crustal imaging of the fowler domain...high gravity and north-east trending magnetic highs, which...
TRANSCRIPT
Chapter 5
Crustal imaging of the Fowler Domain
5.1 Introduction
Archaean and Proterozoic Cratons, as well as their margins and surrounding mobile belts, have
been the focus of a large number of interdisciplinary geoscientific surveys involving the mag-
netotelluric (MT) method (Cagniard, 1953). For example, in North America, Ferguson et al.
(2005) studied the Western Superior Province in Canada as part of the Lithoprobe project to
map deeper structures of the granite-greenstone and metasedimentary belts and subprovinces.
Evans et al. (2005) report an electrically resistive Archean Rae Craton, Canada, bounded by
a more conductive belt (Piling Group). In India, a similar electrical response has been re-
ported by Harinarayana et al. (2006), with the Archaean Dharwar Craton being resistive and
the Southern Proterozoic granulite terranes being more conductive . Economic resources such
as diamond and gold occurrences in the mobile belts surrounding the South African Kaap-
vaal Craton are another incentive to pursue the understanding between surface expressions of
Archaean subdomains and the deep crust/upper mantle underneath (Hamilton et al., 2006).
Magnetotelluric surveys conducted in Namibia describe enhanced conductivity due to possible
graphite enrichment in shear zones within the Damara mobile belt between the Congo and
Kalahari Cratons (Ritter et al., 2003).
Central and western Australia is comprised of numerous Archaean to Proterozoic cratons
that had accreted to form a single continent by the Meso-Proterozoic (Betts et al., 2002).
South Australia is dominated by the Archaean to Palaeoproterozoic Gawler Craton (Daly et al.,
1998)(Figure 5.1). Although different geological domains have been identified within the Gawler
Craton (Figure 5.1), its overall tectonic evolution and the relationships between these domains
are very poorly understood, due largely to extensive cover of younger sediments. In many
cases the geological domains and their associated boundaries have been defined purely through
interpretations of potential field data. MT is ideal for investigating the structure of the Gawler
54
5.1. Introduction 55
130˚E
130˚E
132˚E
132˚E
134˚E
134˚E
136˚E
136˚E
35˚S 35˚S
34˚S 34˚S
33˚S 33˚S
32˚S 32˚S
31˚S 31˚S
30˚S 30˚S
29˚S 29˚S
−1000
−500
0
0
−6000−5500−5000−4500−4000−3500−3000−2500−2000−1500−1000−500
0
m
WILGENA
?
?
Karari
FZ
BathymetryYa
rlb
rin
da
SZ
Coorabie SZ
Munjeela Granite
St. Peters Suite
Granitoids
Fowler Domain
Sleaford Complex
Hiltabe Suite
intrusives
Station locations
Koonibba FZ
22
2120
1918
1715 14 13
12
11 10 09
08 0706
05
04
03 01
−1500
−2000−2500
NAWA
FOW
LER
CHRISITIE
NUYTS GAWLER RANGE
VOLCANICS
Tallacootra
SZ
EUCLA
BASIN
120˚E 140˚E
40˚S
20˚S
120˚E 140˚E
40˚S
20˚S
120˚E 140˚E
40˚S
20˚S
Gawler
Yilgarn
Pilbara
02
25 24 23
Southern
Ocean
Figure 5.1: Long-period MT stations on top of the regional geology of the field area and shadedbathymetry relief of the Southern Ocean. The profile extends over 400 km, crossing the Nuytsand Fowler Domains and numerous major framework shear zones. The passive continentalmargin is relatively wide with the bathyal zone approximately 150-200 km away from the coast.FZ - fault zone, SZ - shear zone
Craton due to its ability to penetrate sedimentary cover and to image lithospheric-scale fea-
tures and due to the absence of cultural electrical noise throughout most of the craton. Three
major events have defined the present-day architecture of the Gawler Craton, the oldest being
the late Archaean to Palaeoproterozoic Sleafordian Orogeny (2440− 2400 Ma), followed by the
Palaeoproterozoic Kimban Orogeny (1730−1690 Ma) and the Mesoproterozoic Karari Orogeny
(1570 − 1540 Ma).
The Gawler Craton is not only interesting from a tectonic point of view, but it has also gen-
erated a substantial economic interest. It hosts numerous mineral deposits of which the iron ox-
ide copper-gold Olympic Dam deposit is the world’s largest uranium producer (Hitzman et al.,
1992). The Olympic Dam deposit is situated along the eastern margin of the Gawler Craton and
MT studies show a zone of enhanced conductivity connecting the deposit with the lower crust,
suggesting upward movement of CO2-bearing volatiles followed by precipitation of graphite
5.2. Geological setting 56
130˚E
130˚E
132˚E
132˚E
134˚E
134˚E
136˚E
136˚E
33˚S 33˚S
32˚S 32˚S
31˚S 31˚S
5007501000125015001750200022502500275030003250350037504000
nT
130˚E 132˚E 134˚E 136˚E
33˚S 33˚S
32˚S 32˚S
31˚S 31˚S
−750−650−550−450−350−250−150−5050150250350
g.u.
St Peter Suite25
2321
19 1714 12 10 8
64
2
2523
21
19 1714 12 10 8
64
2
Figure 5.2: MT stations superimposed on gravity (top) and total magnetic intensity (bottom)image of the survey area. The Nuyts and Fowler Domain show distinctively different gravityand magnetic responses due to the magmatic emplacement of the St. Peter Suite of the NuytsDomain vs the shear zone-intersected Fowler Domain.
along grain boundaries (Heinson et al., 2006).
The MT survey presented in this chapter was designed to investigate the electrical structure
of the Fowler Domain and the St. Peter Suite, which are situated on the western side of the
Gawler Craton, and of the major shear zones which act as bounding structures. The Fowler
Domain, also referred to as Fowler Orogenic Belt or Fowler Suture Zone (Daly et al., 1998), is
poorly outcropping and aeromagnetic images show north-east trending magnetic highs that are
separated by shear zones (Figure 5.1,5.2).
5.2 Geological setting
High gravity and north-east trending magnetic highs, which are separated by shear zones, are
the potential field signatures of the Fowler Domain (Figure 5.1 and 5.2) (Daly et al., 1998).
The Nuyts Domain is defined by the extent of the magmatic St. Peter Suite, which has an
5.2. Geological setting 57
associated gravity low (Figure 5.1 and 5.2). The lithologies that comprise the Fowler Domain
are only very poorly known since it contains very limited outcrop.
During the 2000−1690 Ma interval a number of large rift-basins developed across the Gawler
Craton. The deposited sedimentary basins have bimodal magmatic suites associated with them.
The Fowler Domain contains both pelitic metasediments (Daly et al., 1998), and 1726±9 Ma old
mafic metagabbros (Fanning et al., 2007). The 1730−1690 Ma Kimban Orogeny had a profound
metamorphic influence on deposited metasediments and metamorphism reached amphibolite
facies in the Fowler Domain (Teasdale, 1997).
At 1630 Ma the alkaline, porphyritic rhyodacite of the Nuyts Volcanics erupted and were
subsequently intruded by the 1620−1610 Ma St. Peter Suite (Flint et al., 1990). Between 1595
and 1575 Ma a large-scale magmatic event formed the Hiltaba Suite and the Gawler Range
Volcanics, and form one of the largest felsic volcanic systems in the world (Daly et al., 1998).
The Gawler Range Volcanics have a maximum thickness of about 1.5 km. The Hiltaba Suite
comprises strongly fractionated granites to granodiorites (>70 wt % SiO2), which consist of
more mantle-derived material than the host rock in which they reside (Stewart and Foden,
2003). Several major shear zones, such as the N-S trending Yarlbrinda Shear Zone (Figure 6.2),
are believed to have been reactivated during the emplacement of the Hiltaba Suite granites
and a general NW-SE shortening at 1590 − 1570 Ma. At 1585 Ma the St. Peter Suite was
subsequently intruded by the unfractionated Munjeela Granite, as indicated by potential field
data (see Figure 5.2). The Karari Fault Zone in the north-western part of the Gawler Craton
was formed during the Karari Orogeny (1570 − 1540 Ma)
5.2.1 Shear zones
Shear zones form an important link between the histories of different subdomains within the
Gawler Craton and hold important information about the architecture of orogenic belts. There
are three major generations of shear zones that have been recognised across the Western Gawler
Craton (Teasdale, 1997, e.g.):
1. (SZ1) east-west trending shear zones, which are largely parallel to the survey line,
2. north-east trending craton-scale shear zones (SZ2), dominating the geology of the Western
Gawler Craton (e.g. Tallacootra and Coorabie SZ),
3. the Karari Fault Zone (SZ3).
The tectonic events resulting in the SZ2 shear zones also caused the emplacement of the 1540 Ma
granulites and the 1490 Ma amphibolites and pegmatites in the Fowler Orogenic Belt, as well as
the 1590 Ma Hiltaba intrusives. The north-east trending shear zones coincide with low intensity
magnetic anomalies (Figure 5.2) and form the Fowler Belt extending to 200 km width at the
5.3. MT data 58
intersection with the survey line. The Coorabie Shear Zone represents the easternmost SZ2
shear zone and separates the Fowler Orogenic Belt in the west from the St. Peter Suite to the
east. The Coorabie Fault and the Colona Fault form the eastern- and westernmost boundaries
of the Coorabie Shear Zone, respectively (Parker, 1993). Another major SZ2 shear zone is the
high-grade mylonitic Tallacootra Shear Zone which extends over 700 km and forms the western
boundary of the Fowler Domain.
New 40Ar/39Ar data of the major framework shear zones suggest a single fault system com-
prising the Tallacootra, Coorabie and Karari Shear Zones between 1460-1440 Ma, and possi-
ble earlier metamorphism and deformation between ≈ 1530-1550 Ma (Fraser and Lyons, 2006).
Reshuffling of already adjacent crustal blocks is therefore more likely than a composition of far-
away terranes (Fraser and Lyons, 2006). Direen et al. (2005) reports sinistral strike-slip offset
of tens of kilometers. Gravity forward modelling suggests a 70° dip to the north-west of the
major framework shear zones, with penetration depths of at least 15 km.
The SZ3 Karari Fault Zone postdates all SZ2 shear zones and is characterised by a significant
high intensity magnetic anomaly. It truncates the SZ2 shear zones to the west and north.
There are no known outcrops of the Karari Fault Zone and only two drill cores have intersected
it. Thus, this survey seeks to provide further constraint on its southern extension and depth
extent.
5.3 MT data
In 2005, long-period MT data were recorded at 24 stations, sites 01-25 (site 16 did not record),
along an approximately 450 km long transect from ESE to WNW following the only highway
that runs east-west and crossing the major shear zones at right angles (Figure 5.1). Information
about the bathymetry of the Southern Ocean is important due to the parallel orientation of
the survey line with respect to the adjacent coastline, as it provides a quantitative measure of
conductance outside the profile. The abyssal plains of the passive margin toward the Southern
Ocean are 150 km away, while the continental shelf waters have a depth of around 100-200 m
(Figure 5.1). Therefore, the shelf waters contribute a conductance of 300-600 S. The MT profile
extends from the Gawler Range Volcanics in the east, across the Nuyts Domain comprising the
granite-diorite St. Peter Suite in the center (stations 03-15) to the Fowler Domain to the west
(stations 17-21). Stations 22 to 25 cover the adjacent sedimentary Eucla Basin with thicknesses
up to 3 km , which are expected to have an inductive effect due to the lower resistivity of the
sediments (for a detailed discussion see Chapter 6.3.3). Stations 1-10 have an approximate
spacing of 20 km, which was reduced to 15 km across the western part of the St Peter Suite
and the Fowler Orogenic Belt for stations 11-25. This was to reach a compromise between
covering the main domains and to ensure enough spatial resolution for mid- to upper crustal
5.3. MT data 59
10−1
100
101
102
103
104
105
App
. re
sist
ivit
y
101 102 103 104
fow 01
0
30
60
90
Ph
ase
101 102 103 104
fow 02
101 102 103 104
fow 03
101 102 103 104
fow 04
10−1
100
101
102
103
104
105
Ap
p. r
esi
stiv
ity
101 102 103 104
fow 06
0
30
60
90
Ph
ase
101 102 103 104
fow 08
101 102 103 104
fow 10
101 102 103 104
fow 11
10−1
100
101
102
103
104
105
Ap
p. r
esi
stiv
ity
101 102 103 104
fow 12
0
30
60
90
Ph
ase
101 102 103 104
fow 13
101 102 103 104
fow 14
101 102 103 104
fow 15
Figure 5.3: Apparent resistivity ρa(T ) in [ Ωm] and phase φ(T ) in [°] plots for stations 1-15.Shaded triangles and squares denote ρa and φ of the Zxy and Zyx component of the impedancetensor, respectively. Open circles and inverse triangles represent the Zxx and Zyy componentsof the impedance tensor.
5.3. MT data 60
10−1
100
101
102
103
104
105
App
. res
istiv
ity
101 102 103 104
fow 17
0
30
60
90
Pha
se
101 102 103 104
fow 18101 102 103 104
fow 19101 102 103 104
fow 20
10−1
100
101
102
103
104
105
App
. res
istiv
ity
101 102 103 104
fow 21
0
30
60
90
Pha
se
101 102 103 104
fow 22101 102 103 104
fow 23101 102 103 104
fow 24
10−1
100
101
102
103
104
105
App
. res
istiv
ity
101 102 103 104
fow 25
0
30
60
90
Pha
se
Figure 5.3: (cont’d) Apparent resistivities and phase plots for stations 17-25. For a detaileddescription see previous page.
5.3. MT data 61
structures. Hiltaba Suite granites 1600-1585 Ma of age crop out near site 01 and 02 at the
boundary between the Nuyts Domain and Gawler Range Volcanics (Figure 5.1). The Munjeela
Granites intrude the St. Peter Suite between sites 09 and 11 within the Nuyts Domain.
The long-period MT instruments, developed by Adelaide University, recorded data at 10 Hz,
averaged and downsampled into blocks of 1 s giving responses between 10-6000 s. Each station
recorded two horizontal components of the electric field (Ex, Ey with x-north and y-east) and
three components of the magnetic field (Hx, Hy, Hz) for around 70 h. All time-series data were
processed using a robust remote reference code (Chave et al., 1987; Chave and Thomson, 1989)
and produced MT impedances Z ∈ C and geomagnetic transfer functions T ∈ C.(
Ex
Ey
)
=
(
Zxx Zxy
Zyx Zyy
)(
Hx
Hy
)(
Hz
)
=(
Txz Tyz
)(
Hx
Hy
)
(5.1)
Most of the time, two or three stations recorded simultaneously to allow remote-referencing
where necessary (Gamble et al., 1979); however, coherence thresholding with a higher cut-off
produced better results in some cases. Geomagnetic declination of the survey area lies between
5° and 6°, and has been corrected for, so that the x- and y-coordinates denote geographic north
and east, respectively. Apparent resistivity and phases were calculated from MT impedance
components (Figure 5.3, 5.3). Stations commonly show a split in the phases and apparent resis-
tivities at around 200 s, even more so for stations 8-24. The yx-components of the impedance
tensor decrease in apparent resistivity and increase in phase across all stations, which is most
likely the result of the inductive ocean. Static shift is most prominent for stations 1-13 with
almost equal phases and a similar shape in the resistivity curves of the xy-component.
5.3.1 Induction arrows
Induction arrows (Chapter 1.5) show the regional resistivity distribution without being affected
by static shift like the MT impedance (Jones, 1988), but are sensitive to only lateral resistivity
variations. In the Parkinson convention, used here, real arrows will point away from resisi-
tive blocks and towards zones of highest conductivity. The robust remote reference code by
Chave and Thomson (1989) uses a time dependence in the Fast Fourier transform of e+iωt and
therefore the real and imaginary arrows need to be reversed in the Parkinson convention. Figure
5.4 illustrates both real and imaginary components of the induction arrows for three different pe-
riods, where small periods (high frequencies) indicate influences of shallow and local structures
and longer periods display the influence of the regional and deep resistivity distribution.
At a period of 42 s real induction arrows are large and point consistently south-west. Excep-
tions are stations 01-04, where the magnitude of the arrows is smaller, presumably due to the
longer distance of the stations to the ocean. Stations 20-24 show a 90° deflection of the real
arrows, indicating a north-south trending resistivity interface between station 22 and 23; this
5.3. MT data 62
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
ysz
ysz
kfzcrfz
clfztsz
2523
21
19 1714
12 10 8
64 2
yszkfzcrfz
clfztsz
130˚E 132˚E 134˚E 136˚E33˚S
32˚S
31˚S
0.5
2730s
682s
42s
ysz
kfzcrfz
clfztsz
Figure 5.4: Real (red) and imaginary (blue) components of the induction arrows in the Parkin-son convention. Localities: tsz-Tallacootra Shear Zone; clfz-Colona Fault Zone; crfz-CoorabieFault Zone; kfz-Koonibba Fault Zone; ysz-Yarlbrinda Shear Zone.
possibly represents the margin of the Gawler Craton. Arrows are generally larger for stations
16-21, corresponding to the Fowler Domain, and decrease in size across the St. Peter Suite
(stations 4-15). These two domains also correspond spatially to areas of gravity highs and lows,
respectively (Figure 5.2). For 682 s period, the magnitude of the arrows decreases and arrows
in the western part of the profile rotate by about 20° towards the conductive boundary near
station 22. Arrows rotate back towards their original orientation for long periods (e.g 2730 s)
due to the deep sea water of the southern ocean located approximately 150-200 km to the south.
To summarise, arrows do not show great variation in the orientation across the profile and are
influenced by the continental shelf/conductor and the deep sea from shorter to longer periods.
Imaginary components of the arrows are often parallel to the orientation of the real arrows for
682 s, indicating two-dimensionality. However, for periods of 42 s and 2730 s some imaginary
arrows are no longer parallel suggesting three-dimensionality in some areas.
5.3. MT data 63
101
102
103
01234689101112131415171819202122232425 01234−12−10−8−6−4−2024681012
beta
Figure 5.5: Skew angle β pseudo section for stations along the Fowler profile. Small values forβ for stations 1-14 indicate two-dimensionality. Stations 17-25 show 2-D/3-D or purely 3-Dbehaviour for periods larger than 200 s.
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
0 5 10 15 20 25 30 35 40 45 50 55 60
Minimum phase
85 s
341 s
0.5
1024 s
Figure 5.6: Phase tensors and real induction arrows for 85, 341 and 1024 s. Phase tensor ellipseshave been filled according to minimum phase value.
5.3. MT data 64
−45
−30
−15
0
15
30
45
Strike
N°
E
−45
−30
−15
0
15
30
45
Str
ike
N°
E
123456789101112131415171819202122232425
Station No.
Eucla
basin
Fowler
Domain
Nuyts
Domain
Gawler Range
Volcanics
Figure 5.7: Geoelectric strike of MT stations across the Fowler Domain. The strike angles areperiod-averaged value for periods between 20 and 4000 s. Dashed line: common strike of 16°chosen to rotate the impedance tensor elements prior to 2-D inversion. Solid lines: averagestrike directions of adjacent terraines.
5.3.2 Dimensionality and electrical strike
Before modelling and inversion can be attempted, I analyse both the dimensionality and also the
geoelectric strike of the subsurface using the phase tensor approach developed by Caldwell et al.
(2004). As pointed out in Chapter 2.2 on page 22, the phase tensor skew β defines the asymmetry
of the tensor and is therefore a measure of dimensionality. In the one- (1D) or two-dimensional
(2-D) cases the phase tensor Φ is symmetric and β = 0. A departure from two-dimensionality
leads to an increase in skew β and plotting β over all periods and for all stations indicates the
dimensionality of the subsurface (Figure 5.5).
Figure 5.5 shows that |β| < 4° for most periods between stations 1-14. Empirical observations
indicate that a 5° threshold can be applied in order to discriminate between a 2-D and 3-D
resistivity distribution. Stations 20 and 23 clearly show three-dimensionality between periods
of 100 and 1000 s. Furthermore, a general trend of large skew values for adjacent stations can
be observed for stations 17-25 for periods larger than approximately 200 s. Compared to the
behaviour of the induction arrows, the skewed responses are most likely due to the superposition
of influences of the north-south trending resistivity boundary to the west and the conductive
sediments/ocean water to the south.
Orientation of phase tensor ellipses are approximately north-south, indicating major current
flow perpendicular to the profile (Figure 5.6). Increasing ellipticity of the phase tensor el-
lipses with period suggest an increasing phase split between the xy- and yx-components of the
impedance tensor. Minimum phase values are generally large for the eastern stations, and de-
crease for stations overlying the Fowler Domain. This suggests that the underlying structure is
more resistive across the Nuyts Domain and the Gawler Range Volcanics. Low minimum phase
values of about 10° for the three westernmost stations 23-25 suggest a conductive subsurface,
which is expected from a conductive sedimentary basin underlying these sites.
Phase tensor ellipse orientations are parallel or perpendicular in the case of a 2-D distribution.
5.4. Inversion 65
Here the ellipses are aligned with the strike of the fault zones (compare Figure 5.1). The
orientation of the ellipses has subsequently been used to determine the geoelectric strike of the
profile. Given that most of the stations and periods display 2-D behaviour, the strike angle
obtained can be utilised in the 2-D inversion. Figure 5.7 shows the strike angles θ obtained from
the phase tensor analysis by averaging over period. The strike angle is defined as θ = α − β
(see Figure 2.2). The average strike direction is approximately θ = 16 N°E. There are 4 distinct
groups of stations that show a very small variance in averaged strike. Those groups spatially
coincide with major geological domains. Whereas strike directions of stations above the Fowler
Domain and the Nuyts Domain have similar strike to 16 N°E, stations 23-25 across the Eucla
Basin have a strike direction of about 0°N.
5.4 Inversion
The rotated off-diagonal elements of the impedance tensor and the geomagnetic transfer func-
tions were inverted in a 2-D sense for the period range between 10 s and 2000 s using the 2-D
inversion code of Rodi and Mackie (2001). The starting model was a 100 Ω m half-space, with
a 5 % error floor on log ρa of the TE- and TM-mode and a 1° error floor on the TE- and
TM-mode phases. The error floor of the real and imaginary parts of the geomagnetic transfer
functions was set to 0.02. Static shifts were allowed to freely adjust during the inversion. While
the corrections are small during the first few iterations, the code increases the allowable static
shift correction with each iteration. Starting from the homogeneous half-space, the smoothing
parameter τ (see Chapter 4.7 on page 47) was initially set to 5. Once the inversion converged
to a minimum misfit, τ was set to a value of 3 and the static shifts were reset. This allows
the model to jump out of local convergence minima. The inversion eventually converged to a
misfit of rms = 4.63. The horizontal smoothing factor α was set to 1.2, achieving a higher
smoothness in the horizontal direction than in the vertical direction.
Figure 5.8 shows the results of the NLCG inversion. As the colour scale can be deceptive,
the apparent resistivities are plotted on a logarithmic scale with three different colour scales to
emphasise different features in the model. Features labelled A and B are electrically resistive,
about 1000−10000 Ω m at depths between 20−80 km and 20−60 km, respectively. The resistor
labelled A extends to the surface underneath station 08, coinciding with the location of the
Koonibba Fault Zone. Independent modelling, by omitting station 08, support the findings and
suggests that the resistor crops out at the surface and is not a model artefact of station 08.
Features labelled A and B spatially coincide with the Nuyts Domain on the surface, but are
overlain by a more conductive layer with a resistivity of 100 Ω m in the top 10 km. The feature
labelled B is truncated to the west by a conductive (200 Ωm) sub-vertical feature (labelled
C in Figure 5.8), slightly dipping to the east underneath station 23. The top few kilometers
5.4. Inversion 66
fow25
fow24
fow23
fow22
fow21
fow20
fow19
fow18
fow17
fow15
fow14
fow13
fow12
fow11
fow10
fow08
fow07
fow06
fow05
fow04
fow03
fow02
fow01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
Dep
th (
m)
10000
5810
3376
1962
1140
662
385
224
130
75
44
25
15
9
5
Ohm.m
fow
25
fow
24
fow
23
fow
22
fow
21
fow
20
fow
19
fow
18
fow
17
fow
15
fow
14
fow
13
fow
12
fow
11
fow
10
fow
08
fow
07
fow
06
fow
05
fow
04
fow
03
fow
02
fow
01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
De
pth
(m
)
1000
720
518
373
268
193
139
100
72
52
37
27
19
14
10
Ohm.m
fow
25
fow
24
fow
23
fow
22
fow
21
fow
20
fow
19
fow
18
fow
17
fow
15
fow
14
fow
13
fow
12
fow
11
fow
10
fow
08
fow
07
fow
06
fow
05
fow
04
fow
03
fow
02
fow
01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
De
pth
(m
)
5000
3053
1864
1138
695
424
259
158
97
59
36
22
13
8
5
Ohm.m
Distance (Km)
ABC
DE
Fowler St Peter SuiteTallacootra sz Coorabie sz Koonibba fz
Figure 5.8: 2-D resistivity model from the NLCG inversion by Rodi and Mackie (2001). Thethree sub-figures show the same model with different color scales to highlight different featuresin the model. The colour scale denotes the resistivity of the subsurface evenly distributed inlog-space.
5.4. Inversion 67
100
101
102
103
104
105
App
. re
sist
ivit
y
101 102 103 104
site 25 (rms 3.53)
0
30
60
90
Ph
ase
−1.0
−0.5
0.0
0.5
1.0
Hz
101 102 103 104
101 102 103 104
site 23 (rms 5.11)
101 102 103 104
101 102 103 104
site 21 (rms 3.47)
101 102 103 104
101 102 103 104
site 19 (rms 3.56)
101 102 103 104
101 102 103 104
site 17 (rms 4.46)
101 102 103 104
100
101
102
103
104
105101 102 103 104
site 15 (rms 3.97)
0
30
60
90
−1.0
−0.5
0.0
0.5
1.0
101 102 103 104
100
101
102
103
104
105
Ap
p. r
esi
stiv
ity
101 102 103 104
site 13 (rms 3.14)
0
30
60
90
Ph
ase
−1.0
−0.5
0.0
0.5
1.0
Hz
101 102 103 104
101 102 103 104
site 11 (rms 2.85)
101 102 103 104
101 102 103 104
site 08 (rms 3.09)
101 102 103 104
101 102 103 104
site 06 (rms 3.16)
101 102 103 104
101 102 103 104
site 04 (rms 3.10)
101 102 103 104
100
101
102
103
104
105101 102 103 104
site 02 (rms 3.24)
0
30
60
90
−1.0
−0.5
0.0
0.5
1.0
101 102 103 104
Z (obs)yx Z (obs)xy Z (calc)yx xyZ (calc) T (obs)real T (obs)imag T (calc)real T (calc)imag
Figure 5.9: Apparent resistivity, phases and geomagnetic transfer functions of approximatelyevery second station along the profile. For each station apparent resistivities, phases andgeomagnetic transfer functions are plotted from top to bottom, respectively. Open symbolsdenote observed data, overlain by calculated responses from 2-D inversion (reference). RMSvalues show individual fits of TE-, TM-mode and geomagnetic transfer functions.
5.4. Inversion 68
fow25
fow24
fow23
fow22
fow21
fow20
fow19
fow18
fow17
fow15
fow14
fow13
fow12
fow11
fow10
fow08
fow07
fow06
fow05
fow04
fow03
fow02
fow01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
Dep
th (
m)
10000
5810
3376
1962
1140
662
385
224
130
75
44
25
15
9
5
Ohm.m
NLCG Model (RMS = 4.63)
a
0 0.5 1 1.5 2 2.5 3 3.5 4
x 105
−2
0
2
4
6
8
10
12
x 104 TE & TM Occam Model (RMS = 3.35)
Horizontal Distance (m)
De
pth
(m
)
Lo
g1
0 R
esi
stiv
ity
(oh
m−
m)
1
1.5
2
2.5
3
3.5
4
fow25 fow24fow23fow22
fow21fow20
fow19fow18
fow17 fow15fow14
fow13fow12
fow11 fow10 fow08 fow06 fow04 fow03 fow02 fow01
b
ABCD
E
Figure 5.10: Comparison of inversion results using a) the 2-D NLCG inversionroutine of Rodi and Mackie (2001) and b) the 2-D Occam inversion routine ofdeGroot Hedlin and Constable (1990). Both inversions were performed using the TE-, TM-mode of the impedance tensor Z and for the NLCG inversion the vertical magnetic field Bz
was also used. Capital letters denote features labelled according to Figure 5.8.
underneath stations 24 and 25 are very conductive with resistivities of less than 5 Ω m, as
would be expected from the conductive sediments in the Eucla Basin. Underneath stations 17-
22, spatially coinciding with the Fowler Domain, the upper crust is conductive with a resistivity
of 100 Ωm in the top 10 km. To the east of the profile, the crust has a resistivity of around
100−500 Ωm beneath stations 01 to 04 (labelled E in Figure 5.8). The Gawler Range Volcanics
crop out in the vicinity of these sites.
The inversion solution fits well both the two modes of the impedance tensor and also the
vertical magnetic field (Figure 5.9). Pseudosections of the TE- and TM-mode and the real
and imaginary parts of the geomagnetic transfer functions are also shown in Appendix B in
Figure B.1 and Figure B.2 on page 109.
In order to validate the inversion results further, the data have been inverted using the Oc-
cam approach (deGroot Hedlin and Constable, 1990). The Occam inversion follows a different
inversion scheme to the NLCG inversion approach of Rodi and Mackie (2001) by fitting the
data to a desired misfit followed by a stage of increasing smoothness and maintaining the mis-
5.5. Sensitivity studies and model validation 69
fow25
fow24
fow23
fow22
fow21
fow20
fow19
fow18
fow17
fow15
fow14
fow13
fow12
fow11
fow10
fow08
fow07
fow06
fow05
fow04
fow03
fow02
fow01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
Distance (Km)
De
pth
(m
)
5000
3053
1864
1138
695
424
259
158
97
59
36
22
13
8
5
Ohm.m
A
B
C
Figure 5.11: Model features which have been tested for robustness by changing their resistivitiesand calculating the forward response of the model. Feature labelled C, was set to 200 Ωm whilethe top edge was moved up to shallower levels.
fit. Obtaining similar features in both models would therefore increase confidence in the final
results. For the Occam inversion only the TE- and TM-modes were inverted between periods
of 10 − 5000 s using a starting model of a 100 Ω m half-space. Similar to the NLCG inversion,
static shifts were allowed to change freely during the inversion. The Occam model converged
to a misfit of rms = 3.35. The Occam model shows many of the same features produced in
the NLCG inversion (see Figure 5.8). Underneath stations 06 and 08 the Occam model also
shows a resistor of approximately 1000 Ω m (labelled A in Figure 5.10). The resistor labelled
A also extends to the surface underneath station 08 in the Occam model. The feature labelled
B in Figure 5.8 and 5.10 appears at shallower depth in the Occam inversion result. Features
C, D and E are also reproduced in the Occam inversion with similar resistivities to the NLCG
inversion.
5.5 Sensitivity studies and model validation
In order to validate the extent and resistivity of the most prominent features found from the
NLCG inversion, tests were undertaken by performing forward modelling runs and plotting the
rms misfit over the resistivity (Nolasco et al., 1998). Features A and B are in the vicinity of
major fault zones (Karari and Koonibba, respectively) and therefore represent important parts
of the model. The resistivities of A and B were altered to determine whether alternate models
are acceptable in the sense of fitting the data. Feature A near the surface beneath station 08
is insensitive to resistivity values between 5000 − 200000 Ω m. The rms misfit remains close to
rms = 4.63 of the 2-D inverse model in Figure 5.8. The rms misfit increases for resistivities
below 5000 Ωm and reaches an rms misfit of about 7 for 10 Ωm. Feature B in Figure 5.11 is a
crustal conductor with a resistivity of around 200 Ω m. The resistivity of B has been replaced
5.5. Sensitivity studies and model validation 70
4
5
6
7
8
9
Da
ta m
isfi
t (r
ms)
100 101 102 103 104 105 106
Resistivity in Ohm.m
4
5
6
7
8
9
Da
ta m
isfi
t (r
ms)
100 101 102 103 104 105 106
Resistivity in Ohm.m
4
5
6
7
8
9
Da
ta m
isfi
t (r
ms)
100 101 102 103 104 105 106
Resistivity in Ohm.m
4
5
6
7
8
9
100 101 102 103 104 105 106
Resistivity in Ohm.m
4
5
6
7
8
9
100 101 102 103 104 105 106
Resistivity in Ohm.m
4.0
4.5
5.0
5.5
6.0
Da
ta m
isfi
t (r
ms)
0 20 40 60 80 100
Depth of conductor
4.0
4.5
5.0
5.5
6.0
Da
ta m
isfi
t (r
ms)
0 20 40 60 80 100
Depth of conductor
Feature A Feature B Feature C
Figure 5.12: Data rms misfits, calculated from forward modelling of features labelled A and Bin Figure 5.11, versus altered resistivities of those features. The feature labelled C was set to200 Ωm and shows the data misfit versus depth of the top boundary of the feature C.
with values between 1 − 100000 Ω m. Low resistivities between 1 and 20 Ω m generate a high
rms misfit, whereas values between 50 and 500 Ωm achieve the best fit. For resistivities higher
than 500 Ωm, the rms misfit increases slightly but is still less than rms = 5. The findings
support the notion that EM fields are more sensitive to conductive structures (see discussion
on page 9), i.e. perturbations around small resistivity values (e.g. 10 Ω m) have more influence
on the model response than perturbations around large resistivity values (e.g. 10000 Ωm).
Feature C represents the conductive upper mantle beneath the resistive crust, which underlies
the Nuyts Domain. In order to constrain the depth at which the model becomes insensitive
to changes in conductivity, the top edge of the conductivity gradient has been shifted upwards
from 100 km to a depth of 20 km, while the bottom edge of the 200 Ω m block has been fixed at
a depth of 120 km. At each step, the rms misfit has been computed from 2-D forward modelling.
Results are given in Figure 5.12. The resulting curve of rms misfit as a function of the depth
of the top edge of the conductor shows an increase in gradient at around 65 km for decreasing
depth. This suggests that the skin depth of the maximum period used (2000 s) lies at around
60 − 70 km underneath the Nuyts Domain.
Constrained inversions were conducted to test the model structures discussed above. Specif-
ically, the conductive crustal/upper mantle structure labelled B in Figure 5.11 shows only a
marginal change in rms misfit between 100 and 100000 Ω m. Therefore, model feature B has
been set to 20000 Ωm while the remaining cells were set to the same value as in Figure 5.8 for
the starting model. One way to conduct constrained inversions is to fix the cells comprising
feature B (Bedrosian, 2007), but here all model cells are free to change with each iteration. If
the crustal structure is insensitive to higher resistivities, it should remain resistive. However,
Figure 5.13 suggests that the crustal conductor underneath station 23 has indeed a resistivity
of about 200 Ω m. The same test has been undertaken with feature C, which was set to 200 Ωm
between depths of 20 and 120 km. The constrained inversion result (shown in Figure 5.13
5.6. 3D inversion of the 2-D profile 71
fow25
fow24
fow23
fow22
fow21
fow20
fow19
fow18
fow17
fow15
fow14
fow13
fow12
fow11
fow10
fow08
fow07
fow06
fow05
fow04
fow03
fow02
fow01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
Dep
th (
m)
5000
3053
1864
1138
695
424
259
158
97
59
36
22
13
8
5
Ohm.m
fow
25
fow
24
fow
23
fow
22
fow
21
fow
20
fow
19
fow
18
fow
17
fow
15
fow
14
fow
13
fow
12
fow
11
fow
10
fow
08
fow
07
fow
06
fow
05
fow
04
fow
03
fow
02
fow
01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
De
pth
(m
)
5000
3053
1864
1138
695
424
259
158
97
59
36
22
13
8
5
Ohm.mfo
w2
5
fow
24
fow
23
fow
22
fow
21
fow
20
fow
19
fow
18
fow
17
fow
15
fow
14
fow
13
fow
12
fow
11
fow
10
fow
08
fow
07
fow
06
fow
05
fow
04
fow
03
fow
02
fow
01
50 100 150 200 250 300 350 400 450
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
20000
10000
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
-90000
-100000
-110000
-120000
Distance (Km)
De
pth
(m
)
5000
3053
1864
1138
695
424
259
158
97
59
36
22
13
8
5
Ohm.m
Original
Inversion from
feature B
Inversion from
feature C
Figure 5.13: Constrained inversion results of the Fowler profile. From top to bottom: uncon-strained inversion result (top); constrained inversion starting from the unconstrained inversionresult with feature B set to 20000 Ωm (middle); constrained inversion result setting feature Cto 200 Ω m between dephs of 20 and 120 km (bottom).
bottom) replicates the structures obtained from the unconstrained inversion, but with slightly
lower resistivity values. It can be concluded that the features extracted are required by the
model in order to fit the data.
5.6 3D inversion of the 2-D profile
The MT data collected during the Fowler survey show some three-dimensionality, especially
for stations 19-25 at periods longer than 200 s, due to the conductive shelf waters to the
5.6. 3D inversion of the 2-D profile 72
100
101
102
103
104
App
. res
istiv
ity
101 102 103
fow01
0
30
60
90
Pha
se
101 102 103
fow02101 102 103
fow03101 102 103
fow04101 102 103
fow06
100
101
102
103
104
App
. res
istiv
ity
101 102 103
fow08
0
30
60
90
Pha
se
101 102 103
fow10101 102 103
fow11101 102 103
fow13101 102 103
fow17
100
101
102
103
104
App
. res
istiv
ity
101 102 103
fow18
0
30
60
90
Pha
se
101 102 103
fow19101 102 103
fow20101 102 103
fow21101 102 103
fow22
100
101
102
103
104
App
. res
istiv
ity
101 102 103
fow24
0
30
60
90
Pha
se
101 102 103
fow25
Figure 5.14: Observed and modelled responses of the Zxy and Zyx components of the impedancetensor plotted against period T . Triangles - observed Zxy; squares - observed Zyx, solid line -modelled Zxy, dashed line - modelled Zyx.
5.6. 3D inversion of the 2-D profile 73
south and the thick sedimentary basins to the east in the vicinity of these sites. In order
to accommodate the skewed tensor data, a 3-D inversion of the MT data set has been under-
taken (Siripunvaraporn et al., 2005a). The size of the 3-D grid was 21 × 70 × 24 cells in the
north, east and vertical downwards direction, respectively. The model space dimensions are
3200 × 5000 × 2000 km. Prior to modelling, the data were rotated by 14° to the line of profile.
Inter-site cell spacing in the east-west direction is 8 km, and 10 km in the north-south direction.
Horizontal expansion was applied in both the x- and y-directions outside the grid of interest,
increasing each layer by a factor of 1.6. Cell sizes in the z-direction increase by a factor of 1.4
starting from 250 m for the first cell at the surface.
For the 3-D inversion a subset of 17 stations were chosen, omitting a few stations due to lower
quality of the data. The 3-D code of Siripunvaraporn et al. (2005a) requires the input of the
same period range for all MT stations. It is therefore crucial to have a subset of stations that
covers a possibly wide range of periods without loss of quality. This has led to the inversion
of 7 periods between 32 and 2048 s, representing skin-depths between 9 km (28 km) and 72 km
(225 km) for a subsurface resistivity of 10 Ω m (100 Ω m). All impedance tensor components
were inverted, which improves the accuracy of images of off-profile structures compared to 3-D
inversion of off-diagonal impedance tensor components alone (Siripunvaraporn et al., 2005b).
The error floors were set at 10 % and 15 % on the off-diagonal and diagonal impedance tensor
elements, respectively. The large error floors accommodate static shift in the data, especially
for the diagonal impedance tensor components. Furthermore, the data have been static shift
corrected from visual inspection of the apparent resistivity and phase curves and from com-
parison with the static shift factors obtained from the 2-D inversion results. The ocean was
included as a constraint into the starting model (Figure 5.16). The smoothness parameter τ
was set to 5, and the model length scale set to 0.3 in the x-, y-, and z-directions. The inversion
converged to a lowest misfit, producing a model which was used a starting model for a second
inversion run with a model length scale of 0.1 in all directions. After three inversions, the final
model converged to a rms misfit of 1.79.
Figure 5.14 shows the model responses for the off-diagonal components of the impedance ten-
sor (diagonal components are shown in Appendix B on page 111). The model fits the data well,
with only a few stations exhibiting underfitting of the phase values of the yx-component. This
also leads to a rotation of the phase tensor ellipses for short periods at sites 20-22 (Figure 5.15).
Otherwise, the modelled data follows the trend of the observed data and illustrates the three
different domains as indicated by the phase tensors. Stations 1-15 have higher phase values than
stations 17-22, while stations 24 and 25 have very low phases. Apart from the shortest period
(32 s) the phases between sites exhibit less fluctuation compared to the observed data, which
is a result of the smooth inversion procedure. The inversion tends to reduce the discrepancy
between the orthogonal components of the off-diagonal components of the impedance tensor,
5.6. 3D inversion of the 2-D profile 74
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
33˚S
32˚S
31˚S
33˚S
32˚S
31˚S
33˚S
32˚S
31˚S
33˚S
32˚S
31˚S
33˚S
32˚S
31˚S
33˚S
32˚S
31˚S
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
130˚E 132˚E 134˚E 136˚E
33˚S
32˚S
31˚S
0 5 10 15 20 25 30 35 40 45 50
Minimum phase
Observed data Modelled data
32s 32s
128s 128s
512s 512s
2048s 2048s
Figure 5.15: Comparison of phase tensor ellipses of observed (left) and modelled (right) datafor the Fowler profile.
5.6. 3D inversion of the 2-D profile 75
−80
−60
−40
−20
0
z in
km
Layer 13 between −15.00 to −31.00 kilometer
1
2
3
−80
−60
−40
−20
0
z in
km
Layer 12 between −5.00 to −15.00 kilometer
1
2
3
−80
−60
−40
−20
0
z in
km
Layer 11 between 5.00 to −5.00 kilometer
1
2
3
−80
−60
−40
−20
0
z in
km
Layer 10 between 15.00 to 5.00 kilometer
1
2
3
−200 −100 0 100 200
y in km (east to west)
−80
−60
−40
−20
0
z in
km
Layer 9 between 31.00 to 15.00 kilometer
1
2
3
fow
01
fow
02
fow
03
fow
04
fow
06
fow
08
fow
10
fow
11
fow
13
fow
17
fow
18
fow
19
fow
20
fow
21
fow
22
fow
24
fow
25
−50
0
50
x in
km
(so
uth
to
no
rth
)
Layer 5 at depth −1.78 to −2.74 kilometer
1
2
3
−50
0
50
x in
km
(so
uth
to
no
rth
)
Layer 11 at depth −17.45 to −24.68 kilometer
1
2
3
−50
0
50
x in
km
(so
uth
to
no
rth
)
Layer 15 at depth −68.81 to −96.59 kilometer
1
2
3
−50
0
50
x in
km
(so
uth
to
no
rth
)
Layer 21 at depth −522.26 to −731.46 kilometer
1
2
3
−50
0
50
x in
km
(so
uth
to
no
rth
)
Layer 1 at depth 0.00 to −0.25 kilometer
1
2
3
Top View Cross section
log(rho_a)log(rho_a)
Figure 5.16: Slices of the 3-D inversion results from a 100 Ω m starting model in plan view (leftcolumn) and cross-section (right column). The conductive ocean was included as a a-priorifeature and fixed during the inversion (see top left figure). Model features become smootherwith depth as a result of skin-depth (see bottom left figure).
which can be seen from the less elliptical shape of the phase tensor ellipses.
The 3-D inverse model benefits from the additional information provided by inclusion of
the diagonal impedance tensor elements, which more accurately image off-profile features
(Siripunvaraporn et al., 2005b). Another obvious advantage is the differentiation between true
2-D structures (cross-cutting the profile) and 3-D structures (tangential to the profile) due to
the extra dimension. A comparison with the 2-D results across the line of profile as well as par-
allel to either side will be able to reveal the different structures. Figure 5.16 shows the results
of the 3-D inversion for a starting model of 100 Ω m in plan view and in cross-section, while
Figure 5.17 displays cross-sections of the inverted models for different conducting half-spaces
5.6. 3D inversion of the 2-D profile 76
−80
−60
−40
−20
0
z in
km
Layer 11 between 5.00 to −5.00 kilometer
1
2
3
−200 −100 0 100 200
y in km (east to west)
−80
−60
−40
−20
0
z in
km
Layer 9 between 31.00 to 15.00 kilometer
1
2
3
−80
−60
−40
−20
0
z in
km
Layer 13 between −15.00 to −31.00 kilometer
1
2
3
fow
01
fow
02
fow
03
fow
04
fow
06
fow
08
fow
10
fow
11
fow
13
fow
17
fow
18
fow
19
fow
20
fow
21
fow
22
fow
24
fow
25
fow
01
fow
02
fow
03
fow
04
fow
06
fow
08
fow
10
fow
11
fow
13
fow
17
fow
18
fow
19
fow
20
fow
21
fow
22
fow
24
fow
25
−80
−60
−40
−20
0
z in
km
Layer 13 between −15.00 to −31.00 kilometer
1
2
3
fow
01
fow
02
fow
03
fow
04
fow
06
fow
08
fow
10
fow
11
fow
13
fow
17
fow
18
fow
19
fow
20
fow
21
fow
22
fow
24
fow
25
−80
−60
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−20
0
z in
km
Layer 11 between 5.00 to −5.00 kilometer
1
2
3
−200 −100 0 100 200
y in km (east to west)
−80
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0
z in
km
Layer 9 between 31.00 to 15.00 kilometer
1
2
3
−80
−60
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0
z in
km
Layer 13 −15.00 to −31.00 kilometer
1
2
3
−80
−60
−40
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0
z in
km
Layer 11 between 5.00 to −5.00 kilometer
1
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−200 −100 0 100 200
y in km (east to west)
−80
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z in
km
Layer 9 between 31.00 to 15.00 kilometer
1
2
3
10 Ohm.m start model 100 Ohm.m start model 1000 Ohm.m start model
log(rho_a) log(rho_a) log(rho_a)
Figure 5.17: Cross-sections of the 3-D inverse models of the Fowler profile for different startinghalf-spaces of 10, 100, 1000 Ωm from left to right, respectively. For each model, three cross-sections are shown. The top row denotes cross-sections about 25 km north of the profile, middlerow underneath the profile and the bottom row denotes cross-sections about 25 km south ofthe profile.
(10, 100, 1000 Ωm).
A common feature of all model slices in Figure 5.16 is the conductive zone underneath
sites 01 to 04, which is also present in the 2-D inversion results. Furthermore, the middle to
lower crust is resistive between sites 06 and 17 with resistivities of 500 to more than 5000 Ωm.
However, in contrast to the 2-D inverse result, the resistive crust only extends to the surface
north of the profile. Underneath sites 18 and 19, the 3-D model introduces a crustal conductor
(50−200 Ωm), which is not depicted so clearly in the 2-D model results. However this feature is
more prominent to the north of the profile line, and does not extend south of the profile. Further
to the west however, is another conductor beneath stations 24 and 25; it is more prominent
south of site 24 and 25. The top 5 km of subsurface underneath sites 11 to 25 is conductive
in the order of around 10 Ω m, decreasing in resistivity towards the western part of the profile
near the Eucla sedimentary basin.
Figure 5.17 suggests the same features for the 10 Ω m starting model, which due to the lower
resistivity of the starting model, has more emphasis on the conductive shallow structures. The
3-D model obtained from the 1000 Ω m starting model also replicates the main features, but
is generally biased towards higher resistivities for the model cells. The final rms misfit of the
models presented is lowest (rms = 1.79) for the model generated with the 100 Ω m starting
model. The 10 Ω m and 1000 Ω m models yield final rms misfits of 1.95 and 1.98, respectively.
5.7. Discussion 77
5.7 Discussion
Given the different modelling algorithms used (2-D vs 3-D, NLCG vs data-space method), the
inversion results show similar features:
i) Conductive sediments cover the top 2− 3 km in the Eucla Basin in the western part of the
profile, adjacent to the western boundary of the Gawler Craton. The Fowler Domain to
the east also shows a higher conductivity, however, it cannot be ruled out that hydration
of shear zones causes the enhanced conductivity rather than conductive sediments.
ii) 2-D modelling suggest a crustal conductor dipping to the east underneath the western
boundary of the Gawler Craton. While the Occam model indicates an increase in conductiv-
ity beneath site fow22, the NLCG inverse model shows a moderately resistive crust/upper
mantle to depths of 80 km. Furthermore, the 3-D inversion results confine the position of
the conductor to the south, but do not show a northern extent. It is believed that the
conductive crustal feature is a superposition of the inductive effect of the sediments at the
top, the conductive ocean water to the south and possibly a conductor in the deep crust.
iii) The Fowler Domain is conductive in the top 5 − 10 km and the extent of the near-surface
conductive feature spatially matches the north-east trending (SZ2) shear zones in the
Fowler Domain, as can be seen in Figure 5.2. 3-D modelling suggests a crustal structure
underneath site 18 and 19. However, the model suggests that this structure is not truly
2-D and only extents to the north outside the line of profile. Inter-site spacing is too large
and the periods used are too long to be able to distinguish between individual fault zones
in the Fowler Domain in case they have an electric response.
iv) The Nuyts Domain comprises the St. Peter Suite Granitoids, which have been emplaced
around 1630 Ma. The granitoids cover only the top few kilometers and have a resistivity
of around 10 − 100 Ωm. The granitoids are underlain by a resistive feature (> 1000 Ω m)
spatially coinciding with the extent of the Nuyts Domain at the surface. Resistive middle
crusts have been reported in the North American Slave Craton (Jones et al., 2001) and are
typical for Archaean crust (Gough, 1986). It is surprising that the resistive feature extents
to large depths of 80 km. However sensitivity analysis carried out in Chapter 5.5 indicate
that the model is not very sensitive to structure below 60 km depth.
v) Structure beneath the outcrop of the Gawler Range Volcanics are conductive in the top
5 km and are more resistive with depth. However, the resistivities are lower than the
St Peter Suite granitoids.