AusLAMP MT Over Victoria: New Insight from 3D Modelling Highlights Regions of Anomalously Conductive Mantle and Unexpected Linear Trends in the Crust

Abstract

The Australian Lithospheric Architecture Magnetotelluric Program (AusLAMP) is a multi-year collaborative project aimed at resolving the first order electrical structure of the Australian continental lithosphere through the acquisition of long-period magnetotelluric data at ~55×55 km spacing. Here we present the results of the first deployment of AusLAMP which was in Victoria in 2014. Previous MT coverage over Victoria comprised limited 2D profiles in the western portion of the State. Three-dimensional inversion of AusLAMP data provides a context for these isolated profiles while revealing interesting and unexpected results with evident correlations with the mapped geology and well established seismic tomography trends. Along the eastern and southern edge of Victoria, the resistivity structure is resolvable into the asthenosphere, in contrast beneath the central and western part the State the base of the thicker lithosphere is not resolved. Resistivity of the asthenosphere beneath the Victorian eastern highlands conforms with global values (~1,000 Ωm) and becomes more conductive (~200 Ωm) beneath the Newer Volcanic province. The seismologically defined lithospheric mantle beneath the central and western part of the State is relatively resistive (~200 Ωm) compare to the east (~20 Ωm). This anomalously conductive lithospheric mantle we tentatively attribute to metasomatism during Palaeozoic accretion of oceanic terranes. Vertically, this conductive lithospheric mantle merges upwards into a series of northeast trending conductive anomalies within the mid to lower crust. These trends correspond with the surficial distribution of Devonian granite intrusions suggesting they represent fossil metasomatised ascent pathways of the granitic melts, which cross cut the older dominant north-south structural trend. The western limit of these linear conductive trends maps out the boundary of the Delamerian and Lachlan Orogens.