The granitegreenstone 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
granitegreenstone 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
granitegreenstone 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 granitegreenstone 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 granitegreenstone 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 granitegreenstone 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.
The gravity surveys have been compiled and integrated by the Gravity Section of AGSO. The
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 20 mm 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 granitegreen-
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 granitegreenstone 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 struc- tures of the Pilbara Craton, by Proterozoic struc-
tures parallel to, and outside, the craton margin. Early Archaean granitegreenstone 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 northwest 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 granitegreenstone 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.
The estimates of the margin of the Pilbara Craton from mapped geology, gravity anomalies
and magnetic anomalies are roughly consistent Fig. 1c. The Pilbara Craton is a discrete oval
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
