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.

The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique-Application et Recherche or Critical Zone Observatories-Application and Research) is a National Research Infrastructure (RI). OZCAR-RI is a network of instrumented sites, bringing together 21 pre-existing research observatories monitoring different compartments of the zone situated between "the rock and the sky," the Earth's skin or critical zone (CZ), over the long term. These observatories are regionally based and have specific initial scientific questions, monitoring strategies, databases, and modeling activities. The diversity of OZCAR-RI observatories and sites is well representative of the heterogeneity of the CZ and of the scientific communities studying it. Despite this diversity, all OZCAR-RI sites share a main overarching mandate, which is to monitor, understand, and predict ("earthcast") the fluxes of water and matter of the Earth's near surface and how they will change in response to the "new climatic regime." The vision for OZCAR strategic development aims at designing an open infrastructure, building a national CZ community able to share a systemic representation of the CZ , and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. OZCAR articulates around: (i) a set of common scientific questions and cross-cutting scientific activities using the wealth of OZCAR-RI observatories, (ii) an ambitious instrumental development program, and (iii) a better interaction between data and models to integrate the different time and spatial scales. Internationally, OZCAR-RI aims at strengthening the CZ community by providing a model of organization for pre-existing observatories and by offering CZ instrumented sites. OZCAR is one of two French mirrors of the European Strategy Forum on Research Infrastructure (eLTER-ESFRI) project.

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.

/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.

S U M M A R YWe report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon (44.4 • N, 103.8 • E) and • 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, α 95 = 5.5 • ) for Shovon and Dm = 12.1 • , Im = 66.4 • (n = 27 flows, k = 53.0, α 95 = 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 tiltcorrected 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, α 95 = 4.8 • ). The corresponding palaeopoles computed from these HTC lie at λ = 84.7 • N, φ = 195.0 • E, d p/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, d p/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 MongolOkhotsk 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 JurassicEarly 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, A 95 = 5.8 • ).

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[1] We have developed a new scanning magnetic microscope to image with micrometric resolution magnetic fields originating from room temperature polished samples. This microscope is based on a giant magnetoresistance (GMR) sensor working at room temperature. These magnetic sensors are sensitive to the in-plane components of the magnetic field. The size of the sensing element is 9 mm  36 mm. The noise of the GMR sensor is dominated by a low-frequency 1/f noise. The field equivalent noise of the sensors is 10 nT= ffiffiffiffiffi ffi Hz p at 1Hz and decreases to 0.3 nT= ffiffiffiffiffi ffi Hz p above 1 kHz for a 1 mA sensing current. The spatial resolution of the system is $20 mm, and its peak-to-peak noise during operation is $250 nT. Its high spatial resolution and a minimum sensor-to-sample distance of 30 mm compensate for its rather moderate field sensitivity. This room temperature small-sized and rugged magnetic microscope appears as a powerful instrument for small-scale rock magnetic investigations.Components: 5185 words, 6 figures.

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