Remanence Vectors Research Articles

Improved anisotropy of magnetic remanence results from vectorial readings using novel refinement method

SUMMARYAnisotropy of magnetic remanence (AMR) is typically obtained by imparting a remanence through a sequence of reference directions and subsequent measurement of the remanent magnetic moment with a separate device. This gives rise to potential misalignments which can affect the resultant anisotropy tensor. Vectorial measurements from rock magnetometers are favourable to obtain the best estimate of the anisotropy tensor but make the results highly dependent on the exact knowledge of the reference directions. This can be partly overcome by using the projection of the remanence vector on the reference direction instead of using the full vector in the calculation of the best-fitting anisotropy tensor. Since AMR experiments are laborious, it is troublesome to compensate the loss in information with additional measurements. This paper demonstrates an iterative refinement method to recover the original reference directions and therefore to improve the results of AMR experiments by using full vectorial readings. The method works well with a six-axis protocol for misalignment of the reference directions up to 25° and for degrees of anisotropy up to 250 per cent, which covers most experimental and geological cases.

An Abelian Higgs model of pulsed field magnetization in superconductors

Pulsed field magnetization leads to trapped magnetic field persistent for long times. We present a one-dimensional model of the interaction between an electromagnetic wave and a superconducting slab based on the Maxwell-Ginzburg-Landau (Abelian Higgs) theory. We first derive the model starting from a Lagrangian coupling the electromagnetic field with the Ginzburg-Landau potential for the superconductor. Then we explore numerically its capabilities by applying a Gaussian vector potential pulse and monitoring usual quantities such as the modulus and the phase of the order parameter. We also introduce defects in the computational domain. We show that the presence of defects enhances the remanent vector potential and diminishes the modulus of the order parameter, in agreement with existing experiments.

Separation of magnetic anomalies into induced and remanent magnetization contributions

ABSTRACTInversion of magnetic data is complicated by the presence of remanent magnetization, and it provides limited information about the magnetic source because of the insufficiency of data and constraint information. We propose a Fourier domain transformation allowing the separation of magnetic anomalies into the components caused by induced and remanent magnetizations. The approach is based on the hypothesis that each isolated source is homogeneous with a uniform and specific Koenigsberger ratio. The distributions of susceptibility and remanent magnetization are subsequently recovered from the separated anomalies. Anomaly components, susceptibility distribution and distribution of the remanent and total magnetization vectors (direction and intensity) can be achieved through the processing of the anomaly components. The proposed method therefore provides a procedure to test the hypotheses about target source and magnetic field, by verifying these models based on available information or a priori information from geology. We test our methods using synthetic and real data acquired over the Zhangfushan iron‐ore deposit and the Yeshan polymetallic deposit in eastern China. All the tests yield favourable results and the obtained models are helpful for the geological interpretation.

Archaeomagnetic investigations in Bolgar (Tatarstan)

The objective of this study is to provide a well-dated point for a future palaeosecular variation (PSV) reference curve for western Russia. For this purpose archaeomagnetic and magnetic property analyses were carried out on a pottery kiln unearthed at the UNESCO World Heritage site of ancient Bolgar, having a rather precise age dating. The archaeological context provided an age between 1340 and 1360 C.E. The characteristic remanence vector was determined through alternating field demagnetisation and Thellier-Thellier palaeointensity experiments. Some innovations were introduced regarding palaeointensity. The check testing the equality of blocking and unblocking temperature was redefined. This allowed waiving the commonly used additional zero-field cooling steps during the Thellier-Thellier experiment. Another innovation concerns the calculation of archaeointensity at structure level. A Bayesian approach was introduced for averaging individual specimen archaeointensities using a prior probability distribution of unknown uncertainties. Next, an additional prior probability distribution was used to correct for cooling rate effects. This resulted in a lower uncertainty compared to common practice and in eluding time consuming cooling rate experiments. The complex magnetic mineralogy consists of maghaemite, multi-domain haematite and Al-substituted haematite. Some samples contained also some non-stoichiometric magnetite. The magnetic mineralogy was determined through hysteresis loops, backfield and remanence decay curves, measurements of the frequency dependence of magnetic susceptibility and through low temperature magnetisation curves. Accompanying high-temperature thermomagnetic analyses revealed an excellent thermo-chemical stability of the studied specimens. Directions obtained from alternating field demagnetisation and those extracted from archaeointensity experiments are congruent and have low uncertainties. The obtained archaeomagnetic results are fairly in agreement with global geomagnetic field models and contemporary PSV data of the wider area. The geomagnetic field vector obtained for ancient Bolgar is of high quality, deserving thus its inclusion in a future PSV reference curve for European Russia.

Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites

Dusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with “bulk” spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies.

Multidisciplinary approach to constrain kinematics of fault zones at shallow depths: a case study from the Cameros–Demanda thrust (North Spain)

Thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts (e.g., mylonites, sheath folds, shear bands). In this paper, a combination of techniques (including structural analysis, magnetic methods, as anisotropy of magnetic susceptibility and paleomagnetism, and paleothermometry) is used to define thrusting conditions, deformation, and transport directions in the Cameros–Demanda thrust (North Spain). Three outcrops were analyzed along this intraplate, large-scale major structure having 150 km of outcropping length, 30 km of maximum horizontal displacement, and 5 km of vertical throw. Results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis. Mixed-layer illite–smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation, thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust. Kinematic indicators (foliation, S/C structures, and slickenside striations) indicate altogether a dominant NNW movement of the hanging wall in the western zone and NE in the eastern zone of the thrust, thus implying strain partitioning between different branches of the main thrust. The study of AMS in fault rocks (nearly 400 samples of fault gouge, breccia, and microbreccia) indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max/k int plane in sites with strong shear deformation. Paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting, in spite of the shallow depth for deformation, and a strong deformation or scattering of the magnetic remanence vectors in the fault zone. The application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels.

Palaeomagnetic and geochronological data from Late Mesoproterozoic redbed sedimentary rocks on the western margin of Kalahari craton

Abstract Redbeds of the Aubures Formation constitute the uppermost stratigraphic unit in the Mesoproterozoic Sinclair succession of southern Namibia. Aubures palaeomagnetic remanence vectors, held almost exclusively by hematite, document at least one geomagnetic polarity reversal in the stratigraphy, a positive intraformational conglomerate test indicating primary magnetization and greatest concentration of characteristic directions at 50–60% untilting, indicating that deformation was coincident with sedimentation. The new Aubures palaeomagnetic pole, at 56.4°N and 018.0°E with A 95 =11.3°, is located on the apparent polar wander path of the Kalahari craton, between poles of the 1110 Ma Umkondo igneous event and the c. 1090 Ma Kalkpunt redbeds of the Koras Group near Upington, South Africa. This distinctive concordance suggests that Aubures sediments have an age of approximately 1100 Ma, that the Sinclair region was probably part of Kalahari at that time and that the Aubures and Kalkpunt redbeds are broadly correlative. New laser-ablation inductively coupled plasma mass spectrometry detrital zircon results from the Aubures Formation, including a youngest age component (1108±9 Ma) that is coincident with the Kalahari-wide Umkondo large igneous province, conform to this interpretation. Palaeomagnetism and geochronology of the Sinclair succession can provide kinematic constraints on the tectonic evolution of Kalahari as it approached other cratons in the growing Rodinia supercontinent.

New evidence on an episode of geomagnetic instability, recorded in middle Miocene lava flows in Northwest Iceland

Information on the character of paleomagnetic polarity transitions and major excursions is important to understanding of the generation of the geomagnetic field. The main aim of this study was to extend previous observations on a complex excursion event at around 13 Ma, recorded in lava flows south of the Isafjarðardjup fjord in the Northwest peninsula of Iceland. Core samples were collected in four short hillside profiles, for measurement of their remanence vectors after conventional alternating-field treatment. Despite minor hydrothermal alteration in the area, reliable estimates of primary geomagnetic directions were obtained in 49 of the 51 sites sampled. It is shown that large directional variations were taking place between at least 25 successive flows. Judging from the overall rate of buildup of the local lava pile, the duration of this event may reach 100 kyr. The results support findings in the literature on the occurrence of other episodes of geomagnetic instability in the Neogene. The excursion sequence in Isafjarðardjup will also be of value for stratigraphic correlation in the Northwest peninsula (and even farther afield), as it is probably contemporaneous with an excursion found 30 km away.

Analysis of petrophysical properties of rocks from the Bathurst Mining Camp: Constraints on gravity and magnetic modeling

Understanding the physical rock properties of different lithologies within a mining district allows one to link geologic observations with geophysical interpretations. This paper presents a physical rock property database for the Bathurst Mining Camp (BMC). Density–magnetic susceptibility bivariate plots are used to illustrate patterns indicative of changes in the concentration of paramagnetic versus ferrimagnetic mineral phases. Q-Q plots and histograms are used to determine if a lithology is characterized by a unimodal or multimodal physical property population. To use the physical property data in geophysical models, the rock classification was reduced to five lithological groups and three subgroups. The results of two geophysical modeling exercises, using lithological and petrophysical data as input constraints, are presented. Late-stage deformation of the BMC resulted in two large-scale plunging folds: the Nine Mile Synform (NMS) and the Tetagouche Antiform. The subsurface geometry of the NMS along the model profile was initially estimated from surficial geology maps projected down the plunge of the fold axis. Geophysical data requires dense and magnetic volcanics on the east limb of the Nine Mile Syncline to be nearer to the surface than previously expected. The magnetic anomaly associated with the Armstrong B mineral deposit was modeled using constrained discrete object source geometry. To achieve a satisfactory match between the observed and calculated data requires a significant component of magnetic remanence. The orientation of the calculated remanence vector is similar to a paleomagnetically determined in situ direction and to the direction estimated for Laurentia during the Paleozoic. The source body estimated by the inversion is significantly larger than the known thickness of the ore zone. This is a consequence of computing the inversion on a gridded aeromagnetic data set, which has a cell size that is larger than the known thickness of the ore body.

Detailed analysis of permanent magnets by means of free field measurements

A measurement technique is depicted that allows one to analyze the direction as well as the distribution of the remanence (or remanent magnetization) of permanent magnets. To this end, the magnetic field is measured at several positions around the magnet, and the magnet is subdivided into multiple cells. Then, an efficient method is presented how the remanence vectors of these cells are computed from the measurement values by means of finite element simulations. Finally, the measurement technique is applied to permanent magnets used in magnetically biased chokes which demonstrates the new possibilities and the practical applicability.

Analyses of primary remanence vector data from a large collection of lava flows: towards improved methodology in paleo-geomagnetism

Research into the properties of past geomagnetic fields (paleo-geomagnetism) has been carried out worldwide for over half a century. This research utilizes rocks which are to a varying degree inadequate for that purpose, and evaluation of the results is therefore not always on firm ground. One resource which can be utilized to constrain several aspects of the interpretation of other paleo-geomagnetic data, consists of stable primary remanence vectors in large homogeneous collections of dated fresh lava flows. We outline how the available remanence data from several thousand lavas in Iceland may be analysed in a semi-quantitative way to test some of current results, methodology and concepts of paleo-geomagnetism. Among topics where such analysis provides new insights, are the geomagnetic polarity time scale, the frequency distribution of virtual geomagnetic poles in latitude and longitude, and the relative intensity of the magnetic field as a function of virtual pole latitude. Comparison of the scatter in remanence intensities and in absolute paleointensity determinations on Icelandic lavas indicates, along with other evidence, that quality criteria for the latter are in need of revising. It is also confirmed here that long-term changes have occurred in the amplitude of the geomagnetic secular variation; they should be taken into account in studies on other properties of the paleo-geomagnetic field.

Quantitative constraint on footwall rotations at the 15°45′N oceanic core complex, Mid‐Atlantic Ridge: Implications for oceanic detachment fault processes

The subsurface geometry of detachment faults at slow spreading mid‐ocean ridges is debated: are they planar features that form and slip at low angles, as often inferred for their continental equivalents, or do they initiate at steep angles and then flatten in response to flexural unloading as displacement proceeds, as predicted in “rolling hinge” conceptual models? An essential difference is that significant rotation of the footwall should occur in the rolling hinge but not the planar fault model. This can be tested using paleomagnetism. Previous attempts to address this question have relied upon data from azimuthally unoriented drill cores. Although results are consistent with large rotations having occurred, these interpretations are very nonunique, and other solutions that require minimal rotations are equally permissible. We here present a rigorous analysis of paleomagnetic and structural data from a unique set of azimuthally oriented cores, collected using a seabed rock drill, from the 15°45′N oceanic core complex on the Mid‐Atlantic Ridge. By considering the full paleomagnetic remanence vector in combination with kinematic data from the detachment fault shear zone we are able to quantitatively constrain the geometrically permissible axes and magnitudes of rotation of the detachment fault footwall, for the first time without having to make a priori assumptions about the orientation of the axis. We show that significant rotations (64° ± 16°) have indeed occurred, about a gently plunging, near‐ridge‐parallel axis, robustly supporting the rolling hinge models. We further discuss the geological implications of this result for oceanic detachment fault processes.

Paleomagnetic observations at three locations in the Pleistocene lava sequence of southwest and south Iceland

Magnetic polarity measurements on lava samples were introduced in the 1950s as an aid in stratigraphic research in Iceland, and applied by T. Einarsson especially on Pleistocene sequences in southwest and south Iceland. However, the stratigraphy of these sequences is often complex, and Einarsson’s mapping has only been followed up to a limited extent. The present detailed laboratory study on magnetic remanence directions in lava flows is focused on three locations within the above region. In two of these locations (in southwest Iceland), profiles spanning the boundary between the polarity zones named R2 and N2 by Einarsson were sampled. The potential of using this boundary for correlation over short distances was confirmed. In a third location (in south Iceland), three normal-polarity zones are present in a 500 m thick lava and sediment sequence of dominantly reverse polarity. The stability and within-lava agreement of primary remanence vectors is generally excellent. Remanence directions in successive lava units are in many cases very similar, indicating that the volcanism was episodic. In comparison to geomagnetic poles obtained in previous paleomagnetic collections of older lava series in Iceland, the poles derived from these and other Pleistocene lavas are rarely situated in low latitudes.

Paleomagnetic evidence for clockwise rotation and tilting in the eastern Tethyan Himalaya (SE Tibet): Implications for the Miocene tectonic evolution of the NE Himalaya

Crustal movement around and away from the Namche Barwa syntaxis is indicated in the Asian velocity field inferred from GPS data and Quaternary fault slip rates. Nevertheless, there is a limited field-based control on the rotational history of the north-eastern Himalayan arc. Exploring the poly-phase nature of deformation, within the Cretaceous diorite dykes and their host-rock (Triassic flysch), in the eastern Tethyan Himalaya (90°–92°E), combined with new remote sensing data and existing thermo-geochronological data, allow us to unravel the kinematic relationship between paleomagnetic remanence vectors and the deformation phases. Decay at 325 °C in high temperature susceptibility curves and in the thermal demagnetization of the SIRM indicate that the characteristic remanent magnetization in the Cretaceous diorite dykes is carried by pyrrhotite. The pyrrhotite component unblocks at 280–350 °C, revealing normal and reverse polarities. It is of post-folding origin with a mean remanence direction of 019°/28° and 001°/20° in the eastern (Qonggyai valley) and western (Nagarze) part of the sampling area, respectively. The ~ 22 Ma K–Ar age of the last metamorphic event support that the remanence is post-Eohimalayan folding and likely of thermoremanent or thermo-chemical origin. Comparison of the declination with respect to the Early-Miocene reference direction, yields a trend from no apparent rotation in the west to 20° clockwise rotation in the east with respect to the stable Indian plate. This result can be kinematically related to the Middle to Late-Miocene strain partitioning between far-field southeast extrusion of SE Tibet and near-field strike-slip faulting and E–W extension. Furthermore the observed pattern of tilting around horizontal axis may reflect concealed North Himalayan doming.

Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus

Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron–titanium oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance, strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover, mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation. The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia terrane boundary and the Arakapas-Transform fault form an approximate E–W composite boundary that we term the Troodos Tectonic Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor metamorphism, perhaps 7–15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal circulation. The gabbros and mantle rocks commonly bear young ( 8 Ma), the remagnetizing effects of penetrative strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in present-day coordinates, Troodos rocks moved ~1,000 km South; then ~4,500 km East and finally ~900 km North at an approximate rate of 75 km/Ma [1 km/Ma = 1 mm/a]. This true motion path commenced ~88 Ma ago and rates of motion since 65 Ma may be too high due to the limited precision of strain-corrections of the ChRM orientations in limestone. This true motion path is compatible with the eastward and then northward rotation of Africa relative to Europe although other workers show relative motion paths.

Simulations of remanence and field distributions of permanent magnet array with rectangular cavity

The magnetic phenomena of a Halbach-type permanent magnet array under a strong self-field such as demagnetization, saturation, deflection, and rotation of remanence vectors occur deep in the interior of the magnets, and thus lead to an inhomogeneous field even for a hollow cylindrical permanent magnet array with circular cavity. Considering the anisotropic magnetization loops of commercially available permanent magnets, the self-consistent remanence and field distributions of a 16 piece hollow cylindrical permanent magnet array with rectangular cavity (HCPMARC) are presented here to simulate the magnetic phenomena inside the magnets. The field distribution of a HCPMARC can be modulated by array structure while keeping both the cross sectional areas of the cavity and the magnets constant. At the expense of field uniformity, a HCPMARC acquires greater center field uniformity than a HCPMACC does. Furthermore, we simulated the magnetization and field distributions for a yoked HCPMARC obtained by replacing the partial permanent magnets with soft ones on both magnetic poles of a yokeless HCPMARC. Such substitution always enhances field uniformity but depresses the center field. This, in turn, causes changes in the average magnitude of the field in the entire rectangular cavity.

Late Proterozoic reconstructions of North-West Scotland and Central Canada: Magnetic fabrics, paleomagnetism and tectonics

Red-beds dominate the Stoer Group and the unconformably overlying Torridon Group, having accumulated between ∼1400 and ∼1000 Ma, in a rift of the Hebridean foreland. The sequences were weakly strained and shortened E–W, and each underwent successive diagenetic changes which altered their magnetic properties and caused their different characteristic post-depositional magnetizations. Anisotropy of magnetic susceptibility (AMS) reveals an N–S vertical tectonic foliation superimposed on the bedding-planar AMS sub-fabric. The tectonic AMS sub-fabric was isolated by comparing normalized and non-normalized mean tensors of multiple specimens. The N–S vertical AMS tectonic foliation postdates warps of bedding; thus the high-susceptibility minerals, including those carrying palaeomagnetic signals re-oriented or recrystallized after deposition and diagenesis. Thus characteristic remanence vectors (ChRM) were acquired long after deposition, first in the Stoer Group, then in the Torridon. ChRMs were isolated for 143 Stoer specimens and 94 Torridon specimens using two cycles of low-temperature demagnetization followed by at least 12 steps of thermal demagnetization. For the Stoer Group, two structurally integral groups of sites yield mean magnetizations of 317.0/+43.1 α 95 = 11.3 ( n = 49) and 309.9/52.1 α 95 = 10.6 ( n = 23). Some Stoer specimens bear a Torridon age overprint 133.3/+525 α 95 = 9.6 ( n = 19). The younger Torridon Group yields 138.4/+52.2 α 95 = 7.4 ( n = 42); 134.9/+45.5 α 95 = 16.0 ( n = 15) and 134.4/+55.1 α 95 = 11.3 ( n = 19) from three structurally integral clusters of sites. These late chemical magnetizations postdate bedding warps, faults and tectonic AMS fabrics; they do not warrant local “rigid-body” tilt corrections. However, we restored the ChRM directions for the regional post-Cambrian eastwards tilt which affected the entire foreland from the Lewisian basement to the overlying Proterozoic strata. Paleopoles calculated from the strata's restored vectors were rotated into Laurentian coordinates for comparison with Canadian Proterozoic palaeopoles. The Stoer Group was magnetized in a normal polarity geomagnetic field whereas the Torridon Group was magnetized in a reversed polarity epoch. However, both sequences include at least one polarity transition, indicating long duration chemical magnetizations successfully averaging secular variation. At a minimum, red-bed magnetization processes exceeded 0.1 Ma and may have encompassed several million years. During Stoer Group magnetization, the Hebridean craton was at paleolatitudes of 26–44°N; when the Torridon Group magnetized it was at 23–27°S. Accepting published ages, this defines a minimum southward latitudinal displacement rate of 27–40 km/Ma. Restored Stoer Group paleopoles lie in the present-day north-central Pacific Ocean (∼190E/25N); corresponding to the Laurentian APWP dated at between 1090 and 1109 Ma. Torridon Group paleopoles lie in the south-central Pacific (∼210E/25S) near the poorly documented (<900 Ma) southernmost part of the Laurentian APWP.

Remagnetization (∼1100 Ma) of Voosges lake tuff (1542 Ma) and other mid-proterozoic rocks, Nipigon graben, northern Ontario

Recently discovered horizons of non-deformed felsic welded crystal tuff in the Proterozoic rift system of northern Ontario yield a geologically stable paleomagnetic vector. Using alternating field (AF), thermal (TD) and low-temperature demagnetization (LTD) techniques, a single stable characteristic remanence (ChRM) vector is found in all specimens (declination/inclination = 115°/−73.5°; α 95 = 2.1°, n = 58). It is overprinted by a Bruhnes’ epoch VRM. The tuffs paleopole position coincides with that of ubiquitous sub-horizontal diabase sheets which intruded at ∼1110 Ma during Late Proterozoic Keewanawan Volcanism. The latter produced numerous volcanic and intrusive rocks along the margins of the Lake Superior paleo-rift system, during the interval 1087–1120 Ma, but the sills are most extensive. Although the Voosges Lake Tuff shares the same paleopole location as the diabase sheets (1109 Ma), the nearby Seagull pluton (1112 Ma) and the Keweenawan volcanic rocks, its primary zircons yielded a Mesoproterozoic crystallization age of 1542 ± 3 Ma. This is similar to the nearby English Bay felsic intrusions dated at 1537 − 2 + 10 Ma [Davis, D.W., Sutcliffe, R.H., 1985. U-Pb ages from the Nipigon “Plate” (sic) and northern Lake Superior. Geol. Soc. Am. Bull. 96, 1572–1579]. Whereas the paleomagnetic magnetic signal appears primary in the 1542 Ma Voosges Tuff, it was completely reset by the Late Proterozoic Keewanawan paleofield (∼1100 Ma). The Tuff units are separated from the diabase sills by stratigraphic thicknesses of ≤50 m and show no traditional petrographic evidence of annealing or thermal metamorphism. Since it is improbable that the 1542 Ma tuff returned to the same paleolatitude at 1109 Ma, we postulate that metasomatic processes caused a ∼1109 Ma remagnetization. A similar event could explain unusual ChRMs reported for certain other pre-Keweenawan Proterozoic sites with adjacent Keewanawan diabase sheets. For example, some paleopoles reported for the Gunflint Formation (1878 Ma) and Sibley Formation (>1339 Ma, perhaps ∼1537 Ma by association with the English Bay intrusions) might have been acquired due to secondary remagnetization.

A 640 kyr geomagnetic and palaeoclimatic record from Lake Baikal sediments

SUMMARY Magnetic remanence vectors for 1737 samples from two ∼100 m cores of Lake Baikal sediments are reported along with complete magnetic susceptibility profiles obtained from a passthrough system. Chronological control is established by means of two independent correlations; first, by matching susceptibility variations to the oceanic oxygen isotope record and second, by matching the relative palaeointensity variations to the SINT-800 global reference curve. These both imply an average deposition rate of 15 cm kyr ‐1 and a basal age of ∼640 ka. Spectral analysis reveals the presence of Milankovitch signals at ∼100 kyr (eccentricity), ∼41 kyr (obliquity) and ∼23 and ∼19 kyr (precession). Stable remanence vectors are almost all of normal polarity. The few exceptions comprise brief intervals of low and/or negative inclinations which probably represent geomagnetic excursions. However, these are far less numerous than the high sedimentation rate would lead one to expect. Furthermore, only four of them can be readily matched to the—still poorly understood—global pattern. These are the Laschamp, the Albuquerque, the Iceland Basin and perhaps the West Eifel excursions which occurred at ∼38 000, ∼146 000, at 180 000‐190 000 and at 480 000‐495 000 yr ago, respectively.

Self-modification of the remanence distribution of a hollow cylindrical permanent magnet array

A hollow cylindrical permanent magnet array (HCPMA) used for refrigeration generates a strong magnetic field in the access hole and also inside the permanent magnets. The generated field modifies the remanence distribution of the magnets. Here, we describe a self-consistent calculation method for the remanence vectors and magnetic inductions using the Biot-Savart law. The method takes into account the anisotropic magnetization loops of commercial permanent magnets. Results obtained for the modified remanence distribution of a long HCPMA show demagnetization and deflection, including the reversal of original remanence vectors, when the generated field of the HCPMA exceeds the coercivity of the permanent magnets. The reversed remanence vectors lead to the formation of remanence vortices. In comparison with the field obtained from the ideal model assuming constant remanence vectors, remanence demagnetization and deflection result in obvious decreases in both the field magnitude and field homogeneity in the access hole. However, the field distribution in the access hole can be improved by using the magnet segments of specially high coercivity in places where large inverse or transverse fields exist. The method reported here is applicable for studying the magnetic fields, including the demagnetization, saturation, deflection, and rotation of the remanence vectors, of other permanent magnet arrays.