Structural pattern of the Zagros fold-and-thrust belt in the Dezful Embayment (SW Iran).

The structural style of the southeastern region of the Dezful Embayment (Zagros fold-and-thrust belt) was defined through the interpretation of satellite images, field surveys, well stratigraphy, and magneto-telluric and seismic profiles. The compressional setting of the area derives from the last phases of Cenozoic continental collision between the Arabian Plate and Eurasia. The shortening affected an _13 km thick sedimentary succession comprising Proterozoic units (Hormuz Formation?), Cambrian to Early Miocene platform sediments (mainly carbonates), the Gachsaran evaporites, and Mio-Pliocene foredeep clastic deposits. Although in the Dezful Embayment there is no evidence for outcropping Hormuz salt, the structural features of the area (the style of folds, their lack of consistent vergences, the low-angle tectonic taper) suggest the existence of a lower ductile detachment level of possible great thickness and low frictional resistance. The rounded folds in the carbonates are forced folds detached over a deep ductile level, subsequently faulted with progressive deformation by characteristic steepening up reverse faults. Second-order asymmetric folds affecting the limbs of the main structures indicate a flexural slip mechanism of folding in a multilayered and mechanically heterogeneous (carbonates and shales) succession. The Gachsaran Formation is deformed by shorter, strongly disharmonic folds and thrusts, and acted as a decoupling level for the overlying folds, whose synclines often directly overlie the carbonate anticlines. A regional restored cross section indicates a shortening of 11.5% in the area. The involvement of the crystalline basement in thrusting and the deep structural style of the area are discussed in terms of earthquake mechanisms, focal depths, structural analysis, and stratigraphic considerations.