Tectonics of the zagros orogenic belt of iran: new data and interpretations

Abstract

The Zagros orogenic belt of Iran is the result of the opening and closure of the Neo-Tethys oceanic realm, and consists, from northeast to southwest, of three parallel tectonic subdivisions: 1.(1) the Urumieh-Dokhtar Magmatic Assemblage;2.(2) the Sanandaj-Sirjan Zone; and3.(3) the Zagros Simply Folded Belt. Recent detailed and reconnaissance mapping and stratigraphie-structural analyses in various parts of the belt indicate that the tectonics of the Sanandaj-Sirjan Zone is dominated by either large-scale composite duplex structures or low- and high-angle NE-dipping imbricate systems. These thrust systems have transported numerous horses and sheets (nappes) of metamorphosed and non-metamorphosed Phanerozoic rocks for tens or possibly hundreds of kilometers. Stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the Late Cretaceous and has continued episodically since that time. Shear sense indicators in various parts of the zone reveal a NE-SW direction of transport. Thrust systems generated in each episode are superimposed upon the previous structures, cutting across and displacing them and producing complicated stacks of thrust sheets. Southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10–15 km in the thickness of the continental crust in the southwestern part of this zone.The so-called z“Main Zagros Thrust”, which has been traditionally considered as the boundary between the Sanandaj-Sirjan Zone and the Zagros Simply Folded Belt, is by no means a single “high-angle reverse fault”, nor is it a narrow zone of “crushed rocks” but a series of low-angle thrusts locally cut by normal faults. These thrusts are indistinguishable from those mapped in the Sanandaj-Sirjan Zone, and are geometrically part of the Sanandaj-Sirjan Zone structures. There is no single thrust among them that can be singled out and referred to as the “Main” one.The boundary of the Sanandaj-Sirjan Zone with the Urumieh-Dokhtar Magmatic Assemblage to the northeast is characterized by a series of structural depressions formed by compression. Rock sequences (including ophiolite complexes) exposed along this boundary are strongly dismembered, sheared and mylonitized. Shearing planes and foliations are either subvertical or dipping at high angles to the northeast. This zone is suggested to be the suture between the Afro-Arabian and the Iranian plates.Overall, the intensity of deformation increases northeastward from the Persian Gulf-Mesopotamian basin, where strata are undeformed and horizontal, through the Zagros Simply Folded Belt which is characterized by en échelon, doubly plunging flexural slip folds, gentle to open in the southwest and close to locally SW-overturned in the northeast. The intensity of deformation continues to increase further northeastward through the Sanandaj-Sirjan zone where thrust faulting is dominant. This pattern suggests that the deformational front has migrated through time from the northeastern part of the Sanandaj-Sirjan zone (i.e., the suture zone) to its present position in the central portion of the Persian Gulf-Mesopotamian foreland basin.