Moro, Polyanna1; Giraud, Jérémie1,2; Ogarko, Vitaliy3; and Jessell, Mark1,2.
1Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, Perth, Australia; 2Mineral Exploration Cooperative Research Centre, School of Earth Sciences, The University of Western Australia, Perth, Australia; 3International Centre for Radio Astronomy Research, Faculty of Engineering and Mathematical Sciences, The University of Western Australia, Perth, Australia.
We performed 2D gravity inversions along the reflection seismic line 18GA-KB1, crossing the Eastern Pilbara Craton, the Paterson Orogen, and the Kidson sub-basin, based on the Alternating Direction Method of Multipliers (ADMM). We aimed to reduce the uncertainties in defining properties and the Moho in the region to better understand its lithospheric signature. These arise from the weak constraints imposed by seismic tomography models at such depths, given the poor seismic coverage in parts of the basin, and the ambiguity between structure and mass variation of unconstrained gravity inversions.
In our inversion scheme, densities are allowed to vary within a set of predefined intervals defining bounds at each location, in conformity with geologically-reasonable petrophysical distributions (based on global datasets, with densities varying at depth as a function of pressure, temperature, and compositional changes). We used geological interfaces interpreted from the reflection seismic line as hard constraints and density iso-surfaces from a preceding seismic-gravity study as soft constraints to restrict bounds locally, thus reducing uncertainty and narrowing the model search space. The interfaces from the reflection seismic included the depths of the basin and the Moho. These were retrieved by assigning constant P wavespeeds of 6 km/s for the crystalline crust and 3.7 km/s for the basin. During the “picking” of the Moho, a bounding ribbon around this interface was ascribed to an “ambiguous” or transitional zone. From the ADMM constraints, densities were allowed to vary laterally and vertically throughout inversion, while remaining within, or as close as possible to, the values assigned to each interpreted geologic unit. The permitted densities within the “ambiguous” zone comprised the intersection between values termed for the lower crust and upper mantle.
Inversions were performed using distinct a priori models. These included upper mantle densities derived from the AuSREM model and an alternative homogeneous upper mantle model of 3300 kg/m3. Notwithstanding the different choice of priors, resulting models converged to a similar end-member, indicating consistency between results obtained in different fashions. These results suggest that, while the initial model is sensitive to the mantle density model, its influence is resolved within the bounds of the inverted models.
Our results corroborate with the physical characteristics suggested by previous seismic models in the region, with a low uppermost mantle density of ca. 3280 kg/m3 within the Paterson Orogen and low uppermost mantle densities ranging from 3240 to 3260 kg/m3 within the Canning Basin. The eastern margin of the Pilbara Craton shows a bulk crustal density that is lower than the adjacent tectonic provinces, such as expected for Archean cratons, with a high density lower crust of ca. 3145 kg/m3 and a complementary uppermost mantle density of 3320 kg/m3. The bulk crust of the Paterson Orogen and southern Kidson sub-basin is denser than the adjacent crustal domains and may be a result of convergent tectonics and subsequent magmatic additions to the lower crust related to Proterozoic-Cambrian LIPs in the region. This interpretation is supported by the reflectivity patterns at the crust-mantle boundary observed in the reflection seismic section.
Polyanna is a PhD student at the CET UWA. Her project is entitled “Geodynamics and basin evolution of the Paterson Orogen based on 3D modelling and geophysical inversion” that is part of the granted MRIWA M521 project entitled “Lithospheric and crustal-scale controls on multi-stage basin evolution: Impacts on Mineralising Systems”.