Paleomagnetism of early paleozoic geological complexes of Mongolia

Most of the studied Early Phanerozoic rocks of West Mongolia have undergone repeated remagnetization. Secondary magnetization components with normal and reversed polarity are isolated. The magnetization components with normal polarity are associated with the Mesozoic remagnetization of the rocks. The components with reversed polarity were probably formed during the Carboniferous–Permian superchron of reversed polarity. The analysis of the distribution of the reversed-polarity magnetization component in the structure of Mongolia permits some zonation. Within Mongolia, the regions with insignificant post- Permian deformations and complicated post-Permian deformations are identified; also the area of rotations of large geological blocks about the horizontal axis (Khan-Khukhei Ridge) is distinguished. It is hypothesized that in the Ordovician rocks of West Mongolia, the magnetization component that is close to primary was identified. If this is the case, the paleolatitude calculated from this magnetization direction corresponds to the interval 14°–17°–20° (minimal–mean–maximal) of probably northern latitude     

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.     

Early Yanshanian post-orogenic granitoids in the Nanling region Petrological constraints and geodynamic settings

Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt. They consist mainly of monzogranite and K-feldspar granite. There occur associations of early Yanshanian A-type granitoids (176 Ma-178 Ma) and bimodal volcanic rocks (158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts (177 Ma-178 Ma) in southern Hunan in the central sector of the belt. Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite, while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt. Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations. Post-orogenic suites mark the end of a post-collision or late orogenic event and the initiation of Pangaea break-up, indicating that a new orogenic Wilson cycle is about to start. Therefore it may be considered that the early Yanshanian geodynamic settings in the Nanling region should be related to post-orogenic continental break-up after the Indosinian orogeny and the break-up did not begin in the Cretaceous.     

Early Yanshanian post-orogenic granitoids in the Nanling region-- Petrological constraints and geodynamic settings

Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt.They consist mainly of monzogranite and K-feldspar granite.There occur associations of early Yanshanian A-type granitoids(176 Ma-178 Ma) and bimodal volcanic rocks(158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts(177 Ma-178 Ma) in southern Hunan in the central sector of the belt.Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite,while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt.Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations.Post-orogenic suites mark the end of a post-collision or late orogenic event and the initiation of Pangaea break-up,indicating that a new orogenic Wilson cycle is about to start.Therefore it may be considered that the early Yanshanian geodynamic settings in the Nanling region should be related to post-orogenic continental break-up after the Indosinian orogeny and the break-up did not begin in the Cretaceous.     

新疆塔里木地台晚古生代古地磁极移曲线及其地质构造含义

用热退磁辅以交变退磁方法对采自塔里木盆地阿克苏地区四石厂剖面47个采样点518块标本进行了逐步磁清洗和测试。由本征剩磁方向统计得到塔里木地台晚古生代的古地磁极位置(晚泥盆世φ=10.5°S、λ=151.2°E;晚石炭世φ=52.2°N、λ=179.5°E;早二叠世φ=56.5°N,λ=190.1°E)。古地磁结果表明:塔里木地台在晚古生代是北方大陆的块体之一。从晚石炭世至早二叠世塔里木地台已和北方的哈萨克斯坦板块、西伯利亚地台、俄罗斯地台等连成一片,并且从中生代以来它们之间的相对位置没有发生过大规模的变动     

Paleomagnetism of the Lower Ordovician and Cambrian sedimentary rocks in the section of the Narva River right bank: For the construction of the Baltic Kinematic model in the Early Paleozoic

The paleomagnetic study of the Lower Ordovician and Cambrian sedimentary rocks exposed on the Narva River’s right bank revealed a multicomponent composition of natural remanent magnetization. Among four distinguished medium- and high-temperature magnetization components, the bipolar component, which carries the reversal test, is probably the primary component and reflects the geomagnetic field direction and variations during the Late Cambrian and Early Ordovician. The pole positions corresponding to this component have coordinates 22°N, 87°E (dp/dm = 5°/6°) for the Late Cambrian, and 18°N, 55°E (dp/dm = 5°/7°) for the Early Ordovician (Tremadocian and Arenigian). Together with the recently published paleomagnetic poles for the sections of the Early Ordovician in the Leningrad Region and the series of poles obtained when the Ordovician limestones were studied in Sweden, these poles form new key frameworks for the Upper Cambrian-Middle Ordovician segment of the apparent polar-wander path (APWP) for the Baltica. Based on these data, we propose a renewed version of the APWP segment: the model of the Baltica motion as its clockwise turn by 68° around the remote Euler pole. This motion around the great circle describes (with an error of A₉₅ = 10°) both variations in the Baltic position from 500 to 456 Ma ago in paleolatitude and its turn relative to paleomeridians. According to the monopolar components of natural remanent magnetization detected in the Narva rocks, the South Pole positions are 2°S, 351°E (dp/dm = 5°/9°), 39°S, 327°E, (dp/dm = 4°/7°), and 42°S and 311°E (dp/dm = 9°/13°). It is assumed that these components reflect regional remagnetization events in the Silurian, Late Permian, and Triassic.     

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).     

Uranium in volcanic rocks from the central Andes

The distribution of U in volcanic rocks from two transects across the Central Andes (latitudes 27°–28° S and 16°–18° S, respectively) differs from that of K. For a given SiO₂, content of the rocks, K systematically increases with the distance from the trench, while the highest U abundances are found in the rocks overlying the thickest segments of the continental crust, which are situated in the middle parts of the transects. It is suggested that this variation of U reflects crustal contamination.     

Paleomagnetic and gravimetrical reconnaissance of Cretaceous volcanic rocks from the Western Colombian Andes: paleogeographic connections with the Caribbean Plate

The reconstruction of the tectonic evolution of the oceanic crust, including the recognition of ancient oceanic plumes and the differentiation between multiple and single oceanic arcs, relies on the paleogeographic analysis of accreted oceanic fragments found in orogenic belts. Here we present paleomagnetic and gravity data from Cretaceous oceanic basaltic and gabbroic rocks, the continental metamorphic basement, and their associated cover from northwestern Colombia. Based on regional scale tectonic reconstructions and geochemical constraints, such rocks have been interpreted as remnants of an oceanic large igneous province formed in southern latitudes, which was accreted to the sialic continental margin during the Late Cretaceous. Gravity analyses suggest the existence of a coherent high density segment separated by major suture zones from a lower density material related to the continental crust and/or thick sedimentary sequences trapped during collision. A characteristic paleomagnetic direction in Early and Late Cretaceous oceanic volcano-plutonic rocks, revealing a southeastern declination (D) and a negative inclination (I), may be interpreted in two different ways: (1a primary magnetization (tilt-corrected direction D = 130.3°, I = -23.3°, k = 23.4, α₉₅ = 26.4°), suggesting clockwise rotation around 130°, and magnetization acquired in southern latitudes (range of 4°S to 21°S); or (2) a remagnetization event during a reverse interval of the Earth’s magnetic field in the Cenozoic (in situ direction D = 128.7°, I = -6.2°, k = 23.1, α₉₅ = 26.1°), suggesting a counter-clockwise rotation around 50°. The first scenario seems more plausible, as it is consistent with previous paleomagnetic studies at other localities; it is compatible with a southern paleogeography for this block, and when integrated with other regional geological and paleomagnetic studies, supports a southern Pacific origin of a major oceanic block, formed as a part of a broader Cretaceous plateau that may have extended south or southwest of Galapagos. After its initial accretion, this block was subsequently fragmented due to the oblique SW-NE approach to the continental margin during the Late Cretaceous.     

Palaeomagnetism of the Early Carboniferous Deer Lake Group,western Newfoundland: no evidence for mid-Carboniferous displacement of “Acadia”

Early Carboniferous (Viséan to possibly earliest Namurian) sedimentary rocks of the Deer Lake Group of western Newfoundland rest unconformably on Grenvillian basement rocks of the Canadian Shield which form the western border of the Early Palaeozoic Appalachian orogen. In addition to magnetically soft magnetizations directed along the present field, three families of magnetization directions are found. Two of them (referred to as N (north) and S (south)) are antiparallel and prefolding, and were probably acquired during the depositional process. N and S are roughly of equal frequency. They have a mean direction irrespective of sign of 0.7°, ?35.2°, k = 40, α₉₅ = 8.9°, and a palaeopole 21.5°N, 121.8°E (10.3°, 6.0°) corresponding to a palaeolatitude of 20 ± 6°S. This agrees with the palaeolatitude (17 ± 5°S) determined from the somewhat older Early Carboniferous (Tournaisian) Terenceville Formation of the Avalon Platform on the eastern side of the Appalachian orogen in Newfoundland. The third magnetization, referred to as H (roughly horizontal), has a mean direction 156.8°, ?13.3°, k = 37, α₉₅ = 10.1°, and a palaeopole 45.4°N, 140.3°E (10.3°, 5.3°) corresponding to a palaeolatitude of 7 ± 4°N; we interpret this to be an early Kiaman (latest Carboniferous to early Permian) overprint probably acquired chemically. The palaeolatitude determined from the H overprint agrees with that determined from Early Carboniferous rocks of cratonic North America west of the Appalachians. Therefore, we argue, Early Carboniferous palaeofield determinations for cratonic North America have been strongly biased by unremoved Kiaman overprints. Because of this, and because of the good agreement between Early Carboniferous palaeolatitudes obtained from opposite margins of the Appalachian orogen, we suggest that there is, at present, no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the Appalachians (“Acadia”) relative to cratonic North America during the Carboniferous.     

Palaeomagnetism and palaeogeography of Mongolia from the Carboniferous to the Cretaceous—final report

The present study summarizes the results of palaeomagnetic investigations of Carboniferous to Cretaceous rocks with respect to global-tectonic interpretations of Eurasia. Normal and reverse directions of stable remanence components were found for the Carboniferous to Cretaceous rocks within single outcrops, sometimes even within individual samples. Triassic, Permian and Carboniferous pole positions for Mongolia are strikingly different from those of the Siberian platform. The conclusion about the different palaeogeographical development of Mongolia with respect to the Siberian platform is well seen from differences in palaeolatitudes (up to 50°) and palaeorotations (up to 150°). The apparent polar wander paths for Mongolia and the north China block are almost identical since the Carboniferous. This way, palaeomagnetic and palaeogeographical data are contributing to the global-tectonics model of the regions under study. During the Carboniferous, Permian and Triassic, the regions of Mongolia investigated were not part of Eurasia. Together with the north China block, they show a palaeogeographical affinity with the Pacific Plate.     

New Cambrian paleomagnetic pole for Yangtze Block

A total of 120 samples from 12 sites were collected from two flanks of a fold. Stepwise thermal demagnetization has successfully revealed characteristic magnetization components from the rocks in each case. A well-defined component determined from red fine-grained sandstone is clustered in the northeasterly direction with shallow upward inclination (D = 29.3°,I= -19.2°,k = 283.7, α₉₅ = 7.3°. tilt-corrected). The pole position (39.5°N, 247.3°E,dp = 4.0°,dm = 7.6°) derived from this component is close to the Permian pole for the Yangtze Block, indicating that the red fine-grained sandstone has been overprinted. The red mudstone reveals two characteristic components Component A with lower unblocking temperature, characterized by northerly declination and moderate to steep inclination corresponds to a pole position overlay with the present North Pole. Component B (D = 129.1°,I=-23.6°,k = 44.6, α₉₅ = 7.8°, tilt-corrected) with higher unblocking temperature, passes fold test, and yields a pole position (39.5°S, 185.l°E,dp = 4.4°,dm = 8.3°) different from the other poles for the Yangtze Block. It is therefore suggested that component B was probably a primary magnetization and the Yangtze Block was situated at low latitudes in the Southern Hemisphere in the Middle Cambrian.     

Afternoon Pc5 geomagnetic pulsations on the Earth’s surface and in the ionosphere (STARE radars)

Two cases when Pc5 geomagnetic pulsations were registered at the IMAGE Scandinavian network of stations and with STARE radars in the afternoon sector (1700–1800 MLT) during the recovery phase of the moderate magnetic storm are analyzed in detail. Using the ground-based observations, it has been indicated that classical quasimonochromatic resonance Pc5 pulsations were observed in the first case (on October 12, 1999; Kp = 5); in this case the maximal amplitude of the spectral maximum at a frequency of 2.5 mHz was registered at Φ ～ 65°. Two maximums were observed in the spectrum in the second case (on October 13, 1999; Kp = 4): ～2.5 mHz (the same maximum) and 2.9 mHz; in this case the maximal oscillation amplitude (2.5 mHz) shifted to Φ > 67°. These results were compared with the echo signal intensity simultaneously registered with the STARE Finland radar on a beam oriented along the 105° geomagnetic meridian. The spatial-temporal maps of the Pc5 pulsation amplitude latitudinal distribution (“keograms”), constructed based on the radar measurements in the wide range of geomagnetic latitudes (63°–70°) where the resolution was substantially higher than that of the ground-based observations, made it possible to detect two regions spaced in latitude (Φ ～ 65° and Φ ～ 67°–68°) with the simultaneous excitation of oscillations (double resonance?), between which the plasmapause projection was supposedly located.     

Magnetic anomalies — India and Antarctica

Magnetic anomalies over the continental shelf off the east coast of India (Orissa) suggest the presence of a highly magnetic rock type magnetized with an intensity of 900 nT in a direction, azimuth(A) = 150° and inclination(I) = +65°. This suggest the occurrence of igneous volcanic rocks which is confirmed from samples found below Tertiary sediments from a few boreholes in this region. The depth of this rock type as estimated from magnetic anomalies varies from approximately 1–2 km near the coast to 4–4.5 km towards the shelf margin. This direction of magnetization is the reverse of the reported direction of magnetization for the Rajmahal Traps of the Cretaceous period (100–110 m.y). A small strip of the body near the continental shelf margin appears, however, to possess normal magnetization suggesting the occurrence of normal and reversed polarities side by side, a characteristic typical for oceanic magnetic anomalies. The reversed polarity of the rocks on the continental shelf suggests that they correspond probably to the MO reversal (115 m.y.) on world magnetostratigraphic scale and provide a paleolatitude of 47°S for the land mass of India which agrees with the palaeoreconstruction of India and Antarctica. In this reconstruction, the Mahanadi Gondwana graben on the Indian subcontinent falls into line with the Lambert Rift in Antarctica, suggesting a probable common ancestry. The volcanic rocks on the continental shelf off the east coast of India might represent a missing link, that is, rocks formed between India and Antarctica at the time of the break-up of Gondwanaland. Satellite magnetic anomalies (MAGSAT) recorded over the Indian shield and interpreted in terms of variations in the Curie point geotherm provide a direction of magnetization which also places this continent close to Antarctica. As such MAGSAT anomalies recorded over eastern Antarctica are found compatible with those recorded over the Indian shield.     

Paleomagnetic results from Permian rocks of the northern Saharan craton and motions of the Moroccan Meseta and Pangea

The characteristic magnetization of redbed samples from the upper part of the Série d'Abadla (probably Early Permian 31°N, 2.7°W) has a mean direction derived from 13 sites of D=129°, I=11°, k=59, α₉₅=6° and a corresponding south paleopole at 29°S, 60°E, A₉₅=5°. All directions have reversed polarity. The paleolatitude of the northern fringe of the Saharan craton was 6°±3°S, which is in excellent agreement with that for the Moroccan Meseta. Therefore, in all probability, there has been no paleolatitudinal displacement greater than about 500 km of the Moroccan Meseta relative to Africa since Permian time. Comparison of results from sedimentary rocks shows no evidence for relative rotation of the Moroccan Meseta since Permian time. Small apparent rotations are indicated by evidence from massive trachyandesite lavas from Morocco, but we argue that these could have arisen from the incomplete averaging of secular variation and uncertainties in estimates of paleohorizontal, rather than from true tectonic rotations. The combined latest Carboniferous/Early Permian paleopole for the Saharan craton and the Meseta differs form the path of apparent polar wandering for North America when the continents are assembled in Wegener's Pangea (Pangea A, in which northwest Africa is opposite North America). It is in reasonable agreement when the continents are assembled in the Pangea B configuration (northwest Africa opposite Europe).     

Paleomagnetic study on orogenic belt: An example from Early Cretaceous volcanic rocks,Inner Mongolia,China

Since the 1990s, a large number of paleomagneticstudies have been carried out in the North China block(NCB) and Tarim block[1-8], and more and more geo-physicists recently believe that the last collision andconvergence between Siberia and the Mongolia-NorthChina plate happened in the Late Jurassic, which wascontributed to a paleomagnetic study on these areas byZhao and his colleagues[2]. However, we lack paleo-magnetic results obtained directly from the orogenicbelt between Siberia and th…     

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).     

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO₂ shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial ¹⁴³Nd/¹⁴⁴Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNd_t (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.