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 200 mm s
− 2
. 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 granitegreenstone 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.
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 outcropsubcrop, 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
