TECTONOSTRATIGRAPHIC SYNTHESIS AND STRUCTURAL STYLE OF THE ALBORZ MOUNTAIN SYSTEM IN NORTHERN IRAN

Based on lithologic, structural and stratigraphic analyses of the rocks exposed in the Alborz poly-orogenic system of northern Iran, seven tectonostratigraphic and a number of metamorphic rock assemblages, including remnants of the Paleo-Tethys oceanic realm, are distinguished. The tectonostratigraphic units are: (1) an uppermost Precambrian to lower Ordovician epicontinental sequence; (2) an Ordovician to Devonian mafic magmatic assemblage; (3) a Devonian to upper Triassic continental shelf sequence; (4) an upper Triassic and lower Jurassic foreland siliciclastics succession; (5) a middle Jurassic to upper Cretaceous discontinuous epicontinental/continental shelf succession; (6) an upper Cretaceous and Cenozoic arc-type magmatic assemblage; and (7) the Tertiary and Quaternary synorogenic mostly siliciclastic molasse deposits. The metamorphic assemblages include the upper Paleozoic metamorphosed carbonates, volcanics, and siliciclastics of the Masuleh region, the upper Triassic-lower Jurassic slates and metagraywackes of the Binalood region, and the Paleozoic metavolcanics and metacarbonates of the Gorgan area (“the Gorgan schists”), which are all correlatives of the above tectonostrati-graphic units, as well as the Paleo-Tethys remnants of the Mashhad-Torbat Jam region in eastern Alborz, the Shanderman complex in western Alborz, and the Gasht complex in the vicinity of Masuleh. These rocks, structurally, form either single thrust sheets or complex duplex systems which are transported generally from north-northeast to south-southwest by numerous thrust faults. The thrust faults, which collectively constitute a composite antiformal stack, formed during the Cimmeride and Alpide orogenies.