Post-Triassic normal faulting and extensional structures in Central Alborz, Northern Iran

This paper presents structural evidence of extensional activity in Central Alborz during Mesozoic. The structural evidence of homogenous early stage stretching such as layer-parallel to oblique boudinage of Permian and Triassic rocks in various portions of the study area accompanied by extensional-fibrous fractures were followed with more advanced extensional features. These extensional structures include tension gashes in brittle-ductile shear zones, large to medium scale slickensides with normal movements, distinct normal faults, assemblage of high angle faults with primary normal component of displacement, majorly in Permian to Early-Middle Jurassic rocks and large initially normal faults within the Shemshak Group. Development of later extensional structures has also influenced the former Early Cimmerian structures, where some of north-verging folds were cut and displaced by high angle transverse faults with normal components. Partial continuation of the normal faulting in the Shemshak Group is an evidence for the Jurassic timing of these normal faulting. Many of the faults that are interpreted as originally normal faults, have experienced a reactivation as reverse or strike-slip faults, during the Late Cretaceous-Early Paleocene regional compressional tectonic activity.