Principal features of the crustal structure of the south-caspian basin and the conditions of its formation

Tectonophysics, 69 (1980) 113-121 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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PRINCIPAL FEATURES OF THE CRUSTAL STRUCTURE OF THE SOUTH-CASPIAN BASIN AND THE CONDITIONS OF ITS FORMATION

E.Sh. SHIKALIBEILY

and B.V. GRIGORIANTS

Institute of Geology, Academy

of Sciences of the A~er~a~an S.S. R,, Baku @J.S.S R.)

(Received August 29, 1979; revised version accepted January 8, 1980)

ABSTRACT Shikalibeily, E.Sh. and Grigoriants, B.V., 1980. Principal features of the crustal structure of the South-Caspian basin and the conditions of its formation. Teetonophysics, 69: 113-121. The Earth’s crust within the South Caspian basin has a condensed thickness and lacks a granite layer. However at the margins the thickness reaches 50 km. The South Caspian uplift, surrounded by troughs, is recognized in the southern part of the basin. The thickness of a sedimentary cover in basins is up to 25 km and on the uplift only 12-15 km. The age of the sedimentary cover is Cenozoic and that of the underlying consolidated basement is Mesozoic and even older. The South Caspian uplift and surrounding troughs show latitudinal strike while complicating local folds are both latitudinal and longitudinal. The strike of local folds is apparently the result of horizontal displacement of sediments along steep slopes of troughs and complicating major structures. It is suggested that the South Caspian basin occupied the zone where the intersection of rift troughs, which differ in age and direction, took place.

The South Caspian basin is a very complex structural element which has long attracted widespread attention. Situated within the Alpine geosynclinal area extending as far west as the Mediterranean and as far east as the Himalayas, it is responsible for a general sublatitudinal trend of major elements and their local folding. The South Caspian basin is isometric in outline and lacks a distinct trend not only in the folded structure of its surficial infilling but in the tectonic elements of the crystalline basement or consolidated crust as well. To a first approximation this basin is interpreted as part of the South Caspian trough bounded by the Apsheron sill in the north and the Elburz in the south. Structural boundaries of the depression are more extensive eastward and westward as compared to coast lines, because both the Lower-Kura and the West Turkmenian lowlands are characterized by a thick Cenozoic and particularly Pliocene-Anthropogene infilling due to the fact that these zones were involved in overall subsidence. This Cenozoic subsidence resulted in isolation of sublongitudinal faults bounding the South OO~O-195~/80/0000-0000/$02.25

0 1980 Elsevier Scientific

Publishing Company

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Caspian basin to the east and the west and separating it from the large buried Talysh-Vandam uplift and Mesozoic folded structure of the Kopet-Dag. The inner structure of the South Caspian basin was investigated repeatedly (Khain, 1958; Shikahbeily, 1960; Aksenovitch et al., 1962; Godin et d., 1969). The authors of this paper do not intend to analyse these studies to show the evolution of ideas concerning the structure of the South Caspian basin. Suffice it to say that only during the nineteen sixties were the DSSCMRW * data obtained. They gave a better knowledge of the structure at different elevations and stratigraphic levels as well as of the relationships between the surficial and deep-sea elements within the basin. Naturally, it is difficult to imagine that everybody holds the same view. Only one fact is evident: that in the case of the two-layered crust, as in depressions of other inland seas when a granitic layer is absent, there is a great thickness, in some places over 50 km. Hence, the crust under the South Caspian basin shows some sim~a~ty with both the oceanic and continental crust. An attempt to explain this peculiar pattern gave rise to a discussion resulting in three quite different statements. They were subject to somewhat closer scrutiny by Mdlovitsky (1976) and so we shah not dwell upon them. The South Caspian uplift and fringing troughs are clearly distin~ished in the surface structure of the consolidated crust, defining the distribution of sediment thicknesses and apparently the nature of the folding. The uplift occupies a central part of the basin, but shifted eastward in the direction of the T~kmenian shelf of the Caspian Sea. Within the uplift the surface of the consolidated crust lies at depths of about 12-15 km. The northern trough of the consolidated crust surface, about 20 km deep, comprises the Shemakha-Kobystan and Apsheron regions in the west and stretches eastward to the Tcheleken and Balkhan zones. Within the belt of the Apsheron sill the trough is narrow, as if being squeezed between Karabogaz arch or epi-Hercynian platform and the South Caspian uplift, but widens eastward (Cis-Tcheleken water area) and mainly westwards (in the sea southeast of the Apsheron Peninsula), where the depth increases up to 25 km. The Pre-Elburz trough is recognized in the southern part of the above trough where it coincides with the southern coast of the Caspian. The trough is divided into two parts by the Sefidrud sublongitudinal projection. The western part of the trough actually forms a southern structural element of sublongitudinal strike separating the South Caspian and Ta.lysh-Vamdam uplifts, its axial zone almost coinciding with the western continents slope of the South Caspian trough. The eastern fringe of the South Caspian basin is represented by the trough where the surface of the consolidated crust occurs at depths of about 15-20 km. This trough comprises the West Turkmenian lowland and the adjacent South Caspian shelf. * DSS = Deep Seismic Sounding, CMRW = Correlational Method of Refracted Waves.

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The nature of the folding and its orientation, at least in the uppermost part of a sedimentary cover - Pliocene to Quaternary strata (Fig. l), are consistent with the above-mentioned features of the consolidated crust surface and thus follow the distribution pattern of sediment thicknesses within the South Caspian Sea. In most papers dealing with the South Caspian Sea the following zones are proposed : sublatitudinal ( Apsheron-Balkhan belt and Pre-Elburz trough); sublongitudinal (Kura trough, Baku archipelago and the adjacent deep-sea part of the basin as well as the Gogran’dag-Okareme zone with the adjacent part of the sea) and even a zone of northeast cross-folding traced southeast of the Apsheron Peninsula and confined to the intersection of the northern Apsheron-B~khan trough and the western trough related to the Baku archipelago (Ismailov and Gasanov, 1966, Alikhanov et

Fig. 1. Scheme of structural South Caspian basin. 1 = Cenozoic, mainly erogenic tour lines of the consolidated sedimentary cover. I-I and respectively.

relationships between basement and sedimentary cover of the Pre-Alpine complex; 2 = Alpine geosynclinal complex; 3 = complex; 4 = zone where the granitic layer is absent; 5 = concrust surface; 6 = major crustal faults; 7 = fold axes within a II-II are positions of cross-sections given in Fig. 3 and Fig. 2,

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al., 1968; Ah-Zade et al., 1968; Volvovsky and Shlezinger, 1975). It should be emphasized that the cross-folding zone coincides with the deepest part (over 20 km) of the consolidated crust surface. Such a sharp difference in orientation of the isolated structural elements of the South Caspian basin is explained by its tectonic setting. The depression is located at the junction of two zones of intensive Cenozoic warping and of mutually perpendicular strike: nearly longitudinal and nearly latitudinal. Within the Alpine geosynclinal area the tectonic movements which resulted in the formation of major and local structures - sublatitudinal and sublongitudinal - were different at different times. We can therefore speak about periods of time, of different duration, when the formation of structures of a certain trend was occurring. So, when one considers the Alpine cycle of tectogenesis, in the Mesozoic it means the formation of a structural framework with a sublatitudinal trend of isolated elements following that of the geosynclinal trough. The reconstruction of this structural framework took place in Cenozoic times and was caused by movements which defined the sublongitudinal structures. The superimposed nature of the Cenozoic tectonic movements was reflected in the extension of the South Caspian basin and its overlapping by the Cenozoic strata represented by the Mesozoic folded structures of the Great and Minor Caucasus in the west and Kopetdag in the east. In this connection it would be reasonable to speak of a fit of the Apsheron-Balkhan and Pre-Elburz troughs to the Mesozoic structural framework and to consider the troughs of the western and eastern margins of the South Caspian basin as new structures formed in Cenozoic times. However, it is only the trough encompassing the West Turkmenian lowland and adjacent Caspian shelf that is not reflected in the structure of the Moho discontinuity. The crustal thicknesses there and in the South Caspian uplift are about 30 km, whereas in the northern, southern and western margins of the basin they increase to more than 50 km. Thus, among the isolated peripheral troughs of the South Caspian basin, only the eastern one should be considered as a newly-formed structure. The striking similarity of the structure of the surface with that of the base of the consolidated crust provided the foundation for the widely accepted interpretation. This says that the development of the South Caspian basin has taken place at least later than the Hercynian stage and that within the basin the structural framework has not undergone great changes (Malovitsky, 1968; Rezanov and Shevchenko, 1978). This opinion is confirmed, at first sight, by the conditions of deposition of the sedimentary strata which remained the same throughout the whole Alpine cycle, or, more precisely, during the Jurassic, Cretaceous and Cenozoic. The conformity of sedimentary strata of the Earth’s crust to the complex of the deposits from the Jurassic up to the Quaternary inclusive was considered generally accepted.

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The statements discussed below concern the crustal structure of the South Caspian basin and are based on the DSS~~R~ data obtained both in the open sea and on the coast. Such DSS profiles as Ogurtchin-Sarykamysh (Fig. 2), Atrek--Sadyz (the eastern setting of the South Caspian), SafaralievCaspian Sea (Kura depression) are the most important. The main idea is that the contact of a sedimentary cover and basement corresponds to the boundary between the Cenozoic and Mesozoic. Such a high stratigraphic position of the consolidated crust surface is easily explained by the presence of basic volcanic rocks in the Mesozoic section both in the South Caspian and Kura basins. It was proved by drilling data from the area of the buried Talysh-Vandam uplift near Saatly. The surface of the consolidated crust within the above-mentioned gravity high is established at depths of 3-5 km at velocity boundary having V, = 5.8--6.8 km/set (Rajabov, 1975). Cretaceous volcanic rocks occur, at the depths mentioned, near Saatly where a well was drilled up to a depth of 6.3 km. The basaltic layer if probably Jurassic. Geophysical data (V, = 6.57.3 km/see) show its surface to lie at a depth of 7-8 km. It should be noted that earlier studies dealing with the deep structure of the South Caspian basin have not taken into consideration the drilling data from the zone of Talysh--Vandam gravity maximum and so the Pre-Jurassic basement was thought to exist at a depth of 4 km, on the basis of an abrupt decrease in thickness of the Mesozoic (Cretaceous-Jurassic) sediments (Malovitsky, 1968; Rezanov and Shevchenco, 1978). A drastic subsidence of the consolidated crust surface east of the TalyshVandam uplift i.e. within the South Caspian basin proper could result from the intense down-warping in Cenozoic times. The sublongitudinal faults

SW

NE 50

0

100

200

!GO

300

10 20 30

_LJ WI??@ 3 El 4-

_ -.,

A

1

Fig. 2. Seismic cross-section of the Earth’s crust along Ogurtchin-Sarykamysh (II-II). 1 = Cenozoic complex; 2 = Mesozoic complex; 3 = surface of the basement; 4 = discontinuity; 5 = major crustal faults.

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bounding the South Caspian to the east and to the west imply first of all a spasmodic increase in the apparent thickness of Cenozoic deposits towards an abyssal part of the basin. It suffices to say that according to seismic data the base of the Upper Pliocene lies at a depth of 7 km in the open sea southeast of the Apsheron Peninsula, where the maximum depth (up to 25 km) for the consolidated crust surface was obtained. Interesting drilling data from the southwestern Apsheron, Kobystan, Baku archipelago and the Balkhan zone can be presented as evidence of large true and apparent thicknesses of Cenozoic deposits on the eastern and western margins of the basin. These data argue for the presence of gentle thrusts within Cenozoic formations and strong disharmony of the Cenozoic folding which is responsible for the frequent occurrence of the same horizons within a vertical section, thus causing an abrupt increase in the apparent thickness of the entire Cenozoic sequence. It is noteworthy that in the Balkhan and Gograndag-Okareme zones along the eastern margin and in the Apsheron--Kobystan and Kura regions along the western margin of the South Caspian, the folding in Cenozoic time was synchronous with the downwarping and was directly dependent upon a degree of downwarping resulting from horizontal mass displacement inside the troughs. In fact, folding on the margins of the South Caspian, which caused the inversion relationships between the structure of the base and the ‘Cenozoic surface (Fig. 3), is a compensatory process with respect to the downwarping. Hence, the strike of the fold structure of the sedimentary infilling of the troughs should follow the trends of the latter, It is due to this fact that within the Apsheron--Balkhan trough and particularly in its narrow central part (Apsheron sill) the folding, as in the Pre-Elburz troughs, has sublatitudinal and, in the Kura and Gograndag-Okareme zones, sublongitudinal trends. Thus it is easy to explain a NE strike of folds in the sea southeast of the Apsheron Peninsula at the intersection of troughs having latitudinal and longitudinal trends and so the trend of folding should be intermediate between these two.

Fig. 3. Shemakha-Kobystan

region.

Geological

cross-section.

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Different physical properties of Cenozoic and Mesozoic rocks are chiefly due to their belonging to molasse and geosynclinal complexes respectively. The density of flysch Cretaceous rocks in the southeastern Caucasus usually does not exceed 2.3-24 g/cm3 and that of Jurassic shales is of the order 2.7 g/cm3. Mesozoic volcanic rocks within the Kura and the South Caspian troughs have a mean density of about 2.8 g/cm3 owing to their basic composition. It should be noted that on average the low density of molasse Cenozoic formations (less than 2.3 g/cm3) persists to a depth of more than 20 km. This is mainly due to the argillaceous composition of the Paleogene-Neogene deposits, widespread clayey formations in Pliocene strata, and also accounts for their plasticity because the rocks are very porous and saturated with fluids. It is this high plasticity that even now causes active folding and fracture formation within Cenozoic strata. This is vividly exemplified by manifestations of mud volcanism all over the South Caspian basin. The analysis of mud volcanic breccia fragments suggested a Cenozoic age for the rocks. The same is true also for volcanoes (rather large judging by ejected blocks) of the southeastern Apsheron which are confined to Paleogene exposures and in the vicinity of Cretaceous outcrops. Moreover, mud volcanic breccias often contain rocks which, on the basis of their microfossils, may be assigned to deposits younger than those cut by the volcanic pipe. Thus the Cenozoic, mainly molasse, series of sedimentary cover of the South Caspian basin differ not only from pre-Alpine formations but also from the Alpine geosynclinal complex of Jurassic and Cretaceous age because DSS-CMRW profiles show them to have uniform physical properties, even at great depths. The difference increases with the depth of Mesozoic deposits. The lower Upper Cretaceous strata may be assigned to these deposits because volcanic formations or Lower Paleogene deposits appear at this strati~aphic level, in the southern part of South Caspian. The thickness of the Cenozoic complex increases in the northeastern South Caspian basin southeast of Bolshoy Balkhan. This increase in thickness of Cenozoic strata is due to the fact that sections of lower horizons become more complete southwestwards. Thus the sedimentary infilling of the South Caspian basin is beyond doubt Cenozoic in age despite the great apparent thicknesses, reaching 25 km. It is more difficult to prove the Mesozoic age for the upper part of the consolidated crust, although it is quite evident for the periphery of ,the basin (Balkhan zone, T~ysh-~~d~ buried uplift). The solution of the problem acquires great importance because it helps to explain why the granitic layer is absent from the consolidated crust. The Mesozoic rocks within the South Caspian basin should form part of the structure of the consolidated basement. It is quite possible that the upper part of the consolidated basement within a large area of the South Caspian basin is made up exclusively of Jurassic volcanics and even older rocks, and their considerable consolidation may result from high grade metamorphism and abundance of basic magmatic rocks. The erosion of the Upper Cretaceous part of the Mesozoic section and, in a central part of the basin

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within the South Caspian uplift, of all the Mesozoic strata and older deposits probably provides a reason for the absence of the granitic layer. However, some other explanations are also possible. On the one hand, depths of over 20 km provide conditions for considerable consolidation. However, in the southern part, consolidation of Mesozoic and also Cretaceous rocks may be caused by metamorphism. which resulted from intense igneous activity in the early Paleogene. All this means that crust under the South Caspian basin was developed as continental crust and its similarity to oceanic-type crust is due to superimposed processes in Cenozoic time, i.e. intense, contrasting, tectonic movements which on the one hand, led to the erosion of the granitic layer and on the other hand to metamorphism and considerable consolidation of rocks. Thus, with some degree of certainty, we can speak about the Cenozoic stage of development of the South Caspian basin when tectonic movements were contrasting in nature and when displacements along major fractures in the Earth’s crust led to the formation around the basin’s periphery of deep troughlike depressions surrounding a central uplift. Movements along faults bounding these depressions are clearly reflected in intensely folded Cenozoic sedimentary cover. As to the earlier stages of development, they are not well known because both geophysical and geological data indicate only structural relationships between the Cenozoic sedimentary cover and pre-Cenozoic consolidated basement. However the heterogenous structure of the crust under the basin in pre-Cenozoic times Y--at least in the Mesozoic, should not be in doubt, otherwise it would be difficult to explain the abundance of volcanic formations in the Mesozoic section. One can suggest that the intense overall, but rather irregular, downwarping in the Cenozoic was also preceded by irregular upwarping of consolidated crustal blocks in the Mesozoic when there was most activity within the South Caspian uplift. The latter could be explained by the confinement of the South Caspian uplift to a mantle diapir. The intense rise in the zone of South Caspian uplift in early Alpine (Jurassic, Cretaceous) and probably in Hercynian times might be accompanied by magmatism along the periphery of the depression where the subsidence took place periodically. As a result, the granitic layer was removed by erosion in the area of the South Caspian uplift, where the crust is marked by a drastic decrease in thickness and its absence in the peripheral part might be due to intense basic igneous activity which might have resulted in metamorphism and consolidation of strata up to the state characteristic of the basaltic layer. So we can conclude that the South Caspian basin occupies the zone of multiple rifting since the Baikal stage. The absence of the granitic layer in the section of South Caspian crustal structure may be explained by its confining to the region of intersection of rift zones which differ in age and direction. This region is characterized by intensive magmatism in the pre-Cenozoic and by warping with heavy sedimentation in the Cenozoic.

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