Electrical conductivity profiles and implications for the absence or presence of partial melting beneath central and southeast Australia

A previous paper presented measurements by Australian magnetometer arrays of Earth response to the magnetic daily-variation source field, and interpreted these data by simple representative models of one-dimensional structure found by “forward model-fitting”. This paper now supplements the earlier interpretation by seeking to clarify the ranges of acceptable models which fit the data; that is, to demonstrate the extent of the non-uniqueness of the interpretation. Searches for models which fit the data have been carried out on both a systematic and a random basis, with similar results.The major conclusion of the earlier paper is confirmed; that there is a substantial difference in conductivity structure between central and southeast Australia. Beneath central Australia, the structure is consistent with a traditional continental geotherm and published laboratory measurements on the temperature dependence of the electrical conductivity of recognized upper-mantle crystalline olivine materials. Beneath southeast Australia, a higher conductivity by an order of magnitude in the depth range 200–300 km is most directly interpreted in terms of a small degree (perhaps 5%) of basalt melt.Such a partial-melt zone under southeast Australia is consistent with previous natural electromagnetic measurements for the area; has earlier and independently been indicated by a variety of seismic studies; and correlates with proposed thermal models which involve crustal intrusion from some sub-lithospheric magma source.