Uncover: MT transect across the western Gawler Craton and Eucla Basin

Abstract

The Eucla Basin in Southern Australia is a Tertiary Basin which covers Proterozoic crust between the Yilgarn Craton in Western Australia and the Gawler Craton in South Australia. However, very little is known of the crustal framework of this major orogenic belt, and as a result, the geological evolution of the area is poorly understood. In 2014, a deep reflection seismic and magnetotelluric (MT) transect was undertaken to provide new constraints on the survey area. The MT profile was 830 km long, with 167 stations 5 km apart. Broadband MT responses were obtained at all sites in the bandwidth of 200 – 0.0005 Hz (0.005-2,000 s) which records data from the top 100 m, up to 100 km in depth. The MT responses showed different characteristics along the line. In the western 500 km of the profile, the responses were approximately 1D and were more sensitive to the presence of thick sedimentary sequences with high porosity. For much of the Eucla Basin, the sedimentary thickness was about 500 m, but in places reached depths of around 2 km. Two dimensional inversion revealed a generally very electrically resistive upper crust of >1,000 Ohm.m, but more conductive lower crust of < 100 Ohm.m. However, the lower crustal conductive regions were not continuous, indicating that there are significant crustal domains with different thermal and fluid evolutions. In the eastern 300 km of the survey, the Eucla basin sediments thin and the profile crosses major shear zones and the western extent of the Gawler Craton. The MT responses here are much more three dimensional and the crust appears to be much more electrically heterogeneous.