Syncline

Abstract: The majority of folds in extensional tectonic settings are associated with normal fault systems over a wide range of scales. The hinges of longitudinal folds are subparallel to associated faults, whereas those of transverse folds are oriented at a high angle to the related normal faults. Longitudinal folds include drag folds (hanging-wall synclines and footwall anticlines), reverse-drag folds (hanging-wall anticlines and footwall synclines), and rollover folds (hanging-wall anticlines). Drag folds form as a result of fault propagation into monoclinal warps present at the fault tips; some may form as a result of frictional drag and differential compaction. Reverse-drag folds develop because displacement decreases with distance from the fault surface. Rollover folds are pro uced by movement along gravity-driven listric faults in thick sedimentary successions. Drag folds have a smaller wavelength than reverse-drag or rollover folds and may be superimposed on these larger structures. Basin-scale synclines are the largest type of transverse folds and are manifested in plan view by basin outlines that are concave toward the border fault. These folds form because displacement is greatest near the center of the map trace of the border fault and decreases toward its along-strike ends; this is a scale-invariant feature of most normal faults. Transverse folds are also associated with segmented fault systems: hanging-wall synclines are located near the centers of fault segments, whereas hanging-wall anticlines are found at segment boundaries where the fault segments ommonly overlap. Some transverse folds may be caused by movement along an undulatory fault surface. Many folds in extensional tectonic settings form syndepositionally and control stratal thickness and facies relationships.

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 symmetry 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.

Abstract: Two distinct phases in the structural evolution of normal faults can be identified in the Miocene Gulf of Suez rift: (1) an initial growth fold stage when the fault is a buried structure and (2) a subsequent surface faulting stage. During the growth fold stage, strata thin and become truncated toward the fault zone and are rotated and diverge away from the buried fault into growth synclines. In contrast, once the fault breaks surface, strata form a divergent wedge, which is rotated and thickens into the fault. The two tectono-stratigraphic styles also occur contemporaneously along the length of a single fault segment. Growth folding characterizes deformation around the ends of fault segments where the fault is blind, whereas the center of fault segments are characterized by surface faulting. These observations suggest that marked along-strike variation in stratal surfaces and facies stacking patterns will occur in depositional sequences in areas of normal faulting.