Crustal signature of Late Archaean tectonic episodes in the Yilgarn craton, Western Australia: evidence from deep seismic sounding

Abstract

Deformation in the greenstone supracrustal rocks of the Eastern Goldfields Province of the Archaean Yilgarn Craton in Western Australia is delaminated from the underlying basement along a regional detachment surface presently at 3–7 km depth. This might suggest that the history of crustal deformation cannot be inferred with any certainty from that of the greenstones. However, seismic images of the crust below the greenstones show structures that can be interpreted in terms of a series of tectonic events similar to those within the greenstones. The upper crust (below and to the west of the greenstones) is largely unreflective, with interpreted west-dipping reverse faults. The middle crust is reflective, with reflector geometry implying thickening by west directed thrust stacking, and the lower crust has a fabric indicative of ductile deformation. These reflection fabrics imply crustal shortening, probably during the Late Archaean regional D2 ENE–WSW shortening event. They were subsequently overprinted and disrupted by structures consistent with regional NNW–SSE strike slip D3 faulting, and probably younger, more localised D4 faulting. The seismic images of the crust therefore show that the crust suffered tectonic events in which both the order and direction of deformation are similar to those of the greenstones. This is evidence that the whole crust deformed when the greenstones deformed. However, the scale and style of deformation vary with depth through the crust, and include thrusting and probably folding in the upper crust, thrust stacking in the middle crust, and ductile deformation in the lower crust. The length scale (wavelength) of structures in the middle and lower crust is greater than that in the greenstones.