GOLONKA, JAN, Krakow, Poland

Abstract: Geodynamic Evolution of the South Caspian Basin

The South Caspian Basin with its present shape and petroleum systems was formed as a result of the interaction of major Eurasian, Indian and Arabian plates and numerous microplates starting from the Triassic. In the Late Triassic, the Paleo-Tethys Ocean continued subducting under the southern margin of Eurasia, and the Transcaucasus,Talesh - Alborz (Western and Central Iran), Central Caspian, Herat and North Pamir Blocks collided with the Eurasian Continent. Subduction of the Paleo-Tethys produced the Early Cimmerian volcanic arc along the southern margin of Eurasia, and resulted in back-arc extension which influenced most of the Scythian-Turan platform.

During the Late Triassic time the older Permian-Triassic rifts turned into oceanic and aulacogenic basins. Marine deposits have been identified in Crimea, Fore-Caucasus, Manych, and Mangyshlak and in western and southern Turkmenia. The collision of the microplates with the southern margin of Eurasia resulted in he compressional events which were recorded in major deformation of the Permian-Triassic deposits, the formation of the Mangyshlak and Badkhyz-Karabil fold zones and the general uplift of the Fore-Caucasus and Middle Asia regions.

The Lut (Eastern Iran), Farah (Central Afghanistan) and South Pamir microcontinents were approaching the Eurasian margin from the south.The Central Turkey (Sakhariya and Kirsehir), Lesser Caucasus, and Southern Iran (Sanandaj-Sirjan) were rifted from the Gondwanian,African-Arabian margin.

During the latest Triassic-Early Jurassic the Lut, Farah, and Southern Pamir microcontinents collided with Eurasia and were sutured to the Turan Platform.The compressional events were recorded in Southern Kopet Dagh (Aghdarband) area in northwestern Iran, in the Herat area in Afghanistan and in the Pamir Mountains between CIS,Afghanistan and Western China (Trim). The remnants of Paleo-Tethys still existed between Tarim and Qantang microcontinents. The northward drift of Central Turkey (Sakhariya and Kirsehir), Lesser Caucasus, Southern Iran (Sanandaj-Sirjan) and south central Afghanistan (Helmand) plates from the African-Arabian-Indian margin toward Eurasia divided Neo-Tethys into two branches.The northern - Sevan-Akera- Qaradagh ocean was subducting under the Eurasia. The southern branch was spreading between the Gondwanian margin and series of microplates.

After the phase of Late Triassic-Early Jurassic compression, the rifting regime was re-established between Eastern Black Sea and Western Turkmenia and continued through Middle Jurassic time. Most of the former rift systems that had developed during Late Permian-Triassic time were reactivated and new rift systems originated.The Early-Middle Jurassic rifting influenced the Greater Caucasus, Amu-Dar'ya and Afghan-Tadjik regions.

From Middle Jurassic Bajocian time, sedimentary basins within the Caucasus-Caspian-West Turkmenistan area underwent post-rift subsidence with marine clastic deposits.The fluvio-lacustrine and lacustrine clays and sandstones were deposited in the southern part of the region. Shoulder uplift and deformation accompanied the post rift and pre-back arc spreading. Post-rift subsidence of sedimentary basins, which had started within the platform in the Middle Jurassic Bajocian, continued during the Callovian-Oxfordian and was accompanied by a global rise of the sea level.

The tectonic setting changed in the Kimmeridgian-Tithonian, when the are underwent general uplift as a result of the collision of the south-central Afghanistan (Helmand) and.Qantang microcontinents with the southern edge of Eurasia.

The subduction trench pulling effect along the southern margin of East Pontides, Transcaucasus,Talesh and Alborz plates formed Greater Caucasus-Proto Caspian back-arc basin underlain by the oceanic crust.

The depositional environment during the Callovian-Oxfordian was favorable for accumulation of oil-prone source rocks in the Indol-Kuban, East Manych, Mangyshlak,Terek-Caspian, Murghab, Afghan-Tadjik and possibly Proto-South Caspian basins. The configuration of troughs and uplifted sedimentary platforms created conditions for restricted sea water circulation and anoxic depositional environments. The Callovian section in the basins is composed of marine mudstones and shales with lenses of organic limestone and sandstone. Organic rich shales accumulated in deeper troughs, which were surrounded by barrier reefs. A carbonate platform developed to the south of the East Manych and Mangyshlak basin, at the northern margin of the Greater Caucasus-South Caspian deep-water basin.

Kimmeridgian-Tithonian time was manifested by evaporite sedimentation. Evaporites form a perfect regional seal over Callovian-Oxfordian reservoirs, and provide exceptionally favorable conditions for preservation of hydrocarbon accumulations.

During Tithonian-Berriasian time rifting started along the northern and eastern margin of the Lut block.This rifting was followed by sea-floor spreading during Barremian-Hauterivian and formation of the Sistan Ocean.The Neo-Tethys Ocean was subducting under the southern Eurasian margin. The Jurassic Greater Caucasus - Proto Caspian and Cretaceous western Black Sea oceanic basins, separated subduction-related volcanic arcs, which successively developed along the Neo-Tethys northern margin.The Greater Caucasus - Proto-Caspian ocean was connected with Sistan Ocean, which separated Lut from Afghanistan and Kopet-Dagh area. The Greater Caucasus ocean achieved the maximum width, approximately 400 - 600 km during Late Cretaceous time.

During the Late Cretaceous-Paleocene time Kirsehir and Sakhariya plates collided with Pontides. Lesser Caucasus approached Transcaucasus and Talesh area. The northeastward movement of Arabian Plate and Sanandaj-Sirjan terrane significantly reduced and partly closed the Sevan-Akera-Qaradagh Ocean.At the same time obduction (Oman ophiolites among the others) occurred on the Northeastern margins of the Arabian plate.

During the Eocene time Lesser Caucasus, Sanandaj-Sirjan and Makran plates were sutured to Transcaucasus-Talesh-Southern Caspian-Lut system. The subduction zone was locked and jumped to the Scythian-Turan margin.The western segment of this subduction was located along the northern margin of the Eastern Black Sea, on the Greater Caucasus area and south off the Apsheron Peninsula and ridge. The major transform fault system in the Western Turkmenistan basin area separated eastern and western segment of subduction. This fault system is buried deeply below West Turkmen Basin Neogene sediments. The eastern segment was located along the South Kopet Dagh margin approximately 200-300 km south from the Apsheron ridge.

The subduction jump produced the trench pulling force, which influenced all plates between Black Sea and Sistan Ocean in Afghanistan.The timing of movement initiation and movement velocity is different for different plates.This difference caused the origin of several major strike slip faults of SW-NE direction, which cut both continental crust and Jurassic-Cretaceous oceanic crust. The most important are Arks fault, which separates Lesser Caucasus block and Transcaucasus block from Talesh plate, and Lahijan fault within Alborz Maintains. The extension of Lahijan fault, which separated South Caspian Microcontinent from South-West Caspian Basin, is buried deeply below South Caspian Neogene sediments. The northward movement of South Caspian Microcontinent resulted in rifting between SCM and Alborz plate.

Beginning with Eocene time the southeastern part of Turan Plate and adjacent Afghanistan AAPG International Conference and Exhibition 209.area was strongly affected by the India-Eurasia collision.To the south, the Arabian plate slowly converged with Eurasia closing the remnants of the Pindos Ocean along the Sanandaj-Sirjan margin (Zagros Suture).The Jurassic-Cretaceous oceanic crust of the Eastern Black Sea - Greater Caucasus Basin was subducted under the overriding Scythian plate.The northward movement of the Shatski Rise block caused opening of the Eastern Black Sea. The northward movement of the Transcaucasus block caused the collision and formation of the Greater Caucasus orogenic belt.

The most recent stage of tectonic history of the South Caspian region and adjacent platform started in Oligocene time and was related to the collision of the Indian and Arabian continents with Southern Eurasia. Compressional events in Central Asia and the Neo-Tethys region resulted in the general uplift of the Scythian-Turan platform.

The South Caspian part of the Greater Caucasus - Sistan oceanic system underwent the reorganization during Oligocene Middle Miocene time. The northward movement of the South Caspian Microcontinent resulted in rifting between SCM and Alborz plate. The southwestern part of the South Caspian basin was reopening, while the northwestern part was gradually reduced in size. The South Caspian Microcontinent separated the southwestern part of the South Caspian basin and the Western Turkmenistan area. The clastic sedimentation was predominant during this time.The plate's convergence in the Greatest Caucasus area caused the influx of the large amounts of sediments in form of proximal and distal flysch including large submarine slumps. The sedimentation rate exceeded subsidence and basins were filled.

The main part Neotethys was closed.The Neotethys remnants, foreland basin of the Alpine orogens and reorganized Greater Caucasus - Caspian basin with the adjacent parts of the Scythian-Turan platform formed the Paratethys sea. The Paratethys was isolated from the world ocean. This isolation and persistent low pressure system during Oligocene-Early Miocene time generated favorable condition for deposition and preservation of the organic-rich shales. The Makeup formation containing several layers of organic-rich shales was deposited in north of Greater Caucasus, in the Terek-Caspian Basin, Kura Basin, and perhaps in the parts of South Caspian Basin. The South Caspian Microcontinent was probably emerged during the Oligocene-Early Miocene and received a minimum amount of sediments, so we can expect the absence of the Makeup formation in this area.

During the Pliocene-Quaternary time the collision of Indian continent and Lut plate with Eurasia caused deformation of the Central Asia region.The system of NW-SE transform faults was developed.The deformation connected with the SE-NW strike slip faults were observed in Great Balkhan Area, Apsheron ridge, South Caspian area, Alborz Mountains, and Kura Basin. The N-S strike slip movement system was probably still active, but dramatically reduced. The subduction zone south of the Apsheron ridge became passive perhaps at the end of Miocene because of this SE-NW movement of the lithospheric plates. The collision between South Caspian Microcontinent and Scythian-Turan plate was never concluded. The remnants of the Jurassic-Cretaceous back-arc system oceanic and attenuated crust in the Cheleken and South West Caspian basin as well as Tertiary oceanic and attenuated crust in the Alborz basin and part of the South West Caspian Basin were locked between adjacent continental plates and orogenic systems.

Maximum subsidence of the South Caspian Basin took place mainly during the Pliocene, when more then 8000 m to 10000 m of sediment known as Productive Series and Variegated Series were deposited. The isolation of the Paratethys - Caspian Sea caused changes in the water salinity. Generally the sediments in the South Caspian area, as well as in the adjacent basin on the Scythian-Turan platform, in West Turkmenia and Kura Basin well deposited in the marginal marine environment.The Paleo-Volga, Paleo Amu-Daria and Paleo-Kura rivers delivered majority of sediments.