Fold Development in Zagros Simply Folded Belt, Southwest Iran

Abstract In the simply folded belt of the Zagros Mountains, a sequence of Precambrian to Pliocene shelf sediments about 12 km thick has undergone folding from Miocene to recent time. Much of the section (6,000 to 7,000 m), consisting of Cambrian to Miocene rocks, forms a single structural lithic unit, the Competent group. It is bounded above and below by detachment zones in evaporite deposits. Structures in the Competent group are typical of parallel folds formed by buckling and developed by a combination of flexural-slip and neutral-surface mechanisms. They include bedding-plane slickensides, extension structures on anticlinal crests, and congested anticlinal and synclinal fold cores. The neutral-surface component of folding has had an important influence on fluid migration. The asymmetry of Competent group folds reflects shearing in the lower detachment zone. The enormous size of the folds is the result of many factors acting together; chief among these is the great thickness of the structural unit. Folding induced by salt movement may have occurred in the Competent group but is unrelated to the Cenozoic buckle folds; it provides a mechanism for salt diapirism through competent strata, and an explanation of how room was made for diapirs and why they rarely contain relics of country rocks. Preexisting diapirs have been reactivated in anticlines by the tectonic stresses causing buckling, but their movement generally has been halted in synclines. Diapirs are unlikely to have been initiated during buckle folding. The basement has not taken part in the folding, but instead has been deformed by strike-slip faulting.