Upper crust of the Pilbara Craton, Australia; 3D geometry
of a granite
/
greenstone terrain
Peter Wellman *
Australian Geological Sur6ey Organization,PO Box378,Canberra ACT,Australia Received 14 September 1999; accepted 19 May 2000
Abstract
The Pilbara Craton in Northwest Australia is a 600×550 km region of early-mid Archaean granite/greenstone terrain, dominated by granite domes, and in part covered by younger rocks. Gravity and magnetic anomalies are used to map the granite/greenstone surface under cover, and infer the depth extent of the granite/greenstone structures. A published seismic refraction interpretation gives a two layer crust for the Pilbara Craton, with the layers separated by a velocity gradient at about 14 km. Some magnetic anomalies have a 1000 – 3600 nT amplitude, a width at one-half amplitude of 9 km, and a strike length of \100 km. Their causative bodies have a top at 1 – 2 km, an average
apparent susceptibility of 0.1 – 0.2 (SI), and importantly a base about 14 km. The magnetic material is thought to be a small proportion of banded iron formation within the greenstone belts. Gravity anomalies are interpreted to indicate that granite margins are generally steep, and many granites have a base at a similar level to one another. The shape of the gravity anomalies over the granite/greenstone boundaries, and the amplitude of the anomalies (up to 650
mm s−2) together with the inferred granite/greenstone density contrast, are consistent with both the granites and
greenstones extending to a depth of 14 km. The domes are therefore vertical cylinders extending to mid-crustal depths. The great depth of the greenstone belts is consistent with the domal structure being due to convective crustal overturn. The Pilbara Craton may be unusual, because greenstone belts elsewhere in the world have smaller amplitude gravity anomalies (commonly 200 – 400mm s−2), a shallower inferred base to the greenstone belt (generally B8 km),
and the base of the greenstone belt is thought to be truncated. Crown Copyright © 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords:Upper crust; Gravity anomalies; Magnetic anomalies; Archaean; Greenstone belts; Batholiths
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1. Introduction
Archaean rocks in Australia consist of scattered exposures of generally Late Archaean within the
mainly Proterozoic basement of central Australia, and two large areas of Archaean in western Aus-tralia — the Yilgarn and Pilbara Cratons. The Pilbara Craton, the subject of this paper, com-prises early-mid Archaean granite/greenstone rocks (basement), which are partly overlain by cover rocks of the Late Archaean (Hamersley Basin) and Phanerozoic age.
* Present address: 17 Warragamba Avenue, Duffy ACT 2611, Australia.
The granite/greenstone terrain of the Pilbara Craton differs from other areas of Australian crust, in its relatively old age (ca. 3660 – 2800 Ma) and in its structure, being mainly domal granitoid complexes 50 – 100 km diameter, with intervening synformal greenstone belts (Hickman, 1983). The greenstone belts include a variety of sediments, intrusive rocks, and felsic, mafic and ultramafic lavas, that are often of only greenschist metamor-phic grade, and are coeval with episodes of gran-ite emplacement. Most granitoid complexes consist of numerous intrusions of a range of compositions and ages, with the older intrusions strongly deformed and highly metamorphosed, and incorporating some greenstone belt material. The granitoid complexes comprise approximately 60% of the craton.
There are differences between the eastern and western parts of the Pilbara Craton (Hickman, 1999). From geological mapping, the eastern side has a well developed dome and syncline structure, ages of the granites and greenstones are mainly in the range 3.51 – 2.9 Ga, and greenstone belts are in the form of synclines containing multiple vol-canic – sedimentary packages. The western and possibly northern sides have elongate granitoid complexes, the ages of the granites and green-stones are mainly in the shorter range 3.27 – 2.9 Ga, major west northwest shears are an important part of the structure, many greenstone belts do not have the form of synclines, and some sections of belt have only one group of sediments.
Most previous studies of the geology of the Pilbara have mapped the geology at outcrop level, and have inferred structure above or below this level by extrapolation of the exposed geology. There has been only a limited use of gravity or magnetic anomalies to map the geology of the granite/greenstone surface under cover, or to con-strain its 3D structure; in part this is due to the regional nature of the available gravity and mag-netic data.
This paper discusses the 3D geometry of the main geological features of the Pilbara Craton granite/greenstone terrain, using new and more detailed gravity and magnetic data. The magnetic data were acquired in the North Pilbara Project of the National Geoscience Mapping Accord by the
Australian Geological Survey Organisation (AGSO) and Geological Survey of Western Aus-tralia. Most modelling of gravity or magnetic data use complex models with many variables, and it is generally unclear which parameters of the model are accurately determined and which parameters have large errors because of their interrelationship with other parameters of the model. In this study, simple ‘generic’ models are used, with few variable parameters, and the model defines the geometry of only the main features of the upper crust.
The paper mainly discusses a zone across the northern half of the Pilbara Craton where granite/ greenstone terrain rocks are exposed or have thin cover, and are largely unweathered. In the east of this band the exposure of granite/greenstone ter-rain is more continuous, structures are better un-derstood, and gravity and magnetic anomalies are larger; hence many of the ideas have been devel-oped, and most examples given, for these features in the east. The northern margin of the Pilbara Craton is concealed by thick sediments of the Northwest Shelf, and there is only poor quality gravity and magnetic data. The southern half of the Pilbara Craton is covered with thick sequences of Late Archaean Hamersley Basin sedimentary and volcanic rocks, and because of this ‘cover’ it is difficult to interpret the gravity and magnetic data in terms of granite/greenstone structure.
2. Magnetic and gravity data
The magnetic interpretation was carried out on a detailed composite magnetic anomaly grid derived from 14 separate airborne surveys of the Australian Geological Survey Organisation and the Geological Survey of Western Australia. Most of the area of granite/greenstone outcrop is cov-ered by five 1996 airborne surveys. Each survey collected high-resolution magnetic, gamma-ray spectrometric and altitude data, observed at 80 m above the ground level, with a flight-line separa-tion of 400 m (Richardson, 1997). The remaining land area is covered by 1984 – 1992 regional sur-veys with 1.5 km flight-line spacing.
anomalies are based mainly on three surveys — an AGSO shipborne survey with about 16 km spacing over the northern marine part of the craton which unfortunately does not cover a 40 km wide strip seaward of the coast, an AGSO survey covering the whole land area on a grid with 11 km spacing, and a Hamersley Iron Pty Ltd survey which covered the southern part of the land area, on a 5 km grid spacing. The land gravity surveys used a helicopter for transport and barometers for altitude, so the Bouguer anomaly accuracy is about 20mm s−2
.
The geological and geophysical data for the entire Pilbara are presented at 1:1.5 M scale in atlas form in Blewett et al. (2000).
3. Extent of the Pilbara Craton
The full extent of the Early Archaean rocks of the Pilbara Craton is obscured by younger cover rocks; its extent, therefore, is inferred from geophysical anomalies, and the distribution of younger rocks.
Anomalies due to upper crustal effects are partly obscured in the Bouguer anomaly maps due to the isostatic effect of regional topography increasing in altitude to the Southeast. This re-gional is largely removed when the anomalies are expressed as terrain corrected free air anomalies (Faye anomalies) (Fig. 1a). The thick black line on the figure, marking a change in anomaly tex-ture and anomaly value, gives the extent of the Pilbara Craton interpreted from these gravity anomalies. As this is based on gravity anomalies, this craton boundary is at the mean depth of the structures causing the anomalies — possibly 8 – 14 km. Within the defined ovoid shape, the gravity anomalies define irregularly-distributed oval lows; which are due to Early Archaean granite/ green-stone domal structures within the Pilbara Craton. Outside the ovoid the anomalies are very elon-gate, parallel to the Craton margin, and are due to structures in Proterozoic blocks wrapping around the Pilbara Craton. The boundary is a prominent gravity gradient on all margins except the northwest. This gradient is between a high and low anomaly — the dipole gravity anomaly
that commonly forms at the margins of crustal blocks with different crustal history (Gibb and Thomas 1976; Wellman 1978, 1998). The gravity dipole is thought to be an expression of the low density, thin crust of the Pilbara Craton margin relative to the higher density, thicker crust of the margin of the younger surrounding crustal blocks. This is consistent with the interpretation of the one seismic refraction profile across the southern margin of the Pilbara Craton (Drummond, 1979), and seismic refraction work over similar struc-tures elsewhere (Winardhi and Mereu, 1997).
Magnetic anomalies (Fig. 1b) generally reflect structure at the top of the granite/greenstone ter-rain and above — i.e. at shallow crustal levels. The lines in Fig. 1b mark the truncation of anomalies due to Early and Late Archaean tures of the Pilbara Craton, by Proterozoic struc-tures parallel to, and outside, the craton margin. Early Archaean granite/greenstone domal struc-tures are truncated at the Northeast margin. High-amplitude linear anomalies trending gener-ally west, caused by the banded iron formation deposits of the Late Archaean Hamersley Basin, which form Pilbara Craton cover rocks, are trun-cated at the Southwest margin. Immediately out-side the boundary in the Southwest, west, and northwest is a string of elongate magnetic anomaly highs, in places 12 km wide and 1800 nT in amplitude, caused by relatively shallow bodies. In the absence of other strong indications, these anomalies have been taken to define the margin of the Pilbara Craton in the northwest. Earlier inter-pretations (Wellman, 1978, 1998), put the north-west boundary about 50 km northnorth-west on the basis of the gravity anomalies.
Determining the extent of the Pilbara Craton from geology is hindered by Phanerozoic rocks straddling the boundary, and the absence of ex-posed granite/greenstone terrain near the likely margin of the Pilbara Craton. The best estimate of the craton margin from geology is the extent of the Late Archaean rocks of the Pilbara Craton (Fig. 1c).
area (600×550 km) with a characteristic texture given by oval granites. It is surrounded by younger crust with structures subparallel with the margin.
4. Crustal properties within the Pilbara Craton
Drummond (1983) used the seismic refraction method, and iron ore mine explosions, to deter-mine the seismic velocity structure of the crust of
the Pilbara Craton. The crustal structure from eight independent profiles are generally similar. The average profile given in Fig. 2, has been calculated by averaging the various depths and velocities. Differences between profiles in crustal velocities and crustal thicknesses were thought by Drummond (1983) to be caused by the southward dip of the crust mantle boundary across the Pil-bara Craton. This dip is consistent with the in-creased crustal loading by topography to the south, the mean altitude of the land surface being
Fig. 1. Pilbara Craton’s extent and regional anomalies. (a) Terrain corrected free air anomalies, with contour interval 200mm s−2.
Fig. 2. Crustal structure models. On the left is the mean seismic refraction model from Drummond (1983), and on the right is the gravity and magnetic anomaly interpretation.
deeper cover using the general correlation of broad, magnetic lows on granitoid complexes, and narrow linear magnetic highs over greenstone belts.
Where the boundary outcrops, geological ob-servations show that it is generally very steep. The average horizontal position of the boundary be-tween granitoid complex and greenstones, at a depth of about one half way down the boundary, is given by the centre of the maximum gradient of the gravity anomaly. This maximum gravity gra-dient has a horizontal extent of about 10 km, so the position of the centre of this maximum gradi-ent can be mapped to within about 3 km over the whole land area because the gravity station spac-ing is 5 and 11 km.
Fig. 3 shows the position of the granitoid com-plex boundary in outcrop and subcrop, and the inferred position of the granitoid complex margin at depth. The relative position of these two lines gives the average direction of dip of the granitoid complex margin. The granitoid complex margins dip under the granitoid complex (inwards), and under the greenstones (outwards), for approxi-mately equal horizontal distances, so the average dip of the granitoid complex margins to the base is near vertical. The smoother shape of the grani-toid complex margin at depth relative to the margin of the granitoid complex outcrop/subcrop, is due to the maximum gravity gradient showing the average position of the granitoid complex margin at depth, due to both the averaging effect of gravity anomalies, and the low resolution of the gravity survey. The complexity of the grani-toid complex margin at depth is undefined from the gravity anomalies, and it is likely to be similar to that at the surface.
6. Depth extent of upper crustal structures
Estimates of the depth extent of upper crustal structures can be obtained from the shape of the tails of the magnetic and gravity anomalies, and the amplitude of the gravity anomalies.
Within the Pilbara Craton there are high-ampli-tude, long-wavelength magnetic anomalies occur-ring above some of the greenstone belt synclines. near sea level in the north part of the Pilbara
Craton, and about 500 m on the southern margin of the Pilbara Craton. The Pilbara crust is rela-tively thin, with a large increase in velocity at the base of the crust.
The gravity anomalies (Fig. 1a) show only slight changes in average value across the craton. These slight changes are mainly a decrease from the margin to the centre of the craton. The mag-netic anomaly before removal of the regional, shows only a gradual change in level to the north and Northeast. The above geophysical informa-tion, from gravity, magnetic and seismic surveys, shows the Pilbara upper crust appearing to be a single unit, with no regions with different properties.
5. Margins between granitoid complexes and greenstones
The better developed anomalies have an amplitude of 1000 – 3600 nT, a width at one half amplitude of about 9 km, and a strike length of about 20 km (Fig. 4). The depth to the top of the causative bodies has been estimated using the Vacquier et al. (1951) method, and by modelling 2D sections; it averages 1.5 km in the west, and 2.0 km in the east of the Pilbara Craton. Given depths to the top, and anomaly widths about 9 km, 2D modelling shows that the width of a body with vertical sides has to be similar to the anomaly width at half amplitude. The gravity anomalies are interpreted above to indicate steep boundaries between grani-toid complex and greenstone belts; hence for sim-plicity the type of body modelled is a vertical prism, and the observed anomalies were converted to approximately symmetrical anomalies by reduc-tion-to-the-pole using a magnetic inclination of −70 to −80°. (The true magnetic inclination of the area is −56°, but as discussed below the induced and remanent magnetisation is in the plane of the geological formation bedding.) For tabular, vertical sided models with vertical mag-netisation the depth to the base of the bodies can be determined approximately from the distance of
the flanking lows from the steep gradients of the anomaly. For a body of this type, this distance is 3.5 km for a base at 5 km, and 6.5 km for a base at 14 km (Fig. 5). In the area of the largest broad anomalies (Fig. 4) the distance out of these lows averages about 6 km, consistent with the depth to the base of the body of about 14 km. Interpreta-tion of these large magnetic anomalies was also carried out by 2.5D modelling of profiles using the software package ModelVision by Encom. Each profile of reduced-to-the-pole anomaly was mod-elled by a tabular, vertical sided body, with an appropriate depth to the top and width. The apparent susceptibility and depth to the base of the body was then varied until the amplitude, and the tails on both sides of the anomaly, were approximately correct. Fig. 6 gives the interpreta-tion for the largest anomalies. Importantly, for the relatively wide and deep anomalies in both the eastern and western parts of the Pilbara craton, profile modelling showed that the depth to the base of the anomalous bodies is about 14 km. The mean apparent susceptibilities of these tabular bodies was generally about 0.1 – 0.2 (SI). This is discussed below.
Fig. 3. Extent of granites. The trace of the granitoid complex boundary in outcrop and subcrop is shown by the thin continuous line. The maximum gravity gradient is shown by the discontinuous line. The separation of these lines is a measure of the dip of the granite margin. Granite interpreted to be relatively thick (base at 14 km) are shaded with a dotted outline, other granites (not
Fig. 4. Map of large magnetic anomalies under the greenstone belts of the Pilbara Craton. Contours give magnetic anomalies reduced to the pole assuming a magnetic inclination of−80°, and using a contour interval of 250 nT. Thick black lines gives outcrop/subcrop contact between the granites and greenstone belt. The white dotted line gives the steepest gravity gradient, showing the smoothed position of the margin of the granites at depth.
ture at 14 km (Fig. 7). Although this and the other modelled section are consistent with a depth extent of upper crustal structure being about 14 km, the gravity modelling is unconvincing, be-cause of the poor control of 5 km spacing data on gradients, the relative subtlety of the differences in calculated anomaly shape between upper crustal structures 5 and 14 km deep, and the unrealistic simplicity for the constant density differences for such large horizontal and vertical distances.
The amplitude of the gravity anomalies gives information on the depth extent of the upper crustal structures, provided that the average den-sity contrast between a granitoid complex and a greenstone belt can be estimated. I have adopted the following Yilgarn Craton mean densities for the major rock types: granitoid 2.65 t m−3
, sedi-ments 2.72 t m−3, felsic volcanic rock 2.74 t m−3, mafic/ultramafic rocks 2.92 t m−3
(House, 1996). In the area of the larger gravity anomalies in the eastern part of the Pilbara Craton, greenstone belts are composed of mafic/ultramafic rocks plus
Fig. 5. Modelling of large magnetic anomalies by a 2.5D vertical prism. For the same anomaly width, anomaly ampli-tude, and depth to the top of the body, a body with a base at 5 km has a minimum anomaly closer to the body, than a body with a base at 14 km. Arrows show the position of the minimum.
struc-Fig. 6. Large magnetic anomalies over the greenstone belts (crosses), with their modelled anomalies using tabular bodies (lines). The bodies are not shown. For each anomaly the body is a 2.5D vertical prism, with a top at 2 – 4 km, and a base at 14 km. A sloping straight line shows an inferred regional field. The centres of the profiles are at 21°05%S 119°15%E, 20°55%S 120°20%E, 20°55%S 119°45%E, 21°20%S 117°45%E.
0.27 t m−3 (greenstone belts with 100% mafic / ul-tramafic rocks), and is likely to be about 0.21 t m−3
(greenstone belts with 70% mafic/ultramafic rocks and 30% sediment and felsic volcanic rock). The probable limits are 15 – 45% sediment and felsic volcanic rock, so the uncertainty of the density contrast is +20 – 12%. 3D gravity mod-elling was carried out over those greenstone belts with the largest gravity anomaly contrasts be-tween granitoid complexes and greenstone belt. The greenstone belt was represented by a 3D prism with vertical sides, and a plan shape that is similar to the maximum gravity gradient. The two largest observed anomalies were about 650 mm s−2 amplitude, over a body about 30 km wide, but about 15 km from one end of the body. The modelling showed that for these two larger anomalies, the greenstone belt thickness would be about 14 km for a density contrast of 0.21 t m−3. The interpretation of metamorphic assemblages gives some geological support for greenstone belt rocks extending down to a considerable depth in the past. In the Warrawooma Syncline occur both greenschist facies rocks and kyanite bearing schists that have been buried to at least 6 kbar (Collins and van Kranendonk, 1999), and this is interpreted as indicating that the greenstone belts were subvertical to about 20 km below the then surface, and that slivers of the belts were rapidly uplifted (Collins et al., 1998).
Fig. 7. Model of a gravity gradient between a granitoid complex and greenstone belt. Observed profile is continuous line, modelled profile is dashed line. Profile location is 21°24%S 117°06%E. In this location the greenstone belt is likely to be mainly sediments.
Fig. 8. Distribution of banded iron formations and shear zones. Black areas; high-amplitude, wide magnetic anomalies interpreted to be due to banded-iron formation material deep in the upper crust. Thin line; trace of the granitoid complex boundary in outcrop and subcrop. Thick lines; extent of major shear zones crossing the west and north Pilbara Craton, mapped from gravity and magnetic anomalies, except AA the Scholl Shear Zone mapped by geology. Parallel lines; margin of the Pilbara Craton from gravity anomalies.
Using the above arguments and magnetic, grav-ity, and geological data, I conclude that the upper crustal structures extend down to about 14 km, the base of the upper crust indicated by seismic refraction interpretation.
The minimum geographical extent of the green-stone belt synclines that extend well down into the upper crust is shown by the extent of high-am-plitude long-wavelength magnetic anomalies (Fig. 8), and by the axes of the gravity highs. These indicators allow deep synclines to be mapped in areas of cover, and, importantly, show that deep synclines occur in both the eastern and western part of the Pilbara Craton. The deep synclines are displaced by major shear zones in the west and north. The similar amplitude of the gravity anomalies throughout the craton is consis-tent with the granite and greenstone structures extending to the base of the crust throughout the craton.
7. Cause of the large magnetic anomalies
The high-amplitude long-wavelength magnetic anomalies are inferred to be caused by volumi-nous bodies with mean apparent susceptibilities of 0.1 – 0.2 (SI). If these anomalies are due to suscep-tibility alone then this means that in the body the volume percent of magnetite is about 3%. In the Pilbara the three rock types with high susceptibil-ities are large peridotite bodies and small ultra-mafic bodies with modes of about 0.05 (SI), and banded iron formation in cherts, with a mode of about 0.08 and values up to 1.0 (SI) (Wellman 1999). These rock types also have similar values outside the Pilbara granite/greenstone terrain (Clark and Emerson 1991).
whole of the body is of this rock type with a cross section of 9×12 km extending for hundreds of kilometres, and (c) in the area of the greatest magnetic anomaly the residual gravity anomaly is abnormally low.
Clark and Schmidt (1994) and Clark (1997) discuss the magnetic properties of banded iron formation in the Hamersley Basin and Yilgarn Block and integrate this work with that on similar rocks outside Australia. For all areas, bedding parallel susceptibility is typically 0.5 – 2.0 (SI), and the ratio of remanent and induced magnetisation (Q) is typically 1 – 2. Due to geometrical effects both the susceptibility and remanence magnetisa-tion are much stronger in the plane of the bedding than across the bedding. If we assume that the banded iron formations of the Pilbara granite/ greenstone terrain have the average values of the above ranges (susceptibility 1.0 (SI) andQof 1.5), then their apparent susceptibility is 2.5 (SI). If the average apparent susceptibility of the greenstone belts below 2 km is 0.1 (SI) then the banded iron formation would have to occupy an average of 0.1/2.5=4% of the volume of the greenstone belt. This seems a bit high, but it is at least possible. The preferred cause of the long-wavelength magnetic anomalies is thin bodies of banded iron formation within the greenstone belts, with enhanced mag-netisation below 1.5 – 2.0 km depth. This banded iron formation horizon, or horizons, may be struc-turally repeated by isoclinal folding, or faults.
8. Relative thickness of the granites
The gravity anomalies contain information on the 3D shape of the granitoid complexes. To display this information one must first calculate a residual gravity anomaly that reflects only the mass deficiency due to the low density of the granites. This was calculated in two steps. The effect of the Bouguer gravity anomalies correlating with re-gional altitude was removed by subtracting from the Bouguer anomalies a 50×50 km average Bouguer correction, to obtain a terrain-corrected free air anomaly. A second order surface was removed, so the majority of granites had the same minimum value. The resultant residual gravity
anomaly map is thought to primarily indicate variation in granitoid complex thickness.
However, there are two complications. (1) There is a gravity low around the whole margin of the craton due to changes of density and structure at the margin. This marginal low decreases the resid-ual gravity values over both greenstone belts and granitoid complexes, and its effect must be com-pensated for when estimating the thickness of granites. (2) Near the centre of the Pilbara Craton are three large areas of the more-felsic (and there-fore lower density) granites, each about 30 – 50 km across (parts of the Yule, Carlindi and Pippingarra Granitoid complexes). Above these three areas the residual gravity is more negative than over all the other areas of granitoid complexes, with the excep-tion of the margin of the craton. The more negative gravity lows over these extensive felsic granites are thought to be due to their lower density, rather than a greater thickness of the enclosing granitoid complex.
Taking the above effects into account, the resid-ual gravity anomalies were used to map the relative thickness of the granitoid complexes (Fig. 3). About 50% of the granitoid complexes are near the maximum thickness (14 km), and most of the
remainder are greater than half this thickness and must have higher density (?greenstone-belt) mate-rial below.
9. Discussion
The base of the greenstone belts is inferred to be about 14 km in the Pilbara Craton, which is considerably deeper than the generally less than 10 km inferred for greenstone belts elsewhere in the world (House, 1996; Stettler et al. 1997). This difference is thought to be real, because the den-sity contrast between granitoid complex and greenstone belt is likely to be similar around the world, while the maximum range in gravity anomaly is about 650 mm s−2
in the Pilbara Craton, and 200 – 400mm s−2
for greenstone belts elsewhere. Greenstone belts elsewhere are often thought to be truncated at the base; possibly by a sole thrust or granite intrusion (Stettler et al., 1997). This raises the possibility that the Pilbara Craton structures are unusually complete, in that they contain the basal part of the greenstone belt that is generally absent elsewhere.
The depth extent of the granites and green-stones is not only of academic interest. An upper crustal model is an important parameter in delim-iting the volume of rocks of various types that can be scavenged for metals by solutions, and it also controls the depth extent of faults cutting the granite/greenstone rocks, and these faults are im-portant for fluid flow.
10. Conclusions
1. The Pilbara Craton is about 600×550 km in extent, as shown by the reasonable agreement between geological mapping, gravity and mag-netic anomalies. The oval shaped granites and enveloping greenstone belts forming the struc-ture of the Pilbara Craton are truncated at these margins. The seismic, gravity and mag-netic data are consistent with the Craton being a unit with similar upper-crustal properties throughout; that is with no major subdivisions.
2. The inferred shape of the granitoid complexes is a vertical cylinder, with the dip of the boundary commonly 70 – 90°.
3. The greenstone belts and granitoid complexes are thought to extend down, with similar width, from the surface to the base of the upper crust about 14 km depth.
4. The large magnetic anomalies are interpreted to be due to banded iron formations in the greenstone belts.
Acknowledgements
The work was carried out within the North Pilbara Project of the Australian Geological Sur-vey Organisation and the Geological SurSur-vey of Western Australia within the National Geoscience Mapping Accord. I am very grateful for discus-sions on the work with other members of the project, in particular with Richard Blewett. I am grateful for reviews of the paper from R.S. Blewett, D.C. Champion, W.J. Collins, A. Meixner and P.W. Schmidt. The gravity and mag-netic data used are from the Australian National Gravity Database and the Australian National Magnetic Database. The paper is published with the permission of the Executive Director of the Australian Geological Survey Organisation.
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