Gravity study of the Central African Rift system: a model of continental disruption 2. The Darfur domal uplift and associated Cainozoic volcanism

Tectonophysics,

205

94 ( 1983) 205-222

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

GRAVITY STUDY OF THE CENTRAL AFRICAN RIFT SYSTEM: A MODEL OF CONTINENTAL DISRUPTION 2. THE DARFUR DOMAL UPLIFT AND ASSOCIATED CAINOZOIC VOLCANISM

P.M. BERMINGHAM, ~epurtment (Revised

J.D. FAIRHEAD

and G.W. STUART

of Earth Sciences, The University of Leeds, Leeds LS2 9JT (United Kingdoms

version July 26, 1982)

ABSTRACT

Bermingham,

P.M., Fairhead,

System:

a model

volcanism.

J.D. and Stuart,

of continental

In: P. Morgan

G.W.,

disruption.

1983. Gravity

2. The Darfur

and B.H. Baker (Editors),

study

domal

Processes

of the Central

uplift

African

and associated

of Continental

Rifting.

Rift

Cainozoic

Tectonophysics,

94: 205-222. Gravity Bouguer

studies of the Darfur

anomaly,

the Darfur

anomaly,

Kenya and Ethiopia km) or a thinned regional African

setting

using constraints

Sudan, show it to be associated

and 700 km across. A three-dimensional deduced

from geophysical

domes, suggests either a low-density

lithosphere

with emplacement

of the Darfur

uplift

extensional

of the Afro-Arabian tectonics

is described

the Ngaoundere, development

Abu

of passive

associated

Rift System.

and passive rifting, Gabra,

resulting

with the Darfur,

margins Kenya

with a circular

equivalent

of similar

body at mid-lithospheric

between

in the subsiding

than

of Aden

rifts,

the active

and Ethiopia

depth ( - 60

asthenospheric part

these rift systems

sedimentary are more

domal

uplift

of

but more evolved material

The

of the Central

to the early to middle Miocene

Comparisons

negative

model interpretation

in terms of it being an integral

Red Sea and Gulf

continental

laccolithic

studies

at high level of low-density

Rift System which is shown to be broadly

development

features

uplift, Western

SO mGa1 in amplitude

stage in the suggest

that

rift basins associated

with

typical

of the early

and development

stage

of rifted

domes.

INTRODUCTION

Intraplate Cainozoic volcanism of the ~kali-oiivine basalt-trachyte association is widespread over North and Central Africa in non-cratonic areas (Thorpe and Smith, 1974; Fig. 1). Various attempts have been made to explain its distribution in terms of linear volcanic-tectonic zones (Vail, 1978; Francis et al., 1973), thermal plumes and hot spot tracks (Le Bas, 197 1; Burke and Wilson 1972; Gass et al., 1978 and Morgan, 1981) and lithospheric thinning and domal uplift (Fairhead, 1979, 1980; Brown and Girdler, 1980). The latter tectonic model used the fact that the major 0040- 195 1/83/$03.cKl

0 1983 Elsevier Science Publishers

B.V.

EASEMENT

CAIN020

Fig. 1. Regional volcanism domal

geology

and basement

IC

VOLCANISM

of North

and

Central

uplift areas (e.g.. Hoggar,

uplifts are all associated

with negative

Africa

-

Measured

- - -

Predicted

2zzx==

RIFT

showing

the relationship

Tibesti and Darfurj

Bouguer

anomalies

BOUGUER

to the known

less than

ANOMALY

between

Cainozoic

rift structures.

The

- 60 mCal.

volcanic centres of Darfur, Tibesti and Hoggar are situated on broad crustal uplifts which are associated with and linked by long wavelength, negative Bouguer anomalies which have similar proportions to the gravity anomalies associated with the domal uplifts of East Africa. Crough (198 1a, b) has shown, using the available free air gravity data for the Hoggar and the Darfur domes, that these domes have isostatic roots within the depth range 40-80 km. These results are broadly compatible to the results obtained for the Kenya dome (Banks and Swain, 1978) and oceanic swells (Crough, 1978). Crough thus infers that oceanic and continental swells have probably formed by similar mechanisms and have a common thermal origin. To gain a better insight to the deep structure and thermal origin of one such

207

basement

uplift,

University

detailed

geophysical

and geochemical

of Leeds in 1979 on the Darfur

province

in Western

Sudan.

survey and a seismological presentation

The geophysical

interpretation

tectonic setting. Attention is drawn comprising

anomaly

studies

consisted

map of the Darfur

and discussion

to the striking

the Ngaoundere

studies were initiated

uplift and its associated

similarities

and Abu Gaba

together

gravity to the with its

of the dome’s regional

of the Central

rifts (Browne

is restricted

dome

of the significance

by the volcanic

of a detailed

study of the dome. This contribution

of the Bouguer

preliminary

domal

African

and Fairhead,

Rift System 1983, this

volume) and the Darfur-Tibesti-Hoggar domal uplifted arm to the Afro-Arabian Rift System comprising the Red Sea and Gulf of Aden rifts and the East African domal uplifted arm. Previous studies (Rothe, 1954; Sykes and Landisman, 1964; Fairhead and Girdler, 1971) of the East African Rift System have tended to use the continuity of the seismicity and fault structures from the Mid-Oceanic Rift System into East Africa, via the Gulf of Aden, as evidence that the rift structures in East Africa are typical of the structures that develop prior to continental fragmentation. Such studies are considered here to have confused models of continental disruption. We show by the comparison of the Central African Rift System with the Afro-Arabian Rift System that two distinct rift processes are present and ‘are characterised by distinctly different gravity anomalies. GEOLOGY

AND EPEIROGENY

OF THE DARFUR

DOME

The Darfur dome is a broad almost circular area of exposed Precambrian basement (Fig. 2), 700 km in diameter rising from a background height of 500 m to 1100 m (Fig. 3). On an even broader regional basis, this dome is superimposed on a broad area of uplift extending from the Chad basin, in the west, to the Nile basin in the east. To the northwest it includes rocks exposed on the Darfur dome intrusive folding

the Tibesti and Hoggar domes. The basement consist of a variety of gneisses, schists and

bodies and have had a complex and

metamorphism

is thought

tectonic

thermal

to have occurred

history, during

of which the main the Pan-African

period (Vail, 1978). Palaeozoic granite emplacements and undated quartz-muscovite pegmatites, quartz veins and dykes cut across the NE-SW foliation of the gneisses (Vail, 1978). To the north and west of the dome, Cambro-Ordovician sediments rest unconformably on basement and dip northwards towards the Erdis basin (Fig. 2). To the northeast and south of the dome, sediments of the Nubian Sandstone Formation outcrop over large areas. These sediments are mainly sandstones and mudstones and represent subaqueous continental deposits. They were laid down relatively rapidly under changing facies conditions, probably in braided river systems onto delta fans across flood plains (Vail, 1978). The age of these sediments still remains controversial but fossil evidence from Jebel Dirra, to the east of the dome, indicates an early

Fig. 2. Geological

map of the Darfur

domal

.. ‘..

uplift (redrawn

‘_

BASIN

BAGARRA\

from Vail. 1978).

----,-

,

RIFT

BASIN I

209

I

I

I

I

.

I6

//q

7

.........

\

‘El

200Km

’

’

’

-

’

Fasherm,,

SUDAN IO . ‘0, ) I

.

.*

n

.a* &P

26 I

. ..*. .

I/

.*

Central

African

Fig. 3. Topography exposed

basement,

province.

Contour

Cretaceous

I

domal uplift based on heights of gravity

and Fairhead

(1983, this volume).

while topography interval

28 5-

Republic

of the Darfur

see fig. 4 of Browne

\

above

Topography

1100m is volcanics

stations.

between associated

For station

500 and

coverage

1100 m is mainly

with Jebel Marra

volcanic

is 100 m.

age (Vail, 1978; Edwards,

1926). To the south of the dome in the Abu

Gabra basin, Cretaceous and Tertiary sediments have a combined 4500 m in places (see Browne and Fairhead, 1983, this volume). Faulting

-

east of El Fasher has given rise to a NW-SE

thickness

depression

containing

of up to more

than 1200 m of Nubian Sandstone Formation (Hunting, 1970). Near Zalingie a broad zone of faults, displacing basement gneisses and lower Palaeozoic strata, strike NNW-NW (Medani and Vail, 1974). This zone extends into the Abu Gabra basin to the southeast. It is assumed that most of the movement occurred during the Tertiary to more recent times. Tertiary volcanic rocks in the form of lava flows, plugs, vents and craters are widespread over the dome (Fig. 2). Three ring complexes have been recorded on the northwest side near Ega. Two contain gabbroic rocks emplaced in basement gneiss and are considered by Sonet (1963) to belong to the Tertiary igneous activity. The other, Jebel Mun, is located near Genena and consists of syenite with radial dykes (Lofti,

1963). There are also a few occurrences

of extrusive

volcanic

rocks piercing

Lower Palaeozoic may be remnants few isolated Nahud

strata to the northwest

of an earlier period of volcanic

outcrops

in South Kordofan area

volcanic

Meidob

Hills

fields are remnants

Estimates

and

in the area. ‘There are also a

from Gemini

and south of En

Marra-- Jebel

dome

Gurgei

and comprises

complex.

of lava flows, plugs, vents and craters a broad

photographs

covers an area of about

Kordofan

field lies on the Darfur

the Jebel

Plateau:

1964). These

(Vail, 1978).

volcanic

and Teiga

activity

south of Wadi el Milk in North

The most extensive

Hills

of the dome in Chad (Wolff,

region

bounded

indicate

by the dashed

(Francis

et al., 1973). The summit

overlooks

Deriba

Caldera

which

contains

fumarole and hot springs (Hammerton, recovered from air-fall pumice deposits

erupted

volume

of Jebel Marra a recent

these

in the Tagabo line in Fig. 2.

that the Jebel Marra volcanic

13,000 km2 with a calculated

8000 km

Between

the

complex

of the order of

is 3042 m O.D. and

pyroclastic

cone,

an

1966; fig. 2). A sample of carbonised from a major Plinian eruption centred

active wood on the

caldera has yielded a 14C date of 3520 + 100 yr B.P. which is equivalent to about 4000 yr B.P. allowing for the divergence between the radiocarbon and dendrochronological recorded

scales (Francis et al.. 1973). A total of six thermal springs have been on the Darfur dome. Local earth tremors have been recorded and located

by the seismic array study. The location and are shown in Fig. 2. The lavas on Jebel nmoreite-trachyte-rhyolite

of these events group close to Jebel Gurgei

Marra belong to the alkali basaltthawaiite-mugearitebeseries though basanites and phonolites are also present

(I.R. Wilson,

pers. commun.,

provisionally

dated at 13.5 m.y. (K. Smith, pers. commun.,

1982). Basal lavas in the Jebel Marra

the geological estimate by Andrew (1948). Volcanic ple phases. Alkali olivine basalts are volumetrically the earlier hawaiites,

series

forming

mugearites,

a shield

phonolites

with

activity occurred in two princithe most important member of

differentiates

and in particular

area have been

1982) which agrees with

of the basalts

trachytes.

present

as

A period of quiescence

followed, during which time extensive erosion and dissection occurred. with the deposition of alluvial materials over a wide area (Lebon and Robertson, 1961). When volcanic activity resumed, it was on a reduced scale with a series of lavas again of the basalt-trachyte paucity

of dykes

and

association. faults

suggest

Field relations, that

fissure

photo-interpretation type

eruptions

have

and the not

yet

developed and most of the volcanism can be accounted for by central type volcanism (Francis et al., 1973; I.R. Wilson, pers. commun., 1982). The type and nature of the volcanic products of the Darfur volcanic province are similar to those of Tibesti (Vincent, 1970) and Hoggar and Air (Black and Girod, 1970) volcanic provinces as well as to the early stage development of the East African domes (Baker et al., 1972). The epeirogeny of the Darfur dome is difficult to determine due to the lack of detailed geological mapping; however, the major phases of uplift can be determined. Mesozoic sediments (Nubian Formation) rest unconformably on both Palaeozoic sediments to the north of the dome and on basement on the dome itself. South of the

211

dome, there are in excess of 4.5 km thickness the Abu Gabra

Rift basin, which lie directly

of Mesozoic

and Tertiary

on Precambrian

basement.

sediments

in

This testifies

to a pre-Cretaceous period of uplift to which the widespread Palaeozoic granites may be related. Lower Palaeozoic outliers near Geneina (Fig. 2) are currently at an altitude of 700 m and outliers of the Nubian Formation (Cretaceous?) occur at altitudes

of more than

post-Cretaceous. where

1000 m near Jebel Gurgei,

These sediments

they are capped

formed found

GRAVITY

lavas,

with the volcanic

uplift (Crough,

implying

that the present

occur only at high altitude

by erosion-resistant

contemporaneously for the Hoggar

generally

which

activity.

suggests

Similar

swell is

on the dome, that

the swell

epeirogeny

is also

1981a).

STUDY

Data acquisition The

University

of Leeds

in collaboration

with

the Geological

and

Mineral

Resources Department of Sudan carried out a regional gravity survey of central and western Sudan during the period December 1979 to May, 198 1. The survey used a LaCoste Romberg gravity meter (G 471) to make 1197 gravity measurements; 865 in North and South Darfur provinces and 332 in Kordofan and White Nile provinces (Fairhead et al., 1982). The survey was designed to link together a number of existing surveys in Sudan to produce a regional Bouguer anomaly map for Central and Western Sudan (see Fig. 3 of Browne and Fairhead, 1983, this volume). The Darfur gravity survey was tied to the existing gravity base in Nyala (Isaev and Mitwalli, 1974). Gravity ties between this station and the Khartoum University base were found to be in good agreement with the published value. The published value was thus adopted as the base value for the Darfur survey after an adjustment of - 13.75 mGa1, to make it compatible with the IGSN71. An existing base at Kosti (Isaev and Mitwalli, 1974) was adopted, after the same adjustment, for the White Nile/Kordofan Mineral and

survey. Several other gravity bases established

Resources

Department

by hydrocarbon

exploration

(Isaev

and Mitalli,

surveys

were

1974)

by the Geological by Water

re-occupied.

and

Development

Discrepancies

were

generally found to be within kO.2 mGal. Gravity stations in Darfur were established along interconnecting tracks and occasionally cross-country, at intervals of 10 km decreasing to 3 km over the volcanic province. Errors in base station ties were minimised by network adjustment (Smith, 1951). Heights of gravity stations were determined using pairs of Thommen and Baromec altimeters, employing single base and leap frog heighting methods (Biddle; Swain and Khan, 1977). The heighting network was tied into 25 Sudan Survey heights throughout Darfur and heights of individual stations are considered to have an uncertainty within + 5 m (this is equivalent to + 1 mGa1 for the Bouguer anomaly

using

reduction

density

2.67 g/cm3).

Locations

of gravity

stations

were

712

determined

using

measurements station

existing

together

location

1 : 250,000

with satellite

by satellite

maps and odometer

measurements

corrections

are considered

the plotted Bouguer to - 10 mGa1.

province,

compass

and

odometer accuracy

k 200 m while positions to have an accuracy

of

from

within

i 1

of the Bouguer gravity values to be rt 2 mGa1.

have not been applied

maps of the volcanic

air photos,

and star fixes to within

km. This results in a general accuracy Terrain

maps,

and star fixes. This gave a general

due to the lack of detailed

topographic

where the major relief occurs. This will tend to make

values in the vicinity

of Jebel Marra

volcano

too negative

by up

Bouguer anomaly map The Bouguer gravity map for the Darfur Dome and adjacent regions is shown in Figure 4 and is based on the station distribution shown in Fig. 4 of Browne and Fairhead

(1983, this volume).

The character

of the anomalies

associated

with the

Darfur dome can be more readily appreciated using gravity and topography profiles AA’ (Fig. 5) and CC’ (Fig. 6) which cross the dome from W to E and from NNW to

I

.

I

I

l

. *,...

Central

Fig. 4. Bouguer

African

gravity

1

0I

i

I I

1

LOO /

I

Klr,

. . . . . . i

ReDublir.

map of the Darfur

dome contoured

Profile BB’ is the part of profiIe AA’ crossing

the Darfur

at 10 mGal intervals. dome (see Fig. 5).

Area is same as Fig. 3.

213

SSE respectively the central

(see Figs. 1 and 4 for locations

section

of profiles).

of profile AA’.

Profile AA’ (Fig. 5) is 2000 km long and extends Nile and crosses

the dome immediately

The profile indicates centred

Profile BB’ in Fig. 4 is

a very long wavelength

on the dome. This regional

from Lake Chad

south of the volcanic

anomaly

negative

anomaly

centre,

to the White Jebel Marra.

of amplitude

is shown as a dashed

20 mGa1

line in Fig. 5 and

is used to isolate the pronounced - 50 mGa1 anomaly which is 500 km wide for this section across the Darfur dome (stippled part of anomaly in Fig. 5). Profile CC’ (Fig. 6) crosses the volcanic province and the Bagarra basin. The low density

volcanic

pile is associated

mGa1. This anomaly is elongate

with a short wavelength

closely correlates

to the NNE following

negative

anomaly

of 30

with the shape of the volcanic

province

which

approximately

the - 120 mGa1 contour

in Fig. 4.

The southern flanks of the dome are associated with high gravity gradients, with gravity contours trending WSW-ENE, parallel to the Ngaoundere rift. Such a trend is not reflected

in the topographic

shape of the dome (Fig. 3). This trend

and the

higher gravity gradients are considered to be caused by the superposition of the negative anomaly associated with the dome on the broad positive Bouguer anomaly flanking the northern end of the Abu Gabra rift basin and paralleling the Ngaoundere rift (see Browne and Fairhead, 1983, this volume). These effects can be seen in profiles CC’ and DD’ (Fig. 6). The northern end of profile CC’ indicates that the negative anomaly associated with the dome extends to the north into areas not covered by the present survey because of logistic difficulties. In Chad, however, north of 16”N, Louis (1970) shows that the negative Bouguer anomaly trend continues to the north and links with the Tibesti and Hoggar uplifts (Fig. I).

BOUGUER ANOMALY

TOPOGRAPHY

I-

mGal.

------DDARFUR

km.

DOME-,

I

0

so0

A Fig. 5. West-east

gravity

and topographic

I

IS00

2000 km.

B

B

White Nile showing (not the

1000

profile AA’ across

the 500 km wide negative

residual

volcanic centre). For location see Fig. 1.

gravity

A’

the Darfur anomaly

dome from Lake Chad

associated

with the Darfur

to the dome

214

-15oL I 200

Derl ba Calderal,

km

D’

:

I .I. j:‘, ::

‘-_ _-.-__.

(.I.::’

0

-,

I

100

I

I

200

300

I

_.;.:

:,

,,:..

:;..:

I

400

500

km

C’

C Fig. 6. NNW-SSE centre crossing

gravity

of Jebel Marra the Bake-Birao

the Ngaoundere

and topographic

and the Bagarra

profile

rift basin

section of the Ngaoundere

rift. For location

of profiles

Cc“

across

to the south. rift, illustrating

the Darfur

dome

Inset is profile the positive

cutting

the volcanic

LID’, of similar regional

association

trend, with

see Fig. 4.

lnterpretetion

Attempts the Darfur modelling lar prisms reference

have been made here to interpret the Bouguer anomaly associated with dome in a variety of ways using the three dimensional, iterative program

of Cordell

up from, plane

until

down

and Henderson from,

the computed

anomaly

(1966)) best fits the given Bouguer anomaly associated

with the Darfur

(1968). This program

or symmetrically (using

about

builds rectangu-

a specified

the Prism

formula

in an RMS sense. The Bouguer

dome was digitised

horizontal of Nagy anomaly

on a 25 km grid after first removing

the -60 mGa1 regional anomaly (Fig. 5). The anomaly field to the north of the dome was extrapolated using the predicted field after Slettene et al. (1973). In areas of the dome where the gravity field is accurately known, the short wavelength gravity effects of near surface structures, such as those due to the thick pile of low density surface volcanics and the elongate sedimentary feature east of El Fasher, were removed. The versatility of the three-dimensional modelling program enabled four different model types to be generated for the section BB’ (Fig. 7). Model 1 (Fig. 7) represents an assumed 35 km thick crust locally thickened by 4 km, with density contrast of 0.5 g/cm3 while models, 2, 3 and 4 represent a range of low-density asthenospheric bodies intruding and/or thinning the lithosphere by

215

MODEL

1

MODEL

MODEL

2

MODEL

3

4

\ CRUST

I

i

Fig. 7. ark-dimensional the Darfur Density

dome.

contrasts

model inte~retations Models

1-4

represent

cross

of the residual sections

negative

through

_

Bouguer anomaly

different

models

associated

along

profile

with BB’.

are in g/cm3.

50-60 km, with a density contrast of -0.05 g/cm3. The fit between the observed and model gravity fields increases from an RMS error of 2.7 mGa1 (model 1) to 4.7 mGal (model 4). This is considered to be due to the digitised data field, used in the computations, being essentially the observed gravity field rather than a smoothed version of it, such that model anomalies due to deep seated bodies (models 3 and 4) are less able to fit the short wavelength perturbations of the observed anomaly. The available seismological evidence for the similar but more evolved Kenya and Ethiopia domes tends to discount models 1 and 2. In East Africa there is no evidence of either thickening or thinning of the crust towards the centre of the domes (excluding the rift valley) and it would be difficult to visualise a mantle process giving rise to model 1 without density structures still remaining, from the post-Cretaceous process, within the upper mantle. The normal P, velocities for the Kenya dome (Maguire and Long, 1976) and the Ethiopian dome (Berckhemer et al., 1975) suggest the anomalous body does not immediately underlie the crust as model 2 would imply. The large teleseismic delay times (Fairhead and Reeves, 1977) and slowness anomalies (Long and Backhouse, 1976) for these domes support the existence of large low velocity and presumably tow density bodies at depth beneath these uplifts with their isostatic roots within the depth range 40-80 km. Models of type 3 and 4 represent for Darfur a laccolithic body at mid-lithospheric depth and a thinned lithosphere with emplacement at high level of low-density astenospheric material respectively. These latter models are considered viable alternative model types and it is hoped that the analysis of relative P-wave delays and siowness anomalies, currently in progress for teleseismic and local events recorded by seismological arrays over the dome, will limit the non-uniqueness of these preli~n~ model interpretations.

The similarity of the long wavelength, negative Bouguer anomaly connecting the domal uplifts of North and Central Africa to the gravity field associated with the domal

uplifts

of East Africa

and by Brown and Girdler

has already

been discussed

(1980). We draw attention

by Fairhead

(1079,

here to the similarity

1980) of the

Central African and the Afro-Arabian Rift Systems and discuss the implications. The Afro-Arabian Rift System comprises the well developed Red Sea and Gulf of Aden rift arms and the domally evidence

indicates

that

the

uplifted

Red

and rifted arm of East Africa.

Sea and

Gulf

of Aden

initially

Geological

developed

as

subsiding sedimentary basins. with marine transgressions in Jurassic and Cretaceous times (Karpoff, 1957: Whiteman~ 196X; Beydoun, 1970 and Brown. 1970). This favours the extensional rift basin model of McKenzie ( 1978). Salveson ( 1978, 198 l), using petroleum

exploration

data for a wide variety of rift basins

including

the Red

Sea and Gulf of Suez basins. finds that most of the basins studied have pre-rift sediments, indicating that large-scale uplift prior to graben formation did not occur. For the Red Sea and Gulf of Aden.

faulting

began in upper Cretaceous

and graben

controlled sedimentation continued during the Tertiary with the development of sea floor along their axes. possibly in two stages (Girdler et al., 1980). The uplifted shoulders of these rifts are considered by Salveson (198 i) to be the isostatic response of the crust-lithosphere to the post-extensional process and that extension was not the result of uplift. Theoretical studies dence

and graben

by Bott and Mithen

formation

(1981) models are possible of the lithosphere. By comparison,

models

(1981) show that the extensional.

similar

to the McKenzie

if stresses are able to concentrate

the study

of erosion

surfaces

subsi-

(1978) and Salveson

in the upper brittle part

in East Africa

(Saggerson

and

Baker, 1965) show that the Kenya dome developed in uplift phases since Cretaceous times. By the Iate Miocene there was a broad domaf uplift of about 300 m and was the site of phonolite the development 1400 m during fissure

eruptions

eruptions

(considered

here to be similar

to the present

stage in

of the Darfur domal uplift). This was succeeded by major uplift of the Pliocene and mid-Pleistocene times, when the first extensive and rift faults

developed

(Baker

and Wohlenberg,

197 1). These

events are summarised in Fig, 8. Bott and Mithen (1981) have further shown that sufficient stresses can develop in the brittle upper crust, due to the isostatic response of the crust to a low-density body beneath, for rift development to take place across the resulting domal uplifts without the need for external stresses. The Central African Rift System began in early Cretaceous with the Ngaoundere and Abu Gabra rifts developing as subsiding rift basins in stages into the Tertiary (Browne and Fairhead, 1983, this volume). The lack of present-day seismic activity along this rift system together with the non-development of oceanic-type crust indicates the rift process has been slower to develop than the Afro-Arabian Rift

217

AGE

Red

SERIES

Sea

Gulf

of Aden

Ethmpm

Kenya

Mupl/ft

and

Volcanism

zrzL!z Alkali ‘olcanism

60

INITIAL

‘Av.7*Tr’.‘:

Fig 8. Summary Afro-Arabian

RIFT

STAGE

FORMATdON

SIMILAR CENTRAL SYSTEM

of the tectonic

evolution

African

TO PRESENT AFRICAN RIFT

of the main

Rift System. The solid horizontal

system that the Central

OF DEVELOPMENT

rift and dome

arrow indicates

Rift System has presently

structures

associated

the stage in the development

with the OF this rift

reached.

\

-4

.^

,

Tethys

1

AFRO-ARABIAN

I

0

CENTRES

UPLIFT

VOLCANISM,

Fig. 9. The spatial Miocene

00~4~~

OF AND

ALKALI

ZONES +*a

NO RIFTING

configuration

stage in the development

of the present-day of the Afro-Arabian

OF CRUSTAL

ExTms~oN.

SUBSIDENCE

AND

FORMATION

Central

GRABEN

African

Rift System.

Rift System

and

the early-mid

System and/or during

the regional

the Tertiary

stress regime within

the African

such that it does not favour present-day

System. The stage currently

reached by the Central

African

the early to middle

Miocene

stage of the Afro-Arabian

the solid horizontal

arrow in Fig. 8. The time of initiation

the Red Sea and Gulf of Aden is uncertain Thus the Central

African

continent

has changed

development

of the Rift

Rift System is similar

Rift System as indicated of seafloor

spreading

to by in

but may have already started at this time.

Rift System (Fig. 9) is considered

an incipient

system of

CENTRAL AFRICAN RIFTS

Fig. 10. Contrasting

evolution

of different sections of the Central African and the Afro-Arabian

Systems. The Darfur dome is shown as an early stage development

Rift

of the Kenya dome and the subsiding

rift basins of the Central African Rift Systems as early stage development

of the Gulf of Aden. Note the

different character of the regional Bouguer anomaly associated with the two processes.

219

the Afr~Arabian sional

tectonics

domal

uplift

gravity

anomalies

regional

positive

type which is characterised with passive

(Fig.

rifting,

giving

10). The rift processes (Fig.

gravity

ated with a regional

by two distinct are associated

10). The extensional anomaly

negative

whereas

gravity

rift processes:

rise to subsiding tectonic

The former

exten-

and active

with distinctly

process

the active domal

anomaly.

rift arms,

different

is associated

uplift process anomaly

with a is associ-

is caused

by

stretching and necking of the crust and lithosphere. The thinning produces a large positive gravity effect at the surface, whereas necking

of the crust at the base of

the lithosphere

The resulting

produces

only

a small

but

negative

isostatic response of the lithosphere is subsidence. developed into an embrionic ocean, the development

gravity

effect.

Once this rift process has of oceanic crust produces a

broad positive anomaly (Fig. 10). In contrast, the negative anomaly associated with domal uplift can be satisfactorily modelled by a low-density laccolithic body or thinned lithosphere (models 3 and 4, Fig. 7) without any change in crustal thickness. The isostatic response of these bodies is domal uplift. The Darfur, Tibesti and Hoggar uplifts represent an earlier stage of development than the East African uplifts which have progressed to a rifting and more extensive volcanic the axial part of the uplift. Along the axes of the Kenya and Ethiopian positive gravity the crust.

anomalies

The different the extensional

evolutionary character, that these rift processes show, suggest that tectonics and passive rifting is the more likely process that can

eventually

have developed

lead to the development

(Fig.

10) due to basaltic

stage along Rifts small

of passive continental

intrusions

into

margins.

CONCLUSIONS

(1) Geological evidence indicates that the Darfur dome developed contemporaneously with the Cainozoic volcanism. (2) The Darfur domal uplift and its associated volcanism is similar to the envisaged early-middle Miocene stage in development of the East African domes. Its tectonic setting within the Central African Rift System is similar to the tectonic setting of the East African domes within the Afro-Arabian Rift System. (3) Two rift processes are present in Africa and are characterised gravity anomalies. One is a process of extensional tectonic

regional rifting

resulting

in a subsiding

rift which

is characterised

by different and passive

by a regional

positive

gravity anomaly. The second is a process of active domal uplift and relatively minor rifting which is characterised by a regional negative gravity anomaly. Both these rift processes appear to co-exist along different arms within the same Rift System. (4) The extensional tectonic and passive rifting process is the more likely process that can lead to the development of passive continental margins.

ACKNOWLEDGEMENTS

This study has been funded by the Overseas Development Administration and field work in Sudan was in full collaboration with the tieological and Mineral Resources and

Department,

1.1. Mohamed

Khartoum.

Special

thanks

for help with the fieldwork

are extended

to .4.M.

and

R.C. Boud

(Editor),

Agriculture

Bramall

for home of the

cartography. REFERENCES

Andrew,

G.. 1948. The geology

University Baker.

Press, London,

of Sudan.

In: J.D. Tothill

in the Sudan.

Oxford

pp. X4- 128.

B.H. and Wohlenberg,

J.. 1971. Structure

and evolution

of the Kenya

Rift Valley, Nature,

229:

538-542. Baker,

B.H., Mohr,

P.A. and Williams.

Geol. Sot. Am.. Spec. Publ..

L.A.J.,

1972. The geology

of the Eastern

Rift System of Africa.

136.

Banks, R.J. and Swain, C.J.. 1978. The isostatic

compensation

of East Africa.

Proc. R. Sot. London,

Ser.

H.. Gehrande,

H.. Makris. J., Menzel, H.,

A, 364: 33 I-352. Berckhemer.

H., Baies, B., Bartelson,

Miller, Ethiopia. Beydoun.

H.. Behle, A., Burkhardt,

H. and Vees. R. 1975. Deep seismic

soundings

In: A. Pilger and A. Rosler (Editors),

Z.R., 1970. Arabia

and northern

in the Afar

Afar Depression

Somalia:

comparative

region

of Ethiopia.

geology.

and on the Highland Schweizerbart,

Philos. Trans.

of

Stuttgart.

R. Sot. London

Ser. A, 267: 267-292. Biddle, C.A. Heights Black.

R. and

relationship Tectonics. Bott. M.H.P. Abstracts,

by Aneroid

Girod,

Barometer.

M., 1970. Late

to basement

structure.

Conference

D.P.,

igneous

activity

and LG. Gass (Editors).

in West Africa African

and

Magmatism

its and

pp. 185-2 IO.

1981. Mechanism

on the processes

Ltd.

to Recent

In: T.N. Chfford

Oliver and Boyd, Edinburgh, and Mithen,

Tellurometer

Palaeozoic

of graben

of Planetary

formation

Rifting.

and sources

Lunar

Planet,

of causitive

Inst., Contrib..

stress.

No. 457:

11-12. Brown.

G.F..

Trans.

1970. Eastern

R. Sot. London,

Brown, C. and Girdler, of the continents. Browne,

(Editors),

disruption. Processes

of the Red Sea and the coastal

R.W., t980. Interpretation J. Geophys.

SE. and Fairhead,

continental

margin

structures

of African

Gravity

and its implication

J.D.,

1983. Gravity

study of the Central

I. The Ngaoundere Rifting.

and Abu

Gabra

Tectonophysics,

African

rifts.

plate stationary?

Nature.

three dimensional

solution

R.G., 1968. Iterative

Geophysics,

Crough,

S.T., 1978. Thermal ST., 1981a. Free-air

Edwards,

a model of B.H.

Baker

239: 387-390. of gravity

anomaly

data using

origin of mid-plate

hot-spot

gravity over the Hoggar 77: 189-202. Sweil, Africa:

swells. Geophys.

Massif, Northwest gravity

constraints

J.R. Astron.

Sot.. 55: 45 l-469.

Africa: evidence on its isostatic

for alteration

of

compensation.

Res. Lett., 8: 877-879.

W.N.,

London.,

and

33: 5966601.

the lithosphere. Tectonophysics. Crough, S.T., 1981b. The Darfur Geophys.

Rift System:

94: 187-203.

Burke, K. and Wilson, J.T., 1972. Is the African

Crough,

Philos

for the breakup

In: P. Morgan

Cordell.

a digital computer.

Arabia.

Res., 85: 6443-6455

of Continental

L. and Henderson,

in Saudi

Ser. A, 267: 75-87.

1926. Fossil

82: 94- 100.

plants

from

the Nubian

Sandstone

of Eastern

Darfur.

Q.J. Geol.

SOC.

221

Fairhead,

J.D.,

Africa

1979. A gravity

and its similarity

Fairhead,

J.D.,

link between

1980. The intra-plate

East African

the domally

to the East African

Rift System.

volcanic

uplifted

Cainozoic

System anomaly. centres

In: Geodynamic

of North

evolution

Earth. Africa

volcanic

Planet.

centres

and their possible

of the Afro-Arabian

of North

Sci. Lett., 42: 109- 113. relation

Rift System.

to the

Accad.

Naz.

Lincei, 47: 45-48. Fairhead,

J.D.

and

Girdler,

R.W.,

1971. The seismicity

of Africa.

Geophys.

J.R.

Astron.

SOC.. 24:

271-301. Fairhead,

J.D. and Reeves, C.V., 1977. Teleseismic

of the African Fairhead,

lithosphere.

J.D., Bermingham,

Leeds

(U.K.)

1979-1981. Francis,

Report

in the Darfur

Gass, LG., Chapman, Girdler,

with

R.S. and Ahmed, of Western

D.S., Pollack,

volcanism.

R.W., Brown,

Hunting

Geol. Miner. Resour. Karpoff,

at Jebel Marra,

studies

and geophysical

parameters

survey

Sudan.

Nature.

Development

in the Sudan.

de I’Arabie Seoudite.

volcanism,

crustal

Part

of the westernmost

209: 1290.

Corporation,

1 Gravity

Khartoum).

bases.

Bull. Sudan

Bull. Sot. Geol. Fr., 7: 653-697.

swelling and rifting.

of a part of western

Geophys.

Darfur.

J.R. Astron.

geophysique

Darfur

Nature,

230: 85-87.

and its region. Geogr. J.. 127: 30-49.

J. Geol. U.A.R.,

of the western

7: 89-103.

flank of the Gregory

Rift (Kenya),

Sot., 44: 677-688.

a la connaissance

geologique

du bassin du Lac Tchad.

Mem.

42: 311 pp.

P.K.H.

and Long, R.E., 1976. The structure

Part 1 The crust. Geophys. McKenzie,

volcanism

Ser. A, 298: l-43.

R.W., 1976. The structure

Louis, P., 1970. Contribution Maguire,

of

(Sudan)

of Tertiary-Recent

and Styles, P., 1980. A geophysical

1974. Gravity

Department

Ser. A 288: 581-597

V.C., 1961. The Jebel Marra,

Lofti, M., 1963. The geology Long, R.E. and Backhouse,

ORSTOM,

Resources

Dep., 26: 49 pp.

and Robertson,

Part 2 The mantle.

Studies in Sudan by the University

Mineral

R.S., 1978. Geological

R. Sot. London,

and fumaroles

R., 1957. Esquisse geologique J.H.G.

and

Ltd., 1970. Rept. No. 4 (Rural

M.A.,

Le Bas, M.J., 1971. Per-alkaline Lebon,

thickness

243: 30-32.

H.N. and Thorpe,

R. Sot. London,

and Geophysics

Isaev, E.N. and Mitwalli,

and inferred

of Leeds, unpublished.

Sudan. Nature,

C., Noy, D.J.M.

D., 1966. Hot springs

Geology

anomalies

F., 1973. Setting and significance

Philos. Trans.

Gulf of Aden. Philos. Trans. Hammerton,

the Geological

1 data coverage. University

No.

Province

of mid-plate

P.M. and Brown, S.E., 1982. Gravity

in collaboration

P.W., Thorpe,

delay times, Bouguer

Earth Planet. Sci. Lett., 36: 63-76.

J.R. Astron.

D., 1978. Some remarks

on the western

flank of the Gregory

Rift (Kenya),

Sot., 44: 661-675.

on the development

of sedimentary

basins.

Earth

Planet.

Sci. Lett.,

40: 25-32. Medani,

A.H. and Vail, J.R.,

African Morgan,

W.J., 1981. Hotspot

(Editor), Nagy,

1974. Post Cretaceous

Rift System. Nature,

in Sudan

and its relationship

to the East

tracks and the opening

of the Atlantic

and Indian

oceans.

In: C. Emiliani

The Sea. Wiley, New York, N.Y.

D., 1966. The gravitational

Roth&

faulting

248: 133- 135.

J.P.,

attraction

1954. La zone seismique

of a right rectangular

mediane

prism. Geophysics,

Indo-Atlantique.

Proc.

R. Sot.

31: 362-371,

London,

Ser. A., 222:

387-400. Saggerson,

E.P.

deformation Salveson,

J.O.,

International 82-86. Salveson,

J.O.,

Processes Slettene,

and

B.H.,

Symposium

Mapping

Post-Jurassic

Rifting.

in the Geology

tectonic Lunar

Aerospace

of Rift

model. Abstr.,

Planet. Centre,

Papers

Inst., Contrib., J.R.,

2nd ed.

in eastern

Kenya

and

their

Abstracts

for

121: 5 l-72.

Basins-a

tectonic

Rift. U.C. Los Alamos

L.E., Bouse, R.S. and Sandvs, Agency,

erosion-surfaces

Q. J. Geol. Sot. (London),

on the Rio Grande

1981. Rift basins-a

of Planetary

1965.

to rift structure.

1978. Variations

R.L., Wilcox,

Defense

Baker,

in relation

Scientific

presented

model.

Lab., Pub]. LA-7487-C: to the Conference

on the

457: 39-42.

1973. Bouguer

Gravity

Map of Africa.

U.S.

Smith. A.E., 1951. Graphic

adJustmen

Sonet. J.. 1963. Notrce explicative Tchad. Swain,

B.R.G.M..

Paris, 80 pp.

C.J. and Khan.

M.A..

by least squares.

Geophysics,

16: 2222227.

sur la fruille Niese. Carte geol. Recce. I’Echelle I

: 500.000.Rrpuhltc

of

(~nFrench).

1977. Kenya,

a Catalogue

of Gravity

Measurements.

Univ. of Lcicester.

unpublished. Sykes, L.R. and Landisman.

M.. 1964. The seismicity

of East Africa,

the Gulf of Aden and the Arabian

and Red Seas. Bull. Seismol. Sot. 4m., 54: 1927- 1940. Thorpe,

R.S. and Smith,

K.. 1974. Distribution

of Cenozoic

Volcanism

Africa.

Earth.

Planet.

Sci. Lett..

22: 91-95. Vail, J.R.. 1978. Outline and adjacent Vincent,

of the Geology

areas. Overseas

P.M.. 1970. The evolution

I.G. Gass (Editors) Whiteman,

African

A.J.. 1968. Formation

deposits 49: 68

of the Tibetsi volcanic

Magmatism

Wolff. J.P.. 1964. Carte geologique French).

and mineral

Geol. Miner. Resow.,

province,

and Tectonics.

of the Red Sea depression. de la Rtpublique

of the Democratic

Republic

of the Sudan

pp. eastern

Sahara.

In: T.N. Clifford

Oliver and Boyd, Edinburgh,

and

301.-319.

Geol. Mag., 105: 231-246.

du Tchad.

Echelle

I : 500,000.

B.R.G.M..

Paris (in