The Eyre Peninsula conductivity anomaly, South Australia

Abstract

A major electrically conducting structure has been spatially located in the southern Eyre Peninsula, South Australia. The structure extends from the continental margin inland along the eastern margin of the Eyre Peninsula, trending north-northeast for approximately 150 km. In order to provide a two-dimensional image of the crust orthogonal to the conductor’s strike, 39 broadband (1000 to 0.01 Hz) magnetotelluric sites were collected with approximately 2 km separation across the peninsula. A smoothed 2-D inversion model demonstrated that the conductor appears centred beneath a topographic high, structurally bound at the east by the transpressional Kalinjala Shear Zone and resistive Donington Suite granitoids, and the Sleaford Complex to the west. The main features from modelling are: (i) east of the Kalinjala Shear Zone, a region of high resistivity (> 1000 ohm/m) relates to the Donington Suite granitoids; (ii) the late Archaean Sleaford Complex (2480–2420 Ma) bordering the Donington Suite granitoids features a lower, wider resistivity range between 5 to < 600 ohm/m, and is near-vertical in the top 12 km; (iii) the lowest resistivity structure of < 0.1 ohm/m occurs at a depth of 5-10 km, and appears to terminate at a depth of ~15 km; (iv) the low resistivity structure correlates with banded iron formations and is credibly the result of biogenically deposited graphite in marine sediments, which migrated to become concentrated in fold hinges during the Kimban Orogeny; and (iv) the conductor is co-located with a ridge of high gravity (+ 200 to 500 mGals). The origin of this high gravity may be due to a mafic intrusive block of oceanic crust, compressed during the continental collision of the Kimban Orogeny. Utilising the constraints of the 2-D model, a regional 3-D forward model was developed which shows agreement with compiled legacy data sets.