Constraints on the deformation and rupturing of continental lithosphere of the Red Sea: the transition from rifting to drifting

The Red Sea is an ideal locale for testing differing models and hypotheses for rift evolution and the initiation of sea-floor spreading. The Red Sea is an active rift system that formed by the rifting of Precambrian continental lithosphere beginning in the late Oligocene, leading to breakup and sea-floor spreading by approximately 5 Ma in the southern Red Sea to the south of about 198300N. In the northern Red Sea, north of approximately 238300N, organized sea-floor spreading is not observed, although individual volcanoes are located within discrete ‘deeps’ spaced along the axial depression. These have been interpreted as marking the beginning of a transition from amagmatic, mechanical rifting to magmatic, oceanic spreading. Based on seismic reflection and refraction, gravity, magnetic and heat flow data in the northern Red Sea, it has been suggested that rift development occurs via the rotation of large crustal fault blocks that sole into a zone of plastic creep in the lower crust, resulting in a flat Moho and high upper crustal relief. Melt formed within individual rift segments is focused to form small axial volcanoes. That is, the northern Red Sea is on the verge of replacing horizontal translation with focused mantle upwelling and organized sea-floor spreading. In marked contrast, many passive margins (e.g. West Africa, Brazil, and NW and NE Australia) are characterized by the stacking of regional synrift sag packages, the thickness and distribution of which are inconsistent with the minor amounts of brittle deformation mapped from seismic sections. A fundamental implication of this observation is that rifts characterized by large offset fault systems, (i.e. faults that generate synrift accommodation, such as in the Basin and Range province and East Africa) will not proceed to breakup. The challenge is to understand why different portions of the Red Sea show different stages in the development of a spreading centre during continental rifting. Two hypotheses exist: (1) the structural framework deduced in the north simply continues to the south where sea-floor spreading exists and that the two regions are registering a difference in total extension. Thus, the northern Red Sea has experienced insufficient extension to breach the continental lithosphere but, in time, should develop into a spreading centre; or (2) the lithosphere of the northern Red Sea region is rheologically stronger compared with the lithosphere of the southern Red Sea, perhaps as a consequence of the thermal effects of the Afar plume, and the northern Red Sea will never evolve to sea-floor spreading. The existence of large rotated fault blocks, as implied from the inversion of gravity and magnetic anomaly data, favours the latter. Exactly how the lithosphere ruptures to form a rifted continental margin and a new ocean basin (i.e. the transition from extension to sea-floor spreading), and why some rifts succeed in making this transition while others fail, is poorly known. The Red Sea is an ideal locale for testing differing models and hypotheses for rift evolution and the initiation of sea-floor spreading. The Red Sea is an active rift system that formed by the rifting of Precambrian continental lithosphere beginning in the late Oligocene, leading to breakup and sea-floor spreading by about 5 Ma in the southern Red Sea to the south of approximately 198300N (Roeser 1975). In the northern Red Sea, north of approximately 238300N, organized sea-floor spreading is not observed, although individual volcanoes are located within discrete ‘deeps’ spaced along the axial depression. These have been interpreted as marking the beginning of a transition from amagmatic, mechanical rifting to magmatic, oceanic spreading (Pautot et al. 1986; Martinez & Cochran 1988; Cochran 2005). Between these two regions (in the central Red Sea) there is a transition area where sea-floor spreading cells have nucleated, but have not linked to form a continuous axis (Pautot 1983; Cochran 1983; Bonatti 1985; Bicknell et al. 1986). A fundamental question is whether sea-floor spreading is propagating from south to north or whether the mode of extension in the northern Red Sea has in some way prevented the development of sea-floor spreading in that region. Based on data in the northern Red Sea, Cochran (2005) suggested that rift development occurs via the rotation of large crustal fault blocks that sole From: KARNER, G. D., MANATSCHAL, G. & PINHEIRO, L. M. (eds) Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup. Geological Society, London, Special Publications, 282, 265–289. DOI: 10.1144/SP282.13 0305-8719/07/$15.00 # The Geological Society of London 2007. into a zone of plastic creep in the lower crust, resulting in a flat Moho and high upper crustal relief. Initial broadly distributed extension is eventually replaced by focused extension at the rift axis leading to rapid lithospheric thinning and melt generation. Melt formed within individual rift segments is focused to form small axial volcanoes (Cochran 2005). With continued extension and magmatism, these volcanoes may become cells of sea-floor spreading that grow and coalesce to form a continuous axis, as exists in the southern Red Sea. That is, the northern Red Sea is on the verge of replacing horizontal translation by focused mantle upwelling and organized sea-floor spreading. Given that considerably more extension has occurred in the southern than in the northern Red Sea, as determined by the difference in opening rate between Arabia and Nubia (Roeser 1975; Joffe & Garfunkel 1987; Chu & Gordon 1998; Jestin et al. 1998), then why even question that seafloor spreading will simply propagate northwards into the northern Red Sea? Karner et al. (2003) and Karner (2005) have shown that the West African, Brazilian and NW Australian passive continental margins are characterized by the stacking of regional synrift sag basins, the amplitude and distribution of which are inconsistent with the minor amounts of brittle deformation mapped from seismic sections. The sag basins exhibit none of the diagnostic features expected of synrift extensional systems (e.g. normal faults that control accommodation, rotation of crustal blocks, prominent rift onset unconformities, synrift sediment wedges). While the South Atlantic margins do show a number of large, late-rift stage faults, these faults generally fail to explain the regional development of synrift accommodation across the region and are thus relatively unimportant in the extension process. The fundamental implication of this model is that rifts characterized by large normal fault systems controlling synrift accommodation, such as in the Basin and Range province, East African rift system, North Sea Basins, Rhinegraben and Newark Basin, will not proceed to breakup. By analogy, because of the existence of large fault blocks in the northern Red Sea, the implication is that it should never proceed to sea-floor spreading. The challenge is to understand why different portions of the Red Sea show different stages in the development of a spreading centre during continental rifting. Cochran (2005) based his model on data from the northern Red Sea and assumes that the structural framework deduced in the north continues to the south where sea-floor spreading exists. However, it is entirely conceivable that the two areas, separated by a major tectonic boundary (the ‘Zabargad fracture zone’ Crane & Bonatti 1987), have developed differently and observations in the northern Red Sea cannot be extrapolated to the south where few data are available for comparison. Thus, the difference between the two regions may be owing to a fundamental difference in lithosphere rheology, explaining why sea-floor spreading developed in the southern, but not the northern, Red Sea. The counter-argument is that the entire Red Sea is developing similarly, but rifting in the northern and central Red Sea simply has experienced insufficient extension to develop into sea-floor spreading at this time. In this paper, we will compare and contrast the two models presented above by using existing geophysical data from the region to understand why different portions of the Red Sea show different stages in the development of a spreading centre within a continental rift. For a combination of logistic and political reasons, most of the available geophysical data are concentrated in the northern Red Sea. The morphology and other geophysical parameters for much of the central and southern Red Sea are only known in general terms with Backer et al.’s (1975) single-beam sonar survey still the primary data source. First, we briefly review the prerift development of the Red Sea area and how the pre-existing structure influenced the form and distribution of rifting and the influence of the Afar plume. Finally, we will discuss the structure of the northern and central Red Sea and develop a model for how possible differences between these two areas may influence the future development of the Red Sea.