Palaeomagnetic Study Research Articles

Late Cambrian palaeomagnetic data from the Cupala Creek Formation, western New South Wales, Australia

The Tasmanides of south-eastern Australia comprises the Delamerian Orogen, considered to be stable relative to the craton of Gondwana since the mid Cambrian, despite the presence of the Grasmere Knee Zone, a change of structural trends in the Broken Hill area. A palaeomagnetic study has been carried out on the Late Cambrian red sandstones of the Cupala Creek Formation, a post-Delamerian formation gently folded in the latest Ordovician–Early Silurian and/or Early Devonian. Anisotropy of magnetic susceptibility (AMS) measurements show that internal strain is negligible and low Königsberger ratios indicate that lightning effects can be discarded. The magnetisation observed is complex with four components, which are sometimes difficult to isolate clearly. The orientation of the first component corresponds to the present-day field and appears to be associated with weathering effects. The second component (~185°−350°C) is of reverse polarity relative to the third (350°−575°C). These two components are interpreted to represent a chemical remagnetisation, which lasted long enough to record a reversal as the reversal test is positive and classified C. The overprint must be Early Palaeozoic in age. The last component is carried by haematite and/or maghaemite and is believed to be primary, which is supported by the positive ‘pseudo-unconformity’ test with the underlying Early–Middle Cambrian Teltawongee beds, and by the good correspondence of the pole [PLong. 351.3°/PLat.+33.9° (dp = 3.6°; dm = 6.0°); African coordinates] with other poles of the same age for Gondwana. Despite the presence of the Grasmere Knee Zone, these results imply that this area of the Delamerian Orogen did not record any movement or rotation since the Late Cambrian and can be considered as part of the craton of Gondwana.

Neogene rotations in the Jiuquan Basin, Hexi Corridor, China

Abstract Vertical-axis rotations of blocks in/around the Tibetan Plateau can be attributed to the India–Asia collision. Study of the vertical-axis rotations of these blocks will increase our understanding of the mechanisms and kinematics of continent–continent collisions. We report here a new palaeomagnetic study of rotations using data from four localities (five magnetostratigraphy sections) in the Jiuquan Basin. Our study indicates that the mean declinations of each section are different from each other, similar to what has been observed in the other localities in the NE Tibetan Plateau. However, using the mean directions of every 100 m of section, we observe that the four localities have similar sequential patterns of rotations during the last 13 Ma: significant continuous counterclockwise before c. 8.0 Ma, insignificant rotations between 8.0–4.0 Ma, and slight clockwise rotation after 4.0 Ma. This indicates that, rather than being a record of spatially varying declinations, it is a temporal variation in the occurrence of regional rotations. Combined with other geological evidence, the rotation patterns may suggest two major tectonic activity phases of the northeastern Tibetan Plateau during the last 13 Ma: an eastward extrusion and strike-slip dominant phase before 8.0 Ma, a significant shortening and a rapid uplift dominant phase after 8.0 Ma. Supplementary material: Magnetostratigraphic results of the Hongshuiba and Wenshushan sections are available at: http://www.geolsoc.org.uk/SUP18540 .

Mechanisms for oscillatory true polar wander

Palaeomagnetic studies of Palaeoproterozoic to Cretaceous rocks propose a suite of large and relatively rapid (tens of degrees over 10 to 100 million years) excursions of the rotation pole relative to the surface geography, or true polar wander (TPW). These excursions may be linked in an oscillatory, approximately coaxial succession about the centre of the contemporaneous supercontinent. Within the framework of a standard rotational theory, in which a delayed viscous adjustment of the rotational bulge acts to stabilize the rotation axis, geodynamic models for oscillatory TPW generally appeal to consecutive, opposite loading phases of comparable magnitude. Here we extend a nonlinear rotational stability theory to incorporate the stabilizing effect of TPW-induced elastic stresses in the lithosphere. We demonstrate that convectively driven inertia perturbations acting on a nearly prolate, non-hydrostatic Earth with an effective elastic lithospheric thickness of about 10 kilometres yield oscillatory TPW paths consistent with palaeomagnetic inferences. This estimate of elastic thickness can be reduced, even to zero, if the rotation axis is stabilized by long-term excess ellipticity in the plane of the TPW. We speculate that these sources of stabilization, acting on TPW driven by a time-varying mantle flow field, provide a mechanism for linking the distinct, oscillatory TPW events of the past few billion years.

The Ahmeyim Great Dyke of Mauritania: A newly dated Archaean intrusion

A precise U–Pb baddeleyite age of 2733±2Ma has been obtained for the Ahmeyim Great Dyke of Mauritania that intruded into the Tasiast–Tijirit Terrane of the Reguibat Shield, NW Mauritania. This dyke is approximately 1500m wide at the sampling area and extends for more than 150km NNE/SSW. Major and trace element geochemistry of the dyke indicates that the magmas that formed this intrusive body were sub-alkaline, tholeiitic and boninitic, and the presence of a negative Nb anomaly indicates the involvement of subducted oceanic lithosphere during magma genesis, most likely an inherited signature from earlier subduction events and the Mesoarchaean collision of the Tasiast–Tijirit and Choum–Rag el Abiod Terranes. A palaeomagnetic study was also undertaken on samples collected from two different sections across the dyke. However, no within- or inter-site grouping of any palaeomagnetic directions could be identified, thus precluding any palaeographic interpretation. The Ahmeyim Great Dyke is interpreted to be part of the feeder system for a 2733Ma Large Igneous Province (LIP); tholeiitic–komatiitic greenstone belts of this age are absent in the West African Craton (WAC) but are present on many other blocks. However, additional constraints are required to reliably link the Ahmeyim Great Dyke with any other such LIP-type greenstone belts in late Archaean supercontinent reconstructions. The magmas that formed the Ahmeyim Great Dyke were boninitic; this, combined with evidence of crustal contamination, the scale of the dyke and its potential link (as a feeder) to greenstone belts of tholeiitic–komatiitic affiliation within other crustal blocks suggests that it, and cogenetic magmatic units elsewhere, may be prospective for magmatic Ni–Cu–PGE sulphide exploration.

Palaeomagnetism of the West Sakhalin Basin: evidence for northward displacement during the Cretaceous

SUMMARY A significant margin-parallel translation of terranes is postulated by all models for tectonic evolution of the East Asian continental margin, although the timing and magnitude of displacements of individual elements are poorly constrained as yet. The West Sakhalin Basin—a forearc basin associated with the Cretaceous volcanic arcs—is one of the displaced tectonic elements. Our palaeomagnetic study of the basin rocks provides the first quantitative constraints on its Cretaceous history. We identified a characteristic magnetization in the Berriasian–Valanginian basalt representing the oceanic basement of the basin and four characteristic magnetizations in a continuous sequence of the Albian to Maastrichtian siliciclastic fill of the basin. A combination of the positive fold, conglomerate and reversals tests supports the primary origin of the remanence in the sediments. Palaeomagnetic data indicate a gradual shift of the West Sakhalin Basin from subequatorial latitudes during the Early Cretaceous to about 40oN by the Late Cretaceous. The main phase of the margin-parallel migration took place during the Early Cretaceous, and ended by the Cenomanian. The continuous sedimentary sequence records ∼50 degrees of progressive clockwise rotation during the Late Cretaceous.

Palaeomagnetic investigation of Cenozoic volcanic rocks bordering the North Anatolian Fault Zone, Reşadiye and Koyulhisar Districts, central-east Anatolia, Turkey

A palaeomagnetic study is reported from the lavas of Eocene, Miocene and Pliocene age cropping out immediately to the north of the North Anatolian Fault Zone (NAFZ) in the Reşadiye–Mesudiye region of central-eastern Anatolia. Rock magnetic investigations identify a high percentage of multi-domained magnetite as the dominant ferromagnet in these rocks and this probably accounts for a relatively poor response to alternating field and thermal demagnetisation. Thirty of 37 units yielded acceptable groupings of characteristic magnetisation directions. An earlier study indicated small anticlockwise crustal block rotation in this region since Upper Cretaceous times (D/I = 347/50°), and our study indicates that this was overtaken by clockwise rotation in Eocene times (D/I = 40/47°), although sample size control from the Palaeogene is poor. Results from later Miocene (D/I = 2/62°) and Pliocene (D/I = 0/53°) volcanic rocks indicate that no significant tectonic rotation has occurred in the north of the NAFZ in Neogene times. This contrasts with rotations in the weaker crust comprising the Anatolian collage south of the NAFZ, where differential and sometimes large anticlockwise rotations occurred during the latter part of the Neogene.

Palaeomagnetism of Middle Triassic red bedded cherts from southwest Japan: equatorial palaeolatitude of primary magnetization and widespread secondary magnetization

SUMMARY Middle Triassic (Anisian) red bedded cherts were collected from the Ajiro area in Kyushu Island, southwest Japan, for a palaeomagnetic study. Thermal demagnetization showed five distinct remanent magnetization components from the cherts. The lowest temperature component below 200 °C (component A) is a present-day viscous overprint. The second- (up to 420 °C, component B) and third-demagnetized (up to 580 °C, component C) components are secondary magnetizations of thermoviscous and chemical origin, respectively, both of which reside in magnetite. The fourth-removed component (component D) was demagnetized at 630 °C and is a secondary magnetization carried by haematite. Components B and C are considered to be a result of widespread remagnetization in southwest Japan during the Cretaceous; the basis of this consideration is that red chert remagnetizations from an area about 600 km away from the present study area, are of the same origins with unblocking spectra similar to those observed in this study. The last-removed component during thermal demagnetization (up to 695 °C, component E) is a primary magnetization carried by haematite. The directions of this component, after correction for the inclination flattening, yielded a mean palaeolatitude of 2°S at the time of deposition in the Middle Triassic. The Ajiro cherts accreted as part of an accretionary complex to the eastern margin of the South China Block during the Middle Jurassic and were then translated northwards from the continental margin to their present position through a margin-parallel translation process owing to the oblique convergence of the oceanic plate.

Sedimentary environment changes during the Early-Middle Pleistocene transition as recorded by the Daumantai sections in Lithuania

Two sections (Daumantai-1 and Daumantai-3) in East Lithuania expose Early-Middle Pleistocene lacustrine-alluvial, deposits which lie on a thin Neogene layer and on older Devonian rocks. They are overlain by glacigenic deposits. Geochemical, palaeomagnetic, magnetic susceptibility, plant macrofossil, and diatom investigations were performed to determine the changes in the sedimentary environment. Cluster analysis of the geochemical variables (10 major elements, 16 trace elements, total organic and inorganic carbon) revealed the element groups and factors. The lowermost units are characterized by a major influence of local material and significant fluctuations in heavy mineral content. These fluctuations might reflect short-term climate changes. The transitional units have more expressed trends, especially as regards the trace elements and Zr/Ti. The uppermost units are characterized by a slight influence of local material and small fluctuations in trace elements. The data analysed enable the subdivision of the section into units related to changes in sedimentary environment. The boundary of the Bruhnes/Matuyama reversal in the Daumantai-1 and Daumantai-3 sections was detected by palaeomagnetic studies and enables the stratigraphical correlation of these deposits.  

Post-emplacement fluids and pluton thermobarometry: Mount Stuart batholith, Washington Cascades

The effects of post-emplacement infiltration of externally derived, high-temperature fluids into arc-related batholiths are often not well characterized. Such infiltration can have far-reaching effects on the elemental and light isotopic chemistry of a batholith and on its mineral phases. At high temperature, fluid infiltration can be less easily detected, especially if widespread. The Mount Stuart batholith of the Washington Cascades is offered as an example of high-temperature infiltration of high δ18O fluids derived from its contact aureole. Some of the fluid infiltration coincided with and may have been partly derived from a kyanite-grade, post-emplacement metamorphic event that affected northern portions of the batholith. However, the effects of the fluid infiltration were far reaching and affected the entire margin of the batholith, including southerly portions that did not experience post-emplacement metamorphism. The result led to an oxygen isotopic zonation of the batholith, which is viewed as secondary in origin, with expected effects on mineral chemistry, including derived estimates of thermobarometry, a portion of which is also substantially subsolidus in origin. Our revised emplacement barometry of the Mount Stuart batholith, excluding areas affected by fluid infiltration, demonstrates that it was emplaced at ∼350–400 MPa. Soon after emplacement, the batholith was tilted to the north by loading processes and subsequently was righted and unroofed during erosion in the Eocene. Its current near palaeohorizontal orientation has implications for palaeomagnetic studies supporting northward transport (the Baja–British Columbia hypothesis), but these results need further study, given the batholith's complex metamorphic and deformation history and the nature of its dominant magnetic mineralogy (pyrrhotite).

Grenville age deformation of the Sudbury impact structure: evidence from magnetic modelling of the Sudbury diabase dyke swarm

Terra Nova, 24, 213–220, 2012AbstractThe Sudbury Structure is located <10 km from the Grenville Front; the northern boundary of the Grenville metamorphic terrane. Previous geological studies have suggested that late Grenvillian deformation (1000–1070 Ma) of the Sudbury Structure was limited to minor brittle deformation. Olivine diabase dykes, which were intruded at 1238 Ma, before the last pulse of Grenville deformation, cross much of the Sudbury Structure. Any Grenville age deformation of the Sudbury Structure therefore should produce systematic displacements in the geometry of the dykes. This study uses information derived from an aeromagnetic survey of the Sudbury Structure to map and measure fault displacements. Lateral displacements of dyke segments define a progressive increase in fault rotation from NE to SW across the basin. A two‐dimensional inversion modelling of magnetic anomaly profiles across segments of the dykes outlines a series of fault blocks with common dip. The dyke dip detected by the inversion models suggests that displacement on the faults included rotation about an axis perpendicular to the trend of the fault. The resulting model of scissor, or splay faults is compatible with previously published palaeomagnetic studies of the olivine dykes in the Sudbury region.

Palaeointensity and palaeomagnetic study of Cretaceous and Palaeocene rocks from Western Antarctica

SUMMARY A combined palaeodirectional and palaeointensity study of a representative collection of plutonic rocks from the Antarctic Peninsula batholith from the western part of the Antarctic Peninsula, near the Ukrainian Antarctic base 'Academik Vernadsky' were carried out. Petrographically, the collection includes gabbros, diorites and quartz diorites, tonalities, granodiorites and granites. The ages of igneous complex emplacement vary from 50 to 117 Ma with most of the rocks belonging to the Cretaceous Normal Superchron. The characteristic remanent magnetizations were isolated by stepwise thermal demagnetization over the temperature interval 440–590°C and their intensities amount to 95 per cent of the NRM. The geographic positions of palaeopoles do not contradict the ‘key poles’ of the Antarctic Peninsula between 90 and 60 Ma. A significant part of the collection was subjected to Coe-modified Thellier palaeointensity experiments with the pTRM checks, which yielded seven reliable palaeointensity determinations for seven different locations. The obtained VDMs are relatively low for all sites, being on average about half of the present day VDM. The analysis of available palaeointensity data for the Cretaceous, Miocene and Middle Jurassic indicates the existence of strong correlations between the mean VDM and VDM scatter versus the rate of reversals. However, due to the shortage of data, the correlations are not significant at the 5 per cent significance level.

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.

Palaeomagnetism of the Mississippian Barnett Shale, Fort Worth Basin, Texas

Abstract A palaeomagnetic study of four oriented cores was conducted to better understand the timing of diagenetic events in the Mississippian Barnett Shale, a primary source rock and the unconventional gas reservoir in the Fort Worth Basin, Texas. Thermal demagnetization removes a present-field modern viscous remanent magnetization (VRM) as well as a chemical remanent magnetization (CRM) that has shallow inclinations and streaked south–SE-directed declinations. The VRM was used to orient the CRM data for one well and it produced a similar streak of directions. The streaking of directions could represent a mixing trend between two or more CRMs. Specimens from bedding-parallel and NE subvertical mineralized fractures contain a CRM that is interpreted to be of Pennsylvanian age and to have formed in response to burial diagenetic processes. NE- and NW-oriented vertical fractures are common to rocks that contain late Permian to Triassic CRMs. Sr and sulphur isotope results from vein minerals around NE fractures suggest the CRM could be related to fluids sourced from the Ouachita front. The SE directions in the streak could be explained if the northern part of the basin experienced a component of anticlockwise rotation of up to 20° in the Pennsylvanian.

Tectonic and climatic controls on the sequential arrangement of an alluvial fan/fan‐delta complex (Montserrat, Eocene, Ebro Basin, NE Spain)

AbstractA magnetostratigraphy‐based chronological framework has been constructed in the Eocene sediments of the Montserrat alluvial fan/fan‐delta complex (southeast Ebro Basin), in order to unravel forcing controls on their sequential arrangement and to revise the tectonosedimentary history of the region. The palaeomagnetic study is based on 403 sites distributed along an 1880‐m‐thick composite section, and provides improved temporal constraints based on an independent correlation to the geomagnetic polarity time scale. The new chronological framework together with sequence stratigraphy and geohistory analysis allow us to investigate the interplay between factors controlling the sequential arrangement of the Montserrat complex at the different temporal scales and to test for orbitally driven climate forcing. The results suggest that the internal stacking pattern in transgressive and regressive sequences sets within the more than 1000‐m‐thick Milany Composite Megasequence can be explained as the result of subsidence‐driven accommodation changes under a general increase of sediment supply. Composite sequences (tens to hundreds of metres thick) likely reflect orbitally forced cyclicity related to the 400‐kyr eccentricity cycle, possibly controlled by climatically induced sea‐level fluctuations. This study also provides new insights on the deformational history of the area, and shows a correlation between (tectonic) subsidence and forelimb rotation measured on basin‐margin deformed strata. Integration of subsidence curves from different sectors of the eastern Ebro Basin allows us to estimate the variable contribution of tectonic loads from the two active basin margins: the Catalan Coastal Ranges and the Pyrenees. The results support the presence of a double flexure from Late Lutetian to Late Bartonian, associated with the two tectonically active margins. From Late Bartonian to Early Priabonian the homogenization of subsidence values is interpreted as the result of the coupling of the two sources of tectonic load.

Ancient lunar dynamo

The differential rotation between the Moon's core and mantle may have powered the ancient lunar dynamo, either continuously over several hundred million years or intermittently after large impacts. See Letters p.212 & p.215 Recent palaeomagnetic and seismological studies have strengthened the suggestion that the Moon once possessed a core dynamo. Despite its importance as a constraint on lunar evolution, there is currently no consensus on how such a dynamo was driven. Two groups working independently have arrived at the idea that the lunar dynamo was powered by mechanical stirring of the liquid core. Dwyer et al. investigate the mechanism of a dynamo driven by continuous mechanical stirring arising from the differential motion between the solid silicate mantle and the liquid core beneath. They show that the fluid motions and the power required to drive a dynamo operating continuously for more than 1 billion years are readily obtained by such mechanical stirring. Le Bars et al. propose a model whereby the dynamo action comes from impact-induced changes in the Moon's rotation rate. They show that basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation, and that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary, could have powered a lunar dynamo.

Palaeomagnetism of late Cretaceous sediments from southern Tibet: Evidence for the consistent palaeolatitudes of the southern margin of Eurasia prior to the collision with India

Twenty sites were drilled in the late Cretaceous Shexing Formation for palaeomagnetic studies in the Lhasa terrane near the locality of Maxiang (29.9°N/90.7°E). The stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I = 350.8°/32.1° with α 95 = 8.1° and N = 20 sites, corresponding to a paleopole at 75.0°N, 306.7°E with A 95 = 6.8°. Positive fold tests indicate a primary origin for the characteristic remanence. Based on previous Cretaceous data mainly from the Takena Formation and Paleocene data from the Linzizong volcanic rocks near the city of Lhasa, the latitude of the southern margin of Asia is located at about 15°N, and yields a stable position of the Lhasa terrane during Cretaceous and Paleocene. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700 ± 800 km (16.2 ± 7.6°) between southern Tibet and stable Eurasia and 1500 ± 830 km (14.4 ± 7.9°) between southern Tibet and stable India since the collision of India and Eurasia. A collision age between c. 54 and 47 Ma was determined using the results for the southern margin of Eurasia according to our new data and the extent of ‘Greater India’.

Propagating and incorporating the error in anisotropy-based inclination corrections

SUMMARY Sedimentary rock palaeomagnetic inclinations that are too shallow with respect to the ambient field inclination may be restored using anisotropy-based inclination corrections or techniques that rely on models of the past geomagnetic field. One advantage of the anisotropy technique is that it relies on measured parameters (declinations, inclinations, bulk rock magnetic fabrics and particle magnetic anisotropy) that have measurement errors associated with them, rather than relying on a geomagnetic field model and statistical treatment of the data. Sofar,however,theerrorassociatedwiththemeasurementshasnotbeenpropagatedthrough the corrections and the reported uncertainties are simply the α95 95 per cent confidence circles of the corrected directions. In this paper we outline different methodologies of propagating the error using bootstrap statistics and analytic approximations using the case example of the Shepody Formation inclinationcorrection.Bothtechniquesareingoodagreementandindicateamoderate, ∼15per cent, uncertainty in the determination of the flattening factor (f) used in the correction. Such uncertainty corresponds to an ∼0.31 ◦ increase of the confidence cone and a bias that steepens the mean inclination by 0.32 ◦ . For other haematite-bearing formations realistic uncertainties for f ranging from 0 and 30 per cent were used (together with an intermediate value of 15 per cent) yielding a maximum expected increase in the confidence cones and steepening of the inclinationsof ∼1 ◦ .Suchresultsindicatethatformoderateerrorsoff theinclinationcorrection itself does not substantially alter the uncertainty of a typical palaeomagnetic study. We also compare the uncertainties resulting from anisotropy-based corrections to those resulting from the elongation/inclination (E/I) technique. Uncertainties are comparable for studies with a large sample number (>100), otherwise the anisotropy-based technique gives smaller uncertainties. Whenanisotropydataarenotavailable,itispossibletoestimateacorrectionusingflattening factors (f) obtained from the literature. A range of flattening factors has been observed for both magnetite and haematite-bearing rocks (0.4 ≤ f ≤ 1 for haematite and 0.54 ≤ f ≤ 1f or magnetite), but the exact value is specific to the anisotropy of the formation. To evaluate the maximum effects of inclination shallowing, the smallest f (for magnetite or haematite) should be used.

The ∼270 Ma palaeolatitude of Baltica and its significance for Pangea models

To better constrain Baltica's position within Pangea, we conducted a palaeomagnetic study of Permo-Triassic dykes from the Oslo Graben, as a follow-up to an initial, but rather limited, study by Torsvik and colleagues in 1998. The age of these so-called Lunner dykes had previously been determined as ∼240 Ma in that study, but details in their analyses and new 40Ar/39Ar ages reveal that there may have been some argon loss in the initially dated dyke minerals and that a combined (weighted mean) age of 271 ± 2.7 (2σ) Myr for the dykes is preferable. We find two major components of magnetization in our samples: one carried by an Fe-sulphide (likely pyrrhotite) and the other carried by low-Ti magnetite; these magnetization components may be found together (superposed) in a given sample or they may occur apart. Micronmetre-sized crystals of Ti-Fe oxides, observed with a scanning electron microscope (SEM) show exsolution lamellae, formed upon cooling from intrusion temperatures. Assuming that the submicronmetre-sized (Ti)-magnetite grains that carry a stable remanence are of the same generation as the observed larger grains, we interpret the magnetite remanence in the dykes as of primary, thermoremanent origin. The sulphide remanence appears to be slightly younger, as seen by the SEM observations of pyrite framboids and a Fe-sulphide grain invading a Ti-magnetite grain. Moreover, the sulphide mineralization is likely of region-wide hydrothermal origin. The magnetizations carried by the pyrrhotite and magnetite have nearly identical directions and so, must be nearly of the same age. For this study, we sampled 56 sites including 39 dykes, 10 baked-contact rocks and 7 host rocks removed from the immediate dyke contacts. The dykes and the contact rocks have the same SW and up directions of magnetization, and contain the Fe-sulphide or the magnetite magnetization or both, as diagnosed by their relative unblocking temperatures. However, all the sampled carbonate and igneous host rocks far away from the dykes also have the same directions. Thus, all of the 10 originally planned contact tests are inconclusive. The new palaeopoles of this study are a few degrees apart; the magnetite pole (from dykes only, N= 25) is located at 51°N, 164°E, K= 69, A95= 3.5°, whereas the pole calculated from iron sulphide magnetic directions (all rock types, N= 20) is at 54°N, 166°E, K= 112, A95= 3.1°. All directions are of reversed polarity, suggesting that the magnetization was acquired during the Kiaman Reversed Superchron. The palaeomagnetic mean result from the magnetite-bearing sites implies a palaeolatitude of Oslo of 23°N, whereas the palaeolatitude calculated from the pyrrhotite magnetizations is 25–27°N, depending on choice of host lithologies. As noted in many previous publications, the palaeomagnetic poles for the late Palaeozoic and Early-Middle Triassic are in conflict with classical Pangea reconstructions. The poles with ages of 250 ± 10 Ma, in particular, previously showed a discrepancy of some 25° or more, when the Gondwana and Laurussia continents are restored to their juxtapositions in the Pangea-A fit, before the opening of the Atlantic Ocean. Proposed solutions to this conundrum have been controversial, involving doubts about (1) the geocentric coaxial dipole field model, (2) the reliability of the palaeomagnetic results or their ages, or (3) the validity of the Pangea-A reconstruction, leading to proposals of a Pangea B reconstruction in which Gondwana is displaced some 3500 km to the east with respect to Laurussia. The significance of our new result for this Pangea controversy resides in its improved age within an early Guadelupian (mid to late Permian) time interval where few results exist from well-dated igneous rocks in either Baltica or Laurentia. There are quite a few results from sedimentary rocks, but these may be suspected to suffer inclination shallowing, and are therefore less suitable to settle a palaeolatitudinal argument. Our new result of the magnetite magnetization, granted it is primary and acquired at about 270 Ma, combined with a new ∼265 Ma result from Argentina and selected other poles from igneous rocks, leaves enough room for the north–south configuration of Pangea A at 270 Ma and avoids the overlap between Baltica and Gondwana that necessitated Pangea B, at least for the Late Permian.

Geomagnetic secular variation and the statistics of palaeomagnetic directions

In this study, we examine the role of palaeosecular variation (PSV) in the use of statistics for palaeomagnetic studies, and we provide new reliability criteria for palaeomagnetic poles or directions.We first conclude that Fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole (VGP) distributions instead. Secondly, we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets. The latitude-dependent properties (E, S, k) provide useful guidelines for the reliability of a palaeomagnetic data set. A reliable assessment of these properties depends on the (sufficient) number of palaeomagnetic samples being taken. Therefore, as an additional instrument of assessing data sets, we provide a N-dependent A95 envelope, bounded by an upper limit A95max, and a lower limit A95min that helps to ascertain whether or not a distribution has sufficiently well-sampled PSV and therefore geomagnetic field behaviour. Applying these criteria is indispensable for studies of geomagnetic behaviour, or for studies aiming at using TK03.GAD for inclination error correction through the elongation/inclination (E/I) method. For palaeomagnetic studies aimed at geological reconstructions, they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field. An analysis of published Eastern Mediterranean data shows that the vast majority of studies do not conform to the Van der Voo criteria, in particular with respect to N and A95. We have provided criteria that are on the one hand more lenient (lower N may still provide relevant information), and on the other hand more strict (for high N the criterion of A95 < 16° should be adapted to a requirement of lower A95, e.g. A95 80).

Palaeomagnetic constraints from granodioritic plutons (Jiaodong Peninsula): New insights on Late Mesozoic continental extension in Eastern Asia

Mechanism and kinematics of the Late Mesozoic continental extension event of Eastern Asia are still debated. In order to better constrain its evolution, two granodioritic plutons of the Jiaodong Peninsula have been chosen as targets for a time-constrained palaeomagnetic study. Indeed, plutons are devoid of visible deformation, did not experience rotation along horizontal axis and are precisely dated by U/Pb and 40Ar/ 39Ar methods. Multidomain (MD) magnetite has been identified as the principal magnetic remanent carrier. The interpolation of existing and new U/Pb and 40Ar/ 39Ar ages revealed that characteristic remanent magnetisation was acquired in a narrow range of 116 ± 2 Ma. Twenty out of 27 sites present stable magnetic directions calculated from high-temperature or high-coercive components. The observations of the solo normal magnetic polarity for this palaeomagnetic collection and of the magnetic remanent age consistent with the Cretaceous Normal Superchron (CNS) argue that the characteristic magnetic remanence may be considered as primary. Detailed field observations of the intrusive relationship between the plutons and country rocks and Anisotropy of Magnetic Susceptibility (AMS) study reveal the absence of the subsequent deformation of plutons, or rigid rotation of plutons along a horizontal axis. Two palaeomagnetic poles have been therefore calculated from these plutons. Among 12 out 15 Cretaceous palaeomagnetic poles, including the two new poles provided by this study, from the Jiaodong Peninsula and on both sides of Tan-Lu and Muping-Jimo faults are statistically consistent. As a result, the Jiaodong Peninsula behaved as a rigid block as internal deformation appears negligible. The remaining three derived poles are probably due to the secular variation or/and the vicinity of fault zones near of the palaeomagnetic sampling site. Thus, they can not be applied to the peninsula-scaled tectonics. Comparison of these time-constrained Cretaceous palaeomagnetic results of the Jiaodong Peninsula with those of North China Block (NCB) indicate that the Jiaodong Peninsula was rigidly attached to NCB since, at least, the Cretaceous as residual difference is clearly insignificant with respect to the error brackets. These new palaeomagnetic results confirm that the relative clockwise (CW) rotation of Eastern Liaoning-Korea Block (ELKB) with respect to NCB does not affect the Jiaodong Peninsula as a rigid block, nevertheless, some tectonic structures have been reactivated since Cenozoic and local rotations could be produced. This palaeomagnetic study reveals that the Late Mesozoic extension in Eastern Asia is heterogeneously expressed in time and space. The complete understanding of the mechanism(s) of this extensional event needs further multidisciplinary investigations.