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Precambrian Research 104 (2000) 175–186
www.elsevier.com/locate/precamres
Upper crust of the Pilbara Craton, Australia; 3D geometry
of a granite/greenstone terrain
Peter Wellman *
Australian Geological Sur6ey Organization, PO Box 378, 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
−2
mms ) 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
−2
gravity anomalies (commonly 200–400 mms ), 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
1. Introduction mainly Proterozoic basement of central Australia,
and two large areas of Archaean in western Aus-
Archaean rocks in Australia consist of scattered tralia — the Yilgarn and Pilbara Cratons. The
exposures of generally Late Archaean within the Pilbara Craton, the subject of this paper, com-
prises early-mid Archaean granite/greenstone
rocks (basement), which are partly overlain by
* Present address: 17 Warragamba Avenue, Duffy ACT cover rocks of the Late Archaean (Hamersley
2611, Australia. Basin) and Phanerozoic age.
0301-9268/00/$ - see front matter Crown Copyright © 2000 Published by Elsevier Science B.V. All rights reserved.
PII: S0301-9268(00)00092-9
176 P. Wellman/Precambrian Research 104 (2000) 175–186
The granite/greenstone terrain of the Pilbara Australian Geological Survey Organisation
Craton differs from other areas of Australian (AGSO) and Geological Survey of Western Aus-
crust, in its relatively old age (ca. 3660–2800 Ma) tralia. Most modelling of gravity or magnetic data
and in its structure, being mainly domal granitoid use complex models with many variables, and it is
complexes 50–100 km diameter, with intervening generally unclear which parameters of the model
synformal greenstone belts (Hickman, 1983). The are accurately determined and which parameters
greenstone belts include a variety of sediments, have large errors because of their interrelationship
intrusive rocks, and felsic, mafic and ultramafic with other parameters of the model. In this study,
lavas, that are often of only greenschist metamor- simple ‘generic’ models are used, with few variable
phic grade, and are coeval with episodes of gran- parameters, and the model defines the geometry
ite emplacement. Most granitoid complexes of only the main features of the upper crust.
consist of numerous intrusions of a range of The paper mainly discusses a zone across the
compositions and ages, with the older intrusions northern half of the Pilbara Craton where granite/
strongly deformed and highly metamorphosed, greenstone terrain rocks are exposed or have thin
and incorporating some greenstone belt material. cover, and are largely unweathered. In the east of
The granitoid complexes comprise approximately this band the exposure of granite/greenstone ter-
60% of the craton. rain is more continuous, structures are better un-
There are differences between the eastern and derstood, and gravity and magnetic anomalies are
western parts of the Pilbara Craton (Hickman, larger; hence many of the ideas have been devel-
1999). From geological mapping, the eastern side oped, and most examples given, for these features
has a well developed dome and syncline structure, in the east. The northern margin of the Pilbara
ages of the granites and greenstones are mainly in Craton is concealed by thick sediments of the
the range 3.51–2.9 Ga, and greenstone belts are in Northwest Shelf, and there is only poor quality
the form of synclines containing multiple vol- gravity and magnetic data. The southern half of
canic–sedimentary packages. The western and the Pilbara Craton is covered with thick sequences
possibly northern sides have elongate granitoid of Late Archaean Hamersley Basin sedimentary
complexes, the ages of the granites and green- and volcanic rocks, and because of this ‘cover’ it
stones are mainly in the shorter range 3.27–2.9 is difficult to interpret the gravity and magnetic
Ga, major west northwest shears are an important data in terms of granite/greenstone structure.
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. 2. Magnetic and gravity data
Most previous studies of the geology of the
Pilbara have mapped the geology at outcrop level, The magnetic interpretation was carried out on
and have inferred structure above or below this a detailed composite magnetic anomaly grid
level by extrapolation of the exposed geology. derived from 14 separate airborne surveys of the
There has been only a limited use of gravity or Australian Geological Survey Organisation and
magnetic anomalies to map the geology of the the Geological Survey of Western Australia. Most
granite/greenstone surface under cover, or to con- of the area of granite/greenstone outcrop is cov-
strain its 3D structure; in part this is due to the ered by five 1996 airborne surveys. Each survey
regional nature of the available gravity and mag- collected high-resolution magnetic, gamma-ray
netic data. spectrometric and altitude data, observed at 80 m
This paper discusses the 3D geometry of the above the ground level, with a flight-line separa-
main geological features of the Pilbara Craton tion of 400 m (Richardson, 1997). The remaining
granite/greenstone terrain, using new and more land area is covered by 1984–1992 regional sur-
detailed gravity and magnetic data. The magnetic veys with 1.5 km flight-line spacing.
data were acquired in the North Pilbara Project of The gravity surveys have been compiled and
the National Geoscience Mapping Accord by the integrated by the Gravity Section of AGSO. The
P. Wellman/Precambrian Research 104 (2000) 175–186 177
anomalies are based mainly on three surveys — that commonly forms at the margins of crustal
an AGSO shipborne survey with about 16 km blocks with different crustal history (Gibb and
spacing over the northern marine part of the Thomas 1976; Wellman 1978, 1998). The gravity
craton which unfortunately does not cover a 40 dipole is thought to be an expression of the low
km wide strip seaward of the coast, an AGSO density, thin crust of the Pilbara Craton margin
survey covering the whole land area on a grid relative to the higher density, thicker crust of the
with 11 km spacing, and a Hamersley Iron Pty margin of the younger surrounding crustal blocks.
Ltd survey which covered the southern part of the This is consistent with the interpretation of the
land area, ona5kmgrid spacing. The land one seismic refraction profile across the southern
gravity surveys used a helicopter for transport margin of the Pilbara Craton (Drummond, 1979),
and barometers for altitude, so the Bouguer and seismic refraction work over similar struc-
−2
anomaly accuracy is about 20 mms . tures elsewhere (Winardhi and Mereu, 1997).
The geological and geophysical data for the Magnetic anomalies (Fig. 1b) generally reflect
entire Pilbara are presented at 1:1.5 M scale in structure at the top of the granite/greenstone ter-
atlas form in Blewett et al. (2000). 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-
3. Extent of the Pilbara Craton tures of the Pilbara Craton, by Proterozoic struc-
tures parallel to, and outside, the craton margin.
The full extent of the Early Archaean rocks of Early Archaean granite/greenstone domal struc-
the Pilbara Craton is obscured by younger cover tures are truncated at the Northeast margin.
rocks; its extent, therefore, is inferred from High-amplitude linear anomalies trending gener-
geophysical anomalies, and the distribution of ally west, caused by the banded iron formation
younger rocks. deposits of the Late Archaean Hamersley Basin,
Anomalies due to upper crustal effects are which form Pilbara Craton cover rocks, are trun-
partly obscured in the Bouguer anomaly maps cated at the Southwest margin. Immediately out-
due to the isostatic effect of regional topography side the boundary in the Southwest, west, and
increasing in altitude to the Southeast. This re- northwest is a string of elongate magnetic
gional is largely removed when the anomalies are anomaly highs, in places 12 km wide and 1800 nT
expressed as terrain corrected free air anomalies in amplitude, caused by relatively shallow bodies.
(Faye anomalies) (Fig. 1a). The thick black line In the absence of other strong indications, these
on the figure, marking a change in anomaly tex- anomalies have been taken to define the margin of
ture and anomaly value, gives the extent of the the Pilbara Craton in the northwest. Earlier inter-
Pilbara Craton interpreted from these gravity pretations (Wellman, 1978, 1998), put the north-
anomalies. As this is based on gravity anomalies, west boundary about 50 km northwest on the
this craton boundary is at the mean depth of the basis of the gravity anomalies.
structures causing the anomalies — possibly 8–14 Determining the extent of the Pilbara Craton
km. Within the defined ovoid shape, the gravity from geology is hindered by Phanerozoic rocks
anomalies define irregularly-distributed oval lows; straddling the boundary, and the absence of ex-
which are due to Early Archaean granite/green- posed granite/greenstone terrain near the likely
stone domal structures within the Pilbara Craton. margin of the Pilbara Craton. The best estimate
Outside the ovoid the anomalies are very elon- of the craton margin from geology is the extent of
gate, parallel to the Craton margin, and are due the Late Archaean rocks of the Pilbara Craton
to structures in Proterozoic blocks wrapping (Fig. 1c).
around the Pilbara Craton. The boundary is a The estimates of the margin of the Pilbara
prominent gravity gradient on all margins except Craton from mapped geology, gravity anomalies
the northwest. This gradient is between a high and magnetic anomalies are roughly consistent
and low anomaly — the dipole gravity anomaly (Fig. 1c). The Pilbara Craton is a discrete oval
178 P. Wellman/Precambrian Research 104 (2000) 175–186
area (600×550 km) with a characteristic texture the Pilbara Craton. The crustal structure from
given by oval granites. It is surrounded by eight independent profiles are generally similar.
younger crust with structures subparallel with the The average profile given in Fig. 2, has been
margin. calculated by averaging the various depths and
velocities. Differences between profiles in crustal
velocities and crustal thicknesses were thought by
4. Crustal properties within the Pilbara Craton Drummond (1983) to be caused by the southward
dip of the crust mantle boundary across the Pil-
Drummond (1983) used the seismic refraction bara Craton. This dip is consistent with the in-
method, and iron ore mine explosions, to deter- creased crustal loading by topography to the
mine the seismic velocity structure of the crust of south, the mean altitude of the land surface being
−2
Fig. 1. Pilbara Craton’s extent and regional anomalies. (a) Terrain corrected free air anomalies, with contour interval 200 mms .
The thick continuous line traces the dipole gravity anomaly at the margin of the Pilbara Craton. (b) Magnetic anomalies with
regional removed. The dashed line shows the extent of magnetic anomalies characteristic of the Pilbara Craton. (c) Surface geology.
Extent of the outcropping granite/greenstone rock is shown in dark grey. The lines show estimates of the extent of the Pilbara
Craton from gravity (thick continuous line), and magnetic anomalies (short dashed line).
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