This brief note provides an overview of a new Macintosh™ application, PaleoMac, (MacOS 8.0 or later, 15Mb RAM required) which permits rapid processing of paleomagnetic data, from the demagnetization data acquired in the laboratory, to the treatment of paleomagnetic poles, plate reconstructions, finite rotation computations on a sphere, and characterization of relative plate motions. Capabilities of PaleoMac include (1) high interactivity between the user and data displayed on screen which provides a fast and easy way to handle, add and remove data or contours, perform computations on subsets of points, change projections, sizes, etc.; (2) performance of all standard principal component analysis and statistical processing on a sphere [Fisher, 1953] etc.); (3) output of high quality plots, compatible with graphic programs such as Adobe Illustrator, and output of numerical results as ASCII files. Beyond its usefulness in treating paleomagnetic data, its ability to handle plate motion computations should be of large interest to the Earth science community.

Summary This paper presents new data from palaeomagnetic investigations on the Upper Palaeozoic and Mesozoic geological units from the Siberian platform and the Mongol–Okhotsk suture zone. Within the southern portion of the Mongol–Okhotsk suture we collected palaeomagnetic samples from the Late Permian Belektuy formation (Borzya region; 50.7°N, 116.9°E) and the Middle–Late Jurassic Shadaron formation (Unda‐Daya; 51.5°N, 117.5°E). We sampled the Late Permian Alentuy formation (Khilok region; 50.8°N, 107.2°E), the Early to Middle Jurassic Irkutsk sedimentary basin (ISB; 52.0°N, 104.0°E), the Late Jurassic Badin formation (Mogzon region; 51.8°N, 112.0°E), and the Early Cretaceous Gusinoozesk formation (Gusinoe Lake region; 51.2°N, 106.5°E) additionally in the northern region of the Mongol–Okhotsk suture. Apart from the results of the ISB and Gusinoozersk formations, which show very large ellipses of confidence and might be the present‐day geomagnetic field overprint, our results allow us to constrain the evolution of the Mongol–Okhotsk Ocean palaeomagnetically from the Late Permian to the Middle–Late Jurassic. They confirm that this large Permian ocean closed during the Jurassic, ending up in the late Jurassic or the beginning of the Cretaceous in the eastern end of the suture zone, as suspected on geological grounds. However, although geological data suggest a Middle Jurassic closure of the Mongol–Okhotsk Ocean in the west Trans‐Baikal region, our data show evidence of a still large palaeolatitude difference between the Amuria and Siberia blocks. This is interpreted as a result of the quite fast closure of the ocean after the Middle Jurassic. Finally, our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes, which we interpret as probably arising both from collision processes and from a left‐lateral shear movement along the suture zone, due to the eastward extrusion of Mongolia under the effect of the collision of India into Asia.

S U M M A R YWe 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 (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.

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. Thermal demagnetization of the rocks isolated a high-temperature component that we interpret as the primary magnetization in four localities. The paleopoles lie at 52.6øN/352øE (dp/dm=6.0ø/10.7 ø) for Xialaxiu, 61.6øN/211.3øE (dp/dm=9.7ø/16.1 ø) for Xining, 66.0øN/228.6øE (dp/dm=3.6ø/6.9 ø) for Jungong, and 53.9øN/205.4øE (dp/dm=5.

Based on a compilation of 533 Cretaceous to present-day paleomagnetic 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 2 nd -order anomaly, characterized by high paleolatitudes in Indochina and low paleolatitudes over Tibet and Central Asia, is superimposed on a 1 st -order anomaly, characterized by Cenozoic low paleolatitudes found all over north-eastern Asian stable blocks. The analysis herein convincingly shows that the Europe Apparent Polar Wandering Path (APWP) can no longer be used to interpret paleomagnetic 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 paleopoles 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 1 st -order paleolatitude 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 6 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 1 st -order ~10° low paleolatitude anomaly over Asia during the Tertiary.

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.

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.