The Jurassic growth of mountain ranges along the southern edge of the Siberian platform occurred in an active tectonic setting related to the closure of the Mongol–Okhotsk Ocean. The oceanic subduction and subsequent continent collision events induced compressive deformations at the platform boundary. Understanding the paleogeography related to the Mesozoic closure of the Mongol–Okhotsk Ocean requires dating and correlation of the Jurassic Prisayan Formation in the Irkut basin and Tugnuyskaya Formation in southwestern Transbaikalia. This work presents structural and paleobotanic results within both formations. 40Ar/39Ar dating of underlying volcanics from the upper member of the Ichetuyskaya Formation is used to refine the age of the sediment series and provide probable correlation. The results show that the Tugnuyskaya Formation initiated at the end of the Middle Jurassic–beginning of the Late Jurassic and was not coeval with the Prisayan Formation, whose upper fine-grained members were deposited in the early Middle Jurassic. 40Ar/39Ar dating of volcanics from the upper member of the Ichetuyskaya Formation yielded a Middle Jurassic age of 167.7 ± 1.2 Ma (Bajocian to Bathonian). The paleogeographic data analysis based on facies and mineralogical composition of sediments and on a study of source areas from Sm–Nd data and the U–Pb ages of detrital zircons from the deposits in the southern Irkut basin indicates that the deposition of the Prisayan Formation was followed by the intensification of relief building along the southern edge of the Siberian Platform. Our geochronological data show that active tectonic deformations in southwestern Transbaikalia evidenced in the volcanoclastic Ichetuyskaya Formation in the Tugnuy basin also occurred during the Middle Jurassic. The uppermost sediments of the Tugnuy basin were deposited at the end of the Middle Jurassic–Late Jurassic in a quiet tectonic setting with low relief and lacustrine-boggy depositional environments.
Results of the activity of Quaternary glaciations in the western Baikal area are considered based on new factual material. Also, the problem of the formation of “watershed pebbles” in the area is discussed. Data indicating the fluvioglacial origin of the pebbles are reported. Special attention is given to the composition, lithology, and stratigraphy of glacial deposits and their geomorphologic position in the study area, which is one of the most hardly accessible and interesting regions in East Siberia in terms of both the geology of Quaternary deposits and archaeology. Results of spore-and-pollen analyses and archaeological evidence are reported.
—We study Late Jurassic thrusting of the Archean craton basement over Jurassic sediments in Siberia, with the Khamar-Daban terrane as a rigid indenter. The study focuses on deformation and secondary mineralization in Archean and Mesozoic rocks along the thrusting front and the large-scale paleotectonic thrust structure. The pioneering results include the inference that the Angara, Posol’skaya, and Tataurovo thrusts are elements of the Angara–Selenga imbricate fan thrust system and a 3D model of its Angara branch. The history of the Angara–Selenga thrust system consists of three main stages: (I) detachment and folding of the basement under the Jurassic basin and low-angle synclinal and anticlinal folding in the sediments in a setting of weak compression; (II) brecciation and mylonization under increasing shear stress that split the Sharyzhalgai basement inlier into several blocks moving in different directions; formation of an imbricate fan system of thrust sheets that shaped up the thrusting front geometry, with a greater amount of thrusting in the front because of the counter-clockwise rotation of the Sharyzhalgai uplift; (III) strike-slip and normal faulting associated with the origin and evolution of the Baikal rift system, which complicated the morphology of the thrust system.
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