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Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia

Ellen Wolfenden¹, Cynthia Ebinger^,1, Gezahegn Yirgu², Paul R. Renne³ and Simon P. Kelley⁴

¹ Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK
² Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
³ Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA
⁴ Geology Department, Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK

The process of strain localization as rifting proceeds to continental breakup is readily observed along the Oligocene-Recent southern Red Sea rift, yet much of the Red Sea margin in Ethiopia remains unmapped. Rifting initiated above or near a mantle plume, which is marked by the Eo-Oligocene Ethiopia-Yemen flood basalt province. Objectives of this field, remote sensing, and geochronology study are to establish a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing ⁴⁰Ar/³⁹Ar age data along 6 transects. We present new sketch geological maps and cross sections to document the timing of extension in relation to magmatism and its variation along strike. These new data are integrated with plate kinematic, geological, and geophysical data to present a model for evolution of the southern Red Sea margin. Faults commonly marked by eruptive centers initiated between 29 and 26 Ma, coincident with rifting in the Gulf of Aden. The Red Sea rift terminated at 10°N until linkage of the Main Ethiopian rift and southern Red Sea occurred at ca. 11 Ma. Rifting progressed in three distinct stages; each new phase saw a marked change in the style of volcanism and a narrowing of the locus of extension. Stage 1 rhyolites were emplaced from 29 to 26 Ma in basins bounded by a steep border fault system. Between 25 and 20 Ma, strain localized to narrow zones of basaltic fissural eruptions and minor faulting. Stage 2 faults and eruptive centers are located 50 km to the east of the border faults, and they comprise flows spanning at least 16–7 Ma. After ca. 7 Ma, the locus of strain again migrated eastward (Stage 3). Strain in Stage 3 was largely accommodated by dike injection. Plate reconstructions predict high stretching factors (ß 3) in the southern Red Sea, suggesting that Stages 2 and 3 mark the onset of formation of crust transitional between oceanic and continental.

Key Words: passive margin • magmatism • eastern Africa • rift

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