Kimberley Craton magnetotelluric (MT) surveys

The Kimberley Craton is one of several crustal blocks that together form the Archean to Proterozoic North Australian Craton. Its age, composition and geological history are largely unknown as it is concealed by younger sedimentary basins. Seventeen long–period magnetotelluric (MT) survey stations were deployed in 2000 across the Halls Creek Orogen. This survey formed part of a research project from The University of South Australia which aimed to understand the electrical structure of the orogen through three-dimensional (3D) forward modelling. More recently, a MT traverse of the Kimberley region aimed to identify major structures in the deep crust and upper mantle. Soundings collected at 155 locations provided two-dimensional (2D) conductivity models of the crust and uppermost lithospheric mantle beneath four regional transects and four local transects.

2D modelling reveals a near-surface conductive layer up to 5 km thick, interpreted as the volcanic and sedimentary rocks of the Kimberley and Speewah Basins, underlain by a resistive upper crust to depths of 15–35 km. Several steeply dipping, less resistive features revealed in the upper crust correlate with the location of mapped faults in the central Kimberley Basin. They also relate to the major structural boundaries within the King Leopold and Halls Creek Orogens. These may reflect structural boundaries between discrete crustal blocks in the basement. Regionally, a conductive lower crust appears to be discontinuous at, or near, major inferred crustal block boundaries.

Along the eastern margin of the Kimberley Basin (see image below), a northwesterly dipping resistive slab extending from the surface to at least 60 km in depth is interpreted as ancient lithospheric material subducted during collisional orogenesis, probably associated with the Halls Creek Orogeny. Three dimensional (3D) modelling of the data is being undertaken to address variable electric strike directions.

Details of the 2011 East Capricorn Orogen MT traverse and its interpretation are available in Report 136 A magnetotelluric survey across the Kimberley Craton, northern Western Australia.
All MT data are now available for download from the Airborne Geophysics Index (MAGIX) layers using the Geological Survey of Western Australia’s (GSWA) interactive map-viewing application GeoVIEW.WA.