Unravelling the D1 event: evidence for early granite-up, greenstone-down tectonics in the Eastern Goldfields, Western Australia

The early tectonic history of the Eastern Goldfields Terrane (EGT) is poorly understood, but in places ca 2800 Ma mafic-ultramafic sequences are conformably overlain by ca 2720-2670 Ma sequences (e.g. Leonora and Laverton districts), suggesting minimal early deformation. The first significant angular unconformities occur at the base of the ca 2670-2655 Ma late basins and indicate that deformation was contemporaneous with deposition of the late basins. These basins mark the end of the volcano-sedimentary record in the EGT and typically grade upwards from polymictic mafic-dominated conglomerates to more siliciclastic compositions with abundant granitic clasts. The clastic sequences record the uplift and exhumation of granite-cored domes. There is a distinct lack of clasts with internal deformation fabrics (e.g. schist or gneiss). The timing of late basin formation overlaps with the ca 2672-2660 Ma D-1 event, which has been variously described as extensional or compressional. D-1 structures comprise a bedding-parallel S-1 that is axial planar to F-1 folds. The late basins and early D-1 fabrics are overprinted by upright north-trending F-2 folds and a sub-vertical S-2. A period of crustal thickening was achieved by autochthonous processes with deposition of greenstone sequences into local basins between ca 2720 and 2670 Ma. Partial convective overturn (or granite-up, greenstone-down tectonics) developed as a result of gravitational instability. As the large granite-gneiss bodies rose, solid-state extensional D-1 shears developed around the granites with a radial pattern of L-1 extension lineations. Areas of compression developed within the sinking greenstone sequences. A final phase of subsidence in the central parts of the greenstone belts produced depocentres (late basins). A shift from dominantly vertical tectonics to horizontal tectonics at ca 2655-2650 Ma is marked by the onset of east-west D-2 compression and the change in granite compositions from high-Ca to low-Ca granites.

Automated summary

The early tectonic history of the Eastern Goldfields Terrane (EGT) is unclear, but in some areas, mafic-ultramafic sequences around 2800 Ma are conformably overlain by sequences around 2720-2670 Ma. The formation of late basins at around 2670-2655 Ma coincides with the D-1 event around 2672-2660 Ma, indicating a shift in tectonic processes.