?-ecto~ophysics, 192 (1991) 97-101 Elsevier Science Publishers B.V., Amsterdam

97

Susceptibility and Koenigsberger Ratio of the oceanic crust deduced from magnetic reversals southeast of the Agulhas Bank

R.W. Day a and A. du Plessis b n Joint GeoIogical Survey / University of Cape Town Marine Geoscience Section, University of Cape Town, Private Bag,

Rondebmch 7700, South Africa

b Geological Survey of South Africa, Privaie Bag X112, Pretoria 0001, South Africa

(Received by publisher July 24, 1989)

ABSTRACT

Day, R.W. and Du Pies&s, A., 1991. Susceptibility and Koenigsberger Ratio of the oceanic crust deduced from magnetic reversals southeast of the Agulhas Bank. In: P. Wasilewski and P. Hood (Editors), Magnetic Anomalies-Land and Sea. Tecfonophysics, 192: 97-101.

The Cape Slope Anomaly marks the contact between mid-Cretaceous oceanic crust and the southeastern edge of the Agulhas Bank, which is composed of weakly magnetic continental crust. Amplitude changes in the Cape Slope Anomaly reflect reversals in the direction of remanent ma~e~tion with the oceanic crust. However, the consistent shape of the Cape Slope Anomaly indicates that the anomaly is dominated by “induced” magnetization (which may include a component of viscous remanent magnetization indistinguishable in effect from true induced magnetization). This observation supports the interpretation of the anomaly bounding the oceanic crust off the west coast of South Africa as a product of contact geometry, and not the result of a magnetic reversal. Similarly, other magnetic anomalies along the boundaries between continental and oceanic crust may be largely due to contact geometry.

Introduction

The sheared margin at the tip of Africa (Fig. 1) provides a rare opportunity for studying the mag- netization within the oceanic crust. The separation of the Falkland Plateau, part of the South Ameri- can plate, from Africa along a transform fault (the Agulhas Fracture Zone) juxtaposed oceanic and continental crust as the sea floor spread parallel to the continents margin. This event gave rise to the linear magnetic anomaly first noted by Oguti (1964) and now called the Cape Slope Anomaly (Du Plessis and Simpson 1974). The anomaly, which is consistent in shape but variable in ampli- tude, extends sou~westward from near Durban on the east coast of South Africa to the southern tip of the Agulhas Bank (Fig. 2). Because the continental crust is only weakly magnetic (Talwani

q>

AFRICA

T-

Fig. 1. Location of Agulbas Fracture Zone shown by pecked hne, isobaths by unbroken lines. The zone developed as the Falkland Plateau moved southwestward relative to Africa, Ieav- ing a series of sea-floor spreading anomalies oriented per-

pendicular to the continental ma@.

oO40-1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.

98 R.W. DAY AND A. DU PLESSIS

Fig. 2. Individual magnetometric profiles iUustrating the variation in anomaly amplitude along the Cape Slope Anomaly. Areas of enhanced amplitude shown by iufii of the line schematically portraying the Cape Slope Anomaly. Pecked line shows 1000 m

&bath.

SOUTH AFRICA

Fig. 3. Traverse from the Natal Valley to the Agulhas Basin. Variations observed along the Cape Slope Anomaly are evident further

offshore. Isochron data is that given by Larson and Ladd (1973) and Goodlad et al. (1982).

SUSCEPTIBlLITY OF ‘THE OCEANIC CRUST FROM MAGNETIC REVERSALS, SOUTHEAST OF AGULHAS BANK 99

and Eldholm, 1973; Du Plessis and Simpson,

1974), the cause of the Cape Slope Anomaly lies in the oceanic crust. Similarly, the root of the ampli- tude variations along its length must also lie within the oceanic crust.

Talwani and Eldholm (1973) believed that the Agulhas Fracture Zone was characterised by ultra- mafic rocks in which the ma~et~ation is prim- arily induced. If so, the Cape Slope Anomaly is a product of induced magnetization, and its varia- tions are reflections of physical changes within the oceanic crust. However, another source of ampli- tude variations is reversals in remanent magnetiza- tion, a possibility which can be proved by compar- ing the slope anomaly with the magnetic field further offshore.

Variations along the Cape Slope Anomaly can be traced southeastward away from the continent into the Natal Valley and the Transkei and Agul- has Basins (Figs. 2 and 3) (see Martin et al., (1982, fig. l), and compare Du Plessis and Simpson (1974, fig. 3) with Du Plessis (1979, figs. 4.2 and 4.3)). This sequence of anomalies also closely mir- rors data collected off the west coast of South Africa (e.g. Rabinowitz, 1976; Du Plessis, 1979) where the variations are attributed to magnetic reversals (Larson and Ladd, 1973). Both Du Ples- sis (1979) and Goodlad et al. (1982) invoke rema- nent ma~et~atio~ to explain the changes in mag- netic field over the open ocean southeast of the Agulhas Bank, although there are differences be- tween their interpretations. Du Plessis related the whole string of variations to the Mesozoic M-series of reversals, whereas Goodlad suggested that only the anomalies in the Natal Valley are caused by the M-series. Goodlad’s correlation agrees better with palaeontological information (McLachlan and McMillan, 1979), but if his interpretation is accepted, the anomalies observed southwest of the Natal Valley would have originated during the so-called Cretaceous Normal-Polarity Epoch. Goodlad’s correlation would seem to throw doubt on the source of the anomalies. Incoherent linea- tions have long been recognised in this period (Hayes and Rabinowitz, 1975), while Srivastava et al. (1988) have recently traced a continuous anomaly present on both sides of the North Atlantic. In any event, the anomalies observed

southeast of the Agulhas Bank are indistinguisha- ble in amplitude and shape from acknowledged

reversals off the Natal coast. It is concluded here that the amplitude varia-

tions observed along the Cape Slope Anomaly are linked to magnetization reversals, and that the amplitude of the Cape Slope Anomaly is depen- dent on the direction of remanent ma~etization. Accordingly, a model for the boundary of the oceanic crust can be established and the contribu- tions of remanent and induced magnetization estimated.

Modelling the eontiRent-beg boundary

Two papers published in the seventies sug- gested models for the Cape Slope Anomaly. The first, by Talwani and Eldholm (1973) utilised induced magnetization only. Their model parame- ters describe a 2 km thick horizontal plate, 4 km below sea level, with a susceptibility of 0.5 S.I. units. The second model appeared in Du Plessis and Simpson (1974) who demonstrated that the Cape Slope Anomaly could be modelled by a 2.9 km thick remanently magnetized horizontal plate. The base of the magnetic plate was set to coincide with the projected bottom of acoustic unit 2 of Ludwig et al. (1968).

Rabinowitz (1976) reproduced Talwani and Eldholm’s model in his paper but pointed out that the field direction in the Early Cretaceous, when the crust was extruded was not significantly differ- ent to that of the present day (during periods of normal polarity). Today, the residual remanent ma~et~ation would either directly oppose or en- hance the induced effect, depending on the polar- ity prevailing when the crust was extruded. The amplitude of the anomaly varies according to whether the remanent magnetization exactly rein- forces or opposes the induced component. Thus the relative components of induced and remanent magnetization can be deduced because the magni- tude of the change is twice the remanent magneti- zation contribution.

From the profiles shown in Fig. 2 it may be seen that the Cape Slope Anomaly varies between approximately 300 nT and 800 nT in amplitude. From this it follows that the remanent component

loo R.W. DAY AND A. DU PLESSIS

nT

600 ,

NW

Fig. 4. Modelling of the induced component of the Cape Slope Anomaly by means of a 1.8 km thick horizontal plate and with a

susceptibility contrast of 0.4 S.I. units. A closer fit can be achieved by taking account of the sea-floor topolyaphy.

is responsible for *250 nT of the anomaly’s am-

plitude and the (apparent) induced component for

550 nT. Induced magnetization predominates,

hence the consistent anomaly shape which de-

pends only on the geometry of the causative body.

The Koenigsberger Ratio, the ratio between the

effects of the induced and remanent magnetiza-

tion, appears to be 0.45.

The next step is to calculate the susceptibility

of the oceanic crust from the induced component

of the anomaly. This calculation is dependent on

the chosen thickness of the plate as both thickness

and susceptibility determine the amplitude of the

anomaly. Application of Qureshi and Nalaye’s

method (1978) to the same data as that used by

Du Plessis and Simpson (1974) gives two sets of

results for dip, depth to centre, and body thick-

ness. These results average 176 f 1” (i.e. a hori-

zontal plate extending southeastward), 5.4 f 0.3

km and 3.2 + 1.8 km respectively. The imprecise

thickness reflects the sensitivity of the master

thickness curves-small changes in diagnostic val-

ues are magnified into large changes in thickness

estimates (Qureshi and Nalaye, 1978, fig. 2). How-

ever, the thickness estimate can be better con-

strained, because the top of the body obviously

cannot extend above the sea floor. In following

Du Plessis and Simpson (1974) by placing the top

of the body at the (4.5 km deep) sea floor, a body

thickness of 1.8 + 0.6 km is produced. Forward

modelling using these estimates indicates an ap-

parent susceptibility contrast of 0.40 S.I. units

once the remanent component is taken into

account (Fig. 4).

Discussion

Although there is leeway in the exact values of

the induced and remanent magnetization, it seems

that the induced component dominates. The fact

that the induced magnetization is twice as im-

portant as the remanent magnetization contrasts

with Talwani and Eldholm’s (197311 work on the

marginal escarpment off the Norwegian coast.

There they necessarily modelled the anomaly over

the continent-ocean boundary (which is of early

Tertiary age) using remanent magnetization alone.

Although variations in the intensity of remanent

magnetization with age have been noted by Bleil

and Petersen (1983), enhanced and not reduced

values are expected for Early and mid-Cretaceous

oceanic crust. Extrapolating from Talwani’s model

with dominant remanent magnetization, the re-

sults here should show subordinate induced mag-

netization. It may be that susceptibility has been

overestimated; the seemingly induced component

of the Cape Slope Anomaly may be due not only

to a susceptibility contrast but also to viscous

SUSCEPTIBILITY OF THE OCEANIC CRUST FROM MAGNETIC REVERSALS, SOUTHEAST OF AGULHAS BANK 101

remanent magnetization oriented parallel to the

present-day field.

Regardless of how apt the speculation is to the

true situation, the juxtaposition of oceanic and

continental crust must result in a magnetic anom-

aly the shape of which is controlled by the contact

geometry. This conclusion accords with Larson

and Ladd’s alternative interpretation of the “M-

13” anomaly on the southwestern edge of the

continent as a magnetic edge effect (Larson and

Ladd, 1973). It follows that the effects of domi-

nant induced magnetization must be considered

when deciphering anomalies along continent-oc-

ean boundaries.

Acknowledgements

Thanks are due to three anonymous referees for

their comments and to Dr. S.P. Srivastava for

providing manuscripts of his latest papers.

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