[3] Mugodzhar, the southern end of South Ural, notably differs in a number of features of tectonic structure, magmatism and metamorphism from the structures of Ural folded belt. It does not have marginal structures of the East-European platform opposite it in the west because to the south of latitude 51o N these structures are replaced by edge zone of the Caspian Sea basin, which is in essence the old continental passive margin, and farther to the south by structures of the central part of this basin. This area, which is defined as the junction zone of the Caspian Sea basin and Mugodzhar hereinafter, shows peculiar features of the geological structure and geodynamic evolution. Many of them had been known long before but were not adequately interpreted owing to the conventional understanding of the Caspian Sea basin nature assumed to be a part of the East-European platform.
[4] This basin due the lack of reliable geological and geophysical data on its deep structure was considered as continuation of the basement of East-European platform. Such an approach to this most difficult problem keeps up to the present and is reflected in the latest geodynamic reconstructions for the South Ural [Puchkov, 2000].
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| Figure 1 |
[6] Not all of the so-called structural formation zones of South Ural have continuation in Aktyubinsk area of Ural and Cis-Ural region including Mugodzhar. Moreover, none of these zones from Uraltauskiy anticlinorium to Kizilo-Urtazymskaya zone of Magnitogorsk megasynclinorium is traced to the south of city Aktyubinsk latitude. They successively thin out (are structurally truncated) along the major Ural fault, major suture of the folded belt, dividing it into two sectors: paleocontinental (externides) and paleooceanic (internides). The southern continuation of this suture, i. e. Kempirsaysko-Daulskiy fault (and Prisakmarsko-Voznesenskaya macro melange zone accompanying it) are also truncated by straight West Mugodzharskyi fault, which extends obliquely from north-northeast at latitude somewhat father to south of 50o N and is a continuation of dislocation system in the axial part of Magnitogorskiy megasynclinorium. Owing to this fact at the front of the collision were brought structures of Magnitogorskiy megasynclinorium eastern side and Mugodzharskiy microcontinent (median massif), which is located farther to the east. In this case, neither structures typical of sutures, no indications of folding and cleavage accompanying it common in intense compression zones are noted. In West Mugodzharskaya zone abutting the fault, which predominantly is composed of Middle Devonian greenstone modified basaltic lavas, such dislocations are practically missing and uniform monoclinal bedding prevails; among the rocks, primary volcanogenic structures and specifically fragments of sea rift valley are still retained [Ivanov et al., 1973]. Generally West Mugodzharskaya zone is a large terrain of oceanic crust (in modern structural plan of South Ural it is an autochthonous tectonic unit). A feature of the zone is relatively low extent of island-arc stage volcanic rocks; on the other hand, in a number of syncline plunges (primarily in Berchogurskaya syncline) Late Givetian-Early Frasnian beds of volcanogenic molasses and olistostromes overlay oceanic basalts with sharp angular unconformity; up the section the former are replaced by the thick flyschoid formation of Zilairskaya suite, whose upper horizons belong to Early Carboniferous. These facts suggest that the first episode of the terrain accretion with the Caspian Sea basin margin took place as early as at the end of the Middle Devonian and subsequently common conditions of sedimentation took shape at the junction vast area under which thick beds of volcanogenic-terrigenous sediments were formed.
[7] To the west of the suture of major Ural fold at the area of junction of the Caspian Sea basin and Mugodzhar to the south of 51o N latitude, Uraltauskaya zone, which is very important from geodynamics point of view, tectonically wedges out (anticlinorium or antiform of the same name). At its continuation after the "neck" of Khabarninskiy synclinorium, Kempirsayskiy anticlinorium (antiform) is located, which wedges out to the south of the Oysylkara river valley and similar to Uraltauskiy anticlinorium shows combined scaly and nappe structure.
[8] Thus a structural zone similar to them is not traced to the south of city Aktyubinsk latitude (Figure 1). Zilairskiy synclinorium (synform) structurally conjugated with Uraltauskiy anticlinorium to the south in Aktyubinsk area of Ural continues as Kosistekskiy synclinorium (synform). Similar to Zilairskiy synclinorium, the ophiolite and island-arc complexes of the Ordovician-Middle Devonian occur there that compose tectonic plates in Sakhmarian margin allochthon. The southern continuation of Kosistekskiy synclinorium up to 48o N latitude has allochthon structure as well, where under Mesozoic-Cenozoic cover, ophiolite complex rocks and ultrabasites of Daul'skiy rock mass were established from drilling and geophysical research data. Thick accumulations of greywacke-flyschoid formation of Zilairskaya suite of the Upper Devonian-Lower Carboniferous are abundant there, thus marking the position of the foredeep of that time. This foredeep and autochthon formation of Zilairskaya suite were formed at the accretion stage of Ural mobile belt structures at the East-European continent margin. However to the south between Mugodzhar and the Caspian Sea basin the rocks of greywacke-flyschoid formation are widespread in the junction zone up to South Embinskoye uplift. It suggests that in the junction area accretion processes went on as well but this time above the zone of the Caspian Sea basin oceanic plate subduction under the complex of tectonically scaled oceanic and island-arc structures joined with Mugodzhary micro continent.
[9] In more western zone, the so-called linear folding zone framing the paleocontinental sector of South Ural a bit to the north from city Kuvandyk, the subsidence is noted of rock complex of greywacke-flyschoid formation in the southward direction under thick beds of flyschoid terrigene-siliceous-carbonaceous formation of the Lower-Middle Carboniferous and marine carbonaceous terrigene molasses of the Upper Carboniferous-Lower Permian. Farther to the south on the left bank of the river Ural, Carboniferous deposits in the molasse formation section are replaced by Lower Permian deposits, subsiding under them in the southward direction. From there and to latitude of the River Ilek (in the upstream) in town Kandagach latitude area, Aktyubinskiy trough was separated in this band [Avrov and Dal'yan, 1970; Gridasov et al., 1976]. It was assumed to be a part of gigantic South Ural periclinal trough encircling in the Late Paleozoic the southern continuation of the Ural mountain structure [Yanshin, 1962]. In the west, it is separated from the Caspian Sea basin by Martukskiy buried fault established from geophysical data [Avrov and Dal'yan, 1970] as a border line, to the west of which the linear orientation of folded structures is not noted but mosaic pattern of geophysical anomalies distribution prevails, which is typical of areas of salt-dome tectonics. The total thickness of sediments of flyschoid and lower (marine) molasses in this trough reaches 4.5-5.0 km [Zamarenov, 1970], which considerably exceeds the thicknesses of synchronous structures in Cis-Ural foredeep and the eastern area near the side of the Caspian Sea basin. For example, the thickness of Upper Carboniferous rocks in the east of the basin (in areas Zhanazhol and Alibekmola) is not more than 260 m, and in Cis-Ural trough it amounts about 2200 m. In Aktyubinskiy trough, Paleozoic sediments underlying molasse formation form a system of gently sloping steps divided by faults and descending from east to south towards the Caspian Sea basin and also gently sloping southwards. It is significant that this trough is at 80 km to the south from city Aktyubinsk latitude and in its turn is replaced by Ostansukskiy trough, where thickness of synchronous formations considerably decrease (in Ostansukskiy area they are less by factor 2.5-3) and farther to south at settlement Kenkiyak latitude Pre-Kungurian shearing starts to manifest itself markedly and the thickness of Pre-Kungurian rocks decreases to the first hundreds of meters. In Ostansukskiy trough, it is established that terrigenous rocks of early orogenic marine C2m-P1 molasse of thickness up to 2600 m overlap shallow-water shelf carbonaceous (C1v-C2b) deposits in such structures as Baydzharykskaya, Karnakskaya and Alibekmolinskaya synclines belonging to the eastern edge of the Caspian Sea basin. To the east of this zone, in the eastern Ostansukskiy trough, the same molasses overlays more deep-water sediments of the same age. Thickness distribution of Kungurian stage and overlaying Permian structures seems to be absolutely different. It increases from east to west and from north to south from 3-4 km to 5-6 km, suggesting the growth of the Caspian Sea basin as the result of subsidence of the framing outer zones of Aktyubinsk area of Ural region, Aktyubinskiy and Ostansukskiy troughs [Gridasov et al., 1976].
[10] Aktyubinskiy trough similar to Cis-Ural trough was formed as orogenic structure; Upper Paleozoic sediments that filled it at the end of Permian-beginning of the Triassic were crumpled into linear folds overturned westwards under lateral pressure from the east. A feature of Ostansukskiy trough is gentle folds of platform type that were formed in all probability at the same time; they are located along the same linear faults as folds located to the north of Aktyubinskiy fault. This may mean that in the absence of lateral pressure of any significance in Ostansukskiy trough, step-like lateral subsidence prevailed with block rotation moving on listric faults. Along the plane of each listric fault, the rotating block (step) edge lifted up and above this edge swell-form complication of step fold formed in the sedimentary beds overlapping the steps. In swell-form bends local anticline typical structures were formed. At the same time in the eastern part of Ostansukskiy trough more significant rises took place, which formed deep pre-Kungurian gap.
[11] To the west of Ostansukskiy trough, an interruption in sedimentation of great duration, which was accompanied by deep erosion of sediments accumulated before, is characteristic of the whole eastern edge zone of the Caspian Sea basin. This interruption embraced a considerable part of the Carboniferous, the whole Late Carboniferous (locally with the beginning of the Early Permian) and intervals of Middle Carboniferous, which is indicated by Artinskian-Sakmarian rocks of the Lower Permian overlapping different horizons of carbonaceous complex of Moscovian and Bashkirian stages. This carbonaceous complex forms large "island" uplifts, which are plateau-form areas of shallow shelf; carbonaceous accumulations thickness there ranged from 400 m to 600 m (areas Zhanazhol, Tortkol', Aransay, Alibekmola) and reached 2000 m at Kozhasay and Sinel'nikovskaya areas. Many of them can be compared in their size with the so-called carbonaceous "platforms".
[12] These areas of abundant carbonaceous sediments correspond to fairly large
homogeneous geological bodies of considerable thickness and size of the order of
30
100 km (Temirskiy "ledge", Enbekskiy and Zharkamysskiy "arches").
Together they form Zharkamyssko-Enbekskaya zone bent eastwards in a kind of arc
[Botneva et al., 1990;
Krylov et al., 1994].
Its overall size in north-south is up to 300 km and in easr-west direction
it amounts to 100 km. Just to the southeast on the Ostansukskiy
trough continuation, one more zone, where Carboniferous-Early Permian
carbonaceous sediments are abundant, is located. It is Zhanazholskaya zone
extending for 300 km as a narrow band to the southwest and passing there into
external structures of South Embinskoye marginal uplift. To the west towards
the Caspian Sea basin, carbonaceous platforms of Zharkamyssko-Enbekskaya zone
are cut off with ledges reaching the height of several hundreds meters with
dip angles of 10-15o and more. Bodies of clinoforms composed of Artinskian
terrigenous rocks of gray color formation with conglomerate in the basement generally
lean against the ledges. The thickness of individual bodies of those clinoforms
reaches 800 m. The total thickness of Pre-Kungurian terrigenous formation of
C3-P1 age reaches 3-4 km in the east of carbonaceous platforms where rocks
of this formation compose Ostansukskiy trough abutting the folded structures of Mugodzhar.
In the axial area of the trough, the section of this formation from top to bottom is
increased by uniform terrigenous rocks of C2m-C3 age of thickness up to 2 km.
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| Figure 2 |
[14] To the west, in the zone of Ostansukskiy trough, the surface of reflecting horizon P3, which is interpreted as the base of complex D 1-2, which is likely to overlay the Lower Paleozoic complex, is located at depths of 7-8 km going down to the east towards the tectonic suture with Kosisteksko-Aksuyskaya zone of uralides. Similar subsidence is noted of the reflecting horizon P2 - P21 corresponding to the surface of Visean-Bashkirian sediments. Paleozoic sediments bed confined between the horizons P1 (the top of Sakmarian-Artinskian sediments P1 ) and P2 - P12 belongs to the early orogenic marine molasse of C3-P1. Its rocks are opened by deep boreholes ( P -11 Northern Ostansuk, P -26 Baydzharyk, P -27 Dzhurun, P -30 Dzhilansaid, G -15 Karnak and others). Visean-Bashkirian rocks of carbonaceous shelf formation occur in the base of this rock bed. The total thickness of the Middle Paleozoic preorogenic complex in this area is 4000-5000 m, which testifies to considerable subsidence of its territory and apparently to a rapidly developing trough before the beginning of the collision. From data obtained with the use of seismic refraction and common depth point methods the surface of base of preorogenic complex or the surface of "crystalline basement" as it is given by the cited author subsides stepwise in the eastern direction from Temirskiy arch, reaching the depth of 11-12 km [Kan, 1994].
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