Download Conductive structures in southernmost Africa: a magnetometer array

Ceophys. J . R. astr. Soc. (1980) 63, 479-495
Conductive structures in southernmost Africa:
a magnetometer array study
J. H. de Beer National Physical Research Laboratory, Council for Scientific
and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa
D. I.
G O U g h Institute of Earth and Planetary Physics, Department of Physics,
University o f Alberta, Edmuiitoil. A l b u t a , Canndc, T6G ZJ'i
Received 1980 March 4; in original form 1979 October 1
Summary. A magnetometer array study in 1971 led to the discovery of a
zone of electrically conductive material in the crust or upper mantle,
elongated east-west and located beneath the Cape-Karroo Basin of South
Africa. In 1977 a second array study was made, with 53 magnetometers
distributed over the tip of Africa south of 30" S . T h s array recorded
magnetic storms and substorms w h c h provided signals of good amplitude in
the period range 24-1 58 min and of various polarizations. Magnetograms for
three time sequences are presented and used in locating the edges of the
conductive region. Six sets of maps of Fourier transform amplitudes and
phases, at periods for which the events supply adequate energy, show
anomalies related to currents flowing in three conductive structures. These
are the continental edge structures off the south-east and west coasts, and an
intracontinental conductive strip running approximately east-west from
coast to coast. The use of several magnetic variation events, with various
phase differences between X and Y components, proves highly effective in
isolating the intracontinental conductor from the continental edge (including
oceanic) induction effects. The former is named the Southern Cape Conductive Belt and has been well mapped from the Fourier transform maps and the
magnetograms. Anomalous fields, normalized to the horizontal field at a
station within the craton, are used to locate the induced currents where they
cross three profiles, and t o yield maximum depth estimates between 46 and
52 km.The Southern Cape Conductive Belt is thus in the crust or uppermost
mantle. It correlates closely with a static magnetic anomaly, whose source
must lie above the Curie isotherm and so no deeper than 38 km. The Conductive Belt is strongly related to geochronologic provinces, and lies outside
the edge of the Namaqua-Natal Mobile Belt which forms the edge of the
craton. Heat-flow data indicate that the Conductive Belt is probably not
thermally related. It is provisionally associated with an accumulation of
J, H. de BeerandD. I. Cough
480
oceanic crustal rocks, containing hydrated minerals such as serpentine, in
either a Proterozoic zone of subduction or a marginal sea of Proterozoic
Gondwanaland.
Introduction
Observations with a magnetometer array in 1971 discovered a zone of highly conductive
material, elongated east-west and located in the crust or upper mangle, beneath the Cape16'
180
200
220
24'
I
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'
2 o'
22O
24'
26'
26'-
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26"
HBH.
I
30"
I -
280-
30'-
32"-
34O-
36'-
16'
30'
NDlAN OCEAN
Karoo Supergroup
Cape Supergroup
Namaqua - Natal
Granite Gneiss Terrain
-
Figure 1. The location of the magnetometer array that operated in southern Africa during 1977 (top).
At the bottom the array is shown in relation to the surface geology (Haughton 1969; Scholtz 1946). The
bathymetric contours and the contours of sediment thickness in the Karroo basin are given in kilometres.
The position of the Agulhas Fracture Zone is shown schematically.
Conductive structures in southernmost Africa
481
Karroo Basin of South Africa (Cough, de Beer & van Zijl 1973). The 1971 array was
triangular, with its southern corner station on the axis of the induced currents. In 1977, 53
three-component magnetometers were deployed in an array covering the southern end of the
continent south of latitude 30" S, with the aim of mapping and investigating conductive
structures in this area. The station locations are shown in Fig. 1. Fifty-one magnetometers
were of Cough-Reitzel type (Gough & Reitzel 1967) and two were the standard magnetic
observatories at Hermanus (HER) and Grahamstown (GRH). The northern two-thirds of the
region is covered by Karroo sediments of Mesozoic age, with Palaeozoic sediments folded in
Mesozoic time to form the Cape Fold Belt to the south. The surface geology is indicated in
outline in Fig. 1. The Kaapvaal Craton (of age greater than 2600 Myr) and the NamaquaNatal Belt (of age approximately 1000 Myr) constitute the basement under the northern
half of the array. In the south-western corner of the continent, Proterozoic Malmesbury
Group rocks, intruded by Cape Granites 600 Myr old, underlie the Palaeozoic rocks of the
Cape Supergroup. The location of the boundary between the Namaqua-Natal Belt and the
Malmesbury Croup, which lies under the Cape-Karroo sediments, is uncertain within several
hundreds of kilometres. The extent of subsurface rocks of Malmesbury age is correspondingly
uncertain.
Submerged continental rocks form the Agulhas Bank off the south coast (Fig. 1). The
western continental edge of southern Africa is a rifted margin, whereas the south-eastern
continental edge is a transform faulted sheared margin (Scrutton 1973; du Plessis 1977).
Observations
The instruments were installed by two independent field parties from 1977 August 31 to
September 20. Recording commenced on 1977 September 29 and ended on December 5 .
Over the whole period, 88 per cent of the available data was recorded; over the second half,
the recording efficiency exceeded 96 per cent. The data from three selected sequences, of
durations 6, 1 4 and 3 hr, were processed in Pretoria. The analogue traces, optically enlarged
x 10, were digitized at 1-min intervals by means of a trace-follower digitizing table.
The magnetograms are reproduced in Figs 2 , 3 and 4. The 6-hr sequence 1977 November
10, 13-19 hr GMT, was chosen largely for its high-frequency content (Fig. 2). The storm
sequence of 1977 November 14,08-22 hr GMT, has considerable energy at periods of a few
hours (Fig. 3). The polar magnetic substorm between 1977 November 1 4 , 2 3 hr and November 15, 0 2 hr contained substantial energy at periods of order 1 hr. The magnetograms are
arranged in nine stacks, one for each of nine approximately north-south lines of stations
(Fig. 1).
Magnetograms from the standard observatory at Hartebeeshoek (HBH) are shown at the
top of the line 8 stack in each of Figs 2-4. The position of this observatory is shown in
Fig. 1 : it is on the Kaapvaal Craton far from local anomalies, and records the normal field
with good approximation (Cough et al. 1973). The magnetograms at HBH resemble those
from the array in X and Y , but the 2 variations are much smaller at HBH. To a reasonable
approximation, therefore, the X and Y magnetograms from the array represent an essentially
uniform normal field and the 2 fields are anomalous and related to currents induced, in
effect, by X and Y . External 2 is, as usual, nearly cancelled by normal internal 2.
The locations of the larger concentrations of induced current can be observed by
inspection of the magnetograms. Near the axis of a concentration of current running eastwest, the amplitude of the X component is enhanced, that of 2 is reduced and the phase of
2 is shifted through an angle which may reach 180" at short periods. One or more of these
indications in the magnetograms of Figs 2-4 shows the presence of an approximately east16
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Figure 2. Magnetogramsof a 6 hr event on 1977 November 10,1300-1900 hr GMT.
17
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Conductive structures in southernmost Africa
1977 N O V E M 8 ; t R
483
ILi
<, i
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0
L
NE 9
Figure 3. Magnetograms of a storm sequence that occurred on 1977 November 14,0800-2200 hr GMT.
484
ja.
H. de Beer and D.I. Gough
1977 N O V E M B E R
21
82
21
lif
RND
15
ez
Figure 4. Magnetograms of a polar magnetic substorm that occurred between 1977 November 14,2300 hr
and November 15,0200 hr.
west current concentration with its axis between PAP and SWA (line l), just south of SUT
(line 3), and near to AMA, BVL, JNV, QNT and BLY in lines 4-8. Clear reversals of Z can
be seen in the short-period variations between PAP and SWA in line 1 and between KWK and
BVL in line 5 (Figs 2 and 3). Lines 2 and 9 give no clear indication of concentrated eastwest current.
The northern edge of the conductive strip can easily be traced from the large Z ampljtudes just north of it and the attenuation of Z south of the edge and above the conductor.
Stations just north of the conductor are PAP, S A N , BEA, KWK, GFR, JAM and ELL in lines
1 and 3-8. It is more difficult to locate the southern limit of the conductor from the
magnetograms, because of the superimposed continental edge anomaly. Short-period
variations of small amplitude often convey, by inspection of the magnetograms, information
which is absent from maps of Fourier transform parameters at longer periods.
It is noteworthy that Z is attenuated, at stations above the conductive strip, over a wide
range of periods, from a few minutes to several hours. The point is evidelit when one compares the Z magnetograms at KWK, BVL and OLR in Figs 2 and 3, or those at GFR, JNV
and STV. Equally consistent is the time-shift of maximum Z deflection at SWA versus PAP,
DUT versus BOF, BLV versus KWK for the bay disturbance shown in Fig. 4. This suggests
that the currents producing the intracontinental anomaly differ in phase from the currents
associated with the continental edge anomaly.
Maps of Fourier transform parameters
Contoured maps of the amplitudes and phases of Fourier transforms of magnetograms have
been found useful in delineating and locating induced current concentrations beneath a
two-dimensional array (Reitzel et al. 1970; Camfield, Gough & Porath 1971;Gough 1973a;
Alabi, Camfield & Gough 1975). Six maps, of the amplitudes and phases of X , Y and Z ,
Conductive struchtres in southernmost Africa
485
provide a complete representation of a magnetic variations event at one frequency. Phases
are relative to an arbitrary zero time at the beginning of the analysed time interval. The best
results are generally obtained from an event of duration comparable to the longest period
which is prominent in the magnetogram: thus it is better to map transforms of sequences
2-4 hr long, from the magnetograms of Fig. 3, than to use transforms of the whole 14-hr
sequence shown. X and Y amplitude spectra of a sequence are used to select one or more
periods at which one of these components is near a maximum throughout the array. Details
of the procedure are given by Camfield et al. (1971). A variety of polarizations of the
horizontal field should be provided by the selected periods in the sequence used to maximize
induction in conductors of all orientations. These and other points are discussed in the
references cited.
Thirteen sets of maps of this kind have been drawn for spectral peaks at periods ranging
from 24 to 158 min, from Fourier transforms of five sequences selected from the magnetograms shown in Figs 2-4. The current concentrations revealed by these maps depend
strongly upon the period of the incident field and its polarization. If the normal disturbance
field has components
x=
cos wt
Y = Y, cos (wt + 8)
then the anomalous currents depend upon X,,, Y, w and 6 . To study current concentrations,
and hence conductive structure in a region, several sets of maps with different values of these
parameters are necessary.
Six sets of contoured maps of Fourier transform amplitudes and phases are presented in
Figs 5-10, for periods ranging from 24 to 158 min. The character of the maps depends
much more, however, on the polarization of the incident horizontal fields in the various
events. In Figs 5, 6 and 7 the phase differences 6 between X a n d Yare large (Table 1). In
Figs 8, 9 and 10, 6 is small, so that the polarization ellipse is narrow and has its major axis
in the north-east quadrant. The polarization ellipses represent mean Xand Y for all stations.
The maps show anomalies related to currents flowing in three conductive structures.
These are the continental edge structure off the south-east and west coasts, and the conductive strip, already described, which runs approximately east-west from coast to coast.
At the south coast the continental edge anomaly is complicated by the intracontinental
anomaly, whch is considered below.
Continental edge anomalies
The continental edge anomalies are most easily understood by considering the phase maps
first. It should be remembered that the numbers on the phase maps are related to the starting
time chosen for the data set, which is the same for the three components. Absolute values of
the phase have no significance, but phase differences within a map, and between the maps of
a set, are profoundly significant.
It has been shown, from the HBH magnetograms, that Z at the array stations is essentially
related to anomalous distribution of induced currents. In all six events (and in others not
shown), the phase of Z is close to that of Y near the west coast, and close to the phase of X
near the south-east coast, as can be seen by comparing the three phase maps in each of
Figs 5-10. Where X and Y differ strongly in phase (Figs 5-7), the 2 phase maps thus show
a marked gradient between the west and south-east coasts, so that the continental edge
anomalies appear strongly in the phase of 2. The east-west intracontinental anomaly is
J. H. de Beer and D.I. Gough
486
x
Y
x
AMP
AMP
AMP
PHASE
,,J
i
PHASE
Figure 5. Fourier transform amplitudes (arbitrary units, equal for the three components) and phases
(min) at period 82 min for a sequence 0950-1350 hr GMT on 1977 November 14.The magnetograms of
Fig. 3 include this sequence. The ellipse represents the horizontal field polarization.
superimposed. In Figs 8-10 there is no sign of Z phase continental edge anomalies, because
X and Y differ so little in phase that the Z phase changes negligibly between the edges at
which induction is by X and Y. The intracontinental anomaly therefore appears essentially
alone in t h e Z phase maps of Figs 8-10. These maps enable this current concentration to be
traced from coast to coast.
The anomaly at the south-eastern continental edge is present in all Z amplitude maps,
and the west continental edge anomaly in most of them, with complications in both cases
from the intracontinental anomaly where it reaches the continental margins.
The Southern Cape intracontinental anomaly
The anomaly is present in all six sets of maps, but is clearest in Figs 7-10 which correspond
to ellipses of polarization with major axes in the north-east quadrant.
Conductive structures in southernmost Africa
487
Figure 6. Fourier transform amplitudes (arbitrary units, equal for the three components) and phases
(min, 2 contour interval three times that of X and Y ) at period 137 min for a sequence 1700-1900 hr
GMT on 1977 November 14. The magnetograms of Fig. 3 include this sequence. The ellipse represents the
horizontal field polarization.
The X amplitude maps consistently show an elongated positive anomaly extending
through lines 4-8, with two maxima at AMA and J N V . This anomaly suggests the presence
of a concentration of east-west current beneath it. Turning to the 2 phase maps, one finds
contours closely spaced beneath the X maxima, indicating a steep gradient in phase as
expected above a current concentration. The maps of 2 amplitude show maxima to the
north of the X maxima, and minima south of them. The anomalies in X and Z amplitudes
and 2 phase are all consistent with currents concentrated in an elongated conductive
structure under the positive anomaly in X , together with offshore continental edge currents.
Between the intracontinental conductor and the south coast, the two currents produce a
minimum in Z amplitude as discussed below.
The Y amplitude maps complicate the picture by contributing a positive anomaly very
similar to that in X and of comparable magnitude, with two maxima at the same stations,
AMA and JNV. These Y anomalies require either a north-south component of current or a
vertical component. The latter would certainly produce local anomalies in Y phase. The
J. H de Beer and D. 1. Gough
488
x
AM?
Y
AM?
i
x
PHASE
_<
Y
PHASE
> Y
z
AMP
4
c
z
PHASE
Figure 7. Fourier transform amplitudes (arbitrary units) and phases (rnin) at period 158 min for a
sequence 0950-1350 hr GMT on 1977 November 14. The Z-phase contour interval is three times that for
X and Y . The magnetograms of F i g . 3 include this sequence. The ellipse represents the horizontal field
polarization .
absence of such anomalies from the Y phase maps rules out substantial vertical currents.
Bends in t h e conductor near JNV and AMA would account qualitatively for the Y amplitude
anomalies and would introduce no local phase anomaly in Y if X and Y were nearly in phase
(Figs 8-10). Between lines 3 and 4 and between lines 6 and 7 the 2 phase contours are
consistent with the presence of a north-south component of current, and the Z amplitude
anomaly terminates.
A positive anomaly in amplitudes of X and 2 is centred at PAP on the west coast, and is
associated with a large phase change, approximating a reversal, between PAP and SWA.
Contours o f the Z phase link this anomaly to the intracontinental anomaly east of line 3,
and the phase reversal suggests that much of the intracontinental current crosses the
continental margin between PAP and SWA. The anomaly patterns suggest a dispersed and
intricate current distribution through lines 2 and 3.
It is interesting t o note that a small but real Y phase anomaly follows the intracontinental
conductor in Figs 6 , 7 and 10. In these events there are considerable phase differences
Conductive structures in southernmostAfrica
Y
AM?
AMP
,-<
i
Y
489
PHASE
PHASE
i
Figure 8. Fourier transform amplitudes (arbitrary units) and phases (min) at period 24 min for a sequence
1300-1900 hr GMT on 1977 November 10. The magnetograms are shown in F i g . 2. The ellipse represents
the horizontal field polarization.
between X and Y components. In each case the phase of Y changes toward that of X above
the conductor. This is consistent with induction by X in a conductor running generally
east-west, but with north-south sections.
Profiles of anomalous field component amplitudes, normalized to the appropriate
horizontal amplitude at FRS (Fauresmith) which is on the Kaapvaal Craton, are shown in
Fig. 11 for the six sets of anomaly maps (Figs 5-10). The sections to which the profiles
refer are marked on the key map (Fig. 1) and the stations, whose fields are plotted, are
identified in Fig. 11. For the western and central sections, AA' and BB' (Fig. I), the
quantities plotted in Fig. 11 are:
X , - X(station) - X(FRS)
_
Xn
X(FRS)
and
2, - Z(station) - Z(FRS)
--
X"
X(FRS)
J. H. de Beer and D. I. Gough
490
Y
AMP
i
Y
PHASE
,
<
z
PHASE
i
4
AMP
J
Figure 9. Fourier transform amplitudes (arbitrary units) and phases (min) at period 48 min for a substorm
1977 November 14, 2300 hr-November 15, 0200 hr. The magnetograms are shown in Fig. 4. The
polarization of the horizontal field is linear.
For section CC', Fig. 11 shows
T, _ T(station) - T(FRS)
-_
T"
and
-
T(FRS)
2,- Z(station) - Z(FRS)
-_
T"
T(FRS)
where T is the amplitude of the horizontal component along the section CC' transverse to
the X anomaly; CC' has azimuth 163". The use of normalized amplitudes, without regard to
phase differences, would be unjustified for quantitative modelling. In the present discussion
only approximate current locations and widths are deduced, and for this purpose the
normalized amplitudes suffice.
The normalized Z, anomalies are antisymmetric, with maxima on the north side and
minima south of the X , maximum. As Z,/X, is a normalized amplitude without regard to
Conductive structures in southernmost Africa
49 1
J
Y
i
AMP
Y
PHASE
Figure 10. Fourier transform amplitudes (arbitrary units) and phases (min) at period 85 min for a substorm sequence 1977 November 14, 2300 hr-November 15, 0200 hr. The magnetograms are in Fig. 4.
The ellipse represents the horizontal field polarization.
Table 1. Phase difference 6 X - y at FRS.
Date
November
November
November
November
November
November
14
14
14-15
14
14-15
10
Period (min)
6
J 58
137
85
82
48
24
- 33
~
-43
-4
-26
0
-1
Y(min)
-
6
~
y(deg)
-
- 75
-113
- 17
-114
0
- 15
sign, maxima would be expected to north and south if a single current concentration were
involved. However, 2 is rising t o the south as a result of induced currents off the continental
edge. At stations between the intracontinental current and the continental edge there are
parallel, nearly in-phase currents contributing opposed 2 components and producing the
492
J. H. de Beer and D.I. Cough
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Figure 11. Normalized anomalous field components Z,/X, and X,/X, along section lines AA' and BB',
Fig. 1 and Z,/T, and T,/T, along section line CC', Fig. 1. The polarization of the horizontal field for
each period is indicated.
observed minimum in Z. Along section CC', w h c h is nearest the continental edge, the rise of
Z,/Tn in t h e continental edge anomaly is most obvious.
Under section BB' there is clearly a single current concentration less than 90 km wide. It
is clear that the maximum response is at the shortest period represented (24 min), although
polarization must affect the amplitudes. X maxima are at JNV and 2 maxima at GFR, at
all periods.
The anomaly profdes along the western section AA' (Fig. 1) indicate a broader diffusion
of the current system. At some periods the Xa/Xn profiles show two maxima, suggesting
splitting of the currents, in agreement with the 2 phase contours of Figs 7, 8 , 9 and 10.
The anomaly profdes for section CC' indicate a single current concentration wider than
that under BB', with its centre between ELL and BLY.
In section CC', values of Z,/T, exceeding unity occur at several periods. It must be
remembered that the continental edge anomaly in 2 is large at the stations concerned. At
JNV in section BB', X,/X, = 1.54 at period 24 min. Such a high value might be taken as
evidence of current channelling. Alternatively X at JNV may be increased by a bend in the
Conductive structures in southernmost Africa
493
current, as suggested in the discussion of the Y anomaly. Since the intracontinental conhctivity anomaly crosses both west and south-east coasts, it is very probable that it does
channel currents induced in and under the oceans. This cannot, however, be proved from the
results reported here.
A maximum depth estimate for the currents can be estimated at section BB' (Fig. l),
where the X anomaly is quite symmetrical, from its width at half maximum value. This
width is twice the depth of a line current which would give the observed X component
anomaly. The limiting depth is better estimated from X than from the distance between the
Z extrema, which is also twice the depth, because the distorting effect of the continentaledge anomaly is much larger for 2 than for X . Estimates made from the half-amplitude
width of the Xa/Xn profiles at BB' in Fig. 11 give limiting depths between 40 and 52 h,
with mean 46 km. These estimates are made at periods 24-158 min. The real current must
have finite width and depth less than 46 km. The conductive structure thus appears to be in
the crust or uppermost mantle.
In many magnetovariational studies, induction arrows representing transfer functions
from horizontal to vertical field components are used t o locate induced currents. In the first
magnetometer array study in South Africa (Cough et al. 1973) induction arrows were used
in addition to Fourier transform maps. and the two methods gave consistent results. In the
present investigation it has been found that the use of induction arrows gives a distribution
of induced currents differing strongly from the Fourier transform maps. The discrepancy
is related to correlation between X and Y in most of the events used in the present work
(Schmucker 1970). It will be discussed in detail elsewhere.
The Southern Cape Conductive Belt
The best indication of the margins of the conductive belt is provided by the attenuation of
short-period Z at stations above the conductor, and the enhancement of 2 just outside it.
The margins have been mapped in Fig. 12 on these criteria, by inspection of the magnetograms. Current concentrations within the conductive belt are schematically represented in
Fig. 12(a). These have been located with reference to the 2 phase and X amplitude
anomalies of Figs 8 , 9 and 10.
Fig. 12(b) presents a map of basement age provinces (Burger & Coertze 1973). The extent
of the Namaqua-Natal Mobile Belt is well defined, by numerous age determinations in the
range 830-1200 Myr, near the west and east coasts. In the centre, between longitudes
20" E and 30" E, only three age determinations exist, and the southward extent of the
Mobile Belt is therefore poorly defined. No basement age determination exists from the
Southern Cape Conductive Eelt. There is a strong presumption that the Conductive Belt
lies just outside the southern limit of the Namaqua-Natal Mobile Belt, which has been
found to be essentially cratonic in deep Schlumberger electrical soundings (van Zijl 1977).
The induced currents skirt the cratonic lithosphere.
Measurements of heat flow at four points near the axis of the Southern Cape Intracontinental Anomaly gave a mean value of 60 mW m-2 (Cough 1963). Six measurements in
the Namaqua-Natal Mobile Belt gave mean value 59 mW m-' (Carte & van Rooyen 1969).
There is thus no evidence to support a thermal origin of the high conductivity, and the possibility of a ridge of partial melt in the mantle, considered earlier by Gough (1973b), must be
abandoned.
The central part of the conductive belt coincides with the deepest part of the Karroo
Basin, and it is natural to ask whether the currents flow in the sedimentary rocks. At both
494
J. H de Beer and D.I, Gough
26'
24'
3n0
28'
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I
-
32'-
18"
I
I
2:"
I
72*
I
I
28"
26'
74'
I
I
30"
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Figure 12. (a) The Southern Cape Conductive Belt with the cunent concentrations within the belt
indicated by density of stippling. (b) The conductive belt in relation to basement age provinces (Burger
& Coertze 1973). The line of crosses indicates the axis of the Beattie static magnetic anomaly.
ends, however, the magnetovariational anomaly leaves the Karroo Basin, so that it cannot
be ascribed to currents confmed to the sediments (compare Figs 1 and 12a).
To the south of the western half of the Conductive Belt, Cape Granite of age 600 Myr is
intrusive into Malmesbury Group rocks. All outcrops of the Cape Granite lie to the south
of the Conductive Belt. It is a reasonable hypothesis that the Southern Cape Conductive
Belt is made up of ophiolitic rocks compressed in a subduction zone or a marginal geosyncline in Proterozoic time. A prominent positive anomaly in the static magnetic field,
known as the Beattie Anomaly, correlates with the Conductive Belt. Its axis is indicated in
Fig. 12(b). An accumulation of oceanic lithospheric rocks outside the cratonic margin could
provide both high electrical conductivity and the static magnetic anomaly. The high
electrical conductivity could be associated with sea-water in porous basalt (Brace 1971 ;
Hyndman & Drury 1976) or with hydrous minerals such as serpentinite (Hyndman &
Hyndman 1968; Stesky & Brace 1973). The source of the static magnetic anomaly must lie
above the Curie isotherm, whch should lie between depth limits 25 to 38 km assuming that
the thermal conductivity lies between 2.5 and 3.8 W m-* "C-' and taking heat flow 60
mW m-2 (Gough 1963). A crustal depth for the electrically conductive rocks is therefore
consistent with the observed correlation between magnetovariational and static magnetic
acomalies.
The southern limit of the Namaqua-Natal Mobile Belt presumably formed part of the
continental edge of Gondwanaland in early Proterozoic time. The presence of ancient oceanfloor material along the Southern Cape Conductive Belt could be associated with Proterozoic
subduction, or with a marginal sea. The Namaqua-Natal thermal event at 1OOOMyr
supports the subduction alternative ; the conductive belt is consistent with either possibility.
Tectonic implications of the Southern Cape Conductive Belt will be discussed elsewhere.
Conductive structures in southernmost Africa
495
Acknowledgments
We are grateful t o J. S. V. van Zijl, R. M. J. Huyssen, D. T. Barlow, R. Meyer and J. Blume
for willing help with the field work. Dr van Zijl also gave administrative support to the
project. Ms Janette Fourie digitized the data with patience and precision. DIG thanks
Professors J. A. Gledhill and E. E. Baart and many members of the Physics Department of
Rhodes University for laboratory and other facilities during the preparation of the Canadian
magnetometers for the field. The staff of the Magnetic Observatory of the CSIR kindly
provided data from the observatories at Hermanus and Grahamstown. DIG was supported
by a research grant from the Natural Sciences and Engineering Research Council of Canada,
and by a Hugh Kelly Fellowship from Rhodes University.
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