Remanence Directions Research Articles

Forward modelling of oceanic lithospheric magnetization

We construct a model of remanence for the oceans, combine it with a model of induced magnetization for the whole Earth from a previous study, compute the predicted lithospheric geomagnetic field and compare the result with a model, MF7, that is based on satellite data. Remanence is computed by assigning magnetizations to the oceanic lithosphere acquired at the location and time of formation. The magnetizing field is assumed to be an axial dipole that switches polarity with the reversal time scale. The magnetization evolves with time by decay of thermal remanence and acquisition of chemical remanence. The direction of remanence is calculated by Euler rotation of the original geomagnetic field direction with respect to an absolute reference frame, significantly improving previous results which did not include realistic oceanic magnetization computed this way. Remanence only accounts for 24 per cent of the energy of the oceanic magnetization, the induced magnetization being dominant, increasing slightly to 30 per cent of the part of the magnetization responsible for generating geomagnetic anomalies and 39 per cent of the Lowes energy of the geomagnetic anomalies. This is because our model of oceanic crust and lithosphere is fairly uniform, and a uniform layer magnetized by a magnetic field of internal origin produces no external field. The largest anomalies are produced by oceanic lithosphere magnetized during the Cretaceous Normal Superchron. Away from ridges and magnetic quiet zones the prediction fails to match the MF7 values; these are also generally, but not always, somewhat smaller than the observations. This may indicate that the magnetization estimates are too small, in which case the most likely error is in the poorly-known magnetization deep in the crust or upper mantle, or it may indicate some other source such as locally underplated continental lithosphere or anomalous oceanic crust, or even small-scale core fields.

The Ionian Sea: The oldest in situ ocean fragment of the world?

It is well known that the Ionian Sea is characterized by thin (8–11 km) crystalline crust, thick (5–7 km) sedimentary cover, and low heat flow, typical for a Mesozoic (at least) basin. Yet seismic data have not yielded univocal interpretations, and a debate has developed on the oceanic versus “thinned continental” nature of the Ionian basin. Here we analyze the magnetic anomaly pattern of the Ionian Sea and compare it to synthetic fields produced by a geopotential field generator, considering realistic crust geometry. The Ionian basin is mostly characterized by slightly negative magnetic residuals and by a prominent positive (150 nT at sea level) “B” anomaly at the northwestern basin margin. We first test continental crust models, considering a homogeneous crystalline crust with K = 1 × 10−3, then a 5 km thick deep crustal layer of serpentinite (K = 1 × 10−1). The first model yields insignificant anomalies, while the second gives an anomaly pattern anticorrelated with the observed residuals. We subsequently test oceanic crust models, considering a 2 km thick 2A basaltic layer with K = 5 × 10−3, magnetic remanence of 5 A/m, and a unique magnetic polarity (no typical oceanic magnetic anomaly stripes are apparent in the observed data set). Magnetic remanence directions were derived from Pangean‐African paleopoles in the 290–190 Ma age window. Only reverse polarity models reproduce the B anomaly, and among them the 220–230 Ma models best approximate magnetic features observed on the abyssal plain and at the western basin boundary. The Ionian Sea turns out to be the oldest preserved oceanic floor known so far.

Anisotropy of thermoremanent magnetisation of Cryogenian glaciogenic and Ediacaran red beds, South Australia: Neoproterozoic apparent or true polar wander?

Determining the effects of compaction-related inclination shallowing of remanence directions is crucial for ascertaining the validity of low palaeolatitudes for Neoproterozoic red beds in South Australia that are central to the debate concerning low-latitude Proterozoic glaciation. The inclination correction (or flattening) factor, f, is defined as tan(ID)/tan(IF), where ID and IF are the inclinations of the measured detrital remanence and the ancient inducing field, respectively. The anisotropy can be estimated using anisotropy of magnetic susceptibility and the anisotropy of high-field isothermal remanence (hf-AIR). The elongation–inclination (E–I) method has also been used to infer inclination shallowing. We add the anisotropy of thermoremanent magnetisation (ATR) to these methods. For the late Cryogenian Elatina Formation arenites, which constitute the bulk of the Elatina data set, the inclination correction using f=0.738 derived from ATR increases the palaeolatitude of the Elatina Formation from 6.5±2.2° to 8.8±3.2°, which confirms that the Elatina glaciation occurred near the palaeoequator. Inclination corrections for the Ediacaran argillaceous Brachina and Wonoka formations, using f=0.35–0.38 derived from ATR, are significantly greater than for the more arenaceous Elatina Formation, which increases their palaeolatitudes from ~12° to ~30°. Carbonates from the basal Ediacaran Nuccaleena Formation yielded f=0.8 from ATR, which represents only a small palaeolatitude correction from 19° to 23°. The anisotropy results imply that the characteristic remanent magnetisations carried by all these units were acquired early as depositional remanent magnetisations, essentially at the time of deposition. The shift of the palaeopoles from argillaceous units indicating significantly higher palaeolatitudes introduces a distinctive loop into the late Cryogenian–Ediacaran–Cambrian pole path for Australia. This loop shows similarities with the North American pole path for this period, for which true polar wander (TPW) has been inferred. However, until ages of Neoproterozoic strata in South Australia are better constrained uncertainty persists on whether the similarities of the Australian and North American pole paths reflect TPW.

Analysis and interpretation of natural remanence directions at the upper Olduvai boundary at Lingtai, Chinese Loess Plateau

Analysis and interpretation of natural remanence directions at the upper Olduvai boundary at Lingtai, Chinese Loess Plateau

Age constraints on the India–Asia collision derived from secondary remanences of Tethyan Himalayan sediments from the Tingri area

We report paleomagnetic results from the Zhepure Shan Formation and the Youxia Formation of the Tingri section (28°42′N/86°43′E) and the Longjiang section (28°27′N/86°40′E) in southern Tibet. A lower coercive component demagnetized below 30mT and a higher coercive component demagnetized between 30mT and 100mT were separated. Rockmagnetic analyses reveal magnetite as the remanence carrier. The differences in coercivity are probably due to a wider grain size spectrum and therefore different domain states. Remanence directions of the low-coercive component are highly scattered and were not considered for further analysis. Directions of the high coercive component of Members A–C of the Zhepure Shan Fm are fairly well grouped within sites (k>8 for most sites) whereas those of Member D and the overlying Youxia Fm are highly dispersed. Applied fold tests indicate a postfolding remanence for Members A–C. The latest possible time for remanence acquisition of Members A–C is constrained to a mean of 48Ma by comparing the determined paleolatitudes to the curve of expected paleolatitudes versus age, calculated from the APWP of India. Deformation of the rocks is likely related to India–Asia collision and therefore the remanence age yields a constraint on the latest possible age of the India–Asia collision. In consideration of an extended pre-collisional northern Indian margin (Greater India) the determined values of 48Ma (mean) and 37Ma (youngest possible age within 95% confidence limits) underestimate the age of the continental collision.

Upper-crustal, basement-involved folding in the East Range of the Sudbury Basin, Ontario, inferred from paleomagnetic data and spatial analysis of mafic dykes

Tilting of crystalline basement rocks associated with folding strain at uppermost crustal levels is difficult to recognize if basement rocks are devoid of traceable marker planes. Here we use the spatial variation in strike of Paleoproterozoic mafic dyke segments complemented by compiled paleomagnetic data to identify tilting in Archean basement rocks associated with kilometre-scale folds of the eastern Sudbury Basin, Ontario. Spatial analysis of the strike of dyke segments is consistent with generation of the NE lobe and a newly identified anticline, referred to as the West Bay Anticline, in the layered Sudbury Igneous Complex (SIC). This anticline accounts better for the structural characteristics of the eastern Sudbury Basin than a previously proposed anticline with west-plunging hinge line. The West Bay Anticline is characterized by abrupt plan-view thickness variations in the lower SIC and curved faults displaying significant strike separations of SIC contacts. These structural characteristics are consistent with folding strain imparted to the SIC and adjacent Archean rocks during formation of the West Bay Anticline. Sublayer embayments and associated quartz diorite dykes likely served as zones of mechanical weaknesses, at which the higher-order folds localized. Unfolding magnitudes of the NE lobe based on primary paleomagnetic remanence directions are significantly smaller than inferred magnitudes that are based on the assumption that the basal SIC contact was initially planar. Thus, the basal SIC contact in the NE lobe likely had a trough-like geometry at the time of remanence acquisition. We advocate a scenario for the formation of the NE lobe, in which the trough geometry of the SIC is primary rather than a consequence of tilting prior to solidification of, and remanence acquisition in, the SIC. Finally, we caution the interpretation of photo lineaments in eroded basement terranes purely as a consequence of faulting.

Insights into deposition and deformation of intra-caldera ignimbrites, central Nevada

Magnetic studies, together with structural and 40Ar/39Ar age analyses, were used to investigate the development and deformation of ignimbrites of the Caetano Tuff formation in central-north Nevada. Magnetic susceptibility, susceptibility anisotropy (AMS) and magnetic remanence approaches were used to decipher events accompanying emplacement and subsequent deformation of that tuff, for both intracaldera tuffs and outflow deposits. 40Ar/39Ar ages, both those presented here and previously published ages, indicate about 33.8Ma for the age of Caetano ignimbrites, approximately at the Eocene/Oligocene boundary. Although extensive faulting associated with regional extension of the Great Basin region has dissected these deposits, the thickness of the intra-caldera ignimbrites has been estimated at 3.5km or more. Thermal modeling for cooling of a caldera filling of 3.5km thickness indicates acquisition of remanent magnetization can persist up to 25ka, sufficient to average secular variation. This contrasts with rapid cooling and remanence acquisition in outflow ignimbrites typically only tens of meters thick. A 750m thick reference section was selected for more detailed study about 10km to the west of the type area. This reference section, fault-bounded at both base and top, represents the upper part of the formation. Its lower member includes the uppermost 250m of the lower massive resistant unit of ignimbrites, a middle member of 375m of well stratified pyroclastic deposits, vitrophyres and sediments, and an upper member of about 125m of recessive ignimbrite and thin-bedded clastic and pyroclastic deposits. Structural and magnetic remanence measurements suggest 13° eastward tilting of the caldera floor occurred early during deposition of the middle member. Flow directions inferred from AMS Kmax axes in the middle member diverge about 45° relative to those in the underlying member and suggest a change in vent source accompanying the deposition of the middle member. Correcting for E–W extension, the outflow ignimbrites reached approximately 20km to the west and south of the caldera. North of the caldera, ignimbrites formerly assigned to the Caetano Tuff but now assigned to the tuff of Cove Mine, exhibit variable flow directions from an indeterminate source. Significantly however, the intracaldera AMS magnetic lineations, conventionally used as proxies for flow lines, trend mainly WNW parallel to a proposed major paleodrainage which stretches from the southern Cortez Range through 40km to the Tobin Range. Caetano Tuff magnetic remanence is entirely reversed, and corresponds to magnetization acquired in the younger part of magnetochron 13R. The magnetic remanence directions of both the intra-caldera and outflow ignimbrites are similar, D=151°, I=−60°, significantly different from those predicted by a coeval reference pole for North America, raising the possibility of a regional counterclockwise rotation of the sampling area of 15±6°. Presumably this began during faulting and major regional extension about 16Ma ago, during the Middle Miocene, and possibly continued later.

Palaeointensity determinations from historical and Holocene basalt lavas in Iceland

SUMMARY Palaeomagnetic measurements were made for 11 post-glacial basalt lavas in Iceland: three pre-2 ka lavas, seven historical lavas and one present-day lavas. Palaeodirections were easy to determine at most sites, but three of the sites seem to have suffered from some block movement at the lava front, making determination of palaeodirections difficult. Reliable site mean palaeodirections were successfully obtained at two of these sites by analysing the remanence components with low-blocking temperature, but one site was rejected due to low convergence of remanence directions, indicating a possible post-cooling block movement. A total of 45 samples (52 specimens) from nine lava flows were selected for palaeointensity experiments using the Thellier method based on the thermal stability of remanence and other rock magnetic features together with hysteresis parameters to show single-domain to pseudo-single-domain natures. 26 of the 45 samples (29 of the 52 specimens) produced successful results. Subsidiary experiments using Shaw’s method were also carried out on one specimen from each of eight lava flows, and all of these produced successful results. Combining these Thellier and Shaw results, eight site mean palaeointensities were obtained, including one 1984 AD lava, giving a specimen level success rate of 56 per cent for the experiments using the Thellier method and 100 per cent for those using Shaw’s method. Our results together with those of previous studies indicate that variations in palaeointensity and palaeodirection in Iceland are basically similar to those in NW Europe for the last 2000 yr. Variations in palaeointensity, converted to virtual dipole moment or virtual axial dipole moment, are in good agreement with both NW Europe and global models. Some differences in inclination and declination are recognized between Iceland and NW Europe, but overall variation curves are similar and good agreement with the global models is also observed.

Remanent magnetization of maghemitized basalts from Krafla drill cores, NE-Iceland

We investigated the natural remanent magnetization (Jr) of hydrothermally altered basalts from two drill cores KH1 (200 m) and KH3 (400 m) situated at the rim of the Krafla caldera in NE Iceland, where a geothermal field (>150°C) is still active. Low temperature oxidation along with mineral reactions in the chlorite zone (<350°C) is the prevailing cause for the maghemitization and a strong decrease of Jr to occur in our study. Despite a significant decrease of Jr with respect to fresh basalts in surface outcrops of the same area, the stepwise demagnetization analyses of Jr show the presence of a stable magnetic component with the expected inclination of 77° in Iceland. Because the alteration temperature (<350°C) is above the Curie temperatures of most of the original titanomagnetite (40°–350°C), we suggest that a normal direction of remanence is chemically acquired during the low temperature alteration. We observed only one reliable negative inclination at 293.2 m in the KH3 core, which we rather interpret to be acquired during a geomagnetic excursion with reverse polarity than caused by a self-reversal mechanism.

Pyrrhotite remagnetizations in the Himalaya: a review

Abstract This paper reviews results on the nature and thermo-tectonic interpretation of widespread pyrrhotite remagnetizations in the Himalaya. Throughout the last two decades a large dataset has been acquired, in particular from low-grade metamorphic rocks of the Tethyan Sedimentary Series (TSS). The nature of this magnetization is a thermoremanence when the peak metamorphic temperature T max exceeded the Curie temperature ( T c c. 325 °C) of pyrrhotite; in this case the remanence age can be related to last metamorphic cooling. For T max &lt; T c , the remanence can be of chemical, thermochemical or thermoremanent origin. Cooling ages show a systematic trend of c. 50–20 Ma from the western to the eastern Himalaya. The pyrrhotite remagnetizations post-date main Himalayan folding and record late orogenic long-wavelength rotations and tiltings around vertical and horizontal axes. Remanence directions in the western Himalaya are well matching with large-scale deformations of rotational shortening and oroclinal bending, while in the central and eastern Himalaya they are predominantly controlled by meso-scale effects due to crustal doming. Stable pyrrhotite remanences are especially typical for low-grade marly limestones in the TSS, but were also found in medium-grade rocks of the Lesser Himalaya, highly metamorphic rocks of the Higher Himalayan Crystalline and diorite dykes intruding into the TSS.

Paleomagnetic study of the late Neoproterozoic Bull Arm and Crown Hill formations (Musgravetown Group) of eastern Newfoundland: implications for Avalonia and West Gondwana paleogeography1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

A paleomagnetic study of subaerial volcanic rocks and associated siltstones of the Ediacaran Bull Arm Formation in the Avalon Zone of Newfoundland revealed a stable bipolar, hematite-borne primary remanence supported by positive conglomerate, contact, and fold tests. Mean remanence directions in two distal areas (Bonavista and Argentia) are similar, indicating a low paleolatitude position of Avalonia at ∼570 Ma. Redbeds of the overlying ∼550 Ma Crown Hill Formation also carry a primary bipolar hematite-borne remanence with moderate inclination, indicating that Avalonia remained at low to medium paleolatitudes through the end of the Ediacaran. Combining our results with previously published paleomagnetic data of Avalonia suggests moderate-scale drift of Avalonia through low southern paleolatitudes through the latter half of the Ediacaran, providing a paleogeographic context for the development of the first complex metazoan life.

Paleomagnetism of the Quaternary Bandelier Tuff: Implications for the tectonic evolution of the Española Basin, Rio Grande rift

We present newly acquired paleomagnetic data from Bandelier Tuff exposures in the Jemez Mountains (New Mexico) that show no statistically significant tectonic rotation over Quaternary time. Cooling units of the tuff were mapped in detail and correlated using new geochemical data, allowing us to confidently sample isochronous units for paleomagnetic remanence directions. In total, 410 specimens were subjected to step-wise thermal and alternating field demagnetization. Of the 40 accepted site means, 30 have α95 values ≤5°. Analysis of the geographic distribution of the site-mean declinations of the data set reveals no statistically significant tectonic rotation either across (northwest/southeast) the northeast-striking Jemez fault or across (east/west) the north-striking Pajarito fault zone. Similarly, our data do not record any measurable relative change in declination difference (−1.1° ± 1.6°) that could be interpreted as a rotation over the ∼0.36 m.y. time duration between deposition of the two principal stratigraphic members of the Bandelier Tuff. The step-over discussed in this paper is an area of exceptional structural complexity and, as such, meets the definition of “accommodation zone.” We propose the name “Jemez-Embudo accommodation zone” for this composite of structural and volcanic features in recognition of its regional importance in the evolution of the Rio Grande rift.

A paleomagnetic study from the Late Jurassic volcanics (155 Ma), North China: Implications for the width of Mongol–Okhotsk Ocean

A new paleomagnetic study has been carried out on the Tiaojishan Formation effusives in the Chengde area, North China Block (NCB). The age of the Tiaojishan Formation volcanics was recently dated from 153 Ma to 161 Ma by U–Pb and 40Ar– 39Ar. A total of 154 samples from 16 sites within the Tiaojishan Formation were collected. Detailed stepwise thermal demagnetization allows us to isolate two components from 77 oriented drill-core samples. A low-temperature component identified in most samples falls close to the present local Earth field direction. Characteristic remanent directions obtained from high-temperature pass-reversal test yield a pole position at 240.3°E, 59.9°N with α95 = 6.8°. Combined with coeval paleopoles obtained from the Siberia Block (SIB), these results show a significant paleolatitude displacement between the NCB and SIB, which indicates that the Mongol–Okhotsk Ocean was still open with ~ 3000 km width in its eastern part at ca. 155 Ma.

Diverse, discrete, mantle-derived batches of basalt erupted along a short normal fault zone: The Poison Lake chain, southernmost Cascades

The Poison Lake chain consists of small, monogenetic, calc-alkaline basaltic volcanoes located east of the Cascade arc axis, 30 km ENE of Lassen Peak in northeastern California. This chain consists of 39 distinguishable units in a 14-km-long and 2-km-wide zone trending NNW, parallel to nearby Quaternary normal faults. The 39 units fall into nine coherent groups based on stratigraphy, field characteristics, petrography, and major-element compositions. Petrographic differences among groups are expressed by different amounts and proportions of phenocrysts. MgO-SiO 2 , K 2 O-SiO 2 , and TiO 2 -SiO 2 variation diagrams illustrate clear differences in compatible and incompatible elements among the groups. Variation of K 2 O/TiO 2 and K 2 O/P 2 O 5 with MgO indicates that most of the basalts of the Poison Lake chain cannot be related by crystal fractionation at different pressures and that compositions have not been affected significantly by incorporation of low-degree silicic crustal melt or interaction with sialic crust. Limited trace-element and whole-rock isotopic data also suggest little if any incorporation of upper-crustal material, and that compositional variation among groups primarily reflects source compositional differences. Precise 40 Ar/ 39 Ar determinations show that the lavas were erupted between 100 and 110 ka. The migration of paleomagnetic remanent directions over 30° suggests that the entire Poison Lake chain could represent three short-lived episodes of volcanism within a period as brief as 500 yr. The diverse geologic, petrographic, chemical, paleomagnetic, and age data indicate that each of the nine groups represents a small, discrete magma batch generated in the mantle and stored briefly in the lower crust. A NNW normal fault zone provided episodic conduits that allowed rapid ascent of these batches to the surface, where they erupted as distinct volcanic groups, each aligned along a segment of the Poison Lake chain. Compositional diversity of these primitive magmas argues against widespread, long-lived ponding of uniform basalt magma at the base of the crust in this region and against interaction with a zone of melting, assimilation, storage, and homogenization (MASH) in the lower crust.

Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite

[1] Temporal and spatial variabilities of mantle upwelling and melt supply in mid-ocean ridges (MORs) have long been documented. Such variabilities span a range of scales and have a profound effect on the structure as well as the composition of the oceanic crust. Previous seismic and gravity studies have suggested that the lower oceanic crust plays a major role in accommodating these changes in melt supply. Here we report the first direct evidence for a sharp transition from coherent sub-horizontal to near vertical magma flows frozen in the lower oceanic crust of the Troodos ophiolite at the segment edge near a fossil ridge-transform intersection. We constrain the preferred petrofabric lineation directions at 13 gabbroic sites using anisotropy of magnetic susceptibility (AMS) verified by electron backscatter diffraction. Pre-emplacement accretion-related rotations were corrected using magnetic remanence directions. We identify two provinces of nearly uniform susceptibility directions (principal axes) and attribute them to two magmatic episodes. A more focused mantle upwelling and melting episode near the segment midpoint may have resulted in lower crustal lateral magma flows along the fossil segment-edge, whereas uniform mantle upwelling and melt supply along the entire axis may have resulted in vertical magma flows at the segment-edge. Overall, our data verify the vital role of the lower oceanic crust in accommodating changes in mantle upwelling and melt supply beneath MORs.

An attempt to constrain the age, duration, and eruptive history of the Karoo flood basalt: Naude's Nek section (South Africa)

We have carried out paleomagnetic sampling of a similar to 750 m sequence of the Karoo large igneous province (Naude's Nek Pass, South Africa). K-Ar dating (Cassignol-Gillot) has been performed on four samples from the 650 m upper unit (mean age 179.2 +/- 1.8 Ma) and a sample from the lower unit (184.8 +/- 2.6 Ma). A succession of two phases of volcanism is suggested. The lower 25 flows (115 m thick) have recorded a reversed polarity; the next 23 flows (135 m thick) are transitional and contribute a detailed record of the Van Zijl (1962) Jurassic reversal. The upper 38 flows (500 m thick) have normal polarity. Directional groups (DGs) of lava flows with quasi-identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events. Altogether, 19 DGs and 10 sheet lobes yield a sequence of 29 distinct directions. This could correspond to a total eruptive activity shorter than 3000 years, less than one per mil of the total duration over which the section was emplaced. We obtain a new paleomagnetic pole for South Africa at similar to 180 Ma (lambda = 75.2 degrees N, phi = 276.4 degrees E, A(95) = 5.8 degrees, N = 19), which is consistent with earlier reports.

Paleosecular variation during the Kiaman Superchron: a 1150 year record from glacial varves of the Seaham Formation, New South Wales

Glacial varves of the Seaham Formation at Abercrombie Quarry in eastern New South Wales, Australia, preserve a reverse-polarity characteristic remanence acquired at high paleolatitude during the early stages of the Kiaman Superchron, the longest known interval of constant geomagnetic polarity. Normal polarity overprinting, including the pervasive Cretaceous overprint commonly observed on the southeastern Australian seaboard, is present but does not obscure the primary magnetisation. Ferrimagnetic grains are in the pseudo-single domain size range, confirming the reliability of the varves as paleo-remanence recorders. Rip-up clasts maintain the original direction of the chararacteristic remanence despite their random orientation following redeposition, a form of conglomerate test that restricts the timing of remanence acquisition to within at most a few years of original sedimentation. Characteristic remanence directions from 11 sites spanning 1150 years of annual layers move through a simple, systematic cycle, resembling patterns of secular variation observed for the modern geomagnetic field. Mean virtual geomagnetic poles (VGPs) calculated for the sites trace part of a secular variation cycle around the Upper Carboniferous paleomagnetic pole for the region. The mean of site poles at this locality is at longitude 132.8°E, latitude 31.1°S, with A 95 = 21.3°, although this pole does not span sufficient secular variation to represent a true paleomagnetic pole. The angular standard deviation of the site VGPs exceeds both the standard Model G phenomenological description of the modern field and the Time-Averaged Field Initiative results for comparable latitudes, and appears to agree with the original interpretation of non-suppressed paleosecular variation from late Kiaman, low latitude sites in France. Large scatter in the Abercrombie Quarry poles supports the interpretation that the primary (dipole) family dominates the geomagnetic field during a superchron, and that this condition was already established within the first 11 million years of its 52 million year extent.

Paleomagnetic constraints on an Archean–Paleoproterozoic Superior–Karelia connection: New evidence from Archean Karelia

Charno–enderbitic granitoids in the Karelia craton of the Fennoscandian shield have been studied paleomagnetically. The characteristic remanence component has a steep negative inclination and is interpreted to record magnetization at a maximum age of 2684±2Ma. Consistently stable results were obtained from 12 sites in the Koitere area, corresponding to regions with high positive magnetic anomalies and high remanence intensities. Petrographic studies, coupled with rock magnetic investigations, indicate that the remanence resides in fine SD/PSD magnetite grains formed during Neoarchean clinopyroxene alteration. Cross-cutting vertical/subvertical Paleoproterozoic dolerite dykes suggest that the Koitere granitoids are in their original orientations and were not affected by Svecofennian deformation at ca. 1.9–1.8Ga.The Koitere granitoids have an opposite polarity compared to the steep positive inclination remanence direction of the previously studied ca. 2.63Ga Varpaisjärvi enderbites and granulites. The data from Koitere and Varpaisjärvi imply that at ca. 2.7–2.6Ga the Karelia craton was located at high latitudes of 80–60°, whereas previous paleomagnetic data from ca. 2.5Ga formations in the Vodlozero terrane in NW Russia indicate a near-equatorial position.Comparison of paleomagnetic data from the Koitere and Varpaisjärvi granulite-grade rocks with rocks of similar age in the Superior craton shows that at ca. 2.7–2.6Ga the Superior and Karelia cratons were located at high latitudes and in close proximity, although the present data cannot demonstrate that the cratons were amalgamated. However, during the Archean–Paleoproterozoic transition at ca. 2.50Ga both cratons experienced significant rotation and drifting to near-equatorial paleolatitudes, suggesting that the Superior and Karelia cratons may have been attached at that time.

New Palaeoproterozoic palaeomagnetic data from the Kaapvaal Craton, South Africa

Abstract Palaeomagnetic data from the well-dated 2060.6±0.5 Ma Phalaborwa Complex in South Africa (Kaapvaal Craton) are of excellent quality. High unblocking components are carried by magnetite and single polarity remanence directions (mean declination 5.0°, inclination 57.3°, α 95 = 5.2°) yield a palaeomagnetic pole (latitude 27.7°N, longitude 35.8°E, A95 = 6.6°) that overlaps with existing poles from the near coeval 2054.4±1.3 Ma Bushveld Complex. The Phalaborwa and Bushveld complex poles, along with poles from the well-dated Vredefort impact (2023±4 Ma) and Post-Waterberg Dolerites (1874.6±3.9 Ma), define the most reliable poles for the Kaapvaal Craton during this time interval ( c. 2060–1875 Ma) and witness low rates of Mid-Palaeoproterozoic apparent polar wander. Poorly dated NE–NNE-trending dyke swarms that intrude the Phalaborwa and Bushveld complexes both yield dual-polarity remanence components that share a common mean at the 95% confidence level. Primary palaeomagnetic poles (Phalaborwa dykes pole latitude 7.6°, longitude 12.1°, A95 = 11.8°; Bushveld dykes pole latitude 12.6°, longitude 24.1°, A95 = 10.8°) suggest that they are of the same age as the Post-Waterberg dolerites ( c. 1875 Ma). They could also be as old as the Phalaborwa and Bushveld Complexes, however; high-precision geochronology is required to resolve this issue and to enlarge the number of Palaeoproterozoic key poles for the Kaapvaal Craton.

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.