northeastward subduction of tethyan oceanic crust continued along this portion of the eurasian plate until continental crust of the arabian plate became involved in this convergent boundary. the exact timing of the onset of the subsequent collision is uncertain although there is evidence of some deformation during the deposition of the asmari formation in the oligocene, or possibly as early as the late eocene. deformation within the zagros ftb has continued from then until the present day, although unconformities of regional extent suggest that there have been several distinct phases of deformation. unconformities have been recognised at the base and top of the asmari formation (late eocene and early miocene), at the base of the agha jari formation (late mid-miocene) at the base of the bakhtyari formation (latest pliocene) and mid pleistocene, interpreted to date these separate phases. these deformation pulses were associated with southwestward migration of the active deformation front.