AusMT / surveys / Broken Hill 2002

Broken Hill 2002 GDS

Geomagnetic depth sounding survey · New South Wales, South Australia · University of Adelaide

Part of the Australia legacy GDS collection

The Curnamona Province of eastern South Australia and western New South Wales is a sub-circular block of outcropping and shallowly buried Proterozoic basement, divided since Thomson's work of 1976 into the Olary and Broken Hill domains.

About this survey

The Curnamona Province of eastern South Australia and western New South Wales is a sub-circular block of outcropping and shallowly buried Proterozoic basement, divided since Thomson's work of 1976 into the Olary and Broken Hill domains. Where the boundary between them lay was contested. The conventional line was a magnetic low west of Cockburn on the state border, but Crooks had argued in 2001 that it had little geological validity, and Geoscience Australia's seismic reflection line had shown the Mundi Mundi Fault, which separates the Barrier Ranges from the alluvial Mundi Mundi Plain, to be a major crustal structure dipping about 30 degrees east that might itself be the domain contact. Little was known of the resistivity of the crust there beyond a magnetotelluric survey of 1982 along the Menindee road and a second of 1984, which had found a highly resistive region around Broken Hill. In the winter of 2002 Rob Gill, for an honours project at the University of Adelaide under Graham Heinson with funding from the university, PIRSA and the ASEG, recorded a magnetotelluric traverse roughly perpendicular to the fault, from Kalkaroo north-west of Broken Hill to about 40 km south-east of the town, to test whether the Mundi Mundi Fault marks the contact between the two domains.

Seventeen sites were chosen at about 10 km spacing, fifteen along a line of about 130 km from 140.50 degrees east in South Australia across the border past Broken Hill to 141.78 degrees east, and two near Kalkaroo. Four Flinders University high-frequency marine magnetotelluric instruments, converted to land use by Flinders technical staff, were used. Each was a fluxgate magnetometer with two ceramic-pot electrode pairs 120 m apart. They were buried about 10 cm deep and logged at 20 Hz for a day or two, all set to start together so that the sites served as remote references for one another, and a practised crew could turn four sites around in a day. The electric channels failed. The electrodes lost their electrolyte to evaporation or seepage, or their lids to native animals, and leakage in the amplifiers where contact impedance was high left only the magnetic data usable. Gill processed the recordings with Alan Chave's robust remote-reference program RRRMT, with jackknife errors, derived depth-sounding responses and Parkinson induction arrows from the magnetic recordings, and inverted the vertical-field transfer function alone in two dimensions between 10 and 10,000 seconds. The section shows a contrast of about 500 ohm metres between the western and eastern parts of the profile, with the surface trace of the transition just east of station 9, at the Mundi Mundi Fault. The section images the fault as listric and dipping about 25 degrees, close to the 30 degrees read from the seismic reflection line. The Broken Hill Domain east of the fault is highly resistive, about 1,000 ohm metres, to a depth of 15 km, and denser than its surrounds in gravity; Gill proposed that granulite-facies metamorphism and compression in the Delamerian Orogeny had driven the crustal fluids out of it, and concluded that the fault more likely than not marks the domain boundary. At the western end the profile caught the edge of the Flinders Conductivity Anomaly, a north-south conductor with no surface expression in the Olary basement about 50 km west of the Broken Hill Domain, at about 10 km depth, consistent with the 10 to 20 km depth Sudhir Paul had found, and Gill merged his induction arrows with Paul's to refine the conductor's path near Kalkaroo.

Seventeen sites were deployed and fifteen are served. Site B-12 was never tabulated; the two Kalkaroo sites have no arrow data in the holding, and none is known to survive elsewhere.

The served numbers are vertical-field transfer functions for fifteen sites, at 18 to 24 periods from about 11 to 2,048 seconds. They were converted by AusMT from the induction-arrow tables of a 2007 re-processing of the 2002 field recordings, from the University of Adelaide geophysics collection, and the sites match the positions Table 1 prints to the second of arc, apart from the B-06 longitude, which the thesis misprints by twenty minutes; Table 1 is at printed p. 42 of Gill (2002), A Magnetotelluric Profile across the Broken Hill and Olary Domains, honours thesis, University of Adelaide, hdl:2440/113040. The arrow tables carry no uncertainty column, so no uncertainties travel with the served values, although Gill's own 2002 processing computed jackknife errors and the thesis plots them in Figure 9. The coordinates and elevations come from a station table in the same collection; the coordinates agree with the positions Table 1 prints, apart from the B-06 longitude the thesis misprints, and the original recordings are not held.

Data and downloads

Stations (15)

Contributors and organisations

Organisation

University of Adelaide

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0