Crustal and lithospheric structure of the Albany-Fraser Orogen, Western Australia, from passive-source seismology

Abstract:

The Paleoproterozoic to Mesoproterozoic Albany-Fraser Orogen is situated along the southeastern margin of the Archean Yilgarn Craton in Western Australia. The orogen records a long history of extension and magmatism, dominantly in a rift or back-arc setting, inboard of the collision zone between the West Australian and South Australian Cratons. The extensional structures were inverted during formation of a fold and thrust architecture during the Mesoproterozoic, which must have left its mark on the orogen's deep crustal and lithospheric structure.

In November 2013, a 40-station passive seismic array was installed across the east Albany-Fraser Orogen, which was shifted southeast, along strike of the orogen, in October 2014. The goal of this project is the retrieval of three-dimensional models of crustal and mantle lithospheric structure for the east Albany-Fraser Orogen, thereby extending recently acquired active seismic profiles into the third dimension. First results from analyzing the data recorded by the northern sub-array are presented, exploiting ambient noise, receiver functions and information from occasional local events.

Ambient noise tomography yields a three-dimensional S-wave velocity model of the upper and middle crust. The obtained velocity distribution shows a marked contrast between faster upper crustal velocities throughout the Yilgarn margin and the Albany-Fraser Orogen and markedly slower velocities in the Eucla Basin further east. The Fraser Zone, a ~450 km long body of metamorphic gabbros in the Albany-Fraser Orogen, shows up as a prominent upper crustal high-wavespeed anomaly. At mid-crustal levels, the average seismic velocitiy decreases, and the basement beneath the Eucla Basin appears to be faster than the regions further west. P receiver functions have been used for the estimation of bulk crustal Vp/Vs (H-K stacking) as well as for Bayesian inversion that yields a 1D S-wave velocity profiles. The westernmost stations, which lie on the unmodified margin of the Yilgarn Craton, show a crustal thickness of around 35-38 km, and a very sharp Moho. Crustal thickness increases southwestwards, reaching maximum values of up to 50 km beneath the Biranup and Fraser Zones of the Albany-Fraser Orogen. Towards the Eucla Basin in the east, the crust thins again to about 40 km.