Upper crust of the Pilbara Craton, Australia; 3D geometry of a granite/greenstone terrain

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 μm s −2) 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 gravity anomalies (commonly 200–400 μm s −2), a shallower inferred base to the greenstone belt (generally <8 km), and the base of the greenstone belt is thought to be truncated.