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.

Assessments of the extension directions and their variations are critical for understanding rifting processes. This study provides an overview of the extension directions along the axes of the Main Ethiopian Rift and the Red Sea Rift (or propagator) of Afar, two of the three rifts meeting at the Afar triple junction. This overview is based on new and published field data on the opening direction of significant (width >0.2 m) Holocene extension fractures along the rift axis. The data show that the Red Sea propagator axis opens orthogonally, both in northern and central Afar, even though a significant strike-slip component is recognized at the rift margins in central Afar. The Main Ethiopian Rift axis also opens orthogonal to the trend of the rift, which varies between the different rift segments. Therefore, the axes of two of the three rifts meeting in Afar are characterized by orthogonal extension. However, given the variable orientations of the rift segments, the obtained opening directions are usually not uniform along the rift. Current plate-motion models suggest slightly different divergence directions, especially along the Main Ethiopian Rift, which shows a significant oblique component. The discrepancy between the data along the rift axis and those from plate-motion models suggests an across-rift strain partitioning. The observed orthogonal extension along the rift axis may be magma-induced, provided that a depth-dependent variation in the kinematics exists, at least below the Main Ethiopian Rift axis.