GEOLOGY | March 2014 | www.gsapubs.org 219 INTRODUCTION The nature of crustal extension across the Red Sea Rift, long regarded as one of the best locations to investigate continental breakup, has been debated in studies for more than 30 years (e.g., Cochran, 1983, 2005; Voggenreiter et al., 1988; Bohannon, 1989; Dixon et al., 1989; Rihm et al., 1991). Asymmetric structure across the rift, inferred from seismic refraction profiles (e.g., Gaulier et al., 1988; Egloff et al., 1991; Voggenreiter et al., 1988; Rihm et al., 1991) and the greater amounts of Cenozoic volcanism and uplift along the eastern side of the Red Sea, have been used to argue for extension via simple shear (Fig. 1A) (e.g., Voggenreiter et al., 1988; Bohan- non, 1989; Dixon et al., 1989; Rihm et al., 1991). Interpretations of asymmetric crustal structure have also been used to support a pull-apart mod- el of the northern Red Sea with oblique exten- sion along strike-slip faults (Makris and Rihm, 1991). In contrast, the seismic images of crustal structure have been used to argue for symmetric structure across the northern Red Sea and pure shear extension by diffuse ductile flow below block faulting in the uppermost crust (Fig. 1B) (e.g., Cochran, 1983, 2005; Martinez and Co- chran, 1988). As these models all derive from interpretations of the same seismic data, new constraints on crustal structure across the Red Sea margins are necessary to evaluate them and advance our understanding of how the rifted crust has been extended. Here we present new estimates of crustal thickness from southeastern Egypt, using P- wave receiver functions obtained from broad- band seismograms recorded on the Egyptian National Seismic Network, that when combined with previous estimates of crustal thickness re- veal a symmetric pattern of crustal thickness beneath the northern Red Sea conjugate mar- gins. This finding supports a pure shear model of extension, and suggests that the greater amounts of uplift and volcanism on the eastern side of the Red Sea compared to the western side may be the result of deeper flow in the mantle asso- ciated with the African superplume, and not a direct consequence of the rifting process. BACKGROUND The Red Sea Rift formed by the rupturing of Proterozoic lithosphere starting in the Late Oligocene. Seafloor spreading began ca. 5 Ma in the southern Red Sea, and in the central and northern Red Sea the transition from continen- tal to oceanic rifting is ongoing (Martinez and Cochran, 1988; Cochran et al., 1991). The Gulf of Suez to the northwest of the Red Sea was part of the Red Sea until it was cut off by the development of the Dead Sea transform in the Middle Miocene. Uplift and unroofing began along the entire Red Sea at ca. 34 Ma (Omar and Steckler, 1995), well before the onset of rifting, and Proterozoic basement is exposed on both sides of the rift (Fig. 2). Flood basalt volcanism in the southern Red Sea region (Ethiopia and Yemen) occurred at ca. 30 Ma (Coulié et al., 2003), dike intrusion along the entire length of the Red Sea on the Arabian side occurred between 24 and 21 Ma *E-mails: aha13@psu.edu; nyblade@psu.edu. Crustal structure in southeastern Egypt: Symmetric thinning of the northern Red Sea rifted margins Ahmed Hosny 1 * and Andrew Nyblade 2 * 1 Seismology Department, National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Cairo, Egypt 2 Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA ABSTRACT Crustal structure in southeastern Egypt has been investigated to elucidate the nature of crustal thinning across the northern Red Sea. P-wave receiver function modeling for 7 stations in southeastern Egypt yields typical Proterozoic crustal thicknesses of 35–38 km around Lake Aswan, and thinner crust (25–26 km) within 50 km of the Red Sea coast. The Vp/Vs ratios are on average 1.78 and indicate an intermediate-composition crust. These results, when com- bined with other estimates of crustal thickness in the region, reveal a symmetric pattern of crustal thickness beneath the conjugate margins of the northern Red Sea. Such a pattern is consistent with a pure shear model of extension, and suggests that the greater amounts of uplift and volcanism on the eastern side of the Red Sea compared to the western side may be the result of deeper flow in the mantle associated with the African superplume and not a direct consequence of the rifting process. GEOLOGY, March 2014; v. 42; no. 3; p. 219–222; Data Repository item 2014075 | doi:10.1130/G34726.1 | Published online 10 January 2014 © 2014 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Moho Lithosphere Asthenosphere Convecting Mantle Pure Shear Lithosphere Asthenosphere Moho Simple Shear Convecting Mantle A B Figure 1. A: Simple shear model. B: Pure shear rift model. Models redrawn from Dixon et al. (1989). 32˚ 32˚E 34˚ 34˚ 36˚ 36˚ 38˚ 38˚ 40˚ 40˚ 42˚ 42˚ 44˚ 44˚ 22˚ 22˚ 24˚ 24˚ 26˚ 26˚ 28˚ 28˚ 30˚N 30˚ Precambrian basement Paleozoic Triassic Mio-Pliocene Cretaceous Eocene Egypt Saudi Arabia Red Sea Quaternary 0 300 km 100 200 N Gulf of Suez L. Aswan HRG BDAS JMQS TBKS KBRS YNBS HILS ARSS NABD BRNS NMAN NAHD NNAL MABD Figure 2. Simplified geo- logical map of regions surrounding northern Red Sea. Black triangles show seismic station lo- cations. Mio—Miocene.