Palaeomagnetic Record Research Articles

Thermo-tectonic history of the Tethyan Himalayas deduced from the palaeomagnetic record of metacarbonates from Shiar Khola (Central Nepal)

Palaeomagnetic measurements were carried out on low-grade metamorphic carbonates, of Mesozoic age from the Shiar area (85.1°E, 28.6°N) of the Tethyan Himalaya (TH) in north central Nepal. Two characteristic remanence components carried by pyrrhotite (ChRM1) and magnetite (ChRM2) could be identified by their unblocking temperature spectra of 270–340 and 430–580°C, respectively. Fold tests are not significant, due to the uniform bedding of all sites. However, according to results from other areas of the TH, the pyrrhotite component has been probably acquired as a secondary (p)TRM during exhumation and cooling; thus the age of remanence acquisition can be related to the last cooling event (25–17Ma in the surrounding areas). The inclination of the magnetite component matches the value expected from the Indian APWP. This may the primary origin of the ChRM2.Pyrrhotite site-mean directions show a small-circle distribution, with a best fit parallel to the N–S direction. Backtilting to the expected inclination (Iexp) by intersection of the remanence small-circle with the small-circle of constant Iexp yields a clockwise block rotation of 30–35° with respect to the Indian Plate. Characteristics of the pyrrhotite component (small-circle distribution of site-means, secondary origin, (p)TRM with unblocking temperatures below about 300°C), allow the interpretation of the chronologic order of the thermo-tectonic history: (i) an earlier main folding phase at elevated temperatures; (ii) a later event of cooling through about 300°C coinciding with the acquisition of ChRM1; (iii) clockwise block rotations with respect to the Indian Plate and (iv) long-wave folding as the youngest tectonic event.

An Upper Pliocene lacustrine environmental record from south-Western Australia — preliminary results

A crater created by a supposed meteor impact and located about 30.5°S latitude in Western Australia contains a well-preserved lake sequence of the Upper Pliocene age. This paper reports on the palaeomagnetic record, sediment patterns and selected chemical analyses, and alludes to some of the preliminary results from pollen, charcoal and diatom analyses. The chemical analyses indicate that the lake varied in water depth and that salinity and diatom production were related to climate change rather than changes in major nutrients. The palynological analysis, for the upper 20 m of sediment, shows a woodland and heath similar to today, interspersed with some chenopod shrub land phases, and minor elements include Nothofagus, Araucaria and Phyllocladus which are known from other Tertiary records from southern Western Australia. Charcoal is abundant in the record indicating that fire was a major environmental feature, long before Aboriginal people inhabited the continent. The palaeomagnetic record shows a good match with the Gaussian chron and includes the Kaena and Mammoth subchrons; and these were used as a basis for developing an age-depth model for the sequence. The sediment was subdivided into an upper unit of four lithozones and a lower unit. The upper unit, that dates from ca. 2.5 to 3.51 Ma was interpreted to have formed under a generally wetter climate than today, and with some relatively short intervals of semi-arid climate identified around 2.56 and 2.59 Ma. They were interpreted from episodes of chenopod shrub land that were associated with assemblages of saline habitat diatoms and carbonate and gypsum in the sediments. The sediment profile also included sections of laminated sediments which indicate that around 2.82, 2.86 and 2.96 Ma the climate was more variable and possibly seasonal compared to the rest of the record. These data indicate that the last million years of the Pliocene for south-western Australia endured several episodes of climate variability, thus lending support to the idea that the Pliocene was not a period of a uniform cooling trend gradually leading into the Quaternary glacial episodes.

Neotectonics at the Arabian plate margins

Opening of the Red Sea is accompanied by convergence between the Arabian plate and Eurasia. Regional topography and structure favour gravity glide as the main driving force of plate translation. At the leading edge of the plate, the Zagros Mountains undergo coseismic serial folding which is equivalent to Holocene shortening by ∼20 mm/year and which has led to major episodes of coastal uplift of which the last was ∼1700 years BP. At the Jordan Rift transform, which bounds the Arabian plate on the west, a recurrence interval of ∼1600 years is reported for events of M L≥5.5. The palaeomagnetic record for the last 3.2 Ma indicates an average spreading rate for the Red Sea of ∼20 mm/year; there is some evidence that hydrothermal activity in the Red Sea is pulsatory, with a period of ∼2000 year, and that it reflects discontinuous spreading. The Holocene neotectonic records of the Zagros, the Jordan Rift and the Red Sea are the product of complex plate interactions and of the accumulation and release of strain in the crust along the plate margins. But they also reflect elastic strain energy storage and release within the Arabian plate, whence parallels in the period of major deformation episodes in the three deforming zones and the apparent discrepancy between the seismic moment predicted by plate kinematics and that recorded in the Zagros. Any associated intraplate deformation, if detected geodetically, would thus help the assessment of seismic hazard.

Palaeomagnetism, rock magnetism and magnetic fabrics in Precambrian metamorphic terranes of the Huabei Shield, China

This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a 95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a 95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a 95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.

Studies of post-depositional remanent magnetization and their relevance to the palaeomagnetic record

Abstract The results of experiments on the watering of different types of continental Quaternary sediments in the anomalous magnetic field show that it can cause strong changes in the direction of remanence. The direction of acquired post-depositional remanent magnetization is stable to thermal and alternating field (AF) demagnetization. These changes are not related to the chemical remagnetization and are probably caused by mechanical rotations of magnetic particles in the pore spaces. Post-depositional remagnetization, both in nature and after collection of sediment samples, can distort the palaeomagnetic record of geomagnetic excursions, events and polarity transitions, as well as altering the finestructure features, such as the palaeomagnetic record of secular variations.

Inconsistent palaeomagnetic recording of the Blake event in Chinese loess related to sedimentary environment

Three loess sequences located east of the Liupan mountain region, China, were studied to obtain records of the Blake polarity event. Palaeomagnetic records from the appropriate stratigraphic interval of these three sequences, however, record the Blake event poorly in one case and not at all in the other two cases. It is important to understand why these loess sequences miss the Blake event, whereas others record it quite well. We conclude that regional variations in the sedimentary environment over the Chinese loess plateau are responsible for the inconsistent recording of the Blake event. The palaeomagnetic records suggest a threefold subdivision of the Chinese loess plateau into (1) the central and southern portions of the part of the plateau that lies east of the Liupan Mountains, where the primary record of the Blake event has been severely smoothed or wiped out by pedogenic processes during palaeosol (S1) formation; (2) the northern margin of the loess plateau to the east of the Liupan Mountains, where frequent, severe dust storms have resulted in discontinuous records with a high probability of missing the Blake event; and (3) the region west of the Liupan Mountains, where loess sequences are deposited rapidly and nearly continuously with only minor pedogenesis, and are thus capable of recording short events like the Blake.

A Late Weichselian geomagnetic record from Lake Tamula, SE Estoniah

A Late Weichselian palaeomagnetic record has been obtained from sediments deposited in Lake Tamula in SE Estonia, and detailed mineral magnetic studies have been undertaken to identify the carriers of remanence. The palaeomagnetic record can be compared to records from southern Sweden and Karelia in NW Russia. A westerly declination swing in the Swedish and Russian records, between 13,300 and 12,100 calender years BP is also found in the Lake Tamula sediments. Based on a floating varve chronology in the varved clay unit the deglaciation can be dated to 14,400 calender years or 12,600 radiocarbon years BP. The palaeomagnetic evidence supports the presently accepted deglaciation chronology of southeastern Estonia, which is based on radiocarbon dates of the Haanja and Otepää ice‐marginal stages. Sandgren, P., Hang, T. & Snowball, I.F., 1997: A Late Weichselian geomagnetic record from Lake Tamula, SE Estonia. GFF, Vol. 119 (Pt. 4, December), pp. 279–284. Stockholm. ISSN 1103–5897.

Palaeomagnetic study of Neoproterozoic glacial rocks of the Yangzi Block: palaeolatitude and configuration of South China in the late Proterozoic Supercontinent

Regional glaciation is recorded at two stratigraphic levels in the Neoproterozoic Sinian succession of the Yangzi (South China) Block. A palaeomagnetic study is reported here of the upper sequence (∼ 730-670 Ma) which contains red beds. An ‘A1’ component ( D I = 291 −57° , α 95 = 5.0°, Palaeopole 151.2°E, 0.2°N) is resolved at high unblocking temperatures and resides in specular haematite. A study of contemporaneous deformation structures suggests that this component was acquired at an early stage of lithification and yields a representative palaeolatitude (37°) for the glaciation. Lower blocking temperature magnetizations referred to as ‘A2’ ( D I = 297 −19° , α 95 = 7.6°, Palaeopole 171.8°E, 19.2°S) and ‘A3’ ( D I = 319 −1° , α 95 = 3.2°, Palaeopole 169.1°E, 41.8°S) are later overprints correlating with probable Late Precambrian or Cambrian palaeofields. In other regions, the ‘A’ magnetizations were found to be partially or completely overprinted by a Recent field and/or a ‘B’ remanence ( D I = 4 13° ); the latter correlates with a remagnetization widely recorded elsewhere in South China and is attributed to the Mesozoic Indosinian Orogeny. An ∼90° apparent polar wander (APW) swathe is identified from the Yangzi Block between ∼730-670 Ma and 540 Ma. It is not yet clear whether poles from older Sinian rocks record earlier magnetizations or were overprinted during this APW motion. Comparison of the Sinian-Cambrian palaeomagnetic record from South China with the contemporaneous record from Australia implies that the former block was located immediately to the north-west of Australia during these times. This reconstruction is compatible with Early Palaeozoic biogeographic data. Continuity of the ∼900 Ma Cathaysian-Yangzi suture with the ∼ 1000 Ma Albany-Fraser belt in Australia suggests an extension of this reconstruction back to the earlier part of Neoproterozoic times. A proposed location of South China adjacent to western North America (Li et al., 1995) does not conform with the palaeomagnetic data. No support is also found for the Rodinia reconstruction during the critical 750-550 Ma interval required by the stratigraphic correlation from which the reconstruction was originally derived (Jefferson, 1980). We conclude that the large-scale three-fold sedimentary cycle, which formed the basis for this reconstruction, resulted from global eustatic changes and cannot be used to infer close spatial proximity.

A palaeomagnetic study of the Sivas Basin, central Turkey: Crustal deformation during lateral extrusion of the Anatolian Block

The Sivas Basin is a complex collage of Eocene and younger rocks located within the wedge-shaped eastern margin of the Anatolian Block between the (dextral) North Anatolian Fault Zone and the (sinistral) Eastern Anatolian Fault Zone. It has been subject to ongoing deformation by movement of the Arabian Block into Eurasia and concomittant sideways expulsion of the Anatolian Block. Post-collisional deformation since mid-Miocene times has been dominated by NS to NWSE compression expressed by thrusting and strike-slip faulting. Cretaceous and Eocene rocks were magnetically overprinted to variable degrees during the collisional phase although these overprints have since been rotated mostly anticlockwise. Rocks emplaced during the neotectonic history are high-fidelity palaeomagnetic recorders of subsequent block movements. Regional anticlockwise rotation is recognised across the basin with differential rotation of fault and thrust-bounded blocks. An absence of perceptible differences between group mean rotations identified from Miocene, Pliocene and Quaternary units shows that most regional rotation has been concentrated within the latest phase of the neotectonic history during Quaternary times at an average rate of ∼ 10°/Ma. Commencement of this rotation postdates initiation of the North Anatolian Fault Zone implying that compression following collision was accomodated initially by crustal thickening during Late Miocene and Pliocene times. Subsequent anticlockwise rotations have resulted from sideways expulsion of blocks to the south of the Central Anatolian Thrust along major NESW sinistral faults to achieve the crustal shortening resulting from NS compression. These fault orientations and their sense of motion are explained by a Prandtl model involving deformation of a triangular plastic terrane (the Anatolian Block) between two rigid plates (Eurasia and Afro-Arabia). The variations in regional rotation identified by palaeomagnetism show that average contemporary anticlockwise rotation of Anatolia revealed by GPS data (∼ 1.2°/Ma) is achieved by variable, and locally large, block rotations between major thrusts and strike-slip faults.

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.

A palaeomagnetic secular variation record from late Pleistocene-Holocene lacustrine sediments from Chalco Lake, Basin of Mexico

Palaeomagnetic secular variation is reported for the past 25,000 years from the lacustrine sequence of Chalco Lake, Basin of Mexico. The rock magnetic and palaeodirectional record is obtained from a 11.27 m core drilled in the central sector of the lake (19.25 ° N, 99 ° W). Detailed stratigraphy, pollen and diatom studies provide a reconstruction of the environmental evolution. The depth-time scale transformation is derived from nine 14C dates, which give depositional rates of 0.28 mm/year for the first 3.5 m and 0.80 mm/year between 3.5 m and 11.27 m. Spectral analysis of intensity and directional records shows low frequency components with characteristic periods of 10,500, 3200–3400, 2900–3000, 1400–1500 and 800–900 years. In phase oscillations of inclination and intensity records point to drifting non dipole field anomalies. Mean inclination anomaly is −2.4 °, which is similar to results from volcanic rocks of the Basin of Mexico and Hawaii, and which support a zonal character of the non dipole field. The palaeomagnetic vectors describe characteristic loops, with dominant clockwise sense in the intervals 2000–5600, 12,000–15,000 and 18,600–24,000 years, and dominant counterclockwise sense in the intervals from 6500 to 11,000 and 16,600–18,500 years. Clockwise vector rotation may represent westward drift of non dipole field anomalies, though westward drift may also result in counterclockwise vector motion. Major declination swings are related to environmental and depositional disturbances, and without further confirmation they cannot be considered as geomagnetic variations.

Palaeomagnetic study of the Xitle-Pedregal de San Angel lava flow, southern Basin of Mexico

A detailed study of the ∼ 2000 years old Xitle-Pedregal de San Angel volcanic field in the southern Basin of Mexico was undertaken to assess the reliability of the palaeomagnetic record as derived from fresh well-preserved and exposed lava flows. The Xitle vent is on the slope of the Ajusco volcano, which results in a topographic difference of over 800 m in less than 12 km of horizontal distance. Most sites present a mean direction, with a small within-site dispersion, around the dipolar direction for the locality, but some sites, particularly in the basin sector of flat relief away from the vent, show shallow inclinations. Anisotropy of magnetic susceptibility (AMS) shows normal ‘flow’ fabrics with horizontal foliation planes and small anisotropy degree. AMS lineations correlate with observed flow directions. The magnetic properties vary systematically across flow units, but directions do not show a consistent pattern. Secular variation effects do not apparently contribute to the shallow inclinations or directional scatter. Some lava structures like pressure crests and blocky fronts give shallow inclinations and scattered directions, respectively. The resulting overall mean direction is well defined and close to the dipolar direction ( B = 26, Dec = 359.8°, Inc = 32.8°, k = 167, α 95 = 2.2°), but this excludes two apparent directional groups. The mean direction for one group, with B = 19, Dec = 359.0°, Inc = 35.1°, k = 247, α 95 = 2.1°, may be the representative estimate for the field. Shallow inclinations are considered anomalous and associated to a characteristic specific to given sectors of lava flows. Palaeointensity determinations have been obtained for six samples from five sites by using the Thellier and Shaw methods. Results agree well with previous studies, however, the standard deviation calculated for the mean value remains high after incorporation of the new data. Mean palaeointensity based on five new determinations and eight early data is N = 13, 56.50 ± 6.16 mT (and the mean for the overall data set is N = 20, 60.50 ± 9.21 mT). Results illustrate some practical considerations and limitations in methodology and the reliability of the palaeomagnetic record in volcanic rocks.

Palaeomagnetism of the Rajmahal Traps of India: Implication to the Reversal in the Cretaceous Normal Superchron.

The Upper Gondwana Rajmahal Traps in northeast India were studied over the last four decades and these are referred to as normally magnetized rocks. Our study of these rocks from thirty-five sites has brought into light a palaeomagnetic record revealing the existence of a reverse magnetization event also. In the light of the 40Ar/39Ar age data of the Rajmahal Traps and the geomagnetic polarity time scale during the middle Cretaceous period, the observed magnetic reversal in the Rajmahal Traps has been interpreted to constitute a geomagnetic reversal in the Cretaceous Normal quiet magnetic interval (Cretaceous Normal Superchron, K-N). Similar short magnetic reversals have also been reported at other periods in this superchron from some other places.

A detailed palaeomagnetic record for the last interglacial period

A palaeomagnetic investigation of a late Quaternary loess-palaeosol sequence in central China has revealed two episodes of anomalous geomagnetic field behaviour in the lower and upper parts of soil complex S1, which spans the last interglacial interval. Comparison of the magnetic susceptibility record with the deep-sea oxygen isotope stratigraphy provides a chronology which has been used to estimate the age and duration of these episodes. The age of the younger episode has been estimated at 89-75 ka, comparable with the age of a geomagnetic event recognised from sediments in the Norwegian-Greenland Sea and Arctic Ocean. The older episode is associated with a broad low in relative palaeointensity which has been assigned to the time interval 130-110 ka. This is consistent with existing age estimates of the so-called Blake Event. In contrast to a published record of the Blake Event from the loess at Xining, which shows three short episodes of almost full reversed polarity, in the current record the Blake Event is represented by a loop in the VGP path which passes through shallow southern latitudes. Although both episodes occur within the same part of soil complex S1, the difference between the two records implies a greater degree of smoothing of the geomagnetic signal in the Huanxian record. This is probably a reflection of the lower accumulation rate at Huanxian and may also indicate a greater time constant for the remanence aquisition at this site.

Neoproterozoic of the Mackenzie Mountains, northwestern Canada

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km thick, and comprise two supergroups of differing style. The Mackenzie Mountains Supergroup (4–6 km thick), an epicratonic succession of mainly shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-778 Ma. The overlying Windermere Supergroup (5–7 km thick) comprises basal rift deposits and glacial diamictites that are overlain by three, kilometre-scale, siliciclastic to carbonate “Grand Cycles” deposited on a passive margin. “Grand Cycles” in the eastern Mackenzie Mountains were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope; mapping of successive shelf-margin positions reveals southwestward progradation. The Windermere Supergroup is unconformably overlain by the Ingta Formation, which contains the Proterozoic-Cambrian boundary. Subtrilobite Cambrian strata are approximately 1.5 km thick, and are overlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi Formation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and microbial structures occur throughout the entire succession, but are especially well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Edicara-type fossils first appear below the second Windermere glacial unit (Ice Brook diamictite), and range upwards through 2.5 km of strata to near the top of the Windermere; three distinctive assemblages corresponding to the Twitya, Sheepbed, and Blueflower formations can be recognized. The Neoproterozoic-Cambrian boundary occurs within the Ingta Formation, and is marked by an abrupt change from simple, subhorizontal burrows ( Planolites-Torrowangea Assemblage) to complex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glacial diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record shows three hair-pin curves that should be recognizable in continents attached to Laurentia. Carbonate interbeds throughout the succession typically yield relatively unaltered carbon isotope ratios; C and Sr chemostratigraphy shows marked excursions that appear to be globally correlatable.

Ordovician palaeogeography of Siberia and adjacent continents

Ordovician palaeomagnetic data from the upper reaches of the Lena River, Southern Siberia, confirm and refine the earlier reported data sets. Ordovician palaeomagnetic poles from Siberia define a systematic southwesterly apparent polar wander (APW) trend during Ordovician times (mean south poles: 500 Ma: 42°N, 310°E; 467 Ma: 27°N, 314°E; 460 Ma: 23°N, 313°E; 448 Ma: 22°N, 301°E and 437 Ma: 0N, 290°E). A primary or early magnetization age is verified by the reversal stratigraphy. Siberia was geographically inverted at low southerly latitudes during the Latest Cambrian and Early Ordovician and drifted slowly northward and across the equator at an average palaeo-latitudinal velocity of c. 5cm/year. In Early Ordovician times, Avalonia and the European Massifs (e.g. Armorica and Bohemia) were located together with Gondwana in high southerly latitudes, Laurentia was positioned in equatorial latitudes whereas Baltica was located at intermediate southerly latitudes. Siberia was probably located north of Baltica in latest Cambrian–Early Ordovician times. Subduction-related, eclogite-facies metamorphism in latest Cambrian–Early Ordovician time in the Scandinavian Caledonides occurred in an ocean-continent transition zone marginal to Baltica but facing northern Siberia, and thus throws doubt on traditional Baltic-Laurentian correlations during this particular time period. With Baltica rotating counterclockwise during the Ordovician, the plate scenario allows for a Siberian source for Late Ordovician sedimentation in some areas of maritime Laurentia and perhaps even northern Norway. It also helps to explain the imposition of a deep-seated, sinistral strike-slip, fault regime between the obliquely converging Baltica and Laurentia, a transcurrent system which may have led to the permissive ascent of calc-alkaline granitoid magmas in favourable sites prior to the main stages of Scandinavian orogenic deformation. Recent proposals that Laurentia formed a conjugate margin to the South American part of Gondwana during Ordovician times are permissible from palaeomagnetic data, but a tight continental fit during the entire Ordovician is contradicted by biogeographic data. The tight palaeomagnetic fit could perhaps be an artefact of inaccuracies in the palaeomagnetic record for Gondwana.

Magnetic recording in nature: the medium, the mechanism and the message

The capacity of rocks to magnetically record Earth history has played a crucial part in transforming the way we look at our planet. It has also provided much stimulus for the study of the medium which carries the record, and the mechanisms by which the record has been implanted. The medium consists of compounds of iron (oxides and sulphides) often with high concentrations of titanium. Oxides with spinel structure, which may be highly nonstoichiometric, are abundant. Particle sizes are often too large to be ideal recording materials but seem to carry the palaeomagnetic record effectively. This has led to a concentration of effort on understanding the magnetic state and the magnetization process in particles with few domains. Several recording mechanisms operate in nature: thermoremanent magnetization may be acquired as hot rocks cool to ambient temperature; chemical remanent magnetization can be acquired as particles grow in size or change intrinsic magnetic properties due to a change in composition; and a sedimentary rock may acquire a depositional remanent magnetization as the magnetic particles settle in the forming sediment. The requirements on the medium and mechanism are somewhat stringent: the message must be accessible after exposure of the recording to an environment which may be physically and chemically hostile for times up to hundreds of millions of years. The success of palaeomagnetism testifies to the robustness of nature's magnetic recordings.

Magnetization history of the Upper Silurian Dingle Group, SW Ireland

SUMMARY The Dingle Group, a continental Upper Silurian redbed succession in SW Ireland, has yielded a multicomponent palaeomagnetic record. The oldest magnetization retrieved, the B component, with in situ mean direction of D/I= 191/+9; K= 40; α95= 3.6°; N= 41, post-dates the local late Silurian-mid-Devonian (Acadian) tectonic deformation and is in very good agreement with the prevailing remanences in other Caledonian rocks of western Ireland. An overprinted ‘A’ component with in situ mean direction D/I= 194/-16; K= 53; α95= 3.1°; N= 41, is about as prevalent as the B component and was probably acquired in late Palaeozoic (Kiaman) times. By comparison with other published data, including a positive fold test for corresponding magnetization of Upper Silurian redbeds in Clare Island, it is concluded that the flat-lying B field (relative to Britain and Ireland) was in existence at least from the late Silurian to late Middle Carboniferous, i.e. covering a minimum time span of 100 M. yr.

Palaeomagnetism, rock magnetism and evolution of the Great Barrier Reef

Palaeomagnetic and rock magnetic properties have been determined for sediment cores collected during ODP Leg 133 from the continental slope adjacent to the Great Barrier Reef. The magnetic remanence properties of the sediments are poor, with a pervasive viscous overprint that obscures most reversal boundaries. However, an inter-core magnetic susceptibility stratigraphy has been established that permits some refinement of the existing biostratigraphy. The quality of the palaeomagnetic remanence record relates to the residence time of magnetic particles in the corrosive, near-surface, sulphate-reducing zone. Magnetic susceptibility variations appear to reflect glacio-eustatic sea level cycles, and show a strong inverse correlation with both d18O and carbonate content. For the last several glacial maxima there are marked changes in magnetic properties, largely grain size-related, with sharp peaks in susceptibility and a reversal of the d18O-susceptibility phase relationship. One possible explanation can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop during times of low sea level and become reworked during transgressions. Changes in the pattern of susceptibility variation occur at approximately 0.8 Ma and 0.4 Ma. Boundaries at these ages are seen in seismic profiles, they occur in the d18O frequency spectrum, and there is a rise in sea surface temperature at ~0.4 Ma. We suggest that these boundaries relate through palaeoclimatic and palaeoceanographic changes to stages in evolution of the Great Barrier Reef.

The coast-parallel dolerite dykes of east Madagascar; age of intrusion, remagnetization and tectonic aspects

Palaeomagnetic and whole rock K-Ar data are reported from dolerite dykes from the east coast of Madagascar. The radiometric dates indicate that the dykes were injected 94.5 ± 1.2 Ma ago, during the earliest phase of Cretaceous magmatism on the island. The remanence structure is principally single-component having been acquired in a normal polarity field. The magnetization carrier is inferred to be titanomaghaemite, a product of extensive low-temperature oxidation of original Fe-Ti oxides. When viewed in the context of the African palaeomagnetic reference frame, the characteristic magnetization is consistent with a late Cretaceous-Early Tertiary age, i.e., some 40 million years younger than the emplacement age of the dykes. This low-temperature magnetic resetting is associated with reactivation of the fault/dyke system, this being the consequence of an early Tertiary shear régime provided by 25° anticlockwise rotation of an African/eastern Indian Ocean lithospheric block system. The overall remanence direction yields a pole at 65.5° S, 38° E ( K = 73, α 95 = 1.8), but this pole is regarded as providing only crude paleomagnetic information since the total palaeomagnetic record was most likely acquired at various times during the assumed lithospheric rotation.