Growth faults and avalanches: Reconstructing Paleoarchean basins in the Pilbara and Kaapvaal cratons

Abstract

Four new volcano-sedimentary complexes (VSC's), respectively c. 3510, 3460, 3430, and 3320 Ma old, are identified in the East Strelley, Coongan, and Kelly belts of the East Pilbara craton and compared with the extraordinarily complete c. 3450 Ma Buck Reef-VSC in the southern African Kaapvaal craton. The VSC's reveal an intricate relationship between volcanic deposition, sedimentation, development of syndepositional fault arrays, both extensional and contractional, and magmatism. The geometry and kinematics of these fault systems were analyzed after restoring the tilt of the stratigraphic sequences back to the depositional horizontal. The growth fault arrays are interpreted to have been generated by lack of lateral support of depositional basin margin prisms as known from present-day deltas and passive margins. This ‘basin margin collapse’ scenario for deformation and kinematics of the topmost section of the crust relies on topographic relief combined with vertical crustal oscillation. The relationship between supracrustal collapse and coeval deformation on deeper-seated detachments within the basement of the Pilbara remains as yet unsolved. The restoration corroborates the (semi-)circular basin architecture we proposed in 2017, which preceded, and is unrelated to, the present-day configuration of granitoid complexes and greenstone belts. The new findings also assess an early, syndepositional presence of the east–west Warrawoona Lineament as a major dividing line of as yet unknown structural character between two areas of basin superposition. For the Coonterunah Subgroup a multistorey architecture is established: the Table Top/Coucal-VSC with its near-water level chert top is overlain by regularly bedded Double Bar Basalt. The ensemble is truncated by the newly introduced tectono-stratigraphic Bergamina Unit, interpreted as a mega-avalanche emplaced somewhere between 3496 and 3466 Ma, possibly time-equivalent to the Duffer Fm. The crustal depths of the detachments below the (water-level) tops of all VSC's identified, in other words the thicknesses of the VSC's, are used as proxy for determining minimum basin centre depths ranging from 1000 to 3800 m. The greatest depth was reached in the North and South Coongan Basins, where voluminous bimodal volcanism of the Duffer Fm, maximum subsidence and deposition rates resulted in maximum basin margin instability.