Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana

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l Research 170 (2008) 247–261www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geotherma

Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm,Karoo igneous province, northern Botswana

C. Aubourg a,⁎, G. Tshoso b,c, B. Le Gall b, H. Bertrand c, J.-J. Tiercelin b,A.B. Kampunzu d, J. Dyment e, M. Modisi d

a Laboratoire de Tectonique, CNRS, Université Cergy Pontoise, 8 Le Campus, 95031 Cergy Cedex, Franceb Domaines Océaniques CNRS, Institut Universitaire Européen de la Mer, Place Nicolas Copernic 29280 Plouzané, France

c Ecole Normale Supérieure de Lyon, CNRS, Université Claude Bernard, 69364 Lyon, Franced Department of Geology, University of Botswana, Gaborone, Botswana

e UMR-CNRS 7097, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75005 Paris, France

Received 13 July 2005; accepted 19 October 2007Available online 17 November 2007

Abstract

To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy ofmagnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km fromthe presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use theimbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magneticfoliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysisof normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, apoorly defined imbricated magnetic foliation also suggests lateral flow to the west.© 2007 Elsevier B.V. All rights reserved.

Keywords: magnetic fabric; AMS; imbrication; magma flow direction; Karoo; Okavango dyke swarm

1. Introduction

Giant dyke swarms are related to Large Igneous Provinces(LIP) (Ernst and Buchan, 1997). In most cases, giant swarmshave been interpreted as horizontally fed, 300 to 1000 km longdyke systems radiating from a common source (Ernst andBaragar, 1992; Ernst and Buchan, 1997). Giant dyke swarmshave been used to constrain geotectonic processes, e.g. locatingmantle plumes, reconstructing ancient supercontinents (Yale andCarpenter, 1998; Gudmundsson and Marinoni, 1999) or definingthe driving force behind crustal accretion (Bjornsson, 1985).

The study of the magma flow direction during theemplacement of the giant Jurassic (Karoo) Okavango dykeswarm (ODS) of northern Botswana and the related sills andflood basalts is important for constraining the geotectonic

⁎ Corresponding author.E-mail address: [email protected] (C. Aubourg).

0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2007.10.013

models related to this LIP. The ODS has been interpreted as afailed branch of the triple junction centred on the proposedNuanetsi mantle plume (Reeves, 1978; Uken and Watkeys,1997; Elliot and Fleming, 2000). Preliminary magnetic fabricdata are provided by ErErnst and Duncan (1995) in the ODS.These authors collected 440 samples from 50 km to 400 kmaway from the Nuanetsi triple junction. Basing their analysis onthe correspondence of magnetic lineation to magma flow, theysuggested that vertical flow occurred close (b∼300 Km) to thetriple point and lateral flow further from this source. However, itwill be demonstrated that the magnetic lineation is not a reliablemagma flow indicator.

The objective of this paper is to present additional magneticfabric data (386 samples, 23 dykes) in the ODS. Our work addsto the preliminary work of Ernst and Duncan (1995) because 1)we principally used the imbrication of magnetic foliation alongthe dyke margins to better constrain the magma flow orientation(Geoffroy et al., 2002) and 2) we provide the actual sense offlow.

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http://dx.doi.org/10.1016/j.jvolgeores.2007.10.013

248 C. Aubourg et al. / Journal of Volcanology and Geothermal Research 170 (2008) 247–261

2. The Okavango dyke swarm

A number of dyke swarms (Okavango, Orange River, OlifantsRiver, Lebombo) converge at Nuanetsi in southern Zimbabwe(Fig. 1a). The ODS is ∼2000 km long and 110 km wide, formedby N110°-trending dykes that are easily visible on the aero-magnetic map (Fig. 1b). They form a giant linear structure acrossBotswana (Le Gall et al., 2002). It was previously interpreted as afailed branch of a rift triple junction centred over Nuanetsi(Bristow and Cox, 1984; White and McKenzie, 1989). Since itsdiscovery in 1967 (Crockett, 1968), Reeves (Reeves, 1978) relatedit to a plume assumed to be located beneath Nuanetsi. This authorsuggested that the lateral propagation of the dykes was controlledby the release of stress between adjacent rigid plates rather thanbeing driven by magmatic pressure caused by a mantle plume.Recent dating of the ODS shows that it was emplaced between178–181 Ma (Elburg and Goldberg, 2000; Le Gall et al., 2002;Jourdan et al., 2004), therefore preceding the fragmentation ofGondwana. The discovery of Proterozoic dykes (≥880 Ma)subparallel to Karoo dykes implies that the latter were emplaced ina weak zone already existing in the basement (Le Gall et al., 2005;Jourdan et al., 2006). Several recent data, including Ar/Ar dating,geochemistry and dilation directions led Le Gall et al. (Le Gallet al., 2005) and Jourdan et al. (2004, 2006) to conclude that themantle plume hypothesis could be ruled out for the Nuanetsi triplepoint.

Fig. 1. a) Karoo tectono-magmatic framework, southern Africa. 1 magmatic complexbasins. C, Chilwa; L, Lesotho; LDS, Lebombo dyke swarm; MTZ, Mozambique thinnswarm; RRS, Rooi Rand suite; SBDS, South Botswana dyke swarm; SLDS, Sabi–Ldyke swarm; ZF, Zoetfontain fault (Le Gall et al., 2002). (b) First Vertical Derivative M(c) Simplified geological cross-section X–X’ across the ODS and the Tuli basin.

The ODS cross cuts both Precambrian basement and Karoosedimentary rocks (Carney et al., 1994) and is localized betweenthe Zimbabwe (Wilson et al., 1995; Blenkinsop et al., 1997) andKaapvaal cratons (De Wit et al., 1992). Both cratons stabilizedbefore 2.5 Ga. The ODS is hosted by basement rocks (granites,gneiss and amphibolites) around the Francistown area (ShasheRiver). In the Thune River, the ODS is hosted by Permo-TriasicKaroo sedimentary and volcanic rocks. The basement, volcanicand sedimentary rocks are exposed at the same topographic level,approximately 700 m above sea-level (see cross-section Fig. 1c).The basement rocks are highly fractured, faulted and folded. TheTuli half-graben is fault bounded, which is evident in the north,and comprises a sequence of gently inward dipping Karoo lavaflows which are cut by N110°E-trending dykes of the ODS(Vail et al., 1968) around the Bobonong and Thune River areas(Fig. 1c) (Le Gall et al., 2002).

The sedimentary and volcanic rocks of the Karoo Super-group in Botswana were deposited in the Tuli intracratonicbasin from the Late Carboniferous to the Triassic (Smith, 1984).They uncomformably overlie the Precambrian basement, andare ∼1000–1500 m in thickness.

3. Sampling

We sampled a total of twenty-three dykes (386 samples)from the Okavango dyke swarm. Thirteen dykes were sampled

es with 1a, flood basalts; 1b, dykes and sills, 1c, eruptive centres; 2, sedimentaryed zone; N, Nuanetsi; ODS, Okavango dyke swarm; ORDS, Olifants River dykeimpopo dyke swarm; SleDS, South Lesotho dyke swarm; SMDS, South Malawiap of total Field Aeromagnetic Data, Southern and Eastern Africa (Sahu, 2000).

249C. Aubourg et al. / Journal of Volcanology and Geothermal Research 170 (2008) 247–261

along the Shashe River (Fig. 2a; Table 1). We collected samplesfrom an additional dyke (4B) in the Francistown Quarry locatedeast of the Shashe River. The Shashe River provides relativeeasy access to a profile almost perpendicular to the dyke swarmand traverses the entire width of the dyke swarm (Le Gall et al.,2002; Tshoso, 2003). In the eastern part of Botswana, dykeexposure is poor and the only section which could be studiedeffectively was in the Thune River. Along this section, wesampled nine dykes (Fig. 2b; Table 1).

There are numerous differences between dykes from the Shasheand Thune sections: 1) The Thune and Shashe sections are∼300 km and ∼400 km away from the triple point at Nuanetsirespectively (Fig. 1a); 2) The country rocks in the Shashe sectionare basement gneisses, while those along the Thune River areKaroo sandstones; 3) Sampling along the Shashe River covered∼50 km (Fig. 2a) while it is limited to only ∼200 m along theThune section (Fig. 2b); 4) Sampled dykes are thicker on average(1.2 m–30 m) along the Shashe section, compared to those fromthe Thune section (0.4 m–3 m) (Table 1); 5) Dykes from theShashe section are nearly vertical (Fig. 3a), whilst there is a 10° to20° southward tilt of the Thune dykes (Figs. 1d, 3b). This tilt post-dated the cooling of dykes (Le Gall et al., 2002).

We sampled using a gasoline-driven converted chainsaw,drilling cores of 2.5 cm in diameter and 5 to 10 cm long. Thesecores were oriented in the field using a magnetic compass, whichwas checked for magnetic deviation by taking measurements at

Fig. 2. Location of analysed AMS dykes along: (a) the Shashe section (b) t

∼30 cm from the dyke. Where possible, 6 cores were drilled closeto the contact between the dyke and country rocks as proposed byTauxe et al. (Tauxe et al., 1998). On average, we drilled 13 orientedcores per dyke (Table 1). We sampled two profiles across dyke 9B(1.2 m wide, Shashe section) and dyke 23B (3 m wide, Thunesection). Dyke 5B was sampled in several places (Shashe section)in order to check the consistency of magma flow direction(Table 2). Across the Shashe River, 6 dykes out of 14 show wellexposed dyke margins in contact with the country rocks. Thecontact between dyke margins and country rocks is poorlyconstrained in 8 dykes. It was found that plagioclase phenocrystswere sometimes oriented parallel to the dyke plane. Despite thismacroscopic evidence of the magmatic foliation, their 3Dorientations are more difficult to evaluate. The imbrication ofphenocrysts along the margins was never observed.

Across the Thune River site, the dyke margins are wellexposed (Fig. 3b). The dykes are b1 meter apart and b3 m wide.A 20 m gabbro exposure is situated approximately in the middleof the Thune section. Substantial heating of the country rockand dykes indicate that this small intrusive body post-dated theemplacement of the dykes (Le Gall et al., 2002).

4. Magnetic fabric

Since the seminal study of Elwood (1978), magnetic fabricsare now of common use to infer magma flow directions in dykes

he Thune section. T: Tonotha; B: Bobonong; Th: Thune River section.

Table 1Tabulated dyke parameters for the Shashe and Thune river sampling areas

Dyke Field ID No. Age(Ma)

Width(m)

Orientation Latitude Longitude

Shashe River1B BOT02 12 0.5 N100° 21°00V05.0q 27°20V54.7q2B BOT03 16 179.6 20.0 N110° 21°00V05.0q 27°20V54.7q3B BOT110 13 2.0 N95° 21°00V05.0q 27°20V54.7q4B BOT19 16 173.2 1.4 N100°–N80° 21°16V48.5q 27°30V90.2q5B BOT20 34 176.3 20.0 N100° 21°22V17.0q 27°25V42.4q6B BOT44 18 20.0 N110° 21°25V20.0q 27°27V42.3q7B BOT45 15 10.0 N80° 21°25V20.0q 27°27V42.3q8B BOT46 18 20.0 N100° 21°25V20.0q 27°27V42.3q9B BOT54 45 1.2 N95° 21°22V17.0q 27°25V42.4q10B BOT55 12 178.8 3.0 N60° 21°22V21.3q 27°25V31.6q11B BOT59 13 30.0 N100° 21°03V34.9q 27°21V35.2q12B BOT60 17 30.0 N100° 21°03V32.7q 27°21V26.5q13B BOT88 8 14.0 N100° 21°09V08.3q 27°21V08.06q14B BOT89 19 23.5 N100° 21°09V08.3q 27°21V08.06q

Thune River15B 1BOT2 14 3.0 N115°–60S 22°11V220q 28°40V542q16B 2BOT2 11 1.0 N90°–85S17B 3BOT2 12 1.7 N95°–80S18B 4BOT2 13 2.0 N105°–80S19B 5BOT2 12 0.4 N100°–80S20B 6BOT2 12 0.5 N70°21B 7BOT2 12 1.8 N110°–85S22B 8BOT2 12 3.0 N100°–75N23B 9BOT2 19 2.0 N100°–80S

Field identification numbers are shown with the dyke numbering system used. No. refers to the number of samples drilled for each dyke sampled. Ages shown are Ar/Ar ages (Tshoso, 2003). Finally, the dyke widths and azimuth orientations are provided, as well as the geographic location of the dykes. Note that in places, the localmargin orientation was found to be different to that of the dyke as a whole.


(Knight and Walker, 1988; Rochette et al., 1991; Ernst andBaragar, 1992; Staudigel et al., 1992; Varga et al., 1998). In maficigneous rocks, Fe-bearing minerals, especially primary ilmenite-magnetite solid solutions, control the magnetic fabric. These

Fig. 3. Characteristics of ODS dykes. a) Broken bridge of basement block from dykNote the tilt to the South of the dyke and the sampling scheme, relative to the marg

grains are the last to crystallise and it is generally accepted thattheir orientation directly or indirectly mimics the shapeorientation of plagioclase (Borradaile, 1988; Hargraves et al.,1991). Krasa and Herrero-Bervera (2005) showed that

e 5B, a feature indicative of vertical flow. (b) Dyke 16B along the Thune River.in. Country rocks are Karoo sandstones.

Table 2AMS results after tensorial analysis. We distinguish south and north margins, as well as the center of a dyke, when a profile is performed

K1 K3

Dyke Margins n Km (10−3) K1 K3 E1 E2 E1 E2 P′ (10−3 sd) T (10−2 sd)

1B North 6 32.0 258/3 139/84 16 5 49 9 1.007(3) −0.56(23)South 6 29.0 292/10 173/70 31 11 22 3 1.006(2) −0.08(32)

2B North 7 22.5 273/31 11/12 48 26 34 14 1.012(11) 0.41(25)South 7 29.3 305/22 35/1 33 7 17 10 1.015(3) 0.22(46)

3B North 7 30.7 46/16 138/7 40 24 39 18 1.007(7) 0.40(33)South 6 27.8 353/39 125/39 77 17 66 52 1.004(5) 0.25(33)

4B North 10 46.5 177/5 267/3 18 11 36 17 1.022(25) −0.35(49)South 13 53.8 192/24 286/8 26 13 21 15 1.015(10) −0.02(38)

5B1 North 6 38.7 282/39 162/32 48 18 48 21 1.010(10) 0.40(13)South 5 37.6 171/75 342/14 24 9 31 20 1.024(10) 0.08(46)

5B2 North 4 38.4 349/81 189/8 64 2 64 12 1.018(7) 0.42(23)5B3 South 6 45.8 139/45 12//31 78 17 34 14 1.015(8) 0.59(26)5B4 South 6 39.4 160/19 51/44 61 23 57 40 1.005(5) −0.06(44)6B North 6 21.3 206/6 304/52 65 20 62 48 1.004(9) 0.16(47)

Center 5 14.9 100/41 331/36 17 6 18 6 1.016(5) 0.28(24)South 10 25.0 276/50 31/20 64 14 34 11 1.006(6) 0.72(43)

7B North 7 16.4 16/7 273/62 45 12 19 13 1.006(4) 0.36(43)South 8 19.3 44/31 178/49 24 14 45 12 1.005(5) 0.32(46)

8B North 7 31.0 241/58 350/12 63 33 38 22 1.008(6) 0.46(44)Center 4 30.9 333/66 89/11 59 18 62 7 1.005(2) −0.27(22)South 7 34.2 186/7 309/77 42 23 25 13 1.005(8) 0.33(47)

9B1 North 7 21.6 142/7 22/75 54 16 33 18 1.027(32) −0.08(44)South 5 20.6 293/9 35/55 42 6 74 21 1.019(12) −0.49(44)

9B2 North 7 22.3 339/2 69/1 15 10 15 9 1.071(17) −0.467(200)South 6 11.9 311/10 209/48 62 8 66 40 1.006(18) −0.43(59)

9B3 transect 19 30.3 344/0 77/81 9 7 43 7 1.035(30) −0.72(39)10B North 6 30.4 332/27 224/32 55 15 30 14 1.037(8) 0.13(39)

South 6 26.8 296/24 199/15 42 22 23 13 1.031(14) 0.01(43)South ⁎ 12 28.2 92/0 2/9 22 16 27 19 1.02(18) 0.02(26)

11B North 7 31.0 228/59 333/9 24 10 15 9 1.013(8) 0.20(57)South 4 46.2 301/62 207/2 52 16 84 46 1.007(4) −0.56(31)

12B North 10 30.7 70/21 187/50 26 14 25 13 1.007(4) 0.15(30)South 7 35.0 231/34 354/39 66 38 73 36 1.001(1) −0.51(39)

13B North 3 26.9 42/70 203/19 76 35 66 27 1.020(1) 0.35(13)South 5 18.5 237/58 352/15 55 4 23 3 1.011(3) 0.61(28)

14B North 6 22.5 285/15 24/29 52 37 66 40 1.005(2) −0.17(41)Center 6 41.1 277/21 179/20 14 7 27 8 1.007(2) 0.16(31)South 7 34.8 358/81 206/8 9 8 26 8 1.008(1) −0.20(20)

15B North 7 24.1 292/1 201/64 33 5 12 4 1.030(6) 0.78(14)South 7 30.4 15/26 196/64 22 3 18 3 1.043(18) 0.48(20)

16B North 5 22.2 110/61 258/25 48 15 38 20 1.006(2) −0.30(39)South 5 12.0 3/11 269/21 26 16 47 10 1.005(3) −0.11(30)

17B North 7 28.2 254/55 5/14 15 6 37 8 1.019(4) −0.24(28)South 5 22.1 239/78 120/6 26 22 61 23 1.013(4) −0.62(48)

18B North 6 25.8 146/84 19/4 22 14 45 17 1.010(7) −0.44(20)South 6 14.5 11/28 214/60 23 8 53 11 1.010(11) −0.36(18)

19B North 6 12.9 167/85 279/2 22 4 7 3 1.010(4) 0.16(34)South 6 19.8 50/80 263/9 65 9 19 12 1.005(2) 0.58(45)

20B North 6 11.6 213/76 211/2 13 7 29 6 1.016(5) −0.09(42)South 6 16.2 246/73 27/13 39 12 70 19 1.012(8) −0.85(27)

21B North 6 19.1 327/56 132/33 38 23 30 15 1.005(3) 0.38(52)South 6 22.2 291/72 107/18 19 7 58 6 1.007(5) −0.61(54)

22B North 6 15.7 294/57 173/19 17 11 52 11 1.014(9) −0.61(36)South 5 25.6 140/68 18/12 67 20 63 40 1.005(8) 0.66(48)

23B North 10 32.3 60/21 252/69 44 35 35 26 1.010(14) 0.24(42)Center 6 47.8 266/64 56/22 39 10 25 6 1.013(8) −0.21(35)South 9 53.9 2/20 124/56 41 12 40 15 1.011(8) −0.13(38)

N: number of samples measured. Km: mean magnetic susceptibility Km=(K1+K2+K3)/3. Units: SI. Declination/inclination of K1 and K3 AMS axes. E1 and E2:Half-confidence angles at 95% level for K1 and K3 provided by tensorial analysis. Corrected degree of anisotropy P′ and its standard deviation at 95%.

P0 ¼ exp

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 g1 � gmð Þ2þ g2 � gmð Þ2þ g3 � gmð Þ2h ir� �

; gi ¼ 1nKi; gm ¼ g1 þ g2 þ g3ð Þ=3:

Shape anisotropy parameter T and its standard deviation at 95°.

T ¼ 2 g1 � g2ð Þ= g2 � g3ð Þ � 1:



subsequent forms of alteration do not affect the orientation of themagnetic fabric axes. Several studies have shown that magneticlineation (K1) provides information on the magma flow direction(Rochette et al., 1991; Ernst and Baragar, 1992; Varga et al.,1998). One major breakthrough came from (Knight and Walker,1988) who proposed that the study of the slight imbrication ofmagnetic lineations along the dyke margins provided the sense ofmagma flow (Tauxe et al., 1998). However, several authorswarned against the sole use of magnetic lineation (Aubourg et al.,2002; Geoffroy et al., 2002; Callot and Guichet, 2003). There areseveral complications arising from the interpretation of themagnetic lineation (Geoffroy et al., 2002). Firstly, the magneticlineation can be parallel or perpendicular to magma flow (Knightand Walker, 1988; Dragoni et al., 1997). Secondly, inversemagnetic fabrics are common in dolerite dykes. Interchange ofmagnetic fabric axes effectively renders the meaning of themagnetic lineation useless (Rochette et al., 1999) ; Thirdly, anapparent magnetic lineation can result from the intersection ofmagnetic foliations (Callot and Guichet, 2003).

Geoffroy et al., 2002 considered the imbrication of magneticfoliation with respect to the dyke plane, and found that thegeometry of the magnetic foliation is better constrained than themagnetic lineation because it must be parallel or slightly obliqueto the dyke plane (Rochette et al., 1991). It is this concept ofimbricated magnetic foliations which we use to infer the senseof magma flow in the ODS.

We measure the magnetic fabric using the anisotropy of lowfield magnetic susceptibility (AMS). We measured the principalaxes K1 (maximum), K2 (intermediate) and K3 (minimum)using the Kappabridge KLY-3 spinner (Agico, Brno). AverageAMS tensors were obtained using the tensorial statisticalapproach (Jelinek, 1978). The scalar AMS parametersexpressed are the mean magnetic susceptibility (Km), theshape parameter (T) and the corrected degree of anisotropy (P′)(see Table 2 for definition). The magnetic susceptibility isproportional to the concentration of ferromagnetic grains whenits magnitude is larger than 10−3 SI (Rochette et al., 1991). Inmafic dykes, Callot and Guichet (2003) compiled AMS dataand found that the shape parameter is generally positive,indicating oblate shape (1NTN0) and that the degree ofanisotropy is often weak (P′b1.1). We plot AMS data ingeographic coordinates on an equal area stereonet. Samplesfrom the northern and southern dyke margins are distinguishedfor the purposes of interpretation. The pole of the magneticfoliation is defined by the grouping of K3 axes. The magneticlineation is defined by the grouping of K1 axes.

To get additional information regarding theAMS at the scale ofthe dyke swarm, the AMS data is plotted in dyke coordinates(Rochette et al., 1991). This approach involves 1) rotating the dyketo the vertical around a strike parallel pole; 2) rotating the dyke (ordyke margin) to an arbitrary North direction about a vertical pole.The normal and inverse fabrics are plotted in dyke coordinates asdensity diagram according to Callot et al. (2001).

Several samples yielding inverse fabric (Rochette et al.,1999) were subjected to an alternating demagnetisation field(AF) at 100 mT to clean the natural remanent magnetization(NRM). The anisotropy of anhysteretic remanent magnetization

(AARM) was also measured according to the method describedby Aubourg and Robion (2002). Twenty oriented thin sectionsfrom dykes 1B and 9B (Shashe section) were cut in the planesK1–K2, K1–K3 and K2–K3 (AMS planes before AFdemagnetization). Photomicrographs from these dykes (23 for1B and 22 for 9B) were used for analysis of plagioclaseorientation using the “INTERCEPT” program (Launeau andRobin, 1996).

5. Results

5.1. Petrography and magnetic mineralogy

ODS dykes along the Shashe River are mainly coarse-graineddolerites displaying interstitial/intergranular texture. However,one dyke is a fine-grained rhyolite (13B, Shashe section) and hasmore reflective Fe–Ti oxides which might be hematite-richopaque minerals. The mafic dyke 6B (Shashe section) containsplagioclase megacrysts and several dolerite dykes host plagio-clase phenocrysts aligned parallel to the dyke margins (e.g. 10Bdyke; Shashe section). Imbrication of the phenocrysts along themargin of these dykes was never observed. The dominantminerals in the dykes are plagioclase (∼35–45 vol.%),clinopyroxene (∼20–35%), Fe–Ti oxides (b5% up to 10%maximum), glass, with or without olivine. Plagioclase andclinopyroxene form glomeroporphyritic clusters in some dykes.The groundmass contains fresh glass or cryptocrystallinematerial,feathery augite and skeletal plagioclase and Ti-magnetite. Theopaque minerals are titanomagnetite and locally pyrite (e.g. indyke 10B). They are often dendritic or cryptocrystalline grainsand are therefore inferred to represent late crystallising mineralphases. Fresh glass preserved in some samples (e.g. 2B; Shashesection) indicates that these dykes crystallized at shallow depth.Fe-Ti oxides in the mafic dyke 11B (Shashe section) are notdendritic. They consist of titanomagnetite cores rimmed bymagnetite containing ilmenite exsolution lamellae.

The mean magnetic susceptibility of all samples ranges from10−2 to 7.10−2 SI (Table 2). Histograms of Km (Fig. 4a) displaysimilar distribution in the Shashe and Thune sections. However,a few samples from dykes along the Thune section display alower magnetic susceptibility. The high magnitude of Kmvalues (N10−3 SI) suggest that ferromagnetic minerals are theprincipal magnetic carriers. We carried out thermomagneticcurves of low field magnetic susceptibility on four samples fromthe dyke margins from the Shashe section using a CS2 coupledwith KLY2 (Agico apparatus) (Fig. 4b). Variable magneticassemblages were observed. A Curie temperature is observedbetween 300 °C and 350 °C for dykes 1B, 4B and 9B. ThisCurie temperature is consistent with maghemite or ironsulphides (pyrrhotite). Interestingly, all samples displayingthis Curie temperature have inverse magnetic fabric. A secondset of Curie temperatures between 500° to 580 °C is observedfor dykes 1B, 4B and 5B. This is consistent with the occurrenceof Ti-poor magnetite. It is noteworthy that a sample from dyke5B, which has only magnetite, also has normal fabric. Theabsence of decay in K–T curves between room temperature and∼300 °C show that paramagnetic susceptibility is negligible. A

Fig. 4. Rock magnetism results. a) Histograms of mean magnetic susceptibility (Km) from the Shashe and Thune Rivers sections. (b) Thermomagnetic curves underargon atmosphere for representative samples from dykes 1B, 4B, 5B and 9B. All samples are from the margins. We specify the type of magnetic fabric (normal orinverse). We also indicate the hysteresis ratio Hcr/Hc and Mrs/Ms.


study of hysteresis loops was carried out on four samples(results are not shown). The low magnitude of high fieldmagnetic susceptibility also indicates a weak contribution ofparamagnetic susceptibility. Hysteresis ratios (Mrs/Mr ∼0.16

and Hcr/Hc ∼2.20) put the titanomagnetites in the pseudo-single domain (PSD) range. For samples which carry inversemagnetic fabric, it was noted that no hysteresis parameters fallin the single domain grain (SD) range.

Fig. 5. Representative magnetic fabrics for dykes exposed along Shashe and Thune sections. AMS K1 (squares) and K3 (circles) axes are plotted in geographiccoordinates in equal area stereographic projections (lower hemisphere). Filled symbols are the mean tensor within ellipses of statistical confidence. Black and greycorrespond to the northern and southern margins respectively. Also shown are AMS scalar data (corrected degree of anisotropy P' vs. shape parameter T).

Fig. 6. AMS before and after AF demagnetisation at 100 mT. a) dyke 1B (blacksymbols: southern margin; grey: northern margin) b) dyke 19B (both marginsare shown without colour indication). Legend as per Fig. 5.


5.2. Magnetic fabric data

5.2.1. General characteristicsIn this study, we define a normal magnetic fabric when its

magnetic foliation is close to the dyke plane, at an angle lowerthan ∼40°. The threshold value at 40° is chosen because it isgenerally observed that imbricated magnetic foliations reachangles up to ca. 30° (Knight and Walker, 1988; Tauxe et al.,1998; Varga et al., 1998; Moreira et al., 1999; Callot et al., 2001;Correa-Gomes et al., 2001; Herrero-Bervera et al., 2001;Aubourg et al., 2002). Conversely, inverse magnetic fabricsare characterised by a magnetic foliation strongly oblique(N40°) to the dyke plane and generally perpendicular to it. Dyke2B is a good example of a normal fabric at the scale of the dyke(Fig. 5a). However, a few samples display inverse magneticfabric in the southern margin of this dyke. The magneticfoliations are imbricated with respect to each other, and alsowith respect to the dyke. Angles of imbrication are ∼10° and∼20° for the north and south margins respectively. A typicalexample of inverse magnetic fabric is observed at site 15B(Fig. 5b). Here, the pole of magnetic foliation is containedwithin the dyke plane. Dyke 12B displays a mixture of normaland inverse magnetic fabrics (Fig. 5c). AMS along the northernmargin of dyke 12B is well defined and oblique to the dyke by∼40°. By contrast, AMS from its southern margin is scattered,and not interpretable. On average, the shape of AMS is triaxial(T ∼0,Fig. 5a,c). Some notable exceptions exist such as inversemagnetic fabric at dyke 15B which has an oblate shape(Fig. 5b). The magnitude of the degree of anisotropy (P′) is low(P′b1.05) for both sections (Table 2). These low values arecomparable with values reported in basaltic rocks.

An overall analysis of samples shows that 46% of sampleshave normal magnetic fabric. The remaining 54% has inverse orscattered magnetic fabric. When comparing the Shashe and

Thune sections, it appears that the ratio of normal magneticfabric is higher (54%) for Shashe dykes than their homologuesfrom Thune (32%). The AMS pattern across two profiles at 9B(1.2 m thick) and 23B (3 m thick) provide the following results:(1) for the dyke 9B, the magnetic fabric is inverse throughout.(2) For the dyke 23B, there are inverse and normal fabricsrespectively in the margins and the center. We observe in several


dykes that the magnetic fabric of samples close to the margin(b20 cm) is often scattered.

Following Park et al. (1988) who reported substantialchanges in AMS after AF demagnetisation, selected sampleswere submitted to 100 mT peak AF magnetic field. We reportnoticeable differences for the dykes 1B and 19B. Before AFtreatment, AMS data of the dyke 1B yields an inverse magneticfabric (Fig. 6a). The initial magnitude of NRM is particularlyhigh (∼20 A/m) compared to the other dykes studied (∼A/m).After AF demagnetisation, the NRM lost about one order ofmagnitude. AMS also changes and becomes normal. K3 axesflip to horizontal while K1 remains more or less in the same

Fig. 7. Microtextural analysis of plagioclases from thin section observation. Left: prusing the method of Launeau and Robin method (Launeau and Robin, 1996). (a) and (bfor comparison with the long axis of the intercept roses.

position (Fig. 6a). Scalar data remains comparable before andafter AF treatment. At dyke 19B, the magnetic foliations areinverse (Fig. 6b). The magnitude of NRM is ∼1 A/m and dropsby one order of magnitude after AF demagnetisation at 100 mT.The magnetic fabric remains inverse after AF demagnetisation,but the magnetic lineation groups better and the magneticfoliation rotates by about 90°. The comparison of AMS beforeand after AF demagnetisation suggests that inverse magneticfabric recorded in dykes 1B and 19B originates from complexrock magnetism interaction, including shape and domainanisotropies (Park et al., 1988). AF demagnetisation tests failedfor other dykes with abnormal AMS (dykes 9B and 15B). We

ocessed inverted image (plagioclase are in black). Right: roses of INTERCEPT): sample from dyke 1B. (c) and (d): sample from dyke 9B. AMS axes are shown


have also tested the measurement of the anisotropy ofanhysteretic remanent magnetization (AARM) for inversesamples of dyke 19B. Similar orientations of the AMS andAARM axes were found. This implies that inverse fabrics arenot solely due to SD grains (Rochette et al., 1999).

Petrofabric analysis of plagioclase was undertaken to test ifthe inverse magnetic fabric reflects a true preferred orientationof grains. Dykes 1B and 9B were chosen because of theircomplex magnetic fabric behaviour. Thin sections were cut inthe orientation of the AMS axes in three orthogonal planesbefore AF treatment. 60% of plagioclase grains from the dyke1B are parallel to K1 (14±4°) within the plane K1–K2(Fig. 7a). Within the vertical plane K2–K3, it was found thatthe long axes of plagioclase grains were parallel to K3 by anangle of 71±27° (Fig. 7b). These data confirm that thehorizontal magnetic foliation obtained on the dyke 1B is anartefact, and that AMS after AF demagnetisation (K3interchanges with K2) is consistent with the plagioclaseorientation. The long axes of plagioclase grains in the dyke 9Bare mostly parallel to K1 (20±15°) in the plane K1-K2, andparallel to K2 (15±12°) in the plane K2–K3. This results ingood agreement between AMS and plagioclase fabrics.Abnormal magnetic fabric for the dyke 9B possibly resultsfrom a complex arrangement of grains, either flow related

Fig. 8. Magma flow around the Shashe Dam site (dykes 5B and 10B). AMS axes areimbrication of magnetic foliation. Note the consistency of magma flow direction in tlateral and opposite (east and west) in the western part. Legend as per Fig. 5.

(Dragoni et al., 1997) or due to superimposition of severalfabrics imprinted at different times.

Therefore, AF demagnetisation and petrographic inspectiondemonstrates that inverse magnetic fabric can have two origins:1) inverse behaviour of strongly magnetized samples; 2) planarpreferred orientation of ferromagnetic grains perpendicular tothe magmatic foliation.

5.2.2. Flow directionTo determine the flow direction, we consider the imbrication

of magnetic foliation of normal fabric. We also check that theplunge of magnetic lineation is consistent with flow direction.Key localities in which several dykes have been sampled arepresented first.

We sampled two dykes (5B and 10B) from the Shashe Damsite (Fig. 8). At dyke 5B, four margins have been studied.Margins 5B1 and 5B2 display normal magnetic fabrics onaverage. The strike of the magnetic foliation follows the changeof dyke azimuth. The imbrication of the mean magneticfoliation is consistent with an upward magmatic flow. Thesteep magnetic lineations are also consistent with vertical flow.Field inspection of a sub-horizontal surface along the southernmargin of dyke 5B reveals that mafic magma percolated into thecountry rock. This percolation can be interpreted as evidence of

in geographic coordinates. The star indicates the sense of flow inferred from thehe eastern part of the Shashe dam site (upward, steep) while flow directions are


vertical flow. The mean magnetic foliation of margin 5B3,which is ∼100 m from margins 5B1 and 5B2, suggestswestward lateral flow. This is at variance with the fieldobservation of a broken bridge that suggests vertical flow(Figs. 3a and 8). The magnetic fabric between the block and themargin (site 5B4) is scattered (Fig. 8). At dyke 10B, the twomargins display an imbrication of the mean magnetic foliationconsistent with lateral flow to the east (Fig. 8). The weak plungeof the magnetic lineation also supports lateral flow. Note thatmagnetic fabric of samples close to the margin (b20 cm) arescattered.

Around Shashe–Mooke, dyke 13B intrudes dyke 14B. Theimbrication of mean magnetic foliation of both margins of dyke13B gives a flow direction towards the east. In contrast, thesouthern part of dyke 14B suggests a flow towards the west.Dyke 13B is felsic whilst dyke 14B is mafic. This difference incomposition might indicate a difference in emplacement time,and/or different magma source, a plausible explanation for thedifference in flow directions.

At Borolong (Fig. 9), the dyke 1B (∼0.5 m thick) intrudesthe dyke 2B (∼20 m thick), and an additional dyke (3B)associated with 2B was also sampled. Dykes 1B and 2B displaywell imbricated AMS magnetic foliations (after AF demagne-tisation for dyke 1B), both giving a lateral flow towards the west

Fig. 9. Magma flow around the Borolong site (dykes 1B, 2B, 3B). For dykes 1B andshown after AF treatment. Only the northern margin of dyke 3B can be used to infe

(Fig. 9). For dyke 3B, only AMS from the northern margin canbe used to determine the magma flow sense (Fig. 11). Theimbrication of the mean magnetic foliation suggests a lateralflow to the west. Note that the plunge of the magnetic lineationsis low, consistent with lateral flow for all dykes. Therefore,lateral magma flow to the west is consistently observed for theBorolong dykes.

The magnetic fabric in the Thune section is poorly definedwith the majority of dykes giving an inverse fabric (Table 2).Although poorly defined, the imbrication of the magneticfoliation in four margins suggests lateral flow to the west. Dyke22B is a good example of two margins that are consistent withlateral flow to the west (Fig. 10a). The magnetic lineations aresteeply dipping in 12 of the 16 sampled margins. Steepmagnetic lineations contrast with shallow magnetic lineationsobserved in the Shashe section. The origin of this magneticlineation is debatable. Henry (1997) proposed that it isnecessary to check the relationship between the intersection ofmagnetic foliations and the magnetic lineation. If the correlationis positive, then the magnetic lineation can be simply the resultof the intersection between magnetic foliations (Callot andGuichet, 2003). At dyke 18B, the magnetic lineation is welldefined and parallel to the intersection of the magnetic foliations(Fig. 10b). This is also true for the magnetic lineation of the

2B: black symbols: southern margin; grey: northern margin. AMS of dyke 1B isr the magma flow direction. Legend as per Fig. 9.

Fig. 10. a) Magnetic fabric of dyke 22B. Note the imbricated magnetic foliationon the northern margin. Great circles are the mean magnetic foliations.b) Magnetic fabric of dyke 18B. In both margins, the magnetic lineation isparallel to the intersection of magnetic foliations. Legend as per Fig. 5.


northern margin at dyke 22B (Fig. 10a). Six margins exhibitpositive correlation between magnetic lineations and magneticfoliations.

6. Discussion

Of the 26 dyke margins sampled in the Shashe section,, 13reveal an AMS fabric which is consistent with lateral flow, and 3margins are consistent with vertical flow. 5 dykes exhibit mirrorgeometry of imbricated magnetic foliation on both margins.Lateral flow to the west is dominant (10 out of 13 margins). Theremaining vertical flow is upward. In the Thune section, only 4margins show imbricated magnetic foliations consistent withlateral flow to the west. The plunge of magnetic lineations isgenerally in agreement with the flow direction in the Shashesection, whilst magnetic lineations are mostly parallel to theintersection between magnetic foliations in the Thune section.

To get an overview of the general characteristics of themagnetic fabric, AMS axes are plotted in density diagrams afterthe separation of normal and inverse magnetic fabrics using theobliquity angle of 40° (Fig. 11). The northern and southernmargins are also distinguished. When considering the normalmagnetic fabrics, the differences between Shashe and Thune

Fig. 11. Normal and inverse magnetic fabrics from the Shashe and Thune sectionsNorth). Density diagrams are plotted in the lower hemisphere (1% level). Dashed greaflow inferred from these diagrams according to the imbrication. Note that normal apattern when exchanging K1 and K3. The dominant sense of flow is shown for the

sections are evident particularly for the plunge of magneticlineations: shallow in the Shashe section, and steep in the Thunesection. In the Shashe section, the girdle represented by themagnetic lineations also illustrates the main imbrication of themagnetic foliations (∼15°). The main imbrication suggests apredominance of lateral flow to the west. In the Thune section,only the imbrication of the northern margin makes a consistentpattern, also indicating a lateral flow to the west.

The origin and uses of inverse magnetic fabric is nowconsidered. It is likely that the inverse fabric in this study hasseveral origins. The strong magnetization (several A/m) leadsapparently to the interchange of AMS axes (Fig. 6). One possibleexplanation is that small multi-domain grains, while saturated,behave as ‘SD’ grains. However, it would also seem that thetexture of grains is ‘inverse’ as shown by petrography (Fig. 7)and AARM. Dragoni et al. (1997) suggest that ‘inverse’ fabricmay originate from flow textures. Besides these considerations,can we use K1 axes from the inverse magnetic fabrics as a pole of‘restored’ magnetic foliation? In the Shashe section, it is evidentthat the inverse K1 axes from the northern margins (Fig. 11) arein symmetry with normal K3. The interpretation of inverse K1 ofthe southern margins is less clear. Interestingly, there is also amirror geometry between inverse K3 and normal K1. Insummary, a lateral flow to the west is evident from the inversemagnetic fabric. In Thune section, the interpretation of inversemagnetic fabrics is not clear and no dominant flow direction canbe inferred.

The mirror image of normal and inverse magnetic fabric atShashe is interesting because it confirms that inverse fabric can beused in regional study, or even at the dyke scale. Often, inversemagnetic fabrics, although often representing about fifty percent ofthe data, are simply neglected. To explain the poor quality of AMSdata in Thune section, we propose a late thermal complicationowing to the proximity of a gabbroic magma chamber.

It is interesting to compare our data to the preliminary AMSresults of Ernst and Duncan (1995). These authors collected 440samples from the ODS in southern Zimbabwe (50 km fromNuanetsi) up to Botswana (400 km from Nuanetsi). Close toNuanetsi (where we do not have data), Ernst and Duncan (1995)reported steep magnetic lineations and interpreted this as beingconsistent with vertical flow. However, it is our view that thisinterpretation should be confirmed by an analysis of imbricatedmagnetic foliations. At a distance comparable to that of theThune section from Nuanetsi, they mostly observed compactionmagnetic fabric. A compaction fabric is characterized by a sub-horizontal magnetic foliation, which is also inverse according toour criteria. The magnetic fabric from Thune section isessentially inverse and is therefore comparable to Ernst andDuncan's results. In the Shashe area, they observed a shallowlyplunging magnetic lineation and interpreted this as an indicationof lateral flow. Although the sole use of the magnetic lineationin determining magma flow should be done with discretion,

plotted in the dyke coordinate system (all dykes are vertical and are parallel tot circles illustrate the dominant imbrication. Arrows are used to indicate the mainnd inverse magnetic fabrics from Shashe section display the same imbricationShashe and Thune River sections, as discussed in the text.



their interpretation of lateral flow is consistent with our resultsfrom the Shashe section.

Previous authors have suggested that the ODS represents abranch extending from the triple junction over the proposedNuanetsi mantle plume (Burke and Dewey, 1973; Reeves, 1978;Uken and Watkeys, 1997). These authors suggested that thisdyke swarm propagated westwards basing their arguments onits westwards narrowing. The predominance of lateral flow tothe west documented in this paper is compatible with thisinterpretation and it is therefore possible that the ODS was fedfrom a magma source in the Nuanetsi region.

7. Conclusion

The AMS investigation of 374 samples from 23 Jurassic(179 Ma) dykes, in the Okavango dyke swarm reveals thefollowing characteristics:

1. Titanomagnetites, maghemite, and iron sulphides (?) are themagnetic carriers of magnetic susceptibility in the dykes.

2. Normal (46%) and inverse (54%) magnetic fabric are equallyrepresented. However, the thin (b3 m) dykes emplaced inKaroo sedimentary rocks display a larger proportion ofinverse magnetic fabric (68%) compared to larger dykesemplaced in basement rocks (46%).

3. Some inverse magnetic fabric is significantly reduced whensamples are subjected to AF demagnetization. This suggestsan effect of domain anisotropy, imposed by strong naturalremanent magnetization.

4. Some inverse magnetic fabric reflects a true preferredorientation as supported by the preferred orientation ofplagioclase grains.

5. Magnetic lineations are predominantly horizontal in thickdykes from the Shashe section while they are dominantlyvertical in thin dykes from the Thune section. A correlationbetween steep magnetic lineations and the intersection ofmagnetic foliations is observed in the Thune section.

6. The imbrication of magnetic foliations is observed in 16 outof 26 margins along the Shashe section. The imbricationangle is lower than 30°. Five dykes exhibit mirror geometryof imbricated foliations. In the Thune section, 4 marginsshow poorly defined imbrication.

7. Multiple injections of magma at the dyke scale are inevidence, particularly at the Shashe Dam site.

8. An overall analysis of normal and inverse magnetic fabricssupports a predominance of westward lateral flow, a resultcompatible with a magma source to the east, beneath theNuanetsi triple point.

Acknowledgements

This work is part of the PhD research of G.T. funded jointly bythe French and the Botswana Governments within the co-operative agreement between the University of Botswana(Gaborone) and the University of Western Brittany (Brest). Weacknowledge the financial support of the University of Botswana(Kaapvaal Craton Project R#442), the SUCRI 2E of the

University of Western Brittany and the University of CergyPontoise. We are grateful to C. Tonani, Head of the Cultural andCooperation Service (French Embassy in Botswana) for hissupport to this project. We thank J. Autin for analyses of theThune samples. Finally, we thank L. Geoffroy, two anonymousreviewers and W. Hastie for their very helpful comments.

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Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana