Structural geology and 4D evolution of a half-graben: New digital outcrop modelling techniques applied to the Nukhul half-graben, Suez rift, Egypt

Abstract

LIDAR-based digital outcrop mapping, in conjunction with a new surface modelling approach specifically designed to deal with outcrop datasets, is used to examine the evolution of a half-graben scale normal fault array in the Suez rift. Syn-rift deposition in the Nukhul half-graben was controlled by the graben-bounding Nukhul fault. The fault can be divided into four segments based on the strike of the fault, the morphology of hangingwall strata, and the variation in throw along strike. The segments of the fault became geometrically linked within the first 2.5 m.y. of rifting, as evidenced by the presence of early syn-rift Abu Zenima Formation strata at the segment linkage points. Fault-perpendicular folds in the hangingwall related to along-strike variations in throw associated with precursor fault segments persist for a further 1.8 m.y. after linkage of the segments, suggesting that the fault remains kinematically segmented. We suggest this occurs because of sudden changes in fault strike at the segment linkage points that inhibit earthquake rupture propagation, or because displacement is geometrically inhibited at fault linkage points where the orientation of the intersection line of the segments is significantly different from the orientation of the slip vector on the fault system. Length/throw plots and throw contour patterns for minor faults show that some faults initiated in pre-rift strata, whereas late east-striking faults initiated in the syn-rift basin fill. The late initiating faults are spatially associated with the east-striking Baba–Markha fault, which was active throughout the rift history, but developed as a transfer fault between major block-bounding fault systems around 6–7 Ma after rift initiation.