Late Quaternary reorientation of stress field and extension direction in the southern Gulf of Suez, Egypt: Evidence from uplifted coral terraces, mesoscopic fault arrays, and borehole breakouts

Uplifted Pleistocene coral terraces and modern earthquakes show that several large normal faults are presently active in the southern Gulf of Suez rift basin. The footwall of one of these faults is exposed at Gebel el Zeit, where terraces at elevations of+10-18 and +42 m have been radiometrically dated as 125 ka and 426 ka, respectively. After correcting for eustatic sea-level changes, this results in net tectonic uplift rates of about 0.1 m/kyr. Published interpretations and our own observations indicate that the average extension direction during the Miocene to Pliocene synrift history was 055°. Analysis of borehole breakouts and published earthquake fault plane solutions, however, suggests that the present-day stress field in the southern Gulf has a 010°-020° Shmin orientation. Detailed structural observations show that a change in extension direction occurred in the late Pleistocene, with rotation of the stress field beginning prior to formation of the 125 ka terraces but after formation of older Pleistocene terraces whose ages are less tightly constrained. Using a horizontal slip direction of 015° and our observed net footwall uplift rate, we calculate a separation velocity between Sinai and Africa of 0.8-1.2 m/kyr. The proposed Pleistocene change in extension direction in the Gulf of Suez corresponds closely with the post-500 ka change in extension direction documented in the Kenyan rift system and a similar change in extension direction recorded in the central Red Sea. These regional similarities in tectonic history suggest that the underlying causes of these events may be a plate-scale phenomenon affecting the entire Afro-Arabian rift system, rather than local changes in the Quaternary stress field.