Palaeomagnetic Pole Research Articles

Late Permian palaeomagnetism of Northern Eurasia: data evaluation and a single-plate test of the geocentric axial dipole model

How the first-order structure of the magnetic field changed through geological time is one of the most long-living and hotly debated questions in geomagnetism. For the last 5 Ma, the geomagnetic field is found to be essentially dipolar with about 5 per cent quadrupole term and, possibly, still smaller octupole term. Proving this became possible after accumulation of a large set of globally distributed reliable palaeomagnetic data and their analysis in the coordinates tied to the rotation axis. For older times, the data are less reliable, scarce and unevenly distributed, while tectonic motions destroyed the global coordinates. In this paper, a method to determine the geomagnetic field structure for a single plate is discussed. It is suggested that the coordinate system for analysis can be derived from great circles, each being drawn through a sampling point and the corresponding palaeomagnetic pole (SP-circle). Such circles are to intersect at a true (i.e. pure dipole) pole that is largely purified from the contributions of non-dipole zonal terms and inclination shallowing. The scrutiny of late Permian (270–250 Ma) palaeomagnetic poles from Northern Eurasia revealed two distinct groups: one includes several poles from a limited area in South France (SF), and the other consists of tightly clustered nine poles from locations spread over a large part of Eurasia. Tentatively, the former group can be explained by either rotation of SF with respect to main Eurasia, or less likely, remagnetization in Early Triassic time; irrespectively, these poles cannot be used for calculation of the true dipole pole nor for computation of the apparent polar wander path of Eurasia. The poles from the main part of Eurasia were used to find the position of the true dipole pole of this plate and for analysis of the field. The minimum of residual dispersion of the observed data is found for the quadrupole and octupole terms of less than 10 per cent when each term is varied separately. When both terms are varied together, the best fit is observed for the quadrupole and octupole terms of -4 and 12 per cent, respectively. Our analysis shows that the position of the true dipole pole is robust, while the estimates of non-dipole terms are not. We argue that these estimates should be regarded as the upper limits on the non-dipole contributions to the Late Permian geomagnetic field; at the same time, we are pretty sure that octupole term of 20 per cent or more are not permitted by the data.

Uplift deduced from remanent magnetization of a proterozoic basic dyke and the baked country rock in the Hoting area, Central Sweden: a palaeomagnetic and40Ar/39Ar study

Remanent magnetization in dyke contact zones in the Hoting area of the Fennoscandian Shield in the central part of Sweden has been studied in order to establish the ambient temperature of the host rock and the depth of burial of the present erosion surface at the time of intrusion. A positive baked contact test for two Subjotnian dykes demonstrates the primary nature of the dyke magnetization. From the magnetic properties and the palaeomagnetic data, it can be concluded that the overprinting in the hybrid zone of one of the basic dykes is a partial thermoremanent magnetization. Reliable results were also obtained from a palaeointensity study of samples from the hybrid zone in the baked host rock. The study was performed in the laboratories at Luleå and at Scripps and a mean intensity of the Earth's magnetic field of 5.8 ± 1.9 μT was determined. The maximum temperature due to the dyke intrusion in the hybrid zone has been defined and from that an ambient temperature of 375 °C in the host rock at the time of intrusion has been calculated. This calculated temperature is not contradicted by the 40Ar/39Ar data. A palaeothermal gradient in the crust at ca. 1.6 Ga is calculated at ca. 34 °C km−1 and yields a depth of burial of the present erosion surface at ca. 10.4 km. This implies an uplift rate of 0.65 km (100 Ma)−1. A slow cooling of the gabbroic host rock (ca. 3.5 °C Ma−1) has been calculated from the difference in the U-Pb age of zircon (1.786 ± 0.010 Ga) and the 40Ar/39Ar biotite ages (1.648 ± 0.012 Ga; 1614 ± 0.024 Ga) of this study. This slow cooling resulted in a palaeomagnetic age of ca. 1.7 Ga for the gabbro, which is also the age of the determined palaeointensity. The calculated ca. 1.614 Ga palaeomagnetic pole from the basic dykes fulfils most of the criteria for a reliable pole and may be regarded as a new key-pole for Fennoscandia.

Palaeomagnetism of the 2054 Ma Bushveld Complex (South Africa): implications for emplacement and cooling

SUMMARY The Kaapvaal Craton (South Africa) was the host of several major magmatic events around 2000 Ma, including the Bushveld Complex, the world's largest known layered mafic intrusion (∼0.5–1 × 106 km3). The Bushveld Complex has been the subject of numerous palaeomagnetic studies, which yielded a large spread in palaeomagnetic pole positions for the different Zones, and interpreted to indicate that the Bushveld Complex was emplaced and cooled over a time span of ∼50 million years. New palaeomagnetic data collected from 100 sites (996 drill-cores) from all Zones of the Rustenburg Layered Suite of the Bushveld Complex, yield exceptional palaeomagnetic results with high unblocking (HB) components carried by magnetite. Comparable palaeomagnetic poles from all Zones (mean pole: latitude = 19.2°N, longitude = 030.8°E, A95 = 5.8°) [correction made after online publication 22 September 2009: the mean pole values have been corrected] eliminates the previously noted large spread in poles, and this observation concurs with precise age data that constrain the time period of emplacement of the Bushveld Complex to a few million years at around 2054 Ma. Bedding-corrected HB components from all zones produced better directional groupings, which together with at least seven reversals, strongly points to a primary magnetic signature. This imply that cooling of the Bushveld Complex below the blocking-temperature of magnetite (<580 °C) occurred in a near-horizontal position, and based on the maximum observed reversal rates in the geological history (∼5 Ma−1), we estimate a minimum cooling-interval for the Bushveld Complex of 1.4 million years.

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, rock magnetism and AMS of the Cabo Magmatic Province, NE Brazil, and the opening of South Atlantic

SUMMARY Reconstruction of the South Atlantic opening has long been a matter of debate and several models have been proposed. One problem in tracing properly the Atlantic history arises from the existence of a long interval without geomagnetic reversals, the Cretaceous Normal Superchron, for which ages are difficult to assign. Palaeomagnetism may help in addressing this issue if high-quality palaeomagnetic poles are available for the two drifting continental blocks, and if precise absolute ages are available. In this work we have investigated the Cabo Magmatic Province, northeastern Brazil, recently dated at 102 ± 1 Ma (zircon fission tracks, Ar 39 /Ar 40 ). All volcanic and plutonic rocks showed stable thermal and AF demagnetization patterns, and exhibit primary magnetic signatures. AMS data also support a primary origin for the magnetic fabric and is interpreted to be contemporaneous of the rock formation. The obtained pole is located at 335.9 ◦ E/87.9 ◦ S( N = 24; A95 = 2.5; K = 138) and satisfies modern quality criteria, resulting in a reference pole for South America at ∼100 Ma. This new pole also gives an insight to test and discuss the kinematic models currently proposed for the South Atlantic opening during mid-Cretaceous.

New paleomagnetic results from imbricated Adria: Ist Island and related areas

For the purpose of this work samples for palaeomagnetic analysis were taken from Upper Cenomanian and Lower Senonian shallow water limestones, as well as from Senonian pelagic limestones from both Ist and the surrounding islands. This area belongs to Imbricated Adria, which is characterized by gently folded and faulted strata with a Dinaridic (NW–SE) trend. An exception is Premuda island where the beds are strongly folded and are subvertical. A total of 96 samples were drilled from 10 localities distributed between eight islands. The samples were then subjected to standard palaeomagnetic laboratory analysis and statistical evaluation. Eventually, six localities yielded statistically well-defined palaeomagnetic directions, which were shown pre date the folding in age. The overall mean palaeomagnetic direction obtained for the study area, characterizing the Cenomanian–Early Senonian time period had a Declination of 334°, Inclination=+46°, with statistical parameters k=188, a 95 =4.9°, defining a palaeomagnetic pole at l (N)=63°, f (E)=254°, d p=4.0°, d m=6.2°. This was compared with palaeomagnetic directions obtained for rocks of similar ages from stable Istria and the Kvarner islands. As the three palaeomagnetic directions are statistically identical, we conclude that there was no significant relative movement between the three areas after the Early Senonian. The palaeomagnetic declination for the study area, which characterizes the post-Early Senonian rotation of the Adriatic microplate, is the same as the declination for the Pannonian–Pontian age group from the South Pannonian basin. As the palaeomagnetic signals in both cases are primary, the results of the present paper not only support the conclusion that the rotating Adriatic microplate triggered rotations in the South Pannonian basin, but also suggest that the Adriatic platform did not change its orientation between the late Cretaceous and the Early Pontian.

Distributed neotectonic deformation in the Anatolides of Turkey: A palaeomagnetic analysis

The palaeotectonic phase of deformation in Anatolia accompanied closure of the Tethyan Ocean and was finally completed with sealing of the Bitlis Suture bordering the Arabian Shield at ~ 12 Ma. Arabia has since continued to indent the collage of terranes accreted to the Eurasian margin by differential northward movement relative to Africa along the Dead Sea Fault Zone and counterclockwise (CCW) rotation accompanying the opening of the Red Sea. This neotectonic phase of deformation has accompanied uplift of the Anatolian Plateau and establishment of intracontinental transforms comprising the North and East Anatolian Fault zones. Deformation has included extrusion and rotation of blocks away from the Arabian Syntaxis by tectonic escape. We produce an updated summary of palaeomagnetic results from this region to show that deformation has been distributed so that block rotations vary systematically across Anatolia ranging from strongly CCW in the zone of high strain north of the Arabian indenter to near zero in central Anatolia and then progressively CW in western Anatolia. New palaeomagnetic results from the Ankara igneous district are also reported and help to constrain the east to west change from CCW to CW rotation within central Anatolia. The transition of tectonic regimes from compression in the east to extension in the west involves the expulsion of blocks and their rotation along arcuate strike slip faults at an acute angle to the maximum compressive stress ( σ 1) between Arabia and Eurasia. The comparable distribution of rotations in palaeotectonic and neotectonic units indicates that the bulk of the observed palaeomagnetic rotations have been concentrated within the last (Plio-Pleistocene) phase of the neotectonic era as is found in the Aegean domain to the west, and has involved blocks of the order of ~ 100 km in size. The weak collage of accretionary terranes “sandwiched” between the Arabian Indenter and the Eurasian margin, now defined by the North Anatolian Fault Zone, has evidently taken up the bulk of the strain imparted by the continuing northward motion of Arabia to expand the radius of the Tauride Arc and the perimeter of extruded crust bordering the limit of the extensional province in western Turkey. The distributed character of neotectonic deformation is illustrated by equivalent polar distributions: both palaeotectonic and neotectonic palaeomagnetic poles show arcuate distributions with poles to best-fitting small circles focussed close to the study area. Differences between the distributed neotectonic deformation resolved from palaeomagnetism and the contemporary GPS record reflect the contrasting time periods being evaluated: the palaeomagnetic record incorporates evolving and changing tectonic regimes whereas the GPS signature has no long term validity within the weak Anatolian collage; hence the full spectrum of techniques for resolving neotectonic deformation is relevant to unravelling deformation in such regions.

Palaeomagnetic Study on the Precambrian-Cambrian Boundary Candidate Stratotype Section at Meishucun, Yunnan

The Meishucun secton has been recommended as an international candidate stratotype secton of the Precambrian-Cambrian boundary. The paper deals with the palaeomagnetic study on the section. A total of 159 palaeomagnetic samples were successively collected from the platform-facies sequence of carbonates and phosphates at the section. Thermal demagnetization results indicate a great majority of the rocks at the section have been strongly overprinted by recent magnetic field, but 57 samples have preserved remanent magnetization with antipodal directions (mean D /1 = 4.2 ° / 7.1 °, K= 9, α95= 6.6 ° ). Baaed on calculation, the location of the palaeomagnetic pole was at 68.8 ° N and 270.7 ° E, which is different from any palaeopoles obtained from younger Phanerozoic rocks in South China. The results reveal a polarity zonation which includes at least 9 reversal events. A comparison of China' s magnetostratigraphic records with those from Siberia, Australia and the western U.S.A. shows that all the sections are characterized by frequent polarity reversals'.

A palaeomagnetic and40Ar/39Ar study of late precambrian sills in the SW part of the Amazonian craton: Amazonia in the Rodinia reconstruction

A new key palaeomagnetic pole (Plat. = 64.3 degrees S, Plon. = 271.0 degrees E, N = 14, A(95) = 9.2 degrees; Q = 5) is calculated from a primary magnetization isolated in early Neoproterozoic Aguapei basic sills and dykes hosted by 1.3-1.0 Ga sedimentary rocks from the southwestern part of the Amazon craton. The characteristic remanence carried by stable, pseudo-single domain titanomagnetite shows two antipodal polarities that pass a reversals test. Magnetic anisotropy for most sites shows fabric orientations that are typical of sills, with horizontal magnetic foliations concordant to the flat-lying bedding of the host sedimentary rocks. Ar-40/Ar-39 analyses for one of the sills reveal a well-defined plateau age at 981 +/- 2 Myr. A tectonic reconstruction for Amazonia relative Laurentia based on this new pole 'is consistent with' a position of the present northwestern part of Amazonia attached with eastern Laurentia close to Greenland at ca. 981 Ma. On basis of palaeomagnetic and geological data, we propose a scenario where Amazonia moved northeastwards along the present southeast coast of Laurentia from ca. 1200 to 980 Ma. By 980 Ma, Amazonia is placed alongside Laurentia and Baltica, in a position similar to other reconstructions of Rodinia but with a significantly different orientation. (Less)

New palaeomagnetic results from Late Paleozoic volcanic units along the western Gondwana margin in La Pampa, Argentina

Abstract Preliminary palaeomagnetic results from the Paleozoic volcanic units along the Gondwana margin in Argentina are presented. Even though we are still working on the palaeomagnetic data of the sampled section, these data are consistent with data from former studies carried out in the area. More than 90 specimens were measured from the Sierra Chica locality (37°52′S; 65°27′W) computed in 11 sites (Sc1-Sc11). These rocks are carriers of a characteristic remanence magnetization with reverse polarity, acquired during the Early Permian and assigned to the Kiaman Superchron. The resulting palaeomagnetic pole (PP) is Lat.: 64°S; Long.: 017°E, A95 = 15°, K = 12, N = 10. Others interpretations are also possible if the directions of two sites (SC9 and SC10) are considered to have a different magnetic field record. In that case, the PP calculated for sites Sc1-Sc7 is Lat.: 65°S; Long.: 44°E, A95 = 8°, K = 54, N = 7, and the virtual geomagnetic pole (VGP) calculated for sites Sc9 and Sc10 is Lat.: 48.5°S; Long.: 315°E, A 95 = 8°, N = 2. The last possibility to consider Sc9 and Sc10 after structural correction, and the resulting PP position is Lat.: 66.5°S; Long.: 034°E, A 95 = 8°, K = 41, N = 9. This deformation episode could be related to the San Rafaelic orogenic phase, found for the first time in La Pampa province, but previously observed in other neighboring areas during Permian time. However, more detailed palaeomagnetic sampling, rock magnetism studies, and age dating are necessary to complete the palaeomagnetic study of this area.

Palaeomagnetism and U-Pb dates of the Palaeoproterozoic Akitkan Group (South Siberia) and implications for pre-Neoproterozoic tectonics

Abstract We present new geochronological and palaeomagnetic results from the late Palaeoproterozoic Akitkan Group in South Siberia. The zircon U–Pb conventional age of the rhyodacite from the upper part of the group is 1863±9 Ma and the age of the dacite from the lower part of the group is 1878±4 Ma. Palaeomagnetic study of sedimentary and some igneous rocks from the upper part of the group isolated a high-temperature characteristic component (D=193°, I=19°, k=51, α 95 =7°) which is supported by two of three applied conglomerate tests. However, the third intra-formational conglomerate test demonstrates a contaminating overprint of uncertain nature for a part of our collection. The analysis of data suggests that this overprint occurred at time when the geomagnetic field's direction was similar to that at the time of the deposition. Therefore the corresponding palaeomagnetic pole (22.5 °S, 97.4 °E, dp=1.5°, dm=2.8°) may be considered as representative for the deposition time. Palaeomagnetic study of the sediments in the lower part of the Akitkan Group isolated a stable primary remanence (D=189°, I=8°, k=111, α 95 =5°) supported by positive intra-formational conglomerate and fold tests. The palaeomagnetic pole (30.8 °S, 98.7 °E, dp =2.5°, dm=5.0°) is nearly coeval with the 1879 Ma Molson B pole from the Superior craton. We used these two poles to compare the relative position of Siberia and the Superior craton in the late Palaeoproterozoic. It is different from their reconstruction around 1000 Ma. This demonstrates their relative movements in the Mesoproterozoic.

The palaeomagnetically viable, long-lived and all-inclusive Rodinia supercontinent reconstruction

Abstract Palaeomagnetic apparent polar wander (APW) paths from the world's cratons at 1300–700 Ma can constrain the palaeogeographic possibilities for a long-lived and all-inclusive Rodinia supercontinent. Laurentia's APW path is the most complete and forms the basis for superposition by other cratons' APW paths to identify possible durations of those cratons' inclusion in Rodinia, and also to generate reconstructions that are constrained both in latitude and longitude relative to Laurentia. Baltica reconstructs adjacent to the SE margin of Greenland, in a standard and geographically ‘upright’ position, between c . 1050 and 600 Ma. Australia reconstructs adjacent to the pre-Caspian margin of Baltica, geographically ‘inverted’ such that cratonic portions of Queensland are juxtaposed with that margin via collision at c . 1100 Ma. Arctic North America reconstructs opposite to the CONgo+São Francisco craton at its DAmaride–Lufilian margin (the ‘ANACONDA’ fit) throughout the interval 1235–755 Ma according to palaeomagnetic poles of those ages from both cratons, and the reconstruction was probably established during the c . 1600–1500 Ma collision. Kalahari lies adjacent to Mawsonland following collision at c . 1200 Ma; the Albany–Fraser orogen continues along-strike to the Sinclair‐Kwando‐Choma‐Kaloma belt of south-central Africa. India, South China and Tarim are in proximity to Western Australia as previously proposed; some of these connections are as old as Palaeoproterozoic whereas others were established at c . 1000 Ma. Siberia contains a succession of mainly sedimentary-derived palaeomagnetic poles with poor age constraints; superposition with the Keweenawan track of the Laurentian APW path produces a position adjacent to western India that could have persisted from Palaeoproterozoic time, along with North China according to its even more poorly dated palaeomagnetic poles. The Amazonia, West Africa and Rio de la Plata cratons are not well constrained by palaeomagnetic data, but they are placed in proximity to western Laurentia. Rift successions of c . 700 Ma in the North American COrdillera and BRAsiliano-Pharuside orogens indicate breakup of these ‘COBRA’ connections that existed for more than one billion years, following Palaeoproterozoic accretionary assembly. The late Neoproterozoic transition from Rodinia to Gondwanaland involved rifting events that are recorded on many cratons through the interval c . 800–700 Ma and collisions from c . 650–500 Ma. The pattern of supercontinental transition involved large-scale dextral motion by West Africa and Amazonia, and sinistral motion plus rotation by Kalahari, Australia, India and South China, in a combination of introverted and extroverted styles of motion. The Rodinia model presented here is a marked departure from standard models, which have accommodated recent discordant palaeomagnetic data either by excluding cratons from Rodinia altogether, or by decreasing duration of the supercontinental assembly. I propose that the revised model herein is the only possible long-lived solution to an all-encompassing Rodinia that viably accords with existing palaeomagnetic data.

Neoproterozoic timescales and stratigraphy

Abstract The Infracambrian is a term for mostly Neoproterozoic successions in North Africa and areas to the east. Its base lies within the middle Neoproterozoic period, or Cryogenian, includes the youngest Neoproterozoic period, or Ediacaran, and continues into the early Cambrian to the level at which trilobites first appear. The Cryogenian lacks any biostratigraphic zonation; and no global biostratigraphic schemes exist for the Ediacaran. The formal classification of the Neoproterozoic is currently under review. The Cryogenian–Ediacaran (CE) interval includes at least three prominent diamictite horizons that are clearly linked to penecontemporaneous glaciations. The oldest is ‘Sturtian’, next oldest is ‘Marinoan’ (probably the most extensive), both names are Australian in origin but are used internationally. The Gaskiers glaciation is the youngest and probably the least extensive. There are important unresolved problems of the precise number, age, extent and nomenclature of the Neoproterozic glaciations in Australia. Several palaeomagnetic poles in the age range 600–550 Ma place glacial deposits of that age range in Australia in tropical latitudes. These data, together with older poles from Laurentia, gave rise to the notion of the Snowball Earth, in which the Earth froze over completely, but the profound refrigeration required appears to have had very little effect on biological evolution. Biostratigraphic zonation with the precision attainable throughout the Phanerozoic does not appear possible for the CE interval. Thus, most correlations are based on about 40 U–Pb and Re–Os dates. These confirm the existence of at least three glacial sequences (sequence is used here as an informal term), but it is possible that the Sturtian and Marinoan were essentially one glacial unit. Deglaciation was accompanied by the unique ‘cap carbonates’. The glacial sequences all show a characteristic δ 13 C pattern, but present knowledge is inadequate to use these patterns for detailed global correlation. The most reliable chemostratigraphic correlations are likely to be based on strontium isotope variations. Black shale horizons commonly follow deglaciation. A few basins produce Neoproterozoic hydrocarbons; others are potential producers. As a whole, the Neoproterozoic represents both a scientific and an exploration frontier.

The drift history of Iran from the Ordovician to the Triassic

Abstract New Late Ordovician and Triassic palaeomagnetic data from Iran are presented. These data, in conjunction with data from the literature, provide insights on the drift history of Iran as part of Cimmeria during the Ordovician–Triassic. A robust agreement of palaeomagnetic poles of Iran and West Gondwana is observed for the Late Ordovician–earliest Carboniferous, indicating that Iran was part of Gondwana during that time. Data for the Late Permian–early Early Triassic indicate that Iran resided on subequatorial palaeolatitudes, clearly disengaged from the parental Gondwanan margin in the southern hemisphere. Since the late Early Triassic, Iran has been located in the northern hemisphere close to the Eurasian margin. This northward drift brought Iran to cover much of the Palaeotethys in approximately 35 Ma, at an average plate speed of c . 7–8 cm year −1 , and was in part coeval to the transformation of Pangaea from an Irvingian B to a Wegenerian A-type configuration.

Palaeomagnetism of the Salla Diabase Dyke, northeastern Finland, and its implication for the Baltica-Laurentia entity during the Mesoproterozoic

Abstract New palaeomagnetic and rock magnetic results are presented for the 1122±7 Ma Salla Diabase Dyke in NE Finland. A positive baked-contact test proves that the dyke has a primary natural remanent magnetization carried by magnetite. The characteristic remanent magnetization direction (D=42.2°, I=73.9°, k=75.7°, α 95 =4.8°) of 13 sites along the large single dyke provides a virtual geomagnetic pole (VGP) position of Plat=71°N, Plon=113°E (A 95 =8.1°). Although secular variation may not have been fully averaged out, the new VGP provides an important result to define the late Mesoproterozoic position of Baltica. The VGP is not close to any known Proterozoic palaeopoles of Baltica, and therefore the pre-Sveconorwegian apparent polar wander path (APWP) of Baltica must be modified. The pre-Sveconorwegian ( c . 1.3–1.0 Ga) APW swathes of Baltica, Laurentia (including the Logan Loop) and Kalahari cratons show similar shape, but new well-dated palaeomagnetic poles for c . 1.25–1.12 Ga interval from these continents are required to test the similarity. The Salla dyke VGP provides hints that the Mesoproterozoic Baltica–Laurentia connection in the Hudsonland supercontinent assembly lasted until 1.12 Ga.

Palaeomagnetism and magnetostratigraphy of the Permian and Triassic of Spitsbergen: a review of progress and challenges

Permian and Triassic sediments from Svalbard provide a testing ground for evaluating concepts about the polarity and configuration of the geomagnetic field during the Palaeozoic–Mesozoic transition. This review examines existing palaeomagnetic and magnetostratigraphic data, and also re-examines the issue of partial remagnetization of these sediments. Permian and Triassic palaeomagnetic poles from Spitsbergen demonstrate a close similarity to the stable Europe apparent polar wander path. Magnetostratigraphy from the Gipsdalen and Tempelfjorden groups demonstrates the dominance of reverse polarity (the Permo-Carboniferous Reverse Superchron), with evidence for three normal magnetozones of short duration, one of which may provide a useful marker near the base of the Permian. The other two normal magnetozones in the middle and lower parts of the Kapp Starostin Formation are most likely to represent equivalent magnetozones in the Wordian and Capitanian (mid- Permian). Magnetostratigraphy of the Lower Triassic up to the lowest parts of the Upper Triassic in central and western Spitsbergen provides greater chronostratigraphic detail than has been available hitherto from biostratigraphy alone. This demonstrates the scale of a number of suspected hiatuses, and also hints at improved chronostratigraphic correlations between east and west Spitsbergen. Upper Triassic sediments provide the best palaeomagnetic properties, indicating that future magnetostratigraphic studies in this interval may improve chronostratigraphic understanding of this poorly dated interval. A persistent partial remagnetization affecting both Permian and Triassic sediments is not of a single origin. It may be both a Brunhes-age viscous remanence and a Palaeogene heating/burial-induced magnetization, or a fluid flow related remagnetization locally in western Spitsbergen. The remagnetization is often composite (Cenozoic plus older) in nature.

Palaeomagnetic and rock magnetic study of charnockites from Tamil Nadu, India, and the ‘Ur’ protocontinent in Early Palaeoproterozoic times

Palaeomagnetic and magnetomineralogical results are reported from charnockites in basement terrane at the eastern sector of the WSW–ENE granulite belt of South India. Magnetite is the dominant ferromagnet identified by rock magnetic and optical study; it is present in several phases including large homogeneous titanomagnetites and disseminated magnetite in microfractures linked to growth stages ranging from primary charnockite formation to uplift decompression and exhumation within the interval ∼2500–2100 Ma. Several components of magnetization are resolved by thermal demagnetization and summarized by four pole positions; in the northern (Pallavaram) sector these are P1 (33°N, 99°E, d p/d m = 8/9°) and P2 (79°N, 170°E, d p/d m = 3/6°), and in the southern (Vandallur) sector they are V1 (23°N, 116°E, d p/d m = 8/9°) and V2 (26°S, 136°E, d p/d m = 5/10°). These magnetizations are linked to uplift cooling of the basement and unblocking temperature spectra suggest acquisition sequences P1 → P2 and V1 → V2 in each case implying movement of the shield from higher to lower palaeolatitudes sometime between 2500 and 2100 Ma. Palaeomagnetic poles from the cratonic nuclei of Africa, Australia and India all identify motion from higher to lower palaeolatitudes in Early Palaeoproterozoic times, and this is dated ∼2400 and ∼2200 Ma in the former two shields. The corresponding apparent polar wander (APW) segments match the magnetization record within the charnockite basement terranes of southern India to yield a preliminary reconstruction of the ‘Ur’ protocontinent, the oldest surviving continental protolith with origins prior to 3000 Ma. Although subject to later relative movements these nuclei seem to have remained in proximity until the Mesozoic break-up of Gondwana.

Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: Implications for the field variation during the Cretaceous normal superchron

An integrated palaeomagnetic and 40Ar/ 39Ar dating study was carried out on an Early Cretaceous volcanic lava sequence from the Suhongtu section, Inner Mongolia, to determine the field behavior within the Cretaceous normal superchron (CNS). 40Ar/ 39Ar ages were obtained from 12 lava flows, indicating that the studied lava was formed around 114.1 ± 0.3 Ma for the lower interval and 110.6 ± 0.1 Ma for the upper interval. Rock-magnetic experiments and electron microprobe analyses indicate that the primary Fe–Ti oxides are the main magnetic carriers. All lava flows carry normal palaeomagnetic directions, which can be grouped into 31 units by an F-test, with a Fisher mean characteristic remanent magnetization (ChRM) of D/ I = 12.8/58.6° ( α 95 = 2.3°). The corresponding palaeomagnetic pole is located at 80.3°N and 200.3°E ( A 95 = 3.2°, K = 64.4), which is indistinguishable at the 95% confidence level from the Eurasia pole derived from the apparent polar wander path for the early Cretaceous. Using a modified Thellier palaeointensity method with stringent acceptance criteria, we obtained two time-series of palaeointensity records from 15 independent palaeomagnetic units (total 136 samples). The virtual dipole moment (VDM) values varied from 2.53 × 10 22 Am 2 to 9.92 × 10 22 Am 2. The mean VDMs for the upper and lower intervals are (5.38 ± 2.06) × 10 22 Am 2 and (4.61 ± 2.67) × 10 22 Am 2, respectively. The observed time-series of palaeointensity, together with the previously available data for the CNS, suggest that magnetic field strength during the CNS fluctuated significantly with time.

Widespread remagnetization of late Proterozoic sedimentary units of Uruguay and the apparent polar wander path for the Rio de La Plata craton

A palaeomagnetic study was performed on three Neoproterozoic sedimentary successions exposed in central and eastern Uruguay in order to better constrain the palaeogeographic evolution of the Río de la Plata craton by the latest Proterozoic. These units comprise the latest Ediacaran to Early Cambrian calcareous Cerro Victoria Fm., exposed in the central Nico Pérez terrane, the late Ediacaran clastic Yerbal Fm. and the calcareous Polanco Fm., exposed in the Dom Feliciano belt, and the Ediacaran clastic Rocha Fm. exposed in the easternmost Punta del Este suspect terrane. The study showed that none of the units carry the original remanence and that they were affected by remagnetizations. The most widespread remagnetization is shown by the Cerro Victoria, Yerbal and Rocha Formations. A dual-polarity post-folding remanence, carried by hematite, was isolated in all these units. Mean directions from them are CV (Cerro Victoria): Dec: 179.1°, Inc: 59.0°, α95: 2.9°, n = 79 samples; Y (Yerbal): Dec: 182.4°, Inc: 61.9°, α 95: 4.5°, n = 38 samples; R (Rocha): Dec: 4.2°, Inc: −64.9°, α 95: 2.7°, n = 42 samples. Their corresponding pole positions are virtually identical (CV: 82.6°S, 309.3°E, A95: 3.9°; Y: 77.0°S, 298.4°E, A95: 5.9°; R: 76.6°S, 291.0°E, A95: 4.2°) suggesting the same magnetization age and process. Comparison with the South American path suggests the Late Permian–Early Triassic or the latest Cretaceous–Palaeogene as the most likely times for its acquisition. These poles are also identical to the pole of the Late Proterozoic La Tinta Fm., which is interpreted as affected by the same remagnetization, and therefore, invalid for palaeogeographic reconstructions. A different magnetic component, also dual polarity and post-folding, was defined at two sites of the Cerro Victoria Fm. and other two of the Polanco Fm., possibly carried by magnetite. The mean geomagnetic poles from both units (CVc: 4.2°N, 343.2°E, A95: 13.8° and P: 3.2°N, 325.8°E, A95: 15.2°) fall on the Middle Cambrian to Early Ordovician segment of the Gondwana APWP, suggesting that age for the remagnetization. This permits to establish the age of the tectonic event affecting the Neoproterozoic units of the Río de la Plata craton as Early Cambrian (ca. 525 Ma.). The available palaeomagnetic poles for this craton permit to constrain its palaeogeographic evolution only since approximately 600 Ma.

Palaeomagnetism and the age of the Cracow volcanic rocks (S Poland)

SUMMARY Permian rhyodacites, melaphyres and tuffs from the Cracow area (South Poland) were sampled for the palaeomagnetic and isotope studies. Single-grain U-Pb dating of most zircon grains separated from the rhyodacites gave mean age of magma emplacement of 294.2 ± 2.1 Ma. Some zircons, however, displayed younger ages (268.7 ± 3.4 Ma), probably related to the metasomatic alterations of these rocks. Two Permian components of magnetizations related to these processes were isolated and together with previously defined Late Carboniferous‐ Permian palaeomagnetic poles from South Poland were used for construction of the regional apparent polar wander path (APWP). The Early Permian segment of this APWP shows a certain departure from the coeval part of the Fennoscandian APWP due to anticlockwise rotations of studied rocks most probably caused by mid-Permian sinistral tectonic movements along reactivated prominent Variscan faults of Central Europe. This sense of tectonic mobility does not support the hypothesis about transformation from Pangea ‘B’ to Pangea ‘A’ along an intra-Pangea dextral megashear during the Permian. Older than previously assumed ages of the post-Variscan igneous rocks of Central Europe reduce overlap of Gondwana’s and Laurussia’s parts of the Early Permian Pangea ‘A’.