Early Reactivation of Basement Faults in Central Zagros (SW Iran): Evidence from Pre-folding Fracture Populations in Asmari Formation and Lower Tertiary Paleogeography

Abstract

Early reactivation of basement faults and related development of flexures/forced-folds in the Central Zagros are discussed based on fracture populations observed in outcrops and aerial photographs/satellite images and paleogeographic maps. The presence of pre-folding joint sets slightly oblique to anticline axes and observed even within synclines or the occurrence of N-S (and E-W) trending fracture sets near N-S trending basement faults and strongly oblique to cover folds are not compatible with simple fold-related fracture models in this region. These early fractures are proposed to have formed within the cover above deep-seated basement faults in response to the formation of flexures/forced folds whose geometries and orientations may be different from the present-day folds in the Central Zagros. This early stage of intraplate reactivation of the NW-SE and N-S trending basement faults likely marks the onset of collisional deformation and stress build-up in the Zagros basin. This reactivation led to facies variations and development of different sub-basins in the Central Zagros during the sedimentation of the Oligocene- Miocene Asmari Formation. The evaporitic series of the Kalhur Member within the Asmari Formation resulted from the development during Aquitanian times of a long and narrow restricted lagoon environment, between two main basement faults (i.e., DEF and MFF), and provide one of the main key constraints on the beginning of deformation in the region. Finally, based on observed fracture populations and proposed geodynamic evolution in the Central Zagros basin, it is suggested that partitioning of N-S Arabia-Eurasia convergence into a belt-perpendicular NE-SW shortening and a beltparallel right-lateral strike-slip motion (as currently along the Main Recent Fault) in the Central Zagros may have started as early as Oligocene (?)-Lower Miocene times.