AusMT / surveys / Adelaide Geosyncline 1983-92

Adelaide Geosyncline 1983-92 GDS

Geomagnetic depth sounding survey · South Australia · Flinders University

Part of the Australia legacy GDS collection

The Flinders Anomaly was the first regional conductivity anomaly found in Australia, by the 1970 array of Ian Gough, M.

About this survey

The Flinders Anomaly was the first regional conductivity anomaly found in Australia, by the 1970 array of Ian Gough, M. W. McElhinny and Frederick (Ted) Lilley, whose stations at Partacoona and Waukaringa carried induction arrows almost opposed to one another, suggesting they straddled a conductor. The earlier work and the southern pilot of 1981 to 1982 led Flinders University to map the anomaly in detail from March 1983. The arrays used a new instrument, the microprocessor-controlled digital fluxgate magnetometer built at Flinders by Francois Chamalaun and Bob Walker, which recorded three components once a minute into memory and ran for three to five months on gel batteries, buried in the ground. Chamalaun ran four arrays in two years: nineteen stations at about 50 km spacing across the central Flinders Ranges in the autumn of 1983, two lower Flinders arrays across the Mount Lofty and Olary ranges, the Eyre Peninsula margin and the Murray Basin in 1983 and 1984, and fourteen stations around Lake Frome over the summer of 1984 to 1985, each array with a reference station well away from the anomaly. The 1983 array showed the anomaly following the fold belt eastward rather than the north-south grain of the ranges, at a depth of less than about 30 km, and Chamalaun took the saline fluids of the evaporitic Callanna beds, the lowest sequence of the geosyncline sediments, as the likely origin of the conductor. Chamalaun (1986) reported the two flanking arrays, tracing the conductor south to where it enters Gulf St Vincent between Wallaroo and Port Victoria and north to the east of Lake Frome, where it turns to a northerly trend, and judged it a narrow crustal conductor shallower than 10 km that cuts across tectonic terrains and follows different units in turn.

Sudhir Paul reanalysed the whole dataset for his doctorate and added three arrays of his own in 1991 and 1992, the most recent sited by GPS. Thirteen magnetometers at 15 km spacing along a 180 km line across the Birdsville Basement High near Moomba tested whether the anomaly linked northward to the south-west Queensland anomaly, and found no anomaly there. A Birdsville array tested a westward continuation, and a north-south Broken Hill array, run without a reference station because all its sites lay close to the anomalous zone, showed that the anomaly did not continue east. Reprocessing all 86 stations with robust regression and the Sompi method over periods from 360 to 9,420 s, Paul found the Flinders Conductivity Anomaly continuous over about 700 km, from the coast of Gulf St Vincent near Edithburgh, across the Gawler Block on Yorke Peninsula, along the Houghton Anticlinal Zone at the centre of the geosyncline, to a termination at the northern edge of the Benagerie Ridge. It does not link to the Queensland anomaly, so the two crossed conductors of 1970 became two separate ones. The anomaly is prominent between 600 and 6,780 s of period, peaks between 1,080 and 1,800 s, and shows most clearly in the real component. Paul modelled the conductor as a funnel-shaped body in the middle and lower crust with a root into the mantle, its stem about 96 to 128 km wide and beginning at about 34 km depth, an order of magnitude more conductive than its host. Above it lies a more conductive overburden less than 20 km thick and most likely 10 to 14 km, whose resistivity changes across the boundary between deep-water and shallow-water sediments. The trend correlates with no earthquake, heat flow, gravity or magnetic signature. Paul proposed two tectonic explanations, a ductile shear zone taking up motion between two intercratonic plates or crust rifted apart and later compressed together, and two conduction mechanisms, free water or fluid in the lower crust or graphite films along grain boundaries. The anomalous currents behave as though channelled, and he expected one end of the channel to be the Southern Ocean. Three-dimensional AusLAMP models have since re-imaged the structure as three separate conductive regions within the Nackara Arc and the Curnamona Province.

Paul's thesis prints single-station transfer functions for all 86 stations at five periods, each value with one standard deviation. Chamalaun (1985) Table 1 prints induction-vector parameters for the central Flinders stations across five to eight frequency bands out to 60,000 s, far longer periods than the five the served values carry. The 1983 central Flinders array fielded twenty magnetometers and a further instrument at the university's underground observatory; nineteen sites carry through into Paul's table and into this record.

The served numbers are single-station vertical-field transfer functions at five periods, 654, 1,097, 1,807, 3,964 and 5,120 s (10.90 to 85.33 minutes in Table 3A.1), taken from the ROBUST panel of that table in Appendix II of Paul (1994), Geomagnetic Induction Study of the Adelaide Geosyncline, PhD thesis, Flinders University of South Australia. The thesis prints one standard deviation on every quantity, and those uncertainties travel with the served values for all 86 stations. Coordinates are the site positions printed in Paul's Table 2.1, pages 48 to 50, which, on AusMT's reading, agree with Liejun Wang's Australian Electromagnetic Database for 83 of the 86 stations; the latitudes printed for Rhynie and Truro are wrong in both the thesis and the register. The original one-minute magnetometer recordings are not held.

Data and downloads

Stations (86)

Contributors and organisations

Organisation

Flinders University

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0