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Alpine-Himalayan Orogens |
United Nations, Geological Survey Institute, Box 1555, Tehran, Iran
The Alborz Mountains, forming a gently sinuous east-west range across northern Iran south of the Caspian Sea, constitute a northern part of the Alpine-Himalayan orogen in western Asia. They face the depressed Caspian block on the north and to the south grade into the plateau of Central Iran, which in turn has had a long tectonic history continuing into the Mesozoic and Tertiary. A brief description of Central Iran is included in item 135 below.
The southern part of the main Alpine-Himalayan belt beyond the Central Iranian plateau, the Zagros Mountains, is described by Falcon (this volume, p. 199).
The northern margin of the Alborz belt is a sharp mountain border against a Quaternary coastal plain. The southern margin is arbitrary. The width of the belt described here is 120 km.
Segment: the segment described in this article has a length, measured along strike, of 340 km on average (Fig. 1).
Zones: the Alborz belt is here divided into six structural zones, from north to south (Fig. 1): 1. Gorgan Spur—a relatively rigid area of crystalline Basement overlain by thin Mesozoic sediments. 2. Northern Neogene zone—a normally folded belt of Mesozoic and Neogene rocks, the latter in molasse facies. Southern margin thrust. 3. North-Central zone—characterized by shelf sedimentation nearly continuously from Infra-Cambrian to U. Cretaceous; minor volcanic episodes. Main deformation in Tertiary. 4. South-Central zone—pre-Tertiary shelf deposits as in zone 3, overlain by very thick Tertiary (mainly Eocene) volcanics; strong post-Eocene thrusting. 5. Southern Tertiary zone—very thick Eocene volcanics
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