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

Western Tien Shan

2007ES000223-fig03
Figure 3
[9]  The Northern Fergana, Southern Fergana, and Alay nappe ensembles formed in the Carboniferous and Early Permian, occur in the western Tien Shan. Every nappe ensemble and its neoautochthons occupy an area that is described below as a tectonic zone that is named as a certain nappe ensemble (Figure 3). The Gissar tectonic zone where nappes are insignificant or absent, also occurs in the western Tien Shan. Tectonic zones are separated by Late Permian thrust faults and strike-slip faults.

Northern Fergana Zone

2007ES000223-fig04
Figure 4
[10]  The Northern Fergana tectonic zone includes the Baubashata mountain group and the Atoinak Range. The zone is bounded by the following Late Permian faults: Talas-Fergana strike-slip fault on the northeast, Karaunkur strike-slip fault on the south, Atoinak strike-slip thrust fault on the north, and Chat (West Karasu) thrust fault on the west (Figure 4).

2007ES000223-fig05
Figure 5
[11]  The geologic section begins with the Baubashata unit that is overlain by the Ontamchi, Kerey, and Shaydan units (Figure 5). Stratigraphic sections in the Northern Fergana zone were described by Biske and Porshnyakov [1974], Burtman [2006a], Burtman and Klishevich [1971].

Baubashata.
[12]  The Baubashata unit is exposed in cores of antiform folds. Its basal part is composed of 2000-m-thick sequence bearing Silurian faunal remains. The sequence is characterized by tectonic relationships between polymictic and quartz sandstones and clayey shales bearing Llandovery, Wenlock, and Ludlow graptolites. The sequence includes basalts, andesites, volcanic tuffs and breccias as well. In the upper part of the sequence shales alternate with the limestone that contains Ludlow-Pridoli brachiopods, corals, and trilobites (Karaunkur Formation).

[13]  The discussed sequence is unconformably, with tectonic contact overlain by the Late Silurian and Devonian carbonate and volcanogenic rocks. The stratigraphic section begins with the 1500-m-thick limestone bearing Late Ludlow, Pridoli and Early Devonian corals and brachiopods. Volcanogenic rocks (Bossogotash and other formations, up to 2000 m thick) wedge in the Devonian limestones along their extension. In the lower part of the formation volcanites are of contrasting composition and include dacite, rhyolite, and basalt. The upper part of the formation is composed of subalkalic andesite-basalt and basalt. The limestones occurring among volcanites yield remains of the Early Devonian, Eifelian, and Givetian organisms. Beds of volcanic rocks disappear upward from the base, whereas accumulation of 3000-m-thick carbonate sediments bearing corals, brachiopods, goniatites and foraminifers, continued up to the Early Bashkirian. The upper part of the stratigraphic section is composed of conglomerates with limestone pebble and blocks and coarse flysch bearing Bashkirian foraminifers (Konurtyube Formation, 400 m thick). The erosion that preceded the conglomerate accumulation reached in places the Visean rocks.

Ontamchi.
[14]  Sediments of the Ontamchi unit were mostly accumulated in deep-water conditions. Clayey, clayey siliceous and silty shales bearing Late Llandovery and Wenlock graptolites are exposed at the base of the visible stratigraphic section. Among the shales sandstone and conglomerate beds as well as dacite and basalt flows occur. Petrochemical properties of the lavas indicate their island arc origin [Biske, 1996]. The Upper Silurian section is composed of clayey and clayey siliceous shales and polymictic sandstones alternating with reef limestones bearing Ludlow and Pridoli corals and brachiopods and with basalts, andesites, and tuffs (Kyzkurgan, Seresu, Turasu, and other formations, 1500 m thick). The Lower Devonian is represented by tuffs and shales with tentaculitids; the Eifelian, by basalts, andesites, tuff-breccias, and tuffs interbedded with limestone bearing brachiopods and corals (Airytash Formation, 500 m thick).

[15]  Upward from the base siliceous siltstone and bedded limestone with conodonts and radiolarians (Chichar and other formations) are recorded. This sequence is 100 m to 300 m thick and corresponds to the Givetian-Serpukhovian interval. Its is unconformably overlain by the 300-m-thick Late Serpukhovian-Early Bashkirian conglomerates, sandstones, clayey shales, cherts and siliceous limestones bearing foraminifers. The sequence contains olistostromes with the Visean limestone olistoliths.

[16]  Rocks of the Ontamchi unit compose thrust sheets and slices. The stratigraphic succession of rocks described above cannot be observed in an uninterrupted section. The rocks are in tectonic interrelationships and are partly transformed into mixtite with Silurian shales as a matrix.

Kerey.
[17]  The Kerey unit is formed by weakly metamorphosed spheroidal basalts and picrites that are up to 2000 m thick in the Alashtau oreade (Figure 4, 1). Lavas alternate with chert beds and tectonic lenses of serpentinized ultrabasite. Volcanogenic rocks are of Early-Middle Devonian age. They are overlain by the Givetian-Serpukhovian deep-water siliceous carbonate sediments.

Shaydan.
[18]  The basal part of the Shaydan unit is formed by a thrust sheet composed of gabbro and ultrabasite rocks. The lower part of the sheet is formed by serpentinized harzburgite and dunite along with serpentine mélange bearing blocks of basic volcanites, metamorphic schists, and gabbroids. Pb-Pb zircon age from dunite derived by a thermoion emission method, is 532 pm 12 Ma old. These rocks are overlain by alternated wehrlites and lherzolites that are replaced upwards by pyroxenite and flaser gabbro grading into gabbro amphibolite. U-Pb and Pb-Pb zircon ages from gabbro were estimated in the interval of 395-475 Ma. The upper part of the section is composed of a complex of parallel diabase and gabbro diabase dikes. According to petrochemical properties, the dike rocks are intermediate between komatiite and oceanic tholeiite [Hristov et al., 1986; Komarevtsev et al, 1987; Kurenkov et al., 2002].

[19]  The 2500-m-thick gabbro ultrabasite complex is overlain by a 1500-m-thick thrust sheet that is mostly composed of volcanogenic rocks metamorphosed to greenschists including glaucophane schists (Maylisu and Akdzhol formations). Petrochemical properties of metamorphic rocks correspond to oceanic tholeiite basalts. Chert beds among metamorphic schists contain conodonts of likely Silurian age. The metamorphosed rocks are overlain by clastic and siliceous carbonate sediments bearing conodonts of all Devonian epochs and Visean-Serpukhovian foraminifers and goniatites. Volcanogenic rocks of the oceanic crust in the Shaydan unit were metamorphosed not later than the Early Devonian.

Neoautochthon.
[20]  Neoautochthon rocks occur in synform folds formed at later stages of deformation.

[21]  Neoautochthon-1 overlaps the Serpukhovian rocks of the Shaydan unit and transgressively overlies the lower beds of the unit. Neoautochthon-1 is composed of conglomerate, sandstone, and limestone beds bearing Early Moscovian foraminifers and brachiopods (Chaak and other formations, 1000 m thick).

[22]  Neoautochthon-2 transgressively overlies the Shaydan, Kerey, and Ontamchi units and Neoautochthon-1. The section begins with conglomerate, sandstone, and limestone members bearing Late Moscovian brachiopods and foraminifers (Suoktyube Formation, 1500 m thick). Upward from the base they are replaced by a sequence mainly composed of proximal terrigenous flysch with limestone beds (Bekechal, Turduk, and other formations, 3000 m thick). Foraminifers and brachiopods from this sequence are of Late Moscovian, Kasimovian, Gzhelian, Asselian, and Sakmarian age.

[23]  Neoautochthon-3 unconformably overlaps the eroded Shaydan, Kerey, Ontamchi, and Baubashata units and Neoautochthon-2. It is represented by continental sediments bearing Late Permian flora (Kelematy and Karasu formations, 2000 m thick).

Southern Fergana Zone

2007ES000223-fig06
Figure 6
[24]  The Southern Fergana tectonic zone occupies northern slopes and foothills of the Turkestan and Alay ranges and the Karaunkur region of the Fergana Range (Figures 4, 6 and 7). The zone is bounded by Late Permian faults, namely, by the Karaunkur strike-slip fault on the north and by the Uzgen-Sanzar thrust fault on the south and on the east.

[25]  Nappes of the lower storeys are formed by rocks that were accumulated on shelf and continental slope of the Alay-Tarim terrane (Isfayram and Abshir units); nappes of the upper storeys, by oceanic crust rocks (Taldyk and Shankol units). The Kan unit holds a peculiar place among them likely representing a fragment of the Turkestan oceanic basin's suture. The Karaunkur region also includes the Toguzbulak unit that contains sediments probably accumulated and metamorphosed in an accretionary prism.

[26]  Stratigraphic sections representing units of the Southern Fergana tectonic zone were described by Biske [1996], Burtman [1976, 1984, 2006a], Burtman and Klishevich [1971], and Porshnyakov [1973].

Isfayram.
[27]  The Isfayram unit is composed of a thick shallow-water carbonate sequence and of pelagic sediments that underlie and overlie the carbonate rocks.

[28]  In the Alay and Turkestan Ranges the basal part of the visible stratigraphic section is formed by clayey and siliceous shales and sandstones bearing Llandovery, Wenlock, and Ludlow graptolites. Basalt and tuff beds occur as well. The shales and overlying limestones contact along a tectonic boundary owing to different physical properties of the rocks on their deformation.

[29]  Base of the carbonate sequence occurs at various levels from the Ludlow to Middle Devonian in different sections. In the northern sections it is commonly older. The carbonate sequence is 2000 m to 4000 m thick and is rich in fossil macro- and microfauna. Relation of thicknesses of the Devonian and Carboniferous parts, as well as the structure and composition of carbonate rocks differ in certain sections; various section types were distinguished [Porshnyakov, 1973]. The Alay type of stratigraphic section corresponds to an internal area of carbonate accumulation; the Yauruntuz and Aktur types are formed by rocks accumulated in external parts of the area.

[30]  The carbonate sequence is mainly composed of shallow organogenic and organoclastic rocks. The upper Silurian part of the section contains quartz sandstone beds; the Early Devonian includes chert, alkalic basalt, and tuff layers. In the Sokh-Isfayram interfluve the Givetian carbonate coral-bearing rocks include a 500-m-thick lens of alkalic lavas and acidic and basic tuff. In the Serpukhovian-Early Moscovian relatively deeper conditions provided accumulation of clayey and clastic bedded limestone with chert layers. In these sediments (Gaz and other formations, 50 m to 500 m thick) a hiatus at the Bashkirian base is commonly recorded; Serpukhovian limestones are occasionally missing in the section. The top of the carbonate sequence rises southward within the Lower Moscovian Substage.

[31]  The Lower Moscovian limestone is overlain by a flysch and olistostrome sequence of variable structure and thickness (Tolubay, Malyaran, and other formations, 50-1000 m thick). Its lower part is composed of finely rhythmic calcareous sandy clayey flysch. Upward from the base the flysch becomes coarser and conglomerate and olistostrome beds are recorded. The largest olistoplaques reach hundreds of meters. Large blocks and olistoplaques are composed of shallow authigenic limestone. The form and trains of olistoliths indicate that they entered the basin from the south (in modern system of coordinates). The olistostrome section demonstrates a reversed stratigraphic succession of authigenic limestone olistoplaques, which corresponds to consecutive destruction of the limestone massif; the Early Moscovian olistoplaques occur at the base and are followed by the Early Carboniferous, Devonian, and Late Silurian, respectively. The olistoliths and conglomerate pebble include cherts and pelagic limestone. The upper part of the sequence contains blocks bearing fragments of condensed siliceous carbonate section of the overlying Abshir unit. The youngest foraminifers from olistoliths are of Early Moscovian age, whereas flysch sediments yield the Early and Late Moscovian foraminifers. The Early Moscovian time was characterized by the authigenic clastic material accumulation, in the Late Moscovian it was supplemented with pelagic rock fragments from the overlying Abshir nappe.

[32]  In the Alay Range on the south of the discussed tectonic zone the Early Moscovian flysch is conformably overlain by conglomerate sequence with siliceous shale, siltstone, and sandstone beds bearing fragments of the Moscovian-Kasimovian foraminifers and Late Carboniferous plants (Surmetash Formation, 2000 m thick). Pebble and boulders in the conglomerate are of siliceous and carbonate composition.

[33]  In the Karaunkur region of the Fergana Range the Isfayram rocks are exposed in tectonic windows (Figure 4). The carbonate sequence is about 2000 m thick there. The lower part of the section contains Wenlock and Late Silurian corals and brachiopods; the higher beds, Devonian and Early Carboniferous forms; and sediments nearby the top, Serpukhovian foraminifers. The limestones are overlain by 300-m-thick conglomerate and sandstone member that includes wildflysch beds bearing limestone blocks. Clastic limestone beds yield probably redeposited Bashkirian foraminifers.

[34]  Sediments of the Alay type are autochthonous on the most part of the described tectonic zone. Near a margin of the Southern Fergana zone they were overthrusted onto the rocks of the adjacent tectonic zone. Carbonate and siliceous sediments in sections of the Yauruntuz and Aktur types, which were accumulated on the outer shelf, compose olistoplaques in the tectonized olistostrome of the Abshir unit and klippes of secondary nappes and thrust faults.

Abshir.
[35]  Accumulation area of the Abshir unit included a part of outer shelf and extended continental slope and foot of the Alay-Tarim passive margin. Stratigraphic sections of the unit are mainly composed of pelagic sediments.

[36]  On northern slopes of the Alay and Turkestan Ranges the clayey carbonaceous shales with phthanite strata are recorded at the base of the Abshir nappe thrust sheets. The shales upward from the base and along the strike grade into a distal terrigenous flysch with olistostrome beds (Syuget, Pulgon, and other formations). The shales and flysch yield Llandovery, Wenlock, and Ludlow graptolites. In places the sequence includes Pridoli sediments. Quartz sandstones were recorded in the Wenlock part of the section. Stratigraphic succession of Silurian sediments is disrupted, which makes difficult a real estimation of their thickness. It is evaluated for each stage at several hundred meters. Some sections include volcanogenic rocks. A calcareous alkalic volcanite sequence is distributed in the Alay Range. It is represented by basalts, dacites, rhyolites, and their tuffs (Karatyube Formation, 600 m thick) bearing shale beds with Late Llandovery-Wenlock graptolites.

[37]  Sections of several types, namely, carbonate, carbonate siliceous, siliceous, and volcanogenic siliceous, are distinguished in the Pridoli, Devonian, and Lower Carboniferous sediments. Most of sections are condensed and include sediments of low thickness, which were accumulated during a long-term time interval.

Carbonate type
[38]  (Kokbeles) is known from tectonic slices and olistoplaques. The 100-m-thick Pragian-Eifelian sequence of conglomerates, sandstones, and limestones is overlain by a 200-400-m-thick dolomite and limestone member corresponding to the Givetian-Moscovian. In other sections the condensed Givetian-Moscovian carbonate rocks overlie the thicker Late Silurian-Early Devonian limestones. Carbonate type of the Abshir unit section is transitional to sections of the Isfayram unit.

Carbonate siliceous type
[39]  (Shakhimardan) is composed of carbonate flysch and siliceous rocks. The Alay Range in the Hamzabad oreade (Figure 6, 3) is characterized by flysch with clastic limestone strata bearing Pridoli and Lochkovian graptolites, corals, and brachiopods (Krukkel Formation, 500 m thick). Flysch is overlain by rhythmically alternated bioclastic limestones and cherts with clayey shale beds in the lower part of the section (Talbulak and other formations, 1000 m thick). Graptolites, brachiopods, tentaculutids, ammonoids, and corals indicate the Lochkovian to Givetian age of the sequence. The higher part of the section is formed by the relatively thin (100 m to 500 m) Upper Devonian and Lower Carboniferous limestones.

[40]  In the eastern part of the Turkestan Range, in the Kyk oreade section (Figure 6, 8) several conodont zones of the Emsian, Eifelian, Givetian, Famennian, Tournaisian, and Serpukhovian are absent, likely owing to stratigraphic hiatuses. The upper part of the limestone contains Early Bashkirian conodonts. The section of the discussed type is crowned by the Late Bashkirian or Early Moscovian conglomerates, sandstones, and clayey shales.

Volcanogenic siliceous type
[41]  (Okzhatpes) is characteristic of the Dzhindy oreade in the Karaunkur region (Figure 4, 2). The oreade is composed of several tectonic slices. Their basal part is represented by the Lower Devonian cherts, lavas, and basic and intermediate tuffs with coral- and brachiopod-bearing limestone beds. This sequence is overlain by limestone with Givetian corals and cherts and limestones bearing Serpukhovian and Bashkirian foraminifers. The upper part of the section is composed of proximal terrigenous flysch with olistostrome and conglomerate strata that contain limestone fragments with Bashkirian foraminifers.

Siliceous type
[42]  (Shalan) of section is the most widespread. It is composed of variegated radiolarite, spongolite, phthanite, and limestone members, with siliceous clayey shale and turbidite beds (Tomasha, Bidana, Talbulak, and other formations). According to conodont and foraminifer records, the base of the sequence is dated in various sections as the Lochkovian, Pragian, or Emsian, and the top corresponds to the Serpukhovian or Early Bashkirian. In some sections hiatuses were recorded. Siliceous rocks strongly prevail in the Devonain and Tournaisian; proportion of the pelagic limestone increases upward from the base. The total thickness of the sequence ranges from 40 m to 400 m.

[43]  Upper part of the Shalan-type section is formed by a thick flysch and olistostrome sequence. The limestone beds in the coarse flysch yield Serpukhovian-Bashkirian and Early Moscovian foraminifers. The oldest fossil fauna in the olistoliths is of Cambrian age and the youngest, of Early Moscovian age.

Abshir mixtite.
[44]  A considerable part of the Abshir unit was changed into mixtite with a matrix of intensely deformed Silurian shales. The widespread mixtite contains larger and smaller authigenic blocks of Silurian, Devonian, and Carboniferous rocks derived from the siliceous, volcanogenic siliceous, and other types of sections of the Abshir unit.

[45]  Another kind of mixtite includes blocks of authigenic and allothigenic origin. Silurian graptolite shales and blocks of Shalan rocks are mixed in it with fragments of the olistostrome formed in the Late Carboniferous prior to and during the nappe formation. Allothigenic blocks are composed by (a) limestone bearing a shallow-water Late Silurian, Devonian, and Early Carboniferous fauna from the Isfayram unit; (b) andesite, trachyandesite, andesite-dacite, and their tuffs of authigenic origin or derived from the Isfayram unit; (c) ophiolite fragments, namely, basalt and tuffs from the Taldyk unit, gabbro-pyroxenite cumulates from the Taldyk or Shankol units, and apobasite metamorphic greenschists from the Shankol unit.

[46]  Among the Abshir mixtite large and gigantic olistoplaques are recorded. Part of them are authigenic, formed by siliceous rocks of the Shalan section. Others, of hundred meters in size, are composed of the Taldyk basalts. A great block of volcanogenic rocks, likely a tectonically transported gigantic olistoplaque, occurs in Karachatyr Mountains among the Abshir mixtite (Figure 6). The block is formed by rhyolite, dacite, basalt, tuffs, and by an olistostrome with volcanomictic cement and Cambrian limestone olistoliths (Dedebulak Formation, 500 m thick). According to petrochemical properties, the volcanogenic rocks correspond to island arc volcanites [Vanina, 1988].

[47]  Most of gigantic olistoplaques are composed of an outer shelf carbonate sediments derived from the Isfayram unit and Kokbeles-type section of the Abshir unit. The limestones have tectonic contacts with the Abshir mixtite. They compose the Katran, Yauruntuz, Aktur, Katranbashi (Figure 2), and other large mountain chains. The limestones are overthrusted onto the Abshir mixtite and are overlain by it. These limestone massifs, several kilometers thick and tens of kilometers long, probably are gigantic olistoplaques. During the overthrusting and subsequent deformations the olistoplaques' base was a surface along which the massifs were squeezed out. The process resulted in formation of secondary overthrust and thrust faults observed nowadays at boundaries of the limestone massifs.

[48]  Mixtites were formed mainly owing to gravitational processes initiated or intensified by the Alay-Tarim continent and Kazakh-Kyrgyz microcontinent collision that began in the Moscovian. The flysch and olistostrome sequence was accumulated in the Bashkirian and Moscovian on the gentle and extended Alay-Tarim continental slope. During the collision of the Alay-Tarim and Kazakh-Kyrgyz terranes the width of the Abshir facies zone decreased and the grade of continental slope rose. This resulted in gravitational intermixing of the accumulated sediments. In the Moscovian fragments of shallow limestone and ophiolites entered the mixtite from the Alay-Tarim continent (from the south in modern system of coordinates) and from the set of ophiolitic nappes that overthrusted from the opposite side, respectively. Subsequently the gravitational mixtite was overlain by ophiolitic nappes and underwent a tectonic processing. Its matrix was fragmented and secondary overthrust and thrust faults together with drag folds were formed. Finally the gravitational mixtite was transformed to tectonite.

Taldyk.
[49]  The Taldyk unit is composed of weakly metamorphosed ophiolites and overlying pelagic sediments. The Taldyk unit rocks overlie the Abshir unit and compose blocks in the Abshir mixtite.

[50]  The Sarytal oreade (Figure 6, 9) represents an allochthonous body located in a synform fold. The lower 500 m of the allochthon are formed by dunite, harzburgite, pyroxenite, cumulative gabbro-norite rocks and ophicalcite breccias. Pb-Pb zircon age from pyroxenites obtained by a thermoion emission method, is 1330 pm 12 Ma old [Komarevtsev et al., 1987]. These members are overlain by volcanogenic sedimentary rocks beginning with 25-m-thick hematite cherts bearing Early Ordovician conodonts. Among them are conglomerate lenses with gabbride pebble. Contact of these rocks with the underlying ophicalcite breccia is complicated by a thick ultrabasite-basite sill but is retained in places beneath the sill. The cherts are overlain by 150-m-thick spheroidal amygdaloidal olivine basalts and picrites with chert layers bearing Early-Middle Ordovician radiolarians and conodonts. Further above are 500-m-thick pillow tholeiitic basalts with beds of hyaloclastite, sandstone, and cherts bearing Llandovery graptolites and Late Silurian-Early Devonian radiolarians.

[51]  The Batken oreade (Figure 6, 2) is a deformed allochthonous thrust sheet. Its basal portion is composed of 150-m-thick olivine basalts overlain by pillow-pipe tholeiitic basalts with chert, hyaloclastite, and volcanomictic sandstone beds. Blocks of siliceous and carbonate rocks found among the lavas contain Silurian graptolites, Early-Middle Devonian foraminifers, and Late Devonian conodonts.

2007ES000223-fig07
Figure 7
[52]  The Kyrgyzata oreade (Figure 6, 6) is composed of several thrust sheets crumpled in a synform fold (Figure 7). This allochthon is 90 km long and up to 20 km wide; the number of thrust sheets and slices vary in its different parts. On the west the basal part of the unit is formed by a thrust sheet (0.5 km to 7 km) composed of gabbro and ultrabasite rocks that are mostly transformed into a serpentinite mélange. The ultrabasites are overlain by a 200-m-thick mélange formed by basalt, ultrabasite, chert, and limestone blocks.

[53]  Volcanic rocks and reef limestones are recorded further upward from the base. Secondary overthrust faults separate the section into several tectonic slices with a total thickness of 1000 m. Extrusive rocks are represented by pillow and pipe basalts. The Early-Middle Devonian and Tournaisian conodonts were found in limestone and chert beds among basalts and clastites.

[54]  A higher stratigraphic position in the Kyrgyzata oreade section is occupied by a thick thrust sheet which base is composed of the 100-m-thick tuff sandstone and tuff siliceous rocks bearing Silurian radiolarians and Devonian conodonts. They are overlain by a 1000-m-thick basalt hyaloclastite of the Yash Formation which is in turn covered by pipe-pillow lavas of the 1000-m-thick Aravan Formation including basalts, picrites, and komatiites with limestone lenses bearing Emsian conodonts and Middle Devonian radiolarians. Volcanogenic rocks are cut by sills and swarms of picrite and basalt dikes, part of which refer to a dike-into-dike type. The lavas are overlain by a 500-m-thick sequence of variegated siliceous shales with tuff, sandstone, and limestone beds. The Middle Devonian radiolarians and Early Carboniferous foraminifers were encountered at the base and in the middle part of the sequence, respectively.

[55]  The Hodjagoir oreade (Figure 6, 4) consists of the two allochthonous thrust sheets. The lower, 1000-m-thick sheet is composed in its basal and middle parts of phthanite and hyaloclastite with reef limestone bearing Devonian conodonts. These rocks are overlain by a 100-m-thick tuff sandstone and red jasper member bearing Givetian, Frasnian, and Tournaisian conodonts and, in the upper part, Serpukhovian goniatites.

[56]  The section of the upper thrust sheet begins with a phthanite and siliceous siltstone member that contains Llandovery and Lochkovian-Pragian conodonts. It is overlain by 600-m-thick hyaloclastites and 1500-m-thick pillow-pipe tholeiitic basalts with gabbro, gabbro-diabase, and diabase dikes and thick gabbro-peridotite sills. The volcanites are covered with 100-m-thick siliceous shales and tuffs bearing Middle-Late Devonian radiolarians and Late Devonian-Early Carboniferous ammonoids and goniatites.

[57]  The Akbura oreade (Figure 6, 1) section consists of basalts interbedded with cherts, tuffites, and limestones bearing Middle Devonian corals. They are overlain by a 180-m-thick variegated chert member containing Givetian and Late Devonian radiolarians. Above is a 50-m-thick member of breccia, sandstone, limestone, and cherts with the Visean foraminifers.

[58]  The Kuroves oreade (Figure 4, 3) is located in the Fergana Range. The lower part of its visible section is composed of clayey shales with the Late Llandow graptolites, Wenlock and Ludlow coral-bearing limestones, and sandstones and clayey shales bearing Ludlovian graptolites. Upward from the base lies a 1500-m-thick member of basalts, picrites, and tuffs interbedded with cherts and limestones containing the Early-Middle Devonian corals. The upper part of the section is formed by variegated cherts and terrigenous flysch of the 2000-m-thick Teleksay and Kirkichin formations. Scarce foraminifers define the age of the sequence as the Early Carboniferous. The rocks form several tectonic slices that include serpentinized ultrabasite lenses.

[59]  Klippes and olistoliths composed of the Taldyk unit rocks are also known in other areas of the Southern Fergana tectonic zone.

[60]  In the Alay Range the basalts that are the oldest in the Western Tien Shan fragments of rocks likely derived from the Turkestan oceanic crust, are recorded. The tectonic slice includes basalts interlayered with phthanites, sandstones and limestones yielding Toyonian (Early Cambrian) archaeocyathids and brachiopods. In another exposure basalt fragments form a breccia with carbonate and carbonate tuff cement. The breccia beds alternate with a limestone bearing Tommotian (Early Cambrian) archaeocyathids. These rocks, about 300 m thick, compose a block in the mixtite with a matrix formed by Silurian shales of the Abshir unit.

Origin of the rocks and conditions of sediment formation.
[61]  The rocks of the layered, dike and volcanogenic complexes of the Taldyk unit belong to the tholeiitic petrochemical series. The lower part of the visible section is composed of serpentinized harzburgite, peridotite, dunite, and lherzolite. The layered complex that represents an alternation of gabbro and peridotite, is characteristic of a marginal subduction-related basin or of island arc [Abakumova and Nenakhov, 1988].

[62]  The dikes are composed of basalt, picrite, and gabbro. Picrite dikes are attributed to the early generation, gabbro-diabase and basalts to the later one. The dike swarms and sills are irregularly distributed among lavas, part of dikes is of a dike-into-dike structure. This dike complex demonstrates a dispersed type of spreading characteristic of the oceanic bottom beyond a mid-oceanic ridge and of marginal basins. Petrochemical and geochemical properties of dikes from the Kyrgyzata ophiolithes indicate that they were formed in a mid-oceanic ridge or on oceanic islands [Kurenkov et al., 2002].

[63]  The lavas consist of picrite, tholeiitic and subalkalic basalt, and hyaloclastite. Discrimination of tholeiitic basalts in various diagrams shows that according to petrochemical and geochemical properties they are close to volcanites from mid- oceanic ridges and to intraplate oceanic basalts [Abakumova and Nenakhov, 1988].

[64]  Basalts of the Sarytal oreade were erupted from the Early Ordovician to Early Devonian with great interruptions; in the Akbura, Kyrgyzata, Kuroves, and Hodjagoir oreades eruptions occurred in the Early-Middle Devonian; in the Batken oreade, in the Late Devonian.

[65]  In the Sarytal oreade section the volcanite composition changes from low-Ti picrites and olivine basalts at the base to tholeiites in the mid-section and subalkalic high-K and high-Ti basalts and tuffs at the top. The rare-earth element spectrum in the Ordovician low-Ti lavas is characteristic of marginal sea spreading zones. The graywacke sandstones lying among the volcanites in the upper part of the section contain andesite-basalt, andesite, and dacite fragments suggesting the occurrence of a volcanic arc. Geochemical peculiarities of the graywackes indicate that the clastic material was produced by an oceanic island arc [German and Budyanskii, 1990]. The presence of Early Ordovician silicites at the base of the basalts and their stratigraphic contact with the underlying ophiolitoclastic breccia in the Sarytal section testifies the oceanic crust spreading beyond a mid-oceanic ridge in the Early Ordovician [Kurenkov et al., 2002]. The spreading likely occurred in the basin separated by a volcanic island arc, which rocks later were a source of the graywacke material.

[66]  Consequently, the Taldyk unit includes fragments of the Turkestan oceanic crust, namely, ultrabasites and cumulative gabbroids, and of basalts and the overlying pelagic sediments. The Ordovician lavas of the Sarytal unit were erupted onto oceanic bottom in a marginal basin separated by an oceanic island arc. Other allochthonous sheets contain Devonian lavas that were erupted in a mid-oceanic ridge and outside it. Among peculiarities of ophiolitic sections in the discussed area are a fragmentary dike stratum and occurrence of thick basaltic hyaloclastites erupted at a relatively small depth.

[67]  The condensed sediments overlying the lavas are composed of siliceous rocks and tuffaceous clastites bearing pelagic faunal remains. These pelagic sediments were accumulated in the Givetian-Serpukhovian interval. Reef limestones occurring among the lavas were likely formed on oceanic islands.

Shankol.
[68]  The Shankol unit is composed of greenschists, glaucophane schists, amphibolites and the overlying weakly metamorphosed sediments. Primary composition of metamorphic rocks was represented by basic lavas, tuffs, sandstones, and pelites. Tectonic serpentinite lenses in places occur at the base of the unit.

[69]  A high-baric metamorphism occurred at the earlier stage; subsequently diaphthoresis in the accretionary prism produced a greenschist formation [Bakirov and Sakiev, 1999]. The Rb-Sr age of the rock metamorphism is 475 pm 49 Ma old. The Pb-Pb zircon age from metabasites derived by a thermoion emission method, is 935 pm 60 Ma old [Duk, 1995].

[70]  The Kumbel oreade (Figure 6, 7) is a deformed thrust sheet over 1000 m thick, lying in the Kyrgyzata synform fold on the Taldyk rocks. The sheet is composed of schists that are unconformably overlain by nonmetamorphosed proximal carbonate terrigenous flysch bearing Late Silurian corals, brachiopods, and graptolites (Kaindy Formation, 150 m thick).

[71]  In the Namazdek Mountains (Figure 2) metamorphic schists are overlain by conglomerates with schist boulders. Conglomerates alternate with sandstone, siltstone, and limestone beds yielding Early Devonian corals. Nearby the schists in the Aldyyar Mountains a conglomerate sequence with metamorphic schist pebble is exposed. The conglomerates are interlayered with sandstones, siltstones, and limestones bearing Late Silurian corals, brachiopods, and graptolites.

Kan.
[72]  The Kan unit is formed by serpentinite mélange exposed at the foothills of the Alay Range in a stripe 25 km long and up to 4 km wide (Figure 5). The mélange is composed of serpentinites that contain numerous blocks and boulders of ophiolitic rocks (pyroxenite, ophicalcite, gabbroids, basalts, and cherts), rocks metamorphosed to greenschists and glaucophane schists, as well as boulders of weakly metamorphosed clastic, siliceous, and carbonate sediments. Basalts, basalt tuffs, gabbro, cherts, and pelites are recorded among metamorphic rocks. The largest blocks are over 1000 m in size. The Early-Middle Devonian and Famennian conodonts were encountered in metamorphosed cherts that occur among metabasites.

[73]  Initial relationships between the Kan and other units are not retained. They are separated by a neoautochthon, young fault, and young sediments. North of the Kan unit the Paleozoic Kazakh-Kyrgyz microcontinent rocks are located. The structure and position of the Kan serpentinite mélange make it possible to consider it as the Shankol nappe root zone which marks the position of the Turkestan oceanic basin suture. West and east of the Kan mélange area the Turkestan oceanic suture is hidden beneath younger sediments.

Toguzbulak.
[74]  This unit situated in the Karaunkur region (Figure 4) is formed by terrigenous sediments irregularly metamorphosed to phyllites and greenschists that contain beds of cherts, quartz sandstones, and marble (Toguzbulak Formation, 1500 m thick). Serpentinized ultrabasite lenses occur among the schists. Contrary to the Shankol metamorphic section, volcanic rocks were not recorded in the Toguzbulak unit. Age of the rocks is unknown and primary tectonic contacts of the unit with other ones are not retained. It is quite probable that the Toguzbulak unit represents rocks of the Kazakh-Kyrgyz continental rise, which were metamorphosed in the Devonian or Carboniferous on formation of accretionary prism near the microcontinent margin.

Neoautochthon.
[75]  The neoautochthon sediments transgressively overlie the nappe ensemble of the discussed tectonic zone.

Neoautochthon-1.
[76]  The Kan serpentinite mélange is overlain with stratigraphic contact by serpentinite sandstones and other ophiolitoclastic rocks (gravelstone, conglomerate, and breccia) with olistostromes. The olistoplaques and olistoliths in olistostrome and the blocks in sedimentary breccias are composed of schists, cherts, and limestones. The olistoliths and blocks contain Devonian and Early Carboniferous foraminifers, conodonts, and corals. The Serpukhovian conodonts, goniatites, and foraminifers were identified in the serpentinite sandstones and chert beds. This clastite and olistostrome sequence is over 500 m thick. It was accumulated in accretionary prism on a continental rise or in oceanic trench. Upward from the base these rocks grade into carbonate flysch with beds of olistostromes, conglomerates, and limestones bearing Serpukhovian, Bashkirian, and Early Moscovian foraminifers, goniatites, and conodonts (Shuran, Karatanga, and other formations, 2000 m thick).

[77]  Thus the Kan oceanic crust rocks were transformed to mélange and in the Early Carboniferous were overlain by the Neoautochthon-1 sediments that were deposited on the continental rise up to the Early Moscovian.

[78]  The section of that kind is known in the Karachatyr Mountains (Koksarai Formation, Kalmakbulak, 1000 m thick) where Neoautochthon-1 overlies rocks of the Shankol unit. The basal part of the section is composed of serpentinite and limestone conglomerates and breccias with schist fragments. Further upwards they are followed by a carbonate terrigenous flysch bearing foraminifers and ammonoids of the Visean (likely redeposited) to Early Moscovian age.

Neoautochthon-2.
[79]  In the Alay Range Neoautochthon-2 unconformably overlies the Shankol, Taldyk, and Abshir units. The lower part of the section consists of conglomerate with pebble of underlying rocks. Upward from the base it grades into terrigenous flysch with limestone strata bearing Late Moscovian-Kasimovian, and in the upper part, Gzhelian-Asselian foraminifers (Aldykin, Muyankol, and other formations, 2500 m thick).

[80]  At the northern foothills of the Alay Range Neoautochthon-2 unconformably onlaps onto Neoautochthon-1 that overlies the Kan unit. Here the basal part of Neoautochthon-2 is formed by thick conglomerate that contains pebble of basalts, tuffs, schists, sandstones, and limestones. Further upwards conglomerate beds alternate with sandstone, siltstone, argillite, and limestone yielding Late Moscovian-Kasimovian foraminifers (Kunyakul, Shunkmazar, and Uchbulak formations, 3000 m thick).

[81]  In the Karachatyr Mountains Neoautochthon-2 unconformably overlies rocks of the Shankol and Taldyk units and Neoautochthon-1. The lower portion of the section is composed of proximal carbonate terrigenous flysch bearing Late Moscovian-Early Kasimovian foraminifers (Akterek and Dzhilginsay formations, 2000 m thick). The overlying flysch sediments include wildflysch beds with olistoliths of authigenic limestones. The conglomerate strata commonly contain pebble of granites and extrusive acidic and intermediate rocks transported from the Kazakh-Kyrgyz microcontinent which margin was a place of intense volcanism in the Moscovian and Kasimovian. The derived foraminifer fauna estimates the age of the sequence from the Upper Kasimovian to Sakmarian (Uchbulak, Dastar, Kerkidon, and Dangibulak formations, 3000 m thick). Along with foraminifers the sediments yield numerous remains of shallow benthic fauna, namely, corals, bryozoans, mollusks, and trilobites. The upper part of the section is mainly composed of organogenic limestones.

[82]  Consequently, the Neoautochthon-2 sediments overlie the Abshir nappe, Shankol and Taldyk ophiolitic nappes, and the root zone of ophiolitic allochthons (Kan unit). North of this suture, in the Fergana Karatau and Guzan Mountains (Figure 2) Neoautochthon-2 onlaps onto the Kazakh-Kyrgyz microcontinent rocks.

Neoautochthon-3.
[83]  Continental sediments bearing Permian and Triassic flora and Early Permian foraminifers in conglomerate pebble (Tuleikan, Madygen, and other formations) unconformably overlie all units and Neoautochthon-2.

Alay Zone

2007ES000223-fig08
Figure 8
[84]  The Alay tectonic zone is separated from the Southern Fergana zone by the Uzgen-Sanzar thrust fault of Permian age. The Alay zone includes the southern part of the Fergana Range, the East Alay, Karategin, Zeravshan ranges, and part of the Alay, Turkestan, and Gissar ranges. Being wide in the western and eastern parts, the zone is narrowed between the 72o and 73o meridians and is overlain by younger sediments and by the overthrusted in the Cenozoic Pamirs. The narrowed area separates the Alay zone into two regions, Yassy-Alay on the east and Zeravshan-Alay on the west (Figures 3, 8, and 9).

2007ES000223-fig09
Figure 9
[85]  Geological section of the Alay zone is formed by thrust sheets and slices divided by primary and secondary overthrust faults and thrust faults. A great territory of the Alay tectonic zone is occupied by rocks of the Tar unit. The zone includes the Chekantash, Sugut, and Dzhirgatal units.

[86]  Various stratigraphic sections of the Alay zone were discussed by Biske [1996], Burtman [1968, 1976, 2006a], Porshnyakov [1973], and Yagovkin [1974].

Chekantash.
[87]  The Chekantash unit is the lowest in the geological section of the zone. In the Yassy-Alay region it can be seen in tectonic windows, namely, in the Kipchalma oreade, Fergana Range (Figure 8, 8) and in the Kulgedzhele oreade, East Alay Range (Figure 8, 10). Rocks of the unit most likely occupy the same position in the Birguzy, Chekantash, and Uchat oreades (Figure 8, 4, 5, and 12). Allochthonous bodies composed of rocks of this unit also occur among the sediments of the overlying Tar unit as thrust sheets limited by secondary overthrust faults, and in olistoplaques.

[88]  In the East Alay Range the lower part of the visible section is formed by 2000-m-thick carbonate rocks with abundant coral and mollusk fauna of the Ludlow, Pridolin, and all Devonian series. The Tournaisian and Visean are represented by a 300-m-thick limestone member bearing foraminifers, brachiopods, and corals. These rocks are overlain by a limestone sequence with chert, argillite, and sandstone beds, accumulated from the Serpukhovian to Late Moscovian as evidenced from foraminifer fauna (Dogdul, Dongurama, and other formations, 1000 m thick). It is conformably overlain by sandstones, siltstones, and argillites with conglomerate and clastic limestone strata yielding Late Carboniferous foraminifers (Kashkasu and Tuyuk formations, 1000 m thick).

[89]  The Kipchalma and Birguzy oreades in the Fergana Range represent a series of thrust sheets formed by terrigenous and carbonate rocks with the Late Silurian and Devonian corals and tentaculites. The Uchat oreade includes the Middle Paleozoic carbonate rocks and the overlying carbonate terrigenous sediments bearing Late Carbonoferous and Asselian foraminifers.

[90]  In the Zeravshan-Alay region the Chekantash rocks are exposed in tectonic windows and compose olistoliths and olistoplaques in the Upper Paleozoic sediments referred to the Tar unit.

[91]  In the Dukdon and Ziddy tectonic windows (Figure 9, 1 and 5) the Chekantash unit is overthrusted by the Tar rocks from the north and is thrusted by rocks of the Gissar tectonic zone from the south. The Llandovery to Moscovian sediments of the Dukdon oreade are represented by 3000-m-thick limestones and dolomites yielding abundant coral, brachiopod and foraminifer fauna. Carbonate rocks of the Silurian, Middle Devonian, Visean, and Moscovian sections contain strata of terrigenous sediments; that of the Lower Devonian, Serpukhovian, and Bashkirian include chert beds and lenses. A carbonate terrigenous flysch sequence with basal conglomerates (Darakhtisur and other formations, 1000 m thick) onlaps the carbonate rocks. The polymictic basal conglomerates contain pebble and boulders of Carbonaceous limestone with the youngest fauna of Late Moscovian age. Moscovian foraminifers were found in the flysch sediments as well. This stratigraphic section is complicated by secondary overthrust faults and thrust faults; part of carbonate rocks is of para-autochthonous and allochthonous character.

[92]  The Kurganak tectonic window (Figure 9, 2) is specular in reference to the Dukdon window. Its southern boundary is the overthrust fault at the base of the Tar nappe; the northern, the Karavshin fault that is the western link of the Uzgen-Sanzar thrust fault which separates the Southern Fergana and Alay tectonic zones. An over 3000-m-thick limestone and dolomite sequence is exposed in the Kurganak oreade. The enclosed corals, mollusks, and foraminifers indicate that the sequence corresponds to the whole Devonian, Lower Carboniferous, Bashkirian, Lower Moscovian, and part of the Upper Moscovian. In the Early Devonian part of the section carbonate rocks alternate with terrigenous sediments; in the Middle Devonian and Serpukhovian-Bashkirian parts, with chert beds. Different Devonian and Carboniferous beds are unconformably overlain by a 3000-m-thick conglomerate and coarse flysch sequence with numerous strata of wildflysch and olistostromes. The limestone pebble and boulders bear the Early and Late Carboniferous foraminifers. It is likely in this sequence in the Sokh River basin that the Gzhelian foraminifer specimen was encountered in the conglomerate cement and the Early Permian foraminifers were identified from the limestone beds.

Tar.
[93]  In the Yassy-Alay region the basal part of the visible stratigraphic section of the Yassy oreade (Figure 8, 6) in the Fergana Range is composed of 400-m-thick clayey shales with beds of quartz sandstones, siltstones, and limestones bearing Pridoli corals and brachiopods. They are overlain by distal terrigenous flysch (Yassy and Dzhartyube formations, 3000 m thick). The sandstones and lenses of clastic limestone contain redeposited Devonian corals, tentaculitids, and plant remains. The flysch is overlain by a thin condensed sequence (Kendysu and Dzhurektash formations, from 20 m to 300 m thick) of red and green cherts, clayey shales and bedded limestones bearing foraminifers and conodonts of Late Devonian to earliest Gzhelian age. It is conformably overlain by a proximal, two- and three-component, largely rhythmical flysch composed of sandstone, siltstone, and argillites and enclosing strata of fluxoturbidites and olistostromes (Turgaityube, Mazardon, Mamat, and other formations, 3000 m thick). The calcareous sandstone and clastic limestone beds yield Gzhelian, Asselian, and Early Sakmarian foraminifers.

[94]  The Akbogus oreade (Figure 8, 1) occupies the East Alay Range, Gulchi River valley, and, partly, the southern slope of the Alay Range. The lower part of the oreade section is represented by siliceous and clayey shales with Early and Late Silurian graptolites (Burusundy and Tekelik formations, 1000 m thick). The upper part of the section is formed by carbonate terrigenous flysch (Azvan and other formations, 500 m thick) bearing Lower and Middle Devonian corals, tentaculitids, foraminifers, conodonts, and graptolites. In the Belauli and Irkesh oreades (Figure 8, 3 and 7) the basalts, andesites, and tuffs are included among Silurian and Early Devonian sedimentary rocks in the Archabulak, Karavankul, and Ailyama formations.

[95]  In the Akbogus oreade the Devonian flysch is conformably overlain by a sequence of variegated cherts (radiolarites and phthanites), argillites, siliceous siltstones, and siliceous and clastic limestones (Kalmakasu and other formations, 20 m to 300 m thick). The sequence represents a great stratigraphic interval. In the East Alay Range it yields the Famennian, Visean, Serpukhovian, Bashkirian, and Moscovian ammonoids and foraminifers. In the Alay Range nearby the Daraut Village and in the Sokh River basin the similar sequence contains the Givetian, Frasnian, Tournaisian, Visean, Serpukhovian, Bashkirian, and Early Moscovian conodonts. These condensed sediments are conformably overlain by a terrigenous and carbonate terrigenous flysch sequence bearing Moscovian, Kasimovian, and Gzhelian foraminifers (Akbogus, Demeney, and Oital formations, 2000 m thick).

[96]  The Late Carboniferous flysch reaches maximum thickness of 3500 m in the Kugart oreade (Figure 8, 9) in the Fergana Range. It is represented there by coarse flysch with wildflysch strata containing limestone and basalt boulders.

[97]  The Yassy and Akbogus oreades are analogous in structural position and had similar geotectonic depositional environments. They differ in a time span needed for accumulation of the condensed pelagic sediments. The conodont studies in the Yassy oreade showed that these conditions occurred from the Late Devonian to Gzhelian. In the Akbogus oreade the condensed sediments are similar in composition and appearance but were accumulated in a shorter interval from the Givetian or Late Devonian to the Early Moscovian. This indicates that in the Late Moscovian-Gzhelian the deep basin in the western Tien Shan was retained only in the Yassy oreade area.

[98]  In the Zeravshan-Alay region (Figure 9) the Tar unit also consists of thrust sheets and slices [Leonov, 1979; Rogozhin, 2004]. Clayey, carbonaceous clayey, and siliceous shales, sandstones, tuffstones, and siltstones bearing Early Silurian and Ludlow graptolites, are widespread in the Turkestan Range and on the northern slope of the Zeravshan Range (Turkestan, Lyangar, and other formations). Most part of the sequence is composed of a distal flysch. It contains basalt, trachybasalt, and andesite sheet bodies. Terrigenous carbonate sediments of the same age are known in the Zeravshan Range. Section of that type is formed by quartz sandstones, clayey shales, clayey limestones, and dolomites with corals and brachiopods (Zinakh, Shing, and other formations, 1000 m thick).

[99]  The area of Silurian zonal thermal metamorphism occurs in the axial part of the Turkestan Range. Eight metamorphic zones, from greenschists to sillimanite garnet gneisses, were distinguished in the 15 times 120 km stripe. This metamorphism is likely of Silurian age as the isograds cut the Wenlock rocks and the fragments of schists similar to those of the discussed metamorphic zone were recorded in the Upper Silurian sediments. The obtained K-Ar ages of the metamorphic rocks do not contradict this inference [Bakirov, 1978].

[100]  The Pridoli and Early Devonian are known in the siliceous terrigenous, carbonate siliceous, and reef facies. Siliceous terrigenous sediments bearing graptolites occur on the northern slope of the Zeravshan Range. Radiolarites, phthanites, jaspers, and clayey siliceous siltstones yielding Lochkovian, Pragian, and Emsian conodonts occur in the range as well (part of the Akbasay Formation). On the northern slope of the Gissar Range and in the western part of the Zeravshan Range reef limestones with chert beds are recorded (Kupruk, Khavzak, Shut, and other formations, 2000 m thick). They contain Ludlow, Pridoli, and Early Devonian conodonts and shallow faunal remains.

[101]  The Middle and Late Devonian, Early Carboniferous, Bashkirian, and, partially, Moscovian are represented in the Zeravshan Range by siliceous and carbonate condensed sections.

[102]  The siliceous type of section is composed of jasper, siliceous clayey shales, and siliceous acidic tuffites bearing conodonts of all the Middle and Late Devonian ages. The sediments are several tens (part of the Akbasay Formation) to several hundreds (Vashan and other formations) meters thick.

[103]  The carbonate type of section is known in the Urmetan oreade (Figure 9, 4) in the Zeravshan River valley. The layer-by-layer study of conodonts from the 300-m-thick limestone showed the presence in the section of all Devonian stages excluding the Givetian, all Lower Carboniferous stages, the Bashkirian, and Lower Moscovian.

[104]  The Late Carboniferous-Early Permian is represented by a flysch olistostrome sequence (Obizard and other formations, 1000 m thick) that crowns the section of the Tar unit in the discussed region. The conglomerate and olistostrome beds with large limestone olistoplaques and olistoliths are recorded among the terrigenous flysch. The olistoplaques, olistoliths, and limestone pebble yield the Late Silurian, Early and Middle Devonian, Visean, and Serpukhovian corals, brachiopods, and foraminifers. Carbonate rocks of the Chekantash unit were the source of clastic material for the olistostrome. The conglomerate pebble mainly consists of siliceous rocks derived from the underlying sequence, as well as of sandstone, limestone, and granite. The conglomerate and sandstone cement bears the Late Carboniferous foraminifers.

[105]  In the east of the region the olistostrome was studied north of the Daraut Village. Olistoplaques are 300 m thick and several kilometers long there. Olistoliths and olistoplaques are composed of (a) limestone and dolomite similar to the Chekantash rocks and bearing Devonian, Early Carboniferous, Bashkirian, and Moscovian foraminifers; (b) condensed pelagic siliceous carbonate sediments with Devonian and Carboniferous conodonts; (c) sandstones and argillites with the Silurian and Early Devonian graptolites. West of the Daraut Village the olistostrome sequence underwent a strong tectonic processing. Along with the above-mentioned rocks the mixtite contains serpentinite boulders and olistoplaques of: (d) tholeiitic basalts geochemically similar to rocks of a mid-oceanic ridge; and (e) basalts with petrochemical properties of oceanic island arc rocks [Nenakhov et al., 1992; Pai, 1991]. The possible source of boulders of extrusive rocks was the Taldyk unit. A lot of mixtite boulders are composed of Lower Paleozoic sediments and of sandstone and conglomerate yielding Asselian foraminifers.

[106]  Part of rocks in this region underwent a dynamothermal metamorphism at 300-400o C and at a pressure of 5-10 kbar (Yagnob type of section). These rocks are distributed in the Zeravshan Range and, to a smaller extent, in the Gissar and Karategin Ranges. A gradual transition was recorded between metamorphic and non-metamorphosed Paleozoic rocks. The lower part of the Yagnob type of section is composed of a metamorphosed quartz and quartz arkose siltstone, sandstone, and gravelstone sequence bearing beds of rocks of likely volcanic and volcanogenic sedimentary origin (Yagnob and other formations, 3000 m thick). Most part of the rocks is metamorphosed in greenschist facies. In the upper part of the sequence the metamorphization decreases and the sediments contain Ashgillian and Early Silurian corals. Among these rocks sills and dikes of metamorphosed basalts, andesites, and dacites occur. Andesites and dacites are of high-alkalic composition. The Devonian and Carboniferous rocks and Late Paleozoic flysch also underwent a greenschist metamorphism, which indicates its Permian age.

[107]  The Tar unit as a whole is mainly composed of pelagic sediments. In the Silurian the turbidites with a considerable role of distal facies were accumulated and volcanic centers occurred in the region. In the Silurian and Early Devonian bioherms appeared on the margin of the facies zone and, probably, on volcanic mountains. In the Late Silurian, Devonian, and Early Carboniferous most part of the territory was a place of slow accumulation of deep-water sediments including common radiolarites. In the Late Carboniferous the formation of turbidites, including distal and proximal facies, fluxoturbidites, and olistostromes, resumed.

Sugut.
[108]  The Sugut unit is located in the East Alay Range (Figure 8) and is composed of metamorphic rocks (Sugut and Suuktor formations, 2500 m thick). The lower part of the section is formed by metamorphosed sandstones, pelites, cherts, basalts, and tuffaceous rocks along with metamorphosed andesite basalts, andesites, and liparites. Petrochemical properties of metavolcanites correspond to those of island arc extrusive rocks [Shvanov, 1983]. The up to 100-m-thick tectonic lenses of brecciated and serpentinized dunites are recorded in the section. The upper part of the section is composed of metamorphosed sedimentary rocks that contain Silurian-Middle Devonian corals and foraminifers. The rocks are irregularly metamorphosed from the phyllite to epidote-amphibolite facies with prevailing greenschists.

[109]  The rocks of the Sugut unit occur in a trough and on a synform flank. They onlap onto a flysch sequence (Terek Formation) of the Tar unit that bears the Carboniferous floral remains. The Sugut unit occupies the uppermost position in the geologic section of the Alay tectonic zone.

[110]  Accumulation of the Sugut sediments occurred in a volcanic island arc or on continental slope that underwent volcanic eruptions. This complex gained its modern appearance owing to tectonic decoupling and metamorphism, likely, in the accretionary prism nearby the Kazakh-Kyrgyz microcontinent margin.

Dzhirgatal.
[111]  This unit is located in the eastern part of the Karategin Range and consists of two tectonic sheets 10 and 5 km thick, respectively (Pizan and Khait oreades). The sheets are formed by polymictic and arkose carbonaceous sandstones, siltstones, and argillites that underwent zonal metamorphism from the biotite to sillimanite zone, and are nowadays represented by schists and gneisses (Khait, Chubai, Karagushkhon, and other formations).

[112]  Metamorphosed pelites prevail in the Pizan oreade (Figure 9, 3). Less metamorphosed rocks contain Early and Late Silurian graptolites; limestone beds in the upper part of the section, the Early Devonian and Eifelian crinoids. The Khait oreade is mainly composed of metasandstones. Marble bearing Late Silurian-Early Devonian corals and crinoids is recorded in the upper part of the section. The attribution of beds with fossil fauna to the stratigraphic section of metamorphic rocks has been questioned and their age was suggested to be older. The Dzhirgatal rocks were metamorphosed prior to the intrusion of diorites with Pb-Pb age of 368-395 Ma [Melnichuk, 1989].

Neoautochthon.
[113]  The neoautochthon sediments unconformably overlie the Tar rocks and are represented by a conglomerate and sandstone sequence bearing Early Permian, evidently redeposited, foraminifers (Toktash and Karasuran formations, 1000 m thick). The sediments are not widely distributed. They likely correspond to Neoautochthon 3 in the Southern Fergana tectonic zone. In the southern Zeravshan-Alay region in the Fan Mountains, the Late Carboniferous and still older rocks of the Tar unit are unconformably overlain by a barren sequence of trachytes, phonolites, and other alkalic lavas, of tufflavas and tuffs with conglomerate, sandstone, and limestone beds (Kaznok Formation, 1500 m thick). These sediments are likely of Permian age.

Correlation of Units

2007ES000223-fig10
Figure 10
[114]  Geologic sections in the Northern Fergana and Southern Fergana zones are of similar four-storeyed structure (Figure 10 and Table 1). The lower (I) structural storey is formed by the Baubashata and Isfayram units that mostly consist of shallow carbonate sediments. The II structural storey is occupied by the Ontamchi and Abshir units, which sections are mainly composed of turbidites and deep-water siliceous sediments. The III storey is formed by the Kerey and Taldyk units made up of weakly metamorphosed oceanic volcanites. The IV storey is represented by nappes of the rocks that underwent metamorphic transformation in a subduction zone and accretionary prism prior to their overthrusting onto the Alay-Tarim continent. The Shaydan and Shankol units are composed of oceanic rocks; the Toguzbulak unit, of continental sediments.

[115]  In the Alay tectonic zone the two lower nappe storeys are similar to the lower structural storeys of the Fergana setting (Figure 10 and Table 1). The Paleozoic ophiolitic nappes that form III and IV storeys in the Fergana setting, are unknown in the Alay zone. In the Yassy-Alay region the highest position in the section is occupied by the Sugut unit composed of rocks of a volcanic island arc, which were metamorphosed prior to overthrusting. It is a probable analog of nappes of the IV structural storey in the Fergana setting. The Dzhirgatal unit is likely such an analog in the Zeravshan-Alay region.

[116]  Nappes of the upper storey represent an accretionary complex that was formed nearby the Kazakh-Kyrgyz microcontinent margin prior to the collision. Ophiolites of the III storey were separated from the oceanic crust prior to collision and were included in the accretionary prism. They became an allochthonous body during the collision when the accretionary prism was overthrusted onto the Alay-Tarim continent. Formation of nappes of the II storey resulted from the collision of the Alay-Tarim and Kazakh-Kyrgyz terranes.

[117]  In the Carboniferous and Early Permian the following ensemble of the autochthon and primary nappes appeared (downward from the top): the Shankol-Shaydan, Sugut, and Toguzbulak accretionary nappes (IV storey); the Taldyk-Kerey accretionary collisional nappe (III storey); the Abshir-Ontamchi-Tar collisional nappe (II storey); the Isfayram-Baubashata-Chekantash autochthon (I storey). In the Permian the Karaunkur strike-slip fault and Uzgen-Sanzar thrust fault subdivided the nappe ensemble of the western Tien Shan into parts described above as the Northern Fergana, Southern Fergana, and Alay tectonic zones.

Deformation Stages

D-1.
[118]  Several deformation stages can be recognized in the formation of nappes in the region. The earliest are the deformations of metamorphic schists of the upper structural storey. In the Shankol and Shaydan units metamorphic rocks are strongly crumpled and these folds appeared prior to the overthrusting. They occurred not later than the Silurian since the deformed metamorphic schists are unconformably overlain by slightly modified Late Silurian sediments. In the Shaydan unit Silurian conodonts were encountered in the deformed metamorphic rocks, which reduces the age interval of D-1 deformations to the Silurian.

[119]  Among the deformation structures of stage D-1 several generations were revealed in the Shankol unit. The earliest were recumbent isoclines; the second generation is represented by vertical folds with axial-plane cleavage. They were followed by thrust faults which movement resulted in plastic deformations of folds of the second generation [Duk, 1995]. At stage D-1 the recumbent folds were formed in metamorphic rocks of the Dzhirgatal unit. The isograds of the Devonian zonal metamorphism cut these folds [Melnichuk, 1989].

2007ES000223-fig11
Figure 11
[120]  The next three deformation stages correspond to three overthrust episodes that resulted in formation of the western Tien Shan nappe ensemble (Figures 10 and 11). The nappe ensemble had been formed with interruptions during 60-70 million years, from the Serpukhovian to Artinskian.

D-2.
[121]  At stage D-2 the Shankol-Shaydan nappe was overthrusted onto the Taldyk and Kerey units which were separated from the subducting oceanic crust. The youngest rocks in the Taldyk and Kerey sections are Serpukhovian in age defining the oldest age limit of stage D-2 lower boundary. Neoautochthon-1 composed of Serpukhovian-Early Moscovian rocks onlaps onto the Shankol and Shaydan nappes and on the Shaydan nappe root zone in the Kan unit. These relationships indicate the Serpukhovian age of the overthrusting. Accumulation of the Neoautochthon-1 sediments on the Shankol-Shaydan nappe could occur concurrently with underthrusting of the Taldyk-Kerey sheet beneath the Shankol-Shaydan metamorphic schists. The upper age limit of stage D-2 is defined by the beginning of Neoautochthon-2 accumulation in the Late Moscovian. This neoautochthon onlaps onto Neoautochthon-1, the Shankol-Shaydan, Taldyk-Kerey, and Abshir-Ontamchi nappes, and on the Kan unit.

D-3.
[122]  At stage D-3 the Taldyk unit together with the overlying Shankol nappe and Neoautochthon-1 were overthrusted onto the Abshir unit; the Kerey unit together with the Shaydan nappe and Neoautochthon-1 were overthrusted on the Ontamchi unit. The youngest sediments of the Ontamchi and Abshir units are of Bashkirian and Early Moscovian age, respectively. The Neoautochthon-2 base corresponds to the Late Moscovian. This defines the Moscovian age of stage D-3 deformations.

[123]  The overthrusting at stage D-3 was accompanied by deformations of the allochthon and of rocks overlain by nappes. A large part of the gabbro-ultrabasite complex in the allochthon was transformed to mélange and the allochthon was decoupled by secondary overthrust faults. Twinned sections of the allochthon produced by these processes are described above in the Kyrgyzata and Hodjagoir oreades and were recorded in other objects. The rocks of the Abshir and Ontamchi units underwent tectonic impact as well. A part of tectonic mixtite occurring at the top of the Abshir unit was most likely formed at stage D-3.

D-4.
[124]  At stage D-4 the Tar-Abshir-Ontamchi nappe with the overlying nappes and neoautochthons was overthrusted onto the Chekantash-Isfayram-Baubashata autochthon. The youngest rocks at the autochthon top in the Northern Fergana, Southern Fergana, and Alay tectonic zones are of Bashkirian, Kasimovian, and Asselian age, respectively. Neoautochthon-3 that unconformably overlies the rocks deformed at stage D-4, was dated as the Kungurian-Ufimian. These records indicate that the most probable (or the most intense) nappe movement occurred in the Sakmarian-Artinskian (Figures 10 and 11). The earlier nappe movements at stage D-4 probably took place concurrently with flysch formation on the moving allochthon beginning from the Moscovian. These movements, if they ever occurred, were less intense.

[125]  The Abshir-Ontamchi-Tar nappe underwent a significant tectonic transformation during its movement. This nappe is composed of easily deformed flysch and shale sequences that were tectonically processed, differently in various parts. Tectonic structures produced at this deformation stage are of the following origin.

[126]  1. During the nappe movement the allochthon mass was tectonically subdivided by secondary overthrust faults to sheets that overthrusted one another.

[127]  2. Recumbent and flexure-drag folds at the boundaries of tectonic sheets and the flow folds were formed in the moving allochthon. In the Tegermach oreade (Figure 6, 10) the tectonic flow folds occur throughout the 2-km-thick allochthon sequence. The magnitude of over 100 studied folds varies there from 0.5 m to 10 m; of 160 folds, from 10 m to 100 m; and that of 13 folds, from 100 m to 400 m [Burtman, 1976]. The folds commonly form cascades.

[128]  3. The most intense tectonic processing resulted in formation of the mixtite containing rocks of the whole stratigraphic range of the allochthonous sheet. Its matrix is composed of the deformed Silurian rocks. Such mixtite was discussed above in the Abshir unit description.

Direction of Nappe Movement

[129]  Direction of the nappe movement was investigated in the Abshir and Tar units which form the second structural storey of the western Tien Shan nappe ensemble. For this purpose the folds formed in tectonic sheets in the course of nappe movement were studied.

2007ES000223-fig12
Figure 12
[130]  In the Abshir unit this research was conducted in the Tegermach oreade (Figure 6, 10). It represents a gently curved allochthonous sheet, about 2 km thick and 18 times 20 km in size, that overlies the Carboniferous rocks of the Isfayram unit. The most part of the sheet is composed of Silurian flysch. The folds do not expand beyond the allochthonous sheet, they were formed at stage D-4 during the Abshir nappe movement. Stereograms in Figure 12 demonstrate the F 4 fold vergence as it was at stage D-4. In the stereogram compilation the corrections removing the effect of later deformations were made. Stereogram "a'' shows records on 53 isoclines and stereogram "b'' is based on the parameters of 220 tight folds [Burtman, 1976]. The research revealed that the Abshir unit was overthrusted southwards. The data on other parts of the unit also indicate the prevailing southward vergence of F 4 folds and the southward movement of the Abshir nappe (in modern system of coordinates).

2007ES000223-fig13
Figure 13
[131]  In the Tar unit the F 4 fold vergence was studied in the Belauli oreade (Figure 7, 3) representing an allochthonous sheet, about 3 km thick, that overlies the Archaltur oreade (Figure 8, 2) referred to the Chekantash unit. The upper part of the Archaltur oreade is composed of Lower Carboniferous carbonate flysch and limestones. The lower part of the Belauli oreade is formed by clayey and siliceous shales with tuff and limestone beds bearing Middle and Upper Devonian fossil fauna. They are overlain by the Carboniferous terrigenous flysch. At later stages of deformation in the Permian the Belauli nappe together with the underlying rocks was crumpled in a synform fold and then was curved in a horizontal fold. The parameters of 188 isoclines and flexures of magnitude from 0.2 to 30 m were measured at and nearby the synform centrocline. Figure 13 shows stereograms of the F 4 fold vergence with corrections removing effects of deformations that took place after the stage D-4 [Burtman, 2006a; Klishevich and Klishevich, 1983]. The vergence of F 4 folds favors the inference on south- or southeastward movement of the Tar nappe (in modern system of coordinates).


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

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