GRAVITY STUDY OF A GRANITE-GREENSTONE BELT IN THE EASTERN PILBARA CRATON, WESTERN AUSTRALIA

This study was conducted to investigate whether crustal-scale features in the >3.2 billion year old East Pilbara region of Western Australia are consistent with vertical, gravity-driven tectonics or with horizontal, plate-like tectonics. The Pilbara craton – similar to many cratons in the early Earth – consist of large (>40 km in diameter) ovoid granite bodies separated by greenstone belts. Because the greenstone is significantly more dense than the granitic rocks, it is possible to determine the vertical extent of the greenstone belts using gravity surveys. Gravity data was collected along two nearly perpendicular transects that extend, from the N to S, across the granitic Mt Edgar pluton to the Warrawoona Group, and W to E from the granitic Coruna Downs pluton to the Warrawoona Group. The Warrawoona Group is composed of greenstone and metamorphosed sedimentary rocks. The North-South transect is 23 km long and the East-West transect 17 km long. Gravity measurements, with position constrained by precise GPS measurements, were collected approximately every 500 m along both transects. Gravity measurements were made with LaCoste-Romberg gravimeters, models EG and G, that have a repeatability better than 0.1 mGal. High-precision GPS data has a vertical accuracy of ~1 cm. Instrumental and tidal drift, latitude, free air, and Bouguer corrections were made on the gravity data. Two dimensional models of the subsurface were created using EMIGMA software, to determine the sub-surface geometry of the greenstone belt. The NS-oriented line records an 11 mGal gravity increase along a 4.5 km span across the granite-greenstone boundary. The EW-oriented line has a gravity increase of 8 mGal over a 2 km span, across the granite greenstone boundary. Measurements are currently underway to determine the density of the rocks, so an appropriate inversion model can be made. However, the gravity data and initial models suggest a moderately dipping greenstone body with limited vertical extent into the crust.