Geotektonika

Tectonic mixtites, or mélanges, with clay cement are specific to the western part of the Paleozoic Southern Tien Shan in the Turkestan‒Alai mountain system. The South-Fergana type mélanges, or Tul`‒Sartalian, have a sandy-clayey matrix of Silurian (less often ‒ Ordovician) age and block inclusions of sedimentary, igneous and partially metamorphic rocks with an age from the late Precambrian to the middle Carboniferous. The mélanges were formed as a result of the destruction of water-saturated sandy-clayey deposits, which occurred in the upper part of the subduction channel, on the southern active margin of the Kazakhstan paleocontinent. At the beginning of the collisional stage at the turn of the Early and Middle Carboniferous, they were brought to the surface and became part of the accretionary prism, which also included ophiolite sheets. In the time interval the Moscovian‒the beginning of Permian, the mélanges, together with other accreted formations, were involved in a series of collisional thrust sheets and moved to the south. The reverse northern thrust of tectonic plates with mélanges, observed within the Southern Fergana, arose as a result of the general collision at the beginning of the Permian between the Kazakhstan and Karakum‒Tajik paleocontinents, with subsequent orogenic collapse and granite magmatism. The consequence of this was the disintegrated position of individual fields and structural units with mélanges. The South-Fergana type clay mélanges contain block inclusions in island arcs and marginal basins that formed in the early Turkestan paleo-ocean during the terminal Precambrian and the Early Paleozoic.

The Andrew-Bain transform fault separates two parts of the Southwest Indian Ridge (Indian Ocean), different in their structure and evolution. It stands out among other transform faults by its complex structure, which includes troughs and areas of oblique extension. To identify the geodynamic conditions of formation of the modern structural pattern of the Andrew-Bain transform fault, experimental modeling was carried out, which reproduced the formation of the modern structure of the transform fault. In experiments, we obtained several strike-slip zones, which changed their position, and also were obtained overlapping strike-slip zones and extension axes, corresponding to the currently inactive area of the oblique extension of the transform fault eastern part. The main factors that determined the formation of the structural pattern of the fault zone are (i) the obliquity of the adjacent spreading segments relative to the spreading direction, and (ii) the initially specified lens-like shape of the transform domain. The formation of the lens-like shape was reproduced in a separate experimental series. It is assumed that the thermal influence of the Marion plume under transtension conditions could have led to local compression in the area of the northeastern edge of the transform fault. These conditions differ significantly from other similar examples, where the formation of a complex structural pattern occurred exceptionally under the influence of kinematic reorganization of the boundaries of lithospheric plates without a significant influence of thermal anomalies. The combined influence of these two factors and, as a consequence, the formation of the lens-shaped structure for the Andrew-Bain transform fault was possible in the period 52‒40 Ma ago during a change in the spreading direction on the Southwestern Indian Ridge, which coincided with the pulse of magmatic activity of the Marion plume.