Transpression in the Sanandaj-Sirjan Zone (Zagros Orogen, Iran) during the Jurassic and Early Cretaceous: Evidence from the North Shahrekord Shear Zone

Abstract

Deformation style associated with the early stage of subduction of the Neo-Tethys under Iranian microcontinent is poorly understood. Utilizing field observations, microscopic thin-section interpretation, and recently published 40Ar-39Ar ages, we conducted a structural analysis of the North Shahrekord Shear Zone. The study area is located in the central Sanandaj-Sirjan Zone in western Iran, in an area that includes high-grade eclogite-bearing and low-grade metamorphic units of the North Shahrekord Metamorphic Complex. Two generations of deformation were observed in the study area, including D1 and D2. D1 structures are rare and involve folds and axial-plane foliations (S1), which have been almost obliterated by D2. D2 is characterized by a dextral transpressional zone, where strain is partitioned into distinct simple shear and pure shear domains. The pure shear domain is observed mainly in low-grade zone and some high-grade metamorphic rocks, and is characterized by NW-striking close to isoclinal folds (F2) and axial-plane foliations (S2). Folds are mostly inclined and their vergence is towards the southwest and northeast in the northern and southern parts of the study area, respectively. In the central part of the area, F2 includes approximately upright folds. These mesoscopic folds are associated with map-scale folds. The simple shear domain is restricted to the high-grade metamorphic rocks of the North Shahrekord Shear Zone, defined by NW-striking steeply dipping mylonitic foliation with low-angle stretching lineation. Kinematic indicators, such as quartz sigmoids, feldspar porphyroclasts, mica fish, and C- and C′-type shear bands, reveal a dextral strike-slip movement with a minor reverse component along the North Shahrekord Shear Zone. Recrystallized phengite from mylonitic eclogite and metagranite have been recently dated (40Ar-39Ar) at 178–170 Ma and ~110 Ma, respectively, indicating that the Sanandaj-Sirjan Zone was dominated by dextral transpression phases in the Early-Middle Jurassic and Early Cretaceous. The transpressional phases are interpreted to have occurred in response to the oblique subduction of the Neo-Tethys Ocean under the Iranian microcontinent.