Fault-block tilting: the Gebel Zeit example, Gulf of Suez

Abstract

The Gebel Zeit consists of the eroded crest of a tilted block in the southern part of the Suez rift. The Zeit block displays a typical asymmetrical geometry: it is bordered to the east by a 35–45° E-dipping normal fault with kilometric throw and has a 30° SW-dipping homoclinal flank. Part of the pre-rift sedimentary sequence has been preserved on this flank and is unconformably covered by Upper Burdigalian (NN4) Globigerina marls. In the southern part of the block crest, the complete pre-rift series has been eroded and evaporites of Langhian age (NN5) rest directly on the Precambrian basement. Field evidence indicates alternations from erosion to sedimentation at the crest of the Zeit block. In an attempt to characterize its tilting, subsidence curves were computed along a cross-section with the backstripping method. Results indicate three stages in the evolution of the block: (1) a rapid subsidence between 22 and 16 My; (2) a pause between 16 and 10 My; and (3) a slow subsidence until present time. During the tectonic quiescence, the sedimentary loading effect alone produced an increase of 8° in the tilt angle. A simple kinematic model of tilting along a circular fault is proposed to quantify the Zeit rotation. Depth of the brittle-ductile transition is estimated at 10 km to explain the tilting. Strong driving of the tilting by the listric fault induces conjugate movements between the crest and trough of the block and explains the discrepancy between the Zeit and regional tectonic subsidence.