Depth extent of upper crustal structures Margins between granitoid complexes and greenstones

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

Estimates of the depth extent of upper crustal structures can be obtained from the shape of the tails of the magnetic and gravity anomalies, and the amplitude of the gravity anomalies. Within the Pilbara Craton there are high-ampli- tude, long-wavelength magnetic anomalies occur- ring above some of the greenstone belt synclines. near sea level in the north part of the Pilbara Craton, and about 500 m on the southern margin of the Pilbara Craton. The Pilbara crust is rela- tively thin, with a large increase in velocity at the base of the crust. The gravity anomalies Fig. 1a show only slight changes in average value across the craton. These slight changes are mainly a decrease from the margin to the centre of the craton. The mag- netic anomaly before removal of the regional, shows only a gradual change in level to the north and Northeast. The above geophysical informa- tion, from gravity, magnetic and seismic surveys, shows the Pilbara upper crust appearing to be a single unit, with no regions with different properties.

5. Margins between granitoid complexes and greenstones

The position of the boundary between granitoid complexes and greenstones has been mapped us- ing traditional geological methods over the central band of the Pilbara Craton where granitegreen- stone is exposed Hickman, 1983. This boundary has been mapped under shallow cover using the texture of the short-wavelength magnetic anoma- lies which reflects shallow geology, and under The better developed anomalies have an amplitude of 1000 – 3600 nT, a width at one half amplitude of about 9 km, and a strike length of about 20 km Fig. 4. The depth to the top of the causative bodies has been estimated using the Vacquier et al. 1951 method, and by modelling 2D sections; it averages 1.5 km in the west, and 2.0 km in the east of the Pilbara Craton. Given depths to the top, and anomaly widths about 9 km, 2D modelling shows that the width of a body with vertical sides has to be similar to the anomaly width at half amplitude. The gravity anomalies are interpreted above to indicate steep boundaries between grani- toid complex and greenstone belts; hence for sim- plicity the type of body modelled is a vertical prism, and the observed anomalies were converted to approximately symmetrical anomalies by reduc- tion-to-the-pole using a magnetic inclination of − 70 to − 80°. The true magnetic inclination of the area is − 56°, but as discussed below the induced and remanent magnetisation is in the plane of the geological formation bedding. For tabular, vertical sided models with vertical mag- netisation the depth to the base of the bodies can be determined approximately from the distance of the flanking lows from the steep gradients of the anomaly. For a body of this type, this distance is 3.5 km for a base at 5 km, and 6.5 km for a base at 14 km Fig. 5. In the area of the largest broad anomalies Fig. 4 the distance out of these lows averages about 6 km, consistent with the depth to the base of the body of about 14 km. Interpreta- tion of these large magnetic anomalies was also carried out by 2.5D modelling of profiles using the software package ModelVision by Encom. Each profile of reduced-to-the-pole anomaly was mod- elled by a tabular, vertical sided body, with an appropriate depth to the top and width. The apparent susceptibility and depth to the base of the body was then varied until the amplitude, and the tails on both sides of the anomaly, were approximately correct. Fig. 6 gives the interpreta- tion for the largest anomalies. Importantly, for the relatively wide and deep anomalies in both the eastern and western parts of the Pilbara craton, profile modelling showed that the depth to the base of the anomalous bodies is about 14 km. The mean apparent susceptibilities of these tabular bodies was generally about 0.1 – 0.2 SI. This is discussed below. Fig. 3. Extent of granites. The trace of the granitoid complex boundary in outcrop and subcrop is shown by the thin continuous line. The maximum gravity gradient is shown by the discontinuous line. The separation of these lines is a measure of the dip of the granite margin. Granite interpreted to be relatively thick base at 14 km are shaded with a dotted outline, other granites not shaded generally have a base at about 8 – 12 km. Parallel lines give margin of the Pilbara Craton from gravity anomalies. Y, C, and P indicate the Yule, Carlindi and Pippingarra Granitoid complexes. Fig. 4. Map of large magnetic anomalies under the greenstone belts of the Pilbara Craton. Contours give magnetic anomalies reduced to the pole assuming a magnetic inclination of − 80°, and using a contour interval of 250 nT. Thick black lines gives outcropsubcrop contact between the granites and greenstone belt. The white dotted line gives the steepest gravity gradient, showing the smoothed position of the margin of the granites at depth. ture at 14 km Fig. 7. Although this and the other modelled section are consistent with a depth extent of upper crustal structure being about 14 km, the gravity modelling is unconvincing, be- cause of the poor control of 5 km spacing data on gradients, the relative subtlety of the differences in calculated anomaly shape between upper crustal structures 5 and 14 km deep, and the unrealistic simplicity for the constant density differences for such large horizontal and vertical distances. The amplitude of the gravity anomalies gives information on the depth extent of the upper crustal structures, provided that the average den- sity contrast between a granitoid complex and a greenstone belt can be estimated. I have adopted the following Yilgarn Craton mean densities for the major rock types: granitoid 2.65 t m − 3 , sedi- ments 2.72 t m − 3 , felsic volcanic rock 2.74 t m − 3 , maficultramafic rocks 2.92 t m − 3 House, 1996. In the area of the larger gravity anomalies in the eastern part of the Pilbara Craton, greenstone belts are composed of maficultramafic rocks plus Fig. 5. Modelling of large magnetic anomalies by a 2.5D vertical prism. For the same anomaly width, anomaly ampli- tude, and depth to the top of the body, a body with a base at 5 km has a minimum anomaly closer to the body, than a body with a base at 14 km. Arrows show the position of the minimum. The depth extent of a vertical contact can, in theory, be determined from the shape of the 2D gravity profile across the contact Fig. 7. Two profiles were modelled, one in the east, and one in the west part of the Pilbara Craton. The gravity profiles were taken where observed gravity was better determined 5 km observation grid, where the greenstone belt and granitoid complex were relatively wide so that both tails could be ob- served, and the granitegreenstone contact was near linear. The granitoid complex was modelled in three dimensions, as a 3D prism with a plan shape of the maximum gravity gradient, a vertical margin, a thickness of 14 km, and a constant horizontal density contrast with the surrounding greenstone belts. The observed and calculated profiles across a modelled boundary are consistent to within the accuracy of the 5 km station spacing for a model with the base of upper crustal struc- Fig. 6. Large magnetic anomalies over the greenstone belts crosses, with their modelled anomalies using tabular bodies lines. The bodies are not shown. For each anomaly the body is a 2.5D vertical prism, with a top at 2 – 4 km, and a base at 14 km. A sloping straight line shows an inferred regional field. The centres of the profiles are at 21°05S 119°15E, 20°55S 120°20E, 20°55S 119°45E, 21°20S 117°45E. 0.27 t m − 3 greenstone belts with 100 maficul- tramafic rocks, and is likely to be about 0.21 t m − 3 greenstone belts with 70 maficultramafic rocks and 30 sediment and felsic volcanic rock. The probable limits are 15 – 45 sediment and felsic volcanic rock, so the uncertainty of the density contrast is + 20 – 12. 3D gravity mod- elling was carried out over those greenstone belts with the largest gravity anomaly contrasts be- tween granitoid complexes and greenstone belt. The greenstone belt was represented by a 3D prism with vertical sides, and a plan shape that is similar to the maximum gravity gradient. The two largest observed anomalies were about 650 mm s − 2 amplitude, over a body about 30 km wide, but about 15 km from one end of the body. The modelling showed that for these two larger anomalies, the greenstone belt thickness would be about 14 km for a density contrast of 0.21 t m − 3 . The interpretation of metamorphic assemblages gives some geological support for greenstone belt rocks extending down to a considerable depth in the past. In the Warrawooma Syncline occur both greenschist facies rocks and kyanite bearing schists that have been buried to at least 6 kbar Collins and van Kranendonk, 1999, and this is interpreted as indicating that the greenstone belts were subvertical to about 20 km below the then surface, and that slivers of the belts were rapidly uplifted Collins et al., 1998. Fig. 7. Model of a gravity gradient between a granitoid complex and greenstone belt. Observed profile is continuous line, modelled profile is dashed line. Profile location is 21°24S 117°06E. In this location the greenstone belt is likely to be mainly sediments. some sediments and felsic volcanic rocks, so the density contrast between granitoid complex and greenstone belts must be significantly less than Fig. 8. Distribution of banded iron formations and shear zones. Black areas; high-amplitude, wide magnetic anomalies interpreted to be due to banded-iron formation material deep in the upper crust. Thin line; trace of the granitoid complex boundary in outcrop and subcrop. Thick lines; extent of major shear zones crossing the west and north Pilbara Craton, mapped from gravity and magnetic anomalies, except AA the Scholl Shear Zone mapped by geology. Parallel lines; margin of the Pilbara Craton from gravity anomalies. Using the above arguments and magnetic, grav- ity, and geological data, I conclude that the upper crustal structures extend down to about 14 km, the base of the upper crust indicated by seismic refraction interpretation. The minimum geographical extent of the green- stone belt synclines that extend well down into the upper crust is shown by the extent of high-am- plitude long-wavelength magnetic anomalies Fig. 8, and by the axes of the gravity highs. These indicators allow deep synclines to be mapped in areas of cover, and, importantly, show that deep synclines occur in both the eastern and western part of the Pilbara Craton. The deep synclines are displaced by major shear zones in the west and north. The similar amplitude of the gravity anomalies throughout the craton is consis- tent with the granite and greenstone structures extending to the base of the crust throughout the craton.

7. Cause of the large magnetic anomalies