Cape Fold Belt - Agulhas Bank Transect Across Gondwana Suture, Southern Africa

28 Pages·1.45 MB·English

GLOBAL GEOSCIENCE TRANSECT 9
CAPE FOLD BELT - AGULHAS BANK
TRANSECT ACROSS GONDWANA SUTURE,
SOUTHERN AFRICA
&4 American Geophysical Union
Publication No. 202 of the International Lithosphere Program
ISBN 0-87590-804-7
Copyright 1993 American Geophysical Union. Figures, tables and
short excerpts may be reprinted in scientific books and journals if
the source is properly cited; all other rights reserved.
Printed in the United States of America.
AMERICAN GEOPHYSICAL UNION
2000 FLORIDA AVENUE, N.W.
WASHINGTON, D.C. 20009
Foreword
The Global Geoscience Transects Project (GGT) is surface-bound field geologist, can attest to the excite­
an ambitious international effort that draws together ment of working with geophysicists from around the
geoscientists in a variety of disciplines to produce the world to consider the nature and evolution of deeper
best possible portrayal of the composition and struc­ parts of the crust and to attempt to integrate these data
ture of the Earth's crust. Since its inception in 1985, with surface features.
GGT has encouraged geoscientists in all countries of GGT compilation also plays a significant role in the
the world to compile cross sections of the Earth up to education of geoscientists. The problems being inves­
a few thousands of kilometers in length and drawn to tigated are global and interdisciplinary in scope, and
the base of the crust using all available geological, international cooperation between geologists, geo­
geophysical, and geochemical information. Transects physicists, geochemists, and geodesists is required to
are drawn to common scales and formats so that the resolve them. Transects cannot be compiled without
Earth's crust in different parts of the world can be such interdisciplinary research.
directly compared. The guidelines developed for the transects had to be
flexible enough to accommodate the variations in data
GGT was conceived by the Inter-Union Commis­
available in different parts of the world. Some areas
sion on the Lithosphere (ICL), a "child" of the
have very complete, freely available data bases; others
International Union of Geological Sciences and the
have at best only gravity or aeromagnetic maps as the
International Union of Geodesy and Geophysics
geophysical data base from which to construct crustal
(IUGG), at the IASPEI conference in Tokyo in August
cross sections.
1985. Project coordinators James W. H. Monger and
The first publications of the GGT Project, a transect
Hans-Jurgen Gotze organized a multidisciplinary, mul­
chart and accompanying booklet, are a joint activity of
tinational committee that coordinated the global proj­
ICL and AGU. The seven transects from China (2),
ect. Meeting at the IUGG XIX General Assembly in
South America (2), Australia (2), and Syria (1), were
Vancouver in August 1987, the committee developed
drafted and hand colored by the compilers. Transects
preliminary guidelines for transect compilations, fol­
combine the available data into interpretive sections
lowing suggestions from transect compilers. Proposals
drawn to the base of the Earth's crust. The product is
for transects were solicited from ICL national commit­
a large display—approximately 1 meter by 2 meters—
tees, government geoscience agencies, universities,
with the bottom line cross section as a vertical tectonic
and scientists. A total of 140 proposals were received
map of the crust. The booklet includes an analysis of
from Africa (41), South America (30), China (18),
the transect by the compilers and references to data
USSR (12), USA (12), Europe (12), Australia and New
sources.
Zealand (11), India (3), and Southeast Asia (1).
The GGT Project has generated a special intellectual The GGT Project is now embarking on a new
excitement among participants by encouraging geosci­ direction that will make this wealth of data even more
entists to understand the evolution and physical be­ useful to the scientific community. ICL and AGU plan
havior of the lithosphere by bridging the gap between to publish future transects in electronic format (dis­
surface geology and deeper geophysics. Monger, as a kettes, CD-ROM) that can be manipulated using small
"low-end" computers. A GGT Digitization Group was wide network of scientists cooperating on transects
established in 1988 to prepare guidelines for digitizing would be able to work interactively with these files,
transects; the guidelines and sample will be published adding data to them, analyzing, reconfiguring, and
by AGU. interpreting them.
Digitization of geoscience material will not only ease
This endeavor breaks new ground and goes a long
the task of publishing transects and encourage the
way toward a goal of the ICL of closely linking
exchange of data bases, it promises to allow quanti­
geological, geochemical, and geophysical data bases
tative comparisons of elements of different parts of the
and breaking down present barriers between different
Earth's crust. The use of digitized transect material is
Earth science disciplines. With the digitized transects,
the perfect training ground for the new breed: geosci­
geologists, for example, could manipulate gravity data
entists!
to test the validity of their structural models. A world­
James H. Monger
Geological Survey of Canada
100 West Pender Street
Vancouver V6B 1R8
Canada
Hans-Jurgen Gotze
Institut fur Geologie, Geophysik und Geoinformatik
Freie Universitat Berlin
Malteserstrasse 47 100
D-1000 Berlin 45 Germany
THE CAPE FOLD BELT-AGULHAS BANK TRANSECT
Across the Gondwana Suture in Southern Africa
COMPILED BY
I.W. HALBICH1
COLLABORATORS AND CONTRIBUTORS TO GEOPHYSICS
J.H. DE BEER,2 A. DU PLESSIS,4R.J. DURRHEIM,3 M.J. MAHER,2B.E. PITTS,2 AND G. SMITH5
CONTRIBUTORS TO GEOLOGY
D.S. COETZEE,1 D.H. CORNELL,1 F.J. RTCH,6P.G. GRESSE,1 I.W. HALBICH,1 J.R. KRYNAUW,1
D.M. LE ROUX,1 J.P. LE ROUX,1 J.A. MILLER,7 A.P.G. SOHNGE,1 AND J. SWART1
Geology Department, University of Stellenbosch, Stellenbosch 7600, South Africa.
2 Earth, Marine and Atmospheric Science and Technology, C.S.IR., Pretoria 0001, South Africa.
3 Geophysics Department, University of the Witwatersrand, Johannesburg 2001, South Africa.
4 Geological Survey, Pretoria 0001, South Africa.
5 Southern Oil Exploration Corporation, Parow 7500, South Africa.
6 Department of Geology, Birbeck College, London, England.
7 Department of Earth Science, Bullard Laboratory, Cambridge, England.
THE CAPE FOLD BELT-AGULHAS BANK TRANSECT 1
INTRODUCTION data will serve only as a basis for discussion. I t sets
objectives for future investigations in this highly
This N-S trending, 600 km long on-land, offshore significant region.
crustal transect was chosen because some unusual
geological and geophysical features near the southern DATA BASE
tip of Africa permit analysis of the long (Proterozoic
to Recent) crustal evolution of this region (Fig. 1). During the last 10 to 15 years several new
These features are: geological investigations were conducted over the area
(i) The Southern Cape Conductive Belt (S.C.C.B.) covered by transect 9 (Geological Survey of South
(Fig. 1A). Africa, 1979, 1979a; Rowsell and De Swardt, 1976;
(ii) The Beattie Magnetic Anomaly, (Fig. 1 A, B). Dingle et al., 1983; Sohnge and Halbich, 1983). They
(iii) A northern suite of Late Proterozoic sedi­ include detailed investigations of structural and
mentary rocks (Kango Group) separated from a stratigraphic aspects (Coetzee, 1983; Gresse, 1983;
southern coeval suite (Kaaimans Group) by the Halbich, 1983, 1983a, 1983b, 1983c; Halbich and
southern boundary of the S.C.C.B. (compare Swart, 1983; Le Roux, 1983; Le Roux and Gresse ,
Fig. 1A with Fig. IE). 1983), timing of events (Halbich et al., 1983) and the
(iv) The E-W structural trend of the pre-Cape (Pan metamorphic history (Halbich and Cornell, 1983).
African; 900-450 Ma) mobile belt consisting of Southern Oil Exploration Corporation (SOEKOR)
stacked, north-verging thrust slices (Fig. IF). released geological borehole data along the seismic
(v) E-W trending, syntectonic and thrust-bounded line (geological strip map) and their file no. 10E was
granite sheet intrusions of Pan African age (Fig. extensively used for constructing sea bed geology on
1G). horizon "D" = top of the pre-Mesozoic "Basement" on
(vi) Superimposed E-W trending sedimentary basins the strip map.
of Palaeozoic (Cape) and Mesozoic (Karoo) age Geophysical investigations on the continental shelf
(respectively Figs. lHand II). have been intensified since the 1970s because of the
(vii) A north-verging Cape Fold Belt deforming these search for oil. Most of the results obtained refer to
basins (Fig. 1J). This belt has E-W trending shallow seismic structure in the Cretaceous basins
metamorphic zones with grades increasing from (Fig. 1L; Dingle et al., 1983). The composite shallow
anchizonal to epizonal southwards. There is no seismic profile across the continental platform and the
known contemporaneous magmatic phase on time/depth graph (Geophysical section on display)
land. were supplied and explained by SOEKOR personnel
(viii) A belt of Jurassic Karoo dolerites is confined to (courtesy G. Smith). The bedding attitudes shown
the northernmost part of the transect, and has a below reflector "D" on the geological and seismic
sharp E-W trending southern boundary (Fig. reflection profiles at sea are a free interpretation of
IK) that closely coincides with the northern data supplied by SOEKOR. Some shallow seismic
boundary of the S.C.C.B. in its central part on-land information was taken from Fatti and Du Toit
(Figs. IK, 1A). (1970) and Fatti (1978), as well as from SOEKOR
(ix) Major E-W trending Cretaceous normal faults report no. 9C of 1969. To date, no deep seismic work
branch from the Cape Megashear and follow has been conducted within the on-land segment of
pre-existent thrust faults. This occurred during transect 9.
the break-up of Gondwana, and gave rise to A combined refraction seismic and gravity profile
large E-W trending, fault controlled, asymmetric of the Agulhas Bank along 21°50'E longitude by
sedimentary basins mainly on the Agulhas Bank Hales and Nation (1972; Fig. 1L) was projected
(Fig. 1L). eastwards with adjustments, especially in the southern
(x) An oblique NE-SW megashear zone (marginal part, to accommodate attenuation of the crust at lower
offset) of global proportions is called the Cape latitudes. The general style, however, was assumed to
Megashear or Agulhas Fracture Zone; along it remain the same. Restricted seismic reflection work
Gondwana split into parts by tensional shear by Durrheim (1987) from 23°15' to 23°25' longitude
from the Late Jurassic onwards (Fig. 1L). See on the Agulhas Platform helped in identifying deeper
also both index maps on display. horizons and characterising the crust beneath the shelf
The crustal segment crossed by the Cape Fold at about 34°25'S (Geophysical profiles on display).
Belt-Agulhas Bank transect represents part of former To some degree these results agree with what is
Gondwanaland and has been subjected to at least four known from seismic work in Namaqualand and
consecutive cycles of crustal compression (thickening) Bushmanland (see local index map and Fig. 1 A) to the
and extension (thinning) since 1600-1800 Ma. north of the transect (Green and Durrheim, 1988).
Until adequate seismic data on land become Bouguer gravity on land was extracted from the
available, any present model explaining the available 1:106 geological map of the Republic of South Africa
GLOBAL GEOSCIENCE TRANSECT 9
S.C.C.B.= SOUTHERN CAPE CONDUCTIVE BELT
B.M.A = Trend of BEATTIE MAGNETIC ANOMALY
Dots= magnetometer positions
RESIDUAL TOTAL FIELD AEROMAGNETIC MAP
delineating the B.M.A. C.I. = 100 nT
BOUGUER GRAVITY RESIDUALS
C.I. = 200 g.u.
ISOSTATIC GRAVITY MAP
C.l.= 200 g.u.
PROTEROZOIC SEDIMENTS
northern facies S=southern facies
PAN AFRICAN THRUST STACKS
dipping south in the south
and west in the west
PAN AFRICAN GRANITES
P=late tectonic plutons
3= syntectonic sheets
BASIN of the CAPE
SUPERGROUP
KAROO BASIN
CAPE FOLD BELT
distribution, trend and vergency of ^ W=Worcester Fault
and thrusts
H=Refraction and
gravity line
KAROO DOLERITES
K= Kango Fault
sharp southern boundary
CRETACEOUS BASINS Cape Megashear
fault controlled , branching from right lateral - Basins
CAPE MEGASHEAR -Cape slope magnetic anomaly
-Gravity anomalies
Seamounts
Fig. 1. Geological and geophysical features of the region.
THE CAPE FOLD BELT-AGULHAS BANK TRANSECT 3
(Gravity edition, 1984, Fig. 1C), and modelled by Farther south and mainly on Agulhas Bank, thre e
Maher and Pitts (1989). The isostatic anomaly map of deep fault controlled basins are crossed until the
Smit et al. (1962; Fig. ID) was used. Free-air gravity transform-related Agulhas Fracture Zone (Cape
data at sea were taken from published information by Megashear) is reached (Fig. 1L; maps and sections o n
Hales and Nation (1972), Talwani and Kahle (1975), display). The transect ends just beyond the
Siesser et al. (1974), Udintsev (1975) and Scrutton Megashear in Cretaceous oceanic crust. Gravity
(1975). B.E. Pitts modelled the Bouguer gravity anomalies coinciding with several chains of
profile along the transect (20°30Ti) on land. This is seamounts, and the Cape slope magnetic anomaly
shown on the display. parallel to the Megashear (Fig. 1L) mark the boundary
For magnetics on land we have mainly relied on between continental and oceanic crust.
aeromagnetic data supplied by the Geological Survey,
and a map by P. Bahnemann as published by De Beer CRUSTAL EVOLUTION
etal. (1982); see Figure IB.
An extended and updated version of an Major features of the crust of the transect corridor
upward-continued total field aeromagnetic profile appear to result from four major cycles of crustal
across the well-known Beattie Static Magnetic thinning (deposition of sediments) and thickening
Anomaly (De Beer, 1978, 1983; De Beer and Gough, (orogeny), in the region of the late Proterozoic
1980; De Beer et al., 1982) was recently modelled by Gondwana Suture. The reader is referred to Figures 2
Maher and Pitts (1989) along longitude 22°30'E. It is to 5 (Crustal evolution across the Gondwana Suture)
used on the display. which are also reproduced on the transect display.
Magnetic profiles across the Agulhas Platform as
published by Du Plessis and Simpson (1974), Rabino- First Cycle
witz (1976) and Udintsev (1975) were used to construct
the total magnetic field profile along 22°30' at sea. The Gondwana Suture formed by continental
The Southern Cape Conductive Belt, an electrically collision (Fig. 2, stages 1 and 2). An ocean is
conductive zone (Figs. 1A and B) was discovered by assumed to have opened up between ca. 1600 and
Gough et al. (1973) and mapped and modelled by De 1200 Ma ago and closed somewhere between 1200
Beer (1978, 1983), De Beer and Gough (1980) and De and 900 Ma ago to form the suture. Obduction of
Beer et al. (1982), employing simultaneous observa­ oceanic crust to the north occurred either by
tions by 53 three-component magnetometers covering southward subduction (Model A) or by northward
the southern tip of the African continent south of subduction (Model B). It is also possible that the two
latitude 30°S (Fig. 1A). The electrical resistivity of colliding continents were not previously related.
the crust was determined by an analysis of induction
effects produced by temporal changes in the Second Cycle
geomagnetic field.
Some 900-600 Ma ago, during the Pan African
GENERAL REGIONAL SETTING Depo-Cycle, material eroded from the uplifted suture
zone fills basins to the south with proximal Kango
Transect 9 is located between longitudes 22°E and Group and distal Kaaimans Group sediments (Fig. 3,
23°E (Index maps and Fig. 1) in the middle of the stage 3 and Fig. IE). The Pan African Orogeny
known E-W extent of rocks deposited and deformed (600-450 Ma) culminated in a southward dipping
during the Pan African episode (900-500 Ma), the Megadecollement across the Gondwana Suture with
Gondwanide episode (Cape and Karoo sequences of the generation of granite sheet intrusions following
respectively 450-300 Ma and 280-150 Ma and their some of the numerous north verging thrust slices that
deformation from 450 to 225 Ma), and the branch off the decollement. This took place above a
Cretaceous-Tertiary sedimentary deposits on the zone of delamination of continental lithosphere from
continental shelf surrounding southern Africa. continental crust because intra-continental subduction
The strip map extends from the high inland plateau to the north was accompanied by rising asthenospheric
underlain by undeformed Permian (Karoo) strata (Fig. diapirs (Fig. 3, stage 4).
II) intruded by extensive Jurassic dolerite sheets and
dykes (Fig. IK), and across the "Southern Cape Third Cycle
Conductive Belt" (Fig. 1A) located in the crust below
the northernmost zone of the Cape Fold Belt (Fig. 1 J). Stage 5 (Fig. 4) depicts the 450-300 Ma Cape
The positive Beattie Magnetic Anomaly (Figs. 1A, Depo-Cycle, during which mainly intra-continental
IB) is a narrow E-W trending regional feature within clastic marine sediments accumulated, and stage 6
the Conductive Belt, and is flanked by magnetic lows (Fig. 4) comprises the 280-235 Ma Cape Orogeny,
on either side. which resulted in a northward verging "back folded"
4 GLOBAL GEOSCIENCE TRANSECT 9
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THE CAPE FOLD BELT-AGULHAS BANK TRANSECT 5