Kinematic vorticity flow analysis and 40Ar/39Ar geochronology related to inclined extrusion of the HP–LT metamorphic rocks along the Zagros accretionary prism, Iran

The Zagros accretionary prism in southwestern Iran is exposed along the NW–SE trending of the Zagros Thrust System and inclined Zagros transpression zone. This accretionary prism consists of two units: the upper sedimentary mélange unit on top and the high-pressure metamorphic mélange unit at the bottom. Both units show characteristics of a tectonic wedge. The upper unit consists of type-I, II and III mélanges which display S-, C- and C′-type shear-band cleavages, quartz ribbons and rectangular or fish-head boudins. The lower unit fabrics display σ-, and δ-type porphyroclasts and quartz ribbon mylonites. These fabrics formed from a combination of 60.5% simple shear and 39.5% pure shear. Both components were involved in a lateral exhumation of the high-pressure/low-temperature metamorphic rocks in an inclined transpression wedge-shaped geometry. The estimated kinematic vorticity number (Wk) was calculated from quartz c-axis patterns, rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries. Using the mean estimated Wm value of 0.84, the inclination angle for the thrust wedge on top of the NE-subducting Neo-Tethyan oceanic lithosphere is 18°. The 40Ar/39Ar plateau ages of early generations of biotite from the lower metamorphic mélange are 119.95 ± 0.88 Ma and 112.58 ± 0.66 Ma. This late Aptian age is related to early thrusting and formation of HP–LT metamorphic rocks. The dating of two amphibole samples from the amphibolite yields a weighted mean age of 91.23 ± 0.89 Ma. This Turonian–Cenomanian age suggests a later metamorphic event associated with subduction and obduction of the Neyriz ophiolite and later lateral extrusion of HP–LT metamorphic rocks along the inclined Zagros accretionary prism.