The rift system of West Siberia is the extreme east part of the Arctic-North Atlantic rift mega-system. The Vendian-Paleozoic tectonic-magmatic mega-cycle in this part of the Earth's crust resulted in formation of new continental crust. In Siberia there was a tendency for uplift beginning in the Middle Paleozoic. In particular, large domal uplifts formed at the end of the Paleozoic in the region of the present West Siberian Lowland.
In the Early Triassic a new stage of thermal action of the mantle on the lithosphere led to broad destruction of crust in the northwest segment of the Earth. Continental rift systems formed, above which sedimentary basins developed later in the Mesozoic (basins of the North Sea, Norwegian Sea, Barents Sea. Kara Sea, Paris, West Siberia, and others).
The structure of the Mesozoic-Cenozoic sediments, their thickness, and facies composition as well as the oil-gas potential of these basins were determined greatly by this rifting. This is clearly apparent in the example of the rift system of West Siberia. In the relief of the basement surface there is a net of branching grabens and intervening uplifted basement blocks. At least six linear en echelon graben-rift zones are recognized. Their width increases toward the north to 60-70 km. Vertical displacement is 1-5 km, also increasing toward the north. The rift fills are basic igneous rocks overlain by volcanic- clastic deposits of the Lower and Middle Triassic. The upper part of the rift complex is essentially Middle and Upper Triassic sedimentary rocks. These rocks have been penetrated by the drill in several places.
The graben zones show up as strong positive gravity and magnetic anomalies. This is well illustrated along the deep seismic sounding profile that extends across the West Siberian platform at the latitude of Urengoy. In the central part of this profile is a large lithospheric block some 750 km wide. Here the upper mantle has lower velocity values. Beyond it on both east and west velocity values are higher. Thickness of this low-velocity layer exceeds 70 km.
The consolidated crust above this low-velocity layer is cut into blocks, which have different velocity characteristics. In the lower part of the consolidated crust there is a layer some 10-16 km thick in which formation velocity is 6.9-7.25 km/sec. It is interpreted as a transition layer, or a mixed crust-mantle layer. To the east and west of this anomalous block the crust is more homogeneous. In the upper part of the crust here are three blocks, which are characterized by higher values of velocities, density, and magnetic susceptibility. These coincide with deep grabens in the relief of the basement surface. Crustal thickness on this anomalous block according to seismic data does not exceed 36 km; this is minimum for the West Siberian platform.
Elevated temperature anomalies are found above the rift zones. These values are not particularly high, however.
Part 2 will be News Letter No. 103, June 25, 2001
Taken from Surkov (1993); digested in Petroleum Geology, Vol. 30, No. 2,
1996, two maps, one cross section.
Copyright 2001 James Clarke. You are encouraged to print out this News
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