Australia's lithospheric density field, and its isostatic equilibration

Abstract

Density is a key driver of tectonic processes, but it is a difficult property to define well in the lithosphere because the gravity method is non-unique, and because converting to density from seismic velocity models, themselves non-unique, is also highly uncertain. Here we use a new approach to define the lithospheric density field of Australia, covering from 100 • E to 165 • E, from 5 • N to 55 • S and from the crust surface to 300 km depth. A reference model was derived primarily from the recently released Australian Seismological Reference Model, and refined further using additional models of sedimentary basin thickness and crustal thickness. A novel form of finite-element method based deterministic gravity inversion was applied in geodetic coordinates, implemented within the open-source escript modelling environment. Three spatial resolutions were modelled: half-, quarter-and eighth-degree in latitude and longitude, with vertical resolutions of 5, 2.5 and 1.25 km, respectively. Parameter sweeps for the key inversion regularization parameters show that parameter selection is not scale dependent. The sweep results also show that finer resolutions are more sensitive to the uppermost crust, but less sensitive to the mid-to lower-crust and uppermost mantle than lower resolutions. All resolutions show similar sensitivity below about 100 km depth. The final density model shows that Australia's lithospheric density field is strongly layered but also has large lateral density contrasts at all depths. Within the continental crust, the structure of the middle and lower crust differs significantly from the crystalline upper crust, suggesting that the tectonic processes or events preserved in the deep crust differ from those preserved in the shallower crust. The lithospheric mantle structure is not extensively modified from the reference model, but the results reinforce the systematic difference between the density of the oceanic and continental domains, and help identify subdivisions within each. The lithospheric static pressure field was resolved in 3D from the gravity and density fields. The pressure field model also highlights the fundamental difference between the oceanic and continental domains, with the former possessing lower pressure through most of the model. Overall pressure variability is large in the upper crust (60 MPa) but reduces significantly by −30 km elevation (20-30 MPa). By −50 km elevation, thick lower-crust generates further disequilibria (25-35 MPa) that are not compensated until −125 km elevation (10-20 MPa). Beneath −125 km elevation higher pressure is observed in the continental domain, extending to the base of the model. This indicates a lithosphere that is to a large degree isostatically compensated near the base of the felsic-intermediate continental crust, and again near the theoretical base of mature oceanic lithosphere.

The Australian Continent: A Geophysical Synthesis

“…The AuSREM density distribution was taken as starting point by Aitken et al (2015), who undertook a 3-D inversion to fit the observed gravity field across Australia. The gravity data employed combine both satellite and near-surface gravity observations, with the removal of a continental-scale gravity field linked to the density variations in the upper mantle.…”

A global reference model of the lithosphere and upper mantle from joint inversion and analysis of multiple data sets

“…A global reference model of the lithosphere 1603 models in regional gravity and/or isostatic studies (e.g. Mechie et al 2013;Levandowski et al 2014;Aitken et al 2015), as input in global circulation/convection models and dynamic topography studies (e.g. Flament et al 2013;Becker et al 2014;Steinberger 2016), as anchor points in the interpretation of a number of geochemical data (cf.…”

Weighted cross-gradient function for joint inversion with the application to regional 3-D gravity and magnetic anomalies

“…We define density anomaly ρ = ρ 0 m 1 and magnetic susceptibility χ = χ 0 e m 2 for unknown property functions m 1 and m 2 , reference density ρ 0 and reference magnetic susceptibility χ 0 . Reference quantities ρ 0 and χ 0 can be constant or functions of their location if prior information is available (Aitken et al 2015). The PDEs are…”

“…However, this weighting is spatially variable and consistent with nonlinear conjugate gradient solvers. In the next section various options for weighting are introduced within the inversion framework of , based on a coherent formulation using partial differential equations (PDEs) and the finite element method (FEM; see also Aitken et al 2015;Codd & Gross 2018). Several approaches to modify the cross-gradient function are presented in Section 3.…”