The archaean whim creek belt, an ensialic faultbounded basin in the pilbara block, Australia

Abstract

Abstract The Whim Creek Belt in the western Pilbara Block, Australia, is an elongate structural basin ∼ 80 km long and up to 15 km wide. It contains a c. 2950 Ma old sequence of bimodal volcanics, volcaniclastics and terrigenous sediments, which unconformably overlies granitoids and older deformed and metamorphosed supracrustal sequences. The volcano-sedimentary sequence was deposited in terrestrial to marine environments, in a fault-bounded basin, with approximately the same configuration as the present Whim Creek Belt. Alluvial and subaqueous fanglomerate facies were deposited along the northern margin of the basin, with submarine-fan and basinal facies, deposited longitudinally in the centre of the basin. The basin-margin faults, are now part of a regional pattern of strike-slip faults which were active both prior to and after basin formation, and probably had a significant lateral component during basin evolution. The Whim Creek Belt is thus interpreted as an Archaean rift or pull-apart basin. Similar basins, containing bimodal volcanics and terrigenous sediments occur at the head of the present Gulf of California (e.g. Salton Trough). Although a direct tectonic analogy cannot be assumed, this interpretation implies that the Archaean tectonic pattern in the western Pilbara involved externally imposed deformation, and was not solely the result of gravity-driven uprise of granitic batholiths.