TECTONICS OF THE ZAGROS OROGENIC BELT OF IRAN - NEW DATA AND INTERPRETATIONS

The Zagros orogenic belt of Iran is the result of the opening and clos ure of the Neo-Tethys oceanic realm, and consists, from northeast to s outhwest, of three parallel tectonic subdivisions: (1) the Urumieh-Dok htar Magmatic Assemblage; (2) the Sanandaj-Sirjan Zone; and (3) the Za gros Simply Folded Belt. Recent detailed and reconnaissance mapping an d stratigraphic-structural analyses in various parts of the belt indic ate that the tectonics of the Sanandaj-Sirjan Zone is dominated by eit her large-scale composite duplex structures or low- and high-angle NE- dipping imbricate systems. These thrust systems have transported numer ous horses and sheets (nappes) of metamorphosed and non-metamorphosed Phanerozoic rocks for tens or possibly hundreds of kilometers. Stratig raphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the Late Cretaceous and has continued episodically si nce that time, Shear sense indicators in various parts of the zone rev eal a NE-SW direction of transport. Thrust systems generated in each e pisode are superimposed upon the previous structures, cutting across a nd 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 ''Main Zagr os Thrust'', which has been traditionally considered as the boundary b etween 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 cu t by normal faults. These thrusts are indistinguishable from those map ped in the Sanandaj-Sirjan Zone, and are geometrically part of the San andaj-Sirjan Zone structures. There is no single thrust among them tha t can be singled out and referred to as the ''Main'' one. The boundary of the Sanandaj-Sirjan Zone with the Urumieh-Dokhtar Magmatic Assembl age to the northeast is characterized by a series of structural depres sions formed by compression. Rock sequences (including ophiolite compl exes) exposed along this boundary are strongly dismembered, sheared an d mylonitized. Shearing planes and foliations are either subvertical o r dipping at high angles to the northeast. This zone is suggested to b e 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 ech elon, doubly plunging flexural slip folds, gentle to open in the south west and close to locally SW-overturned in the northeast. The intensit y of deformation continues to increase further northeastward through t he Sanandaj-Sirjan zone where thrust faulting is dominant. This patter n suggests that the deformational front has migrated through time from the northeastern part of the Sanandaj-Sirjan zone (i.e., the suture z one) to its present position in the central portion of the Persian Gul f-Mesopotamian foreland basin.