The Impact of Cover Rock Rheology on Deformation of the Zagros Mountain Belt

Sepehr, Mohammad¹, John Cosgrove¹, Mohsen Moeini² (1) Imperial College, London, United Kingdom (2) NIOC Exploration, Tehran, Iran

The present day morphology of the Zagros fold-thrust belt, one of the world’s most important hydrocarbon provinces, is dominated by magnificent exposures of NW-SE trend­ing folds. These folds accommodate most of the NE-SW shortening across the belt and host the entire discovered hydrocarbon reservoirs of the region. The Zagros folds different in their size and geometry and these differences are related mainly to the rheological profile of the cover rock. The cover rock succession of the Zagros, which is up to 14 km thick, con­sists of a sequence of competent and incompetent units which vary both along and across the belt. Field based study combined with the use of satellite images reveal that, the thick­ness and facies distribution of the cover rocks, particularly the incompetent units, has a sig­nificantly impact on the style of deformation in the Zagros. These incompetent units, some of which provide the source for the hydrocarbon (e.g. Kazhdumi shale) and others which seal the underlying reservoirs (e.g. Gachsaran evaporate), also act a detachment for the overlying folds. In addition, where these incompetent units are very thick, they decouple the underlying folds and faults from those above it and allow for significant changes in style, size and location of these structures. As a result, the underlying folds including the compe­tent units (reservoirs) have a different geometry to the overlying folds. An understanding of the various processes that influence the geometry of the Zagros folds is crucial for future hydrocarbon exploration in the region.