MODELING GRAVITY DATA IN THE EASTERN PILBARA CRATON, AUSTRALIA

The Pilbara region in Western Australia is one of Earth’s oldest preserved continents, with rocks up to 3.8 billion years old. The type of tectonic motion preserved by the granite-greenstones is debated as being either primarily horizontal, as observed in modern plate motion, or primarily vertical due to gravitational instability. Gravity measurements were taken along a northeast-southwest, 13.5-kilometer transect with LaCoste-Romberg gravimeters (models EG and G), which have an accuracy of better than ~0.1 mGal and a GPS vertical precision of approximately 1 cm. Inner terrain corrections were recorded while in the field. Twenty-nine (29) gravity and high-precision GPS measurements were recorded at approximately a 500-meter spacing. The transect, from southwest to northeast, traverses 4.5 kilometers of the granitic Corunna Downs pluton, 1 kilometer of greenstone rocks from the Warrawoona Group, and 8 kilometers of the granitic Mt. Edgar pluton. Instrumental drift, tidal, latitude, free air, and Bouguer corrections were applied to the gravity data. The results show a 10-15 mGal positive anomaly, which peaks when crossing over the Warawoona group, then decreases by ~5 mGal and stabilizes over Mt. Edgar toward the northeastern end of the transect. Notably, the gravity anomaly is wider than the traversed greenstone belt. Gravity modeling of the data, undertaken with EMIGMA software, suggests the boundaries between the units dip moderately but are not vertical. The observed anomaly magnitude is likely smaller than expected if motion of units was due to vertical, gravity driven tectonics. These results contribute to the understanding of the geology of the Pilbara region and will inform future studies.