and Truba sections) and 54 flows from the Norilsk region (Sunduk and Ergalakh sections) were studied.
The procedure previously described (Chenet et al., 2008, 2009) was used to estimate the time interval over which the trap sections being studied were formed. Based on the secular variations that have been recorded in the lava flows, this procedure has permitted the identification in the composite Kotuy section of 17 directional groups (DGs) and 13 individual directions (IDs), and 4 DGs and 3 IDs in the Ergalakh section, and 9 DGs and 6 IDs in the Sunduk section.
The same approach was applied to the composite section that contains the Listvjanka, Icon and Abagalakh sections in the Norilsk region that have been previously studied (Heunemann et al., 2004). Pavlov et al. say they have isolated 23 DGs and 12 IDs in this composition section containing 76 flows representing the majority of trap volcanic sequences in the Norilsk region. The Talnakh section was not considered in their analysis as it is located in the immediate proximity to the Listvjanka section and virtually repeats its lower part.
They concluded that the duration of active volcanism that produced the composite Kotuy and Norilsk sections was no more than 9,000 and 11,000 years respectively, taking into account the time constraints discussed (Chenet et al., 2009), and correlating directional groups (individual groups) with volcanic pulses (individual eruptions).
Of course, quiescent periods that separate volcanic pulses and individual eruptions were not included in the estimates. A clear indication that such quiescent periods were brief is provided by the absence of sedimentary layers and developed weathering crusts between the flows, though Pavlov et al. mention that an absence of any evidence can mark quiescent periods.
The rather high values of the non-random ordering factor also support this conclusion (Biggin et al., 2008), which had been calculated for the Kotuy and Norilsk sections (0.9999 and 0.8750 respectively). The non-random ordering factor was suggested (Biggin et al., 2008) as a measure of serial correlation of the successive directions in lava sections. A high correlation of ID and DG directions in the studied sections point towards by the calculated values which therefore indicate brief time gaps between them.
The composite Norilsk section (Listvjanka, Icon and Abagalakh) has been subdivided (Heunemann et al., 2004; Gurevitch et al., 2004) into consecutive R, T E and N sections, each of which records reversed, transitional, excursional and normal states, respectively, of the geomagnetic field. They also note that the geomagnetic field reaches normal polarity briefly between the transitional and excursional intervals (flows gd5, tk7, tk6). Pavlov et al. suggest further that the excursional interval may correspond to a post-transitional rebound effect (Merrill et al., 1996; Valet et al., 2012).
Pavlov et al. observe that the same features in the Ergalakh and Sunduk sections. The Sunduk section contains almost all of the intervals isolated in the composite Listvjanka-Icon-Abagalakh section (Heunemann et al., 2004), while only the reversed and transitional intervals can be identified (levels DG1, DG2, DG3,ID1, ID2, ID3, DG4, respectively in the Ergalakh section.
The 2 lowest reversely magnetised flows forming the volcanic pulse DG1are included in the reversed interval of the Sunduk section. The transitional interval is made up of the next 17 flows – ID1, DG2, DG3, DG4, DG5. Excursional directions, that have been identified as it was previously (Heunemann et al., 2004; Gurevitch et al., 2004) are recorded by flows in DG8 and ID6. Between the transitional and excursional sections in the Sunduk section Pavlov et al. observed several flows (C20-C31) marking the first arrival of the geomagnetic field to a “precursor” normal state, similar to the section studied by Heunemann et al., (2004), which was succeeded by a post-transitional excursion (DG8, ID6, DG9(?)). The final arrival of the field to a fully normal state is possibly recorded in the Sunduk section by the 3 uppermost flows (DG9).
Clear correlations among these sections, however, have been allowed by peculiarities of the geomagnetic field that has been recorded in the sections. At least ¼ of the Norilsk region volcanic sequences were probably formed during a time interval that was relatively short at the time of a geomagnetic field reversal. The duration of a geomagnetic field reversal of between 5,000 and 10,000 years has been suggested by the convergence of results from studies, though there have also been a suggestion of 20,000-30,000 years (Merrill et al., 1996). Similarities of several reversals of different ages that have been well-recorded have recently been analysed (Valet et al., 2012). During a reversal there are 3 phases that can be observed, a precursor, the transit, and a rebound. Only the transit and rebound are present, as is observed in the Karoo Traps (Mountain et al., 2011), in this study by Pavlov et al. It has been estimated (Valet et al., 2012) that the transit and the rebound may have not been more than 1,000 years and 2,500 years, respectively. Therefore it is indicated by the data of the study of Pavlov et al. that a significant part, up to 1,200 m thick, of the volcanic sequence of the Norilsk region may have formed in time intervals of several thousand years, or even shorter.
Pavlov et al. say it must be noted that at this stage of the study, the possibility cannot be excluded by them that the transitional and excursional intervals observed by them are linked with geomagnetic events that are independent and are not parts of the same reversal. The time estimate above should be considered to be very conservative if this is the case,
Pavlov et al. say it is interesting to note that the transitional interval, which is referred to as Group 2 (Gurevitch et al., 2004)), corresponding to the Gudchikhinsky and Syverminsky formations, is characterised by directions that have, on average, inclinations that are smaller than those of the overlying intervals. Therefore, attempts can be made to make correlations between the measurements in this interval by Pavlov et al. and those from magnetostratigraphic borehole studies (Gurevitch et al., 2004); Mikhal’tsov et al., 2012). In 2 of the 3 boreholes that have been studied palaeomagnetically that cross the Gudchikhinsky and Syverminsky formations (CD28 and HS59), the mean inclinations that have been calculated for the Gudchikhinsky and Syverminsky formations are close to those that have been obtained for the transitional interval (Heunemann et al., 2004; Gurevitch et al., 2004) and are smaller than the mean inclinations that have been calculated for overlying formations at a confidence level of 95 %. A magmatic event has therefore been suggested that corresponds to the transitional and, most probably, excursional intervals that occurred over a large region, that include at least the Kharaelkh, Norilsk and Imangda troughs. Pavlov et al. obtained a minimum volume estimate for the lava of 20,000 km3 (the greater thickness in the centre of the troughs is not considered, therefore this is considered a minimum estimate), using the total area of these troughs, more than 18,000 km2 and the thickness, about 1,100 m, of the transitional and excursional intervals in the sections that were studied. Pavlov et al. say that all this volume was erupted during only a few volcanic pulses and individual eruptions, providing the subdivision of the studied sections is correct.
The majority of volcanism in the Norilsk and Maymecha-Kotuy regions formed during a limited number of volcanic pulses and individual eruptions, as indicated by palaeomagnetic studies of several key sections of the traps from the Permo-Triassic of the northern Siberian platform. The calculations of Pavlov et al. specifically for the composite Kotuy section reveal 17 pulses and 13 individual eruptions, and 23 pulses and 12 individual eruptions for the composite Norilsk section. This conservatively implies that the duration of active volcanism in the Kotuy and Norilsk sections did not exceed a time interval on the order of 10,000 years. The quiescence intervals separating pulses of volcanism and individual eruptions are not included in this estimate. A clear indication that the periods of quiescence were brief is the general absence of sedimentary layers and weathering crusts between the flows in the areas that were studied.
Pavlov et al. say their study has confirmed the occurrence of thick transitional and excursional intervals in sections of the Norilsk region, as is suggested (Heunemann et al., 2004; Gurevitch et al., 2004). These intervals can be traced across the entire Norilsk region. It is indicated by this observation that at least ¼ of the volcanic sequences in the Norilsk region may have formed during a time interval that was relatively short at the time of a reversal of the geomagnetic field, i.e. within several tens of thousands of years or even more rapidly. Pavlov et al. stress, however, that there is not sufficient data to affirm that transitional and excursional intervals that are observed are parts of the same geomagnetic reversal. As has been suggested by recent petrological and geodynamical studies (Sobolev et al., 2009, 2011) the period of time between observed transitional and excursional intervals may have been significantly larger. It is implied in the study by Pavlov et al. that by the tracing of the transitional interval through the Kharaelkh, Norilsk and Imangda troughs, as well as subdivision of sections suggested in the study, that the more than 20,000 km3 of lava may have been erupted in as little as a few pulses of volcanism and individual eruptions,.
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