Strain partitioning during oblique convergence: the Aghdarband case-study, NE Iran

The Aghdarband Basin, consisting of a strongly deformed arc-related Triassic marine succession, is a key-area for the study of the Cimmerian events, as the Cimmerian deformational structures are sealed by the Middle Jurassic to Cenozoic poorly deformed units of Kopeh Dagh, NE Iran. Aghdarband is located to the north of the Palaeotethys suture zone, which records a Devonian to early Mesozoic convergence. The integrated stratigraphic and structural analyses of the Triassic successions of Aghdarband give new insights on the Cimmerian collision. Based on detailed fieldwork, we refined the previous stratigraphic framework and we performed meso- to megascopic structural analyses of the upper Palaeozoic to Triassic units of the area. Our data suggest that the Triassic sedimentation occurred in an extensional/transtensional basin characterized by important synsedimentary faulting activity. During the latest stages of the Cimmerian deformation, the Triassic successions, including the Upper Triassic Miankuhi Fm. were severely deformed into a N- to NNE-verging imbricate thrust fan, interacting with an ESE-WSW left-lateral wrench fault zone, the Northern Fault, producing positive flower structures with high angle reverse faults. The thrust stack, occurring to the south, extends for some tens of kilometres, involving Upper Permian conglomerates. It can be interpreted as a compressional retro-wedge formed in an arc/back-arc position with an opposite vergence with respect to the main collisional zone deriving from N-directed subduction of the Palaeotethys. One of the most peculiar characters of the area, given by the occurrence of megascopic plunging to vertical folds, is closely associated with the transpressional regime due to the activity of the left-lateral shear zone occurring in the northern part of the area. Curvilinear vertical fold axes result from hinges rotation during progressive deformation and to dragging occurring along the main strike-slip faults. Preliminary results from kinematic modelling of passive hinge rotation in triclinic transpression suggest that any hinge line formed at the onset of deformation will rotate during progressive deformation towards the extrusion direction of the pure shear component of transpression. This occurs with different combinations of the characteristic transpressional angles: transpression obliquity and extrusion obliquity. It appears that for any of these combinations, higher kinematic vorticity values produce larger rotations and thus will explain wider distributions of fold axes orientation. This peculiar setting results from nearly complete strain partitioning between mainly reverse and strike-slip faults and can be related to the reactivation of previous tectonic discontinuities affecting the Eurasian basement of the Kopeh Dagh region, resulting from oblique convergence during the final stages of the Cimmerian collision.