Paleomagnetic timing of Mesozoic Mongol-Okhtotsk Ocean closure

International audienceWe present reconstructions of the Mongol-Okhotsk (MO) Ocean, based on a review of paleomagnetic results from Transbaikalia and Amuria terranes. In the late Permian, localities situated on each side of the MO suture, at the present-day longitude of 110°E, exhibit a very large paleolatitudinal difference, which we interpret as an evidence for a widely open ocean, the width of which could reach 3500-5300 km. More surprisingly, this ocean might still be largely open in the middle-late Jurassic, with paleolatitude difference between Siberia south margin and Amuria north margin amounting to 26.2° 11.8°. This somewhat contradicts geological evidence such as the lack of any marine sediments of this age in Transbaikalia. In contrast, early Cretaceous paleomagnetic data from both sides of the suture at 110 -120 E reveal a paleolatitude consistency, implying a final closure of the ocean at 130 5 Ma, at an average speed of 13-15 cm.yr-1. We finally show that this timing of the MO closure history is corroborated by paleomagnetic results from more remote regions to the south, in North and South China blocks. Importantly, we underline that very short time delay is allowed between the end of converging plate margins episode and late Jurassic / early Cretaceous N-S intracontinental extension, as demonstrated by recent field studies in Mongolia and Transbaikalia. This rapid change in tectonic regime from converging to diverging continental plates motion could be linked to the jump of Izanagi plate kinematics at that time, preventing from convergence-induced high topography construction in the region, as suggested by some authors

Seismic and magnetic anisotropy of serpentinized ophiolite: Implications for oceanic spreading rate dependent anisotropy

International audienceCompressional and shear wave anisotropy, shear wave birefringence, and the anisotropy of magnetic susceptibility were measured on a series of dunites sampled from the Dinardic–Hellenic ophiolites. The densities of these materials ranged from 3330 kg/m3 to 2620 kg/m3 and indicate degrees of serpentinization from 2.3% to 87.9%, respectively. Magnetic susceptibility increases and the compressional and shear wave velocities decrease in proportion to the degree of serpentinization as has been observed by other workers. In all cases the magnetic susceptibility tensor is described by an oblate spheroid whose minor axis is closely aligned to the pole of the foliation, but the magnetic anisotropy is not related to the degree of serpentinization. The compressional wave anisotropy ε monotonically decays from 12% for nearly pure olivine dunite to less than 2% for the most serpentinized sample; this observation strongly suggests that serpentinization progressively destroys the original anisotropy by consuming the preferentially aligned olivines and replacing them with randomly oriented serpentines. Typical serpentine mesh textures seen in microscopic thin section examinations support this suggestion. This loss of anisotropy with serpentinization may partly explain the apparent relationship between seismic compressional wave anisotropy of the oceanic lithosphere. The more complex geological structure of slow spreading ridges may admit more sea water via faults for deep circulation which increases serpentinization and consequence of which is decreased seismic anisotropy

Preliminary dating of the Viluy traps : eruption at the time of Late Devonian extinction events ?

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Preliminary dating of the Viluy traps (Eastern Siberia) : eruption at the time of Late Denovian extinction events ?

International audienceIn this short note, we report new age determinations from four samples of the Middle-Paleozoic Viluy Traps in Siberia, east of the more famous Permo-Triassic Siberian Traps. These samples, which were collected from three drill cores, have been analyzed in parallel and independently in the Orsay (France) and Berkeley Geochronology Center (BGC; USA) geochronology laboratories, using respectively the Cassignol-Gillot K-Ar and the Ar-40/Ar-39 techniques. Dating these samples as a concerted effort in two independent laboratories working jointly on their interpretation is a rather rare yet very valuable exercise. With the K-Ar technique, ages ranging from 338 to 367 Ma with uncertainties on the order of 5 Ma were obtained. With the Ar-40/Ar-39 technique, integrated ages range from 344 to 367 Ma, with uncertainties on the order of 1 Ma, and two samples yielded plateaus, i.e. the best determined ages, at 360.3 +/- 0.9 and 370.0 +/- 0.7 Ma. Three out of four ages yielded by the two separate methods are in agreement within uncertainties. One sample yields incompatible ages and could be from a later, altered dyke event. The Ar-40/Ar-39 plateau age of 370.0 0.7 Ma (conventional calibration) or 373.4 +/- 0.7 Ma (recalculated per Renne et al., 2010), the most reliable age obtained in this study, is compatible with recent determinations of the Late Devonian extinction events at the end-Frasnian (similar to 376 +/- 3 Ma). These results underscore a need for further work, in progress. (C) 2010 Elsevier B.V. All rights reserved

Paleomagnetism and K/Ar dating of two Cretaceous and Tertiary localities from Mongolia : implications on the large scale deformation of Eurasia

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Palaeomagnetism and K–Ar dating of Cretaceous basalts from Mongolia

International audienceWe report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon (44.4°N, 103.8°E) and Arts-Bogd (44.3°N, 102.2°E) localities in the Gobi Desert, south Mongolia. K—Ar dating based on seven rock samples, with two independent measurements for each sample, allows us to propose an age of 94.7 ± 1.3 Ma for Shovon locality and a 98.2 ± 1.4 to 118.3 ± 1.7 Ma age range for Arts-Bogd. Stepwise thermal and AF demagnetization generally isolated a high temperature component (HTC) of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component (LTC) in some samples. The HTC directions display normal polarity, consistent with the Cretaceous Long Normal Superchron. Rock magnetic analysis identifies fine-grained pseudo-single domain (PSD) magnetite and titanomagnetite as primary carriers of the remanence. Mean HTC palaeomagnetic direction is Dm = 8.2°, Im = 63.7° (n = 18 flows, k = 41.1, α95 = 5.5°) for Shovon and Dm = 12.1°, Im = 66.4° (n = 27 flows, k = 53.0, α95 = 3.9°) for Arts-Bogd. Because of their similar ages, we combine data from Shovon and data previously obtained from Khurmen Uul (92.0 ± 4.0 Ma), recomputed in geographic coordinates, and not in tilt-corrected ones as in our previous interpretation, at the Shovon locality. The combined final average palaeomagnetic direction for Shovon-Khurmen Uul is Dm = 7.4°, Im = 62.7° (n = 23 flows, k = 41.4, α95 = 4.8°). The corresponding palaeopoles computed from these HTC lie at λ = 84.7°N, φ = 195.0°E, dp/dm = 5.8/7.5 for Shovon-Khurmen Uul (average age: 93.4 ± 2.6 Ma) and λ = 80.5°N, φ = 159.0°E, dp/dm = 5.2/6.3 for Arts-Bogd (average age: 104.6 ± 6.6 Ma). These poles are consistent with those from the European apparent polar wander path (APWP) at 90, 100 and 110 Ma, and other published pole from the Mongol-Okhotsk suture zone, Amuria and North China blocks. This confirms the lack of a discernable latitudinal motion between Amuria and Siberia since their final accretion by the Late Jurassic—Early Cretaceous, and reinforces the idea that Europe APWP can be used as a reference for Siberia by the mid-Cretaceous. We finally propose a mid-Cretaceous mean palaeomagnetic pole for the Siberia-Amuria-North China Block assemblage which lies at: λ = 86.4°N, φ = 191.1°E(n = 10, k = 74.9, A95 = 5.8°)