R. V. Veselovskiy

Secular Geomagnetic Variations and Volcanic Pulses in the Permian-Triassic Traps of the Norilsk and Maimecha-Kotui Provinces

Detailed paleomagnetic studies have shown that the effusive Permian-Triassic traps in the Kotui River valley were formed as the result of volcanic activity, which occurred in the form of volcanic pulses and individual eruptions with net duration of at most 7000–8000 years, excluding the periods of volcanic quiescence. According to the analysis of the paleomagnetic data earlier obtained by Heunemann and his coauthors [2004b] on the Abagalakh and Listvyanka sections in the Norilsk region, those geological units were formed during 25 volcanic pulses and separate eruptions, which all lasted up to 8000 years altogether, whereas the total time of formation (including the periods of volcanic quiescence) exceeded 10000–100000 years for the Norilsk section and was probably a bit shorter for the Kotui section. Comparison of the positions of virtual geomagnetic poles calculated for the Norilsk and the Kotui sections provides no grounds to suggest that these sections were formed at different geological times. The scatter in the positions of the virtual geomagnetic poles (VGP) for the directional groups and individual directions (58 altogether) jointly for the two sections (more than 160 lava flows) indicates that the secular geomagnetic variations at the Permian-Triassic boundary had similar amplitudes to those that occurred in the past 5 Ma.

Magnetostratigraphy of the Ordovician Angara/Rozhkova River section: Further evidence for the Moyero reversed superchron

We present new magnetostratigraphic results obtained from a well-dated Ordovician key section located along the Angara River, near the terminus of the Rozhkova River (southern Siberian platform). More than 220 samples were thermally demagnetized up to 670°C in order to isolate their characteristic ancient magnetization. Samples from the Arenig, the Llanvirn and the Llandeilo stages are all (but two) of reversed magnetic polarity. In contrast, samples dated of the Caradoc yield a sequence of several magnetic polarity intervals. These new data therefore confirm the occurrence of a long reversed magnetic polarity interval during the Ordovician, the so-called Moyero superchron, which ended during the middle or late Llandeilo.

New paleomagnetic data on the Anabar Uplift and the Uchur-Maya region and their implications for the paleogeography and geological correlation of the Riphean of the Siberian Platform

The results of paleomagnetic studies of the reference sections of the Riphean and Late Proterozoic intrusive bodies of two remote areas of the Siberian Platform are presented. Within the limits of the Uchur-Maya region the sedimentary rocks of the Gonam, Omakhta, Ennin and Konder formations were studied; and the Riphean sedimentary rocks of the Burdur and Kotuykan formations on the northern and western slopes of the Anabar Uplift and, also, the Late Proterozoic intrusive complexes, located in the basins of the Fomich, Magan, Dzhogdzho and Kotuykan Rivers were studied. The paleomagnetic poles obtained in the course of this work and the present-day geochronological data give grounds to assume that: (1) the accumulation of the Riphean of the Anabar Uplift occurred after the formation of the Uchurskaya series of the Uchur-Maya region and was completed in approximately 1.5 Ga; (2) the Konder layers, compared according to the correlation pattern accepted at the present time [Semikhatov and Serebryakov, 1983] with the bottoms of the Totta formation, can be related to the appreciably more ancient stratigraphic level; (3) the intrusion of the studied intrusive bodies of the northern and western slopes of the Anabar Uplift occurred nonsimultaneously, although within close time intervals of approximately 1.5 Ga. The estimates of the kinematic parameters of the drift of the Siberian Platform within an interval of 1.7–1.0 Ga is carried out.

Paleomagnetism, geochronology, and magnetic mineralogy of Devonian dikes from the Kola alkaline province (NE Fennoscandian Shield)

The new paleomagnetic data on forty dikes and two intrusive plutons of Devonian age located in different parts of the Kola Peninsula, which have not been previously covered by systematic paleomagnetic studies, are reported. We describe the results of the rock magnetic, petrographic, and microprobe investigations of the Devonian dikes and present their isotopic ages (40Ar/39Ar, stepwise heating). Within the studied area, almost all the Devonian dikes, metamorphic Archaean-Proterozoic complexes of the Fennoscandian Shield, and Proterozoic dikes have undergone low-temperature hydrothermal-metasomatic alteration, which resulted in the formation of new magnetic minerals with a secondary (chemical) component of magnetization. The comparison of the paleomagnetic poles indicates the Early Jurassic age of the secondary component. We suggest that regional remagnetization event was caused by endogenic activity genetically related to the formation of the Barents Sea trap province 200–170 Ma ago. On the basis of the obtained data, the preliminary Devonian paleomagnetic pole of the East European Platform is determined.

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

between them. Discussed in this paper are four potential factors that had caused the observed dierences between the paleomagnetic poles of Siberia and Europe: (1) the large-scale relative movements of these cratons in post-Paleozoic time, (2) the dierent ages of the compared paleomagnetic poles, (3) the substantial contribution of non-dipole components to the geomagnetic field at the Paleozoic-Mesozoic boundary, and (4) the shallowing of the magnetic inclination in the European sedimentary rocks. Also discussed is the adequacy of the data selection. Arguments are advanced to prove that the possibility of the post-Paleozoic large-scale relative displacements of the cratonic blocks discussed as well as considerable age dierence of their mean poles is unlikely. Also estimated were the input quadrupolar and octupolar sources in the total time-average geomagnetic field and also values of the inclination shallowing factor, which might have explained the observed discordance of the Siberian and European poles. The best agreement of the European and Siberian paleomagnetic data was achieved for the octupolar coecient g3 = 10% or for the inclination shallowing factor f = 0.62. Our calculations showed that the statistically significant dierence between the Siberian and European average poles can be removed assuming a very small value of the inclination shallowing corresponding to the f values of 0.9 to 0.95, potentially associated with some compaction of the studied sedimentary rocks. This gives grounds for interpreting the low inclinations in the European objects as the most probable source of the observed disagreement between the European and Siberian paleomagnetic data. INDEX TERMS: 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics: regional, global; 3040 Marine Geology and Geophysics: Plate tectonics; KEYWORDS: paleomagnetism, Siberian traps, Stable Europe, non-dipole field, inclination shallowing.

Mesozoic orogens of the Arctic from Novaya Zemlya to Alaska

Mesozoic orogenic belts fringe the Alaska and eastern Russia portion of the Arctic Basin. From west to east, these include the fold belts of Novaya Zemlya, Taimyr Peninsula, northern Verkhoyansk–Kolyma, Chukotka and the Brooks Range, as well as their continuations onto the continental shelves. The Taimyr and Novaya Zemlya structures were traditionally interpreted as the continuation of the late Palaeozoic Uralian orogenic belt. This is probably correct for Taimyr, but not for Novaya Zemlya, where shortening post-dates Uralian deformation. The Mesozoic tectonic evolution of the Verkhoyansk–Kolyma, Chukotka and Brooks Range orogens relates to the accretion of numerous continental and arc terranes to the Siberian and North American margins starting in the Late Jurassic and driven by palaeo-Pacific dynamics. This history is complicated by the opening of the Amerasia Basin of the Arctic, which displaced the Arctic Alaska–Chukotka microplate from a position adjacent to Arctic Canada towards the palaeo-Pacific. Although the Chukotka fold belt and the Brooks Range both formed along the southern edge of Arctic Alaska–Chukotka, most shortening took place prior to Amerasia Basin opening. The remoteness of this region and the complexity of its geology has left numerous questions regarding its tectonic evolution unresolved, providing rich avenues for future research.

Evidence for the Mesozoic endogenous activity in the northeastern part of the Fennoscandian Shield

Paleomagnetic study of dykes and intrusions remanent in the central part of the Kola Peninsula has been carried out; the Devonian age of these objects has been confirmed by isotopic-geochronological studies. The component analysis of the magnetization vector in the samples has shown that there are two magnetization components in most samples. The paleomagnetic pole corresponding to the direction of a more stable component is located in the close vicinity of the Middle Devonian segment of the apparent polar wander path (APWP) for the East European Craton, so this enables us to estimate its age to be as old as the Devonian. The second magnetization component was found in Devonian dykes of both northern and southern parts of the Kola Peninsula; the paleomagnetic pole corresponding to this component is located close to the Mesozoic (Early Jurassic) part of the APWP for the East European Craton. It is suggested that the extensive remagnetization of Devonian intrusions in the Kola Peninsula was caused by the thermal effect of the Barents-Amerasian superplume and by the appearance of an extensive area with trap magmatism within the modern Arctic Basin region. Discovery of a significant thermal event that covered the Fennoscandian northeast allows us to explain the geochronological problem concerning the Mesozoic ages of particular singular zircon grains from Precambrian rocks of the shield derived via the SHRIMP method.

Siberian Traps volcaniclastic rocks and the role of magma-water interactions

The Siberian Traps are one of the largest known continental flood basalt provinces and may be causally related to the end-Permian mass extinction. In some areas, a large fraction of the Siberian Traps volcanic sequence consists of mafic volcaniclastic rocks. Here, we synthesize paleomagnetic, petrographic, and field data to assess the likely origins of these volcaniclastic rocks and their significance for the overall environmental impact of the eruptions. We argue that magma-water interactions, including both lava-water interactions and phreatomagmatic explosions in vents, were important components of Siberian Traps magmatism. Phreatomagmatic episodes may have generated tall water-rich eruption columns, simultaneously promoting removal of highly soluble volcanic gases such as HCl and potentially delivering additional sulfur to the upper atmosphere.

Paleomagnetism of the trap intrusive bodies in arctic Siberia: Geological and methodical implications

New paleomagnetic data are reported for the dikes, sills, and lava flows in the Arctic part of the Siberian Platform, which has not been covered by previous systematical paleomagnetic investigations. The analysis of the newly obtained and previously published data provides important time constraints for the duration and character of evolution of the Permian-Triassic magmatic events in the studied regions. Our results once again illustrate the conclusion that, in order to obtain an exact estimate for the location of the paleomagnetic pole in the northern paleolatitudes, at least 30–40 rapidly cooled magmatic bodies (dikes, flows, or minor sills) should be sampled if secular variation is commensurate with the intensity of the presentday variations.

Short intense bursts in magmatic activity in the south of Siberian Platform (Angara-Taseeva depression): the paleomagnetic evidence

Based on the paleomagnetic study of intrusive and explosive Permian-Triassic traps in the Angara River basin, Siberian Platform, it is established that the formation of the traps was marked by three short and highly intense bursts in magmatic activity, which resulted in the intrusion of three large dolerite sills (Tolstomysovsky, Padunsky and Tulunsky) and the deposition of the tuffs of the Kapaevsky Formation. These magmatic bursts occurred against the long-lived less intense background magmatism, which caused the formation of small intrusive bodies and tuff sequences. The geochronological data and correlation of the Angara traps to the effusive trap sequences in the north of the Siberian Platform (Norilsk and Maymecha-Kotuy regions) indicate that intrusion of the Tolstomysovsky sill and eruption of its comagmatic tuffs of the Kapaevsky Formation occurred in the Early Triassic. The obtained paleomagnetic data contradict the existing idea that the Padunsky and Tulunsky sills are coeval. Moreover, these data show that the magmatic bodies of different ages were mistakenly referred to the same sill.