AusMT / surveys / Tamar Anomaly 1983-84

Tamar Anomaly 1983-84 GDS

Geomagnetic depth sounding survey · Tasmania · University of Tasmania

Part of the Australia legacy GDS collection

Nazhar Buyung's study of 1978 to 1979 had found a conductive zone beneath the Tamar River, and Frederick (Ted) Lilley's earlier array across southern Victoria and Bass Strait had already pointed the Devonport and Bridport vectors towards the Tamar, so that later writers credit him with the first sighting.

About this survey

Nazhar Buyung's study of 1978 to 1979 had found a conductive zone beneath the Tamar River, and Frederick (Ted) Lilley's earlier array across southern Victoria and Bass Strait had already pointed the Devonport and Bridport vectors towards the Tamar, so that later writers credit him with the first sighting. From early 1983 to the middle of 1984 Rahmat Hermanto, a Master's candidate under Wilfred Parkinson and R. J. G. Lewis at the University of Tasmania, occupied thirty-two temporary magnetograph sites across the north, north-east and centre of Tasmania to trace that zone's southern extension and characterise it; the thesis abstract counts thirty-five stations, the three extra rows of its station table being, on AusMT's reading, Parkinson's earlier Golconda and Myrtle Bank sites and N. L. Bindoff's North Scottsdale magnetotelluric site. The Tamar Fracture Zone marks the junction between the folded Siluro-Devonian Mathinna beds of the east and the Precambrian and Cambrian rocks of the west, but dolerite and Tertiary cover hide its nature at the surface, and refraction seismics and gravity had shown the crust thickening along it.

Two three-component EDA fluxgate magnetometers, modified in the Geology Department to record digitally onto Memodyne cassette and fitted with temperature sensors, were moved from site to site. Most sites were occupied for about two weeks at a 30 second reading interval, 6 seconds at Ross, and each site was analysed on its own. With only two instruments in the field the work was done by plotting induction arrows rather than by synoptic contour maps. Three traverses were drawn perpendicular to the strike, and two of the new sites, West Frankford and Tayene, lay within a few hundred metres and two kilometres of Buyung's stations. At periods of 240 to 1,200 s the in-phase arrows at neighbouring stations on either side of the zone pointed in opposite directions, as they do where current concentrates in a highly conductive zone. Hermanto found that zone coincident with the Tamar Fracture Zone, striking about 25 degrees west of true north, with the smallest arrows on its axis. At periods of 6,000 s and more the oceans dominate and every arrow points south-south-east; as the period shortens the arrows on the north-east side turn west-south-west and those on the south-west side turn east-north-east, both pointing at the lineament below 1,800 s. Two-dimensional modelling put the conductor about 2 km below the surface, 2 to 3 km thick, at 0.5 ohm m between resistive blocks to east and west, in agreement with the magnetotelluric observations of previous workers. He rejected partial melt and solid conduction for want of any thermal evidence and attributed the zone to saline water in a deep fault zone or extensive fracturing. Parkinson, Hermanto, J. Sayers and Bindoff, with the analogue models of the surrounding oceans made by H. W. Dosso and W. Nienaber, published the result in 1988 as current flowing in a channel about 40 km wide along the Tamar lineament, and the feature became the Tamar Conductivity Anomaly. Hermanto's doctoral magnetotellurics of 1992 put its top at 500 to 700 m and its western limit at the Tiers Fault rather than the river, and Tom Ostersen's AusLAMP model of 2021 images it as a crustal-scale conductor along the boundary between eastern and western Tasmania, offset east of the river, with the same induction-arrow pattern the 1980s array recorded.

Hermanto's thesis printed its induction arrows as ten maps and tabulated no value from any of them. Seven of the ten periods served here, 240 to 1,200 s, fall inside the band the interpretation rests on, where the release before this one carried two.

The served numbers are vertical magnetic field transfer functions at up to ten periods from 240 to 7,680 s at each of the 35 stations (340 of 350 station-periods carry a value), each carrying an uncertainty propagated from the digitiser's own error measured against control plates whose truth is known to the pixel. They are AusMT's digitisation, by code and with no value read off a figure by a person, of the ten induction arrow maps printed as Figures 4.11 to 4.20 of Hermanto (1985), Underground conductivity structure in Tasmania, MSc thesis, University of Tasmania, Chapter 4, and they replace the four periods served before, which came from an earlier hand digitisation of the same plates quantised to 1/180 in radius and half a degree in azimuth and carrying no uncertainty. Eight station periods are withheld, both registers left empty: six on which this reading and the earlier one point more than a right angle apart, so that one of them has read a different line and nobody has yet ruled which, and TRM and VPF at 7,680 s, where the earlier reading carries a documented defect, the first with its real and quadrature parts interchanged and the second an azimuth of 48 degrees where the plate draws about 149. A third defect is a coordinate: BOT's latitude sits about 16.6 km from the position its marker occupies on every plate, and since the earlier files remain the position authority for this survey that coordinate is published unchanged. On AusMT's reading these are the tipper values of the 1983 to 1984 University of Tasmania array; three of the stations carry earlier University of Tasmania work, Wilfred Parkinson's own observations at Golconda and Myrtle Bank and Bindoff's magnetotelluric site at North Scottsdale, as Chapter 1 of the thesis records at page 2. The coordinates are those in the earlier files themselves, checked against Table 1 of Hermanto (1985), whose printed positions are in error at five sites. The original recordings are not held.

Data and downloads

Stations (35)

Contributors and organisations

Organisation

University of Tasmania

Licence

CC BY 4.0

Identifiers and provenance

Machine-readable survey record · catalogue schema mtcat 2.0