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

Kyzylkum

[132]  The Kyzylkum geologic section similar to that of the western Tien Shan is four-storeyed and is composed of (downward from the top): accretionary Tamdy nappe (IV storey); accretionary collisional Kulkuduk nappe (III storey); the Bukan collisional nappe (II storey); the Murun autochthon and para-autochthon (I storey).

[133]  Stratigraphic sections of the Kyzylkum units were described by Burtman [1973, 2006a] and Mukhin et al. [1991].

Murun

2007ES000223-fig14
Figure 14
[134]  This unit occupies a vast territory (Figure 14). The visible basal part of the Murun unit in the Kuldzhuktau Mountains is composed of sediments bearing the Late Ordovician benthic fauna. The Silurian rocks conformably overlie the Ordovician. The Llandovery in the South Kuldzhuktau Mountains is represented by offshore sediments, namely, conglomerates, cross-bedded quartz sandstones, and volcanic tuffs, interbedded with limestone containing brachiopods, tabulates, and stromatoporids (Darbaza Formation, 500 m thick). In the North Kuldzhuktau Mountains this sequence is replaced by limestones bearing trilobites, brachiopods, and tabulates (Yangikazgan Formation, 500 m thick).

[135]  Upward from the base these sediments grade into a carbonate sequence that is 2000-3000 m thick in the Kuldzhuktau and Tamdytau Mountains and half as much in the North Bukantau Mountains. In the Kuldzhuktau Mountains this sequence corresponds to a time span ranging from Wenlock to Early Carboniferous. A hiatus is recorded in the Early Devonian. The carbonate sediments contain benthic fauna, chert nodules and quartz sandstone beds. The upper part of the autochthon is composed of flysch and olistostrome sequence (Taushan and Kamysty formations, 1000 m thick). The youngest fauna in the olistoliths is of Moscovian age.

[136]  North of the Kuldzhuktau Mountains the basal part of carbonate sediments is of Early Devonian age; the uppermost, of Early Moscovian. This sequence overlies the Silurian rocks with angular unconformity. Hiatuses are widespread in the Serpukhovian-Early Bashkirian interval that is characterized by formation of karst hollows and bauxite accumulation in them.

[137]  In the Tamdytau and Bukantau Mountains the Early Moscovian limestones are overlain by carbonate terrigenous flysch sequence that composes the upper part of the Murun section (Azhrikty, Keriz, and other formations, 500 m thick). Upward from the base of the sequence the clastic material gets coarser and olistoliths and conglomerate beds appear. The lower part of the sequence bears Early Moscovian foraminifers and the youngest fauna from limestone fragments is of Late Moscovian age. In the western Tamdytau Mountains at the top of the Murun unit beneath the Bukan overthrust fault, the 250-m-thick tectonic mixtite was recorded and described [Burtman, 1973, 1975].

[138]  In the South Nuratau Mountains the Murun rocks are exposed in an antiform core in the Debelyand tectonic window (Figure 14, 4).

[139]  In the North Nuratau Mountains the Murun rocks compose several allochthonous massifs overlying the Late Carboniferous flysch and olistostrome sequence that terminates the stratigraphic section of the Bukan unit. These allochthonous massifs most likely represent the erosionally prepared gigantic olistoplaques transported a second time during the autochthon and allochthon deformation. The largest massif, Basragata oreade (Figure 14, 1), represents a set of thrust sheets that overlie the flysch bearing Early Moscovian foraminifers. The thrust sheets together with the underlying rocks compose flanks and trough of the Shokhtau synform fold. The Basragata oreade base is marked by a breccia formed by chert and limestone fragments. At the allochthon base the 400-m-thick carbonaceous siliceous shales, siltstones, limestones and dolomites containing the Wenlock and Ludlow graptolites and Pridoli brachiopods, are recorded. They are overlain by an over 2500-m-thick limestones and dolomites. The limestones bear Early-Middle Devonian and Frasnian brachiopods, corals, and conodonts. The geologic section of the Basragata oreade is crowned by a thin thrust sheet composed of cherts and clastic limestones with the Visean brachiopods and foraminifers.

[140]  The Dzhalpak and Kyzkol oreades (Figure 14, 3 and 5) are the allochthonous bodies of smaller size. They occur in synform troughs over the Late Carboniferous flysch and olistostrome sequence that terminates the Bukan section. The Dzhalpak oreade is composed of Silurian, Devonian, and Lower Carboniferous rocks; the Kyzkol oreade, of Silurian sediments.

[141]  In the Murun facies zone the shallow, mainly biogenic carbonate sediments were accumulated in the Middle Paleozoic. In the inner, southern, part of the zone carbonate accumulation started in the Ordovician and Early Silurian; in the outer part, in the Late Silurian and Lochkovian. It continued with rare interruptions up to the Moscovian when all the facies zone area was submerged to pelagic depths and the turbidity accumulation began.

Bukan

[142]  The Bukan unit is characterized by incomplete and condensed Devonian and Carboniferous sections and by the Late Carboniferous flysch and olistostrome sequence.

[143]  The Silurian sediments composing isolated fragmentary sections are known in the Nuratau Mountains. The sediments bearing Llandovery and Wenlock graptolites are represented by argillites, carbonaceous siliceous clayey shales and mainly quartz sandstones. They are commonly rhythmical and contain basalt flows. The Late Silurian sediments are represented by terrigenous and carbonate rocks bearing benthic fauna.

[144]  The Devonian sediments unconformably overlie the underlying rocks. In the western North Nuratau Mountains, on the flanks of the Shokhtau synform (Figure 14) the Silurian sediments are overlain with angular unconformity by a carbonate sequence bearing abundant corals, brachiopods, and foraminifers. The sequence base is of Lochkovian age, the top, of the Early Moscovian. A lot of hiatuses are recorded in the section. It lacks the Eifelian, Upper Devonian, Lower Tournaisian, Serpukhovian, and partly Bashkirian rocks. Thickness of the carbonate sequence in the Shokhtau synform is about 1000 m. Within the same stratigraphic range it decreases eastwards down to 400 m in the Merishkor synform and to 200 m in the Daristan synform. In the same direction a number and range of hiatuses increase. In the Daristan and Sentyab synforms the Middle and Late Devonian and Early Carboniferous rocks are missing. In the southeastern North Nuratau the 150-m-thick Late Bashkirian-Early Moscovian limestones with basal conglomerates overlie Silurian shales and sandstones.

[145]  In all the sections the carbonate sequence is conformably overlain by a finely rhythmical flysch bearing rare conglomerate beds. In the Daristan synform the flysch member yields Moscovian foraminifers. Its visible thickness is over 500 m. In the Sentyab synform the carbonate terrigenous flysch is overlain by a 300-m-thick mixtite formed as a result of olistostrome tectonic processing.

[146]  The flysch and olistostrome sequence (Murynkuduk and other formations) is widespread in the Bukan nappe. It was studied in the Tamdytau, Bukantau, Sangruntau, Dzhetymtau, and North Nuratau Mountains. The olistostromes contain large olistoliths and olistoplaques of organogenic limestones bearing fauna of different age and of sandstones, cherts, alkalic basalts, metamorphic schists, and rocks of all oceanic crust beds. Some limestone olistoliths are of authigenic origin, another part is composed of shallow-water rocks derived from the Murun facies zone. Fragments of oceanic crust rocks and metamorphic schists arrived at the flysch from the oncoming Kulkuduk and Tamdy nappes.

[147]  The blocks are commonly tens of meters in size. In the North Nuratau and Tamdytau Mountains some olistoplaques are as great as kilometers in size. Gigantic olistoplaques in the North Nuratau Mountains which are composed of the Murun rocks are described above in the Murun unit discussion. In the Tamdytau Mountains gigantic olistoplaques are formed by metamorphic schists (Uchkuduktau Formation), Early Paleozoic trachybasalts (Elmesashchi Formation), and Middle Paleozoic limestones (Balpantau and Dzhamankyngyr Mountains).

[148]  In the western Tamdytau the mixtite formed as a result of tectonic processing of a flysch and olistostrome sequence compose a tectonic sheet overthrusted onto the Bukan and Kulkuduk rocks along the secondary overthrust fault. Characteristic of the mixtite are gabbroid olistoliths and plagiogranite olistoplaques, the greatest of which is 800 m long.

[149]  The youngest organic remains found in the flysch and olistostrome sequence are the Moscovian foraminifers. They were identified in an olistolith from the Bukantau Mountains, in olistostrome cement in the Sangruntau Mountains, and in the Tamdytau Mountains flysch. The total thickness of the discussed sequences is several kilometers, however, their true stratigraphic thickness is difficult to estimate owing to widely distributed secondary overthrust faults and tectonic slices.

[150]  The Middle Paleozoic sediments of the Bukan unit were formed on the continental crust in unstable conditions. In the Early Silurian pelagic sediments containing terrigenous input were accumulated. Subsequently the area of the Bukantau facies zone repeatedly occurred below and above the carbonate compensation depth. The deposits were also washed out by submarine currents. Fractures of the basement were accompanied by local volcanism. In the Moscovian the collision and beginning of overthrusting led to a more contrast bottom topography and generated conditions for accumulation of thick flysch and olistostrome sediments.

Kulkuduk

[151]  The Kulkuduk unit is composed of slightly transformed ophiolites and the associated sediments. It is of comparatively limited distribution in the Tamdytau, North Bukantau, and North Nuratau Mountains.

[152]  The Bassumar oreade (Figure 14, 2) in the western Tamdytau Mountains is formed by thrust sheets of the Kulkuduk and Tamdy units gently overlying the Bukan rocks. Ophiolites compose the lower tectonic sheet of the Bassumar oreade. Its base is represented by a 50-m-thick serpentinite mélange with peridotite, pyroxenite, and gabbro blocks. The mélange is overlain by 120-m-thick cumulates composed of dunite, peridotite, wehrlite, lherzolite, websterite, and pyroxenite stripes. Upward from the base they grade into a 150-m-thick olivine gabbro, gabbro-norite, gabbro-diabase, and gabbro-pyroxenite member. Still upwards occurs the 200-m-thick complex of parallel basic dikes and leucogabbroids with prevailing plagiogranites [Burtman, 1973; Mukhin et al., 1991; Sabdyushev and Usmanov, 1971].

[153]  The above members are overlain by 300-m-thick lavas that along the tectonic contact onlap all parts of the gabbro-ultrabasite complex. A breccia consisting of gabbro and pyroxenite fragments in a tuffaceous carbonate cement is retained in places at the lavas base. The breccia is of sedimentary origin and indicates the erosion of the gabbro-ultrabasite complex before lava eruption. The lavas are represented by amygdaloidal basalts and andesite-basalts. The structure and petrochemical properties of the lavas suggest a highly differentiated magma in a chamber, which occurs with low spreading rate. This rate estimated from the titanium oxide content and dike morphology ranges from 0.5 to 2 cm year -1. The vesicular lavas, eroded underlying sediments, and the occurrence of carbonate deposits among them indicate a high level of the speading range. The lavas are of low-titanium composition and belong to different series, namely, alkalic, tholeiitic, and calc-alkalic. They were likely formed in a marginal sea or oceanic island arc [Mukhin et al., 1991].

[154]  In the Sangruntau and Aristantau Mountains the volcanogenic terrigenous sequence of the Kulkuduk unit is composed of calc-alkalic basalts, andesite-basalts, andesites, dacites, and tuffs referred to island arc volcanites according to their petrochemical properties (Sangruntau Formation, 500 m thick). Clastic rocks of the sequence yield cherts with Late Devonian-Early Carboniferous conodonts and limestone pebble bearing Bashkirian foraminifers.

[155]  In the Tamdytau Mountains, in the Kynyr oreade (Figure 14, 6) the Bukan olistostrome is covered by a 500-m-thick sheet composed of amygdaloidal basalts, tuffaceous conglomerates, and tuffstones with chert beds containing Frasnian conodonts.

[156]  On the northern slope of the North Nuratau Mountains, the Sentyab and other synforms are formed by the Kulkuduk thrust sheets composed of lavas and tuffs of alkalic pyroxene and olivine-pyroxene high-titanium basalts. Among them gabbro-diabase sheet bodies and chert beds with Famennian conodonts are recorded (Shavaz and other formations, 500 m thick).

[157]  The Kulkuduk volcanites are of different age in certain Kyzylkum regions. The Cambrian, Ordovician, Silurian, Devonian, Early Carboniferous, and Bashkirian fossil fauna was found in various sites in lahar limestone fragments and in pebble occurring among basic lavas and tuffs. Carbonate rock fragments could have been derived from reef constructions on volcanic mountains.

[158]  The Kulkuduk thrust sheets overlie the Bukan Carboniferous rocks and are overlain by schists of the Tamdy unit. In many areas the Kulkuduk unit is represented only by serpentinite mélange slices squeezed between the Bukan and Tamdy nappes.

Tamdy

[159]  The schists of the Tamdy unit compose thrust sheets that in the North Nuratau and Tamdytau Mountains overlie the Kulkuduk and Bukan rocks in synform troughs (Madzherum, Ittynusay, Kumbulak, and other formations). In the North Nuratau Mountains, in the Khissar oreade (Figure 14, 8) the Tamdy unit overlies the Kulkuduk lavas. The thrust sheet is composed of 2000-m-thick basic volcanites and graywackes metamorphosed to greenschists that contain high-baric crossite and winchite. In the Ustakhan oreade (Figure 14, 7) tectonic lenses of amphibolites and garnet-biotite gneisses occur at the base of the Tamdy section. In the Tamdytau Mountains tectonic sheets of metamorphosed basalts and gabbro of the Tamdy unit are the uppermost element in the geologic sections of the Bassumar and Kynyr oreades. The Tamdy rocks are also included in olistoliths of the Bukan olistostrome.

[160]  Composition of most of metabasites corresponds to that of oceanic tholeiites. The rocks also include volcanites assigned to the alkalic trap and calc-alkalic island arc series. The rocks were metamorphosed in the garnet-amphibolite facies and underwent a low-temperature diaphthoresis. Geochemical properties of the metabasites are within the oceanic portion of discrimination diagrams and in the transitional area between oceanic and island arc series.

[161]  The rocks of the unit contain Ordovician chitinozoans. The oldest K-Ar ages of metamorphic rocks correspond to Neoproterozoic.

Neoautochthon

[162]  A thick sequence of coarse molasse (Arkhar, Kynyr, Takhtatau, and other formations, 3000 m thick) unconformably overlies the Tamdy and Kulkuduk rocks. In the North Bukantau Mountains the base of the neoautochthon is formed by nonsorted block-bearing conglomerates. They are overlain by a sandstone, argillite, siltstone, gravelstone, and conglomerate sequence characterized by cross-bedding, traces of sediment erosion, suspending and slides, and by mud cracks. Early Moscovian goniatites and foraminifers were found in the lower part of the sequence; in other points Late Moscovian foraminifers and Late Carboniferous brachiopods, pelecypods, and floral remains were recorded. The neoautochthon is intruded by granites of Permian K-Ar biotite and amphibole age. The Kyzylkum molasse, according to clastic material composition and geologic position, corresponds to Neoautochthon-2 in the western Tien Shan where it is of Late Moscovian-Sakmarian age. The formation of Neoautochthon-2 in the Kyzylkum probably began earlier than in the western Tien Shan, or the Early Moscovian organic remains were redeposited.

Deformation Stages

D-1.
[163]  At stage D-1 the Murun unit underwent tectonic flow accompanied by formation of large and small recumbent isoclinal folds, thrust faults, and overthrust faults and by distribution of cleavage and tectonic lenses. Deformations were followed by greenschist metamorphism and occurred before the Early Devonian limestone accumulation. In the Kuldzhuktau Mountains the upper age limit for D-1 deformations is the Early Silurian. The K-Ar ages of the metamorphism were estimated as ranging from the Ordovician to Silurian [Babarina, 1999; Mukhin et al., 1991].

[164]  In the Tamdy unit, North Nuratau Mountains, one can see three generations of structural forms in metamorphic schists, namely, the oldest recumbent isoclinal folds and two subsequent generations of superimposed vertical folds with different trends of axial surfaces [Burtman, 1973]. The two earliest generations of these structures refer to stage D-1. In the Tamdytau Mountains two generations of isoclines were recorded in the Tamdy unit fragment [Mukhin et al., 1991].

D-2.
[165]  At stage D-2 the Kulkuduk rocks moved beneath the Tamdy metamorphic schists and were decoupled from the subsiding oceanic crust. The thrust sheets therewith underwent internal deformations.

[166]  In the Kulkuduk unit the youngest lavas were erupted on an island arc in the Early Carboniferous. Subsequently the rocks were included in the accretionary prism that was formed near a Kazakh-Kyrgyz microcontinent margin. The upper age limit for the stage D-2 deformations is the Moscovian, i.e. the beginning of the accretionary prism overthrusting onto the Alay-Tarim continental slope.

D-3.
[167]  At stage D-3 the accretionary prism composed by Kulkuduk and Tamdy rocks was overthrusted on the Bukan unit. Flysch and olistostrome sediments bearing rock fragments of this accretionary prism occur at the top of the Bukan unit and are of Moscovian age.

[168]  In the Tamdytau Mountains, in the Bassumar oreade, the lower part of the Kulkuduk unit was transformed to a 50-m-thick serpentinite mélange and the overlying ultrabasites attained a pressurized appearance. In the Nuratau Mountains similar rock deformations are recorded at the Kulkuduk overthrust fault in tectonic sheets that form the Khissar and Ustakhan oreades. These deformations probably occurred at stages D-2 or D-3.

[169]  The upper part of the Bukan unit was also subjected to deformations at stage D-3 and was intensely deformed at later stages as well. The drag folds beneath the base of the upper nappe can be reliably referred to stage D-3. These folds described in the Bassumar oreade indicate a southward shift of material [Burtman, 1973].

D-4.
[170]  At stage D-4 the Bukan unit together with the overlying allochthons was overthrusted onto the Murun unit. The top of the Murun section underlying the Bukan nappe contains Late Moscovian foraminifers. The youngest organic remains in the Bukan rocks are of Moscovian age. These records define the Late Moscovian as the lowest age limit of the Bukan nappe formation. The upper limit is not determined. The Bukan nappe likely moved prior to formation of analogous nappes in the southern Fergana region, where this process was the most intense in the Sakmarian-Artinskian.

[171]  The Bukan nappe is the largest in the Kyzylkum and is characterized by a complicated structure owing to intense inner deformations. They mainly resulted from a tectonic flow in flysch and olistostrome sequences. Recumbent isoclinal folds with amplitude up to several kilometers are widespread there [Mukhin et al., 1991]. Hinge trends and vergence of folds vary in different areas reflecting an irregular tectonic flow.


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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