RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES1006, doi:10.2205/2007ES000223, 2008

Formation of the Southern Tien Shan Nappe Ensembles

[225]  The collision of the Alay-Tarim continent and Kazakh-Kyrgyz microcontinent started in the Moscovian. By that time the oceanic part of the Turkestan basin was closed but its marginal parts persisted as a vast marine basin over a thin continental crust. The Alay-Tarim crust corresponded to the most portion of the basin and the Kazakh-Kyrgyz one, to the lesser part. This intracontinental marine basin, the successor of the Turkestan ocean, is convenient to name the Turkestan sea. The Turkestan sea occurred from the Moscovian to Permian and occupied the area between the northern Turkestan and Tarim landmasses. Flysch bearing olistostrome beds was accumulated in most of the Turkestan sea area. Carbonate sediments entered the flysch from the Alay-Tarim shelf and the fragments of approached nappes, from the opposite side. The sediment accumulation was interrupted in areas overthrusted by nappes and continued on moving thrust sheets and in front of the nappes represented by Neoautochthon-2. The vast Turkestan sea survived up to the Sakmarian; at the northern Tarim margin the residual marine basin occurred up to the Late Permian. The drying up of the territory resulted from the collisional emergence of the Turkestan sea bottom.

[226]  With the beginning of the collision, the oceanic crust subduction beneath the Kazakh-Kyrgyz microcontinent was replaced by subduction of the Alay-Tarim continental crust in the same direction. The collision and continental subduction resulted in the formation of collisional nappes at the Turkestan sea bottom.

[227]  Let us outline the age limits for tectonic processes caused by or associated with the microcontinent collision. The ophiolitic nappes are overthrusted onto the Early Moscovian sediments of the Alay-Tarim continental slope. The turbidite formation in the Turkestan sea completely terminated in the Sakmarian. Radiologic ages of the orogenic granites are within the Permian period; the continental orogenic molasse is dated as the Late Permian. These records indicate the Moscovian-Permian interval of the tectonic activity.

2007ES000223-fig21
Figure 21
[228]  The four-storeyed nappe ensemble was gradually formed at the margin of the Alay-Tarim terrane; the upper nappes arose earlier than the lower ones (Figure 21). The nappes of storey IV, i.e. the Tamdy, Sugut, Shankol, Shaydan, and Atbashi units, are of accretionary origin and were formed prior to the terrane collision. The Toguzbulak and Balykty units likely originated in the accretionary prism as well. The ophiolitic nappes of storey III, namely, the Kulkuduk, Taldyk, Kerey, and Keltubek units are probably of accretionary collisional genesis. That is, the isolating of ophiolite sheets from the subducting oceanic crust took place nearby the accretionary prism at the Kazakh-Kyrgyz margin of the oceanic basin (at deformation stage D-2), whereas the ophiolites were overthrusted onto the Alay-Tarim continental slope after the collision began, in response to underthrusting of the Alay-Tarim continental slope beneath the accretionary prism (at deformation stage D-3). This was occurring in the Moscovian. As a result of the described processes the nappes of storeys IV and III were found to be overlying the rocks that accumulated on the Alay-Tarim continental slope (Figure 21).

[229]  During or after the formation of the upper nappes the tectonic decoupling of sediments at the Alay-Tarim terrane margin began. The decollement fault surface occurred in Silurian shales. It became the base of future nappes of storey II. The tectonic decoupling took place in higher beds as well and affected to the largest extent flysch sequences of the Bukan, Tar, Abshir, Ontamchi, Chatyrkul, and Maidantag units.

[230]  The decoupled sediment complex of the former Alay-Tarim continental slope, together with the overlying ophiolitic nappes and Neoautochthon-1, was displaced inside the terrane and overthrusted on the Murun-Chekantash-Isfayram-Baubashata-Kokkiya-Muzduk facies megazone in the form of the Bukan, Tar, Abshir, Ontamchi, Chatyrkul, and Maidantag nappes (deformation stage D-4). This took place in the Sakmarian-Artinskian but could have begun still earlier (Figures 9 and 17). The secondary overthrust faults, recumbent folds and flexures were formed in the moving nappe owing to tectonic flow and drag processes.

On the overthrusting mechanism.
[231]  Much research of nappe formation conditions, including the author's work [Burtman, 1973], is dedicated to the mechanics of allochthonous sheets' generation (decoupling) and shifting. Investigations showed the lack of physical constraints of an allochthon transportation for long distances. By an overthrusting mechanism discussed below is meant an allochthon displacement mode.

[232]  The upper storey (IV) allochthons of the southern Tien Shan nappe ensemble were formed in accretionary prism through the underthrusting. The decoupling of ophiolitic sheets which make up the storey III nappes, likely also occurred in the accretionary prism on underthrusting of the subducting oceanic crust beneath the sheets. With the beginning of the collision in the Moscovian both tectonic and gravitational ways of nappe formation were possible. The accretionary prism nearby a margin of the Kazakh-Kyrgyz terrane was elevated relative to the Turkestan sea bottom composed of deep-water sediments overlying the Alay-Tarim continental basement. This made possible a gravitational input of the prism rocks to the sea bottom. The continental subduction, i.e. underthrusting of the Alay-Tarim crust beneath the accretionary prism and Kazakh-Kyrgyz terrane, occurred concurrently. The extent of participation of each mechanism in the ophiolite nappe formation is difficult to recognize. The cases when a storey IV nappe directly overlies a storey II one (Sugut and Chekantash units in the western Tien Shan or Tamdy and Bukan units in the Kyzylkum) were probably provided by gravitational movement of allochthonous sheets.

[233]  The decoupling and movement of the allochthon making up the storey II of the nappe ensemble directly resulted from the Alay-Tarim continental crust subduction under the Kazakh-Kyrgyz terrane. There is no parental territory that could be a source of vast gravitational sheet and the parental territory could not disappear on subduction. The subduction of the continental crust resulted in underthrusting of the autochthon beneath the decoupled allochthonous sheet. The allochthon was deformed in the course of overthrusting, which provided the formation of secondary tectonic and gravitational nappes, numerous inside the units of storey II in the nappe ensemble.

[234]  In the Permian the nappes and autochthon of the southern Tien Shan were folded (Figures 5, 7, 16 and 20) and experienced other deformations [Burtman, 2006a].


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Citation: Burtman, V. S. (2008), Nappes of the southern Tien Shan, Russ. J. Earth Sci., 10, ES1006, doi:10.2205/2007ES000223.

Copyright 2008 by the Russian Journal of Earth Sciences

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