Tectonic model of the Limpopo belt: Constraints from magnetotelluric data

Full text

Precambrian
Research
226 (2013) 143–
156
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lists
available
at
SciVerse
ScienceDirect
Precambrian
Research
journa
l
h
omepa
g
e:
www.elsevier.com/locate/precamres
Tectonic
model
of
the
Limpopo
belt:
Constraints
from
magnetotelluric
data
D.
Khozaa,b,∗, A.G.
Jonesa, M.R.
Mullera, R.L.
Evansc, S.J.
Webbb,
M.
Miensopustd,
the
SAMTEX
team
aDublin
Institute
for
Advanced
Studies,
5
Merrion
Square,
Dublin,
Ireland
bUniversity
of
the
Witwatersrand,
1
Jan
Smuts
Avenue,
Johannesburg,
South
Africa
cDepartment
of
Geology
and
Geophysics,
Woods
Hole
Oceanographic
Institution,
Woods
Hole,
MA
02543-1050,
USA
dInstitut
fur
Geophysik,
Westfalische
Wilhelms
Universitat,
Munster,
Germany
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
25
June
2012
Received
in
revised
form
24
November
2012
Accepted
26
November
2012
Available online xxx
Keywords:
Limpopo
belt
Archaean
Magnetotelluric
a
b
s
t
r
a
c
t
Despite
many
years
of
work,
a
convincing
evolutionary
model
for
the
Limpopo
belt
and
its
geometri-
cal
relation
to
the
surrounding
cratons
is
still
elusive.
This
is
partly
due
to
the
complex
nature
of
the
crust
and
upper
mantle
structure,
the
significance
of
anatectic
events
and
multiple
high-grade
meta-
morphic
overprints.
We
use
deep
probing
magnetotelluric
data
acquired
along
three
profiles
crossing
the
Kaapvaal
craton
and
the
Limpopo
belt
to
investigate
the
crust
and
upper
mantle
lithospheric
struc-
ture
between
these
two
tectonic
blocks.
The
20–30
km
wide
composite
Sunnyside-Palala-Tshipise-Shear
Zone
is
imaged
in
depth
for
the
first
time
as
a
sub-vertical
conductive
structure
that
marks
a
fundamental
tectonic
divide
interpreted
here
to
represent
a
collisional
suture
between
the
Kaapvaal
and
Zimbabwe
cratons.
The
upper
crust
in
the
Kaapvaal
craton
and
the
South
Marginal
Zone
comprises
resistive
granit-
oids
and
granite-greenstone
lithologies.
Integrating
the
magnetotelluric,
seismic
and
metamorphic
data,
we
propose
a
new
tectonic
model
that
involves
the
collision
of
the
Kaapvaal
and
Zimbabwe
cratons
ca.
2.6
Ga,
resulting
in
high-grade
granulite
Limpopo
lithologies.
This
evolutionary
path
does
not
require
a
separate
terrane
status
for
each
of
the
Limpopo
zones,
as
has
been
previously
suggested.
© 2012 Elsevier B.V. All rights reserved.
1.
Introduction
Precambrian
regions
hold
the
key
to
understanding
the
tec-
tonic
processes
that
prevailed
during
the
early
and
middle
Archaean.
Many
questions
still
remain
to
be
answered
regarding
the
amount
of
heat
available
at
the
time,
the
onset
and
domi-
nance
of
early
plate
tectonic
processes
and
crustal
generation
these
processes
and/or
by
plumes.
The
greatest
impediment
is
the
rel-
ative
paucity
of
preserved
Archaean
rocks,
compared
to
inferred
crustal
generation
during
the
Archaean,
and
the
identification
of
Precambrian
structures
is
often
masked
by
secondary
tectonic
events.
The
highly
complex
Limpopo
belt
in
Southern
Africa
provides
a
natural
laboratory
to
investigate
these
questions
and
elucidate
the
possible
geological
processes
taking
place,
where
structural,
metamorphic
and
geochemical
data
are
available.
The
Limpopo
belt
is
an
Archaean-aged
high-grade
metamorphic
complex
located
∗Corresponding
author
at:
Dublin
Institute
for
Advanced
Studies,
5
Merrion
Square,
Dublin,
Ireland.
Tel.:
+27
(0)
71
623
9168.
E-mail
addresses:
davidkhoza@cp.dias.ie,
david.khoza2@gmail.com
(D.
Khoza).
1Members
of
the
SAMTEX
team
include:
L.
Collins,
C.
Hogg,
C.
Horan,
G.
Wallace,
A.D.
Chave
(WHOI),
J.
Cole,
P.
Cole,
R.
Stettler
(CGS),
T.
Ngwisanyi,
G.
Tshoso
(GSB),
D.
Hutchins,
T.
Katjiuongua
(GSN),
E.
Cunion,
A.
Mountford,
T.
Aravanis
(RTME),
W.
Pettit
(BHPB),
H.
Jelsma
(De
Beers),
P.-E.
Share
(CSIR),
and
J.
Wasborg
(ABB).
between
the
Kaapvaal
and
Zimbabwe
cratons
(Fig.
1).
It
has
an
ENE–WSW
trend
and
comprises
three
zones,
the
Northern
and
Southern
Marginal
Zones
and
the
Central
Zone,
separated
from
each
other
by
major
thrust
faults
or
strike-slip
shear
zones.
Metasedi-
ments,
granitoids
and
gneisses
comprise
a
significant
component
of
the
rock
outcrop.
Due
to
its
Archaean
affinity
and
relatively
good
surface
exposure,
the
Limpopo
belt
has
been
a
focus
of
a
number
of
geological,
structural,
metamorphic
and
geophysical
studies
(Van
Reenen
et
al.,
1987;
Roering
et
al.,
1992;
De
Beer
and
Stettler,
1992;
Rollinson,
1993;
Durrheim
et
al.,
1992).
More
recently,
a
Geologi-
cal
Society
of
America
Memoir
(207)
on
the
origin
and
evolution
of
high-grade
Precambrian
gneiss
terranes
focused
specifically
on
the
Limpopo
belt
(van
Reenen
et
al.,
2011).
Several
models
have
been
suggested
regarding
the
formation
and
deformation
of
the
Limpopo
belt
and
these
are
presented
and
discussed
in
Section
3.
Seismic
tomography
models
suggest
that
the
lithospheric
mantle
beneath
the
Limpopo
belt
is
gener-
ally
similar
to
that
of
the
Kaapvaal
and
Zimbabwe
cratons,
i.e.,
fast
velocities,
implying
thick,
cold
lithosphere
(Li,
2011;
James
et
al.,
2001).
In
contrast
the
crustal
structure
of
the
Limpopo
belt
is
highly
complex,
with
evidence
of
polytectonic
events
that
include
meta-
morphism,
magmatism,
crustal
uplift
and
structural
deformation
(Kramers
et
al.,
2011).
The
nature
of
the
horizontal
movements
dur-
ing
continental
accretion,
including
the
orientation
and
rate
of
plate
movements
during
the
Archaean,
is
not
fully
described.
In
addition,
the
deep
geometry
of
the
margin
between
the
Kaapvaal
craton
and
0301-9268/$
–
see
front
matter ©
2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.precamres.2012.11.016

144 D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156
Fig.
1.
SAMTEX
stations
(black
circles)
overlain
on
the
regional
tectonic
map
of
Southern
Africa,
modified
from
Begg
et
al.
(2009)
and
Webb
(2009).
The
yellow,
purple
and
green
filled
circles
show
locations
of
the
MT
stations
for
the
LOW,
KAP
and
LIM-SSO
profiles
respectively
that
are
the
focus
of
this
work.
The
bathymetry
data
are
from
ETOPO1
courtesy
of
Anamte
and
Eakins
(2009).
The
blue
rectangle
shows
the
area
of
interested,
shown
in
Fig.
2.
the
Limpopo
belt,
including
the
shear
zones
separating
each
zone,
is
not
fully
known.
In
this
article
we
attempt
to
address
several
of
these
issues,
particularly
the
nature
of
the
geometry
of
the
shear
zones
sepa-
rating
the
cratonic
units,
and
investigate
the
exotic
terrane
status
of
the
Limpopo
belt,
particularly
for
the
Central
Zone.
To
this
end
we
use
magnetotelluric
(MT)
data
to
image
the
crust
and
the
man-
tle
lithosphere
beneath
the
Limpopo
belt
and
Kaapvaal
craton.
The
deep-probing
MT
technique
has
been
used
successfully
in
Precam-
brian
regions
all
over
the
world
to
elucidate
their
tectonic
history
(Heinson,
1999;
Davis
et
al.,
2003;
Selway
et
al.,
2006,
2009;
Spratt
et
al.,
2009).
In
Southern
Africa,
the
MT
method
has
been
used
to
image
Archaean
and
Proterozoic
boundaries
and
to
understand
the
tectonic
history
of
Archaean
lithosphere
(Jones
et
al.,
2005,
2009;
Hamilton
et
al.,
2006;
Muller
et
al.,
2009;
Evans
et
al.,
2011;
Miensopust
et
al.,
2011;
Khoza
et
al.,
2011).
The
primary
physical
parameter
being
investigated
is
electrical
resistivity
of
sub-surface
materials.
Given
its
sensitivity
to
resistivity
contrasts,
the
crustal
models
derived
from
MT
data
are
very
informative
in
mapping
basement
features,
the
location
of
deep
seated
fault
blocks
(Selway
et
al.,
2006;
Spratt
et
al.,
2009)
and
crustal
melts
(Wei
et
al.,
2001;
Unsworth
et
al.,
2004,
2005;
Le
Pape
et
al.,
2012).
In
the
Earth’s
crust
the
primary
conducting
mineral
phases
are
saline
waters,
graphite,
sulphides,
iron
oxides
and,
in
active
regions
like
Tibet,
partial
melt.
Mantle
electrical
resistivity
is
primarily
sensitive
to
temperature
variation
and
water
content
(Jones
et
al.,
2012;
Evans,
2012)
and,
to
a
lesser
extent
chemical
composition
and
pressure.
As
part
of
the
highly
successful
Southern
African
Magneto
Telluric
EXperiment
(SAMTEX)
we
have
collected
MT
data
along
several
profiles
crossing
the
Limpopo
belt
and
its
bounding
terranes:
the
Kaapvaal
craton
to
the
south,
Zimbabwe
craton
to
the
north
and
the
Magondi
belt
to
the
west
(Fig.
1).
The
LOW
profile
(yellow
filled
circles,
Fig.
1)
crosses
the
northern
Kaapvaal
craton,
over
the
Hout
River
Shear
Zone,
which
is
thought
to
represent
the
southern
limit
of
the
Limpopo
belt,
into
the
Central
Zone.
The
KAP
profile
(pink
filled
circles,
Fig.
1),
part
of
which
was
the
focus
of
the
two-dimensional
Kaapvaal
craton
study
of
Evans
et
al.
(2011),
is
re-modelled
here
using
newly
developed
three-
dimensional
(3D)
techniques
and
also
to
do
a
holistic
focused
study
of
the
Limpopo–Kaapvaal
boundary.
The
LIM-SSO
profile
(green-
filled
circles,
Fig.
1)
crosses
the
Kaapvaal
craton,
Bushveld
complex,
Limpopo
belt,
Magondi
belt
and/or
southern
Zimbabwe
craton
and
terminates
close
to
Orapa
kimberlite
field.
The
Martin’s
Drift
kim-
berlite
cluster
(Fig.
2)
is
about
50
km
from
MT
site
SSO103
(Fig.
2).
The
Orapa
and
Martin’s
Drift
kimberlites
erupted
about
93
Ma
and
1350
Ma
respectively
(Haggerty
et
al.,
1983;
Jelsma
et
al.,
2004).
These
profiles,
crossing
the
Limpopo
belt
and
its
surrounding
ter-
ranes,
were
picked
to
provide
a
spatially
adequate
database
to
perform
3D
magnetotelluric
inversion
and
to
understand
the
nature
of
the
crustal
and
upper
mantle
geometry
of
the
geology
along
the
Limpopo
belt.
2.
Geological
background
We
define
in
Table
1
some
acronyms
that
will
be
referred
to
consistently
in
the
text.
The
Limpopo
belt
is
an
ENE–ESW
trending
high
grade
Archaean
metamorphic
complex
situated
between
the
lower-metamorphic
grade
granite-greenstone
Zimbabwe
and
Kaapvaal
cratons.
The
three
geologically-defined
zones
that
make
up
the
complex,
the
Northern
Marginal,
Central
and
Southern
Marginal
zones,
are
sep-
arated
from
each
other
by
variously
dipping
shear
zones
(Fig.
2).
The
Northern
Marginal
Zone
(NMZ),
which
comprises
granite-
greenstone
material
(magmatic
enderbites),
is
separated
from
the
Zimbabwe
craton
to
the
north
by
the
southward-dipping
North

D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156 145
Fig.
2.
Geology
of
Limpopo
belt
(modified
from
Kramers
et
al.,
2011)
showing
major
structural
and
geologic
features.
The
MT
station
locations
of
the
LOW
(yellow
circles),
KAP
(purple-filled
circles)
and
LIM-SSO
(green-filled
circles)
are
shown.
The
red
dots
indicate
the
location
of
known
kimberlites,
including
the
Orapa,
Martin’s
Drift
and
Venetia
kimberlite
fields.
CZ
=
Central
Zone;
KC
=
Kaapvaal
craton;
NMZ
=
North
Marginal
Zone.
Limpopo
Thrust
Zone
(NLTZ).
The
southern
limit
of
the
NMZ
(the
northern
limit
of
the
CZ)
is
marked
by
the
south-dipping
Trian-
gle
Shear
Zone
(TSZ)
(Roering
et
al.,
1992;
Kamber
et
al.,
1995a;
Kramers
et
al.,
2011).
The
Central
Zone
(CZ)
is
a
3.3–2.5
Ga
high
grade
zone
that
com-
prises
metasediments,
S-type
granitoid
gneisses
and
supracrustal
rocks.
Two
periods
of
high
metamorphic
grade
metamorphism
are
recorded
in
the
CZ
between
2.7–2.6
Ga
and
the
other
at
2.0
Ga
(Smit
et
al.,
2011).
The
530
Ma
diamondiferous
Venetia
kimberlite
cluster
is
located
within
the
CZ
of
the
Limpopo
belt
(see
Fig.
2).
The
Palala-Tshipise
Straightening
Zone
(PTSZ)
separates
the
CZ
from
the
SMZ
in
NE
South
Africa
(McCourt
and
Vearncombe,
1992)
and
has
an
ENE
to
NE
trend.
However,
in
SE
Botswana
the
southern
margin
of
the
CZ
is
marked
by
a
composite
40
km
wide
NW–SE
striking
linear
structure,
made
up
of
gneisses
of
the
Sunnyside
Shear
Zone
and
mylonites
of
the
Palala-Tshipise
Shear
Zone.
This
complex
composite
structure
is
thus
referred
to
as
the
Table
1
Description
of
acronyms
referred
to
in
the
article.
Acronym
Description
KC
Kaapvaal
craton
SMZ
South
Marginal
Zone
CZ
Central
Zone
NMZ
North
Marginal
Zone
ZC
Zimbabwe
craton
TSZ Triangle
Shear
Zone
PTSZ
Palala-Tshipise
Shear
Zone
SPTSS
Sunnyside-Palala-Tshipise
Shear
System
NLTZ North
Limpopo
Thrust
Zone
HRSZ
Hout-River
Shear
Zone
SSZ Sunnyside
Shear
Zone
Sunnyside-Palala-Tshipise
Shear
System
(SPTSS)
and
is
inferred
from
mineral
lineation
studies
to
have
sub-vertical
dip
(Horrocks,
1983;
McCourt
and
Vearncombe,
1992).
The
Southern
Marginal
Zone
(SMZ)
is
a
60
km
zone
that
con-
sists
chiefly
of
enderbitic
and
charnokitic
gneisses
and,
unlike
the
CZ
and
NMZ,
the
SMZ
experience
a
single
metamorphic
event
at
2.72–2.65
Ga
only.
The
north-dipping
Hout-River
Shear
Zone
(HRSZ)
is
thought
to
mark
the
boundary
between
the
SMZ
and
the
Kaapvaal
craton
to
the
south.
The
1.9
Ga
Soutpansberg
basin,
which
forms
a
40
km
wide,
300
km
long,
7
km
thick
volcano-sedimentary
trough,
partially
cross-cuts
the
PTSZ
and
developed
as
a
graben-
like
basin
(Tankard
et
al.,
1982;
Kamber
et
al.,
1995a,b;
Schaller
et
al.,
1999).
Granitoid
gneisses
and
NE-trending
greenstone
belts
(i.e.,
Murchison,
Pietersburg
and
Giyani
belts)
make
up
the
geolog-
ical
composition
of
the
north-eastern
Kaapvaal
craton
(Rollinson,
1993).
The
SMZ
and
the
Kaapvaal
craton
show
uniform
and
low
207Pb/204 Pb
and206Pb/204 Pb
isotope
ratios
in
metapelites
and
leu-
cocratic
granitoids
(Kreissig
et
al.,
2000)
suggesting
that
the
SMZ
is
a
high-grade
equivalent
of
the
Kaapvaal
craton.
This
result
is
in
con-
trast
to
a
separate
terrane
model
for
the
SMZ
proposed
by
Rollinson
(1993),
who
argued
that
the
different
crustal
histories
and
the
sep-
aration
of
the
SMZ,
CZ
and
NMZ
by
major
thrust
faults
pointed
to
separate
Limpopo
belt
terranes.
Similarly,
the
Zimbabwe
craton,
the
NMZ
and
the
CZ
show
elevated 207Pb/204 Pb
and 206Pb/204 Pb
iso-
tope
ratios,
implying
that
these
three
terranes
cannot
be
regarded
as
separate
units
from
each
other
and
were
derived
from
poten-
tially
the
same
source,
either
in
the
mantle
or
crust,
having
high
U/Pb
ratios
(Barton
et
al.,
2006;
Andreoli
et
al.,
2011;
Kramers
et
al.,
2001,
2011).
The
geometry
of
the
shear
zones
bounding
and
separating
the
marginal
zones
and
central
zones
from
the
cratonic
blocks
have
been
central
to
some
of
the
proposed
collisional
models
of
the

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Precambrian
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226 (2013) 143–
156
evolution
of
the
Central
belt
(Durrheim
et
al.,
1992;
De
Beer
and
Stettler,
1992;
Van
Reenen
et
al.,
1987;
Roering
et
al.,
1992;
Treloar
et
al.,
1992).
The
Kaapvaal
craton
forms
the
core
of
the
composite
Kalahari
craton
and
formed
in
Archaean
times
ca.
3.7–2.6
Ga
(de
Wit
et
al.,
1992).
Post-stabilization
processes
include
the
development
of
sedimentary
basins
(e.g.,
Witwatersrand
basin)
at
ca.
3
Ga
and
were
followed
by
extensive
volcanism
(Ventersdorp
magmatism)
(Eglington,
2004).
The
very
widespread
platform
sedimentation
of
the
Transvaal
Supergroup
that
overlies
the
Ventersdorp
Super-
group
represents
exceptional
early
stability
of
the
Kaapvall
craton.
Bushveld
complex
(Fig.
2),
which
intruded
into
the
Kaapvaal
craton
and1
upper-most
Transvaal
sequence,
is
the
largest
known
layered
intrusion
on
Earth
and
its
emplacement
significantly
altered
the
thermal
and
chemical
structure
of
the
Kaapvaal
craton
at
2.06
Ga
(James
et
al.,
2001;
Evans
et
al.,
2011).
The
Zimbabwe
craton,
which
is
also
part
of
the
greater
Kalahari
craton,
is
itself
thought
to
be
com-
posed
of
a
number
of
distinct
tectonostratigraphic
terranes,
which
consist
of
3.5–2.95
Ga
gneissic
rocks
overlain
by
2.92
Ga
assemblage
of
mafic
and
felsic
volcanic
rocks
at
its
core
(Blenkinsop
and
Vinyu,
1997;
Kusky,
1998).
Several
3.5–2.6
Ga
greenstones
belts
complete
the
lithological
profile
of
the
Zimbabwe
craton
(Blenkinsop
and
Vinyu,
1997).
3.
Tectonic
models
of
Limpopo
belt
The
complex
nature
of
the
geological
and
structural
relation-
ships
in
the
Limpopo
belt
has
led
to
several
plate
tectonic
and
non-plate
tectonic
models
being
proposed
for
its
evolution.
These
were
detailed
and
reviewed
by
Kramers
et
al.
(2011)
and
are
sum-
marized
here.
3.1.
The
Neoarchaean
Himalayan
model
Treloar
et
al.
(1992)
were
the
first
to
propose
a
model
of
the
Limpopo
belt
involving
continental
growth
by
accretion
followed
by
shortening,
which
is
similar
to
the
Mesozoic
evolution
of
Tibetan
plateau
during
India–Asia
collision.
Central
to
this
model
was
the
observation
of
a
regional
structural
pattern
that
suggest
NW–SE
compression,
resulting
in
crustal
thickening
that
also
involved
fold-
ing
and
NW-directed
thrusting
and
lateral
extrusion
of
crustal
blocks
along
SW-
to
WSW-trending
shear
zones.
Treloar
et
al.
(1992)
thus
argued
that
terrane
status
(i.e.,
that
each
unit
formed
separately
as
unique/discrete
crustal
or
lithospheric
blocks
prior
to
amalgamation
and
accretion
as
the
Limpopo
belt)
for
the
SMZ,
CZ
and
NMZ
was
not
required,
and
that
the
Limpopo
belt
was
a
result
of
a
crustal
deformation
event
that
included
much
of
the
Kaapvaal
and
Zimbabwe
cratons.
Roering
et
al.
(1992)
argued
that
the
gran-
ulite
terrane
was
a
result
of
crustal
thickening
in
response
to
the
northward
thrusting
of
the
Kaapvaal
craton
over
the
Zimbabwe
cra-
ton
along
the
Triangle
Shear
Zone,
ca.
2.7–2.6
Ga.
This
was
followed
by
a
metamorphic
event
and
subsequently
isothermal
decompres-
sion
during
which
rocks
moved
upward
and
spread
outward
onto
the
adjacent
cratons,
during
a
period
of
widespread
anatexis,
cre-
ating
what
has
since
being
called
a
pop-up
structure
(Roering
et
al.,
1992).
3.2.
Terrane
accretion
models
Rollinson
(1993)
and
Barton
et
al.
(2006)
proposed
models
describing
the
Limpopo
belt
formed
by
accretion
of
separate
ter-
ranes
of
unrelated
origin
that
constitute
the
NMZ,
CZ
and
SMZ.
In
Rollinson
(1993)’s
model
the
distinct
crustal
evolutions
of
the
zones,
supported
by
prominent
shear
zones
separating
them,
implied
these
blocks
accreted
together
prior
to
the
collision
of
between
the
Kaapvaal
and
Zimbabwe
cratons
in
Neoarchaean
and
warrant
their
consideration
as
discrete
terranes.
Barton
et
al.
(2006)
proposed
a
similar
accretion
model,
but,
unlike
Rollinson
(1993),
the
process
involved
a
complex
assem-
bly
of
a
large
number
of
terranes
between
ca.
2.7
and
ca.
2.04
Ga,
where
the
SMZ,
CZ,
NMZ,
Zimbabwe
craton,
Phikwe
and
Beit
Bridge
complexes,
accreted,
in
subduction
settings,
to
form
migrating
arcs
that
led
to
development
of
juvenile
crust.
This
Turkic-type
accretion
was
proposed
by
Sengor
and
Natal’in
(1996)
as
the
principal
craton
building
process
through
Earth’s
history.
In
this
model
the
Beit
bridge
and
Phikwe
complexes,
in
addition
to
the
terranes
defined
by
Rollinson
(1993),
show
distinct
P–T–t
(pressure–temperature–time)
paths
and
metallogenic
signatures,
that
suggest
they
are
separate
terranes.
Furthermore,
the
lack
of
S-type
granitoid
magmatism,
ophiolites
and
syntectonic
sed-
imentary
basins
led
Barton
et
al.
(2006)
to
argue
against
the
continent–continent
collision
model
(Kramers
et
al.,
2011).
3.3.
Transpression
model
for
the
central
zone
In
contrast
to
McCourt
and
Vearncombe
(1992),
who
argued
for
a
dip-
or
oblique-slip
movement
along
shear
zones
(see
below),
Kamber
et
al.
(1995a,b)
interpreted
the
movement
along
the
TSZ
and
PTSZ
as
being
dextral-transcurrent
that
recorded
the
Paleopro-
terozoic
transpressive
collisional
event
which
resulted
in
crustal
thickening
and
uplift
of
the
CZ.
This
model
was
later
expanded
by
Holzer
(1998)
and
Schaller
et
al.
(1999).
3.4.
Other
models
Other
models
that
have
been
invoked
include
that
of
McCourt
and
Vearncombe
(1992)
who,
based
on
Pb
isotopic
and
struc-
tural
grounds,
interpreted
the
CZ
to
be
an
exotic
terrane
that
was
emplaced
from
NE
to
SW
as
a
thrust
sheet
facilitated
by
the
Triangle-Tuli-Sabi
and
Sunnyside-Palala
structures
which
acted
as
complimentary
lateral
ramps.
Based
on
Pb
isotope
data
on
igneous
rocks
and
the
dip
slip
shear
movement
along
these
structures,
which
resulted
in
the
CZ
being
the
structurally
the
highest
in
rela-
tion
to
the
marginal
zones,
McCourt
and
Vearncombe
(1992)
argued
that
the
CZ
may
have
represented
a
part
of
the
overriding
plate
during
the
Neoarchaean
Limpopo
orogeny
(Kramers
et
al.,
2011).
4.
Previous
geophysical
studies
of
the
Limpopo
belt
Several
studies
have
been
performed,
each
attempting
to
image
the
geometry
and
structure
of
the
Limpopo
belt.
These
are
summa-
rized
in
Fig.
3.
The
geoelectric,
seismic
reflection
and
gravity
results
of
De
Beer
and
Stettler
(1992)
and
Durrheim
et
al.
(1992)
formed
the
basis
from
which
many
of
the
collisional
models
were
formulated
(Fig.
3D).
The
northward
and
southward
dip
of
HRSZ
and
the
NLTZ
respectively
lend
support
to
the
‘pop-up’
model
derived
by
Roering
et
al.
(1992),
as
outlined
above.
Furthermore,
the
gravity
results
delineated
high
density
rocks
in
the
upper
crust
of
the
NMZ
and
SMZ.
The
PTSZ
did
not
correspond
with
any
seismic
reflection
signa-
ture
which
led
De
Beer
and
Stettler
(1992)
to
propose
that
it
is
a
near
vertical
fault
that
penetrates
the
entire
crust.
The
work
of
James
et
al.
(2001)
used
P-wave
and
S-wave
delay
times
from
a
broad-
band
seismic
array
to
map
high
velocity
mantle
roots
extending
to
depths
of
250–300
km
beneath
the
Kaapvaal
and
Zimbabwe
cra-
tons
and
the
Limpopo
belt
(Fig.
3B).
In
a
related
crustal
study,
Nguuri
et
al.
(2009)
analyzed
receiver
functions
to
map
the
crustal
struc-
ture
of
the
Limpopo
belt
(Fig.
3A).
The
NMZ
was
found
to
have
a
37
km
thick
crust,
similar
to
the
Zimbabwe
craton,
while
the
SMZ
had
a
40
km
thick
crust
similar
to
Kaapvaal
craton
and
the
Moho
in
both
zones
generated
strong
P-to-S
conversions.
However,
the
Moho
structure
from
Nguuri
et
al.
(2009)’s
study
appeared
more

D.
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Precambrian
Research
226 (2013) 143–
156 147
Fig.
3.
Summary
of
geophysical
studies
undertaken
over
the
Limpopo
belt.
The
study
of
Nguuri
et
al.
(2009)
focussed
on
defining
the
crustal
structure
across
the
Kaapvaalcraton,
Limpopo
and
Zimbabwe
craton
using
seismic
receiver
functions
(A),
while
the
tomography
work
of
James
et
al.
(2001)
focused
on
the
deep
mantle
lithospheric
structure
(B).
De
Beer
and
Stettler
(1992)
and
Durrheim
et
al.
(1992)
used
reflection
seismic
data
to
study
the
crustal
shear
zones
bounding
the
marginal
zones
of
Limpopo
belt
(D)
and
more
recently
Gore
et
al.
(2010)
used
seismic
receiver
functions
to
study
the
crustal
structure
of
the
Limpopo
Central
Zone.
complex
beneath
the
CZ,
corresponding
with
weaker
P-to-S
con-
versions.
In
a
second
attempt,
using
additional
seismic
stations,
Gore
et
al.
(2010)
analyzed
receiver
function
data
and
derived
a
Moho
map
(Fig.
3C)
beneath
the
entire
Limpopo
belt
which
appears
to
show
thick
(more
than
50
km)
crust
for
the
Kaapvaal
and
Zimbabwe
cratons,
which
are
consistent
with
estimates
from
Nguuri
et
al.
(2009)’s
results.
As
noted
by
Gore
et
al.
(2010),
the
Limpopo
belt
has
a
low
elevation
relative
to
the
adjacent
cratons,
which
is
puzzling
on
isostacy
grounds
given
the
deeper
Moho.
The
authors
explain
this
discrepancy
by
interpreting
the
CZ
as
a
remnant
of
a
deep-
rooted
crustal
block
that
did
not
fully
rebound
during
denudation.
The
notion
of
a
dense
lower-crustal/upper-mantle
root
beneath
the
CZ
is
supported
by
the
positive
Bouger
anomaly
and
could
possibly
be
the
result
of
magmatic
underplating
(Gore
et
al.,
2010;
Gwavava
et
al.,
1992;
Ranganai
et
al.,
2002;
Kramers
et
al.,
2011).
5.
The
magnetotelluric
(MT)
method
and
data
The
magnetotelluric
method
is
an
electromagnetic
(EM)
sound-
ing
technique
that
has
evolved
rapidly
since
its
first
theoretical
description
in
the
1950s.
By
measuring
the
time
variations
on
the
surface,
of
the
horizontal
electric
(Ex,
Ey)
and
horizontal
and
verti-
cal
magnetic
(Hx,
Hyand
Hz)
fields
induced
in
the
subsurface,
we
can
derive
the
lateral
and
vertical
subsurface
variations
of
electrical
resistivity.
The
ratios
of
the
EM
fields
are
related,
in
the
frequency
(ω)
domain,
by
an
impedance
Zxy(ω)
=
Ex(ω)
=
Hy(ω),
from
which
the
apparent
resistivity
(i.e.,
the
resistivity
of
a
homogeneous
half
space)
a,xy(ω)
=
1
ω




Ex(ω)
Hy(ω)




(1)
and
impedance
phase
xy(ω)
=
arctan(Ex(ω))
Hy(ω)(2)
can
be
estimated
(Chave
and
Jones,
2012).
In
Southern
Africa,
we
have
acquired
broadband
(periods
from
0.001
s
to
8000
s)
and
long
period
(15
s
to
over
10,000
s)
magne-
totelluric
data
as
part
of
the
SAMTEX
project
(Fig.
1).
More
than
750
stations
of
data
were
collected
in
Namibia,
Botswana
and
South
Africa
over
four
field
seasons
along
various
profiles
from
2003
to
2008,
with
station
spacings
of
approximately
20
km
and
60
km
for
broadband
and
long
period
data,
respectively.
The
orientations
of
the
profiles
were
chosen
to
transect
over
specific
geological
fea-
tures
of
interest.
Magnetotelluric
broadband
data
were
collected
using
Phoenix
Geophysics
(Toronto)
MTU5
instruments,
and
long
period
data
were
acquired
with
LVIV
(Ukraine)
LEMI
systems.
In
this
work
we
model
MT
data
collected
along
three
profiles:
LOW,
LIM-SSO
and
KAP
(Fig.
2).
The
NE–SW
KAP
line
was
the
focus
of
the
Kaapvaal
lithospheric
study
of
Evans
et
al.
(2011).
We
focus
here
specifically
on
the

148 D.
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Precambrian
Research
226 (2013) 143–
156
Fig.
4.
MT
responses
curves
for
4
sites,
showing
apparent
resistivity
and
phase
plotted
against
increasing
period
(proxy
for
depth).
The
red
squares
show
the
transverse-electric
(TE)
mode
data,
and
the
blue
squares
show
the
transverse-magnetic
(TM)
mode
data.
northern
half
of
the
profile
traversing
over
the
Limpopo
belt.
There
were
several
motivations
for
remodelling
this
part
of
the
profile.
Firstly,
the
work
of
Evans
et
al.
(2011)
focussed
principally
on
defining
the
Kaapvaal
craton
lithosphere
not
the
Limpopo
belt.
Secondly
Evans
et
al.
(2011)
developed
lithospheric
models
using
2D
techniques.
While
2D
modelling
is
able
produce
first-order
regional
features,
it
cannot
account
for
3D
data
complexities,
particularly
off-profile
features.
To
address
this
deficiency
we
apply
a
newly-developed
3D
inversion
algorithm
(Egbert
and
Kelbert,
2012)
to
derive
information
on
the
3D
structure
of
the
Limpopo
belt.
To
this
end
only
14
stations
from
the
KAP
profile
were
modelled,
crossing
the
northern
limb
of
the
Bushveld
Igneous
Complex
(BIC),
the
SMZ
and
the
CZ
(purple
sites
in
Fig.
2).
The
almost
NNW-SSE
LOW
profile
comprises
12
stations
cross-
ing
the
northern
Kaapvaal
craton
in
the
south,
extending
into
the
SMZ
and
part
of
the
CZ
(yellow
sites
in
Fig.
2).
Vertical
magnetic
field
(Hz)
data
were
recorded
at
3
stations
only
on
the
LOW
profile.
Due
to
logistical
and
security
concerns
of
going
into
Zimbabwe
at
the
time
of
the
survey,
the
LOW
profile
was
terminated
close
to
the
South
Africa–Zimbabwe
border.
The
NW–SE
LIM-SSO
profile
comprises
25
stations
(13
of
which
recorded
Hz
data)
crossing
(from
SE
to
NW)
the
Kaapvaal
craton,
the
northern
limb
of
the
BIC,
the
western
Limpopo
belt
and
the
south-
western
margin
of
the
Zimbabwe
craton
(green
sites
in
Fig.
2).
5.1.
MT
data
and
processing
The
recorded
electric
and
magnetic
time
series
data
were
processed
using
standard
robust
processing
methods
of
Jones
and
Jodicke
(1984),
Egbert
(1997)
and
Chave
and
Thompson
(2004)
(methods
6,
7
and
8
in
Jones
et
al.,
1989).
Given
that
multiple
sites
were
recorded
simultaneously,
we
employed
remote
referencing
methods
(Gamble
et
al.,
1979)
to
reduce
bias
effects
and
improve
the
quality
of
the
estimated
MT
responses.
The
resulting
responses
are
shown
in
Fig.
4
for
four
rep-
resentative
stations
on
and
off
the
Limpopo
belt,
where
variation

D.
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/
Precambrian
Research
226 (2013) 143–
156 149
Fig.
5.
Pseudosections
of
TE
and
TM
mode
data
for
the
three
profiles,
showing
apparent
resistivity
and
phases
as
a
function
of
period
increasing
downwards
(proxy
for
depth).
Some
features,
like
the
Kaapvaalcraton
upper
crust
and
the
resistive
lithologies
of
the
Limpopo
belt
Central
Zone
are
readily
recognizable.
in
apparent
resistivity
is
plotted
as
a
function
of
period,
the
latter
being
a
proxy
for
depth
(i.e.,
the
longer
the
period
the
deeper
the
depth
of
penetration).
Data
quality
was
generally
very
good
for
most
stations
on
the
KAP
and
LIM-SSO
profiles.
The
LOW
profile
sites
suffered
from
long
period
distortion
and
at
most
stations
we
were
only
able
to
model
periods
up
to
300
s.
However,
the
general
resistive
nature
of
the
crust,
inferred
from
resistivity
studies
of
De
Beer
et
al.
(1991)
in
the
SMZ
and
northern
Kaapvaal
craton
(high
and
low
grade
granitoids),
implied
that
depth
of
penetration
up
to
100
km
was
assured
(also
estimated
with
1D
Niblett-Bostick
approximations).
Fig.
5
shows
pseudo-section
plots
of
apparent
resistivity
and
phases
for
the
LOW,
KAP,
LIM-SSO
profiles
giving
an
indication
of
the
lateral
variation
of
resistivity
with
period
(i.e.
depth).
The
appar-
ent
resistivity
and
phase
pseudo-sections
are
shown
for
both
the
transverse
magnetic
(TM)
and
transverse
electric
(TE)
modes.
High
resistivity
values
are
represented
by
blue
(cold)
colours,
whereas
red
(hot)
colours
indicate
low
resistivity
values.
Although
these
images
are
distorted
due
to
the
representation
of
distances
along
the
abscissa
versus
log
(period)
instead
of
true
depth,
some
major
features
can
already
be
recognized.
The
LOW
profile
in
particular
reveals
the
resistive
lithologies
of
the
Kaapvaal
craton,
SMZ
and
Central
Zones
of
the
Limpopo
belt.
We
will
discuss
key
features
that
are
resolved
from
3D
inversion
modelling
of
these
data
in
a
Section
7.
6.
3D
inversion
modelling
The
motivation
for
doing
3D
instead
of
2D
inversion
is
that
no
assumption
about
the
dimensionality
of
the
data
had
to
be
made.
Furthermore,
modelling
the
four
components
of
the
impedance
together
with
vertical
transfer
functions
(Hz)
enable
us
to
define
the
nature
of
the
structures
that
would
otherwise
not
be
resolved
by
applying
2D
inversion
modelling,
thus
we
are
able
to
gain
added
information
about
the
resistivity
distribution
at
depth.
In
total,
MT
data
from
a
total
of
51
stations
were
modelled.
The
modular
EM
code
of
Egbert
and
Kelbert
(2012)
was
used
to
gen-
erate
3D
models.
The
apparent
resistivity
error
floors
were
set
to
10%
for
the
diagonal
and
15%
for
the
off-diagonal
elements
of
the
impedance
tensor
(i.e.,
if
the
errors
were
less
than
10%
or
15%
of
the
amplitude
of
the
impedance,
they
were
set
to
that
level,
if
they
were
more,
they
were
unchanged).
The
phase
error
floors
were
set
to
5%
and
a
constant
absolute
error
floor
in
Hz
was
set
at
0.01%.
The
size
of
the
3D
grid
was
79,
72
and
52
cells
on
the
north,
east
and
vertical
downwards
direction,
respectively.
The
impedance
elements
(Zxx,
Zxy,
Zyy,
Zyx)
and
Hzwere
modelled
with
the
smoothing
parameter
tau
()
set
to
3
and
using
a
100
ohm-m
half-space
as
an
input
model.
The
final
model
produced,
converged
to
an
average
RMS
of
3.61
(Fig.
9).
In
order
to
validate
the
features
observed
in
the
3D
model
and
test
the
resolution
of
nearby
off-profile
conductors,
we
con-
ducted
3D
inversion
of
each
of
the
2D
profiles
separately
(3D/2D).
Siripunvaraporn
et
al.
(2005)
demonstrated
the
advantages
of
mod-
elling
data
this
way
using
synthetic
examples
and,
in
a
more
recent
study,
Patro
and
Egbert
(2011)
applied
similar
technique
to
model
data
from
the
Deccan
Volcanic
Province.
The
principal
advantage
is
that
off-profile
features
are
correctly
located
spatially,
and
are
not
artificially
placed
beneath
the
profile,
as
can
be
the
case
in
2D.
In
order
to
maintain
consistency
we
use
the
same
inversion
parame-
ters
(i.e.,
four
impedance
elements
Zxx,
Zxy,
Zyy,
Zyx were
modelled,
using
a
tau
value
of
3
and
100
ohm-m
half-spaces
as
input).

150 D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156
Fig.
6.
3D
E-W
perspective
view
showing
the
variation
in
resistivity
laterally
and
depth
across
the
Limpopo
belt.
The
location
of
the
Orapa
and
Martin’s
Drift
kim-
berlite
fields
are
projected.
The
dark
solid
line
shows
the
approximate
trace
of
the
Palala-Tshipise-Sunnyside
Shear
system.
SPSZ:
Sunnyside-Palala
Shear
Zone.
The
conductive
Feature
A
is
explained
more
in
the
text.
7.
Results
Our
final
3D
model
(in
perspective
view)
from
inverting
all
data
simultaneously
is
shown
in
Fig.
6,
and
horizontal
depth
slices
through
the
volume
are
shown
in
Fig.
7.
The
blue
triangles
show
the
locations
of
the
MT
stations
and
the
off-profile
extent
of
the
sec-
tions
is
limited
to
the
corresponding
depth-footprint.
Highlighted
on
both
figures
are
the
SMZ,
CZ,
PTSZ,
HRZ,
Bushveld
complex
and
Kaapvaal
and
Zimbabwe
cratons.
On
comparing
the
10
km
depth
slices
to
the
locations
of
major
shear
zones,
it
is
clear
that
the
PTSZ
corresponds
to
a
major
resistivity
contrast
down
to
lower
crustal
level.
In
order
to
obtain
an
indication
of
the
geometry
and
extent
of
the
structures
at
depth,
2D
sections
were
extracted
from
the
3D
model
and
are
shown
in
Fig.
8.
We
will
now
highlight
the
major
features
resolved
for
each
profile.
7.1.
LOW
profile
The
crust
in
the
southern
part
of
the
model
is
dominated
by
resistive
features
in
the
SMZ
and
the
Kaapvaal
craton.
There
is
a
significant
resistivity
break,
up
to
20
km
in
lateral
distance,
in
the
model
at
sites
LOW003
and
LOW004
that
correlates
spatially
with
the
Soutpansberg
basin
(see
Fig.
2).
The
1.9
Ga
elongated
Soutpansberg
Basin
occurs
within
the
SMZ
on
the
southern
side
of
the
PTSZ
and
partly
transcends
the
location
of
the
PTSZ.
The
3D/2D
models
suggest
that
the
basin
extends
off-profile
to
the
east
and
west
confirming
its
E–W
orientation.
The
location
of
the
PTSZ
is
characterized
by
a
conductive
signature.
Based
on
its
lack
of
seismic
response,
Durrheim
et
al.
(1992)
and
De
Beer
and
Stettler
(1992)
interpreted
the
PTSZ
to
have
a
sub-vertical
dip.
The
sites
KAP074
and
LOW001
are
clustered
around
the
2.57
Ga
Bulai
pluton.
Compositionally
the
pluton
is
charnokitic,
made
up
of
granites
and
granodiorites
and
as
a
result
it
appears
as
a
resistive
feature.
Conductive
Feature
B
beneath
the
SMZ
occurs
at
35
km
depths
and
similar
to
the
enigmatic
feature
imaged
by
De
Beer
et
al.
(1991)
using
DC
resistivity
and
LOTEM
methods.
Moho
Fig.
7.
Crustal
depth
sections
derived
from
3D
inversion
model.
Three
crustal
depth
are
shown.
The
10
km
section
is
overlain
on
the
geological
map
of
the
Limpopo
belt
(after
Kramers
et
al.,
2011).
The
main
features
are
highlighted,
including
the
locations
of
the
Orapa
and
Martin’s
drift
kimberlite
fields
which
are
in
close
proximity
to
the
LIM-SSO
profile.

D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156 151
Fig.
8.
LOW,
KAP
and
LIM-SSO
profiles
derived
from
3D
inversion
model.
The
Moho
depth
on
the
LOW
profile
was
derived
from
the
seismic
receiver
function
study
of
Gore
et
al.
(2010)
(note
the
profile
depth
is
100
km).
The
location
of
Venetia
kimberlite
is
projected
on
the
KAP
profile
and
is
shown
as
red
arrow.
Similarly,
the
Orapa
and
Martin’s
Drift
kimberlite
clusters
are
shown
on
the
LIM-SSO
profile.
Features
A
and
B
are
referred
to
in
the
text.
PTSZ:
Palala-Tshipise
Shear
Zone,
SPTSS:
Sunnyside-Palala-Tshipise
Shear
System.
depth
was
estimated
by
Gore
et
al.
(2010)
from
seismic
receiver
functions,
and
infers
a
relatively
thicker
crust
beneath
the
CZ.
7.2.
KAP
profile
Similar
features
to
the
LOW
profile
are
observed
on
the
KAP
pro-
file.
The
resistive
granite-greenstones
lithologies
of
the
Kaapvaal
craton
to
the
south
are
mapped
in
the
upper
crust.
A
significant
con-
ductivity
break
in
crustal
structure
is
observed
with
a
conductive
anomaly
corresponding
to
the
northern
limb
of
the
Bushveld
com-
plex.
The
PTSZ
is
situated
along
sites
KAP068,
KAP069
and
KAP070
and
is
similarly
evident
as
a
conductive
feature.
The
resistive
upper-
crustal
Beitbridge
Complex
lithologies
extend
to
a
depth
of
10
km
and
are
underlain
by
conductive
Feature
A.
Given
the
tectonic
impli-
cations
of
the
location
and
geometry
of
the
PTSZ,
several
3D
forward
models
were
generated
in
order
to
obtain
a
geometrical
model
that
matches
the
observed
resistivity
responses.
To
this
end,
we
tested
models
where
a
15–20
km
conductive
zone
(approximately
10
m,
akin
to
PTSZ)
is
embedded
in
a
resistive
media
(10,000
m,
akin
to
the
CZ
and
Kaapvaal).
Various
dip
angles
were
tested
and
the
model
with
the
conductive
zone
having
a
sub-vertical
dip
returned
almost
similar
responses
to
those
of
KAP068
and
KAP069
(Fig.
8).
7.3.
LIM-SSO
profile
The
SE
part
of
this
profile
is
characterized
by
resistive
low
grade
lithologies
related
to
the
Kaapvaal
craton
and
a
conductive
fea-
ture
extending
to
depths
of
over
100
km
attributed
to
the
northern
limb
of
the
Bushveld
Igneous
Complex.
The
prior
2D
isotropic
and
anisotropic
models
of
Evans
et
al.
(2011)
mapped
the
Bushveld
complex
as
a
mantle
conductive
feature
extending
to
depths
in
excess
of
150
km.
The
composite
SPTSS
is
positioned
on
the
geo-
logical
map
between
MT
sites
LIM002
and
SSO101.
This
region
corresponds
to
a
vertically-dipping
conductive
feature
on
the
MT

152 D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156
Fig.
9.
RMS
misfit
between
modelled
data
and
resulting
model
plotted
from
each
station.
model.
The
SPTSS
is
composed
of
gneisses
and
mylonites
(McCourt
and
Vearncombe,
1992),
therefore
the
observed
elevated
conduc-
tivities
are
interpreted
to
map
the
lateral
and
depth
extent
of
the
SPTSS.
(McCourt
and
Vearncombe,
1992)
mapped
the
SPTSS
as
a
40
km
wide
linear
shear
system
that
maps
the
boundary
between
the
CZ
and
the
SMZ.
From
the
resistivity
models
we
infer
that
the
SPTSS
has
approximately
50
km
lateral
extent
and
that
it
represents
a
fundamental
crustal
suture
zone.
The
locations
of
the
Martin’s
drift
and
Orapa
kimberlite
clusters
are
projected
on
the
model
and
the
lithosphere
beneath
it
is
characteristically
resistive
and
thick.
8.
Interpretation
Several
features
are
resolved
from
3D
inversion
modelling:
(1)
the
upper
crustal
resistive
granite-greenstone
of
the
Kaapvaal
and
SMZ,
(2)
the
lower
crustal
conductors
labelled
A
and
B
on
the
LOW
profile
and
A
on
the
KAP
profile,
(3)
the
conductive
feature
associ-
ated
with
the
composite
Sunnyside-Palala-Tshipise
Shear
System
(SPTSS).
On
the
LOW
and
KAP
profiles
(in
South
Africa)
this
shear
zone
is
labelled
the
Palala-Tshipise
Shear
Zone
(PTSZ)
and
it
extends
west-
ward
(into
NE
Botswana)
where
is
links
with
the
Sunnyside
Shear
Zone
(SSZ)
and
becomes
a
composite
SPTSS.
In
the
region
of
sites
KAP071
to
KAP075
(Fig.
2),
the
upper
crust
is
composed
of
Beit
Bridge
Complex
lithologies,
which
comprises
quartzo-feldspathic
rocks
with
a
granitic
bulk
compositions
(Klemd
et
al.,
2003).
These
are
evident
in
the
LOW
and
KAP
profiles
(Fig.
8)
as
resistive
features.
Below
these
rocks,
at
10
km
depth,
is
a
conductive
(approximately
10
m)
Feature
A.
Seismic
results
(De
Beer
and
Stettler,
1992;
Durrheim
et
al.,
1992)
indicate
a
reflector
at
10
km
depth
below
the
Beit
Bridge
complex,
corresponding
to
the
top
of
the
conductive
Feature
A.
Conductive
middle
to
lower
crust
is
observed
globally
(Jones
et
al.,
1992;
Hyndman
et
al.,
1993)
and
in
the
Canadian
Cordillera,
for
example,
coincident
reflective
and
conductive
mid-
dle
to
lower
crust
is
observed
(Marquis
et
al.,
1995),
the
causes
of
which
are
thought
to
be,
principally,
trapped
saline
pore-fluids
for
Phanerozoic
regions
or
graphite
for
Precambrian
regions.
Pretorius
(2003)
combined
xenolith-derived
P–T
mineral
equilibria
data
with
seismic
information
to
derive
the
crustal
and
upper
mantle
lithol-
ogy
structure
beneath
the
520
Ma
Venetia
kimberlite
cluster.
This
structure
consists
of
supracrustal
rocks
down
to
10
km
depth,
over-
lying
mafic
amphibolite
rocks
and
restites
(garnet-quartz
rocks,
granulite
and
eclogite).
According
to
Pretorius
(2003),
the
amphibolites/restites
were
derived
from
partial
melting
of
a
remnant
subducted
Fe–O
rich
Archaean
oceanic
crust
and
gravitationally
settled
at
deeper
levels
due
to
their
higher
densities
(3.2–4.5
g/cm3).
However
it
is
unlikely
that
Feature
A
corresponds
to
the
amphibolite
in
that
silicate
rocks
are
usually
resistive
(Chave
and
Jones,
2012;
Evans,
2012);
there-
fore
another
conducting
mineral
phase
must
be
present
to
account
for
the
observed
high
conductivities.
Conductive
Feature
A
extends
from
10
km
depth
to
about
50
km,
although,
given
the
shielding
effect
of
conductive
features,
the
bottom
depth
is
possibly
overes-
timated.
The
top
of
the
conductor
overlain
by
a
resistor
is
usually
wellresolved
with
the
MT
method.
There
is
a
clear
spatial
corre-
lation
between
the
location
of
Feature
A
with
high
density
rocks
(Ranganai
et
al.,
2002)
and
the
interpreted
thickened
crust
(Gore
et
al.,
2010).
In
their
review
of
age
determinations
in
the
Limpopo
Complex,
Kramers
and
Mouri
(2011)
have
commented
on
the
sharp
age
peak
of
the
2.0
Ga
event
in
the
Central
Zone,
and
they
and
Kramers
et
al.
(2011)
have
suggested
that
there
may
have
been
underplating
by
Bushveld
complex
related
mafic
magmas.
They
argued
that
this
could
also
explain
the
gravity
anomaly
and
poorly
defined
Moho
in
the
region.
Given
the
known
conductive
signature
of
the
Bushveld
complex,
this
could
provide
an
alternative
expla-
nation
of
Feature
A.
It
is
thus
worthwhile
to
investigate
the
possible
causes
and
tectonic
significance
of
this
Feature
A.
In
the
Earth’s
crust,
fluids,
interconnected
sulphides/oxides,
graphites,
or
high
conducting
metamorphic
rocks
are
potential
candidates
for
material
that
can
give
rise
to
observed
elevated
conductivities.
The
presence
of
fluids,
particularly
in
Precambrian
terranes,
is
diffcult
to
discern
due
to
the
complex
evolution
of
metamorphic
rocks.
There
is
evidence
of
prograde
and
retrograde
metamorphism,
where
rocks
have
undergone
more
than
one
defor-
mation
event
in
the
Limpopo
belt.
For
this
reason,
Barnes
and
Sawyer
(1980)
argued
that
it
is
unlikely
that
fluids
will
have
remained
stable
in
the
crust
since
Archaean
times,
given
their
short
residence
times.
Also
argued
by
Yardley
and
Valley
(1997),
with
interesting
discussion
by
Wannamaker
(2000)
and
response
by
Yardley
and
Valley
(2000).
Goldfarb
et
al.
(1991)
gives
evidence
for
at
least
70
Ma
residence
times
of
water.
Fluid
inclusion
studies
in
the
Limpopo
belt
have
focused
in
the
Central
Zone
(Hisada
and
Miyano,
1996;
Hisada
et
al.,
2005;
Tsunogae
and
van
Reenen,
2007;
Huizenga
et
al.,
2011)
and
the
South
Marginal
Zone
(van
Reenen
and
Hollister,
1988;
van
Reenen
et
al.,
1994;
van
den
Berg
and
Huizenga,
2001;
Touret
and
Huizenga,
2001).
In
the
SMZ,
fluid
inclusion
studies
(and
high-temperature
reaction
texture)
in
granulites
(Touret
and
Huizenga,
2001;
van
den
Berg
and
Huizenga,
2001)
indicate
presence
of
brines
and
CO2rich-
fluids
during
peak
granulite
facies
metamorphism.
van
den
Berg
and
Huizenga
(2001)
suggest
that
the
brines
represents
remnants
of
preserved
connate
water.
Studies
by
Huizenga
et
al.
(2011)
in
the
Central
Zone
confirmed
the
presence
of
CO2-rich-fluids
in
high-
temperature
Mg-rich
garnets
co-existing
with
brines,
similar
to
the
SMZ.
While
presence
of
fluids
in
the
CZ
and
SMZ
is
known,
questions
can
be
asked
as
to
(1)
which
tectonic
process
introduced
CO2in
the
crust,
(2)
how
widespread
are
they,
and,
more
importantly
for
elec-
trical
conductivity,
(3)
what
is
the
nature
of
the
inter-connectivity
of
the
fluids?
The
fluid
inclusion
studies
undertaken
on
material
from
the
Limpopo
belt
are
unable
to
estimate
absolute
fluid
con-
tent
and,
as
such,
questions
(2)
and
(3)
are
beyond
the
scope
of
this
study,
but
we
address
here
the
first
question
and
suggest
a
possible
mantle
source
for
CO2,
in
a
subduction
setting.
Sm–Nd
and
Lu–Hf
results
suggest
that
the
southern
Zimbabwe
craton
was
an
active
magmatic
arc
from
the
south
to
the
south-
west,
characterized
by
subduction
of
oceanic
lithosphere
ca.
2.7–2.6
Ga
(Bagai
et
al.,
2002;
Kampunzu
et
al.,
2003;
Zhai
et
al.,
2006;
Zeh
et
al.,
2009;
Kramers
et
al.,
2011;
Kramers
and
Zeh,
2011).
Furthermore,
the
CZ,
NMZ
and
Zimbabwe
craton
have
sim-
ilar
elevated
U/Pb
ratios
and
U,
Th
concentrations
(Kramers
et
al.,

D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156 153
2001;
Barton
et
al.,
2006;
Andreoli
et
al.,
2011),
implying
that
all
three
represent
parts
of
the
same
plate
where
the
CZ
is
the
shelf,
the
NMZ
the
active
margin
and
the
Zimbabwe
craton
being
the
overriding
plate
(Kramers
et
al.,
2011).
These
results
contra-
dict
the
Turkic-type
model
that
requires
separate
terrane
status
for
the
CZ,
NMZ
and
Zimbabwe
craton.
The
presence
of
subduct-
ing
carbonate-rich
oceanic
lithosphere
would
release
CO2fluids
into
the
lower
crust
and
react
with
the
mantle
peridotite
of
shelf
region
(CZ),
evidence
of
the
latter
process
in
the
CZ
stems
from
zircon
Lu–Hf
data
on
the
charnockitic
Bulai
pluton
sample
which
shows
remelting
of
older
crust
(Zeh
et
al.,
2007).
However,
the
SM-Nd
studies
of
Harris
et
al.
(1987)
suggested
that
the
pluton
might
have
been
derived
from
anetexis
of
juvenile
crust.
In
addi-
tion,
petrographic
study
of
65
samples
exposed
in
a
10
km2area,
approximately
13
km
west
of
site
KAP074
(Fig.
2),
indicated
the
presence
of
graphite
and
magnetite
(Klemd
et
al.,
2003).
In
the
same
study,
CO2-rich
inclusions
were
found
in
garnet
rims
of
metapelites.
It
is
therefore
reasonable
that
graphite
was
precipitated
from
the
CO2-rich
fluids.
We
therefore
propose
that
the
observed
con-
ductivity
of
Feature
A
is
due
to
the
presence
of
graphite
and
minor
accessory
minerals
like
magnetite,
in
the
crust.
The
graphite
reduc-
tion
mechanism
was
invoked
to
explain
the
high
conductivity
signatures
of
the
Fraser
fault
and
also
proposed
for
Yellowknife
Fault.
The
conductive
feature
B
is
located
in
the
lower
crust.
De
Beer
and
Stettler
(1992)
mapped
it,
but
provided
no
explanation
at
the
time
as
to
its
possible
cause
and
significance.
In
our
models
and
those
of
De
Beer
et
al.
(1991)
conductive
Feature
B
coincides
with
the
position
of
the
HRSZ
at
depth;
it
is
not
known
if
Features
A
and
B
is
are
tectonically
related.
However
given
that
two
independent
studies
using
different
techniques
have
now
confirmed
its
presence
it
is
reasonable
to
infer
that
conductor
B
is
a
pervasive
feature
in
the
crust.
Conductivity
in
the
Earth’s
lower
crust
has
been
observed
from
MT
studies
and
debated
for
many
decades
(Edwards
et
al.,
1981;
Gregori
and
Lanzerotti,
1982;
Jones
et
al.,
1992;
Touret
and
Marquis,
1994;
Glover,
1996;
Yang,
2011),
but
there,
is
yet,
no
consensus.
van
Reenen
and
Hollister
(1988)
suggested
a
subdivision
of
the
SMZ
into
a
northern
granulite
zone
and
a
zone
of
retrograde
hydration
in
the
south,
the
former
being
the
result
of
prograde
melting
reac-
tions
without
involving
fluids
(Stevens,
1997)
whereas
the
latter
is
associated
with
CO2and
brine
rich
fluids
sourced
from
devoli-
tisation
reactions
(van
den
Berg
and
Huizenga,
2001).
Feature
B
is
located
within
this
hydrated
subzone
but
it
is
unlikely
that
fluids
are
responsible
for
the
observed
conductivities
given
their
short
residence
times
in
the
crust,
therefore
graphite
(and
minor
con-
ductive
phases
like
FeO
and
sulphides)
are
suggested
to
be
likely
candidates
for
increased
conductivities.
Central
to
the
pop-up
model
proposed
for
the
Limpopo
develop-
ment
was
the
geophysical
mapping
studies
by
De
Beer
and
Stettler
(1992)
and
Durrheim
et
al.
(1992),
who
imaged
the
northward
and
southward
dip
geometry
of
the
HRZ
and
TSZ
respectively.
The
PTSZ
exhibited
no
seismic
response
in
the
study
of
Durrheim
et
al.
(1992),
leading
to
the
suggestion
that
it
was
a
vertically-dipping
feature.
While
our
results
do
not
dispute
the
previous
results
of
De
Beer
and
Stettler
(1992)
and
Durrheim
et
al.
(1992),
we
map,
for
the
first
time,
the
PTSZ
as
a
conductive
feature
and
suggest
that
it
in
fact
rep-
resent
a
fundamental
suture
between
the
Kaapvaal
and
Zimbabwe
cratons.
The
PTSZ
appears
to
be
sub-vertical
on
the
KAP
and
LIM-
SSO
profiles
and
dips
slightly
to
the
north
on
the
LOW
profile.
On
a
mantle
lithospheric
scale,
the
PTSZ
correlates
with
the
disconti-
nuity
observed
between
the
300
km
thick
Kaapvaal/SMZ
block
and
the
250
km
thick
Zimbabwe/NMZ/CZ
block
(Fouch,
2004).
The
PTSZ
comprises
conductive
mylonite
and
ultramylonite
rocks,
similar
to
those
found
in
some
parts
of
the
Bushveld
complex
(McCourt
and
Vearncombe,
1992).
The
conductive
region
in
the
Bushveld
crust
is
related
to
metal-
lic
sulphides
and
oxides
widespread
in
the
complex
(the
Bushveld
complex
is
the
largest
resource
for
platinum
group
metals).
The
location
of
the
Northern
Limb
of
the
BIC
is
on
a
junction
between
the
Hout
River
Shear
Zone
and
the
Palala-Tshipise
Straightening
zone.
These
shear
zones
are
both
characterized
by
high
conductivity
sig-
nature.
Kamber
et
al.
(1995b)
noted
that
high
grade
metamorphism
at
2.0
Ga,
which
was
a
result
of
collision,
was
coeval
with
dextral
transcurrent
shear
movement
of
the
PTSZ,
suggesting
a
transpres-
sive
collisional
event.
Given
that
the
Soutpansberg
basin
is
located
in
the
SMZ
just
south
of
the
PTSZ,
it
is
possible
that
the
deposition
of
volcano-clastic
sediments
in
the
trough
developed
on
the
exten-
sional
side
of
the
transpressive
shear
zone
system
(post
CZ
uplift),
an
observation
suggested
by
Kramers
et
al.
(2011).
This
model
is
in
contrast
with
the
aulocogen
model
proposed
by
Jansen
(1975),
and
is
in
agreement
with
a
half-graben
setting
suggested
by
Bumby
et
al.
(2002)
and
Tankard
et
al.
(1982).
The
geometry
of
inter-cratonic
sutures
have
played
a
significant
role
in
the
evolution
of
the
African
tectonic
landscape,
by
focussing
ascending
magmas
and
areas
of
localized
rifting
(Begg
et
al.,
2009).
The
close
spatial
proximity
of
the
PTSZ
and
the
Venetia
and
Martin’s
drift
kimberlite
suggest
that
these
shear
zones
could
have
possibly
acted
as
conduits
to
ascending
magma,
leading
to
the
emplacement
of
kimberlite
volcanic
material.
The
projection
of
the
Orapa
kimber-
lite
on
the
LIM-SSO
MT
model
suggests
that
it
plots
on
the
part
of
resistive
thick
lithosphere
that
is
an
extension
of
the
Zimbabwe
craton,
as
was
suggested
by
Miensopust
et
al.
(2011).
The
resistive
lithosphere
in
this
region
on
the
LIM-SSO
profile
extends
to
150
km
depth.
Seismic
tomography
maps,
however,
infer
a
seismically
slow
mantle
beneath
the
Orapa
kimberlite
field
(James
et
al.,
2001)
which
was
attributed
to
intrusion
events
related
to
mid-Proterozoic
colli-
sion
of
the
Okwa
and
Magondi
belts
(Shirey
et
al.,
2002;
Griffin
et
al.,
2003)
that
significantly
modified
the-then
Archaean
lithosphere.
9.
Towards
a
tectonic
model
From
the
discussions
above
and
combining
the
resistivity
mod-
els
and
the
recent
metamorphic
results,
we
propose
a
model
for
the
evolution
of
the
Limpopo
belt
(illustrated
in
Fig.
10)
that
involves
three
main
tectonic
processes,
namely
(1)
subduction
phase
at
2.7–2.6
Ga,
(2)
collision
phase
at
2.6–2.5
Ga
and
(3)
transpression
phase
at
2.2–1.9
Ga.
In
this
model,
the
Neoarchaean
collision
of
the
Kaapvaal
and
Zimbabwe
cratons,
preceded
by
oceanic
lithospheric
subduction
beneath
the
latter,
is
followed
by
Paleoproterozoic
tran-
scurrent/transpression
along
shear
zones.
The
SPTSS
represents
a
major
suture
zone
between
the
Kaapvaal
and
Zimbabwe
cra-
tons.
The
Subduction
phase
(Fig.
10A):
petrological
results
suggest
that
the
southern
Zimbabwe
craton,
in
effect
the
NMZ,
was
an
active
magmatic
arc
characterized
by
subduction
of
oceanic
lithosphere
ca.
2.7–2.6
Ga
(Bagai
et
al.,
2002;
Kampunzu
et
al.,
2003;
Zeh
et
al.,
2009).
Magmatism
was
during
convergence
but
prior
to
collision
of
the
Kaapvaal
craton
with
the
Zimbabwe
craton.
At
2.7
Ga,
however
there
is
no
evidence
that
the
SMZ
was
an
accretionary
margin,
but
there
is
an
indication
that
high
grade
metamorphism
was
prevalent
(Kramers
et
al.,
2011).
The
Collision
phase
(Fig.
10B):
the
ensuing
collision
between
the
Kaapvaal
and
Zimbabwe
cratons
gave
rise
to
the
observed
high-
grade
metamorphism
in
the
CZ
and
resulted
in
thickened
crust.
Shallow
syntectonic
melting
resulted
in
the
intrusion
of
the
gran-
odiorites,
such
as
the
2.6
Ga
Bulai
pluton,
which
shows
signatures
of
old
and
juvenile
crustal
anetexis
at
this
time.
Deeper
melting
of
remnant
oceanic
crust
resulted
in
CO2-rich
fluids
migrating
into
the
crust
and
precipitating
graphite.
The
collisional
suture
zone
between
the
Kaapvaal
and
Zimbabwe
cratons
is
located
in
the
CZ
as

154 D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156
Fig.
10.
Schematic
illustration
of
the
tectonic
evolution
of
the
Limpopo
belt
from
2.7
Ga
derived
from
the
combination
of
the
MT
results
and
the
metamorphic
studies
(not
to
scale).
The
resistivity
image
of
the
LOW
profile
is
projected
in
the
background.
the
composite
conductive
mylonitic
feature:
the
Sunnyside-Palala-
Tshipise-Shear
zone
system.
The
Transpression
phase
(Fig.
10C):
the
PTSZ
was
reactivated
at
2.04
Ga
with
transcurrent
movement
at
2.02
Ga
(Holzer,
1998)
as
a
result
of
the
eastward
movement
of
the
Zimbabwe
craton
(coevally
with
Magondi
belt)
relative
to
Kaapvaal
craton.
Crustal
exhuma-
tion
and
uplift,
a
result
of
Bushveld
age
magmatic
underplating
ca.
2.03–1.95
Ga,
of
the
CZ
was
followed
by
erosion
and
the
subsequent
deposition
of
the
Soutpansberg
basin
at
1.9
Ga
in
an
extensional
graben-like
setting
(Kamber
et
al.,
1995a,b;
Schaller
et
al.,
1999).
10.
Conclusions
The
Limpopo
belt
has
been
given
many
geological
tags
(i.e.,
mobile
or
orogenic
belt,
complex,
terrane),
in
essence
to
separate
it
from
the
Kaapvaal
and
Zimbabwe
cratons.
From
the
discussions
above,
the
Limpopo
belt
is
perhaps
best
viewed
as
a
plate
tec-
tonic
manifestation
of
polytectonic
structural
and
metamorphic
activities
that
resulted
from
the
horizontal
collision
between
the
Kaapvaal
and
Zimbabwe
cratons.
Geophysical
studies
presented
in
this
work,
supported
strongly
by
metamorphic
results,
appeal
to
an
evolutionary
path
involving
the
collision
between
the
Kaapvaal
and
Zimbabwe
cratons,
with
the
Palala
Shear
Zone
representing
a
fundamental
suture.
To
this
end
the
Zimbabwe
craton
represent
an
overriding
plate
margin
with
the
NMZ
being
the
active
margin
and
the
CZ
the
leading
shelf.
Thus,
evolutionary
models
proposing
the
separation
of
the
Limpopo
belt
into
separate
terranes
are
not
required.
Questions
still
remain
however
regarding
the
timing
of
the
metamorphic
events
(particularly
in
the
SMZ),
orientation
and
rate
of
plate
of
movement
in
the
Archaean.
However
the
presented
model,
based
on
all
available
data
including
the
new
MT
data,
sug-
gests
that
horizontal
collisional
movement
is
the
most
plausible
of
all
the
models
that
have
been
presented
thus
far.
Acknowledgements
The
SAMTEX
consortium
members
(Dublin
Institute
for
Advanced
Studies,
Woods
Hole
Oceanographic
Institution,
Council
for
Geoscience
(South
Africa),
De
Beers
Group
Services,
The
Univer-
sity
of
theWitwatersrand,
Geological
Survey
of
Namibia,
Geological
Survey
of
Botswana,
Rio
Tinto
Mining
and
Exploration,
BHP
Billiton,
Council
for
Scientific
and
Industrial
Research
(South
Africa)
and
ABB
Sweden)
are
thanked
for
their
funding
and
logistical
support
during
the
four
phases
of
data
acquisition.
Other
members
of
the
SAMTEX
team
include:
L.
Collins,
C.
Hogg,
C.
Horan,
G.
Wallace,
M.
Mienso-
pust
(DIAS),
A.D.
Chave
(WHOI),
J.
Cole,
P.
Cole,
R.
Stettler
(CGS),
T.
Ngwisanyi,
G.
Tshoso
(GSB),
D.
Hutchins,
T.
Katjiuongua
(GSN),
E.
Cunion,
A.
Mountford,
T.
Aravanis
(RTME),
W.
Pettit
(BHPB),
H.
Jelsma
(De
Beers),
P.-E.
Share
(CSIR),
and
J.
Wasborg
(ABB).
This
work
is
also
supported
by
research
grants
from
the
National
Science
Foundation
(EAR-0309584
and
EAR-0455242
through
the
Conti-
nental
Dynamics
Programme
to
R.L.
Evans),
the
Department
of
Science
and
Technology,
South
Africa,
and
Science
Foundation
of
Ireland
(grant
05/RFP/GEO001
to
A.G.
Jones).
The
magnetic
data
courtesy
of
the
Council
for
Geoscience
South
Africa.
The
Irish
Cen-
tre
for
High
Performance
Computing
(ICHEC)
is
thanked
for
availing

D.
Khoza
et
al.
/
Precambrian
Research
226 (2013) 143–
156 155
the
STOKES
cluster
to
carry
out
the
numerical
computations.
Gary
Egbert
and
Weerechaii
Siripunvarapon
are
thanked
for
providing,
respectively,
the
ModEM
and
WSINV3DMT
codes
and
NaserMeqbel
and
Jan
Vozar
for
installing
the
code
on
our
clusters.
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