Evidence for non-Andersonian faulting above evaporites in the Nile Delta

Abstract

Abstract This study examines present-day stress orientations from borehole breakout and drilling-induced fractures in 57 boreholes in the Nile Delta. A total of 588 breakouts and 68 drilling-induced fractures from 50 wells reveal sharply contrasting present-day maximum horizontal stress ( S Hmax ) orientations across the Nile Delta. A typical deltaic margin-parallel S Hmax exists in parts of the Nile Delta that are below or absent from evaporites (NNE–SSW in the west, east–west in the central Nile, ESE–WNW in the east). However, a largely margin-normal (NNE–SSW) S Hmax is observed in sequences underlain by evaporites in the eastern Nile Delta. The margin-normal supra-salt S Hmax orientations are often subperpendicular to the strike of nearby active extensional faults, rather than being parallel to the faults as predicted by Andersonian criteria. The high angle between S Hmax and strike of these extensional faults represents a new type of non-Andersonian faulting that is even less-suitably oriented for shear failure than previously described anomalous faulting such as low-angle normal faults and highly oblique strike-slip faults (e.g. San Andreas). While the mechanics of these non-Andersonian faults remains uncertain, it is suggested that the margin-normal supra-salt orientation generated by basal forces imparted upon rafted blocks sliding down seawards-dipping evaporites.