D.V. Alexeiev

Tectonic models for accretion of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (c. 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge–trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tuffs). They offer a promising perspective for future investigations.

Tectono-Stratigraphic framework and Palaeozoic evolution of the Chinese South Tianshan

A stratigraphic and structural study was carried out in the central part of the Chinese South Tianshan (STS) within a 50–100 km-wide transect centered on the Dushanzi-Kuqa road (83°–85° E). Our data elucidate the tectonic structure and evolution of the Palaeozoic sedimentary basin, document overthrust structures in the late Carboniferous-early Permian orogenic belt and suggest correlations between the western and eastern segments of the STS in Kyrgyzstan and China.We recognise a series of lithotectonic units in the study area that have different stratigraphic characteristics and were formed within (a) continental margin and slope of the Kazakhstan continent, (b) Turkestan (South Tianshan) ocean, (c) intra-oceanic carbonate sea-mounts, which at least partly evolved on top of an extinct island arc, (d) a back-arc oceanic-crust basin, (e) external deeper marine and internal shallow-marine areas of the Tarim shelf and (f) Tarim craton. The overall structure of the basin was similar within Kyrgyzstan and China. The main distinction of the western areas is a lack of ophiolites on the southern flank of the belt, a poorly expressed arc in the axial part, and a more complicated facial setting of the central area, where carbonate banks were separated by deeper marine depressions with cherty deposits. The eastern sector is defined by a continental arc that evolved on the northern margin of the Tarim craton in the Silurian and became separated from the continent in the latest Silurian-early Devonian. There is also a middle Palaeozoic metamorphic belt on the southern flank of the STS.A pre-Carboniferous unconformity, previously assumed throughout the study area, is only confirmed within the continental massifs of Kazakhstan and Tarim. As in the western areas, the unconformity does not exist within the STS. Continuous sedimentation in the STS occurred from the Early Devonian to the early Bashkirian in marginal parts of the belt and up to Gzhelian age in the axial part. Convergence began in the Bashkirian (320–315 Ma) and caused subduction of oceanic crust in the northern and southern areas of the STS to the north and south, respectively. A back arc basin in the south was closed in the Moscovian, and since that time top-to-the-south thrusting and overthrusting prevailed throughout the STS.The time of onset of collision of Kazakhstan with Tarim was not younger in age than Kasimovian, based on the age of initiation of a turbidite foreland basin on the northern margin of Tarim. Thrust deformation during the Late Pennsylvanian to early Permian was synchronous within Kyrgyzstan and China; it occurred in a collisional setting and was accompanied by accumulation of turbidites and olistostromes. Broad termination of thrusting, followed by folding and uplift of the area in the middle Asselian indicates the beginning of a rigid collisional phase. Emplacement of early Permian stitching granite plutons in the STS and adjacent areas of Kazakhstan and Tarim completes the formation of the collisional orogen within Kyrgyzstan and northwestern China.

The stratigraphic, sedimentological and structural evolution of the southern margin of the Kazakhstan continent in the Tien Shan Range during the Devonian to Permian

Abstract The passive margin carbonate platform in the Middle Tien Shan rests on Givetian–Frasnian red siliciclastic strata. It evolved from an attached carbonate platform in the Famennian and early Tournaisian to an isolated carbonate platform in the late Tournaisian to early Bashkirian. The open-ocean side of the platform was reef-rimmed, whereas the continental side was both reef- and shoal-rimmed. Platform interiors exhibit low-energy facies during the Famennian to early Visean and high-energy facies during the late Visean to Bashkirian. Eustatic sea-level rises in the middle Tournaisian, early Visean and near the Visean and Serpukhovian boundary caused major reorganizations in platform architecture. Deformation in the middle Bashkirian reflects the onset of a convergent margin. Flexural loading by an orogenic thrust wedge controlled basin subsidence along the southern edge of the Middle Tien Shan in the Late Pennsylvanian to Asselian. Cessation of deposition in the Asselian followed by folding and granitoid plutonism reflects the onset of a rigid collision. Devonian to Permian carbonates represent outcrop analogues of coeval oil- and gas-rich carbonate platforms in the North Caspian basin and can be used for comparative and predictive sedimentological studies. Palaeozoic carbonate reservoir facies may host subsurface Cenozoic oil fields in the Fergana Basin.

Structures of the late palaeozoic thrust belt in the Chinese South Tian Shan

Aiming to resolve contradictions in tectonic models and to establish a correlation between Chinese and Kyrgyz sectors of the South Tian Shan we carried out stratigraphic and structural studies in Chinese part of the belt along the Bayinbuluk—Kuqa transect. New data indicate that Chinese South Tian Shan is dominated by top-to-the-south structures, which were formed during the latest Carboniferous and Early Permian. Major allochthons of the Devonian carbonates, thrusted on the Gzhelian and Asselian turbidites, are revealed in the northern part of the belt. Imbricated thrust packages and recumbent folds in deeper marine Devonian and Carboniferous rocks are common in the South. Postkinematic granites yield U-Pb ages of 285–275 Ma, which indicate that thrust deformation ceased by the middle of the Early Permian. The same direction of motion and similar age of deformations in Kyrgyz and Chinese sectors of the South Tian Shan prove, that top-to-the-south structures were formed during the same structural episode, which corresponds to the main collisional stage within entire belt.

Palaeozoic evolution of the North Tianshan based on palaeomagnetic data – transition from Gondwana towards Pangaea

ABSTRACT We present new palaeomagnetic data for Cambrian and Ordovician volcanic and sedimentary rocks from the Kyrgyz North Tianshan (NTS) and review available data from the southwestern Central Asian Orogenic Belt (CAOB) to elucidate the tectonic history and evolution of this region during the early Palaeozoic. We observed a coherent evolution of the NTS and the Kazakhstan continent (or Kazakhstania) with a constant northwards movement between the Cambrian and Devonian at ∼5 cm/a. After the northwards movement ceased in the Devonian, the accreted terrane assemblage of Kazakhstania occupied a stable latitudinal position at ∼30°N until the final amalgamation of Eurasia occurred in the late Carboniferous to early Permian. Amalgamation of the Tarim and Turan blocks caused a counterclockwise bending within the southwestern segment of the CAOB, which occurred in an inconsistent way by a brittle-like response of the upper crust with a large variety of rotational movement. We suggest an evolution of the Kyrgyz CAOB terranes by steady migration away from Gondwana and subsequent capture in a zone of global downwelling at ~30°N, where accretion and subsequent amalgamation of Eurasia occurred with the CAOB terranes in its centre.

Kinematic analysis of Jurassic grabens of soulthern Turgai and the role of the Mesozoic stage in the evolution of the Karatau–Talas–Ferghana strike-slip fault, Southern Kazakhstan and Tian Shan

The Karatau–Talas–Ferghana Fault (KTF) extending for 1500 km from Turgai to western Tarim is one of the world’s largest intracontinental strike-slip faults. This paper overviews the evolution of the KTF, providing insight into its relatively poorly studied northern segment in the Karatau Range and Turgai, known as the Main Karatau Fault (MKF). The right-lateral strike-slip along the KTF developed during three stages in the late Permian–Triassic, Early–Middle Jurassic, and late Cenozoic. The total strike-slip decreases northward from 200 km in the Ferghana Range to 100 km in the Karatau Range and decreases to zero in southern Turgai. Kinematic analysis of Jurassic grabens compensating the strike-slip in southern Turgai shows that strike slip along the KTF in the Jurassic, previously regarded as insignificant, actually measures tens of kilometers and 50% of the total strike slip in the northern segment of this fault.

New Data on the Age of Detrital Zircons from the Kokdjot Group and Bolshekaroi Formation, Malyi Karatau (Southern Kazakhstan)

U–Pb dating of detrital zircon from the Kokdjot Group and Bolshekaroi Formation (Malyi Karatau) yielded for all the samples age clusters at about 800–805, 855–890, 1980–2100, and 2440–2470 Ma. The Kolmogorov–Smirnov test suggests they have identical provenance sources. The Kokdjot Group and Bolshekaroi Formation ages are presumably Neoproterozoic, not older than 800 Ma, though a somewhat younger age (not older 770 Ma) cannot be ruled out.

Continuous Sections of the Devonian and Carboniferous Carbonates and Timing of Collision in the Chinese South Tianshan

New stratigraphic data indicate that carbonate sedimentation in the axial part of the Chinese South Tianshan (STS) occurred continuously from the Early and Middle Devonian to the latest Carboniferous. This reflects steady subsidence in quiet tectonic regime and excludes major collisional events in the STS during this time. Pre-Carboniferous structural events, previously identified in the marginal parts of the Kazakhstan and Tarim continental massifs, did not extend to the STS basin. Continental collision in the Chinese STS occurred during the latest Carboniferous and early Permian, as in adjacent areas to the west in Kyrgyzstan.

ABSTRACT: Development of an Isolated Carbonate Platform from an Underlying Attached Platform: A Newly Discovered Devonian-Carboniferous Carbonate Platform in the Tian Shan Mountains of Kyrgyzstan

Preliminary field studies of this carbonate platform show that: 1) a 4,500 m-thick platform evolved stratigraphically upward from a 1,000 m-thick attached carbonate platform in the Famennian-Early Tournaisian into a 3,500 m-thick isolated carbonate platform in the Late Tournaisian-Bashkirian; 2) extensional normal faulting probably caused the isolation of the carbonate platform in the Late Tournaisian; 3) reef and grainstone-rimmed platform margins occur in the Late Devonian; ramps with skeletal mud mounds appear in the Tournaisian-Lower Visean; and platforms with grainstone-rimmed margins and slopes with massive skeletal mud mounds characterize the Middle Visean-Bashkirian; 4) platform interiors are comprised mainly of lower energy facies in the Famennian-Lower Visean and of higher energy ooid-bioclastic facies in the Middle Visean-Bashkirian; and 5) major, relative rises of sea-level observed in the Bolshoi Karatau carbonate platform 600 km northwest in Kazakhstan occur in coeval Kyrgyzstan platform facies. These data suggest that the observed relative sea-level events may have a common or related origin over wide areas including the North Caspian Basin.