THE WEST SIBERIAN SEDIMENTARY BASIN. AN ORIGIN WITHOUT STRONG CRUSTAL STRETCHING – THE SUPERDEEP DRILLING DATA ANALYSIS

Mechanism of formation of large sedimentary basins on the continents is among the main problems in geodynamics. The formation of such structures far from convergent plate boundaries is most often explained by intense stretching of the lithospheric layer - rifting. This should be followed by a large and prolonged crustal subsidence due to cooling of the stretched lithosphere. A characteristic feature of the post-rift subsidence is a decrease in its rate in time - several times over a hundred million years. Rifting in the West Siberian Basin was associated with the formation of Permian-Triassic graben-rifts. The subsequent (post-rift) subsidence is considered by many researchers as a cause of formation of a thick layer of Mesozoic and Cenozoic sediments in the basin. Over the 86 Ma that passed from the beginning of the Middle Triassic to the end of the Middle Jurassic, the rate of post-rift subsidence should have decreased by about a factor of three. In fact, the opposite trend took place. Judging by the data of the Tyumen (SG-6) and En-Yakhin (SG-7) superdeep wells drilled in the north of the basin, the rate of crustal subsidence increased several times during this period of time. Such a sharp difference between the expected post-rift subsidence and the actual subsidence of the crust indicates that lithospheric stretching was not the main subsidence mechanism, and it could provide only a small part of the subsidence. Under such circumstances, the main cause of the subsidence should be contraction of rocks in the Earth’s crust due to prograde metamorphism.