Palaeomagnetism of Neoproterozoic glacial rocks of the Huabei Shield: the North China Block in Gondwana
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The Late Paleozoic-Early Mesozoic apparent polar wander path of Gondwana is largely constructed from relatively old paleomagnetic results, many of which are considered unreliable by modern standards.Paleomagnetic results derived from sedimentary sequences, which are generally poorly dated and prone to inclination shallowing, are especially common.Here we report the results of a joint paleomagneticgeochronologic study of a volcanic complex in central Argentina.U-Pb dating of zircons has yielded a robust age estimate of 263.0 +1.6/-2.0Ma for the complex.Paleomagnetic analysis has revealed a pretilting (primary Permian) magnetization with dual polarities.Rock magnetic experiments have identified pseudosingle domain (titano)magnetite and hematite as the mineralogic carriers of the magnetization.Lightninginduced isothermal remagnetizations are widespread in the low-coercivity magnetic carriers.The resulting paleomagnetic pole is 80.1°S, 349.0°E,A 95 = 3.3°, N = 35, and it improves a Late Permian mean pole calculated from a filtered South American paleomagnetic data set.More broadly, this new, high-quality, igneous-based
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Apparent polar wander
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The paper summarized the studies on Tarim paleomagnetism. Based on the paleomagnetic data worked for last ten years, an apparent polar wandering path was presented. There are three major motions and rotations of the Tarim block since early Paleozoic, which controls the tectonic evolution and oil and gas accumulation in Tarim plate. The author believed that compaction may be the reason for paleomagnetic inclination shallowing both in Cretaceous and Tertiary epochs.
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Paleomagnetic poles from the Upper Proterozoic Mackenzie Mountains supergroup (MMs) of northwestern Canada define an apparent polar wander path lying to the west of the Grenville Loop. This path is suggested from an analysis of the quartzitic Katherine Group, whose probable primary pole lies at the beginning of the sequence, near the younger end of the Grenville Track (0.88 Ga). The end of the apparent polar wander (APW) sequence may be defined by a primary pole from sills intruding the Tsezotene Formation below the Katherine. We relate the sills, dated at about 0.77 Ga, to the rifting event that led to "Copper cycle" and Rapitan sediments above the MMs, and we suggest that the exposed part of the MMs has an age between 0.88 and 0.77 Ga. The APW path is apparently not affected by rotations: pole evidence indicates little if any relative rotation between thrust sheets of the fold belt or between the fold belt and the craton.Paleomagnetic analysis of the Katherine Group data, obtained by alternating field, thermal, and chemical methods, revealed three magnetizations. The probable primary remanence, K A , carried by mainly detrital hematite grains, has a direction of D, I = 267°, +21 °(N = 13 specimens, k = 33, α 95 = 7°) and a pole at 9°N, 210°W (δ p , δ m = 4°, 8°). A secondary component, K B , carried by hematite pigment, has a direction of D, I = 258°, +42 °(N = 4 sites, k = 326, α 95 = 5°) and a pole at 17°N, 196°W (δ p , δ m = 4°, 6°). It documents further a pervasive overprint magnetization found in most MMs rocks. A similar hematite magnetization is probably primary in the overlying Copper cycle rocks. The youngest component, K C , is found partly in a second, probably largely post-folding pigment phase (post-Late Cretaceous or Paleocene) and has a direction of D, I = 007°, +84 °(N = 9 sites, k = 77, α 95 = 6°) and a pole at 77°N, 122°W (δ p , δ m = 11°, 12°).
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The decade from 1951 to 1961 witnessed the birth of a new geophysical subdicipline, paleomagnetism. Early studies in Europe, North America, and Australia led to the following conclusions: (1) rocks could preserve directions of magnetiziation for hundreds of millions of years in red beds, (2) late Cenozoic lavas had directions of magnetiziation that led to the conclusion that the mean geomagnetic field was a geocentric dipole aligned along the axis of rotation, (3) rocks of Triassic age and older yield directions which depart widely from the present axis of rotation, (4) if these directions are used to calculate pole positions, then poles for older and older rocks fall farther and farther from the present pole of rotation, (5) these data may be used to construct polar wander curves, (6) polar wander curves from different continents do not coincide with one another, (7) they may be reconciled if the continents move with respect to each other, and (8) the distribution of climatic indicators show that the pole of rotation of Earth and the paleomagnetic pole for the same periods coincide for Phanerozoic time. These observations changed the perspectives of many Earth scientists and paved the way for seafloor spreading and plate tectonics.
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Jurassic fast polar shift rejected by a new high-quality paleomagnetic pole from southwest Greenland
A selective compilation of paleomagnetic data from North America indicates that a vast amount of rapid polar motion occurred in Late Jurassic time. The over 30° polar shift that accumulated during a relatively short time interval (~160–145 Ma) suggests an episode of fast true polar wander (TPW) and was referred to as the Jurassic "monster polar shift" by some workers. However, this rapid TPW event is not supported by paleomagnetic data on a global scale. Here, we scrutinize the Jurassic apparent polar wander path (APWP) by virtue of a new paleomagnetic and 40Ar/39Ar geochronology study of Mesozoic coast-parallel dykes exposed in southwest Greenland. Combined with existing geochronological data, our results show that the dykes were emplaced during a prolonged period centered at 147.6 ± 3.4 Ma (2σ). A primary nature of the characteristic remanent magnetization is supported by multiple positive baked contact tests and a reversal test. The paleomagnetic pole calculated from 40 site-mean paleomagnetic directions is located at Plat = 69.3°S, Plong = 5.0°E (A95 = 4.6°), or at Plat = 73.9°S and Plong = 0.4°E when reconstructed to North America. Our new high-quality paleomagnetic pole and an updated global APWP do not support the fast Jurassic polar shift but instead indicate steady polar motion with moderate rates of about 0.7°/Myr. The new pole effectively reduces the mismatch between the APWPs for Laurentia and Europe. Our critical reassessment of the monster polar shift indicates that it may be an artifact of paleomagnetic and geochronological data that were previously used to argue for its existence.
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Palaeomagnetic studies have been carried out in a Stephano-Autunian formation of the Saharan craton. Despite the presence of a strong recent magnetic overprint, some samples give a well-defined characteristic remanent magnetization direction. Moreover, a new approach using the remagnetization circles has been used to confirm the significance of this direction. The obtained Stephano-Autunian pole (35.3°S, 60.3°E) allows a substantial improvement of the apparent polar wander path (APWP) for stable Africa. This APWP clearly shows that the clockwise rotation of Africa, which occured during the Hercynian orogeny, was stopped during the Stephano-Autunian period.
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