Crustal structure of the Curnamona Province in Australia by ambient noise tomography

Abstract

The Curnamona Province in southern Australia is a 90,000 km² Paleoproterozoic to Mesoproterozoic craton that rifted from the Gawler Craton in the Neoproterozoic and is now separated by the Adelaide Rift Complex sediments in the Flinders Ranges. Three-component passive seismic data were collected over 30 days at 135 stations spaced approximately 25 km with sample rate of 2 ms and a natural frequency of 5 Hz. Cross-correlation functions of ambient noise were calculated between station pairs to determine dispersion curves at a period bandwidth of 3 to 9 s, from which a shear-wave velocity model was constructed to a depth of 20 km. In the top 5 km, low shear-wave velocity < 3400 m/s correlates with low electrical resistivity <100 Ohm.m from a broadband MT inversion of 134 co-located sites, primarily due to fluid-porosity of sediments. In the depth range 5–20 km there are two notable features. Firstly, across the southern part of the Curnamona Province shear-wave velocity increases by >200 m/s from west to east, over ∼200 km, correlated with a trend in Bouguer gravity that increases from −200 μm/s² to +200 μm/s². Secondly, there is weaker correlation in dip and spatial extent between a zone of low resistivity <10 Ohm.m and low shear-wave velocity < 3600 m/s. Higher density crust in the eastern margin of the Curnamona Province may be due to Paleoproterozoic-aged mafic crust that was originally subducted and then upthrust to shallower depths during the Olarian Orogeny from 1620 to 1580 Ma. Reduced shear-wave velocity coincident with low-resistivity in the crust may be related to grain-boundary graphitic Paleoproterozoic sediments that would be ductile with slightly lower shear-modulus and be electrically conductive.