Palaeomagnetic Pole Position Research Articles

Palinspastic restoration of Variscan oroclines – implications for dextral transpression and terrane affinities

Abstract The Paleozoic Variscan orogen in Europe has a markedly circuitous trace for which several different origins have been postulated, including deformation around promontories on the colliding continental margins, extrusion within the collision zone, folding of a ribbon continent and collision with a substantial dextral component. Adopting the latter assumption, unfolding of the large, steeply plunging folds (‘oroclines’) of Iberia and the Moroccan Meseta requires more than 4000 km of dextral lateral translation of Laurussia with respect to Gondwana. Constraints on the age of the folding require that this lateral translation occurred in mid-late Carboniferous time. Significant dextral translation of Laurussia with respect to Gondwana late in the Variscan collision is supported by palaeomagnetic data for the two supercontinents, although the exact timing of this relative motion is not well constrained due to large uncertainties in the palaeomagnetic pole positions. Our palinspastic reconstruction of the major Variscan folds of Iberia places the rocks of the South Portuguese Zone of western Iberia adjacent to the Rhenish Massif; for instance, as part of Avalonia. By the same token, the Sehoul Block in the Moroccan Meseta probably originated at the western end of Cadomia, although nothing in our analysis precludes it also being derived from Avalonia.

Late Miocene to late Pleistocene geomagnetic secular variation at high northern latitudes

SUMMARYWe report a palaeomagnetic study of Icelandic lavas of late Miocene to late Pliocene age to test the geocentric axial dipole hypothesis at high northern latitudes. Cores were sampled from 125 sites in the Fljótsdalur valley in eastern Iceland, and hand samples were taken for 17 new incremental heating 40Ar/39Ar age determinations. 96 per cent of the cores were oriented using both a Brunton compass and a sun compass. Comparison of the magnetic and sun azimuths reveals deviations of ±5°, ±10° and ±20°, respectively, for 42, 16 and 3 per cent of the data points, indicating that core sampling intended for palaeosecular variation (PSV) studies at high northern latitudes should be oriented by sun. A total of 1279 independent specimens were subjected to AF- and thermal-demagnetization for palaeodirectional analysis, and well-grouped site mean directions were obtained for 123 sites of which 113 were found to be independent sites. Applying a selection criteria of k > 50 and N ≥ 5 (Nmean = 9.5), we obtain a combined grand mean direction for 46 normal and 53 reverse (for VGPlat > ±45°) polarity sites of declination = 5.6° and inclination = 77.5° that is not significantly different from that expected from a GAD field. The corresponding palaeomagnetic pole position (VGPlat = 86.3°N, VGPlon = 21.2°E, dp/dm = 4.0°/4.3°) is coincident with the North Pole within the 95 per cent confidence limits. An updated age model is constructed based on the 40Ar/39Ar ages, showing that the majority of the Fljótsdalur lavas fall within 2–7 Ma. We combine the Fljótsdalur data with existing data from the nearby Jökuldalur valley. The 154 palaeodirections are well-dispersed between 1 and 7 Ma and constitute a high-quality data set for PSV analysis. Our results partly support previous conclusions of a generally higher dispersion during reverse polarity intervals. However, when comparing our Matutayma data with Brunhes age data from Jan Mayen, we find no evidence of a higher VGP scatter during the Matuyama as previously suggested. When comparing our VGP scatter to the two commonly used models for VGP dispersion: Model G and TK03, we find a good fit for all 1–7 Ma VGP scatter data SB(1–7) to Model G, whereas SB(1–7) is not fitted by TK03, even when considering the uncertainty of SB(1–7). We also find that all VGP scatter estimates, except that for the Gilbert subset, are consistent with Model G, while the discrepancy with TK03 is generally larger.

Reorientation of the early lunar pole

An active core dynamo may have operated on the early Moon. Extraction of palaeomagnetic pole positions on the Moon from magnetic anomalies measured by the Lunar Prospector and Kaguya orbiters suggests that the ancient lunar dynamo experienced reversals and an ancient reorientation of the Moon rotated the geographic locations of the poles.

Palaeomagnetic Study of Dated Pre-Cambrian Rocks of the Front Range, Colorado-Wyoming

Summary The mean direction of remanent magnetization, after partial a. c. demagnetization, of Pre-Cambrian rocks from the Front Range of Colorado and Wyoming gives a palaeomagnetic pole position at 8°N, 29°E. Rock types sampled include andesite dykes, diorite inclusions in granite, and a highly oxidized red granite, all with a radiogenic age of 1.41 G. Y. This preliminary study suggests there are at least two and probably as many as four field reversals recorded in these rocks, and that remanent directions from andesite dykes may be useful in determining tectonic rotation of the Front Range.

New time constraints on lithospheric-scale oroclinal bending of the Ibero-Armorican Arc: a palaeomagnetic study of earliest Permian rocks from Iberia

Abstract: The Palaeozoic Variscan orogeny was a large-scale collisional event that involved amalgamation of multiple continents and microcontinents. Previous palaeomagnetic and structural analyses of the western Variscan orogen, notably the Ibero-Armorican Arc, suggest that this region underwent oroclinal bending of an originally near-linear orogen during the latest stages of Variscan deformation in the late Palaeozoic. These analyses necessitate a two-stage tectonic history with east–west compression forming a linear belt in the Carboniferous, followed by a change in regional compression directions resulting in crustal block rotation and formation of the present-day arc. This study presents new palaeomagnetic analyses of Early Permian samples from the Sotres and Cabranes formations (41.3°N, 222.5°W, α 95 = 13.1°) in the northern inner limb and the Viar Basin (51.8°N, 199.2°W, α 95 = 6.7°) in the southern outer limb of the arc. Both locations yield expected Early Permian palaeomagnetic pole positions for stable Iberia, implying that measured magnetizations have undergone no significant vertical-axis rotation since acquisition, and thus were acquired subsequent to oroclinal bending. This new result places a time constraint of about 10 Ma for oroclinal bending of the Ibero-Armorican Arc, which agrees well with recent geodynamical models that relate oroclinal bending to lithospheric delamination in this region.

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.

Early Miocene magnetostratigraphy and a new palaeomagnetic pole position from New Zealand

Abstract We report palaeomagnetic results from a 26.5 m sequence of early Miocene sediments in NW Nelson, New Zealand. Analysis of the strong, stable characteristic component of natural remanent magnetization has yielded an important record of the part of Chron 5 from ca. 18.5 to 16.5 Ma, including positive identification of the cryptochron or “tiny wiggle” C5Dr-1, and a pole position ([{«mbda _k} = {78.4^ circ }]; [{☎i _p} = {283.0^ circ }]; dp = 2.2°; dm = 2.8°) for NW Nelson that is indistinguishable from the contemporaneous published pole for the Australian Plate. We infer that this portion of NW Nelson has undergone negligible rotation with respect to the main part of the Australian Plate over the past 17.5 Myr.

Tectonic rotations in the Late Palaeozoic continental margin of southern South America determined and dated by palaeomagnetism

SUMMARY Detailed palaeomagnetic studies of Late Palaeozoic rocks exposed in the UspallataCalingasta Valley region, located at about 32s 69.5W in the Argentine Andean Chain, have been recently accomplished. Previous results are discussed in Valencio & Vilas (1985) and Rapalini et al. (1989). The most recent studies were carried out mainly on the thick sequences of late Early Permian to Late Permian rhyolites and ignimbrites assigned to the Tambillos and Horcajo Formations. Both formations yielded palaeomagnetic pole positions (P15: 319.6E, 78.9S7 N = 16, k = 33.5, AgS = 6.5 and P17: 264.8E, 72.4S, N = 26, k = 6.4, Ag5 = 12, respectively) concordant with the Late-Palaeozoic path of South America. Preliminary palaeomagnetic results from the Middle Carboniferous Hoyada Verde Formation, exposed in the same region, suggest that these rocks carry a synfolding magnetization acquired during the Late Carboniferous, as the palaeomagnetic pole position computed from partially corrected remanence directions (C6: 356.2E, 41.9S, D1 = 8.3, 02 = 6.0) agrees with the Late Carboniferous poles from cratonic areas of South America. This concordant position of C6 definitely rules out the possibility of a Late Palaeozoic allochthony of this section of the Argentine Andean Chain. The discordant positions previously found in Late Carboniferous and early Early Permian rocks of this region are interpreted as caused by large clockwise crustal block rotations that occurred not later than the late Early Permian. A tectonic model is proposed to explain these rotations which suggests that they were associated with strike-slip displacements parallel to the western continental margin of South America caused by an oblique subduction of the Proto-Pacific Plate during the Early Permian. Many tectonic and geologic observations fit this model. The concordant position of C6 also suggests that a complex pattern of crustal block rotations should have developed in this continental margin. Other palaeomagnetic data suggest that similar rotations may have taken place in other areas of the Argentine Andean Chain in the Early Permian.

Palaeomagnetism and K-Ar Ages of the South-west African Basalts and their Bearing on the Time of Initial Rifting of the South Atlantic Ocean

Palaeomagnetic and isotopic results from the Kaoko lavas, Hoachanas basalts and dolerite sills of South-West Africa indicate that the Upper Triassic-Lower Jurassic Stormberg flows of South Africa may have extended into SW-Africa and that younger igneous events of Lower Cretaceous age were simultaneous with the Serra Geral volcanism in Brazil. Five analyses on three samples of the Keetmanshoop sills gave K-Ar ages between 178 ± 4 and 199 ± 4 Ma, four analyses of two samples of the Hoachanas basalts gave ages between 161 ± 3 and 173 ± 2 Ma and eight analyses of five samples of Kaoko basalt gave ages between 110±4 and 128 ± 2 Ma. The components of remanent magnetization (RM) used to compute palaeomagnetic pole positions for the Kaoko lavas (48° N, 93° W, A95 = 3°) and for the Hoachanas basalts (61° N, 106° W, A95 = 7° are stable to alternating field (AF) and thermal demagnetization. Correlation on a pre-drift map and on a map reconstructed for 112 Ma BP (before present) between the palaeomagnetic poles from the Kaoko and Serra Geral lavas suggests that the South Atlantic had not opened appreciably by 112 Ma BP. Cretaceous pole positions for S. America and Africa on a map reconstructed for 80 Ma BP are also discussed.

Palaeomagnetism of Devonian ring complexes from the Bayuda Desert, Sudan-new constraints on the apparent polar wander path for Gondwanaland

SUMMARY The approach of Gondwana, and/or microplates derived from its northern margins, towards Laurentia and the closure of the intervening Palaeo-Tethys is a controlling factor in the geodynamic evolution of the Atlantic bordering continents during the early and mid-Palaeozoic. However the currently available palaeomagnetic data for Gondwana, especially for the geodynamically critical Silurian to Devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios. In order to address this issue 114 samples (18 sites) have been collected from a series of high level acidic intrusive rocks, rhyolitic lavas and tuffs of mid-Devonian age (377 ± 5 Ma, Rb/Sr) along the southern escarpment of the Gilif Hills, Sudan (17.83°N, 32.67°E). Detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization, labelled A, B and C. The overall mean direction of component A (Dec. 0.9°, Inc. 29.8°, α95= 4.6°, k= 91.1, N= 12 sites) is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern Sudan. Mixed polarities suggest a Recent or Pleistocene age of this component. Component B also displays mixed polarities. The site mean directions for this component average to 6.5° declination, -40.2° inclination with α95= 7.0° and k= 48.1 (based on 10 sites) and yield a palaeomagnetic pole at 48.8°S, 23.5°E (dp= 5.1°1, dm= 8.4°). Maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component A and/or B. Both A and B are the probable results of low-temperature oxidation of magnetite. Although the age of component B can not be directly determined, it is inferred by correlation with published data to reflect a late Carboniferous overprint. Component C, identified in 11 sites (39 samples), is characterized by a very steep downward magnetization (Dec. 296.2°, Inc. 79.3°, α95= 10.8°, k= 18.7, n= 11 sites) which corresponds to a palaeomagnetic pole position in the southwestern corner of Libya (25.9°N, 11.6°E, dp= 19.6°, dm= 20.6°). Maximum blocking temperatures of component C, generally below 580°C, are indicative of magnetite as carrier of this remanence. The results of thermal remanent magnetization experiments demonstrate that component C is a thermo-remanent magnetization, probably primary, and suggest this component to be representative for the direction of the Earth's magnetic field during emplacement of the Gilif Hills volcanic rocks in mid-Devonian times. The position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide (approx. 5000 km) Pre-Hercynian ocean during Devonian times. The simplest reconciliation of component B with the reference apparent polar wander path is achieved by assuming it to be of Carboniferous age and by invoking a westward cusp in the Carboniferous part of the apparent polar wander path. Taken at face value, the age of this cusp coincides with large-scale post-orogenic right lateral strike slip movements in Hercynian Europe and North Africa. This interpretation does not require the apparent polar wander (APW) loop and very high rates of plate motion implied by some alternative models. However, if it is valid it will require reinterpretation of the remanence-age of one West African intrusive suite and of some eastern Australian Palaeozoic rocks.

A palaeomagnetic study of the lower part of the Palaeoproterozoic Waterberg Group, South Africa

Pole positions from previous palaeomagnetic work in the late Palaeoproterozoic Waterberg Group, South Africa [Jones, D.L., McElhinny, M.W., 1967. Stratigraphic interpretation of paleomagnetic measurements on the Waterberg Red Beds of South Africa. Journal of Geophysical Research 72, 4171–4179] seemed to indicate that Waterberg Group sedimentation commenced during emplacement of the ca. 2.06 Ga Bushveld Complex, and continued intermittently through numerous tectonic events affecting the preserved Transvaal Basin to just before the ca. 1.1 Ga Umkondo thermal event of southern Africa. However, from these studies no consistent directions could be determined. Instead, a pattern was identified and interpreted in terms of apparent polar wander during the deposition and consolidation of the Waterberg sediments within South Africa and Botswana. The Swaershoek Formation, the basal unit of the Waterberg Group in the Nylstroom Protobasin, has been tentatively correlated with the Wilge River Formation, which is the only unit in the Middelburg basin. A new palaeomagnetic study on the Swaershoek and Wilge River Formations is reported here in an attempt to re-determine the palaeomagnetic pole positions for these two formations and to confirm their correlation. A total of 49 sites across both basins were sampled, both within the sedimentary succession and in the associated diabase intrusions. The calculated anti-poles for the Swaershoek Formation (18.9°N, 7.7°E, A 95 = 21.9°) and the Wilge River Formation (16.9°N, 0.2°E, A 95 = 22.4°) cannot be distinguished at the 95% level of confidence, although scattering is high. This either confirms the geological correlation or indicates simultaneous magnetic overprinting. The results from the two basins are provisionally combined to present a mean pole position for the lower-Waterberg Group. The mean anti-pole for the lower-Waterberg Group is 17.9°N, 3.9°E, A 95 = 16.2°. The diabase in both basins was sampled to test for thermal overprinting of the magnetic direction of the sedimentary rocks. Although the calculated anti-pole for the diabase in the Nylstroom Protobasin (63.3°N, 53.2°E and A 95 = 35.7°) is poorly determined, its circle of 95% confidence includes the confidence region of the diabase in the Middelburg basin (anti-pole 69.3°N, 28.5°E and A 95 = 14.2°). These two poles cannot be distinguished from the results of previous studies on post-Waterberg diabase [Jones, D.L., McElhinny, M.W., 1966. Paleomagnetic correlation of basic intrusions in the Precambrian of Southern Africa. Journal of Geophysical Research 71, 543–552] nor from results of the Umkondo diabase [McElhinny, M.W., Opdyke, N.D., 1964. The palaeomagnetism of the Precambrian dolerites of eastern South Rhodesia. An example of geologic correlation by rock magnetism. Journal of Geophysical Research 69, 2465–2475]. The pole positions from the Waterberg sediment and associated diabase are sufficiently displaced from each other to rule out any overprinting by these intrusions. It does not, however, rule out magnetic overprinting at some earlier or later age. The calculated mean pole position for the lower-Waterberg Group is tentatively interpreted to represent a direction of magnetic overprinting due to the ∼1.88 Ga Palaeoproterozoic intraplate magmatism associated with the Eburnean Event, which means that the cumulative of palaeomagnetic data can no longer connect the onset of Waterberg sedimentation with the emplacement of the Bushveld Complex.

A synthesis of Cretaceous palaeomagnetic data from South Korea: tectonic implications in East Asia

SUMMARY Tectonic provinces in South Korea include the Precambrian Gyeonggi Massif, Palaeozoic Okcheon Belt, Precambrian Yeongnam Massif and Cretaceous Gyeongsang Basin, from northwest to southeast. The Cretaceous strata mainly occur in the Gyeongsang Basin as well as in several small basins along the boundaries of the Okcheon Belt and on the Gyeonggi Massif. This paper reviews and analyses previously published palaeomagnetic data from the Cretaceous successions in South Korea. A total of 23 Cretaceous palaeomagnetic poles, from 18 studies, satisfy more than four standard reliability criteria. The palaeomagnetic pole positions, from the Gyeongsang Basin and from small basins in the Gyeonggi Massif and the Okcheon Belt, are consistent with one another, indicating that the Korean Peninsula has been a single terrane since the Cretaceous. However, based on the comparison of the palaeomagnetic pole positions and the palaeolatitudes of blocks in the Gyeongsang Basin, it was observed that the Gyeongsang Basin had experienced tectonic adjustments during the Cretaceous Period. Within the Gyeongsang Basin, the geographically northern area (Yeongyang block) was rotated counterclockwise by 16.3°± 4.6° with respect to the southern area (Milyang and Uiseong blocks), based on the comparison of palaeomagnetic pole positions between the two areas. This palaeomagnetic result and some geological features in the Gyeongsang Basin collectively indicate that the Yeongyang block underwent counterclockwise rotations, accompanied by northwestward protrusion, into the Yeongnam Massif during the Late Cretaceous. These relative tectonic movements within the Gyeongsang Basin were probably due to the northwestward subduction of the proto-Pacific oceanic plate during the Late Cretaceous. The gradual eastward displacement of the Korea's Cretaceous palaeopoles, from the Eurasian pole, increases with stratigraphic age, indicating that the Korean Peninsula underwent progressive clockwise rotations, with respect to Eurasia, during the Cretaceous Period. The clockwise rotation of Southwest Japan, with respect to Eurasia, is ascribed to the Miocene opening of the East Sea, and has no connection with the Cretaceous clockwise rotations of the Korean Peninsula, indicating that the Korean Peninsula and Southwest Japan may have been independent terranes during the Cretaceous Period.

Magnetostratigraphy of Tertiary sediments from the Hoh Xil Basin: implications for the Cenozoic tectonic history of the Tibetan Plateau

SUMMARY We conducted an integrated palaeomagnetic and stratigraphic study on a 5452.8 m thick sedimentary sequence of the Hoh Xil Basin in northern Qinghai-Tibet Plateau to obtain a chronostratigraphic framework for these sediments. A total of 966 individual oriented palaeomagnetic samples (spaced at stratigraphic intervals) were collected from six measured sections in the Hoh Xil Basin. Magnetic directions in these samples were obtained by progressive thermal (mainly) and alternating-field demagnetization experiments. Most samples exhibit two components of magnetization. The lower unblocking temperature component is an overprint resembling the present-day geocentric axial dipole field direction at the sampling locality. The most stable, characteristic remanence (ChRM) appears to be an early chemical remanent magnetization residing mainly in haematite. The positive results of fold and reversal tests indicate that the ChRM is a record of the palaeomagnetic field close to the time of formation of these sediments. Further evidence for the magnetization of these sediments acquired close to their time of deposition is the fact that patterns of magnetic reversals can be matched with the established polarity timescale. On the basis of the distinct interval of magnetic reversal zones and biostratigraphic datums, 13 magnetozones can be recognized at the Hoh Xil Basin that range from chrons C11 to C23 (30.1‐51.0 Ma). The age of the Fenghuoshan Group is palaeomagnetically dated as 51‐31 Ma (Early Eocene‐Middle Early Oligocene), and the age of the Yaxicuo Group is between 31 and 30 Ma (Middle Early Oligocene‐Late Early Oligocene). The new palaeomagnetic data from the Fenghuoshan Group suggest that it has undergone no significant rotation since the Oligocene. In contrast, declination data from the Yaxicuo Group in Wudaoliang area imply a vertical-axis clockwise rotation (29.1 ◦ ± 8.5 ◦ ) since the Late Oligocene. The Tertiary palaeomagnetic pole position of the Hoh Xil Basin implies a significant northward convergence of the Hoh Xil Basin (∼1600 km) with respect to Eurasia (Siberia) since Early Eocene‐Late Oligocene time. Our results are consistent with the pattern of disturbingly low palaeolatitudes derived from a large number of high-quality palaeomagnetic studies of Tertiary rocks from sites that reach all the way from eastern China to Kyrgyzstan. Future work is needed to separate the influences of sedimentary inclination shallowing and tectonic shortening.

Chemical remagnetization of the Upper Carboniferous-Lower Triassic Pyeongan Supergroup in the Jeongseon area, Korea: fluid migration through the Ogcheon Fold Belt

SUMMARY Palaeomagnetic and rock magnetic investigations have been carried out for the Upper Carboniferous‐Lower Triassic Pyeongan Supergroup, exposed in the Jeongseon area in eastern South Korea. A total of 302 independently oriented core samples were drilled from 24 sites for the study. The mean direction in stratigraphic coordinates (D/I = 345.3 ◦ /38.4 ◦ , k = 4.8, α95 = 18.2 ◦ ) is more dispersed than the mean direction in geographic coordinates (D/I = 354.9 ◦ /61.5 ◦ , k = 58.0, α95 = 4.7 ◦ ), and the stepwise untilting of the characteristic remanent magnetization (ChRM) reveals a maximum value of k at 0 per cent untilting. Furthermore, authigenic magnetic mineral grains accompanied with altered clays are identified by electron microscope observations. These results collectively imply that the ChRMs of this study were acquired by the formation of authigenic magnetic minerals after tilting of the strata. The palaeomagnetic pole position (97.3 ◦ E, 82.4 ◦ N, A95 = 6.8 ◦ ) of the Pyeongan Supergroup calculated from the site mean directions of the ChRMs in geographic coordinates is close to those of the Late Cretaceous and Tertiary periods of the Korean Peninsula. The presence of many hydrothermal vein deposits, reportedly formed between 94 and 73 Ma, near the study area and the evidence for extensive alteration of clay minerals under the influence of fluids shown in the X-ray diffraction study strongly suggest that the chemical remanent magnetization was acquired by a fluid-mediated process during the Late Cretaceous‐Early Tertiary. Considering the different remagnetization timing of several areas in the Ogcheon Fold Belt, which extends from NE to SW diagonally across the Korean Peninsula, it is interpreted that the fluids and magmatism associated with this remagnetization originated from the subduction of the Kula/Pacific plates under the Eurasian Plate during the Cretaceous period. The fluids might have migrated northward or northeastward through the fault systems within the Ogcheon Fold Belt.

Palaeomagnetic and rock-magnetic studies of Cretaceous rocks in the Gongju Basin, Korea: implication of clockwise rotation

Palaeomagnetic and rock-magnetic studies have been carried out for Cretaceous non-marine sedimentary rocks (Gongju Group) and volcanic rocks in the Gongju Basin, located along the northern boundary of the Ogcheon Belt, Korea. K-Ar age dating for the volcanic rocks was also performed. It is found that the Gongju Group was remagnetised during the tilting of the strata with the characteristic remanent magnetisation (ChRM) direction of D/I = 23.9°/50.6° (k = 95.5, α 9 5 = 3.9 ) at 30 per cent untilting of the strata with a maximum value of precision parameter (k), while the volcanic rocks are revealed to acquire primary remanence with the direction of D/I = 204.2°/-43.8° (k = 36.6, α 9 5 = 8.6°) after the tilt-correction. The K-Ar ages of the volcanic rocks range from 81.8 ′ 2.4 to 73.5 ′ 2.2 Ma, corresponding to the Campanian stage of the Late Cretaceous. Electron microscope observations of samples from the Gongju Group show authigenic iron-oxide minerals of various sizes distributed along the cleavage of chlorite and in the pore spaces, indicating that the strata acquired the chemical remanent magnetisation due to the formation of secondary magnetic minerals under the influence of fluids. The palaeomagnetic pole positions are at Lat./Long. = 69.6°N/224.3°E (dp = 3.5°, dm = 5.2°) calculated for the 30 per cent tilt-corrected direction of the Gongju Group and at Lat./Long. = 67.2°N/235.3°E (A 9 5 = 8.9°) for the volcanic rocks. Based on the results of this study, it is interpreted that the volcanic rocks acquired the primary magnetisation almost at the same time as the remagnetisation of the Gongju Group in the Late Cretaceous. Comparisons of Cretaceous palaeomagnetic poles from the Korean Peninsula with those from Eurasia implies that the Korean Peninsula underwent clockwise rotation of 21.2° ′ 5.3° for the middle Early Cretaceous, 12.6° ′ 5.4° for the late Early Cretaceous, and 7.1° ′ 9.8° for the Late Cretaceous with respect to Eurasia, due to the sinistral motion of the Tan-Lu Fault.

A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstruction

A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China’s largest dyke swarm, which trends for about 1000 km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U–Pb dating of primary zircon, is 1769.1 ± 2.5 Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D = 36°, I = − 5°, k = 63, α95 = 4°, yielding a palaeomagnetic pole at Plat=36°N, Plong=247°E, dp = 2°, dm = 4° and a palaeolatitude of 2.6°S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800 Ma

Palaeomagnetic constraints on the age of the Red Arch Formation and associated sandstone dykes (Northern Ireland)

The Red Arch Formation is a continental red bed succession of uncertain age exposed on the northeast coast of Ireland. There are lithological similarities with the Old Red Sandstone of the Highland Border region, although unconformities within the succession, the presence of reworked aeolian sand grains and variations in the induration of the rocks have prompted earlier workers to suggest that the upper part of the succession may be Triassic in age. This paper reports palaeomagnetic results from the Red Arch Formation. The characteristic magnetizations (ChRMs) are exclusively of reversed polarity and the palaeomagnetic pole position for the sequence bears close comparison with other results from the Old Red Sandstone in Scotland and southern Britain. These poles are regarded as magnetic overprints formed during the Kiaman Superchron because they fall in the Permo‐Carboniferous segment of the apparent polar wander path. Sandstone dykes that cut the Red Arch Formation on a NNE–SSW and NNW–SSE trend have also been studied palaeomagnetically. The dykes show more steeply inclined ChRMs predominantly of normal polarity. The dykes are therefore interpreted to be post‐Kiaman in age, a conclusion consistent with our structural observations. The age of the dykes cannot be determined precisely because they have also been, at least partially, remagnetized. Lithological and regional geological considerations suggest the dykes are Triassic in age and formed in an extensional stress regime prior to the opening of the North Atlantic. They were remagnetized in Tertiary times, possibly associated with uplift and emplacement of the British Tertiary Igneous Province.

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E (dp/dm=5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event. An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age (c. 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician—Cambrian—Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula (c. 580 Ma) towards our new Arenig pole (c. 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement. The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.

Palaeomagnetism and aeromagnetic modelling of the Mesoproterozoic Ntimbankulu pluton, Kwazulu-Natal, South Africa: mushroom-shaped diapir?

The aeromagnetic signature of the Mesoproterozoic Ntimbankulu porphyritic granite-charnockite pluton, southern KwaZulu-Natal, is conspicuously circular. Surface geological mapping of the area broadly confirms such a geometry: the pluton is expressed topographically as an annulur ring of marginal hills, composed of porphyritic granite, surrounding a low-lying central core underlain by charnockite. The pluton forms one of the least deformed of ten major granite-charnockite intrusions known collectively as the Oribi Gorge Suite. The suite, which has been dated at ∼1050 Ma, intrudes the Mzumbe and Margate Terranes of the ∼1.1 Ga Natal Metamorphic Province. A palaeomagnetic study was performed on samples from the pluton and the surrounding country rock gneisses. The measured directions of magnetisation of the different sites were combined to calculate the virtual palaeomagnetic pole position of the pluton, which was determined at 27°N and 327°E. This is consistent with previously published poles from the Namaqua-Natal Metamorphic Belt. The magnetic parameters derived during the palaeomagnetic study were integrated into the aeromagnetic data to model three selected magnetic profiles across the pluton. The aim was to test the hypothesis that the pluton has a mushroom-shaped 3D form with remnants of Mapumulo Group gneiss situated underneath, and dipping towards, the pluton. It is concluded that these remnants of gneiss with their negative inclination are responsible for the ring dyke like signature of the aeromagnetic data.

Palaeomagnetism of the ejecta-bearing Bunyeroo Formation, late Neoproterozoic, Adelaide fold belt, and the age of the Acraman impact

A new palaeomagnetic study has been conducted on haematitic shales and siltstones of the late Neoproterozoic Bunyeroo Formation in the Adelaide fold belt (Geosyncline), South Australia, which host an extensive horizon of shock-deformed rock fragments and microtektite-like material of probable impact origin. Thermal step demagnetisation of 116 samples of red shale and siltstone from six sections (sites) revealed a high-temperature component with a bedding-corrected site-mean direction of remanence of D = 56.6°, I = 29.3° (α95 = 10.7°) that gives a pole at 18.1°S, 16.3°E (dp = 6.5°, dm = 11.8°). The high-temperature component provides a positive tectonic-fold test (99% level of confidence). The Bunyeroo high-temperature remanence direction is near the remanence direction (D = 50°, I = 40°) indicated by modelling the subsurface magnetic source of the central high-amplitude anomaly at Acraman, Australia's largest confirmed meteorite impact structure 220–350 km west of the Adelaide fold belt, and also is close to the mean direction (D = 48.3°, I = 54.7°, α95 = 5.2°) determined for surface melt rock from Acraman. Statistical tests show that the virtual geomagnetic poles indicated by the directions for the subsurface central magnetic source and surface melt rock at Acraman may be regarded as subsets of the Bunyeroo palaeomagnetic pole position, indicating that the three pole positions are statistically indistinguishable. The results imply that the subsurface magnetic source and surface melt rock acquired their remanence in the ambient geomagnetic field during cooling, after the impact when structural disturbance had ceased, while the Bunyeroo Formation was accumulating. The agreement among the various remanence directions argues strongly that the ejecta horizon in the Bunyeroo Formation was derived from Acraman.The present findings confirm that the Acraman impact occurred in the late Neoproterozoic, about 590 Ma, which is the age of the Bunyeroo Formation provided by RbSr whole-rock shale dating of equivalent and contiguous strata in the Adelaide fold belt region. The Bunyeroo palaeomagnetic data give a palaeolatitude of ∼ 15°, indicating a low palaeolatitude for the Acraman impact and supporting other findings that the Adelaide fold belt occupied low to equatorial palaeolatitudes during the late Neoproterozoic.