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Morphotectonics of the Lake Albert Rift Valley and its significance for continental margins
JOURNAL OF GEODYNAMICS11, 343--355 (19901
343
M O R P H O T E C T O N I C S OF THE LAKE ALBERT RIFT VALLEY A N D ITS SIGNIFICANCE FOR C O N T I N E N T A L MARGINS
C. D. OLLIER
Department ~[" Geography and Planning, University ~f New England, Armidale 2351, Australia (Received July 4, 1989; accepted November 9, 1989)
ABSTRACT Ollier, C. D., 1990. Morphotectonics of the Lake Albert Rift Valley and its significance for continental margins. In: N. A. Logatchev and H. J. Zwart (Editors), Intracontinental Mountainous Terranes. Journal ~[' Geodynamics, 11:343 355. The Lake Albert region is described as an example of a rift valley. It is far more complex than the typical graben used by rift valley modellers. The northern section is a complex half graben with en echelon faults and warps on the west. The central section is a graben, but with uplift axes remote from the faults. The southern section is complicated by the partly faulted but mainly upwarped tilt block of the Ruwenzori massif. Many features of the landscape~drainage patterns, erosion surfaces, regolith-pre-date the rift valley tectonics, and provide insight into the dating and evolution of tectonic features. As rift valleys are precursors of seafloor spreading, the rift geomorphology shows which features may pre-date the formation of new continental edges. Many modern passive margins have morphotectonic features that date back to the rift valley stage of crustal evolution.
INTRODUCTION
The continent of Africa is divided into a number of huge basins by swells (Fig. 1). Some of the swells are the site of rift valleys, which roughly follow the line of the swell. The most instructive basin is that of Lake Victoria (Fig. 2) which is bounded almost completely by rifts. Two important aspects of rift morphotectonics will be examined in this paper: 1. The swells and rifts post-date the major river systems and erosion surfaces. 2. The rifts are not simple grabens, but exhibit complex tectonic and geomorphic relationships. As an example, the Lake Albert Rift will be discussed in more detail.
0264-3707/90/$3.00
(¢, 1990 Pergamon Press plc.
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Basins and swells of Africa. L.V. = Lake Victoria.
RELATIONSHIP
BETWEEN RIFTING AND DRAINAGE
PATTERNS
The drainage pattern associated with Lake Victoria is shown in Fig. 3. In the southern half of the area, the original drainage was from east to west, right across what is now Lake Victoria. The Mara was continuous with the Kagera, and the rivers that now enter the northeast corner of Lake Victoria were continuous with the Katonga. This dendritic pattern is repeated further north by the Kyoga-Kafu River, also flowing west. The drainage lines of the Katonga and Kagera are continuous between Lake Victoria and Lake Edward, as is that of the Kafu from Lake Kyoga to Lake Albert. Where the Kafu, Katonga and Kagera valleys cross axes of
M O R P H O T E C T O N I C S O F THE LAKE ALBERT RIFT VALLEY
345
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L, KE
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/ LAKEE Y A S / ~ /
• LAKE ~ TAA/GANY/KA
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/
f
The pattern of rift valleys around the Lake Victoria Basin.
uplift they are broad and swamp-filled, with imperceptible drainage. Much of the drainage is now reversed (as shown by arrows indicating the present direction of flow on Fig. 3) because of uplift along an axis parallel to the rift valley. The middle course of the Mara-Kagera river was drowned when downwarping created the Lake Victoria basin. The basin filled up, creating Lake Victoria which covers 70 000 km 2 but is less than 100 m deep. Eventually the lake overflowed at the lowest point on the divide separating the Mara-Kagera drainage from the Kyoga drainage, along the present course of the Nile. The stretch of the Nile between Victoria and Kyoga is very dif-
346
OLLIER
Axis of uplift
N
.
)
0 I
Fig. 3.
Vsctorta
200 km t
D r a i n a g e p a t t e r n in the L a k e Victoria - Lake Albert region.
ferent from any other tributary of Lake Kyoga, being incised and occupied by a major river, whereas most other tributaries are filled by papyrus swamp. Lake Kyoga was itself formed by back tilting of the Kafu River, which made a lake with the distinctive dendritic pattern of a drowned valley. The defeated drainage overflowed along a northern tributary and then into the Albert Rift Valley, where the line of uplift died out to the north.
M O R P H O T E C T O N I C S O F T H E LAKE ALBERT RIFT VALLEY
347
The valleys lie on a series of broad plains or planation surfaces. (For map see Ollier, 1981, Fig. 11.8), The Buganda surface is the highest and consists of plateau remnants along the divide between the Victoria and Kyoga drainage, and some broad plateaus in the south west. Much of the Kyoga basin lies on the African surface, which is cut across weathered rock. In the north the Acholi surface is present where all the earlier (Mesozoic?) regolith has been eroded away, and this surface is cut across fresh bedrock (Ollier, 1957: 1981, p. 159). The three surfaces together provide a large-scale example of etchplanation. Preservation of the Buganda surface on the major watershed suggests it is pre-tectonic. The African Surface is also pre-tectonic, and its deep weathered mantle has been stripped since the formation of the rift valley. The relationship of the Acholi surface to the Nile-Aswa drainage suggests it is post-rift tectonics. Thus the history of tectonics and the history of surfaces are intimately linked. The actual age of erosion surfaces, major drainage patterns, and warping are difficult to derive except by reference to the rift valley itself. Once formed, the rift valley started to accumulate sediment. The oldest rift sediment is Miocene, which is therefore the minimum age for the rift valley. The major drainage is older than this.
ASPECTS O F T H E RIFT VALLEY T E C T O N I C S A N D G E O M O R P H O L O G Y
For descriptive purposes, the Lake Albert Rift valley may be divided into three sections: a northern section, a central graben, and a southern section (Fig. 4).
The northern section
The northern section of the Lake Albert Rift has a series of faults and warps on the west and a single fault on the east; together these make up a complex half graben. The main fault on the western side of Lake Albert dies out to the north and is replaced by a series of en echelon faults with downthrow to the east, each dying out to the north. The main fault east of the Nile has downthrow to the west: this fault is continuous in direction with a fault to the south with downthrow in the reverse direction, the two faults making a scissors fault. The geomorphology of the northern section can be considered in terms of four units: the Plateau; the Escarpment Zone; the Lowland Plain; and the Rift Valley Sediments (Fig. 5a).
348
OLLIER
Aswa
R.
PLAIN
L.KYOGA
~.- PLAIN
PLATEAU
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Semlilk Flats
:.GEORGE
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Fig, 4.
4
M a i n features of the Lake Albert Rift Valley.
a
0
0
° o
o
o
MORPHOTECTONICS OF THE LAKE ALBER'r RIFT VALLEY
349
E
W Congo Nd( ¢~ters",ed SLt, TEAU Escarpment Zone ~-~E
Inselberg Line
J Monochne
~i.
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RED PLAIN
PLAIN PLAIN
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•
En Echelon Faults
a
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UGANDA
Lake Albert
b
RUWENZORI
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RIFT VALLEY
[
~ AFRICAN SURFACE
Lake George
*
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f
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Fig, 5. a) Diagrammatic cross-section of the Nile half graben, north of Lake Albert. b) Diagrammatic cross-section of Lake Albert. c) Diagrammatic cross-section of Ruwenzori and associated features, south of Lake Albert.
350
OLLIER
The Plateau This relatively flat area reaches 1600 m in the south, and 1200 m in the north. The Congo (Zaire)-Nile watershed runs north-south along the highest part of the plateau.
The Escarpment Zone The Plateau terminates to the east at a monocline, now dissected into an escarpment zone which becomes higher and narrower to the south, where it is 3 km wide with a relief of 600 m. To the north, the escarpment zone is replaced by a fault scarp. An "Inselberg Line" marks the eastern side of the Escarpment Zone (Hepworth, 1964).
The Lowland Plain The Lowland Plain, which is cut across bedrock, starts at the foot of the Escarpment Zone at about 1030 m and slopes down to about 720 m, where the Rift Valley sediments begin. The Lowland Plain often terminates at fault escarpments, and the Rift Valley sediments start at the foot of these escarpments. The western part of the Lowland Plain is the Red Plain. This refers to the colour of the soils, several metres thick, that cover the plain, indicating greater age here than on the eastern side. The Red Plain provides an important geomorphic datum. Its altitude ranges from 1030 m to 980 m, varying less than 50 m over a distance of 80 km. The flatness of the plain indicates very little tectonic movement since its formation. The eastern side of the Lowland Plain consists of a step-like sequence of minor erosion surfaces. The area east of the Nile, beyond the Rift Valley Sediments, is essentially a plain correlated with the Lowland Plain. This plain lies at an elevation only slightly above the Rift Valley Sediments, but the two units are separated by a major Rift Valley fault. This has little topographic expression but has a throw of over 1000 m.
The Rift Valley Sediments The Rift Valley 690 m and 660 m, present-day rivers. Red Plain (300 m
Sediments have three well-marked surfaces at 720 m, with a veneer of modern alluvium along the line of The sediments are faulted to some extent, but as the higher) appears unaffected by tectonic movements, the
M O R P H O T E C T O N I C S O F T H E L A K E ALBERT RIFT VALLEY
351
relatively recent faulting must be confined to the sedimentary area (Hepworth, 1964).
Summary of the morphotectonics of the northern section The Plateau and the Lowland Plain are parts of a single former surface that has been displaced by warping and faulting, and then modified by erosion. The broad structure is a half-graben, with a rise to the rift on the Congo-Nile watershed, downwarping at the Escarpment Zone monocline, downfaulting at a hinge-zone of en echelon faults, and a single, simple fault east of the Nile with no associated warping (Fig. 5a). The combination of scissors fault, en echelon faults, and warping suggests the action of a major tectonic axis that is oblique to the strike of rock structure. The tectonic movement is clearly reflected in the morphology.
The central section The central section is that part of the Rift where Lake Albert itself is situated (Fig. 4, 5b). It is a simple graben, rather asymmetrical, with higher
b
Fig. 6. a) Diagram of a rift valley with a "rise to the rift," b I Diagram of a rift valley with the rise some distance from the rift valley faults.
352
OLLIER
uplift and fault scarps on the western, Congo side, and lower uplift on the east. The faults are tectonically simple with only minor offsetting in plan. Sediments have accumulated in this section of the rift to a thickness of over a kilometre. Geomorphically, the fault scarps are very clear and rivers draining to the rift have not yet cut their valleys down to the level of the lake. The "rise to the rift" is a c o m m o n expression used to suggest uplift of the edges of the rift (Fig. 6a). In the central section of the Lake Albert Rift, however, the rise is some distance (about 30 km) from the actual fault line. The effect of this rise on the drainage pattern has already been described. On the rift side of the axis of uplift, the old regolith has been stripped, and a dense drainage pattern with much adjustment to structure replaces the broad, widely-spaced, swampy valleys preserved on the other side of the divide. The southern section
The southern section involves a change in direction of the rift. The western side of the rift consists of several fault scarps which swing around so that the Lake Edward Fault (downthrow to the east) is almost in line with the fault east of Lake Albert (downthrow to west). The situation on the eastern side is more complex, because of the Ruwenzori massif. Ruwenzori is a block of basement rocks that has been thrust to a height of three kilometres above the surrounding plateau. The highest mountain, after much erosion, still lies at over 6000 m. Ruwenzori is essentially a tilt block where the U g a n d a Plain (African Surface) had been upwarped (Fig. 5c). To the north, the Ruwenzori block is reduced to a narrow horst or "nose" that declines in height northwards. Further south, a number of en-echelon faults take up the displacement. These border the lowland that includes Lake George, which drains into Lake Edward, which in turn drains north across the Semliki Flats into Lake Albert.
TECTONIC SUMMARY
The Lake Albert Rift Valley is very much more complex than a simple graben. In plan (Fig. 4) it appears that the graben is developed along one general line, but perhaps because of bedrock structure it has a more complex form that splits it into three sub-parallel sections. The structure of the rift valley is not merely the result of subsidence of.a
M O R P H O T E C T O N I C S O F THE LAKE ALBERT RIFT VALLEY
353
graben, but also results from the uplift of marginal areas. Sometimes, this uplift is fairly symmetrical, as across Lake Albert itself, though even here there is a difference of 600 m in the amount of uplift on the opposite sides of the rift valley. In contrast, the northern section uplift is very asymmetrical with the formation of a half graben: the eastern erosion surface is essentially undeformed, while the western side has uplift along a major axis (CongoNile watershed), a monocline, and a fault zone. The southern section is even more complex, with the upwarp of Ruwenzori which consists of a horst in the north and a tilt-block to the south. West of Ruwenzori is a fairly normal rift valley, but to the east is the Lake George Fault, which marks the eastern side of a wider rift valley which continues to the south. The Ruwenzori block is uplifted to an extraordinary degree, three kilometres above the nearby plateau. Elsewhere, there is a simple rise towards the rift, with the axis of uplift about 30 km from the actual faults. The axes of uplift, like the faults, are arranged en echelon.
S I G N I F I C A N C E O F THE M O R P H O T E C T O N I C S O F THE LAKE ALBERT RIFT VALLEY
In modern theories of global tectonics, rift valleys are seen as precursors of seafloor spreading sites. It is thought that after the formation of a Lake Albert type rift valley, oceanic-type crust may be intruded to form a Red Sea type of rift, and with further seafloor spreading this may turn into a typical ocean such as the symmetrical Altantic. Numerous studies have been made on the morphotectonics of continental passive margins to see how they have evolved (e.g. Jessen, 1943; Godard, 1982; Ollier, 1985a, b). It has often been found that the land surface rises towards the continental margin, and this rise has been explained in various ways, including isostatic response to erosion of the continental margin (King, 1955), passage of the continent over heated areas (Smith, 1982: Karner and Weissel, 1984), and underplating (Wellman, 1987). Ollier (1985a, b) has suggested that in part the rise at the continental margin may be inherited from uplift at the rift valley stage, before the formation of a true continental margin (Fig. 6b). In Africa, the basin and swell structure included a swell that rifted to become the Atlantic seafloor-spreading site. The ancient rift valley fault scarps have eroded into Great Escarpments. The Great Escarpment of Namibia and Angola, for instance, is thought to have resulted from backwearing from a rift (now the South Atlantic) that separated Africa from South America (Ollier and Marker, 1985). The argument can be extended to other continents. In Brazil, the Serro do Mar is a Great Escarpment that also results from erosion after the formation of the Atlantic (Maack, 1969). J ( ) ( i I I 4~6
354
OLLIER
Swells and escarpments have also been described from North Atlantic continental margins (Godard, 1982; Brooks, 1985). The Great Escarpment of New South Wales (Australia) is thought to result from erosion from the rift that separated Australia from a landmass now dispersed in the Pacific (Ollier, 1982; Pain, 1985). It is further possible that more complex continental margins, such as that of Queensland, Australia, may be the result of Lake Albert style uplift. Both the Great Divide and the Great Escarpment in Queensland possess a series of offset, en-echelon sections, juts like the axis of uplift and the fault scarps in Uganda (Ollier and Stevens, 1988). Numerous models have been proposed for the tectonics of passive continental margins (e.g. Smith, 1982; Karner and Weissel, 1984; Meissner and Kopnick, 1988), invoking thermal rises, isostasy and other factors, but commonly assuming that the rises found at continental margins developed after the margin had been created. This discussion of the Albert Rift Valley suggests that at least some of the major geomorphic features of continental margins may be created during the rift valley phase, before the continental margins are formed. Several "big bang" theories for the development of continental margins have a sudden start that virtually ignores the rift phase, such as "sudden stretching" (McKenzie, 1978); "sudden intrusion" (Royden etal., 1980); or "sudden heating" (Bott, 1980). None of these models investigate the rift precursors of continental margins. They are generally strong on fairly remote geophysical data, but ignore the obvious and accessible morphological data which provides direct evidence of the geometry of rifting, which should surely be a precursor to speculations about the causal mechanism. In understanding the surface of the Earth, it is important to realize that many of our landforms are very old. The major valleys of Central Africa pre-date the tectonics of the rift valleys. In eastern Australia, there are remnants of major drainage lines that pre-date the opening of the seas to the east (Ollier, 1981). Some of the uplift of eastern Australia may date back to the time of continental breakup. At present, few rift valleys have been studied from the morphotectonic viewpoint as much as the Lake Albert rift, and few continental margins have been examined as thoroughly as that of eastern Australia. There is a need for many more such studies to relate the tectonics and geomorphology rift valleys and continental margins.
REFERENCES Bott, M. P. H., 1980. In." Dynamics of Plate Interiors (Bally, A., Bender, P., McCretchin, T. and Walcott, R., eds.). Geodynamics Series 1, 27 32.
MORPHOTECTONICS OF THE LAKE ALBERT RIFT VALLEY
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Brooks, C. K., 1985. Vertical crust movements in the Tertiary of central East Greenland: a continental margin at a hot spot. In: Oilier, C.D. (ed.), Morphotectonics of Passive Continental Margins. Z. Geomorph. Supp. Bd. 54, 101 117. Godard, A. (ed)., 1982. Les bourrelets marginaux des hautes latitudes. Bull. Assoc. Geog. Franc., 489, 239-259. Hepworth, J. V., 1964. Explanation of the Geology of Sheets 19, 20, 28 and 29 (Southern West Nilet. Geological Survey of Uganda, Report No. 10, pp, 127. Jessen, O., 1943. Die Randschsellen der Kontinente. Pet. Geogr. Mitt. Erganzugsh, 241. Karner, G. D. and Weissel, J. K., 1984. Thermally induced uplift and lithospheric flexural readjustment of the eastern Australian highland. Geol. Soc. Austr. Abstr. 12, 293 294. King, L. C., 1955. Pediplanation and isostasy: an example from Southern Africa. Quart. J. Geol. Soc. Lond. 111, 353 359. Maack, R., 1969. Die Sierra do Mar im Staate Parana. Die Erde, 100~ 327-347. McKenzie, F., 1978. Some remarks on the development of sedimentary basins. Earth and Planet. Sci, Lett., 40, 25 32. Meissner, R. and Kopnick, M., 1988. Structure and evolution of passive margins: the plume model again~ J. Geodynamics, 9, 1-13. Ollier, C. D., 1959~ The Soils of Northern Province, Uganda. Memoir of the Soil Survey of Uganda, No. 2, pp. 47. Ollier, C. D., 1981. Tectonics and Landforms. Longman, London. Oilier, C. D. (ed.), 1985a. Morphotectonics of continents with Passive Margins. Zeitschrift fur Geomorphologie, Supplementband 54, pp. 117. Ollier, C. D., 1985b. Morphotectonics of continental margins with Great Escarpments. Ch. 1, pp. 3 25 in "Tectonic Geomorphology," eds. M. Morisawa and J. T. Hack~ George Allen & Unwin, London. Oilier, C. D. and Marker, M. A., 1985. The Great Escarpment of Southern Africa. In: Ollier, C. D. (ed.), Morphotectonics of Passive Continental Margins. Z. Geomorph. Supp. Bd. 54, 37 56. Ollier, C. D. and Stevens, N. C., 1990. The Great Escarpment in Queensland. E.S. Hills Memorial Volume "Pathways in Geology," ed. R. Le Maitre. Blackwell, Oxford. Pain, C. F., 1985. Morphotectonics of the continental margins of Australia. In: Oilier, C.D. led), Morphotectonics of Passive Continental Margins. Z. Geomorph. Supp. Bd. 54, 23-36. Royden, L., Sclater, J. G. and yon Herzen, R. P., 1980. Continental margin subsidence and heat flow: important parameters in formation of petroleum hydrocarbons. Am. Assoc. Petr. Geol. Bull., 64, 173 187. Smith, A. G., 1982. Late Cenozoic uplift of stable continents in a reference frame fixed to South America. Nature, 296, 40(Y404. Wellman, P., 1987. Eastern Highlands of Australia: their erosion and uplift. BMR J. Aust. Geol. Geophys., 10, 277-286.
343
M O R P H O T E C T O N I C S OF THE LAKE ALBERT RIFT VALLEY A N D ITS SIGNIFICANCE FOR C O N T I N E N T A L MARGINS
C. D. OLLIER
Department ~[" Geography and Planning, University ~f New England, Armidale 2351, Australia (Received July 4, 1989; accepted November 9, 1989)
ABSTRACT Ollier, C. D., 1990. Morphotectonics of the Lake Albert Rift Valley and its significance for continental margins. In: N. A. Logatchev and H. J. Zwart (Editors), Intracontinental Mountainous Terranes. Journal ~[' Geodynamics, 11:343 355. The Lake Albert region is described as an example of a rift valley. It is far more complex than the typical graben used by rift valley modellers. The northern section is a complex half graben with en echelon faults and warps on the west. The central section is a graben, but with uplift axes remote from the faults. The southern section is complicated by the partly faulted but mainly upwarped tilt block of the Ruwenzori massif. Many features of the landscape~drainage patterns, erosion surfaces, regolith-pre-date the rift valley tectonics, and provide insight into the dating and evolution of tectonic features. As rift valleys are precursors of seafloor spreading, the rift geomorphology shows which features may pre-date the formation of new continental edges. Many modern passive margins have morphotectonic features that date back to the rift valley stage of crustal evolution.
INTRODUCTION
The continent of Africa is divided into a number of huge basins by swells (Fig. 1). Some of the swells are the site of rift valleys, which roughly follow the line of the swell. The most instructive basin is that of Lake Victoria (Fig. 2) which is bounded almost completely by rifts. Two important aspects of rift morphotectonics will be examined in this paper: 1. The swells and rifts post-date the major river systems and erosion surfaces. 2. The rifts are not simple grabens, but exhibit complex tectonic and geomorphic relationships. As an example, the Lake Albert Rift will be discussed in more detail.
0264-3707/90/$3.00
(¢, 1990 Pergamon Press plc.
344
OLLIER
ioj ,
fl
EL-JUF
or CHAD
t>
N CUBANGO
o I
Fig. 1.
looo I
',~ : .rKAR , , O ,-,
Basins and swells of Africa. L.V. = Lake Victoria.
RELATIONSHIP
BETWEEN RIFTING AND DRAINAGE
PATTERNS
The drainage pattern associated with Lake Victoria is shown in Fig. 3. In the southern half of the area, the original drainage was from east to west, right across what is now Lake Victoria. The Mara was continuous with the Kagera, and the rivers that now enter the northeast corner of Lake Victoria were continuous with the Katonga. This dendritic pattern is repeated further north by the Kyoga-Kafu River, also flowing west. The drainage lines of the Katonga and Kagera are continuous between Lake Victoria and Lake Edward, as is that of the Kafu from Lake Kyoga to Lake Albert. Where the Kafu, Katonga and Kagera valleys cross axes of
M O R P H O T E C T O N I C S O F THE LAKE ALBERT RIFT VALLEY
345
/
Y
•
%
L, KE
II
/ LAKEE Y A S / ~ /
• LAKE ~ TAA/GANY/KA
LAKEMWERU Fig. 2.
/
f
The pattern of rift valleys around the Lake Victoria Basin.
uplift they are broad and swamp-filled, with imperceptible drainage. Much of the drainage is now reversed (as shown by arrows indicating the present direction of flow on Fig. 3) because of uplift along an axis parallel to the rift valley. The middle course of the Mara-Kagera river was drowned when downwarping created the Lake Victoria basin. The basin filled up, creating Lake Victoria which covers 70 000 km 2 but is less than 100 m deep. Eventually the lake overflowed at the lowest point on the divide separating the Mara-Kagera drainage from the Kyoga drainage, along the present course of the Nile. The stretch of the Nile between Victoria and Kyoga is very dif-
346
OLLIER
Axis of uplift
N
.
)
0 I
Fig. 3.
Vsctorta
200 km t
D r a i n a g e p a t t e r n in the L a k e Victoria - Lake Albert region.
ferent from any other tributary of Lake Kyoga, being incised and occupied by a major river, whereas most other tributaries are filled by papyrus swamp. Lake Kyoga was itself formed by back tilting of the Kafu River, which made a lake with the distinctive dendritic pattern of a drowned valley. The defeated drainage overflowed along a northern tributary and then into the Albert Rift Valley, where the line of uplift died out to the north.
M O R P H O T E C T O N I C S O F T H E LAKE ALBERT RIFT VALLEY
347
The valleys lie on a series of broad plains or planation surfaces. (For map see Ollier, 1981, Fig. 11.8), The Buganda surface is the highest and consists of plateau remnants along the divide between the Victoria and Kyoga drainage, and some broad plateaus in the south west. Much of the Kyoga basin lies on the African surface, which is cut across weathered rock. In the north the Acholi surface is present where all the earlier (Mesozoic?) regolith has been eroded away, and this surface is cut across fresh bedrock (Ollier, 1957: 1981, p. 159). The three surfaces together provide a large-scale example of etchplanation. Preservation of the Buganda surface on the major watershed suggests it is pre-tectonic. The African Surface is also pre-tectonic, and its deep weathered mantle has been stripped since the formation of the rift valley. The relationship of the Acholi surface to the Nile-Aswa drainage suggests it is post-rift tectonics. Thus the history of tectonics and the history of surfaces are intimately linked. The actual age of erosion surfaces, major drainage patterns, and warping are difficult to derive except by reference to the rift valley itself. Once formed, the rift valley started to accumulate sediment. The oldest rift sediment is Miocene, which is therefore the minimum age for the rift valley. The major drainage is older than this.
ASPECTS O F T H E RIFT VALLEY T E C T O N I C S A N D G E O M O R P H O L O G Y
For descriptive purposes, the Lake Albert Rift valley may be divided into three sections: a northern section, a central graben, and a southern section (Fig. 4).
The northern section
The northern section of the Lake Albert Rift has a series of faults and warps on the west and a single fault on the east; together these make up a complex half graben. The main fault on the western side of Lake Albert dies out to the north and is replaced by a series of en echelon faults with downthrow to the east, each dying out to the north. The main fault east of the Nile has downthrow to the west: this fault is continuous in direction with a fault to the south with downthrow in the reverse direction, the two faults making a scissors fault. The geomorphology of the northern section can be considered in terms of four units: the Plateau; the Escarpment Zone; the Lowland Plain; and the Rift Valley Sediments (Fig. 5a).
348
OLLIER
Aswa
R.
PLAIN
L.KYOGA
~.- PLAIN
PLATEAU
~O44~Z ~
%•,- ,,L.ALBERT
Semlilk Flats
:.GEORGE
L.EDWARD O
100 km
Fig, 4.
4
M a i n features of the Lake Albert Rift Valley.
a
0
0
° o
o
o
MORPHOTECTONICS OF THE LAKE ALBER'r RIFT VALLEY
349
E
W Congo Nd( ¢~ters",ed SLt, TEAU Escarpment Zone ~-~E
Inselberg Line
J Monochne
~i.
\
RED PLAIN
PLAIN PLAIN
\~~.!-.~.....~
•
En Echelon Faults
a
SONSO
UGANDA
Lake Albert
b
RUWENZORI
iI
~ ?
RIFT VALLEY
[
~ AFRICAN SURFACE
Lake George
*
/
f
C
Fig, 5. a) Diagrammatic cross-section of the Nile half graben, north of Lake Albert. b) Diagrammatic cross-section of Lake Albert. c) Diagrammatic cross-section of Ruwenzori and associated features, south of Lake Albert.
350
OLLIER
The Plateau This relatively flat area reaches 1600 m in the south, and 1200 m in the north. The Congo (Zaire)-Nile watershed runs north-south along the highest part of the plateau.
The Escarpment Zone The Plateau terminates to the east at a monocline, now dissected into an escarpment zone which becomes higher and narrower to the south, where it is 3 km wide with a relief of 600 m. To the north, the escarpment zone is replaced by a fault scarp. An "Inselberg Line" marks the eastern side of the Escarpment Zone (Hepworth, 1964).
The Lowland Plain The Lowland Plain, which is cut across bedrock, starts at the foot of the Escarpment Zone at about 1030 m and slopes down to about 720 m, where the Rift Valley sediments begin. The Lowland Plain often terminates at fault escarpments, and the Rift Valley sediments start at the foot of these escarpments. The western part of the Lowland Plain is the Red Plain. This refers to the colour of the soils, several metres thick, that cover the plain, indicating greater age here than on the eastern side. The Red Plain provides an important geomorphic datum. Its altitude ranges from 1030 m to 980 m, varying less than 50 m over a distance of 80 km. The flatness of the plain indicates very little tectonic movement since its formation. The eastern side of the Lowland Plain consists of a step-like sequence of minor erosion surfaces. The area east of the Nile, beyond the Rift Valley Sediments, is essentially a plain correlated with the Lowland Plain. This plain lies at an elevation only slightly above the Rift Valley Sediments, but the two units are separated by a major Rift Valley fault. This has little topographic expression but has a throw of over 1000 m.
The Rift Valley Sediments The Rift Valley 690 m and 660 m, present-day rivers. Red Plain (300 m
Sediments have three well-marked surfaces at 720 m, with a veneer of modern alluvium along the line of The sediments are faulted to some extent, but as the higher) appears unaffected by tectonic movements, the
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relatively recent faulting must be confined to the sedimentary area (Hepworth, 1964).
Summary of the morphotectonics of the northern section The Plateau and the Lowland Plain are parts of a single former surface that has been displaced by warping and faulting, and then modified by erosion. The broad structure is a half-graben, with a rise to the rift on the Congo-Nile watershed, downwarping at the Escarpment Zone monocline, downfaulting at a hinge-zone of en echelon faults, and a single, simple fault east of the Nile with no associated warping (Fig. 5a). The combination of scissors fault, en echelon faults, and warping suggests the action of a major tectonic axis that is oblique to the strike of rock structure. The tectonic movement is clearly reflected in the morphology.
The central section The central section is that part of the Rift where Lake Albert itself is situated (Fig. 4, 5b). It is a simple graben, rather asymmetrical, with higher
b
Fig. 6. a) Diagram of a rift valley with a "rise to the rift," b I Diagram of a rift valley with the rise some distance from the rift valley faults.
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uplift and fault scarps on the western, Congo side, and lower uplift on the east. The faults are tectonically simple with only minor offsetting in plan. Sediments have accumulated in this section of the rift to a thickness of over a kilometre. Geomorphically, the fault scarps are very clear and rivers draining to the rift have not yet cut their valleys down to the level of the lake. The "rise to the rift" is a c o m m o n expression used to suggest uplift of the edges of the rift (Fig. 6a). In the central section of the Lake Albert Rift, however, the rise is some distance (about 30 km) from the actual fault line. The effect of this rise on the drainage pattern has already been described. On the rift side of the axis of uplift, the old regolith has been stripped, and a dense drainage pattern with much adjustment to structure replaces the broad, widely-spaced, swampy valleys preserved on the other side of the divide. The southern section
The southern section involves a change in direction of the rift. The western side of the rift consists of several fault scarps which swing around so that the Lake Edward Fault (downthrow to the east) is almost in line with the fault east of Lake Albert (downthrow to west). The situation on the eastern side is more complex, because of the Ruwenzori massif. Ruwenzori is a block of basement rocks that has been thrust to a height of three kilometres above the surrounding plateau. The highest mountain, after much erosion, still lies at over 6000 m. Ruwenzori is essentially a tilt block where the U g a n d a Plain (African Surface) had been upwarped (Fig. 5c). To the north, the Ruwenzori block is reduced to a narrow horst or "nose" that declines in height northwards. Further south, a number of en-echelon faults take up the displacement. These border the lowland that includes Lake George, which drains into Lake Edward, which in turn drains north across the Semliki Flats into Lake Albert.
TECTONIC SUMMARY
The Lake Albert Rift Valley is very much more complex than a simple graben. In plan (Fig. 4) it appears that the graben is developed along one general line, but perhaps because of bedrock structure it has a more complex form that splits it into three sub-parallel sections. The structure of the rift valley is not merely the result of subsidence of.a
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graben, but also results from the uplift of marginal areas. Sometimes, this uplift is fairly symmetrical, as across Lake Albert itself, though even here there is a difference of 600 m in the amount of uplift on the opposite sides of the rift valley. In contrast, the northern section uplift is very asymmetrical with the formation of a half graben: the eastern erosion surface is essentially undeformed, while the western side has uplift along a major axis (CongoNile watershed), a monocline, and a fault zone. The southern section is even more complex, with the upwarp of Ruwenzori which consists of a horst in the north and a tilt-block to the south. West of Ruwenzori is a fairly normal rift valley, but to the east is the Lake George Fault, which marks the eastern side of a wider rift valley which continues to the south. The Ruwenzori block is uplifted to an extraordinary degree, three kilometres above the nearby plateau. Elsewhere, there is a simple rise towards the rift, with the axis of uplift about 30 km from the actual faults. The axes of uplift, like the faults, are arranged en echelon.
S I G N I F I C A N C E O F THE M O R P H O T E C T O N I C S O F THE LAKE ALBERT RIFT VALLEY
In modern theories of global tectonics, rift valleys are seen as precursors of seafloor spreading sites. It is thought that after the formation of a Lake Albert type rift valley, oceanic-type crust may be intruded to form a Red Sea type of rift, and with further seafloor spreading this may turn into a typical ocean such as the symmetrical Altantic. Numerous studies have been made on the morphotectonics of continental passive margins to see how they have evolved (e.g. Jessen, 1943; Godard, 1982; Ollier, 1985a, b). It has often been found that the land surface rises towards the continental margin, and this rise has been explained in various ways, including isostatic response to erosion of the continental margin (King, 1955), passage of the continent over heated areas (Smith, 1982: Karner and Weissel, 1984), and underplating (Wellman, 1987). Ollier (1985a, b) has suggested that in part the rise at the continental margin may be inherited from uplift at the rift valley stage, before the formation of a true continental margin (Fig. 6b). In Africa, the basin and swell structure included a swell that rifted to become the Atlantic seafloor-spreading site. The ancient rift valley fault scarps have eroded into Great Escarpments. The Great Escarpment of Namibia and Angola, for instance, is thought to have resulted from backwearing from a rift (now the South Atlantic) that separated Africa from South America (Ollier and Marker, 1985). The argument can be extended to other continents. In Brazil, the Serro do Mar is a Great Escarpment that also results from erosion after the formation of the Atlantic (Maack, 1969). J ( ) ( i I I 4~6
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Swells and escarpments have also been described from North Atlantic continental margins (Godard, 1982; Brooks, 1985). The Great Escarpment of New South Wales (Australia) is thought to result from erosion from the rift that separated Australia from a landmass now dispersed in the Pacific (Ollier, 1982; Pain, 1985). It is further possible that more complex continental margins, such as that of Queensland, Australia, may be the result of Lake Albert style uplift. Both the Great Divide and the Great Escarpment in Queensland possess a series of offset, en-echelon sections, juts like the axis of uplift and the fault scarps in Uganda (Ollier and Stevens, 1988). Numerous models have been proposed for the tectonics of passive continental margins (e.g. Smith, 1982; Karner and Weissel, 1984; Meissner and Kopnick, 1988), invoking thermal rises, isostasy and other factors, but commonly assuming that the rises found at continental margins developed after the margin had been created. This discussion of the Albert Rift Valley suggests that at least some of the major geomorphic features of continental margins may be created during the rift valley phase, before the continental margins are formed. Several "big bang" theories for the development of continental margins have a sudden start that virtually ignores the rift phase, such as "sudden stretching" (McKenzie, 1978); "sudden intrusion" (Royden etal., 1980); or "sudden heating" (Bott, 1980). None of these models investigate the rift precursors of continental margins. They are generally strong on fairly remote geophysical data, but ignore the obvious and accessible morphological data which provides direct evidence of the geometry of rifting, which should surely be a precursor to speculations about the causal mechanism. In understanding the surface of the Earth, it is important to realize that many of our landforms are very old. The major valleys of Central Africa pre-date the tectonics of the rift valleys. In eastern Australia, there are remnants of major drainage lines that pre-date the opening of the seas to the east (Ollier, 1981). Some of the uplift of eastern Australia may date back to the time of continental breakup. At present, few rift valleys have been studied from the morphotectonic viewpoint as much as the Lake Albert rift, and few continental margins have been examined as thoroughly as that of eastern Australia. There is a need for many more such studies to relate the tectonics and geomorphology rift valleys and continental margins.
REFERENCES Bott, M. P. H., 1980. In." Dynamics of Plate Interiors (Bally, A., Bender, P., McCretchin, T. and Walcott, R., eds.). Geodynamics Series 1, 27 32.
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Brooks, C. K., 1985. Vertical crust movements in the Tertiary of central East Greenland: a continental margin at a hot spot. In: Oilier, C.D. (ed.), Morphotectonics of Passive Continental Margins. Z. Geomorph. Supp. Bd. 54, 101 117. Godard, A. (ed)., 1982. Les bourrelets marginaux des hautes latitudes. Bull. Assoc. Geog. Franc., 489, 239-259. Hepworth, J. V., 1964. Explanation of the Geology of Sheets 19, 20, 28 and 29 (Southern West Nilet. Geological Survey of Uganda, Report No. 10, pp, 127. Jessen, O., 1943. Die Randschsellen der Kontinente. Pet. Geogr. Mitt. Erganzugsh, 241. Karner, G. D. and Weissel, J. K., 1984. Thermally induced uplift and lithospheric flexural readjustment of the eastern Australian highland. Geol. Soc. Austr. Abstr. 12, 293 294. King, L. C., 1955. Pediplanation and isostasy: an example from Southern Africa. Quart. J. Geol. Soc. Lond. 111, 353 359. Maack, R., 1969. Die Sierra do Mar im Staate Parana. Die Erde, 100~ 327-347. McKenzie, F., 1978. Some remarks on the development of sedimentary basins. Earth and Planet. Sci, Lett., 40, 25 32. Meissner, R. and Kopnick, M., 1988. Structure and evolution of passive margins: the plume model again~ J. Geodynamics, 9, 1-13. Ollier, C. D., 1959~ The Soils of Northern Province, Uganda. Memoir of the Soil Survey of Uganda, No. 2, pp. 47. Ollier, C. D., 1981. Tectonics and Landforms. Longman, London. Oilier, C. D. (ed.), 1985a. Morphotectonics of continents with Passive Margins. Zeitschrift fur Geomorphologie, Supplementband 54, pp. 117. Ollier, C. D., 1985b. Morphotectonics of continental margins with Great Escarpments. Ch. 1, pp. 3 25 in "Tectonic Geomorphology," eds. M. Morisawa and J. T. Hack~ George Allen & Unwin, London. Oilier, C. D. and Marker, M. A., 1985. The Great Escarpment of Southern Africa. In: Ollier, C. D. (ed.), Morphotectonics of Passive Continental Margins. Z. Geomorph. Supp. Bd. 54, 37 56. Ollier, C. D. and Stevens, N. C., 1990. The Great Escarpment in Queensland. E.S. Hills Memorial Volume "Pathways in Geology," ed. R. Le Maitre. Blackwell, Oxford. Pain, C. F., 1985. Morphotectonics of the continental margins of Australia. In: Oilier, C.D. led), Morphotectonics of Passive Continental Margins. Z. Geomorph. Supp. Bd. 54, 23-36. Royden, L., Sclater, J. G. and yon Herzen, R. P., 1980. Continental margin subsidence and heat flow: important parameters in formation of petroleum hydrocarbons. Am. Assoc. Petr. Geol. Bull., 64, 173 187. Smith, A. G., 1982. Late Cenozoic uplift of stable continents in a reference frame fixed to South America. Nature, 296, 40(Y404. Wellman, P., 1987. Eastern Highlands of Australia: their erosion and uplift. BMR J. Aust. Geol. Geophys., 10, 277-286.