BREAKING OF IRANIAN BLOCK DURING CRETACEOUS AND OPENING OF NEW OCEANIC BASINS WITHIN THE TETHYS OCEAN EXAMPLE, SABZEVAR-NAIN BASIN AND ITS GEODYNAMIC HISTORY

The Cretaceous fragmentation of Iranian Block and evolving of inner oceanic Basins such as Sabzevar – Nain is the most important event in the Iranian Tethys Ocean. The ophiolitic belts surrounding Central Iran are the indicator of the closure of these basins and continent – continent collision between the adjacent blocks.

In this paper, the authors tried to review the geochronology of processes of evolving and closing of the Sabzevar – Nain Basin. This basin has been formed due to an extensional system, along with the crustal thinning, which is accompanied with the north-south direction rifting. This event is expressed with pillow basalt lavas within the Middle Cretaceous sediments. The pillow basalts within the Aptian – Albian flysch enable us to determine the Cenomanian age of the establishment of the oceanic crust. The most extreme development of Sabzevar – Nain Basin has occurred between Santonian and Campanian. The silicified flyschs and their calcareous layers which have been deposited in Campanian- Maastrichtian are the youngest sediments over the oceanic crust.

The general convergence between Arabian Plate and Iranian Block and the assembling of inner lithospheric fragments, has led to the fastening of the entire basins around Central Iran, including Sabzevar – Nain Basin. The subduction of oceanic crust under the Alborz Mountains has also conducted to the creation of an East-West trending volcanic arc during Paleocene – Eocene, currently, is exposed on the southern margin of Alborz Mountains. Both petrology and geochemical studies of volcanic rocks indicate that, this arc is the result of a northward subduction of oceanic crust of Sabzevar – Nain Basin.

The definitive collision between both Central Iran and Alborz Mountains occurred during Middle Eocene. The wide collision zone is currently composed of accretionary prism and slices of ophiolites, which are overridden on the Central Iran margin. The other members of this collision zone are the ophiolitic belt of Sabzevar, the post collision of intrusive masses, the volcanic arc of Sabzevar, and the back arc basin of Joghatay.