An automated superconducting magnetometer and demagnetizing system

Summary. A novel 3-axis superconducting magnetometer has been constructed using just one SQUID (Superconducting Quantum Interference Device) sensor. The magnetometer has been placed in a large mu-metal shield together with a non-magnetic oven and 3-axis af demagnetizer. Once a sample is entered into the system a microprocessor controller executes a predetermined measuring and demagnetizing/remagnetizing sequence. The system was originally designed automatically to determine field magnitude values from ceramics and igneous rocks but has proved itself to be a useful general purpose palaeomagnetic instrument.     

On the interpretation of frequency response functions for oceanic gravity and bathymetry

Summary. In 1972–78, the late N. D. Watkins and others carried out ajoint field programme of geological mapping in the Mio-Pliocene flood basalts of Iceland, including sampling for K-Ar dating and palaeomagnetic research. The major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile. This paper deals with various statistical properties of this data set.
It is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean-floor polarity time-scale. There is no evidence for significant asymmetries between normal and reverse polarity states of the field, neither as regards chron lengths, secular variation, or virtual dipole moment magnitude. Intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution. The latitude distribution of virtual magnetic poles can be fitted with a Bingham function having k' ∼ 4.5, and low-latitude poles do not occur preferentially in any particular longitude interval.     

The response of palaeomagnetic data to Earth expansion

Summary. Attempts to estimate palaeo-radii of the Earth, using palaeomagnetic data have necessarily been based on simplistic Earth models. It has been asserted that real geological processes are too complex to enable us to approach the problem quantitatively, and such attempts yield invalid results. We examine this and argue that, to the contrary, it appears that errors introduced by allowing for more realistic behaviour of the continents, e.g. 'orange-peel effect' and crustal extension, are smaller by an order of magnitude than the response of palaeomagnetic data to simplified expansion models.
From a qualitative argument, it is shown that the observed Late Palaeozoic and Early Mesozoic palaeomagnetic data are not what should be expected from an expanded Earth. We conclude that it appears unlikely that the Earth has expanded significantly since the Early Mesozoic.     

The magnitude of the palaeomagnetic field: a new non-thermal, non-detrital method using Sundried bricks

Summary. A new method for determining the magnitude of the palaeomagnetic field is described. The material used is adobe Sun-dried mud brick, and some of the magnetic properties of these bricks are described. The magnetization acquired by the mud in the higher coercive force region seems to be caused by shear strain (squeezing) alone. This magnetization is called an SRM. The method is tested on a modern adobe brick from Lima, and it is shown to be accurate and repeatable. The method may also be used to determine values of D and I , if the adobe brick is found in situ . Three results from old adobe bricks — two from Peru and one from Egypt — are presented and are compared with the world-wide average of published ancient VDM's.     

Palaeomagnetic study of Permocarboniferous glacial varves from the Itarare Subgroup, southern Brazil

Summary. A sequence of 167 glacial varves from Itu, State of São Paulo (Brazil), has been sampled for palaeomagnetic analysis. Since the varve thicknesses permit individual measurements, an attempt was made to compare annual palaeomagnetic results with recent observatory data. Inclination and declination plots show larger variations than those displayed by the present geomagnetic field. This could, however, be due to sedimentation factors since it may be assumed that the pattern of the present geomagnetic field variations is not greatly different from that of the Permocarboniferous field. In the next paper (Ernesto & Pacca) the experimental data are submitted to spectral analysis and the results support some of the hypotheses raised in this work.     

Continental drift and true polar wandering

Summary . Evidence in the form of 75 yr of ILS data is accumulating which suggests that true polar wander may be currently taking place. It seems likely that true wander of some magnitude must always accompany plate motions, but the extrapolated ILS rate is an order of magnitude larger than the rate of true polar wander deduced from palaeomagnetic data over the past 55 Myr. The conflict between palaeomagnetic and latitude data provides the motivation for investigating one possible excitation of polar wander, the mass redistribution which accompanies continental drift.
The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 10⁶yr of plate motions is found to be negligibly small; even consideration of episodic plate movements, anelasticity, or a decoupled lithosphere cannot boost the effect to the ILS rate of polar wander. These conclusions are strengthened by the fact that any one of several absolute plate velocity models, based on extremely diverse assumptions, yields the same results.
In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander.     

Structural analysis of dyke emplacement directions as an aid to palaeomagnetic studies: an example from northern Chile

A major problem in palaeomagnetic studies of intrusive rocks lies in determining whether or not such rocks have been subjected to post-emplacement tilting. Structural analysis of dyke emplacement directions can be used to show the current attitude of the extension direction for the dyke swarm. If the original extension direction, at the time of emplacement, can be deduced from geological evidence, this then provides a field test for post-emplacement tilting of the dyke swarm and its host rocks. In the example given from northern Chile, we were able to make a palaeomagnetic study of and structurally analyse three successively younger dyke swarms that intrude similarly younging plutons. All three dyke swarms yielded extension directions close to horizontal but with markedly different azimuths. It is argued that the similarity in the plunge of the extension directions cannot be coincidental and that the dykes and their host plutons have not suffered significant post-emplacement tilting. This simple technique should be widely applicable in the assessment of post-emplacement tilting of dykes in palaeomagnetic studies.     

Palaeomagnetic record in Late Pleistocene and Holocene dry lake deposits at Tlapacoya, Mexico

Summary. A palaeomagnetic investigation of Carbon-14-dated marsh and near-shore lacustrine sediments deposited between about 25000–5000 yr bp at Tlapacoya, Mexico, reveals normal polarity of the geomagnetic field in all samples measured. At one site, anomalous palaeomagnetic directions in a mud unit dated about 14500 yr bp raised the possibility of a geomagnetic excursion, but subsequent work at six additional sites in the unit revealed no abnormal directions. Thus the anomalous directions are most likely not a true reflection of geomagnetic field behaviour, although no specific alternative explanation is entirely convincing. The preliminary Tlapacoya data of anomalous directions have been cited by others as positive evidence for an excursion. We strongly recommend it no longer be considered as such.     

Magnetostratigraphy of Palaeocene basalts from the Vaigat Formation of West Greenland

A palaeomagnetic study comprising the directional results from 289 individual lava flows, sampled along eight sections in the Palaeocene basalts of West Greenland, is reported. The eight individual sections are correlated using lithostratigraphical marker horizons to form a single composite profile. Generally, the lithological correlation is in good agreement with the record of geomagnetic secular variation.
  The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6  km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism. ⁴⁰Ar/³⁹Ar dates support a high extrusion rate and also indicate that the lower normal polarity zone is Chron C27n and that the upper reverse polarity zone is Chron C26r.
  The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200  m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years.     

Mesozoic palaeomagnetism in Vestfjella, Dronning Maud Land, East Antarctica

Summary. Thermal and alternating field demagnetization of oriented samples of Mesozoic lava flows (200–230 Myr) and dykes (154–172Myr) collected from seven nunataks within the Vestfjella region, Dronning Maud Land, revealed stable directions of magnetization of normal and reverse polarity. Directional distributions of both polarities define tight groups along the same palaeomagnetic axis. Depending on whether the regional westward dip of the lava flows originated prior to, or after the intrusive igneous phase, two or one, respectively, palaeomagnetic pole(s) can be estimated. Both poles, however, are in general accord with previous Mesozoic poles from East Antarctica.     

The geomagnetic field over the past 5 million years

The modern geomagnetic field is usually expressed as a spherical harmonic expansion. Although the palaeomagnetic record is very incomplete in both space and time, sufficient data are available from a span of ages to generate time-averaged spherical harmonic field models with many degrees of freedom. Here three data sets are considered: directional measurements from lavas, inclination measurements from ocean sediments, and intensity measurements from lavas. Individual data are analysed, as well as site-averages, using the same methods that have been developed for the modern field, to give models for the past 5 Myr. The normal-polarity field model has an axial-dipole intensity similar to that of the modern-day field, whilst the equatorial-dipole component is very much smaller. The field is not axisymmetric, but shows flux concentrations at the core's surface under Canada and Siberia similar to those observed in the field over historical timescales. Tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples. The reverse-polarity field model does not show such obvious features, but this may be due to the sparsity of the data.
The patterns observed in the normal-polarity field, with persistent features in the northern hemisphere and a smooth southern hemisphere, could be explained if the present pattern of secular variation is typical of the past several million years. This would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation, with relatively little change over quieter regions. However, we have been unable to find any evidence for a geographical pattern of secular variation in the data.     

Palaeomagnetism of some Indian Rocks

i
Hitherto, no palaeomagnetic data for India are available earlier than the Jurassic, 130 M.y. The remanent magnetic directions of oriented samples from one Jurassic and four pre-Cambrian rock formations in India have now been determined. One pre-Cambrian formation gave very scattered direction: the other four formations gave well-defined directions and their magnetic stability was demonstrated by a.c. and thermal demagnetization. The three pre-Cambrian results enable values of the ancient latitude and orientation of India to be estimated at 500, 600 and 750 M.y., assuming the ancient field to have been an axial dipole.
By making use of these new results, together with similar data by other workers for Australia, North America and Europe, it can be shown that, as far as the scanty data goes, the rate of drift in latitudes and orientation of the four continents has been of roughly the same magnitude throughout the period since 750 M.y.     

An investigation of the magnetic variation within two recent lava flows

Samples collected in vertical sections from two recent (1169 AD and 1971 AD) lava flows show significant variations in both the palaeomagnetic vector and the rock magnetic characteristics. The age and the short cooling time of the flows (both are less than 3 m thick) are such that the observed range of palaeomagnetic variation is more than can reasonably be explained by secular variation. The variations in palaeomagnetic and rock magnetic characteristics appear in part to be a response to the relative proportions of two coexisting ferrimagnetic phases, one that displays a primary coarsegrained and Ti-rich composition and a second that has finer-grained, Ti-poor characteristics. The use of AF demagnetization in this study means that the latter phase will have a disproportionate influence on the palaeomagnetic results. It has been suggested elsewhere that in such a magnetic system the Ti-poor phase may represent a CRM formed at some temperature below its final Curie temperature; if so, the results of this study imply that not only does it affect the palaeointensity estimate (which requires a pure TRM) but it appears also to influence the palaeomagnetic direction to an extent that far exceeds the normal secular change that might be expected to occur during delayed acquisition. Such a result has important implications for palaeomagnetic studies that use lavas, both in the selection of a suitable sampling strategy and in the geomagnetic interpretation of palaeomagnetic variation within single flow units.     

Late Mesozoic and Cenozoic palaeomagnetism of Australia–II. Implications for geomagnetism and true polar wander

Summary. 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.     

New palaeomagnetic result from Vendian red sediments in Cisbaikalia and the problem of the relationship of Siberia and Laurentia in the Vendian

A palaeomagnetic study of Vendian red sediments from the Lena River section on the western margin of Lake Baikal in the region of Cisbaikalia (54°N, 108°E) has isolated a stable remanence with direction D = 296.3°, I = −27.7° (high-temperature component) and a corresponding pole of 2.7°S, 168.2°E. The primary nature of this remanence is confirmed from a positive fold test, dual polarities and the presence of detrital haematite. This result, together with all late Precambrian–Early Cambrian palaeomagnetic data from Siberia, indicates that Siberia occupied low latitudes during that time. It has been proposed on the basis of palaeomagnetic data that Laurentia occupied high latitudes during the Vendian, so it would appear that there cannot have been any Laurentia–Siberia connection at that time. A review of Vendian to Cambrian Laurentian palaeomagnetic data shows that such an interpretation is ambiguous. An alternative interpretation places Laurentia in low latitudes and confirms the Laurentia–Siberia fit of Hoffman (1991 ) and Pelechaty (1996 ). However, the lack of Late Vendian palaeomagnetic data for Siberia still allows the possibility that it could have occupied high latitudes during that time.     

Magnetic stratigraphy observed in ferromanganese crust

Summary. A sample of ferromanganese crust deposit (SCHW-1D) was cut into two sets of 1 mm slices for palaeomagnetic study. Alternating field and thermal demagnetization, and isothermal remanent magnetization analyses were performed on each thin slice. The results reveal the presence of a stable magnetism and both normal and reversed polarity intervals in the specimens. The observed polarity intervals cannot be confidently correlated with the geomagnetic polarity time-scale of the last 10 Myr due to the polarity overlap inherent in the sampling technique. But the results confirm the slow accretion rate of ferromanganese deposit determined by ¹⁰Be method and suggest potential use of ferromanganese deposits in palaeomagnetic and tectonic studies.     

Palaeosecular variation in Central Mexico over the last 30 000 years: the record from lavas

13 lava flows of known age (ages from ¹⁴C dating), which have been erupted in the last 30 000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 × 10²² A m². Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.     

On the British Siluro-Devonian palaeomagnetic field problem: reply to comments by T. H. Torsvik et al.

We appreciate this opportunity to clarify our position on a few central issues of this debate. By our rejection of a number of Palaeozoic palaeomagnetic data from Scotland (Storetvedt et al. 1990), Torsvik et al. accuse us of disregarding all classical field tests. We have never disregarded any classical field test, but would argue that such tests have sometimes been misapplied. Also, we obviously have not rejected all earlier British 'Siluro-Devonian'data, including Scottish results (e.g. table 3, Storetvedt et al. 1990).     

Palaeomagnetism of alkaline lava flows from El Salto-Almafuerte, Córdoba Province, Argentina

Summary. Stable components of magnetization have been isolated in 15 lava flows (mean K-Ar age 123 ± 4 Myr) from the alkaline sequence outcropping at El Salto-Almafuerte, Province of Cordoba, Argentina. Magnetic and geologic stratigraphy, as well as K-Ar ages indicate that this sequence was probably extruded in the Lower Cretaceous during the first volcanic cycle of the Sierra de los Cóndores Group (Vulcanitas Cerro Colorado Formation).
The palaeomagnetic pole-position for El Salto-Almafuerte lava flows, computed from the mean of 15 virtual geomagnetic poles and denoted SAK₇, is: 25° E, 72° S ( k = 35, α₉₅= 6.5°); it is fairly close to other Lower Cretaceous palaeomagnetic poles for South America. The elongated distribution of Cretaceous palaeomagnetic poles suggest recurrent drift for South America in early Cretaceous time.
The palaeomagnetic and radiometric data for the igneous rocks from El Salto-Almafuerte support the magnetic reversal time-scale for the early Cretaceous suggested by oceanic magnetic lineations.     

Remanent magnetization of Precambrian and Cretaceous kimberlites in South Africa

Summary. Middle Precambrian and Cretaceous kimberlites were collected from three sites (Premier, Montrose and National) and two sites (Wesselton and Koffyfontein) in South Africa respectively. The natural remanent magnetization of these rocks remains stable to both alternating field and thermal demagnetization. The virtual geomagnetic pole-positions derived from the directions of stable remanence of the Precambrian rocks can be correlated with palaeomagnetic poles obtained from other Middle-Late Precambrian rocks in Africa. The Cretaceous poles for the Wesselton and the Koffyfontein rocks coincide with other Cretaceous poles.