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Abstract: 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. [Copyright &y& Elsevier]

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 (∼55 Ma) and locus (∼5–10°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 ∼10° 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 ∼10° low palaeolatitude anomaly over Asia during the Tertiary. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

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, d p/d m= 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, d p/d m= 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. [ABSTRACT FROM AUTHOR]

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 = 8.2°, Im = 63.7° ( flows, ) for Shovon and Dm = 12.1°, Im = 66.4° ( flows, ) 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° ( flows, ). The corresponding palaeopoles computed from these HTC lie at , , for Shovon-Khurmen Uul (average age: Ma) and , , 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: , . [ABSTRACT FROM AUTHOR]

We present new palaeomagnetic results from the Transbaïkal area (SE Siberia), from the Mongol-Okhotsk suture zone, the boundary between the Amuria and Siberia blocks. In order to better constrain the time of closure of the Mongol-Okhotsk Ocean in the Mesozoic, we collected 532 rock samples at 68 sites in six localities of basalts, trachy-basalts and andesites, from both sides of the Mongol-Okhotsk suture: at Unda river ( J3; 51.7°N, 117.4°E), Kremljevka peak (K1; 51.8°N, 117.5°E) and Torey lakes ( K1; 50.1°N, 115.9°E) on the southern side of the suture, and at Monostoy river ( J1; 51.1°N, 106.8°E), Ingoda river ( K1; 51.2°N, 112.2°E) and Bichura town ( K1; 50.6°N, 107.6°E) on the northern side. Progressive thermal demagnetization enabled us to resolve low (LTC) and high (HTC) temperature components of magnetization at most sites. Jurassic palaeopoles computed from the HTCs show a large discrepancy with respect to the Apparent Polar Wander Path of Eurasia, which we interpret in terms of 1700–2700 km of post-Late Jurassic northward movement of Amuria with respect to Siberia. Although geological data suggest a middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data give evidence of a large remaining palaeolatitude difference between the Amuria and Siberia blocks. In contrast, Early Cretaceous sites cluster remarkably well along a small-circle, which is centred on the average site location. This implies the absence of post-Early Cretaceous northward motion of Amuria relative to Siberia, and demonstrates the pre-Early Cretaceous closure of the Mongol-Okhotsk Ocean. Finally, we interpret the very large tectonic rotations about local vertical axes, evidenced by the small-circle distribution of poles, as arising both from collision processes and from left-lateral shear movement along the suture zone, due to the eastward extrusion of Amuria under the effect of the collision of India into Asia. [ABSTRACT FROM AUTHOR]

Abstract: We present new paleomagnetic results obtained at ten sampling sites from six Upper Cretaceous (92±4 Ma) basaltic lava flows from the westernmost part of Amuria block. The samples were collected in 1999 in the Khurmen Uul region (44.0°N, 103.0°E), Gobi desert (S. Mongolia). Stepwise thermal demagnetization isolated a high temperature component (HTC) carried by magnetite. Five out of ten sites exhibit anomalously high Koenigsberger ratios and low inclinations, which likely resulted from lightning remagnetization. However, three of these sites were only partially remagnetized, allowing us to use remagnetization great-circles to help determine a characteristic magnetization direction for this formation. HTC directions of the eight remaining sites from six distinct flows all possess normal polarity and exhibit better clustering after tilt correction of the flows (ks/kg=129.3/36.6, n =6). This clustering is ascertained by a positive fold test at the specimen scale within a single flow. We therefore propose that the tilt-corrected HTC represents a primary magnetization. The paleopole computed from the six lava flows HTC mean directions lies at λ =71.5°N, ϕ =227.2°E (dp/dm=5.6°/8.2°). We discuss the possibility that our pole might not have completely averaged paleosecular variation, because it is based on only 6 independent flows. Finally, this pole is far-sided with respect to the 90 Ma reference pole of Eurasia APWP (Δλ =10.2°±6.1°), but it is consistent, albeit somewhat far-sided too, with other coeval paleopoles from Amuria, and east China. [Copyright &y& Elsevier]

In order to better understand the tectonic evolution of central Asia under the influence of the India-Asia collision, we carried out a paleomagnetic study of 1500 cores from 106 sites along the Altyn Tagh fault, in the Qaidam and Tarim basins, and on the Tibetan plateau. Samples were mainly collected from Jurassic to Neogene siltstones and sandstones. In most cases stepwise thermal demagnetization unblocks low and high temperature components carried by magnetite and hematite. Low temperature components are north and down directed and lie close to the recent geomagnetic field. High temperature components from 10 of 13 age/locality groups pass fold and/or reversal tests and likely represent primary remanent magnetizations. The ten overall mean directions display a complex pattern of vertical-axis block rotations that are compatible with a tectonic model of clockwise rotation of the Qaidam Basin and concomitant left-lateral slip on the Altyn Tagh fault. Two of the ten localities are rotated significantly counterclockwise; they lie adjacent to the Altyn Tagh fault zone, consistent with the idea that left-lateral strike-slip motion occurred along it. The age of counterclockwise rotation near the eastern extremity of the fault was dated as younger than 19 Ma. Three widely spread areas within the Qaidam Basin exhibit similar and significant clockwise rotations, on the order of 20°, with respect to the North China Block, Tarim and Eurasia. The mean of the three values is thought to represent the total rotation of Qaidam. Because the youngest rocks displaying clockwise rotations are Oligocene, the main phase of Qaidam Basin rotation, and hence shear on the Altyn Tagh fault, took place after or near the end of the Oligocene (∼24 Ma). Upper Neogene strata located on the Qaidam Basin are not significantly rotated, thus tectonic deformation acting since the Upper Neogene (∼5 Ma) is not resolvable by paleomagnetic methods. Given a 20° ± 5° clockwise rotation of the Qaidam Basin with respect to the Tarim Basin, the maximum left-lateral displacement on the Altyn Tagh fault since 24 Ma is 500 ± 130 km. [ABSTRACT FROM AUTHOR]

Abstract: 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°C, displays a direction consistent with the present-day geomagnetic field. The locality-mean directions related to the high temperature component (above 350°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. [Copyright &y& Elsevier]

Stepwise demagnetization isolates a stable magnetic component in 13 sites of basalt flows and baked sediments dated at 113.3±1.6 Ma from the Tuoyun section, western Xinjiang Province, China. Except for one flow from the base of the ∼300 m thick section, the rest have exclusively reversed polarity. The sequence correlates with chron M-0 in some geomagnetic polarity time scales, which potentially places the section just before the start of the Cretaceous Long Normal polarity superchron. Five of 11 sites of Early Cretaceous red beds that underlie the basalts possess coherent directions that pass both fold and reversals tests. Six sites of Upper Jurassic red beds have a magnetic component that was likely acquired after folding in the Tertiary. The mean paleolatitude of the Lower Cretaceous red beds is 11° lower than that of the Lower Cretaceous basalts suggesting the red beds underestimate the true field inclination. We further test this result by calculating the paleolatitudes to a common point of the available Early Cretaceous to Present paleomagnetic poles from red beds and volcanic rocks from central Asian localities north of the Tibetan Plateau. We find that paleolatitudes of volcanic rocks roughly equal the paleolatitudes calculated from the reference Eurasian apparent polar wander path (APWP) and that paleolatitudes of red beds are generally 10–20° lower than the paleolatitudes of volcanic rocks and those predicted from the reference curve. Our study suggests that central Asian red beds poorly record the Earth’s field inclination, which leads to lower than expected paleolatitudes. Good agreement in paleolatitudes from volcanic rocks and the Eurasian APWP argues against proposed canted and non-dipole field models. [Copyright &y& Elsevier]