Geometry and Origin of Fault-Related Folds in Extensional Settings

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 produced 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 commonly 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.