The East Pilbara Terrane in Western Australia is one of the best exposed examples of early Earth’s continents. A widely accepted model for formation of the East Pilbara Terrane, is convective overturn driven by mechanical instability between buoyant granites below volcano-sedimentary greenstone sequences that constituted the uppermost crust. Models of convective Rayleigh-Taylor style crustal overturn predict steeply dipping to subvertical boundaries and compositional zonation of the crust on a comparable scale to the current surface pattern. The first deep seismic reflection survey in the East Pilbara Terrane (Doublier et al., 2020) produced a surprising result, by revealing a stark contrast in structure between what is exposed at Earth’s surface and what is imaged at depth. The present day East Pilbara crust is horizontally layered and of predominantly felsic composition, with dense greenstone sequences largely restricted to the top 10 km. There is no geophysical evidence at depth of structural variations similar in shape or scale to the archetypical dome-and-keel geometry exposed at Earth’s surface. Instead of being the product of convective overturn, we propose that the defining feature of Earth’s early continents was extreme mechanical decoupling during incremental reworking of evolving continental crust. This rheological architecture governed how volcanic basins forming in the brittle-elastic upper crust interacted with flow of highly mobile, viscous mid- to lower crust, and how surface-derived volatiles were introduced to crustal and lithospheric melt sources.

Reference(s)

Doublier, MP, Johnson, SP, Gessner, K, Howard, H, Chopping, R, Smithies, RH, Martin, DMcB, Kelsey, DE, Haines, PW, Hickman, AH, Czarnota, K, Southby, C, Champion, DC, Huston, DL, Calvert, AJ, Kohanpour, F, Moro, P, Costelloe, R, Fomin, T and Kennett, BLN 2020. Basement architecture from the Pilbara Craton to the Aileron Province: new insights from deep seismic reflection line 18GA-KB1. In: Czarnota, K, Roach, ., Abbott, S, Haynes, M, Kositcin, N, Ray, A, Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.