Subsurface distribution of granites and greenstones in the Paleoarchean East Pilbara Terrane from 3-D gravity inversion

Abstract

The Paleoarchean East Pilbara Terrane contains some of the oldest rocks on Earth, and has a distinctive surface architecture of granite domes surrounded by arcuate greenstone belts. Explanations for creation of the domes include fold interference, core complex formation and the subsidence of a dense, mafic crust between relatively buoyant granitic complexes. In the latter “sagduction” process, greenstones are often viewed as descending to the base of the crust. Here we present a 3-D inversion of Bouguer gravity data for density contrast constrained by shear wave velocities and a seismic reflection profile. We show that the East Pilbara Terrane is predominantly characterized by a two-layer crust with high-density greenstone rocks largely confined to the upper crust, which is inconsistent with vertical tectonic models requiring large-scale greenstone preservation in the lower crust. The eastern margin of the terrane is marked by a west-to-east transition from low-density ovoid bodies to linear northwest-trending sources associated with the Proterozoic Paterson Orogen. We identify the previously unknown 10–15 km wide Tumbinna Pool Dome, between the Mount Edgar and Yilgalong domes. Higher density sources related to the surface distribution of greenstone rocks largely occur above 10–15 km depth, though they may locally reach >15 km depth on the north side of the Mount Edgar Dome. Some granitic domes have higher density cores related to distinct granite suites, while the Split Rock Supersuite appears to exhibit systematically lower density at depths of 5–10 km. Low-density sources due to small intrusions such as the North Pole Dome only extend to depths up to 10 km, while the large Mount Edgar Dome can be identified at depths as great as 15–20 km. Some low-density bodies in the lower crust, one of which underlies the Yilgalong Dome, correlate with regions of reduced seismic reflectivity, which we interpret to be possible relict migmatisation zones or melt pathways. A weakly constrained ESE-trending high-density region immediately below the Moho correlates with isolated sub-Moho reflectors, and may be a relict zone of rifting and delamination.