Systematics of Early Cambrian Paleomagnetic Directions from the Northern and Eastern Regions of the Siberian Platform and the Pr

ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2018, Vol. 54, No. 5, pp. 782–805. © Pleiades Publishing, Ltd., 2018. Original Russian Text © V.E. Pavlov, A.M. Pasenko, A.V. Shatsillo, V.I. Powerman, V.V. Shcherbakova, S.V. Malyshev, 2018, published in Fizika Zemli, 2018, No. 5, pp. 00000–00000.

Systematics of Early Cambrian Paleomagnetic Directions from the Northern and Eastern regions of the Siberian Platform and the Problem of an Anomalous Geomagnetic Field in the Time Vicinity of the Proterozoic–Phanerozoic Boundary V. E. Pavlova, b, *, A. M. Pasenkoa, A. V. Shatsilloa, V. I. Powermana, b, d, V. V. Shcherbakovae, and S. V. Malysheva, c aSchmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, 123242 Russia bGeological Faculty, Kazan Federal University, Kazan, Republic of Tatarstan, 420008 Russia cSt. Petersburg State University, Institute of Earth Sciences, St. Petersburg, 199034 Russia dInstitute of the Earth Crust, Siberian Branch, Russian Academy of Sciences, Irkutsk, 664033 Russia eBorok Geophysical Observatory, Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Borok, 152742 Russia *e-mail: [email protected] Received April 9, 2018

Abstract—Representative paleomagnetic collections of Lower Cambrian rocks from the northern and eastern regions of the Siberian platform are studied. New evidence demonstrating the anomalous character of the paleomagnetic record in these rocks is obtained. These data confidently support the hypothesis (Pavlov et al., 2004) that in the substantial part of the Lower Cambrian section of the Siberian platform there are two stable high-temperature magnetization components having significantly different directions, each of which is eligi- ble for being a primary component that was formed, at the latest, in the Early Cambrian. The analysis of the world’s paleomagnetic data for this interval of the geological history shows that the peculiarities observed in Siberia in the paleomagnetic record for the Precambrian–Phanerozoic boundary are global, inconsistent with the traditional notion of a paleomagnetic record as reflecting the predominant axial dipole component of the geomagnetic field, and necessitates the assumption that the geomagnetic field at the Proterozoic–Pha- nerozoic boundary (Ediacaran–Lower Cambrian) substantially differed from the field of most of the other geological epochs. In order to explain the observed paleomagnetic record, we propose a hypothesis suggesting that the geomagnetic field at the Precambrian–Cambrian boundary had an anomalous character. This field was characterized by the presence of two alternating quasi-stable generation regimes. According to our hypothesis, the magnetic field at the Precambrian–Cambrian boundary can be described by the alternation of long periods dominated by an axial, mainly monopolar dipole field and relatively short epochs, lasting a few hundred kA, with the prevalence of the near-equatorial or midlatitude dipole. The proposed hypothesis agrees with the data obtained from studies of the transitional fields of Paleozoic reversals (Khramov and Iosi- fidi, 2012) and with the results of geodynamo numerical simulations (Aubert and Wicht, 2004; Glatzmayer and Olson, 2005; Gissinger et al., 2012).

DOI: 10.1134/S1069351318050117

INTRODUCTION The recent studies (Pavlov et al., 2004; Abrajevitch It is well known (e.g., (Merril et al., 1996)) that the and Van de Voo, 2010; Biggin et al., 2012; Bazhenov Earth’s magnetic field during the geological history et al., 2016; Halls, 2015; Gallet and Pavlov, 2016, etc.) could exist in two states: indicate that in the history of the Earth there probably were sufficiently long periods (on the order of a few (1) a stable state (of normal or reversed polarity), Ma and longer) when the state of the Earth’s magnetic with a predominant dipole geometry and wide spec- field differed from the two regimes noted above. The trum of the lengths of geomagnetic polarity intervals main distinctive feature of this new fundamental state ranging from hundreds of ka to dozens of Ma; was hyperactivity, i.e., extreme variability of the main (2) a transitional (reversal) state with the complex parameters of the field (the direction, intensity, ampli- geometry of the field and a duration ranging from a tude of secular variation, etc.) and/or significant devi- few hundred years to the first few kA. ation from the axial dipole geometry.

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The epoch of the transition from the Proterozoic to the catalog (Paleomagnitnye…, 1973) under the editor- the Phanerozoic corresponding to the end of the Edi- ship of A.N. Khramov. This result was determined acaran (Vendian)–beginning of the Cambrian was from the section in the middle reaches of the Olenek perhaps one such period. In this work we consider the River where E.P. Osipova (E.P. Sidorova) studied sev- paleomagnetic data for a number of geological objects eral exposures of the Emyaksa Formation. In 1984, predominantly pertaining to the Early Cambrian and V.P. Rodionov presented the paleomagnetic results located in the north, northeast, and east of the Sibe- derived by him for the Emyaksa Formation of the rian platform, which allows us to test this hypothesis to Udzha River valley (Rodionov, 1984), and two years a certain extent. later the Paleomagnetic catalog (Paleomagnitnye…, 1986) was added by the new data on the paleomagnetism of the Pestrotsvetnaya formation of the Uchur– THE PROBLEM OF THE PALEOMAGNETISM Maya region studied in the valley of the Iniken River— OF THE LOWER CAMBRIAN the left tributary of the Maya River. In (Komissarova FROM THE SIBERIAN PLATFORM and Osipova, 1986) the authors again touched upon The Lower Cambrian rocks are widespread across the question concerning the paleomagnetism of the the platform and are mainly represented by carbonate rocks of the Pestrotsvetnaya formation from the Maya facies, which were formed within warm shallow epi- River section and presented a new paleomagnetic cratonic seas. The Siberian Lower Cambrian sections result which fairly well agrees with the previous results frequently contain a significant amount of red and determined from this formation. green beds which, as experience shows, are fairly The research carried out by S.A. Pisarevskii et al. favorable for recording and preserving the paleomag- (Pisarevskii, 1986) at the beginning of the 1980s at the netic signal. VNIGRI Paleomagnetic Laboratory is perhaps the Except for the formations composing some sec- most comprehensive and detailed study conducted in tions in the platform’s marginal parts, the Lower that period of studying the Lower Cambrian from the Cambrian rocks of the platform are typically only Siberian platform. In the cited work, the authors barely metamorphosed and have never subsided to explored more than 300 stratigraphic levels from 12 out- below a depth of 1–3 km. The fact that these sections crops exposed over more than 200 km along the lower are extensively outcropped and have been excellently reaches of the Olenek River. explored from the biostratigraphic standpoint (Rozanov et al., 1992) makes them even more attrac- As a result of the conducted studies, up to the mid- tive for paleomagnetologists. The Siberian Lower 1980s, the paleomagnetic poles were obtained from a Cambrian sections often contain numerous faunal number of Lower Cambrian objects representing dif- remains, allowing the researchers to track the evolu- ferent regions of the Siberian Platform. These poles tion of the organic world during that time interval, to were located near the southern termination of Austra- conduct a detailed biostratigraphic subdivision of the lia and fairly closely agreed with each other. They were sections, and to correlate the sections. It would not be also closely consistent with the younger Lower Paleo- an overstatement if we say that the Lower Cambrian zoic paleomagnetic poles suggesting a more or less sta- sections of the Siberian platform rank among the best ble (quiet) drift of the Siberian platform at the begin- ones, and in many cases are indeed the best ones of ning of the Phanerozoic. The problem of the position this age in the world. of the Early Cambrian paleomagnetic pole for the Siberian Platform seemed to be close to being com- Therefore it is not surprising that the Lower Cam- pletely solved. In his book of 1982 (Khramov et al., brian rocks from the Siberian Platform have attracted 1982), Khramov generalized these data and calculated the attention of researchers since the very first years of the average pole which is hereinafter referred to as the development of paleomagnetology in our country. Khramov’s pole. Perhaps the first published work that presented the results of these studies was the paper of E.P. Sidorova, Against this background, the result published in the researcher with the Paleomagnetic laboratory at 1984 by J. Kirschvink and A.Yu. Rozanov in Geologi- the All-Russia Petroleum Research Exploration Insti- cal Magazine (Kirschvink and Rozanov, 1984) was tute (VNIGRI), where she reported the data obtained extremely surprising. At the beginning of the 1980s, by her by studying the sub-red-bed formations in the Kirschvink, with the involvement of A.Yu. Zhuravlev middle reaches of the Lena River and Chara formation and Rozanov, carried out detailed studies on the of the Olekma River (Sidorova, 1963; Paleomagnit- Lower Cambrian stratotype reference sections in the nye…, 1971). The papers published a year later by lower reaches of the Lena River and obtained the Davydov and Kravchinskii (1965) and Sidorova (1965) paleomagnetic pole for the Lower Cambrian which addressed the paleomagnetism of the Ust-Tagul for- differs from the closest (in age) Middle Cambrian pole mation of the Biryusa area of the Sayan region and by an angle of about 70° (Kirschvink and Rozanov, Pestrotsvetnaya formation of the Maya River. In 1973, 1984). The results were obtained with the use the cryo- the new paleomagnetic result obtained for the Lower genic magnetometer and the new, at the time, method Cambrian of the Siberian platform was presented in for calculating the magnetic components—PCA

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(Kirschvink, 1980), which allowed Kirschvink and a In 1997, Pisarevskii et al. published a paper in the number of other authors to consider this result as more Journal of Geophysical Research (Pisarevsky et al., 1997) substantiated compared to the previous ones. where Khramov’s directions were confirmed based on reinvestigating the old Lower Cambrian collection The primary nature of Kirschvink’s direction was (Kessyusin and Erkeket formations) from the lower fairly soundly supported by the fact that the zones of reaches of the Olenek River with the up-to-date magnetic polarity identified in the remote exposures instruments and techniques. However, following reasonably well agreed with each other. The obtained A.Yu. Kazanskii (2002), we note that the data pre- paleomagnetic data were reliably correlated to the sented on some stereograms in (Pisarevsky et al., 1997) Tommotian–Atdabanian biostratigraphic scale which can also be considered as indicating the probable pres- had been well developed up to that time, and this was ence of Kirschvink’s component in the magnetization certainly a strong point of the cited work. In his study, of the studied rocks. Kirschvink examined about 500 samples from four exposures of the Variegated Formation spaced a few Also in 1997, based on analyzing a number of the km to a few dozen km apart from each other. Interest- paleomagnetic data (including the Lower Cambrian ingly, in his analyses Kirschvink used rather large ones) for several continents, Kirschvink et al. (1997) demagnetization steps (30°–40°–50°), including in proposed the well-known Inertial Interchange True the high-temperature domain; i.e., the degree of detail Polar Wander (IITPW) hypothesis suggesting a huge, in these demagnetization experiments was rather low on the order of 90°, shift of the Earth’s crust and man- from the present-day standpoint. tle relative to the Earth’s rotation axis from the end of the Atdabanian to the beginning of the Middle Cam- When surveying the banks of the Lena River in brian (15–20 Ma). 1994, T. Torsvik, together with V.M. Moralev and The data used in the cited work immediately drew J. Tait (Torsvik et al., 1995), resampled the outcrops sharp criticism (Torsvik et al., 1998). The heated studied by Kirschvink. The subsequent laboratory debate (see also (Evans et al., 1998; Meert, 1999; investigations conducted by Torsvik have shown that the Meert and van der Voo, 2001; Pisarevsky et al., 2001)), paleomagnetic signal in the studied rocks has an inter alia, has led to the understanding that no com- extremely low quality and is unsuitable for interpretation. monly accepted paleomagnetic poles for the Late Ven- In 1995, in the outcrops that were explored by dian–Early Cambrian existed for any of the ancient Kirschvink, we carried out confirmatory sampling of continents at that time (this fully applies for the pres- the intervals of the section that were described as most ent as well). We note, however, that some of the dis- favorable in Kirschvink’s paper. Overall, 200 samples cussed poles, mainly the Vendian ones, satisfied the were acquired. Our working hypothesis was that due to present-day reliability criteria quite well. The contro- the difference in the Soviet and western systems of versies that have become apparent suggested that the measuring the attitude of the samples, an error proba- way out of the problem should perhaps be sought bly occurred in the orientation of the cores (the sam- beyond the range of the traditional paleomagnetic ples for Kirschvink were largely acquired by Zhurav- postulates (Khramov et al., 1982). However, we lev) causing the discrepancy of the directions obtained started approaching this conclusion only after study- by Kirschvink from Khramov’s directions. In 1996, ing quite a few Lower Cambrian reference sections in 32 samples of this collection were investigated at the the Siberian platform. Paleomagnetic Laboratory of the Institut de Physique Thus, up to the beginning of the 2000s and mid- du Globe de Paris. Kirschvink’s direction was not 2010s, things have come to such a pass that two more revealed in any of the thoroughly examined samples or less reliable strongly different paleomagnetic poles which, by the way, gave very good Zijderveld dia- were suggested for the Lower Cambrian of Siberia, grams. Moreover, at that time, we did not even observe leading to significantly dissimilar conclusions about unambiguous circles tending to concentrate towards the geodynamical history of the Siberian Platform and Kirschvink’s direction. The single clearly identified com- the Earth overall. ponent (the present-day component) coincided within 1°–2° with its counterpart obtained by Kirschwink, Resolving this contradiction needed a further which indicated that it was all right with the orientation of research into the Lower Cambrian rocks with the the samples in Kirschvink’s collection. expansion of the study over the neighboring time intervals and other rock types that were different from However, to be fair, we should note that the Zij- the previously studied ones. It was important to obtain derveld projections tended sometimes slightly and an Early Cambrian paleomagnetic record in the igne- seemingly nonsystematically to miss the origin of the ous rocks which acquire magnetization by a different Zijderveld diagrams, which might have indicated the mechanism compared to the sedimentary formations. existence of a certain extremely weak ancient compo- This could help avoid the difficulties associated with nent; however, in our works conducted in 1995–1996, the probable duration of the paleomagnetic record’s we failed to find any other signs of the presence of formation in sedimentary rocks and the ensuing com- Kirschvink’s component. plicated superimposition of the magnetic components.

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N Age Anabar 2 Period

1 1 71° N Olenek Yenisei Olenek Uplift Len Anabar Atdabanian ~521 Ma Shield a Nizhnyaya Tunguska Aldan 1 4 4 5 3 Siberian 5 2

Platform Tommotian ~525 Ma Lower Cambrian Lower Aldan Shield ° 60 N 127 3

° Daldyn

в.д. Nemakit–

100 ~541 Ma

Lake Baikal Ediacaran

Fig. 1. Geographic positions of studied objects: (1) Bol’shaya Kuonamka river valley; (2) Neleger river valley; (3) Aldan river valley, Kyllakh uplift; (4) Belaya river valley; (5) Maya river valley. Dashed line outlines contours of Siberian platform.

In subsequent years we carried out extensive higher in the sequence, and again greenish gray in the research of the Lower Cambrian and Vendian (Edi- uppermost layers. The samples from paleomagnetic acaran) rocks from the Siberian platform. The results analysis were handed over to us by A. Kuchinskii who of a part of these studies have been published (Gallet studied the Emyaksa sequences of the Bol’shaya et al., 2003; Shatsillo et al., 2005). In this work we Kuonamka River at the end of the 1990s. The rocks in present the results of the few past years of studying the these sections have a subhorizontal bedding; the abun- Lower Cambrian sedimentary and igneous rocks from dant faunal remains found in this formation (Rozanov the northeast and east of the Siberian platform. et al., 1992) date it to the Tommotian–Atdabanian; the upper half of the Emyaksa formation corresponding to the upper Tommotian and the Atdabanian BRIEF OUTLINE OF STUDY OBJECTS stages was sampled with an interval of 1 to 1.5 m. Fifty- Below we present the results determined from the four oriented samples were acquired here from a 70-m Lower Cambrian rocks of the Emyaksa formation out- thick stratigraphic level. cropping in the middle reaches of the Bol’shaya The Pestrotsvetnaya formation is widespread along Kuonamka river valley on the eastern slope of the the eastern framing of the Siberian platform. In the Anabar uplift, of the Pestrotsvetnaya formation out- studied exposures of the Belaya and Maya rivers, this cropping in the valleys of the lower reaches of the formation is represented by clayey floglike limestones, Belaya and Maya rivers in the east of the Siberian plat- sometimes greenish gray, gray, pink, red, and wine- form, from the Nemakit–Daldyn rocks of the Sardana colored marls. In the Maya sections, the Pestrotsvet- formation within the Kyllakh Ridge, and from the naya formation has an almost horizontal bedding, a basic sills of the Chekurovka anticline (northeastern thickness of about 50 m, and is only partially exposed Siberian platform, valley in the lower reaches of Lena in separate outcrops with a thickness of 2 to 5 m. At the River, Fig. 1). beginning of the 2000s, we studied four outcrops: two The Emyaksa Formation of the Bol’shaya on the left bank of the Maya River upstream of the Kuonamka River is composed of clayey flaglike lime- Inikan River mouth, one on the right bank of the stones, gray and greenish gray in the bottom part, red Inikan River 3 km upsttream of the river mouth, and

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 54 No. 5 2018 786 PAVLOV et al. one on the right bank of the Maya river 5 km upstream In the absence of direct isotopic age determination of the Chaya cliff. Overall, more than 130 oriented of the sills, their relationship with the trachybasaltic samples were acquired from these outcrops. lava flow that outcrops at the base of the Cambrian The Pestrotsvetnaya formation in the valley of the section four km downstream of the Chekurovka village Belaya River has a limited distribution. We have only immediately downstream of the mouth of the Biskeibit found a single outcrop located on the left bank brook becomes key to their dating. The data of the approximately 20 km upstream of the Mutula River chemical and isotopic studies obtained in (Prokop’ev mouth. The outcrop is exposed in the low coastal cliffs et al., 2016) quite definitely show that the flow and the composed of flag-like gently dipping (up to 15°–20°) sills are comagmatic bodies which are very likely to red and green clayey limestones. During the field have been formed simultaneously. If this is so, the age works of 2014, we acquired about 30 oriented samples constraints existing for this flow can be applied to the from here. The thickness of the studied interval of the sills of the Chekurovka anticline. formation is at most 10–15 m. The considered flow has the model age TNd(DM) = The Sardana Formation corresponds to the upper 532–629 Ma and is located within the Tommotian part of the Yudoma group (the Yudomian), it is wide- Tyuser Formation (Rozanov et al., 1992). The flow spread in the region of the Kyllakh uplift, and it under- overlays the conglomerate with occurrences of rhyolite lies the layers of Pestrotsvetnaya formation. The data pebbles. The concordant U-Pb age of the youngest of provided by the studies of the small shelly fauna show the studied pebbles is 525.6±3.9 Ma. Thus, the age of that the upper part of the Yudoma group (and, hence, the the flow (and, naturally, the age of all the Chekurovka Sardana formation) pertains to the Nemakit–Daldyn sills) is limited by this date from below and should fall horizon (Khomentovskii and Karlova, 1994; 2002). within the time limits of the Tommotian age which, according to the present-day notions (Geological…, The Sardana formation was studied by us in 2014 in 2012) correspond to 525–521 Ma. the section of the Kyllakh ridge which outcrops in the An additional argument in favor of this age is pro- right bank of the Aldan River 10 km upstream of the vided by the data that were recently obtained by Belaya River mouth. We sampled there the Pes- V.I. Powerman et al. (2018). These authors have sepa- trotsvetnaya (red and green) layers of siliceous silt- rated and dated detrital zircons from the terrigenous stones and limestones in the stratigraphic sequence layers of Neleger Formation directly bordering the sill with an interval of 1 to 2 m. About 30 oriented samples that outcrops on the right bank of Lena River in the overall were collected from there. Ukta River mouth. These studies have shown that The region of the Chekurovka anticline (the Tuor– quite a few of these zircons have a U-Pb age of 520– Asis Ridge) is marked by a wide distribution of sills 525 Ma. This age does not reflect the time of zircon and dike bodies of basic composition which intrude crystallization since the terrigenous layers hosting the the Riphean–Vendian sedimentary strata. The intru- zircons are much older; instead, this date probably sions most frequently occur in the rocks of the Upper corresponds to a certain event that had disturbed the Riphean (?)–Early Ediacaran Neleger Formation and U-Pb system in the clastic zircons of the Neleger For- Upper Vendian Kharayutech formation (Khabarov mation at a significantly later time than the time of its and Izokh, 2014). A few dozen large sills are revealed, deposition. It is logical to hypothesize that the rees- with many of them tracked for 10–20 km. The thick- tablishment of the age of the zircons is probably due to ness of the individual sills ranges from 10 to 120 m their active hydrothermal alteration during the intru- (Geologicheskaya…, 1983; Oleinikov, 1983), whereas sion of the sill. These phenomena have been described the total (cumulative) thickness of the sills is more in the literature (Pidgeon et al., 1966). than 250 m. During the field works of 2016, we sampled seven sills which outcrop in the Neleger River valley (the right tributary of the Lena River) and along PALEOMAGNETIC ANALYSIS the left bank of the Lena River downstream of the All the described collections were investigated by Chekurovka village. Overall, 138 oriented samples the standard paleomagnetic techniques (Khramov from these sills were taken for paleomagnetic studies, et al., 1082; Butler, 1998; Tauxe, 2010). The magneti- on average 15 to 20 samples from each sill. The sam- zation components were identified based on the pled igneous bodies are typically composed of trachy- results of the detailed thermal demagnetization; and dolerites (Oleinikov, 1983; Prokop’ev et al., 2016). the directions of these components were calculated Until recently, there have been only two K-Ar age using the PCA method (Kirschvink, 1980). determinations for the sills of the Chekurovka anticline which were obtained more than 30 years ago and Emyaksa Formation dated these rocks to 449 ± 13 and 508 ± 13 Ma (Oleinikov et al., 1983). Recently, for one of the sills in The natural remanent magnetization (NRM) of the Lena river valley, A.V. Prokop’ev et al. obtained the gray varieties of the Emyaksa Formation varies the model age TNd(DM) = 577–648 Ma (Prokop’ev within (4–12) × 10–4 A/m and the magnetization of et al., 2016). red rocks is (1–5) × 10–3 A/m. In the first approxima-

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(a) (b)

Sample 7485 N Sample 765 Sample 5215 NN NRM 120° NRM ° NN ° 120 190 N N 250° 250° NRM ° 120° 380 300° 430° 250° 380° 340° 480–650° E Up E Up ° ° 670 520–560 380–670° E Up HTC HTC 580–600° Scale = 1 mA/m HTC Scale = 0.5 mA/m Scale = 0.2 mA/m Sample 7275 NN N NRM N 120° Sample 7515 250° 380° NN 430–560° E Up 600–670° NRM 120° HTC E Up Scale = 1 mA/m 250° 340–670° Scale = 0.5 mA/m

Sample 711 N

NRM NN N 120° Sample 7295 250° NRM NN 340° 120° E Up 250°

Scale = 0.2 mA/m HTC 340° A-component E Up ° 480–650 HTC Scale = 0.5 mA/m A-component

Fig. 2. Bol’shaya Kuonamka river section. Behavior of NRM vectors during demagnetization. Zijderveld diagrams: filled and open circles denote vector projections on horizontal and vertical plane, respectively. Stereograms: filled and open circles denote vector projections on lower and upper hemispheres, respectively. data are presented in ancient coordinate system. tion, the component content of the magnetization in the diagram, suggesting the probable presence of the rocks composing this formation can be described another high-temperature magnetization component by the superimposition of two components. One is a (Fig. 2, sample 711). The presence of this component relatively weakly stable low temperature component (hereinafter, referred to as B) in a number of samples (LTC) which is mainly destroyed at 300°–350°. By its quite clearly manifests itself in the stereograms by a direction this component is close to the present geo- chain of remagnetization circles diverging from the magnetic field and is likely to have been acquired rela- projection of the A-component (Fig. 2, sample 7275) tively recently (Fig. 2, samples 765, 5215, etc.). or from the projection of the present-day LTC compo- Another component is the ancient one, hereinafter nent. In the second case, the NRM is the result of the referred to as the A-component. This component is superimposition of the LTC- and B-components. significantly more stable, has the maximal unblocking temperatures of 600–680°C, and is characterized by a Almost the entire diversity of the obtained Zij- NNW declination and moderate inclination. The derveld diagrams and stereograms can be explained by example of this component content is presented in a certain combination of the LTC-, A-, and B-compo- Fig. 2, samples 765 and 5215. nents. It is important that, firstly, the B-component is never isolated in a pure form: due to the overlapping However, a more thorough examination of the Zij- spectra of the unblocking temperatures of the magne- derveld diagrams shows that component A in the high- tization components, we failed to obtain any end point temperature area quite frequently misses the origin of corresponding to this component; and, secondly, the

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Table 1. Paleomagnetic directions and paleomagnetic poles of Lower Cambrian rocks outcropped along Kuonamka, Maya, Belaya, and Neleger rivers and within Kyllakh uplift Outcrop, component, Geographic coordinate system Stratigraphic coordinate system N formation DIKα95 D I K α95 Kuonamka River section (Emyaksa Formation, 70.6° N, 112.8° E) HTC (KHR), 47 342.3 57.6 57.2 3.9 342.3 57.6 57.2 3.9 A-component Plat = –56.3° Plong = 138.3° A95 = 4.9° Maya River section (Pestrotsvetnaya formation, 59.3° N, 135.0° E) Maya 99, HTC, 9 7.9 52.5 55.3 7.0 7.9 52.5 55.3 7.0 Inikan, HTC(KHR) 11 358.7 55.2 35.0 7.8 356.8 51.5 44.3 6.9 Outcrop 2,HTC(KHR) 24 0 52.5 59.4 3.9 1 52.4 48.3 4.3 Outcrop 4, HTC(KHR) 8 353.8 57.6 42.6 8.6 354.3 56.2 40.7 8.8 Average over 4 outcrops 4 0.3 54.4 375.3 4.7 0.1 53.3 388.4 4.7 Plat = –64.6°; Plong = 134.8°; A95 = 5.4 Belaya River section (Pestrotsvetnaya formation, 61.5° N, 136.5° E) HTC (KHR) 27 4.3 48.4 35.4 4.7 3.5 47.7 36.2 4.7 Plat = –57.2; Plong = 130.8°; A95 = 4.9° Kyllakh Ridge section (Sardana Formation, 61.6° N, 135.6° E) HTC (KHR) 14 342.6 35.3 25.0 8.1 24.0 51.3 28.3 7.6 Plat = –56.5°; Plong = 96.8°; A95 = 8.5° Neleger River Section (71.2° N, 127.7° E) Sill 1, HTC component 10 213.9 –18.8 139.7 4.1 218.6 –16.1 139.7 4.1 (KRS) Sill 2, HTC component 9 224.2 –22.6 50.7 7.3 229.0 –16.1 50.7 7.3 (KRS) Sample average 19 218.6 –22.7 54.2 4.6 223.5 –16.2 54.2 4.6 Sample average disre- 17 217.2 –22.2 76.7 4.1 222.1 –16.1 89.5 3.8 garding two extremes Plat = –21.8°; Plong = 82.1°; A95 = 2.8° Kirschvink’s pole: Plat = –17°; Plong = 65°; A95 = 5° (Kirschvink and Rozanov, 1984) Khramov’s pole: Plat = –44°; Plong = 157°; A95 = 8° (Khramov et al., 1982) D, I, K, а95 are parameters of Fisher’s distribution; Plat, Plong, A95 are coordinates of paleomagnetic poles and their confidence circles.

A- and B-components are likely to be contaminated by ant fact is that practically all the circles diverge from the present-day component. The presence of this con- the vicinity or interspace between the LTC- and tamination is demonstrated, e.g., by samples 711, A-components and frequently go in the opposite direc- 7485, and 7515 (Fig. 2), where the existence of the tions, indicating the bipolar character of the B-compo- present-day component is perceived up to the highest nent (Fig. 2, sample 7515; Fig. 3, sample 7435). Here, temperatures. By analyzing the Zijderveld diagrams, we the projections of the NRM vectors sliding along the managed to isolate the A-component (although not circles during the demagnetization process move fully devoid of the admixture of the other components) towards the expected Kirschvink direction, which sug- and calculate its mean direction (Fig. 3, Table 1). gests that the sought B-component is likely to corre- In the case of the B-component, by using the spond to Kirschvink’s paleomagnetic pole. At the remagnetization circles, we can only constrain the same time, the A-component, which is separated fairly probable direction of this component. As seen in Fig. reliably, corresponds to Khramov’s paleomagnetic 4, there is a certain regularity in the distribution of the direction (Fig. 4, Table 1). Several observed remagne- circles across the diagram: the circles are predomi- tization circles fall out of the described predominant nantly oriented in the NE–SW direction. An import- trend (Fig. 3, sample 735). These circles could proba-

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(а) Sample 7435 N N N

NRM 120° 250°

E UP 480–650° HTC Scale = 0.5 mA/m B-component

N Sample 735

N N NRM 120° 300° 380° E UP 430–680°

Scale = 0.5 mA/m

(b) 0

A-component LTC

270 90

180

Fig. 3. Bol’shaya Kuonamka river section: (a) behavior of NRM vectors during demagnetization. Left: Zijderveld diagrams; right: corresponding stereograms. Designations for Zijderveld diagrams are hereinafter same as in Fig. 2; (b) distribution of normals of demagnetization circles (right-hand convention). Filled (open) squares denote projections of normals on lower (upper) hemisphere; filled circle and oval around it denote normal of “average” circle and its confidence oval, respectively; asterisks denote expected Kirschvink’s directions with allowance for closing of Vilyui rift system (with filled circle) and without it. Black circle with white cross denotes direction of A- (Khramov’s) component; white circle with black cross denotes direction of low-temperature LTC component.

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(a) (b) 0 0

270 90 270 90

A-component LTC component and remagnetization circles 180 180 (c) (d) 0 0

270 90 270 90

B-component 180 180 Kirschvink’s direction and remagnetization circles

Fig. 4. Bol’shaya Kuonamka river section: (a), (d) remagnetization circles; (b), (c), vector distributions. Data are presented in ancient coordinate system. Stereograms with remagnetization circles show relationship between orientations of circles and directions of A- (a) and B-components (d). bly be formed by the B-component of a different the maximal unblocking temperatures of 400° and polarity. higher; its direction highly accurately coincides with Section Summary that of the present dipole field. The high-temperature (1) The A-component with the direction corre- characteristic component (HTC) has a northern dec- sponding to the expected Khramov direction is reli- lination and the inclinations of ~50° (Fig. 5, samples ably established in the studied rocks of the Bol’shaya 974, 944, etc.). Considering the closeness of this com- Kuonamka River section. ponent to the expected Khramov direction, we will (2) During the demagnetization, a significant part refer to it as the KHR-component. In some cases, sep- of the samples shows the trends that can be considered arating the components is challenged by the evident as indicating the presence of a bipolar component with overlapping of their spectra and relative closeness of the Kirschvink direction. This component in a pure their directions. The directions of the high-tempera- form is not separated. ture component were only calculated in the cases when the Zijderveld diagrams had a distinct bend indi- Pestrotsvetnaya Formation of the Maya River cating the destruction of the main part of the low-tem- Most samples have NRM in the interval from (4– perature component. Clearly, with this approach, the 5) × 10–4 to (2–3) × 10–3 A/m and magnetic suscepti- vectors pertaining to that part of the sought distribu- bility varying from a few to a few dozen 10–6 SI units, tion where inclinations are relatively high can be lost. which is typical of this type of rock. As a rule, two However, this separation procedure is unlikely to draw magnetization components are revealed during the the calculated mean direction significantly away from demagnetization (the samples not yielding a regular the true direction because the separated vectors of the signal, which are predominant in some outcrops, are high-temperature component are probably contami- the exception). The low-temperature component has nated, to some extent, by the present-day component.

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In the studied collection there is a significant set of The direction of this component is close to the samples where the evidently present high-temperature direction of the present geomagnetic field. This leads component differs from the described one. In practi- us to suggest a recent origin of this component and, cally none of the samples can this component be sep- hence, exclude it from the further analysis. The more arated in a pure form; however, its presence is defi- stable high-temperature characteristic magnetization nitely suggested by the respective Zijderveld diagrams component (HTC) is typically destroyed to 640– and the stereograms. In a number of cases this compo- 670°C. We note however that there are some samples nent is present in the samples that, besides it, only in which the characteristic component is almost com- contain a fairly stable present-day component (Fig. 6, pletely destroyed to 580°C. This component has a sample 945). However, as a rule, this component par- moderate inclination and northern declination, and ticipates in a three-component system made up, in this is close to the expected Khramov direction. addition to this one, by the relatively low-temperature Therefore, hereinafter we refer to this component as present-day magnetization component and the KHR- KHR. The mean direction of the KHR-component component (Fig. 6, samples 912, 924, and 987). for the Sardana Formation is slightly rotated towards Examining the respective remagnetization circles the east relative to the mean direction of the KHR- (Fig. 7), we do not find any clear regularity which component of the Pestrotsvetnaya formation from the would allow us to anyhow constrain the direction of Belaya river valley, which is quite natural to attribute to the sought component. In our opinion, the existing a certain difference in the age of the respective rocks. data at least do not conflict with the hypothesis that No signs of the Kirschvink component were found this component is actually the KHR one but having an during the demagnetization of the samples of the Pes- opposite (direct) polarity. trotsvetnaya formation from the Belaya River and the However, we cannot but note that in some samples Sardana Formation from the Kyllakh Ridge. (Fig. 6, samples 924 and 945) the direction of rema- Section Summary nent magnetization is shifted towards the presumed (1) The studied Lower Cambrian rocks of the Pes- Kirschvink direction during the demagnetization. trotsvetnaya formation from the Belaya River valley Section Summary and the rocks of the Sardana Formation from the Kyl- (1) The studied Lower Cambrian Pestrotsvetnaya lakh Ridge contain only one ancient KHR-compo- Formation rocks from the Maya River valley contain nent of magnetization which is close to the expected the ancient magnetization component KHR that is Khramov direction. close to the expected Khramov direction. (2) During the detailed demagnetization, some Sills of the Chekurovka Anticline samples demonstrate peculiarities that can be under- stood as indicating the presence of the Kirschvink Overall, we have studied seven sills in the valley of component in these samples. However, these pecu- the Neleger River and lower reaches of the Lena River. liarities can also be interpreted as the result of the In five of them the paleomagnetic record is either cha- superimposition of the KHR components having a otic or demonstrates, more or less explicitly, the pres- different polarity. ence of the magnetization component with a steep positive inclination and northeastern–eastern declination (Fig. 10). By its direction this component is PestrotsvetnayaFormation of the Belaya River close to the direction of the Mesozoic remagnetiza- and Sardana Formation of the Kyllakh Ridge tion, which is widespread within the Chekurovka anti- The samples of the Pestrotsvetnaya formation from cline (Pavlov et al., 2004). Based on this we assume the outcrop studied in the valley of the Belaya River have that this component is metachronous and reflects the magnetization varying from 1–2 to 10–15 mA/m, which Mesozoic magnetic field. is by many orders higher than the magnetization of the The two remaining sills, both sampled by us in the rocks of this formation from the Maya River valley. The valley of the Neleger River, contain a distinct paleo- samples of the Sardana Formation are magnetized, on magnetic record. This record is carried by titanomag- average, weaker than the samples of the Pestrotsvetnaya netite with the Curie points in the interval from 480 to formation from the Belaya River valley and stronger than 520°C, as suggested by the values of the maximal the samples of this formation from the Maya River valley. unblocking temperatures of the NRM (Fig. 10). Their magnetization is 3–8 mA/m. The same also follows from the temperature depen- The behavior of the NRM of the samples of the dence of the saturation remanent magnetization illus- Sardana Formation is similar to that of the samples of trated in Fig. 11. We reject the possibility that the drop the Pestrotsvetnaya formation from the valley of the in the saturation remanent magnetization in the inter- Belaya River (Figs. 8, 9): in both cases, two magneti- val of 480 to 520°C is due to the disintegration of zation components are clearly detected. The first, maghemite because in this case the value of saturation low- to medium-temperature (LTC) component is remanent magnetization should be expected to drop relatively less stable and destroyed to 350–400°C. after heating rather than to increase by a factor of 14.5

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Sample 974 N N Sample 944 N N

110° 110°

350° 310° HTC 450° ° 530 HTC E UP E UP 655° 590° Scale = 0.5 mA/m Scale = .5 mA/m

N N N N Sample 575 Sample 904

° 150 110°

410° 340°

470° 560° 550° E UP E UP 680° 660° Scale = .5 mA/m Scale = 0.2 mA/m

N Sample 922 N N 110°

340°

440°

E UP 670° HTC Scale = 0.5 mA/m KHR

Fig. 5. Demagnetization results for samples of Pestrotsvetnaya formation from Maya River’s lower reaches, KHR component. Data are presented in ancient coordinate system.

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N N N Sample 912

110° 340° 470° E UP ° Scale = 0.5 mA/m 690

N N N Sample 945 110° LTC

390° 470–690° E UP Scale = 0.5 mA/m

N N N Sample 987

120°

LTC 390° E UP 430–680° Scale = 0.5 mA/m KHR

Sample 924 N N N 110°

LTC 340° 500°

E UP 615–690° Scale = 0.1 mA/m KHR Variegated formation, Maya River

Fig. 6. Demagnetization results for samples of Pestrotsvetnaya formation from Maya River’s lower reaches, KRS component. Data are presented in ancient coordinate system.

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Maya 99 N Outcrop 2 N

Outcrop 4 N Inikan N

Variegated formation, Maya River

Fig. 7. Vector directions of characteristic (high-temperature, HTC) magnetization component and remagnetization circles in four outcrops of Pestrotsvetnaya formation in Maya River’s lower reaches. Lower stereogram shows that revealed remagnetization circles have no predominant orientation. Data are presented in ancient coordinate system.

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Sample BEL332 0 NRM 0 400° N, Up

480° 180 HTC A-component 270 90 NRM = 8.31e–03 A/m “КНR” 0 W E 670° S, Down

180 NRM Sample BEL333 N, Up 400° 0 0

480°

HTC 180 A-component “КНR” 270 90

NRM = 1.44e–02 A/m 0 W E 640° S, Down 180

270 90

180

Fig. 8. Demagnetization results for samples of Pestrotsvetnaya formation from Belaya River. Zijderveld diagrams: filled and open circles denote vector projections on horizontal and vertical plane, respectively. Stereograms: filled and open circles denote vector projections on lower and upper hemisphere, respectively. Data are presented in ancient coordinate system.

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Sample 406 N, Up 0 NRM 400° 480° NRM = 5.42e–03 A/m 270 90 W E 580° S, Down 0

Sample 398 180 N, Up 0 NRM

400° 470° NRM = 6.01e–03 A/m 270 90 W E ° HTC 650 S, Down

180 Sample 391 0 N, Up

400° NRM

490° 270 90 NRM = 4.88e–03 A/m W E 640° 0 S, Down

0 180

270 90

180

Fig. 9. Demagnetization results for samples of Sardana formation from Aldan River. Zijderveld diagrams: filled and open circles denote vector projections on horizontal and vertical plane, respectively. Stereograms: filled and open circles denote vector projections on lower and upper hemisphere, respectively. Data are presented in ancient coordinate system.

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 54 No. 5 2018 SYSTEMATICS OF EARLY CAMBRIAN PALEOMAGNETIC DIRECTIONS 797 as is actually observed in the results of the performed This manifestly qualifies the second high-temperature experiment. component as the Kirschvink component. The samples of these sills have two clearly distinct A similar component content (combination of magnetization components. The less stable one, the KHR and KRS with a relatively smaller contribution low- to medium-temperature LTC-component is of KRS) is also observed in the rocks of the Pes- removed by heating up to 350–400°C and, judging by trotsvetnaya formation exposed in the valley of the its direction (Table 1), has either the Mesozoic or Maya River, more than 1500 km away from the out- present age. The second, high-temperature compo- crops of the Emyaksa Formation in the valley of the nent (JHTC) is mainly destroyed in the temperature Kuonamka River. interval from 400 to 520°C and is the most stable in the The magnetization component corresponding to studied samples. the Kirschvink’s direction was identified by us in an The direction of this high-temperature component explicit form in the sills of the Neleger River. The pres- significantly differs from the expected Khramov direc- ence of this component in both the sedimentary and tion (i.e., the one recalculated from the Khramov pole igneous rocks is a strong argument in favor of its real to the coordinates of the sampling sites) and has a very existence. low negative inclination and southwestern declination. For completeness we recall that the KRS compo- However, the direction of this component is very close nent was detected in the Lower Cambrian rocks of the to the expected Kirschvink direction, and, in the Tyuser Formation in the Chekurovka reference section understanding defined above, this component should (the lower reaches of the Lena River) (Pavlov et al., be considered as the Kirschvink component (KRS). 2004) and Erkeket formation along the Khorbusuonka The virtual geomagnetic pole calculated from the River (Gallet et al., 2003). The presence of this compo- characteristic magnetization of the sills from the Nele- nent in the Late Vendian–Early Cambrian rocks from ger River is close to the geographical position of the the southern region of the Siberian Platform was also Early Cambrian pole determined by Kirschvink and reported in (Kravchinsky et al., 2001; Rodionov, 2014). Rozanov (Kirschvink and Rozanov, 1984), Table 1, Fig. 12). Thus, there is a fairly large body of data supporting the existence of the Kirschvink component (KRS) in We also note that any signs indicating the presence the Late Vendian–Early Cambrian rocks of Siberia. of the Kramov component in the studied sills have not The existence of the Khramov component (KHR) been found. which was repeatedly described in a series of previous Section Summary works (e.g., (Pisarevskii et al., 1998)) also does not (1) The characteristic component separated in the raise any doubt. The data obtained in our present work Lower Cambrian sills of the Neleger River valley cor- validate its reality once again. responds to the expected Kirschvink direction. The results of these studies suggest the following (2) No other stable components besides the Meso- conclusions. zoic of the present-age low- to medium-temperature (1) The magnetization of the Lower Cambrian component were found in the studied objects. rocks from the Siberian platform typically has two ancient highly stable components: the Khramov (KHR) component and the Kirschvink (KRS) com- DISCUSSION ponent, significantly different by their directions. Anomalous Character of the Paleomagnetic Record (2) The KHR component is predominantly monopo- in the Lower Cambrian Rocks of the Siberian Platform lar and is fairly confidently identified in a significant part Let us summarize some results. Extensive studies of the studied objects by the linear segments trending were carried out with the analysis of quite a few sam- towards or going past the origin of the Zijderveld dia- ples from different outcrops of the Lower Cambrian grams. The second component typically manifests rocks in the northern and eastern regions of the Sibe- itself by the remagnetization circles (frequently rather rian Platform. weakly pronounced) in the stereograms and by some peculiarities in the behavior of the vector projections In the outcrops of the Emyaksa Formation, the on the Zijderveld diagrams. During the present work presence of two high-temperature magnetization we have for the first time found the Lower Cambrian components was detected. One component is clearly objects (the sills of the Neleger River) carrying the identified in many samples by the linear segment in KRS component in the direct form. the Zijderveld diagrams and has the direction corresponding to the Khramov direction (KHR). The pres- (3) Each of these components exists objectively and ence of the second high-temperature component is is not an artifact of data processing. suggested by the distinctly pronounced circles in the (4) Both components are frequently present in the stereograms. During the demagnetization, the projec- sections spaced by a thousand or many hundred kilo- tions of the NRM vectors migrate along these circles meters from each other and representing different towards the expected Kirschvink direction (KRS). regions of the Siberian Platform with a different geo-

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Sample 6666 0 (a) N, Up NRM = 47.7e–03 A/m 570° W E 520° 500°HTC 270 90 KRS 480°

400° NRM S, Down LTC 180 0 (b) Sample 6664 N, Up NRM = 245.7e–03 A/m 570° W 520° E ° 500 270 90 480°

S, Down 400° NRM 180 Sample 6655 0 (c) N, Up NRM = 63.6e–03 A/m W E 520° 500° 540° 270 90

480° S, Down NRM 180 0

(d)

270 90 Average direction over two sills

180

Fig. 10. Demagnetization results for samples from sills of Neleger River. Zijderveld diagrams: filled and open circles denote vector projections on horizontal and vertical plane, respectively. Stereograms: filled and open circles denote vector projections on lower and upper hemisphere, respectively. Data are presented in ancient coordinate system.

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Irs(t)/Irs0(t) – Sample’s ﬁrst heating 1.2 Nel666 1.0 0.8

0.6

0.4

0.2 Magnetic moment, arb. units moment, Magnetic 0 100 200 300 400 500 600 700 Temperature, °C

1.2 Irs(t)/Irs0(t) – Sample’s second heating Nel666 1.0 0.8

0.6

0.4

0.2 Magnetic moment, arb. units moment, Magnetic 0 100 200 300 400 500 600 700 Temperature, °C

Fig. 11. Sills of Neleger River. Sample NEL666. Temperature dependence of saturation remanent magnetization. Top: first heating; bottom: second heating. After first heating, magnetization increases by a factor of 14.5. logical history, e.g., the area of the Anabar block, can Kirschvink’s component be the result of subse- Uchur–Maya region, and Cis-Sayan south of the quent remagnetization since the respective pole is Siberian Platform (Kravchinsky et al., 2001). quite distant from the Phanerozoic segment of Sibe- (5) In a part of the samples, these components are ria’s apparent polar wander path (Torsvik et al., 2012), present in the form of a single high-temperature char- Fig. 12. acteristic component. In another part of the samples, However, the hypothesis of the metachronous these components are present jointly, in the form of nature of Khramov’s component implying its Middle- two high-temperature components with relatively or Late Cambrian age also faces a very serious chal- strongly overlapping spectra of their unblocking tem- lenge. This is because the KHR component is fre- peratures. In these cases, KRS is, as a rule, the high- quently present in the Vendian–Cambrian sections est-temperature (end) component. located in different segments of the Siberian Platform, (6) The studied sections are largely composed of having different geological histories and composed of sedimentary, mainly carbonate rocks. However, in this dissimilar rocks. The discussed magnetization compo- study, one of the discussed components was isolated in nent occurs in the Cambrian rocks which are distrib- the igneous rocks. uted across vast territories and, being a secondary We note that the results of this work completely component, it should have been formed as a result of a rule out the interpretation of the Kirschvink compo- certain large-scale tectonic or magmatic event that hit nents as an artifact caused by the superimposition of the Siberian platform in the last half of the Cambrian. the Khramov components with the normal and However, we find no signs of this event in the geolog- reversed polarities by the superposition of the ical history of the Siberian Platform. Moreover, if such Khramov and younger remagnetizing components an event had really occurred, why hasn’t it affected the because the Kirschvink component is identified in the more ancient rocks?. Indeed, we do not see the signs sills of the Neleger River clearly and unambiguously, of Khramov’s component the Riphean rocks of the without any relation to the Khramov component. Nor Uchur–Maya region (Pavlov et al., 2000), in the Sibe-

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P-T

D3-C1

Equator 30° E 90° E 150° E

Kirschvink’s pole O2-O3 7 1 Khramov’s pole Cm3-O1

° S 2 30 Cm2

3 4 6

° S 5 60

Fig. 12. Comparison of obtained poles corresponding to KHR and KRS components with Paleozoic poles (squares) of Siberian platform (Smethurst et al., 1998). Dashed line marks curve of = apparent wander path of paleomagnetic poles for Siberian platform. Black asterisk indicates pole from (Kirschvink and Rozanov, 1984). Ages of Paleozoic poles: Cm2, Middle Cambrian; Cm3-O3, Late Cambrian–Early Ordovician; O2-O3, Middle to Late Ordovician; D3-C1, Late Devonian–Early Carboniferous; P-T, Permian–Triassic. Pole numbers: (1) Usatovskaya Formation (Rodionov, 2014); (2) Shamanskaya Formation (Kravchinskii et al., 2001); (3) Sardana Formation; (4) Pestrotsvetnaya formation of Belaya River; (5) Pestrotsvetnaya formation of Maya River; (6) Emyaksa Formation of Kuonamka River; (7) sills of Tuor-Asis Ridge (poles 3–7 are obtained in this work). rian platform’s northwest (Gallet et al., 2000), north objectively existing stable high-temperature magneti- (Ernst et al., 2000; Veselovskii et al., 2006; 2009), north- zation components, each of which can be considered east (Rodionov, 1984), and south (Komisarova, 1983). as the primary one that had not been formed later than the Early Cambrian. The statistically significant difference of the respective paleomagnetic poles (Fig. 12) is another However, the presence of two different primary strong argument disproving the Middle–Late Paleo- components evidently contradicts the traditional zoic remagnetization as a source of origin of the KHR notion of the paleomagnetic record as mainly reflect- component. ing the axial dipole character of the geomagnetic field. This leads us to seek an alternative interpretation of the Thus, the obtained data strongly point to the valid- observed facts. ity of our previous conclusion (Pavlov et al., 2004) that in a significant part of the transitional Vendian–Cam- We suggest the following hypothesis as a probable brian layers in the Siberian Platform there are two explanation. Both the considered components in the

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 54 No. 5 2018 SYSTEMATICS OF EARLY CAMBRIAN PALEOMAGNETIC DIRECTIONS 801 sedimentary rocks are primary in the sense that they KRS component alone—the case observed in the sills have been acquired either during the deposition of the of the Neleger River. studied sediments or shortly after this. These components can have both syndepositional and early diage- netic origin. The difference of their directions is due to Analysis of the Global Data the fact that the geomagnetic field at the end of Late If the anomalous field actually existed during the Vendian and at the beginning of the Cambrian had an considered time interval, this phenomenon should be anomalous character with relatively long periods of the on a global scale and should manifest itself on the predominant axial molopolar dipole field recorded in other continents. In (Shatsillo et al., 2005), we carried the KHR component alternating with the relatively out a detailed analysis of the Vendian–Cambrian short epochs dominated by the reverting subequatorial determination contained in the Global Paleomagnetic dipole recorded in the KRS component. Database (Pisarevsky and McElhinny, 2003). In the The data obtained in this work substantially expand cited work, we have shown that the presence of two the body of evidence supporting this hypothesis. nonidentical directions during the Vendian–Lower Cambrian can indeed be considered as a global-scale In our opinion, this behavior of the magnetic field event apparently reflecting the anomalous behavior of could explain most of the peculiarities of the paleo- the Earth’s magnetic field. In this case, the character- magnetic record that we observe in the studied sec- istic angular distance between the corresponding poles tions and to reconcile the seemingly antagonistic is typically close to 50°, which is also the case for the results obtained by the different researchers. Siberian KHR and KRS poles. In the case the magnetization in the sediments was A conceptually similar study was recently con- formed virtually simultaneously with the sediment, ducted by A. Abrajevitch and R. Van der Voo (2009). the deposited sequences will record the alternation of These authors analyzed the entire existing data for the the KHR and KRS components. This character of the Ediacaran of Laurentia and Baltica to come to practi- paleomagnetic record is observed in that part of the cally the same conclusions as we obtained a few years Chekurovka section which has not been remagnetized earlier mainly based on the Siberian data (e.g., (Pavlov by the Mesozoic field (Pavlov et al., 2004). et al., 2004; Shatsillo et al., 2005)). Abrajevitch and In the case the magnetization was formed rapidly Van der Voo have shown that the paleomagnetic but still with a certain delay commensurate with the results determined from different Ediacaran objects of duration of the KRS epochs, the KHR-component on Baltica and Laurentia definitely point to the coexis- some levels of the section will be observed alone, tence of two magnetization components, one shallow whereas on the other levels this component will be and the other steeply inclined. Besides, there are superimposed on the relatively weaker KRS-compo- sound arguments (including positive field tests) indi- nent. The presence of the latter can probably be only cating that both components are primary and very perceived through the trends of the Zijderveld dia- close in age. The traditional interpretation of these grams and through the remagnetization circles tending data in the scope of the geocentric axial dipole towards Kirschvink’s component. Clearly, the KHR- hypothesis requires implausibly high velocities of con- component in this case will frequently be present in tinental migration which are neither attained by the the samples as an intermediate component. This char- plate tectonics nor by the true polar wander hypothe- acter of the record is observed, e.g., in the Lower sis. The authors assert that the observed data can only Cambrian sections of the Bol’shaya Kuonamka and be accounted for by the extremely irregular, anoma- Maya rivers. In the case when the present-day compo- lous behavior of the geomagnetic field at that time, nent significantly contributes to the magnetization, which can probably be described by the alternation of these trends will be masked and blurred; and the rela- the co-axial and quasi-equatorial dipole. This behav- tively close positions of the present-day and Kramov’s ior of the geomagnetic field can emerge under certain components will impede their separation. specific conditions in the core and/or at the core/mantle boundary which, in turn, strongly con- If the recording of the paleomagnetic signal in the strains the thermal evolution of the inner shells of the rocks lasted significantly longer than the duration of Earth. KRS epochs, it should be expected that during the In the recent work of Halls et al. (2015), based on demagnetization these rocks will show the presence of the Grenville dikes dated to ~585 Ma, it was shown only one magnetization component. This is observed that the studied dike swarms that are scattered in age in the Lower Cambrian rocks of the Pestrotsvetnaya by at most 4 Ma (perhaps even by a much shorter formation from the Belaya River and Sardana Forma- period) carry the primary magnetization with sharply tion of the Kyllakh Ridge. different (by ~90°) directions. The latter cannot be However, if the paleomagnetic record was formed explained by the ordinary geomagnetic variations, and very fast (as typically occurs in the sills after their the obtained paleomagnetic data cannot be inter- intrusion) and the direction of the field corresponded preted as reflecting the tectonic motions. The authors to Kirschvink’s pole, we should see the record of the propose that one of the two directions obtained by

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 54 No. 5 2018 802 PAVLOV et al. them corresponds to some quasi-stable state of the many points in common with the model of the geo- subequatorial dipole, which substantially determines magnetic reversals developed by Khramov and his col- the geometry of the field during the change of its leagues based on the studies of these reversals in the polarity. The role of this subequatorial dipole signifi- Early Paleozoic (Khramov and Iosifidi, 2012). cantly increases during the periods when the geomag- According to their model, the field of the geocentric netic field is in an hyperactive state with extremely fre- axial dipole diminishes, up to completely vanishing, quent reversals. It is the combination of highly fre- during the geomagnetic reversals. However, the geo- quent reversals and the presence of a relatively stable magnetic field does not disappear altogether but is subequatorial dipole that is suggested by the authors to determined by the superimposition of the equatorial explain the two sharply different paleomagnetic direc- dipole and nondipole components. In this case, tions in the Ediacaran rocks of a similar age. according to (Khramov and Iosifidi, 2012), the contri- The possibility of the existence of epochs with a bution of the nondipole components can make up 15– hyperactive geomagnetic field characterized by an 20% of the dipole axial field. Actually, this model extremely low intensity and very high variability in the could fully describe the peculiarities of the Late Ven- directions is substantiated in the recent work (Scher- dian–Early Cambrian paleomagnetic record observed bakova et al., 2017). by us if we assume the reversals of the nonaxial dipole. In this case, the axial dipole field which is recorded in Thus, the world data fairly well agree with our the KHR-type components would be the normal, results for the Siberian Platform and definitely support ordinary stable field, whereas the KRS-type compo- our conclusion about the anomalous character of the nents would reflect a certain transitional state resem- geomagnetic field in the Ediacaran–Lower Cambrian. bling the one described in the literature as the excursions of the geomagnetic field. In this approach, it Anomalous Field at the Precambrian/Cambrian should also be assumed that these excursions (let us Boundary refer to them as superexcursions) have the following important feature: certain predominant approximately In order to explain the observed paleomagnetic antipodal positions of the magnetic poles associated record, we suggest the hypothesis that the geomag- with the reversals of the nonaxial dipole would be netic field at the Precambrian/Cambrian boundary observed during these events. had an anomalous character induced by two alternating quasi-stable generation regimes. In the prelimi- The possibility of the existence of this dipole is val- nary form this hypothesis was formulated in our work idated by the results of analyzing the set of bipolar (Pavlov et al., 2004). According to this hypothesis, the paleomagnetic determinations contained in the Inter- magnetic field of the Earth at the Precambrian/Cam- national Paleomagnetic Database (Khramov and Iosi- brian boundary can be described by the alternation of fidi, 2012). The considered data support the model of long periods of predominance of the axial, mainly the paleomagnetic field according to which the field monopolar dipole field, which is recorded in the KHR includes a long-surviving component corresponding component and the relatively short epochs when the to the equatorial dipole. This dipole is responsible for magnetic field was mainly determined by the reversing the nonantipodality of the paleomagnetic directions in subequatorial or midlatitude dipole and recorded in the zones of direct and reversed polarity in the sedi- the form of the KRS component. mentary and volcanic sequences. During the interval A conceptually similar two-dipole model was pro- from 359 to 207 Ma, the equatorial dipole preserved its posed by L. Pesonen and H. Nevanlinna (Nevanlinna intensity at a level of 5 to 8% of the geocentric axial and Pesonen, 1983) to explain the asymmetric rever- dipole but flipped its polarity several times. The posi- sals detected by these authors in the Keweenawan sections of its northern poles on the surface of the Earth tions (1100–1000 Ma). formed two antipodal groups located within or close to the subduction zones in the periphery of the Pangaea Importantly, we do not insist that the observed supercontinent. It is assumed that this localization of paleomagnetic record necessarily needs the imple- the equatorial dipole is associated with the descending mentation of a two-dipole model. The observed phe- branches of the mantle convection and with the nomenon can be alternatively explained, e.g., by the topography of both boundaries of the Earth’s core hypothesis of the significant (at times) contribution of outer part. nondipole components in the geomagnetic field at the Precambrian/Phanerozoic boundary. In any case, the Superexcursions should have another important performed analysis of the Siberian and global paleo- feature: in order to leave a sufficiently distinct imprint magnetic data indicates that the geomagnetic field in on the paleomagnetic record, they should last notice- the latest Vendian and Early Cambrian was signifi- ably longer than the ordinary excursions. cantly different from the geomagnetic field of most of In this case, the hypothesis of the anomalous Late the subsequent epochs. Vendian–Early Cambrian geomagnetic field can be A very interesting and, perhaps, not coincidental formulated in a somewhat different form: the Earth’s fact is that the model suggested by has surprisingly magnetic field at the Precambrian/Cambrian bound-

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 54 No. 5 2018 SYSTEMATICS OF EARLY CAMBRIAN PALEOMAGNETIC DIRECTIONS 803 ary was far less stable than in the Cenozoic, and the Foundation for Basic Research under project nos. 17- normal state of the field corresponding to the axial 35-50068 and 17-05-00021. We are grateful to dipole was frequently interrupted by the geomagnetic A.G. Iosifidi and V.G. Bakhmutov for their careful excursions. The latter had the following distinctive review of our manuscript. The thermomagnetic analy- features: (a) the virtual poles associated with the sis was carried out using the instruments developed at excursions were predominantly concentrated in two the Borok Geophysical observatory by a team led by coarsely antipodal regions of the globe, which were Yu.K. Vinogradov. located in the middle-to-low latitudes; (b) the geomagnetic excursions were more frequent and lasted longer than in the Cenozoic. 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