Step Demagnetization Research Articles

Paleomagnetism of a Neoarchean-Paleoproterozoic carbonate ramp and carbonate platform succession (Transvaal Supergroup) from surface outcrop and drill core, Griqualand West region, South Africa

Detailed paleomagnetic study across the Archean-Paleoproterozoic boundary interval preserved in the Ghaap Group, Transvaal Supergroup, of the Kaapvaal craton reveals a complex series of viscous and thermo-chemical magnetic overprints. Despite this complex history, a potential primary single-polarity remanence direction was revealed at high-temperature demagnetization steps in about 10% of orientated drill core samples only. This component (declination = 132°, inclination = 69°) was not identified in any of the outcrop sites. Overprint directions include a present geomagnetic field remanence and a very well developed thermo-chemical overprint (declination = 359°, inclination = 54°), which is believed to be associated with a ∼2.0 Ga regional thermal event and the development of extensive epigenetic Pb–Zn deposits. At higher levels of demagnetization, two shallow west-directed components appear to be related to the extrusion of the Ongeluk Formation lava at 2.22 Ga (declination = 268°, inclination = −20°; eastern parts of the studied region) and to the post-1.92 Ga Kheis orogeny (declination = 272°, inclination = 16°; exclusive to the western margin of the study area), respectively.

Paleomagnetic Evidence for an Early Permian Age of the Lisheen Zn-Pb Deposit, Ireland

The Lisheen Zn-Pb deposit is one of the major Lower Carboniferous carbonate-hosted base metal deposits of the Irish Midlands. The timing of ore mineralization and, therefore, its genesis remains much debated because of a lack of absolute age determinations. Paleomagnetic analysis of 432 specimens from 12 sites in ore mineralization and 10 sites in host rocks at Lisheen isolated a well-defined stable characteristic remanent magnetization (ChRM) direction using AF step demagnetization, except in one site of massive sulfide mineralization and two sites of postore pink dolomite. Thermal step demagnetization and saturation remanence tests show that the ChRM is carried dominantly by single domain magnetite. A paleomagnetic fold test is negative, indicating that the ChRM postdates burial lithification and deformation. The ChRM directions from eight host-rock and 12 Zn-Pb mineralized sites are indistinguishable at 95 percent confidence. The combined ChRM directions give a mean paleopole at 41.6°S, 18.8°W (semiaxes of the oval of 95% confidence along and perpendicular to the site-pole great circle, dp = 1.7, dm = 3.3) that, when compared to the robust apparent polar wander path for Laurentia rotated into European coordinates, gives an age of 277 ± 7 (2σ) Ma for the ChRM. This 277 ± 7 Ma age is close to the paleomagnetic ages of 290 ± 9 (2σ) Ma for Zn-Pb mineralization at Galmoy and 269 ± 4 (2σ) Ma for the Ba mineralization at Silvermines, which are ~10 km northeast and ~30 km west of Lisheen, respectively. This Early Permian magnetization postdates peak Variscan orogenic heating that is observed in all Carboniferous strata in Ireland. Our preferred interpretation is that the paleomagnetic data, in conjunction with other thermal data, support an entirely epigenetic origin in contrast to the pre-Variscan syn-genetic or syndiagenetic models favored by previous authors. We propose a topographically driven fluid flow, either from the south of Ireland or from closer to the deposit, which was modified by localized convection during the Variscan orogeny.

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.

Optimization of apparent polar wander paths: An example from the South China plate

Paleomagnetically derived apparent polar wander paths form the foundation of quantitative plate tectonic reconstructions. However, deformation leading to vertical axis block rotations displaces paleomagnetic poles away from their original positions, leading to an ambiguity as to which pole, or group of poles, best approximates the “true” reference pole position for a given time. Here we show that the best estimate of the “true” reference pole will match the observed paleolatitude (pλ) for each point on the plate. This means that the expected pλ from the “true” reference pole minus the observed pλ, derived from each individual study, will average to zero. Histogram plots and associated parameters help further discriminate between candidate reference poles when more than one of them fulfills the zero-average requirement within prescribed uncertainty limits. Our analysis of 44 Late Permian to Middle Triassic paleomagnetic poles from the South China plate corroborates previous assumptions that the poles from Sichuan Province best represent the “true” reference for the South China plate. To better understand the age of the rotations, we studied the paleomagnetism of Lower and Upper Triassic rocks from the Shiwandashan region in Guangxi Province. Early Triassic paleomagnetic directions isolated at high temperature demagnetization steps are of dual polarity and pass the fold test. This magnetization component is indistinguishable at 95% confidence limits from Middle Triassic paleomagnetic directions from other parts of Guangxi Province. The corresponding pole for this component lies within the swath of Early to Middle Triassic paleomagnetic poles from the South China plate, confirming that Guangxi has been a part of the South China plate since at least the Early Triassic. Late Triassic paleomagnetic data require further study before their complex magnetizations can be interpreted.

On the possibility of recovering paleo-diurnal magnetic variations in transitional lava flows: I. Constraints from thermoremanence modelling for an experimental protocol

One of the tenets in paleomagnetism is that perturbations of the ground magnetic field due to magnetospheric or ionospheric current systems are too small to leave a detectable paleomagnetic signature in lava flows. As suggested by recent work in paleomagnetosphere modelling, however, external-field perturbations may be significantly enhanced during periods of transitional field behavior, particularly when the dipole-field axis is strongly tilted towards the equator, which then leads to an extremely dynamic magnetosphere on the diurnal time scale even for quiet solar wind conditions. We here demonstrate that thin (rapidly cooled) lava flows ( ∼ 50 cm thick) with high magnetic blocking temperatures (within ∼ 50 –100 ° C below the Curie temperature) indeed have the potential to record such diurnal perturbations. Further, an experimental protocol is suggested to paleomagnetically extract these perturbations. Our proof-of-concept is based on numerical modelling of thermoremanence (TRM) acquisition and simulation of thermal demagnetization surfaces for discrete temperature steps in function of vertical position in the flow. The TRM direction recovered at a given thermal demagnetization step varies with vertical position in the flow and thereby reveals the wave form of the external-field variation. Characteristically, the vertical position of a captured signal changes systematically with unblocking temperature, which reflects the oblique orientation of cooling isochrons, along which the signals are blocked. The signals have their largest amplitudes at the maximum unblocking temperatures, but decay away at lower temperatures. It is by these systmatic trends that external-field perturbations, if trapped, can be paleomagnetically identified and distinguished from a secondary overprint. The experimental procedure requires a sample spacing of 1 cm (with 1 cm drill cores) and small thermal demagnetization steps (15 ° C) at elevated temperatures.

A paleomagnetic test for northward tectonic transport of the eastern Selwyn Basin, northern Canadian Cordillera: data from the mid-Cretaceous Ragged Pluton

Ragged Pluton is a member of the Tungsten Plutonic Suite in the eastern Selwyn Basin. This circular massive 97 Ma granitic pluton is ∼5 km in diameter. It intrudes deformed and metamorphosed clastic strata of the Neoproterozoic to Lower Cambrian Vampire Formation. Thermal and alternating field step demagnetization data for 216 specimens from 21 plutonic sites isolated a stable characteristic remanent magnetization (ChRM) direction at 20 sites of declination D = 325.9°, inclination I = 80.4° (α95 = 2.7°, k = 142). Thermal demagnetization and saturation remanence analysis show that the ChRM is carried by both single-domain pyrrhotite and magnetite. Ragged Pluton’s paleopole at 73.9°N latitude, 191.4°E longitude (dp = 5.0°, dm = 5.2°) is concordant with its co-eval 97 Ma North American reference paleopole, showing at 95% confidence that the pluton has not been significantly rotated or translated relative to North America. This paleomagnetic result favours tectonic models in which the eastern Selwyn Basin is autochthonous, or nearly so, since the mid Cretaceous.

Holocene Sediment Magnetic Properties along a Transect From Ísafjardardjúp to Djúpáll, Northwest Iceland

ABSTRACTHolocene changes in terrestrial provenance and processes of sediment transport and deposition are tracked along a fjord-to-shelf transect adjacent to Vestfirdir, Iceland, using the magnetic properties of marine sediments. Magnetic susceptibility (MS) profiles of 10 cores (gravity and piston) were obtained onboard using a Bartington MS loop. Remanent magnetizations were measured at 1-cm intervals from u-channel samples taken from six cores on a cryogenic magnetometer. Between six and nine alternating field demagnetization steps were used to isolate the characteristic magnetization directions. The chronologies of the cores used in this study were determined from AMS 14C dates on mollusks and foraminifera and constrained by the regional occurrence of the 10,200 ± 60 cal yr. BP Saksunarvatn tephra. Correlative fluctuations in magnetic concentration are noted between the fjord and shelf sites, though these fluctuations are partially masked by regional variations in carbonate content. The onset of Neoglaciation is interpreted by changes in magnetic properties including an increase in mass magnetic susceptibility that began approximately 3000 cal yr. BP. The maximum angular deviation and the median destructive field (generally <20 mT) suggest that the natural remanent magnetization is carried by a coarse ferrimagnetite mineralogy, likely magnetite or titano-magnetite. Reproducible paleomagnetic inclination values are observed in several records, including a nearly vertical inclination around 8000 cal yr. BP, suggesting that the magnetic pole may have been proximal to Iceland, followed by an interval of much shallower inclination (6000–7000 cal yr. BP).

SP-grain production during thermal demagnetization of some Chinese loess/palaeosol

SUMMARY Progressive thermal demagnetization of natural samples to above 300 °C may cause significant alterations of magnetic mineral phases. Such changes are commonly monitored by magnetic susceptibility (MS) determinations at every demagnetization step. In an attempt to characterize in more detail alteration products created during thermal demagnetization of some Chinese loess/palaeosols (L1/S1), MS was measured at both room temperature (MS293K) and at liquid Nitrogen temperature (MS77K) between each demagnetization step. We have used the difference between MS77K and MS293K (temperature dependent susceptibility, kTD= MS293K− MS77K) to monitor production of superparamagnetic (SP) magnetic grains. kTD is a function of; (1) paramagnetic susceptibility according to the Curie law (k∼ 1/T) that predicts a theoretical 3.83 times increase in MS at 77K. (2) SP grains at 293 K that passes into the single-domain (SD) state on cooling to 77 K causing a dramatic reduction in MS77K. (3) magnetite grains exhibiting magnetic changes when passing through the Verwey transition at 120 K will also reduce MS77K. We observe a linear relationship between frequency-dependent MS (kFD) and temperature dependent susceptibility (kTD) indicating that SP-SD blocking of magnetite may be the dominant factor of MS change during cooling. High values of kTD indicates high SP concentrations, and an increase of kTD with increasing demagnetization temperature is mainly attributed to the production of SP grains since the amount of magnetite exhibiting changes in MS at the Verwey-transition is unlikely to increase during heating of loess/palaeosol to 600 °C in air. This interpretation is also based on results from Zero Field Heating IRM77K(1T), Zero Field Cooling of IRM77K(1T) and short-time (100 s) viscous decay of IRM500mT. Our findings suggest that SP-grains are produced continuously in both loess and palaeosol with thermal treatment, and that SP-production in palaeosol apparently commence at the ‘surprisingly’ low temperature of around 200 °C.

Paleomagnetism of the HYC Zn-Pb SEDEX Deposit, Australia: Evidence of An Epigenetic Origin

The HYC Zn-Pb-Ag deposit of the McArthur River Mines Ltd. is in the late Paleoproterozoic HYC Pyritic Shale Member of the 1639 ± 2 Ma (2σ) Barney Creek Formation in the McArthur Basin of the Northern Territory, Australia. This deposit is often cited as the archtypical syngenetic-syndiagenetic McArthur-type sedimentary exhalative (SEDEX) deposit. Paleomagnetic analysis of 273 specimens from 27 sites in seven of eight ore zones (20 sites) and their host rocks (7 sites) using mostly alternating field step demagnetization isolated a well-defined stable characteristic remanent magnetization (ChRM) in all sites but one tuff site. Thermal step demagnetization, saturation remanence, and artificial mill-concentrate specimen tests show that about two-thirds of the ChRM is carried by single- and pseudosingle-domain pyrrhotite and about one-third by single-domain magnetite, which are interpreted to occur as submicroscopic inclusions in ore-stage minerals including sphalerite and galena. Paleomagnetic conglomerate and fold tests are overwhelmingly negative, showing that the ChRM postdates lithification, burial, and fold deformation. The ChRM of the host rocks and ore gives a paleopole at 77.0° S, 167.0° E ( n = 20, α95 = 3.6°). This pole is coincident with that of the 1636 ± 4 Ma (2σ) Linott Formation, providing a direct date for the ChRM. Relative dating using the apparent polar wander path for the southern McArthur Basin dates the ChRM at 1637 ± 1 Ma (1σ). Thus both dating methods show that the ChRM is ~2–3 m.y. younger than the primary age of the host rocks. Using available vitrinite reflectance values, time-temperature calculations indicate that the ore lenses have been heated to 310° ± 30°C for ~105 yr, whereas nearby and remote host rocks have been subjected to much lower temperatures. The results of this study suggest that primary sea-floor sediments, perhaps with syngenetic pyrite, were buried to a depth of ~800 m over 2–3 m.y., with attendant lithification and synclinal folding during rift fault tectonics. The preferred model discussed in this paper is that ascending 310° ± 30°C hydrothermal fluids in the Emu fault zone subsequently penetrated the pyritic zones or other metal-bearing sediments by dissolution and/or replacement, precipitating sphalerite, galena, secondary pyrite, and other ore-stage minerals with pyrrhotite and magnetite inclusions to form an epigenetic ore deposit.

Cryptochron C2r.2r-1 recorded 2.51 Ma in the Koolau Volcano at Halawa, Oahu, Hawaii, USA: Paleomagnetic and 40Ar/ 39Ar evidence

New paleomagnetic measurements, coupled with Argon–Argon ( 40Ar/ 39Ar) radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcano's buttressed flank within Halawa Valley. Prior paleomagnetic and Potassium–Argon (K–Ar) investigations of the Koolau Volcanic Series revealed excursional directions The alkaline composition of lava flows, easy access, and close geographical proximity to K–Ar dated lava flows made this newly studied 120-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 28 successive flow sites, were stepwise demagnetized by both alternating field (5 mT to 100 mT) and thermal (from 28 °C to 575–650 °C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps, with thermal and AF results differing insignificantly. Low-field susceptibility vs. temperature ( k– T) analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point determinations from these analyses revealed a temperature close to or equal to 580 °C, indicative of almost pure magnetite ranging from single domain (SD) to pseudosingle domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼ 10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeastern part of Africa, close to Madagascar. 40Ar/ 39Ar incremental heating experiments on groundmass from nine flow sites at different stratigraphic levels yields isochrons between 2.64 ± 0.23 to 2.37 ± 0.17 Ma with a weighted mean age of 2.514 ± 0.039 Ma, which, combined with the overall reversed polarity and the absence of polarity reversals, strongly suggests that the excursion corresponds to Cryptochron C2r.2r-1 [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093–6095.]. Current dating is insufficiently precise to unambiguously define this excursion as a Microchron (< 10 kyr). This is potentially the first terrestrial record of Cryptochron C2r.2r-1 and the age is 2–3% older than in the reported timescale [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093–6095.]. Our finding places important constraints on the evolution of the entire Koolau shield edifice, since most of the lavas recorded a reversed polarity.

Paleomagnetic and mineral magnetic constraints on Zn–Pb ore genesis in the Pend Oreille Mine, Metaline district, Washington, USA

Zinc–lead mineralization in the Metaline mining district of northeastern Washington, USA, is hosted by the Cambrian Metaline Formation and is classified into Yellowhead-type (YO) and Josephine-type (JO) ore based on texture and mineralogy. Paleomagnetic results are reported for four Cambrian Metaline Formation sites, one Ordovician Ledbetter slate site, 12 YO and 13 JO (including two breccia sites) mineralization sites in the Pend Oreille Mine, and eight sites from the nearby Cretaceous Kaniksu granite batholith. Thermal and alternating field step demagnetization, saturation isothermal remanence analysis, and synthetic specimen tests show that the remanence in the host carbonates and Zn–Pb mineralization is carried mostly by pseudosingle (PSD) to single domain (SD) pyrrhotite and mostly by PSD to SD magnetite in the Kaniksu granite. Based on thermomagnetic measurements, sphalerite and galena concentrates and tailings from the mine’s mill contain hexagonal and monoclinic pyrrhotite. The postfolding characteristic remanent magnetization (ChRM), known thermal data, and paleoarc method of dating suggest that the Zn–Pb mineralization carries a primary chemical remanent magnetization (CRM), and Metaline Formation carbonates a secondary CRM that were acquired during the Middle Jurassic (166 ± 6 Ma) during the waning stages of the Nevadan orogeny. A paleomagnetic breccia test favours a solution-collapse origin for the Josephine breccia. Finally, the Kaniksu paleopole is concordant with the North American Cretaceous reference paleopole, suggesting the Kootenay terrane has not been rotated since emplacement of the batholith at ~94 Ma.

Paleomagnetism of the Fort Knox Stock, Alaska, and rotation of the Yukon–Tanana terrane after 92.5 Ma

The 92.5 Ma Fort Knox granodiorite stock, near the western end of the Fairbanks Belt in the Yukon–Tanana terrane (YTT) of central Alaska, hosts a world-class gold mine. The stock has been analysed paleomagnetically using thermal and alternating-field step demagnetization and isothermal remanence methods. This pluton retains a primary thermoremanent magnetization at 18 sites (232 specimens) that resides mainly in single-to pseudosingle-domain magnetite with a direction of D = 228.8°, I = 84.3° ( N = 18, k = 130, α 95 = 3.0°), giving a paleopole at 56.5°N, 197.1°E ( dp = 5.9°, dm = 5.8°). The pluton's host rock, the Fairbanks schist, does not retain a stable coherent remanence. Relative to the North American craton, the stock's paleoinclination indicates that the Fairbanks Belt has undergone nonsignificant poleward (northwesterly) translation of 25 ± 750 km only. Analysed in concert with the few available paleoinclinations available for the YTT in Yukon, the paleoinclination suggests further that the YTT has undergone only ∼ 250 to 450 km of dextral displacement along the Tintina fault in the past ∼ 100 Ma and, therefore, is parautocthonous since the mid-Cretaceous. The stock's paleodeclination records 121 ± 35° of counterclockwise rotation relative to the North American craton. Consideration of models published for Alaska's tectonic evolution suggests that this paleodeclination discordance is caused by rotations associated with the opening of the Canada Basin, with dextral displacement on the Tintina fault, and with development of the western Alaskan orocline. Thus the paleomagnetic results for the Fort Knox stock support a thin-skin tectonic model for the accretion of the YTT and Intermontane Belt terranes to the northern Cordillera.

Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes

Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE–SW and N–S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over ∼400 km 2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation ( k MAX– k INT) parallel to dike walls and a lineation ( k MAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility ( k MAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow ( k MAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced ∼4 km parallel to the spreading axes and ∼6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for ∼240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ≥ 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, α 95 = 3.9; mean T UB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, α 95 = 8.7; mean T UB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N–S and NE–SW striking dikes. A-component unblocking temperatures ( T UB) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as ∼274 /+ 33 (88 Ma).

User‐friendly software for vector analysis of the magnetization of long sediment cores

New software is described that is designed for easy visualization and treatment of stepwise demagnetization of the natural remanent magnetization measured along sediment cores with pass‐through magnetometers. The software displays demagnetization diagrams (Zijderveld plots) at successive horizons for any depth interval selected by the user. Paleomagnetic directions are automatically calculated using principal component analysis, with an anchored or a free origin. The demagnetization steps to be considered for the diagram visualization and paleodirection calculations are selected by the user. Maximum angular deviation angles and median destruction fields are also calculated. All the results are stored in a single data sheet, which can be easily exported. Demagnetization diagrams can be printed and copied. This Microsoft Excel software can be used by PC or Macintosh microcomputers.

Paleomagnetic and geochronological constraints on the post-collisional northward convergence of the southwest Tian Shan, NW China

A combined paleomagnetic and geochronological study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield 40Ar / 39Ar isochron ages of 58.5 ± 1.3 Ma (2 σ, MSWD = 0.9) and 60.4 ± 1.3 Ma (2 σ, MSWD = 1.7). These compare well with a previously published K–Ar dilution age of 61.7 ± 2.3 Ma for comparable Paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250–350 °C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk of the collection imply that paleomagnetic data from the upper and lower (∼ 115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth's spin axis for ∼ 50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at ∼ 60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya.

Oligocene magnetostratigraphy from Equatorial Pacific sediments (ODP Sites 1218 and 1219, Leg 199)

An Oligocene magnetostratigraphy from ODP Sites 1218 and 1219 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 221 discrete samples. U-channel samples were measured at 1 cm intervals and were stepwise demagnetized in alternating fields (AF) up to a maximum peak field of 80 mT. The magnetization directions were determined at 1 cm intervals by principal component analysis of demagnetization steps in the 20 to 60 mT peak field range. A similar treatment was carried out on the discrete samples, which confirmed the results obtained with u-channel measurements. Sites 1218 and 1219 were precisely correlated based on multisensor track, paleontological and shipboard magnetostratigraphic data; this correlation is substantiated by u-channel measurements. Although the magnetostratigraphy obtained from the u-channels is similar to the interpretation deduced from shipboard measurements based on blanket demagnetization at peak AF of 20 mT, the u-channel results are substantially more robust since many interpretative uncertainties are resolved by the stepwise demagnetization and higher stratigraphic resolution. The temporal resolution of u-channel-based magnetic stratigraphy in the Oligocene section of Sites 1218 and 1219 is better than 5 kyr, and it is therefore suitable for detection of brief polarity subchrons. However, in spite of the high resolution, we did not find any reversals corresponding to the numerous cryptochrons identified in this time span by Cande and Kent [S.C. Cande, D.V. Kent, Revised calibration of the geomagnetic polarity time scale for the Late Cretaceous and Cenozoic, J. Geophys. Res. 100 (1995) 6093–6095].

Miocene magnetostratigraphy from Equatorial Pacific sediments (ODP Site 1218, Leg 199)

Pliocene and Miocene magnetostratigraphy from ODP Site 1218 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 50 discrete samples. U-channel samples were measured at 1 cm intervals and stepwise demagnetized in alternating fields up to a maximum peak field of 80 mT. The component magnetization directions were determined by principal component analysis for demagnetization steps in the 20–60 mT peak field range. A relatively small number of discrete samples were subject to both thermal and alternating field (AF) demagnetization and gave results compatible with u-channel measurements.Magnetostratigraphy from u-channel samples are compared with shipboard data that were based on blanket demagnetization at peak AF fields of 20 mT. U-channel measurements add more detail to the magnetostratigraphic record and allow identification of thin polarity zones especially in the upper part of the section were the sedimentation rates are very low (∼2 m/Myr). The component magnetization directions determined from u-channel measurements also gave more reliable and precise estimates of inclination (paleolatitude). The magnetostratigraphy from Site 1218 can be unambiguously correlated with the reference geomagnetic polarity time scale and gives a means of dating the sedimentary sequence. Both Miocene–Pliocene and Oligocene–Miocene stage boundaries were easily identified from the magnetostratigraphic record.Although calculation of paleomagnetic poles is hindered by the low precision of the cores' azimuthal orientation, the data from both u-channel and discrete samples allow determination of the paleolatitude of the Site through time with good precision. Paleomagnetic data indicate that the paleolatitude of Site 1218 has increased form nearly equatorial latitude in the Oligocene to its present-day latitude close to 9°N. Within the precision of the paleomagnetic data, this is in agreement with current predictions of plate motion models based on fixed hotspots.

Paleomagnetism and 40Ar/39Ar Chronology of Lavas from Meseta del Lago Buenos Aires, Patagonia

As part of a larger project investigating paleosecular variation in South America, 36 paleomagnetic and geochronology sites were sampled on Meseta del Lago Buenos Aires (47°S, 289°E), 200 km east of the active plate boundary in southern Patagonia. Basaltic lavas sampled range from late Miocene to late Pleistocene yet most of the individual lavas are younger than 3.3 Ma. Thirty‐eight isochron ages determined via 40Ar/39Ar incremental‐heating experiments frame a detailed stratigraphy. The isochron ages range from 66.9 ± 4.1 ka for scoria cones and a youthful lava flow in the Rio Pinturas valley to 10.12 ± 1.35 Ma for thick plateau‐forming lavas exposed along the SE edge of the Meseta. The last 3.3 myr is characterized by seven episodes of volcanism at ca. 3.2–3.0 Ma, 2.4 Ma, 1.7 Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka, and &lt;110 ka. The bulk of lavas forming the surface of the Meseta erupted in the last 1.7 my. All sites have stable magnetization, and after step demagnetization using either thermal or alternating field techniques, yield characteristic directions held by magnetite and/or titanomagnetite. Ten sites have distinct transitional directions (defined by pole latitudes &lt;55°), and associated ages indicate possible connections to known reversals within the Matuyama Chron, including the onset of the Jaramillo subchron (1.016 ± 0.01 Ma), the Cobb Mountain subchron (1.25 ± 0.03 Ma), the Ontong‐Java 1 event (1.37 ± 0.03 Ma), and the termination of the Olduvai subchron (1.72 ± 0.02 Ma). Remaining sites are divided into normal (14 sites) and reversed polarity (12 sites). At 95% confidence mean normal and reversed directions overlap. The mean direction for 26 sites is I = −63.0°, D = 3.4°, α95 = 5.4°, which is indistinguishable from the expected geocentric axial dipole. Paleosecular variation, measured by the dispersion of virtual geomagnetic poles about the rotation axis, is higher than expected at 20.0°. Model G, using data for the past 5 Myr, predicts a dispersion of 17°. This discrepancy may be due to true dispersion of the field in southern South America or it may be an artifact of inadequate sampling.

New 42 Ma cratonic North American paleomagnetic pole from the Yukon underscores another Cordilleran paleomagnetism-geology conundrum

Eocene posttectonic plutons of the Beaver River alkalic complex in southeastern Yukon intruded Devonian–Mississippian and Triassic sandstones in the Foothills of the Canadian Cordillera. A paleomagnetic collection of 27 sites from three separate plutons produced 326 specimens that were analyzed using alternating field and thermal step demagnetization methods. The A component characteristic remanent magnetization (ChRM) resides in magnetite with normal polarity in the 42.6 ± 0.8 Ma Beaver River pluton, reversed polarity in the 42.1 ± 0.7 Ma Larson Creek East pluton, and both polarities in the 41.3 ± 0.4 Ma Larson Creek West pluton, corresponding with magnetic polarity chrons 20n, 19r, and the boundary between chron 19r and 18n, respectively. The ChRMs of the plutons are indistinguishable (2σ) with a mean for the 42.0 ± 0.5 Ma complex of D = 158.8°, I = –73.1° (N = 21 sites, α95 = 3.0°, k = 116.8). A positive paleomagnetic contact test shows the A component to be primary, and the poorly isolated B component suggests the host rocks for Larson Creek West are Early to Middle Devonian. The paleopole for the Beaver River complex at 79.2°N, 145.8°E (N = 21, dp = 4.8°, dm = 5.4°; Q = 7) is concordant with interpolated 42 Ma reference poles for the North American craton. In contrast, paleopoles from the accreted Intermontane and eastern Coast Belt terranes record clockwise rotations of 24° ± 10° (Eocene) and 13° ± 5° (Oligocene–Pliocene), indicating that the allochthonous Intermontane terranes have been progressively driven ~240 ± 120 km eastwards up and over pericratonic and cratonic North American lower crust by Pacific plate subduction since the mid-Eocene.

Absolute paleointensities and paleodirections of miocene and pliocene lavas from Fernando de Noronha, Brazil

Forty-four volcanic units were sampled on the Equatorial Atlantic Island of Fernando de Noronha, Brazil. Twenty-seven sites are from the 1.8–3.3 Myr old Quixaba formation, and the others are from the approximately 8–12 Myr old Remedios formation. Analyses of paleodirections yielded normal and reverse polarities in both formations with virtual geomagnetic poles (VGPs) and VGP dispersion ( S f) of VGP lat: 86.9°, VGP long: 136.3°, S f: 11.2° for the Quixaba formation and VGP lat: 86.8°, VGP long: 196.7°, S f: 15.5° for the Remedios formation. Successful paleointensity determinations were obtained for 9 of the 27 sampled units in the Quixaba formation. A modified Thellier method was used that includes checks to monitor alterations and repetition of demagnetization steps to detect multidomain tails of partial thermoremanent magnetizations (pTRM) and magnetization stabilization processes. Furthermore, the additivity of pTRMs was monitored during this modified technique of the Thellier experiment. These additivity checks provide a quality test for correcting the paleointensity determination for magnetomineralogical changes which occur during laboratory treatment. Such correction methods were used for analyzing approximately 50% of the determinations and lead to a reduction of the within-site scatter. The mean paleointensity result of 29.3 μT, corresponding to a virtual axial dipole moment (VADM) of 7.5×10 22 A m 2, yields approximately the present day field strength. Mean values of the paleodirections for volcanic units of reversed and normal polarity are closer to a dipolar field configuration than previously observed. Today’s geomagnetic field in the Equatorial Atlantic shows a significant dipole anomaly. Our results indicate that this anomaly was far less pronounced in the geological past.