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The coastline fit of Africa and South America
Palaeogeography, Palaeoclimatology, Palaeogeography Elsevier Publishing Company, Amsterdam
Printed in The Netherlands
T H E C O A S T L I N E FIT OF A F R I C A A N D S O U T H A M E R I C A R. MESERVEY
Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.) (Received June 12, 1970)
ABSTRACT MESERVEY, R., 1971. The coastline fit of Africa and South America. Palaeogeography, Palaeoclimatol., Palaeoecol., 9: 233-243. It has become axiomatic that the pre-drift reconstruction of Africa and South America is to be made by fitting together the continental shelves. Although this procedure is very plausible, actually the detailed fit of the coastlines is better than that of the shelves and the overall fit is remarkable, provided the angle of the Gulf of Guinea is decreased by 16 °. A past increase in this angle is implied by the rift valleys in the vicinity of the Niger and Benue rivers. A comparison with the Red Sea shows a striking similarity in the nature of the fracture and gives some clues as to the significance of the coastline fit in both of these cases. This method of fitting (which may correspond to the early stages of the pre-separation rifting) is consistent with, but does not prove, the hypothesis of earth expansion. INTRODUCTION
WEGENER (1929) and other early advocates of continental drift were greatly impressed by the geometrical fit of South America and Africa. This fit has remained the classic example and thus at the center of the long argument over the reality of drift. JEFFRIES (1959, p.370), who at the time was convinced that the hypothesis of drift was untenable on other grounds, pointed out quite correctly that the near right angle of the coast of Brazil was about 15 ° less than the obtuse angle of the African coast of the Gulf of Guinea into which it was supposed to fit. This argument was answered by CAREY (1955), who showed by carefully matching transparent spherical overlays on a globe that the continental shelves of Africa and South America fit very well. BULLARD et al. (1965) have used a computer-aided technique to show that the overlap and void areas in this fit are indeed small. Carey, after trying to fit the rest of the continents together on a globe, finally came to the conclusion that they would only fit on a smaller-sized earth. For this and other reasons CAREY (1958) adopted the hypothesis of an expanding earth. However, it has since been pointed out by WESTOLL (1965) and others that the exactness of the fit of the continental shelves of South America and Africa apparently limited the amount of expansion to much less than that assumed by Carey. The original purpose of the present work was to place quantitative limits on the possible rate of earth expansion by examining this fit. Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233-243
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R. MESERVEY
FIT OF COASTLINESAND CONTINENTALSHELVES The suggestion that the geometrical fit be made on the continental shelves was so plausible that, since it was made, it does not seem to have been questioned. In spite o f this great plausibility, careful comparison of the coastlines show that over large sections they fit better than the continental shelves. The present contribution is to demonstrate this fact and to try to understand its significance. The amazingly detailed fit o f the coastlines can be seen in Fig.l. Here are shown the Atlantic coastal outlines o f South America (solid line) and Africa (dotted line)
i
..... A F R I C A
I~ \ " 16 °
i ....
.....
® P
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Fig.1. Coastline of South America (solid line) compared with the coastline of Africa (dotted line) and the coastline of Africa west of the Niger river rotated 16 ° (dashed line). The Niger delta (A), the Ogoou6 delta (B), and the promontory near the mouth of the Cuanza river (C) are marked as areas of recent origin. with the west coast o f Africa positioned on the east coast of South America to give very nearly their o p t i m u m fit. The coastal outlines were made by tracing on a transparent spherical plastic shell f r o m a 40 cm globe. Fig.1 is a cylindrical projection from the spherical shell o f each segment of the coast and because o f the near coincidence o f the outlines no significant relative distortion is introduced. It is to be noted that with the southern segments o f coast in coincidence, the African coastline west o f the Niger river does not at all fit northern South America, just as JEFFRIES (1959) pointed out. However, the remarkable fact is that if we rotate this section o f the African coastline as a rigid b o d y 16 ° about point P, we obtain the detailed fit shown by the dashed line. The fit o f the two segments is even more impressive when we eliminate the deltas o f the Niger (A) and the Ogoou6 (B) rivers,
Palaeogeography, Palaeoclirnatol., Palaeoecol., 9(1971) 233-243
COASTLINE FIT OF AFRICA AND SOUTH AMERICA
235
which are of comparatively recent origin and should be neglected in making the fit. The promontory near the mouth of the Cuanza River (C) should perhaps also be eliminated. The probability that such a detailed fit should have occurred randomly seems very small. One might object that although the coastal fit is very good, so is the fit on the continental shelves. Certainly the gross fit (without a conjectured rotation) is much better. However, the detailed tit in the two cases is a different story. Fig.2 shows the details of the coastlines, the 1,000 fathom level, and the 2,000 fathom level. In the lower part of Fig.2 the contours at the three different levels are for the
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Fig.2. Separate comparison of the detailed fit of the coastline and the 1,000 and 2,000 fathom levels of each segment of coast of South America and Africa.
African coast south of the Niger delta (dotted) and the South American coast south of Natal, Brazil (solid). In each case the lines are slightly offset to make visual comparison easier. In the upper part of the figure are the equivalent contours for the coast of Africa west of the Niger and the coast of South America west of Natal. in each case the very striking congruence of the coasts becomes increasingly blurred at 1,000 fathoms and 2,000 fathoms. It is hard to attribute this simply to chance and Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233-243
236
R. MESERVEV
there are two lines of evidence which indicate strongly that the detailed fit of the coastlines is not by chance nor is it secondary to the fit of the continental shelves. W E S T A F R I C A N RIFTS
First, there are known to have been active rift valleys in the region of Africa near the mouth of the Niger. Such rifts would tend to give the required angular displacement and they were active during the period of continental separation. Fig.3, which is adapted from WRIGHT (1968), shows some of the rift structure in the region. Wright suggests that the Benue trough (1 in Fig.3) is the principal rift
Fig.3. Rift system in the vicinity of the Niger river as adapted from WRIGHT(1968), Benue trough (1), Cameroun rift valley (2), Niger river valley fracture (3). and developed in Early Cretaceous or Late Jurassic as Africa and South America were progressively being wedged apart by a spreading axis moving from the south. Subsidence was most rapid during the Albian to Late Cenomanian when it slowed up and the sea advanced westward along the coast west of the Niger. The Cameroun rift valley (2) is approximately parallel and appears to be prolonged seaward in the volcanic chain of islands in the Gulf of Guinea. FURON (1950) has suggested a fracture formed in the Cretaceous along the Niger valley (3) almost perpendicular to the other faults. This summary of the major faults in this part of Africa and their time sequence is in essential agreement with KING (1950), CRATCHLEVand JONES (1965),
Palaeogeography, PalaeoclimatoL,Palaeoecol., 9(1971) 233-243
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237
SUTTON (1968), and McCoNNELL (1969). Such a rift structure apparently increased the angle between the two segments of the African coasts at about the time when separation was taking place. How great an increase in angle took place is yet to be determined, but here, at least in a qualitative way, is an explanation of the relative angular displacement of the coastlines. Although the major rifting of these faults took place in the Cretaceous, SUTTON (1968) has suggested that they existed in kate Precambrian and Early Cambrian 900-500 million years ago and have been intermittently active from then until their period of greatest activity in the Cretaceous. Thus it appears that separaiion took place along very old established faults. A N A L O G Y W I T H THE RED SEA
Additional evidence concerning the validity of coastline reconstructions comes from considering the Red Sea. Fig.4 shows the well-known fact that in the northern part of the Red Sea the African and Arabian coasts can be made to coincide with great accuracy. The dotted line from A' to B' is the outline of the African coast A to B rotated (approximately 6 ° about a center O, whose position is shown on the inset) to coincide with the coast of Arabia (solid line). The fit is evidently very good. South of B and B' the coastlines do not fit, but the 1,000 m contour CD rotated by the same amount becomes the dash-dot line C'D' which again fits the Arabian coast very well. The coasts of the Gulf of Aden also show a remarkable fit, G H being displaced to the dotted outline G ' H ' by a 6 ' rotation about O plus 7 c' rotation from Q (or the equivalent single rotation of 1 2 about P). Again the 1,000 m contour E F is displaced to the dot-dot-dash outline E'F'. The geological rational for using the 1,000 m contour of the edges of the Afar triangle is that this land area is basaltic and of recent origin and is thought to have been recently elevated. This interpretation and the resulting geometrical fit were suggested by GIRDLER (1958). If we accept this reconstruction, the 7 ° rotation of the Gulf of Aden in addition to that associated with the Red Sea is easily interpreted as connected with the opening of the East African system. This rift system, which is continuous with the rift in the Gulf of Aden, provides the sort of differential rotation implied by the reconstructed coastlines. These observations and interpretations are all well known, but the detailed fit of the coastline or some higher elevation is impressive. The Red Sea was considered by WEGENER(1929) to be an incipient ocean and recent studies support this conclusion. The actual connection of the Red Sea with the Gulf of Aden and the Indian Ocean took place about 8 million years ago but there was previously a long history of rifting in this region as recently reviewed by GIRDLER (1969). Actual rifting began as early as 50 million years ago and the matin Red Sea graben was formed by 26 million years ago. There is even evidence of a depression in this region in the Lower Carboniferous 335 million years ago. In Palaeogeo
238
R. MESERVEY
)
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Fig.4. Map showing the fit of the Arabian coastline and the coastlines of the Red Sea, the Gulf of Aden, and the 1,000 m contour defining the Afar triangle. The fit on the west coast of Arabia is made with a rotation of 6 ° about point O (see inset); the fit on the south coast of Arabia by an additional rotation of 7 ° about point Q. the recent p a s t a n d p r e s u m a b l y at present, the rate o f sea-floor s p r e a d i n g in the R e d Sea is given b y PHILLIPS et al. (1969) to be a b o u t 1.6 c m / y e a r a n d if this is sustained over a geologically significant p e r i o d p r e s u m a b l y A f r i c a a n d A s i a will be c o m p l e t e l y separated. Since j u s t such a s e p a r a t i o n t o o k place with S o u t h A m e r i c a a n d A f r i c a some 150 million years ago it seems r e a s o n a b l e to see if we can u n d e r s t a n d the details o f the f o r m e r s e p a r a t i o n f r o m the one which is t a k i n g place at present a l o n g the R e d Sea rift. In Fig.5 the a p p a r e n t average p a t h o f s e p a r a t i o n o f the A r a b i a n coastline relative to the A f r i c a n coastline, as i n d i c a t e d by their geometrical fit, is
Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233-243
COASTLINE FIT OF AFRICA AND SOUTH AMERICA
239
500f o
Fig.5. Map showing section of the Red Sea and the directions of displacement as indicated by the coastline fit (long solid arrows), by the 500 fathom level (short solid arrows), and the sl~reading direction indicated by the magnetic anomalies (dashed arrows).
shown by the large solid single-headed arrows. The equivalent average path of separation of the 500 fathom contour (which might be considered as near the edge of an incipient continental shelf) is in quite a different direction as shown by the small solid arrows. The dotted double-ended arrows show the direction of spreading as indicated by the magnetic anomaly pattern. This spreading is in a third direction. It is apparent that, if these motions and apparent displacements have recorded the early history of Arabian separation, the process of separation is not simple. Still there seems to be more correlation between the coastlines than even the much less separated 500 fathom contours. It seems probable that the coastline fit is to be attributed actually to the fact that the coastline follows closely the shape of the coastal mountain ranges. If these ranges originally were the side walls of a developing rift valley they would have been rather closely spaced ( ~ 35 kin) and reasonably parallel. This hypothesis at least seems to give a plausible explanation for the remarkable coastline fit, remarkable because the coastline itself is of no obvious tectonic importance. The great difference between the direction of present spreading and apparent past displacements presumably has to do with the constraints imposed by the partial attachments of Africa and Eurasia and perhaps by the effect of other crustal plates. Conclusion from the discussion above will be summarized in the form of reasonable hypotheses concerning continental separation and fit. (1) The original separation of two continents takes place along a rift fault Palaeogeography, Palaeoclirnatol., Palaeoecol., 9(1971) 233 243
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R. MESERVEY
whose sides develop into coastal ranges. Thus the coastline is often a good measure of the original location of the start of the separation. (2) The initial stages of the separation are very slow and subject to very strong constraints. Once the continents are separated the rate of drift may be very much greater. This implies the coastline fit may apply to a much earlier epoch than the fit of the continental shelves. (3) The coastline fit is faithfully preserved over long segments separated by regions of rifting which introduces an angular displacement of the segments. APPLICATION TO AFRICA AND SOUTH AMERICA
Returning to the separation of Africa and South America it seems likely that the coastlines mark the position where the original graben formed. The time given for the separation of the continents is about 150 million years ago and is based on the first influx of ocean fauna. However, if we judge by the Red Sea case, the original few hundred kilometers of rifting probably started much earlier. To emphasize the essential identity of the two cases Fig.6 shows a schematic representation of the fracturing. Here the South American case is shown with north at the N
( ~' :)I ~ -I~)
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V
S. AMERICA
'~
AFRICA
(N)
Fig.6. Schematic diagram showing the similarity of the fractures separating Arabia and South America from Africa. The coastal outlines of South America (dotted line) and Arabia (dashed line) are included to show how close they approximate a right angle.
top; by turning the figure upside down we have the Arabian case also with north at the top. The light solid line represents the original fracture, where a right angle is originally formed probably as a transform fault at right angles to a spreading axis in the manner observed at present at the mid-ocean ridges. Once formed, this fault evidently became a new spreading axis. The rifting of the reentrant angle is perhaps to be explained mechanically because the continental masses are prePalaeogeography, Palaeoclirnatol., Palaeoecol., 9(1971) 233-243
C O A S T L I N E FIT OF A F R I C A A N D S O U T H AMERICA
241
sumably stronger in compression than in tension and the spreading forces are in each case normal to the sides adjacent to the angles, as indicated by the small arrows. If we take the average direction of the two adjacent coasts of the corner of South America with the best fitting great circles we find that they form an accurate right angle and the same is true of Arabia. The dashed and dotted contours of Fig.6 demonstrate this fact. The above discussion makes it very plausible to follow the clue of the detailed fit of the two segments of coastline with Africa and South America, rotate them into coincidence and make them fit on that basis in reassembling the original continent. However, if we accept this new sort of fit, how can we explain the fact that the continental sheh'es fit so well? To begin with it should be pointed out that the gross fit of the continental shelves of South America and Africa, which accounts for effectively increasing the angle of South America by about 16 ~', is very dependent on the two regions of the shelf farthest from the angle, both of which have somewhat questionable credentials. On the north coast of South America the coastline and continental shelf outlines differ most strongly where the continental shelf suddenly widens in the region near the mouth of the Amazon and it seems doubtful that this widening is independent of the river's presence. In the extreme sonth below the Plata river the continental shelf is broad and the coastlines do not fit at all. However, this section of South America is narrow and liable to large distortions in the process of separation. It has often been suggested that the proper reconstruction of southern South America is curled around the southern tip of Africa. Relia nee on these sections of the continental shelf is therefore probably over-optimistic. In spite of these objections the fit of the shelves is still rather good and it is perhaps possible to retain both fits as applying to different times in geological history. This is made plausible by the history of the Red Sea that was outlined above. Here the original rift, which may be connected to the present coastlines or even higher contours, possibly dates back to Carboniferous times and the process of initial separation has been extremely slow and intermittent. On the other hand if the process continues the final separation will presumably be along what will be called the continental shelves. So that perhaps we actually have here two proper fits which apply to widely separated periods of time. The question remains as to the significance of the above reinterpretation of the facts. The proponents of earth expansion would say that this is exactly what would happen if the diameter of the earth h~td greatly increased since the time of formation of the original rift valley. To see this in the simplest case consider a circular continent whose area does not change as the earth expands. Assuming the continent nmst conform to the changing curvature it is clear that the [~erimeter of the continent must be stretched or rifted as the radius of the sphere increases. In the case of the African continent, it is intuitively plausible that the rifting would take place particularly at the reentrant angles. This is actually what happens when a thin sheet of lead of the conjectured original shape is forced to conform to ~ larger Palaeoffeoffraph.v, Palaeoclimato[., Palaeoecol.,
9( 1971 ) 233 243
242
R. MESERVEY
sphere. If we accepted such a model we could, by eliminating the material from the region of the rift valleys and closing up the gap, make a semiquantitative reconstruction which would give a value for the radius of the earth before rifting. Although the fit of the coastlines and the existing rift systems seems to be consistent with earth expansion, do they necessarily imply expansion? The answer seems to be no, at least on the basis of the present evidence. On a nonexpanding earth, the observed rifts may simply have resulted from the mechanics of rift lines extending themselves into the continental masses somewhat as a wedge. The right angle corners, which appear to be characteristic, are probably connected with the formation of transform faults perpendicular to an axis of spreading. Where the axis of spreading does not coincide with the rift direction, there is a possibility of the rift propagating along a transform fault. The characteristic opening up of reentrant corners might simply result from the wedging action of the rifting mechanism. A careful analysis of all the rifts and compressions of Africa should eventually be able to decide between the two hypotheses suggested here. CONCLUSIONS
In general, this study indicates that fitting together coastal (or higher) contours is sometimes a valid method for reconstructing pre-drift continental masses. The coastal fit probably gives information concerning the early history of continental rifting and separation; it should supplement and correct conclusions drawn from the fit of continental shelves and give us insight into the rifting mechanism. In particular, it is concluded that the fit of the continental shelves of South America and Africa cannot be used to disprove the hypothesis of earth expansion. On the other hand neither does the fit of the coastlines necessarily prove that the earth has expanded, since the exact nature of the rifting process is not yet understood. REFERENCES BULLARD, E., EVERETT, J. E. and SMITH, A. G., 1965. The fit of the continents around the Atlantic. Phil. Trans. Roy. Soc. London Ser. A: 258: 41-51. CAREY, S. W., 1955. Wegener's South American-African assembly, fit or misfit. Geol. Mag., 92: 196-200. CAREY,S. W., 1958. A tectonic approach to continental drift. In: S. W. CAREY(Editor), Continental Drift, a Symposium. University of Tasmania, Hobart, Tas., pp.177-355. CRATCHLEY,C. R. and JONES, (3. P., 1965. An interpretation of the geology and gravity anomalies of the Benue valley, Nigeria. Overseas Geol. Surv., Geophys. Paper, 1 : 25 pp. EURON, R., 1950. G~ologie de I'Afi'ique. Payot, Paris, 350 pp. GIRDLER, R. W., 1958. The relationship of the Red Sea to the east African rift system. Quart. J. Geol. Soc. London, 114: 79-105. GIRDLER, R. W., 1969. The Red Sea--A geophysical background. In: E. T. DEGENS and D. A. Ross (Editors), Hot Brines and Recent Heavy Metal Deposits in the Red Sea. Springer, New York, N.Y., pp.38-58. Palaeogeography, Palaeoclimatok, Palaeoeeol., 9(1971 ) 233-243
COASTLINE FIT OF AFRICA AND SOUTH AMERICA
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JEFFRIES, H., 1959. The Earth, 4th ed. Cambridge University Press, Cambridge, 438 pp. KuNc~, L. C., 1950. Speculations upon the outline and mode of disruption of Gondwanaland. Geol. Mag., 87:353 359. McCONNELL, R. B., 1969. Fundamental fault zones in the Guiana and west African shields in relation to presumed axes of Atlantic Spreading. Geol. Soc. Am. Bull., 80:1775 1782. PHILLIPS, J. D., WOODS1DE,J. and BROWN, C. 0., 1969. Magnetic and gravity anomalies in the central Red Sea. In: E. T. DEGENS and D. A. Ross, (Editors), Hot Brines atul Recent Heavy Metal Deposits in the Red Sea. Springer, New York, N.Y., pp.98 I 13. SUTTON, J., 1968. Development of the continental framework of the Atlantic. Proc. Geoh~gists" Assoc. (Engl.), 79:275 303. WEGENER, A., 1929. Die Entstehang der Kontinents and Ozeatte, 4th ed. Vieweg, Braunschweig, 231 pp. WESTOLL, T. S., 1965. Geological evidence hearing upon continental drift. Phil. Trans. Roy..S'oc. London Set'. A. 258: 12-25. WRIq:;HT, J. B., 1968. South Atlantic continental drift and the Benue trough. Tectom;physics. 6(4): 301-310.
Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233 243
Printed in The Netherlands
T H E C O A S T L I N E FIT OF A F R I C A A N D S O U T H A M E R I C A R. MESERVEY
Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.) (Received June 12, 1970)
ABSTRACT MESERVEY, R., 1971. The coastline fit of Africa and South America. Palaeogeography, Palaeoclimatol., Palaeoecol., 9: 233-243. It has become axiomatic that the pre-drift reconstruction of Africa and South America is to be made by fitting together the continental shelves. Although this procedure is very plausible, actually the detailed fit of the coastlines is better than that of the shelves and the overall fit is remarkable, provided the angle of the Gulf of Guinea is decreased by 16 °. A past increase in this angle is implied by the rift valleys in the vicinity of the Niger and Benue rivers. A comparison with the Red Sea shows a striking similarity in the nature of the fracture and gives some clues as to the significance of the coastline fit in both of these cases. This method of fitting (which may correspond to the early stages of the pre-separation rifting) is consistent with, but does not prove, the hypothesis of earth expansion. INTRODUCTION
WEGENER (1929) and other early advocates of continental drift were greatly impressed by the geometrical fit of South America and Africa. This fit has remained the classic example and thus at the center of the long argument over the reality of drift. JEFFRIES (1959, p.370), who at the time was convinced that the hypothesis of drift was untenable on other grounds, pointed out quite correctly that the near right angle of the coast of Brazil was about 15 ° less than the obtuse angle of the African coast of the Gulf of Guinea into which it was supposed to fit. This argument was answered by CAREY (1955), who showed by carefully matching transparent spherical overlays on a globe that the continental shelves of Africa and South America fit very well. BULLARD et al. (1965) have used a computer-aided technique to show that the overlap and void areas in this fit are indeed small. Carey, after trying to fit the rest of the continents together on a globe, finally came to the conclusion that they would only fit on a smaller-sized earth. For this and other reasons CAREY (1958) adopted the hypothesis of an expanding earth. However, it has since been pointed out by WESTOLL (1965) and others that the exactness of the fit of the continental shelves of South America and Africa apparently limited the amount of expansion to much less than that assumed by Carey. The original purpose of the present work was to place quantitative limits on the possible rate of earth expansion by examining this fit. Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233-243
234
R. MESERVEY
FIT OF COASTLINESAND CONTINENTALSHELVES The suggestion that the geometrical fit be made on the continental shelves was so plausible that, since it was made, it does not seem to have been questioned. In spite o f this great plausibility, careful comparison of the coastlines show that over large sections they fit better than the continental shelves. The present contribution is to demonstrate this fact and to try to understand its significance. The amazingly detailed fit o f the coastlines can be seen in Fig.l. Here are shown the Atlantic coastal outlines o f South America (solid line) and Africa (dotted line)
i
..... A F R I C A
I~ \ " 16 °
i ....
.....
® P
\ C~
Fig.1. Coastline of South America (solid line) compared with the coastline of Africa (dotted line) and the coastline of Africa west of the Niger river rotated 16 ° (dashed line). The Niger delta (A), the Ogoou6 delta (B), and the promontory near the mouth of the Cuanza river (C) are marked as areas of recent origin. with the west coast o f Africa positioned on the east coast of South America to give very nearly their o p t i m u m fit. The coastal outlines were made by tracing on a transparent spherical plastic shell f r o m a 40 cm globe. Fig.1 is a cylindrical projection from the spherical shell o f each segment of the coast and because o f the near coincidence o f the outlines no significant relative distortion is introduced. It is to be noted that with the southern segments o f coast in coincidence, the African coastline west o f the Niger river does not at all fit northern South America, just as JEFFRIES (1959) pointed out. However, the remarkable fact is that if we rotate this section o f the African coastline as a rigid b o d y 16 ° about point P, we obtain the detailed fit shown by the dashed line. The fit o f the two segments is even more impressive when we eliminate the deltas o f the Niger (A) and the Ogoou6 (B) rivers,
Palaeogeography, Palaeoclirnatol., Palaeoecol., 9(1971) 233-243
COASTLINE FIT OF AFRICA AND SOUTH AMERICA
235
which are of comparatively recent origin and should be neglected in making the fit. The promontory near the mouth of the Cuanza River (C) should perhaps also be eliminated. The probability that such a detailed fit should have occurred randomly seems very small. One might object that although the coastal fit is very good, so is the fit on the continental shelves. Certainly the gross fit (without a conjectured rotation) is much better. However, the detailed tit in the two cases is a different story. Fig.2 shows the details of the coastlines, the 1,000 fathom level, and the 2,000 fathom level. In the lower part of Fig.2 the contours at the three different levels are for the
"~
S.A
AFRICA
~
,
~
~
SEALEVEL
IO00f
2000f
2000f
IO00f
]
SEA LEVEL
• AFRICA
/S.AM ~,CA~I
iii
Fig.2. Separate comparison of the detailed fit of the coastline and the 1,000 and 2,000 fathom levels of each segment of coast of South America and Africa.
African coast south of the Niger delta (dotted) and the South American coast south of Natal, Brazil (solid). In each case the lines are slightly offset to make visual comparison easier. In the upper part of the figure are the equivalent contours for the coast of Africa west of the Niger and the coast of South America west of Natal. in each case the very striking congruence of the coasts becomes increasingly blurred at 1,000 fathoms and 2,000 fathoms. It is hard to attribute this simply to chance and Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233-243
236
R. MESERVEV
there are two lines of evidence which indicate strongly that the detailed fit of the coastlines is not by chance nor is it secondary to the fit of the continental shelves. W E S T A F R I C A N RIFTS
First, there are known to have been active rift valleys in the region of Africa near the mouth of the Niger. Such rifts would tend to give the required angular displacement and they were active during the period of continental separation. Fig.3, which is adapted from WRIGHT (1968), shows some of the rift structure in the region. Wright suggests that the Benue trough (1 in Fig.3) is the principal rift
Fig.3. Rift system in the vicinity of the Niger river as adapted from WRIGHT(1968), Benue trough (1), Cameroun rift valley (2), Niger river valley fracture (3). and developed in Early Cretaceous or Late Jurassic as Africa and South America were progressively being wedged apart by a spreading axis moving from the south. Subsidence was most rapid during the Albian to Late Cenomanian when it slowed up and the sea advanced westward along the coast west of the Niger. The Cameroun rift valley (2) is approximately parallel and appears to be prolonged seaward in the volcanic chain of islands in the Gulf of Guinea. FURON (1950) has suggested a fracture formed in the Cretaceous along the Niger valley (3) almost perpendicular to the other faults. This summary of the major faults in this part of Africa and their time sequence is in essential agreement with KING (1950), CRATCHLEVand JONES (1965),
Palaeogeography, PalaeoclimatoL,Palaeoecol., 9(1971) 233-243
COASTLINE FIT OF A F R I C A A N D S O U T H AMERICA
237
SUTTON (1968), and McCoNNELL (1969). Such a rift structure apparently increased the angle between the two segments of the African coasts at about the time when separation was taking place. How great an increase in angle took place is yet to be determined, but here, at least in a qualitative way, is an explanation of the relative angular displacement of the coastlines. Although the major rifting of these faults took place in the Cretaceous, SUTTON (1968) has suggested that they existed in kate Precambrian and Early Cambrian 900-500 million years ago and have been intermittently active from then until their period of greatest activity in the Cretaceous. Thus it appears that separaiion took place along very old established faults. A N A L O G Y W I T H THE RED SEA
Additional evidence concerning the validity of coastline reconstructions comes from considering the Red Sea. Fig.4 shows the well-known fact that in the northern part of the Red Sea the African and Arabian coasts can be made to coincide with great accuracy. The dotted line from A' to B' is the outline of the African coast A to B rotated (approximately 6 ° about a center O, whose position is shown on the inset) to coincide with the coast of Arabia (solid line). The fit is evidently very good. South of B and B' the coastlines do not fit, but the 1,000 m contour CD rotated by the same amount becomes the dash-dot line C'D' which again fits the Arabian coast very well. The coasts of the Gulf of Aden also show a remarkable fit, G H being displaced to the dotted outline G ' H ' by a 6 ' rotation about O plus 7 c' rotation from Q (or the equivalent single rotation of 1 2 about P). Again the 1,000 m contour E F is displaced to the dot-dot-dash outline E'F'. The geological rational for using the 1,000 m contour of the edges of the Afar triangle is that this land area is basaltic and of recent origin and is thought to have been recently elevated. This interpretation and the resulting geometrical fit were suggested by GIRDLER (1958). If we accept this reconstruction, the 7 ° rotation of the Gulf of Aden in addition to that associated with the Red Sea is easily interpreted as connected with the opening of the East African system. This rift system, which is continuous with the rift in the Gulf of Aden, provides the sort of differential rotation implied by the reconstructed coastlines. These observations and interpretations are all well known, but the detailed fit of the coastline or some higher elevation is impressive. The Red Sea was considered by WEGENER(1929) to be an incipient ocean and recent studies support this conclusion. The actual connection of the Red Sea with the Gulf of Aden and the Indian Ocean took place about 8 million years ago but there was previously a long history of rifting in this region as recently reviewed by GIRDLER (1969). Actual rifting began as early as 50 million years ago and the matin Red Sea graben was formed by 26 million years ago. There is even evidence of a depression in this region in the Lower Carboniferous 335 million years ago. In Palaeogeo
238
R. MESERVEY
)
f-" fJ
( /~D
D' ..~
•
_~._
(=) a
~-"
-~G'
~,~
H'
~ o u ~ H .~
u
....:/." J
Fig.4. Map showing the fit of the Arabian coastline and the coastlines of the Red Sea, the Gulf of Aden, and the 1,000 m contour defining the Afar triangle. The fit on the west coast of Arabia is made with a rotation of 6 ° about point O (see inset); the fit on the south coast of Arabia by an additional rotation of 7 ° about point Q. the recent p a s t a n d p r e s u m a b l y at present, the rate o f sea-floor s p r e a d i n g in the R e d Sea is given b y PHILLIPS et al. (1969) to be a b o u t 1.6 c m / y e a r a n d if this is sustained over a geologically significant p e r i o d p r e s u m a b l y A f r i c a a n d A s i a will be c o m p l e t e l y separated. Since j u s t such a s e p a r a t i o n t o o k place with S o u t h A m e r i c a a n d A f r i c a some 150 million years ago it seems r e a s o n a b l e to see if we can u n d e r s t a n d the details o f the f o r m e r s e p a r a t i o n f r o m the one which is t a k i n g place at present a l o n g the R e d Sea rift. In Fig.5 the a p p a r e n t average p a t h o f s e p a r a t i o n o f the A r a b i a n coastline relative to the A f r i c a n coastline, as i n d i c a t e d by their geometrical fit, is
Palaeogeography, Palaeoclimatol., Palaeoecol., 9(1971) 233-243
COASTLINE FIT OF AFRICA AND SOUTH AMERICA
239
500f o
Fig.5. Map showing section of the Red Sea and the directions of displacement as indicated by the coastline fit (long solid arrows), by the 500 fathom level (short solid arrows), and the sl~reading direction indicated by the magnetic anomalies (dashed arrows).
shown by the large solid single-headed arrows. The equivalent average path of separation of the 500 fathom contour (which might be considered as near the edge of an incipient continental shelf) is in quite a different direction as shown by the small solid arrows. The dotted double-ended arrows show the direction of spreading as indicated by the magnetic anomaly pattern. This spreading is in a third direction. It is apparent that, if these motions and apparent displacements have recorded the early history of Arabian separation, the process of separation is not simple. Still there seems to be more correlation between the coastlines than even the much less separated 500 fathom contours. It seems probable that the coastline fit is to be attributed actually to the fact that the coastline follows closely the shape of the coastal mountain ranges. If these ranges originally were the side walls of a developing rift valley they would have been rather closely spaced ( ~ 35 kin) and reasonably parallel. This hypothesis at least seems to give a plausible explanation for the remarkable coastline fit, remarkable because the coastline itself is of no obvious tectonic importance. The great difference between the direction of present spreading and apparent past displacements presumably has to do with the constraints imposed by the partial attachments of Africa and Eurasia and perhaps by the effect of other crustal plates. Conclusion from the discussion above will be summarized in the form of reasonable hypotheses concerning continental separation and fit. (1) The original separation of two continents takes place along a rift fault Palaeogeography, Palaeoclirnatol., Palaeoecol., 9(1971) 233 243
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R. MESERVEY
whose sides develop into coastal ranges. Thus the coastline is often a good measure of the original location of the start of the separation. (2) The initial stages of the separation are very slow and subject to very strong constraints. Once the continents are separated the rate of drift may be very much greater. This implies the coastline fit may apply to a much earlier epoch than the fit of the continental shelves. (3) The coastline fit is faithfully preserved over long segments separated by regions of rifting which introduces an angular displacement of the segments. APPLICATION TO AFRICA AND SOUTH AMERICA
Returning to the separation of Africa and South America it seems likely that the coastlines mark the position where the original graben formed. The time given for the separation of the continents is about 150 million years ago and is based on the first influx of ocean fauna. However, if we judge by the Red Sea case, the original few hundred kilometers of rifting probably started much earlier. To emphasize the essential identity of the two cases Fig.6 shows a schematic representation of the fracturing. Here the South American case is shown with north at the N
( ~' :)I ~ -I~)
-.-.-~J! (VIBV~V)
V
S. AMERICA
'~
AFRICA
(N)
Fig.6. Schematic diagram showing the similarity of the fractures separating Arabia and South America from Africa. The coastal outlines of South America (dotted line) and Arabia (dashed line) are included to show how close they approximate a right angle.
top; by turning the figure upside down we have the Arabian case also with north at the top. The light solid line represents the original fracture, where a right angle is originally formed probably as a transform fault at right angles to a spreading axis in the manner observed at present at the mid-ocean ridges. Once formed, this fault evidently became a new spreading axis. The rifting of the reentrant angle is perhaps to be explained mechanically because the continental masses are prePalaeogeography, Palaeoclirnatol., Palaeoecol., 9(1971) 233-243
C O A S T L I N E FIT OF A F R I C A A N D S O U T H AMERICA
241
sumably stronger in compression than in tension and the spreading forces are in each case normal to the sides adjacent to the angles, as indicated by the small arrows. If we take the average direction of the two adjacent coasts of the corner of South America with the best fitting great circles we find that they form an accurate right angle and the same is true of Arabia. The dashed and dotted contours of Fig.6 demonstrate this fact. The above discussion makes it very plausible to follow the clue of the detailed fit of the two segments of coastline with Africa and South America, rotate them into coincidence and make them fit on that basis in reassembling the original continent. However, if we accept this new sort of fit, how can we explain the fact that the continental sheh'es fit so well? To begin with it should be pointed out that the gross fit of the continental shelves of South America and Africa, which accounts for effectively increasing the angle of South America by about 16 ~', is very dependent on the two regions of the shelf farthest from the angle, both of which have somewhat questionable credentials. On the north coast of South America the coastline and continental shelf outlines differ most strongly where the continental shelf suddenly widens in the region near the mouth of the Amazon and it seems doubtful that this widening is independent of the river's presence. In the extreme sonth below the Plata river the continental shelf is broad and the coastlines do not fit at all. However, this section of South America is narrow and liable to large distortions in the process of separation. It has often been suggested that the proper reconstruction of southern South America is curled around the southern tip of Africa. Relia nee on these sections of the continental shelf is therefore probably over-optimistic. In spite of these objections the fit of the shelves is still rather good and it is perhaps possible to retain both fits as applying to different times in geological history. This is made plausible by the history of the Red Sea that was outlined above. Here the original rift, which may be connected to the present coastlines or even higher contours, possibly dates back to Carboniferous times and the process of initial separation has been extremely slow and intermittent. On the other hand if the process continues the final separation will presumably be along what will be called the continental shelves. So that perhaps we actually have here two proper fits which apply to widely separated periods of time. The question remains as to the significance of the above reinterpretation of the facts. The proponents of earth expansion would say that this is exactly what would happen if the diameter of the earth h~td greatly increased since the time of formation of the original rift valley. To see this in the simplest case consider a circular continent whose area does not change as the earth expands. Assuming the continent nmst conform to the changing curvature it is clear that the [~erimeter of the continent must be stretched or rifted as the radius of the sphere increases. In the case of the African continent, it is intuitively plausible that the rifting would take place particularly at the reentrant angles. This is actually what happens when a thin sheet of lead of the conjectured original shape is forced to conform to ~ larger Palaeoffeoffraph.v, Palaeoclimato[., Palaeoecol.,
9( 1971 ) 233 243
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R. MESERVEY
sphere. If we accepted such a model we could, by eliminating the material from the region of the rift valleys and closing up the gap, make a semiquantitative reconstruction which would give a value for the radius of the earth before rifting. Although the fit of the coastlines and the existing rift systems seems to be consistent with earth expansion, do they necessarily imply expansion? The answer seems to be no, at least on the basis of the present evidence. On a nonexpanding earth, the observed rifts may simply have resulted from the mechanics of rift lines extending themselves into the continental masses somewhat as a wedge. The right angle corners, which appear to be characteristic, are probably connected with the formation of transform faults perpendicular to an axis of spreading. Where the axis of spreading does not coincide with the rift direction, there is a possibility of the rift propagating along a transform fault. The characteristic opening up of reentrant corners might simply result from the wedging action of the rifting mechanism. A careful analysis of all the rifts and compressions of Africa should eventually be able to decide between the two hypotheses suggested here. CONCLUSIONS
In general, this study indicates that fitting together coastal (or higher) contours is sometimes a valid method for reconstructing pre-drift continental masses. The coastal fit probably gives information concerning the early history of continental rifting and separation; it should supplement and correct conclusions drawn from the fit of continental shelves and give us insight into the rifting mechanism. In particular, it is concluded that the fit of the continental shelves of South America and Africa cannot be used to disprove the hypothesis of earth expansion. On the other hand neither does the fit of the coastlines necessarily prove that the earth has expanded, since the exact nature of the rifting process is not yet understood. REFERENCES BULLARD, E., EVERETT, J. E. and SMITH, A. G., 1965. The fit of the continents around the Atlantic. Phil. Trans. Roy. Soc. London Ser. A: 258: 41-51. CAREY, S. W., 1955. Wegener's South American-African assembly, fit or misfit. Geol. Mag., 92: 196-200. CAREY,S. W., 1958. A tectonic approach to continental drift. In: S. W. CAREY(Editor), Continental Drift, a Symposium. University of Tasmania, Hobart, Tas., pp.177-355. CRATCHLEY,C. R. and JONES, (3. P., 1965. An interpretation of the geology and gravity anomalies of the Benue valley, Nigeria. Overseas Geol. Surv., Geophys. Paper, 1 : 25 pp. EURON, R., 1950. G~ologie de I'Afi'ique. Payot, Paris, 350 pp. GIRDLER, R. W., 1958. The relationship of the Red Sea to the east African rift system. Quart. J. Geol. Soc. London, 114: 79-105. GIRDLER, R. W., 1969. The Red Sea--A geophysical background. In: E. T. DEGENS and D. A. Ross (Editors), Hot Brines and Recent Heavy Metal Deposits in the Red Sea. Springer, New York, N.Y., pp.38-58. Palaeogeography, Palaeoclimatok, Palaeoeeol., 9(1971 ) 233-243
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JEFFRIES, H., 1959. The Earth, 4th ed. Cambridge University Press, Cambridge, 438 pp. KuNc~, L. C., 1950. Speculations upon the outline and mode of disruption of Gondwanaland. Geol. Mag., 87:353 359. McCONNELL, R. B., 1969. Fundamental fault zones in the Guiana and west African shields in relation to presumed axes of Atlantic Spreading. Geol. Soc. Am. Bull., 80:1775 1782. PHILLIPS, J. D., WOODS1DE,J. and BROWN, C. 0., 1969. Magnetic and gravity anomalies in the central Red Sea. In: E. T. DEGENS and D. A. Ross, (Editors), Hot Brines atul Recent Heavy Metal Deposits in the Red Sea. Springer, New York, N.Y., pp.98 I 13. SUTTON, J., 1968. Development of the continental framework of the Atlantic. Proc. Geoh~gists" Assoc. (Engl.), 79:275 303. WEGENER, A., 1929. Die Entstehang der Kontinents and Ozeatte, 4th ed. Vieweg, Braunschweig, 231 pp. WESTOLL, T. S., 1965. Geological evidence hearing upon continental drift. Phil. Trans. Roy..S'oc. London Set'. A. 258: 12-25. WRIq:;HT, J. B., 1968. South Atlantic continental drift and the Benue trough. Tectom;physics. 6(4): 301-310.
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