Inverse Magnetic Fabric Research Articles

Inverse magnetic fabric of remagnetized limestones in the Zaduo area, Eastern Qiangtang Terrane: Implications for oroclinal bending in the Eastern Himalayan Syntaxis

Magnetic fabric analysis is a common technique to assess the strain regime during mountain building processes. Here, we use this approach to evaluate the tectonic evolution of the Tibetan Plateau and the Eastern Himalayan Syntaxis by analyzing the limestones of the Jurassic Buqu Formation in the Zaduo area, Eastern Qiangtang Terrane (China). These limestones were chemically remagnetized during the Cenozoic. For a proper assessment, it is relevant to understand how the mineralogy of the remagnetized limestones affects their magnetic fabric and how the magnetic fabric can improve our understanding of the tectonic strain and regional deformation. The role of the authigenic magnetite in the development of the magnetic fabric should thus be explored. Comparison of the bulk susceptibility (Km) with various natural and laboratory rock magnetic properties (Km versus natural remanent magnetization, Km versus saturation isothermal remanent magnetization, and Km versus saturation magnetization) indicates that susceptibility and remanences are both carried by authigenic magnetite. Most of the magnetite grains show axial ratios <1.3:1 according to the Néel diagram, and fall within the single-domain range based on the mass magnetic susceptibility (χ) and DC field-normalized anhysteretic remanent magnetization (χARM) ratio, giving rise to the inverse magnetic fabric observed. Twelve sites (120 specimens) are divided into four groups based on the magnetic fabric and rock magnetic behaviors. Overall, there is a clear trend of decreasing Km, natural remanent magnetization, saturation isothermal remanent magnetization, ferromagnetic percentage and shape parameter from Group I to IV. The K1 axis of all four groups documents a NNE-SSW oriented compression during remagnetization, contrasting with the Eocene NE-SW compression in the Gongjue area farther east. This different compressional regime likely resulted in different rotations and structural trends surrounding the Eastern Himalayan Syntaxis.

Inverse Magnetic Fabrics Caused by Magnetofossils in the Northwestern South China Sea Since End of the Last Glacial

AbstractThe relationships among the abundance of magnetofossils, the ensuing magnetic properties, and the controlling paleoenvironmental factors in marine sediments remain broadly unexplored. Here, we identify magnetofossils in core XB1 from the northwestern South China Sea (SCS) since end of the Last Glacial. Using rock magnetic and electron microscopic data, we propose a model that links the anisotropy of magnetic susceptibility fabric and the abundance of magnetofossils. The magnetofossil concentration in sediments increases significantly during the 14.7–4.7 ka period, which in turns leads to inverse magnetic fabrics and near‐horizontal of the minimum magnetic susceptibility axes. Further, we show that the abundance of magnetofossils is linked to paleoenvironmental changes in the northwestern SCS. The production and preservation of magnetofossils during the 14.7–4.7 ka period are promoted by an intensified East Asian summer monsoon and sluggish deep‐water ventilation, while the paucity of magnetofossils after 4.7 ka is attributed to high oxygen content.

ПЕТРО- И ПАЛЕОМАГНЕТИЗМ МАФИТОВЫХ ДАЕК СЕРГЕЕВСКОГО ТЕРРЕЙНА (ЮЖНЫЙ СИХОТЭ-АЛИНЬ)

The paper presents the results of paleomagnetic and rock magnetic studies of rocks from the mafic dikes of the Sergeevka terrane that intruded the basement rocks. Currently, they are widespread in coastal outcrops of the eastern part of the Peter the Great Bay of the Sea of Japan between Anna and Priboinaya bays in the south of Primorsky Krai. Structural information allows us to conclude that dikes of both the eastern (Srednyaya Bay-Priboinaya Bay) and the western (Anna Bay-Cape De Livron) clusters intruded from the igneous centers of the same strike. Rocks of the dikes have undergone significant secondary alterations. Inverse magnetic fabric in the dikes is the result of high-temperature decomposition and granulation of primary titanomagnetites during postmagmatic deformations and metamorphism, traces of which are clearly visible in the microscopic and microprobe study of secondary alteration products. All this testifies to a significant secondary change in the rocks of the dikes after their intrusion. This is the secondary heating of the dikes above a temperature of 600 °C, that is, above the Curie point of magnetite (578 °C). The nature of the isolated high-temperature NRM component of the dikes is not primary, but secondary, that is, metachronous. The paleomagnetic data constrained the time of the metamorphic event leading to the remagnetization of mafic dikes of the Sergeevka terrane to ca. 250 Ma. The calculated paleolatitude of the Sergeevka terrane at the time of acquisition of the metachronous NRM component by mafic dikes (21.8 ± 4.2° N) is consistent with the paleolatitudes of the northeastern edge of the North China Craton at the Late Permian/Early Triassic boundary.

Magnetic anisotropy of an ancient volcanic system: Flow dynamics of post-collisional Ediacaran volcanism in southernmost Brazil

Knowledge about flow dynamics of volcanic sequences is fundamental for understanding their emplacement and consequently the evolution of the associated volcanic terrain. Despite this importance, studies that apply different approaches to ancient volcanic systems are still rare. In this paper, we study the case of silicic volcanic sequences in southernmost Brazil, contributing to the interpretation of the post-collisional Ediacaran volcanic settings of the Sul-riograndense shield. Rock magnetism analyses, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM), were performed on 32 sites of silicic volcanic rocks integrated with fieldwork observations. Magnetic mineralogy data indicate that magnetite or Ti-poor magnetite and high-coercivity phases (e.g., hematite) are the main magnetic carriers for the studied volcanic deposits. AARM results reveal an inverse magnetic fabric when single-domain grains are present, strongly affecting the interpretation of flow directions of lavas and ignimbritic deposits. AMS scalar results integrated with ignimbrite lithofacies analyses showed different fabric imbrication styles between stratified lower units and rheomorphic upper ignimbrites, allowing their separation in the emplacement model. Flow directions based on AMS, AARM data and field observations show a potential correlation of these volcanic deposits with an intrusive complex located on the southeastern border of the ignimbritic plateau. The emplacement of pyroclastic flow deposits was probably associated with a complex fissure system, where discontinuities within the basement plateau border may have served as feed conduits for these deposits. Our results highlight the importance of applying a regionally distributed AMS sampling coupled with a strong mineralogical and field control to the study of ancient volcanic systems.

Magnetic Anisotropy of Rocks: A New Classification of Inverse Magnetic Fabrics to Help Geological Interpretations

AbstractInverse magnetic fabrics defined by anisotropy of magnetic susceptibility with the longest principal axis perpendicular to bedding were identified in the studied samples of Ordovician sedimentary rocks from the Barrandian area, Czech Republic. These fabrics are related to ankerite fibers in association with cone‐in‐cone structures. To understand the meaning of such inverse magnetic fabrics for geological interpretations, a new, more detailed classification of inverse magnetic fabrics' types is formulated in this paper. Distinguishing the structural (i.e., shape) and magnetocrystalline anisotropies in rocks, three new (sub)types of magnetic fabrics were defined in this study based on the combination of structural normal/inverse and magnetocrystalline normal/inverse anisotropies. As a result, normal magnetic fabric is extended by new fabric types associated with inverse anisotropy: simple inverse, pure inverse, and pseudo‐normal magnetic fabrics. The studied fibrous ankerite shows simple inverse magnetic fabric, which results from the combination of normal magnetocrystalline and inverse structural anisotropies.

Interpreting Inverse Magnetic Fabric in Miocene Dikes From Eastern Iceland

AbstractAnisotropy of Magnetic Susceptibility (AMS) is a valid tool to investigate magma flow direction within dikes. However, geometrically inverse magnetic fabric characterized by maximum magnetic susceptibility axis (kmax) perpendicular to the dike wall may complicate the interpretation of flow trajectories. To better understand the nature of this fabric, we present a multiscale study on 19 dikes (383 samples) in the Miocene Alftafjordur volcanic system (Iceland), where 80% of the samples show a geometrically inverse magnetic fabric. We carried out (1) AMS measurements at different magnetic fields and temperatures, along with Anisotropy of Anhysteretic Remanent Magnetization (AARM) analysis; (2) hysteresis loops and FORC diagrams; (3) thin section analysis; (4) structural fieldwork. A variable Ti‐content (0.1 &lt; x &lt; 0.6, Fe3‐xTixO4) titanomagnetite is the main magnetic carrier, and the contribution of the paramagnetic elongated crystals to the magnetic fabric is negligible. Single domain is not the prevailing domain state of the magnetic particles, suggesting that its occurrence cannot be the main cause for the inverse fabric. AMS analysis at different fields and temperatures along with AARM allow us to exclude any mineral phase change of the titanomagnetite across the dike. Nevertheless, kmax is parallel to a diffuse horizontal column‐like fracture pattern perpendicularly oriented with respect to the dike strike. This suggests that the Ti‐magnetite mineral orientation during dike cooling was affected by the fracture network progressively developing columnar basalts. This study demonstrates that the interpretation of AMS data on old and deep volcanic bodies is not straightforward and observations at different scales are required.

Anisotropy of out-of-phase magnetic susceptibility as a potential tool for distinguishing geologically and physically controlled inverse magnetic fabrics in volcanic dykes

Among the mechanisms of the mass transport within the Earth's crust and the upper mantle the magma movement within the dykes is very important. In some dykes, the magma movement is represented by almost free flow infilling more or less open fractures in the host rocks, while it has character of forceful injection of the host rocks in the other dykes. The magnetic fabric originated by the free flow is characterized by the magnetic foliation and magnetic lineation approximately parallel to the dyke plane (normal fabric) whereas the forceful injection gives rise to the magnetic foliation perpendicular to the dyke plane defining the intermediate or inverse magnetic fabric. It is known, however, that the inverse magnetic fabric can also originate through the free flow of magma in the case when the minerals carrying the AMS are represented by single-domain (SD) grains. In order to distinguish between the two mechanisms controlling the intermediate/inverse fabric, the anisotropy of magnetic remanence (AMR) is often used, because it indicates the shape preferred orientation of ferromagnetic minerals regardless of their domain state. We show that not only AMR, but also the anisotropy of out-of-phase magnetic susceptibility (opAMS) is conformable with particle shape regardless of the magnetic state of the particle. Consequently, the opAMS, which is measured automatically with the standard AMS using the KLY-5 Kappabridge, may substitute the AMR at least in some cases of volcanic dykes.

Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment: Implications for Magnetofossil Abundance and Alignment

AbstractSingle‐domain magnetite particles exhibit minimum susceptibility along their elongation, resulting in so‐called inverse fabric of the anisotropy of magnetic susceptibility (AMS). We report the discovery of inverse AMS fabrics from pelagic clay recovered by a ∼12 m long piston core from the western North Pacific. A previous study identified fossil single‐domain magnetite produced by magnetotactic bacteria (magnetofossils) as the dominant ferrimagnetic mineral in the sediment. The inverse AMS fabrics were found in a ∼2 m zone. The ∼6 and ∼4 m of sediment above and below this zone showed normal, horizontal AMS fabrics. Rock magnetic data and ferromagnetic resonance spectroscopy indicated that magnetofossils account for most of the mean susceptibility regardless of normal or inverse AMS. This was explained by the mixing models where the inverse fabric from magnetofossils is nearly balanced by the normal fabrics of terrigenous minerals. The corrected degree of AMS carried by magnetofossils in the sediment was estimated to be ∼1.01, which is comparable to that of typical pelagic sediment at shallow depth. On the other hand, terrigenous minerals in the sediment were estimated to have higher degree of anisotropy, possibly reflecting burial and subsequent erosion of &gt;80 m of sediment, which was also suggested by a subbottom acoustic stratigraphy. This suggests that inverse AMS fabrics due to magnetofossils may be widespread in pelagic clay without strong compaction.

Simultaneous Free Flow and Forcefully Driven Movement of Magma in Lamprophyre Dykes as Indicated by Magnetic Anisotropy: Case Study from the Central Bohemian Dyke Swarm, Czech Republic

A composite lamprophyre dyke from the Central Bohemian Dyke Swarm (Czech Republic) shows both indications of magma free flow (normal magnetic fabric with magnetic foliation and lineation parallel to the dyke plane) as well as those of forcefully driven magma movement (intermediate and inverse magnetic fabrics with magnetic foliation perpendicular to the dyke plane). The overall characteristics of the magnetic parameters across the dyke indicate the existence of at least two slightly differing parts that probably represent two magma pulses. The marginal part of the dyke is formed by kersantite, while toward the axial part, the composition gradually changes to spessartite, and obtains an increasing degree of amphibolization. The rocks of the dyke are inhomogeneous, both compositionally and structurally. It is likely that some portions of ascending magma were more viscous than the others, and the magnetic minerals in the more viscous magma portions may have oriented according to their longer dimensions perpendicular to the dyke, creating an inverse fabric. The lengthening perpendicular to the dyke was compensated by the vertical escape of neighboring more fluid magma, creating a normal magnetic fabric. The frequent oblique magnetic fabrics may represent transitions between the above two mechanisms.

AMS studies on a 450 km long 2216 Ma dyke from Dharwar craton, India: Implications to magma flow

Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma), 450 km long N–S striking dyke in the Dharwar Craton, to determine the magma flow direction along the dyke length. In order to use the imbrication of the magnetic foliation, forty eight samples were collected from 13 locations along the length of the dyke. Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite. The corrected anisotropy degree (Pj) of the samples was found to be low to moderate, between 1.007 and 1.072, which indicates primary magnetic fabric. The magnetic ellipsoid is either triaxial, prolate or oblate and clearly defines normal, intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement. The maximum susceptibility axes (Kmax) of the AMS tensor of the dyke is predominantly inclined at low angles (<30°), with no systematic variation in depth along the N–S profile, indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source (mantle plume?), flow dynamics together with the compressive stresses exerted by the overlying crust.

Inflation and collapse of the Wai\u2019anae volcano (Oahu, Hawaii, USA): implications from rock magnetic properties and magnetic fabric data of dikes

In order to investigate the role of dikes in the volcanic evolution and the triggering mechanisms of catastrophic mass wasting volcanoes, we have sampled for a pilot study, seven dikes within the Wai’anae volcano, Oahu, Hawaii. The width of the dikes ranged between 0.4 and 2.5 m. This work focuses on the characterization of the magma flow directions using anisotropy of magnetic susceptibility (AMS) data in dikes of the inner part of the Wai’anae volcano. This part is now exposed, because this volcano experienced destabilization and flank collapse. Rock magnetism data show composite magnetic mineralogy, corresponding when plotted on the Day diagram to be dominated by single domain (SD) and pseudo-single domain particles of pure titanomagnetite, suggesting possible inverse magnetic fabric associated with the SD grains. The obtained magnetic fabric does not reflect such grain sizes and is probably partly related to the presence of different magnetic phases, resulting in part of our samples as having “abnormal” fabrics. We therefore used a simple criterion to eliminate most of the abnormal fabrics in order to analyze the magnetic fabric data in a clearer way. After rejection of most of the abnormal data, the determination of the magnetic zone axis, which underlines the effect of imbrication in dike margins, yielded reliable magma flow directions in most of the studied dikes, with a predominance of vertical to subvertical AMS directions. The inferred dominantly vertical to subvertical magma flow of dikes (feeding from below) within the most internal parts of the volcano, suggests a process of accumulation of new magma at different levels within the inner part of the edifice. This process was enhanced by subhorizontal magma flow toward the volcano center in two other dikes. Such accumulation helps to explain the inflation, subsequent destabilization, and flank collapse of the Wai’anae volcano.

Inverse magnetic fabric in unconsolidated sandy event deposits in Kiritappu Marsh, Hokkaido, Japan

Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AARM) were evaluated for samples collected from units of two unconsolidated sandy event layers, presumed to be paleo-tsunami deposits, from Kiritappu Marsh, northeastern Japan. The AARM technique isolates the fabric of fine-grained (titano)magnetite from AMS fabric of all minerals in deposits. Bulk susceptibilities of the older event layer were weaker than those of the younger event layer. Our AMS results show that the principal minimum AMS axes (Kmin) were distributed along the horizontal bedding plane, suggesting the presence of an AMS “inverse fabric” where magnetic axes are interchanged. Our AARM results indicated the principal maximum AARM axes (kmax) are parallel to the AMS Kmin, whereas the AARM intermediate and minimum axes (kint and kmin) are related to the AMS Kmax and Kint. Moreover, the shape of anisotropy parameters revealed that the AMS is oblate (Kmax≈Kint), whereas the AARM is prolate (kint≈kmin). We found the presence of single-domain sized titanomagnetites in the mud matrix based on electron microscopy observations. AARM is regarded as a complementary tool to estimate paleocurrent directions using grain fabrics of unconsolidated sediments. However, these lines of evidence confirm that our AMS fabrics showed “inverse fabric”, providing no information about flow directions at this locality. Although this effect is uncommon in soft sediments, it requires caution to estimate paleocurrent directions using AMS.

Role of single-domain magnetic particles in creation of inverse magnetic fabrics in volcanic rocks: A mathematical model study

The role of single-domain (SD) magnetic particles in creation of inverse magnetic fabrics is investigated on simple mathematical models using a realistic estimate for SD intrinsic susceptibility. In contrast to the fraction created by multi-domain (MD) particles, in which the anisotropy of magnetic susceptibility (AMS) is controlled by both the grain AMS and intensity of the preferred orientation of the particles, the AMS of the SD fraction is controlled solely by the intensity of the preferred orientation. The degree of AMS of ensemble of SD grains with a preferred orientation is therefore much higher than that of the same ensemble of MD particles implying the existence of frequent inverse magnetic fabrics. However, the occurrence of inverse magnetic fabrics due to SD particles is more the exception than the rule. Consequently, the amounts of SD particles is probably in general low. Nevertheless, the presence of SD particles in amounts insufficient to create inverse magnetic fabrics may diminish the whole rock AMS substantially. This can be one of the reasons for relatively low AMS in volcanic rocks whose magnetic particles may be really small obeying the conditions for the existence of SD particles.

Magnetic properties of Triassic rocks of Svalbard Archipelago - preliminary results

Studies were carried out in three rock formations: the Vardebukta Formation (Early Triassic) and the Bravaisberget Formation (Middle Triassic) from Spitsbergen and the Botneheia Formation (Middle Triassic) from Edgeoya (Eastern Svalbard). Collected samples are diversified and represent various lithologies with different thermal and tectonic histories. Magnetic properties of the specimens were studied using rock-magnetic methods including: thermal demagnetization of saturated isothermal remanent magnetization (SIRM), hysteresis loops and analysis of magnetic susceptibility variations at high temperatures. Subsequently anisotropy of magnetic susceptibility (AMS) and analysis of the structure of natural remanent magnetization (NRM) using thermal and alternating field (AF) demagnetization were investigated. The obtained results show that magnetite represents the main magnetic mineral in all studied rocks, whereas pyrrothite, as a second magnetic information carrier, was noted in some of specimens only. The samples from Edgeoya possess the highest magnetic susceptibility (ca. 240∙10−6 SI) which tends to decrease in sites located further to the west and reaches the minimum value for samples from western Spitsbergen (ca. 30–40∙10−6 SI). AMS studies revealed that the Middle Triassic samples are characterized by inverse magnetic fabric and prolate AMS ellipsoids, whereas the Early Triassic specimens represent normal sedimentary fabric and oblate AMS ellipsoids. Inverse magnetic fabric can be carried by many minerals including: tourmaline, cordierite or goethite (Chadima et al., 2006). For investigated samples two models proposed by Rochette (1988) are highly probable: (1) presence of single-domain elongated magnetite grains or (2) presence of ferroan carbonates whose maximum susceptibility is parallel to the c-axis. Paleomagnetic results of the Botneheia Formation revealed only one characteristic NRM component characterized by steep inclination and maximal unblocking temperatures in range of 425–450°C. In turn, the Early Triassic rocks record several NRM components with partially overlapping unblocking temperatures. We have concluded that different magnetic properties observed in studied rocks might be related to various lithologies but also to distinct tectonic and thermal histories associated with complex tectonic evolution of the Barents Sea Shelf.

AMS–NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic fabrics in flood basalts

AbstractThe evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies. A statistically significant sample set of titanomagnetite‐bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India, has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar anisotropy of magnetic susceptibility (AMS) parameter and the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an alternating field (AF) tumbling demagnetizer (peak fields of 100 mT) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic fabrics, carrying a higher portion of single‐domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic fabrics) and therefore a more precise and better reflection of the “actual” mineral fabric.

Internal structure of basalt flows: insights from magnetic and crystallographic fabrics of the La Palisse volcanics, French Massif Central

We present a new interpretation of anisotropy of magnetic susceptibility (AMS) fabrics in basaltic lava flows based on the detailed study of magnetic mineralogy and silicate crystallographic fabric of a Quaternary lava flow from the French Massif Central (La Palisse). We consider the model of AMS fabric imbrication between magnetic foliation and flow surface, as initially proposed for dykes. At the two sampling sites, the concordance between the flow direction deduced from the AMS foliation and that deduced from field observations indicates that the imbrication model could apply to the lava flows. However, the flow senses inferred fromAMSare systematically opposed between the two sampling sites suggesting permutations between K1 and K3 AMS axes, a configuration referred to as inverse fabric. Electron backscatter diffraction (EBSD) measurements show strong lattice-preferred orientations (LPO) for plagioclase, especially the (010) plagioclase plane, which tends to be parallel to the flow. Clinopyroxene LPO remains less marked than plagioclase LPO, whereas titanomagnetite does not display a significant LPO. Comparison between magnetic and crystallographic fabrics suggests that the AMS fabric of the lava flow results from the distribution of titanomagnetite grains, which is in turn controlled by the fabric of the silicate framework. Magnetic hysteresis parameters and anisotropy of remanent magnetization (ARM) measurements exclude a significant contribution from single-domain grains, often called upon to explain inverse magnetic fabrics. The origin of the observed inverse magnetic fabric may relate to the dip of the palaeosurface, which is the only remarkable difference between the two sampling sites. AMS appears as a good tool to determine the direction of basaltic lava flows and coupling with local crystallographic fabric data provides a valuable control of relationships between magnetic fabrics and flow and thus contributes to better constrain the AMS signature of lava flows.

Magnetic fabric and its significance in the sills and lava flows from Taimyr fold-belt, Arctic Siberia

Magnetic fabric research on the emplacement of the Taimyr fold-belt is still absent. Therefore, we have performed magnetic fabric studies on samples collected from 23 sites from mafic igneous rocks (75°N, 100°E) in South Taimyr. This study indicates a correlation between the anisotropy of magnetic susceptibility (AMS), magma flow and fold-related compression in the rocks. AMS measurements on 183 unheated and 122 heated samples reveal the magma flow directions. The magma flow direction is mainly parallel to the ESE–WNW and secondarily parallel to the SSW from the NNE in the mafic sills. The unheated and heated basaltic flows reveal a NE-trending flow vector. In some of the sills, AMS shows a tight clustering of the maximum K 1 axes close to the bedding pole, which is not thought to define the flow direction. Corresponding AMS measurements on igneous rocks allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main folding episode that occurred subsequent to the Mid-Triassic magmatic event in latest Triassic–Early Jurassic times. In the sills, the distribution of most maximum K 1 axes is close to NS that corresponds to the maximum compressive stress or folding directions; contrarily, the minimum K 3 axes have an elongated distribution along the EW direction or parallel to the fold axis before exchanging K 1 and K 3 axes for inverse fabrics, but close to the NS stress direction after exchanging K 1 and K 3 axes for inverse fabrics. In the basaltic flows, the minimum K 3 axes almost parallel to the NS folding direction. The structural interpretation of all AMS data taken from the igneous bodies is in accordance with a NNW–SSE stress-related folding taking place around 198 Ma in the Taimyr Peninsula (Arctic Siberia). The relationship of susceptibility axes to bedding surfaces and magnetic foliation planes is a criterion that permits differentiation of normal magnetic fabrics from inverse fabrics in the igneous samples. Single-domain (SD) and small pseudo-single-domain (PSD) magnetite crystals in the sill samples are the main carriers of the AMS fabrics, implying the abundance of inverse magnetic fabrics. PSD and multidomain (MD) titanomagnetite (TM) grains as well as minor hematite are identified in the basalt flows, suggesting the occurrence of more normal magnetic fabrics in the basalts than in the sills.

On the interpretation of normal and inverse magnetic fabric in dikes: Examples from the Eger Graben, NW Bohemian Massif

Recent studies of igneous rocks indicate that the predominant occurrence of normal/inverse fabric in dikes may either reflect the presence of multi-domain (MD)/single-domain (SD) grains or it may result from different orientation mechanisms of magnetic minerals in magmas of different viscosities. The ambiguity in physical vs. geological cause of normal/inverse magnetic fabric must be answered before any successful geological interpretation of magnetic fabric can be made. In order to address this problem, we studied magnetic fabric of selected dikes associated with the SW–NE trending Eger Graben (NW Bohemian Massif). The studied area offered very extensive collection of rock types: basanite, bostonite, camptonite, tinguaite, and trachybasalt. Magnetic susceptibility varies according to rock type and reflects the relative contents of magnetic minerals. In most cases, titanomagnetite with variable Ti content was identified as main magnetic carrier. The degree of anisotropy is relatively low, in most cases less than 10%, the shape of anisotropy ellipsoid ranges from slightly prolate to neutral and oblate. Several different types of magnetic fabric (using anisotropy of low-field magnetic susceptibility, AMS) were observed in studied dikes: so-called normal and inverse magnetic fabrics and anomalous magnetic fabric. Comparing all studied sites it seems that the type of magnetic fabric is lithology-dependent. Normal magnetic fabric with magnetic foliations and subhorizontal magnetic lineations both parallel to the dike margins was found in bostonite and trachybasalt dikes. Inverse magnetic fabric with magnetic lineations and magnetic foliations perpendicular to the dike margins was found in camptonite dikes. Anisotropy of anhysteretic remanent magnetization (AMR) indicate that the observed inverse magnetic fabric may be caused by the presence of SD magnetic grains; AMR fabric being normal with respect to dike margins. In contrast to that no single-domain particles were revealed using frequency dependence and anhysteretic susceptibility measurements. The AMS measured in variable weak magnetic fields is field dependent for camptonite dike and field independent for other rock types, i.e. bostonite, basanite, tinguaite, and trachybasalt. For further flow direction and tectonic interpretations of magnetic fabric in dikes it is suggested to use preferably the AMR fabric (at least for dikes which demonstrate significant field dependence of AMS) as it reflects the ‘true’ rock fabric more accurately than AMS fabric.

Petromagnetic features of sediments at the Mesozoic-Cenozoic boundary: Results from the Gams section

The paper continues a cycle of petromagnetic investigations of epicontinental deposits at the Meso- zoic-Cenozoic (K/T) boundary and is devoted to the study of the Gams section (Austria). Using thermomag- netic analysis, the following magnetic phases are identified: goethite ( T C = 90-150°C ), hemoilmenite ( T C = 200-300°C ), metallic nickel ( T C = 350-360°C ), magnetite and titanomagnetite ( T C = 550-610°C ), Fe-Ni alloy ( T C = 640-660°C ), and metallic iron ( T C = 740-770°C ). Their concentrations are determined from M ( T ) . In all samples, ensembles of magnetic grains have similar coercivity spectra and are characterized by a high coerciv- ity. An exception is the lower coercivity of the boundary clay layer due to grains of metallic nickel and iron. With rare exceptions, the studied sediments are anisotropic and generally possess a magnetic foliation, which indicates a terrigenous accumulation of magnetic minerals. Many samples of sandy-clayey rocks have an inverse magnetic fabric associated with the presence of acicular goethite. The values of paramagnetic and dia- magnetic components in the deposits are calculated. According to the results obtained, the K/T boundary is marked by a sharp increase in the concentration of Fe hydroxides. The distribution of titanomagnetite reflects its dispersal during eruptive activity, which is better expressed in the Maastrichtian and at the base of the layer J. The along-section distribution of metallic iron, most likely of cosmic origin, is rather uniformly chaotic. The presence of nickel, most probably of impact origin, is a particularly local phenomenon as yet. The K/T bound- ary is not directly related to an impact event.

Magnetic fabric variations in Mesozoic black shales, Northern Siberia, Russia: Possible paleomagnetic implications

A 28-m-long section situated on the coast of the Arctic Ocean, Russia (74°N, 113°E) was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the Jurassic/Cretaceous boundary. The section consists predominantly of marine black shales with abundant siderite concretions and several distinct siderite cemented layers. Low-field magnetic susceptibility ( k) ranges from 8 × 10 − 5 to 2 × 10 − 3 SI and is predominantly controlled by the paramagnetic minerals, i.e. iron-bearing chlorites, micas, and siderite. The siderite-bearing samples possess the highest magnetic susceptibility, usually one order of magnitude higher than the neighboring rock. The intensity of the natural remanent magnetization ( M 0) varies between 1 × 10 − 5 and 6 × 10 − 3 A/m. Several samples possessing extremely high values of M 0 were found. There is no apparent correlation between the high k and high M 0 values; on the contrary, the samples with relatively high M 0 values possess average magnetic susceptibility and vice versa. According to the low-field anisotropy of magnetic susceptibility (AMS), three different groups of samples can be distinguished. In the siderite-bearing samples (i), an inverse magnetic fabric is observed, i.e., the maximum and minimum principal susceptibility directions are interchanged and the magnetic fabric has a distinctly prolate shape. Triaxial-fabric samples (ii), showing an intermediate magnetic fabric, are always characterized by high M 0 values. It seems probable that the magnetic fabric is controlled by the preferred orientation of paramagnetic phyllosilicates, e.g., chlorite and mica, and by some ferromagnetic mineral with anomalous orientation in relation to the bedding plane. Oblate-fabric samples (iii) are characterized by a bedding-controlled magnetic fabric, and by moderate magnetic susceptibility and M 0 values. The magnetic fabric is controlled by the preferred orientation of phyllosilicate minerals and, to a minor extent, by a ferrimagnetic fraction, most probably detrital magnetite. Considering the magnetic fabric together with paleomagnetic component analyses, the siderite-bearing, and the high-NRM samples (about 15% of samples) were excluded from further magnetostratigraphic research.