Year

1997

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

The Sanandaj-Sirjan Zone is part of the Zagros Orogen in western Iran. The study area (June area) occurs in the middle part of this zone and consists of: Permian marble and metadolomite; Middle to Late Triassic marble, metadolomite, schist, quartzite, and amphibolite; Late Triassic - Jurassic phyllite and metaigneous rocks; Late Jurassic to Early Cretaceous intermediate volcanic rocks; and Aptian-Albian limestone.

Two major episodes of deformation are recognisable in the study area and in adjoining regions. The first is characterised by east-northeasterly plunging tight folds with axial plane schistosity and associated prograde amphibolite to greenschist metamorphic facies. The second deformation is the major event and is characterised by west-northwesterly-trending tight folds with an axial plane schistosity dipping steeply towards the north-northeast. These folds are overturned with south-southwest vergence and plunge moderately to the east. Low-grade (greenschist) metamorphism is associated this event. This deformation was caused by dextral transpression with the deformation partitioned into two domains. One domain contains schist and marble, which are ductilely folded and cut by abundant thrusts. In the second domain, a mylonitic foliation is formed in mylonitic rocks and syn-deformational granitic plutons. Dextral shearing is defined by abundant shear-sense indicators. The Galeh-Doz pluton has had its emplacement controlled by S-shaped fissures produced by dextral transpression.

A new subdivision is recognised for the Sanandaj-Sirjan Zone and consists of from southwest to northeast: (1) the radiolarite sub-zone with Late Triassic to Late Cretaceous shallow to deep-marine rocks; (2) the Bistoon sub-zone with Late Triassic to Late Cretaceous shallow-marine carbonates; (3) the ophiolite sub-zone with Late Cretaceous ophiolites; (4) the marginal sub-zone with a Late Jurassic - Early Cretaceous volcanic arc succession deposited in shallow-marine environments; and (5) the complexly deformed sub-zone with the late Palaeozoic - Mesozoic passive margin succession of the northeastern side of Neo-Tethys and overlying convergent margin assemblages. Deformation in the complexly deformed and marginal sub-zones was related to subduction of Neo-Tethyan oceanic crust and was associated with arc volcanism during the Late Jurassic - Early Cretaceous. Subsequently, oblique collision occurred in the Late Cretaceous producing dextral (pure-shear dominated) transpression and deformation partitioning. Collisional tectonics in a regime of dextral transpression was renewed in the Neogene.

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