Active versus passive continental rifting: Evidence from the West African rift system

473

Tec~ono$zysic.s, 94 (1983) 473-481 Elsevier Science Publishers

ACTIVE

VERSUS

THE WEST

J. GODFREY

B.V., Amsterdam

PASSIVE

AFRICAN

- Printed

in The Netherlands

CONTINENTAL

RIFT

RIFTING:

EVIDENCE

FROM

evidence

from the West African

rift system.

Rifting.

Tectonophysics, 94: 473-481.

SYSTEM

FITTON

Grani Institute of Geology, University of Edinburgh (Great Britain) (Revised

version received

May 1, 1982)

ABSTRACT

Fitton,

J.G.+ 1983. Active versus passive continental

In: P. Morgan

and B.H. Baker, Processes

The West African volcanic

superimposed stages

rift system

Cameroon

line. The

perfectly

in the geological

lithosphere

stretching

South Atlantic. otherwise

history

asthenosphere-lithosphere

volcanism systems

one with respect

of the system

rotation

to the other

are postulated.

by 7“ about

Cameroon

period of clockwise

the lithosphere

coupling

were

and the Gulf

restored

allowing

and

to Recent

size and

may

a pole in Sudan.

was produced

of Guinea. the hot

rotation

by

of the

interrupted

from the asthenosphere.

the

The hot

relative to the lithosphere (3) Anticlockwise

zone

to manifest

be

Three

the early stages of the opening

the Benue trough thus became displaced

beneath

and the Tertiary

in shape

(1) The Benue trough during

a short-lived

of Africa and decoupled

beneath

similar

and

rotation itself

and

as the

line.

The Cameroon thermal

Benue trough

are remarkably

(2) At about 80 Ma (Santonian)

anticlockwise

to a new position

Cameroon

the Cretaceous

as one arm of an RRR triple junction

zone in the asthenosphere moved

comprises

two features

by rotating

rifting:

of Continental

anomaly

line is possibly

a unique

in the asthenosphere

and uplift, the Cameroon with graben

structures

example

of what a “rift

would look like. It is si~;f~cant

line has not developed

are produced

a graben

passively by lithosphere

system”

produced

that, despite

structure.

acriuefy

by a

a long history

The implication

of

is that rift

stretching.

INTRODUCTION

Continental rifting is usually accompanied by magmatism and regional uplift. The large negative Bouguer anomalies associated with modern rifts are best explained by thinning of the lithosphere and its replacement by lower density, hotter asthenosphere (e.g. Brown and Girdler, 1980). Partial melting of the upwelling asthenosphere can account for rift magmatism while the regional uplift is an isostatic response to the lower density of the upwelling material. The processes responsible for lithosphere thinning are central to the current debate on the origins of continental rifting. Is it caused by the actioe (convective?) 0040- 195 1/83/$03.oLI

0 1983 Elsevier Science Publishers

B.V.

474

upwelling

of asthenosphere

nied by passive

upwelling

volcanism

should precede

volcanism

in “passive”

and fracture

of asthenosphere

material?

rifting whereas rifting rifts (Sengor

point out, the end products and the differences

or by stretching

of the lithosphere In “active”

and, possibly,

and Burke.

accompa-

rifts uplift

and

uplift should precede

1978). However.

as these authors

of active and passive rifting are likely to be very similar

between

the two not easy to recognise

in the geological

record.

The purpose of this paper is to show that the tectonic and magmatic development of the West African rift system provides unique evidence in support of a passive origin for continental

rifts.

THE WEST AFRICAN

RIFT SYSTEM

The West African rift system as defined here comprises the Cretaceous Benue trough and the Tertiary to Recent volcanic Cameroon line (Fig. 1). The Benue trough is a linear depression filled with up to 6000 m of marine sediments whose deposition was terminated by a period of mild deformation in the Santonian Ma). Rift faulting has not been observed directly in the trough because

I

I

I

5”

IO”

15”

80 of the (c.

Km

Fig. 1. The West African

rift system.

trough

The Cameroon

is shown stippled.

The outcrop

of Cretaceous

line volcanic

sedimentary

rocks are shown in black.

rocks defining

the Benue

475

overlying

sediments

these sediments ments

though

suggests

along basement

Wright

(1981) has argued

that the folds were caused

that the style of folding by differential

vertical

in

move-

faults.

The Benue trough is often cited (e.g., Burke and Wilson, 1976) as one of the best examples of a failed arm of an RRR triple junction. In this case the other two arms gave rise to the South Atlantic

Ocean. At its northeastern

two smaller rift structures

striking

(Chad rift). The northern

end of the Chad rift is obscured

in the Chad (Wright, The

basin.

Volcanic

respectively

rocks

have

1976) but are not extensively Cameroon

line

is a chain

eastwards

been

reported

end the trough

splits into

(Yola rift) and northwards by Quaternary from

sediments

the Benue

trough

developed. of transitional

to strongly

alkaline

volcanoes

extending 1600 km from the Atlantic island of Pagalu (formerly Annobon) across the African continental margin towards the centre of the continent (Fig. 1). The earliest magmatism is represented age from 65 to 35 Ma (Cantagrel

by syenite and granite ring complexes ranging in et al., 1978). The oldest extrusive rocks have been

dated at 45 Ma on the continental sector (PI. Okeke, unpublished data) and 31 Ma on the oceanic sector (Dunlop and Fitton, 1979). The most recent volcanic activity was on Mt. Cameroon which last erupted in 1982, though morphologically recent volcanic cones can be found in virtually all parts of the line. The Cameroon line has been almost continuously active over the past 65 Ma (Grant et al., 1972; Gouhier et al., 1974; Cantagrel et al., 1978; Dunlop and Fitton, 1979; H.M. Dunlop, unpublished data). The volcanism shows no consistent migration with time.

16

c 1

’

0 Oceanic

sector

volcanic

rocks

I

40

45

50

55 Weight

Fig. 2. Alkali-silica

60

65

70

75

% SiO,

diagram for the Cameroon line volcanic rocks. The line separating Hawaiian alkaline

and tholeiitic rocks (Macdonald

and Katsura, 1964) is shown for reference.

476

The compositional alkali-silica phonolite

diagram and

nephelinite

systems (cf. Williams, activity

of oversaturated

complexes).

Basaltic

is very similar

1972, Fig. 2; Upton.

on the continental

composed 10 Ma the Biu basaltic during

range of the Cameroon

line volcanic

in Fig. 2. The association

of alkali

to that

rocks is illustrated basalt,

found

trachyte,

in many

continental

rift

1974, figs. 3 and 4). The earliest volcanic

sector of the line (65-- 10 Ma) produced salic rocks

on an rhyolite,

(e.g. Bambouto,

rocks have only been erupted

Oku

central and

in large quantity

volcanoes

the early during

ring

the last

and are generally associated with fissure eruptions (e.g., Mt. Cameroon and and NgaoundCrC Plateaux). Volcanism in the oceanic sector has been mostly with more evolved rocks (undersaturated trachytes and phonolites) erupted the waning phases of activity (Fitton and Hughes, 1977).

Continental sector volcanism Precambrian basement of about

has been accompanied by regional uplift of the 1 km but there is no evidence of rift faulting and

graben formation. Nor is there any evidence that the siting of volcanic centres is controlled by basement structure. Volcanic fissures, often marked by lines of cinder cones, are generally aligned along the Cameroon line. These fissures run SW-NE on Mt. Cameroon and Manengouba, N-S on the Biu Plateau and NW-SE on the Ngaoundere

Plateau.

Swarms of dykes oriented

SW-NE

are found in the older lavas

on the island of Principe. In places, volcanic fissures can be seen to cut across lines of weakness in the basement as on the Ngaoundere Plateau where lines of cinder cones intersect large basement fractures at angles of about 70”. The oceanic sector is likewise unaffected by the transform faults which it crosses (Sibuet and Mascle, 1978). In their compositional range and association with regional uplift, the Cameroon line volcanoes have much in common with those found in rift valleys. However, the lack of graben structures and the restriction of basaltic fissure eruption to the most recent phases of volcanism suggest that for much of its long history the Cameroon line was not an extentional feature. In this respect it contrasts sharply with the Kenya rift where fissure eruptions have formed a large proportion of the volcanic activity

from the earliest

Though rift-like

not a rift system in the accepted feature which has implications

stages in its development

(Baker

and Wohlenberg,

1971).

sense of the phrase, the Cameroon line is a for the origin of more conventional rift

systems. ORIGIN

OF THE CAMEROON

LINE

The Cameroon line and Benue trough are complementary features. The former is composed of volcanic rocks of rift valley affinities but lacks rift faulting whereas the latter is a rift valley containing relatively few volcanic rocks. An explanation for the origin of the Cameroon line may lie in its relationship with the Benue trough (Fitton, 1980). The two features are so remarkably similar in shape and size that they may be superimposed perfectly by rotating one with respect

to the other about a pole in Sudan (Fig. 3). This geometrical coincidence cannot be accidental but probably

results from a displacement

of the African lithosphere

relative to the underlying asthenosphere. Thus the “Y”-shaped hot zone in the asthenosphere which would have been situated beneath the Benue trough in the Cretaceous became displaced (relative to the lithosphere) so that it now lies beneath Cameroon and the Gulf of Guinea. Magmas originally destined for the Benue rift, therefore, reached the surface as the Cameroon line instead. The postulated sequence of events leading to the development of the Cameroon line are illustrated schematically in Fig. 4. The model presented in these diagrams implies that for the past 100 Ma the West African lithosphere has been coupled to its underlying

asthenosphere

apart from a brief period around

70 Ma ago when the two

were decoupled. Theoretical studies on mantle convection (Davies, 1977; Parmentier and Turcotte, 1978; Hager and O’Connell, 1981) show that moving hthosphere plates will impose their motion on the upper parts if not all of the asthenosphere. However, it is likely that the inertia of asthenosphere motion will prevent its responding to short-lived changes in lithosphere motion. Such a change in plate motion during the development of the Benue rift may have been responsible for the lithosphere-asthenosphere

decoupling

+ 0

proposed

1

lb

Km

in the model.

*

500 4

. r),

Fig. 3. The Cameroon

line superimposed

the latter by 7’ about

a pole at 12.2”N, 30.2”E (from Fitton,

on the Benue trough

by rotating 1980).

the former clockwise

relative to

This

decoupling

must have

occurred

l3enue rift (80 Ma) and the earliest best

explained

Fig. 4. Evolution upper

mantle

by postulating

of the West African

measuring

between

magmatism

a short-lived

rift system.

the cessation

in the Cameroon period

of activity

of clockwise

The block diagrams

represent

in the

line (65 Ma), It is rotation

segments

which

of crust and

1000 km square by 200 km deep.

interrupted the generally anticlockwise rotation implied by the South Atlantic hot-spot traces. Evidence in support of such a wobble in the motion of Africa at this time is not hard to find. The late Cretaceous was a period of rapid sea-floor 1972) and major changes were taking place all spreading (Larson and Pitman, around the African plate. The change in orientation of the South Atlantic transform faults at magnetic anomaly 34 points to a change in the relative motion between Africa and South America 79 Ma ago (Sibuet and Mascle, 1978). The separation of

479

India

from Madagascar

the Atlantic

Ocean

and Africa (Barron

north

and Harrison,

1980) and the opening

of the Bay of Biscay (Pitman

and Talwani,

of

1972) both

occurred at about this time. The most convincing evidence, however, comes from the Walvis Ridge which has been interpreted as a hot-spot trace. This ridge is off-set at anomaly

34 in the sense predicted

(Fitton, 1980). The erratic behaviour

of the African

event. There is strong evidence but involves

by this model and by roughly the required

the jostling

plate required

that plate motion

of plates

much as ice floes do (Girdler,

a particularly

large sideways

phase

in the development

formation geological

IMPLICATIONS

FOR CONTINENTAL

If this interpretation then it has implications rift provides a unique development

line.

and

This

alternately

jammed

1980). The coincidence

lurch in the motion of a major

of the Cameroon record.

by this model is not a unique

is not a smooth, continuous

which become

then released,

amount

of the African

continental

rift

coincidence

may

process

together

and

in the timing of

plate with a critical

was responsible well be unique

for the in the

RIFTING

of the evolution of the West African rift system is correct for general models of continental rifting. The West African example of a rift system interrupted in the course of its

allows

us to examine

the

effects

of the underlying

thermal

disturbance in isolation from the rifting which produced it. The Benue trough was produced as part of the much larger rift system which gave rise to the South Atlantic course governed

Ocean.

This rift system may have been initiated

by hot spots (Burke and Dewey,

1973) but most probably

through stretching of the lithosphere in the areas around spots. An inevitable consequence of this stretching would linear zones where hot asthenosphere resulting thermal disturbance would

and its evolved

and between these hot be the development of

welled up passively into the lithosphere. The extend down into the asthenosphere. At this

point a geological accident decoupled the rift system from the deeper portions of this thermal disturbance and brought it to rest beneath what is now the Cameroon line. The disturbance could then assert itself in an active role and rise into the overlying lithosphere (Fig. 4). In this way an image of the Benue trough thermal disturbance has been imprinted on the lithosphere beneath Cameroon and its magmatic effects are still being felt today. It is significant that, despite a long history of rift-valley-type magmatism and associated uplift, the Cameroon line has never developed a graben structure. There is no reason why the Cameroon line should be under regional tension as the tension which created the Benue trough would have been relieved with the opening of the South Atlantic. The character of volcanism on the Cameroon line (mostly central volcanoes) argues against a tensional regime during most of its history. The shift to basaltic fissure eruptions over the past 10 Ma is probably the result of local tension

associated

with doming

but insufficient

line may well be the only example system”.

It follows

produced

as a passive response

to cause graben

formation.

The Cameroon

of a truly active (as opposed

that rift systems

which do develop

to lithosphere

graben

to passive) structures

“rift

must

be

stretching.

ACKNOWLEDGEMENTS

Field

work in Cameroon,

research

grant

from

Nigeria

the U.K.

and the Gulf

Natural

of Guinea

Environment

carried out with the support of the General Delegation Research, Cameroon, the Geological Survey of Nigeria cial dos Services

Geologia

e Minas,

was financed

Research

Council

by a

and

was

for Scientific and Technical and the Reparticao Provin-

Sao Tome and Principe.

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