Seven new primary poles upgrade the apparent polar wander path for northern Australia for the period 1730–1590 Ma. This brings the total number of primary poles to sixteen, of which nine have well‐defined SHRIMP U–Pb zircon ages. The upgraded path provides a tool for dating and correlation of tectonic events as well as fluid migration episodes for one of the most important periods of base‐metal mineralisation in Australia. Other new primary and overprint poles allow the path to be extended tentatively forward in time to ca 1500 Ma and back to ca 1770 Ma. The 1730–1590 Ma path comprises five quasilinear segments or tracks, each terminated by a sharp bend. The bends can be related to major (supersequence) boundaries in the northern Australian superbasins and to the initiation of sub‐basins that host the base‐metal deposits. Most bends are also associated with magnetic overprints related to episodes of fluid circulation, and three appear to be associated with the expulsion of metal‐rich brines that formed the world‐class Broken Hill, Mt Isa and HYC Pb–Zn deposits. According to the pole path, the movement of the region in the period 1730–1590 Ma can be described as series of clockwise‐counterclockwise rotations about mostly similar Euler poles located southwest of Australia, suggesting that the lithosphere may have been coupled alternately to oppositely convecting cells in the mantle. This mode of movement was terminated at the onset of the Isan Orogeny. The oldest track spans a period of igneous activity from 1740 Ma to 1710 Ma that left a 1000 km‐long trail of volcanics with a decreasing age trend to the northwest. Palaeomagnetic data are consistent with the interpretation that this trail was caused by the movement of the plate over a thermal anomaly in the upper mantle.

A largely convergent setting is proposed for crustal, tectonic and basin evolution of the intracratonic regions of north‐central Australia between 1800 and 1575 Ma. The new geodynamic model contrasts with previous proposals of widespread extension during the Leichhardt, Calvert and Isa intervals. Local transtensional to extensional structures exist, but these are best explained by a combination of flexural, thermal and dynamic processes related to an active southern margin. The development of thick accumulations of sediments (superbasins) is linked geodynamically to interpreted active margin processes (subduction and magmatic arcs) in central Australia. A synthesis of geochemical data from the 1870–1575 Ma igneous units from the Arnhem, McArthur and Mt Isa regions of north‐central Australia confirms the intracratonic setting of these units and suggests that a long‐lived thermal anomaly was responsible for the generation of both mafic and felsic magmas. The geochemical characteristics suggest the igneous units are derived from the lithospheric mantle and are not typical rift‐ or plume‐related melts. A review of the U–Pb SHRIMP ages for the entire region demonstrates the minimum distribution of correlative igneous rocks was widespread. Exotic populations in the 207Pb/206Pb isotopic data provide insights into the nature and evolution of the crust throughout north‐central Australia. Archaean inheritance is found to be nearly ubiquitous. The data support the temporal subdivision of north‐central Australia into the Leichhardt (1800–1750 Ma), Calvert (1750–1690 Ma) and Isa (1690–1575 Ma) intervals which are marked by superbasins and concomitant episodes of igneous activity. A highly heterogeneous pre‐superbasin crust is interpreted from regional, newly processed geophysical data. The cratonic portion of north‐central Australia is interpreted to consist of three broad northwest‐trending belts or elements that are further distinguished into western, central and eastern geophysically distinct provinces. A map of the superbasin distribution is derived and integrated with structural and stratigraphic data to assess the evolution of the basins and the crust through time. The superbasin successions of north‐central Australia are synchronous and widespread, although not necessarily interconnected. The tectonic model incorporates dynamic tilting of the craton during episodes of subduction and transmission of compressive intraplate stresses through the craton during intervening episodes of orogeny. These processes resulted in flexure, strike‐slip deformation and a complex thermal structure. These mechanisms account for the subsidence and basin evolution that results in widespread ramp and strike‐slip basins. The model also accounts for the thermal history recorded by magmatic events. The proposed geodynamical model provides a unifying crustal evolution scenario for central and northern Australia for approximately 225 million years of the Proterozoic.

Sequence-stratigraphic interpretations of outcrop, drillcore. wireline and seismic datasets are integrated with SHRIMP zircon and palaeomagnetic determinations to provide a detailed chronostratigraphic basin framework for the base-metal-rich Palaeoproterozoic rocks of the southern McArthur, Lawn Hill and Mt isa regions. The analysis forms a basis for future correlations across northern Australia. Nine second-order unconformity-bounded supersequences are identified. Supersequences have a duration of 10-20 million years: some hitherto-unrecognised unconformity surfaces record up to 26 million years of missing rock record. The second-order supersequences contain a series of nested third-. fourth- and fifth-order sequences many of which can be correlated across the Mt Isa, Lawn Hill and southern McArthur regions. The analysis relates accommodation history to major intraplate tectonic events evident on the apparent polar wander path for northern Australia. Major tectonic events at approximately 1735 Ma, 1700 Ma, 1670 Ma, 1650 Ma, 1640 Ma, 1615 Ma, 1600 Ma and 1575 Ma impacted on accommodation rates and basin shape in northern Australia. Sub-basin depocentres, the hosts for major sulfide mineralisation, are attributed to reactivated faults that controlled local subsidence. Pb/Pb model ages of 1653 Ma, 1640 Ma and 1575 Ma for the Mt Isa, McArthur River and Century Pb-Zn-Ag deposits, suggest that changes to intraplate stresses at tectonic events of like age resulted in the migration of metal-bearing fluids into the sub-basins. A Pb/Pb model age of 1675 for the Broken Hill deposit suggests that intraplate stresses manifest in northern Australia also affected rocks of similar age further south. Magmatic events close to 1700 Ma (Weberra Granite) and 1675 Ma (Sybella Granite) coincide with times of regional incision and the formation of supersequence-bounding unconformity surfaces.

We attempt to correlate the Browns Creek Clay type section--one of the most important Palaeogene marine reference sections in southern Australia--with the geomagnetic polarity time scale. The correlation depends on the choice of bioevent ties between the Browns Creek Clay and other sections with well-defined polarity reversals at similar mid to high southern latitudes. Of the various possible ties, we discuss two that best fit the available evidence, nominating our preference. The first correlation is a one-to-one match of reversals between the Browns Creek Clay record and the Geomagnetic Polarity Time Scale interval Chron 18n.1 to Chron 16n.2. This is achieved via the Weddell Sea record (ODP Leg 133 Site 689), using as the baseline tie the disappearance event of the foraminiferid Acarinina collactea. Inherent in this correlation are misfits with other more reliable bioevents and the utilisation of two short normal polarity events in the Browns Creek record, each based on a single stratigraphic sampling level. The second and preferred correlation is via the South Atlantic record (DSDP Leg 73 sites) and uses two reliable calcareous nannofossil bioevents: the appearance of Isthmolithus recurvus and the disappearance of Cyclicargolithus teticulatus. It suggests a match between the Browns Creek Clay polarity reversal record and a Geomagnetic Polarity Time Scale interval that starts in Chron 16n.2 and ends in Chron 15n. Although in this correlation the two brief normal polarity events have no counterparts in the Geomagnetic Polarity Time Scale as it is currently recognised, it is preferred because of stronger biostratigraphic links. The correlation suggests that the lower part of the Browns Creek Clay (the Turritella clays and Notostrea greensand) was deposited in similar to 1.3 million years, about half the time suggested by the first correlation, and dates at ca 35.0 Ma the Hantkenina alabamensis primitiva interval which is an important biostratigraphic marker widespread in southern Australia.

Approximately 1800 samples from volcanic, elastic and carbonate sequences of the southeastern McArthur Basin in northern Australia were analysed palaeomagnetically to define in detail the Australian apparent polar wander path (APWP) for a period centred at similar to 1670 Ma and to develop a Proterozoic reversal stratigraphy for correlation and dating. Twelve palaeomagnetic poles are interpreted as primary, including two that are preliminary. These increase the Australian palaeomagnetic database for the Proterozoic by similar to 30%. Another ten pales record three, or possibly four, periods of overprinting. The study also yields one of the oldest reversal records reported so far.The ten better-established primary poles were obtained from the lower three of the four main stratigraphic subdivisions of the Basin: the Tawallah, McArthur and Nathan Groups. These poles define an angular trajectory of about 120 degrees for a time span of at least 75 m.y., and possibly exceeding 100 m.y. They confirm earlier preliminary results that suggested a major revision of the Proterozoic APWP for Australia. The two preliminary poles were obtained from the youngest sub-division of the Basin, the Roper Group, and confirm a large age gap between the Nathan and Raper Groups.The poles for the oldest overprints were from igneous units in the Tawallah Group and appear to record metasomatism at the end of the last major period of volcanism in the basin. The pole for the next overprint falls on the path near the apex of a hairpin bend; it coincides with a likely period of rifting and high heat flow that accompanied the deposition of the upper McArthur Group. The third overprint, apparently acquired during the long time-break separating the Nathan and Roper Groups, was found mostly in red dolomitic rocks.The geomagnetic reversal record from this study represents an accumulated stratigraphic thickness of 2100 m, excluding gaps where the polarity remained undetermined because of overprinting, unstable remanences, intervals not sampled, or other causes. Reversal patterns from duplicate sections at different localities were compared for two formations, and were found to be consistent. Several parts of the reversal column appear to be distinctive enough for use in stratigraphic correlation.

Comparison of a new ∼1700–1600 Ma segment of the Proterozoic apparent polar wander path (APWP) for Australia with the time-equivalent segment of the North American APWP that has been rotated clockwise by 117° about the Euler pole, long 100°E, lat 38°N, superposes the North American APWP onto the Australian APWP and shows the segments to be similar. The same rotation makes the Pacific margins of the North American and Australian cratons adjacent, as predicted by the Southwest U.S.–East Antarctic (SWEAT) hypothesis, but North America is located farther north relative to Australia than originally suggested. However, the reconstruction is consistent with the identification of western sediment sources for the Belt-Purcell basin in western North America, with matching of basement provinces, and with correlation of major lineaments of the two continents. The rotation gives only very broad agreement between the younger Proterozoic pole sets. This is probably partly due to the sparseness of poles on the Australian APWP whereby prominent features that are found on the North American APWP, such as the Grenville loop, are eliminated on the Australian path by smoothing. Nevertheless, some discrepancies between the younger Proterozoic poles cannot be accounted for without revision of the APWPs.

A new Late Eocene (approximately 36.5 Ma) South palaeomagnetic pole based, on 33 sampling levels in the Browns Creek Formation of the Otway Basin, south-eastern Australia, lies at 65.5-degrees-S, 112.5-degrees-E with A95 = 2.5-degrees. This pole fills a 30 Myr gap in the Palaeogene part of the Australian apparent polar wander pole path, and appears well suited for calibrating the path because of a well-defined age and evidence that the sequence was magnetized during a depositional period long enough to thoroughly average out secular variations. The remanence seems to have been acquired during post-depositional re-working of the sediment. The pole agrees with one biostratigraphically dated and three undated poles that constrain the Palaeogene trajectory.All Palaeogene poles on the path lie west of an alternative apparent polar wander path. The latter is modelled by low-order curve fitting of much more widely scattered poles from basalts in which the scatter appears to be due to incomplete time-averaging of remanence directions. The new pole also reinforces a previously observed systematic westward displacement, by a few degrees, of the early Tertiary palaeomagnetic poles relative to hotspot-derived poles. This displacement may be caused by true polar wander and/or departures from the geocentric axial dipole model. It again highlights the approximateness of the available palaeogeographic reference systems.

A palaeomagnetic study was carried out on Proterozoic dolerite dykes, plugs and some of their host rocks in the intrusion-rich Mount Isa Inlier for which only one other similar study has been reported (Duff and Embleton, 1976). After rejection of intrusions with unstable remanences and internally inconsistent directions, the results from eleven dykes and five plugs remain. These yield two main palaeomagnetic directions: a direction with both polarities and moderate northerly upward (southerly downward) inclination, obtained from metamorphosed dykes and plugs; and a near-vertical downward direction, obtained from unmetamorphosed dykes. The first appears to be associated with a period of regional metamorphism in the Mount Isa Inlier, estimated at ∼ 1620-1500 Ma, and having the last major metamorphic peak near 1554 Ma. A moderate directional dispersion suggests that this magnetization post-dates periods of major deformation of the region, estimated to have ended at around 1550 Ma. Thus the likely age of the magnetization is 1550-1500 Ma or younger, depending on the rate of cooling. Its pole (IM) plots at 110.6°E, 79.0°S (A95=8.4°). The second direction is concordant with the IA group of Duff and Embleton (1976), which includes results from the unmetamorphosed Lakeview Dolerite dyke dated at 1116 Ma. The combined results from previous and present studies on the unmetamorphosed dykes give a pole (IAR) at 311.1°E, 9.5°N (A95=17.4°). Poles IAR and IM suggest very high and middle palaeolatitudes at around 1100 and 1500 Ma, respectively. The new results agree with the most recent version of the Australian pole path by Idnurm and Giddings (1988), and do not challenge the concept of a single path for the continent.

Click to increase image sizeClick to decrease image sizeMcArthur Basinmagnetic overprintspole pathreversal record

Palaeomagnetic measurements indicate that the uraniferous Radium Ridge Breccias near Mt Painter in the northern Flinders Ranges of South Australia have been magnetized twice, both times in the Permo-Carboniferous. The palaeomagnetic south pole for one remanence was estimated as 165.1-degrees-E, 65.7-degrees-S (A95 = 11.5-degrees). The direction for the other remanence was not well defined; however samples collected from diamictite bodies within the breccia gave 133.9-degrees-E, 33.1-degrees-S (A95 = 6.9-degrees) as the probable pole for the overprint. The latter is similar to overprint poles published from central Australia, which are generally attributed to the Alice Springs Orogeny.Magnetization directions interpreted as Permo-Carboniferous were obtained also from the Mt Gee Sinter (a quartz-hematite-rich chemical sedimentary rock which overlies the Radium Ridge Breccias) and from U-mineralized hematitic ironstone bodies within the breccia. The magnetizations of both units were probably coeval with the younger magnetization of the Radium Ridge Breccias. A positive fold test demonstrates that the remanence of the sinter is primary, indicating a major hydrothermal event in the Permo-Carboniferous.It is not clear by how much this event post-dates the deposition of the underlying Radium Ridge Breccias and hematitic ironstone. The latter may be Ordovician or older if earlier monazite U-Pb data are correct, and may have been formed by granite-related hydrothermal fluids. In that case. the older remanence in the breccia is also an overprint. Epithermal sinter formation and chemical (rather than purely thermal) resetting of the remanence in the underlying breccia were probably due to deep circulation of oxidized fluids during Permo-Carboniferous tectonic activity. Chemical reaction of these fluids with pre-existing magnetite-bearing ironstone may have been responsible for uranium mineralization during the Permo-Carboniferous. This interpretation is consistent with published textural, isotopic and fluid inclusion data, but an older age for the uranium concentration, as a primary part of the ironstone formation, cannot be excluded. Alternatively, if the monazite data are discarded, the entire hydrothermal process including iron, uranium and silica deposition could have occurred in the Permo-Carboniferous.

The highlights of a palaeomagnetic study of the Palaeoproterozoic Kombolgie Formation, western McArthur Basin, are reported. We show that the Kombolgie Formation preserves a remarkable record of seven distinct periods of magnetic overprinting, and that magnetic overprinting can be used as a sensitive tracer of fluid passage. The overprint record also increases our understanding of the geological evolution of the Basin.

"Palaeomagnetism of Ferricrete From Vale of Belvoir, Western Tasmania: Implications For Tasmanian Cenozoic Glacial History." Exploration Geophysics, 24(2), pp. 301–302Glacial depositsdatingCenozoicTasmania

"Uraniferous quartz — hematite breccias at Mt Painter (South Australia): Palaeomagnetic dating of hydrothermal activity." Exploration Geophysics, 24(2), pp. 275–276Uraniumhydrothermal brecciasMount Painterpalaeomagnetic dating

The age of the NSW coastal lowland from Tuross to the Victorian border can now be shown to be at least mid-Tertiary. By this time the coastal plain had twice been partially blanketed by terrestrial sediments. Palaeomagnetic determinations on the more recent of these sedimentary accumulations, the Long Beach Formation, reveal a minimum depositional age of Early Miocene. Eustatic influences may be responsible for the aggradation of the Long Beach Formation and as a consequence the diversion of the lower Bega River from its former Tertiary valley. Marked differences in the pattern of weathering between the older sediments, the Quondolo Formation, and the inset Long Beach Formation are evident, with the former largely silicified and the latter ferruginized. Similarities to the Quondolo and Long Beach Formations in terms of their stratigraphic relationships, chronology and weathering styles are evident within other Tertiary sedimentary formations north along the coastal plain to Ulladulla.

With the recent publication of Musgrave’s paper (Musgrave 1989), two Late Mesozoic-Cainozoic pole paths that differ substantially in age calibration have become available for Australia. The earlier path (path 1) is age calibrated almost entirely by remanence directions of sedimentary sequences (Idnurm 1985); the new path (path 2) is calibrated by the remanence directions of volcanics. Path 2 has the special merit that again it puts to use data which fell out of favour in the late 1970s and early 1980s because of serious discrepancies with the Indian data base (e.g., Embleton 1981; Embleton & McElhinny 1982). The two approaches to Australia’s Late MesozoicCainozoic APWP differ fundamentally in the treatment of scatter in pole data. The approach adopted for path 1 attempts to eliminate abnormally large scatters at the source-believed to be the incomplete averaging out of geomagnetic secular variations (for discussion, see Idnurm 1985tby utilizing the time-averaging characteristics of sedimentary sequences. The approach for path 2 relies heavily on statistics to eliminate the scatter. The differences between the paths are of concern especially when they are used for age dating of weathered profiles (the age estimates may differ by up to about 15Ma in the Tertiary, corresponding to an angular difference of So), and Musgrave has attempted to test the paths by comparing their respective age estimates on such units with independent geological constraints. The results are in favour of path 2, but he appears to have made some errors in these comparisons-errors that invalidate the finding. More seriously, the method devised for age calibration of path 2 seems unsound. The first error occurs when the palaeomagnetic age estimates for a New Caledonian laterite are compared with stratigraphic evidence that its age is Early Miocene or older. Musgrave finds that this stratigraphic age limit is consistent with path 2, but not consistent with path 1, and concludes that the test has provided ‘a clear discrimination in favour of the new APWP’. This is not correct. Fig. 1 shows path 1 with the New Caledonian pole AA. Since the 95 per cent confidence circle for AA encloses the 26 Ma calibration pole, path 1 is consistent with an Early Miocene age for the laterite. (As a separate issue, all three New Caledonian laterite poles in Musgrave’s paper have values of A,, that are excessively high for such material-see, for example, his table 1-suggesting unresolved complexities in their remanence.)

The Precambrian apparent polar wander path for Australia is revised in the light of new results. As previously, a single path can be drawn through the poles even though these have been derived from different cratonic units. The poles and the path are critically reviewed in an historic context.

Palaeomagnetic results from Part I of this study and their analysis in Part II are combined to eliminate bias from the Cenozoic apparent polar wander path for Australia — a bias due to non-dipole components in past geomagnetic fields or, for poles calculated from hot-spot data, due to the motion of hot spots relative to the Earth's rotational axis. This path is extended in approximately bias-free form to the late Mesozoic, and indicates a significant change in the drift direction of the continent between 26 and about 60 Ma. The bias-corrected Australian path is used, first, with seafloor spreading data for the Southern Ocean to derive a corresponding late Mesozoic—Cenozoic pole path for Antarctica. The latter shows that the Antarctic drift direction reversed in the early Tertiary. It is suggested that the early Tertiary directional changes of both Australia and Antarctica are part of a global reorganization of plates during the Eocene, postulated by Rona & Richardson, Cande & Mutter and Patriat & Achache. Next, the Australian path is compared with hot-spot data from the African and Australian plates, indicating a movement of the hot spots relative the Earth's rotational axis during the Cenozoic. The direction of this movement is found to be consistent with previous results from other parts of the world. Finally, the Australian path is used together with non-dipole components in the geomagnetic field to explain a prominent westward displacement of the mid- and late Cenozoic poles of India relative to those of Australia. Because of uncertainties in the original poles and in the analysis, the present results are likely to contain appreciable errors. Nevertheless, their consistency with independent findings supports the dipole-quadrupole model of Part II for mid- and late Cenozoic geomagnetic fields.

New palaeomagnetic results from Australia indicate that throughout the Cenozoic era the continent lay further south than suggested by hot-spot data. Moreover, while hot spots give a uniform rate of drift during most of the Cenozoic, the drift rate obtained from apparent polar wander varies considerably. The discrepancies between the palaeomagnetic and hot-spot results are analysed by comparing the Australian data with those of Europe and the central Pacific. The analysis suggests that the discrepancies are due to: (1) departures of the Earth's magnetic field from the geocentric axial dipole model, and (2), either true polar wander or a non-axial inclined dipole component. It is found that since the mid-Tertiary the dominant non-dipole component has been a quadrupole, and that during this period both the quadrupolar field and the true polar displacement/non-axial dipole component decreased progressively. During the Quaternary, and also at the earliest Tertiary, the non-dipole components appear to have been moderate or small. The comparison of data sets demonstrates that considerable errors may be incurred when Cenozoic, and presumably earlier, poles from one geographic region are used to derive those of another, widely separated, region. The results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors, and that hot-spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes. Finally, the new early Tertiary pole for Australia is used in conjunction with updated early Tertiary poles from other lithospheric plates to reapply the McKenzie test for true polar wander. The results indicate a small true polar displacement since the beginning of the Tertiary. The amount and direction of the displacement, however, differ from those generally obtained from hot-spot data.