RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6002, doi:10.2205/2005ES000187, 2005

The Prealpine History of the South Tien Shan Orogenic Belt

[8]  The pre-Alpine geological history of the region was discussed in many papers and monographs (see the references in [Leonov, 1996]), its Paleozoic evolution being discussed briefly in this paper.

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Figure 3
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Figure 4
[9]  Almost throughout the whole of the Paleozoic, the South Tien Shan territory was an oceanic space with a complex, dissected morphostructure (Figures 3 and 4). The early (Riphean-Early Silurian) periods of its geological history were dominated by general extension which resulted in the opening of the Turkestan paleoocean and was associated with the lateral movements of the oceanic lithosphere and with the formation of diffused spreading zones. This period of time also witnessed the formation of positive morphostructures, such as volcanic sea-floor and island-arc ridges. These morphostructures experienced the operation of the mechanism of gravitational instability, which caused the formation of specific structural-metamorphic rock assemblages with the formation, accompanied by the origin of synform structural features, plastic flow zones, and other specific features. This means that the early stage was marked by two independent geodynamic processes: the general motion of the oceanic lithosphere and the local process of the structural and lithologic reworking of the volcanic geomorphic structures under the conditions of gravitational instability.

[10]  The Middle Silurian time was marked by the beginning of the morphostructural differentiation of the oceanic basin with the formation of linear uplifts (carbonate platforms and island chains with reef buildings). The origin of the morphostructural differentiation seems to have been associated with changes in the state of the oceanic lithosphere, namely, with the replacement of extension by compression. This involved the mechanism of bending instability [Lobkovskii, 1988], which stimulated the lateral subplastic redistribution of rock masses in the crustal layer, the formation of outflow zones (lows) and of discharge zones (highs), as well as the shaping of the respective topographic features and structural parageneses (see Figure 4).

[11]  The mid-Carboniferous time witnessed the general space contraction and the successive accretion of morphostructural elements to the northern side of the paleoocean (see Figure 3). Under the compression conditions its embryonic forms grew more complex and were transformed, together with the basin deposits, to the complex nappe-fold structural feature of the region as whole. Some morphostructural elements were replaced by intrabasin collision sutures and by the zones of shallow self-closing subduction (see Figure 4). By the end of the Carboniferous to the beginning of the Permian a complex nappe-fold area was formed in the oceanic space. The main structure formation mechanism operating during that period of time was submeridional compression and space shrinking. A nappe-fold area with dissected topography was formed in the area of the former ocean. The newly formed erosion areas supplied the terrigenous material of the Late Paleozoic molasse to the remaining basins of the region. This period of time was marked by some granitoid magmatism.

[12]  It follows that the Paleozoic structure of the Tien Shan Mountains was formed mainly at the expense of the transverse shortening of the space and horizontal submeridional compression. However, at the background of this geodynamic activity, associated with the paleooceanic lithosphere evolution (initially during its extension and later during its compression), the paleoocean experienced the effects of independent geodynamic effects, such as the gravitational instability of the rock masses in the intraoceanic positive morphostructures, and also the mechanism of the bending instability of the rheologically stratified lithosphere, which resulted in the initial warping of the ocean floor, and in the formation of new intrabasin topography. These mechanisms produced specific structural assemblages which affected substantially the morphostructural evolution of the region during the Mesozoic and Cenozoic periods.

[13]  To sum up, a complex heterogeneous nappe-fold structural feature was formed in the region of the former Turkestan paleoocean. Taking into account the specific structure of the paleoocean, its evolution, and present-day structure, namely, its fold-nappe structure, the structural combination of different morphostructural elements, rocks, and crust types, the presence of lenses and bands of metamorphic rocks, the significant volumes of orogenic volcanic rocks, the tectonic layering of the crust, the partial removal of the sedimentary cover, and other factors, it can be assumed that the final result of the Paleozoic evolution of the region was the formation of a thick crustal layer. However, this layer differed radically from its present-day analog, because it had not experienced any separation into a "granite-metamorphic" and a "basic" rock layer. By the end of the Paleozoic, this crustal layer seems to have been represented by some intricately bedded, heterogeneous structural melange of igneous and metamorphic rocks reworked during the long-lasting evolution of the fragments of the basic crust, the granite metamorphic rock layer of the microcontinents (such as the well known Alai microcontinent), the rocks of the Paleozoic volcanogenic sedimentary cover, and early metamorphic rocks. It is obvious that the heterogeneous combination of these rock masses must have existed in a thermodynamically and isostatically unstable state with a variable with a variable and complex internal stress field.


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Citation: Leonov, M. G. (2005), The Post-Oceanic Geodynamics of the South Tien Shan Region, Russ. J. Earth Sci., 7, ES6002, doi:10.2205/2005ES000187.

Copyright 2005 by the Russian Journal of Earth Sciences

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