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Content archived on 2022-12-23

The last unknown Mesozoic ocean: palaeomagnetic studies in south Siberia

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Results : Mainly based on palaeomagnetic results from South and North China blocks (SCB & NCB), the northeastern part of the Asian mosaic is generally considered to have been accreted to Siberia prior to the Cretaceous. In order to check this hypothesis, two field trips have been performed during which have been collected Jurrassic and Cretaceous effusives north and south ofthe Mongol-Okhotsk suture which separates the Mongol block to the south from Siberia to the north. South of the Mongol-Okhotsk suture, i.e. in the Mongol block, have been collected one Jurassic locality in the Transbaikal region along the Unda river at (51.7 N, 117.4 E), and three lower Cretaceous effusives formations: the first one in Amur region around the town of Taldan (53.8°N, 124.5°E), and the two others in the Transbaikal it theKriemlovka peak locality (51.8°N, 117.5°E) and the Torey lakes locality (50.2°N, 11 5.8°E). North of the Mongol-Okhotsk suture, Le. in the Siberian continent, two Cretaceous localities have beeri sampled. The first one along the Ingoda river at (51.2°N, 112.3°E) and the second in the vicinity of Bitchura town(50.6°N, 107.6°E). The Jurassic locality and three out the five Cretaceous localities yielded consistent paleomagnetic results deserving, attention, which can be summarized as follows: We have determined an upper Jurassic and two lower Cretaceous poles for the Mongolian block, and anew Cretaceous pole for the Siberian block. When compared on an equal area projection, the Cretaceous poles from the north and the south of the Mongol-Okhotsk suture lie on small circle containing the Cretaceous poles of the reference apparent polar wander path of Eurasia demonstrating that Mongolia and Siberia were accreted by the lower Cretaceous. The North China block-South China block-Mongolia-Siberia assemblage was already formed in the Cretaceous and did not Stiffer any relative N-S motion since that time Important relative rotations between the north (clockwise) and the south(anticlockwise) of the suture could indicate that it has acted as left lateral shear zone allowing, a post Cretaceous eastward extrusion of Mongolia. Based on the Jurassic pole of Mongolia, we estimate the width of the Mongol-Okhotsk ocean to between 2500 and 4000 kin at that time The MongolOkhotsk ocean seems to be entirely closed between the upper Jurassic and the lower Cretaceous.

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