Precambrian Research 104 2000 175 – 186
Upper crust of the Pilbara Craton, Australia; 3D geometry of a granitegreenstone 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 granitegreenstone terrain, dominated by granite domes, and in part covered by younger rocks. Gravity and magnetic anomalies are used
to map the granitegreenstone surface under cover, and infer the depth extent of the granitegreenstone 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 granitegreenstone boundaries, and the amplitude of the anomalies up to 650
m m s
− 2
together with the inferred granitegreenstone 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 – 400 mm s
− 2
, a shallower inferred base to the greenstone belt generally B 8 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 www.elsevier.comlocateprecamres
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
granitegreenstone 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.
0301-926800 - see front matter Crown Copyright © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 0 9 2 - 9
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
