The lithospheric transition between the Delamerian and Lachlan orogens in western Victoria: new insights from 3D magnetotelluric imaging

ABSTRACT

The magnetotelluric (MT) method was used to image the crust and upper mantle beneath the Delamerian and Lachlan orogens in western Victoria, Australia. During the Cambrian time period, this region changed from being the extended passive margin of Proterozoic Australia into an Andean-style convergent margin that progressively began to accrete younger oceanic terranes. Several broadband MT transects, which were collected in stages along coincident deep (full crust imaging) seismic reflection lines, have now been combined to create a continuous 500 km east–west transect over the Delamerian–Lachlan transition region in the Stawell Zone. We present the electrical resistivity structure of the lithosphere using both 3D and 2D inversion methods. Additionally, 1D inversions of long-period AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) MT data on a 55 km regionally spaced grid were used to provide starting constraints for the 3D inversion of the 2D profile. The Delamerian to Lachlan Orogen transition region coincides with the Mortlake Discontinuity, which marks an isotopic discontinuity in Cenozoic basalts, with higher strontium isotope enrichment ratios in the Lachlan Orogen relative to the Delamerian Orogen. Phase tensor ellipses of the MT data reveal a distinct change in electrical resistivity structure near the location of the Mortlake Discontinuity, and results of 3D and 2D inversions along the MT profile image a more conductive lower crust and upper mantle beneath the Lachlan Orogen than the Delamerian Orogen. Increased conductivity is commonly ascribed to mantle enrichment and thus supports the notion that the isotope enrichment of the Cenozoic basalts at least partially reflects an enriched mantle source rather than crustal contamination. Fault slivers of the lower crust from the more conductive Lachlan region expose Cambrian boninites and island arc andesites indicative of subduction, a process that can enrich the mantle isotopically, and also electrically, by introducing carbon (graphite) and water (hydrogen).

KEYWORDS:

• magnetotellurics
• electrical resistivity
• Delamerian Orogen
• Lachlan Orogen
• lithosphere
• Newer Volcanic Province

Acknowledgements

Thank you to the many personnel who collected the five different MT datasets used in this paper. Many thanks to Zara Dennis and Michael Stepan, for making their data available, and to Geoscience Australia and the Geological Survey of Victoria, for making the Victorian AusLAMP dataset available. Comments from two anonymous reviewers significantly improved the manuscript. Figures were produced using GMT (Wessel, Smith, Scharroo, Luis, & Wobbe, 2013), MTpy (Krieger & Peacock, 2014) and 3D grid (kindly provided by Naser Meqbel). Thank you to Alison Kirkby, for providing scripts for the 1D interpolation, and to Phil Skladzien, for providing geophysical maps for the project. The gravity, magnetics and elevation data included in the figures were all from Geoscience Australia's Geophysical Archive Data Delivery System (GADDS). AuScope provided equipment, maintained by Goran Boren. Funding was generously provided by AuScope, the University of Adelaide and the Victorian Geological Survey. This publication is TRaX record number 364.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

NCRIS/AuScope [grant number Earth Imaging].