Structural characteristics and analysis of the Gebel El Halal fold, Northeast Sinai, Egypt

Abstract

Gebel El Halal fold is a NE-SW oriented doubly plunging, asymmetric, SE-vergent anticline with parallel folding style. The two noses of this anticline have semi-elliptical open shape, with conical form. This anticline developed mainly by buckling except for the central part of its forelimb, which was formed by flexural-slip. Flexural-slip folding is related to the rotation of beds forming the forelimb due to their drag over a reverse fault. Folding in the central part was developed by fault-propagation. The blind fault underneath is thought to be reverse with steep dip angle. This fault did not originate during folding, but it is an old fault that originated during the Early Mesozoic rifting of the North African plate margin and was rejuvenated later as a reverse fault during Syrian Arc deformation. Gebel El Halal area passed through two main phases during the Early Mesozoic-Early Tertiary. The earlier phase was extensional and resulted in the development of a rift basin. The second phase was compressional and inverted that basin in the Coniacian-Santonian, where its climax was at the end of the Late Santonian. This phase continued mildly till the Early Tertiary.

The analysis of the stress markers in the northeastern and north-central parts of Gebel El Halal anticline shows N75°W oriented compression. On the other hand, the central and south-central parts of the anticline indicate N38°W and N42°W oriented compression directions respectively. The northeastern and north-central parts exhibit different compression direction (N75°W) owing to their rotation. Strain analysis indicate that shortening values of the conical parts of the main anticline and the en echelon folds affecting its southern steep limb are similar and range between 15% and 17.5%. These values are relatively larger than those calculated for the en echelon synclines, which range from 6.0% to 9.5%.

The increase in amplitude of the Gebel El Halal anticline in its central part led to contemporaneous lengthening parallel to the hinge line. This lengthening produced SW-dipping, NW oriented normal faults in the northeastern fold nose and NW to E-W oriented normal faults with NE-dipping planes in the southwestern nose. The normal faults of the two fold noses form step faulted blocks but in the central part of the anticline they form horsts and grabens.