Evolution of the North Caucasus foredeep: constraints based on the analysis of subsidence curves

Transcription

1 ELSEVIER Tectonophysics 307 (1999) Evolution of the North Caucasus foredeep: constraints based on the analysis of subsidence curves Valentine O. Mikhailov a,ł, Ludmila V. Panina b, Riccardo Polino c, Nikolay V. Koronovsky b, Elena A. Kiseleva a, Natalya V. Klavdieva b, Ekaterina I. Smolyaninova a a United Institute of Physics of the Earth RAS, B. Gruzinskaya 10, Moscow, , Russia b Moscow State University, Vorobjevy Gory, MGU, Moscow, , Russia c Centro di Studi sulla Geodinamica delle Catene Collisionali, Via Accademia delle Scienze 5, Torino, 10123, Italy Received 18 September 1998; accepted 22 February 1999 Abstract Using a database of more than 130 wells the Alpine evolution of the North Caucasus foredeep can be described in three main periods. (1) The Early Jurassic to Middle Late Cretaceous (including the Cenomanian) relates to the initial rifting phase and was characterised by succession of comparatively high rates of subsidence and uplift. During this stage, many events in the eastern and western parts were synchronous, whereas some of them appeared to be smoothed in the Stavropol high. The southern border of the area had a more complex behaviour often moving in the opposite direction relative to the rest of the area. The main peculiarities of evolution of the Great Caucasus region can be explained if we adopt the hypothesis of Stamply and Pillevuit (1993) and suppose that the Early Jurassic extension of the trough was accompanied by a strong left-lateral transform movement. As a result the central part of the Great Caucasus trough formed as a pull-apart basin. Analysis of the style of movements of the southern border of the Scythian plate as well as data on tectonics and volcanism in the Great and Lesser Caucasus showed that other regional compressional and extensional Mesozoic events could also have a transform component. Shear stresses within the lithosphere of the Great Caucasus can be due to oblique subduction or even transform movements at the plate boundary to the south of the Caucasus region (Dercourt et al., 1993). (2) The period from the Late Cretaceous to the Middle Eocene (from the Turonian to the Bartonian) relates to the oceanic suture of the Lesser Caucasus and was characterised in the Great Caucasus area by alternating subsidence and uplift events of considerably lower amplitude (at least up to the Maastrichtian). Beginning in the Late Paleocene, subsidence curves for almost all the area reflect the same events, but in the western part and also in the north of the central part the rate of movements was considerably higher and since the Late Paleocene short term events in this area took place on the background of rather fast subsidence at nearly constant rate. We believe that this subsidence and formation of the East Black Sea depression have the same origin. We consider strong differences in evolution of the eastern, western and central parts during the second stage to be due to closure of the Lesser Caucasus oceanic basin and arrival of the Nakhichevan block. This led to changes of configuration of the plate boundary and resulted in reorganisation of stress and displacements within the Caucasus region. This reorganisation was a reason for the opening of the East Black Sea depression and rapid subsidence of the western and central parts of the area at the end of the second stage. (3) The period from the Middle Eocene to the present relates to the development of a foreland basin coeval with shortening and uplift in the adjacent Great Caucasus range. It was characterised by alternation of relatively short uplift and longer subsidence events. An important feature of this stage is that although the amplitude of movements varied from place to place, these events were synchronous Ł Corresponding author. mikh@uipe-ras.scgis.ru /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S (99)

2 362 V.O. Mikhailov et al. / Tectonophysics 307 (1999) in all parts of the foredeep (at the Stavropol region sediments preserved only for the beginning of this stage). Formation and evolution of the foredeep during the third stage can not be explained exclusively by elastic flexure. To do this we used the model of a small-scale convection within the asthenosphere (Mikhailov et al., 1996, 1999). By the comparison of numerical results with the data on the evolution of the North Caucasus foredeep we concluded that there were four main stages of compression in the processes of formation of the Great Caucasus mountain belt. The first compression took place before the Maykopian (between 39.5 and 36.0 Ma). The other three were in the Tarkhanian ( Ma), Konkian Early Sarmatian ( Ma) and Pontian ( Ma). The different width of the Great Caucasus trough by the beginning of the compression, as well as variations in thickness of the lithosphere and a different thermal state can cause interruption of the formation of the foredeep at the Stavropol high Elsevier Science B.V. All rights reserved. Keywords: Great Caucasus; foredeep; subsidence curves; geodynamics 1. Introduction The North Caucasus foredeep occupies the southern part of the Scythian platform north of the Great Caucasus mountain belt. The main thrusts of the Great Caucasus are S-vergent and situated along the southern slope of the belt (e.g. fig. 4 in Philip et al., 1989), so this foredeep is considered as a conjugate one. The modern structure of the North Caucasus foredeep was formed mainly during the late Sarmatian, concurrently with the onset of the uplift of the Caucasus. At present the two troughs are identified within the foredeep: the Azov Kuban to the west and the Terek Caspian to the east. These troughs are separated by a structural high (Stavropol high, Fig. 1). The Azov Kuban trough is characterised by a relatively simple asymmetrical structure with a gentle and wide northern flank and a very narrow and steep southern flank (Fig. 2). In the south the trough appears more complex a west east directed anticline developed in the Mesozoic sediments (Anastasiev Troitsky anticline zone in Fig. 1). The Stavropol high occupies the central part of the area. Deepening to the north, the Mesozoic and Cenozoic sedimentary layers do not form any depression here (Fig. 3). The structure of the Terek Caspian trough (Fig. 4) is more complex than that of the Azov Kuban. It incorporates the north-vergent Terek and Sundja thrust ridges, and to the east it is partly overthrust by the Dagestan nappe (Fig. 1). The distinctive feature of the North Caucasus region is that foreland basins developed in its western and eastern flanks, where mountains are considerably low, and not in the front of the highest central part of the mountain belt. The structure and kinematics of the Great Caucasus region changed considerably during Mesozoic and Cenozoic times; thus, a question of nomenclature arises. During the Mesozoic, this area formed the transition zone from the Scythian platform to the Great Caucasus trough. The structural pattern of the area was rather complicated. The following structures are recognised: the West and East Kuban troughs, the Timashev step, the Kanev Berezan zone of gentle basement folds. Fig. 1 shows the recent limits of the West and East Kuban troughs (since the Late Miocene), while in the Mesozoic the East Kuban trough stretched far to the east up to the region of well 7 (Fig. 1). To the south it was bordered by the Laba Malka monocline zone, so that the Adigeya swell appeared to be a structural high between the West Kuban and East Kuban troughs. The Stavropol area existed as a relatively uplifted area during almost all the Jurassic and since the end of the Middle Miocene. The eastern part of the study area incorporates the Terek Caspian and Manych troughs separated by the Prikumsky uplift. All these structures are manifested in Mesozoic sediments and they can be hardly recognised in Cenozoic deposits (see Figs. 2 and 4). In order to simplify the descriptions we divided the area from west to east into three main domains: (1) the West and East Kuban troughs, the Adigeya swell, the Timashev step and the Kanev Berezan zone of basement folds referred to below as the Azov Kuban trough or the Western domain; (2) the Stavropol high and the Laba Malka monocline zone (the Central domain); (3) the Terek Caspian

3 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 1. Sketch map of main structural units of the study area. K-B D Kanev Berezan system of basement folds; WK and EK D the West and East Kuban troughs; A-T D Anastasiev Troitsky anticline zone; AS D Agigeya swell; TS D Timashev step; T and S D Terek and Sundja ridges; DN D Dagestan nappe. Wells: 1 D Medvedovskaya, 2 D Hadyzhenskaya, 3 D Lovlenskaya, 4 D Temirgoevskaya, 5 D Yu. Sovetskaya, 6 D Labinskaya, 7 D Urupskaya, 8 D Maykopskaya, 9 D Anastasievsko Troitskaya, 10 D Kalujskaya, 11 D Karskaya, 12 D Koshehablskaya, 13 D Uspenskaya, 14 D Tulskaya, 15 D Ipatovo, 16 D Stavropolskaya, 17 D Nagutskaya, 18 D Kavminvodskaya, 19 D Buivolinskaya, 20 D Padinskaya, 21 D Ozernaya, 22 D S. Kochubey, 23 D Datyh, 24 D Pervomayskaya, 25 D Chanty Argun, 26 D Dagestanskiye Ogni, 27 D Pravokumskaya, 28 D Chervlennaya, 29 D Arak Dalatarek, 30 D Argudan, 31 D Oktyabrskaya, 32 D Kayakent; A,B,C D position of profiles shown in Figs and Manych troughs, the Prikumsky uplift and the Dagestan nappe referred to below as the Eastern domain. These domains are not tectonic structures. Each of them incorporates the southern edge of the Scythian platform, a part of the northern slope of the Great Caucasus ridge and the related part of the foredeep. The discussion about the boundaries between the tectonic structures of the region is not matter of debate in this paper. It should be mentioned also that the level of geological knowledge of the region is rather dissimilar. A great number of wells and seismic reflection profiles characterise the structure of the upper part of sedimentary cover of the Eastern and Western domains while the data on the structure of the Middle Jurassic and older layers for deep parts of the domains are very scarce, especially below the strongly deformed Terek and Sundja ridges (Figs. 2 and 4). Our present knowledge of the Earth s crust structure is based mainly on the results of gravity field modelling and few rather old DSS profiles crossing the North Caucasus region and the Great Caucasus which can hardly be considered as providing reliable data on the structure of the lower crust and the uppermost part of the mantle. The aim of this paper is to investigate the spatial and temporal features of the evolution of the North Caucasus region and correlate them with the main regional tectonic events in the Great and Lesser Caucasus. Joint analysis of the data on the North Caucasus foredeep and the Great and Lesser Caucasus makes it possible to gain new insight on the Mesozoic and Cenozoic geodynamics of the region.

4 Fig. 2. Cross-section of the Eastern domain based on borehole and reflection seismic data (profile A in Fig. 1). 364 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 3. Cross-section of the Central domain based on borehole and reflection seismic data (profile B in Fig. 1).

5 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Initial data and assumptions Fig. 4. Cross-section of the Western domain based on borehole and reflection seismic data (profile C in Fig. 1). To investigate the evolution of different parts of the area subsidence curves for more than 130 wells were constructed and analysed. These wells are situated in the central part of the foredeep, at the northern slope of the Great Caucasus and far to the north on the Scythian platform and characterise the different domains of the study area. Results of the study of outcrops at the northern slope of the Great Caucasus and the Stavropol high and reflection seismic data were also incorporated. The most representative subsidence curves for the different domains are given in Figs For the location of the corresponding wells see Fig. 1. When digitising the borehole data, the stratigraphic age (subdivided into early, middle and late if possible) was accepted as the main time unit. For the Mesozoic and the lower part of the Cenozoic (including the Priabonian) we used the international geologic timescale (Haq et al., 1988), and from 36.0 Ma the local timescale presented in Table 1 (Chumakov et al., 1992; Sherba, 1993). To simplify the description of the main geological events the period from the Triassic up to the present was subdivided into 29 time intervals (see Table 1 and Figs. 6 11). To take into account the depth-dependent porosity, we used the results of the statistical analysis of data on more than 30,000 samples processed by the North Caucasus Geological Survey during (Stet uha, 1964). On the basis of these investigations we chose eight different equations in relation to particular lithology and region studied. These dependencies are linear or exponential. They incorporate four different lithological types (clay, marl, sand, limestone) and two different sets of coefficients, the first one for the Eastern domain and the second for the Central and Western domains (Table 2). Porosity of evaporites is set equal to zero. A major problem was to estimate the palaeobathymetry for every layer. As palaeodepth usually can not be defined precisely, we assigned the upper and lower limits of palaeobathymetry for each layer using an approach similar to Stam et al. (1987). Thus, we present minimal, maximal, and average tectonic subsidence curves, obtained after correction of burial depth on compaction (Table 2), depth of sedimentation, sea-level changes and isostatic re-

6 366 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Table 1 Time intervals and average depth of sedimentation used for subsidence curves analysis Number of event Absolute age Age Average depth of sedimentation in domains (m) (Ma) Western Central Eastern Lias Aalenian Bajocian E. Bathonian L. Bathonian Callovian Oxfordian Kimmeridgian Tithonian Berriasian Hauterivian Barremian Aptian Albian Cenomanian Turonian Campanian Maastrichtian E. Paleocene L. Paleocene E. Eocene M. Eocene L. Eocene Maykopian Tarkhanian Tschokrakian Karaganian Konkian E. Sarmatian M. Sarmatian L. Sarmatian Meotian Pontian Kimmerian Akchagilian Apsheronian L. Quaternary bound (local compensation). Depth of sedimentation for every layer vary from place to place. The average values for the domains are given in Table 1. For some time intervals the well data appear to be rather rough, especially for the beginning of the Alpine stage, because of the lack of data on Table 2 Equations used for correction for depth-dependent porosity in relation to different lithology and domains (Oz axis is downward directed) Rock type Type of dependence Eastern domain Western and Central domains A B (1=km) A B (1=km) clay A Ł exp. Bz/ marl A Bz sand A Bz limestone A Bz

7 V.O. Mikhailov et al. / Tectonophysics 307 (1999) the deep-buried Jurassic sediments in the central parts of the Terek Caspian and West Kuban troughs. Moreover, there is not a single well in which all the 29 events can be distinguished. Thus, some intervals of subsidence curves appear smoothed and do not reflect the main peculiarities of the Alpine evolution of the region (Figs. 6 11). The age of sedimentary layers ranges from Triassic to Quaternary (Fig. 5). According to seismic data total thickness of sediments in the deepest parts of the foredeep is more than 12 km. The Triassic and Jurassic successions have been explored by drilling only at the periphery of the foredeep. The Triassic succession comprises carbonate, clastic and volcano-detrital rocks of which the thickness ranges from 0.5 to 1.5 km. The lower Jurassic succession consists of: (1) lacustrine coal-bearing sediments of which the thickness varies from few tens of meters in the north to several hundred meters in the south in the Eastern domain; (2) continental lacustrine-marsh series up to 0.8 km thick in the East Kuban; (3) shallow marine argillites up to 1 km thick in the West Kuban. In the Terek Caspian trough, Aalenian shallow marine and deltaic sediments of more than 1 km thick are overlaid by Bajocian Bathonian marine terrigenous sediments of which the thickness varies from few meters in the north up to 1 3 km in the south. The upper part of the Middle Jurassic succession consists of coarse-grained clastic sediments of Callovian age, 0.1 km thick. In the Western domain the Middle Jurassic succession consists mainly of marine argillites with sandstones. Their total thickness reaches 1.3 km in the East Kuban. Lower to Middle Jurassic sediments are not known in the Stavropol area. The Oxfordian succession consists mainly of carbonate-platform sediments, with interlayers of clays in the Western domain. The thickness of the Oxfordian sediments is up to 700 m in the Terek Caspian and up to 250 m in the Azov Kuban. The Kimmeridgian Tithonian interval includes evaporites, carbonates and clastics. Their thickness changes from several meters in the north to 1.8 km in the centre of the Terek Caspian and East Kuban. In the Stavropol area thickness of the Upper Jurassic sediments decreases to 100 m or less. Early Cretaceous glauconite sandstones and clays are widespread all over the region. Their thickness reaches km in the West Kuban and Terek Caspian troughs and decreases to several hundred meters in the East Kuban depression and Stavropol area. In the Terek Caspian the lower part of these sequences contains shallow water carbonate rocks (see Fig. 5). The Upper Cretaceous is characterised by transgressive limestones with a maximum thickness of as much as 1.6 km in the Eastern domain. In the Western domain this sequence contains interlayers of clastics and flysch, the average thickness being 600 m. During this time the central parts of the West Kuban and Stavropol areas are considered emerged, because of the lack of Upper Cretaceous sediments. Considerable differences exist between the Palaeocene Eocene sedimentary successions of the Eastern, Western and Central domains. A rather thick flysch sequence developed in the Western (about km thick) and Central (0.7 1 km thick) domains, while in the Eastern domain only m thick carbonate rocks were deposited. The Oligocene Early Miocene (Maykopian) succession consists of clays with interlayers of sandstones. These series are widespread throughout a broad area from the Pannonian Basin to Iran, including the North Caucasus foredeep and the South Caucasus intramontane basins. In the central parts of the Terek Caspian and West Kuban the thicknesses reach 1.7 km and more, and in the East Kuban depression and Stavropol area the thickness is from 0.6 to 1.4 km. The Middle Miocene Quaternary period is characterised by the deposition of molasses. The Middle Miocene comprises shallow water clastic sediments with scattered interlayers of carbonate and lagoon deposits of km thick in the West Kuban and Terek Caspian troughs and m thick in the Stavropol high and East Kuban depression. The Upper Miocene marine clays with interlayers of limestones are developed in the Terek Caspian (about 2.5 km thick), in the West Kuban (up to 1.2 km) and in the East Kuban (up to 400 m). The Pliocene Quaternary molasse includes marine and continental clastics, more coarse-grained conglomerates, pebbles, sands (in the southern part of the study area). Its thickness varies from 1 to 1.2 km and more in the south of the West Kuban and Terek Caspian, and to 100 m in the East Kuban and Stavropol areas.

8 368 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 5. Generalised stratigraphic columns for the Eastern, Central and Western domains.

9 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Analysis of subsidence curves The subsidence curves show three main periods of subsidence characterised by different styles of subsidence. Each period contains many events and almost every event coincides with periods of deformation and=or volcanic activity in the Caucasus region. The first period corresponds to the time intervals from 1 to 13 in Table 1, the second to intervals from 14 to 19 and the third one to intervals from 20 to 29. The wells in the central parts of the Terek Caspian and West Kuban troughs penetrated only into the upper part of the Jurassic successions. Let us consider the main features of these periods of subsidence. (I) At the northern edge of the Eastern domain, in the Manych trough, the subsidence curves show the Late Triassic Early Jurassic slowing down subsidence, similar to thermal one (wells 21, 22, Fig. 8, for location of wells see Fig. 1). Wells 3, 5 and 7 (Fig. 6) that reached the Lower Jurassic sediments in the East Kuban trough and in the northern edge of the Laba Malka zone reveal subsidence during the Lias. The Stavropol high appeared to be exposed. From the Early Jurassic to the beginning of the Late Cretaceous (including the Cenomanian, i.e. age intervals 1 13 in Figs. 6 8) a succession of subsidence and uplift events of relatively high rate occur simultaneously in the Eastern and Western domains, whereas the rate of uplift or subsidence in the Central domain is less intense. The southern border of the area shows a more complicated behaviour: the vertical component of movements was often greater and sometimes opposite to the movements of the adjacent regions of the Scythian platform. For the first period many subsidence curves are straight or convex upward. During the Aalenian (2nd interval) the Manych trough was uplifted (well 22, Fig. 8), while at the same time subsidence took place in the East Kuban (well 5 in Fig. 6) and to the east of the present Dagestan nappe (well 26, Fig. 8). With the beginning of the Bajocian (3rd interval) the Terek Caspian subsided. Next was the Bathonian uplift (interval 4) of the southern border of the Terek Caspian and a slowing down subsidence of the Manych trough (Fig. 8). As the Bajocian and Bathonian layers were not distinguished in the wells of the East Kuban, the total movement for the 3rd and 4th periods appeared to be subsidence (wells 5 and6infig.6). During the Callovian (5th interval) slight subsidence of the main part of the whole area occurred (wells6infig.6,17infig.7and22and30in Fig. 8) with the exception of the southern border of the Terek Caspian (uplift in wells 24 and 25 in Fig. 8). From the Oxfordian to the Hauterivian (intervals from 6 to 9) the whole area subsided, but the amplitude of subsidence differed considerably from place to place and sometimes sedimentation simulates a relative uplift. These uplifts were local and did not incorporate large territories. On the southern border of the Terek Caspian where it is possible to distinguish the Kimmeridgian sediments (7th interval) from the Tithonian ones (8th interval), the subsidence curves show relative uplift in the Kimmeridgian, followed by Tithonian (late Tithonian) subsidence (wells 25 and 24 in Fig. 8). The only regional uplift and erosion occurred during the Berriasian (beginning of the 9th interval). It took place on the Stavropol high, in the western part of the Terek Caspian (well 30 in Fig. 8) and possibly in the whole territory of the Western domain (subsidence curves 1 8 in Fig. 6 do not contradict this assumption). There was an uplift in the Barremian (10th time interval) of almost all the study area except the southern edge of the Azov Kuban trough and some parts of the Adigeya swell (wells 2 and 7 in Fig. 6). In the Eastern and Central domains this uplift was moderate, but in the center and especially at the peripheral parts of the East and West Kuban troughs it was much stronger; thus, a considerable part of Barremian sediments was eroded there. Erosion removed sediments since the Berriasian (well 6), Tithonian (wells 4 and 1), Callovian (well 5) and even Aalenian (well 3; all curves for the above-mentioned wells are presented in Fig. 6). Well 8 (Fig. 6) shows that possibly part of these sediments was eroded earlier, before the Hauterivian. In the Aptian (11th interval) all the area including the Stavropol high (wells 17 and 18, Fig. 7) subsided, but the amplitude of subsidence again varied considerably. During the Albian (12th interval) the rate of subsidence slowed down, and in many boreholes of the area it appeared to be close to zero. The

10 370 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 6. Subsidence curves for the Western domain. For the location of wells see Fig. 1. Dots mark age intervals for which the data were specified. H stands for hiatus.

11 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 7. Subsidence curves for the Central domain. For explanation see Fig. 6. Albian sediments were eroded in the southern edge of the West Kuban trough (wells 2 and 10 in Fig. 6) and also in some places of the East Kuban (well 5). The Cenomanian (interval 13) was included into the first period of subsidence because all of the wells of the Eastern domain where it was identified, show rapid subsidence (wells 23, 25 and 30 in Fig. 8). From the beginning of the Turonian until the Middle Eocene the subsidence rate appeared to be considerably slower. (II) From the Late Cretaceous to the Middle Eocene (from Turonian to Bartonian, i.e. time intervals 14 19) several subsidence and uplift events also occur, but their amplitude was considerably smaller than during the first period (for the Azov Kuban trough at least up to the beginning of the Paleocene). Since the beginning of the Late Paleocene (16th time interval), subsidence curves for almost the whole study area reflect the same events, but in the Western domain and in the north of the Central domain, the rate of movements was considerably higher than that in the Eastern domain, and since the Late Paleocene short-term events in these domains took place in the background of fast subsidence at nearly constant rate. This subsidence appears to be coeval with the opening of the Eastern Black Sea. One of the most important features of the second period is that the style of tectonic development of the Eastern domain (slow movements) differs strongly from that for the Western and Central domains (fast movements). The

12 372 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 8. Subsidence curves for the Eastern domain. For explanation see Fig. 6.

13 V.O. Mikhailov et al. / Tectonophysics 307 (1999) subsidence curves for the Terek Caspian are convex downward, sometimes being flat. For the rest of the area subsidence curves are straight or convex upward. The beginning of the second period (14th time interval) is characterised by slow subsidence of the Eastern domain. Sediments of this age were eroded in the northern part of the Central domain and southern border of the Western domain (wells 15 and 16 in Fig. 7, and wells 2 and 8 in Fig. 6). In the vicinity of wells 5 and 10 (Fig. 6) erosion lasted till the Coniacian. The Maastrichtian (15th interval) was a period of slow subsidence. Further, during the rest of the second period the rate of movements in the Eastern domain remained nearly zero. On the contrary, movements in the Western and Central domains were more significant. The main events there were: 16th interval (Early Paleocene) uplift of the northern edge of the area (wells 15, 16 in Fig. 7 and possibly wells 1, 3, 4 in Fig. 6); 17th interval (Late Paleocene) rapid subsidence of the northern part of the Central domain and the whole Western domain (with the exclusion of short hiatus in well 2 in Fig. 6) and slow uplift of the Eastern domain and the southern part of the Central one; 18th interval (Early Eocene) uplift of the southern border of the Western domain, no data for the rest of the area; 19th interval (Middle Eocene) slow subsidence. It should be mentioned that similarly to the first period, the development of the southern edge of the Azov Kuban trough during the second period was more complex with few periods of erosion and higher rate of subsidence in comparison to the rest of the area (wells 2, 5, 8, 10, 11 in Fig. 6). (III) The Middle Eocene to the present is characterised by succession of relatively short events of uplift and generally continuous stages of subsidence (see Figs. 9 11). During this period the rate of movements increased and although the amplitude of movements varied from place to place, these events were synchronous in almost all parts of the region (for the Stavropol high at least for the beginning of this period of which the sediments were preserved). In the Late Eocene (20th time interval) subsidence of the whole study area slowed down. Uplift and somewhere erosion took place near the northern slope of the Great Caucasus trough and in the East Kuban trough (wells 8, 13, 14, Fig. 9). A very impressive feature of the subsidence curves is the rapid Maykopian subsidence (interval 21) that occurred in the whole area. After that the twofold event consisting of short-term uplift or nearly zero subsidence followed by more long-term rapid subsidence at nearly constant rate, took place at least three times (see Figs. 9 11): The Tarkhanian (interval 22) uplift and erosion of almost the whole area were followed by the Tschokrakian Karaganian subsidence (interval 23). Since the Tarkhanian the main part of the Stavropol area became emerged land. Nearly zero subsidence or a slight uplift and=or erosion occurred during the Konkian and early Sarmatian (24th interval) and subsidence until the middle late Sarmatian (25th interval). After the Sarmatian the period of slow subsidence during the Meotian (time interval 26) took place. The subsidence appeared to be small here, not due to the lack of sediments but because of the fact that a value of uplift of the sea level up to 100 m was assigned to this stage according to (Haq et al., 1988). Since the Meotian all the Central domain has been uplifted above sea level (Figs. 7 and 10). It should also be mentioned that since the late Sarmatian Meotian a considerable part of the southern edge of the area (i.e. of the northern slope of the Great Caucasus) has been subjected to erosion. Uplift during the Pontian (27th time interval) gave way to subsidence. In the Prikumsky uplift and Sundja ridge the period of uplift lasted till the end of the Kimmerian (first part of 28th interval). Erosion of the Meotian sediments was possibly a result of the uplift during the Pontian Kimmerian (well 12 in Fig. 9, wells 19 and 20 in Fig. 10, and wells 27 and 32 in Fig. 11). Boreholes data for the last 1.6 m.y. are not detailed enough to investigate movements of the area during the Apsheronian and the Late Quaternary. 4. Interpretation To compare the peculiarities of evolution of different parts of the North Caucasus with main tectonic and volcanic events in the Great and Lesser Caucasus it is necessary to know the structure and thermal state

14 374 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 9. Subsidence curves for the time period 0 40 Ma. Western domain. For explanation see Fig. 6.

15 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 10. Subsidence curves for the time period 0 40 Ma. Central domain. For explanation see Fig. 6. of the North Caucasus lithosphere. These factors determine the lithosphere s response to external effects. The borehole data available are not detailed enough to investigate the Triassic and Early Jurassic history of the study area. Thus, for these periods we incorporated the results of the study of the Pre-Alpine and Early Jurassic evolution of the Great Caucasus region (Panov, 1976; Dercourt et al., 1993). These investigations show, in particular, that by the beginning of the Jurassic the lithosphere of the region was inhomogeneous: it seems to be thicker at the Stavropol high than in the neighbouring areas. Moreover, the analysis of the distribution of sediments thickness and facies makes it possible to conclude that the Liassic extension of the Great Caucasus trough was not uniform everywhere. At least two deep sedimentary basins were formed to the east and west from relatively wide and shallow basins in the central part of the trough (Panov, 1976). Presence of diabase dikes stretching approximately from north to south, as well as strong variations of thickness of the Lower Jurassic sediments are consistent with the idea that extension of the trough was accompanied by a considerable transform displacement (Stamply and Pillevuit, 1993). Taking into account the distribution of thickness of the Lower Jurassic sequences and the direction of stretching of dikes, we can conclude that extension, which led to formation of the Great Caucasus trough, was accompanied by left-lateral transform displacement; thus, a pull-apart basin developed in the central part of the Great Caucasus trough. This non-uniform extension predetermined the whole Mesozoic and Cenozoic evolution of the Great Caucasus region. Analysis of the subsidence curves revealed a more complex behaviour of the southern border of the study area. The amplitude of movements of the southern border was often considerably higher in comparison to the rest of the area; sometimes separate blocks (for example the Adigeya swell) moved upward when the rest of the area subsided and vice versa. Movements like that usually coincided with periods of folding and=or volcanic activity within the Great and Lesser Caucasus region. This style of movements of the southern border of the Scythian platform in conjunction with strong variations of amplitude and sometimes of the sign of movements and alternation of areas of compression and extension within the Great Caucasus trough also shows that events of regional compression or extension could be accompanied by a strong transform component. The reconstruction of the Palaeo-Tethys supports this hypothesis. According to Dercourt et al. (1993) the subduction zone to the south of the Caucasus region

16 376 V.O. Mikhailov et al. / Tectonophysics 307 (1999) Fig. 11. Subsidence curves for the time period 0 40 Ma. Eastern domain. For explanation see Fig. 6.

17 V.O. Mikhailov et al. / Tectonophysics 307 (1999) has never been parallel to the corresponding oceanic rift zone, so during some periods it was a zone of oblique subduction, sometimes a transform boundary. Such movements at the active margin were able to produce considerable shear stresses within the lithosphere of the Caucasus region. This suggestion can also help to solve the problem of the Jurassic calc-alkaline volcanism. Many authors associate this volcanism with a hypothetical subduction zone at the northern edge of the Great Caucasus trough (see, for example, Gamkrelidze, 1986). Taking into account that this volcanism took place within a relatively small area in the central part of the Great Caucasus and that there are no remnants of oceanic crust within the folded Great Caucasus belt, Lomize (1987) suggested that extension within the Great Caucasus trough was small and the oceanic basin and continuous subduction zone did not develop. Transform movements along the Great Caucasus trough could result in local underthrusting in the area of transition from the thinned crust of the trough to the thicker crust of the Stavropol high. This local underthrusting could result in calc-alkaline volcanism. Such a scheme can explain the Bathonian and Kimmeridgian uplifts concentrated mainly at the southern border of the Eastern and possibly Western domains. During these periods folding and volcanism within the Great Caucasus trough occurred. As a consequence of the Bathonian folding (usually referred as pre-callovian phase of folding), the area of flysch sedimentation within the Great Caucasus troughs considerably decreased (Panov, 1976). As far as the Kimmeridgian is concerned it was a period of amplification of back-arc volcanism in the Lesser Caucasus and calc-alkaline volcanism in the Dzerula massif (Lomize, 1987; Dercourt et al., 1993). Subsidence of the broad area of the southern slope of the Scythian platform during the Jurassic Cretaceous seems to be related to thermal subsidence of the flanks of the extensional Great Caucasus basin (Mikhailov, 1993). Back-arc extension above the corner flow that developed under the top of the subducting plate may also have contributed to the subsidence of the area since the Bajocian. The Low Jurassic slowing down subsidence of the Manych trough also seems to be due to cooling of the lithosphere after extension, manifested in the formation of the Middle and Upper Triassic volcano-sedimentary series. Uplift and subsidence during the Aalenian followed by the Bajocian subsidence of the southern border of the Scythian platform are compatible with the late Aalenian=beginning of Bajocian phase of folding in the Great Caucasus trough followed by the Bajocian phase of intensive andesite volcanism which occurred in the Lesser Caucasus region (Lomize, 1987). The folding within the Great Caucasus trough and upward and downward movements of different parts of the Scythian platform can be due to regional compression in the Aalenian which came before the formation of the Lesser Caucasus volcanic arc. If so, the Bajocian subsidence coincided with the beginning of subduction at the active margin to the south of the Caucasus region. During the Aptian Cenomanian amplification of volcanism occurred in the Lesser Caucasus and within the flysch troughs in the western part of the Great Caucasus. Formation of ophiolites in the Lesser Caucasus took place between the Albian and the Santonian. So, the beginning of the closure of the Lesser Caucasus trough is dated as Albian Cenomanian in age (Knipper, 1979). The closure of the Lesser Caucasus basin was accompanied by docking of the Nakhichevan block at the southern border of the Caucasus region (Dercourt et al., 1993). The change of configuration of the southern border of the continent could be responsible for reorganisation of the regional stress field and opening of the Eastern Black Sea and possibly the South Caspian basin. Since that time the Eastern domain has been enclosed inside a continental block, far from the active margin, being a possible explanation of the very slow movements in this area during the second period. If we suppose that the long-term component of the relatively fast subsidence of the Western and Central domains (time intervals 17 19) was governed by the same mechanism as the opening of the Eastern Black Sea, then the beginning of its opening should be dated as Early Paleocene. Taking into account the peculiarities of the development of the three different domains at the stage of continental collision, one could hardly explain them by elastic flexure exclusively. The main thrusts are S-directed and are situated at the southern slope of the Great Caucasus. The weight of the back thrusts

18 378 V.O. Mikhailov et al. / Tectonophysics 307 (1999) especially in the Western domain is not enough to produce as much as 1 km of the tectonic subsidence. That is hardly surprising, as for many mountain belts (Apennines, Pyrenees, for example) it was found that elastic flexure is not the only mechanism of foredeep formation (for discussion see Mikhailov et al., 1999). To explain the evolution of the foredeep basin during the third period, in addition to elastic flexure, we took into account a small-scale convection within the asthenosphere below a compressional belt (see Mikhailov et al., 1999). According to the model of a small-scale convection, compression within orogenic belts disturbs the mechanical and thermal equilibrium within the Earth s outer shell. This gives rise to a small-scale convection within the asthenosphere, causing long-term uplift of the compressional belt and subsidence at its periphery. As it was shown by numerical calculations these phases of regional compression should be expressed by slight uplift of the area neighbouring the compressional belt and nearly zero subsidence far from the belt. After the end of external (regional) compression, subsidence of the peripheral part of the compressional belt occurred. The rate of the subsidence slowly decreases with time. If two compressional stages or more took place, uplift inside the foredeep during the next stages of external compression will increase. According to the geodynamic model the first compressional events should be pre-maykopian in age (Late Miocene, 21th time interval). Numerical calculations have shown that compression of the order of 10 15% within a structure similar to the Great Caucasus trough is enough to cause subsidence of a Maykopian-like amplitude. Such compression of a wide sedimentary trough will not result in a considerable uplift on the surface. According to Sedenko (1976), during the Maykopian the Great Caucasus area was a low highland, where the Paleocene and Eocene sediments were being destroyed. This conclusion coincides with the recent results of studying recycled nannofossils at the base of the Maykopian series in the central slope of the Northern Caucasus. It showed that at the beginning of the Maykopian the source of terrigenous material was located on the present Great Caucasus where Upper Cretaceous and Palaeogene sediments were eroded (Lozar and Polino, 1997). According to the subsidence curves the next compressional stages could be dated as Tarkhanian, Konkian early Sarmatian, and Pontian. Formation of molasse conglomerates which are a manifestation of high-gradient topography started in the Sarmatian. The topography similar to the modern one was formed during the Pontian compressional stage. The compression seems to be nearly orthogonal to the former direction of stretching of the Great Caucasus trough, so the evolution of different parts of the foredeep at least at the first half of the third period was approximately the same. The role of elastic flexure in the domains was different. The structure of the Central domain (no thrusts in the northern slope of the Great Caucasus) and high heat flow (reducing flexural rigidity) make it possible to conclude that the role of elastic flexure at least at the end of the third stage was negligible here. As a result, subsidence of the Central domain at the third stage was more smooth and in the course of time it was slowed down. According to the small-scale convection model, contribution of asthenospheric convection to the process of foredeep formation critically depends on thickness of the lithosphere and asthenosphere as well as on the width of the compressional area. As it was already mentioned, by the beginning of the Early Eocene the central part of the Great Caucasus trough was wider and shallower than its western and eastern parts. If the Cenozoic compression shortening was uniform and orthogonal to the ridge trend, then the compression ratio in the central part should be lower. All this enables us to explain the interruption of the foredeep formation at the Stavropol high. 5. Conclusion The analysis of subsidence curves for the broad area including the northern slope of the Great Caucasus and the southern part of the Scythian platform reveals a close correlation between evolution of this area and main tectonic and volcanic events in the Great and Lesser Caucasus. The main peculiarities of the subsidence curves can be explained if we accept the hypothesis of Stamply and Pillevuit (1993) that the Early Jurassic extension of the Great Caucasus trough was accompanied by a strong left-lateral transform movement. As a result a pull-apart basin was formed in the central part of the Great Caucasus trough. Analysis of the peculiarities of the

19 V.O. Mikhailov et al. / Tectonophysics 307 (1999) movements of the southern border of the Scythian platform as well as data on the tectonic evolution and volcanism showed that other Mesozoic regional compression and extension events could also be accompanied by a transform component. The style of extension and compression was also controlled by the inhomogeneous structure of the lithosphere of the southern slope of the Scythian platform at the beginning of the Alpine stage; at least it seems clear that the lithosphere appeared to be thicker in the Central domain than in the Western and Eastern ones. This conclusion calls for further testing with data on the development of the Great Caucasus and especially the Kura and Riony intramontane basins. Acknowledgements The first version of the manuscript was considerably extended and improved according to numerous corrections and constructive comments of two anonymous reviewers. They are gratefully acknowledged. This work was supported by INTAS grant number and Russian Foundation for basic research (grant ). References Dercourt, J., Ricou, L.E., Vrielynck, B. (Eds.), Atlas Tethys Paleoenvironmental Maps. Gauthier-Villars, Paris, 397 pp. Chumakov, I.S., Byzova, S.L., Ganzey, S.S., Geochronology and Correlation of the Late Cenozoic in the Paratethys (in Russian). Nauka, Moscow, 95 pp. Gamkrelidze, I.P., Geodynamic evolution of the Caucasus and adjacent areas in Alpine time. Tectonophysics 127, Haq, B.U., Hardenbol, J., Vail, P., Mesozoic and Cenozoic chronostratigraphy and cycles of sea level changes. Soc. Econ. Paleontol. Mineral. Spec. Publ. 42, Knipper, A.L., The tectonic position of ophiolites of the Lesser Caucasus. In: Panayiotou, A. (Ed.), Ophiolites. Proc. Int. Ophiolite Symp. Cyprus, IGC Project 39, Lomize, M.G., The regional and global events in the development of the Great Caucasus geosynclyne (in Russian). In: Koronovsky, N.V., Milanovsky, E.E. (Eds.), The Geology and Mineral Resources of the Great Caucasus. Nauka, Moscow, pp Lozar, F., Polino, R., Early Cenozoic uprising of the Great Caucasus revealed by reworked calcareous nannofossils. EUG 9 Abstr. Suppl. N 1, Terra Nova 9, 141 pp. Mikhailov, V.O., Crustal control on the Terek-Caspian trough evolution: constraints based on a new paleotectonic analysis method. Tectonophysics 228, Mikhailov, V.O., Myasnikov, V.P., Timoshkin, E.P., Dynamic evolution of the Earth s outer shell under extension and compression. Izv. Phys. Solid Earth 32 (6), Mikhailov, V.O., Timoshkina, E.P., Polino, R., The foredeep basins: the main features and model of formation. Tectonophysics 307, (this issue). Panov, D.I., Stratigraphy, magmatism and tectonics of the Great Caucasus at the Early Alpine stage (in Russian). In: Ajgirey, G.D. (Ed.), Geology of the Great Caucasus. Nedra, Moscow, pp Philip, H., Cisrernas, A., Gvishiani, A., Gorshkov, A., The Caucasus: an actual example of initial stages of continental collision. Tectonophysics 161, Sedenko, S.M., Stratigraphy, magmatism and tectonics of the Great Caucasus at the Middle Late Alpine stages of the development (in Russian). In: Ajgirey, G.D. (Ed.), Geology of the Great Caucasus. Nedra, Moscow, pp Sherba, I.G., Stages and phases of the Cenozoic development of the Alpine region (in Russian). Nauka, Moscow, 228 pp. Stam, B., Gradstein, F.M., Loyd, P., Gillis, D., Algorithms for porosity and subsidence history. Comput. Geosci. 13, Stamply, B., Pillevuit, A., An alternative Permo Triassic reconstruction of the kinematics of the Tethyan realm. In: Dercourt, J., Ricou, L.E., Vrielynck, B. (Eds.), Atlas Tethys Paleoenvironmental Maps. Gauthier-Villars, Paris, pp Stet uha, E.I., Equations of the correlation connections between physical properties and depth of rocks (in Russian). Moscow, 134 pp.