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 and Q of 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
0.12.5 = 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.
The seismic refraction model of Drummond 1983 shows an increase in velocity in the mid
crust at about 14 km Fig. 2. This seismic model implies that near the mid-crustal boundary the
average lower crust is more mafic than the average upper crust. At the surface of the granitegreen-
stone terrain about 40 by area of the upper crust is greenstone belt material and 60 is granitoid
complex material, and the proportion of dense material increases with depth, so the upper part of
the lower crust is on average much more mafic than the granitoid complexes. Hence, at the present day,
if the greenstone belts extend down to the base of the upper crust, and the upper part of the lower
crust is fairly uniform horizontally, then the gran- itoid complex material is not continuous below the
greenstone belts. The figures of Collins et al. 1998 and Van Kranendonk 1998 suggest that it contin-
ued below in the past.
9. Discussion
