ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2019, Vol. 55, No. 3, pp. 471–487. © Pleiades Publishing, Ltd., 2019. Russian Text © The Author(s), 2019, published in Fizika Zemli, 2019, No. 3. Systematics of Paleomagnetic Directions from Early–Middle Devonian Rocks of Minusa Troughs: New Data and Old Problems A. V. Shatsilloa, * and V. E. Pavlova aSchmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, 123242 Russia *e-mail: [email protected] Received December 29, 2017; revised May 23, 2018; accepted May 28, 2018 Abstract—Paleomagnetic results from numerous Early–Middle Devonian volcanic sequences of the Minusa trough, southern Siberia, are presented. The analysis of these data definitely indicates that the geomagnetic field in the Devonian had a specific character, different from both the present field and the field of more ancient geological epochs, and was extremely variable (hyperactive). The anomalies in the paleomagnetic record of the Early–Middle Devonian are not local, peculiar to a particular region, but have a global occurrence. The synthesis of the obtained results with the paleomagnetic data from the coeval volcanics from Scotland shows that during a relatively short time (10–20 Ma), the geomagnetic pole repeatedly changed its location, significantly deviating from the Earth’s rotation axis up to the (paleo)equator and assuming some quasi-stable positions. The arguments suggesting that the specific features of the Devonian paleomagnetic record could probably be induced by the significant contribution of the equatorial dipole to the main geomagnetic field are presented. Keywords: the Devonian, paleomagnetism, equatorial geomagnetic dipole, axial geomagnetic dipole, Sibe- rian platform, true polar wander DOI: 10.1134/S1069351319030091 INTRODUCTION quasi-stable paleomagnetic directions, which are peri- odically repeated in the sections and which were inter- The problem of the Devonian paleomagnetism preted as the transition zones with respect to the DII dates far back to the beginning of the paleomagnetic directions of direct and reverse polarity (Sallomy and research when this problem was noted by K. Creer, Piper, 1973; Kono, 1979). who studied the Lower Devonian Old Red Sandstone Just as many problems also appeared in the paleo- (ORS) volcanic–sedimentary formation of British magnetic studies of the Devonian volcanics from the Caledonides (Creer and Embleton, 1967 and refer- Asian part of the former Soviet Union (here, we will ences there). Initially, this problem only consisted of not specially focus on the old results that were the presence of two significantly different paleomag- obtained with the use of low-intensity and low-detail netic directions recorded in the ORS rocks. These demagnetization). In particular, the studies of the Late directions, which were referred to as DI and DII in the Devonian–Early Carboniferous kimberlites and traps literature (Creer and Embleton, 1967), were detected of the Siberian platform (Kravchinsky et al., 2002; in the same sections and yielded significantly different Konstantinov and Stegnitskii, 2012; Orlov and Shat- paleolatitudes. A similar situation was revealed by the sillo, 2011; Shatsillo et al., 2014; etc.) have clearly Russian researchers in the European part of the terri- shown that the range of the paleomagnetic directions tory of the former Soviet Union (Paleomagnetism…, contained in these objects is not limited to the DI and 1974). The discussion on the nature of the DI and DII DII directions in Creer’s terminology but contains directions resulted in the view, shared by most many other anomalous directions (including the sys- researchers, according to which the DI direction came tematic ones) whose existence cannot be explained by to be considered as the result of the Late Paleozoic the local tectonics or by the hypothesis of the remag- remagnetization. This interpretation was induced by netisation. Furthermore, it turned out that the number the fact that, on one hand, the paleomagnetic pole of the anomalous Devonian paleomagnetic directions corresponding to this direction was close to the Car- obtained from these objects is commensurate with the boniferous–Permian poles and, on the other hand, 1 the DII direction was more consistent with the paleo- number of the expected ones, which unequivocally climatic zonality for the Devonian of the East Euro- 1 Based on the position of the respective paleomagnetic poles rel- pean Platform (Creer, 1968; Paleomagnetism…, 1974). ative to the Paleozoic segment of the apparent polar wander path At the same time, the ORS studies also revealed other (APWP) curve for the Siberian platform. 471 472 SHATSILLO, PAVLOV suggests some specificity of the geomagnetic field in tions. The Byskar group overlies the subjacent forma- the Devonian. In fact, a similar result was obtained tions of different ages with angular unconformity, ero- from the Late Devonian lavas of the Northern Tien sion, and conglomerates at the base. The thickness of Shan (Bazhenov and Levashova, 2011): the authors the Byskar group in its most complete sequences attribute the anomalies fixed in the paleomagnetic reaches 1.5–2 km. The Byskar layers are concordantly record to the secular variations of the geomagnetic and with erosion overlain by terrigenous-carbonate field and, based on the example of the studied objects, variegated units with a total thickness ~0.5 km, con- conclude that the amplitude of secular variations is taining in their upper part (the Beya Formation) pele- not constant in time. In any case, the set of the paleo- magnetic data accumulated to date for the Devonian cypods, brachiopods, and bryozoans characteristic of rocks strongly suggests that the behavior of the field in the upper parts of the Givetian faunal stage of the Mid- the Devonian was significantly different from both the dle Devonian (Geologicheskaya…, 1957) and Eifelian- present field and from the field of the other geological Lower Givetian flora at the base (the Toltak Formation) epochs. For studying the Devonian geomagnetic phe- (Geologicheskaya …, 1959). The section of the Minusa nomenon and developing the corresponding dynamo troughs is completed with the Late Devonian red rocks models, first of all, new paleomagnetic data are and Tournaisian and Visean volcanic-sedimentary needed. strata, which are erosionally overlain by the Early In this work, we present new paleomagnetic results Permian coal-bearing formations. A number of the from the Early–Middle Devonian objects of the modern geochronological determinations are Minusa troughs, their systematics, and comparison obtained from the Byskar volcanics (Ar-Ar and U-Pb with the coeval volcanics from the ORS formation of ages reported in (Vorontsov et al., 2012)), which fall in British Caledonides. the interval 407.5 ± 0.2 to 386 ± 4 Ma; younger datings The fact that valid paleomagnetic data for Early– are obtained from the dikes crosscutting the volcanic Middle Devonian of the Siberian platform are practi- units (385 ± 4 and 364 ± 5.5 Ma). Thus, the existing cally absent attaches additional relevance to this study. geochronological and paleontological data constrain Obtaining these data in the conducted works would the time of accumulation of the main volume of Bys- make it possible to solve a range of geodynamical and kar volcanics to the Emsian and Eifelian, which covers tectonic questions associated with the Paleozoic evo- an interval of ~20 Ma. lution of the structures of North Eurasia. During the field studies of 2010–2012 within the GENERAL INFORMATION Minusa troughs, we tested the main sections of the ABOUT THE GEOLOGY OF THE REGION Byskar volcanics (the samples were taken only from AND OBJECTS OF STUDY the sections where it was possible to establish the pri- mary bedding of the strata). The study covered the The object of this study was the Early–Middle sections in the southwestern (along the Abakan and Devonian volcanogenic formations composing the Dzhebash rivers), eastern (Tuba River), and northern lower part of the Minusa troughs sedimentary sequence. According to the existing data on the tectonics of the parts (Koksa Bay, Krasnoyarsk Reservoir) of the region (Zonenshain et al., 1990; Berzin et al., 1994), South Minusa trough; the south of the Syda-Yerba the Minusa troughs were formed on the heterogeneous trough (Krasnoyarsk Reservoir, upstream of the Bel- basement of the south-Siberian Caledonides, which lyk village); and southeast (Sisim Bay), north (Kras- constituted a single structure with the craton part of noyarsk Reservoir, downstream of the Ezagash Bay, Siberia since the end of the Ordovician. The Minusa the Truba (Pipe) section), and the central part (Kras- troughs are a series of isometric synclines (centro- noyarsk Reservoir, in the vicinity of Novoselovo village) clines) separated by the anticlinal uplifts and include of the Chebaki–Balakhta trough (Fig. 1). The studied the South Minusa, Syda-Yerba, Chebaki–Balakhta sections represented the fragments of the limbs of the (North Minusa), and Nazarovo troughs (Fig. 1). The large folds or the monoclines with a consistent bed- studied volcanic sequences have a bimodal composi- ding attitude within the each section. As the attitude tion (rhyolites, dacites, basalts and their tuffs) with elements, the dip angles of the overlying sedimentary alkaline specificity. Within the Minusa troughs, the strata, the rare sedimentary interbeds within volca- Devonian volcanics are laterally variable, both by the nics, and the planes of magmatic