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Aeromagnetic tests for continental drift in Africa and South America
EARTH AND PLANETARY SCIENCE LETTERS 7 (1970) 429-435. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM
AEROMAGNETIC
TESTS FOR CONTINENTAL
DRIFT
IN AFRICA AND SOUTH AMERICA D. W. STRANGWAY
Department of Physics, University of Toronto, Toronto, Canada and P. R. VOGT
US Naval Oceanographic Office, Washington, DC, USA Received 30 October 1969 (Revised version received 23 December 1969) Aeromagnetic data from parts of the east coast of South America and the west coast of Africa have been compiled and compared. The data available include detailed surveys in Africa over Sierra Leone, the Ivory Coast and Ghana and in South America over Guyana and Surinam. These show a distinct character difference along a boundary between the east and west of the middle of the Ivory Coast. The western pattern is a pronounced, banded pattern of highs and lows trending approximately N 20-40°E, with wavelengths of about 30 km and total amplitudes of 500-800 gammas. To the east, the pattern is quite different as no banding is present and only more local features can be detected. In South America a series of anomalies strike N 45°E, with wavelengths and amplitudes similar to those found in the Ivory Coast. New data from high altitude aeromagnetic surveys have been flown over Africa and South America. These confirm the patterns found in the detailed surveys and show that the Brazilian shield area is quite different magnetically than the Guiana shield. Instead it resembles the eastern part of the African area under study. It therefore appears that the shield in the Ivory Coast in Africa and in the vicinity of the Amazon trough in South America can be divided into two portions. When the two continents are refitted these lines become one continuous ancient geologic "tie line" between the continents. This sort of tie line based on magnetic anomaly character provides yet another constraint in precise pre-drift continental reconstructions.
1. Introduction In recent years airborne magnetic surveying has played a very important role in several fields o f earth sciences; examples include the search for mineral resources, seamount mapping and the study o f sea floor spreading. Since the information derived is essentially a form of geologic mapping, the present study was initiated in an attempt to study the problem o f continental drift by supplementing geologic mapping with airborne magnetic methods. In general, the anomalous features found in continental areas are due to magnetic igneous and metamorphic basement. The recent publication of an
aeromagnetic anomaly map o f Canada [6] has very clearly illustrated some o f the various magnetic patterns that are associated with different blocks o f the continental crust. Specifically, aeromagnetic mapping o f the basement on the continents o f Africa and South America could reveal evidence of a similar character on blocks that may have once been together. Detailed geologic mapping, one way of looking for such similarities, has been impeded by the extensive jungle cover, particularly in west Africa. Carey [ 1] and Bullard et at. [2] showed that Africa and South America could be fitted together with a minimum o f discrepancy at the 500 fathom contour. More recently, Hurley et al. [3] studying age dates from the Gulf
430
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X Fig. 1. Location map showingavailable detailed aeromagnetic surveys.Major anomalies shown in black. of Guinea and from NE Brazil showed that similar age boundaries existed on both continents. A11ard and Hurst [4] have presented evidence of similar synclinal features both in Brazil and in Gabon, and Herz [5] has discussed the similarity of anorthosite distribution from the continents around the Atlantic. Aeromagnetic data are avilable from a variety of sources. Detailed surveys for mineral resource exloration have been made in Guyana and Surinam in South America and in Sierra Leone, the Ivory Coast, Ghana and Nigeria in Africa. Some of the surveys cover quite limited areas while others cover very large regions. We have studied these detailed surveys to establish whether regional magnetic characteristics exist and, if so, to see whether these can be used to categorize the basement.
about 30 km and total amplitudes of about 500-800 gammas. Essentially no regional gradient is present in this area due to its proximity to the magnetic equator. Patterns somewhat similar to this occur on the aeromagnetic map of Canada over some of the Precambrian provinces [6] although the highs and lows are somewhat further apart in that case. It is difficult to account for these broad anomalies on the basis of the available geological evidence. The geological map of Africa [7] and the more detailed map of the Ivory Coast [8] show that the region is predominantly granite, with a few narrow bands of north-easterly striking Precambrian sediments. No particular correlation with the known surficial geology is obvious and it is probable that the pattern represents a fundamental crustal character of the granitic Precambrian basement of this region.
2. Detailed aeromagnetic surveys
2.1.2. Ghana A survey has also been conducted in Ghana using similar flight line spacing and flying heights (fig. 1). The area surveyed is confined to a small zone of later Precambrian sediments and greenstones [9]. The magnetic map, not reproduced in this report, has an altogether different and rather nondescript regional character. No conspicuous regional anomalies such as those of the Ivory Coast are found.
2.1. Africa 2.1.1. Ivory Coast The coverage available from the Ivory Coast (fig. 1) was based on flight lines 1 km apart at a height of 500 feet above sea level. The most prominent character of the data is (fig. 2) a series of high and low anomaly bands characterized by wavelengths of
AEROMAGNETIC TESTS FOR CONTINENTAL DRIFT IN AFRICA AND SOUTH AMERICA
431
eastern Sierra Leone, however, a well-developed lineation striking NE to NNE gives the basement very distinct character. 2.2. South America
Fig. 2. Aeromagnetic anomaly map of the southwestern part of the Ivory Coast. (Location of survey shown in fig. 1). Shaded areas represent anomalous fields over 3000 gammas (arbitrary base) and unshaded areas represent anomalous fields under 3000 gammas.
2.1.3. Sierra Leone Portions of Sierra Leone have also been covered by aeromagnetic surveys (fig. 1). Much of the area covered here is designated as granitic Precambrian basement. Although the pattern of the survey is not as clear as in the Ivory Coast, some distinct NE magnetic banding is present. This suggests that the Precambrian basement in Sierra Leone is similar in its magnetic character to that in the western part of the Ivory Coast. Taken together, the available data from West Africa suggest that the basement there can be divided magnetically into two major subdivisions. In Ghana the field is fairly uniform with only a few superimposed local anomalies. In the western Ivory Coast and
2.2.1. Surinam Unfortunately no detailed surveys are available from Brazil or French Guiana, countries where some of the most interesting correlations might exist. Surinam, however, located on the Guianan shield has had an extensive and detailed survey over the Precambrian basement [10]. Geological mapping, extremely difficult in that part of the world, suggest that the Guiana shield is mainly characterized by a series of granitic rocks. These are shown on the geological map of Surinam [11 ] granite. The regional magnetic character is shown in fig. 3. A strong gradient across the area is present since Surinam is about 30 ° north of the magnetic equator but the most conspicuous superimposed feature of the map is a series of high and low magnetic bands which trend about N 45°E. The amplitude of the features and their separation makes them similar to the bands found in the Ivory Coast. Evidently the similarity of magnetic anomaly character implies that the Guiana shield and shield in the western Ivory Coast may have had a common geologic history. 2.2.2. Guyana Extensive surveys have also been conducted in Guyana as part of the United Nations effort in exploration for mineral resources (McConnell, 1962). The location of these surveys is shown in fig. 1. The pattern of NE striking highs and lows is found here also so that this seems to be a characteristic pattern for the Guiana shield. Again granitic basement predominates, particularly in the southern part of the country [12].
3.
Regionalaeromagneticsurveys
The magnetic map of Canada [6] has clearly established that different portions of the continental crust can have quite different magnetic signatures. These changes in character are often associated with well-defined age boundaries. In a cooperative program
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Fig. 3. Aeromagneticanomaly map of Surinam. (Location of survey shown in fig. 1). Shaded areas represent positive magnetic anomalies and unshaded areas represent negative magnetic anomalies. Due to a large regional gradient in the original survey no single contour line could be picked for a datum on the map. with Project Magnet of the US Navy, it was decided to conduct a series of high level airborne magnetic traverses over selected portions of the shields of NE Brazil and West Africa. A fluxgate magnetometer was used for the survey. The target regions initially selected were designed to intersect age boundaries outlined by Hurley et al. [3], by Cahen and Snelling [13] and others. Anomaly profiles cross an age boundary in Ghana [14] dividing rocks of about 2000 m.y. age on the west from rocks of about 550 m.y. on the east (fig. 1). On the South American side, profiles in the region south-east of the mouth of the Amazon (near Sao Luis) were planned for comparison. A relatively high flight elevation of 6-9000 feet above sea level attenuated much of the detail, found in the low level surveys discussed in the previous section. One of the most interesting results (fig. 4) is an abrupt change in the magnetic pattern on the traverse along the east
coast of French Guiana, over the mouth of the Amazon and over the Brazilian shield. The character north of the Amazon is similar to that found in the detailed studies and reflects a similar geologic character for the whole Guiana shield. This is to be expected from the limited geologic data available [11, 15]. An abrupt change of magnetic character occurs south of the mouth of the Amazon River. The well-developed banding and long wavelength anomalies terminate there. Surprisingly, no particular change in character is found off the mouth of the Amazon trough even though drilling has penetrated 4000 m of sediments on Marajo Island in the mouth of the Amazon. This indicates that the sources of the anomalies are sufficiently deep or regional that variations in thickness of the sedimentary cover have little attenuating effect on the anomalous field. Just south of the mouth of the Amazon, w.here the
AEROMAGNETIC TESTS FOR CONTINENTAL DRIFT IN AFRICA AND SOUTH AMERICA
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Fig. 4. Aeromagnetic profiles collected on Project Magnet with regional field removed. Heavy lines mark boundaries of postulated magnetic provinces. Short profiles taken from detailed aeromagnetic surveys in Surinam and the Ivory Coast.
profile crosses the coastline in the vicinity of exposed Precambrian rocks [16], the character of the magnetic field changes drastically and remains that way across most of the Brazilian shield. Thus, either the banded, long wavelength anomalies of the Guiana shield are not present at all in this part of the shield or, if simialr features are present, they strike nearly parallel to the flight lines. In either case, the regional magnetic character of the Brazilian shield is different from that of the Guiana shield. Additional profiles parallel to the Brazilian leg of the first flight line reveal a similar rather irregular character for the Brazilian shield. Surprizingly no real difference was observed from line to line even though they traversed over sedimentary basins such as the Sao Luis Basin known to be as much as 4000 metres deep, and the Maranhao basin, known to be 3000 metres deep. Again we must conclude that the anomaly character is due to deepseated features of the basement rock. Some anomalous regions striking N 45°E, normal to the flight lines, are detected in the Brazilian shield. In summary, the Guiana and the Brazilian shields differ markedly in their magnetic character but no major difference was found across the age boundary near Sao Luis. At the southeastern end of the flight lines there is another abrupt change in character. This pattern was
found at the end of all three lines and on a fourth track flown west from Recife. A second major magnetic boundary striking N 45°E can be followed for 500 km. South and east of this boundary large amplitude banded anomalies strike N 45°E suggesting a separate magnetic province. The boundary strikes N 45°E can be followed for at least 500 km. Unfortunately, no data are available on the African side, where the corresponding boundary should be found in the vicinity of Biafra. Future effort should be directed toward delineating the possible continuation of this major magnetic boundary. This pattern is very much in accord with a suggestion of Guimaraes [17] who discussed the possibility of an Archaeo-Guiana, an Archaeo-Brazil and an Archaeo-Africa as separate continental nuclei. The postulated boundaries are rather like the magnetic boundaries reported here. On the African side a series of traverses was flown across the age boundary between rocks 500 m.y. old and rocks 2000 m.y. old [3, 14]. This boundary appears to intersect the coast line close to 0 ° longitude near Accra, Ghana. As on the South American side no major character difference can be found in the magnetic traverses across this boundary. The lines show some features that can be traced to the NE for distances of a few hundred miles which
434
D.W.STRANGWAY and P.R.VOGT
might tentatively be correlated into Brazil. The anomaly character, however, over most of the area of flight line coverage is quite like that found on the Brazilian shield and it is therefore probable that these two portions of shield had a similar Precambrian geologic history. A pronounced band of magnetic highs and lows intersecting the coast line at about 4°W and striking NNE is probably related to a series of greenstones and basic rocks which crop out in the southeastern Ivory Coast and extend NE into Ghana. Several profiles extended well onto the shelf but no major difference in character was encountered in spite of known thick sediments [18]. This one again supports the conclusions that these regional geologic features reflect the basement rocks only and obvious surficial features such as sedimentary basins have little influence on the general anomaly pattern. Unfortunately, the profiles were terminated at about 5°W and no data were obtained over the western Ivory Coast. If the pattern observed in the latter area were present in any of the region surveyed, however, it would have been distinctly recognizable. We conclude that there is a major character difference between the detailed coverage in the western Ivory Coast and the regional coverage in the eastern Ivory Coast. A distinct geologic boundary is observed in this general vicinity on the geologic maps [8]. To the east, Birrimian rocks, mainly sedimentary, are exposed, while to the west the rocks are mainly granitic. Although no well-defined age boundary is located here, this does appear to be a dividing line between two kinds of basement, hence different magnetic signatures. It appears therefore that a significant magnetic character line is present in both Africa and South America. Although this line does not correlate with the known age boundaries, it is nevertheless a useful marker line. In Africa it lies in the middle of the Ivory Coast and in South America, it lies at the southern edge of the Amazon trough. When the continents are reconstructed, these marker lines are seen to fit with each other. Basement provinces delineated by magnetic anomaly character thus match up when the continents are reconstructed. 4. Conclusions
Detailed surveys from parts of west Africa and
northeastern South America show a remarkable similarity in the nature of banded magnetic features. This pattern characterizes the Guiana shield and the portion of Africa west of the central Ivory Coast. The Brazilian shield and the African shield east of the central Ivory Coast exhibit a different pattern without the strong banding found in the Guiana shield. A distinct magnetic boundary along the southern edge of the Amazon basin seems to match with a similar boundary in the central Ivory Coast, although additional coverage is necessary to confirm this. Additional traverses on both the South American and the African side are required. A second major boundary was located in South America striking about N 45°E through Recife. Future surveys should be conducted on the African side to search for the continuation of this boundary. The character of magnetic anomalies as well as specific trends appear to be useful constraints on pre-drift continental reconstruction.
Acknowledgements It is a real pleasure to acknowledge the help of many interested individuals whom we have corresponded or who willingly supplied us with data. Dr. A.S.Sasraku of the Ghana Geological Survey, Mr. Van de Graaf of the U.N. Development Program in the Ivory Coast, Mr. L.Zanone of Sodemi, Ivory Coast, Dr. E.M.Laing of the Geological Department of Sierra Leone, Dr. I.Gombe of the Federal Government of Nigeria, Dr. Snijderes, Dr. Loemban Toebing and Dr. L.O'Herne of the Ministerie van Opbouw in Surinam, Dr. Singh of the Geological Survey in Guyana, Sr. Hans Setz of Brazil, Sr. Gildo Sa, Sr. J.A. Medeiros Ferreira, and Sr. E.F. Suczyznski of Sudene, Brazil, M.Barruol of French Guiana, M.Chatue-Kamga, M.Laplaine and M.Soba of the Cameroons and Mr. A.E.Nyema Jones of the Geological Survey in Liberia. Mr. R.Morrison of the University of Toronto provided much helpful advice on the geology of South America and pointed out the work by Guimaraes. The work with the US Navy was ably organized and conducted by Dr. C.C.Bates, Mr. H.P.Stockard and Mr. Charles Gunn, party chief. All of these people were extremely helpful and cooperative and we take this opportunity to acknowledge their assistance.
AEROMAGNETICTESTS FOR CONTINENTALDRIFT IN AFRICA AND SOUTH AMERICA Partial support was provided by the Office of Naval Research under contract No. Nonr 1841 (74) with MIT.
References [ 1] E.Bullard, J.E.Everett and A.G.Smith, The fit of the continents around the Atlantic, In: A Symposium on Continental Drift (Royal Society of London, 1965). [2] S.W.Carey,Continental drift, A symposium (Geology Department, University of Tasmania, Hobart, 1958). [3] P.M.Hurley, F.F.M.de Almeida, G.C.Melcher, U.G. Cordani, J.R.Rand, K.Kawashita, P.Vandoros, W.H. Pinson Jr. and H.W.Fairbairn, Test of continental drift by comparison of radiometric ages, Science 157 (1967) 495 -500. [4] G.O.Allard and V.J.Hurst, Brazil-Gabon geologic link supports continental drift, Science 163 (1969) 528-532. [5] N.Herz, Anorthosite belts, continental drift and the anorthosite event., Science 164 (1969) 3882, 944-947. [6] L.W.Morley,A.S.MacLaren and B.W.Charbonneau, Aeromagnetic anomaly map of Canada, Scale 1:5 000 000, Map 1255A (Geol. Surv. of Canada, 1967). [7 ] Geological map of Africa.
435
[8] Geological map of the Ivory Coast, 1:1 000 000 (Sodemi, 1963). [9] Geological map of Ghana, Scale 1:1 000 000 (D.A. Bates, 1955) reprinted 1966. [10] Isaacs, Geophysical case history of the Rosebel-Bonidoro group, Surinam, South America, In: MiningGeophysics, Vol. 1 (Soc. of Exptl. Geophysicists, 1966). [11] L.O'Herne, A short introduction to the geology of Surinam (Mijnbouwkundige Dienst, Surinam, Geologic map, 1962, scale 1:500 000). [ 12 ] Geological map of British Guiana, scale 1:1 000 000, (R.B.McConnell, 1962). [13] L.Cahen and N.Snelling, The geochronology of equatorial Africa (North Holland Publ. Co., Amsterdam, 1966). [14] N.K.Grant, Complete late Precambrian to early Paleozoic orogenic cycle in Ghana, Togo and Dahomey, Nature 215 (1967) 609-610. [15] B.Choubert, French Guiana, Geological Map. [ 16 ] W.Kegel,A estrutura geologica do nordeste do Brasil, Bull. no. 227 (Departamento nacional da produc~o mineral, Divis~lode geologia e mineralogia, 1965); A.R.Lamego, Mapa geologico do Brasil, scale 1:5 000 000 (DNPM, Rio de Janeiro, 1960). [17] D.Guimaraes, Arqui-Brasil e sua evoluc[o geologica, Bull. 88 (DNPM, Divisao de formento da producao mineral, 1951). [18] A.De Spengler et J.R.Delteil, La bassin secondairetertiaire de C6te d'Ivoire, Report 94 (Sodemi, 1963).
AEROMAGNETIC
TESTS FOR CONTINENTAL
DRIFT
IN AFRICA AND SOUTH AMERICA D. W. STRANGWAY
Department of Physics, University of Toronto, Toronto, Canada and P. R. VOGT
US Naval Oceanographic Office, Washington, DC, USA Received 30 October 1969 (Revised version received 23 December 1969) Aeromagnetic data from parts of the east coast of South America and the west coast of Africa have been compiled and compared. The data available include detailed surveys in Africa over Sierra Leone, the Ivory Coast and Ghana and in South America over Guyana and Surinam. These show a distinct character difference along a boundary between the east and west of the middle of the Ivory Coast. The western pattern is a pronounced, banded pattern of highs and lows trending approximately N 20-40°E, with wavelengths of about 30 km and total amplitudes of 500-800 gammas. To the east, the pattern is quite different as no banding is present and only more local features can be detected. In South America a series of anomalies strike N 45°E, with wavelengths and amplitudes similar to those found in the Ivory Coast. New data from high altitude aeromagnetic surveys have been flown over Africa and South America. These confirm the patterns found in the detailed surveys and show that the Brazilian shield area is quite different magnetically than the Guiana shield. Instead it resembles the eastern part of the African area under study. It therefore appears that the shield in the Ivory Coast in Africa and in the vicinity of the Amazon trough in South America can be divided into two portions. When the two continents are refitted these lines become one continuous ancient geologic "tie line" between the continents. This sort of tie line based on magnetic anomaly character provides yet another constraint in precise pre-drift continental reconstructions.
1. Introduction In recent years airborne magnetic surveying has played a very important role in several fields o f earth sciences; examples include the search for mineral resources, seamount mapping and the study o f sea floor spreading. Since the information derived is essentially a form of geologic mapping, the present study was initiated in an attempt to study the problem o f continental drift by supplementing geologic mapping with airborne magnetic methods. In general, the anomalous features found in continental areas are due to magnetic igneous and metamorphic basement. The recent publication of an
aeromagnetic anomaly map o f Canada [6] has very clearly illustrated some o f the various magnetic patterns that are associated with different blocks o f the continental crust. Specifically, aeromagnetic mapping o f the basement on the continents o f Africa and South America could reveal evidence of a similar character on blocks that may have once been together. Detailed geologic mapping, one way of looking for such similarities, has been impeded by the extensive jungle cover, particularly in west Africa. Carey [ 1] and Bullard et at. [2] showed that Africa and South America could be fitted together with a minimum o f discrepancy at the 500 fathom contour. More recently, Hurley et al. [3] studying age dates from the Gulf
430
D.W.STRANGWAY and P.R.VOGT
• °
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X Fig. 1. Location map showingavailable detailed aeromagnetic surveys.Major anomalies shown in black. of Guinea and from NE Brazil showed that similar age boundaries existed on both continents. A11ard and Hurst [4] have presented evidence of similar synclinal features both in Brazil and in Gabon, and Herz [5] has discussed the similarity of anorthosite distribution from the continents around the Atlantic. Aeromagnetic data are avilable from a variety of sources. Detailed surveys for mineral resource exloration have been made in Guyana and Surinam in South America and in Sierra Leone, the Ivory Coast, Ghana and Nigeria in Africa. Some of the surveys cover quite limited areas while others cover very large regions. We have studied these detailed surveys to establish whether regional magnetic characteristics exist and, if so, to see whether these can be used to categorize the basement.
about 30 km and total amplitudes of about 500-800 gammas. Essentially no regional gradient is present in this area due to its proximity to the magnetic equator. Patterns somewhat similar to this occur on the aeromagnetic map of Canada over some of the Precambrian provinces [6] although the highs and lows are somewhat further apart in that case. It is difficult to account for these broad anomalies on the basis of the available geological evidence. The geological map of Africa [7] and the more detailed map of the Ivory Coast [8] show that the region is predominantly granite, with a few narrow bands of north-easterly striking Precambrian sediments. No particular correlation with the known surficial geology is obvious and it is probable that the pattern represents a fundamental crustal character of the granitic Precambrian basement of this region.
2. Detailed aeromagnetic surveys
2.1.2. Ghana A survey has also been conducted in Ghana using similar flight line spacing and flying heights (fig. 1). The area surveyed is confined to a small zone of later Precambrian sediments and greenstones [9]. The magnetic map, not reproduced in this report, has an altogether different and rather nondescript regional character. No conspicuous regional anomalies such as those of the Ivory Coast are found.
2.1. Africa 2.1.1. Ivory Coast The coverage available from the Ivory Coast (fig. 1) was based on flight lines 1 km apart at a height of 500 feet above sea level. The most prominent character of the data is (fig. 2) a series of high and low anomaly bands characterized by wavelengths of
AEROMAGNETIC TESTS FOR CONTINENTAL DRIFT IN AFRICA AND SOUTH AMERICA
431
eastern Sierra Leone, however, a well-developed lineation striking NE to NNE gives the basement very distinct character. 2.2. South America
Fig. 2. Aeromagnetic anomaly map of the southwestern part of the Ivory Coast. (Location of survey shown in fig. 1). Shaded areas represent anomalous fields over 3000 gammas (arbitrary base) and unshaded areas represent anomalous fields under 3000 gammas.
2.1.3. Sierra Leone Portions of Sierra Leone have also been covered by aeromagnetic surveys (fig. 1). Much of the area covered here is designated as granitic Precambrian basement. Although the pattern of the survey is not as clear as in the Ivory Coast, some distinct NE magnetic banding is present. This suggests that the Precambrian basement in Sierra Leone is similar in its magnetic character to that in the western part of the Ivory Coast. Taken together, the available data from West Africa suggest that the basement there can be divided magnetically into two major subdivisions. In Ghana the field is fairly uniform with only a few superimposed local anomalies. In the western Ivory Coast and
2.2.1. Surinam Unfortunately no detailed surveys are available from Brazil or French Guiana, countries where some of the most interesting correlations might exist. Surinam, however, located on the Guianan shield has had an extensive and detailed survey over the Precambrian basement [10]. Geological mapping, extremely difficult in that part of the world, suggest that the Guiana shield is mainly characterized by a series of granitic rocks. These are shown on the geological map of Surinam [11 ] granite. The regional magnetic character is shown in fig. 3. A strong gradient across the area is present since Surinam is about 30 ° north of the magnetic equator but the most conspicuous superimposed feature of the map is a series of high and low magnetic bands which trend about N 45°E. The amplitude of the features and their separation makes them similar to the bands found in the Ivory Coast. Evidently the similarity of magnetic anomaly character implies that the Guiana shield and shield in the western Ivory Coast may have had a common geologic history. 2.2.2. Guyana Extensive surveys have also been conducted in Guyana as part of the United Nations effort in exploration for mineral resources (McConnell, 1962). The location of these surveys is shown in fig. 1. The pattern of NE striking highs and lows is found here also so that this seems to be a characteristic pattern for the Guiana shield. Again granitic basement predominates, particularly in the southern part of the country [12].
3.
Regionalaeromagneticsurveys
The magnetic map of Canada [6] has clearly established that different portions of the continental crust can have quite different magnetic signatures. These changes in character are often associated with well-defined age boundaries. In a cooperative program
432
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Fig. 3. Aeromagneticanomaly map of Surinam. (Location of survey shown in fig. 1). Shaded areas represent positive magnetic anomalies and unshaded areas represent negative magnetic anomalies. Due to a large regional gradient in the original survey no single contour line could be picked for a datum on the map. with Project Magnet of the US Navy, it was decided to conduct a series of high level airborne magnetic traverses over selected portions of the shields of NE Brazil and West Africa. A fluxgate magnetometer was used for the survey. The target regions initially selected were designed to intersect age boundaries outlined by Hurley et al. [3], by Cahen and Snelling [13] and others. Anomaly profiles cross an age boundary in Ghana [14] dividing rocks of about 2000 m.y. age on the west from rocks of about 550 m.y. on the east (fig. 1). On the South American side, profiles in the region south-east of the mouth of the Amazon (near Sao Luis) were planned for comparison. A relatively high flight elevation of 6-9000 feet above sea level attenuated much of the detail, found in the low level surveys discussed in the previous section. One of the most interesting results (fig. 4) is an abrupt change in the magnetic pattern on the traverse along the east
coast of French Guiana, over the mouth of the Amazon and over the Brazilian shield. The character north of the Amazon is similar to that found in the detailed studies and reflects a similar geologic character for the whole Guiana shield. This is to be expected from the limited geologic data available [11, 15]. An abrupt change of magnetic character occurs south of the mouth of the Amazon River. The well-developed banding and long wavelength anomalies terminate there. Surprisingly, no particular change in character is found off the mouth of the Amazon trough even though drilling has penetrated 4000 m of sediments on Marajo Island in the mouth of the Amazon. This indicates that the sources of the anomalies are sufficiently deep or regional that variations in thickness of the sedimentary cover have little attenuating effect on the anomalous field. Just south of the mouth of the Amazon, w.here the
AEROMAGNETIC TESTS FOR CONTINENTAL DRIFT IN AFRICA AND SOUTH AMERICA
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I
Fig. 4. Aeromagnetic profiles collected on Project Magnet with regional field removed. Heavy lines mark boundaries of postulated magnetic provinces. Short profiles taken from detailed aeromagnetic surveys in Surinam and the Ivory Coast.
profile crosses the coastline in the vicinity of exposed Precambrian rocks [16], the character of the magnetic field changes drastically and remains that way across most of the Brazilian shield. Thus, either the banded, long wavelength anomalies of the Guiana shield are not present at all in this part of the shield or, if simialr features are present, they strike nearly parallel to the flight lines. In either case, the regional magnetic character of the Brazilian shield is different from that of the Guiana shield. Additional profiles parallel to the Brazilian leg of the first flight line reveal a similar rather irregular character for the Brazilian shield. Surprizingly no real difference was observed from line to line even though they traversed over sedimentary basins such as the Sao Luis Basin known to be as much as 4000 metres deep, and the Maranhao basin, known to be 3000 metres deep. Again we must conclude that the anomaly character is due to deepseated features of the basement rock. Some anomalous regions striking N 45°E, normal to the flight lines, are detected in the Brazilian shield. In summary, the Guiana and the Brazilian shields differ markedly in their magnetic character but no major difference was found across the age boundary near Sao Luis. At the southeastern end of the flight lines there is another abrupt change in character. This pattern was
found at the end of all three lines and on a fourth track flown west from Recife. A second major magnetic boundary striking N 45°E can be followed for 500 km. South and east of this boundary large amplitude banded anomalies strike N 45°E suggesting a separate magnetic province. The boundary strikes N 45°E can be followed for at least 500 km. Unfortunately, no data are available on the African side, where the corresponding boundary should be found in the vicinity of Biafra. Future effort should be directed toward delineating the possible continuation of this major magnetic boundary. This pattern is very much in accord with a suggestion of Guimaraes [17] who discussed the possibility of an Archaeo-Guiana, an Archaeo-Brazil and an Archaeo-Africa as separate continental nuclei. The postulated boundaries are rather like the magnetic boundaries reported here. On the African side a series of traverses was flown across the age boundary between rocks 500 m.y. old and rocks 2000 m.y. old [3, 14]. This boundary appears to intersect the coast line close to 0 ° longitude near Accra, Ghana. As on the South American side no major character difference can be found in the magnetic traverses across this boundary. The lines show some features that can be traced to the NE for distances of a few hundred miles which
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D.W.STRANGWAY and P.R.VOGT
might tentatively be correlated into Brazil. The anomaly character, however, over most of the area of flight line coverage is quite like that found on the Brazilian shield and it is therefore probable that these two portions of shield had a similar Precambrian geologic history. A pronounced band of magnetic highs and lows intersecting the coast line at about 4°W and striking NNE is probably related to a series of greenstones and basic rocks which crop out in the southeastern Ivory Coast and extend NE into Ghana. Several profiles extended well onto the shelf but no major difference in character was encountered in spite of known thick sediments [18]. This one again supports the conclusions that these regional geologic features reflect the basement rocks only and obvious surficial features such as sedimentary basins have little influence on the general anomaly pattern. Unfortunately, the profiles were terminated at about 5°W and no data were obtained over the western Ivory Coast. If the pattern observed in the latter area were present in any of the region surveyed, however, it would have been distinctly recognizable. We conclude that there is a major character difference between the detailed coverage in the western Ivory Coast and the regional coverage in the eastern Ivory Coast. A distinct geologic boundary is observed in this general vicinity on the geologic maps [8]. To the east, Birrimian rocks, mainly sedimentary, are exposed, while to the west the rocks are mainly granitic. Although no well-defined age boundary is located here, this does appear to be a dividing line between two kinds of basement, hence different magnetic signatures. It appears therefore that a significant magnetic character line is present in both Africa and South America. Although this line does not correlate with the known age boundaries, it is nevertheless a useful marker line. In Africa it lies in the middle of the Ivory Coast and in South America, it lies at the southern edge of the Amazon trough. When the continents are reconstructed, these marker lines are seen to fit with each other. Basement provinces delineated by magnetic anomaly character thus match up when the continents are reconstructed. 4. Conclusions
Detailed surveys from parts of west Africa and
northeastern South America show a remarkable similarity in the nature of banded magnetic features. This pattern characterizes the Guiana shield and the portion of Africa west of the central Ivory Coast. The Brazilian shield and the African shield east of the central Ivory Coast exhibit a different pattern without the strong banding found in the Guiana shield. A distinct magnetic boundary along the southern edge of the Amazon basin seems to match with a similar boundary in the central Ivory Coast, although additional coverage is necessary to confirm this. Additional traverses on both the South American and the African side are required. A second major boundary was located in South America striking about N 45°E through Recife. Future surveys should be conducted on the African side to search for the continuation of this boundary. The character of magnetic anomalies as well as specific trends appear to be useful constraints on pre-drift continental reconstruction.
Acknowledgements It is a real pleasure to acknowledge the help of many interested individuals whom we have corresponded or who willingly supplied us with data. Dr. A.S.Sasraku of the Ghana Geological Survey, Mr. Van de Graaf of the U.N. Development Program in the Ivory Coast, Mr. L.Zanone of Sodemi, Ivory Coast, Dr. E.M.Laing of the Geological Department of Sierra Leone, Dr. I.Gombe of the Federal Government of Nigeria, Dr. Snijderes, Dr. Loemban Toebing and Dr. L.O'Herne of the Ministerie van Opbouw in Surinam, Dr. Singh of the Geological Survey in Guyana, Sr. Hans Setz of Brazil, Sr. Gildo Sa, Sr. J.A. Medeiros Ferreira, and Sr. E.F. Suczyznski of Sudene, Brazil, M.Barruol of French Guiana, M.Chatue-Kamga, M.Laplaine and M.Soba of the Cameroons and Mr. A.E.Nyema Jones of the Geological Survey in Liberia. Mr. R.Morrison of the University of Toronto provided much helpful advice on the geology of South America and pointed out the work by Guimaraes. The work with the US Navy was ably organized and conducted by Dr. C.C.Bates, Mr. H.P.Stockard and Mr. Charles Gunn, party chief. All of these people were extremely helpful and cooperative and we take this opportunity to acknowledge their assistance.
AEROMAGNETICTESTS FOR CONTINENTALDRIFT IN AFRICA AND SOUTH AMERICA Partial support was provided by the Office of Naval Research under contract No. Nonr 1841 (74) with MIT.
References [ 1] E.Bullard, J.E.Everett and A.G.Smith, The fit of the continents around the Atlantic, In: A Symposium on Continental Drift (Royal Society of London, 1965). [2] S.W.Carey,Continental drift, A symposium (Geology Department, University of Tasmania, Hobart, 1958). [3] P.M.Hurley, F.F.M.de Almeida, G.C.Melcher, U.G. Cordani, J.R.Rand, K.Kawashita, P.Vandoros, W.H. Pinson Jr. and H.W.Fairbairn, Test of continental drift by comparison of radiometric ages, Science 157 (1967) 495 -500. [4] G.O.Allard and V.J.Hurst, Brazil-Gabon geologic link supports continental drift, Science 163 (1969) 528-532. [5] N.Herz, Anorthosite belts, continental drift and the anorthosite event., Science 164 (1969) 3882, 944-947. [6] L.W.Morley,A.S.MacLaren and B.W.Charbonneau, Aeromagnetic anomaly map of Canada, Scale 1:5 000 000, Map 1255A (Geol. Surv. of Canada, 1967). [7 ] Geological map of Africa.
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[8] Geological map of the Ivory Coast, 1:1 000 000 (Sodemi, 1963). [9] Geological map of Ghana, Scale 1:1 000 000 (D.A. Bates, 1955) reprinted 1966. [10] Isaacs, Geophysical case history of the Rosebel-Bonidoro group, Surinam, South America, In: MiningGeophysics, Vol. 1 (Soc. of Exptl. Geophysicists, 1966). [11] L.O'Herne, A short introduction to the geology of Surinam (Mijnbouwkundige Dienst, Surinam, Geologic map, 1962, scale 1:500 000). [ 12 ] Geological map of British Guiana, scale 1:1 000 000, (R.B.McConnell, 1962). [13] L.Cahen and N.Snelling, The geochronology of equatorial Africa (North Holland Publ. Co., Amsterdam, 1966). [14] N.K.Grant, Complete late Precambrian to early Paleozoic orogenic cycle in Ghana, Togo and Dahomey, Nature 215 (1967) 609-610. [15] B.Choubert, French Guiana, Geological Map. [ 16 ] W.Kegel,A estrutura geologica do nordeste do Brasil, Bull. no. 227 (Departamento nacional da produc~o mineral, Divis~lode geologia e mineralogia, 1965); A.R.Lamego, Mapa geologico do Brasil, scale 1:5 000 000 (DNPM, Rio de Janeiro, 1960). [17] D.Guimaraes, Arqui-Brasil e sua evoluc[o geologica, Bull. 88 (DNPM, Divisao de formento da producao mineral, 1951). [18] A.De Spengler et J.R.Delteil, La bassin secondairetertiaire de C6te d'Ivoire, Report 94 (Sodemi, 1963).