Геодинамика и тектонофизика (Sep 2015)
THE SOUTHERN FRAGMENT OF THE SIBERIAN CRATON: “LANDSCAPE” HISTORY OVER TWO BILLION YEARS
Abstract
In the state-of-the-art geology, concepts of evolution of interrelated geodynamic and biotic events throughout the history of the Earth have been developed (Fig. 1). Research results on sediments, bio-stratigraphy and geodynamics of the southern fragment of the Siberian craton (SSC, Fig. 2) provide for more or less reliable assessments of the status and evolution of ancient landscapes and biotas from the Lower Proterozoic to the Cenozoic.In the Lower Proterozoic, the geodynamic regime of the Urik-Iyskiy graben was similar to those of the westernpacific island-arc systems, which resulted in the orogen formation and established post-orogen granitoids of 1.86 bln years of age. At the beginning of the Early Riphean, volcano-sedimentary masses were accumulated in continental basins (Fig. 2, 3A). Collision orogenesis also resulted in the occurrence of the terrigeno-volcanogenic complex of the Akitkanskaya suite in the Western Pribaikalie and the transecting Irelskiy granitoids, aged 1.86 bln years, at the edge of the craton. Later on, most probably before the Riphean, peneplanation took place, and a shallow peripheral sea was formed with highly-mature sediments of the Purpolskaya suite. Different environments are reconstructed in the KodarUdokan zone. Sediments of the Udokanskaya suite, varying in thicknesses from 11 to 14 km, suggest a complicated evolution of sedimentation in the peripheral marine basin. Dozens of radiochronological datings of granitoids of the Chuiskiy and Kodarskiy complex which transect the Udokanskaya suite are within the range from 1.7 to 2.0 bln years. From the deposit composition and texture, it can be suggested that the middle, Chineiskaya sub-suite was formed under island-arc conditions; and glacial phenomena occurred in the late Udokan time.Further geological history of the SSC can be described only within the period after the Late Riphean sedimentations (see Fig. 3Б, В). The SSC evolution in the Neo-Proterozoic began with divergence events, which most probably occurred in the period of 1000–850 mln years in the east, and in the interval of 780–730 mln years in the west of the territory. The latest period is logically aligned with disintegration of Rodini, the super-continent. The period of 780–680 mln years in the eastern part of the region can defined by the beginning of convergency processes, formation and evolution of the island arc and the back-arc basin. It is supposed that basal layers of the Baikalskaya and Oselokskaya suites and their analogues occurred 730 mln years ago, and evidences of glacial processes in these series correlate with the global Sturtian glaciations. The period of 680–630 mln years was characterized by formation of the foreland-type peripheral basin which was then replaced by a system of orogen-type submontain troughs in the Early Vendian (from 630 mln years, see Fig. 3Г). The second half of the Vendian in various zones of SSC was distinguished by shallow-water carbonate-terrigenous sediments of a similar type. Compensatory sedimentation occurred in residual valleys of the basin. Fast infill of the basin and leveling of the relief resulted in the stationary regime of the relatively shallow, yet vast basin. In the Early Cambrian, carbonate sedimentation occurred throughout the Siberian Platform and in the area adjacent to the SSC (see Fig. 3Д).The Paleozoic sediments preserved mainly in the central and northern regions of the Siberian Platform reflect a complex evolution of internal and epicontinental seas and shallower basins of the Siberian continent named Angarida. In the Ordovician, predominating were carbonate rocks with marine fauna. In the Silurian was characterized by a variety of sediments formed in different marine environments, ranging from distal shelf to shallow water and salted gulfs. In the Late Silurian and the Early Devonian, the territory of Angarida was land. Local volcanism with mafic lava eruptions through fractures took place at the background of sub-continental sedimentation. In the Late Paleozoic, the geologic development was marked by major transformation of the pattern of tectonic structures, that was most likely related to inside-plate extension and thinning of the continental crust. In the Mid and Late Carbon (Fig. 4A), the integrated Tungusskiy sedimentation basin was formed as a result of continuous and uniform bending. In the Early Permian (see Fig. 4Б), positive tectonic movements led to significant dewatering of the Paleozoic basins, so that they turned into a washed-out area. Overall raising of the Siberian Platform preconditioned climate changes, such as aridization and climate cooling. In the Mesozoic, landscapes were presented by a combination of flat uplands, wide river valleys with swampy plains and lakes wherein carbonous sediments were accumulated. Basic volcanism with shield eruptions and sub-volcanic rocks was typical then. In the Jurassic (see Fig. 4B), elements observed in the recent topography of the Siberian Platform were formed. In that period, major structural transformation occurred in association with the largest diastrophic cycles in the territory of the Eastern Asia, including formation of the Baikal rift and its branches.From the analyses of the available data which are briefly presented above, it is obvious that the period of two billion years in the Earth history includes numerous epochs of diastrophic processes of tremendous destructive capacity. Unconformities of formations differing in ages by millions and even hundreds of million years, as those dating back to the Pre-Cambrian, suggest quite realistic yet astounding visions. At the background of scenarios of floods, rock up-thrusts, volcanic explosions and earthquakes evidenced from the very remote past, the current geological and climatic phenomena may seem quite trivial.
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