Matching conjugate volcanic rifted margins : 40 Ar/ 39 Ar chrono-stratigraphy of pre- and syn-rift bimodal £ood volcanism in Ethiopia and Yemen Ingrid A. Ukstins a;b; Ã , Paul R. Renne c;d , Ellen Wolfenden a , Joel Baker b , Dereje Ayalew e , Martin Menzies a a Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK b Danish Lithosphere Centre, Òster Voldgade 10 L, DK-1350 Copenhagen K, Denmark c Berkeley Geochronology Centre, 2455 Ridge Road, Berkeley, CA 94709, USA d Department of Earth and Planetary Science, University of California, Berkeley, CA 94709, USA e Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia Received 4 September 2001; accepted 6 February 2002 Abstract 40 Ar/ 39 Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40 Ar/ 39 Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that flood volcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen by V0.2^2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyolitic ignimbrite unit in Yemen at V30 Ma. Accurate and precise 40 Ar/ 39 Ar dates on initial rhyolitic ignimbrite eruptions suggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out a causative link between these explosive silicic eruptions (with individual volumes v 200 km 3 ) and climatic cooling which produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive and systematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift flood volcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease in volcanic activity in Ethiopia between 25 and 20 Ma correlates with a prominent break-up unconformity in Yemen (26^19 Ma), both of which mark the transition from pre- to syn-rift volcanism (V25^26 Ma) triggered by the separation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation and preservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmatic starvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angular unconformity in the northern Main Ethiopian Rift is evident at 10.6^3.2 Ma, and is also observed throughout the Arabian plate in Jordanian, Saudi Arabian and Yemeni intraplate volcanic fields and is possibly linked to tectonic re- organization and initiation of sea floor spreading in the Gulf of Aden and the Red Sea at 10 and 5 Ma, respectively. ß 2002 Elsevier Science B.V. All rights reserved. 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0012-821X(02)00525-3 * Corresponding author. Tel.: +45-38142634; Fax: +45-33110878. E-mail address: iau@dlc.ku.dk (I.A. Ukstins). Earth and Planetary Science Letters 198 (2002) 289^306 www.elsevier.com/locate/epsl