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1 Geology Department, NIOC Exploration, Yaghma Alley, Jomhouri-e Islami Avenue, Tehran, Iran, (e-mail: zagrosftb{at}gmail.com)
2 Department of Earth Science and Engineering, Royal School of Mines, Imperial College, Prince Consort Road, London SW7 2BP, UK
The present-day morphology of the Zagros Fold-Thrust Belt is dominated by magnificent exposures of NW-SE-trending folds. This mountain belt is also characterized by a series of belt-parallel and belt-oblique (transfer) fault zones. Not all the fault zones have a clear exposure at the surface because of the presence of thick incompetent overlying sedimentary successions, but they can be identified from a study of the present-day seismicity of the belt and by the spatial organization of the overlying folds. The folds are not often cross-cut by the faults but plunge towards, or are deflected and end against, these blind transfer fault zones. As a result, shortening is accommodated by different fold trains on either side of the fault. This decoupling in turn causes the step-like offset of the major, belt-parallel thrust zones and the related topographic elements. These offsets occur on all scales, from regional (e.g. across the Kazerun and Izeh fault zones) to local. These transfer faults-lateral ramps may be influenced by pre-existing structures or by facies boundaries, which in particular influenced the Zagros Basin in the Cretaceous. In addition to controlling sedimentation and the compartmentalization of deformation, these fault zones also act as paths of fluid migration, as indicated by the present-day concentration of oil seepages and thermal or sulphur springs.
