Matching conjugate volcanic rifted margins: Ar chrono ...esmenzies/PUBLICATIONS/2002_Ukstins_et...Matching conjugate volcanic rifted margins: 40Ar/39Ar chrono-stratigraphy of pre-

Matching conjugate volcanic rifted margins:40Ar/39Ar chrono-stratigraphy of pre- and syn-rift bimodal

£ood volcanism in Ethiopia and Yemen

Ingrid A. Ukstins a;b;�, 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, UKb Danish Lithosphere Centre, Òster Voldgade 10 L, DK-1350 Copenhagen K, Denmark

c Berkeley Geochronology Centre, 2455 Ridge Road, Berkeley, CA 94709, USAd 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

40Ar/39Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from thepre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemenconjugate rifted volcanic margins. Sixteen new 40Ar/39Ar dates confirm that basaltic flood volcanism in Ethiopia wascontemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that floodvolcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen byV0.2^2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyoliticignimbrite unit in Yemen at V30 Ma. Accurate and precise 40Ar/39Ar dates on initial rhyolitic ignimbrite eruptionssuggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out acausative link between these explosive silicic eruptions (with individual volumes v 200 km3) and climatic coolingwhich produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive andsystematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift floodvolcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease involcanic 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 theseparation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation andpreservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmaticstarvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angularunconformity in the northern Main Ethiopian Rift is evident at 10.6^3.2 Ma, and is also observed throughout theArabian 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: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 5 2 5 - 3

* Corresponding author. Tel. : +45-38142634; Fax: +45-33110878. E-mail address: [email protected] (I.A. Ukstins).

EPSL 6161 26-4-02

Earth and Planetary Science Letters 198 (2002) 289^306

www.elsevier.com/locate/epsl

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

1. Introduction

The Red Sea margins contain the best pre-served continental rift system on Earth, and therelative youthfulness of the conjugate volcanicmargins permits precise comparison of the volca-no-stratigraphy. The temporal and stratigraphicrelationships between £ood volcanism in Yemenand Ethiopia, however, are not yet well under-stood. In this study, a north^south section ofthe rifted margin in Ethiopia was targeted for40Ar/39Ar dating to complement a detailed under-standing of the temporal and spatial evolution ofthe conjugate Yemen margin [1,2].

This chronological study was motivated bythree objectives : (1) to examine the temporallink between magmatism and tectonics in anevolving rift margin with a continuous preservedstratigraphy representing £ood volcanism, the de-velopment of seaward dipping re£ector series andthe transition to sea£oor spreading. (2) To com-pare and contrast the chrono- and volcano-strat-igraphies of Ethiopia and Yemen in an attempt tolink the conjugate rifted margins, and to evaluateany spatial variation in timing of volcanic activityacross the Afro-Arabian £ood volcanic province.(3) To carefully date silicic units that form a ma-jor component of this bimodal £ood volcanicprovince. Earlier studies [3,4] have linked the ex-plosive silicic volcanism occurring in Ethiopia tothe Oi2 global cooling event which is character-ized by one of the maxima in the Cenozoic N

18O(oxygen isotope) record [5], the largest sea-leveldrop in the Cenozoic, and signi¢cant glaciationin Antarctica [6]. Major lower Oligocene tephralayers linked to this Oi2 cooling event are foundin ODP Leg 115 cores in the Indian Ocean at theChron C11r boundary [7], and were postulated byRochette et al. [4] to be temporally correlatedwith the ¢rst silicic volcanic units found in theLima Limo and Wegel Tena sections from Ethio-pia, 2600 km to the northwest of this IndianOcean ODP site.

2. Regional setting

Continental £ood volcanism in Afro-Arabia ex-tends over an area of at least 600 000 km2,stretching from southwestern Ethiopia throughEritrea and Djibouti to Yemen, with an estimatedvolume s 350 000 km3 [8] (Fig. 1). Volcanism wasassociated with rifting of the Afro-Arabian con-tinent in late Oligoceneêarly Miocene times [9]and with the initiation of sea £oor spreading inboth the Gulf of Aden (10 Ma) and the Red Sea(5 Ma) [10,11]. Continental £ood volcanism inEthiopia^Yemen is presumed to be associatedwith the Afar plume and comprises voluminoussub-aerial basaltic lavas and silicic pyroclasticrocks. Flood volcanism produced a ca. 4 km thickvolcanic pile, the erosional remnants (ca. 2 km) ofwhich are well-preserved on the Ethiopian andYemeni plateaux [1,2,12^15].

2.1. Ethiopia

Flood volcanic rocks in Ethiopia were uncon-formably emplaced on a regional lateritized sand-stone horizon. This sandstone unit covers an areaof 90 000 km2 [16] and represents a period of pa-leosol development prior to initiation of £ood vol-canism [17^20]. Mohr and Zanettin [13] con-structed a general volcanic stratigraphy for theEthiopian £ood volcanic province based on typelocalities in northeastern Ethiopia, and then ex-trapolated these formation divisions to sectionsof volcanic rocks to the south. Based on previ-ously published 40Ar/39Ar data, the most volumi-nous eruptive stage was during the late Oligo-ceneÊarly Miocene (V32^21 Ma) with most ofthe volcanic activity occurring between 30 and29.5 Ma [3], followed by a central Ethiopian vol-canic episode from 13 to 9 Ma and a period ofvolcanism in central Afar from 4.5 to 1.5 Ma [21].Signi¢cantly older volcanic rocks have been datedat 45 Ma in southern Ethiopia [18,22^28], butthese are interpreted as a smaller volume, unre-

EPSL 6161 26-4-02

I.A. Ukstins et al. / Earth and Planetary Science Letters 198 (2002) 289^306290

lated volcanic event and are not discussed furtherhere.

2.2. Yemen

The pre-volcanic stratigraphy is better exposedin Yemen because the margin was uplifted anderoded after break-up, in contrast to the Ethio-pian margin where parts of this stratigraphy havebeen buried by continued volcanism. In Yemen,

pre-volcanic rocks exhibit a change in deposition-al environment from continental in the west toshallow marine in the east and have thus beenused to de¢ne an Oligocene palaeo-coastline[29]. In the east, there was a change from deposi-tion of shallow marine to sub-aerial continentalclastic rocks immediately preceding volcanism,with development of regional lateritic paleosolsand overlying localized lacustrine deposits [29].Flood volcanic rocks were disconformably em-placed onto these relatively £at-lying sedimentaryrocks. According to the stratigraphy and 40Ar/39Ar age data of Baker et al. [1], the oldest £oodvolcanic rocks in Yemen consist of a 50^1000 msequence of basaltic lavas overlying basementsandstone erupted at 30.9 Ma in the south, inthe north the oldest lavas have an age of 29.4Ma (Fig. 1). The ma¢c stage of £ood volcanismpeaked just prior to 29 Ma, shortly before bimo-dal volcanism commenced throughout the regionat 29.5^29.2 Ma [1]. The main £ood volcanic pile,comprising V1.5 km of bimodal basaltic lavasand silicic ignimbrites, airfall tu¡s and calderacollapse breccias, was emplaced in V3.5 Myrand £ood volcanism ceased regionally acrossYemen by 27.1^26.7 Ma. An unconformity (here-in referred to as ‘break-up unconformity’) formedduring rift initiation and is represented by a dis-crete erosional surface cut into basaltic lava £owsdated at 26.7 Ma and overlain by a trachytic lava£ow dated at 19.9 Ma [1]. This coincides with aperiod of rapid cooling and erosion of the Yemenrifted margin proximal to the Red Sea [2] and theemplacement of dike swarms near the Gulf ofAden (25.5 and 18.5^16 Ma) [30] and the RedSea (24^21 Ma) [9].

3. Sampling strategy and objectives

3.1. Sampling rationale

A detailed 40Ar/39Ar chrono-stratigraphy [1]and volcano-stratigraphy [15,31] available forthe Yemen margin provides a template for com-paring the timing and evolution of volcanismalong the conjugate Ethiopian margin. Five strati-graphic pro¢les were sampled which span V250

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

EPSL 6161 26-4-02

I.A. Ukstins et al. / Earth and Planetary Science Letters 198 (2002) 289^306 291

km of the western escarpment along the Ethio-pian Plateau and into the northern Main Ethio-pian Rift (11.27‡N 39.507‡E to 9.023‡N 39.559‡E)(Fig. 2). These sections were chosen to cover themajor tectono-volcanic events which a¡ected theregion since the onset of £ood volcanism: (i) theinitiation of continental rifting and break-up ofAfro-Arabia, (ii) the formation of the Ethiopianrift, and (iii) the initiation of sea £oor spreadingin both the Gulf of Aden and the Red Sea. Addi-tionally, the northernmost pro¢les cover the £oodvolcanics in Ethiopia which were juxtaposed withnorthern Yemen, near Sana’a, prior to rifting.

Pro¢les A and B are from two sections alongthe northern sector of the western escarpment(Fig. 2), which exposes bimodal £ood volcanicsthought to be contemporaneous with the oldestcontinental £ood volcanism in Yemen. Overlyingalkaline shield lavas and rift-related silicic vol-canics were also sampled to constrain the timingof di¡erent modes of volcanic activity. Pro¢les Cand D are located at the bend in orientation ofthe 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 northernMain Ethiopian Rift and samples younger vol-canics including Pliocene eruptions. These sec-tions are thought to span the range of pre- tosyn-rift volcanism related to break-up of Afro-Arabia, the initiation of volcanism in Afar, andthe establishment of sea£oor spreading in theGulf 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 of137 samples. Potassium feldspar (both sanidineand anorthoclase), plagioclase, and phlogopitewere hand-picked from crushed and sieved sepa-rates of ignimbrites and basaltic lava £ows (125 to250 Wm sieve size). In addition, groundmass washand-picked from two basaltic units. The sampleswere screened to determine suitability for 40Ar/39Ar radiometric dating on the basis of petrogra-phy, major and trace element geochemistry andmicroprobe analysis of phenocryst phases. The

Fig. 2. Digital elevation model of the northern Ethiopian plateau and escarpment and northern Main Ethiopian Rift, based on a1 km horizontal grid with þ 30 m vertical resolution. Section A: Desi-Bati, Section B: Ataye, Section C: Robit, Section D: An-kober, Section E: Kessem River Gorge. All pro¢les initiate on the plateau and traverse across the escarpment except E, which islocated in a river gorge 2 km south of the upper escarpment edge.

EPSL 6161 26-4-02


degree of alteration and, for ignimbrites and tu¡s,homogeneity of feldspar populations were alsoconsidered, to minimize possible xenocrystic con-tamination.

Samples were loaded into wells in an Al diskand irradiated for 20 h in the TRIGA reactor atOregon State University following the methodsestablished by Renne et al. [32]. Fish Canyon sa-nidine (FCs) was used as a neutron £ux monitor(FCs: 28.02 Ma [32]) and irradiated in four posi-tions horizontally spanning the unknowns in theirradiation vessel. For comparative purposes, alldata referred to here from other literature sour-ces have been recalculated to re£ect the age ofFCs at 28.02 Ma as recommended by Renne etal. [32]. The calculated mean J value was0.0051555 þ 0.0000170 where the uncertainty re-£ects total variation in neutron £ux over the Aldisk. Samples were analyzed at the Berkeley Geo-chronology Center using CO2 and Ar-ion lasers infour run batches. Laser heating of multi-grainsamples probably results in less uniform heatingthan of single crystals, but we estimate that ex-traction temperatures within each sample werehomogeneous to within þ 50‡C for each step dur-ing these experiments. Homogeneous extractiontemperatures are evidenced by the progressive in-crease in %40Ar* throughout each experiment (seeTable 3 in the Background Data Set1). Argonisotopic analysis was performed on MAP 215Cand MAP 215^50 mass spectrometers.

The sample analysis methods included single-crystal total fusion (11 experiments), single- andmulti-crystal step-heating of 5^30 grains of sani-dine, plagioclase and phlogopite (12 experiments),and multi-grain step-heating of 8^12 grains forgroundmass samples (six experiments). A totalof 90 grains of sanidine, plagioclase and phlogo-pite were analyzed by single-crystal total fusion.Four samples of sanidine, plagioclase and phlogo-pite were analyzed by multiple methods to con-strain variations in accuracy and precision for dif-ferent techniques and mineral phases. Whenmultiple experimental methods were applied tothe same sample, calculated ages were weighted

mean isochron ages of all steps from all tech-niques used (excluding two xenocrystic grainsof sanidine and plagioclase from single-crystal to-tal fusion experiments, EEWB22 and EEWB9).Mass discrimination for the four runs, monitoredby analysis of 177 air pipettes interspersed withthe unknowns, ranged from 1.0021 þ 0.0012 to1.0048 þ 0.0025 per atomic mass unit. Averageprocedural blanks for these analyses were compa-rable to those reported by Renne [33] and Renneet al. [34] (for laser samples, and were typically6 1% of the measured signal for 40Ar). One sam-ple, EIU99010, produced age spectra representa-tive of bimodal mixing of components of di¡erentages even during single-crystal step-heating ex-periments and was thought to contain xenocrysticfeldspars which had partly re-equilibrated with ahost magma, and as such was excluded from fur-ther consideration, as precise age information isnot provided by the data. All other samples pro-duced precise plateau ages, described below andpresented in Tables 1 and 2 and Background DataSet Table 31.

4. Analytical results: 40Ar/39Ar mineral andwhole-rock ages

Flood volcanic rocks in Ethiopia contain a sig-ni¢cant component of rhyolitic airfall tu¡s andignimbrites intercalated with basaltic lava £ows.For the silicic units, potassium feldspar from 11ignimbrite samples provides precise and reliable40Ar/39Ar dates of marker units within otherwisedi⁄cult to date sections of aphyric or altered ba-saltic units. Furthermore, the silicic units areclearly identi¢able as extrusive eruptions and notintrusions, whereas for some of the ma¢c units itis di⁄cult to distinguish lava £ows from sills. Forthe basaltic units, separates of plagioclase (n = 2),phlogopite (n = 1) and groundmass (n = 2) fromrelatively unaltered lavas were selected to con-strain the age of ma¢c magmatism spanning thepre- and syn-rift volcanic stratigraphy.

All samples produced stratigraphically consis-tent ages and yielded plateaux for step-heatedsamples or homogeneous distributions for single-crystal total fusion samples (Tables 1 and 2 and1 http://www.elsevier.com/locate/epsl

EPSL 6161 26-4-02


Tab

le1

40A

r/39

Ar

dati

ngre

sult

sfr

omE

thio

pian

pre-

and

syn-

rift

volc

anic

unit

s

Sam

ple

No.

Pha

seD

epos

itL

ocat

ion

(‡N

,‡E

)A

naly

sis

met

hod

Las

erT

otal

gas

age

Pla

teau

age

Pla

teau

step

s/N

o.of

grai

ns

%A

rra

d.A

gepr

obab

ility

Isoc

hron

age

Des

i-B

ati:

Pro

¢le

AE

EW

B1

phlo

trac

hyte

11.1

93,

40.0

25si

ngle

-xtl

tfA

rio

n30

.84

þ0.

1130

.86

þ0.

128

of8

na30

.86

þ0.

1130

.92

þ0.

12E

EW

B1

phlo

mul

ti-x

tlst

epC

O2

31.1

þ0.

231

.0þ

0.2

13/2

091

.931

.06

þ0.

1831

.05

þ0.

18E

EW

B1

phlo

mul

ti-x

tlst

epC

O2

32.0

þ.0

831

.6þ

0.7

9/9

100

31.0

6þ

0.18

31.0

5þ

0.18

EE

WB

1ph

low

eigh

ted

aver

age

30.9

5þ

0.12

31.1

5þ

0.34

30.8

8þ

0.11

30.9

2þ

0.11

EIU

9903

5sa

nig

nim

b11

.268

,39

.516

mul

ti-x

tlst

epC

O2

30.0

7þ

0.10

30.0

8þ

0.12

11/1

110

030

.08

þ0.

1030

.16

þ0.

13

EIU

9902

9sa

nig

nim

b11

.269

,39

.516

sing

le-x

tltf

CO

229

.73

þ0.

1429

.65

þ0.

176

of7

na29

.69

þ0.

1429

.91

þ0.

11E

IU99

029

san

sing

le-x

tlst

epC

O2

29.0

þ0.

429

.6þ

0.4

6/15

85.6

729

.4þ

0.27

29.4

7þ

0.14

EIU

9902

9sa

nsi

ngle

-xtl

step

CO

229

.1þ

0.5

29.1

þ0.

415

/15

100

29.4

þ0.

2729

.47

þ0.

14E

IU99

029

san

wei

ghte

dav

erag

e29

.46

þ0.

1729

.45

þ0.

3229

.63

þ0.

1329

.61

þ0.

12

EE

WB

7sa

nig

nim

b11

.092

,39

.766

sing

le-x

tltf

CO

229

.43

þ0.

1329

.43

þ0.

156

of6

na29

.43

þ0.

1229

.68

þ0.

15

EE

WB

20w

rba

salt

11.2

7,39

.507

mul

ti-g

rain

step

Ar

29.4

8þ

0.14

29.3

6þ

0.17

7/9

81.2

29.4

8þ

0.14

29.4

þ0.

3E

EW

B20

wr

mul

ti-g

rain

step

Ar

29.4

3þ

0.15

29.4

þ0.

27/

988

.329

.43

þ0.

1529

.4þ

0.3

EE

WB

20w

rm

ulti

-gra

inst

epA

r29

.55

þ0.

1429

.28

þ0.

196/

971

.429

.55

þ0.

1429

.4þ

0.3

EE

WB

20w

rw

eigh

ted

aver

age

29.4

9þ

0.11

29.3

5þ

0.19

29.5

3þ

0.11

29.3

4þ

0.15

EE

WB

22pl

basa

lt11

.088

,39

.633

sing

le-x

tltf

Ar

25.6

þ1.

425

.6þ

1.6

10of

11na

25.6

þ1.

328

.5þ

1.1

EE

WB

22pl

mul

ti-x

tlst

epC

O2

24.2

þ0.

424

.9þ

0.4

29/3

093

.124

.9þ

0.3

25.1

þ0.

2E

EW

B22

plm

ulti

-xtl

step

CO

2

EE

WB

22pl

wei

ghte

dav

erag

e24

.30

þ0.

425

.1þ

0.80

24.9

0þ

0.3

25.1

þ0.

2

EE

WB

9pl

basa

lt11

.083

,39

.683

sing

le-x

tltf

Ar

25.6

þ0.

625

.0þ

0.8

11of

12na

24.9

þ0.

525

.5þ

1.2

EE

WB

9pl

mul

ti-x

tlst

epC

O2

24.1

þ0.

525

.0þ

0.6

14/1

589

.624

.5þ

0.3

24.9

þ0.

2E

EW

B9

plm

ulti

-xtl

step

CO

221

.8þ

0.5

24.5

þ0.

414

/15

87.7

24.5

þ0.

324

.9þ

0.2

EE

WB

9pl

wei

ghte

dav

erag

e23

.3þ

0.3

24.8

þ0.

624

.5þ

0.2

25.0

þ0.

2

Ata

ye:

Pro

¢le

BE

EW

B25

san

igni

mb

10.3

62,

39.9

32si

ngle

-xtl

tfC

O2

25.3

1þ

0.11

25.2

8þ

0.13

7of

7na

25.2

8þ

0.11

25.3

0þ

0.13

Rob

it:

Pro

¢le

CE

EW

R1

san

igni

mb

9.96

7,39

.918

mul

ti-x

tlst

epC

O2

19.7

0þ

0.07

19.6

9þ

0.07

7/11

86.9

19.7

0þ

0.07

19.7

7þ

0.07

EE

WR

1sa

nsi

ngle

-xtl

tfC

O2

19.6

8þ

0.08

19.6

7þ

0.09

7of

7na

19.6

7þ

0.08

19.5

7þ

0.14

EE

WR

1sa

nw

eigh

ted

aver

age

19.6

9þ

0.07

19.6

8þ

0.08

19.6

9þ

0.07

19.7

6þ

0.07

EE

WR

4sa

nig

nim

b9.

845,

39.7

68m

ulti

-xtl

step

CO

214

.91

þ0.

0614

.91

þ0.

079/

1283

.414

.89

þ0.

0614

.90

þ0.

06

EPSL 6161 26-4-02


Tab

le1

(Con

tinu

ed)

Sam

ple

No.

Pha

seD

epos

itL

ocat

ion

(‡N

,‡E

)A

naly

sis

met

hod

Las

erT

otal

gas

age

Pla

teau

age

Pla

teau

step

s/N

o.of

grai

ns

%A

rra

d.A

gepr

obab

ility

Isoc

hron

age

Ank

ober

:P

ro¢l

eD

EIU

9905

5sa

nig

nim

b9.

562,

39.7

83m

ulti

-xtl

step

CO

211

.69

þ0.

0411

.69

þ0.

058/

1198

.111

.7þ

0.04

11.7

0þ

0.04

EIU

9907

5sa

nig

nim

b9.

558,

39.7

61si

ngle

-xtl

tfA

r11

.63

þ0.

0411

.64

þ0.

057

of7

na11

.64

þ0.

0411

.73

þ0.

05

EE

WA

4sa

nig

nim

b9.

646,

39.5

8si

ngle

-xtl

tfA

r11

.61

þ0.

0511

.61

þ0.

057

of7

na11

.61

þ0.

0511

.59

þ0.

06

EE

WA

3w

rba

salt

9.91

7,39

.733

mul

ti-g

rain

step

Ar

10.6

9þ

0.19

10.7

3þ

0.16

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EPSL 6161 26-4-02


Fig. 3. 40Ar/39Ar age spectra and age probability diagrams obtained from mineral separates and groundmass from all datedEthiopian volcanic rocks. All mineral separates have relatively undisturbed spectra. Few have minor quantities of excess Ar. Twosamples of feldspar, analyzed by single-crystal total fusion experiments, contain a signi¢cantly older xenocryst phase, which wasremoved from the sample sets for age calculations. The most precise ages were obtained from the potassium-feldspars (K-feld-spar); a phlogopite (EEWB1) from a trachytic lava also provided a very well-constrained plateau and isochron age date. Plagio-clase and whole-rock mineral separates have similar 2c errors and in many cases were comparable to K-feldspar. Age spectrumdiagrams are shown for multi-crystal and multi-grain step-heating experiments, steps used to calculate plateau age are indicatedwith an arrow and plateau age is shown. Age probability diagrams are shown for single-crystal total fusion experiments. The agefor 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 diagramfor EIU99029 sanidine single-crystal step-heating experiment. (C) Plateau diagram for EEWB20 whole-rock basalt groundmassmulti-grain step-heating experiment. (D) Plateau diagram for EEWR4 sanidine multi-crystal step-heating experiment. (E) Plateaudiagram for EIU99055 sanidine multi-crystal step-heating experiment. (F) Plateau diagram for EEWA3 whole-rock basalt ground-mass multi-grain step-heating experiment. (G) Isochron diagram of all analyses of EEWB1 phlogopite, eight single-crystal totalfusion and two multi-crystal step-heating experiments of 20^30 grains each. Age probability diagrams: (H) Single-crystal total fu-sion of six sanidine grains from EEWB7. (I) Single-crystal total fusion of plagioclase from EEWB22. (J) Single-crystal total fu-sion of seven sanidine grains from EEWB25. (K) Single-crystal total fusion of sanidine from EEWR1. (L) Single-crystal total fu-sion of seven sanidine grains from EIU99075. (M) Single-crystal total fusion of seven sanidine grains from EEWA4. (N) Single-crystal total fusion of 11 plagioclase grains from a basalt, EEWK2. (O) Single-crystal total fusion of seven sanidine grains fromEEWK5. (P) Single-crystal total fusion of 12 grains of plagioclase from a basalt, EEWB9, showing an average age of 25.0 Mafor 11 mineral grains and a xenocrystic grain producing an age of 51.37 Ma.

EPSL 6161 26-4-02


Background Data Set Table 31). Of 18 single-crys-tal, multi-crystal and multi-grain step-heating ex-periments, 16 had plateaux incorporating 80% orgreater 39Ar and ¢ve of them had concordantspectra for 100% of the gas released. Calculatedisochron ages were within error (2c) of plateauages for all samples and con¢rmed the presenceof only minor excess Ar in some mineral sepa-rates.

Fig. 3 illustrates all 40Ar/39Ar dating results ob-tained from 16 mineral separates and whole-rockgroundmass from the Ethiopian pre- and syn-riftvolcanic rocks, including multi-crystal and multi-grain step-heating and single-crystal total fusionexperiments. An isochron diagram of all analysesperformed on phlogopite separated from a tra-chytic lava £ow EEWB1 is also shown, which in-cludes eight single-crystal total fusion experimentsand two multi-grain (V30 grains each) step-heat-ing experiments of 14 and 15 steps each.

Fig. 3P presents an important analytical resultfrom a single-crystal total fusion experiment ofplagioclase grains from a basaltic lava £ow(EEWB9). This experiment identi¢ed a single pla-

Fig. 3 (Continued).

Table 2Summary of 40Ar/39Ar age data

Sample Phase Age(Ma)*

Dese-Bati: Pro¢le AEEWB1 phlogopite 30.92 þ 0.11EIU99035 sanidine 30.16 þ 0.13EIU99029 sanidine 29.61 þ 0.12EEWB7 sanidine 29.43 þ 0.12EEWB20 wr groundmass 29.34 þ 0.15EEWB22 plagioclase 25.1 þ 0.2EEWB9 plagioclase 25.0 þ 0.2Ataye: Pro¢le BEEWB25 sanidine 25.30 þ 0.13Robit: Pro¢le CEEWR1 sanidine 19.76 þ 0.07EEWR4 sanidine 14.90 þ 0.06Ankober: Pro¢le DEIU99055 sanidine 11.70 þ 0.04EIU99075 sanidine 11.73 þ 0.05EEWA4 sanidine 11.59 þ 0.06EEWA3 wr groundmass 10.87 þ 0.06Kessem River: Pro¢le EEEWK2 sanidine 10.58 þ 0.07EEWK5 sanidine 3.19 þ 0.04

* þ 2c S.D.

EPSL 6161 26-4-02


gioclase crystal that produced an age of 51.37 Ma.This is signi¢cantly older than the average appar-ent age of 25.0 Ma for 11 other plagioclase grains.The plagioclase grains in this sample appear to behomogeneous and do not exhibit any visible xen-ocrystic cores or other disequilibrium features.However, this anomalous age is interpreted torepresent a xenocrystic grain that has been mixedinto the basaltic magma and not fully degassedprior to eruption. Typically, plagioclase mineralseparates from basalts are 40Ar/39Ar dated bymulti-crystal step-heating, not single-crystal totalfusion. If this sample had been measured by mul-ti-crystal step-heating the average apparent ageproduced from these 12 grains would have been28.0 þ 0.6 Ma, 3 Myr older than the age calcu-lated after removing the xenocryst ; this has highlysigni¢cant implications for using 40Ar/39Ar datingto establish precise chronostratigraphies of riftedvolcanic margins. If this xenocrystic grain hadbeen accidentally included in an analysis, it wouldhave added an extra 11% error to the averageapparent age. Moreover, the error associatedwith the average apparent age (excluding the xen-ocryst) of þ 0.5 Myr for this single-crystal total

fusion experiment on EEWB9 is larger than theaverage error for all other samples in this studyof þ 0.09 Myr. This larger error may be attrib-uted to other xenocrystic plagioclase grains in thissample which had degassed but still not com-pletely re-equilibrated with surrounding magmaprior to eruption.

5. 40Ar/39Ar chrono-stratigraphy of the Ethiopianmargin

5.1. Flood basalt volcanism: initiation andduration

The 40Ar/39Ar chrono- and volcano-stratigra-phies for northern Ethiopia are summarized inFig. 4. Dated samples for each pro¢le are placedin a regional volcanic stratigraphy based on ¢eldwork and tectonic reconstructions [31,35,36]. Theoldest dated pre-rift £ood basalt unit is a phlogo-pite-bearing trachytic lava £ow sampled near thebase of the volcanic section exposed in the north-ern 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 foot-wall of a fault block exposing riftward-dipping silicic volcanic rocks east of the marginal graben. Volcanic rock packages andstratigraphy thickness are presented as a general representation. Pro¢le thickness is based on ¢eld stratigraphy and tectonic re-construction [36]. Dates presented here from Hofmann et al. [3] for the Lima Limo pro¢le are only those sampled from a contin-uous stratigraphic section and do not include samples whose locations were extrapolated from s 10 km away. Errors for all datapresented are þ 2c.

EPSL 6161 26-4-02


Ma (EEWB1). It should be noted that in thispro¢le the base of the volcanic pile is not exposedand thus the earliest lava £ow was not sampled.EEWB1 was collected from the lower portion ofan exposed basaltic lava £ow sequence V200 mbelow the ¢rst ignimbrite. Based on a generalizedstratigraphy of Ethiopia [8,13,21], and a well-con-strained basal volcanic stratigraphy for Yemen[1,2,15,31], we estimate that the basal £ood basaltpackage underlying the ¢rst ignimbrite in this areacould be between 500 and 1000 m thick. Possiblyas much as 800 m of lava £ows could be belowthis lowest dated basalt sample. Basaltic lava£ows in this section have an average thicknessof 30^40 m, and by using well-constrained erup-tion rates from Yemen (£ood basalt lavas eruptedevery 10^100 kyr [1]) and Ethiopia (£ood basaltlavas erupted every 50 kyr [4]), 800 m of £oodbasalt lavas may have been emplaced overV200 kyr to V2 Myr. Consequently, Ethiopianbasaltic £ood volcanism could have begun signi¢-cantly earlier than 30.9 Ma.

An earlier 40Ar/39Ar dating study of northernEthiopian £ood volcanism [3] was based mainlyon samples from the Lima Limo area (Fig. 1),V300 km to the northwest of our northernmostpro¢le A. Lima Limo is located towards the mar-gin of the volcanic province and thus may notrecord the full pre-rift stratigraphy that is pre-served closer to the rift margin. The 40Ar/39Ardates [3] are reported in relation to the laboratorymonitor standard Hb3Gr (for which an age of1072 Ma was reported), in good agreement withthe ages of 1074 þ 4 and 1075 þ 4 relative to FCsat 28.02 Ma reported in Renne et al. [32]. Whenthese 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 recalcu-lated to a 2c level, the dates are comparable tothe oldest dated £ood volcanics in this study,although their associated analytical errors are sig-ni¢cantly larger than those presented here (Fig. 4).Whereas their data suggest that the bulk of thetraps were erupted at approximately 30 Ma withina period of 1 Myr or less, our volcano-strati-graphic pro¢les and dating indicate that alongthe rift margin in north central Ethiopia volcan-ism may have initiated signi¢cantly earlier, poten-

tially as early as 32^33 Ma, with the greatest erup-tion rates and volumes occurring from V31 to 28Ma, and the total duration of £ood volcanismwas at least 4 Myr. The di¡erences in the durationof volcanism preserved in the Lima-Limo sectionand in our pro¢les along the escarpment indicatethere may have been larger £ow volumes or afocusing of volcanism towards the rift marginduring the initial stages of £ood volcanic erup-tions.

5.2. Onset of silicic volcanism

The ¢rst silicic ignimbrite in this part of Ethio-pia was erupted at 30.2 Ma and bimodal £oodvolcanism continued until 25.3 Ma (Fig. 4: pro-¢les A and B). The initiation of bimodal volcan-ism in the Ethiopian £ood volcanic province oc-curs at least 700 kyr after the start of basaltic£ood volcanism. Silicic volcanism as young as24 Ma in the region of pro¢le B is recorded byxenocrysts and conglomerate cobbles of reworkedignimbrite found in Pliocene deposits down-stream, in the Central Awash Complex [34], butthese ignimbrites have not been found in situ.

Recent work on £ood volcanism in Ethiopia[3,4] has mentioned the striking coincidence be-tween the timing of silicic volcanism during Ethio-pian trap emplacement and the Oligocene (Oi2)global cooling event. This event occurred at thebase of Chron C11r and is marked by a suddenshift to higher marine N

18O values, major develop-ment of the Antarctic ice shelf, and the largestTertiary sea-level drop [5,6]. Tephra layers foundin ODP leg 115 drill cores in the Indian Ocean,biostratigraphically located at the base of ChronC11r [7], have been linked to Afro-Arabian rhyo-litic volcanism because of this temporal coinci-dence [3,4]. However, our data and that of Bakeret al. [1] establish that silicic £ood volcanism ini-tiated in both Yemen and Ethiopia at 30.2 þ 0.1Ma, which post-dates the Oligocene cooling eventat the base of Chron C11r (31.2 þ 0.7 Ma [37,38]).Depending on the accuracy of the time scale cal-ibration, and assuming that we have analyzed the¢rst silicic ash £ow erupted, this asynchrony sug-gests that the cooling event would need to betriggered by the earlier basaltic volcanism if in-

EPSL 6161 26-4-02


deed there is a genetic link between these phenom-ena. However, the massive silicic pyroclastic erup-tions with volumes s 200 km3 [15,31] may havereinforced already turbulent climatic conditions.

5.3. Initiation of syn-rift volcanism

Following a dramatic decrease in volcanism atV25 Ma, sporadic syn-rift bimodal volcanism ispreserved in pro¢les CÊ (Fig. 4). Pro¢le C spansan area where the tectonic regimes of the Red SeaRift and the northern Main Ethiopian Rift inter-sect, resulting in a bend in the orientation of thewestern escarpment. EEWR1 is the lowermostignimbrite in the footwall of a westward-dippingfault block that comprises riftward-dipping syn-rift silicics. EEWR4 unconformably overlies pla-teau £ood volcanics and may represent some ofthe youngest deposits of syn-rift silicic volcanicserupted onto and capping the signi¢cantly olderplateau stratigraphy. These two silicic ignimbritesproduced isochron ages of 19.76 þ 0.07 and14.90 þ 0.06 Ma (EEWR1 and EEWR4).

The Kessem pro¢le (E), located in the northernMain Ethiopian Rift, represents a well-con-strained stratigraphic section containing a majorangular unconformity and volcanic hiatus. A bi-modal section of silicic lavas and ignimbritesplus basaltic lavas dips 20^25‡ to the N-NE.This is capped by an angular unconformity andoverlain by a £at-lying coarse £uvial conglomer-ate and ignimbrite. The ignimbrite contains V30^50% loose pebbles and cobbles from the under-lying conglomerate in the lowermost 1.5 m, indi-cating that the sedimentary unit was uncon-solidated when the ignimbrite was emplaced.Two ignimbrites were dated in this pro¢le, onefrom beneath the unconformity and the conglom-erate-clast-bearing ignimbrite directly overlyingthe unconformity. These yield isochron ages of10.58 þ 0.07 and 3.19 þ 0.04 Ma, respectively,which bracket the unconformity (EEWK2 andEEWK5).

5.4. Temporal and spatial variation of magmatismalong the Ethiopian margin

Fig. 5 illustrates the temporal and spatial sys-

tematics of volcanism along the Ethiopian escarp-ment. The timing of volcanism represented inpro¢les A through E reveals a progressive northto south decrease in age. The apparent linearyounging of laterally equivalent volcanics alongthe escarpment may be a re£ection of continuedand more focused volcanism approaching thenorthern Main Ethiopian Rift as well as enhancedpreservation through volcanic loading of theEthiopian margin. This systematic youngingmay re£ect shifts in loci of volcanism that arerelated to rifting episodes and basin formationduring the initiation of continental break-up (see[36]).

Pro¢le A contains the oldest £ood volcanicrocks with an age range from 31 to 25 Ma. Pro¢leB has only one dated sample but contains £oodvolcanic rocks that stratigraphically correlate withthe upper part of pro¢le A. The apparent break involcanism between pro¢les B and C is related tothe initiation of syn-rift volcanism. There is anobservable change in eruption and emplacementstyle from the pre- to syn-rift volcanics. Pre-riftlavas and ignimbrites are laterally extensive, large-volume £ood volcanic deposits, whereas syn-riftvolcanics are small-volume, localized deposits

Fig. 5. Temporal and spatial variation in volcanism alongand across the main Ethiopian escarpment. Note that fromnorth (A) to south (E) a systematic and progressive decreaseis observed in the age of the exposed laterally equivalent vol-canic rocks. Oligocene volcanism is dominant in the northand Miocene to recent volcanism in the south.

EPSL 6161 26-4-02


usually associated with volcanic centers which oc-cur in margin-parallel rift segments ranging from60 to 100 km in length.

Pro¢le C is composed of younger, sporadicallyerupted syn-rift volcanics (19.8^14.9 Ma). Thesevolcanic deposits may be the southernmost ex-pression of Red Sea rifting and are tectonicallyoverprinted by northern Main Ethiopian Riftfaulting [35]. Pro¢le D is composed of near-ventsyn-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 un-conformity that represents a 7.5 Myr break involcanism. The hiatus occurs during a periodwhen sea£oor spreading initiated in the Gulf ofAden (10 Ma) and the Red Sea (5 Ma) [10,11]. Avolcanic hiatus from 9 to 4 Ma is also foundthroughout the Arabian plate, in intraplate vol-canic ¢elds in Jordan, Saudi Arabia and Yemen[39^42]. Chernet et al. [12] found that from 7 to 3Ma, during the earliest phase of modern rift mar-gin development throughout the central andnorthern Main Ethiopian Rift and southernAfar, there was a period of focused volcanism insmall silicic centers which formed in a marginalgraben along the eastern escarpment of the north-ern Main Ethiopian Rift. WoldeGabriel et al. [43]also report interbedded hominid fossil-bearingsediments with a few basaltic lava £ows andthin silicic (3^20 cm) and basaltic (V20 cm to2^3 m) tephra layers dated at 5^6 Ma from theMiddle Awash area of Afar. Also, by the lateMiocene, both basaltic and silicic eruptions werecon¢ned to both sides of the rift margin in thenorthern and central sectors of the Main Ethio-pian Rift [20,21,44^46]. A shift from a di¡usestrain regime in Afar and Arabia during earlysyn-rift volcanism (20^10 Ma) to localized strainwhen sea£oor spreading initiated ¢rst in theGulf 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 magmatismalong the conjugate margins and resulted in local-ized magmatic activity feeding volcanic centersalong rift margins which were active during con-tinental rifting.

5.5. Matching conjugate margins in Yemen andEthiopia

The extent to which di¡erent 40Ar/39Ar datasets can be critically compared depends upon ac-curate intercalibration between samples, as well asto primary and laboratory standards. For accu-rate and precise comparisons, ages which are cal-culated with standards other than FCs at 28.02Ma (which was used in our study) must have pri-mary standard ages re-calculated to re£ect a con-sistent age for FCs [32]. All 40Ar/39Ar dates fromother studies used for comparative purposes herehave been recalculated to re£ect a monitor miner-al age in accordance with that of FCs at 28.02Ma. Baker et al. [1] and George et al. [23] usedmonitor standard MMhb-1 at 520.4 Ma, and re-calculation 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 re-calculate to approximately 0.19% older; andChernet et al. [12] used FCs at 27.84 which makestheir data 0.6% older.

Fig. 6 summarizes a total of 96 40Ar/39Ar datesfor Afro-Arabian volcanism. The data from thisstudy are presented and compared with publisheddata from Ethiopia, Djibouti, Sudan and Yemen[1,3,4,12,22,23,30,47,48]. The break-up uncon-formity found in Yemen (26.5^19.7 Ma) and thedramatic decrease in volcanism shown in our da-taset (24^20 Ma) appear to represent a regionalevent. Apatite ¢ssion track studies reveal that rap-id crustal cooling occurred between 25 and 17 Main rocks underlying the £ood volcanic units prox-imal to the rift margin in Yemen [2]. This periodof break-up, extension and presumably tectoni-cally driven exhumation matches very well withthe 26.7^19.9 Ma break-up unconformity in Ye-men [1] and the observed decrease in volcanism inEthiopia from 25.3 to 19.8 Ma (Figs. 6d and 7). Itshould be noted that we cannot rule out samplingbias, preferential erosion, or burial of volcanicunits to explain the apparent gap in volcanismfrom V24 to 20 Ma. However, our preferred ex-planation is a dramatic decrease in volcanism, andthis is corroborated by compiled 40Ar/39Ar datingstudies in the same region which support our ¢nd-ings and indicate sampling bias is not a factor. No

EPSL 6161 26-4-02


evidence has been observed for signi¢cant erosion,and near the bend in the western escarpment vol-canic rocks in contact with basement have beendated at V24 Ma [12], indicating that the com-plete volcanic stratigraphy is preserved. Our mainsampling pro¢les are laterally spaced from 20 to30 km along the Main Ethiopian Rift and, giventhe challenges of ¢nding continuous sections dueto the generally poor exposure and access di⁄cul-ties, this study represents the best available data

set to put a complete pre- to syn-rift volcanic suiteof 40Ar/39Ar data into volcano- and tectono-stratigraphic context.

Comparing the volcano-stratigraphy of the con-jugate margins of Ethiopia and Yemen highlightsthe similarities of the province (Fig. 7). In Yemen,the basal volcanic rocks, in contact with basementsandstones, vary in age from 30.9 Ma (south) to29.4 Ma (north) in tandem with a variation inthickness from 50^300 m (north) to 1000 m(south) [1]. In Ethiopia, basal volcanic rocks

Fig. 7. Schematic stratigraphic diagram illustrating a general-ized volcano-stratigraphy for Ethiopia as presented in thiswork and related to the £ood volcanic stratigraphy of Ye-men. The break-up unconformity preserved in Yemen corre-lates with a decrease in volcanism in Ethiopia during earlysyn-rift volcanism. Apatite ¢ssion track data shows a con-temporaneous period of rapid uplift and exhumation inYemen from 25 to 17 Ma [2]. Sporadic syn-rift volcanism ispreserved in Ethiopia but not in Yemen.

Fig. 6. 40Ar/39Ar age probability plots for 96 compiled min-eral separate and whole-rock dates of pre- and syn-rift vol-canic activity in Afro-Arabia. Flood volcanism spans 31^24Ma, with a peak in ages, and inferred peak in volcanism, at30 Ma. The largest peak contains 19 ages. The Yemen un-conformity and hiatus in Ethiopian volcanism correspondwell at 25^20 Ma. Sporadic syn-rift volcanic activity occursfrom 25 to 11 Ma, and a regional hiatus in Ethiopia corre-sponds to the timing of the Kessem River Gorge unconform-ity from 10 to 3 Ma. Chernet et al. [12] recognized small sili-cic centers active from 7 to 3 Ma, formed in a marginalgraben along the Main Ethiopian Rift. This localized volcan-ism falls in the time represented by the angular unconformityfound in the Kessem River Gorge section. Data compiledfrom Ethiopia, Djibouti, Sudan and Yemen age dating proj-ects [1,3,4,12,22,23,30,47,48].

EPSL 6161 26-4-02


vary from s 30.9 Ma (pro¢le A, not in contactwith basement) to 24 Ma (near pro¢le D and incontact with basement [12]). When additional un-dated £ood basalt lavas are taken into account inthe northern Ethiopian sections, initiation of vol-canism in northern Ethiopia predates that ofYemen, by several hundred thousand years andpotentially up to V2 Myr based on well-con-strained eruption rates and estimated sectionthickness. The di¡erence in timing of initiationof basaltic volcanism for comparable localitieson the conjugate margins could be V1.5 Myror more, depending on plate reconstruction mod-els juxtaposing northern Yemen and Ethiopia.Baker et al. [1] proposed that initial silicic volcan-ism commenced at 29.5^29.2 Ma throughoutYemen. However, new ¢eld data (Ukstins unpub-lished data) indicate there is an older poorly pre-served silicic volcanic component (ignimbrite andairfall tu¡s). Based on dated basaltic lavas (from[1]) intercalated with the oldest ignimbrite unitnow recognized in northern Yemen, silicic volcan-ism initiated at V30 Ma, which is comparable tothe date of 30.2 Ma for the ¢rst rhyolitic ignim-brite in northern Ethiopia. Bimodal volcanismended at 26.7 Ma in Yemen, whereas basalticand rhyolitic volcanism continues to the presentday in Ethiopia.

The initiation of break-up of Afro-Arabia atV26 Ma resulted in a volcanic hiatus in Ethiopiaduring the earliest stages of syn-rift volcanism(24^20 Ma) and represents a change from thelarge-volume pre-rift £ood volcanism exposed inthe north to smaller volume syn-rift volcanism inthe south. Syn-rift volcanics were sporadicallyerupted and associated with laterally restrictedvolcanic centers. The transition from pre- tosyn-rift volcanism is preserved in Yemen as abreak-up unconformity that juxtaposes 26.7 Mabasalts with a 19.9 Ma trachytic lava £ow [1].While we have not found evidence for such a dis-tinct contact in Ethiopian volcanics, the decreasein volcanic activity coupled with changes in erup-tive mechanisms, rift geometry and architectureindicates that break-up of Afro-Arabia initiatedat V26 Ma.

The conjugate margins of Ethiopia and Yemenshare a similar volcano-stratigraphy representing

the primary stages of £ood volcanism, an initialsequence of basaltic lava £ows overlain by inter-calated silicic ignimbrites, airfall tu¡s and basalticlava £ows. The duration of eruption for thephases of volcanism are similar also: initial basal-tic £ood volcanism spanned s 0.7 Myr in Ethio-pia and V1.0 Myr in Yemen, bimodal volcanismspanned 5.2 Myr in Ethiopia and s 3.3 Myr inYemen (with an estimated 2 km of erosion ofoverlying material [2]). Di¡erences in syn-rift tec-tono-volcanic activity resulted in continued bimo-dal volcanism and preservation of these depositsin Ethiopia, while Yemen may have had only mi-nor syn-rift volcanism, indicated by two periodsof dike emplacement at 25.5 and 18.5^16 Ma [30],which could have been removed during a periodof major denudation and erosion from 25 to 17Ma [2]. The di¡erences in tectono-volcanic evolu-tion of the conjugate rifted margins can be ac-counted for in large part by the disposition ofthe Afar plume with respect to the rifting plates.As Afro-Arabia broke up, Arabia rifted awayfrom the plume axis while the Ethiopian marginand Afar remained above it. The syn-rift volcan-ism in Ethiopia is a re£ection of the continuingplume in£uence under Afar. In Yemen, minorsyn-rift volcanism and regional denudation anderosion re£ects the continued rifting of Arabiaaway from the axis, and sphere of in£uence, ofthe Afar plume.

6. Summary

A detailed 40Ar/39Ar chronological study ofpre- and syn-rift £ood volcanism in Ethiopiaand a comparison to the tectono-volcanic evolu-tion of the Yemen conjugate rifted margin haveallowed us to address the timing and duration of£ood volcanism in relation to major tectonicevents during rifting at the Afro-Arabian triplejunction. Our main conclusions are:

1. Basaltic volcanism initiated in northern Ethio-pia prior to 30.9 Ma, perhaps by as much as200 kyr to 2 Myr, whereas initiation of basalticvolcanism is well-constrained in Yemen to beno earlier than 30.9 Ma.

EPSL 6161 26-4-02


2. Bimodal ma¢c^silicic volcanism initiated innorthern Ethiopia at 30.2 Ma, similar to Ye-men at V30 Ma, and lasted for 5.2 Myr,switching to syn-rift volcanism by 25.3 Ma.The duration of preserved £ood volcanism inYemen is 3.3 Myr, but erosion of V2 km ofmaterial [2] may have removed additional pre-rift as well as any syn-rift volcanism eruptedthere. Silicic volcanism initiated from 200 kyrto 1.8 Myr after the Oligocene Oi2 global cool-ing event.

3. Volcanism along the Ethiopian escarpmentshows a broadly linear decrease in age fromnorth to south, which may re£ect focusing ofcontinued syn-rift volcanic activity towards thenorthern Main Ethiopian Rift.

4. A dramatic decrease in volcanism in Ethiopiafrom 24 to 20 Ma correlates with a break-upunconformity in Yemen from 26.7 to 19.9 Maand represents the shift in eruption rates andmechanisms during early syn-rift volcanismand the initial stages of Afro-Arabian conti-nental rifting.

5. Contrasting margin evolution is expressedthrough syn-rift volcanism: Ethiopia is charac-terized by sporadic bimodal silicic and basalticsyn-rift volcanism associated with small vol-canic centers; in Yemen syn-rift volcanism isabsent, although two sets of dike swarms alongthe Red Sea margin dated at 25.5 Ma and18.5^16 Ma [30] indicate small-volume syn-rift volcanism may have been emplaced andlater eroded. Di¡erences in volume and preser-vation of syn-rift volcanism may be, in largepart, related to the location of the rifting con-jugate margins with respect to the Afar plumeaxis.

6. An angular unconformity in the northernMain Ethiopian Rift spanning 10.6^3.2 Mamay be related to the initiation of sea£oorspreading in the Gulf of Aden and southernRed Sea.

Acknowledgements

I.A.U. acknowledges support from Royal Hol-loway and the Danish Lithosphere Centre in a

joint Ph.D. studentship. Tim A. Becker, WarrenD. Sharp and Abdur-Rahim Jaouni are thankedfor assistance with sample preparation and 40Ar/39Ar dating at BGC. Zemenu Geremew, Geza-hegn Yirgu (University of Addis Ababa), CindyEbinger (Royal Holloway University of London)and Ketsela Tadesse (Ethiopian Petroleum Insti-tute) are thanked for logistical help and generos-ity of their time in Ethiopia. P.R.R. acknowledgessupport from NSF grant EAR-9909517; E.W. ac-knowledges support from NER/T/S/2000/00647.This manuscript has bene¢ted from discussionswith David W. Peate and Kim B. Knight. Wethank Robert Duncan, Gilbert Fe¤raud, AndySaunders and Giday WoldeGabriel for commentson an earlier version of this manuscript.[BW]

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