39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen

Earth and Planetary Science Letters 198 (2002) 289^306 www.elsevier.com/locate/epsl

40

Matching conjugate volcanic rifted margins: Ar/ Ar chrono-stratigraphy of pre- and syn-rift bimodal £ood volcanism in Ethiopia and Yemen 39

Ingrid A. Ukstins a;b; , Paul R. Renne c;d , Ellen Wolfenden a , Joel Baker b , Dereje Ayalew e , Martin Menzies a a

d

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 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 km3 ) 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 reorganization 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.

* Corresponding author. Tel.: +45-38142634; Fax: +45-33110878.

E-mail address: [email protected] (I.A. Ukstins).

0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 5 2 5 - 3

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Keywords: £ood basalts; volcanism; rifting; Ar-40/Ar-39; Ethiopia; Yemen

1. Introduction

2. Regional setting

The Red Sea margins contain the best preserved continental rift system on Earth, and the relative youthfulness of the conjugate volcanic margins permits precise comparison of the volcano-stratigraphy. The temporal and stratigraphic relationships between £ood volcanism in Yemen and Ethiopia, however, are not yet well understood. In this study, a north^south section of the rifted margin in Ethiopia was targeted for 40 Ar/39 Ar dating to complement a detailed understanding of the temporal and spatial evolution of the conjugate Yemen margin [1,2]. This chronological study was motivated by three objectives : (1) to examine the temporal link between magmatism and tectonics in an evolving rift margin with a continuous preserved stratigraphy representing £ood volcanism, the development of seaward dipping re£ector series and the transition to sea£oor spreading. (2) To compare and contrast the chrono- and volcano-stratigraphies of Ethiopia and Yemen in an attempt to link the conjugate rifted margins, and to evaluate any spatial variation in timing of volcanic activity across the Afro-Arabian £ood volcanic province. (3) To carefully date silicic units that form a major component of this bimodal £ood volcanic province. Earlier studies [3,4] have linked the explosive silicic volcanism occurring in Ethiopia to the Oi2 global cooling event which is characterized by one of the maxima in the Cenozoic N18 O (oxygen isotope) record [5], the largest sea-level drop in the Cenozoic, and signi¢cant glaciation in Antarctica [6]. Major lower Oligocene tephra layers linked to this Oi2 cooling event are found in ODP Leg 115 cores in the Indian Ocean at the Chron C11r boundary [7], and were postulated by Rochette et al. [4] to be temporally correlated with the ¢rst silicic volcanic units found in the Lima Limo and Wegel Tena sections from Ethiopia, 2600 km to the northwest of this Indian Ocean ODP site.

Continental £ood volcanism in Afro-Arabia extends over an area of at least 600 000 km2 , stretching from southwestern Ethiopia through Eritrea and Djibouti to Yemen, with an estimated volume s 350 000 km3 [8] (Fig. 1). Volcanism was associated with rifting of the Afro-Arabian continent in late Oligocene^early Miocene times [9] and with the initiation of sea £oor spreading in both the Gulf of Aden (10 Ma) and the Red Sea (5 Ma) [10,11]. Continental £ood volcanism in Ethiopia^Yemen is presumed to be associated with the Afar plume and comprises voluminous sub-aerial basaltic lavas and silicic pyroclastic rocks. Flood volcanism produced a ca. 4 km thick volcanic pile, the erosional remnants (ca. 2 km) of which are well-preserved on the Ethiopian and Yemeni plateaux [1,2,12^15]. 2.1. Ethiopia Flood volcanic rocks in Ethiopia were unconformably emplaced on a regional lateritized sandstone horizon. This sandstone unit covers an area of 90 000 km2 [16] and represents a period of paleosol development prior to initiation of £ood volcanism [17^20]. Mohr and Zanettin [13] constructed a general volcanic stratigraphy for the Ethiopian £ood volcanic province based on type localities in northeastern Ethiopia, and then extrapolated these formation divisions to sections of volcanic rocks to the south. Based on previously published 40 Ar/39 Ar data, the most voluminous eruptive stage was during the late Oligocene^Early Miocene (V32^21 Ma) with most of the volcanic activity occurring between 30 and 29.5 Ma [3], followed by a central Ethiopian volcanic episode from 13 to 9 Ma and a period of volcanism in central Afar from 4.5 to 1.5 Ma [21]. Signi¢cantly older volcanic rocks have been dated at 45 Ma in southern Ethiopia [18,22^28], but these are interpreted as a smaller volume, unre-

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lated volcanic event and are not discussed further here. 2.2. Yemen The pre-volcanic stratigraphy is better exposed in Yemen because the margin was uplifted and eroded after break-up, in contrast to the Ethiopian margin where parts of this stratigraphy have been buried by continued volcanism. In Yemen,

Fig. 1. Distribution of Cenozoic volcanism in the Red Sea and Gulf of Aden region (modi¢ed after [1,49]). Volcanism is divided into two groups according to age: the older group ( s 25 Ma) is concentrated near the Afro-Arabian triple junction and comprises transitional to sub-alkaline £ood basalts and associated rhyolitic pyroclastic £ow and fall deposits; a younger group ( 6 25 Ma) is found primarily in the Ethiopian-Afar province and comprises silicic and basaltic volcanic rocks. The samples from this study come from the northern escarpment and northern Main Ethiopian Rift in Ethiopia, the boxed area shows location of the detailed digital elevation model with pro¢le and dated sample locations (Fig. 2).

291

pre-volcanic rocks exhibit a change in depositional environment from continental in the west to shallow marine in the east and have thus been used to de¢ne an Oligocene palaeo-coastline [29]. In the east, there was a change from deposition of shallow marine to sub-aerial continental clastic rocks immediately preceding volcanism, with development of regional lateritic paleosols and overlying localized lacustrine deposits [29]. Flood volcanic rocks were disconformably emplaced onto these relatively £at-lying sedimentary rocks. According to the stratigraphy and 40 Ar/ 39 Ar age data of Baker et al. [1], the oldest £ood volcanic rocks in Yemen consist of a 50^1000 m sequence of basaltic lavas overlying basement sandstone erupted at 30.9 Ma in the south, in the north the oldest lavas have an age of 29.4 Ma (Fig. 1). The ma¢c stage of £ood volcanism peaked just prior to 29 Ma, shortly before bimodal volcanism commenced throughout the region at 29.5^29.2 Ma [1]. The main £ood volcanic pile, comprising V1.5 km of bimodal basaltic lavas and silicic ignimbrites, airfall tu¡s and caldera collapse breccias, was emplaced in V3.5 Myr and £ood volcanism ceased regionally across Yemen by 27.1^26.7 Ma. An unconformity (herein referred to as ‘break-up unconformity’) formed during rift initiation and is represented by a discrete erosional surface cut into basaltic lava £ows dated at 26.7 Ma and overlain by a trachytic lava £ow dated at 19.9 Ma [1]. This coincides with a period of rapid cooling and erosion of the Yemen rifted margin proximal to the Red Sea [2] and the emplacement of dike swarms near the Gulf of Aden (25.5 and 18.5^16 Ma) [30] and the Red Sea (24^21 Ma) [9].

3. Sampling strategy and objectives 3.1. Sampling rationale A detailed 40 Ar/39 Ar chrono-stratigraphy [1] and volcano-stratigraphy [15,31] available for the Yemen margin provides a template for comparing the timing and evolution of volcanism along the conjugate Ethiopian margin. Five stratigraphic pro¢les were sampled which span V250

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Fig. 2. Digital elevation model of the northern Ethiopian plateau and escarpment and northern Main Ethiopian Rift, based on a 1 km horizontal grid with þ 30 m vertical resolution. Section A: Desi-Bati, Section B: Ataye, Section C: Robit, Section D: Ankober, Section E: Kessem River Gorge. All pro¢les initiate on the plateau and traverse across the escarpment except E, which is located in a river gorge 2 km south of the upper escarpment edge.

km of the western escarpment along the Ethiopian Plateau and into the northern Main Ethiopian Rift (11.27‡N 39.507‡E to 9.023‡N 39.559‡E) (Fig. 2). These sections were chosen to cover the major tectono-volcanic events which a¡ected the region since the onset of £ood volcanism: (i) the initiation of continental rifting and break-up of Afro-Arabia, (ii) the formation of the Ethiopian rift, and (iii) the initiation of sea £oor spreading in both the Gulf of Aden and the Red Sea. Additionally, the northernmost pro¢les cover the £ood volcanics in Ethiopia which were juxtaposed with northern Yemen, near Sana’a, prior to rifting. Pro¢les A and B are from two sections along the northern sector of the western escarpment (Fig. 2), which exposes bimodal £ood volcanics thought to be contemporaneous with the oldest continental £ood volcanism in Yemen. Overlying alkaline shield lavas and rift-related silicic volcanics were also sampled to constrain the timing of di¡erent modes of volcanic activity. Pro¢les C and D are located at the bend in orientation of the escarpment : the transition region from the N^ S oriented western escarpment to the NE^SW ori-

entation of the northern Main Ethiopian Rift (Fig. 2). Pro¢le E is located in the northern Main Ethiopian Rift and samples younger volcanics including Pliocene eruptions. These sections are thought to span the range of pre- to syn-rift volcanism related to break-up of AfroArabia, the initiation of volcanism in Afar, and the establishment of sea£oor spreading in the Gulf of Aden and the Red Sea. 3.2. Analytical techniques Seventeen units of silicic ignimbrite and basalt (Table 1) were selected for dating from a set of 137 samples. Potassium feldspar (both sanidine and anorthoclase), plagioclase, and phlogopite were hand-picked from crushed and sieved separates of ignimbrites and basaltic lava £ows (125 to 250 Wm sieve size). In addition, groundmass was hand-picked from two basaltic units. The samples were screened to determine suitability for 40 Ar/ 39 Ar radiometric dating on the basis of petrography, major and trace element geochemistry and microprobe analysis of phenocryst phases. The

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degree of alteration and, for ignimbrites and tu¡s, homogeneity of feldspar populations were also considered, to minimize possible xenocrystic contamination. Samples were loaded into wells in an Al disk and irradiated for 20 h in the TRIGA reactor at Oregon State University following the methods established by Renne et al. [32]. Fish Canyon sanidine (FCs) was used as a neutron £ux monitor (FCs: 28.02 Ma [32]) and irradiated in four positions horizontally spanning the unknowns in the irradiation vessel. For comparative purposes, all data referred to here from other literature sources have been recalculated to re£ect the age of FCs at 28.02 Ma as recommended by Renne et al. [32]. The calculated mean J value was 0.0051555 þ 0.0000170 where the uncertainty re£ects total variation in neutron £ux over the Al disk. Samples were analyzed at the Berkeley Geochronology Center using CO2 and Ar-ion lasers in four run batches. Laser heating of multi-grain samples probably results in less uniform heating than of single crystals, but we estimate that extraction temperatures within each sample were homogeneous to within þ 50‡C for each step during these experiments. Homogeneous extraction temperatures are evidenced by the progressive increase in %40 Ar* throughout each experiment (see Table 3 in the Background Data Set1 ). Argon isotopic analysis was performed on MAP 215C and MAP 215^50 mass spectrometers. The sample analysis methods included singlecrystal total fusion (11 experiments), single- and multi-crystal step-heating of 5^30 grains of sanidine, plagioclase and phlogopite (12 experiments), and multi-grain step-heating of 8^12 grains for groundmass samples (six experiments). A total of 90 grains of sanidine, plagioclase and phlogopite were analyzed by single-crystal total fusion. Four samples of sanidine, plagioclase and phlogopite were analyzed by multiple methods to constrain variations in accuracy and precision for different techniques and mineral phases. When multiple experimental methods were applied to the same sample, calculated ages were weighted

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mean isochron ages of all steps from all techniques used (excluding two xenocrystic grains of sanidine and plagioclase from single-crystal total fusion experiments, EEWB22 and EEWB9). Mass discrimination for the four runs, monitored by analysis of 177 air pipettes interspersed with the unknowns, ranged from 1.0021 þ 0.0012 to 1.0048 þ 0.0025 per atomic mass unit. Average procedural blanks for these analyses were comparable to those reported by Renne [33] and Renne et al. [34] (for laser samples, and were typically 6 1% of the measured signal for 40 Ar). One sample, EIU99010, produced age spectra representative of bimodal mixing of components of di¡erent ages even during single-crystal step-heating experiments and was thought to contain xenocrystic feldspars which had partly re-equilibrated with a host magma, and as such was excluded from further consideration, as precise age information is not provided by the data. All other samples produced precise plateau ages, described below and presented in Tables 1 and 2 and Background Data Set Table 31 .

4. Analytical results: whole-rock ages

40

Ar/39 Ar mineral and

Flood volcanic rocks in Ethiopia contain a signi¢cant component of rhyolitic airfall tu¡s and ignimbrites intercalated with basaltic lava £ows. For the silicic units, potassium feldspar from 11 ignimbrite samples provides precise and reliable 40 Ar/39 Ar dates of marker units within otherwise di⁄cult to date sections of aphyric or altered basaltic units. Furthermore, the silicic units are clearly identi¢able as extrusive eruptions and not intrusions, whereas for some of the ma¢c units it is di⁄cult to distinguish lava £ows from sills. For the basaltic units, separates of plagioclase (n = 2), phlogopite (n = 1) and groundmass (n = 2) from relatively unaltered lavas were selected to constrain the age of ma¢c magmatism spanning the pre- and syn-rift volcanic stratigraphy. All samples produced stratigraphically consistent ages and yielded plateaux for step-heated samples or homogeneous distributions for singlecrystal total fusion samples (Tables 1 and 2 and

EPSL 6161 26-4-02

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wr wr wr wr

pl pl pl pl

pl pl pl pl

EEWB20 EEWB20 EEWB20 EEWB20

EEWB22 EEWB22 EEWB22 EEWB22

EEWB9 EEWB9 EEWB9 EEWB9

EEWR4

san

Robit: Pro¢le C EEWR1 san EEWR1 san EEWR1 san

Ataye: Pro¢le B EEWB25 san

san

EEWB7

ignimb

ignimb

ignimb

basalt

basalt

basalt

ignimb

ignimb

multi-grain step Ar multi-grain step Ar multi-grain step Ar weighted average

9.845, 39.768

9.967, 39.918

CO2

multi-xtl step

CO2

multi-xtl step CO2 single-xtl tf CO2 weighted average

10.362, 39.932 single-xtl tf

11.083, 39.683 single-xtl tf Ar multi-xtl step CO2 multi-xtl step CO2 weighted average

11.088, 39.633 single-xtl tf Ar multi-xtl step CO2 multi-xtl step CO2 weighted average

11.27, 39.507

CO2

tf CO2 step CO2 step CO2 average

11.092, 39.766 single-xtl tf

11.269, 39.516 single-xtl single-xtl single-xtl weighted

CO2

san san san san

ignimb

EIU99029 EIU99029 EIU99029 EIU99029

Laser

11.268, 39.516 multi-xtl step

Analysis method

san

Location (‡N, ‡E)

EIU99035

Deposit

11.193, 40.025 single-xtl tf Ar ion multi-xtl step CO2 multi-xtl step CO2 weighted average

Phase

Desi-Bati: Pro¢le A EEWB1 phlo trachyte EEWB1 phlo EEWB1 phlo EEWB1 phlo

Sample No.

Table 1 40 Ar/39 Ar dating results from Ethiopian pre- and syn-rift volcanic units

29.65 þ 0.17 29.6 þ 0.4 29.1 þ 0.4 29.45 þ 0.32

29.73 þ 0.14 29.0 þ 0.4 29.1 þ 0.5 29.46 þ 0.17

14.91 þ 0.06

19.70 þ 0.07 19.68 þ 0.08 19.69 þ 0.07

25.31 þ 0.11

14.91 þ 0.07

19.69 þ 0.07 19.67 þ 0.09 19.68 þ 0.08

25.28 þ 0.13

25.0 þ 0.8 25.0 þ 0.6 24.5 þ 0.4 24.8 þ 0.6

25.1 þ 0.80

24.30 þ 0.4 25.6 þ 0.6 24.1 þ 0.5 21.8 þ 0.5 23.3 þ 0.3

25.6 þ 1.6 24.9 þ 0.4

29.36 þ 0.17 29.4 þ 0.2 29.28 þ 0.19 29.35 þ 0.19

25.6 þ 1.4 24.2 þ 0.4

29.48 þ 0.14 29.43 þ 0.15 29.55 þ 0.14 29.49 þ 0.11

29.43 þ 0.15

30.08 þ 0.12

30.07 þ 0.10

29.43 þ 0.13

30.86 þ 0.12 31.0 þ 0.2 31.6 þ 0.7 31.15 þ 0.34

Plateau age

30.84 þ 0.11 31.1 þ 0.2 32.0 þ .08 30.95 þ 0.12

Total gas age

81.2 88.3 71.4

na

na 85.67 100

100

na 91.9 100

%Ar rad.

9/12

7/11 7 of 7

7 of 7

83.4

86.9 na

na

11 of 12 na 14/15 89.6 14/15 87.7

10 of 11 na 29/30 93.1

7/9 7/9 6/9

6 of 6

6 of 7 6/15 15/15

11/11

8 of 8 13/20 9/9

Plateau steps/ No. of grains

14.89 þ 0.06

19.70 þ 0.07 19.67 þ 0.08 19.69 þ 0.07

25.28 þ 0.11

24.9 þ 0.5 24.5 þ 0.3 24.5 þ 0.3 24.5 þ 0.2

24.90 þ 0.3

25.6 þ 1.3 24.9 þ 0.3

29.48 þ 0.14 29.43 þ 0.15 29.55 þ 0.14 29.53 þ 0.11

29.43 þ 0.12

29.69 þ 0.14 29.4 þ 0.27 29.4 þ 0.27 29.63 þ 0.13

30.08 þ 0.10

30.86 þ 0.11 31.06 þ 0.18 31.06 þ 0.18 30.88 þ 0.11

Age probability

14.90 þ 0.06

19.77 þ 0.07 19.57 þ 0.14 19.76 þ 0.07

25.30 þ 0.13

25.5 þ 1.2 24.9 þ 0.2 24.9 þ 0.2 25.0 þ 0.2

25.1 þ 0.2

28.5 þ 1.1 25.1 þ 0.2

29.4 þ 0.3 29.4 þ 0.3 29.4 þ 0.3 29.34 þ 0.15

29.68 þ 0.15

29.91 þ 0.11 29.47 þ 0.14 29.47 þ 0.14 29.61 þ 0.12

30.16 þ 0.13

30.92 þ 0.12 31.05 þ 0.18 31.05 þ 0.18 30.92 þ 0.11

Isochron age

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san

wr wr wr wr

EEWA4

EEWA3 EEWA3 EEWA3 EEWA3

EPSL 6161 26-4-02 9.023, 39.559

Ar

Ar

CO2

Laser

single-xtl tf

single-xtl tf CO2

Ar

multi-grain step Ar multi-grain step Ar multi-grain step Ar weighted average

single-xtl tf

single-xtl tf

multi-xtl step

Analysis method

10.55 þ 0.07 3.22 þ 0.05

3.23 þ 0.03

10.73 þ 0.16 10.76 þ 0.15 10.87 þ 0.11 10.78 þ 0.14

11.61 þ 0.05

11.64 þ 0.05

11.69 þ 0.05

Plateau age

10.56 þ 0.06

10.69 þ 0.19 10.68 þ 0.13 10.75 þ 0.17 10.69 þ 0.09

11.61 þ 0.05

11.63 þ 0.04

11.69 þ 0.04

Total gas age

100 51.6 100

na

na

98.1

%Ar rad.

4 of 7

na

11 of 11 na

9/9 4/9 7/7

7 of 7

7 of 7

8/11

Plateau steps/ No. of grains

3.22 þ 0.03

10.53 þ 0.05

10.73 þ 0.12 10.82 þ 0.08 10.87 þ 0.09 10.80 þ 0.06

11.61 þ 0.05

11.64 þ 0.04

11.7 þ 0.04

Age probability

3.19 þ 0.04

10.58 þ 0.07

10.87 þ 0.06 10.87 þ 0.06 10.87 þ 0.06 10.87 þ 0.06

11.59 þ 0.06

11.73 þ 0.05

11.70 þ 0.04

Isochron age

Samples are grouped on the basis of geographically distinct stratigraphic sections and within each group are presented in ascending stratigraphic order from oldest to youngest. Fig. 3 illustrates representative age spectra of the mineral separates and multi-grain experiments. Abundances of ¢ve argon isotopes were measured [36^40], and the 40 Ar/39 Ar ratio corrected for the presence of atmospheric argon as well as interfering isotopes produced by irradiation of K, Ca and Cl. Full details of the analytical technique are given in Renne et al. [32]. Plateaux are de¢ned as three or more steps which are all statistically equivalent at the 2c level and which contain s 50% of 39 Ar released. Plateau ages are calculated from the error-weighted mean of F values (40 Ar*/39 ArK ) for steps de¢ning the plateau. All data for all experiments are presented: integrated spectra, plateau age, age probability and isochron age. For 15 samples, the average isochron age for all experiments is taken as the experimentally determined age, for EEWB7 the age probability has a smaller error and is taken as the measured age. Section lists stratigraphic section of samples, see Fig. 2 for location map. Location is in decimal longitude N and latitude E. Analysis method indicates type of experiment run: single-crystal total fusion, single-crystal step-heating, multi-crystal or multi-grain step-heating. Either an Ar ion or CO2 laser was used for each experiment, this is indicated for each experiment. Plateau steps as a ratio refers to the number of steps used in calculating plateau ages (ie. 7/7 = seven steps out of seven total) and the percent 39 Ar re£ects the amount of 39 Ar in each plateau calculation (100% means all steps with all Ar were used). Samples with text notation in plateau steps column (i.e. 7 of 7) refer to single-crystal total fusion experiments and the number of single crystals used to calculate ‘plateau’ ages for each group of grains analyzed. As such, the percent 39 Ar for these types of experiments is not relevant and has been annotated with ‘na’. ‘san’ = sanidine, ‘wr’ = whole-rock groundmass, ‘pl’ = plagioclase, ‘phlo’ = phlogopite, tf = total fusion.

ignimb

EEWK5

san

9.04, 39.503

9.917, 39.733

Kessem River: Pro¢le E EEWK2 san ignimb

basalt

ignimb

9.646, 39.58

9.558, 39.761

ignimb

san

Location (‡N, ‡E)

EIU99075

Deposit

9.562, 39.783

Phase

Ankober: Pro¢le D EIU99055 san ignimb

Sample No.

Table 1 (Continued)

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Fig. 3. 40 Ar/39 Ar age spectra and age probability diagrams obtained from mineral separates and groundmass from all dated Ethiopian volcanic rocks. All mineral separates have relatively undisturbed spectra. Few have minor quantities of excess Ar. Two samples of feldspar, analyzed by single-crystal total fusion experiments, contain a signi¢cantly older xenocryst phase, which was removed from the sample sets for age calculations. The most precise ages were obtained from the potassium-feldspars (K-feldspar); a phlogopite (EEWB1) from a trachytic lava also provided a very well-constrained plateau and isochron age date. Plagioclase and whole-rock mineral separates have similar 2c errors and in many cases were comparable to K-feldspar. Age spectrum diagrams are shown for multi-crystal and multi-grain step-heating experiments, steps used to calculate plateau age are indicated with an arrow and plateau age is shown. Age probability diagrams are shown for single-crystal total fusion experiments. The age for the data peak is shown, as are individual data points and associated errors, and the average age for all data from the sample. Age spectrum diagrams: (A) plateau diagram for EIU99035 sanidine multi-crystal step-heating experiment. (B) Plateau diagram for EIU99029 sanidine single-crystal step-heating experiment. (C) Plateau diagram for EEWB20 whole-rock basalt groundmass multi-grain step-heating experiment. (D) Plateau diagram for EEWR4 sanidine multi-crystal step-heating experiment. (E) Plateau diagram for EIU99055 sanidine multi-crystal step-heating experiment. (F) Plateau diagram for EEWA3 whole-rock basalt groundmass multi-grain step-heating experiment. (G) Isochron diagram of all analyses of EEWB1 phlogopite, eight single-crystal total fusion and two multi-crystal step-heating experiments of 20^30 grains each. Age probability diagrams: (H) Single-crystal total fusion of six sanidine grains from EEWB7. (I) Single-crystal total fusion of plagioclase from EEWB22. (J) Single-crystal total fusion of seven sanidine grains from EEWB25. (K) Single-crystal total fusion of sanidine from EEWR1. (L) Single-crystal total fusion of seven sanidine grains from EIU99075. (M) Single-crystal total fusion of seven sanidine grains from EEWA4. (N) Singlecrystal total fusion of 11 plagioclase grains from a basalt, EEWK2. (O) Single-crystal total fusion of seven sanidine grains from EEWK5. (P) Single-crystal total fusion of 12 grains of plagioclase from a basalt, EEWB9, showing an average age of 25.0 Ma for 11 mineral grains and a xenocrystic grain producing an age of 51.37 Ma.

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297

Fig. 3 (Continued). Table 2 Summary of Sample

40

39

Ar/ Ar age data Phase

Dese-Bati: Pro¢le A EEWB1 phlogopite EIU99035 sanidine EIU99029 sanidine EEWB7 sanidine EEWB20 wr groundmass EEWB22 plagioclase EEWB9 plagioclase Ataye: Pro¢le B EEWB25 sanidine Robit: Pro¢le C EEWR1 sanidine EEWR4 sanidine Ankober: Pro¢le D EIU99055 sanidine EIU99075 sanidine EEWA4 sanidine EEWA3 wr groundmass Kessem River: Pro¢le E EEWK2 sanidine EEWK5 sanidine

Age (Ma)*

30.92 þ 0.11 30.16 þ 0.13 29.61 þ 0.12 29.43 þ 0.12 29.34 þ 0.15 25.1 þ 0.2 25.0 þ 0.2 25.30 þ 0.13 19.76 þ 0.07 14.90 þ 0.06 11.70 þ 0.04 11.73 þ 0.05 11.59 þ 0.06 10.87 þ 0.06 10.58 þ 0.07 3.19 þ 0.04 * þ 2c S.D.

Background Data Set Table 31 ). Of 18 single-crystal, multi-crystal and multi-grain step-heating experiments, 16 had plateaux incorporating 80% or greater 39 Ar and ¢ve of them had concordant spectra for 100% of the gas released. Calculated isochron ages were within error (2c) of plateau ages for all samples and con¢rmed the presence of only minor excess Ar in some mineral separates. Fig. 3 illustrates all 40 Ar/39 Ar dating results obtained from 16 mineral separates and whole-rock groundmass from the Ethiopian pre- and syn-rift volcanic rocks, including multi-crystal and multigrain step-heating and single-crystal total fusion experiments. An isochron diagram of all analyses performed on phlogopite separated from a trachytic lava £ow EEWB1 is also shown, which includes eight single-crystal total fusion experiments and two multi-grain (V30 grains each) step-heating experiments of 14 and 15 steps each. Fig. 3P presents an important analytical result from a single-crystal total fusion experiment of plagioclase grains from a basaltic lava £ow (EEWB9). This experiment identi¢ed a single pla-

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gioclase crystal that produced an age of 51.37 Ma. This is signi¢cantly older than the average apparent age of 25.0 Ma for 11 other plagioclase grains. The plagioclase grains in this sample appear to be homogeneous and do not exhibit any visible xenocrystic cores or other disequilibrium features. However, this anomalous age is interpreted to represent a xenocrystic grain that has been mixed into the basaltic magma and not fully degassed prior to eruption. Typically, plagioclase mineral separates from basalts are 40 Ar/39 Ar dated by multi-crystal step-heating, not single-crystal total fusion. If this sample had been measured by multi-crystal step-heating the average apparent age produced from these 12 grains would have been 28.0 þ 0.6 Ma, 3 Myr older than the age calculated after removing the xenocryst ; this has highly signi¢cant implications for using 40 Ar/39 Ar dating to establish precise chronostratigraphies of rifted volcanic margins. If this xenocrystic grain had been accidentally included in an analysis, it would have added an extra 11% error to the average apparent age. Moreover, the error associated with the average apparent age (excluding the xenocryst) of þ 0.5 Myr for this single-crystal total

fusion experiment on EEWB9 is larger than the average error for all other samples in this study of þ 0.09 Myr. This larger error may be attributed to other xenocrystic plagioclase grains in this sample which had degassed but still not completely re-equilibrated with surrounding magma prior to eruption.

5.

40

Ar/39 Ar chrono-stratigraphy of the Ethiopian margin

5.1. Flood basalt volcanism: initiation and duration The 40 Ar/39 Ar chrono- and volcano-stratigraphies for northern Ethiopia are summarized in Fig. 4. Dated samples for each pro¢le are placed in a regional volcanic stratigraphy based on ¢eld work and tectonic reconstructions [31,35,36]. The oldest dated pre-rift £ood basalt unit is a phlogopite-bearing trachytic lava £ow sampled near the base of the volcanic section exposed in the northern sector of the western escarpment (Fig. 2), which produced a plateau age of 30.90 þ 0.11

Fig. 4. Dated samples located stratigraphically in sample pro¢les A to E. Ataye pro¢le B sampled an ignimbrite from the footwall of a fault block exposing riftward-dipping silicic volcanic rocks east of the marginal graben. Volcanic rock packages and stratigraphy thickness are presented as a general representation. Pro¢le thickness is based on ¢eld stratigraphy and tectonic reconstruction [36]. Dates presented here from Hofmann et al. [3] for the Lima Limo pro¢le are only those sampled from a continuous stratigraphic section and do not include samples whose locations were extrapolated from s 10 km away. Errors for all data presented are þ 2c.

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Ma (EEWB1). It should be noted that in this pro¢le the base of the volcanic pile is not exposed and thus the earliest lava £ow was not sampled. EEWB1 was collected from the lower portion of an exposed basaltic lava £ow sequence V200 m below the ¢rst ignimbrite. Based on a generalized stratigraphy of Ethiopia [8,13,21], and a well-constrained basal volcanic stratigraphy for Yemen [1,2,15,31], we estimate that the basal £ood basalt package underlying the ¢rst ignimbrite in this area could be between 500 and 1000 m thick. Possibly as much as 800 m of lava £ows could be below this lowest dated basalt sample. Basaltic lava £ows in this section have an average thickness of 30^40 m, and by using well-constrained eruption rates from Yemen (£ood basalt lavas erupted every 10^100 kyr [1]) and Ethiopia (£ood basalt lavas erupted every 50 kyr [4]), 800 m of £ood basalt lavas may have been emplaced over V200 kyr to V2 Myr. Consequently, Ethiopian basaltic £ood volcanism could have begun signi¢cantly earlier than 30.9 Ma. An earlier 40 Ar/39 Ar dating study of northern Ethiopian £ood volcanism [3] was based mainly on samples from the Lima Limo area (Fig. 1), V300 km to the northwest of our northernmost pro¢le A. Lima Limo is located towards the margin of the volcanic province and thus may not record the full pre-rift stratigraphy that is preserved closer to the rift margin. The 40 Ar/39 Ar dates [3] are reported in relation to the laboratory monitor standard Hb3Gr (for which an age of 1072 Ma was reported), in good agreement with the ages of 1074 þ 4 and 1075 þ 4 relative to FCs at 28.02 Ma reported in Renne et al. [32]. When these ages are recalculated to re£ect the 0.19% increase in age in reference to the Renne et al. [32] age for Hb3Gr, and their errors are recalculated to a 2c level, the dates are comparable to the oldest dated £ood volcanics in this study, although their associated analytical errors are signi¢cantly larger than those presented here (Fig. 4). Whereas their data suggest that the bulk of the traps were erupted at approximately 30 Ma within a period of 1 Myr or less, our volcano-stratigraphic pro¢les and dating indicate that along the rift margin in north central Ethiopia volcanism may have initiated signi¢cantly earlier, poten-

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tially as early as 32^33 Ma, with the greatest eruption rates and volumes occurring from V31 to 28 Ma, and the total duration of £ood volcanism was at least 4 Myr. The di¡erences in the duration of volcanism preserved in the Lima-Limo section and in our pro¢les along the escarpment indicate there may have been larger £ow volumes or a focusing of volcanism towards the rift margin during the initial stages of £ood volcanic eruptions. 5.2. Onset of silicic volcanism The ¢rst silicic ignimbrite in this part of Ethiopia was erupted at 30.2 Ma and bimodal £ood volcanism continued until 25.3 Ma (Fig. 4: pro¢les A and B). The initiation of bimodal volcanism in the Ethiopian £ood volcanic province occurs at least 700 kyr after the start of basaltic £ood volcanism. Silicic volcanism as young as 24 Ma in the region of pro¢le B is recorded by xenocrysts and conglomerate cobbles of reworked ignimbrite found in Pliocene deposits downstream, in the Central Awash Complex [34], but these ignimbrites have not been found in situ. Recent work on £ood volcanism in Ethiopia [3,4] has mentioned the striking coincidence between the timing of silicic volcanism during Ethiopian trap emplacement and the Oligocene (Oi2) global cooling event. This event occurred at the base of Chron C11r and is marked by a sudden shift to higher marine N18 O values, major development of the Antarctic ice shelf, and the largest Tertiary sea-level drop [5,6]. Tephra layers found in ODP leg 115 drill cores in the Indian Ocean, biostratigraphically located at the base of Chron C11r [7], have been linked to Afro-Arabian rhyolitic volcanism because of this temporal coincidence [3,4]. However, our data and that of Baker et al. [1] establish that silicic £ood volcanism initiated in both Yemen and Ethiopia at 30.2 þ 0.1 Ma, which post-dates the Oligocene cooling event at the base of Chron C11r (31.2 þ 0.7 Ma [37,38]). Depending on the accuracy of the time scale calibration, and assuming that we have analyzed the ¢rst silicic ash £ow erupted, this asynchrony suggests that the cooling event would need to be triggered by the earlier basaltic volcanism if in-

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deed there is a genetic link between these phenomena. However, the massive silicic pyroclastic eruptions with volumes s 200 km3 [15,31] may have reinforced already turbulent climatic conditions. 5.3. Initiation of syn-rift volcanism Following a dramatic decrease in volcanism at V25 Ma, sporadic syn-rift bimodal volcanism is preserved in pro¢les C^E (Fig. 4). Pro¢le C spans an area where the tectonic regimes of the Red Sea Rift and the northern Main Ethiopian Rift intersect, resulting in a bend in the orientation of the western escarpment. EEWR1 is the lowermost ignimbrite in the footwall of a westward-dipping fault block that comprises riftward-dipping synrift silicics. EEWR4 unconformably overlies plateau £ood volcanics and may represent some of the youngest deposits of syn-rift silicic volcanics erupted onto and capping the signi¢cantly older plateau stratigraphy. These two silicic ignimbrites produced isochron ages of 19.76 þ 0.07 and 14.90 þ 0.06 Ma (EEWR1 and EEWR4). The Kessem pro¢le (E), located in the northern Main Ethiopian Rift, represents a well-constrained stratigraphic section containing a major angular unconformity and volcanic hiatus. A bimodal section of silicic lavas and ignimbrites plus basaltic lavas dips 20^25‡ to the N-NE. This is capped by an angular unconformity and overlain by a £at-lying coarse £uvial conglomerate and ignimbrite. The ignimbrite contains V30^ 50% loose pebbles and cobbles from the underlying conglomerate in the lowermost 1.5 m, indicating that the sedimentary unit was unconsolidated when the ignimbrite was emplaced. Two ignimbrites were dated in this pro¢le, one from beneath the unconformity and the conglomerate-clast-bearing ignimbrite directly overlying the unconformity. These yield isochron ages of 10.58 þ 0.07 and 3.19 þ 0.04 Ma, respectively, which bracket the unconformity (EEWK2 and EEWK5). 5.4. Temporal and spatial variation of magmatism along the Ethiopian margin Fig. 5 illustrates the temporal and spatial sys-

Fig. 5. Temporal and spatial variation in volcanism along and across the main Ethiopian escarpment. Note that from north (A) to south (E) a systematic and progressive decrease is observed in the age of the exposed laterally equivalent volcanic rocks. Oligocene volcanism is dominant in the north and Miocene to recent volcanism in the south.

tematics of volcanism along the Ethiopian escarpment. The timing of volcanism represented in pro¢les A through E reveals a progressive north to south decrease in age. The apparent linear younging of laterally equivalent volcanics along the escarpment may be a re£ection of continued and more focused volcanism approaching the northern Main Ethiopian Rift as well as enhanced preservation through volcanic loading of the Ethiopian margin. This systematic younging may re£ect shifts in loci of volcanism that are related to rifting episodes and basin formation during the initiation of continental break-up (see [36]). Pro¢le A contains the oldest £ood volcanic rocks with an age range from 31 to 25 Ma. Pro¢le B has only one dated sample but contains £ood volcanic rocks that stratigraphically correlate with the upper part of pro¢le A. The apparent break in volcanism between pro¢les B and C is related to the initiation of syn-rift volcanism. There is an observable change in eruption and emplacement style from the pre- to syn-rift volcanics. Pre-rift lavas and ignimbrites are laterally extensive, largevolume £ood volcanic deposits, whereas syn-rift volcanics are small-volume, localized deposits

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usually associated with volcanic centers which occur in margin-parallel rift segments ranging from 60 to 100 km in length. Pro¢le C is composed of younger, sporadically erupted syn-rift volcanics (19.8^14.9 Ma). These volcanic deposits may be the southernmost expression of Red Sea rifting and are tectonically overprinted by northern Main Ethiopian Rift faulting [35]. Pro¢le D is composed of near-vent syn-rift silicic volcanics and spans 10.9^11.7 Ma. The youngest volcanism is found in pro¢le E, furthest to the south, with dates from a basalt (10.6 Ma) and an overlying ignimbrite (3.2 Ma). These two units are separated by an angular unconformity that represents a 7.5 Myr break in volcanism. The hiatus occurs during a period when sea£oor spreading initiated in the Gulf of Aden (10 Ma) and the Red Sea (5 Ma) [10,11]. A volcanic hiatus from 9 to 4 Ma is also found throughout the Arabian plate, in intraplate volcanic ¢elds in Jordan, Saudi Arabia and Yemen [39^42]. Chernet et al. [12] found that from 7 to 3 Ma, during the earliest phase of modern rift margin development throughout the central and northern Main Ethiopian Rift and southern Afar, there was a period of focused volcanism in small silicic centers which formed in a marginal graben along the eastern escarpment of the northern Main Ethiopian Rift. WoldeGabriel et al. [43] also report interbedded hominid fossil-bearing sediments with a few basaltic lava £ows and thin silicic (3^20 cm) and basaltic (V20 cm to 2^3 m) tephra layers dated at 5^6 Ma from the Middle Awash area of Afar. Also, by the late Miocene, both basaltic and silicic eruptions were con¢ned to both sides of the rift margin in the northern and central sectors of the Main Ethiopian Rift [20,21,44^46]. A shift from a di¡use strain regime in Afar and Arabia during early syn-rift volcanism (20^10 Ma) to localized strain when sea£oor spreading initiated ¢rst in the Gulf of Aden and then in the Red Sea [10,11] may have resulted in a change in the active stress ¢eld which disrupted large-volume magmatism along the conjugate margins and resulted in localized magmatic activity feeding volcanic centers along rift margins which were active during continental rifting.

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5.5. Matching conjugate margins in Yemen and Ethiopia The extent to which di¡erent 40 Ar/39 Ar data sets can be critically compared depends upon accurate intercalibration between samples, as well as to primary and laboratory standards. For accurate and precise comparisons, ages which are calculated with standards other than FCs at 28.02 Ma (which was used in our study) must have primary standard ages re-calculated to re£ect a consistent age for FCs [32]. All 40 Ar/39 Ar dates from other studies used for comparative purposes here have been recalculated to re£ect a monitor mineral age in accordance with that of FCs at 28.02 Ma. Baker et al. [1] and George et al. [23] used monitor standard MMhb-1 at 520.4 Ma, and recalculation makes their data approximately 0.5% older. Hofmann et al. [3] and Rochette et al. [4] both used Hb3Gr at 1072 Ma, and their ages recalculate to approximately 0.19% older; and Chernet et al. [12] used FCs at 27.84 which makes their data 0.6% older. Fig. 6 summarizes a total of 96 40 Ar/39 Ar dates for Afro-Arabian volcanism. The data from this study are presented and compared with published data from Ethiopia, Djibouti, Sudan and Yemen [1,3,4,12,22,23,30,47,48]. The break-up unconformity found in Yemen (26.5^19.7 Ma) and the dramatic decrease in volcanism shown in our dataset (24^20 Ma) appear to represent a regional event. Apatite ¢ssion track studies reveal that rapid crustal cooling occurred between 25 and 17 Ma in rocks underlying the £ood volcanic units proximal to the rift margin in Yemen [2]. This period of break-up, extension and presumably tectonically driven exhumation matches very well with the 26.7^19.9 Ma break-up unconformity in Yemen [1] and the observed decrease in volcanism in Ethiopia from 25.3 to 19.8 Ma (Figs. 6d and 7). It should be noted that we cannot rule out sampling bias, preferential erosion, or burial of volcanic units to explain the apparent gap in volcanism from V24 to 20 Ma. However, our preferred explanation is a dramatic decrease in volcanism, and this is corroborated by compiled 40 Ar/39 Ar dating studies in the same region which support our ¢ndings and indicate sampling bias is not a factor. No

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set to put a complete pre- to syn-rift volcanic suite of 40 Ar/39 Ar data into volcano- and tectonostratigraphic context. Comparing the volcano-stratigraphy of the conjugate margins of Ethiopia and Yemen highlights the similarities of the province (Fig. 7). In Yemen, the basal volcanic rocks, in contact with basement sandstones, vary in age from 30.9 Ma (south) to 29.4 Ma (north) in tandem with a variation in thickness from 50^300 m (north) to 1000 m (south) [1]. In Ethiopia, basal volcanic rocks

Fig. 6. 40 Ar/39 Ar age probability plots for 96 compiled mineral separate and whole-rock dates of pre- and syn-rift volcanic activity in Afro-Arabia. Flood volcanism spans 31^24 Ma, with a peak in ages, and inferred peak in volcanism, at 30 Ma. The largest peak contains 19 ages. The Yemen unconformity and hiatus in Ethiopian volcanism correspond well at 25^20 Ma. Sporadic syn-rift volcanic activity occurs from 25 to 11 Ma, and a regional hiatus in Ethiopia corresponds to the timing of the Kessem River Gorge unconformity from 10 to 3 Ma. Chernet et al. [12] recognized small silicic centers active from 7 to 3 Ma, formed in a marginal graben along the Main Ethiopian Rift. This localized volcanism falls in the time represented by the angular unconformity found in the Kessem River Gorge section. Data compiled from Ethiopia, Djibouti, Sudan and Yemen age dating projects [1,3,4,12,22,23,30,47,48].

evidence has been observed for signi¢cant erosion, and near the bend in the western escarpment volcanic rocks in contact with basement have been dated at V24 Ma [12], indicating that the complete volcanic stratigraphy is preserved. Our main sampling pro¢les are laterally spaced from 20 to 30 km along the Main Ethiopian Rift and, given the challenges of ¢nding continuous sections due to the generally poor exposure and access di⁄culties, this study represents the best available data

Fig. 7. Schematic stratigraphic diagram illustrating a generalized volcano-stratigraphy for Ethiopia as presented in this work and related to the £ood volcanic stratigraphy of Yemen. The break-up unconformity preserved in Yemen correlates with a decrease in volcanism in Ethiopia during early syn-rift volcanism. Apatite ¢ssion track data shows a contemporaneous period of rapid uplift and exhumation in Yemen from 25 to 17 Ma [2]. Sporadic syn-rift volcanism is preserved in Ethiopia but not in Yemen.

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vary from s 30.9 Ma (pro¢le A, not in contact with basement) to 24 Ma (near pro¢le D and in contact with basement [12]). When additional undated £ood basalt lavas are taken into account in the northern Ethiopian sections, initiation of volcanism in northern Ethiopia predates that of Yemen, by several hundred thousand years and potentially up to V2 Myr based on well-constrained eruption rates and estimated section thickness. The di¡erence in timing of initiation of basaltic volcanism for comparable localities on the conjugate margins could be V1.5 Myr or more, depending on plate reconstruction models juxtaposing northern Yemen and Ethiopia. Baker et al. [1] proposed that initial silicic volcanism commenced at 29.5^29.2 Ma throughout Yemen. However, new ¢eld data (Ukstins unpublished data) indicate there is an older poorly preserved silicic volcanic component (ignimbrite and airfall tu¡s). Based on dated basaltic lavas (from [1]) intercalated with the oldest ignimbrite unit now recognized in northern Yemen, silicic volcanism initiated at V30 Ma, which is comparable to the date of 30.2 Ma for the ¢rst rhyolitic ignimbrite in northern Ethiopia. Bimodal volcanism ended at 26.7 Ma in Yemen, whereas basaltic and rhyolitic volcanism continues to the present day in Ethiopia. The initiation of break-up of Afro-Arabia at V26 Ma resulted in a volcanic hiatus in Ethiopia during the earliest stages of syn-rift volcanism (24^20 Ma) and represents a change from the large-volume pre-rift £ood volcanism exposed in the north to smaller volume syn-rift volcanism in the south. Syn-rift volcanics were sporadically erupted and associated with laterally restricted volcanic centers. The transition from pre- to syn-rift volcanism is preserved in Yemen as a break-up unconformity that juxtaposes 26.7 Ma basalts with a 19.9 Ma trachytic lava £ow [1]. While we have not found evidence for such a distinct contact in Ethiopian volcanics, the decrease in volcanic activity coupled with changes in eruptive mechanisms, rift geometry and architecture indicates that break-up of Afro-Arabia initiated at V26 Ma. The conjugate margins of Ethiopia and Yemen share a similar volcano-stratigraphy representing

303

the primary stages of £ood volcanism, an initial sequence of basaltic lava £ows overlain by intercalated silicic ignimbrites, airfall tu¡s and basaltic lava £ows. The duration of eruption for the phases of volcanism are similar also: initial basaltic £ood volcanism spanned s 0.7 Myr in Ethiopia and V1.0 Myr in Yemen, bimodal volcanism spanned 5.2 Myr in Ethiopia and s 3.3 Myr in Yemen (with an estimated 2 km of erosion of overlying material [2]). Di¡erences in syn-rift tectono-volcanic activity resulted in continued bimodal volcanism and preservation of these deposits in Ethiopia, while Yemen may have had only minor syn-rift volcanism, indicated by two periods of dike emplacement at 25.5 and 18.5^16 Ma [30], which could have been removed during a period of major denudation and erosion from 25 to 17 Ma [2]. The di¡erences in tectono-volcanic evolution of the conjugate rifted margins can be accounted for in large part by the disposition of the Afar plume with respect to the rifting plates. As Afro-Arabia broke up, Arabia rifted away from the plume axis while the Ethiopian margin and Afar remained above it. The syn-rift volcanism in Ethiopia is a re£ection of the continuing plume in£uence under Afar. In Yemen, minor syn-rift volcanism and regional denudation and erosion re£ects the continued rifting of Arabia away from the axis, and sphere of in£uence, of the Afar plume.

6. Summary A detailed 40 Ar/39 Ar chronological study of pre- and syn-rift £ood volcanism in Ethiopia and a comparison to the tectono-volcanic evolution of the Yemen conjugate rifted margin have allowed us to address the timing and duration of £ood volcanism in relation to major tectonic events during rifting at the Afro-Arabian triple junction. Our main conclusions are: 1. Basaltic volcanism initiated in northern Ethiopia prior to 30.9 Ma, perhaps by as much as 200 kyr to 2 Myr, whereas initiation of basaltic volcanism is well-constrained in Yemen to be no earlier than 30.9 Ma.

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2. Bimodal ma¢c^silicic volcanism initiated in northern Ethiopia at 30.2 Ma, similar to Yemen at V30 Ma, and lasted for 5.2 Myr, switching to syn-rift volcanism by 25.3 Ma. The duration of preserved £ood volcanism in Yemen is 3.3 Myr, but erosion of V2 km of material [2] may have removed additional prerift as well as any syn-rift volcanism erupted there. Silicic volcanism initiated from 200 kyr to 1.8 Myr after the Oligocene Oi2 global cooling event. 3. Volcanism along the Ethiopian escarpment shows a broadly linear decrease in age from north to south, which may re£ect focusing of continued syn-rift volcanic activity towards the northern Main Ethiopian Rift. 4. A dramatic decrease in volcanism in Ethiopia from 24 to 20 Ma correlates with a break-up unconformity in Yemen from 26.7 to 19.9 Ma and represents the shift in eruption rates and mechanisms during early syn-rift volcanism and the initial stages of Afro-Arabian continental rifting. 5. Contrasting margin evolution is expressed through syn-rift volcanism: Ethiopia is characterized by sporadic bimodal silicic and basaltic syn-rift volcanism associated with small volcanic centers; in Yemen syn-rift volcanism is absent, although two sets of dike swarms along the Red Sea margin dated at 25.5 Ma and 18.5^16 Ma [30] indicate small-volume synrift volcanism may have been emplaced and later eroded. Di¡erences in volume and preservation of syn-rift volcanism may be, in large part, related to the location of the rifting conjugate margins with respect to the Afar plume axis. 6. An angular unconformity in the northern Main Ethiopian Rift spanning 10.6^3.2 Ma may be related to the initiation of sea£oor spreading in the Gulf of Aden and southern Red Sea.

Acknowledgements I.A.U. acknowledges support from Royal Holloway and the Danish Lithosphere Centre in a

joint Ph.D. studentship. Tim A. Becker, Warren D. Sharp and Abdur-Rahim Jaouni are thanked for assistance with sample preparation and 40 Ar/ 39 Ar dating at BGC. Zemenu Geremew, Gezahegn Yirgu (University of Addis Ababa), Cindy Ebinger (Royal Holloway University of London) and Ketsela Tadesse (Ethiopian Petroleum Institute) are thanked for logistical help and generosity of their time in Ethiopia. P.R.R. acknowledges support from NSF grant EAR-9909517; E.W. acknowledges support from NER/T/S/2000/00647. This manuscript has bene¢ted from discussions with David W. Peate and Kim B. Knight. We thank Robert Duncan, Gilbert Fe¤raud, Andy Saunders and Giday WoldeGabriel for comments on an earlier version of this manuscript.[BW]

References [1] J. Baker, L. Snee, M. Menzies, A brief Oligocene period of £ood volcanism in Yemen: implications for the duration and rate of continental £ood volcanism at the AfroArabian triple junction, Earth Planet. Sci. Lett. 138 (1996) 39^55. [2] M. Menzies, K. Gallagher, A. Yelland, A. Hurford, Volcanic and non-volcanic rifted margins of the Red Sea and Gulf of Aden: crustal cooling and margin evolution in Yemen, Geochim. Cosmochim. Acta 61 (1997) 2511^2527. [3] C. Hofmann, V. Courtillot, G. Fe¤raud, P. Rochette, G. Yirgu, E. Ketefo, R. Pik, Timing of the Ethiopian £ood basalt event and implications for plume birth and global change, Nature 389 (1997) 838^841. [4] P. Rochette, E. Tamrat, G. Fe¤raud, R. Pik, V. Courtillot, E. Ketefo, C. Coulon, C. Hofmann, D. Vandamme, G. Yirgu, Magnetostratigraphy and timing of the Oligocene Ethiopian traps, Earth Planet. Sci. Lett. 164 (1998) 497^ 510. [5] S. Pekar, K.G. Miller, New Jersey Oligocene ‘Icehouse’ sequences (ODP Leg 150X) correlated with global N18 O and Exxon eustatic records, Geology 24 (1996) 567^570. [6] B.U. Haq, J. Hardenbol, P.R. Vail, The chronology of £uctuating sea level since the Triassic, Science 235 (1987) 1156^1165. [7] H. Okada, Quaternary and Paleogene calcareous nannofossils, Leg 115, ODP Sci. Res. 115 (1990) 129^174. [8] P. Mohr, Ethiopian £ood basalt province, Nature 303 (1983) 577^584. [9] G. Fe¤raud, V. Zumbo, A. Sebai, 40 Ar/39 Ar age and duration of tholeiitic magmatism related to the early opening of the Red Sea Rift, Geophys. Res. Lett. 18 (1991) 195^ 198. [10] J.R. Cochran, The Gulf of Aden: structure and evolution

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