ALEXANDER, ANDREW C., E. IWANIW, S.C. OTTO, Robertson Research International Limited, O.S. TURKOV, Kazakhstan Caspi-Shelf, H.M. KERR and C. DARLINGTON, Robertson Research International Limited
Abstract: Tectonic Model for the Evolution of the Greater Caspian area
This presentation aims to refine previously published tectonic models of the Greater Caspian region (Fig. 1) by using recently acquired geophysical and stratigraphic data from the Kazakh sector of the Caspian Sea, as well as petroleum geological studies of the Precaspian, North Ustyurt, Mangyshlak-Ustyurt, South Caspian, Kura-Kartli, Rioni and Amu-Dar'ya basins. These data provide valuable new constraints on the Devonian to Recent evolution of the central part of the Tethys system. A series of palaeotectonic reconstructions will be presented.
The structural history of the region (summarized in Table 1) reflects the northward drift of rifted Gondwanan continental blocks and the progressive accretion of these and Tethyan arc fragments against the southern margin of the East European and Kazakh plates.
The pre-Devonian history of the Caspian Sea area is poorly understood, having been largely overprinted by subsequent tectonic events and buried under extensive and thick Upper Palaeozoic-Cenozoic sediments. Widespread Middle-Late Devonian rifting occurred within the East European, Siberian and Kazakh plates and probably led to the generation of oceanic lithosphere in the Central Precaspian Depression at this time. Although there was northerly-dipping subduction and arc development on the southern margin of the Ukrainian Shield in the Caucasus area at this time, there is no evidence for active margin processes in the southern Precaspian until the Visean-Bashkirian (Early-early Late Carboniferous). This was marked by an influx of arc volcanics and clastics synchronous with inversion of the South Emba rift.
Closure of the Ural Ocean between the East European and Kazakh plates culminated in uplift during the Bashkirian (early Late Carboniferous)-Early Triassic and the creation of the Eurasian continent. During the Early Permian (Artinskian) the Mangyshlak Block finally accreted to the southern margin of Eurasia causing extensive deformation (NNE-verging folds and thrusts) and uplift, particularly along the Mangyshlak Suture Zone. The effect of these two collisions was to isolate the Precaspian Basin and the oceanic lithosphere in the central depression.
Late Permian (especially Kungurian) rapid subsidence dominated in the Precaspian Basin and may have been associated with cooling-related phase changes within the trapped oceanic lithosphere. The isolated nature of the Precaspian Basin at this time favoured extensive salt precipitation.
The tectonic evolution of the Mangyshlak Block during the Late Permian-Triassic was dominated by post-orogenic extensional and strike-slip faulting. Extensional reactivation of south-dipping low- to moderate-angle convergence-related structures in the Mangyshlak Suture Zone is thought to have allowed hanging-wall sedimentation. Over 7km of Upper Permian to Triassic sediments have been inferred from reflection seismic data. In the Precaspian Basin thick Permian-Triassic sediments were controlled by rapidly growing salt domes and associated halokinetic structures.
The Late Triassic-Lias Cimmerian collision of the Gondwana-derived NW Iran, Sanandaj-Sirjan, Armenia and Lut fragments with Eurasia brought about major inversion of the Mangyshlak rift zone and the development of large-scale strike-slip faults. This event is marked by a major unconformity over most of Paratethys region at this time.
In the northern part of the Greater Caspian area, Jurassic-Cretaceous post-Cimmerian subsidence was interrupted and influenced by localised uplift on strike-slip faults (Mangyshlak) and salt domes (Precaspian). Episodes of uplift affecting the region can be linked to collisional events on the southern margin of the Eurasian continent. For example, the Middle Jurassic unconformity can be attributed to closure of the Kure retro-arc basin or collision of the Kirsehir block in the Pontides of Turkey. End-Jurassic uplift is related to the collision of the Lhasa block.
Post-Cimmerian rifting of the Iran, Armenia and Lut blocks away from the Eurasian margin during the Early Jurassic brought about opening of major basins (including the Greater Caucasus). The present day South Caspian is in part underlain by the sediments of these basins. The complex southern margin of Eurasia experienced contractional deformation during the Late Cretaceous when a change in plate motions brought about an increase in northerly subduction rates and/or ophiolite obduction.
In common with the eastern Black Sea it is believed that the oceanic lithosphere which floors much of the South Caspian Basin formed during Paleocene retro-arc extensional or wrench tectonics. The re-collision of Lut (Paleocene-Miocene) and the collision of India and Arabia brought about Tertiary reactivation of major strike-slip faults in the Greater Caspian. Foreland basins developed peripheral to the uplifting Caucasus, Alborz and Kopet-Dagh mountain belts. The Paleocene oceanic/transitional basin was preserved in the South Caspian and Eastern Black Sea. However, in the Rioni and Kura-Kartli basins there was complete closure in front of the Oligocene-Miocene Arabian indentor. Rapid subsidence throughout the Miocene-Pliocene in the Black Sea and South Caspian may again have been controlled by cooling-related phase changes within trapped oceanic lithosphere.
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