LATE QUATERNARY REORIENTATION OF STRESS-FIELD AND EXTENSION DIRECTIONIN 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 s everal large normal faults are presently active in the southern Gulf o f Suez rift basin. The footwall of one of these faults is exposed at G ebel el Zeit, where terraces at elevations of +10-18 and +42 m have be en radiometrically dated as 125 ka and 426 ka, respectively. Alter cor recting for eustatic sea-level changes, this results in net tectonic u plift 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 degrees. Analysis of boreh ole break-outs and published earthquake fault plane solutions, however , suggests that the present-day stress field in the southern Gulf has a 010 degrees-020 degrees S-hmin orientation. Detailed structural obse rvations show that a change in extension direction occurred in the lat e Pleistocene, with rotation of the stress field beginning prior to fo rmation of the 125 ka terraces but after formation of older Pleistocen e terraces whose ages are less tightly constrained. Using a horizontal slip direction of 015 degrees and our observed net footwall uplift ra te, 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 ex tension direction documented in the Kenyan rift system and a similar c hange in extension direction recorded in the central Red Sea. These re gional similarities in tectonic history suggest that the underlying ca uses of these events may be a plate-scale phenomenon affecting the ent ire Afro-Arabian rift system, rather than local changes in the Quatern ary stress field.