Strain and vorticity analysis in the Zagros suture zone (W Iran): Implications for Neo-Tethys post-collision events

Abstract

Shear zone development in the Zagros orogenic belt is a result of the oblique collision of the Arabian plate with the Iranian plateau. To distinguish areas deformed during the collisional phase from those deformed by post-collisional events, the coaxiality of deformation and the shape of the strain ellipsoid (SES) have been analyzed. The variation of strain intensity in each shear zone has been evaluated by calculating the strain ratio in the principal strain planes, the θ′ angle, and the kinematic vorticity number (Wk). In the post-collisional shear zones, Wk varies from 0.80 to 0.99 and θ′ from 16.7° to 44.01°; the strain component is mostly simple shear, though in some shear zones it is sub-simple shear. Outside of the shear zones, Wk is extremely low, varying from 0.00 to 0.26, and θ′ values are under 5°; a coaxial deformation regime is dominant and the strain component is pure shear. In most of the shear zones, SES is constricted and prolate, indicating that deformation occurred under transtensional conditions. However, in some regions, SES is flattened and oblate, as is common in transpressional zones. Shear zones that have evolved under dual deformation conditions suggest that transtensional events can be superimposed on transpressional structures.