GEODYNAMIC AND TECTONIC FRAMEWORK OF THE SOUTH CASPIAN BASIN EVOLUTION

Marie-Francoise Brunet* and MEBE Teams
Lab. Tectonique, CNRS-Univ. P. & M. Curie, case 129, 4 place Jussieu, 
75252 Paris Cedex 05 France, [email protected]

This study takes place in the frame of the Middle East Basins Evolution Program (MEBE) and results of a cooperation of the Universities of Paris, Keele and Friburg with the Geological Institute of Azerbaijan and the Geological Survey of Iran.

To understand the complex framework of the South Caspian Basin (SCB) evolution, the offshore part of the SCB cannot be considered alone but in addition to the surrounding areas, including SCB margins in Iran and Azerbaijan. A first part of work, performed in the Peri-Tethys Program, dealing with the subsidence evolution of the SCB, showed the lack of precise tectonic information on the tectonic calendar of the opening phases. The age of the oldest sediments of the SCB is unknown as they are inaccessible because of their great depth. This lead us to perform new fieldworks on the former margins of the SCB. The first reconnaissance was made in 2002 in Alborz and now in Kopet Dagh (Iran) as well as in Azerbaijan in autumn 2003. Using brittle tectonic analysis, we aim at characterising the major tectonic events (age, stress pattern, etc..), that occurred, since the Mesozoic, in these margins now inverted after the collision of the Arabian plate with Eurasia. Subsidence analysis inferred from field or exploration data is combined with fieldwork data on the regional tectonic/sedimentary setting and numerical modelling is performed on offshore data.

An extensional phase, at the end of Triassic-early Jurassic, followed the Eo-Cimmerian tectonic phase induced by the collision of Cimmerian blocks with Eurasia. This event developed during the deposition of the Shemshak Formation in Alborz (Late Triassic to Middle Jurassic). We assign this extension to the rifting phase of the South Caspian basin that probably opened in Late Jurassic. Some compressional tectonic events intercalated during the post rifting period of thermal subsidence, especially at the Jurassic-Cretaceous boundary and uppermost Cretaceous. These compressional phases are evidenced by regional unconformities and deposition of clastic sediments. In Eocene, a strong extension, associated with E-W trending normal faulting, originated the thick volcanoclastic Karaj formation in the southern part of the Alborz and the thick series of Talesh (NW Iran and SE Azerbaijan). We assume that this regional sub-meridian extension is related to the back arc opening behind the northward subduction of the Neo-Tethys oceanic lithosphere beneath the Eurasian margin. The next major orogenic period developed during the Late Cenozoic in relationship with the Arabia-Eurasia collision. The thickest series in the SCB is represented by the Pliocene-Quaternary with more than 10 km of sediments deposited in a very short time interval (less than 5 Myr). It occurred within the framework of an intense shortening. The surrounding orogens (Caucasus, Alborz, Kopet Dagh) were uplifted at this time, isolating the SCB and providing by their erosion, a huge source of sediments filling the subsiding SCB.