Stress States Through the Zagros Foreland Folded Belt, Fars Province, Iran

The formation of NW-SE trending Zagros Orogenic Belt was initiated during the convergence of the Afro-Arabian continent and the Iranian microcontinent in the Late Cretaceous. Ongoing convergence is confirmed by intense seismicity related to compressional stresses collision-related in the Zagros Orogenic Belt by reactivation of early extensional faults to latter compressional segmented strike-slip and dip-slip faulting. These activities are strongly related either to the deep-seated basement fault activities (deep-seated earthquakes) underlies the sedimentary cover or gently dipping shallow-seated décollement horizon of the rheological weak rocks of the Infra-Cambrian Hormuz salt. The Compressional stress regimes in the different units plays an important role in controlling the stress conditions between the different units within the sedimentary cover and basement. A significant set of nearly N-S trending right-lateral strike-slip faults exists throughout the study area in the Fars area in the Zagros Foreland Folded Belt. Fault-slip and focal mechanism data were analyzed using the stress inversion method to reconstruct the paleo and recent stress conditions. The results suggest that the current direction of maximum principal stress averages N19°E, with N38°E that for the past from Cretaceous to Neogene, (although a few sites on the Kar-e-Bas fault yield a different direction). The results are consistent with the collision of the Afro-Arabian continent and the Iranian microcontinent. The difference between the current and paleo-stress directions indicates an anticlockwise rotation of the maximum principal stress orientation over the time. This variation resulted from changes in the continental convergence path, but was also influenced by the local structural evolution, including the lateral propagation of folds and the presence of several local décollement horizons that facilitated decoupling of the deformation between the basement and sedimentary cover. The obliquity of the maximum compressional stress into the fault trends reveal a typical stress partitioning of thrust and strikeslip motion in the Kazerun, Kar-e-Bas, Sabz-Pushan, and Sarvestan fault zones, that caused these fault zones behave as segmented strike-slip and dip-slip faults. The maximum principal stress directions obtain from inversion of focal mechanism data has been used to evaluate the movement potential (FMP) of the Kar-e-Bas fault zone. Analysis of the movement potential of the Kar-e-Bas fault zone shows that the northern segment of the fault zone has the higher (0.99) and the southern segment has the lower (0.14) potential of fault movement. A negligible difference between dips of the segments indicates that the strike of fault plane is a controlling factor in the movement potential changes in the study area.