Palaeomagnetic Determinations Research Articles

Problems of Precambrian paleomagnetism on the territory of Ukraine

Some problems of paleomagnetic research of the Precambrian rocks of Ukraine are considered. The results of paleomagnetic researches of the Proterozoic rocks of the Ukrainian Shield, Ediacaran traps of Volyn and sedimentary rocks of Podolia were analyzed. With the emergence of many new isotopic age data for the Ukrainian Shield rocks we have new opportunities to get reliable palaeomagnetic poles. But our studies of the pilot collections of samples from some key outcrops in the Middle Dnieper, Azov, Dniester-Bug, Ingul and North-Western crustal blocks of the Ukrainian Shield with modern geochronological isotopic ages showed their absence of informativity for palaeo-magnetic determinations. On the Ukrainian Shield the gabbro-anorthosite and granite rapakivi of Korosten and Korsun-Novomyrhorod anorthosite-mangerite-charnockite-granite complexes in the northwestern and central part of the shield are good candidates for palaeomagnetic investigations. New paleomagnetic pole (Φ = 22.7°N, Λ = 164.3°E, dp/dm = 4.9°/9.6°) for ages of 1750 Ma were calculated for six sites within Ingul mega-block. Based on the analysis of paleomagnetic poles for Fennoscandia and the Ukrainian Shield the model of relative position and orientation of these blocks at 1770 and 1750 Ma were calculated. About 1770 Ma Fennoscandia and the Ukrainian Shield occupied low latitudinal positions (15° N and 2° S respectively). At the same time the Ukrainian Shield was rotated anticlockwise about 40° relatively to Fennoscandia. In the next 20 Ma Fennoscandia and the Ukrainian shield drifted southward at about 5° and occupied 10°N and 7°S paleolatitudes respectively, but relative rotation of blocks almost did not occur. The average speed of latitudinal drift of both blocks was near to 2.8 cm/yr. The new results of paleomagnetic studies of Volhyn basalts of Ratnenska suite are presented. We assume that the geometry of the geomagnetic field was not dipole as one of the interpretation of these results. In this context we should study of whole Volyn traps sequence from boreholes and Ediacaran—Cambrian sequences of Podolia in order to gather some data about frequency of geomagnetic reversals.

Palaeosecular variation in Java and Bawean Islands (Indonesia) during the Brunhes chron

SUMMARY A palaeomagnetic study has been conducted from Indonesian lava flows and dykes. Sampling was concentrated to the Merapi and Merbabu volcanoes (central Java), the Bromo‐Tengger volcanic complex, the Lurus volcano (both east of Java), and the Bawean Island. Numerical dating indicates that the rock units sampled have ages that are well distributed within the Brunhes chron. Only a few reverse polarity flows were probably emplaced during the late Matuyama chron. Alternating field (AF) and thermal demagnetization treatments were routinely used for all sites. Results from both methods were generally in good agreement, demonstrating the reliability and the reproducibility of the palaeomagnetic determinations obtained from 36 independent sites. The large dispersion of the directions and the age distribution indicate that a large spectrum of the secular variation has been sampled. Our best estimate of the mean Brunhes field direction in Java is characterized by a zero declination and an inclination anomaly of −4.7 ◦ (with a 95 per cent confidence angle of 5.3 ◦ ). This value agrees with an axial dipolar field model, or a model to which a persisting zonal quadrupolar term of a few per cent of the geocentric axial dipole is added. This study adds new constraints to recent results which suggested that a large zone of negative inclination anomalies may have persisted in the southwestern Pacific ocean over the past 500 Kyr, and possibly extended toward the eastern Indian ocean. The present results do not support such an extension. They also suggest that the inclination anomaly may have varied over 100 Kyr, and that it was more pronounced in recent times.

The Late Cretaceous palaeolatitude of the Neotethyan spreading axis in the eastern Mediterranean region

A compilation of available palaeomagnetic data from the Troodos (Cyprus) and Baër–Bassit (Syria) ophiolitic terranes of the eastern Mediterranean Tethyan orogenic belt is presented. The ophiolites represent fragments of oceanic lithosphere generated at a Neotethyan spreading axis in the Late Cretaceous, although debate continues over the tectonic setting of this spreading axis and its position within the eastern Mediterranean palaeogeography. Two types of model reconstructions have been proposed: Type 1—the ophiolites formed in a southerly Neotethyan basin by spreading above an oceanic subduction zone. The Baër–Bassit ophiolite was then emplaced a relatively short distance (tens of kilometers) southwards on to the Arabian continental margin, leaving the Troodos ophiolite isolated in an intra-oceanic setting to the west; and Type 2—the ophiolites formed in a northerly Neotethyan basin by spreading at a ‘normal’ oceanic ridge, with subsequent large-scale thrusting (hundreds of kilometers) to the south of emplaced ophiolites over microcontinental fragments to reach their present positions. Palaeomagnetic determination of the palaeolatitude of the Neotethyan spreading axis is, therefore, of considerable interest.Previous palaeomagnetic analyses have demonstrated the presence of significant, and in some cases extreme, relative tectonic rotations of a variety of origins in both ophiolites. To allow palaeomagnetic data from these rotated units to be combined, an inclination-only formulation of the palaeomagnetic tilt test is employed. This provides unequivocal evidence that both ophiolites retain pre-deformational remanent magnetizations, which are interpreted as original ocean-floor magnetizations acquired close to the time of crustal formation in the Late Cretaceous. The mean inclinations of 37.0±2.6° for the Troodos terrane and 41.1±3.4° for the Baër–Bassit terrane indicate respective palaeolatitudes for the spreading axes of 20.6°N±1.8° and 23.6°N±2.5°, consistent with a Late Cretaceous position between the Arabian and Eurasian margins. These data, together with a well-defined palaeolatitude of 25.5°N±4.5° for the eastern Pontides previously reported in the literature, provide constraints which must be incorporated in any successful tectonic reconstruction of the eastern Mediterranean Tethys. The implications of these constraints for Type 1 and 2 models are discussed using a series of plate tectonic cross-sections constructed along a line extending northwards from the Arabian continental margin. In the absence of palaeomagnetic data from Late Cretaceous rocks of the eastern Taurides, however, it is presently impossible to use these palaeolatitudinal constraints to resolve the root zone debate on a purely palaeomagnetic basis. Solutions which satisfy the constraints may be found for both types of model reconstruction. Additional, published field-based geological considerations, however, strongly support models in which the Troodos and Baër–Bassit (and other southerly) ophiolites were generated in a southern Neotethyan basin, rather than those involving generation in a northerly basin and subsequent large-scale thrust displacement to the south.

Palaeomagnetic study of Oligocene (24–30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean): directional analysis and magnetostratigraphy

We report palaeomagnetic determinations in order to estimate the palaeosecular variation of the geomagnetic field for the upper Oligocene period from a southern site. We measured the anisotropy of magnetic susceptibility (AMS) of basalts from five sections to check the reliability of applying tectonic correction and concluded that no stratigraphic corrections were needed. A positive reversal test implies that secondary components were sufficiently removed from the characteristic remanent magnetization. Also, the sampling was sufficiently random to accurately reflect upper Oligocene palaeosecular variation. Calculations of the between-flow angular standard deviation ( S B ) from the geographic pole show values between 21.2 and 21.8°. This palaeosecular variation estimate agrees with Model G for palaeosecular variation [J. Geophys. Res. 96 (B3) (1991) 3923], which is axisymmetric and assumes an equatorial symmetry of the geomagnetic field. The virtual geomagnetic poles (VGPs) from the Mont des Ruches section seem to exhibit two polarity changes: reverse–normal (R–N) first, and in more detail a second normal–reverse (N–R) one. Finally, assuming that no polarity chrons are missed by any of the sections, we compare the magnetostratigraphy we defined to the global geomagnetic polarity scale of [Geophys. J. Int. 129 (1) (1997) 176] using previously obtained radiometric dates to correlate the sections [J. Petrol. 39 (4) (1998) 711; Earth Planet. Sci. Lett. 174 (2000) 313; J. Petrol. 43 (7) (2002) 1341]. The precise age constraints furnished by these correlations allow an estimation of eruption rates from 0.30 to at least 1.31 km/my, with wide variations from section to section and an increase of eruption rate for younger sections. However, due to the non-uniqueness of the correlations, the age limits of the sections and consequently the extrusion rate estimations can vary. Moreover, assuming the Mont des Ruches section that has probably recorded all the magnetic polarity intervals, the Mont de la Rabouillère and Mont de la Tourmente intermediate VGP directions may indicate unrecorded polarity chrons. The eruption rates calculated can thus be overestimated.

Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730–1575 Ma) in northern Australia and implications for base‐metal mineralisation

Sequence‐stratigraphic interpretations of outcrop, drillcore, wireline and seismic datasets are integrated with SHRIMP zircon and palaeomagnetic determinations to provide a detailed chrono‐stratigraphic 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 25 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.

Low-latitude glaciation in the Palaeoproterozoic era

One of the most fundamental enigmas of the Earth's palaeoclimate concerns the temporal and spatial distributions of Precambrian glaciations. Through four billion years of Precambrian history, unequivocally glacial deposits have been found only in the Palaeoproterozoic and Neoproterozoic record. Nonetheless, some of these deposits are closely associated with tropical— rather than just polar—palaeolatitudinal indicators such as carbonate rocks, red beds, and evaporites. These observations are quantitatively supported by palaeomagnetic results indicating a ~5° latitude for Neoproterozoic glaciogenic rocks in Australia. Similarly reliable palaeolatitudes for the older, Palaeoproterozoic glaciogenic rocks have not yet been obtained, as such deposits commonly suffer from poor preservation and secondary magnetic overprinting. The Archaean–Palaeoproterozoic 'Transvaal Supergroup' on the Kaapvaal craton in South Africa is, however, exceptionally well preserved, and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes. Within this supergroup the ~2.2 billion-year old Ongeluk lavas are a regionally extensive, largely undeformed and unmetamorphosed, extrusive volcanic succession, which conformably overlies glaciogenic deposits (the Makganyene diamictite). Here we report a palaeomagnetic estimate of 11 ± 5° depositional latitude for the lavas, and hence for the underlying contemporaneous glacial rocks. The palaeoclimate enigma is thus deepened; a largely ice-free Precambrian world was apparently punctuated by two long ice ages, both yielding glacial deposits well within tropical latitudes.

Relative ages of loess and till in two Quaternary palaeosols in Gorges Valley, Mount Kenya, East Africa

Two compound palaeosol profiles formed in loess and till were studied in the oldest early Quaternary end moraine (Gorges age) system on Mount Kenya. Although both tills appear to have similar weathering histories, the overlying loessic sediments, serving as secondary parent materials for the palaeosols, are considered to have different ages on the basis of colour, presence or absence of clay films, and field texture (especially percentage of silt). Palaeomagnetic determinations showed both tills to have reversed remanent magnetism, suggesting that they were emplaced during the Matuyama Chron. The overlying loessic sediments in profiles GOR55 and GOR58 show normal magnetism and grade upward into the lower zone of bioturbation in the A horizon complex. However, in GOR58 the surface loessic sediments have normal remanent magnetism with considerable secular variations, suggesting it was emplaced over a longer time frame during the Brunhes Chron (i.e. < 0.78 Ma). Profile GOR55 contains larger amounts of gibbsite, indicating more aggressive leaching over time, and an absence of secular magnetic variations suggests deposition over a shorter time interval. The study of volcanic feldspars and ash, using scanning electron microscopy, in the two profiles, shows that sand grain (63–250 μm) weathering is slightly different in the two tills; relative weathering effects (e.g. corrosion of grain surfaces and neoformation of clay minerals) are greater in profile GOR55 than in profile GOR58. Overall the data indicate that some sites in the lower tropical Afroalpine timberline environment may have aeolian covers that are considerably younger than the deposits they overlie. © 1997 by John Wiley & Sons, Ltd.

Anomalously shallow palaeomagnetic inclinations and the question of the age of the Canarian Archipelago

SUMMARY Extensive geological, geophysical and geochronological data available from the Canary Islands establish conclusively that formation of the oldest subaerial volcanic structure of the islands began during the Miocene. A mid-Cretaceous age for these volcanic formations has been postulated in previous works on the basis of palaeomagnetic determinations. The results obtained in the present palaeomagnetic study of Lanzarote include analysis of the record of variations of magnetic inclination in boreholes that penetrate the whole of the oldest volcanic series. They show that the excessive age previously assigned to these formations is due to the utilization of volcanic units with abnormally low (<15°) magnetic inclinations (LGIs). In Lanzarote, lavas exhibiting LGIs appear interbedded in a volcanic series that, overall, shows a typical Miocene inclination (∼45°). The units stratigraphically beneath and above the LGI horizons give, in fact, directions consistent with the Middle-Upper Miocene field direction (D= 359°, I= 45°, with k= 29, a95= 6.7° and a palaeopole of 87°N, 178°E), thereby confirming the Miocene age of the oldest subaerial volcanics of Lanzarote. Short excursions of the geomagnetic field seem likely to be the explanation for these LGIs, since other factors such as tectonic tilting, post-eruptive modification of the primary remanence or errors in sample orientation can be disregarded. The detection in the Canary Archipelago of volcanic units with abnormally low magnetic inclinations seems to be related to the relatively continuous record of the geomagnetic field in rapidly growing volcanic edifices, as seems to be indicated by the presence of a few (usually one or two) short polarity events in volcanic suites of several hundred metres thickness.

Palaeomagnetic determination of transgressive and regressive movements during the quaternary in the Black Sea

The piston core with a length of 60·8 m from the “Samotino-east” structure of the Bulgarian Black Sea shelf (about 28·2 E, 42·8 N) is studied palaeomagnetically. 129 levels from the interval 2·5–60·2 m, generally represented by clays and aleurolitic muds, were sampled. The age of the sediments of this core was determined by correlation with another core, situated very close to the one concerned (3 km), whose fauna is known very well. Its age is Eopleistocene (Gurinian deposits), Upper Pleistocene (Neweuxinian deposits) and Holocene.

Sedimentology, weathering, age and geomorphological significance of Tertiary sediments on the far south coast of New South Wales

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.

Neogene palaeontology and geochronology of the Baringo Basin, Kenya

The period from 14 to four million years is poorly known in Africa, but during this time the Ethiopian fauna became established and hominids originated. The sedimentary sequence of the Tugen Hills in the Baringo area of Kenya provides important geological, environmental and plaeontological data concerning this interval. Concordant radiometric and palaeomagnetic determinations within the type section of the Ngorora Formation show that it spans more than 2 m.y., from 13 m.y.a. to less than 10 m.y.a., and from chrons 14 to 9. Other dates refine the calibration of the Younger Mpcsida, Lukeino and Chemeron units. Palaeontological results include the collection from the Ngorora Formation of one of the best Neogene macrofloras in Africa, and more fauna, including hominoids. No equids have been recorded older than 10 m.y.a. We also report new fauna from the more recent units.

Palaeomagnetic determination of emplacement temperature of Vesuvius AD 79 pyroclastic deposits

The city of Herculaneum was hurried under 20 m of pyroclastic deposits during the AD 79 eruption of Vesuvius, whose crater is only 7 km to the east. These deposits have been interpreted as the deposits of mudflows1 or hot pyroclastic flows2–5. Maury's studies6 of incinerated wood in Herculaneum demonstrate heating to at least 400 °C. We have studied the variation of remanent magnetism with temperature for specimens taken from the deposits, including specimens of the matrix material and of embedded lithic fragments. We conclude that the temperature of the deposit at emplacement is unlikely to have been greater than 400 °C, which further supports the interpretation of the pyroclastic deposits at Herculaneum as largely ignimbrites (hot pyroclastic flow deposits).

Further palaeomagnetic confirmation of the magellanes orocline

Palaeomagnetic determinations for Jurassic volcanics and a Cretaceous quartz—dolerite sill at three localities around the arcuate Patagonia'n Cordillera show that a bending of 40° to 27° has occurred. Since the structural bend is 90° it is probable that the Mesozoic arc was originally curved. Cainozoic dykes and sills at Lago San Martin in the northwardtrending belt and Sierra Valdivieso in the eastward-trending belt have the same magnetic directions and were intruded after bending. These results essentially confirm the palaeomagnetic determinations of Dalziel et al., (1973a) and further suggest that the collapse of the marginal sea during the Late Cretaceous orogenesis resulted in greater bending in the outer arc than in the inner arc. The orocline must have formed during the Andean orogeny between 80 m.y. and 21 m.y. ago and was accompanied by folding and thrusting. New K—Ar dates for the Dientes de Navarino Sill (105 m.y.), basalt at Isla Carlos in the Straits of Magellan (21 m.y.) and diorite and andesite sills at Lago San Martin (14 m.y.) are also presented.

A palaeomagnetic determination of the emplacement temperature of some South African kimberlites

Summary. Seventeen wall rock samples close to the contact and seven accidental inclusions have been collected from four South African kimberlite pipes. Magnetic measurements on these samples have been analysed to determine the viscous partial thermoremanence (VPTRM) acquired by them at the time of the emplacement of the kimberlite. Numerical integration of the heat flow equation for a thermal model including convection of the emplaced material has been performed to determine an upper limit for the emplacement temperatures required to cause the observed WTRM. It is concluded that in these pipes the kimberlitic material, at least down to the levels studied, was almost certainly emplaced at a temperature of less than 35OoC. The emplacement temperature cannot have been greatly overestimated and it is suggested that the emplacement temperature was about 30OoC.

Faunal Provinces and Possible Planetary Reconstruction of the Middle Cambrian

After a world-wide correlation of Middle Cambrian biostratigraphic units the epoch is divided into three periods of time that may be approximately equal and perhaps be of 10 m.y. duration. Trilobite occurrences for each of these periods for different parts of the world have been recorded. The smallest faunally homogeneous areas with a sufficiently large list of generic occurrences are compared both at the family and generic levels. This comparison, based on Simpson's coefficient, employs cluster, principal component and discriminative analyses. The groups so produced are interpreted as indicating provinciality. Three provinces-Columban (Americas), Viking (Europe, northwestern Africa, and maritime North America), and Tollchuticook (Asia, Australia, and Antarctica)-are denned as being distinct through the Middle Cambrian. It is stressed that although the data analysis provides fairly concrete evidence for faunal similarity the planetary reconstructions are an attempt to express these results within the context of the most reliable palaeomagnetic data available. As the number and reliability of Cambrian palaeomagnetic determinations increases and as more geotectonic data from critical areas become available these reconstructions may be confirmed or modified.

The Intensity of the Ancient Geomagnetic Field: A Review and Analysis

Summary In this review all palaeomagnetic determinations of the intensity of the ancient geomagnetic field known to the author at 1966 June are brought together and analysed. The lists of data cover determinations from both historic (archaeomagnetic) and geological specimens. The problems encountered in the determination of ancient field intensities from rocks, especially that of rock alteration during the formation of artificial remanence and that of the presence of secondary components of magnetization, are discussed, and methods used to overcome them are described. Experimental techniques used hitherto are briefly summarized. Methods of treating data are reviewed; and suggestions for comparing data from different latitudes are made. The main conclusions drawn from an analysis of both the field intensity data and the problems involved in obtaining this data are: (i) that the problem of rock alteration during the heating necessary to induce an artificial TRM may be overcome by heating selected naturally highly-oxidized specimens in air; (ii) that the effects of secondary components of magnetization may be avoided by utilizing that part of the primary moment which remains after the removal of secondary moments by thermal or a.c. demagnetization; (iii) that the geomagnetic dipole is not constant within any given polarity but fluctuates in strength, possibly with a period of the order of lo4 years; (iv) that because of these fluctuations and non-dipole field variations no conclusions regarding the mean strength of the geomagnetic dipole at any given time may be made if only a few rock samples are measured, and hence that future workers should use sufficient samples to average out these effects; (v) that the limited data from transition zones suggest that during a field reversal the dipole moment reduces to zero but the non-dipole field remains.

Palæomagnetism in Some Norwegian Sparagmites and the Late Pre-Cambrian Ice Age

PRE-PLEISTOCENE glacial deposits have been claimed from many ages and latitudes. Palaeomagnetic studies of the Permian tillites in Australia1 suggest that they were near the geomagnetic, and probably also the geographical pole. The late Pre-Cambrian (‘Eo-Cambrian’, Varangian, Sinian, etc.) tillites are of special interest for several reasons: (1) Widespread occurrences have been claimed2 and many are well established, for example, in Australia, China, Greenland, Norway and Spitsbergen. (2) Their stratigraphical position suggests that all are referable to one major glacial period (with, however, interglacial or interstadial intervals). It is therefore a key horizon in international correlation below the fossiliferous Lower Cambrian. (3) The boulder beds usually suggest deposition from floating ice in an apparently continuous marine succession. (4) Palaeomagnetic determinations of Palaeozoic and late Pre-Cambrian rocks from Britain and America3 give pole positions which suggest that the Norwegian and Arctic tillites were deposited not far from the geomagnetic equator. The successions in which they occur show facies consistent with a near equatorial position.