William Bosworth

A high-strain rift model for the southern Gulf of Suez (Egypt)

Abstract Extensive exploratory and development drilling have documented the large extension produced during Miocene rifting in the southern Gulf of Suez. Basement fault blocks are commonly rotated > 40°, with maximum dips of 65°. Early fault planes along the rift axis now dip 23–35° NE, with later extension shifting to complex arrays of rider blocks. Multiple well penetrations show that some fault planes are gently listric between basement hanging wall and footwall cut-offs. Stratigraphic and structural relationships show that deformation shifted between several basin-bounding faults during a complex history of basin evolution and progressive sub-basin abandonment. One low-angle normal fault has been identified in outcrop and a detachment model can explain the Miocene structural development of the rift. Post-Miocene deformation and subsidence focused along the basin axis, with extension probably occurring on a new system of planar normal faults. Restoration and balancing of regional and local cross-sections indicate that the southern Gulf has been extended by 56–62%, and that total separation between Sinai and Africa was 34–36 km. Average regional β factor for the central basins of the rift is 1.9–2.0. These new extension estimates are in agreement with subsidence models and crustal thickness estimates for the Gulf, and help quantify plate reconstructions between Sinai, Africa and Arabia. They also indicate that the southern Gulf of Suez is one of the most highly extended, failed continental rifts preserved in the rock record. As such, it provides an important conceptual link between more numerous low-strain continental rifts and successful oceanic basins.

Cyrenaican “shock absorber” and associated inversion strain shadow in the collision zone of northeast Africa

Far-field compressional stresses resulting from arc collisions with the northeast coast of Africa-Arabia propagated across the African plate in the Santonian (ca. 84 Ma), and later similar pulses occurred through much of the Cenozoic. Waves of inversion-related features including folds, thrusts, and igneous activity developed during the compressional events. Structures generated were uneven in intensity and distribution, and were commonly concentrated in existing rifts. Because of the irregular shapes of the North African continental margin and associated inboard basins, Cyrenaica underwent the most severe shortening, although deformation extended eastward from Libya to the Levant. Areas south and southeast of Cyrenaica (the Sirt Basin and the far Western Desert of Egypt) underwent very little shortening because they occupied a regional strain shadow. The eastern region of the Western Desert and Sinai, which were not sheltered by the stress-absorbing Cyrenaica inversion, recorded strong compressional deformation synchronous with that in Cyrenaica. As closure of the Mediterranean continues, Cyrenaica will become an indenter comparable to those of other orogenic belts. The detailed tectonic evolution of individual northeast African basins has depended on their positions in relation to the Cyrenaican strain shadow. No single subsidence and paleostress history reflects the complete regional development of the converging Mediterranean–North African margin.

Geological Evolution of the Red Sea: Historical Background, Review, and Synthesis

The Red Sea is part of an extensive rift system that includes from south to north the oceanic Sheba Ridge, the Gulf of Aden, the Afar region, the Red Sea, the Gulf of Aqaba, the Gulf of Suez, and the Cairo basalt province. Historical interest in this area has stemmed from many causes with diverse objectives, but it is best known as a potential model for how continental lithosphere first ruptures and then evolves to oceanic spreading, a key segment of the Wilson cycle and plate tectonics. Abundant and complementary datasets, from outcrop geology, geochronologic studies, refraction and reflection seismic surveys, gravity and magnetic surveys, to geodesy, have facilitated these studies. Magnetically striped oceanic crust is present in the Gulf of Aden and southern Red Sea, active magma systems are observed onshore in the Afar, highly extended continental or mixed crust submerged beneath several kilometers of seawater is present in the northern Red Sea, and a continental rift is undergoing uplift and exposure in the Gulf of Suez. The greater Red Sea rift system therefore provides insights into all phases of rift-to-drift histories. Many questions remain about the subsurface structure of the Red Sea and the forces that led to its creation. However, the timing of events—both in an absolute sense and relative to each other—is becoming increasingly well constrained. Six main steps may be recognized: (1) plume-related basaltic trap volcanism began in Ethiopia, NE Sudan (Derudeb), and SW Yemen at ~31 Ma, followed by rhyolitic volcanism at ~30 Ma. Volcanism thereafter spread northward to Harrats Sirat, Hadan, Ishara-Khirsat, and Ar Rahat in western Saudi Arabia. This early magmatism occurred without significant extension or at least none that has yet been demonstrated. It is often suggested that this “Afar” plume triggered the onset of Aden–Red Sea rifting, or in some models, it was the main driving force. (2) Starting between ~29.9 and 28.7 Ma, marine syn-tectonic sediments were deposited on continental crust in the central Gulf of Aden. Therefore, Early Oligocene rifting is established to the east of Afar. Whether rifting propagated from the vicinity of the Sheba Ridge toward Afar, or the opposite, or essentially appeared synchronously throughout the Gulf of Aden is not yet known. (3) By ~27.5–23.8 Ma, a small rift basin was forming in the Eritrean Red Sea. At approximately the same time (~25 Ma), extension and rifting commenced within Afar itself. The birth of the Red Sea as a rift basin is therefore a Late Oligocene event. (4) At ~24–23 Ma, a new phase of volcanism, principally basaltic dikes but also layered gabbro and granophyre bodies, appeared nearly synchronously throughout the entire Red Sea, from Afar and Yemen to northern Egypt. The result was that the Red Sea rift briefly linked two very active volcanic centers covering 15,000–25,000 km2 in the north and >600,000 km2 in the south. The presence of the “mini-plume” in northern Egypt may have played a role somewhat analogous to Afar vis-a-vis the triggering of the dike event. The 24–23 Ma magmatism was accompanied by strong rift-normal extension and deposition of syn-tectonic sediments, mostly of marine and marginal marine affinity. The area of extension in the north was very broad, on the order of 1,000 km, and much narrower in the south, about 200 km or less. Throughout the Red Sea, the principal phase of rift shoulder uplift and rapid syn-rift subsidence followed shortly thereafter. Synchronous with the appearance of extension throughout the entire Red Sea, relative convergence between Africa and Eurasia slowed by about 50 %. (5) At ~14–12 Ma, a transform boundary cut through Sinai and the Levant continental margin, linking the northern Red Sea with the Bitlis–Zagros convergence zone. This corresponded with collision of Arabia and Eurasia, which resulted in a new plate geometry with different boundary forces. Red Sea extension changed from rift normal (N60°E) to highly oblique and parallel to the Aqaba–Levant transform (N15°E). Extension across the Gulf of Suez decreased by about a factor of 10, and convergence between Africa and Eurasia again dropped by about 50 %. In the Afar region, Red Sea extension shifted from offshore Eritrea to west of the Danakil horst, and activity began in the northern Ethiopian rift. (6) These early events or phases all took place within continental lithosphere and formed a continental rift system 4,000 km in length. When the lithosphere was sufficiently thinned, an organized oceanic spreading center was established and the rift-to-drift transition started. Oceanic spreading initiated first on the Sheba Ridge east of the Alula-Fartaq fracture zone at ~19–18 Ma. After stalling at this fracture zone, the ridge probably propagated west into the central Gulf of Aden by ~16 Ma. This matches the observed termination of syn-tectonic deposition along the onshore Aden margins at approximately the same time. At ~10 Ma, the Sheba Ridge rapidly propagated west over 400 km from the central Gulf of Aden to the Shukra al Sheik discontinuity. Oceanic spreading followed in the south-central Red Sea at ~5 Ma. This spreading center was initially not connected to the spreading center of the Gulf of Aden. By ~3 to 2 Ma, oceanic spreading moved west of the Shukra al Sheik discontinuity, and the entire Gulf of Aden was an oceanic rift. During the last ~1 My, the southern Red Sea plate boundary linked to the Aden spreading center through the Gulf of Zula, Danakil Depression, and Gulf of Tadjoura. Presently, the Red Sea spreading center may be propagating toward the northern Red Sea to link with the Aqaba–Levant transform. However, important differences appear to exist between the southern and northern Red Sea basins, both in terms of the nature of the pre- to syn-rift lithospheric properties and the response to plate separation. If as favored here no oceanic spreading is present in the northern Red Sea, then it is a magma-poor hyperextended basin with β factor >4 that is evolving in many ways like the west Iberia margin. It is probable that the ultimate geometries of the northern and southern Red Sea passive margins will be very different. The Red Sea provides an outstanding area in which to study the rift-to-drift transition of continental disruption, but it is unlikely to be a precise analogue for all passive continental margin histories.

Integration of outcrop and subsurface data during the development of a naturally fractured Eocene carbonate reservoir at the East Ras Budran concession, Gulf of Suez, Egypt

Abstract The East Ras Budran Concession is located in the eastern rift shoulder of the Gulf of Suez. Syn- and pre-rift rocks are exposed in the north and east of the concession, and the Markha alluvial plain covers the SW. The Markha plain occupies the hanging wall of a large extensional fault which preserves most of the pre-rift stratigraphic sequence and >3500 m of syn-rift strata. Vertical wells drilled in 1999 indicated the presence of a >200 m oil column in low-porosity naturally fractured limestone beds of the Eocene Darat and Thebes formations. Outcrop, borehole image and core data define NW, WNW, N, NE, and ENE steeply dipping fracture sets. Borehole breakouts and drilling-induced fractures show that the minimum horizontal stress is aligned NNE to NE, so the NW and WNW fractures should be open in the subsurface. Using this structural picture, a near-horizontal well of 300 m length was drilled into the Darat in a NE direction. During testing, the well flowed at a rate of 1900 barrels of oil per day with no water. Future development of the field includes drilling similarly oriented wells with longer horizontal sections.

Seismic hazards implications of uplifted Pleistocene coral terraces in the Gulf of Aqaba

The Gulf of Aqaba transform plate boundary is a source of destructive teleseismic earthquakes. Seismicity is concentrated in the central sub-basin and decreases to both the north and south. Although principally a strike-slip plate boundary, the faulted margins of the Gulf display largely dip-slip extensional movement and accompanying footwall uplift. We have constrained rates of this uplift by measurements of elevated Pleistocene coral terraces. In particular the terrace that formed during the last interglacial (~125 ka) is found discontinuously along the length of the Gulf at elevations of 3 to 26 m. Global sea level was ~7 m higher than today at 125 ka indicating net maximum tectonic uplift of ~19 m with an average rate of ~0.015 cm/yr. Uplift has been greatest adjacent to the central sub-basin and like the seismicity decreases to the north and south. We suggest that the present pattern of a seismically active central region linked to more aseismic areas in the north and south has therefore persisted for at least the past 125 kyr. Consequently the potential for future destructive earthquakes in the central Gulf is greater than in the sub-basins to the north and south.