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

Central Tien Shan

2007ES000223-fig15
Figure 15
2007ES000223-fig16
Figure 16
[172]  Nappe ensembles of the Atbashi-Inylchek and Kokshaal tectonic zones are situated in the central Tien Shan (Figures 15 and 16). The zones are separated by the Borkoldoi secondary overthrust fault, along which the Atbashi-Inylchek nappe ensemble was overthrusted onto the Kokshaal nappe ensemble. Stratigraphic sections of the autochthon and allochthons were described previously [Biske et al., 1985; Biske and Shilov, 1998; Burtman, 1976, 2006a].

The Atbashi-Inylchek Zone

[173]  The Atbashi-Inylchek nappe ensemble is formed by the Kokkiya (storey I), Chatyrkul (storey II), Keltubek (storey III), and Atbashi and Balykty (storey IV) units.

Kokkiya.

2007ES000223-fig17
Figure 17
[174]  The Kokkiya unit mainly includes shallow carbonate rocks. The base of the unit in the Ulanskii, Borkoldoi, and Torugart Ranges is composed of sandstones and clayey shales interbedded with basalt flows and limestones bearing Ludlow and Pridoli brachiopods and tabulates (Karachukur and other formations, 1000 m thick). They are overlain by an up to 4000-m-thick carbonate sequence that yields benthic faunal remains and foraminifers. The lower boundary of the sequence ranges within the Ludlow-Lochkovian; the upper, within the Bashkirian-Early Moscovian (Figure 17). Among the Middle-Upper Devonian carbonate sediments, volcanogenic sedimentary rocks and basalt and andesite-basalt lavas are recorded (Tekelitor and Ashusu formations, 100-1000 m thick). Petro- and geochemical properties of the basalts [Biske and Tabuns, 1996a] indicate an intraplate position of the volcanic manifestations and allow their association with a continental crust fracture. Limestones in the Lower Carboniferous part of the section contain chert beds. The section is crowned by a carbonate terrigenous flysch with olistostromes. The youngest fauna derived from the limestone olistoplaques is of Bashkirian age and the flysch bears Early Moscovian foraminifers.

[175]  The Kokkiya limestones also form olistoliths and olistoplaques in the Upper Paleozoic flysch. In the Kokshaal tectonic zone, in the Aksay oreade (Figure 15, 3) the flysch of the Maidantag unit includes gigantic tectonic blocks of limestones. They are probably olistoplaques that experienced tectonic impact in the Late Paleozoic. The limestones contain brachiopods, corals, and foraminifers indicating that the accumulation took place from the Ludlow to Early Moscovian (Kulsu, Sarybeles, and other formations, 2000 m thick).

[176]  The Kokkiya unit is located at the base of the nappe ensemble and was autochthonous in the Atbashi-Inylchek zone. Subsequently it was overthrusted onto the Kokshaal tectonic zone and now occupies an allochthonous position.

Chatyrkul.
[177]  The Chatyrkul unit is mainly composed of pelagic sediments. Its stratigraphic sections are substantially condensed.

[178]  The lower visible part of the sections is composed of clayey and siliceous shales yielding Llandovery graptolites, and of finely rhythmical flysch and carbonate terrigenous sediments bearing Late Silurian benthic fauna (Belkarasu, Karachukur, and other formations, 1000 m thick). The overlying Devonian and Carboniferous sediments are of variable structure which provides the distinction of several types of stratigraphic sections among them. The major types are carbonate siliceous, volcanogenic siliceous, and siliceous facies types of the sediments.

The carbonate siliceous type
[179]  of sediments is known in the Atbashi and Kokshaaltau Ranges. It is marked by accumulation in the Pridoli-Lochkovian of coral and crinoid limestones (Kargantash, Chirmash, and other formations, 300-1500 m thick). Upward from the base these rocks grade into a condensed sediment sequence (Karagir and other formations). The lower part of the sequence is composed of pelagic limestones bearing Pragian and Emsian tentaculitids and Frasnian conodonts; the upper part, of variegated argillites, siltstones, and radiolarites with Famennian and Tournaisian conodonts and Visean and Early Moscovian foraminifers. In the Atbashi Range the sequence is 200 m thick. These rocks are overlain by a 1000-m-thick terrigenous flysch with olistostromes. Limestone beds in the flysch sequence contain Early Moscovian foraminifers. Olistoliths in the olistostrome are built up of Bashkirian limestones. Sandstones contain schist grains derived from nappes of the upper structural storey.

[180]  Thrust sheets made up of sediments of the volcanogenic siliceous type participate in the structure of synform folds in the Dzhanydzher and Borkoldoi Ranges. The lower part of this type of section is composed of clayey and siliceous shales yielding Late Silurian graptolites. Upward from the base the shales alternate and then grade into volcanogenic sedimentary rocks and lavas of basalt and andesite (Chakyrkorum, Borlutor, and other formations, 1500 m thick) or basalt and rhyolite-dacite (Kainar Formation, 1000 m thick) composition. The contrast extrusive rocks were produced by the mantle and continental crust as evidenced by their geochemical characteristics [Biske and Tabuns, 1996a]. Limestone beds in the volcanogenic sequence contain Pridoli and Lower Devonian tabulates and tentaculitids and Emsian-Eifelian brachiopods and corals. Condensed pelagic sediments are recorded higher in the section. They are represented by cherts with micritic limestone beds bearing Tournaisian and Visean-Serpukhovian foraminifers (Kensu and other formations, 100-300 m thick), and are overlain by a flysch and olistostrome sequence with limestone olistoliths (Charatash Formation, 500 m thick).

[181]  Thrust sheets characterized by a siliceous type of sediments occur in the eastern Atbashi-Inylchek zone. Their stratigraphic section differs from the volcanogenic siliceous one in the lack of lavas, lesser proportion of tuffs, and lesser thickness.

[182]  In the geologic section of the Atbashi-Inylchek tectonic zone the Chatyrkul nappe sheets cover the Kokkiya unit.

Keltubek.
[183]  The Keltubek unit is formed by oceanic crust rocks. The Aigyrbulak oreade (Figure 15, 1) on the eastern slope of the Fergana Range represents a series of tectonic slices composed of mylonitized apoharzburgite serpentinites, flaser gabbro-amphibolites, basic volcanites and cherts. The lavas include limestone beds bearing Early Devonian corals.

[184]  The Akbeit oreade (Figure 15, 2) is located in the western Atbashi Range and represents a set of thrust sheets and slices. Its lower part is built up of serpentinite mélange that contains metamorphic schist and gabbro blocks and serpentinized pyroxenites and peridotites, gabbro and gabbro-amphibolites. Upward from the base they are replaced by cherts, tholeiitic basalts, hyalobasalts and their tuffs metamorphosed to greenschists and blueschists. Geochemical properties of the basalts correspond to that of mid-oceanic ridge rocks [Biske and Tabuns, 1996a]. The cherts contain Devonian conodonts.

[185]  In the Sarybulak oreade (Figure 15, 7), in the Atbashi Range, a thrust sheet composed of serpentinized peridotites and cumulative gabbroids is recorded. They are covered with a tectonic contact by 200-m-thick cherts bearing Middle and Late Devonian conodonts. Further upwards occur 700-m-thick, partially amygdaloidal and pillow tholeiitic basalts with chert beds.

[186]  The Dzhanydzher oreade (Figure 15, 4) is formed by thrust sheets and slices with a total thickness of 3 km on the northern slope of the Dzhanydzher Range. They are composed of ultrabasites and volcanites including pillow basalts. The ultrabasites represent a mélange with a serpentinite matrix and blocks of serpentinized peridotites, gabbro, basalts, cherts, greenschists, and ophiolitoclastic breccia-conglomerates. The cherts bear Middle-Late Devonian conodonts and Tournaisian radiolarians.

[187]  The Karaarcha oreade (Figure 15, 5) in the Kokshaal Range is an up to 2.5-km-thick thrust sheet made up of serpentinized ultrabasites, wehrlite-pyroxenite-gabbro cumulates, a complex of parallel dikes, and of basalts, hyaloclastites and cherts.

Atbashi.
[188]  The Atbashi unit in the Atbashi Range is composed of schists, gneisses, garnet amphibolites, marble, and glaucophane schists. Initially the rocks were metamorphosed to the epidote-amphibolite facies following which they underwent regressive metamorphism to the greenschist facies. The glaucophane schists were likely formed during a later dislocation metamorphism [Bakirov, 1978]. The discussed sequence probably includes crystalline limestones bearing Pridoli tabulates.

[189]  Lenslike eclogite bodies are recorded among the metamorphosed rocks. Geochemical properties of the eclogites correspond to those of mid-oceanic ridge basalts [Sobolev et al., 1989]. The Rb-Sr isochronous age of the eclogite metamorphism, defined by garnet, omphacite, phengite and by the rock, is 267 pm 5 Ma [Tagiri et al., 1995].

Balykty.
[190]  Thrust sheets of the Balykty unit participate in the structure of synform folds in the Dzhanydzher and Inylchek Ranges. They are composed of a 2000-m-thick rhythmical sequence of quartz and volcanic sandstones, siltstones, and clayey shales with limestone beds. The silty pelitic sediments in the Dzhanydzher Range are characterized by graded bedding. The rocks are partially metamorphosed to greenschists. Corals and floral remains indicate the Early Silurian-Early Devonian age of the sequence. In synform folds the Balykty unit overlies the Chatyrkul and Keltubek rocks.

[191]  Structure and geologic position of the Balykty unit suggest the accumulation of its sediments on the Kazakh-Kyrgyz continental rise, where they were partially metamorphosed on the accretionary prism formation.

Neoautochthon.
[192]  On the northern slope of the Atbashi Range the ophiolites are onlapped by nonsorted conglomerates bearing fragments of cherts from the ophiolite section and metamorphic schists of the Atbashi unit. The overlying limestones contain Gzhelian foraminifers. Still above-lying beds of this terrigenous sequence (Arpa Formation, 2000 m thick) include calcareous sandstones and limestones bearing Asselian foraminifers.

[193]  According to the position in the section and age of the rocks, the described sediments are analogous to Neoautochthon-2 in the western Tien Shan.

Kokshaal Zone

[194]  The Kokshaal tectonic zone includes the autochthonous Muzduk unit and allochthonous Maidantag unit (Figure 15).

Muzduk.
[195]  On the Tarim Depression margin, in the Kelpintag Range, the Silurian and Devonian beds are represented by a thick sequence of terrigenous and carbonate terrigenous sediments yielding acidic and intermediate lava beds. A significant part of the sequence is composed of flysch. The Devonian flysch is in places conformably or with an angular unconformity overlain by 1000-m-thick conglomerates and red cross-bedded quartz sandstones that were accumulated on an alluvial plain in the Early Carboniferous. The cross-bedding and conglomerate texture indicate the northern direction of the flows [Carroll et al., 1995]. North of the Kelpintag Range the red sandstones alternate with limestone breccias and still further grade into carbonate rocks bearing brachiopods, corals, and foraminifers.

[196]  In the Kelpintag Range the red sandstones are overlain by limestones with benthic fauna and foraminifers indicating that the limestones, with the thickness of up to 100 m, embrace the Late Visean-Asselian interval. This was the inner part of the northern Tarim carbonate platform. Further northward, in the Muzduk Mountains and other uplands, the Late Carboniferous-Asselian shallow carbonate sediments are 1000 m thick.

[197]  The rocks of the northern Tarim carbonate shelf are exposed in the eastern Kokshaal Range. The base of the carbonate sequence (Dzhangart and other formations, 2000 m thick) is there as old as the Emsian or Pragian and its top is Asselian in age (Figure 17). Limestones in the upper part of the section include beds of rhyolite and trachydacite tuffs and lavas. Upward from the base the limestones grade into carbonate terrigenous flysch containing olistostromes (Sauktor Formation, 700 m thick). The flysch bears Early Permian foraminifers.

Maidantag.
[198]  The Maidantag unit is formed by thrust sheets composed of pelagic sediments that were accumulated on a continental slope and rise. In the Maidantag and Kokshaal Ranges the visible basal part of the section is built up of terrigenous distal flysch with quartz prevailing in the clastic fraction (Airytor, Tysbel, Bozdzhalpak, and other formations, 2000-3000 m thick). The flysch contains Early-Middle Devonian tabulates and tentaculitids and, in the upper part, Late Devonian conodonts and flora. The Lower Carboniferous is represented by a condensed section of pelagic sediments (Karasainyn and other formations, 80-300 m thick), namely, by radiolarites bearing Early-Late Tournaisian conodonts, argillites and silicites with the Visean conodonts, and by variegated clayey micrites and calcarenites yielding Serpukhovian conodonts and goniatites. Among these sediments volcanogenic sedimentary rocks and andesite-basalt lavas are recorded in places.

[199]  Upward from the base the condensed sediments are replaced by terrigenous and carbonate terrigenous flysch with limestone and silicite olistoplaques (Kipchak and other formations). The flysch bears Late Bashkirian, Moscovian, Kasimovian and Asselian foraminifers and conodonts and is several kilometers thick.

[200]  The Ortosu oreade (Figure 15, 6) includes rocks that made up submarine volcanoes. The basal part of the thrust sheet is composed of siliceous and clayey shales bearing Devonian foraminifers. They are overlain by a 300-1500-m-thick sequence of tuff breccias, tuffs, and reef limestones alternating with basalt, andesite and trachyandesite lava flows. Geochemical records indicate the intraplate genesis of the basalts [Biske and Tabuns, 1996a]. The sequence is Givetian-Late Devonian in age as estimated from corals, stromatoporoids, and foraminifers. The Devonian rocks are overlain with a hiatus by 100-m-thick condensed carbonate siliceous sediments yielding Late Visean to Early Moscovian foraminifers. The section is terminated by a 500-m-thick coarse flysch bearing olistostromes and including Early Moscovian foraminifers in the sandstone cement.

[201]  In the northern foothills of the Maidantag Range the upper part of the unit is composed of the Aksay oreade thrust sheets and slices (Figure 15, 3). In the oreade's section the Llandovery-Lochkovian graptolite shales are overlain by a sequence of condensed sediments, among which prevail cherts (Karasainyn, Kuldzhakbashi, Urusai, and other formations, 200-400-m thick). The lower part of the sequence yields Emsian conodonts; the above sediments, Late Devonian-Serpukhovian radiolarians and conodonts; and the uppermost part, Early Baskirian conodonts, respectively. In places the limestone breccia with benthic fauna is recorded, probably as a result of turbidity currents activity.

[202]  Further upward from the base of the Aksay section a terrigenous flysch and olistostrome sequence occurs. The flysch bears Bashkirian and Early Moscovian foraminifers. The flysch sandstones include basalt grains likely derived from the upper nappes of the Atbashi-Inylchek tectonic zone. Several lenslike tectonic slices in the flysch are composed of thick shallow limestones of the Kokkiya unit, which most likely have initially got to the continental slope as gigantic olistoplaques. A brief characteristics of these limestones, the youngest of which are Early Moscovian in age, was given above, in description of the Kokkiya unit.

Correlation of Units

[203]  The Atbashi-Inylchek nappe ensemble is of four-storeyed structure. The Kokshaal ensemble consists of two storeys corresponding to the lower ones of the Atbashi-Inylchek nappe ensemble (Figure 17). In the Muzduk and Kokkiya units of the first storey the section is mostly composed of shelf carbonate sediments. The Muzduk unit in the Kokshaal zone is autochthonous representing a margin of the ancient Tarim massif. The Kokkiya unit, prior to its shifting by the Borkoldoi secondary overthrust fault, was autochthonous in the Atbashi-Inylchek zone. The Muzduk and Kokkiya units formed the inner and outer Tarim shelf, respectively.

[204]  The Maidantag and Chatyrkul units of the second storey are mainly composed of pelagic sediments accumulated on the Tarim continental slope. Composition of extrusive rocks that occur among the pelagic sediments also indicates that they were formed on a continental basement. The Maidantag-Chatyrkul nappe overlying the Muzduk and Kokkiya units resulted from a collision of the Kazakh-Kyrgyz and Alay-Tarim continental terranes.

[205]  Several allochthonous units, namely, Keltubek, Atbashi, and Balykty, located nearby the Turkestan oceanic suture, form the upper structural storeys of the Atbashi-Inylchek nappe ensemble. The primary relationships between these units in many cases were broken by secondary overthrust faults and thrust faults during and after the collision.

Deformation Stages

[206]  Tectonic structure of this region is more compressed compared to the western Tien Shan.

[207]  Prenappe deformations of unclear age are recorded in metamorphic rocks of the Atbashi unit. Several generations of these deformations referred to stage D-1 are recognized [Duk, 1995].

2007ES000223-fig18
Figure 18
[208]  Accretionary nappes were formed at stage D-2. In the course of the accretionary prism growth near the Kazakh-Kyrgyz terrane margin, the Atbashi rocks were overthrusted onto the younger oceanic crust sediments which formed the allochthonous Keltubek unit at a subsequent overthrusting stage (Figure 18).

[209]  At stage D-3 owing to the collision of the Kazakh-Kyrgyz microcontinent and Alay-Tarim continent, the accretionary prism that occurred near the Kazakh-Kyrgyz microcontinent margin was overthrusted on the continental slope of the Alay-Tarim continent, i.e. onto the rocks of the future Chatyrkul unit. The youngest Chatyrkul rocks are of Early Moscovian age. They include fragments of metamorphic rocks derived from the accretionary prism. The neoautochthon is of Gzhelian and younger age. These data indicate the Moscovian-Kasimovian age of stage D-3.

[210]  The collisional Maidantag-Chatyrkul nappe composed of the rocks that were accumulated in the Silurian, Devonian, and Carboniferous on the Alay-Tarim continental slope and rise, was formed at stage D-4. This complex together with the overlying allochthonous Keltubek, Atbashi, and Balykty units was overthrusted onto the autochthonous Muzduk-Kokkiya zone. The youngest rocks in the Muzduk section are estimated as the Asselian, marking the oldest age limit for the Maidantag-Chatyrkul nappe in the Kokshaal tectonic zone. In the Atbashi-Inylchek zone which is closer to the nappe roots, its shifting could have begun earlier (Figure 17).

[211]  The Carboniferous and Early Permian tectonic events provided the formation of an ensemble that included an autochthon and three storeys of primary nappes, among which those of the upper storey originated earlier than the lower ones (Figure 18). The geologic section includes downward from the top: the Balykty and Atbashi accretionary nappes (IV storey), the Keltubek accretionary collisional nappe (III storey), the Maidantag-Chatyrkul collisional nappe (II storey), and the Muzduk-Kokkiya autochthon (I storey).


RJES

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

Powered by TeXWeb (Win32, v.2.0).