New Insight into the Crustal Structures of SE Australian Continent from a Combined Study of Ambient Noise Tomography and Gravity Modelling

Abstract:

The Tasmanide, the eastern one third of Australian continent, experienced long-term lateral continental growth via accretionary processes in the Phanerozoic. The Delamerian and Lachlan Orogens represent the first two phases of such accretions. A detailed 3D crustal Vs model of the orogenic suture zone help 1) decipher the early subduction-accretion processes and 2) explore the crustal responses of the latest intraplate basaltic volcanisms on Australian continent beneath the Newer Volcanic Province (NVP). To construct such model, ambient noise data are collected from 40 short-band seismic stations in the region, from which Rayleigh wave phase velocity maps are generated at 2-18 s periods. A 3D Vs model with a general resolution of 20-30 km is then constructed based on a Monte-Carlo approach, and is interpreted with the aid of 3D gravity modelling.

In the shallow crust of this model, different velocity features are observed on the two sides of the Moyston Fault (MF), which are coincident with distinct magmatism and metamorphism at each side. Especially, high velocities are observed right above amphibolite metamorphic complex and its western edge are perfectly confined to the MF, suggesting the tectonic significance of MF during early accretions. In the middle crust, the MF is associated with ~6-8% velocity differences on the two sides, which indicate large compositional differences even taking 20-30 mW/m² thermal discrepancies of the two areas into account. The compositional differences are probably results of a remanent continental lithosphere in the east of MF and an oceanic lithosphere in the west as proposed by previous seismic and geochemical studies in the area. Meanwhile, no large concentrated low velocities are observed in the upper and mid crust of the NVP; while considerable high velocities are seen in the lower crust. The crustal velocity structures together with strong positive gravity features at NVP suggest large basaltic magmatic reservoir has probably crystallized in the lower crust of the NVP.