GEOPHYSICAL STUDY OF THE SUBSURFACE GEOMETRY OF A GRANITE-GREENSTONE BELT IN THE EASTERN PILBARA CRATON, WESTERN AUSTRALIA

A geophysical study was conducted to determine whether the granite-greenstone (GG) structures in Western Australia's East Pilbara region, >3.2 billion years old, are more consistent with modern horizontal plate-like tectonics or vertical gravity-driven tectonics. The Pilbara craton is among the oldest and least deformed Paleoarchean terrains in the world, consisting of large (>40 km in diameter) ovoid granite bodies separated by greenstone belts. Gravity data was collected along two nearly perpendicular transects: 1) N to S across the granitic Mt Edgar pluton to the greenstone belt; and 2) W to E across the granitic Corunna Downs pluton to the greenstone belt. Gravity measurements, constrained by GPS measurements, were collected approximately every 500 m along both transects (each ~20 km long). The N-S oriented line records an 11 mGal gravity increase and the W-E oriented line an 8 mGal gravity increase across the GG boundary. Forward gravity models created bodies of varying; density, shape, and depth to match the observed gravity anomalies.

The total magnetic intensity (TMI) data used in this study is aeromagnetic data compiled by the Geologic Survey of Western Australia which has an 80 m grid spacing centered over the N-S and W-E gravity lines. The TMI data along the N-S gravity transect has an ~950 nT anomaly while the W-E gravity transect has a ~300 nT anomaly across the GG boundary. Magnetic susceptibility measurements were taken in the field and in the laboratory on collected samples. Magnetic susceptibility of lab measured samples averaged 1.76_x10^-2 for the greenstone and 3.13_x10^-3 for the granites in SI units. Magnetic models are used to constrain the gravity models to further understand the subsurface GG geometry. Gridded TMI data allows for refining the orientation, size, and extent of the greenstone belt within a granitic crust. Initial models of the subsurface suggest moderately dipping greenstone bodies with a depth extent of <4.5 km. The gradual gravity increase across GG boundaries and relatively shallow depth of the greenstone belt inferred from the gravity anomalies suggest that vertical, gravity driven tectonics might not be active in the Paleoarchean Pilbara craton, favoring an interpretation of moderately to shallowly dipping deformation.