The Electrical Structure of the Lithosphere and Asthenosphere

J.
The
Electrical
Structure
Asthenosphere
of
beneath
Institut
fur
G.
the Lithosphere
Corrensstmβe
Federal
Republic
(Received August
The
electrical
of
Scandinavia
tal
spatial
ed
by
structure
has
been
gradient
the
of
the
crust
delineated
(HSG)
by
IMS
811-827,
1983
and
Shield
24,
of
Germany
15, 1983)
and
the
techniques.
Scandinavian
35,
JONES*
Geophysik,
Munster,
Geoelectr.,
the Fennoscandian
Alan
D-4400
Geomag.
upper
mantle
beneath
magnetotelluric
The
analyses
magnetometer
three
(MT)
were
array
and
applied
regions
the
to
horizon-
data
complimented
record-
by
telluric
observations.
Models
and
compatible
northeastern
negligibly
small
quivocal
the
order
a
It
trical
mation,
with
1.
layer
of
at
of
an
response
conducting
is
resistivity
existence
contrast,
with
depth
contrast
beneath
a qualitative
increasing
distance
in
distinctive
by
in
make
the
southern
10-50Ωm)
on
known
indicates
the
due
that
centre
demands
the
of
lower
the
lack
of
north
In
the
highly
and
an
increase
region.
depth
to
long
period
depth
European
a
une-
definite
a
This
asthenosphere
the
(i)
the
crust,
mantle.
for
of
to
(ii)
regions.
Moho
estimate
Finland,
and
the
Sweden
exhibiting:
both
the
seismic
northern
Finland
in
entering
quantitative
southern
towards
Moho;
beneath
in
resistivity
a
measure
seismic
asthenosphere
around
with
to
observed
are
the
observed
increase
compatible
functions
across
electrical
resistivity
possible
asthenosphere
but
response
Finland
function
(of
magnitude
is not
the
Norway/northern
the
elecinfor-
increases
craton.
Introduction
The lithospheric and asthenospheric structure of shield regions is of paramount importance for determining the tectonic processes that gave riseto their
generation. With this knowledge, the geotectonicistcould build physical models
of the processes that are, at present, occuring, and accordingly present a viable
mantle convection model of the earth. One physical parameter that is very sensitiveto variations in certain other parameters, with which it has a strong affinity, iselectricalconductivity. Many workers believe that a conductivity-depth profileof the mantle, at a certain location, can be directlytranslated into a mantle
* Present Address: Earth Physics Branch , Energy, Mines and Resources, Ottawa, Ontario, Canada,
K1A OY3. (Previously Geophysics Laboratory, Physics Department, University of Toronto, Toronto, Ontario, Canada, MSS
1A7).
811
812
A. G.
JONES
geotherm for that location, using laboratory data of the conductivity-temperature
relationshipof samples of pure minerals, or assemblages, thought to be representativeof the dominant mantle constituents(for example, OLDENBURG, 1981). Such
an exerciseis,however, frought with inherent problems (see, for example, DUBA,
1976), and itsvalidityhas been called into question. TOZER (1979) is of the opinion that variationsin electrical
conductivity are more strongly correlated to variations in effective viscosity,rather than temperature, with the inference (TOZER
1981) that the role of meteoric water is of extreme importance. Notwithstanding
these apparent dissensions,itiswithout question that geomagnetic induction studies
over various tectonic regimes can radically constrain mantle models proposed
by workers in other geophysical fields.With these considerations in mind, the
recent ELAS (for ELectrical ASthenosphere) project attempted to focus the attention of the geomagnetic induction community
on the detection and delineation of a possible local maximum
in electricalconductivity in the upper mantle.
Such a maximum
may existswithout corresponding viscosity or seismic velocity
minima, if the somewhat speculative carbon theory of DUBA and SHANKLAND
(1982) is true. However, the converse is not possible-any minima in effective
viscosity or seismic velocity, due to the effects of partial melting and/or water
content, must have a corresponding counterpart as a maximum
in the mantle
conductivity profile.
In this work, the results pertinent to ELAS are collated from the various
induction studies carried out by the author and co-workers utilisingdata from
the IMS (InternationalMagnetospheric Study) magnetometer array in Scandinavia,
of 36 modified Gough-Reitzel variometers (KLTPPERS and POST, 1981; KLTPPERS
et al., 1979) and 6 digitalfluxgate magnetometers (MAURER and THEILE, 1978),
and data from telluricfield observations at two locations (JONES et al., 1983).
The derived response functions from three regions of Scandinavia-northern
Sweden, northeastern Norway/northern
Finland, and southern Finland-certainIy display differing characteristics.Also, they are all markedly dissimilarto the
"generalized"responsecurve, constructedby vANYAN et al. (1977),of the
regional data from the East European Platform. The differences for northern
Sweden and northeastern Norway/northern
Finland from the "typical" shield
response is that beneath both there must be a highly conducting zone in the
upper mantle, at depths of around 100-200km,
to satisfy the observations.
In contrast, for southern Finland there must be a highly conducting lower crustal
layer, which is not present beneath the other regions. This vindicates the previous
comments by KLIPPERS et al. (1979) in their explanation of the observed stronger
attenuation of the verticalmagnetic field component at short periods (i.e.,100
-1000s
period)is southern Finland (south of Oulu) than in northern Finland .
The observations made will be described briefly in the following section,
and the transfer functions estimated from the data sets will be illustrated,both
in terms of the variation of the depth of the eddy current flow with period
and their "Bostick" transformations. Section 3 willpresent the best-fittingmodels
(found by Monte-Carlo
random
searches), together with the more objective D+
The
and
H+
models,
models
acceptable
follows
efficacy
ture
Electrical Structure
of
(Section
2.
Full
be
and
details
of
the
et al.
(1979),
from
the
with
in
with
a
the
of
et
of
at
such
al.
(1983).
the
100s
of
period),
three
groups
near
Sauvamaki
of
Sauvamaki
KIR
tensor
in
both
which
Illustrated
Inductive
or
in
analyses.
real
the
Fig.
depth
(WEIDELT,
be
of
the
points
(i) The
estimated
are
"centre
part
the
in
terms
as
of
responses
of
of
of
the
WEIDELT,
et
real
the
The
in-phase
Platform,
al
. (1977),
i.e.,
as
two
Sweden,
and
herein
are
the
with
any
cited
considered
MT
impedance
considered.
g,
of
from
length,
and
current
current
illustrated
calculated
in
the
Schmucker's
of
induced
described
SAU-
references
eddy
gEEP,
and
response
Finland.
units
Also
on
1980,
northern
maximum
1975).
more
(KEV)and
functions
part,
has
the
the
(NAT)
SAU-MT,
is
the
techni-
(JONES
appropriate
response
the
function
depth
gravity"
and
East
European
of
VANYAN
worthy
This
the
Kevo
southern
of
et
employed
elsewhere
confidence
of
(2)
was
1),
element
estimated
of
(HSG)
Finland,
and
most
of
C(ω).
SCHMUCKER
are
NAT
reducing
estimates
illustrated
the
the
error
JONES
and
of
made
by
at Nattavarra
be
in
tensor
the
displayed
data
gradient
full
of
not
found
responses,
considered
for
the
curve
Several
2
and
MT
characteristic
will
be
places
detecting
Norway/northern
off-diagonal
Function,
can
1972;
the
function
"generalized"
MT
the
are
Response
part
the
for
the
et al. (1983).
representative
can
author
They
JONES
be
themselves
only
the
in
also
in a phase
Fig.
see
between
result
see
made
to
were
derive
of
(HOP,
20s
(1970);
were
in
20s
technique
reported
Brief-
resolution
of
to
HSG
(1981),
et al. (1983).
on
spatial
(KIR,
are
considered
of
thus
Kiruna
believed
functions
and
The
northeastern
included-this
Also,
SCHMUCKER
described
are
describe
response
and
details
are
are
information
used
will
POST
temporal
would
on
horizontal
observations
NAT
to
SAU-MT
1D,
the
and
and
and
be
the
deployment,
observations
scheme
were
on
a
Finland
employed
(1) the
(1969)
JONES
(this
method.
-
(SAU-MT)
HSG
the
al.
centred
(SAU)
is thought
five
et
Magnetotelluric
KEV
to
these
struc-
and
discrepancy
objective
and
earth:
and
field
timing
methods
the
stations
1982b).
above.
an
magnetotelluric(MT)
The
of
concerning
their
KLJPPERS
with
unacceptable
devised
and
southern
telluric
a
hence
induction
of
in
The
was
and
response
functions
drawn
to
(1978),
recorded
However,
data
used,
THEILE
located
1979).
telluric
BERDICHEVSKY
1982a,
discussion
be
lithospheric/asthenospheric
is referred
were
one
10s.
different
ubiquitous
error
al.,
was
Two
reader
and
of
discrepancy
geomagnetic
que
exception
and
72°
the
instrumentation
MAURER
KLJPPERS
any
various
magnetometers
resolution
magnetic
A
will
Functions
interested
ly,
map
and
813
Asthenosphere
functions.
conclusions
studies
Response
here-the
KiJPPERS
10s,
response
finaly
and
Fennoscandia.
given
data
the
and
induction
Observations
not
4),
geomagnetic
beneath
to
of the Lithosphere
JONES
in
flow,
system
Fig.
from
into
four
distinct
"groups",
which
is
the
(1982b).
note:
fall
2
correspond
814
A. G.
Fig. 1.
Map
indicate
showing
the
HSG
JorrEs
the locations
of the five induction
sites, whilst
the
circles depict
the
studies
MT
reported
in the text. The
triangles
ones.
exactly to a geographical grouping: Group 1, northern Sweden response, displayed
by gKIR and gNAT; Group 2, northeastern Norway/northern
Finland response of
gKEV; Group 3, southern Finland response of gSAU-MT
4, the East European Platform response, BEEP.
and gSAU-HSG;
and Group
(ii)Response function gxEV is very alike gXIR and gNAT at the shorter periods,
i.e.<1000s,
but differs from them at the longer periods. This indicates that
the crustal/uppermost mantle structure beneath both regions is similar,but that
the depth to a conducting zone in the upper mantle is less beneath KEV than
beneath KIR and NAT.
(iii)gSAU-HSG
and gsAU-MT
are dissimilar to the other responses throughout
the whole period range. This suggests that both the crust and mantle structure
The
Fig.
2.
Comparison
(EEP)
are
different
gSAU-MT.
data
were
que
where
of
C
(ω) for
the
demands
This,
a
and
Asthenosphere
five analyses
with
whilst
1980);
the
residual
MT
resulted
ρa and
in
Φ
itself,
been
and
the
HSG
were
discrepancy
a large
estimates.
beneath
for the
815
East
European
HSG
phase
and
gKIR
from
source
a
Platform
data
summer
for
MT
MT
studies,
with
gNAT,
(b)
or
and
one
all winter
time
time
events
moderate
and
hence
telluric
an
HSG
and
inaccurate
of
that
HSG
techni-
the
spatial
varies
in
BERDICHEVSKY,
were
the
estimated
gSAU-HSG
(a) the
the
estimation
source
of
the
and
that:
data;
reliable
(DMITRIEV
between
Scandinavia.
considering
the
uniform
distance
error,
northern
remarkable
manner
either
horizontal
(c)
both
is rather
non-uniform
requires
data
timing
has
that
compatible-compare
a different
MT
manner
SCHMUCKER,
in
highly
whilst
linear
very
in
Finland
there
are
analysed
gradients
the
part
southern
functions
with
have
real
beneath
regions
response
any
of the
of the Lithosphere
response.
(iv) in both
ty,
Electrical Structure
events
of
high
activity;
magnetic
estimate
a
1979;
activiand
data
of
(d)
would
g
from
816
A. G. JONES
(v) the depth of the "centre of gravity" of the in-phase induced current system
in the upper mantle, at 3000s say, becomes progressively deeper as one traverses
Scandinavia from north to south, from KEV-KIR/NAT-SAU-EEP,
i.e.,
as one approaches the centre of the North European Craton.
(vi)distortion of the telluricfield,by current channelling effects,does not affect
the HSG responses, only the MT ones. Hence, the similitudes of the HSG and
MT responses infer that there are no appreciable, or detectable, telluricdistortion effects at NAT
and SAU. Thus, no DC-type distortion removal factor, of
the form discussed by LARSEN (1977) and RICHARDs et al. (1982), need be applied to the MT apparent resistivityresponse curves.
That such independent estimations should produce highly compatible responses
vindicates the HSG
analysis, and subsequent interpretation, of data from the
KIR region. It is apparent that any effects due to the high ferromagnetic mineral
content in the crustal rocks at Kiruna were removed by smoothing the magnetic
fields.
To ascertain if the ocean-continent boundary, and/or possible mantle conductivityvariationsbetween the oceanic mantle and the continental mantle, could
give riseto two-dimensionality in the response function observed at KIR, numerical
modelling was undertaken. Two possible 2D geoelectricmodels for northwestern
Scandinavia, taken along a profile running NW/SE
from KIR towards the coast
(as profile AA' in Fig. 12 of JONES 1981a), were studied. Model a (see Fig.
3) is for the case where the structure is the same beneath the Norwegian Sea
as beneath the northwestern edge of the Fennoscandian shield.Model b isperhaps
more geophysically plausiblein that beneath the ocean the crust isthinner (absence
of sialiclayer) and the depth to the asthenosphere is less.The theoretical MT
response functions that would be observed at KIR, at periods of 200s, 1000
s, and 3600s (1 hour), for both the E and H polarizations,and for both models,
are indicated in Fig. 3. As can be seen from the illustration,it is not possible
at KIR to detect any differences,to within the standard errors, between the 1D
best-fittingmodel and either of the 21) models, in the period range of observation. Possible 3D effects due to the irregular coastline were studied by JoNEs
and WEAVER (1981). They showed that, for the profile of interesthere, 2D and
3D thin sheet models gave virtually identical responses.
That gsAu-HsG should be so much like gSAU-MT confirms the confidence
placed by the author in the former (JONES, 1982b). The two longest period
estimates of gSAU-MT
inFFig. 2 are the preliminary estimates quoted in JONES
(1982b), which did not, however, pass the very strictacceptance criteriaapplied
to the MT data in the more rigorous full analysis (see JONES et al., 1983). The
SAU-HSG
data
will not be used
further as the imaginary
part of CSAU-HSG,
i.e.,hSAU-HSG
is not considered well estimated.
First-approximations of the conductivity-depth distributionswere gained from
the estimated response functions for KIR, KEV, NAT, SAU-MT
and EEP by
using the Bostick transformation (BOSTICK, 1977; see also WEIDELT et al., 1980;
JONES, 1983). For the HSG responses, there was sufficient confidence in the
The
Electrical Structure
of the Lithosphere
and
Asthenosphere
817
818
A. G. JONES
phase information that the "Bostick" resistivities
were considered best estimated
from the expression given in WEIDELT et al. (1980), which utilisesthe "approximate phase" determination Of WEIDELT
(1972). In contrast, for the MT
responses, and the EEP "generalized curve", more confidence was placed in
the gradients of the apparent resistivitycurves than in the phases-due
to the
aforementioned problem with relativetiming-and
therefore the conventional
expression was employed for determining the Bostick resistivity.
Figure 4 illustrates
the Bostick inversions of the responses considered here (note: SAU refers to
SAU-MT).
These approximate inversions also suggest various points mentioned above:
(i)beneath KEV there is a conducting zone beginning at around 120 km depth;
(ii)KIR and NAT are very compatible and indicate a conducting zone beginning
at around 170-200km
depth; (iii)beneath SAU there is a conducting lower
crust, and at long periods there is the indication that the SAU
structure approaches that beneath EEP; and (iv)the EEP response is radicallydifferent from
all of the others-note that for 103s period all the Scandinavian responses are
around 125km
depth, whilst the EEP response is at a depth of 250km.
Fig. 4. The "Bostick" inversionsof the response functions.The solid circlesare the responses
at 100s, whilst the open circlesare those at 1000s.
The
3.
Electrical Structure
of the Lithosphere
and
Asthenosphere
819
Models
Acceptable 1D geoelectric models were discovered from the response functions by the Monte-Carlo random search procedure of JONES and HUTTON (1979).
Although the Monte-Carlo approach is known to be impractical if the dimension
of the search space is large, i.e.,if the number of model parameters is Large,
for a small number of parameters itsadvantage of not having to assume linearity
gives it superiority over other available methods which necessitatelinearizingthe
inversion problem (PARKER, 1983). The ranges of the acceptable model parameters
will not be shown here-they
are to be found in the other referenced publications-only
the best-fittingmodels discovered will be considered. For the
Monte-Carlo searchs, the "best" - defined as the models that minimise AMPDIF and PHADIF
as defined in JONES and HUTTON
(1979) - 4-layer models
are illustratedin Fig. 5. These models were discovered with the a priori informa-
Fig. 5. The "best-fitting" 4-layer models to the response functions, with the interpreted EEP
of VANYAN
et al. (1977).
model
820
A. G. JONES
tion,
or
"problem
constants",
WESTERLUND,
1980):
1972)
KEV;
mantle
(same
3000Ωm
(from
Fig.
to
ρ1=104Ωm
interpretation
as
by
VANYAN
as
by
et al.
(1977)
al.,
et al.,
1983).
the
(same
NAT;
and
of
of
et
ρ4=5Ωm
KIR):
KIR):
Joys
data
BUNGUM
and
under
NAT
conducted
AMT
from
found
beneath
measurements
Moho
ρ3=80Ωm
KEV
assumed
(from
to
Moho),
beneath
crust
AMT
KIR;
(depth
(depth
assumed
upper
5 is the
of:
d2=46km
d2=46km
resistivities
104Ωm
in
and
ρ1=
SAU;
ρ1=
Also
shown
"generalized"
EEP
response.
The
inclusion
responses
was
depths
for
were
be
cient
the
forced
found
long
that
This
satisfied
at the
certainly
crustal
data
(longer
below
KIR,
equally
for
due
150km
KIR
the
SAU,
at
However,
the
between
models
of
SAU
of
KEV
Moho
3-layer
because
existence
resistivity
and
known
case.
NAT,
required
the
any
be
and
For
are
to
the
at the
to
KEV
well.
periods
locations
layer),
46km
boundaries
discovered
responses,
the
other
at
if geoelectric
was
information
than
boundary
discover
Scandinavian
period
lower
layer
to
northern
mantle
ting
a
permissable.
all three
could
of
undertaken
insuffi-
for
probing
highly
conduc-
1-200Ωm
is
acceptable.
In
order
diverse
assessments
ascertain
1983)
D+
models,
that
the
Table
misfits
response
analysis
models
H+models
originate
error
infer
10%
were
then
are
then
were
sought
those
at
can
the
data
smoothed
which
with
just
largest
90%
X2
reject
level
of
in
a
900
acceptable
values
1D
the
for
PARKER's
misfit
the
The
in
manner.
s are
to
any
for
(note:
models
Fig.
are
6.
The
rejected
from
inversion).
The
X2
the
statistic.
layer
the
hypothesis
confidence
illustrated
Monte-Carlo
possible
had
independent
response).
are
s and
the
to
EEP
them
190
the
able
interpreted
at
for
were
the
the
the
of
be
be
above
employing
response,
to
for
250km
all responses
after
points
earth
given
dramatically,
undertaken
each
assumed
upper
too
were
for
statistic
1D
constraints
space
responses
Lists,
X2
a
the
is acceptable
(these
the
was
and
that
l
a priori
model
the
from
of
2,
of
Table
with
the
the
inversions
together
data
in
whether
restricting
schemes.
astandard
given
by
and
(1980,
X2
to
effect
The
KEV
the
H+
These
parameters,
Table 1. The misfitofthe responsefunctionsto theD+ models listedinTable 2,and theminimum misfit
to be ableto rejectthe hypothesisthatthe data are from a 1D earth,atthe 90% levelofconfidence.
KEV* isthe KEV response without the two anomalous, and physically unacceptable, gKEV values at
190s and 900s (see Fig. 2).
The
Table
and
are
therefore
be
seen
that
2.
of
section.
the
least
H+
for the response
delta-like.
models
the
points
For
the
electrical
and
functions
These
are
very
models
Asthenosphere
821
of interest in this work.
modles
reproduce
"best-fitting"
satisfy
then
geophysically
the
crust,
however,
lower
As
that
dinavia
the
highly
display
thus
are
significant
crust,
conductivity
Scandinavia.
and
D+models
of the Lithosphere
shown
in
faithfully
illustrated
all
in
Fig.
Fig.
7.
the
pertinent
It can
5.
Discussion
Various
in
The
the
characteristics
4.
Electrical Structure
observed
there
the
does
the
not
responses
conducting
the
3-layer
at KIR,
upper
them
is radically
as
Type
different
Ⅱ
in
the
be
a
lower
Note
that
the
range
crustal
models
These
and
all models
Joys
layer.
previous
variation
for
(1981b).
This
northern
can
be
found
models
would
uppermost
100-300Ωm
to
the
beneath
KEV.
crust
in
substantial
boundary
and
according
lower
to
geoelectric
NAT
crust,
resistivity
presented
seismic
asthenosphere.
electrical
models
appear
Moho
previously,
represent
classifying
there
across
mentioned
an
about
layer
mantle,
northern
Scan-
for
lower
the
Beneath
SAU
is highly
con-
822
Fig. 6.
The D+
models
listed in Table
1.
A. G.
for the
response
JONES
funcitons
discussed
herein.
The
misfits
to the data
are
The
Fig.
7.
The
H+
Electrical Structure
models
that the X2
derived
misfit was
by
of the Lithosphere
inverting
the maximum
and
the response
permissable,
Asthenosphere
functions
i.e., these
with
823
a layer
parameter
such
are the least delta-like acceptable
models.
ducting,
is
of
very
resistivity
difficult
graphitization
very
to
BUNGUM
et
Finland
be
the
very
response,
beneath
not
of
may
upper
similar
SAU
exceptionally
no
there
Moho
their
in
be
to
but
the
one
an
mantle,
of
3).
such
restriction
indication
fixed
was
that
JONES,
H+inversion
locations
where
the
et
a
quite
Fig.
al.,
seismic
for
a
According
is at
(see
(JONES
as
provides
layer.
correlates
inversion
and
theories
layer
Finland
depth
1981b),
speculative
underlying
southern
transition
was
the
This
Parker's
few
Ⅲ,
such
conducting
beneath
depth
well
7),
and
1983).
of
with
the
Hence,
Moho
transi-
zone.
resistivity
value
so.
the
the
(this
is the
highly
Monte-Carlo
electrical
to
it and
Table
in
the
(Type
recourse
between
62km
42km
corresponds
For
to
(1980),
zone
of
10-50Ωm
This
resistivity
(see
depth
southern
tion
in
al.
transition
range
without
serpentinization.
45-49km
average
the
explain
contrast
between
the
or
large
in
to
beneath
in
the
applied
the
KEV,
Monte-Carlo
in
upper
Parker's
mantle
KIR
and
search
H+
is more
NAT
for
appears
the
inversion),
resistive,
KEV
but
but
824
A. G. JONES
The
most
quirements
all
pertinent
of
three
a
(KIR
zone
of
105km
was
the
was
about
200s
Bergen
was
MT
For
ferent
groups
Sweden
ed
that
(such
a
models
2D
can
also
be
infer
towards
the
ahighly
conducting
and
S,
which
integrated
a depth
(see
this
a
stable
implies
SAU,
the
showing
a
not
earlier
at 250km
a
than
the
H+model
of
more
rigourous
et
by
al.'s
gsau
analysis
of
the
3.8Ωm
some
(1977)
NAT,
the
zone
integrated
of
of
8000
criterion
4500S
than
inversion
models
indicates
of
interpretation
the
H+
resistivity
i.e.,
for
KIR
progressively
Beneath
s
best-fitting
However,
product
VANYAN
3000
propos-
beneath
The
moves
thickness,
confirmed
the
true.
178.5km
1982b)
1977;
the
than
one
KIR,
dif-
beneath
NOLET,
to
from
be
as
1000S).
at
models
interpretation).
to
of
18km
depth
is not
from
1979;
NAT
thins
Beneath
(JONES,
zone,
zone
depths
1D
have
by
shallower
well
Spitz-
Three
velocity
Fucxs,
the
a depth
and
not
geolectric
low
a conductivity-thickness
4.1Ωm
is
West
(1982a).
indication,
and
greater
of
higher
estimates
conducting
JONES
beneath
affect
at
around
had
on
acceptable
by
and
is the
craton.
he
highly
comparible
deeper
does
by
of
is somewhat
(1965)
seismic
very
deepens
depth
parameter
a
coninter-
the
the
MT
for
gEEP,
data
MT
response
of
data.
Conclusions
In
this
paper,
studies
quite
able
patible
of
to
The
two
results
shown
giving
delineate
response
possibility
the
have
capable
position.
the
2),
derived
These
be
which
beginning
203km,
asthenosphere
5.
at
There
seriously
conductivity
of
For
Fig.
function
zone.
not
This
the
detail
of
is slightly
the
zone
30km.
conductivity.
an
of
of
a
115km.
of
is a "well-developed"asthenosphere
(adepth
is at
in
all
asthenosphere
centre
thickness
-
electrical
the
of
(CASSELL
1980)
found
than
presence
in
model
to
peninsular
a
which
OELSNER
the
observations
would
this
by
depth
discussed
asthenosphere
effect
a
they existence
their
the
hence
the
nor-
formal
Kola
zone
at
to
beneath
the
at
period
correlation
been
HELMBERGER,
conducting
models,
the
has
proposed
explain
and
highly
ones
have
to
GIVEN
at
the
80Ωm
re-
beneath
closer
transition
from
conducting
1960's
indicating
Sweden,
seismic
the
and
early
beginning
to
longest
1),
the
on
600Ωm
be
than
a
comes
Lovozero
of
for
KEV
the
surface
to
(KEV)
evidence
are
the
appears
Finland
from
Fig.
in
as
1Ωm,
northern
the
his
work
of
zone
beneath
resistivity
however,
200km
This
Vladimirov's
interpreted
resistivity
with
but
(see
Also,
within
at
transition
The
value,
project,
Independent
depth
recorded
A
interpreted.
KEV
resolved.
zone
NAT).
data
(197E)).
ELAS
stations.
100km
MT
VLADIMIROV
the
Norway/northern
and
at around
pretation
than
conducting
northeastern
Sweden
ducting
for
Scandinavian
beneath
thern
of
highly
northern
surface
results
various
estimated
the
methods
functions.
of
of
conclusively
telluric
studies
geomagnetic
functions
asthenosphere
employed,
distortion
induction
response
electrical
This
that
MT
and
effectively
effects
with
and
HSG,
eradicates
on
the
MT
have
been
induction
sufficient
any
with
surprisingly
very
aspersions
responses,
or
are
accuracy
its variations
gave
collated.
techniques
to
lateral
com-
concerning
of
significant
The
magnetic
field
The
Such
zone
existing
at a
the
is that
the
depth
of
of
one
around
the
centre
the
of the Lithosphere
the
iron
beneath
deeper
less
than
300km
on
Spitzbergen,
to
north
this
al.
zone
increases
European
Two
of
distinct
the
existence
impression
increasing
This
theory
given
lateral
gains
a highly
the
surface.
is interpreted
European
analyses,
with
craton.
East
exist
of
and
the
other
must
200km
SAU,
postulate
the
there
beneath
(1977).
Thus,
Kiruna.
within
centre
825
Asthenosphere
at
10Ωm)
the
et
depth.
deposit
150km
beneath
and
Scandinavia
than
VANYAN
West
ore
northern
resistivity
100km
depth
of
(i.e.,
be
and
at
by
that
must
model
peninsular
zone
show
zone
a
from
distortion
results
conducting
Electrical Structurc
one
of
on
the
Kola
a conducting
by
these
distance
weight
as
Platform
from
studies
towards
the
obser-
vations of ADAM et al. (1983) on the Karelian megablock. They report that the
Niblett inversion (which is exactly the same as the Bostick inversion, see JoNEs,
1983) of their data shows a continuous decrease of resistivitywith depth with
aform
very similar to that of the EEP curve illustratedin Fig. 4.
The other result of importance is the requirement of a conducting lower
crust beneath southern Finland. This zone explains the previously observed stronger
attenuation of the vertical magnetic field component at locations south of the
Svecokarelian fault compared to that observed to the north of it (KUPPERS et
al., 1979). It could also explain the observations by PAJuNPAA et al. (1983) of
differences between western and north-eastern stations of their magnetometer
array which straddled the Svecokarelian fault.
The author is indebted to many colleaguesand friendsin Scandinavia and MUnster
who aided in the organizationand executionof the magnetometer array.The listisendless
-please referto the Acknowledgements in KUPPERS et al. (1979).He wishes particularly
to mention the continualhelp and guidance given him by Professor J. Untiedt and Drs.
W. Baumjohann and F. Kuppers whilstat Munster. Three groups in Scandinavia made
the induction work, particularly
the MT phase, both possibleand enjoyable;namely B.
Olafsdottirand Dr. F. Eleman of Uppsala (Swedish GeologicalSurvey),Professor S.-E.
Hjelt,Dr. P ..Kaikkonen, J. Tiikkainen and K. Pajunpaa of Oulu(Geophysics Department, Oulu University),
and Drs. C. Sucksdorffand R. Pirjolaof Helsinki(Geomagnetism
Division,Finnish Meteorological Institute).
Dr. R. L. Parker is thanked for providing the coding of his inversionroutines.
The author was supported by grantsfrom the Deutsche Forschungsgemeinschaftwhilst
in Munster, was a guest of the Swedish GeologicalSurvey for fivemonths, and has been
supported by Natural Sciencesand Engineering Research Council of Canada grants to
Professors G. D. Garland, R. N. Edwards and G. F. West (NSERCC
A2115, NSERCC
GO501) whilst at the Universityof Toronto. He wishes to express his appreciationto
all three bodies.
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