Seismic P-wave anisotropy in the subcrustal lithosphere of north-west Australia

Summary. Reduced P_n travel times from the Archaean Pilbara Craton of north-west Australia show a strong correlation with azimuth, which could be used as evidence of anisotropy. However, the azimuthal correlation could also be explained by a southerly dip of between 1 and 2° on the crust–mantle boundary, although the models from several reversed seismic profiles across the craton suggest a smaller dip.
A time-term analysis of the P_n date yielded several models. The preferred solution, in which the dip on the crust–mantle boundary is similar to that in the models from the reversed profiles, has approximately 2 per cent anisotropy in the uppermost mantle, with the direction of maximum velocity 30° east of north. One possible cause of the anisotropy is that olivine crystals were aligned by syntectonic recrystallization and/or power law creep in the tensional environment caused at the base of the lithosphere by flexure during loading of the lithosphere by the strata of the Hamersley Basin which overlies the Pilbara Craton.
A seismic discontinuity occurs about 15 km below the crust–mantle boundary under the craton. A qualitative analysis of all available seismic data suggests that the velocity below the boundary is probably also anisotropic, with the direction of maximum velocity between north and 40° west of north. The direction of minimum velocity below the sub-Moho boundary correlates loosely with the direction of basement lineaments in the Proterozoic Capricorn Orogenic Belt to the south of the craton, suggesting that the anisotropy under the boundary may be younger than that immediately under the crust/mantle boundary. This is consistent with the notion that the Archaean lithosphere was thinner than the present lithosphere.