Results of paleomagnetic study of Early Proterozoic rocks in the Baikal Range of the Siberian craton

This paper presents paleomagnetic results obtained from the study of Early Proterozoic rocks in the Baikal Range of the Siberian craton, namely, the 1850–1880-Ma volcanicalstic rocks of the Akitkanskian series of the North Baikal volcanic-plutonic belt) and 1674-Ma basic dikes of the Chaya complex within the massif. The data of this work are used to reconstruct the development of the Siberian craton structure in the Early Precambrian. The projections of the inferred paleomagnetic directions onto a sphere form S (southern) and W (western) groups of vectors of characteristic magnetization components. The S group consists of three clusters representing primary magnetization components belonging to different time levels of the end of the Early Proterozoic. The W group is represented by directions associated with a metachronous magnetization probably acquired during the Riphean. Four paleomagnetic poles are obtained. Two of them that can be regarded as key poles correspond to time levels of 1875 and 1670 Ma (the Early Proterozoic). The two other poles can be used for a detailed reconstruction of the Proterozoic segment of the Siberian apparent polar wander path. The data presented in the paper indicate that the formation of the southern Siberian craton structure was accomplished at the end of the Early Proterozoic, which resulted in a synchronous motion of different blocks composing the southern flank of the craton (in particular, the Sharyzhalgai and Baikal Ranges).     

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.     

Paleomagnetism of the Upper Vendian Basu formation of the Bashkirian Meganticlinorium revisited

We have carried out paleomagnetic studies of the Upper Vendian sedimentary rocks from the Bashkirian Meganticlinorium (Southern Ural). The rocks were sampled at three localities spread over more than 100 km. Totally, more than 300 samples were collected from about 40 sampling sites. Stepwise thermal demagnetization up to 700°C revealed a stable component of magnetization of either polarity in 25 sites. The fold test and the reversal test for this component are positive, which is usually regarded as a sound argument in favor of the primary origin of magnetization. However, the Basu paleomagnetic pole (longitude 187.3°E, latitude 1.1°N) is located near the Late Ordovician-Early Silurian segment of the apparent polar wander path for Baltica, which might indicate a Paleozoic remagnetization of Vendian rocks. In this work we analyze different interpretations of the obtained results and evaluate the reliability of the Late Riphean and Vendian paleomagnetic data for Baltica.     

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?C40 rapidly cooled magmatic bodies (dikes, flows, or minor sills) should be sampled if secular variation is commensurate with the intensity of the presentday variations.     

Paleomagnetic studies and estimation of geomagnetic paleointensity at the early/middle Riphean boundary in rocks of the Salmi Formation (North Ladoga area)

The territory of Karelia (Baltic Shield) is virtually not represented in the global paleomagnetic database for the Lower Riphean time interval (1650—1350 Ma). As regards the paleointensity H _an, the huge interval 1–2 Ga in length is represented in the global paleointensity database by only eight determinations concentrated in the interval 1–1.35 Ga. The paper presents results of paleomagnetic studies of volcanic and subvolcanic rocks composing the Early Riphean Salmi Formation, which outcrops in the valley of the lower Tulemaioki River in the northern coast area of Lake Ladoga. Results of the study indicate that, in the Early Riphean time, the East European craton was located in the tropical region of the Southern Hemisphere between 15° S and 40° S. The inferred value of H _an is close to the lower boundary of the interval (1.36–11.56) × 10²² A m², encompassing previously published intensity values of the paleofield 1–1.35 Ga; this supports the hypothesis on the existence of long intervals of a lower field in the period in question [Maquoin et al., 2003].     

Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: new constraints for Late Proterozoic-Early Paleozoic paleocontinental reconstructions of the Americas

New UPb zircon crystallization ages and ⁴⁰Ar/³⁹Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. ε_Nd (t = 1.1 Ga) for these rocks range from −3.9 to +0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga orogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (T_DM) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions.     

The new Permian–Triassic paleomagnetic pole for the East European Platform corrected for inclination shallowing

The results of detailed paleomagnetic studies in seven Upper Permian and Lower Triassic reference sections of East Europe (Middle Volga and Orenburg region) and Central Germany are presented. For each section, the coefficient of inclination shallowing f (King, 1955) is estimated by the Elongation–Inclination (E–I) method (Tauxe and Kent, 2004) and is found to vary from 0.4 to 0.9. The paleomagnetic directions, corrected for the inclination shallowing, are used to calculate the new Late Permian–Early Triassic paleomagnetic pole for the East European Platform (N = 7, PLat = 52.1°, PLong = 155.8°, A95 = 6.6°). Based on this pole, the geocentric axial dipole hypothesis close to the Paleozoic/Mesozoic boundary is tested by the single plate method. The absence of the statistically significant distinction between the obtained pole and the average Permian–Triassic (P–Tr) paleomagnetic pole of the Siberian Platform and the coeval pole of the North American Platform corrected for the opening of the Atlantic (Shatsillo et al., 2006) is interpreted by us as evidence that ~250 Ma the configuration of the magnetic field of the Earth was predominantly dipolar; i.e., the contribution of nondipole components was at most 10% of the main magnetic field. In our opinion, the hypothesis of the nondipolity of the geomagnetic field at the P–Tr boundary, which has been repeatedly discussed in recent decades (Van der Voo and Torsvik, 2001; Bazhenov and Shatsillo, 2010; Veselovskiy and Pavlov, 2006), resulted from disregarding the effect of inclination shallowing in the paleomagnetic determinations from sedimentary rocks of “stable” Europe (the East European platform and West European plate).     

Paleomagnetism of sedimentary strata and the origin of the structures in the western slope of South Urals

Paleomagnetic data may contribute to studying the formation history of orogens; in particular, these data can promote identifying the pattern and scales of deformations at the final stages of orogeny. We have conducted paleomagnetic studies of the Paleozoic and Neoproterozoic sediments in the western part of the Western Ural Megazone in South Urals. The detailed thermal demagnetization revealed the intermediate temperature magnetization component in most samples. This magnetization has a reversed polarity and has been acquired before folding or at the early stages of the deformations. The directions of this component are narrowly grouped in rocks of a different age in all the segments of the studied part of South Urals, and the regional average direction closely agrees with the reference paleomagnetic direction of 270 Ma for the East European Platform. The results of our study suggest the following conclusions: (1) the main magnetization component in the studied sedimentary rocks has a secondary origin; (2) this component has an age of ~270 Ma and has been formed during the Kungur deformations (279–272 Ma ago) of the western part of South Urals; (3) neither a general rotation of the studied part of the Urals relative to the East European Platform nor local rotations of the individual tectonic blocks relative to each other are revealed; (4) the changes in the strike of the structures from NE within the Karatau uplift to the submeridional in the remaining part of the Urals is not an oroclinal bend.     

Paleomagnetism of Devonian and Carboniferous sedimentary rocks of Spitsbergen: to the Paleozoic history of the Barents-Kara basin framing

We present the results of studying the paleomagnetic samples of Carboniferous and Devonian rocks collected in 1986–1987 from the Island of Spitsbergen. The paleomagnetic poles determined from these collections are compared to the poles from the coeval rocks of the Russian Platform. On the basis of these results and the available data of the World paleomagnetic database, the paleogeographic layout of Spitsbergen and the Russian Platform in Early Devonian and Early Triassic are reconstructed. The reconstructions demonstrate a common north-northwestern drift of these units from the southern equatorial latitudes in the Early Devonian to the middle northern latitudes with a small (800 km) convergence, compared to this drift, of Spitsbergen with the Russian Platform by the Early Triassic, which is followed by the subsequent retrograde motion.     

Siberian paleomagnetic data and the problem of rigidity of the Northern Eurasian continent in the post-Paleozoic

The Meso-Cenozoic paleomagnetic poles from the Siberian platform and its folded margins, which comply with the modern technical and methodological standards, are analyzed. The analysis suggests the following conclusions. (1) The geometrical relationship between the Permo-Triassic poles of the Stable Europe and Siberian Platform prohibits the possibility of relative displacements of these platforms in the post-Paleozoic time. (2) The Mesozoic paleomagnetic poles of the Siberian Platform support the hypothesis of rigid Northern Eurasia. (3) The paleolatitudes of the Mesozoic sections located on the folded margins of the Siberian Platform closely agree with the Apparent Polar Wandering Path (APWP) for Europe. (4) The available data indicate that the vertical-axis rotation of separate local blocks within the folded margins of the Siberian Platform was a widespread phenomenon. Therefore, (1) the modern paleomagnetic data quite certainly show that consolidation of the Northern Eurasian continent was completed by the end of Permian, and, since the very beginning of the Mesozoic, the Siberian and East-European platforms have been parts of a single rigid megablock. (2) The Meso-Cenozoic segment of the APWP for Europe can be used as reference for the Siberian platform.     

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.     

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

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 eligible 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–Phanerozoic 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 Iosifidi, 2012) and with the results of geodynamo numerical simulations (Aubert and Wicht, 2004; Glatzmayer and Olson, 2005; Gissinger et al., 2012).     

Paleomagnetism of Permian rocks of the Subpolar Urals,Kozhim River: To the history of evolution of the thrust structures in the Subpolar Urals

The collections of Permian rocks from sections of the Kozhim River (Asselian, Kungurian, and Ufimian stages) and the Kama River (Ufimian and Kazanian stages) are studied. The paleomagnetic directions determined on the studied structures closely agree with the existing data for the Subpolar Urals and Russian Platform (RP). In the Middle Permian red clays of the Kama River region, the paleomagnetic pole N/n = 28/51, Φ = 47° N, Λ = 168° E, dp = 3°, and dm = 5° is obtained. The analysis of the existing paleomagnetic determinations for the Early and Middle Permian of the Russian and Siberian platforms and Kazakhstan blocks (KBs) is carried out. For the Subpolar Ural sections, the estimates are obtained for the local rotations during the collision of the Uralian structures with the Russian and Siberian platforms and KBs. The amplitudes of the horizontal displacements of the studied structures are, on average, 170 ± 15 km per Middle Permian. The scenario describing the evolution of the horizontal rotations of the structures of Subpolar Urals is suggested.     

Middle paleozoic segment of the apparent polar wander path from the Siberian platform: New paleomagnetic evidence for the Silurian of the Nyuya-Berezovskii facial province

The paper presents results derived from study of the Silurian of the Nyuya-Berezovskii facial province. Variegated sedimentary rocks of the Meutian and Kurungian series (Llandoverian, Wenlockian, and lower Ludlovian) are studied. Detailed thermal demagnetizations of the collections revealed two stable magnetization components; one of them (Ds = 193.8, Is = 19.2; k = 10.7, α₉₅ = 6.1) is bipolar and is likely to have formed during or shortly after the rock formation, i.e., in the interval from the Early Silurian to the beginning of the Late Silurian. The second component is unipolar and apparently metachronous, and its formation time can be bounded by the latest Early to the Middle Devonian. Based on the paleomagnetic results of this study, paleolatitudes and kinematics of Siberia are estimated for the Middle Paleozoic. The inferred paleomagnetic poles provide additional constraints on the Middle Paleozoic segment of the apparent polar wander path from the Siberian platform.     

Determining the origin of volcanic rocks in the mélange complex of Karangsambung based on the electrical resistivity imaging

An ENE-WSW-trending localized basalt-diabase outcrop along the SE margin of Luk Ulo Mélange Complex has been suggested as intrusive rocks cut through the Paleogene Totogan and Karangsambung formations. However, the absolute dating of the volcanics is older than the inferred relative age of the sedimentary formations, hence the in-situ intrusion theory is less likely. A subsurface imaging should delineate the possibility of the in-situ nature of volcanic rock by looking at the continuation of the rocks to the depth. In this study, we did a subsurface imaging by electrical resistivity method. The electrical resistivity surveys were conducted at 3 (three) lines across the ENE-WSW trend of the volcanic distribution. From those three measurements, we obtained three inversion models that present the distribution of the resistivity. We could differentiate between the high resistivity of volcanic rocks and the low resistivity of the clay-dominated sediments. Instead of the deep-rooted intrusions, the geometry of the volcanic rocks is concordant with the sedimentary strata. Since we do not observe any spatial continuity of the bodies, both laterally and vertically, the volcanic rocks might be part of broken intrusive rocks. Furthermore, the size and the sporadically distributed of the rocks also indicated that they are more likely as fragments during the olistostrome deposition, transported from its original location.     

Petromagnetic and paleomagnetic study of Jurassic sedimentary rocks of the southeast of Russia

In the present paper the new petro- and paleomagnetic data on the Jurassic terrigenous complexes of the Mesozoic sedimentary basins of the Amur River region, Trans Baikal region, and Yakutia are presented. The magnetic properties of the sedimentary rocks of coastal-marine (paleo-shelf) and lake genesis are investigated in the contemporary intracontinental riftogenic Mesozoic superimposed troughs of the Siberian and Amur plates: the Chulmansk, the Unda-Dainsk, the Sredne-Amur, the Amuro-Zeisk, and the Verkhne-Bureinsk troughs. The statistically significant differences in the magnetic (anisotropic) characteristics of continental and marine deposits were inferred. The correlation of the scalar and tensor characteristics of magnetic susceptibility anisotropy and the parameters of its linearity, which depend indirectly on the intensity of the folding, is established. The preferential directions of regional stress during the fold formation are determined based on the analysis of the distribution of the axes of the tensor ellipsoid of the magnetic susceptibility anisotropy. The Jurassic positions of the paleomagnetic pole, which are close to the Mesozoic section of the trajectory of its apparent motion for the North Chinese plate and which differ from the Jurassic poles of the Siberian plate, are defined more accurately. The intraplate rotations of geoblocks within the limits of the amalgamated to the end of the Jurassic-to the beginning of the Cretaceous terrains as a part of the Amur tectonic plate are inferred. The calculated coordinates of the paleomagnetic pole indicate the larger than the present-day difference of the latitudinal positions of the southern part of the Siberian plate, and also of the Amur and North Chinese plates in the Early-Middle Jurassic time. This can be indicative of the fact that the total width of the shallow residual basins of the Paleo-Asian Ocean, which separated the geoblocks indicated in the Early-Middle Jurassic, attained the first thousands of kilometers, and/or such a difference in the paleolatitudes reflects the total value of the shortening (crowding) of the crust during the accretion and the fold formation. The time of the completion of the accretion of the terrains of the Amur and the North-Chinese plates and their attachment to the Siberian plate is not earlier than the end of the Late Jurassic-the beginning of the Cretaceous.     

Paleomagnetism of carboniferous formations of the Russian platform: Reinvestigation of old collections

The collections of Carboniferous rocks from sections of the Russian Platform (Gzhelian, Moscovian, Bashkirian, and Visean stages) are studied. The new mean paleomagnetic poles are obtained from the Gzhelian, Moscovian, and Visean layers of the Carboniferous of the Russian Platform. In the redbed Gzhelian and Moscovian rocks, the natural remanent magnetization (NRM) components with the inclination shallowing are revealed, which is due to the presence of the large hematite particles or particle aggregates associated with the interaction between the magnetic and clay particles. Based on the obtained determinations and the results contained in the World paleomagnetic database, the trajectory of the apparent polar wander path (APWP) for the East European Platform is constructed in the interval from the Devonian to Early Permian. The Carboniferous kinematics of the East European Platform is estimated.     

东海陆架盆地雁荡低凸起综合地球物理解释及其成因探讨

通过对东海陆架盆地西部地震和重磁资料的综合地球物理解释,对雁荡低凸起展布形态进行了细致刻画,凸起呈NE方向不连续展布于瓯江凹陷和闽江凹陷之间,长约170 km、宽约15~50 km.地震资料揭示雁荡低凸起上广泛发育了侏罗纪与白垩纪地层,厚度约为500~1500 m,展布面积约5000 km²,局部缺失中生界地层.凸起两侧中生代盆地结构差异明显,西侧瓯江凹陷为典型的断陷盆地,东断西超、断裂发育,半地堑、掀斜断块等中生界构造样式发育;东侧闽江凹陷为坳陷型盆地,断裂、火成岩不发育,挤压背斜、断背斜、反转构造等中生界构造样式发育.自由空间重力异常图与剩余重力异常图上,凸起表现为一系列NE向团块状重力高值区,而磁力异常ΔT图上则表现为深部磁场强度低的特征,火成岩影响部位可见磁力高值异常.综合凸起及邻域重磁震、莫霍面深度等地质地球物理资料,认为雁荡低凸起为一元古界组成的古隆起,区别于东部的台北低凸起.同时,结合区域构造演化及沉积特征,推测侏罗纪时期雁荡低凸起与浙闽隆起区可能连为一体,晚白垩世近东西向伸展作用下浙闽隆起发生裂陷进而形成了雁荡低凸起.     

Paleomagnetic results from Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane,Tibet, China,and their tectonic implications

Results of a systematic paleomagnetic study are reported based on Late Carboniferous to Early Permian sedimentary rocks on the north slope of the Tanggula Mountains,in the northern Qiangtang terrane(NQT),Tibet,China.Data revealed that magnetic minerals in limestone samples from the Zarigen Formation(CP^z)are primarily composed of magnetite,while those in sandstone samples from the Nuoribagaribao Formation(Pnr)are dominated by hematite alone,or hematite and magnetite in combination.Progressive thermal,or alternating field,demagnetization allowed us to isolate a stable high temperature component(HTC)in 127 specimens from 16 sites which successfully passed the conglomerate test,consistent with primary remnance.The tilt-corrected mean direction for Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane is D_s=30.2°,I_s=-40.9°,k_s=269.0,a_(95)=2.3°,N=16,which yields a corresponding paleomagnetic pole at 25.7°N,241.5°E(dp/dm=2.8°/1.7°),and a paleolatitude of 23.4°S.Our results,together with previously reported paleomagnetic data,indicate that:(1)the NQT in Tibet,China,was located at a low latitude in the southern hemisphere,and may have belonged to the northern margin of Gondwana during the Late Carboniferous to Early Permian;(2)the Paleo-Tethys Ocean was large during the Late Carboniferous to Early Permian,and(3)the NQT subsequently moved rapidly northwards,perhaps related to the fact that the Paleo-Tethys Ocean was rapidly contracting from the Late Permian to Late Triassic while the Bangong Lake-Nujiang Ocean,the northern branch of the Neo-Tethys Ocean,expanded rapidly during this time.