Maximum Unblocking Temperatures Research Articles

Remagnetization of magnetite-bearing rocks in the Eastern Qiangtang Terrane, Tibetan Plateau (China): Mechanism and diagnosis

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling.

Magnetic signature of sewage polluted river sediments

The present study was designed to characterize the magnetic proxy for sewage-related pollution from river sediments. A suite of mineral magnetic analyses including low-field magnetic susceptibility, isothermal remanent magnetization (IRM) acquisition and demagnetization, back-field IRM treatment, and thermal demagnetization of composite IRMs are carried out for topsoil samples collected from river sediments in Daejeon, South Korea. A 350-m long in situ profile of magnetic susceptibility shows a background mean magnetic susceptibility of 22.4 ± 5.6 × 10−5 SI. The coercivity spectra display dual peaks of lower- and higher-coercivity fractions. The predominance of magnetic signal with maximum unblocking temperatures of 580 °C points to magnetite as the sole magnetic mineral. Hence the lower- and higher-coercivity fractions correspond to coarse-grained and fine-grained magnetite, respectively. Extraordinarily high magnetic susceptibility was observed from BTC02, near to the sewage discharge point. However, its mineral magnetic properties and particle morphologies are not different to those of other soil samples. Then, it is apparent that a high magnetic susceptibility simply reflects the high concentration of magnetites in BTC02. It is likely that a continuous discharge of heavy metal enriched water at the discharging point concentrated significant amount of magnetic particles. It is fortunate that anomalously high magnetic susceptibility (by implication heavy metal concentrations) diminished only several meters away from the discharge point. Perhaps, influences of anthropogenic magnetite were diluted as the high magnetic phases were washed away from the water cycles along the river. In practice, it can be proposed that a periodic replacement of soils near the sewage discharge point is efficient to reduce the heavy metal concentrations.

Unblocking temperature of secondary magnetic component to outline anomaly thermomagnetic maps of archaeological fireplaces from the southern region of Mexico City, Mexico

We report a magnetic study of two preceramic hearths located at the southern region of Mexico City. Thermal demagnetization was applied between 200 to 540 °C to define the remanent magnetization components. The maximum unblocking temperature of the secondary magnetic component reached due to the last heating was identified in most samples. They were used to develop thermomagnetic anomaly source maps. The direction of this component was used to place the blocks to their last cooling spot and configurate a real and primal magnetic anomaly map produced by the thermoremanent magnetization acquired. A protocol to relocate blocks frame is proposed using geomagnetic secular variation curves. The thermomagnetic mapping revealed the position and temperature (220 to 460 °C) of the last heat sources. The distribution of these sources allowed to model an extensional zone of benefit and probable uses of the hearths.

Extensional vs. compressional deformation in the Central High Atlas salt province: A paleomagnetic approach

In this paper we address the problem of the distinction between diapiric, salt-driven and compressional structures, using the outstanding example of the Central High Atlas (Morocco). A remagnetized component carried by magnetite has been isolated in 32 new paleomagnetic sites. It is characterized by: maximum unblocking temperatures around 450 °C, syn-folding behavior and normal polarity.These 33 mean paleomagnetic directions were analyzed together with other 68 from published works around the study area to construct a robust paleomagnetic dataset along a cross-section perpendicular to the main structures. The remagnetization direction (n: 100, Dec: 332.2°, Inc: 34.5°, η: 6.2°, ξ: 2.0°, A/n: 6.427°) and the paleo-dip of beds (the attitude of the beds at the remagnetization occurrence) were calculated through small circle methods. The remagnetization can be dated as ca. 100 Ma. Because of its occurrence between the extensional and compressional periods, this remagnetization offers the possibility of restore the basin to its pre-inversion geometry.Comparison between present-day and pre-inversion structure allows discriminating three different evolutionary patterns: (i) thrusted and welded salt-walls mainly structured during the extensional stage (Ikkou ridge) with steep limbs close to the salt-wall core. (ii) Jurassic salt-walls with weaker deformation, restricted to the areas adjacent to the structure (Tadaghmamt and Timedouine); in this case, Cenozoic compression is limited to welding of the salt-walls and buttressing of the sedimentary sequences against faults. (iii) salt-rollers gently initiated during the Jurassic (Toumliline diapir), thrusted during the Cenozoic compression. Results show the importance of salt tectonics both during extension and compression, as well as the control of the compressional features by the inherited extensional structures. The performed restorations prove that paleomagnetism is a useful, independent tool to obtain palinspastic restorations and to separate, and quantify, the imprint generated during the basinal stage from the inversional features.

Paleomagnetism of the Cretaceous Galula Formation and implications for vertebrate evolution

This study uses magnetostratigraphy to help constrain the age of the paleontologically important Galula Formation (Rukwa Rift Basin, southwestern Tanzania). The formation preserves a Cretaceous vertebrate fauna, including saurischian dinosaurs, a putative gondwanatherian mammal, and notosuchian crocodyliforms. With better dating, the Galula Formation and its fossils help fill a temporal gap in our understanding of vertebrate evolution in continental Africa, enabling better evaluation of competing paleobiogeographic hypotheses concerning faunal exchange throughout Gondwana during the Cretaceous. Paleomagnetic samples for this study were collected from the Namba (higher in section) and Mtuka (lower in section) members of the Galula Formation and underwent stepwise thermal demagnetization. All samples displayed a strong normal magnetic polarity overprint, and maximum unblocking temperatures at approximately 690 °C. Three short reversed intervals were identified in the Namba Member, whereas the Mtuka Member lacked any clear reversals. Given the relatively limited existing age constraints, one interpretation correlates the Namba Member to Chron C32. An alternative correlation assigns reversals in the Namba Member to recently proposed short reversals near the end of the Cretaceous Normal Superchron (Chron C34), a time that is traditionally interpreted as having stable normal polarity. The lack of reversals in the Mtuka Member supports deposition within Chron C34. These data suggest that the Namba Member is no older than Late Cretaceous (Cenomanian-Campanian), with the Mtuka Member less well constrained to the middle Cretaceous (Aptian-Cenomanian). The paleomagnetic results are supported by the application of fold and reversal tests for paleomagnetic stability, and paleomagnetic poles for the Namba (246.4°/77.9°, α95 5.9°) and Mtuka (217.1°/72.2°, α95 11.1°) members closely matching the apparent polar wander path for Africa during the Late Cretaceous. These results confidently indicate a Late Creteceous age assignment for the Namba Member of the Galula Formation, a unit that has yielded key crocodyliform (e.g., Pakasuchus; Rukwasuchus) and dinosaur (e.g., Rukwatitan; Shingopana) fossils from eastern Africa.

Three archaeomagnetic applications of archaeological interest to the study of burnt anthropogenic cave sediments

Recent archaeomagnetic studies carried out on Mid-to Late Holocene burnt anthropogenic cave sediments have shown that under certain conditions, these materials are suitable geomagnetic field recorders. Archaeomagnetic analyses carried out on these contexts constitute a rich source of information not only for geophysical purposes -in terms of reconstructing the variation of Earth's magnetic field in the past- but also from the archaeological point of view, for example by archaeomagnetic dating. Here, we report three different archaeomagnetic applications to the study of burnt cave sediments: (i) archaeomagnetic dating; (ii) determining palaeotemperatures and (iii) assessing post-depositional processes. The first case study is a dating attempt carried out on a Late Holocene (Bronze Age) burnt level from El Mirador Cave (Burgos, Spain). Using the directional European secular variation curve, several dating intervals were obtained for the last burning of this combustion feature. Considering the archaeological evidence and the independent radiometric (14C) dating available the possible ages obtained are discussed. This is the first archaeomagnetic dating obtained in these contexts so far. The second case study is an application of the method to determine the last heating temperatures reached by the carbonaceous facies of these fires. Stepwise thermal demagnetization of oriented samples can be used to quantitatively estimate heating temperatures. An intermediate normal polarity component interpreted as a partial thermo-remanence (pTRM) with maximum unblocking temperatures of 400–450 °C was systematically identified, revealing the last heating temperatures experienced by this facies. These temperatures were confirmed with partial thermomagnetic curve experiments. Finally, archaeomagnetic analyses on a partially bioturbated burning event were performed in order to evaluate until what spatial extent the burnt sediments were affected by post-depositional mechanical alteration processes. For each case study, the archaeological implications are discussed highlighting the potential of archaeomagnetic methods to retrieve archaeological information.

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.

Inversion of titanomaghemite in oceanic basalt during heating

We studied the change of magnetic behaviour upon laboratory heating of altered mid-ocean ridge basalt (MORB) samples in the age range of 16–35 Ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation (NRM) of these basalts. MORB samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or NRM was monitored continuously. After each heating step, hysteresis loops and remanent magnetisation curves between 10 K and room temperature were measured. With this procedure, it is shown that the dominant magnetic remanence carrier in our MORB samples is cation deficient titanomaghemite. Moreover, it is demonstrated that the titanomaghemite is gradually changing to a Ti-poor titanomagnetite as the final inversion product. During inversion, both the Curie temperature as well as the maximum unblocking temperature of the NRM are gradually increasing. We show that the paradox of unblocking temperatures above the Curie temperatures often observed for altered MORBs is an artefact of this gradual, heating induced inversion process.

Counter-clockwise rotation of the eastern part of the Mongolia block: Early Cretaceous palaeomagnetic results from Bikin, Far Eastern Russia

SUMMARY We present palaeomagnetic results from Lower Cretaceous rocks in Bikin area of the Alchan basin (46.5°N, 134.7°E), Sikhote Alin orogenic belt, Far Eastern Russia. A high-temperature magnetization component with maximum unblocking temperatures at about 590°C was isolated from six sites of dacite welded tuffs in the Albian Alchanskaya Formation. The fold tests for these six sites are positive, suggesting that primary magnetization is preserved in the studied rocks. The tilt-corrected mean direction of D= 309.3°, I= 68.7° (α95= 10.1°), with a corresponding palaeopole position at 57.0°N, 76.8°E (A95= 15.1°), indicates a counter-clockwise (CCW) rotation for the studied area. CCW rotation is also indicated from west-directed declinations (D= 249.1°, I= 64.1°, α95= 11.2°) obtained from secondary magnetization of the Berriasian Kultukha Formation. Combining with the previously reported studies, the west-directed Cretaceous palaeomagnetic directions cover widely the eastern part of the Mongolia block. Comparison with 100 Ma palaeomagnetic pole for Eurasia shows that the eastern part of the Mongolia block experienced a CCW rotation of over 36° with respect to the Eurasian continent later than Late Cretaceous. This rotation is ascribed to post-Late Cretaceous extension that affected the continental basins (the Middle Amur, Sanjiang, Razdolnian, Amur-Zeya and Songliao basins) of the northeast Chinese Plain along the eastern margin of the Mongolia block. Contemporaneous with this CCW rotation, similar extension resulted in clockwise rotation of the eastern part of the North China block.

Paleomagnetic study of Upper Jurassic rocks from the Sichuan basin: tectonic aspects for the collision between the Yangtze Block and the North China Block

A paleomagnetic study has been carried out on Late Jurassic redbeds in the Sichuan basin, at the northern part of the Yangtze Block. Upper Jurassic brownish-red sandstones and brownish-red siltstones were collected in the Penglaizhen Formation at 26 sampling sites around Jiangyang city (30.4°N, 104.5°E). Thermal demagnetization isolated a high temperature magnetization component with a maximum unblocking temperature of about 690°C. The primary nature of magnetization acquisition is ascertained through a 95% confidence level positive fold test, as well as a positive reversal test. The tilt-corrected overall mean direction of 24 sites is D=25.9°, I=26.7° ( α 95=4.7°), leading to a paleomagnetic pole situated at 61.3°N/222.7°E ( A 95=4.2°). This pole position agrees with two other Late Jurassic poles from the Sichuan basin, indicating that this basin has behaved as a rigid block since Late Jurassic. These three poles from the Sichuan basin therefore provide a characteristic Late Jurassic pole for the stable part of the Yangtze Block (64.3°N, 231.2°E, A 95=6.7°). Comparison of the Late Jurassic pole of the Yangtze Block with the contemporaneous pole for the North China Block (NCB) identifies a significant difference, implying that the relative movement between the Yangtze Block and the NCB continued during, and probably after, the Late Jurassic. Paleomagnetic and geological evidence reveals that the Yangtze Block collided first with the NCB during the Early Permian, and continued to penetrate into it while undergoing a clockwise rotation until the Early Cretaceous.

La rotation miocene inferieur du bloc corso-sarde; nouvelles contraintes paleomagnetiques sur la fin du mouvement

La rotation miocene inferieur du bloc corso-sarde; nouvelles contraintes paleomagnetiques sur la fin du mouvement

Palaeomagnetism and magnetostratigraphy of uppermost Permian strata, southeast New Mexico, USA: correlation of the Permian-Triassic boundary in non-marine environments

Continental red sandstone and siltstone rocks of the Dewey Lake (Quartermaster) Formation at Maroon Cliffs, near Carlsbad, New Mexico, are characterized by two components of magnetization with partially overlapping laboratory unblocking temperature spectra. Both magnetizations display high coercivities (>100 mT), probably residing in haematite. A north-directed magnetization with steep positive inclination unblocks between 100 and 650 °C, isolating a predominantly northwest-directed magnetization, with shallow inclination, of near uniform normal polarity and maximum unblocking temperatures of 680 °C. We collected samples from 24 palaeomagnetic sites (i.e. individual beds) from a ~60 m thick section of flat-lying strata disconformably overlying carbonate and evaporite rocks of the Rustler Formation. The upper member of the Rustler Formation contains a Late Permian (early Changxingian) marine invertebrate and conodont fauna. Of the sampled sites, four yield only steep magnetizations, interpreted to be recent overprints. Eight sites did not yield well-grouped site means and were excluded from the final calculations. The formation mean (dec=337.7°, inc=9.2°; k=31.6, α95=7.8°, N=12 sites) defines a palaeomagnetic pole located at 55.2°N, 117.5°E, in good agreement with other Late Permian North American cratonic poles. Correlation of the short polarity sequence of this section of Dewey Lake strata is unambiguous. Compared with the polarity stratigraphy of marine sections in Asia, and supported by isotopic age determinations on a widespread bentonite bed in Dewey Lake strata in west Texas (approximately 251 Ma) and fossil data for the underlying Rustler Formation, the magnetostratigraphy is consistent with deposition of the Dewey Lake Formation during the latest Changxingian (Late Permian) stage.

Orogenic remagnetizations in the Front Ranges and Inner Foothills of the southern Canadian Cordillera: Chemical harbinger and thermal handmaiden of Cordilleran deformation

ABSTRACT Paleomagnetic studies of carbonate rocks in mountain belts have been helpful to the understanding of orogenic mational origin. With only one exception, the A component has normal polarity in the Front Ranges and reverse processes, the timing of deformation, and the nature of fluid flow in actively deforming foreland basins. Most studies on shallow-water carbonates indicate that their remanence is not primary, but rather they record remagnetization events contemporaneous with orogeny. By considering the remanent directions, fold test results, and rock magnetic properties, and tying these results to complementary structural, geochronological and geochemical studies, we are able to interpret deformation history, hydrological environment, and orogenic diagenesis. We study the relationship between remagnetization and deformation of Paleozoic carbonates of the Front Ranges and Inner Foothills of the southern Canadian Rockies. The magnetic properties are remarkably constant along 500 km strike length, as sampled at 124 sites through 4 transects. Primary Paleozoic remanent directions which would have shallow inclination, are never observed. Rather, the paleomagnetic signal is dominated by geographically persistent remagnetizations, characterized by steep inclination. As well as a soft present field overprint, we observed two distinctive secondary magnetizations, named the A and B components, carried by fine-grained magnetite. Pervasive diagenesis induced the A component, a total chemical remanent remagnetization. Poles for the A component are better concentrated after bedding correction indicating a pre- or early syndefor-polarity in the Inner Foothills. Pole positions, polarity, and geological and thermal constraints indicate that the A component was acquired diachronously in advance of the eastward migrating Cordilleran tectonic wedge. Subsequently, an intermediate temperature partial thermo-remanent remagnetization, the B component, was superimposed on large regions of the Front Ranges and Inner Foothills. B component directions are brought into optimal concentration by differential untilting of 0% to 50%, indicating that the component was acquired after the rocks were incorporated into the orogenic wedge, but before the end of contractional deformation. The B component is strongest within a couple of kilometers of the frontal thrust of the Front Ranges. The relative magnitude of the B to A components and the maximum unblocking temperature of the B component decrease away from the frontal thrust over about 30 km, both to the west and to the east. The B component thermal overprint was attained by <250°C heating in response to tectonic, or possibly sedimentary, loading. It was preserved by a rapid cooling accompanying a differential uplift and erosion of up to 8 km in the vicinity of the frontal thrust late in, or post-dating, its local tilting history. The likely cause was uplift of the exposed structural panel by contraction of younger underlying thrust structures.

New Late Jurassic palaeomagnetic data from the northern Sichuan basin: implications for the deformation of the Yangtze craton

SUMMARY Upper Jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the Penglaizhen formation. This formation is in the north of Bazhong county (31.8°N, 106.7°E) in the Sichuan basin, which is located in the northern part of the Yangtze craton. Thermal demagnetization isolated a high-temperature magnetic component with a maximum unblocking temperature of about 690°C from 45 layers. The primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities, as well as positive fold and reversal tests at the 95 per cent confidence level. The tilt-corrected mean direction of 36 layers is D=20.0°, I=28.8° with a 95 =5.8°. A Late Jurassic palaeomagentic pole at 64.7°N, 236.0°E with A 95 =7.0° is calculated from the palaeomagnetic directions of 11 localities. This pole position agrees with the two other Late Jurassic poles from the northern part of the Yangtze craton. A characteristic Late Jurassic pole is calculated from the three poles (68.6°N, 236.0°E with A 95 =8.0°) for the northern part of the Yangtze craton. This pole position is significantly diVerent from that for the southern part of the Yangtze craton. This suggests that the southern part of the Yangtze craton was subjected to southward extrusion by 1700±1000 km with respect to the northern part. Intracraton deformation occurred within the Yangtze craton.

Paleomagnetism of the Paleoproterozoic hematitic breccia and paleosol at Ville-Marie, Québec: further evidence for the low paleolatitude of Huronian glaciation

The paleomagnetism of a saprolitic paleosol developed on Archean granite and of an overlying hematitic breccia in the basal Lorrain Formation of the Cobalt Group (Huronian Supergroup, ∼2.4–2.3 Ga) near Ville-Marie, Québec, has been investigated to further constrain the paleolatitude of the Lorrain Formation and the subjacent glaciogenic Gowganda Formation. Only the breccia, whose hematite may be a product of weathering prior to or shortly after deposition, yielded useable results with stepwise thermal demagnetization. A stable component A (maximum unblocking temperature, T ub, of ∼675°C) carried by hematite is directed shallowly to the east-northeast ( D=59.5°, I=2.4°, α 95=6.7°, n=47, dip-corrected), and a less stable component B (maximum T ub of ∼580°C) carried by magnetite is directed steeply down to the northeast ( D=52.1°, I=70.3°, α 95=2.8°, n=75, dip-corrected). Component A is ascribed to a chemical remanent magnetization (CRM) acquired when the hematitic breccia was deposited or soon thereafter. Component B is interpreted as a thermochemical overprint related to the Penokean Orogeny at 1.9–1.8 Ga. Dip-corrected specimen mean directions give the pole positions at latitude = 57.1°N, longitude = 338.3°E (dp = 4.1°, dm = 4.8°) for component B, and latitude = 21.1°S, longitude = 213.1°E (dp = 3.3°, dm = 6.7°) for component A. The more easterly directions for components A and B relative to comparable early and overprint components previously determined for the Lorrain and Gowganda formations at Elliot Lake and Desbarats 260–360 km southwest of Ville-Marie imply relative rotation of ∼30° or more about a vertical axis within the Southern Province since the Penokean Orogeny. The results suggest that the hematitic breccia and the Ville-Marie paleosol formed at a paleolatitude of 1.2°±3.4°, in accord with our earlier inference of equatorial paleolatitudes for the Lorrain and Gowganda formations and Huronian glaciation. Paleomagnetic data from several continents suggest that ferruginous weathering horizons formed over a wide range of paleolatitudes in the Proterozoic.

First paleomagnetic data from the sedimentary cover of the French Penninic Alps: evidence for Tertiary counterclockwise rotations in the Western Alps

We present a paleomagnetic study performed in the Briançonnais sedimentary cover of the western Alpine Arc (France). Sampling was focused on Upper Jurassic rocks of the Briançonnais cover in the Briançon–Guillestre area. More than 100 samples from 11 sites were collected essentially from Ammonitico rosso limestones and sedimentary dykes of the Upper Jurassic. Thermal and alternating field demagnetizations revealed three components of magnetization carried dominantly by magnetite: (1) a low-temperature component with present-day magnetic field direction of viscous origin; (2) a well defined reverse-polarity component with a maximum unblocking temperature of 440°C (A component); and (3) a weak high-temperature component (B component) that experienced a viscous overprint during heating and was difficult to isolate. The A component exhibits a negative fold test and shows a better clustering when corrected for tilting associated with late Alpine extension. This component is interpreted as a secondary remagnetization acquired during the cooling path related to Late Eocene–Early Oligocene metamorphism. The mean A component direction, corrected for late Alpine extensional tilting, is D=142°, I=−57°, k=44, α95=8°. This direction significantly differs by 47° ± 13° from the expected direction for Eurasia. We interpret this difference to be related to a post-Eocene counterclockwise rotation about a vertical axis of the Penninic zone relative to stable Europe.

High-Unblocking-Temperature Haematite Magnetizations of Late Palaeozoic Red Beds From the Okcheon Zone, Southern Part of the Korean Peninsula

The Late Carboniferous and Permo-Triassic red beds of the Pyeongan Supergroup in the Okcheon zone, southern part of the Korean Peninsula, were found to contain three components of remanence. These components, which are resolved by thermal demagnetization, are characterized by high (Hhc for Late Carboniferous and Hhpc for Permo-Triassic), intermediate (Hm) and low (HI) unblocking temperatures. As the maximum unblocking temperature spectra of the Hhc (Hhpt) component often overlapped the Hm spectra above 680° C, 3° to 5° C steps were needed above 670° C to resolve the Hhc-(Hhpt-) component magnetization. The tilt-corrected mean direction of the Hhc component red beds is D=255.5°, 1=-2.4° (α95=20.8°), and that of the Hhpt component is D=271.9°, 1=-33.4° (α95=16.5°). The precisions of mean directions of Hhc and Hhpt components improve after tilt correction and allow the directions to be distinguished from each another. This indicates that both Hhc and Hhpt component magnetizations were acquired during or shortly after the deposition of the red beds. According to micropetrographic observations, the haematite in the red beds can be divided into three types: detrital specularite (A), non-detrital specularite (B), and pigmental haematite (C). Type A grains are tens of micrometres in size, and many of them are martite. The remanent magnetization of type A is possibly depositional remanent magnetization (DRM), post-DRM or chemical remanent magnetization (CRM) acquired close to the deposition time, whereas that of types B and C is of CRM origin acquired during and after folding, respectively. The time of folding is well established as a Triassic to Jurassic event. We believe that the remanent magnetization carried by type A grains is primary, and is correlated to Hhc-(or Hhpt-) component magnetization.

Widespread Neogene remagnetization in Jurassic limestones of the South-Iberian palaeomargin (Western Betics, Gibraltar Arc)

Palaeomagnetic investigations in Upper Jurassic carbonate rocks from the Western Subbetics (Betic Cordillera, Southern Spain) testify to a widespread Neogene remagnetization. Progressive thermal and alternating field demagnetization analyses reveal the presence of two stable components of the natural remanent magnetization (NRM). The NRM is dominated by a pervasive Neogene overprint, which always shows normal polarity and maximum unblocking temperatures of 450°C throughout the whole region studied. The primary Jurassic component, however, can also be isolated in many outcrops. This component has low intensity values and maximum unblocking temperatures of about 550°C, showing both normal and reversed polarities. Palaeomagnetic and rock magnetic analyses indicate that both primary Jurassic and secondary Neogene components are carried by magnetite. The incremental fold test performed in all sampling localities demonstrates that the remagnetization occurred during some stage of the Neogene deformation of the Subbetics, being pre-, syn- or post-folding in the various folds studied.

Thermal demagnetization of VRM and pTRM of single domain magnetite: No evidence for anomalously high unblocking temperatures

Stepwise thermal demagnetization of viscous remanent magnetization (VRM) and partial thermoremanent magnetization (pTRM) of single‐domain magnetite with mean grain size 37 nm reveals only minor deviations from the predictions of Néel theory. Three different initial states were used, following (1) alternating field demagnetization to 100 mT, (2) thermal demagnetization to 610°C, and (3) modified thermal demagnetization, consisting of zero‐field heating to 610°C followed by zero‐field cooling to the VRM or pTRM acquisition or blocking temperature, TB. VRMs were produced by applying a 0.1 mT field for 3.5 hr, starting from initial states 2, 3 and 1, in that order, at TB=283°C and from states 3, 2 and 1 at TB=404°C. In a final experiment, pTRM was produced by applying 0.1 mT during cooling from TB1=414°C to TB2=404°C, starting from state 3. In all but one case, VRM began to unblock significantly (90% of initial intensity) ≈15°C below TB, dropped to ≈50% at TB, and then tailed off, dropping below 10% ≈40°C above TB=283°C or ≈25°C above TB=404°C. Partial TRM behaved in the same way, except that demagnetization was essentially complete by TB1=414°C. The maximum unblocking temperatures (TUB) observed are exactly as predicted by the theory of Pullaiah et al. (1975). Although very long acquisition times cannot be tested, we find no evidence for anomalously high TUBs of VRM in single‐domain grains. In previously reported cases of rocks with unexpectedly high TUBs, the VRM was probably carried by multidomain grains.

Magnetic grain‐size variations through an ash flow sheet: Influence on magnetic properties and implications for cooling history

Rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks, provide a basis for interpretation of aeromagnetic data over volcanic terranes, and yield insights into the depositional and cooling histories of ash flow sheets. A rhyolitic ash flow sheet, the Miocene‐aged Tiva Canyon Member of the Paintbrush Tuff, contains both titanomagnetite phenocrysts, present in the magma prior to eruption, and cubic Fe‐oxide microcrystals that grew after emplacement. Systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the Nevada Test Site. Natural remanent magnetization varies from less than 1 × 10−4 to more than 8 × 10−4 A m3 kg−1, and in‐phase magnetic susceptibility varies from less than 1 × 10−6 to more than 10 × 10−6 m3 kg−1. The microcrystals, which include both magnetite and maghemite, have Curie points and maximum unblocking temperatures between 580°C and 640°C. Rock magnetic data, including in‐phase and quadrature magnetic susceptibilities as well as hysteresis parameters, demonstrate that these microcrystals are of superparamagnetic and single‐domain sizes. Titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section, whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15‐m‐thick zones at the top and bottom. Within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet. The decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break; possibly represented by a concentration of pumice. Laboratory heating produces changes in magnetic properties which vary complexly from sample to sample. Such changes include (1) decrease in saturation magnetization, (2) both increase and decrease in in‐phase and quadrature magnetic susceptibilities, and (3) shifts in coercivity spectra. The complex sample‐to‐sample variation in response to heating is attributed to differences in initial grain‐size distributions.