Mean Palaeomagnetic Pole Research Articles

A new grand mean palaeomagnetic pole for the 1.11 Ga Umkondo large igneous province with implications for palaeogeography and the geomagnetic field

© The Authors 2015. We present a new grand mean palaeomagnetic pole (Plong: 222.1°, Plat: -64.0°, A95: 2.6°, N = 49) for the ca. 1110 Ma Umkondo large igneous province (LIP) of the Kalahari Craton. Newpalaeomagnetic data from 24 sills in Botswana and compiled reprocessed existing data are used to develop a palaeomagnetic pole as the Fisher mean of cooling unit virtual geomagnetic poles (VGPs). The mean and its associated uncertainty provide the best-constrained pole yet developed for the province. Comparing data from individual cooling units allows for evaluation of palaeosecular variation at this time in the Mesoproterozoic. The elongation of the population of VGPs is consistent with that predicted by the TK03.GAD model lending support to the dipolar nature of the field in the late Mesoproterozoic. In our new compilation, 4 of 59 (~7 per cent) of the igneous units have northerly declinations while the rest are southdirected indicating that a geomagnetic reversal occurred duringmagmatic activity. Interpreting which of these polarities corresponds with a normal or reversed geomagnetic field relative to other continents can constrain the relative orientations between cratons with time-equivalent data. This interpretation is particularly important in comparison to Laurentia as it bears on Kalahari's involvement and position in the supercontinent Rodinia. The dominance of southdirected declinations within the Umkondo Province was previously used to suggest that these directions are the same polarity as reversed directions from the early magmatic stage of the Keweenawan Midcontinent Rift of Laurentia. Two Umkondo sills with northerly declinations have U-Pb baddeleyite ages of ca. 1109 Ma that are temporally close to dated Midcontinent Rift units having reversed directions. Based on this comparison, and palaeomagnetic data from younger units in the Kalahari Craton, we favour the option in which the sites with northerly declinations from the Umkondo Province correspond to the reversed polarity directions from the early magmatic stage in the Midcontinent Rift. This interpretation allows for the Namaqua-Natal metamorphic belt of Kalahari to be a conjugate to the Grenville margin of North America and for Kalahari to have become conjoined with Laurentia within the supercontinent Rodinia subsequent to Umkondo LIP magmatic activity.

Tectonic evaluation of the Indochina Block during Jurassic-Cretaceous from palaeomagnetic results of Mesozoic redbeds in central and southern Lao PDR

Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon.=56.0°N/178.5°E (A95=2.6°), 63. 3°N/170.2°E (A95=6.9°), and 67.0°N/180.8°E (A95=4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8±8.8° (16.4±9.0°); −11.1±6.2° (17.8±6.8°); and −5.3±4.7° (13.3±5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.

Palaeomagnetism of the upper volcanic supergroup, southern part of the Sierra Madre Occidental, Mexico

Ash flow tuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 ± 0.2 Ma (the youngest age obtained so far in this area) and 29.2 ± 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8°N; Long = 180.5°E; Kappa = 142; A95 = 6.3°), calculated for the period 28–31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 ± 4° compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12–9 Ma). Palaeointensity estimates, obtained with the Thellier–Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations.

Paleomagnetism of Bhander Sediments from Bhopal Inlier, Vindhyan Supergroup

Abstract: Paleomagnetic investigations have been carried out on poorly determined radiometric age controls of Bhander sandstones within the vicinity of Bhopal Inlier of the Upper Vindhyan Supergroup. Available ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources and methods. Paleomagnetic data generated from the Bhander Group of Bhopal Inlier yielded a mean declination of 357° and mean inclination of 58° (k=17.69, α95 = 16.38) with a Virtual Geomagnetic Pole (VGP) at 74° N, 69.0° E. This pole position is falling close to the Malani Igneous Suite (MIS) mean palaeomagnetic pole of 67.8° N and 72.5° E (A95=8.8°) by Gregory et al. (2009). The results obtained from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone suggest that the Upper Vindhyan sequence may be older than is commonly thought earlier.

The Earth's magnetic field prior to the Cretaceous Normal Superchron: new palaeomagnetic results from the Alto Paraguay Formation

We report a detailed palaeomagnetic investigation of 28 lava flows (221 standard palaeomagnetic cores) collected from the Paraguayan part of the Paraná flood basalts (the Alto Paraguay Formation). The initial aims of our study were to (i) document variability of the Earth's magnetic field during the time interval sampled, (ii) obtain a new Early Cretaceous palaeomagnetic pole (PP) for stable South America, and (iii) estimate the extrusion rate of the Paraná magma. We precisely determined the palaeofield direction for 26 sites for which the characteristic remanent magnetizations exhibit small within-site dispersion and high directional stability. No palaeodirections were determined for two sites because of a very complex and erratic behaviour of the remanence during the palaeomagnetic treatments. Nine sites display normal polarity magnetization, whereas nine others are reversely magnetized and the remaining eight sites yield intermediate palaeodirections. The mean palaeomagnetic direction of normal polarity sites give I = −41.8°, D = 4.9°, k = 112, and α95 = 4.9°, whereas reversely magnetized sites give I = 37.1°, D = 181.4°, k = 23, and α95 = 11.1°. The reversal test as defined by McFadden and McElhinny (1990; Classification of the reversal test in paleomagnetism: Geophysical Journal International, v. 103, p. 725–729) is positive, corresponding to Type B with γ = 8.7° and γ0 = 3.7°. This ensures that the palaeomagnetic treatment successfully removed the secondary natural remanent magnetization and that the sampling adequately averaged the palaeosecular variation (PSV). The mean PP position obtained from 18 sites is palaeolongitude (Plong) = 359.2° and palaeolatitude (Plat) = 86.2° S. We show by means of probability plots and formal testing procedures that a Fisher distribution with a concentration parameter K = 65 satisfactorily fits the distribution of virtual geomagnetic poles (VGPs). The PP obtained in this study agrees reasonably well with coeval pole positions, in particular with those obtained from the Central Paraná Magmatic Province in Brazil, Los Adobes, and Misiones in Argentina. However, some other similar age PPs show significant departure that may be attributed to local tectonic rotations or insufficient sampling needed to overcome the PSV. This new PP differs slightly from the reference poles at 135 Ma for South America given by Besse and Courtillot (2002; Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300). The PSV parameters are in agreement with those obtained from selected data reported for the Cretaceous Normal Superchron (CNS). In contrast, VGP angular dispersion found here is lower with respect to the Jurassic and Plio–Pleistocene data. The intermediate VGPs determined in the present geographical frame show a northern hemisphere cluster of seven VGPs located east of India, whereas one other VGP is located in the vicinity of Australia.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores Archipelago-North Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3°N, 160.7°E, K= 33 and A95= 3.4°) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 ± 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNW-SSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNW-SSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNW-ESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Chapter 36 Neoproterozoic glacial record in the Mackenzie Mountains, northern Canadian Cordillera

Abstract In the Mackenzie Mountains, an arcuate foreland thrust-fold belt of Late Cretaceous–Paleocene age in the northern Canadian Cordillera, two discrete glacial–periglacial sequences of Cryogenian age (the Rapitan Group and the Stelfox Member of the Ice Brook Fm.) are separated by c. 1.0 km of non-glacial strata. The older Rapitan diamictite occurs in an amagmatic rift basin; the younger Stelfox diamictite occurs on a passive-margin continental slope. The Rapitan Group consists of three formations. The lower Mount Berg Fm. is a complex of diamictites and conglomerates of limited extent. The middle Sayunei Fm. is a thick sequence of maroon-coloured mudrocks hosting innumerable graded layers of silt- and fine-grained sandstone. It lacks wave- or traction current-generated bedforms, and is lightly sprinkled with granule aggregates (‘till pellets’) and lonestones of dolostone and rare extrabasinal granitoids. It is capped by a hematitic Fe-formation that was reworked into the disconformably overlying Shezal diamictite. The Shezal Fm. is a complex of olive-green coloured boulder diamictites with subordinate, dark-grey shales, siltstones and parallel-sided sandstones. Some of the boulders are faceted and striated, and include dolostone, quartzite, siltstone and gabbro in declining order of abundance. Diamictite terminates abruptly at the top of the Shezal Fm., which is sharply overlain by dark shales or by <52 m of fetid, dark-grey, 13 C-depleted limestone with graded bedding. The Stelfox Member is dominated by non-stratified, carbonate-clast diamictite with faceted and striated clasts, locally associated with subordinate, well-laminated shales containing till pellets and ice-rafted dropstones. It is thin or absent on the palaeocontinental shelf, but thickens seaward (southwestward) on the palaeocontinental slope. A thin clay drape separates it from a laterally continuous post-glacial ‘cap’ dolostone, which is a very pale coloured, micro- to macropeloidal dolostone with low-angle cross-laminae, giant wave ripples and local bioherms of corrugated stomatolites. In the NW, the dolostone is followed by reddish and greenish marls, followed by black shale of the Sheepbed Fm. In the SE, the dolostone is overlain by pink or grey limestones with well-developed sea-floor cements pseudomorphic after aragonite. In this area, the top of the dolostone is ferruginous and contains digitate rosettes of sea-floor barite cement, variably calcitized. The dolostone–limestone contact is perfectly conformable, and synclinal structures previously intepreted as karst features are tectonic in origin. The grand mean palaeomagnetic pole for the well-studied Franklin Large Igneous Province ( c. 718 Ma) of Arctic Laurentia, coeval with the basal Rapitan Group in the Mount Harper area, Yukon Territory, places the Mackenzie Mountains firmly in the tropics, at 18±3°N palaeolatitude, at the onset of the Rapitan glaciation. Carbon (δ 13 C), oxygen (δ 18 O) and strontium ( 87 Sr/ 86 Sr) isotopes have been measured in carbonates bracketing the Rapitan and Stelfox diamictites. Sulphur isotope data (δ 34 S) have been obtained from carbonate-associated sulphate and barite above the younger diamictite, and calcium isotope data (δ 44 Ca) from the younger carbonate itself. The results are broadly consistent with data from other areas. Iron isotope (δ 57 Fe) and cerium anomaly (Ce/Ce*) values increase systematically upwards through the Sayunei Fe-formation, supporting an interpretation that deposition occurred within a redox chemocline through which the basin floor descended as a consequence of isostatic loading by the advancing Shezal ice sheet. Supplementary material: Data are available at http://www.geolsoc.org.uk/SUP18470 .

A new key pole for the East European Craton at 1452 Ma: Palaeomagnetic and geochronological constraints from mafic rocks in the Lake Ladoga region (Russian Karelia)

Palaeomagnetic and geochronological studies on mafic rocks in the Lake Ladoga region in South Russian Karelia provide a new, reliably dated Mesoproterozoic key paleopole for the East European Craton (Baltica). U–Pb dating on baddeleyite gives a crystallisation age of 1452 ± 12 Ma for one of the studied dolerite dykes. A mean palaeomagnetic pole for the Mesoproterozoic dolerite dykes, Valaam sill and Salmi basalts yields a paleopole at 15.2°N, 177.1°E, A 95 = 5.5°. Positive baked contact test for the dolerite dykes and positive reversal test for the Salmi basalts and for the dykes confirm the primary nature of the magnetisation. Comparison of this Baltica palaeopole with coeval paleomagnetic data for Laurentia and Siberia provides a revised palaeoposition of these cratons. The results verify that the East European Craton, Laurentia and Siberia were part of the supercontinent Columbia from the Late Palaeoproterozoic to the Middle Neoproterozoic.

New palaeomagnetic data supporting the extent of the stable body of the South China Block since the Cretaceous and some implications on magnetization acquisition of red beds

SUMMARY Previous palaeomagnetic studies have demonstrated that a large part of the South China Block (SCB) has behaved as a stable body since the Cretaceous. We undertook a palaeomagnetic investigation of Lower Cretaceous red sandstones at 24 sites within the Ganzhou and Xingguo basins in southern Jiangxi Province in China. The aim of this study was to further constrain the extent of the SCB, which has been stable since the Cretaceous. We isolated the characteristic directions of higher temperature components (HTCs) with an unblocking temperature from 650 to 700 ◦ C, by progressive thermal demagnetization and principal component analysis. The optimal concentration of site mean HTC directions calculated using the direction-correction tilt test was achieved at 51.2 ± 32.4 per cent untilting, indicating syntilting magnetization, suggesting that the remanences were not acquired immediately after sedimentation. Most Cretaceous sedimentary basins on the eastern part of the SCB were controlled by fault movement (extensional basins). Most tilting in such an extensional basin is thought to have progressed contemporaneously with the structural and stratigraphic development of the basin. We concluded that the remanence acquisition of red sandstones on the Ganzhou and Xingguo basins occurred during synsedimentary tilting. We adopted 51.2 per cent untilted directions of the HTCs as the palaeomagnetic field directions during the Early Cretaceous. The mean palaeomagnetic pole (76.3 ◦ N, 224.3 ◦ E, α95 = 3.3 ◦ ), calculated using virtual geomagnetic poles from 23 sites (excluding site 20, which has an insufficient number of samples showing a stable magnetic component), is in good agreement with the Late Cretaceous palaeomagnetic poles from the same region and also with most of Cretaceous poles previously reported from the stable body of the SCB. This agreement indicates that the Jiangxi region has been part of the stable body of the SCB since the Cretaceous. This result also demonstrates that the tectonic influence of the India–Asia collision did not destroy the rigidity of most of the SCB. The global mean pole position (78.8 ◦ N, 214.4 ◦ E, α95 = 2.6 ◦ ), calculated using sixteen Cretaceous poles from the stable body of the SCB, is suitable for use as the reference Cretaceous palaeomagnetic pole of the stable SCB.

Palaeomagnetism and K-Ar dating of Cretaceous basalts from Mongolia

We report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon (44.4°N, 103.8°E) and Arts-Bogd (44.3°N, 102.2°E) localities in the Gobi Desert, south Mongolia. K—Ar dating based on seven rock samples, with two independent measurements for each sample, allows us to propose an age of 94.7 ± 1.3 Ma for Shovon locality and a 98.2 ± 1.4 to 118.3 ± 1.7 Ma age range for Arts-Bogd. Stepwise thermal and AF demagnetization generally isolated a high temperature component (HTC) of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component (LTC) in some samples. The HTC directions display normal polarity, consistent with the Cretaceous Long Normal Superchron. Rock magnetic analysis identifies fine-grained pseudo-single domain (PSD) magnetite and titanomagnetite as primary carriers of the remanence. Mean HTC palaeomagnetic direction is Dm = 8.2°, Im = 63.7° (n = 18 flows, k = 41.1, α95 = 5.5°) for Shovon and Dm = 12.1°, Im = 66.4° (n = 27 flows, k = 53.0, α95 = 3.9°) for Arts-Bogd. Because of their similar ages, we combine data from Shovon and data previously obtained from Khurmen Uul (92.0 ± 4.0 Ma), recomputed in geographic coordinates, and not in tilt-corrected ones as in our previous interpretation, at the Shovon locality. The combined final average palaeomagnetic direction for Shovon-Khurmen Uul is Dm = 7.4°, Im = 62.7° (n = 23 flows, k = 41.4, α95 = 4.8°). The corresponding palaeopoles computed from these HTC lie at λ = 84.7°N, φ = 195.0°E, dp/dm = 5.8/7.5 for Shovon-Khurmen Uul (average age: 93.4 ± 2.6 Ma) and λ = 80.5°N, φ = 159.0°E, dp/dm = 5.2/6.3 for Arts-Bogd (average age: 104.6 ± 6.6 Ma). These poles are consistent with those from the European apparent polar wander path (APWP) at 90, 100 and 110 Ma, and other published pole from the Mongol-Okhotsk suture zone, Amuria and North China blocks. This confirms the lack of a discernable latitudinal motion between Amuria and Siberia since their final accretion by the Late Jurassic—Early Cretaceous, and reinforces the idea that Europe APWP can be used as a reference for Siberia by the mid-Cretaceous. We finally propose a mid-Cretaceous mean palaeomagnetic pole for the Siberia-Amuria-North China Block assemblage which lies at: λ = 86.4°N, φ = 191.1°E(n = 10, k = 74.9, A95 = 5.8°).

Further palaeomagnetic results from the Sierras Australes fold and thrust belt, Argentina

Summary The southwestern Gondwanaland margin during the late Palaeozoic was located between the South America craton and the Patagonia terrane. The Sierras Australes fold and thrust belt of Buenos Aires province (Argentina) has been interpreted as being the result of a crustal collision between these two blocks. A palaeomagnetic study of the less deformed upper part of the Permian Tunas Formation of the Pillahuinco region within the Sierras Australes belt was carried out in order to obtain reliable palaeomagnetic poles to constrain the palaeogeographic and tectonic evolution of this part of Gondwana. More than 300 specimens were measured from the Tunas Formation in the Sierra de Pillahuinco, from 27 sites in four localities of these red sandstones and clay-siltstones. Demagnetization at high temperatures isolated a reversed characteristic remanent magnetization, consistent with magnetization acquired during the Kiaman reverse superchron. Stepwise tectonic tilt correction suggests that the upper part of the Tunas Formation acquired its magnetization before or early in the deformation. The position of the palaeomagnetic pole on the apparent polar wander path of South America suggests that the magnetization was acquired some time between the late Early Permian and the early Late Permian. From the best grouping of the partially tilt-corrected remanence directions, at 90 per cent unfolding for 24 sites, a mean palaeomagnetic pole Tunas II for the Tunas Formation of the Pillahuinco region was calculated. The position is longitude 025.9°E, latitude 74.1°S; A95 = 5.2°, K = 33.6. This pole is consistent with previous poles from South America assigned to a similar age and allows for a more accurate definition of the apparent polar wander path. An earlier study in the lower part of the Tunas Formation (Tunas I) shows syntectonic magnetization acquired during the Early Permian. Therefore the differences in the type and age of magnetization between the lower and upper parts of the Tunas Formation suggest that the deformation was diachronous and propagated more slowly towards the foreland. Palaeogeographically, these two Tunas Formation poles are interpreted as recording a slight counterclockwise movement of South America during the Permian.

The magnetic polarity stratigraphy of a Hauterivian/Barremian carbonate sequence from southern Italy

SUMMARY Remanence directions, measured at 2 cm intervals along a composite 88 m bore-core, enable mean palaeomagnetic poles to be defined at 13.6°S, 25.2°W and 13.6°N, 154.8°E. The directions of remanence vary very smoothly away from each palaeomagnetic pole, extending more than 90° from them. This raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles. For practical purposes, intermediate polarity is defined as directions whose virtual poles lie more than 25° from the mean pole, enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three-quarters of the core. The stratigraphic thickness between these subchrons shows a very high linear correlation (r>0.99) with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age. The analysed sequence contains wavelength spectra which, when transformed to the temporal realm, match periodicities determined for three marine magnetic anomaly profiles of similar age. These also match planetary orbital periodicities for the Cretaceous. These observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits. Such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically.

The drift of the Fennoscandian and Ukrainian Shields during the Precambrian: a Palaeomagnetic analysis

A revised Precambrian (2.85−0.6 Ga) Apparent Polar Wander Path (APWP) for the Fennoscandian Shield, based on a new compilation and analysis of data, is presented. In fitting the APW path to successive Grand Mean Palaeomagnetic poles (GMPs), we applied the spherical spline technique originally developed by Jupp and Kent in 1987. The position and orientation of the Fennoscandian Shield during 2.85−0.6 Ga was determined from the GMPs. Major palaeoclimatological findings are used to constrain the palaeomagnetic interpretation of palaeolatitudes. The general drift of Fennoscandia, from relatively high latitudes in the late Archaean-Early Proterozoic to nearly equatorial latitudes in the Middle Proterozoic, correlates with palaeoclimatological indications that a period of cold climate was followed by one of warm climate during this time interval. From the continuous APWP the APW velocities and latitudinal drift velocities of the shield were calculated. An accumulated APW curve was also calculated. The palaeomagnetic data are irregularly distributed and some periods are rather poorly represented. This means that the calculated velocities can sometimes be artifacts of sampling. Late Archaean and Early Proterozoic (2.85−1.90 Ga) data are too sparse to make these calculations meaningful and velocity calculations are therefore restricted to data of 1.90 Ga and younger ages. The accumulated APW curve shows a number of linear segments with varying slopes, indicating sudden changes in drift rate. During the Middle Proterozoic (1.90−1.35 Ga) there was a period when the rate of APW was constant and low and that of latitudinal drift also was low. This pattern changed at ca. 1.35 Ga, and the following Middle-Late Proterozoic period can be described by rapid APW and strongly fluctuating drift velocities. Jotnian rifting and the intrusion of numerous dyke swarms (at ca. 1.25 Ga) correlate with this shift in rate. These changes are attributed to changes in plate configuration. A new database for the Ukrainian Shield is also presented, and GMPs in the 2.32−1.20 Ga range are defined. The database is still inadequate and the comparison of the Ukrainian and Fennoscandian drift histories is therefore tentative. Similarities in position, latitudinal drift and rotation during the Early-Middle Proterozoic are, nevertheless, evident. A close relationship between the shields in this period is consistent with the low APW rate of Fennoscandia, indicating that Fennoscandia may have been part of a larger continent, including the Ukraine, at that time. At ca. 1.2 Ga, the latitudinal position of Ukraine differed significantly from that of Fennoscandia, suggesting that the large shield split up between ca. 1.35 and 1.2 Ga. This would explain the change in APW rate at 1.35 Ga. The subsequent increase in rate was due to a reduction in the size of the shield. The discrepancy in palaeopositions of Fennoscandia and Ukraine at 1.2 Ga led Mikhailova and Kravchenko to suggest a late Precambrian time (1.07−0.57 Ga) for the accreation of Fennoscandia to the East European Platform (EEP). This may be correct as the rate of APW for Fennoscandia decreased in the late Precambrian, reflecting such a consolidation.

Palaeomagnetism of the Mesozoic Serra Geral Formation, southern Brazil

Abstract Palaeomagnetic results from 20 volcanic sequences and 11 intrusive bodies (sills and dykes) of the Serra Geral Formation (Parana Basin) are reported in this paper. The sequences are widespread all over the basin, while sills and dykes (Ponta Grossa arch) come from the northeastern portion. Three mean palaeomagnetic poles were computed for the Serra Geral Formation, which account for a time interval of ∼ 15 Ma. Pole SG1 is located at 85°S,108°E ( α 95 = 1.1°, N = 18) and represents the main phase of the magmatic activity in the basin, with a mean age of ∼ 135 Ma. Pole SG2 is located at 82°S,38°E ( α 95 = 7.8°, N = 2) and represents a younger magmatic phase, with an associated age of ∼ 130 Ma. Pole SG3 (72° S ,37° E ; α 95 = 6.8°, N = 10) is the youngest pole. It is computed from the intrusive rocks and its age is assumed to be not younger than 118 Ma, the lower limit of the ‘Cretaceous normal magnetic interval’. These three poles describe a shifting path, which suggest that the South American platform moved ∼ 5° southwards and rotated ∼ 10° clockwise during the Lower Cretaceous, preceding the South Atlantic opening.

Petrography and palaeomagnetism of the basalts, southwest Harrat Rabat, Saudi Arabia

SUMMARY A total of 151 oriented samples were drilled out of 14 basaltic flows southwest of Harrat Rahat (22.1°N, 39.3°E). Ar39/Ar40 age determinations for these flows range between 3.3 ± 0.17 and 3.73 ± 0.16 Ma. Microscopic observations of 56 thin, and polished sections indicate a secondary crystallization of very fine grained magnetite and iddingsite at the boundaries and along microfractures in the olivine crystals. These single to multi-domain crystals produce a strong secondary magnetization unstable against alternating field demagnetization. The mean direction of these magnetic overprints is: D= 345.4, I= 28.0, α95 = 16.4, k= 32.3, n= 4 (sites) yielding a mean palaeopole at: 285.5°E, 74.4°N, A95 = 14.1, K= 34.5, N= 4. The stable remanent magnetization of all the samples is of reverse polarity and has a mean direction: D= 168.7, I= 38.8, α95 = 3.7, k= 114.5, n= 14, yielding a mean palaeomagnetic pole position at: 310.4°E, 80.0°N, A95 = 3.9, K= 102.6, N= 14. These data, together with other data for the Arabian plate were used to construct a preliminary apparent pole wander path (APWP) for Arabia during the Tertiary. The APWPs for Africa and Arabia during the Tertiary were constructed. Comparison of the Arabic and African APWPs indicate high mobility during the Miocene to Holocene, but the APWPs are in agreement with each other after proper reconstruction of Arabia to Africa.

Direction and intensity of the geomagnetic field in the Middle Devonian and Lower Ordovician: southern Mugodjary ophiolites (Urals)

A considerable quantity of petromagnetic and palaeomagnetic measurements in the field has allowed us to obtain reliable palaeomagnetic results for such difficult and complex objects as ophiolites. Eifelian baked sediments, diabases (dyke contacts) and gabbros, and Tremadocian baked sediments were studied. Continuous thermocleaning, conglomerate and fold tests were applied. The palaeointensity was investigated by means of several palaeomagnetic methods: Wilson-Burakov, van Zijl-Shaw and Bagina-Petrova. The mean intensities of the geomagnetic fields are 7 and 16.5 μT, with mean palaeomagnetic pole positions of 9°N, 134°E and 6°S, 83°E, respectively. These data demonstrate that southern Urals (Mugodgary) was located near the northern margin of the East European plate during Ordovician-Devonian time. The allochthonous crustal oceanic plate was thrust upon the edge of the plate and deformed as late as the Permian. Changes in the polarity of the geomagnetic field are recorded in the succession of sheeted dykes, a direct corroboration of the Vine-Mathews hypothesis.

Palaeomagnetic results from late Archaean and early proterozoic granites from South Western Kenya

Results of a palaeomagnetic study carried out on the Migori granites exposed in the South Western part of Kenya are presented. The granites of Post-Nyanzian and Post-Kavirondian ages are dated at 2739 ± 111 Ma and 2420 ± 60 Ma , respectively, using the whole rock RbSr isochron method. Alternating frequency and thermal demagnetization techniques have been used to isolate characteristic components of natural remanence. Opaque petrological and thermomagnetic studies indicate titanomagnetite grains as the main carrier of the primary component. The mean palaeomagnetic poles for the Post-Nyanzian and the Post-Kavirondian granites are located at 336°E, 72°N ( δ m = 12°, δ P = 6° ) and 263°E, 41°N ( δ m = 23°, δ P = 15° ), respectively.

Low palaeolatitude of deposition for late Precambrian periglacial varvites in South Australia: implications for palaeoclimatology

The natural remanent magnetisation (NRM) has been determined for periglacial clastic varvites (Elatina Formation) deposited in the Adelaide Geosyncline in South Australia during the Marinoan Glaciation 680 my. ago. The varvites contain abundant silt-sized detrital specular titaniferous haematite and martite that experienced a high-temperature (∼ 600°C) oxidation event prior to deposition, as well as ultrafine haematite and minor goethite pigment. The NRM intensities exhibited little change prior to thermal treatment above 600°C. The mean palaeomagnetic pole lies at latitude = 50.9°S, longitude = 337.1°E and A 95 = 2.2° the inferred palaeolatitude of the sampling locality is 5°. This pole position is similar to other late Precambrian poles for Australia. In view of the nature of the NRM and the lack of metamorphism of the rocks the magnetic remanence is regarded as primary (early diagenetic) and dating closely from the time of deposition. The contradiction of very cold climate near sea level in low palaeolatitudes during the late Precambrian brings into question basic tenets in palaeomagnetism and celestial mechanics.

Implications of a combined biostratigraphic and palaeomagnetic study of the Umbrian Maiolica Formation

The study of a 275.5 m thick section of white, pelagic limestones occupying the valley of the Fonte del Giordano river on the southern slope of Mt. Montagnola has yielded a biostratigraphically controlled clear magnetic reversal pattern after thermal cleaning. The magnetic stratigraphy of the lower 131 m of the section (Calpionellid zones) is correlatable with the M-sequence of oceanic magnetic anomalies between M-19 and M-14. The reversal stratigraphy of the upper 81.5 m of the section (Radiolaria zone) has also been tied to the oceanic polarity time scale by making linear interpolation for a missing 63 m thickness underneath it. Besides the Fonte del Giordano section two Berriasian outcrops each with a different bedding attitude were studied at Gubbio and near Cagli for tectonic tilt test giving positive results. The mean palaeomagnetic pole position for the Late Jurassic/Early Cretaceous after bedding correction is: Φ = 19.1°, Λ = 288.2°, k = 148.7, α 95 = 10.2° ( N = 3), confirming the presence of a large swing in the polar path, a common behaviour of apparent polar wandering for the peri-Adriatic region during this time.

Palaeomagnetism of the Proterozoic Zig-Zag Dal Basalt and the Midsommers� Dolerites, eastern North Greenland

Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersϕ Dolerites of eastern North Greenland. The Zig-Zag Dal Basalt is a typical tholeiitic flood basalt sequence, and dolerite intrusions in the underlying sandstones are thought to be genetically related to the basalts. After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane. Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A95=3.8°; dolerites: 6.9°S, 62.0°E with A95= 5.1°). After correction for Phanerozoic drift of Greenland the two mean poles compare closely to a relevant North American APW-curve for 1250–1350 Ma, in good agreement with Rb-Sr isochron ages of 1250 Ma obtained for the intrusives. The palaeogeographical position of Greenland was near equator with the major geographical axis orientated E-W.