Abstract Segmented, planar, domino-style extensional fault arrays and their associated hanging wall fault-related folds form complex linked basins along the onshore margin of the northwestern Red Sea, Egypt. The extensional fault systems form half-graben basins with kilometre-scale, asymmetrical, doubly plunging longitudinal synclines and narrow, plunging transverse anticlines and synclines. The axial traces of the hanging wall longitudinal folds are curvilinear, sub-parallel to the half-graben Border faults, and bend or are offset at relay ramps and at fault linkage points. Transverse corner fold systems occur at the fault linkage points and fault jogs. The fold geometries, variations in fault displacement, and fault slip indicators indicate that the fold and fault systems are kinematically related and formed during the Late Oligocene–Miocene rifting of the northern Red Sea. The folds were controlled by vertical and lateral fault propagation and by the mechanical anisotropy of the pre-rift strata. The proposed model for these extensional folds is the initial formation of monoclinal flexures above reactivated blind basement faults. Increased displacement, propagation and segment linkage formed hanging wall longitudinal folds and transverse corner folds. The longitudinal folds grew progressively at the expense of the transverse folds and merged along-strike into long hanging wall synclinal basins.

Abstract: Fault-related folds are common structural features found at a variety of scales in extensional settings, and have been recognized in both outcrop and subsurface studies. However, the detailed geometry and origin of complex 3D folds adjacent to normal faults are poorly known, and, in some cases, are interpreted to be due to strike-slip tectonics and post-rift contraction. Here we examine the 3D geometry of seismic-scale folds in a rift margin – the Red Sea – and discuss the interrelationship between the growth of normal faults and the development of their related folds. Detailed field mapping of the NW Red Sea rift system has shown that the rift margin is dominated by two large extensional fault systems formed by a series of linked NNW-, north–south- and NNE-striking fault segments. These linked segments exhibit distinct zigzag fault patterns and combine to form a number of NNW-trending faults that dip NE with dominant hanging-wall stratal dips to the SW. Hanging-wall stratal dips define 3D extensional fault-related synclinal folds in pre- and early synrift strata. The hanging-wall synclines are kilometre-scale, gently doubly plunging, with curved axial surface traces orientated sub-parallel to the bounding faults. Field data demonstrated that these folds are formed by along-strike variations in fault displacements, and they form transverse synclines combined with hanging-wall extensional fault-propagation folds. The complex 3D geometry of the hanging-wall synclines is the result of the along-strike segment linkage. Adjacent to the bounding faults, the stratal dips are sub-parallel to the faults as a result of extensional fault-propagation folding controlled by highly anisotropic pre-rift strata. Palaeo-strain analyses of fault-slip data, together with analysis of the fold geometry, clearly indicate that the faulting and folding in the NW Red Sea are formed by pure NE–SW extension during the Late Oligocene–Miocene rifting, and that contraction or strike-slip tectonics need not be invoked.