Northeast Asia is the greatest area in the world where continental extension is documented, from Transbaikalia to the Pacific coast. The major extensional events occurred during Late Jurassic - Early Cretaceous times and are marked by rift basins filled with continental sediments and volcanic rocks, metamorphic core complexes, and extensive magmatism. Here, we review the different types of extensional structures in the region using our own examples from eastern Mongolia, including new data from the Gobi area, and those previously documented in the adjacent regions. We further provide a compilation of available ages, including new data from Transbaikalia, to constrain crustal stretching throughout northeastern Asia. Our analysis shows that extension is not correlated to orogenic collapse, but is coeval with two major changes in plate kinematics that occurred at ca. 140 Ma: the closure of the Mongol-Okhotsk Ocean and a major change in the subduction direction of the Izanagi oceanic plate towards northeast Asia, from high-angle to low-angle convergence. These major changes in plate kinematics combined with an abnormally hot pre-extension state of the continental lithosphere might have triggered spreading in northeast Asia.

The Aar Massif forms part of the polycyclic basement of the External Crystalline Massifs in central Switzerland. Strong heterogeneous Alpine deformation produced a network of broad, anastomosing shear zones, with deformation strongly localized in mylonitic domains. This study investigates the combined effects of high-strain deformation and synkinematic metamorphism on magnetic fabric evolution in Tertiary shear zones of the Aar granite and Grimsel granodiorite. In transects across several mesoscale shear zones with large strain gradients, magnetic fabric orientations are in excellent agreement with principal strain orientations determined from outcrop fabrics and strain markers. However, the magnitude and shape of the magnetic anisotropy do not change systematically with increasing finite strain, likely as a result of recrystallization and metamorphism. The overall pattern of steeply dipping fabrics is consistent with the main shortening stage of regional Alpine kinematics, while some mylonite structures reflect a local component of dextral shearing.

The widely distributed Cenozoic paleomagnetic inclination anomaly in Asia is usually attributed to either a widespread error of magnetic field recording due to an inclination flattening mechanism in sediments, or to the persistence of an anomalous non-dipolar component of the geomagnetic field throughout the Tertiary. Based on an analysis of the Asian paleomagnetic database for Meso-Cenozoic times, we suggest that instead this puzzling anomaly results from an overlooked global plate tectonics cause where the wide so-called Eurasian plate would have suffered from previously undetected transpressive north–south relative movements between its western and eastern ends since the Cretaceous. These relative movements are most probably accommodated by a component of right-lateral shear movement distributed in the Tornquist–Tesseyre zone, and a localized left-lateral shear movement in the Ural Mountain chain during the Tertiary. Therefore, Eurasia was not the single rigid plate that Cenozoic plate reconstructions have accepted.

Based on a compilation of 533 Cretaceous to present-day palaeomagnetic poles obtained from both sedimentary and igneous rocks, we present a new analysis of the so-called 'Asian inclination anomaly' and demonstrated the anomaly to be twofold: a 2nd-order anomaly, characterized by high palaeolatitudes in Indochina and low palaeolatitudes over Tibet and Central Asia, is superimposed on a 1st-order anomaly, characterized by Cenozoic low palaeolatitudes found all over northeastern Asian stable blocks. The analysis herein convincingly shows that the Europe Apparent Polar Wandering Path (APWP) can no longer be used to interpret palaeomagnetic data East of the Urals, including interpretation of Asian Tertiary deformation related to the India-Asia Collision. We, thus, construct a new APWP for East Asia, based on palaeopoles from blocks assumed to be stable. This new APWP is consistent with and reinforces previous analyses of Asian tectonics, such as the age (similar to 55 Ma) and locus (similar to 5-10 degrees N) of the Indo-Asian collision, the lateral extrusion of SE Asian continental blocks and the intracontinental shortening in Central Asia. Possible origins of the 1st-order palaeolatitude anomaly are: (1) a geomagnetic origin, due to long-lasting non-dipolar contribution to the magnetic field and (2) a tectonic hypothesis, in which a newly defined East Asia Plate was located similar to 10 degrees farther south than expected from the current Europe APWP. Based on a set of six new reconstructions from 90 Ma to present, we show that our tectonic model reconciles geophysical, geological and tectonic observations throughout Eurasia, from Siberia to Europe, including kinematics in the Arctic Ocean, up to northwestern Arctic Alaska. Beyond possible occurrences of non-dipolar field contribution and/or local inclination flattening in the sedimentary data, our model leads us to conclude that Cenozoic tectonics is the dominant contributor to the observed 1st-order similar to 10 degrees low palaeolatitude anomaly over Asia during the Tertiary.

Northwestern China belts result from the Palaeozoic collage of Central Asia and the subsequent reactivations due to far-field effects of the Mesozoic Tibetan and the Cenozoic Himalayan collisions. Triassic is a crucial period to understand and decipher the tectonics related to these two episodes. About 250 oriented palaeomagnetic cores from 43 sites were collected from six sections of Upper Permian to Late Triassic sandstone, in South and West Junggar, Northwestern China. Thermomagnetic, IRM and hysteresis measurements reveal magnetite as the main carrier of the magnetic remanence with minor hematite and maghemite. Stepwise thermal demagnetisation has generally isolated two components. The low temperature component, up to 300-350 degrees C, displays a direction consistent with the present-day geomagnetic field. The locality-mean directions related to the high temperature component (above 350 degrees C) were also calculated. Two out of six sections display intense viscous magnetisation and the occurrence of maghemite reveals a possible Cenozoic chemical remagnetisation for these two localities. For the other four localities, we assume that the magnetisation is primary because: (1) AMS measurements reveal a primary fabric, (2) there are local occurrences of antipodal polarities, and (3) palaeolatitudes of tilt-corrected poles are compatible with previous studies. The consistency between the Early Triassic poles of West and South Junggar indicates that Junggar evolved as a rigid block only since Early Mesozoic. The comparison of the Late Palaeozoic and the Early Mesozoic poles of Junggar and those of Siberia and Tarim shows major rotations between the Late Permian and the Late Jurassic-Early Cretaceous. These periods of discrete rotations are characterized by strike-slip faulting in Tianshan and Altai and they may correlate with the major episodes of coarse-grained detrital sedimentation and uplift of the range. Especially, the counter-clockwise rotations of Junggar relative to Tarim and Siberia, which occurred between the Early and the Late Triassic and between the Late Triassic and the Late Jurassic, are accommodated by transpressive tectonics in the Tianshan and the Altai belts. This reactivation is a far-field effect of Tibetan blocks diachronous collisions. Therefore, these first Triassic palaeomagnetic results from Junggar infer that post-Carboniferous rotations were due to the combined effect of the post-orogenic transcurrent movement and the Mesozoic oblique reactivation. (C) 2013 Elsevier Ltd. All rights reserved.

We have analyzed four sediment cores from the Southern Indian Ocean (ODP sites 757, 758, 1135 and 762) with high carbonate content, in order to reconstruct the neodymium isotopic composition (εNd) of ancient intermediate South Indian seawater from Late Cretaceous (90 Ma) to Early Eocene (40 Ma). The εNdvariations are highly consistent and exhibit reproducible patterns over a very large geographic area, confirming the seawater origin of the signal. Combining geochemical constraints with paleogeographic reconstructions, we highlight the respective roles of (1) large‐scale tectonic events, (2) continental weathering from surrounding Precambrian terrains (90–65 Ma), (3) oceanic circulation changes (50–40 Ma) and, possibly, (4) local volcanism of the ultra‐fast spreading South East Indian Ridge (SEIR) (60–50 Ma) on the Nd isotopic composition of South Indian seawater. Between 60 Ma and 50 Ma, the regional Nd isotopic variations closely mimic changes in SEIR spreading rate. We suggest that the Nd isotopic composition of seawater could be influenced by Nd of volcanic origin in the vicinity of ultra‐fast spreading ridges (>13 cm/yr). The India‐Asia collision closed the Equatorial Seaway between Asia and India and drastically changed oceanic circulation patterns in the Indian Ocean: warm and more radiogenic Pacific equatorial seawater was diverted to the South by the East Indian coast. A stronger mixing of this Pacific seawater with South Indian seawater would explain the rapid shift ofεNd from 50 Ma (−11) to 40 Ma (−8).

New paleomagnetic data from Permian red beds and Middle Jurassic limestones from the Tarim Basin pose a paradox. Their declinations are similar to Upper Carboniferous to Neogene rocks collected from the same sections, and their inclinations parallel present values. When assuming that lower than expected inclinations in continental sedimentary rocks arise from inclination shallowing effects, then the paleolatitudes of all Upper Carboniferous to Present rocks from Tarim are indistinguishable. Local vertical axis block rotations occurring in the last 20 million years explain why declinations vary at different localities in the basin. Our Middle Jurassic data positions Tarim 23.6 ± 8.4° farther south than that predicted from the coeval reference pole for Eurasia; however, no geologic argument exists to support the closure of a large ocean basin between Tarim and Siberia since the Middle Jurassic. Thus the paradox: are the rocks from Tarim totally overprinted, or is the middle Mesozoic part of the reference Eurasian apparent polar wander path erroneous? Several lines of evidence suggest the Tarim rocks are not remagnetized. We conclude that Tarim has experienced little or no apparent polar wander since the Carboniferous. Moreover, our Middle Mesozoic reconstruction of Eurasia using the new Middle Jurassic pole from Tarim results in a more geologically compatible solution for the eastern Asian blocks over previous reconstructions.

We propose to characterize land-ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian. (C) 2008 Elsevier B.V. All rights reserved.

We 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 D 8.2(-), Im D 63.7(-) (n D 8 flows, k D 41.1, fi(95) D 5.5(-)) for Shovon and Dm D 12.1(-), Im D 66.4(-) (n D 27 flows, k D 53.0, fi(95) D 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 D 23 flows, k D 41.4, fi(95) = 4.8(-)). The corresponding palaeopoles computed from these HTC lie at , D 84.7(-)N, ' D 195.0(-)E, dp/dm D 5.8=7.5 for Shovon-Khurmen Uul (average age: 93.4 2.6 Ma) and , D 80.5(-)N, ' D 159.0(-)E, dp/dm D 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: D = 86.4(-)N, D = 191.1(-)E(n D 10, k D 74.9, A(95 D) 5.8(-)).

Fold axis strikes in the Yangtze fold belt of the South China Block (SCB) undergo significant changes over distances of > 1000 km. This large-scale variation provides an ideal opportunity to test the oroclinal-bending hypothesis using palaeomagnetic methods, which we have attempted by drilling the Lower Triassic Daye Formation limestones in western Hubei Province. Thermal demagnetization isolated two components in most samples. A low unblocking temperature component (< 350 degrees C) that does not decay to the origin is interpreted as a drilling induced magnetization. The second, high temperature component (HTC), isolated at temperatures > 400 degrees C, unblocks univectorially towards the origin. The HTC passes the McFadden-fold test with an overall mean tilt-corrected direction of Dec = 255 degrees, Inc = -24 degrees (N = 7, alpha(95) = 9 degrees). Scanning electron microscopy (SEM) observations suggest an early diagenetic, possible (bio)chemical origin for the magnetic extracts dominated by Ti-poor magnetite. Rock magnetic data show no evidence that the HTC has been affected by tectonic or compaction strain. Our data together with previously published results suggest a general clockwise rotation pattern in the Middle Yangtze fold belt, which is probably related to the collision between the North and SCBs. Comparison of palaeomagnetic rotations with fold axis trends in the fold belt suggests that about 30 degrees clockwise rotation occurred in the Middle reaches of the Yangtze River, while a 15 degrees difference in fold axis trends would be due to initial variation within the fold belt. However, since little is known about the timing of the clockwise rotation, whether the Middle Yangtze fold belt is an orocline awaits further studies.

Fatim Hankard,1 Jean-Pascal Cogné,1 France Lagroix,1 Xavier Quidelleur,2 Vadim A. Kravchinsky,1,∗ Amgalan Bayasgalan3 and Purevdorj Lkhagvadorj3 1Laboratoire de Paléomagnétisme, Institut de Physique du Globe de Paris, Université Paris 7, 4 Place Jussieu, 75252 Paris Cedex 05, France. E-mail: hankard@ipgp.jussieu.fr 2Laboratoire de Géochronologie Multi-techniques U.P.S. – I.P.G.P., Bat. 504, Sciences de la Terre, Université Paris Sud, 91405 Orsay, France 3Mongolian University of Science and Technology, PO Box 49/418, Ulaanbaatar 210349, Mongolia

We report results of a paleomagnetic study of 490 cores from 59 sites, corresponding to 52 distinct basaltic flows from Mongolia and Siberia: Khaton Sudal (39.4 Ma, 44.5°N/101.4°E), Taatsyn Gol (1, 31.5 Ma, 45.4°N/101.3°E; 2, 28.0 Ma, 45.5°N/101.1°E), Ust Bokson (19.9 Ma, 52.1°N/100.3°E), and Taatsyn Gol (3, 12.7 Ma, 45.5°N/101.0°E). Stepwise thermal and alternating field demagnetizations isolated a stable high‐temperature component (HTC) of magnetization in most specimens, which we interpret as the primary magnetization of these basaltic lava flows. The four corresponding paleopoles appear consistent with coeval paleopoles from other Asian effusive formations. However, except for the 12.7 Ma paleopole, the paleopoles are systematically far‐sided from the European apparent polar wander path (APWP) with respect to site locations, corresponding to anomalously shallow inclinations in Tertiary Asian effusive formations. In the hypothesis of a dipolar magnetic field in the Tertiary, this indicates a ∼1000–1500 km position of the Siberia craton and Amuria block farther south than expected at 40 and 30 Ma. Tectonically, this interpretation implies decoupling and relative rotations between the western and eastern parts of Eurasia between the Cretaceous and Present. We show that if Siberia were located more to the south, the ∼15°–20° paleolatitude anomaly generally observed in sedimentary formations from central Asia reduces to a more reasonable average value of ∼7°, which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of Asian blocks under the effect of ongoing penetration of India into Eurasia in the Tertiary.

SUMMARY We present the results of a palaeomagnetic study of 277 cores drilled at 35 sites, in 32 basaltic flows from three Early Palaeocene volcanic regions in the Gobi Desert, Mongolia, at Sumber Uul (62.2 Ma; 42.6°N/104.0°E), Tulga (62.0 Ma; 43.2°N/104.1°E) and Khuts Uul (57.1 Ma; 43.2°N/104.6°E) localities. Samples from Sumber Uul (62.2 ± 0.9 Ma) and Khuts Uul (57.1 ± 0.8 Ma) localities were dated using the K-Ar Cassignol–Gillot technique. Stepwise thermal and alternating field demagnetizations isolated a stable A component of magnetization carried by single domain (SD) to nearly SD magnetite. We interpret this A component to be the primary magnetization of these basaltic lava flows.The Sumber Uul and Tulga data were combined and recomputed at the Sumber Uul locality because of their similar ages. The palaeopoles computed from the A components lie at λ= 85.2°N, φ= 92.5°E, dp/dm= 3.9/4.9 (n= 14 flows) for Sumber Uul-Tulga (average age: 62.1 ± 5.9 Ma) and λ= 69.6°N, φ= 148.0°E, dp/dm= 6.3/7.3 (n= 14 flows) for Khuts Uul (average age: 57.1 ± 0.8 Ma). The palaeomagnetic inclinations are steeper than expected at the sites and consequently our palaeopoles occupy a near-sided position with respect to the 60 Ma reference apparent polar wander path (APWP) pole for Europe (Besse & Courtillot 2002). However, they appear to fully conform to the new high-resolution APWP poles for the 65–42 Ma period of Moreau et al. (2007). Following these authors, we interpret this anomalous near-sided position of our poles as arising from a rapid true polar wander (TPW) event in the Palaeocene, highlighted by a cusp at anomalies 26–25 (61–56 Ma), inexistent in the European APWP of Besse & Courtillot (2002). We conclude that our new data do not reveal any anomalous shallow inclinations in the Central Asia Palaeocene effusive rocks which is consistent with the Late Jurassic/Early Cretaceous age of Mongol-Okhotsk ocean closure and amalgamation of Amuria and Siberia, forming a rigid entity since then.