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Mechanically induced calcite twins in veins and host rocks of Late Cretaceous to Miocene age in Iran have been used to determine regional Arabia-Eurasia collisional stresses. A late folding stress regime with a compression oriented 025° (±15°) has been identified across the Zagros belt and the southern Iranian Plateau. This late Neogene stress pattern agrees with the current stress field determined from the focal mechanisms of basement earthquakes and suggests that the Hormuz salt décollement poorly decouples the basement and cover stress fields. Our data show that the collisional state of stress has been relatively constant since ca. 5 Ma. The magnitudes of the stresses obtained from the twinning analysis are unexpectedly low, and, to a first approximation, they are constant across the Zagros simply folded belt. This result supports an overall mechanism of buckling of the detached Zagros cover. Internal viscous-plastic processes help to relieve stress within this cover, thus lowering its seismogenic potential. Beyond these regional implications, this study underlines the potential of paleostress analyses in constraining both the tectonics and the mechanics of ancient and active foreland fold belts. [ABSTRACT FROM AUTHOR]

The Eastern Iranian range is seen as a large scale India-Eurasia collisional event and the subsequent amalgamation of the Afghan and Lut micro-continents, including the Neo-Tethyan accretionary complexes and associated island arcs. Revised examination of the nature of tectono-stratigraphic units distributed in both sides of the N S elongated Eastern Iranian Orogen, reveals two remarkable facts: first, from a general perspective, it is structurally a giant, parallel antiform in profile view; second, lithologically, the most time-equivalent, comparable tectonic units from both northern and southern domains comprising the Eastern Iranian Orogen are connectable, follow the strike of hinge and limbs zones of the antiform and a M-shaped, disharmonic pattern in its core. These characters are thought to be formed in response to two principal deformational phases resulting from two perpendicular stress directions. The first was N S associated with the north-dipping subduction of the Tethyan oceanic lithosphere beneath the roughly ribbon-shaped Cimmerian supercontinent, including the Afghan and Lut blocks, before late Eocene. Afterward, the second was E–W shortening originated by the westward escape of the Afghan block as a consequence of the Indian plate indentation into Eurasia. This orogen-parallel stress was correspondingly accommodated by tens of degrees, counter-clockwise rotation of the Lut block around a vertical axis. The result of such shortening was oroclinal buckling and concomitant retreating of the subduction zone and closure of the Sistan Ocean, a remaining wedge-shaped part of the Neotethys Ocean formed by the ribbon curvature. The development of the Eastern Iranian Orogen already included "the Sistan suture zone", an enormous mass of folded Eocene flysch deposits with a general N S axial surface and some Cretaceous ophiolitic remnants. The plate-scale buckling which was formed by this late-stage orogenic event is hereby considered as an orocline, call "Eastern Iranian Orocline". Lithosphere buckling around a vertical axis caused a large-scale crustal folding coeval with the bending of its mantle part, increasing the lithospheric thickness of the orocline compared to adjacent regions. The syn -orocline deformation, associated delamination and resulting magmatic events took place in eastern Iran during a short period of about 10 to 20 Ma in the Late Paleogene. The orocline hinge zone is occupied by the Khorasan outer arc and the Birjand core, while its limbs are composed of the Baluch inner arc and the southwestern and southeastern terminations of the Lut and Afghan blocks. The Eastern Iranian Orocline seems to be the middle segment of a triple-bending belt with N S two-opposite facing bend including Afghan, Lut, and Sanandaj-Sirjan blocks. This large curvature is possibly an example of secondary oroclines which was formed by collision-related escape tectonics, although the limited available kinematic data show a kind of progressive orocline. [ABSTRACT FROM AUTHOR]