Crustal composition of the Western Australian craton and implications for craton formation tectonics

Abstract:

The Western Australian craton is composed of two Archean blocks, the early-to-mid Archean Pilbara craton and the late Archean Yilgarn craton, and surrounding Proterozoic orogens. Early seismic studies found that these Archean blocks are characterized as a signature type of simple crust and sharp crust-mantle transition, while the orogenic terranes have rather complex crustal structure and much diffused Moho, posing a question whether a possible change in the crustal forming processes occurred between Archean and Proterozoic. Applying receiver functions to available seismic stations, we estimate the bulk crustal thickness and Vp/Vs ratio across the Western Australian craton. The use of the 3D AuSREM velocity models greatly improves the robustness in the bulk crustal property estimates, especially the Vp/Vs ratio that is essential in deriving the crustal composition as well as inferring the tectonic history of individual tectonic units.

Our crustal thickness estimates agree well with early studies. The Pilbara crust is uniformly thin (30 km) with a sharp Moho (20% receiver function relative amplitude), and is inferred as extremely felsic in crustal composition (Vp/Vs ratio 1.70, among the lowest in the craton crust globally). The Yilgarn crust is relatively thick (>35km), and is more toward intermediate (Vp/Vs ratio > 1.73). Lateral variations of the Vp/Vs ratios in Yilgarn correlate well with the sub-terranes, reinforcing the early proposal that individual sub-units had gained their unique seismic originality BEFORE the craton amalgamation. Differences between the Pilbara and Yilgarn crust suggest that the vertical and accretional tectonics may have played different roles in forming the cratons.

Along the Proterozoic orogens, thicker crust and weaker and possibly dipping Moho interfaces are observed, agreeing with that the regions have been repeatedly reworked. A 20-km thick lower crustal layer with >1.86 Vp/Vs ratio is observed in the Musgrave orogen, reflecting a more mafic composition of the layer which is likely the high velocity residue from early magmatic events. As part of the SIEF program, “Distal Footprints of Giant Ore systems”, a 2D seismic array is currently in place in the Capricorn orogen which joined Pilbara and Yilgarn around ~1.8Ga. Results will be presented and discussed at the meeting.