RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES1006, doi:10.2205/2007ES000223, 2008
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Figure 3 |
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Figure 4 |
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Figure 5 |
[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.
[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.
[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.
[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).
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Figure 6 |
[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].
[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.
[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.
[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.
[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.
[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.
[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
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.
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Figure 7 |
[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.
[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.
[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
49 Ma old. The Pb-Pb zircon age from
metabasites derived by a thermoion emission method, is 935
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.
[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.
[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.
[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.
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Figure 8 |
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Figure 9 |
[86] Various stratigraphic sections of the Alay zone were discussed by Biske [1996], Burtman [1968, 1976, 2006a], Porshnyakov [1973], and Yagovkin [1974].
[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.
[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
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.
[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.
[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].
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Figure 10 |
[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.
[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].
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Figure 11 |
[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.
[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.
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Figure 12 |
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Figure 13 |
Citation: 2008), Nappes of the southern Tien Shan, Russ. J. Earth Sci., 10, ES1006, doi:10.2205/2007ES000223.
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