basement faults in the Fars Arc of the Zagros (Iran) Structure, timing and morphological signature of hidden reverse Geological Society, London, Special Publications

Abstract: In the Zagros Fold–Thrust Belt (ZFTB) of Iran it is firmly established that the base-ment is involved in the deformation. The strongest line of evidence for this assertion comes fromthe relatively intense mid-crustal seismic activity. On one hand, the main basement structuressuch as the Main Zagros Fault (MZT) and High Zagros Fault (HZF) reach the surface andare therefore well identified. On the other hand, basement faults south of the HZF are hiddenby sedimentary cover and their location is uncertain. In the Eastern Zagros, basement controlon surface structures occurred only at a late stage of the tectonic evolution. In other words,the current thick-skinned style of Zagros deformation succeeded a more general thin-skinnedphase of orogeny. This chronology is particularly well illustrated by spectacular interference pat-terns, in which early detachment folds are cut by late oblique basement faults. We present acombined morphological and structural analysis of such structures, and we explore theirimpact on the river network. We confirm that basement involvement occurred at a late stageof deformation and we show that thick-skinned deformation progressively propagated towardsthe foreland. An overview of basement steps throughout the Zagros based on published cross-sections allows us to conclude that although basement deformation is concentrated on twomajor faults in the Central Zagros, it is distributed on several segmented faults in the FarsArc. This segmentation increases to the SE towards the so-called Oman line and the transitionto the Makran accretionary prism.