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Paul B. Toft a, Patrick T. Taylor b, Jafar ~kani-famed a and Stephen E. Haggerty ' ... (jr 1 to 6 A m-l) compared to the magnetizations of areally abundantly ...
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Tectonophysics, 212 (1992) 21-32

Elsevier Science Publishers B.V., Amsterdam

Interpretation of satellite magnetic anomalies over the West African Craton a and Stephen E. Haggerty ’ Paul B. Toft a, Patrick T. Taylor b, Jafar ~kani-famed a Department of Geological Sciences, McGill University, Montreal, Que. H3A 2A7, Canada b Geodynamics Branch, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA ’ Department of Geology, University of Massachusetts, Amherst, MA 01003, USA

(Received February 6,199O; revised version accepted August 2,1991)

ABSTRACT Toft, P.B., Taylor, P.T., Arkani-Hamed, J. and Haggerty, SE., 1992. Interpretation of satellite magnetic anomalies over the West African Craton. In: R.R.B. von Frese and P.T. Taylor (Editors), Lithospheric Analysis of Magnetic and Related Geophysical Anomalies. ~ec~~~ophysjcs, 212: 21-32. Satellite magnetic anomaly maps of west Africa display persistent anomalies that are spatially related to the geological structure of the West African Craton. The Reguibat and Man shields to the north and south of the Taoudeni Basin have positive magnetization contrasts whereas the basin, which was disrupted by rifting and volcanism in the Late Proterozoic and Mesozoic, has a negative magnetization contrast. A forward model of the craton is developed, based on the surface geology and including aspects of the evolution of deep-seated thermal, petrological, and magnetization contrasts. Anomalies calculated from the model correspond fairly well to the most reliable observed anomalies. The anomaly associated with the basin is due partly to the anomalies arising from the shields and partly to intra-basin deep-seated demagnetization resulting from Mesozoic thermal and metasomatic demagnetization. Another similar demagnetized zone probably contributes to the Reguibat Shield anomaly. The Man Shield anomaly results primarily from the Archean segment of this shield. The largest magnetization contrast required, for a 70-km-thick shield slab, is + 0.3 A m-‘.

When the first global-scale magnetic anomaly maps were produced, from POGO (Polar Orbiting Geophysical Observatories) satellite data, it was noticed that ~ntinental shields and basins were spatially associated with anomalies that indicate positive and negative induced magnetization contrasts, respectively (Regan et al., 1975; Frey, 1982). Subsequent global-scale maps of Magsat (Magnetic FieId Satellite) data confined that recent continental rifts are commonly regions of negative magnetization contrast, but old rifts may have positive contrasts (Arkani-Hamed and Strangway, 1985a,b). These qualitative rela-

Correspondence to: P.B. Toft, Department of Geological Sciences, McGill University, Montreal, Que. H3A 2A7, Canada.

tionships between surface geology and magnetic anomalies are generally supported by inversions of long-wavelength aeromagnetic and satellite-derived anomalies to crustal-induced magnetization contrasts, though a strongIy magnetic Iower crust is often invoked because the induced magnetization intensities obtained from inversions are large (jr 1 to 6 A m-l) compared to the magnetizations of areally abundantly exposed, upper crustal rocks (Mayhew et al., 1985; Langel, 1985). West Africa is one region where the shields and basins are spatially correlated with sateliitederived magnetic anomalies (Hastings, 1982). A forward model of the Magsat anomaly over the Man Shield of west Africa included a strongly magnetic lower crust (Taft and Haggerty, 19861, but magnetization beneath the shield probably extends into the uppermost mantle (Taft and Haggerty, 1988). A magnetization model that de-

0040-1951/92/%05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

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scribes the relationship between recent rifts and negative magnetization contrasts, as due to high heat flow associated with lithospheric thinning, was based in part on spatial correlations evident

in west Africa (Arkani-Hamed and Strangway, 1985aI. The present paper combines these hypotheses for shield and rift magnetization contrasts. with the objective of achieving a more

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et *I.,

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Fig. 1. Satellite magnetic anomalies over west Africa. Negative (positive) anomalies are shown by horizontal (vertical) shading. The contour interval is 2 nT and the zero contour is omitted. Projections are Mercator, Cylindrical Equidistant, or van der Grinten. There are three robust features over west Africa: a central positive anomaly flanked to north and south by negatives. Main field inclinations are shallow CO”-4O”N),so negative (positive) induced magnetic anomalies result from positive (negative) susceptibility contrasts.

1NTERPRETATlON OF SATELLITE MAGNETIC ANOMALIES OVER THE WEST AFRICAN CRATON

complete dete~ination of the ma~etization of Precambrian cratons. Following an overview of the satellite-derived magnetic anomalies of west Africa and of the regional geology, our hypotheses are combined to calculate the Iithospheric induced magnetic field at satellite altitude and the results are compared to the observed anomalies. Magnetic anomalies A usefuI technique to test the reliabili~ of magnetic anomalies, before interpreting or modeling them, is to assess their repeatability according to different processing methods used with data acquired at different times and by different methods. Nine satellite scalar magnetic anomaly maps of west Africa show essentially the same three anomalies over continental west Africa, namely a central positive anomaly that is flanked to north and south by negatives (Fig. 1). It is important to recognize that scalar magnetic anomalies that result from induced ma~etization are a function of inclination of the Earth’s main magnetic field. The main field is close to horizontal over southern west Africa and it steepens northward to about 4O”N inclination over the northern edge of the craton. In low in&nation areas there is an inverse relationship between susceptibili~ contrast and the sign of an anomaly so that a positive susceptibility contrast causes a predominantIy negative scalar magnetic anomaly and a negative contrast causes a positive anomaly. The earliest satellite magnetic anomaly maps covering west Africa (Fig. lA,B) were made from high-altitude data (400-700 km) from Cosmos 49 and POGO satellites. The data processing used to produce these maps was relatively elementary: a spherical harmonic model (to degree 13) of the main magnetic field was subtracted from quiet time data and the residual values (features with haif wavelengths less than about 1500 km) were contoured (Regan et al;, 1975). Data smoothing and filtering to suppress ionospheric and magnetospheric electrical currents produced better quality maps from POGO data, probably at the expense of some loss of the lithospheric component of the magnetic field (Frey et al., 1979; Fig. lC,D>.

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Magsat was the first satellite flown at low altitudes with a specific objective of resolving the lithospheric field. The satellite orbited the Earth in a near-polar, sun-synchronous, N 400&maltitude orbit for eight months during 1979-1980 collecting scalar and vector data (Lange1 et al., 1982a; Lange1 and Benson, 1987). More sophisticated data processing methods, introduced to deal with the Magsat data on a global scale, included rigorous selection of quiet time data, removal of external fields by multiple linear trend analysis, and statistical justification of rejecting outlying residual values (Lange1 et al., 1982b). The resulting anomalies (Fig. 1E) resemble the higher altitude POGO anomalies (Fig. 1D) but with larger amplitudes. A second giobal-scale Magsat map was produced by Cain et al. (1984) using data processing methods similar to Lange1 et al. (1982b) with a different main field model and a different subset of the Magsat data. This map shows anomalies over west Africa (Fig. 1F) that are intermediate in amplitude to filtered POGO anomalies (Fig. lD> and the preliminary Magsat anomalies (Fig. 1E). Subsequently it was shown that time-dependent differences in ionospheric electrical current polarities lead to differences between Magsat anomaly maps made from data collected at Iocal dawn and dusk times (Arkani-Hamed et al., 1985). A third scalar anomaly map was made from a third subset of Magsat data (Arkani-Hamed and Stran~ay, 1985b,c) using only quiet time data common to both local times and also introducing band-pass spherical harmonic filtering to further suppress external fields. The anomalies in Figure 1G are from an unpublished contour map that is essentially equivalent to the color-coded maps by Arkani-Hamed and Strangway. Additional data processing steps proposed by Ravat (1989) included tie-point and cross-line adjustments following Taylor and Frawley (1987), reduction of the equatorial electrojet by empirical modeling, and statistical gridding by collocation. Including these steps in making Magsat anomaly maps of Africa, Ravat (1989) completely rejected the dusk data because it was found to produce poorer quality maps than the dawn data alone (Fig. 1H).

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Global reprocessing of POGO data divided into midnight, dawn, noon, and dusk local times showed that the local time differences between anomalies at about 500 km altitude are less than for the lower altitude t - 400 km) Magsat data (Langei, 1990). Th e time-averaged POGO anomalies over west Africa (Fig. 11) closely resemble the earlier filtered POGO anomalies (Fig. 1D) and the most recent Magsat anomalies (Fig. lG,H). Thus the nine anomaly maps in Figure 1 were produced by various processing methods from data collected by three different satellites. Other presentations of Magsat data include a filtered version of Figure IE with a topographic overlay (Hastings, 1986) and vector anomaly maps (Langef et al., 1982c; Cain et al., 1984; Yanigasawa and Kono, 1984; Cohen and Achache, 19901, which generally confirm the dominant features seen in Figure 1. The overall similarity among the maps shown in Figure 1 demonstrates that the three anomalies over continental west Africa are

temporally persistent and consistent features 01 the geomagnetic field. Contrasts in lithospheric magnetization within west Africa are the most probable cause of these anomalies but interpretations should recognize that the anomalies have error estimates of about + 1 nT (e.g., Lange1 et af., 1982b; Arkani-Hamed and Stran~ay, 198%; Ravat, 1989). A trend that is apparent in Figure 1 is toward a better mutual agreement among the later maps. The earlier maps (Fig. IA-F) are inconsistent probably because they retain more of the high and low wave number information. The more highly filtered maps (Fig. lD, and G-13, on the other hand, are more consistent with each other and they emphasize the most prominent and repeatable features. Thus the most reliable estimates of anomaly location and amplitude are the most recent Magsat maps (Fig. lG,H) and the latest POGO map (Fig. II). However, we are not aware of objective criteria that would permit the selection from these maps of a single superior

-$_

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;

eraton

q

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Fig. 2. Geological map of west Africa and surroundings. The West African Craton includes the Taoudeni Basin (Tu), the Man (M) and Reguibat (R) shields, and most of the Tindouf (Ti) and Volta (V) basins. Other features are: the Casamance (C) and Essaouria (E) rifts, the Gourma Aulacogen (G), the Senegal (S) and Bove (B) basins and the Canary Islands (a). The western .&&an ( - 2.7 Ga) and eastern Lower Proterozoic (2.2-1.8 Ga) shield provinces are divided by fault zones shown as dashed tines. Compiled from Burke and Whiteman (1973), Bessoies (1977) van Houten (1979). Black (1980) Derry (1980), Clatter et al. (1982) Cahen and Snethng (1984) and Fabre et al. (1989).

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quality map for modeling. Between and among these maps the disparities of a given anomaly probably result from the data reduction techniques, Detailed modeling to reproduce each of these maps would require different bodies or magnetization contrasts, or both. Previous interpretations of the satellite magnetic anomalies over west Africa have associated the negative anomalies (positive magnetization contrasts) with shields and the positive anomaly (negative contrast) with rift or basin structures (Haggerty and Toft, 1981; Frey, 1982; Hastings, 1982; Arkani-Hamed and Strangway, 1985a; Toft and Haggerty, 1986; Ravat, 1989; Ravat et al., 1992-this issue). The regional geology exposed at the Earths surface that is the basis of these associations, and some possible causes of the underlying magnetization contrasts, are examined below. Regional geology The West African Craton (Fig. 2) is dominated by the Taoudeni Basin and by two regions of exposed metamorphic and igneous basement, the Reguibat Shield and the Man Shield (Bessoles, 1977). Both shields are divided by mylonitic shear zones that are about 1.7 Ga old (Caen-Vachette et al., 1984; Cahen and Snehing, 1984). In the Archean provinces to the west of these fauits the rocks are dominantly migmatized granitic and charnockitic gneisses with itabirites, and the main metamorphism, up to granulite facies, occurred * 2.7 Ga ago; in the eastern Lower Proterozoic provinces the gneisses and migmatites are more felsic, granites are more common, itabirites are less abundant, and the main metamo~hism, up to amphibolite facies, occurred w 2.0 Ga ago (Choubert et aI., 1971; Hurley and Rand, 1973; Dillon and Sougy, 1974; Bessoies, 1977; Black, 1980; Cahen and SnelIing, 1984; Boher et al., 1992). Smaller exposures of similar basement rocks, probably below Magsat resolution, occur within the Taoudeni Basin and north and east of the craton within fold belts (Hurley et al., 1967; Dillon and Sougy, 1974; Black, 1980; Cahen and Snelling, 1984; Boher et al., 1992; Fig. 2). The northern boundary of the craton is obscured by

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the Tindouf Basin, and the southern boundary is on the continental slope. The southern tip of the West African Craton prior to Mesozoic continental rifting is represented by the Lower Proterozoic Sgo Luis Craton in northeastern Brazil (Hurley et al., 1967; de Almeida et al., 1981; Torquato and Cordani, 1981; Caby, 1989). Surrounding the craton to the west, north, and east with thrust fault contacts are discontinuous fold belts, dominated by 800-500-Ma-old (PanAfrican erogenic cycle) metamorphic rocks. Geophysical and structural studies, and dismembered ophiolites, suggest that the fold belts are a result of Wilson-cycle accretionary episodes (Hurley et 1971; al., 1967; Choubert and Faure-Muret, Grant, 1973; Leblanc, 1976; Black et al., 1979; Lesquer and ~oussine-Pou~hkine, 1980; Cahen and Snefling, 1984; Viheneuve and Dallmeyer, 1987; Ponsard et al., 1988; LecorchC et al., 1989; Saquaque et al., 1989; Fig. 2). In the extreme north of west Africa, north of the fold belt, are still younger rocks (250-350 Ma old) that are correlated to European Hercynian and Alpine deformation (Cahen and Snelling, 1984; Pique, 1989). These fold belts help to define the edges of the craton for magnetic modeling purposes. In the Taoudeni Basin most of the sediments are less than 3 km thick and are 1100-400 Ma old, but the thickest sediments (N 8 km) are in the Gourma AuIacogen (Bronner et al,, 1980; Clauer et al., 1982; Cahen and Snelling, 1984; Bertrand-Safarti and Moussine-Pouchkine, 1988; Fig. 2). The basin also deepens to the northwest in the Tiris-Tagant Trough (Bronner et al., 1980). The Gourma Auiacogen is a Late Proterozoic (850-800 Ma) rift, interpreted to be the failed third arm of a triple junction: the rift was closed to the east about 600 Ma ago by a Pan-African age collisional suture foi~owing unsuccessful ocean opening (Black et al., 1979; Lesquer and Moussine-Pouchkine, 1980; Cahen and SneIling, 1984). West of the craton, continuing the trend of the Gourma Aulacogen, the Casamance Rift divides the Bov6 Basin from the Senegal Basin microplate (Venkatakrishnan and Culver, 1988; Brun and Lucazeau, 1988;. Ritz and Belhon, 1989). The Casamance Rift and the Essaouria Rift to the north of the craton (Fig. 2) are probably failed

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third arm rifts that developed 220-170 Ma ago in the early stages of Mesozoic continental rifting (Burke and Whiteman, 1973; Burke, 1976; van Houten, 1979). Also associated with Mesozoic continental rifting, and most abundant in the Archean segment of the Man Shield, are basaltic dikes and sills that are 170-2~ Ma old (Triassic-Jurassic) and 90-

12O-Ma-old (Cretaceousf kimberiite pipes and dikes (Haggerty, 1982: Cahen and Snelling, 1984; Wright, 1985; Dupuy et al., 1988). Evidence of other volcanic activity includes Triassic-Jurassic basaltic intrusives within the Taoudeni Basin between the Casamance Rift and the Gourma AuIacogen (Fig. 2) and also within and around the Essaouria Rift (Black and Girod, 1970; van

40*N

A...

A

‘5% 4@N

25*w

5”E 25OW

t

Fig. 3. Model bodies and calculated scalar magnetic anomalies of the West African Craton. Bodies in A represent from south to north: the Man Shield (flecked); the Casamance Rift-Taoudeni Basalts-Gourma Auiacogen corridor (dotted); the Reguibat Shield (flecked); and the Canary IsIands-~aouria Rift block (dotted). Su~ptibiIj~ contrasts, thicknesses and main field values are given in the text. Anomalies are calculated at 400 km altitude. Contour interval is 1 nT. Horizontal (vertical) shading emphasizes negative (positive) anomalies at the resolution of Figure 1.

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Houten, 1979; Cahen and Snelling, 1984; Giraud et al., 1985). The Essaouria Rift is linked by the South Atlas Fault with the Canary Islands (van Houten, 19791, which are the location of the most recent volcanism to have affected west Africa. These islands may be underlain in the east by continental crust but the islands are largely basaltic shield volcanoes that have been active since about 40 Ma ago (Cahen and Snelling, 1984). Minor Cenozoic uplift and subsidence and intraplate Holocene earthquakes (Burke and Wells, 1989; Fabre et al., 1989) have occurred in west Africa but the craton has remained a stable strucN 1.5 Ga ago (Clifford, 1970). ture since Marginally accreted fold belts have not achieved cratonic status. However, rifting and volcanic intrusives in the Gourma-Casamance corridor seem to have compromised the structural integrity of the central part of the craton. The EssaouriaCanary Islands region to the north and northwest of the craton is another zone of rifting, faulting, and volcanism. These two zones may be interpreted as regions of structurally weakened and/or thinned lithosphere, in sharp contrast to the two shields which are very long-lived and stable regions of cratonic lithosphere. Thus the shields and the volcanic rifts are the outstanding tectonic structures that may be expected to be associated with satellite magnetic anomalies over west Africa. The major regional influences on magnetization appear to be met~o~hism, serving to increase the intensity of magnetization particularly in the cratonic nuclei, and the thermal regime, acting to decrease the magnetization in areas of relatively high heat flow (Hastings, 1982; ArkaniHamed and Strangway, 1985a). In detail, however, the anomalies probably represent the integrated effects of many rocks. For example, sources of strong magnetization within the shields may include: (a) low economic grade iron formations; (b) high metamo~hic grade rocks within the upper crust, the lower crust, and the uppermost mantle; and possibly (c> partially serpentinized lower crustal and uppermost mantle rocks (Haggerty, 1979; Toft and Haggerty, 1986, 1988; Toft et al., 1990). A magnetization deficiency in

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rift or volcanic regions may be due to magmatic activity, which is probably greater at depth than is apparent from surface exposures. One mechanism that may reduce magnetization is heating associated with large-scale, deep-seated intrusive activity. Thermal demagnetization caused by such events may persist for up to 400 million years according to numerical models of thermal evolution (Arkani-Hamed and Strangway, 1985aI. Another mechanism of demagnet~ation is metasomatism, due to devolatil~ation of upwelling mantle-derived magmas. ~uid-induced metasomatism is especially prevalent when rift-associated magmas are mechanically or otherwise impeded (Haggerty, 1989). The metasomatic reactions that are involved, in particular the formation of titanite (CaTiSiO,) from Fe-Ti oxides (Haggerty, 19761, are not easily reversible. Therefore, the destruction of magnetic minerals by metasomatism also may impart a long-lived reduction in magnetization intensity. With these underlying sources in mind, the spatial correlations between surface geological features and the satellite magnetic anomalies are examined below by modeling. Magnetic model The Man and Reguibat shields are approximated as vertical-sided bodies with plan views corresponding to the surface exposures of Precambrian basement rocks (Fig. 3A). The magnetic thickness of the Man Shield is estimated to be 70 km (Taft and Haggerty, 1988) and this value is adopted for both shields. The development of these continental nuclei probably included downward thickening of cratonic lithospheric roots (Haggerty, 1986; Toft et al., 1989). The thermal evolution of cratonic root zones shows that 70 km is a reasonable depth for the present-day 600°C isotherm (Ballard and Pollack, 1988) and the limiting depth of magnetization. Previous estimates of the susceptibili~ contrast per unit volume @.I. units) for these shields with a magnetic thickness of 70 km are + 0.02 (Toft and Haggerty, 1986) and about +0.005 (Arkani-Hamed and Strangway, 1985a). In the present modeling a value -1-0.01 is initially adopted for both shields, This susceptibility contrast is equivalent to a mag-

netization contrast of +0.21 to +0.24 A m ’ at the local main field strengths (26 to 30 x 10.’ nT). It is also equivalent to a content of magnetite that is - 0.3 voi.% greater than in the surroundings, according to a summary of the room temperature relationship between vol.% magnetite and initial susceptibility (Taft et al., 1990, fig. 3). The Taoudeni Basin has been a low-lying area since - 1000 Ma ago, but the slow accumulation of its thin sediment veneer was interrupted 850600 Ma ago by aborted rifting and ocean closure to form the Gourma Aulacogen and - 180 Ma ago by basaltic volcanism. These two features seem to define an east-west-oriented corridor of structural weakness that may extend westward off the craton into the Mesozoic Casamance Rift (Fig. 2). The source of the negative susceptibility contrast previously associated with the Taoudeni Basin is here approximated by a vertical-sided body with a plan view corresponding to the Casamance Rift, the Taoudeni Basin basal& and the Gourma Aulacogen (Fig. 3A). The susceptibility contrast of this body is estimated as follows. Suppose that the region had a magnetic thickness and susceptibility equivalent to that of the shields prior to the rifting and volcanism, and that this activity locally thinned the magnetic lithosphere by half due to thermal and metasomatic demagnetization. The susceptibility contrast of the deep demagnetized layer (35 km thick) would then be -0.01 (S.I., per unit volume). This thickness and susceptibility contrast are quite compatible with estimates made from thermal intrusion and evolution models (models 1 and 3, and lithosphere thermal structure Cl) by Arkani-Hamed and Strangway (1985a, fig. 6), taking the age of the Taoudeni Basin basalts and the Casamance Rift to be 200 Ma. This thickness and susceptibility contrast are extended eastward to include the Gourma Aulacogen, and the same values are assumed to represent the zone of rifting and volcanism to the north of the craton. The latter block represents the Essaouria Rift and the Canary Islands (Figs. 2 and 3A). These susceptibility contrasts are relative and are given with respect to the surrounding rocks, which are assumed to be of uniform magnetization. No intrinsic contrast between the continent

and the surrounding oceans is constdercd (Arkani-Hamed and Strangway, 1986). The four blocks seen in Figure 3A are meant to represent. perhaps in a too simple manner, the major magnetization contrasts in west Africa. Making magnetic anomaly simulations from these generalized geologic bodies was done with the method of Plouff (1976). Each body has a thickness, plan shape, and susceptibili~ contrast as described above. The direction and intensity of induced magnetization of each body were found from the Earth’s main field (e.g., Peddie, 1982) for that particular location in 1980 (Magsat time of orbit). Declination of the main field was held constant at 1O”W; the average intensities and inclinations at the locations of the Man, Taoudeni, Reguibat, and Canary-Essaouria bodies were taken to be 26, 28, 30 and 30 X 10” nT and O”, 12S”N. 35”N, and 35”N, respectively. The resulting scalar anomaly field (Fig. 3B-D) represents a vector sum of the fields from the individual bodies computed over an equal area grid at an altitude of 400 km. Results and discussion Figure 3B shows the scalar anomalies from the shields alone. The main field is horizontal for the Man Shield and the positive susceptibility contrast (+O.Ol) causes a strong negative anomaly over the body, with small positive lobes to the north and south. At the steeper inclination of the Reguibat Shield the positive susceptibility contrast ( + 0.01) causes only two significant anomaIy peaks: a negative to the north of the body and a positive to the south. This southern positive peak is enhanced and elongated southward due to its overlap with the northern positive lobe of the Man Shieid anomaly. Thus a positive anomaly occurs over the Taoudeni Basin in the absence of an intra-basin geological feature with negative magnetization contrast (Fig. 3B). This effect of anomaly overlap was suggested on the basis of two-dimensional modeling of the region (Taft and Haggerty, 1986). However, the overlap alone does not produce the magnitude or the Iocation of the observed anomaly over the basin (Fig. I). The two volcanic rift sources which have nega-

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tive susceptibility contrasts ( -0.01) are included in the second calculated anomaly map (Fig. 3C). The calculated positive anomaly over the basin corresponds fairly well to the most reliable of the observed anomalies (Fig. lG-I). The peak of the observed anomaly is a little further north than the calculated anomaly, and it may be that the effects of demagnetization extend northward where the basin deepens (Bronner et al., 1980; D.A. Hastings, pers. commun., 1990). The most northerly of the four model bodies, which represents the Essaouria Rift-Canary Islands zone of negative magnetization contrast, does not appear in the calculated map as a distinct prominent feature. The anomaly from this block is incorporated into the calculated negative anomaly whose observed counterpart has been interpreted as due only to the Reguibat Shield, and the agreement between the calculated and observed anomalies is thereby improved. In both these calculated anomaly maps (Fig. 3B,C) the amplitude of the negative anomaly over the Man Shield is too large in relation to the observed anomalies (Fig. 1). Figure 3D shows a third model in which the negative-contrast volcanic rifts and the western Archean segment of the Man Shield are maintained as before, while the contrast of the eastern Proterozoic segment of the Man Shield is reduced to zero and the entire Reguibat Shield is reduced to a contrast of +O.OOS. The anomaly of the Man Shield cafculated in this case closely resembles the observed anomaly in Figure 1H. However, the amplitudes of the Taoudeni Basin and Reguibat Shield anomalies are less than those observed. Comparing the three calculated anomaly maps (Fig. 3B-D) to the observed anomalies (Fig. 11, the significant results are summarized as follows. The anomaIy over the Taoudeni Basin appears to be due to the combined effects of overlapping anomalies from the shields and intra-basin deepseated demagnetization due to Mesozoic volcanism and rifting. Similar demagnetization probably applies to the region of the Essaouria Rift and the Canary Islands but the effect is absorbed at high altitude into the negative anomaly associated with the Reguibat Shield. This latter anomaly stems primarily from both the Archean and Pro-

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terozoic segments of the shield, and possibly also from the region to the east of the exposed shield. For a magnetic thickness of 70 km the volume susceptibility contrast is required to be at least +O.Ol (S.I.). A similar thickness and susceptibility contrast are indicated as a minimum for the Archean segment of the Man Shield. For this shield, however, the effect of the Proterozoic segment is negligible. The details of any of the reliable observed maps could probably be duplicated by adjustments to the shapes and susceptibility contrasts of the bodies in the present model. However, there are no clear grounds for selecting a particular observed map for detailed modeling. Furthermore, the present objective was to take a forward modeling approach by combining our prior hypotheses for the magnetic properties of shields and of volcanic rifts. The present modeling is constrained more by the geology than by the anomalies, and, although the adopted body shapes could be simplified with little effect on the calculated anomalies, a relatively good agreement between calculated and observed anomalies is achieved. The present model requires only modest magnetization contrasts, about +0.25 A m-i as a m~mum for the 70-~-thick Archean shields. If magnetization were restricted to a conventional crustal thickness of about 35 km, the maximum contrast would still be only +OS A m-‘. This magnetization is equivalent at room temperature to a concentration of magnetite that is about 0.7 vol.% greater than in the surrounding regions. These are more conservative and reasonable rock magnetic properties for cratonic regions than the very high magnetizations that have sometimes been suggested. The mode1 is based on the regional geology of west Africa and it explains relationships among shields, basins, rifts, volcanism and Magsat anomalies that may be applicable to other cratons. Conclusions The dominant geological structures of west Africa are the Archean and Lower Proterozoic Reguibat and Man shields and the central Taoudeni Basin. These parts of the West African

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Craton have remained stable since about 1000 Ma ago, except for Late Proterozoic and Mesozoic rifting and volcanism that disrupted the Taoudeni Basin and the region to the west of the craton. Another zone of rifting and volcanism occurs just to the north of the craton. Comparison of nine different satellite magnetic anomaly maps of west Africa shows that there are three persistent anomalies that are spatially related to the geological structure of the West African Craton. Negative anomalies associated with the shields indicate that these regions are loci of positive magnetization contrasts, whereas a positive anomaly (negative magnetization contrast) overlies the Taoudeni Basin. These spatial associations are tested by a model that addresses the geological evolution of cratonic shields and rifts, including the effects of thermal and metasomatic demagnetization due to deep-seated volcanism. The anomaly over the Taoudeni Basin appears to be due to localized deep-seated demagnetization, along with the effect of overlapping anomaly fields arising from the shields to the north and south. North of the basin, the Archean and the Proterozoic segments of the Reguibat Shield and the demagnetized zone to the north of this shield all contribute to the dominantly negative anomaly associated with the shield. South of the basin, the Man Shield negative anomaly appears to be almost entirely due to just the Archean segment of this shield. The largest magnetization contrast that is required, for a 70-km-thick slab representing the Reguibat Shield, is about +0.25 A m-‘. Acknowledgments

Financial support was provided by NSERC Grant GPO041245 to J.A.-H., a Faculty Grant from Mount Holyoke College to P.B.T., and NASA Contract NAS5-26414 to S.E.H. The anonymous referees and D.A. Hastings, R.W. Girdler, and D.N. Ravat are thanked for critical reviews. References Arkani-Hamed, J. and Strangway, D.W., 1985a. An interpretation of magnetic signatures of aulacogens and cratons in Africa and South America. Tectonophysics, 113: 257-269.

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Note added in proof Similar results to those in Figures lG-I are shown by Baldwin, R. and H. Frey, 1991. MAGSAT crustai anomalies for Africa: dawn and dusk data differences and a combined data set. Phys. Earth Planet. Inter. 67: 237-250.