[14] The main structural and lithologic manifestations of geologic events and the indicators of the geodynamic mechanisms that had operated during the plate stage of the evolution of the mobile belts are the intraplate and orogenic structural basins (discrete sedimentation basins). It is the tectonic structure of the intraplate and inter-mountain basins, their relations with the basement rocks, the specific manifestations of igneous activity and sedimentation, and the character of the secondary structural and diagenetic transformation of the volcanic-sedimentary rock cover provide information for the consolidated crust geodynamics and specific evolution during the platform and orogenic stages in the Earth mobile zones. Some information is provided by the studies of the topography and its association with the internal structure of the rock masses, and also by the data available for the crustal structure of the study region and for the thermal state of its interior.
[15] In the South Tien Shan territory the platform sediments and orogenic rocks are preserved mainly on the southern and northern slopes of these mountains and in the numerous intermontane basins arranged as individual beaded structural features, usually restricted to the suture zones, described briefly above. In this chapter we will discuss the structure of the Mz-Kz intermontane basins at the intersection of the Gissar and Alai ridges (see Figure 1 and Figure 2). Arranged in the northward direction are the following structural elements composed of Mz-Kz rocks: the Afgan-Tajik basin, the zone of the South slope of the Gissar Ridge, the Karakul-Zidda basin system, the Ravat Basin, the Zeravshan Basin, the Nuratau-Kurganak zone on the northern slope of the Turkestan Range, and the Fergana Basin.
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| Figure 5 |
[17] The Triassic rocks were mapped in the margins of the Afgan-Tajik Basin, where they are represented by red conglomerates, and also in the Zidda and Fan-Yagnob basins, where they are composed of the products of weathering. The Jurassic rocks are present in many basins where they consist of gray and black sandstone, clay with coal lenses and seams, conglomerates, and gravelite of continental facies (lake, swarm, and alluvial deposits). The maximum thickness of the Jurassic deposits was found in the Fan-Yagnob River area, ranging from 100 m to 300 m elsewhere.
[18] In contrast to the Jurassic rocks, the Cretaceous deposits showed a wider areal extent, varying greatly in lithology. The Lower Cretaceous rocks are represented by red continental and continental-margin deposits. They overlie transgressively all of the older sediments. One type of the continental deposits is represented mainly by pebble beds, sandstones, and alluvial-proluvial and alluvial-deltaic facies. Those of the second type are terrigenous rocks with interbeds and members of thin layers of dolomite limestone, as well as of their detrital varieties, enriched more or less with a sand material. Also present their are gray clay and siltstone interbeds. All of these rocks characterize the marginal environment of a large epicontinental sea basin. In the internal basins marine deposits may be absent, the Early Cretaceous ones being represented by red sandstones and conglomerates. The thickness of the Lower Cretaceous rocks varies from basin to basin from 0 m to 300-400 m (see Figure 5). The products of the Late Cretaceous sedimentation differ substantially from those described above. They do not contain any coarse terrigenous material, the number of red beds diminishes, whereas the volume of carbonate deposits grows notably. On the whole, the Late Cretaceous rocks are typical of a shallow epicontinental sea basin of intricate configuration, and also of lagoons, sometimes separated from the sea basin. A series of transgression and regression cycles can be traced, yet, later the sea transgression embraced almost the entire Gissar-Alai territory. The thickness of the Late Cretaceous deposits varies greatly amounting to 100-500 m in some basins.
[19] The Paleogene deposits cover all of the older rocks with a distinct erosion. They are missing in some basins. The Paleogene rock sequence is composed of dolomite, limestone (including its algal and oolitic varieties), claystone, some sand and silt material, and bituminous shale with phosphorite and gypsum intercalations. The sedimentation conditions show the oscillations of the basin floor with intermittent, short epochs of transgressions and regressions which developed under the conditions of a shallow, epiplatform sea basin. The thickness of the Paleogene deposits from basin to basin from 0 m to 200-300 m.
[20] All rocks, mentioned above, are overlain, with angular unconformity and erosion, by Oligocene-Neogene red siltstones, sandstones, gritstones, conglomerates of continental origin. The thickness of these rock sequences vary from 500 m to 800 m, amounting to 1500 m in some basins (for example, in the Magian Basin). Quaternary rocks are also widespread in many basins. These are usually sand, sandy loam, and pebble beds, being as thick as 300-500 m.
[21] Apart from the typical deposits of intermontane troughs, the Quaternary sediments include trains of peculiar coarse-clastic carbonate breccias, which were classified in this study as tectonic-gravitational mixtite. These rocks will be discussed here in detail later. The Neogene-Quaternary rocks prove intensive mountain-formation activity in the Gissar-Alai region.
[23] The Karakul Basin is situated at the contact of two large structure-formation zones of the southern Tien Shan region (see Figures 1 and 2), namely, the Zeravshan-Gissar zone in the north and the South Gissar zone (together with the Osmantala zone) in the south. This basin is bounded by large thrust faults, being sort of pressed between them. The northern and southern thrust faults are spaced some distance apart in some areas and merge together in the others producing a system of scalloped features composed of Mesozoic and Cenozoic rocks. The areas of the convergence show the structures of "tectonic linkage". The merging of the thrust faults and the "collapse" of the zone are restricted to high hypsometric levels and are generally found in pass zones at the heights of 3500-4000 m. Along the strike and toward the valleys the faults diverge slowly allowing one to observe the internal parts of the zone. The scalloped outcrops of the Mesozoic and Cenozoic rocks are usually found in relative (yet significant) topographic lows (depressions), which separate the mountain ridges accompanied by longitudinal river valleys filled with alluvial-proluvial, mainly coarse-clastic deposits. The segment discussed includes two Karakul and Zidda basins.
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| Figure 6 |
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| Figure 7 |
[25] The area south of the Karakul Basin is occupied by the Gissar Zone of Lower Carboniferous crystalline limestone, the monotonous argillaceous and carbonate rocks of the Mura Formation, supposedly dated Middle Carboniferous, and by the granites of the Gissar Batholith. All of these rock units are separated from one another by faults, their contacts with the granitoids being often intrusive, and are thrust northward as a system of tectonic slabs over the deposits of the Karakul Basin. The surface of the overthrust is marked by a band, up to 100 m wide, of tectonic breccias and mylonite. The fault plane dip varies from 20o to 80o. The rocks of the hanging wall bordering the fault plane vary greatly up to the Gissar batholith granitoids. In some areas the Paleozoic rocks of the Gissar zone are thrust over not only the Paleozoic rocks, but also over the Mesozoic and Cenozoic deposits.
[26] Similar to the situation south of the Karakul Basin, the carbonate rocks of the Khazret-Dukdon zone were thrust over the rocks of the Karakul Zone, the former corresponding structurally to the tectonic nappes of the Devonian-Carboniferous limestone, resting as allochthon outliers on the Upper Paleozoic flysch-molasse rocks in the area of the Zidda Basin described above, the fault plane dipping northward at angles varying from 20o to 40o, being even steeper in some areas. The carbonate rock massif is broken by a series of faults into individual slabs and tectonic wedges with insignificant displacements. The western edge of the outcrop of the Devonian limestone looks (in the map) as a tectonic nappe outlier resting on the flysch. This overthrust has been dated pre-Early Cretaceous, because it was sealed by Lower Cretaceous rocks. The rocks of the Khazret-Dukdon Zone (and eastward of the Karakul Zone) are, in turn, covered tectonically by the polyfacies rocks of the Zeravshan-Gissar Zone, and by those of the Yagnob Zone in more eastern areas. The rocks of this zone compose a series of tectonic slabs and wedges of different thickness and length, which had experienced dislocation-type greenschist and epidote-amphibolite facies metamorphism. The plastic rocks are deformed to folds with their axial planes inclined in the north direction.
[27] The central part of the region is occupied by the Karakul Zone proper. Its axial segment is composed of Upper Paleozoic flysch deposits with interlayers and lenses of conglomerates, breccias, and tectonic-gravitation mixtite. The flysch sequence includes individual long and thick limestone slabs dated supposedly Carboniferous (mesoliths and synsedimentation nappes). The flysch of these zones is crumpled to small folds opened fan-like downward. In the southern and northern segments of the region the flysch rocks are broken and rolled out, the sandstone beds being broken and boudinaged. The flysch of these zones is transformed to a tectonic mixture, which is a kind of a sedimentary melange. The northern segment of the melanged flysch is restricted to an overthrust where the flysch is overthrust by the limestone of the Khazret-Dukdon type. On the whole, the flysch rocks are deformed to an anticlinal fold with the rock layers in its limbs dipping in opposite directions. However, proceding from the discovery of rock layers with reverse gradational bedding (which is masked by fine bedding) and from its analogy with the Zidda Basin (see below), it can be inferred that here we deal with a fanlike syncline whose limbs are overturned opposite to one another.
[28] The intricately dislocated and melanged flysch deposits are overlain, with an erosion and with an angular (up to 90o ) and stratigraphic unconformity, by Mesozoic and Cenozoic deposits (see Figure 5). The eastern part of the basin includes Jurassic rock fragments which are overlain with a stratigraphic unconformity by the Lower Cretaceous red beds and Upper Cretaceous red beds and Upper Cretaceous terrigenous argillaceous and carbonate rocks. The thickness of these rocks is not higher than 200 m. The Cretaceous rocks are overlain with a stratigraphic unconformity by the Oligocene-Neogene rocks represented by red sandstones and conglomerates, measuring about 600 m in thickness. The Quaternary rocks are represented here by glacial, slope-type deposits of Middle Pleistocene-Holocene age.
[29] Occurring as one rock sequence, the Mesozoic and Cenozoic deposits form an asymmetric syncline with a steeply dipping, fault-cut southern limb. The syncline is complicated by second-order bends which complicate its structure. The bends of this kind are observed along its southern boundary, where small faults of the overthrust type are observed in the Neogene rocks. A pronounced bend was fond also in the northern limb of the syncline, where the Mz-Kz rocks grade to an almost horizontal position with the dip angles lower than 10o. The interbed tectonic deformations of the Mesozoic and Cenozoic rocks are almost absent, though some beds show their lateral thickness variation over insignificant distances. This is especially obvious in the carbonate and gypsum-bearing rock sequences, which can be an indirect indication of the tectonic flow of these rocks, which has been reported for the rocks of similar age and composition [Davidzon et al., 1982].
[30] Worthy of mention is the position of the pre-Mesozoic unconformity plane, which corresponds to the surface of the Paleozoic basement and, at the same time, to the surface of the pre-Mesozoic (pre-Cretaceous in our case) peneplain. The block diagram shows that this plane experienced intensive deformation, which is proved by its incline, amounting locally to 70-80o, and also flexure-type bending without any breaks of its surface. This proves that this plane experienced plication, which can be observed now thanks to the deep erosion of the rocks. It should be noted that the most intensive deformation of the flysch sequences, located below the surface of the unconformity, and their transformation to the melange, are restricted to the region of the maximum deformation of the Paleozoic basement surface.
[31] To sum up, the upper and lower structural stages differ in the deformation intensity and style. The rocks of the lower stage are highly dislocated, folded, and melanged. The rocks of the upper stage are deformed to a relatively simple syncline, the limb of which is complicated by flexure-type folds, any internal deformations being absent. Yet, the plane separating the structural stages, suffered fairly intensive plicate deformation without any breaks. The bending of the pre-Mesozoic peneplane surface without any breaks was possible thanks to the fact the flysch rocks were transformed to melange, lost their coherence, and, hence, were able to experience volumetric brittle-plastic flow.
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| Figure 8 |
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| Figure 9 |
[33] The general regularities of the Zidda tectonic structure can be seen in Figures 8 and 9. One can see that they are similar to those of the Karakul Basin and do not call for any detailed comments. Yet, the main specific features that controlled the tectonic style of the zone in this region are offered below.
[34] The structure of the Zidda Basin in the general plan is a megasyncline filled with the complex rocks of the Paleozoic folded basement and the overlying deposits of the Mz-Kz sedimentary cover. This syncline is pressed between two large extensive faults, namely between the Major Gissar fault in the south and the Anzob reverse fault in the north. The syncline has an asymmetric structure and a different expression in the basement and in the sedimentary cover. The rocks of the basement and those of the sedimentary cover occur as two distinct structural stages of different dislocations.
[35] The structure of the Paleozoic rocks is represented in the general plan by a complex megasyncline with the limbs overturned opposite to each other. The southern limb is more overturned and, hence, has an asymmetric structure. In the south the granites of the Central Gissar batholith and the Paleozoic rocks of the Osmantala Zone are thrust over the Paleozoic rocks. In the north the Zidda basin deposits are restricted by the overthrust surface of the Zeravshan-Gissar (Yagnob) zone. The southern overturned limb of the megasyncline, composed of the flysch deposits of the Middle-Late Carboniferous Maikhura Formation, developed as an upthrust, where the Lower Paleozoic rocks are thrust over the Mesozoic and Cenozoic rocks of the sedimentary cover. The overthrust attenuates in the dip and strike directions in the sedimentary rocks of the Maikhura Formation, yet, it was mapped in the sides of the intermontane basins in the west and east.
[36] The megasyncline discussed is complicated by two asymmetric second-order synclines which are connected with each other by a compressed ridge-like anticline. The axes of these folds are oriented WNW-ESE, being somewhat oblique to the general strike of this structural feature, suggesting the presence of a shear component. The axial planes dip SSW at 60-70o. The large folds are complicated by minor folds, restricted either to the curves of the large folds or complicating the bedding of the thin-bedded arenaceous-argillaceous and carbonate rocks, this producing some intrabed disharmonic folding pattern. The cores of the synclinal folds shape the bodies of the synsedimentation tectonic nappes composed of Lower-Middle Devonian limestone.
[37] The Mz-Kz deposits, resting in the transgressive and discordant manner on the Paleozoic rocks of the folded basement, also form a syncline, though a simple one, with a gently dipping, almost horizontal, floor which shows a gentle anticlinal bend in the middle of the basin. The sides of the syncline are deformed to single tense large-magnitude folds. The central section of the basin shows that in the north the sedimentary rocks are gently dipping under the Anzob reverse fault, and in the south they dip slightly to the north, having been cut off by the Gissar fault. However, westward these two reverse faults converge in the form of a tectonic suture. In these areas, like in the Karakul basin, the sediments occur as a fan-shaped syncline with its limbs overturned opposite to each other. It should be noted, however, that in the eastern direction the Gissar fault deviates from the Mz-Kz boundary into the field of the flysch rocks of the Paleozoic basement and dies out. Yet, eastward it manifests itself again at the boundary between these two rock complexes. The deformation of the sedimentary rocks agrees with that of the pre-Mesozoic peneplane surface. It is important that the floor of the basin is deformed to a lesser extent, compared to its margins.
[38] It should be noted that, like the Karakul Basin, the Mz-Kz and recent basin coincides in space with the region underlain by the Late Paleozoic flysch, this suggesting that the formation of this structural feature had been inherited form the Paleozoic period of its evolution.
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| Figure 10 |
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| Figure 11 |
[40] The Zeravshan-Turkestan Zone is represented in this region by a terrigenous flysch complex (up to 4000 m thick) of Llandoverian-Wenkockian age. The tectonic style of this zone is controlled by a series of size-varying asymmetric folds, overturned to the south, and overthrusts [Rogozhin, 1977]. The morphology of these structural features, as well as their orientation, suggest that these rock masses had been overthrust in the southern direction.
[41] The Zeravshan-Gissar Zone is a highly complicated tectonic unit. Structurally, this is a series of tectonic sheets separated by steep overthrust faults which grow flatter with depth. As follows from the general S-N orientation of the structural elements, such as, the overthrust vergence, the axial surfaces of the folds, and the positions of the corrugation and kink zones, which are opposite to those observed in the Zeravshan-Turkestan Zone, the general trend of the rock mass movement was from the south to the north, that is, opposite to the trend observed in the Zeravshan-Turkestan Zone. The Zeravshan-Turkestan and Zeravshan-Gissar zones contact the Zeravshan Zone, located between them, along large, extensive overthrust faults. These faults dip in the opposite directions, being steep in the upper segments and flattening with depth. The area where the northern and southern faults join together shows a subvertical suture where the rocks of the Zeravshan-Turkestan and Zeravshan-Gissar zones contact each other. Therefore, the Zeravshan Zone is a wedge compressed between two faults, which grows wider downward and eastward. As mentioned above, this zone corresponds to a basin filled with the Alpine deposits.
[42] Mapped in the Zeravshan Zone are two structural stages separated by an erosion surface and an abrupt (up to 90o) angular unconformity. The lower stage corresponds to the Paleozoic basement of folded metamorphic rocks, the lower, to a platform and molasse rock cover. The lower stage is composed of the Carboniferous rocks of the Vasha (C1-2 ) and Darakhtisurkh (C2-3 ) formations [Cherenkov, 1973]. Saltovskaya [1964] dated the rock of the Vasha Formation Namurian and those of the Darakhtisurkh Formation, Early Moscovian, which is not important in the context of this paper. The Vasha Formation is represented by a thick (up to 500 m) sequence of thin-bedded pelitomorphic limestone, polymictic sandstone, and chert, the Darakhtisurkh Formation being represented by terrigenous flysch, more than 500-meter thick, with interlayers of block breccias and less common conglomerates and gravelstones, including individual blocks and slabs (up to more than 1 km long) of limestone older than the enclosing rocks [Rogozhin, 1977], including some detached masses of the Vasha rocks. The deposits of the Darakhtisurkh Formation were identified as flysch including the bodies of olistostromes [Cherenkov, 1973] or of tectonic-gravitational mixtite [Leonov, 1981]. The rocks of the condensed Kshtut-Urmeta sequence, earlier ranked as a bedrock sequence [Saltovskaya, 1964; Torshin, 1970], is also a large mesolith or a detached mass of the synsedimentation nappe imbedded in the flysch deposits. This proves that we deal with a detachment of some zone, unknown in the bedrock and hidden from the observation in some suture zone.
[43] The Paleozoic rocks of the lower structural stage are deformed to steep folds compressed to isoclinal folds with subvertical axial planes and acute hinges. The beds stand on their heads or dip to the south (in the southern side) or to the north (in the northern side), producing a fan-shaped structure, slightly open downward. In some areas the tectonic reworking was intensive enough so that the rocks lost their bedding and stratification. The sandstone beds are boudinaged, broken, and rolled out, their argillaceous varieties being transformed to a structureless mass. This produces a chaotic structure with intricate broken antiform folds, the morphology of which suggests the tectonic flow of the rock material and the forced flow of the rock material to the cores of the anticlinal folds produced by the unconformity plane (the basement surface or the pre-Mesozoic peneplain).
[44] The upper structural stage, corresponding to the sedimentary cover, is composed of Liassic, Upper Cretaceous, Paleogene, and Miocene sediments [Davidzon et al., 1982]. Like the unconformity plane, the sediments of this complex are deformed to large, conjugated synclinal and anticlinal folds, slightly overturned to the north. The thick competent conglomerate deposits of Liassic and Miocene age are deformed to simple structural forms. The core of the anticlinal fold, composed of Cretaceous-Paleogene plastic claystone, limestone, and gypsum, shows a series of second-order strained carinate folds. The gypsum beds show changes in their thicknesses, associated with the sublayered tectonic flow of the plastic rocks.
[45] The Mesozoic and Cenozoic deposits are bounded in the south by an overthrust fault, yet, in the western area thrust over the younger rocks are the Paleozoic rocks of the Zeravshan-Gissar Zone, whereas in the more eastern part of the region the thrust was formed from the limb of a north-overturned fold composed of the Late Paleozoic flysch of the Zeravshan Zone itself. The tectonic pattern here is similar to the pattern observed in the Zidda Basin. The Zeravshan Zone is also bounded by an overthrust in the north, yet, in contrast to the southern area, it was not mapped everywhere, because eastward, in the Aini Village area, it extends under the transgressive Upper Cretaceous deposits.
[46] The morphology of the structural features of the Zeravshan Zone and its relationships with the morphotectonic elements of the South Tien Shan show that the tectonic pattern of this zone during the Alpine (Neogene-Quaternary) period of time was formed under the conditions of its bilateral compression between the opposite overthrusts under the conditions of the active subplastic redistribution of the rock masses. The overthrusting of the rocks masses of the neighboring structure-formation zones resulted not only in the lateral flow of the rock masses, but also in their vertical redistribution, namely, in the pressing them downward with the formation of molasse basins and in the pressing them upward with the formation of pseudodiapirs or, more likely, of protrusions. The southern overthrust was more active than the northern one. The latter was not rejuvenated in many areas during the recent time. The southern fault is bordered by almost recent and recent tectonic-gravitation mixtites which occur as extensive and thick (up to 100 m) fields bordering the front of this overthrust.
[47] To sum up, the transverse spatial reduction of the Zeravshan Zone was compensated, in addition to folding and thrusting, by the squeezing out of the rocks of the Zeravshan-Gissar and Zeravshan-Turkestan zones and by the subsidence (apparently, with lateral spreading) of the Zeravshan rocks proper. In the upper horizons the overthrusts grow steeper and converge to produce a subvertical suture. Broadening downward in a fan geometry, the deposits of the Zeravshan Zone dip under the allochthon masses of the neighboring structure-formation zones.
[48] The movements responsible for the structural reconstruction of the sedimentary cover modified the configuration of the basement surface the bends of which became conformable with the folds in the sedimentary cover. Moreover, the intricately folded rocks of the sedimentary cover were reworked repeatedly, lost their coherence, and were transformed to a structurally complex tectonic mixture, known as tectonic mixtite or as sedimentary melange. The loss of coherence as a result of melange formation provided for the 3D mobility of the basement rocks and for the curvature of the basement surface without any breaks. The melange formation takes place in this case in areas of most intensive form changes. The tectonomixtites compose the cores of the anticlines, as well as diapir-like and protrusive bodies. Similar relationships are characteristic of the other Southern Tien Shan zones (see below).
[49] It should be noted that the mantle-type accumulations of tectonic-gravitational carbonate mixtite were mapped not only in the Zeravshan Zone but also in the Zidda Zone along the overthrusts limiting this zone in the south. They occur as a coarse chaotic breccia up to 100 m thick. The area covered by these "mantles" is about 10 km 2. The rock fragments are poorly rounded or not rounded at all. There is no indications of the rock layering. Both the fragments and the cement of these rocks are composed exclusively of the limestone and dolomite of the Silurian rocks composing the hanging wall of the overthrust. Earlier, proceeding from the lithology of their rock fragments, containing Silurian fauna remains, and from their resting on the Mesozoic deposits, these rocks were interpreted as tectonic nappes. Along with some other indications, the formation of such breccias suggests the modern activity of the nappe-thrust structural features located along the sides of the intermontane basins.
[51] As mentioned above, many intermontane Alpine basins coincide in area with the Late Paleozoic flysch troughs or, to be more exact, with the modern areas of flysch development. Such patterns are observed in the Karakul-Zidda, Zeravshan, and Nuratau-Kurganak zones. These regular patterns can be interpreted as the evidence of their evolution inherited from their Paleozoic history, which seems to be a real fact. At the same time some of the young basins originated outside of the flysch areas.
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| Figure 12 |
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| Figure 13 |
[54] In other words, the intensive plication folding of the pre-Mesozoic peneplain without its breaks (see Figure 13) operated there at the expense of the volumetric mobility of the rocks of the metamorphic basement, provided by the differential brittle-plastic flow of the Paleozoic rocks [Leonov, 1991, 1993].
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| Figure 14 |
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| Figure 15 |
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