A new concept of the junction nature of the Caspian Sea basin and Mugodzhar

Conclusion

[15] The features of the deep structure discussed above make this area markedly different from more northern areas of Ural region and the western slope of the Ural, where foreland fold-thrust complexes and allochthon complexes of Ural internides overlaying the ancient crystalline basement of East European platform are widespread.

[16] Thus along subvertical Sakmarian-Kokpektinskiy fault dividing Ostansukskiy trough and Kosisteksko-Aksuyskaya zone of uralides a marked tectonic contact that is a marginal suture is traced between the largest tectonic structures of the Caspian Sea suboceanic basin (plate) and Ural folded belt. To the north approximately at latitude of 49^o40N, the linear structure of marginal suture widens and is replaced by fold-thrust dislocations of Aktyubinskaya area of Cis-Ural foredeep. At the same time on the eastern side of the suture, fold-thrust structures are encountered of the external zone of uralides, western Ural zone of linear folding, composed of marine molasse rocks C₂-P₁. It is notable that to the south, the molasse formations occur on the other side of the marginal suture in Ostansukskiy trough and are not known in Kosisteksko-Aksuyskaya zone of uralides. Ostansukskiy trough may be assumed to have been set in the Middle Carboniferous on the suboceanic basement of the Caspian Sea plate in the course of converging to and colliding with island arcs that underwent accretion and microcontinents of Ural mobile belt. One can now appreciate the significance of the continental interruption of immense duration that embraced the Late Carboniferous and the Early Permian and is represented at the area of eastern edge zone of the Caspian Sea basin to the west of Ostansukskiy and Aktyubinskiy troughs. It corresponds to the collision time of its plate and Ural mobile belt structures. One may assume that the collision of Ural mobile belt structures to the south of latitude 51^oN went on differently from the collision along the ancient passive margin of the East European continent. It took place at the front of collision with suboceanic plate that later on was transformed into the basement of the Hercynian Caspian Sea basin. The suboceanic lithosphere of this plate moved therewith under island-arc constructions of the southern continuation of Ural mobile belt. Specifically the persisting general eastward tilt of preorogenic complex surface in Ostansukskiy trough testifies to this effect as well as the occurrence in the Kempirsayskiy anticlinorium section base of thick beds of differentiated volcanic rocks of the island arc set as early as in the Early Paleozoic. The deep-sea trench formed in the course of subduction and the marginal swell framing it from outside bordered the basin of the would-be Caspian Sea basin and predetermined the accumulation of a thick series of Middle Paleozoic deep-sea terrigenous sediments. In the marginal swell, rises took place; owing to the rises in the conditions of medium and shallow depths large carbonaceous masses ("platforms") were formed which were deeply eroded at the end of the Middle Carboniferous and Late Carboniferous.

[17] The situation discussed above can be related to the so-called "butt-end" junctions of heterogeneous structural elements of the Earth's crust: the Caspian Sea basin edge zone of east-west extension "is jointed" in the east to the Ural folded belt structures of north-south extension. How does this affect the geological structure of Ural? And why is it so? We tried to draw attention to some features of the geological structure of Ural at the continuation of the Caspian Sea basin edge zone (latitude 51-50^o N) and to show that besides the expected "reflection" of this zone on the western slope of South Ural owing to its continuation at the level of the platform crystalline basement, which may be traced from geophysical data, such features are revealed among others that cannot be accounted for by the zone influence. From geodynamics viewpoint it becomes important that the case of suboceanic plates collision with subduction structures of mobile belt be discussed, its mechanism be established and corresponding models be developed.