Viscous Magnetization Research Articles

Understanding viscous magnetization of multidomain magnetite

Viscous magnetization (VM) and viscous remanent magnetization (VRM) have been measured, as a function of temperature, between room temperature and the Curie temperature using a suite of well‐characterized synthetic and natural multidomain (MD) magnetite samples. Particular attention was given to possible diffusion aftereffects such as dislocation creep (stress relaxation) and disaccommodation (vacancy and ionic reordering) and their contribution to viscous behavior in what has been commonly thought of as a purely thermal fluctuation process. Dislocation creep was examined by measuring viscosity before and after annealing. Annealing was found to reduce the non‐log(t) behavior, where t is time. Non‐log(t) behavior has been associated with diffusion aftereffects, suggesting that these are a major contributor to viscosity and (de)magnetization processes in MD samples. The positive curvature of the non‐log(t) acquisition processes indicates that dislocation creep dominates over disaccommodation. This does not imply that VM and VRM are due solely to dislocation creep, but rather that VRM and VM reflect a number of unrelated temperature‐dependent processes, primarily thermal fluctuations and dislocation creep. This is the first time that dislocation creep has been directly identified as contributing to viscosity at temperature. These findings will have particular implications for paleointensity determinations, as on heating a sample, its dislocation structure may relax, giving rise to demagnetizations not associated with thermal fluctuations. This will lead to incorrect intensity estimates. If no heating is performed on a geological specimen, then it is very likely that laboratory timescale stress relaxation processes will have already occurred in situ.

Palaeomagnetic constraints on continental break-up processes: observations from the Main Ethiopian Rift

Abstract We report the first palaeomagnetic results from the Main Ethiopian Rift (MER), the northernmost sector of the East African rift system. This part of the MER shows an along-axis tectono-magmatic segmentation pattern similar to that of slow-spreading mid-ocean ridges, which developed during the past 1.9 Ma. The aims of our palaeomagnetic, structural and geochronological studies are to test plate kinematic models for the right-stepping, en echelon 60–80 km-long magmatic segments. Twenty palaeomagnetic sites were sampled on either basalt or ignimbrite outcropping in the region adjacent to, and within, the <1.9 Ma-old tectono-magmatic segments of Gademsa-Koka, Boset and Fentale-Dofan. Five K-Ar age determinations were made to bracket the age of units studied in the palaeomagnetic analyses. The natural remanent magnetization intensity possibly exhibited a unimodal distribution with a value of 6.6 A/m (σ = 5.6 A/m) for the basalts and a bimodal distribution with magnetization intensity of 0.69 A/m (σ = 0.55 A/m) and 0.03 A/m (σ = 0.02 A/m), statistically similar to values from previous studies in the Afar triple junction zone (e.g. Kidane et al. 1999, 2002). Progressive heating, alternating field analysis, and susceptibility vs. temperature measurements indicated unblocking temperature ranging between 300 °C−600 °C for basalts and between 500 °C−660 °C for ignimbrites, suggesting the magnetic mineralogy to be titanomagnetite and magnetite for the former and magnetite and titanohematite for the latter. Palaeomagnetic measurements using both TH and AF technique revealed quasi-single component of magnetization with viscous remanent magnetization (VRM) on a few samples. Principal component analysis and statistical averaging resulted in an overall mean palaeomagnetic direction of (D s = 2.3°, I s = 7.8°, α 95 = 7, K = 26.9, N = 17) which is statistically identical to the expected direction (D = 1.9°, I = 13.5°, α 95 = 2.5, K = 105.6, N = 32) from the Apparent Polar Wander Path reference curve for Africa at 1.5 Ma (Besse & Courtillot 2003). The angular difference between the observed and expected directions above with their uncertainty is calculated to be 0.4° ± 7.5°. These results indicate that the Late Pliocene-Pleistocene rocks of the MER in the studied region do not suffer vertical axis rotation, arguing against transtensional and seafloor-spreading-transform kinematic models. We suggest that magma intrusion, rather than large offset faults, produce the right-stepping, en echelon magmatic segments of the MER, which is at the transition from continental to oceanic extension.

Installation age of limestone masonry determined from its viscous remagnetization

AbstractMany rocks passively acquire some time‐dependent or “viscous” remanent magnetism (VRM) at ambient temperatures, without any extraordinary energetic intervention. This magnetization overprints existing remanent magnetization so that it is effectively a remagnetization subparallel to the contemporary geomagnetic field, averaging the geomagnetic field orientation. Certain limestone masonry remagnetizes viscously over an archaeologically useful interval (100 to 8000 Ka) so that the degree of remagnetization is monotonically (but not linearly) related to the construction age. The laboratory unblocking temperature (TUB) that removes the viscous magnetization is a simple monotonic measure of relative age. The longer a piece of masonry remained stabilized in a certain orientation, the greater is its viscous remagnetization and the higher is its TUB. Monuments of known age with a similar limestone source permit us to establish a calibration curve of T UB against historical ages. The resulting calibration curve may then be used to predict the ages of otherwise‐undated masonry. Viscous remanent magnetism dating provides precision of <50a in medieval monuments in England and <150a precision for classical to Neolithic monuments in Cyprus; precision depends on the remagnetization rate of the limestone in question. Our calibration curves, for the Jurassic Oolitic Limestone of England and for the Lefkara‐Pakhna Chalks of Cyprus, allowed us to investigate the authenticity of a medieval English synagogue in Lincoln, England, and of a medieval house in Cyprus. Multiple archaeologic VRMs show that masonry was recycled in historical times. © 2006 Wiley Periodicals, Inc.

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.

Highly magnetic Upper Miocene sandstones of the San Francisco Bay area, California

A high‐resolution aeromagnetic survey of the San Francisco Bay area shows prominent positive anomalies over distinctive blue sandstones of Late Miocene age. The total‐field survey was measured at a nominal height of 300 m above the land surface along flight lines spaced 0.5 km apart. Anomalies with amplitudes up to 200 nT correlate with sandstones of the San Pablo Group, and these anomalies are similar in strength to the magnetic signatures of serpentinites and basalts in the surveyed region. Andesitic sandstone of the Neroly Formation, the upper part of the San Pablo Group, has high magnetic susceptibility (0.013 SI units, volume) and relatively strong natural remanent magnetization (0.29 A/m). Total magnetization of the sandstone is two thirds induced and one third remanent magnetization. The presence of coarse‐grained magnetite detritus, low coercivity of remanence, low thermal stability of remanence, and multidomain properties is consistent with the NRM being a viscous remanent magnetization that grew during the Brunhes normal‐polarity chron. The strong magnetic signature of the Upper Miocene sandstones allows their delineation over distances as great as 100 km, through areas where they are concealed by landslides and younger deposits. The sandstones are important structural markers for understanding the complex folding and faulting associated with active fault systems in the San Francisco Bay area.

Paleomagnetism of Late Paleozoic Strata and Mineralizationin the Tri-State Lead-Zinc Ore District

Paleomagnetic results are reported from five Upper Mississippian and 19 Upper Pennsylvanian limestone and black shale sites (272 samples) and from 42 boulders, oriented with respect to their bedding, of Mississippi Valley-type (MVT) mineralization from three mine waste piles (136 samples) in the Tri-State MVT district and along a northward transect to the Kansas City region. Thermal and alternating field-step demagnetization plus saturation isothermal remanence analysis show that the remanence of the samples is carried mostly by single to pseudosingle domain pyrrhotite and/or magnetite. Both outcrop and boulder samples carry a modern A component from a viscous remanent magnetization in the pyrrhotite and/or magnetite and from hematite produced by weathering. The site mean characteristic remanent magnetization (ChRM) from the five Mississippian outcrops (the D component), when combined with data from seven previously published sites, give a middle to Late Mississippian pole position (330 ± 6 Ma, 1 σ ), which indicates that the host rocks for the Tri-State MVT ores retain a primary chemical remanent magnetization. Similarly the site mean ChRM directions for the 19 Pennsylvanian outcrops (the C component) give a Middle Pennsylvanian pole position (313 ± 8 Ma, 1 σ ). Thus, despite the common presence of trace to minor MVT mineralization in a majority of these sites, they are also deemed to retain a primary chemical remanent magnetization. The MVT mineralization in the boulders yields a ChRM (the M component) with an average inclination of 7.4° ± 1.9°, which constrains ore genesis to an equatorial paleolatitude and which was true only from the Middle Pennsylvanian to the Middle Triassic for the study area. Two interpretations are given, using the paleomagnetic data that support epigenetic MVT ore genesis during either the Late Pennsylvanian (304 ± 6 Ma, 1 σ ) or the Late Permian (257 ± 10 Ma, 1 σ ).

Reliability of geomagnetic paleointensity data: the effects of the NRM fraction and concave-up behavior on paleointensity determinations by the Thellier method

To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies. Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams. We introduce the factor, Q = N〈 q〉, to assess the quality of the weighted mean paleointensity, H w, for each cooling unit.

Absence of natural viscous remanent magnetization in multidomain high-titanium magnetites: evidence for domain-wall interactions

SUMMARY We have studied viscous remanent magnetization (VRM) acquisition in lava clasts from volcaniclastic breccias from Ruapehu, North Island, New Zealand. The age of these deposits ranges from 2.5 to >65 ka BP VRM parallel to the direction of the present Earth’s field is present in most clasts. However, VRM is not recorded in lava clasts in which high-titanium titanomagnetite of composition in the range x = 0.44‐0.65 (∼TM55) is the only or the dominant magnetic mineral present. Experiments demonstrate that these TM55-bearing samples have low blocking/unblocking temperatures, which would have been reset by viscous processes to give VRM if the material was in a single-domain state. Our experiments show that our TM55 samples can acquire low-temperature pTRMs in the laboratory. Analytical electron microscopy confirms the composition and demonstrates that these TM55 grains are up to 100 µm in size and that there are no observable exsolution structures. The grains are therefore in a multidomain state. Previous theoretical work has predicted that multidomain (MD) magnetic material will not acquire VRM if changes in remanence result in a complete reorganization of the whole-grain domain structure, as the activation energy of this process considerably exceeds the kinetic energy available at ambient temperatures (∼20 ◦ C) even over timescales greater than the age of the Earth. Previous laboratory experiments have been reported that show the acquisition of VRM in crushed and sized MD magnetite and titanomagnetite. These results have been used by several authors to conclude that MD VRM proceeds by motion of small segments of individual walls rather than by whole-grain domain reorganization. Our experiments suggest that the previously reported VRM effects in synthetic MD titanomagnetites may be due to contamination by a minute fraction of very fine and very viscous particles rather than due to inherent MD properties. We conclude that, on our natural material, MD TM55 does not acquire VRM. In this material, there is evidently no mode of remagnetization available for VRM acquisition other than thermal activation between whole-grain LEM states, and there is insufficient kinetic energy available at 20 ◦ C to effect this activation. This is the first demonstration of this effect.

Identification of magnetic minerals carrying NRM in pyroclastic-flow deposits

The Ikeshiro pyroclastic-flow deposit at Yufu volcano, Kyushu in Japan, is a typical block-and-ash flow generated by collapse of the Ikeshiro lava dome erupted ca. 2000 years ago. Clasts in the Ikeshiro pyroclastic-flow deposit were found to have a low-temperature magnetic component that is parallel to the present Earth’s magnetic field. We carried out paleomagnetic and rock magnetic experiments in order to examine the origin of the component by identifying magnetic minerals. As a result, the clasts were classified into two types based upon magnetic mineralogy. Type A is carried by titanomagnetite ( T c=480–485°C), whose grain size is mainly MD. Type B is carried by Ti-poor titanomagnetite ( T c=500–580°C), hematite and titanohematite (Fe 2− y Ti y O 3) with y≈0.5 ( T c=215–220°C). The low-temperature magnetic component of Type A was carried by titanomagnetite with very low coercivity, whereas that of Type B was principally carried by titanohematite. The viscous remanent magnetization (VRM) acquisition and demagnetization results indicated that titanomagnetite with low coercivity in Type A acquires VRM easily, whereas titanohematite in Type B hardly acquires VRM. Therefore, the low-temperature magnetic component of Type B originates not from VRM but from thermoremanent magnetization, whereas a part of the low-temperature component of Type A originates from VRM. Emplacement temperature of the Ikeshiro pyroclastic-flow deposit, which is estimated from the unblocking temperature of Type B samples, is between 220 and 400°C. The different magnetic mineral assemblage between the two types reflects the difference in oxidation state: samples of Type B are strongly oxidized, whereas those of Type A are not. Oxidation had probably occurred at the Ikeshiro lava dome before generation of the Ikeshiro pyroclastic flow.

Paleomagnetism of the Newcastle Range, northern Queensland: Eastern Gondwana in the Late Paleozoic

The Newcastle Range is an extensive (2500 km2) and well‐exposed caldera system erupted on the trailing edge of Eastern Gondwana between 325 and 295 Ma. Paleomagnetic samples were collected from ignimbrites and associated microgranitoid intrusions from the central, northern and southern calderas from which three components of magnetization are recognized. Component 1 is considered to be a viscous magnetization acquired during the Brunhes Chron. A presumed Permian component, C2, is found in seven paleomagnetic sites with a mean pole at 30.9°S, 139.7°E (K = 13.9, A95 = 16.8°, ASD = 21.7°), agreeing with previously reported Permian data from Australia. Carboniferous units have a well‐defined characteristic component, C3, distinguished by dual polarity (predominantly reversed) and moderate to steep inclination directions. Paleomagnetic polarities in the Newcastle Range Volcanics are formation dependent and new constraints on the timing of Carboniferous volcanism (∼325–317 Ma) are consistent with recent reanalysis of the base of the Permo‐Carboniferous Reversed Superchron (PCRS). A mean paleomagnetic pole, calculated from 15 VGPs, lies at 63.4°S, 125°E (K = 26.22, A95 = 7.6°, ASD = 15.8°), suggesting that Australia remained at midlatitudes into the Middle Carboniferous. This paleomagnetic pole is consistent with similarly aged poles from Western Gondwana, the conformity of which indicates contributions from nondipole components of the Earth's paleofield were probably not significant in the time immediately preceding the PCRS.

Viscous magnetization, archaeology and Bayesian statistics of small samples from Israel and England

Certain limestones remagnetize viscously and noticeably over archaeological time‐intervals, after their reorientation into monuments. The laboratory demagnetization temperatures (TUB) for the VRM increase with the installation age; with rates of ∼0.07 C0/year for Israel chalk and ∼0.1C0/year for English chalk. The empirical relationship may be used to date enigmatic buildings or geomorphological features (e.g., land slips). Such correlations also give some insight into the viscous remagnetization process over time intervals τ ≤ 4000 years, which are unobtainable in laboratory studies. The TUB‐age relationship for the viscous remagnetization appears to follow a power law, linearized as log10(τ) ≈ b log10 (TUB). Different pelagic limestones follow different curves and, whereas conventional regression estimates the power law exponent b, the small sample size recommends a Bayesian statistical approach. From sites constructed with pelagic chalk from eastern England, precise prior information (b = 0.761) is compared with less precise information for much more ancient sites in northern Israel (b = 0.873). The collective posterior correlation shows a generalized power law exponent b = 0.849. That regression explains 84.9% of the collective variance in age (r2 = 0.849). Of course, site‐specific calibration is required for archaeological age determinations.

Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic glass

Paleointensity estimates based on the high quality Thellier‐Thellier data from the early Brunhes (420–780 ka) are rare (only 30 in the published literature). The Second Hawaiian Scientific Drilling Project (HSDP2) drill hole recovered submarine volcanics spanning the approximate time period of 420–550 ka. These are of particular interest for absolute paleointensity studies owing to the abundance of fresh submarine basaltic glass, which can preserve an excellent record of ancient geomagnetic field intensity. We present here new results of Thellier‐Thellier paleointensity experiments that nearly double the number of reliable paleointensity data available for the early Brunhes. We also show that the magnetizations of the associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true ancient geomagnetic field intensity at the time of extrusion. The viscous contamination is particularly severe because of a combination of low blocking temperatures in the basalts and relatively high temperatures in the deeper parts of the drill core. Our new data, when placed on the approximate timescale available for HSDP and HSDP2, are at odds with other contemporaneous paleointensity data. The discrepancy can be reconciled by adjusting the HSDP timescales to be younger by about 35 kyr.

An Automated High-Sensitivity Vibrating-Coil Magnetometer

A magnetometer allows the measurement of the magnetic moment of samples in a range from 1.3 × 10–8 to 5.45 × 10–4 A m2. The range of variation of the magnetic field strength is from 0 to 56 kA/m; the temperature range is 20–700°C. Measurements are performed in an air atmosphere. The obtained data are automatically inputted into a PC. In addition, the magnetometer can measure the coercive force Hc and the destructive field Hcr. It can also be used to study the formation of thermoremanent and viscous magnetizations and measure the Curie-temperature spectrum and the blocking-temperature spectrum. Measurements of the field, temperature, and time dependences of the magnetic moment can also be performed by the researcher or by special software using a more complicated algorithm.

An example of multicomponent magnetization in welded tuffs: a case study of Upper Cretaceous welded tuffs of eastern Russia

Four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the Upper Cretaceous Kisin Group of the Sikhote Alin mountain range, Russia. In order to contribute toward a basis for an interpretation of multicomponent magnetization, rock magnetic experiments were performed on the welded tuffs. All four magnetization components essentially reside in magnetite. The lowest-temperature component up to 300 °C (component A: D=349.3°, I=60.9°, α 95=7.3°, N=7) is a present day viscous magnetization. The third-removed component (component C: D=41.4°, I=51.8°, α 95=3.5°, N=8), isolated over the temperature range of 450–560 °C, is a primary remanence. The second- and fourth-demagnetized components (component B: D=174.7°, I=−53.1°, α 95=21.2°, N=3 and component D: D=188.1°, I=−64.5°, α 95=4.0°, N=8, respectively) are secondary magnetizations related to a thermal event in Sikhote Alin between 66 and 51 Ma. Components B and D were acquired through different remagnetization processes. Component B is ascribed to a thermoviscous remanent magnetization carried by single-domain magnetite, and component D is a chemical remanent magnetization.

Magnetic mapping and dating of prehistoric and medieval iron‐working sites in northwest Wales

AbstractAs part of a long‐term research project on prehistoric and medieval iron‐working sites in northwest Wales, a technique has been developed for processing magnetic survey data to improve the presentation and recognition of the high‐amplitude dipolar signals that are characteristic of iron‐smelting furnaces. This technique has now been used successfully on some 37 British iron‐working sites. High‐resolution surveys, on a 10‐cm grid, have been made both by caesium magnetometer and fluxgate gradiometer over four prehistoric furnaces and one medieval furnace giving detailed maps of their magnetic signals. Mathematical models of these maps, using multiple dipoles, has given estimates of the directions of total magnetization of the furnaces. Three of the furnaces were also surveyed at subsequent stages of excavation and after the removal of the furnace, giving the background signal. The background signals can then be subtracted from the survey data to give clean residual maps. These are easier to model, giving more reliable results, and they give useful information on the contribution of the different furnace materials to the overall magnetic signals. The corrections required for the influence of induced magnetism and for viscous remanent magnetism are examined. The results are compared with the British archaeomagnetic curve, and with the archaeomagnetic date estimates from three of the furnaces, to assess the value of the modelling technique in giving usable estimates of the date of last firing of the furnaces. Copyright © 2002 John Wiley & Sons, Ltd.

Magnetic constraints on the thermal evolution of a collapsing orogen

SUMMARY The thermal evolution of an orogen undergoing late-stage extension was investigated using rock magnetic properties of a suite of mafic dyke rocks affected by greenschist facies metamorphism in the internal zones of the Betic Cordillera, southern Spain. The natural remanent magnetization (NRM) in the dykes intruded into the lowest geological unit is made up of up to three components. The lowest temperature component (LT) is in the direction of the present day magnetic field and is believed to be a chemical remanent magnetization (CRM) or viscous remanent magnetization (VRM) acquired in the recent magnetic field. The intermediate temperature (IT) component unblocked between 200 ◦ C and 450 ◦ C is thought to be largely a thermoviscous overprint acquired during metamorphism. This component is carried by either primary or authogenic sulfides and low-unblocking temperature magnetite. The component of magnetization with the highest blocking temperature (HT) is isolated above 450 ◦ C and is interpreted as the primary component of remanent magnetization. It is most likely that this component is carried by magnetite that resides in the plagioclase and has been shielded from the metamorphism, which transformed most of the original magnetite to metamorphic amphibole, chlorite and biotite. Thermal demagnetization of these dykes separates the IT overprint from the HT primary remanence at a sharp junction occurring at 450 ◦ C. For single domain grains this translates to a peak palaeotemperature in the natural sample of approximately 370 ◦ C, which is close to the estimated temperature experienced by the greenschist facies country rocks (400 ◦ C). In contrast, results obtained from a dyke that is intruded into an overlying weakly metamorphosed geological unit, indicates that temperatures only reached about 175 ◦ C in this unit. These results are consistent with temperatures deduced from geological constraints and they imply that between 4 and 6 km of section has been removed between the two units during late orogenic extension. These new data are then combined with previously published Ar‐Ar wholerock geochronology and information on vertical axis rotations to put constraints on the thermotectonic evolution of this collapsing orogen.

Paleomagnetism of mid-Cretaceous red beds in west-central Kyushu Island, southwest Japan: paleoposition of Cretaceous sedimentary basins along the eastern margin of Asia

A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240–480°C), and high (480–680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α 95=5°, which yields a paleomagnetic pole at 39°N, 186°E and A 95=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.

Magneto‐gravity response function and its application to the Daito Ridge

A magneto‐gravity response function, which shows a phase relationship between magnetic and gravity anomalies caused by a common source body with a constant density‐to‐magnetization ratio, is derived for determining the magnetization direction of a source body for 2‐D and 3‐D cases. The validity of the method is demonstrated through application to test data and to field anomalies from the Daito Ridge. The Daito Ridge is found to be magnetized in the present main field direction, contrary to shallow inclinations suggested by studies of Deep Sea Drilling Project (DSDP) sediment cores. The strong normal magnetization is ascribed to high magnetic susceptibility and/or viscous remanent magnetization.

Juxtaposed and superimposed paleomagnetic primary and secondary components from the folded middle carboniferous sediments in the Reggane Basin (Saharan craton, Algeria)

A paleomagnetic study was carried out on the Middle Carboniferous sediments of the eastern margin of the Reggane Basin of Algeria. Seven sites (108 samples) in the Lower Serpukhovian and 11 sites (129 samples) in the Upper Serpukhovian, Bashkirian and Lower Moscovian levels were investigated.Besides a common, but generally limited, viscous remanent magnetization (component A) and a recent chemical remanent magnetization of reversed polarity (A′), two main components were identified: one of these (component B), is characterized by a negative fold test and has been identified as a Lower Jurassic remagnetization. The associated paleomagnetic pole obtained in the seven zones by combining characteristic remanent magnetization directions (ChRM) and great circles (λ=71.1°N, ϕ=251.4°E, A95=3.8°, K=254) lies in the vicinity of the NW African poles of similar ages. The second (component C) displays both normal and reversed polarities. Also determined by the combination of ChRM or stable end points and remagnetization circles, it yields a positive fold test which constrains the magnetization acquisition time and a positive reversal test which argues in favor of a “non-composite” nature of the component C. The normal polarities observed in the Lower Serpukhovian levels represent the latest normal event observed in Africa before the Kiaman superchron. The paleomagnetic South pole calculated from 10 sites (n=64 data) gathered in four large areas (λ=26.5°S, ϕ=44.7°E, A95=4.7°, K=383) is the first African Carboniferous pole founded on both positive reversal and fold tests. It lies only slightly apart from other Middle Carboniferous poles previously published for the northern part of Africa where no intraformational test were available to constrain the magnetization age.

Viscous magnetization of loess/palaeosol samples from Bulgaria

The processes of acquisition of viscous remanence and its decay in zero magnetic field are studied for three sets of palaeosol and loess samples. A linear behaviour of VRM vs. log t minus is obtained for 10–104 min. to 30 days of acquisition experiments. Linear correlation between the obtained coefficients of viscous acquisition normalized to saturation magnetization (Sa/Js) or frequency dependent magnetic susceptibility (Sa/χfd%) and the ratio of susceptibility to saturation magnetization (χ/Js), suggests that ultra fine SP/SD grains are mainly involved in the process of viscous acquisition. Non-linear VRM vs. log t behaviour is observed during viscous decay in μ-metal shielded space, being most pronounced for the first 10 min. of the experiment. It is supposed that this phenomena is due to the presence of composite grains (core/shell structure), formed as a result of low-temperature oxidation. The latter increases towards older palaeosol units.