The structure of speke Gulf, Tanzania, and its relation to the East African rift system

Tectonophysics, 23 (1974) 155-175 0 Elsevier Scientific Publishing Company,

Amsterdam

-

Printed

THE STRUCTURE OF SPEKE GULF, TANZANIA, TO THE EAST AFRICAN RIFT SYSTEM BRIAN

in The Netherlands

AND ITS RELATION

W. DARRACOTT*

Department of Geophysics and Planetary Newcastle upon Tyne (Great Britain) (Submitted

April

26, 1973;

accepted

Physics,

School

for publication

of Physics,

January

The

Uniuersity,

18, 1974)

ABSTRACT Darracott, African

B.W., 1974. The structure of Speke Gulf, Tanzania, Rift System. Tectonophysics, 23: 155-175.

and its relation

to the East

A gravity survey has been made of the region between Lake Victoria and the Gregory Rift, in an attempt to establish the relationship of Speke Gulf to the East African Rift System. Regional and residual Bouguer anomaly maps show the main feature of interest to be the negative residual Bouguer anomaly in line with, and over Speke Gulf. The anomaly is trough shaped (width 50 km) and has an amplitude of -200 g.u., extending for nearly 200 km. Two explanations for the negative anomaly are plausible: (1) an elongate, low-density granitic intrusion; (2) Speke Gulf occupies part of a much eroded Precambrian graben, within which a thicker segment of the earth’s crust is preserved. At present, the “graben” model seems to offer a better correlation with the geology and is preferred. Either model implies the presence of fundamental lines of weakness in the crust in this region, and the seismicity along these lines indicates incipient faulting, probably due to stresses arising from north-south variations in the extension across the Gregory Rift. It is possible that a future rift may be developing at Speke Gulf, paralleling the Kavirondo Rift Valley.

INTRODUCTION

Lake Victoria occupies a broad, shallow downwarped region between the eastern and western branches of the Rift System in East Africa. The eastern shoreline of the lake is indented by two large gulfs, the Kavirondo Gulf in the north, and Speke Gulf in the south (fig. 1). Shackleton (1950) has shown that the Kavirondo Gulf is a rift valley, of probable Late-Miocene age, and related genetically to the Gregory Rift in Kenya. Comparatively little is known about the structure of Speke Gulf. In many ways it is similar to the Kavirondo Gulf, and it has been postulated that it occupies part of a graben (Horne,

* Present

address:

Geological

Survey

of South

Africa,

Pretoria,

South

Africa.

156

1962a, b). The reconnaissance gravity survey of Tanzania by Masson Smith ant Andrew (1960) revealed a negative Bouguer gravity anomaly of -200 g.u. (1 g.u. = 1O-6 m set * = 0.1 mGa1) in line with Speke Gulf, and Wohlenberg (1969) has shown that there is a zone of seismicity trending east-northeast in line with the Gulf. In 1968 and 1969, gravity and total intensity magnetic field surveys were made in the region 1-4“s; 33.3-36.5”E. The gravity data are presented and interpreted with special reference to problems connected with the evolution of Speke Gulf and the Gregory Rift. SUMMARY

OF THE GEOLOGY

AROUND

SPEKE GULF

The geology of the area is summarised in Fig. 1, which has been compiled from the work of Dawson et al. (1961), Dundas and Awadallah (1966), Gray and MacdonaId (1966), Gray et al. (1965), Horne (1962a, b), Huddleston (1951), Macfarlane (1967,1968), Mulgrew (1966), Naylor (1962,1965), Naylor and Thomas (1963), Pickering (1958,1960, 1964, 1965), Saggerson (1966), Shackleton (1946), Thomas (1966,1967), Thomas and Kennedy (1966), Williams (1964) and Wright (1967). Much of this part of Kenya and Tanzania is gently undulating, with prominent but relatively smooth hills rising to not more than about 300 m above the level of the surrounding land, consisting of the more erosion-resistant quartzites and granites. The general level of the ground increases from the shores of Lake Victoria (1,134 m) to the high land of the Narok district in the northeast (2,000 m), then drops suddenly to around 700-1000 m in the Gregory Rift. The geology of the area is restricted to the Precambrian, Neogene (25-l m.y.) and Quaternary periods. The Precambrian may be divided into two main tectonic units, the Tanganyika Shield, west of about 35”E, and the Mozambique Orogenic Belt to the east. The Tanganyika

Shield

The Tanganyika Shield is composed predominantly of biotite granites and granodiorites, dated at about 2500 m.y. (Cahen and Snelling, 1966; Edwards and Howkins, 1966; Old and Rex, 1971), with supracrustal remnants of metavolcanics and metasediments of the Nyanzian and Kavirondian Systems, dated at 2,900 m.y. (Robertson, 1969), lying in northeast trending belts. In addition, there is the Bukoban System, which in Tanzania is represented by the Ikorongo Group of sediments and me&sediments (Late Proterozoic, approx. 1000 m.y.). These rest unconformably on the older rocks and form part of a once continuous and extensive sedimentary cover, now broken up by post-Bukoban folding and minor faulting. The maximum thickness of this group is about 1,000 m, though in most places it is much less. In Kenya, the Bukoban System is represented by the Kisii Series, a flat lying series of basaltic and andesitic lavas and tuffs.

Fig. 1. Summary of the geology of the Speke Gulf region inset gives the location of the region under consideration.

of northern

Tanzania.

The

158

The Mozambique

Orogenic Belt (450-700

m.y.)

This consists of “crystalline basement” schists and gneisses. The area is extremely complex structurally and metamorphically, and the sequence of events is still not completely resolved. The Neogene

and Quaternary

The Neogene tectonic activity of the region is mainly confined to the neighbourhood of the Gregory Rift, and the sequence of faulting and volcanic activity has been summarised by Baker et al. (1972) and Fairhead et al. (1972). The major Tertiary features within the region are the Isuria and Utimbara normal faults of Miocene age, with downthrows of at least 500 m to the southeast and south, respectively. Large parts of the region are covered by Neogene and Quaternary volcanics. In the north, there are Miocene phonolitic lavas, which have flowed from the northeast. They are apparently thickest at the base of the Isuria Escarpment, and elsewhere are not much more than 60-70 m thick. In the southeast part of the region, the volcanics come from the Ngorongoro and associated volcanos. Much of the solid geology of the whole region is covered by low-density superficial matter. A medium to fine-grained calcareous tuff, derived from nearby volcanics, covers the whole of the Serengeti Plains in the southeast, and is probably less than 30 m thick. The extensive alluvial deposits over the western part of the region, with granites outcropping here and there, show that the shallow waters of Lake Victoria once extended further in land. THE GRAVITY

SURVEY

During 1968 and 1969, 1,091 gravity measurements were made, and their locations are shown in Fig. 2. Nearly all the observations were along roads and tracks, at intervals of 3-4 km, with an increased spacing at the edges of the survey area. The measurements were made with a Lacoste-Romberg geodetic gravimeter (No. G 167), and were tied to the base station network of Masson Smith and Andrew (1962). Elevations were obtained using “leap-frog” barometric levelling (Searle, 1969), and were tied into trigonometrical points and bench marks wherever possible. The uncertainty in the elevations is about f 4 m over most of the survey, and possibly as high as f 9 m in a few cases. Bouguer anomalies, g,, were calculated according to the relation: g, = go-g,+

3.086h - 0.4191h.s

+ TCg.u.

where: g, = the observed gravity, in g,u., gt = the theoretical gravity given by the 1931 International Gravity Formula, h = the elevation in metres above mean sea level, s = the average crustal specific gravity, and TC = the terrain correction. An average crustal specific gravity of 2.67 was used in the Bouguer reductions;

KENYA

.

.. .:. ._.......q*,--_, *... i .. /

. . .

. .

. .

. . .

.

TOPOGRAPHY AND LOCATION g . . OF GRAVITY STATIONS D/\FlR*COTT: . T&,gyi” : mom FAICHEG,D : 0 . SEARLE : . O.G.5. LEICESTER UN,” :a l

0

.

,A,

4Ukm

Fig. 2. Location

3,4O

of gravity

stations

. ’ . . . .

..

-

and generaked

topography.

the results of density measurements on samples collected from the region show that this is a reasonable choice. The terrain corrections were made for radial distances up to 21.9 km (Hammer’s (1939) zone M) for stations near

160

BOUCUER Contour

ANOMALIES

interval : 5Oq.u

-Assumed specific Major

fault

qravity

: 2.67

/

Fig. 3. Contour map of Bouguer anomalies.

the Gregory Rift. Elsewhere, the topography is relatively smooth and terrain corrections were considered to be less than 5 g.u. and were neglected. The probable accuracy of the Bouguer anomalies is k 15 g.u.

-rim

I - -1600

I

I I t -

-

-1‘00

I

-IL00

--___

L -2000

I )

-KM

,

_

.

-

.

-1400

--___

I

1100 -h-o0

-LO3 -----__ -1600 r-_~+--=--J.Iam WN

BOUCUER

___c-mo 9-u. -Mm

_----

-___-___-____

_---

-

___-c_

SSE

W

ANOMALY

.-;j

wsw

mUOO ‘6% q.u. -km

:

-

REGIONAL

ANOMALY

: -_--_

Fig. 4. Long profiles used in the construction of the regional Bouguer anomaly. All are approximately perpendicular to the ENE trend of Speke Gulf (vertical dashed line).

162

Fig. 3 shows a Bouguer anomaly map with contours at 50 g.u. intervals. For extra control at the edges of the survey, the data have been supplemented by the observations of O.G.S. (Masson Smith and Andrew, 1960), Searle (1970b), Sowerbutts (1971), Fairhead (1973) and the University of Leicester (J. Mansefield, personal communication, 1970).

REGIONAL

BOUCUER

ANOMALY Contour

interval

UOkmS4”

: 50 9,~.

\yyi

40, _-._

Fig. 5. Contour map of the regional Bouguer anomaly.

163 THE GRAVITY

ANOMALIES

Previous studies (Bullard, 1936; Girdler et al., 1969; Girdler and Sowerbutts, 1970) have shown that the Eastern Rift is associated with a broad negative Bouguer anomaly of about 500 km width and amplitude -1000 g.u., which dies out at about 4-5”s. This has been interpreted by Girdler et al. (1969) as being due to the slightly lower density asthenosphere expanding to higher levels and engulfing part of the lithosphere. The lithosphere (uppermost mantle plus crust) is thus thinned under East Africa. The gradients of the Bouguer anomalies in Fig. 3 show that most of the anomalies are caused by relatively near-surface structures. Before a quantitative interpretation can be made, the broad regional anomaly must be delineated and subtracted from the observed Bouguer anomalies, to give the residual Bouguer anomalies, which will then reflect more clearly the crustal features.

Determination

of the regional anomaly

The following procedure was used to obtain a regional Bouguer anomaly map. Ten approximately NNW-SSE profiles were constructed from the contour map of Bouguer anomalies. These long profiles (Fig. 4) are perpendicular to the ENE trend of Speke Gulf. Hand-drawn, smooth “background’‘-curves, representing the regional anomaly, were fitted to the ten profiles. These regional anomaly curves were then sampled at 10 km intervals, and plotted and contoured (Fig. 5). As an independent check on the above result, the regional anomaly was also found using a least-squares polynomial curve-fitting technique (Coons et al., 1963). The result is similar and suggests a second-order polynomial fit to the data. A second-order polynomial implies a major causative structure of about 300 km width and is consistent with the model for the asthenolith beneath the Eastern Rift presented by Darracott et al. (1972).

The residual Bouguer anomalies The difference between the observed Bouguer anomalies and the regional anomaly gives the residual Bouguer anomalies (Fig. 6). It is seen that there is a good correlation between the residual anomalies and the geology of the region. For example, the positive anomalies ZZZand V (Fig. 6) are correlated with the high-density Nyanzian metavolcanics, and the circular positive anomaly ZZcorrelates with a iarge gabbroic intrusion of lopolith shape (Darracott, 1972a). Anomaly Z is due to the high-density Bukoban lavas of the Kisii Series. It will be noticed that most of the anomalies are confined to the Tanganyika Shield, and that the Mozambique Orogenic Belt (east of about 35.3”E) is relatively free of short-wavelength anomalies. The significance of this for regional studies has been discussed by Darracott (1972b). Fig. 7 shows five profiles (B-B’, C-C’, D-D’, E-E’, and F-F’ - for location see Fig. 6) of residual Bouguer anomalies with the schematic geology, and demonstrates the way in which the positive and negative components of the residual anomaly have been separated.

164

c :

RESIDUAL

BOUCUER

ANOMALIES Contour Assumed Major

Q

interval specific fault

: grawty:

2.67’

/ 4,Okm

Fig. 6. Contour map of residual Bouguer anomalies. The profiles discussed in the text are shown, and the more important anomalies are labelled with Roman numerals. Fig. 7. The five profiles used for the interpretation matic geologic sections.

of the Speke Gulf anomaly, with sche-

165

.*

.

P RESIOUAL .ve

AND/OR

BOUGUER -“e

ANOMALY.

COMPONENT OF

..+.***...

KM

-------

Lp NEOGENE

SEDIHENTS

TEPTlAR”

LA”*5

s”YOB*N

LLVLS

ANCMALY

THE RESIDUAL

BOUGUER

ANOMALY

OVER

SPEKE GULF

Fig. 7 shows the nature and extent of the trough-shaped anomaly in line with Speke Gulf. This anomaly (IV, Fig. 6) strikes approximately eastnortheastwards, and has a maximum amplitude of -200 g.u.. It extends for nearly 200 km and its width varies from 45 to 60 km. There is a small degree of uncertainty in the anomaly gradients because of the somewhat subjective way in which the positive and negative components of the residual anomaly were separated, and this must be borne in mind when discussing the interpretation of the anomaly. The regional crustal structure assumed in the interpretation is that derived by Gumper and Pomeroy (1970) from studies of the phase and group velocities of surface waves, and the velocities of the short-period body waves, P,, S,, and L,, observed for this part of Africa. Their model has a three-layer crust, with the top 7 km having s.g. = 2.70 (corresponding to a compressional velocity, VP = 5.90 km/set), the next 10.5 km having s.g. = 2.80 (VP = 6.15 km/see) and the lowest layer of 18.7 km with s.g. = 2.85 (VP = 6.6 km/set). The layering and physical parameters of their structure were taken from the CANSD model of the Canadian Shield (Brune and Dorman, 1963) and the results are in good agreement with other models for the stable region of Africa presented by Rykounov et al. (1972) and Bonjer et al. (1970). Although in reality the density of the crust probably increases more smoothly with depth, the three-layer crust is probably a good approximation and is considered the best available at present. All gravity anomalies were calculated using the 2-D computer program of Takin and Talwani (1966). Three possible causes for the negative gravity anomaly in line with Speke Gulf are considered: (1) Low-density sediments The negative anomaly persists over rocks of widely differing types. In the Speke Gulf locality, the granitic rocks are overlain by lake sediments and alluvial soils, which can be traced up to 50 km eastwards, but the anomaly continues much further to the northeast. Further, on profile B--B’ the minimum in the anomaly is to the northwest of the sedimentary cover. From studies of the drainage reversal patterns associated with the formation of Lake Victoria, Bishop and Trendall (1967) estimate there is about 70 m of sediment in the northwest part of the lake. For the southeast part of the lake, boreholes show not more than 76 m of unconsohdated sediments. The evidence suggests that the maximum thickness of the sediments is about 100 m. Such a thickness would contribute only -35 g.u. to the Bouguer anomaly (i.e., 18% of -200 g.u.), assuming a specific gravity of 1.80 for the sediments. (2) An elongate low-density granite intrusion The negative anomaly lies over a region of Precambrian biotite granites and granodiorites (the Tanganyika Shield). Profile D--D’ is used to illustrate how

167

S.E.

N.W. TOP SURFACE

=

DEPTH

____L

2.1 KM

:KM

I TOP SURFACE

DEPTH _

0

KM

__lKM

LO

A

G.“._2jLL LO

3.30 0

KM

LO

Fig. 8.A. The observed and computed residual Bouguer anomaly along profile D-D’ for the ‘granite intrusion” model, for three density contrasts. B. The observed and computed reiidual Bouguer anomaly along profile D-D for the “graben” model.

168

an elongate, low-density granitic intrusion could possibly cause the observed negative anomaly. Fig. 8a shows the computed gravity anomaly over an intrusion, for three different density contrasts, and assuming a fixed depth of 7 km for the base of the intrusion. This depth corresponds to the base of the upper layer of the crustal model of Gumper and Pomeroy (1970). The interpretations of gravity surveys over most large granitic intrusions show a density contrast extending to the order of 10 km depth (Bott and Smithson, 1967). At all but two of the 22 points, the computed and observed anomalies agree to within 20 g.u. Mineralogical considerations place a lower limit of about 2.58 on the specific gravity of granites (Bott and Smithson, 1967) and this would allow a maximum specific gravity contrast of about -0.12 between the postulated intrusion and the surrounding granitic rocks. With these assumptions, the maximum depth to the top of any intrusion can be shown to be about 2 km. (3) Speke Gulf is part of a graben Horne (1962a, b, and personal communication, 1971) has suggested, on the basis of geological mapping, that Speke Gulf occupies part of a Precambrian graben structure. On this hypothesis, the negative gravity anomaly in line with Speke Gulf is seen to arise from the thicker segment of the earth’s crust preserved within the graben. The observed anomaly crosses the northsouth belt of Bukoban sediments (ca. 1,000 m.y.) and these are not disturbed by major faulting. The proposed graben must therefore be older than this formation. The granitic country rocks have radiometric ages of around 2,500 m.y., and any graben must be younger than this. Fig. 8B illustrates such an interpretation using profile D-D’ and assuming the layered crust. The best fit is obtained (+ 20 g.u.) assuming normal faults curving inwards slightly with depth and with a vertical displacement of 3.5 km. Similar models for profiles B-B’, C-C’, E-E’ and F-F’ indicate a variation of 0.9-4.6 km in the vertical fault displacement over a horizontal distance of about 130 km. This seems geologically reasonable as Precambrian rifts in other shields are known which have vertical displacements of this order (e.g., Kanasewich et al., 1969). DISCUSSION

On the available geological and geophysical data both the “granite” and the “graben” interpretations are plausible, and until more detailed data (e.g., seismic refraction and geological) become available, neither can be unequivocally discarded. Nevertheless, the “graben” alternative is favoured here for the following reasons. The five profiles (B---B’ to F-F’) were used to “map out” the approximate position of the inferred faults for the “graben” model and the approximate limits of the intrusion in the “granite” model (Fig. 9). At the southwestern end of the proposed graben, the northern inferred fault lines up with the prominent northern shore of Speke Gulf, cited by Home (1962a, b) as evidence of faulting there. The northern inferred fault also lines up with the broad Precambrian mylonitic shear zone along the Isuria Escarpment, and

D 0 N

0 -

c

<

<

0’ 0 M

c

I

d

170

elsewhere the proposed faults coincide with mapped shear zones in the granitic rocks. Large parts of the postulated structure are concealed by the Bukoban sedimentary sequence and Neogene sediments and volcanics, and no evidence of Mid-Precambrian faulting would be observed in the field. Otherwise, the proposed faulting is almost entirely in the Precambrian granitic country rocks, and since the geological mapping in large parts of the region is very much of a reconnaissance nature, it is quite possible that if such faulting exists it has so far been unobserved. Finally, rock samples collected during the survey show no systematic differences in petrology or density between granites within and outside the area of the negative anomaly, and whilst this does not negate the “granite” model, it tends, at least, to rule out the possibility of a surface-outcropping intrusion. Thus for the time being, the “graben” model appears to offer a better correlation with the known geology than the “granite” model. REGIONAL

TECTONICS

A future rift at Speke Gulf? The tendency for faults of the East African Rift System to follow older trends or lines of weakness in the pre-Karoo formations has been discussed by McConnell (1951), Harpum (1955), Dixey (1956), Cox (1970) and King (1970). The Isuria and Utimbara side-faults (post-Miocene) also follow preexisting lines of weakness presented by the Precambrian mylonitic shear zone, and the granite Nyanzian metavolcanics contact, respectively. Near Speke Gulf, interpretation of the gravity anomaly suggests the presence of either an elongate low-density granitic intrusion or an ancient graben. Either interpretation implies the presence of fundamental lines of weakness in the crust in this region, and it appears that such lines of weakness are sites for the current release of stress. The earthquake epicentres (for m > 4.0) located by Wohlenberg (1969, 1970) have been plotted on the geological map in Fig. 9. Seven epicentres fall within 20 km of the southern inferred line of weakness, and two more are very close to the northern line, near Speke Gulf. The seismicity indicates incipient faulting, and it may be that a future rift is developing at Speke Gulf, paralleling the Kavirondo Gulf rift valley. The extension

across the Eastern Rift

In terms of plate tectonics, the rift in East Africa may be considered as a developing extensional boundary between the Nubia and Somalia plates (Darracott et al., 1973). On geological grounds, it can be shown that there must be about 4 km of crustal extension in northern Tanzania, and at least 5.5 km of extension in central Kenya. Geophysical data strongly indicate between 15-35 km extension in Kenya (Searle, 1970b; Searle and Gouin, 1972). In a comprehensive review of rotation poles for the three plates (Arabia-

171

1 3

0

km

100

EXTENSIONn, 25km

0

-I

0

,

2:

_

HILLS

i” ,\

NORTHERN VOLCANIC

TANZANIA tlNE

,

/32’ Fig. 10. The relation of the proposed future Speke Gulf Rift to the faulting Rift, and the movement predicted from plate tectonics.

of the Eastern

Nubia-Somalia), Girdler and Darracott (1972) concluded that the rotation pole for describing the motion between Nubia and Somalia plates must lie off the coast of southwestern Africa. As was shown, it is difficult to obtain a quantitative estimate of the extension across the Eastern Rift using poles of rotation, but a pole off southwestern Africa implies that the ratio of the extension in central Kenya to that in northern Tanzania should be much less than appears to be the case. It is suggested that the possible future rift at Speke Gulf, the Kavirondo Rift and the many side faults, especially in northern Tanzania, help to accommodate this variation in the lateral extension across the Eastern Rift. Studies of the seismicity (Wohlenberg, 1969, 1970; Fairhead and Girdler, 1971) show that the Gregory Rift in Kenya is relatively devoid of teleseismic

172

activity, and that most of the stress release occurs in the region of block faulting in northern Tanzania, and to the sides of the Rift. Mapping of the axial positive Bouguer anomaly in the Gregory Rift (Searle, 1970a) and seismic studies (Fairhead and Girdler, 1971) show that the regional tensional stress field in this part of East Africa is approximately 120”N (Fig. 10). If the excess extension in central Kenya over that in northern Tanzania is distributed among all the side faults and rifts, only a small component of strike-slip movement is required along each fault, together with the usual extension associated with normal faulting. The overall movement on the structures to the west of the rift will be sinistral, and dextral for those to the east (Fig. 10). Transcurrent movement along the mapped faults has not often been reported. Pickering (1960) noted a zone of sinistral shear in the Itonjo Hills (Fig. lo), a region broken by Plio-Pleistocene normal faults. To the east of the Natron-Manyara escarpment (longitude 36”E) the volcanic province of northern Tanzania is dominated by the ENE-WSW line of volcanos which includes Meru and Kilimanjaro. Evans et al. (1971) have suggested that this volcanic line is the site of a possible “leaky transform fault”. This may also take up some of the variation in the extension across the Eastern Rift. CONCLUSIONS

The negative Bouguer gravity anomaly in line with Speke Gulf could arise from either (1) an elongate granitic intrusion; or (2) a Precambrian graben-like structure aligned more or less parallel to the present Spe_ke Gulf. Of the two possible interpretations, the latter is preferred because of the somewhat better correlation with the known geology. Either model implies fundamental lines of weakness in the crust of that area and it is thought that these lines are acting as preferential sites for stress release associated with north-south variations in the extension across the Eastern (Gregory) Rift. Much is now known about the structure and evolution of the Gregory Rift, though many problems still exist. From the present work it seems clear that some may be resolved by more detailed geological and geophysical work in the regions flanking the rift. ACKNOWLEDGEMENTS

This study was supported by the Natural Environment Research Council (N.E.R.C.) research grant No. GR/3/481 to Dr. R.W. Girdler. I am indebted to the N.E.R.C. for the award of a research studentship, and to Dr. R.W. Girdler for much advice. I am also grateful to J.D. Fairhead for assistance with the field work and many discussions, and to A.G. Green, S.A. Hall, and K.G. Service who contributed many ideas. I wish also to thank members of the Department of Physics, University of Dar-es-Salaam, for invaluable assistance with the equipment during fieldwork, and also Dr. C.H. Emeleus of the Department of Geology, University of

173

Durham,

who kindly performed

the petrographic

analyses of the rock samples.

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