Paleomagnetism of paleozoic sediments from the Kozhim River section: On the problem of palinspastic reconstructions of the Subpolar Urals and Pai-Khoi

The collections of the Silurian, Devonian, Carboniferous, and Early Permian rocks from the Kozhim River section in the Subpolar Urals are studied. It is established that the pre-Permian Paleozoic deposits from the Kozhim River were remagnetized during the Kiama hyperchron (C₂-P₂) probably due to magnetoviscous processes that were blocked as these sediments were leaving the zone of increased temperature during the collision between the East European Platform and the Urals. The thrusts that took place at the last stage of this collision rendered the paleomagnetic directions of the studied structures different from those extrapolated from the East European Platform. The estimates of local rotations and displacements are obtained and applied in the combined analysis with the paleomagnetic determinations for the Pai-Khoi Ridge and Subpolar Urals. The amplitudes of the thrusts average 260 km and 180 km for the structures of Pai-Khoi and Subpolar Urals, respectively. The reconstruction of the prethrusting layout of the studied structures is presented.     

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     

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.     

鄂尔多斯地块构造演化的古地磁学研究

鄂尔多斯地块与中朝地台其它地区相同时代地层的古地磁结果基本一致表明:晚二叠世以来,中朝地台经历了从低纬度(19°左右)向中纬度的北移过程,并伴有50°左右的逆时针旋转;晚二叠世—中三叠世地台北移10°(1000km)左右,而方位基本未变;中三叠世—中侏罗世主要发生50°左右的逆时针旋转,而北向位移不明显,这一旋转可能与杨子地台和中朝地台碰撞拼合有关,或者说是印支运动在该地区的反应,中侏罗世—早白垩世地块已基本和现代位置一致     

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.     

The absolute paleoposition of the North China Block during the Middle Ordovician

Present-day hot spots and Phanerozoic large igneous provinces(LIPs) and kimberlites mainly occur at the edges of the projections of Large Low Shear Wave Velocity Provinces(LLSVPs) on the earth's surface. If a plate contains accurately dated LIPs or kimberlites, it is possible to obtain the absolute paleoposition of the plate from the LIP/kimberlite and paleomagnetic data. The presence of Middle Ordovician kimberlites in the North China Block provides an opportunity to determine the absolute paleoposition of the block during the Middle Ordovician. In addition to paleobiogeographical information and the results of previous work on global plate reconstruction for the Ordovician Period, we selected published paleomagnetic data for the North China Block during the Middle Ordovician and determined the most reasonable absolute paleoposition of the North China Block during the Middle Ordovician: paleolatitude of approximately 16.6°S to 19.1°S and paleolongitude of approximately 10°W. The block was located between the Siberian Plate and Gondwana, close to the Siberian Plate. During the Cambrian and Ordovician periods, the North China Block may have moved toward the Siberian Plate and away from the Australian Plate.     

Paleomagnetism and magnetic fabrics of Mio-Pliocene hypabyssal rocks of the Combia event,Colombia: tectonic implications

Mio-Pliocene hypabyssal rocks of the Combia event in the Amagá basin (NW Andes-Colombia), contain a deformational record of the activity of the Cauca-Romeral fault system, and the interaction of terranes within the Choco and northern Andean blocks. Previous paleomagnetic studies interpreted coherent counterclockwise rotations and noncoherent modes of rotation about horizontal axes for the Combia intrusives. However, rotations were determined from in-situ paleomagnetic directions and the existing data set is small. In order to better understand the deformational features of these rocks, we collected new paleomagnetic, structural, petrographic and magnetic fabric data from well exposed hypabyssal rocks of the Combia event. The magnetizations of these rocks are controlled by a low-coercivity ferromagnetic phase. Samples respond well to alternatingfield demagnetization isolating a magnetization component of moderate coercivity. These rocks do not have ductile deformation features. Anisotropy of magnetic susceptibility and morphotectonic analysis indicate that rotation about horizontal axes is consistently to the south-east, suggesting the need to apply a structural correction to the paleomagnetic data. The relationships between magnetic foliations and host-rock bedding planes indicate tectonic activity initiated before ~10 Ma. We present a mean paleomagnetic direction (declination D = 342.8°, inclination I = 12.1°, 95% confidence interval α₉₅ = 12.5°, precision parameter k = 8.6, number of specimens n = 18) that incorporates structural corrections. The dispersion S = 27° of site means cannot be explained by secular variation alone, but it indicates a counterclockwise rotation of 14.8° ± 12.7° relative to stable South America. Paleomagnetic data within a block bounded by the Sabanalarga and Cascajosa faults forms a more coherent data set (D = 336.5°, I = 17.4°, α₉₅ = 11.7°, k = 12.5, n = 14), which differs from sites west of the Sabanalarga fault and shows a rotation about a vertical axis of 20.2° ± 10.7°. Deformation in the Amagá basin may be tentatively explained by the obduction of the Cañas Gordas terrane over the northwestern margin of the northern Andean block. However, it can also be related to the local effects of the Cauca-Romeral fault system.     

Paleomagnetism of Carboniferous-Permian and early jurassic geological complexes in Mongolia

Paleomagnetic characteristics of Carboniferous-Permian and Early Mesozoic geological complexes in Mongolia are studied. The studied rocks are shown to possess a multicomponent magnetization. Lowtemperature overprinting components of normal polarity discovered in nearly all of the studied strata were acquired after main deformation stages of the rocks, apparently in the Cenozoic. High-temperature overprinting components of reversed polarity identified in rocks of an active continental margin (ACM) were acquired when bimodal magma melts moved through ACM volcanic sequences. Late Carboniferous and Early Permian paleomagnetic poles of Mongolia calculated from directions of primary magnetization components are, respectively (Λ = 154.6, Φ = 32.2, A = 7.8) and (Λ = 95, Φ = 71, A = 8.7). Apparently, the territory of Mongolia in the Early Permian was a margin of the Siberian craton and was separated from the Northern China block by a basin extending for no less than 2000 km in the E-W direction. The strike of a marginal-continental volcanic belt was submeridional and a plate subducted under the continent from the east. Late Carboniferous-Permian intraplate magmatic complexes of Mongolia formed at various latitudes from various mantle sources during the northward movement of the Mongolian part of the Siberian continent. The oldest bimodal sequences of the Gobi-Tien Shan zone (318–314 Ma) formed at more southern latitudes (40°–47°–54°N) as compared with the 275-Ma complexes of the Gobi-Altai zone (51°–58°–67°N). Thus, sources of the Carboniferous-Permian intraplate magmatism in Central Asia either occupied a vast mantle region (up to 1000 km in the latitude direction) or moved together with the Asian continent.     

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.     

Paleomagnetic study of Middle-Upper Jurassic sediments from the Polish segment of the Pieniny Klippen Belt

Middle-Upper Jurassic carbonates exposed in 7 separate fragments of three tectonic units were sampled in the Polish segment of the Pieniny Klippen Belt in 1981, before construction of the Niedzica-Czorsztyn dam. Demagnetization experiments performed about 20 years ago and analyzed lately with modern program package revealed the presence of Middle Miocene overprints acquired in the normal and reversed geomagnetic field, and pre-folding components of natural remanence acquired during the Middle-Upper Jurassic. A paleolatitude of 36N±7° was obtained for the studied area for this time on the basis of 3 normal and 1 reversed components. Three pre-folding pole positions are situated between pole positions of the normal and reversed Stable European Middle Jurassic field. The obtained paleomagnetic results imply several prefolding (pre-Upper Miocene) and postfolding (post-Upper Miocene) episodes of rotations of the studied fragments.     

Paleomagnetic evidence for the clockwise rotation of Southwest Japan

Over 500 oriented samples of felsic rocks of Cretaceous to Middle Miocene age were collected along the Go¯River in the central part of Southwest Japan, in an attempt to detect the process of tectonic rotation of Southwest Japan from the paleomagnetic view point. Thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples. Reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities. The paleomagnetic results establish that Southwest Japan began to rotate clockwise through58 ± 14° later than 28 Ma and ceased its motion by about 12 Ma. Southwest Japan has undergone no detectable north-south translation since 28 Ma. These results imply that southwest Japan was rotated about the pivot around 34°N, 129°E between 28 Ma and 12 Ma in association with the opening of the Japan Sea.     

Paleomagnetic and rock-magnetic survey of eocene dike swarms from the Tecalitlan area (Western Mexico): Tectonic implications

For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacán block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacán block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α₉₅ = 8.9°. The corresponding mean paleomagnetic pole position is P_lat = 75.7°, P_long = 166.6°, K = 31, A₉₅ = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacán Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence.     

Paleomagnetism of red beds of Early Devonian age from Central Iran

Paleomagnetic results are reported from 13 sites of red beds of Early Devonian age from Central Iran. Detailed paleomagnetic analyses were carried out. Two types of partial progressive demagnetization were applied, one using alternating magnetic fields, the other heating. These procedures resulted in the detection of the characteristic remanences with a mean direction with D = 24.2°, I = 1.3° (α₉₅ = 10.1°). The paleomagnetic pole is located at 51.3°N, 163.7°W. If one shifts the Iranian landmass to its most likely position in the Gondwana configuration, then the position of the paleomagnetic pole coincides with the alternative polar wander path [14,15] which crossed South America in early Middle Paleozoic times.     

Paleomagnetic evidence for the relative positions of Timor and Australia in the Permian

An Upper Permian paleomagnetic pole has been determined for the Cribas Formation in eastern Timor. The co-ordinates for the mean pole are 159.8°E and 56.6°S,α₉₅ = 9.0. The reliability of the pole is ascertained through thermal demagnetization, a fold test, comparison between red beds and a lava flow, and the presence of normal and reversed polarities. The Timor pole is in excellent agreement with the Australian Upper Permian and Triassic poles. From this it is inferred that autochthonous Timor formed part of the Australian continental margin at least since the Upper Permian.     

Inclination shallowing in the Permian/Triassic boundary sedimentary sections of the Middle Volga region in light of the new paleomagnetic data

One of the key challenges which are traditionally encountered in studying the paleomagnetism of terrigenous sedimentary strata is the necessity to allow for the effect of shallowing of paleomagnetic inclinations which takes place under the compaction of the sediment at the early stages of diagenesis and most clearly manifests itself in the case of midlatitude sedimentation. Traditionally, estimating the coefficient of inclination flattening (f) implies routine re-deposition experiments and studying their magnetic anisotropy (Kodama, 2012), which is not possible in every standard paleomagnetic laboratory. The Elongation–Inclination (E–I) statistical method for estimating the coefficient of inclination shallowing, which was recently suggested in (Tauxe and Kent, 2004), does not require the investigation of the rock material in a specially equipped laboratory but toughens the requirements on the paleomagnetic data and, primarily, regarding the volume of the data, which significantly restricts the possibilities of the post factum estimation and correction for inclination shallowing. In this work, we present the results of the paleomagnetic reinvestigation of the Puchezh and Zhukov ravine (ravine) reference sections of the Upper Permian and Lower Triassic rocks in the Middle Volga region. The obtained paleomagnetic data allowed us to estimate the coefficient of inclination shallowing f by the E–I method: for both sections, it is f = 0.9. This method was also used by us for the paleomagnetic data that were previously obtained for the Permian–Triassic rocks of the Monastyrskii ravine (Monastirskoje) section (Gialanella et al., 1997), where the inclination shallowing coefficient was estimated at f = 0.6.     

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.     

塔里木地块侏罗、白垩纪的古地磁

本文对塔里木地块西北缘库车、拜城一带中新生代剖面进行了古地磁研究。库车与拜城两剖面具有不同方向产状,经产状校正之后,均为同一方向,表明磁性是在第三系褶皱之前获得的。热退磁结果表明500℃之前为现代地磁场方向,解阻温度为675℃,说明磁性载体为赤铁矿。 古地磁结果表明,塔里木地块在晚侏罗—晚白垩世之间没有经历大规模的构造运动。有可能自晚白垩世之后相对西伯利亚地块向北东方向移动过     

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.     

Paleomagnetic constraints on the tectonic history of the major blocks of China duing the Phanerozoic

Paleomagnetic study of China and its constraints on Asia tectonics has been a hot spot. Some new paleomagnetic data from three major blocks of China. North China Block (NCB), Yangtze Block (YZB) and Tarim Block (TRM) are first reported, and then available published Phanerozoic paleomagnetic poles from these blocks with the goal of placing constraints on the drift history and paleocontinental reconstruction are critically reviewed. It was found that all three major blocks were located at the mid-low latitude in the Southern Hemisphere during the Early Paleozoic. The NCB was probably independent in terms of dynamics. its drift history was dominant by latitudinal placement accompanying rotation in the Early Paleozoic. The YZB was close to Gondwanaland in Cambrian, and separated from Gondwanaland during the Late-Middle Ordovician. The TRM was part of Gondwanaland, and might be close to the YZB and Australia in the Early Paleozoic. Paleomagnetic data show that the TRM was separated from Gondwanaland during the Late-Middle Ordovician, and then drifted northward. The TRM was sutured to Siberia and Kazakstan blocks during the Permian, however, the composite Mongolia-NCB block did not collide with Siberia till Late Jurassic. During Late Permian to Late Triassic, the NCB and YZB were characterized by northern latitudinal placement and rotation on the pivot in the Dabie area. The NCB and YZB collided first in the eastern part where they were located at northern latitude of about 6°—8°, and a triangular oceanic basin remained in the Late Permian. The suturing zone was located at northern latitude of 25° where the two blocks collided at the western part in the Late Triassic. The collision between the two blocks propagated westward after the YZB rotated about 70° relative to the NCB during the Late Permian to Middle Jurassic. Then two blocks were northward drifting (about 5°) together with relative rotating and crust shortening. It was such scissors-like collision procedure that produced intensive compression in the eastern part of suturing zone between the NCB and YZB, in which continental crust subducted into the upper mantle in the Late Permian, and then the ultrahigh-pressure rocks extruded in the Late Triassic. Paleomagnetic data also indicate that three major blocks have been together clockwise rotating about 20° relative to present-day rotation axis since the Late Jurassic. It was proposed that Lahsa Block and India subcontinent successively northward subducted and collided with Eurasia or collision between Pacific/Philippines plates and Eurasia might be responsible for this clockwise rotating of Chinese continent.