Fundamental differences in crustal character and in age of basementconsolidation allow three major structural units to be distinguished, separated from each other by an ophiolite –bearing suture.Other criteria such as structural styleage and intensity of deformation, age and nature of magmatism, are used to subdivide these major zones into smaller elements.

The three major units and their main constituents are as follows. (1)  A S unit with a crystalline basement consolidated in Precambrian time and a platform –type Paleozoic development. This   unit comprises the Zagros folded belt. (2) The central unit is interpreted as an assemblage of marginal Gondwana fragments that were united with the mother-continent and separated from the N (Eurasian) continent in the Paleozoic, but detached from Gondwana and attached to Eurasia in the Mesozoic, and finally rejoined by Gondwanic Afro-Arabia in the Late Cretaceous. It comprises central Iran and the Alborz. (3) The northern unit sharply separated from the central unit by the North Iran

Suture. It is characterized by continental crust including remnants of more or lees cratonized former (Paleozoic) oceanic crust that seems to reflect a Paleotethys. The N unit represents a marginal strip of the Hercynian realm of Central Asia- broadly overlapped by the Alpine realm. It was deformed and largely consolidated by strong early kimmerian folding and a Late Alpine folding. The N unit comprises the South Caspian Depression and the kopet Dagh Range. 

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