Structure of Yemeni Miocene dike swarms and emplacement of coeval granite plutons

203

Tecronophysics, 198 (1991) 203-221 Elsevier Science Publishers B.V., Amsterdam

Structure of Yemeni Miocene dike swarms, and emplacement of coeval granite plutons Paul Mohr Department of Geology, University College Galway, Galway, Ireland

(Received July 24, 1989; revised version accepted August 24, 1989)

ABSTRACT Mohr, P., 1991. Structure of Yemeni Miocene dike swarms, and emplacement of coeval granite plutons. In: J. Makris, P. Mohr and R. Rihm (Editors), Red Sea: Birth and Early History of a New Oceanic Basin. Tectonophysics, 198: 203-221. Dike swarms in the Yemeni sector of the eastern margin of the Red Sea basin have been mapped and sampled. The greatest concentration of mafic dikes occurs along the Tihama (coastal) escarpment zone; less profuse swarms lace the Plateau interior. The escarpment zone also hosts microgranitic dike swarms related to a chain of closely spaced, Miocene granite plutons. Dolerite irruption overlapped in time and space with this plutonism. Southern Red Sea basin evolution commenced with flood basalt eruptions upon the late-Proterozoic Arabian craton. Subsequent continental rifting and crustal fissuring focussed dike swarms along a zone of coeval anatectic granites that largely blocked further flood basalt eruption. Immediately following this crustal fissuring/diking episode, severe crustal stretching and thinning led to block faulting and tilting. Eventually, rupture near the rift axis initiated sea-floor spreading. The resulting crust beneath the outer parts of the Red Sea basin is new igneous material that has generated two-layer neo-continental crust.

Introduction

Rifted domal topography focussed on the Afar triple junction inspired Hans Cloos (1939) to propose the sequence uplift-rifting-volcanism. This model was developed in a lithospheric dimension by Gass (1970), reinterpreted in terms of plate tectonics by Dewey and Burke (1974), and, with the additional concept of an outwardly convecting mantle plume, by Sengor and Burke (1978). Nevertheless, field evidence reveals that major uplift episodes in the African rift system were later than the initiation of rifting (Baker et al., 1972). Rifting in turn was later than the earliest, most voluminous flood basalt volcanism (Dainelli, 1943; Abbate and Sagri, 1969). This sequence, volcanismrifting-uplift, also applies to the early history of the Red Sea basin (Almond, 1986a,b). The present article examines the Miocene rifting of the eastern margin of the southern Red Sea basin in Yemen Arab Republic (YAR), with particular emphasis on diking and associated granite diapirism. Numerous dike swarms lace the western

half of Yemen, and provide clues concerning the style and timing of crustal extension during early Red Sea rifting. A pioneering study of the geography and chemistry of these dikes by Capaldi et al. (1987a) led to the proposal of three sets of Yemeni dike swarms: those that fed the plateau basalts (l), and an earlier (2) and a later (3) set along the escarpment zone separating plateau and coastal plain. Western YAR comprises three main tectonic zones, and because of the Neogene age of the tectonism the zones remain topographically distinct: (1) the Tihama (coastal plain) strip, some 25-40 km wide and site of recent alluvial sedimentation that blankets bedrock; (2) the flood lava/ignimbrite-capped plateau, now uplifted l-3 km above sea-level, and separated from the Tihama by the third unit; (3) a 20-30 km wide, deeply dissected escarpment zone that forms a structural hinge (Coleman, 1984a,b) along which the majority of dike swarms and plutons have been intruded. For brevity, and in conformity with previous usage (Coleman et al., 1977; Coleman, 1984b;

0040-1951/91/%03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved

204

I’ MOIII1

1960).

Their

respective

plateaux

ment of late-Proterozoic and

volcanics

expose

island-arc

extensively 1973;

intruded

Stoeser

a base-

metasediments by granitoid

plutons

(Beyth,

Stoeser,

1986). This

basement

overlain

by scattered

outliers

of Ordovician

glacial

sediments

Permo-Carboniferous al., 1971; El Nakhal, ward without

unconformity

sandstone,

stone. In Yemen,

Camp,

1985;

is unconformably

grade

into a shallow sequence

limestone

et up-

marine

comprising

and

these respective

and

(Dow

1984). The latter

transgression-regression Jurassic

and

again

Jurassic

sandlitholo-

gies form the Kohlan Sandstone, Amran Limestone and Tawilah Sandstone Formations (Lamare et al., 1930; Geukens, 1960). Rift magmatic activity in Yemen commenced in Eocene time, according to Capaldi et al. (1987a), with the emplacement of ATD (“Ancient Tihama Dikes”) swarms. There is more general agreement that major

Trap

Series volcanism

began

Ma ago, with flood basalts and ordinate silicic ignimbrites covering SW Arabia AL *

(Civetta

about

30

initially subN YAR and

et al., 1978; Schmidt

et al.,

1982; Chiesa et al., 1983a). This activity peaked at 27 Ma, and then waned or locally ceased. The lavas were fed from fissure swarms termed PBD (“Plateau Basalt Dikes”) by Capaldi et al. (1987a). Major volcanism and plutonism was active during

Fig.

1. Simplified

metamorphic stone; flood

geologic

and granitoid

3 = Mesozoic basalts

plutons

and

marine

Blank areas: Quatemary

of Yemen:

volcanic

rocks;

rhyolites; cover:

sedimentary

1978; Roland,

I = Proterozoic

2 = Ordovician

sedimentry

ignimbritic

and cogenetic

and Overstreet,

map rocks;

Wajid Sand-

4 = Trap Series

5 = Miocene

granite

6 = Quatemary

lavas.

cover. (Data from Grolier

1979; Rruck,

1980, 1984; Kruck

et al., 1984.)

the 23-20 Ma interval, but focussed now over central and S YAR (Chiesa et al., 1983a). This second magmatic episode, characterised by granite plutons (Rathjens and Wissmann, 1934) and rhyolitic lavas and ignimbrites, was coeval with 21 + l-Ma-old dolerite

Gdogic

setting(Fig.

1)

Yemen and northern Ethiopia, now separated by the 400~km-wide Red Sea basin, have had a similar geologic history (Dainelli, 1943; Geukens,

(“Tihama diking

Dike

Swarms”)

along the escarp-

ment zone (Capaldi et al., 1987a). 11-10 Ma ago, minor volumes of flood basalt were briefly extruded from scattered localities across Yemen

Capaldi et al., 1987a), the escarpment zone dikes and plutons are prefixed ‘Tihama’ in contradistinction from those of the plateau.

TDS

and microgranite

and central

Ethiopia

(Seife Berhe et

al., 1987; Capaldi et al., 1988). Finally, a new volcanotectonic regime became established in Yemen during Pliocene-Quaternary time with eruption of basalts and rhyolites from both NW- and transverse NE-trending fissures on the central and eastern Plateau (Geukens, 1960; Civetta et al., 1980; Chiesa et al., 1983b; Heyckendorf and Jung, 1989) concurrent with massive flood lava eruptions in Afar.

STRUCTURE

OF YEMEN1

MIOCENE

DIKE

205

SWARMS

Physical parameters of Yemeni Miocene dike swarms

0

50 km

Twenty-four traverses of Yemeni dike swarms have been made across the Tihama escarpment (Tl-T15) and on the Plateau (Pl-P9) (Fig. 2). The 3-D orientation, width, density, and lithology of each swarm was examined. Results are summarised in Table 1 and Figs. 2-4. It is important

’ q SAN'A'

160

150

Fig. 2. Map of western Yemen Arab Republic showing major towns, and roads traversed during the 1988. field program. Sections along which dike swarms have been measured, are marked with heavy trace; each section is allocated a code, T for Tihama and P for Plateau (n.b.: section P7 runs from south of Huth to south of Sa’dah, along which no major swarm was encountered). Also shown are positions of the cross-sections of fig. 4. The regional (UTM zone 38) grid is given around the map margins.

Fig. 3. Yemeni Tertiary dike swarms identified and measured during the 1988 field season. Dolerite swarms are shown with heavy trace at the swarm margins (broken trace where not exposed or observed), and dashed within the swarm. The predominant direction of dike dip is marked with a tick. Microgranite swarms indicated with dots (n.b.: some swarms contain both lithologies). Miocene(?) transverse faults identified during the 1988 program are shown with thin trace and labelled f. The Ai’thayn caldera is prominent some 50 km south of San’s’. Regional grid as for Fig. 2. The area occupied by Fig. 5 is indicated.

to note, where a synthesis is attempted below, that the present work has left large intervening areas of terrane unexamined.

Grid ref.

il

10 85E 140 1

10

394 1785

P7.1

dolerite dolerite


5 4

5+ lo?

351 1739 362 1740

P6.1 6.2

dolerite microgranite


000 130

1 ?

5? 57

Wadi Z&r-d Wadi Zabr-g

409 1707 406 1706

PS.1 5.2

4 20 5 3 1.5 1.5

85E 95w 95w

170 175 175

Hajjah Kuhian

1.5 2

0.3 0.4 0.1

0.1

dolerite

microgranite dolerite dolerite

microgranite microgranite?

miaogranite

doierite dolerite dolerite microgranite dolerite dolerite

7 1.5 1.1

1.5 5

0.1

0.1 _

1 1 2 15 5? l?

2 2 2 3 3 6

lo? ? ?

15-t 10 5? ? 5? 3?

0.2 O.l?

25 5?

2 2

Maghrabab Al Qadam Darajah

360 1670 351 1673 3411675

P4.1 4.2 4.3

1 0.5 0.5?

lo? 5? 5?

90E %W

YaZil Mafhaq Madar

AzZahr Askari Al lthnayn Al Hurmiyah Ku&n J. Babara

Maqwaiab Bani Ah

Oe+I 130 160

dolerite doletite

dolerite dol./microgr dolerite dolerite

0.5? 6 1

1.0

0.2?

Dike hthology

5? 25 5?

393 1684 383 1670 369 1668

P3.1 3.2 3.3

50

10 _

Total extension (km)

100 sow 95E 100 9oE 95w

2.5 1 5 3

Maximum extension (‘k;)

160 050 020 050 060 050

100 85E 85W

loo

&P

Dike width (m)

3-13 3 0.5 2 2 2

380 1624 382 1620 369 1634 366 1637 359 1643 350 1645

P2.1 2.2 2.3 2.4 2.5 2.6

135 170 180 160

strike

95w 90W 9oW

Abd&

370 1501 368 1500 365 1496

1.5 1.6 1.7

5

3 4 4

?

width

10 7 25

length

Swarm

150 175 150

Nagd Ta’izz Hi&ran Hajdah

391 1515 388 1508 384 1504 312 1502

Name (informal}

Pl.1 1.2 1.3 1.4

Plateau swarms

Traverse

Physical parameters of Yemeni dike swarms (summary means for width, strike and dip; J = Jabal, W = Wadi, d = dolerite, g = micro8mnite)

TABLE 1

limestone

sandstone limestone

ignimbrite ignimbrite

basalt/ignim. basah/ignim. PC granite

ignimbrite &nimbrite

basalt basalt PC schist rhyohte basalt basalt

basalt basalt basalt ss, &rim., basalt sandstone sandstone ignimbrite

lithoiogy

Hostrock

0

low

0

0

0

5-3ow 0

2ow 3ow

15SE OSSE

IOE 10E 60E

10E

05E

20E 15E 10E

05E

dip

; 2 z F

360 1787

335 1878

8.2

P9.1

354 tM0

339 1525

335 1530 343 1535

336 1545

336 1568

337 1585 344 1581

342 1623

324 1614 325 1675

315 1698

315 1710 317 1709

310 1716 312 1717

305 1723

308 1728 325 1725

309 1775

303 1822 307 1817

T1.1

T2.1

T3.1 3.2

T4.1

T5.1

T6.1 6.2

T7.1

T8.1 8.2

T9.1

T1O.l 10.2

Ttt.1 11.2

T12.1

T13.1 13.2

Tt4.1

T15.t 15.2

TihamaSW-

386 1795

P8.1

J. Hirab Wadi Ram

W. Bawhal

J. Hajibab Wadi La’ah

AI B&ah

Ghulaysi At Murran

W. Tabab tweak

W, Hatab

k?an w. lzzan

Wadi Ribat

J. Qawnis At Khurab

W. as Sanam

J. Dubas

SHYLY

J. Qafr

Al Uthayb

Marab

W. Gumatay

Al Mahasir J. Shabarah

lo+ 30+

lo?

la? 20?

7

35? 5?

35 ?

35

7 ?

5?

10+

15+ ?

15-l.

2.5

5?

20 to? 5

5 3-t

6

2 11

1

4+ 0.5

5+ ?

3+

10 1

1.5

3-f

9 3

4t

4

3

4

175 165

160

160 170

16s

155 160

000 100

oto

170 150

005

140

170

160

155 170

165

000

130

160 170

E&W 75E

75E

9OE 90W

90E

15E 95E

85W 85N

90W

75E 80E

100

95E

85E?

75E

8% 80E

9OE

8QW

90W

95E 85E

1 2

4

5 10

5

15

7 1.5

10

3 3

4?

5

4

4 5

5

30

4.5

5 3

100 300

5

25 20

10

tOo+ 2?

30 oo

30

5? <5

10

t

30

40 15

80

> 50


2? 5

1.5 3.5

0.3

0.4 1.0-k

0.1 t

0.5 +

1.3 ?

0.7

0.4

0.1

microgranite

_

doterite dolerite

dolerite

ho-dole&e

miCrOgrtiIlite

dot./mkrogr.

microgranite doterite

PC schist PC granite

PC granite

Limestone PC granite

basalt

ignimbrite rhyolite

rhyotite ?

rhyotile

scram te microgranite dote&r?

rhyolite basalt

rhyolite

rhyolite

Mio. granophyre

Mio. granophyn

Mio. granqhyre limestone

Mio. granophyre

ignimbrite

PC granite

basalt/ignim.

timestone

microgranite dolerite

microgranite

micorgranite

dolerite

0.7

2.5 0.4

1.8

1.5?

dolerite

_

~~~~e

dol.,‘microgr.

0.1 0.2

5OW

OSE

1oSw

t5sw

35NW

t0N

1ON OSN

35s

30NW

35w

25w

0

208

P MOHK

Strike (Fig. 3 and Table 1) The Red Sea basin at the latitude of Yemen exhibits two divergent structural trends: (i) the locus of Pliocene-Quaternary sea-floor spreading and its containing axial trough run NW-SE; (ii) the Yemeni escarpment, marking the Miocene basin margin, runs NNW-SSE. These two trends converge on Bab al Mandab, at the southern end of the Red Sea basin. The predominant Miocene dike trend in Yemen is NNW, with subordinate N and NNE trends in the central sector of the Plateau. The parallelism of many Tihama dike swarms with the coeval basin margin indicates a common structural control. Tihama swarms in central and S YAR are bimodal at the structural level exposed. In addition to the intense dolerite diking, microgranitic swarms are intimately associated with a line of Miocene silicic plutons that today form a dramatic mountainous terrane along the western side of the escarpment zone, between Al Qanawis and Al Mukha (Figs. 1 and 2). Tihama dolerite swarms follow the basin margin in the north, but in the south swarms tend to diverge anticlockwise from this trend. For example, intense sheeting within the Jabals Qafr and Dubas granite plutons (swarms T2-T4) runs NW-SE, parallel to the Red Sea axial trough farther west rather than the NNW-trending line of 0’

host plutons. The Qafr-Dubas dikes thus project obliquely into the Tihama coastal plain sector. In the opposite, southeast direction, the Jabal Qafr pluton and its contained dikes become mantled by NNW-trending sheeted silicic dikes (Tl) that fed voluminous rhyolitic ignimbrites. Farther east still, near the top of the escarpment zone, profuse NW-trending dolerite dike swarms (eg. Pl) traverse the block-tilted flood basalt pile in the Ta’izz sector, and terminate in PDR Yemen (‘Aden). Plateau dike swarms are more variable in trend. The central part of the plateau, between Bajil and Dhamar (Figs. 2 and 3) contains a singular domain of NE-trending dolerite and subordinate microgranite dike-swarms (P2.2-P2.6). To the north, in the Manakah region, some of these swarms swing to a N-S trend (e.g. P4). Mafic swarms on the northern plateau mostly trend NNW, parallel to the younger Tihama dikes farther west. Dip (Fig. 4 and Table I) The dip of individual dikes can vary, and even the polarity can switch along strike. Nevertheless, most Yemeni swarms, and particularly the doleritic ones, show a > 90% consistency of polarity. Except in the extreme south, Tihama dikes dip steeply eastward. This, and the west-directed dip of the co-genetic stratoid volcanics (e.g. TlO, Tll, T12), are consistent with an originally vertically injected cl

SHAHARAH 1

AL

HVTH

1

KHUSM

B

B’

SAi;A’

MANAKHAH

1

r = y:: = r ..::

A”’

A” A’

J. QAFR

Fig. 4. Schematic ENE-WSW

TA’IZZ

MAQBANAH

sections across western Yemen (for locations

dike swarms. True dips are indicated.

A

I

I = Amran Limestone;

1Okm

see Fig. 2), showing widths and attitudes of the major

Dolerite swarms are dashed, microgranite

basalt; G = granite (age not distinguished);

0

dotted, as in Fig. 3. Host-rock

r = rhyolite, mostly ignimbrite;

symbols:

b = flood

s = Kohlan Sandstone.

STRUCTURE

OF YEMEN1

MIOCENE

DIKE

209

SWARMS

range 3 to 5 km, and is independent of whether the swarms are mafic or silicic. Swarm margins can be either diffuse or abrupt (e.g. T1.4E, cf. T4.2W). From the nature of the field-program, focussed on traverses of the dike swarms, the lengths of most Yemeni swarms have not been established. The values given in Table 1 are estimates tending to minima, and are based on simple linkage of neighboring traverses, and occasionally from sightings from high vantage points. A typical length: width ratio is estimated at 5 : 1, again without clear distinction among mafic and silicic swarms. Abrupt apparent termination of some Tiiama swarms coincides with a change of hostlithology, in turn associated with transverse faulting that can effect a change of structural level of hundreds of metres.

upper crust that was subsequently block-faulted and tilted. The Yemeni escarpment zone thus mirrors the opposing, Ethiopian escarpment across the Red Sea basin (Abbate and Sagri, 1969; Mohr, 1983). In the few cases where backtilting in Yemen is directed eastward, the dikes dip appropriately westward, retaining an angle of 100 + 15B to the fed lava pile (e.g. Pl). Gross non-perpendicularity of dikes and hoststratoid volcanics is a feature of some silicic swarms (e.g. T3.3, T7.1, T8.1, T1O.l). Possible reasons include fissure-fed ignimbrites that emerged on paleoslopes, and fanning out of microgranite sheets up from shallow silicic plutons. Less common instances of non-perpendicularity for doleritic swarms relate either to inferred predike tilting of Mesozoic host-strata (e.g. P6.2, T13.1), or to uncommon irruption post-dating the stretching and block-tilting of the basin margin. Yemen Plateau dikes dip within 10s of vertical, lying as they do outside the influence of the block-tilted escarpment zone.

Cross-swarm

extension

Individual dolerite dikes tend to be narrower than microgranite dikes. The former are typically l-5 m thick, with modal values of ca. 2 m for Plateau and 4 m for Tihama dikes. Microgranitic Tihama dikes can attain 30 m thickness (e.g. Tl), and 5-10 m is common. However, in mixed

Width and length

Yemeni dike swarms range in width from 0.5 km to at least 15 km. Modal width lies in the

TABLE 2 Mid-Tertiary dating of igneous events in the southern Red Sea basin 1

2

3

4

5

6

Yemen Trap rhyolites

Tihama granites, dikes

Tihama mafic dikes

Ethiopian basalts and rh yoh tes

(Ma) 18 19 20 21 22 23 24 25 26 21 28 29 30 31

Jiddah silicic mafic dikes and volcanics

Tihamat Asir complex

Afar margin dikes, granites ? plateau

gabbro/diorite e=lY

rift ? volcanics

Yemen Trap basalts

I dikes

Ethiopian flood basalts ?

Key: 1 = Jiddah sector; 2 = Jii sector; 3-5 = Yemen sector; 6 = Afar sector. Sources: Barberi et al. (1975) Capaldi et al. (1987a,b), Civetta et al. (1978), Coleman et al. (1977, 1979), Megrue et al. (1972), Pallister (1987).

210

I’ MOIIK

Tihama

swarms,

common

silicic

and

1-5 m thickness,

for Plateau Total critically

extension

dependent

on

express

depth.

extension

The for

northernmost toward

share

a

structural

the extension maximum injection (T15).

degree

of

is 100-3008 Swarm

‘Asir sheeted-dike

in

Composite proximity

dikes

to the

an initial,

sheet

a wider

injected

axially

sheet. The boundary usually

is

hornblende-dolerite

La’ah (T13.2)

whose marginal

sharply

bounded

complex

cognate.

dikes

of

(Coleman

The

Wadi

but

their axes where

commay be

have effected

a

1-3 m deep brecciation of the adjoining Precambrian granitoid host, attesting to pre-irruptive gas injection

during

The Jabals Qajr/

axes,

La’ah

dikes zones en-

xenoliths

these xenoliths

marked offset can occur either to west (e.g. P1.3) or east (e.g. T15.2). Microgranite swarms attain along

phenomenon

in the massive

to their

status

the

revealed

angular,

A separate

is

often feature

at Wadi

most

sheeted-dike

microgranitic

the two lithologies

microgranite.

train

in (e.g.

thin dolerite

marginal

tures into fissures.

almost

by between

sharp, but mafic enclaves

(T2, T3) discussed usually show their

close

plutons

posed of calcic plagioclase;

1984b). Up to 80% crustal for the dolerite swarm in

injection

common

granite

P1.l, T8.2). They comprise

the Jabal Qafr granite pluton below. Yemeni dike swarms intense

are particularly Miocene

T15 strikes

60 km to the NNW

et al., 1979; Coleman, extension is measured

(e.g.

in the uppermost observed

mafic

the Tihamat

level

is

zone, and surely increase

Tihama

SW Saudi Arabia,

a dike-swarm

the values given in Table

crust of the escarpment with

across

1961). Therefore

2 merely

dikes

silicic dikes.

crustal

Walker,

mafic

which is also the range

widening

of the initial

frac-

Dubas diked granites (Fig. 5)

The host granites

Tl, T1l.l).

Miocene plutons along the Tihama escarpment zone are composed predominantly of granophyre/ microgranite. Capaldi et al. (1987b) have dis-

Coeval mafic and silicic intrusion

tinguished two major mineralogic solvus (> 4-5 kb) alkaline biotite

exposed

at a relatively

deep structural

level (eg.

Relations among dolerite and microgranite Dolerite timately

and microgranite if intermittently

Tihama. This intimacy swarms are intersected vice versa (e.g. TlO.l),

dikes

dike-swarms associated

are in-

along

the

suggests that, where dolerite by microgranite swarms, or the time-interval

between

the two irruptive episodes was brief. This is confirmed by the not infrequent occurrence of mafic enclaves in Tihama silicic dikes and lavas/ ignimbrites (cf. Bacon, 1986). On the Plateau, bimodal swarms appear less common (e.g. P8). Within a given bimodal swarm, dolerite and microgranite dikes have identical strike, dip and modal thickness (e.g. T1O.l, T12.1 and P8.2). This is consistent with competition among coexisting mafic and silicic magmas to enter a common fissure system during a crustal rifting episode. The proportion of mafic to silicic dikes in the Tihamat ‘Asir swarm of SW Arabia is estimated at 2 : 1 (Coleman et al., 1979). In Yemen, this ratio is variable but can reach 1: 1 (e.g. T12.1).

types: (1) subgranophyre, and

(2) hypersolvus (< 4-5 kb) peralkaline arfvedsonite granite. The latter is distinguished chemically by lower Ca and Sr, and higher Fe. The present work reveals that this classification may be

oversimplified,

phyres

form

much

for

riebeckite-phyric

grano-

of the Qafr/Dubas

plutons,

and aegirine granite is recognised at Gabal Sabir. Capaldi et al. (1987b, 1988) consider that the granitic magmas formed from mantle melts through fractional crystallisation processes. If this be the case, then the following data are relevant to the nature of the Tihama crust: (1) the plutons, whose margins are almost invariably under volcanic cover, are at least 15-20 km wide; (2) the exposed thickness of granite exceeds 3 km for the largest plutons (e.g. Gabal Sabir); (3) under ideal conditions, olivine basalt magma could yield up to a few percent (< 5%) of its initial mass as granitic magma through crystal fractionation (Barberi et al., 1975). Accordingly, if a 20-km-diameter, 3km-thick Yemeni granite was derived from olivine tholeiite by fractional crystallisation alone, the source magma body, even if it could have attained

STRUCTURE

to

OF YEMEN1

AI Hudaydah

\

MIOCENE

DIKE

J. DUBAS

9

\

E.limi2, dike

531.

*\ 55 <

S

211

SWARMS

f

/

v1 _.

30 I

5 _. 0

TABLE 3 Estimation of diking intensity across the

2

85 “s

i

Terminal location West (Jabal Ghaziyah)

‘akh,ah

154

1 km

RUWAYNAH

JABAL

QAFR

\

\

‘? 153

WADI AL UTHAYB

00%

Total diking Length of section (km) width (km) 1.5 1.5 3 1.0 1.2 0.5 3

0.3 0.4 ca. 0.85 0.3 0.5 0.15 ca. 0.4

Crustal extension (W) 25 35 40 45 70 45 15

East (Wadi Jabal Assuad)

SUALHIRA

\

northern end of the

Qafr pluton

3/+

I

Fig. 5. The Jabals Qafr/Dubas dike swarm in the vicinity of Hays. Percentage values show the calculated degree of crustal extension due to diking across the indicated transect. Arrows mark dips of cover strata mantling the granite plutons (grads). Lithologic abbreviations: g = Miocene granite; I = Amran Limestone; q = Quatemary coastal-plain sands; s = Kohlan Sandstone. f= fault trace. WJA = Wadi Jabal Assuad. Regional grid is given at map margins. Location of section in Fig. 6 is indicated.

the full SO-km-width of the Tihama, would require to have been at least 12 km deep. That is the present thickness of the crust below the Tihama (Mooney et al., 1985). Even allowing that the crust was stretched and thinned during and after granite emplacement, the proportion of added igneous rocks to the Precambrian sialic crust must be such as to significantly influence seismic-section interpretations. Alternatives to a crystal fractionation origin for the Yemeni granites are presented below.

Intensity of diking From a distance, the Jabals Qafr and Dubas plutons have the appearance of gigantic cooling baffles, or, as first reported by Karrenberg (1957, p.180), “einer grosse Anzahl ‘Bretter”’ [a great multitude of ‘planks’]. Both plutons are densely combed with quite regularly spaced dolerite dikes that trend 15s oblique (anticlockwise) to the pluton axis (Figs. 1 and 3). Table 3 gives an estimation of the intensity of diking across the northern end of the Qafr pluton (Fig. 5). The data derive from dike counts, and assume a modal dike-width of 4 m (Table 1, T3.1). In the zone of greatest extension, offset east from the axes of the pluton, 122 dikes were counted along a 1200 m section, yielding nearly 500 m widening, or 70% extension (Fig. 5). The total amount of extension across the Qafr swarm is some 3 km for a 12&m-wide section, a minimum value as neither the east nor west margins of the swarm are exposed. The entire studied section across the pluton therefore widened by about 35%. 15 km farther south, at Wadi al Uthayb (T2), 80% extension is measured across a 0.2-km-wide zone (Fig. 5). This zone strikes NW into the 40% extensional zone of section T3, indicating a dextral offset of the axis of maximum injection. Another, 3 km dextral offset occurs between the axes of the Qafr (T3) and Dubas (T4) swarms (Fig. 5). In this way, the zone of maximum injection retains an overall NNW trend, parallel to the

212

Wadi

escarpment, despite the NW trend of the en-echelon arrangement of individual dike-swarm sectors.

The curvilinear, cuspate and bulged margins of many of the Qafr and Dubas dikes show that dolerite magma was emplaced into a ductile or even liquid granite host, a phenomenon described from elsewhere by Walker (1969) and Hibhard and Walters (1985). Such margins to Qafr/Dubas dikes are typically chilled against a mediumgrained granophyre. Thus, although individual dikes can retain an identity for hundreds of metres along strike and up dip, at a scale of metres to tens of metres dikes can be sinuous, bifurcated and networked. Mechanical admixture of mafic and silicic liquids is evidenced in several Tihama dike swarms (e.g. Tl, T3, T4, T7, T12). For example, near the eastern margin of the Dubas swarm (T4), a dolerite dike translates up into a train of globules that are abundant enough (up to 30% of the granophyredolerite mix) to comprise an emulsion. Mafic magma here rose up a fracture in congealed granite until it encountered a plastic or liquid zone in the upper part of the pluton. It then sprayed as 1-15 cm mafic globules, the two liquids remaining immiscible until the globules had frozen. This dikesquirt coincides with a transverse, NNE-tren~ng fault that deforms both Iithologies, mostly in a ductile fashion that suggests that faulting and crustal fissuring/diking were contemporaneous. The emplacement relationships of granite and mafic dikes are revealed in a crucial exposure at the NE margin of the Qafr pluton (Fig. 5). At Suayhira village, a roof of Precambrian schist and Mesozoic sedimentary rocks is moderately injected (up to 15% extension) with NW-trending dolerite dikes. The Mesozoic strata dip 20gNW, but south toward the exposed margin of the pluton, at Wadi Jabal Assuad (Figs. 5 and 6), these strata swing and steepen to 30-40BSW, and to 70gSW at the granite contact. Steep inward dip of cover strata toward Tihama granitic plutons is also observed at Jabal Dubas (T4, E side), Jabal Bura (PZ, E side), and Jabal Milhan (T7, W side), a structural phenomenon that is reminiscent of the Amaro horst in the Ethiopian rift valley (Levitte et al., 1974).

Fig. 6, Simplified

Jabal

cross-section

suad (see Fig. 5 for lacation), Limestone

(lined)

on Miocene

Assuad

(E-W)

across Wadi

showing

granite (crosses) at the eastern

margin of the Jabal Qafr pluton. Vertical tal scale, but true dips are indicated. subvertical

Jabal As-

thin cover of Amran

scale ca. 2 x horizon-

Dikes are schematised by

heavy lines, and have been reduced in number

clarity.

s = skam at granite-limestone

for

contact.

At Wadi Jabal Assuad, the Precambrian and Kohlan Sandstone rocks preserved 2 km to the north at Suayhira are missing, and decarbonated Amran Limestone lies directly upon chilled, finegrained biotite granophyre (Fig. 6). ~arnetiferous skams are variably developed at the contacts. A remarkable feature of the Wadi Jabal Assuad exposure is an abrupt cut-off of those granitehosted dolerite and microgranite dikes that fail to penetrate the overlying limestone strata. This indicates that diking was partly accomplished hefore the emplacement of the Qafr pluton at its present structural level. The implied sequence of events is (i) major extension and mafic diking of the Qafr pluton during its primary empla~ment and partial consolidation, and (ii) overlapping and immediately followed by rise of the pluton through the land surface, under cover of its own volcanic carapace. The granite-limestone contact, marked by a thin ( < 50 cm) limestone breccia, must therefore be an inward-dipping tectonic one (Figs. 5 and 6). How is this inward dip to be reconciled with pluton uplift? And how were the Precambrian and Kohlan Sandstone rocks removed, which originally underlay the Amran Limestone beds that now rest directly on granite at Wadi Jabal Assuad? The upper part of the dike-injected granite that originally extended above the present contact was also presumabiy removed at the same time. While firm answers to these questions must await detailed field-mapping and structural analysis of the Qafr and other Tihama plutons, constraints can be provided. First, the Qafr granite

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during early rise of the pluton (Fig. 7c), or to terminal cooling and subsidence of the pluton. The Qafr/Dubas (T3, T4), Wadi La’ah (T13) and Harad (T15) swarms from S, central and N Tiiama, respectively, share a common radiometric age of ca. 21 Ma (Capaldi et al., 1987a). Nevertheless, diversity is manifested through the lamprophyric affinities of the Wadi La’ah hornblende dolerites. The Qafr/Dubas dolerites, being relatively severely propylitised within their coeval granite host, have higher K, Rb, Y and Zr, and lower Mg than the Harad dolerites intruded into Precambrian granitoids of N Tihama (Capaldi et al., 1987a). Geochemical study of the dike samples collected during the present program is in progress.

evidently cannot have stoped in a single stage into the cover strata, otherwise the contained dolerite dikes would not be preserved up to a planar contact. Second, the fine-grained marginal facies of the granite indicates that the silicic magma cooled rapidly, though whether against its present limestone cover remains uncertain. Third, the indistinguishable dips of dikes (a) in the granite and (b) those penetrating the cover limestone strata, prove that no block rotation has been responsible for the inward-dipping granite-cover contact. It is tentatively proposed (Fig. 7) that granite emplacement at Jabal Qafr, although within a zone of tectonic extension, was accomplished through forceful hydrostatic injection of mafic magma within a ballooning granite pluton. This resulted in low-angle decollements at the pluton margins in which progressively higher levels of the upper crust were transported away from the intrusive zone. The present inward dip of the upper plate can be related either to lateral gravity collapse

Jabal Milhan granite emplacement

The Jabal Milhan pluton lies immediately north of the Sana’a-Al Hudaydah road (Figs. 1 and 2).

W

E post - P.C. cover

Up. Proterozoic

-

basement

a(

C

I

‘b

d

Fig. 7. A proposed model of granite and dike emplacement at Jabal Qafr, to help explain the field relations observed near Suay’hirah (see Figs. 5 and 6). (a) Forceful diapiric injection of granite magma into the upper crust. Post-P.C. Cooer comprises sedimentary strata of Paleozoic and Mesozoic age, and Oligocene-Miocene stratoid volcanics (dike feeders not shown). Note 2-km-scale vertical and horizontal bars. (b) Irruption of mafic magma, focussed into the solidifying granite, rejuvenates the mobility of the pluton, which rises further toward the surface, ballooning as it does so and initiating lateral intrusion under the adjacent cover rocks. A granite-generated topographic dome encourages slumping off of remaining cover-rocks. Mafic magma (dotted) rises along sectors of the pluton margin. (c) Pluton-crust density differential in a high heat-flow regime facilitates rise of the pluton above the breached surface, with further decollement of the adjacent crust. Late mafic dikes intrude both pluton and the under-injected countryrock. (d) The pluton and its largely volcanic carapace reach a maximum elevation. Mafic intrusion is now restricted to large pipes (the earlier dikes are omitted for clarity).

214

The present topographic summit attains 2520 m elevation. At its ~u~~te~ margin and lowest exposed level (ea. 250 m asl), the granite has been emplaced into subho~ntal Ed-Tertiary flood basalts. However, it is separated from direct contact with these lavas by a ca. 6-m-wide sheath of intrusive basalt that back-veins into the granite for tens of metres. These veins trend NW and dip 75BE, parallel to the Dubas/Qafr dikes. They develop patches of pegmatitic pyroxene, and frequently lead into broader patches of pegmatoid quartz and orthoclase within the adjacent granite. The veins therefore provided channels for volatile transfer. The marginal granite at Jabal M&an is not chilled, attesting to its contact with the hotter, mafic magma. The pluton is considered to owe its to~grap~c eminence to lubrication by a coeval basaltic ma~atic sheath during structurally controlled uplift throu~ the upper crust. Jointing in the SE flanks of the granite strikes NO75g and dips 70% hinting at an upward tapering of the original pluton.

160

Yemen

The Yemen dike-swarm traverses (Figs. 2 and 3) can be linked to yield a regional pattern. This has been attempted in Fig. 8, based on similarities of swarm parameters and dike lithologies, and taking note of changes of structural level across transverse fault-zones that augment or diminish swarm intensity at the present surface. A prominent zone of ~cro~~te dike swarms extends from the YAR-PDR Yemen border, north to where it loses identity at the northern limit of the eroded Tertiary rhyolitic lava pile, not far south of the Saudi Arabian border. The swarm zone overlaps the belt of Miocene granite plutonism (Fig. 8), of which it is evidently a higher level of expression (Capaldi et al., 1987b). On the Plateau, ~~o~~te swarms link exposed Miocene granite plutons, including a presumed pluton beneath the Q-~~~~e~~ ~~yn caldera. For example, swarm P3.2 links this caldera NNW to the Manakhab Granite; and swarm P4.1

150

Fig. 8. Preliminary map depicting proposed full extents of beak dike swan&, lions observed sections (Fig. 3) alon strike (see text). Doleritic swarms shown dashed, microgranite dotted. Individual swarms are not necessarily continuous within the indicated envelopes. Miocene granite plutons shown with fine dots. Regional grid at map margins.

links the Man&u& and As Salafiyah Granites (Kruck, 1984). These linking swarms suwt the plutons to be cupolas on deeper-seated ba~~~~~. drawing further attention to a significant new component in the Yemeni crust. i Dolerite dike swarms form a regional pattern not easily reconcilable with Capaldi et ah’s (1987a)

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tripartite distinction of TDS, ATD and PBD swarms (see above). The single instance of crosscutting dolerite swarms observed during the present work occurs on the Plateau (P2.1 and P2.2; Fig. 8) and even here no significant age-difference can yet be demonstrated. Regarding the Tihama dolerites, Capaldi et al.‘s (op. cit.) own chemical data support no significant distinctions between purported older (ATD) and younger (TDS) dolerites [n.b. Plateau (PBD), cf. Tihama dolerites are clearly richer in Al and Mg, and poorer in Ca and Ti, but Capaldi et al.% PBD samples are restricted to one swarm near Sa’dah]. The dike-swarm map (Fig. 8) demonstrates an apparent 150 km gap in dolerite emplacement between the Al Qanawis bimodal swarm (T12.1) in the north, and the Qafr/Dubas swarms (T2-4) in the south. This gap can be interpreted in terms of the coeval granitic magma chambers acting as density filters that inhibited the ascent of underlying mafic magma. Thus it is considered that mantle melts were generated along the entire length of the Tihama, despite the intermittent development of high-level dolerite dike swarms. Beyond the northern limit of the Tihama Miocene granites, a broad zone of NNW-trending dolerite dike swarms emerges in Wadi La’ah (T13). The abruptness of this emergence may additionally be related to transverse faulting that brings up Mesozoic strata on its southern side. A sinistral jump in the original injection pattern, west from Plateau interior swarms (Fig. 8) is also possible. North from Wadi La’ah, mafic diking continues (T14, T15) via Harad to the Tihamat ‘Asir Complex of SW Arabia; the transition from lamprophyric to ‘normal’ dolerites along this zone remains to be located. The powerful development of mafic diking along the northern Tihama of Yemen, trending parallel (NNW) to the coastal and escarpment zones, demonstrates a strong, upper crustal structural control on mafic irruption in the absence of silicic plutonism. No NW-trending swarms, offset en-echelon, occur here to match the pattern farther south in some of the Miocene granites. Within the northern Plateau, the Hajjah swarms (P6.1, P6.2) may link north to the powerful swarm at Jabal Shaharah (P8.2) and possibly as far as

Wadi Gumayli, west of Sa’dah (P9.1). These and other Plateau dike swarms, together with major centres, are presumed to have fed the late Oligocene-early Miocene flood lava pile in Yemen. By contrast, the younger, Tihama mafic dikes of the proto-Red Sea rift margin have not yet been connected to any extant lava pile. Tihamat ‘Asir (SW Arabian) dike swarms

Dike swarms of various and contrasting styles continue north from YAR for 1500 km along the coastal plain-escarpment boundary of W Saudi Arabia, some 20-35 km inland from the coast (Blank, 1977; Coleman et al., 1977, 1979; Coleman, 1984a, b; Pallister, 1987). In SW Arabia, dikes follow a zone of steeply SW-tilted blocks at the margin of the proto-Red Sea continental rift (Bohannon, 1986b). A particularly intense swarm, the Jabal at Tirf swarm, is exposed along the Tihamat ‘Asir, inland from Jizan and immediately north of YAR. Its dikes share parallelism with the N155gE-trending axis of Red Sea spreading and the SW Arabian escarpment, the latter having turned abruptly from its NNW trend in YAR. Dips of 60-70gNE for many of the dikes indicate that block-tilting followed (vertical) irruption of the swarms, as in YAR. The Jabal at Tirf swarm is exposed across a width of 5-10 km, but aeromagnetic studies extend this westward for at least another 15 km (Blank, 1977; Coleman et al., 1977). Swarm density increases toward the Red Sea (Blank, 1977). Individual dike widths range 0.5-18 m (post-tilting, vertical dikes lie in the lowest sector of this range). The predominating lithology is an andesine plagioclase-clinopyroxene dolerite containing abundant accessory iron-titanium oxides and minor quartz (Coleman et al., 1977; Coleman, 1984b). As in the Yemeni Tihama dolerites, however, the primary mineralogy has typically suffered a degree of hydration, due to vigorous hydrothermal circulation in the upper crust during rift extension and plutonism (Blank, 1977). The Tihamat ‘Asir, like the Yemen Tihama, exposes an intimate association of silicic and dolerite dikes. The former share a similar chemistry with small granophyre stocks in the Jabal at

216

Tirf rift zone (Coleman et al., 1977). Bimodal, dolerite and rhyolite dike swarms also occur farther north near Jeddah (Pallister, 1987). The age-range of the SW Arabian diking is considered by some authors to be as tight as 24-22 Ma (Coleman et al., 1977; Cotonian et al., 1988) though others date a commencement at 27-26 Ma (Blank, 1977; Pallister, 1987). Regional flood volcanism in W Saudi Arabia was concurrent with flood volcanism on the Yemen Plateau at 29-24 Ma (Coleman et al., 1977; Civetta et al., 1978; Capaldi et al., 1987a). On the plateau inland from Tihamat ‘Asir, the Harrat Hadan and As Sarat alkali olivine basalt fields were supplied from pipe-feeders (Schmidt et al., 1982; Bohannon, 1986b). The oldest of these lavas are dated at 31-29 Ma, while the youngest could be coeval with the Tihamat ‘Asir diking (Bohannon et al., 1989). The lava feeders may be related to the 27-Ma-old PBD dikes of the Sa’dah region, northern Yemen Plateau (Capaldi et al., 1987a). The Miocene southern Red Sea stress-field Coleman et al. (1984a) consider that, while early crustal fissuring and diking in SW Arabia had a NW-SE trend, subsequent to tectonic development of the rift margin the preferred orientation became N-S. However, Voggenreiter (1987) proposes a NW-SE trend for both pre- and posttectonic dikes, the N-S dikes in some sectors expressing a postulated post-irruptive 40-20 o clockwise rotation of discrete crustal blocks: Voggenreiter claims that young NW-trending dikes cut purportedly rotated, now N-S dikes. In Yemen too, differing trends are recognised; notable are Tihama doleritic swarms that are NW-SE in the south but NNW-SSE in the north. However, available radiometric data are unable to discriminate between these (Capaldi et al., 1987a), and it has been shown above that the NW-trending dikes are disposed en-echelon to yield an escarpment-parallel, NNW-trending belt. Capaldi et al’s (1987a) identification of more than one episode of pre-tilting diking along the N Tihama of YAR could not be confirmed in the present reconnaissance survey. Capaldi et al.% (1987b) proposal of two episodes of rift-related

f’ MOHK

granite plutonism in YAR, at ca. 26 and 22 Ma, respectively, is rendered dubious. Thus the Jabal Dubas Granite was injected with coeval dolerite sheets (see above): yet Capaldi et al. (op. cit.) present K-Ar apparent ages of 26 Ma for granite and 21 Ma for dolerite. A similar discrepancy occurs in the Tihamat ‘Asir data of Coleman et al. (1977). Why do the Qafr-Dubas dike swarms trend NW-SE, oblique to the NNW-trending contemporary rift margin in YAR, while yet retaining their position along the margin through right-en echelon offsets? It is proposed that a component of sin&al shear acted briefly during dilation of the rift margin in Yemen. This in turn resulted from differential meridional stresses acting on the SW Arabian Peninsula, due to westward propagation of N-S dilatation along the proto-Gulf of Aden (Fig. 9). The Yemen Tihama was thus susceptible to sinistral shear during weakening of its underlying lithosphere by Red Sea rifting. Nevertheless, the abrupt turn of the Red Sea margin, from NNW at Harad (YAR) to NW at Jizan (SW Arabia), still demands a regional structural explanation, The essence of the problem relates to the non-parallelism of spreading axis and basin margins in the southern Red Sea, expressed in a symmetric convergence of the Yemeniand Dane-block margins toward that axis (Fig. 9). If the Danakil block has rotated during opening of the Red Sea, this symmetry is merely a coincidence specific to the present moment in the evolution of the basin (Mohr, 1970). But more than coincidence may be involved. Both the Red Sea and Gulf of Aden spreading axes, as they converge on Bab al Mandab, project closer to the Yemeni than to the opposing sides of their respective basins. If the plan of Red Sea-Gulf of Aden spreading is conforming to a regional plate-tectonic stress-field, then it is the cont~~ng basin development in the junction area that has veered. In both basins the vergence is toward the Afar hot-spot @chilling, 1975), which lies southwest of the projected intersection of the Red SeaGulf of Aden sea-floor spreading lines. This implies that basin development is attracted toward neighboring mantle thermal anomalies. Additionally, the centre of pre-basin Miocene flood

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\

-e Gulf of ‘Adsn rift propogoiion

Fig. 9. Tectonic elements of the present southern Red !3ea basin, including margin of Miocene continental rift (thick broken lines), 1000 m isobath in Red Sea and Gulf of Aden (thin broken lines), Quatemary fault belts in the rift system in Ethiopia (thin solid lines), Quaternary graben at Ethiopian Plateau-Afar margin (double lines), and the erosional margin of the Yemeni Plateau (dotted line). The inset shows the postulated early Miocene ~~st~sion~ zone of Yemen Tihama with its en-echelon fissure pattern.

volcanism in Ethiopia-Yemen, as well as of subss quent major Ethiopian uplift, lay at the southwestem comer of Afar. This in turn suggests that veering of the Red Sea and Gulf of Aden rift basins accommodated only partially to a focus of asthenospheric perturbation situated under the central Ethiopian region. Implications for the nature of the crust under the southern Red Sea basin

There have been numerous discussions on the nature of the crust that links undoubt~ oceanic

crust under the Red Sea axial trough to full-thickness continental crust under the Nubian and Arabian shields (Barbeti et al., 1975; Makris, 1975; Co&ran, 1983). The structural geology and geophysics of the escarpment ‘hinge’ zones are of crucial importance here. Yet in recently reviewing the relatively well-studied SW Arabian Tihamat, Bohannon (1986a) concludes that the precise nature of the crust there remains undetermined. Geophysical profiling of the crust beneath the Red Sea coastal plain in SW Arabia reveals an abrupt thinning, from 40 km under the plateau to 20 km under the eastern sector of the coastal plain (Gettings et al., 1986). Coleman et al. (1979) and Voggenreiter (1987) consider the Tihamat ‘Asir dike complex, emplaced within this zone of abrupt thinning, to mark the boundary between continental and oceanic crust. The degree of dolerite irruption increases abruptly from near-zero at the plateau margin, to 50% dikes (100% extension) only 1 or 2 km westward, and to 75-100X dikes (3~~-infinite extension) after 4 km (Bohannon, 1986b). Screens of plateau crustal rocks within the dike complex, and sialic xenoliths in individual dikes, diminish westward to zero. Nevertheless, a 20-km-thick crust, decreasing to 10 km at the Farasan Islands, cannot be normal oceanic crust. This leads Coleman (1984b) and Bohannon (1986b) to postulate Tertiary igneous crust beneath the Tihamat ‘Asir, continuing west under the Red Sea coastal shelf which Cochran (1983) had described as “stretched continental crust injected with zones of mafic diking”. In eastern Afar, the Gulf of Tajura margins are considered by Gadaha and Varet (1983) to be underlain by igneous crust that separates thinned continental from true oceanic crust. In a review of the nature of the Afar crust, Mohr (1989) concludes that magmatically active continental rifts are sites for the generation of new continental crust comprising an initially mafic lower and a granitic upper layer. In Yemen and NW Afar, this upper layer is commonly exposed as high-level Miocene granite plutons and their volcanic carapace. The crucial source of these granites and related rhyolites is debated: (i) Capaldi et al. (1987b) propose an origin by crystal fractionation of tholeiitic m~tle-delved magma; (ii) Coleman

218

P

(1984b) and Coleman and McGuire (1988) propose an additional input into the fractionating system from anatexis of the enclosing continental crust; radiogenic upper crust is excluded on isotopic grounds from this model (Pallister, 1977; Pallister and Henner, 1989); (iii) it is proposed here that partial melting of underplated Tertiary gabbro sills during mantle thermal pulses has also been important. Summary geologic evolution of the southern Red Sea margins

Continental rifting during the 32-20 Ma interval formed the initial Red Sea basin (Schmidt et al., 1982). The rift filled with lacustrine sediments, and with volcanic pyroelasts and lavas in which the silicic/mafic ratio increased with time. Magmatism was focussed about the Afar triple-junction, including the present-day Ethiopian and Yemeni plateaux, where lavas and ignimbrites overflowed for hundreds of kilometres beyond the proto-Red Sea rift margins. Along the rift, volcanism became more confined NW-ward, abruptly so some 500 km from the Afar focus and then diminishing for a further 600 km to near-zero at Jiddah (Schmidt et al., 1982). Table 2 attempts a temporal correlation along this profile. During the 24-20 Ma interval, bimodal dike swarms were irrupted from Tihamat ‘Asir south to Bab al Mandab, and along the opposing margin of the rift (now the western margin of Afar), in intimate association with granitic plutonism. The main mafic diking episode was followed by faulting and basinward rotation of crustal blocks (Coleman et al., 1977; Mohr, 1983, 1986; Cotonian et al., 1988). The early evolution of the southern Red Sea basin along its eastern margin can be tabulated as: (4) Crustal stretching, block faulting and tilting Intense mafic diking and al(3) kali granite plutonism/volcanism (2) Flood mafic and silicic volcanism peaking at (1) Initiation of rifting ?

ca. 20 Ma

24-20 31-24 27-26 32-30

Ma Ma Ma Ma

MOHH

Along the opposing, western margin of the rift, intense mafic diking was active 26-19 Ma ago. culminating at 24 Ma with accompanying peralkaline granite intrusion (Mohr, 1978). Though these events are apparently 4-2 Ma earher in Afar than in Yemen, the Yemeni margin would have been susceptible to thermal ove~rinting on synchronously set isotopic systems, due to activity along the transtensional zone identified here, If this argument is correct, rather than a diachronous Red Sea margin development. then the listing given above requires appropriate correction. Intense mafic diking, acting as precursor to crustal stretching and attendant block faulting and rotation, is recorded from other magmati~ continental margins. The E Greenland margin provides a clear such example where, in the opening of the Norwegian Sea, profuse diking 53-51 Ma ago preceded faulting and block tifting during 52-50 Ma (Noble et al., 1988). The apparent temporal overlap of these two events expresses the limiting errors of radiometric analysis, for the proportion of unrotated dikes is smail, as in Yemen and Ethiopia (Mohr. 1983) though not in SW Arabia (J.S. Pallister, pers, commun., 1989). A general case is therefore proposed for continental rifting and break-up, in which crustal fissuring immediately precedes crustal stretching and thinning above a decompressing and melting asthenosphere. Finally, the date of ultimate continental disruption and commencement of sea-floor spreading in the Red Sea basin is disputed. Whereas geologic studies of the SW Arabian coastal strip suggest a ca. 20 Ma date (Coleman et al., 1977; Schmidt et al., 1982), inte~retations of magnetic anomalies over the Red Sea basin yield other and differing estimates: 25 Ma (Girdler and Southren, 1987): 10 Ma (Laughton et al., 1970; Izzeldin, 1987) and 5 Ma (Roeser, 1975). A 10 Ma age coincides with minor but widespread basaltic volcanism in SW Arabia, YAR and central Ethiopia (Capaldi et al, 1988; Mohr, 1986). The Pliocene-Pleistocene flood volcanism of internal Afar, and minor volcanotectonic activity along the Yemeni and Ethiopian Plateau margins and interiors, indicate that basin development in the southern Red Sea is continuing.

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Acknowledgements

This work was funded from a BMFT Grant for Red Sea crustal studies awarded to Professor Jannis Makris, University of Hamburg. The author is most grateful for the invitation to participate in this program. In Yemen, essential cooperation and assistance was received from Eng. Ali Jabr Alawi (Deputy Minister, Mineral Resources Sector, Ministry of Oil and Mineral Resources, Sana’a), and in the field from geologist Mohamed Abdul Hamid and drivers Ali As Sabri and Hamoud Ali. Dr. Piero Manetti (Universita degli Studi, Firenze) gave generous indication of certain dike localities prior to the writer’s field program. The author’s arguments have been substantially clarified following a careful review from Dr. John S. Pallister. References Abbate, E. and Sagri, M., 1969. Dati e considerazioni sul margine orientale dell altopiano etiopico nelle province de1 Tigrai e de1 Wollo. Boll. Sot. Geol. Ital., 88: 489-497. Almond, D.C., 1986a. Geological evolution of the Afro-Arabian dome. Tectonophysics, 131: 301-332. Almond, D.C., 1986b. The relation of Mesozoic-Cainozoic volcanism to tectonics in the Afro-Arabian dome. J. Volcanol. Geotherm. Res., 28: 225-246. Bacon, C.R., 1986. Magmatic inclusions in silicic and intermediate volcanic rocks. J. Geophys. Res., 91: 6091-6112. Baker, B.H., Mohr, P.A. and Williams, L.A.J., 1972. Geology of the Eastern Rift System of Africa. Geol. Sot. Am. Spec. Pap., 136, 67 pp. Barberi, F. and Varet, J., 1975. Nature of the Afar crust. In: A. Pilger and A. Roesler (Editors), Afar Depression of Ethiopia. Schweizerbart, Stuttgart, pp. 375-378. Barberi, F., Ferrara, G., Santacroce, R., Treuil, M. and Varet, J., 1975a. A transitional basalt-pantellerite sequence of fractional crystallization: the Boina centre (Afar rift, Ethiopia). J. Petrol., 16: 22-56. Barberi, F., Ferrara, G., Santacroce, R. and Varet, J., 1975b. Structural evolution of the Afar triple junction. In: A. Pilger and A. Roesler (Editors), Afar Depression of Ethiopia. Schweizerbart, Stuttgart, pp. 38-54. Beyth, M., 1973. Correlation of Paleozoic-Mesozoic sediments in northern Yemen and Tigre, northern Ethiopia. Am. Assoc. Pet. Geol. Bull., 57: 2440-2446. Blank, H.R., 1977. Aeromagnetic and geologic study of Tertiary dikes and related structures on the Arabian margin of the Red Sea. Miner. Resour. Bull., Jiddah, 22: Gl-G18. Bohannon, R.G., 1986a. How much divergence has occurred between Africa and Arabia as a result of the opening of the Red Sea? Geology, 14: 10-513.

219 Boharmon, RG., 1986b. Tectonic configuration of the Western Arabian continental margin, southern Red Sea. Tectonics, 5: 477-499. Bohannon, R.G., Naeser, C.W., Schmidt, D.L. and Zimmermann, R.A., 1989. The timing of uplift, volcanism, and rifting peripheral to the Red Sea: a case for passive’rifting? J. Geophys. Res., 94, 1683-1701. Capaldi, G., Manetti, P., Piccardo, G.B. and Poli, G., 1987a. Nature and geodynamic significance of the miocene dyke swarm in the North Yemen (YAR). Neues Jahrb. Mineral. Abh., 156: 207-229. Capaldi, G., Chiesa, S., Manetti, P., Orsi, G. and Poli, G., 1987b. Tertiary anorogenic granites of the western border of the Yemen Plateau. Lithos, 20: 433-444. Capaldi, G., Chiesa, S., Civetta, L., LaVolpe, L., Manetti, P., Orsi, G. and Piccardo, G.B., 1988. Magmatic and tectonic activities in North Yemen during Tertiary and Quatemary times. Mem. Sot. Geol. Ital., 31: 375-393. Chiesa, S., La Volpe, L., Lirer, L. and Orsi, G., 1983a. Geological and structural outline of Yemen Plateau: Yemen Arab Republic. Neues Jahrb. Geol. Palaontol. Monatsh., 11: 641-656. Chiesa, S., La Volpe, L., Lirer, L. and Orsi, G., 1983b. Geology of the Dhamar-Rada’ volcanic field, Yemen Arab Republic. Neues Jahrb. Geol. Palaontol. Monatsh., 8: 481-494. Civetta, L., La Volpe, L. and Lirer, L., 1978. K-Ar ages of the Yemen Plateau. J. Volcanol. Geotherm. Res., 4: 307-314. Civetta, L., De Fino, M., La Volpe, L. and Lirer, L., 1980. Recent volcanism of North Yemen: structural and genetic implications. Atti Convegni Lincei, 47: 617-628. Cloos, H., 1939. Hebung, Spaltung, Vulkanismus. Geol. Rundsch., 30: 401-527. Cochran, J.R., 1983. A model for development of Red Sea. Am. Assoc. Pet. Geol. Bull., 67: 41-69. Coleman, R.G., 1984a. The Red Sea: a small ocean basin formed by continental extension and sea-floor spreading. Proc. 27th Int. Geol. Congr., Moscow, 23: 93-121. Coleman, R.G., 1984b. The Tihamat Asir igneous complex, a passive margin ophiolite. Proc. 27th Int. Geol. Congr., Moscow, 9: 221-239. Coleman, R.G. and McGuire, A.V., 1988. Magma systems related to the Red Sea opening. Tectonophysics, 150: 77100. Coleman, R.G., Fleck, R.J., Hedge, C.E. and Ghent, E.D., 1977. The volcanic rocks of southwest Saudi Arabia and the opening of the Red Sea. Miner. Resour. Bull., Jiddah, 22: Dl-D30. Coleman, R.G., Hadley, D.G., Fleck, R.G., Hedge, CT. and Donato, M.M., 1979. The Miocene Tiiama Asir ophiolite and its bearing on the opening of the Red Sea. In: A.M.S. Al-Shanti (Editor). Evolution and Mineralisation of the Arabian-Nubian Shield. King Abdulaziz Univ., Inst. Appl. Geol., Bull. 3, 1: 173-187. Cotonian, C., Sebai, A., Giannerini, G., Feraud, G., Bertrand, H. and Angelier, J., 1988. Les ednements volcanotectoniques de la bordure ouest de la plaque arabique lies

220 aux stades precoces d’ouverture du rift Mer Rouge (abstract). In: Le Magmatisme Mesozdlque a actuel de la PIaque Afrique et son Corttexte Structural. Centre Int. Formation Changes Geol., Publ. Gccas., 1988/l? 72-77. Dainelli, G., 1943. Geologia dell’Africa Orientate. R. Accad. Ital., four volumes. Dewey, J.F. and Burke, K., 1974. Hotspots and continental breakup: implications for continental orogeny. Geology, 2: 57-60. Dow, D.B., Beyth, M. and Hailu Tsegaye, 1971. Palaeozoic glacial rocks recently discovered in northern Ethiopia. Geol. Mag., 108: 53-59. El-Nakhal, H.A., 1984. Possible late Palaeozoic glaciation in the central parts of the Yemen Arab Republic. J. Glacial.. 30: 126-128. Gadalia, A. and Varet, J., 1983. Les rhyolites miocenes de I’Est de l’Afar Bull. Geol. Sot. Fr., (7) 25: 139-153. Gass, LG., 1970. The evolution of volcanism in the junction area of the Red Sea, Gulf of Aden and Ethiopian rifts. Phil. Trans. R. Sot. London, A267: 369-381. Gettings, M.E., Blank, H.R., Mooney, W.D. and Healey, J.H., 1986. Crnstal structure of southwestern Saudi Arabia. J. Geophys. Res., 91: 6491-6512. Geukens, F., 1960. Contribution a la geologic du Yemen. Mem. Inst. Geol. Louvain, 21: 117-180. Girdler, R.W. and Southren, T.C., 1987. Structure and evolution of the northern Red Sea. Nature, 330: 716-721. Grolier, M.J. and Overstreet, W.C., 1978. Geologic map of the Yemen Arab Republic (San’a’). US Geol. Surv. Miscell. Invest. Ser., Map I-1143-B (1: SOO,OOO), Washington DC. Heyckendorf, K. and Jung, D., 1989. Tertiary trap volcanism of the Yemen Arab Republic-first results of 1988 field campaign., Ann. Geophys., Spec. Iss. Abstr., Eur. Geophys. Sot. 14th General Assembly, 91. Hibbard, M.J. and Walters, R.J., 1985. Fracturing and diking in incompletely crystallized granitic plutons. Lithos, 18: Izzeldin, A.Y., 1987. Seismic, gravity and magnetic surveys in the central part of the Red Sea. Tectonophysics, 143: 269-306. Karren~~, H., 1957. Junger Ma~atismus und Vulkanismus in Siidwestarabien (Jemen). 20th Int. Geof. Congr., Mexico City, Vulcanologia de1 Cenozoico, 1: 171-185. Kruck, W., 1980. Geological map of the Yemen Arab Republic, Sheet Al Harm, 1: 250,000. Federal Institute for Geosciences and Natural Resources, Hannover. Kruck, W., 1984. Geological map of the Yemen Arab Republic, Sheet San’s’, 1: 250,000. Federal Institute for Geosciences and Natural Resources, Hannover. Kruck, W., Al Anissi, A. and Saif, M., 1984. Geological map of the Yemen Arab Republic, Sheet Al Hudaydah, 1: 250,000. Federal Institute for Geosciences and Natural Resources, Hannover. Lamare, P., Basse, E., Dreyfuss, M., Lacroix, A. and Teilhard de Chardin, P., 1930. Etudes gdologiques en Ethiopie,

I’. MOliK

Somalie et Arabie meridionale. Sec. Gtol. Fr. Mem., 6 (14): l-83. Laughton, A.S., Whitmarsh. R.B. and Jones, M.T.. 1970. The evolution of the Gulf of Aden. Phil. Trans. R. Sot. London, A2671 227-266. Levitte, D., Columba, J. and Mohr, P., 1974. Reconnaissance geology of the Amaro horst, southern Ethiopian rift. Bull. Geol. Sot. Am., 85: 417-422. M&is. J., 1975. Afar and Iceland-a geophysical comparison. in: A. Pilger and A. Roesler (Editors), Afar Depression of Ethiopia. Schweizerbart, Stuttgart, pp. 379-390. Mohr, P., 1970. The Afar triple Junction and sea-floor spreading. J. Geophys. Res., 75: 7340-7352. Mohr, P., 1978. Afar. Ann. Rev. Earth Planet. Sci., 6: 145-172. Mohr, P., 1983. The Morton-Black hypothesis for the thinning of ~ntinental crust-revisited in western Afar. Tectonophysics, 94: 509-528. Mohr, P., 1986. Sequential aspects of the tectonic evolution of Ethiopia. Mem. Sot. Geol. Ital., 31: 447-461. Mohr, P., 1989. The nature of the crust under Afar: new igneous. not stretched continental. Tectonophysics, 167: l--11. Mooney, W.D., Gettings, M.E., Blank, H.R. and Healy, J.H., 1985. Saudi Arabian seismic refraction profile: a traveltime interpretation of crustal and upper mantle structure. Tectonophysics, 111: 173-246. Noble, R.H., Macintyre. R.M. and Brown, P.E., 1988. In: AC. Morton and L.M. Parson (Editors). Early Tertiary Volcanism and the Opening of the NE Atlantic. Geol. Sot. London Spec. Publ., 39: 201-214. Pallister. J.S., 1987. Magmatic history of Red Sea rifting: perspective from the central Saudi Arabian coastal plain. Bull. Geol. Sot. Am., 98: 400-417. Pallister, J.S. and Hegner, E., 1989. Isotopic evidence for evolution of sub-continents mantle during Red Sea rifting. U.S. Geol. Surv., Gpen File Rept., USGS-OF-09-6, 25 pp. Rathjens, C. and von Wissmann, H., 1934. Die geologischen Verhlltnisse des in Jemen bereisten Gebiets. Abh. Gebiet Auslandskd., Univ. Hamburg, 40 (3): 155-176. Roeser. H.A., 1975. A detailed magnetic survey of the southern Red Sea. Geol. Jahrb. Hannover. 13: 131-153. Roland, N.W., 1979. Geological map of the Yemen Arab Republic, Sheet Sa’dah, 1: 250,000. Federal Institute of Geosciences and Natural Resources, Hannover. &hilling, J.-G., 1975. Afar mantle plume: rare-earth evidence. Nature Phys. Sci., 242: 2-5. Schmidt, D.L., Hadley, D.G. and Brown, G.F., 1982. Middle Tertiary continental rift and evolution of the Red Sea in southwestern Saudi Arabia. U.S. Geol. Surv. Open-File Rept., 83-641, 56 pp. Seife Berhe, M., Desta, B., Nicoletti, M. and Mengistu Teferra, 1987. Geology, geochronology and geodynamic impiications of the Cenozoic magmatic province in W and SE Ethiopia. J. Geot. Sot. London, 144: 213-226. Sengor, A.M.C. and Burke, K., 1978. Relative timing of rifting

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and volcanism on earth and its tectonic implications. Geophys. Res. Lett., 5: 419-421. Stoeser, D.B., 1986. Distribution and tectonic setting of plutonic rocks of the Arabian Shield. J. Afr. Barth Sci., 4: 21-46. Stoeser, D.B. and Camp, V.E., 1985. Pan-African microplate accretion of the Arabian Shield. Geol. Sot. Am. Bull., 96: 817-826. Voggenreiter, W., 1987. Block rotation in southwest Saudi

Arabia deduced from an investigation of the Tertiary Tiharna Asir mafic dike swarm. Abstr., Int. Symp. Mafic Dykes and Related magmatism in Rifting and Intraplate Environments, IGCP-257 Tech. Rept., 1: 47. Walker, G.P.L., 1961. Zeolite zones and dike distribution in relation to the structure of the basalts of eastern Iceland. J. Geol., 68: 515-527. Walker, G.P.L., 1969. The breaking of magma. Geol. Mag., 106: 166-173.