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

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 deformatio n cannot be inferred with any certainty from that of the greenstones. Howev er, 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 gree nstones) is largely unreflective, with interpreted west-dipping reverse fau lts. The middle crust is reflective, with reflector geometry implying thick ening by west directed thrust stacking, and the lower crust has a fabric in dicative of ductile deformation. These reflection fabrics imply crustal sho rtening, probably during the Late Archaean regional D2 ENE-WSW shortening e vent. They were subsequently overprinted and disrupted by structures consis tent with regional NNW-SSE strike slip D3 faulting, and probably younger, m ore localised D4 faulting. The seismic images of the crust therefore show t hat the crust suffered tectonic events in which both the order and directio n of deformation are similar to those of the greenstones. This is evidence that the whole crust deformed when the greenstones deformed. However, the s cale and style of deformation vary with depth through the crust, and includ e 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 t hat in the greenstones. (C) 2000 Elsevier Science B.V. All rights reserved.