Using 3D seismic data and geochemistry to model magma ascent pathways in the Otway Basin.

Abstract

Modern studies have challenged the traditional view point that magma rises through the lithosphere via dykes and high-angle faults. These studies have suggested that horizontal structures such as inter-connected saucer-shaped sills play a large part in magma transport and result in large horizontal offsets between the source and eruption point of the magma. The South Australian Quaternary Basalts (SAQB) are found in the S.E. of South Australia and are the result of the most recent volcanic activity on the Australian continent and have undergone little alteration. As a result, the SAQB represent a superb natural laboratory to be used as a field area to conduct geochemical and 3D seismic modelling in order to discriminate between competing hypotheses for magma transport through the Earth’s crust. Geochemical modelling suggests that the magma that fed the SAQB was generated by decompressional melting of a secondary mantle plume at 42, 33 and 28 kbar for the Northern Group, Mt. Schank and Mt. Gambier respectively. During ascent the magma underwent between 34 and 41% fractional crystallisation, cooled approximately 200°C and resided in the crust for a time period in the order of days to weeks. 3D seismic modelling and volume visualisation of the Balnaves Seismic Survey revealed a saucer-shaped sill with an unusual and previously undescribed morphology, exhibiting a series of vertical radial steps towards its outer rim. This sill is being fed by high-angle normal faults from a feeder dyke that is hosted by the regional Hungerford-Kalangadoo Fault. These results suggest that the melt that fed the SAQB rose through the crust via dykes and high-angle normal faults with less horizontal offset than previously studied inter-connected sill complexes. This indicates that through the coupling of geochemical and seismic data the plumbing system of a volcanic province such as the SAQB can be effectively modelled.