Archaean basin margin geology and crustal evolution: an East Pilbara traverse

Abstract

A palinspastic reconstruction of a 100 km long traverse through Archaean rocks of the East Pilbara, Western Australia, includes new observations of the deformation preceding the now visible greenstone belt pattern. The restoration is time-calibrated with all available U–Pb datings. Between incompletely preserved basin sequences, two superposed Palaeoarchaean volcano-sedimentary basins (the Coongan and Salgash Basins) are separated by an eastwards time-transgressive interface tentatively interpreted as an onlap surface. For over 140 Ma, the basin margin architecture was structurally controlled by superposed extensional growth fault arrays (D1) with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts. The basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini-subduction of slabs of cooled water-saturated basalt towards the base of an originally c. 45 km mafic crust. At c. 3.31 Ga, the extension was replaced by plate-driven regional NW–SE compression (D2) inferred from NW-over-SE shear and ramp-and-flat thrusts, partly reversing offsets of the D1 extension. The recognition of widespread D2 pre-doming compression is important because it triggered the c. 3.18 Ga start of formation of the dome-and-keel pattern (D3) visible today, which culminated at c. 2.9 Ga.