F. H. Chamalaun

The Bouguer gravity field and crustal structure of eastern Timor

Abstract The results from a recent North—South gravity traverse across eastern Timor show that the Bouguer gravity field is characterized by a strong, 6 mGal/km, gradient on the north coast. This gradient appears to be a fundamental feature of Timor and of the Outer Banda Arc. Preliminary computer models suggest that, to a first approximation, the gradient is due to a vertical fault at the north coast of Timor separating oceanic crust from continental crust. The fit between the computed and observed gradient can be improved significantly by assuming a northward-dipping lithospheric slab, north of Timor. The model further indicates that the Australian continental crust extends at least as far as the north coast of Timor.

Characteristics of the Equatorial Electrojet determined from an array of magnetometers in N-NE Brazil

An array of 29 vector magnetometers was operated in N-NE Brazil from November 1990 until March 1991. We present the analysis of 16 selected quiet days, for which a simple model of an equivalent current distribution for the Sq and EEJ, fits the observed maximum amplitude of the daily variation at midday.In equatorial regions the precise latitude profile of the Sq field is masked by the EEJ. This uncertainty is resolved by assuming that the EEJ, obtained after subtracting the Sq from the daily ranges, should present a ratio of 0.3 for the westward to eastward current. With this constraint, a combined non-linear least squares inversion of Sq and EEJ was used to estimate the parameters of Onwumechili’s model of the EEJ current distribution. The H and Z components of the EEJ are jointly inverted and good agreement obtained between the calculated and observed data for all 16 days.The EEJ’s main parameters averaged for 16 quiet days were: A total positive current intensity equal to 67 ± 20 (103 A) for diurnal range M4 (or 80 ± 20 (103 A) for M3) and a half-width of 403 ± 67 km. The EEJ centre was located at 21 ± 16 km south of the dip equator. The Sq was estimated from several permanent observatories and found to be centred at a mean latitude of 5.5 ± 2 degrees south.

Quiet-day ionospheric currents and their application to upper mantle conductivity in Australia

This study concerns the use of selected geomagnetic field records to establish the 1990 quiet-day current system (Sq) for Australia and to use the ionospheric current source of Sq for a determination of the Earth’s deep electrical conductivity. The primary data set came from a chain of eight, three-component magnetometer stations that was operated along a north-south line in central Australia. Additional records, necessary for boundary conditions, were added to the data set. A regional spherical harmonic analysis (SHA) allowed the separation of the internal and external field contributions to the Sq variations. Mapping of the equivalent ionospheric current from the external field showed that the Sq contour focus passed near the —30° geomagnetic latitude of central Australia with a 5° latitude variation between winter and summer and a corresponding change from about 80 to 200 kA in strength. A special transfer function allowed the computation of an equivalent conductivity-depth profile of central Australia from the paired external and internal coefficients of the SHA. A regression line through the conductivity estimates gives a profile that starts at 0.025 S/m for a depth of 130 km, rising gradually to about 0.045 S/m at 250 km, then steepens to 0.11 S/m at 360 km and rises moderately to 0.13 S/m at 470 km near the base of the upper mantle. No data were obtained through the mantle transition zone. Computations gave 0.18 S/m in the region of 800 km depth. Previous conductivity models for the upper mantle beneath central Australia, although less specific in values, are consistent with our profile. At depths greater than 500 km, the regression profile is in agreement with the conductivity distribution beneath the Tasman Sea determined from seafloor magnetotellurics, although both measurements lack high resolution at such depths.

2-D geoelectrical model for the Parnaiba Basin conductivity anomaly of northeast Brazil and tectonic implications

Abstract A magnetometer array study in the north-northeast of Brazil has revealed a roughly NE–SW-trending conductive structure in the southeastern part of the intracratonic Parnaiba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation. The 2-D numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body. The anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data, the graben structure in the basement is corroborated by the aeromagnetic data. The formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the Brasiliano orogeny or with the Jurassic–Cretaceous magmatic events. The diabase dikes intruded in the basin in association with the Jurassic–Cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting Paleozoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon-saturated sediments.

Geomagnetic deep sounding experiment in the central Flinders Ranges of South Australia

Abstract A closely spaced geomagnetic deep sounding array study was conducted in the central Flinders Ranges of South Australia. The results indicate an anomaly which follows the main fold trend to the east, rather than the north-south trend suggested by earlier studies. Although the coast effect obscures some of the detailed characteristics of the anomaly, the data, particularly at high frequencies, favour an interpretation in terms of a crustal anomaly, probably associated with the lower members of the Adelaide Geosyncline. The Torrens Hinge Zone, which is a major tectonic boundary at the western margin of the Adelaide Geosyncline, has no associated induction anomaly.

Carpentaria Electrical Conductivity Anomaly, Queensland, as a major structure in the Australian Plate

The Carpentaria Conductivity Anomaly of western Queensland is a major element in the electrical conductivity structure of the Australian continent. Investigation of it is significant both for its own sake, and as a case history in the general understanding of continental conductivity structure. Following its earlier discovery by reconnaissance magnetometers arrays, detailed magnetotelluric observations were carried out in 1997 along a transect crossing the anomaly between Cloncurry and Julia Creek. The magnetotelluric results define a good conductor within the crust beneath the sediments of the Eromanga Basin. The conductor extends over a depth range of tens of kilometres. This structure, evidently shown also by aeromagnetic and gravity data, is interpreted as the eastern boundary of the Mt Isa Block at a plate suture, which was later covered by the sediments of the Eromanga Basin. Seismic tomographic results show a major gradient in seismic-wave speed in the region. It appears the potential-field, electromagnetic and seismic methods have detected different characteristics of the same geologic structure, with complementary results. The electromagnetic results, new to this paper, define horizontal position well, and give evidence of highly conducting material from the crust to a depth of tens of kilometres. The seismic results extend the depth of the boundary into the upper mantle. The case history supports the hypothesis that the major conductivity anomalies of the geomagnetic deep-sounding method mark continental sutures of fundamental significance in recording the creation of continents.

On the interpretation of the distinctive pattern of geomagnetic induction observed in northwest India

The geomagnetic variation data from the 1979 Indian array experiment have been reanalyzed and reexamined using the hypothetical event analysis technique. The contour map of the |Z/H ratio replicates distinctive anomaly in northwest India previously delineated in maps of the Fourier coefficients. The anomaly reveals the presence of a significant conductor under the Ganga basin. The contour map has been used to derive a response profile perpendicular to the strike of the anomaly, for comparison with 2-D numerical models. An excellent fit was found for a conductor at a depth of 32 km, with a width of 110 km and a conductivity contrast of 1000. This result places the conductor deep within the lithosphere. In the absence of supporting data the origin of the conductor is difficult to resolve. However, it is thought to be related to pressure-released partial melting, caused by fracturing of the Indian crust during the collision of India with Asia.

Palaeomagnetic reconnaissance result from the maubisse formation, east timorand its tectonic implication

Abstract A preliminary palaeomagnetic investigation of the supposedly allochthonous Permian Maubisse Formation in East Timor yielded a mean direction of magnetization of decl. = 140E, incl. = +45 and α 95 = 13°. This direction is indistinguishable from that of the autochthonous Permian Cribas Formation reported earlier. The apparent agreement between the palaec magnetism of the two Formations is interpreted to imply that both are autochthonous and have been part of the Australian continental margin since the Permianand does not support the supposed Asian origin for the Maubisse Formation.

Comprehensive Mapping of Australia's Geomagnetic Variations

Australian scientists have undertaken a magnetometer array study that will map in detail, for the first time, the natural fluctuations of the geomagnetic field over an entire continent. On November 18, 1989, 57 stations comprising the Australia-Wide Array of Geomagnetic Stations (AWAGS) began recording the geomagnetic variation field across Australia at 1-minute intervals (Figure 1). The full array operated for 8 months, and instruments along a north-south line from Darwin through Alice Springs to Adelaide were kept operating for 12 months. Most of the magnetometers were sited near magnetic repeat stations, where absolute measurements were made of the field to correct for any magnetometer drift.

Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Abstract Magnetovariational (MV) data from 14 sites on a transect of the South Island of New Zealand are presented. The induction arrows clearly show the effects of induction in the surrounding oceans. Comparison with the responses of an analogue model of the effect of the oceans on induction in the New Zealand region indicates that the induction arrows also contain information on anomalous on-land conductivity structure. The use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two- and three-dimensional numerical models pertinent to the New Zealand situation. Difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal, in both real and quadrature arrows, across the South Island. Two-dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the Southern Alps of an anomalously conducting region previously identified by magnetotelluric (MT) sounding. Use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation.