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The Eo-Cimmerian orogenic event is related to the Late Triassic collision of the Iranian Plate with the Eurasian active margin. The collision, which was active east of Chios, has taken to the closure of the Palaeotethys ocean. The Paleotethys suture is well exposed in NE-Iran between Mashad and the Kopet Dag, where a Late Palaeozoic to Triassic accretionary wedge is unconformably sealed by the flat-lying Middle Jurassic Kashaf Rud Fm., postdating the accretion of Iran to southern Eurasia. The extension of the suture to the west across central and western Alborz (Talesh Mountains) is still strongly debated. New field data obtained during three years of work in the Alborz and Talesh Mountains of North Iran indicate that the Late Triassic-Jurassic Shemshak Fm. seals a complex collisional orogen representing the southern portion of the Eo-Cimmerian belt. Several tectono-stratigraphic units representing thrusted slices of continental crust have been identified in the Talesh and Gorgan areas. Eclogitic rocks, possibly related to the Eo-Cimmerian event have also been recognized for the first time in the Talesh Mountains