Magnetotelluric evidence for serpentinisation in a Cambrian subduction zone beneath the Delamerian Orogen, southeast Australia

Abstract

The Delamerian Orogen in southeast Australia represents a Proterozoic continental rift margin, overprinted by convergent margin Andean-style subduction in the Cambrian. A detailed 150km east–west magnetotelluric transect was collected across the orogen to investigate the electrical resistivity structure. The magnetotelluric transect follows an existing full crustal reflection seismic transect, of which interpretations support a westward-dipping Cambrian subduction model as derived from field mapping and geochemistry. A 2D inversion of the data from the 68 station broadband magnetotelluric transect imaged a heterogeneous crust with lateral changes as large as 10,000Ωm occurring over ~15km. The crust within the western Glenelg Zone is resistive, in contrast to the eastern Glenelg Zone and the Grampians–Stavely Zone (above the paleo-subduction zone), which host three conductive pathways. The main low resistivity regions (~1–10Ωm) reside at mid-lower crustal depths (~10–30km), extending up to the surface with a higher resistivity (~300Ωm), but still much less than surrounding resistivity (mantle~1000Ωm, crust~10,000Ωm). Fluids released from the upper mantle during the Cambrian west-dipping subduction are interpreted to have moved up crustal faults to create the observed low resistivity pathways by serpentinisation and magnetite creation in mafic–ultramafic rocks. The electrical conductivity of hand samples of serpentinised mafic–ultramafic rocks in the region was found to be much greater than most other rock types present. In addition to adding insight into the crustal structure, the magnetotelluric data also supports geological surface mapping, as the major Lawloit and Yarramyljup Faults that bound different geological domains also mark domains of different electrical structure.