Anisotropy Of Magnetic Susceptibility Research Articles

Recursive anisotropy: a spatial taphonomic study of the Early Pleistocene vertebrate assemblage of Tsiotra Vryssi, Mygdonia Basin, Greece

By applying advanced spatial statistical methods, spatial taphonomy complements the traditional taphonomic approach and enhances our understanding of biostratinomic and diagenetic processes. In this study, we elaborate on a specific aspect – spatial anisotropy – of taphonomic processes. We aim to unravel the taphonomic history of the Early Pleistocene vertebrate assemblage of Tsiotra Vryssi (Mygdonia Basin, Greece). Circular statistics are used for the fabric analysis of elongated elements; geostatistics (directional variograms), wavelet and point pattern analyses are applied for detecting anisotropy at the assemblage level. The anisotropy of magnetic susceptibility (AMS) of sedimentary magnetic minerals is also investigated. The results, integrated with preliminary remarks about the differential preservation of skeletal elements, sedimentological and micromorphological observations, suggest multiple dispersion events and recurrent spatial re‐arrangement of a lag, (peri)autochthonous assemblage, consistent with the cyclical lateral switching of a braided fluvial system. Furthermore, this study offers a contribution to the building of a spatial taphonomic referential framework for the interpretation of other fossil vertebrate assemblages, including archaeo‐palaeontological ones.

Application of anisotropy of magnetic susceptibility (AMS) in understanding regional deformation, fabric development and granite emplacement: examples from Indian cratons

Abstract In this paper the authors review various applications of analysing fabric in granites from Indian cratons using anisotropy of magnetic susceptibility (AMS). First the general importance of AMS in identifying the internal fabric in massive granitoids devoid of visible foliations/lineations is highlighted. Subsequently, three important applications of AMS in granitoids are discussed. (a) The case of Godhra Granite (southern parts of the Aravalli Mountain Belt) is presented as an example of the robustness of AMS in working out the time relationship between emplacement/fabric development and regional deformation by integrating field, microstructural and magnetic data. (b) AMS orientation data from Chakradharpur Granitoid (eastern India) are compared with field-based information from the vicinity of the Singhbhum Shear Zone to highlight the use of AMS in kinematic analysis and vorticity quantification of syntectonic granitoids. (c) Magnetic fabric orientations from the Mulgund Granite (Dharwar Craton) are presented to document the application of AMS in recognizing superposed deformation in granitoids. Moreover, AMS data from Mulgund Granite are also compared with data from another pluton of similar age ( c. 2.5 Ga) from the Dharwar Craton (Koppal Granitoid; syenitic composition). This highlights the use of AMS from granitoids of similar absolute ages in constraining the age of regional superposed deformation.

Magnetic Anisotropy in Silurian Gas‐Bearing Shale Rocks From the Pomerania Region (Northern Poland)

AbstractAnalysis of anisotropy of magnetic susceptibility (AMS) and anhysteretic remanent magnetization (AARM) was conducted on unconventional gas‐bearing Silurian shale rocks from northern Poland. Samples of these rocks were collected from depths greater than 3,500 m from two drill cores. The aim was to investigate magnetic fabrics to verify current models of depositional conditions, current direction, and/or tectonic evolution. To obtain an in‐depth interpretation, rock magnetic studies, microscopic analyses, and graptolite orientation measurements were performed. The results indicate that the magnetic susceptibility is mainly governed by paramagnetic minerals (phyllosilicates) with a small contribution of ferromagnetic minerals, mostly magnetite. Typically, in the studied mudstones, the AMS and AARM fabrics are characterized by strong bedding‐parallel foliation, resulting mostly from compaction. The foliation is much weaker in the associated early‐diagenetic calcareous concretions. Although the mudstones are almost isotropic in the bedding plane, there is a slight tendency for grouping of magnetic lineation axes with an orientation of approximately NNW‐SSE. The corresponding orientation of the AMS lineation is preserved in the concretions, and the same trend also prevails in the preferred orientation of graptolites. Thus, we interpret this orientation to be related to paleocurrents. The observed directions show that during the Wenlock (Silurian) in the studied part of the Baltic Basin, the dominant currents had an orientation of circa NNW‐SSE along the margin of Baltica. Our results confirm that even in such fine‐grained sediments, in which there are no other unequivocal directional sedimentary indicators, paleocurrent directions can be determined by applying AMS and AARM methodology.

Reconstruction of Magma Flow in Permo–Triassic Intrusions of the Angara–Taseeva Syneclise (Siberian Platform) Based on Magnetic Susceptibility Anisotropy Data

Abstract —Based on detailed studies of the anisotropy of magnetic susceptibility (AMS), the directions of magmatic-melt propagation have been reconstructed in large dolerite sills of the Angara–Taseeva syneclise. Half the sites studied showed a magnetic fabric of “normal” type, i.e., the minimum K3 axis of the AMS ellipsoid is orthogonal to the contact, and the other two axes lie in the plane of the intrusive body. Interpreting the orientation of the maximum K1 axis as the direction of the melt flow at such sites, we compiled a scheme of the main magma flow directions. The obtained results are generally consistent with the hypothesis of magma-feeding zone in the central, most downwarped part of the Angara–Taseeva depression. The distribution of the maximum axes of the AMS ellipsoid is different in sills and points to the existence of local intrusion centers. Intrusions with an AMS ellipsoid of “reverse” type are predominant on the periphery of the depression. The sites with a “normal” magnetic fabric do not demonstrate any prevailing direction of magma propagation, because there is no general direction of the melt flow in the peripheral subsurface zones of large intrusions.

A review of anisotropy of magnetic susceptibility analysis of Indian dykes: Implications for magma emplacement

The analysis of Anisotropy of Magnetic Susceptibility (AMS) is a powerful and rapid technique to examine the preferred orientations of mineral (magnetic) fabrics and can indicate the nature of a magma transport (vertical or lateral). The relationship between magnetic fabric and geometry of a dyke swarm enables us to understand magma emplacement processes. Depending on the mutual relationship of magnetic fabric and individual dyke geometry, mode of magma transport is interpreted. The knowledge on the nature of magma transport combined with information on geometry, magmatic overpressure and geochemistry enable us to comment on dyke emplacement processes, the location of possible feeders, syn-emplacement and post-emplacement deformations and prevailing stress regime during emplacement. A number of dykes and dyke swarms have been emplaced into the Indian shield at different points in time. Their ages vary from the Mesoarchean to Tertiary. We present here a review of three case studies where AMS technique was applied to the samples collected from Indian dykes. Two case studies are on the Proterozoic dykes that intruded into the Dharwar craton and the third case study is on Mesozoic dykes that punctured the South Indian Granulite Terrain (SIGT). The dykes generally show “normal” anisotropy fabric to indicate vertical magma emplacement with few exceptions where lateral/inclined magma flow was suggested or the results were inconclusive. We present here a critical review on the interpretation of such “anomalous” fabrics and comment on further studies that can be carried out to extract more information from such results.

Anisotropy of Stable Single Domain Ferrimagnetic Particles in a Rock Sample from Gyroremanent Magnetization and Comparison with Other Anisotropy Methods

The orientation of stable single domain (SSD) ferrimagnetic particles in an igneous rock sample was determined by a sensitive technique utilizing gyroremanent magnetization (GRM). Components of GRM were measured in the sample upon exposure to an alternating field (AF) at various orientations in 3 orthogonal planes. The major components of GRM exhibited a sin(2θ) dependence on AF orientation in the respective perpendicular planes. This was in accordance with theory [1] and contrary to some previously reported experimental results on magnetic recording tape, which produced a distorted sin(2θ) dependence of the GRM [1]. The explanation is likely due to the SSD ferrimagnetic particles in the rock sample being more dispersed (less interacting) compared to the highly interacting SSD particles in the magnetic tape sample of the previous study. The GRM results were consistent with another remanence anisotropy method, anisotropy of isothermal remanent magnetization (AIRM). This method again measures the anisotropy of the remanence carrying ferrimagnetic particles, but the IRM is also acquired by larger multidomain (MD) particles as well as by the SSD particles. The results were also consistent with the visible rock anisotropy (petrofabric), the anisotropy of magnetic susceptibility (AMS), and the shear wave velocity anisotropy. A comparison of all the methods demonstrated that the fine SSD particles, which make up only a small proportion of the rock, were aligned in quite a similar orientation to that of the main rock forming minerals that constituted the bulk of the sample.

Strain Field Associated With a Component of Divergent Motion Along the Southern Dead Sea Fault: Insights From Magnetic Fabrics

AbstractIn order to reconstruct the strain field along the southern segment of the Dead Sea Fault (DSF) plate boundary, we analyzed the magnetic fabrics of carbonate rocks, outcropping along it. The magnetic fabrics provide a microstructural indicator that help to approximate the principal strain directions in the rocks. Our analysis includes ~900 cores from 58 sampling localities, along ~400 km of the southern DSF. We measured the magnetic fabrics of (1) pure calcite‐bearing limestones that consist diamagnetic fabrics and (2) chalks with composite fabrics, which we further separated into diamagnetic and paramagnetic subfabrics, using measurements of Anisotropy of Magnetic Susceptibility at low temperatures. The results show that 87% of the diamagnetic fabrics and subfabrics are of tectonic origin. The orientations of the maximum Anisotropy of Magnetic Susceptibility axes (K3 axes) approximately align with the maximum horizontal shortening directions along the southern segment of the DSF, differ from the remote stress direction, and are largely parallel to the main trace of the DSF. This parallelism is not related to local variations in the geometry of the faults. We suggest that the deflection of the maximum horizontal shortening parallel to the transform plate boundary is a kinematic consequence of the Sinai‐Arabia relative plate motion, which expresses a component of divergence along the southern segment of the DSF. We conclude that magnetic fabrics of carbonate rocks are sensitive and reliable microstructural indicators for determination of the strain field along major fault systems.

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During the study of Bi2Te3-Sb2Te3 crystals, their magnetic and optical properties were studied depending on the ratios of the components of Bi2Te3 and Sb2Te3 in the composition of the solid solution and temperature. The results obtained allow us to state that a sharp decrease in the anisotropy of magnetic susceptibility at T = 293 K is observed in those samples in which, according to optical studies, electron-plasmon interaction is intensified, which is caused by the convergence of plasmon energies and interband transitions that form the fundamental absorption edge.

Periodic trends and hidden dynamics of magnetic properties in three series of triazacyclononane lanthanide complexes.

In three structurally related series of nine-coordinate lanthanide(iii) complexes (Ln = Tb, Dy, Ho, Er, Tm and Yb) based on triazacyclononane, solution NMR studies and DFT/CASSCF calculations have provided key information on the magnetic susceptibility anisotropy. Both experimental and computational approaches have revealed a poor correlation to Bleaney's theory of magnetic anisotropy. CASSCF calculations suggested that the magnetic susceptibility is very sensitive to small geometric variations within the first coordination sphere, whereas DFT analyses indicate that it is the thermal accessibility of low energy vibrational modes that may lead to distortion. Parallel NMRD and EPR studies on the three Gd(iii) complexes revealed good correspondence in estimating the electronic relaxation time. The Gd(iii) tris-pyridinecarboxylate complex possesses a very long electronic relaxation time making it a promising starting point for responsive gadolinium EPR probe design.

Magnetic characterisation of magnetite and hematite from the Blötberget apatite – iron oxide deposits (Bergslagen), south-central Sweden

Rock magnetic measurements were carried out on drill core material and hand specimens from the Blötberget apatite – iron oxide deposit in the Bergslagen ore province, south-central Sweden, to characterise their magnetic properties. Measurements included several kinds of magnetic susceptibility and hysteresis parameters. Petrographic and scanning electron microscopy were used to independently identify and quantify the amount and type of magnetite and hematite. Two hematite-rich samples were studied with laser ablation inductively coupled plasma mass spectrometry to quantify the trace element chemistry in hematite and investigate the potential influence of trace elements on magnetic properties. Three aspects of this study are noteworthy. (1) Hematite-rich samples display strong anisotropy of magnetic susceptibility, which is likely to affect the appearance and modelling of magnetic anomalies. (2) The magnitude-drop in susceptibility across Curie and Néel temperature transitions shows significant correlation with the respective weight percentage of magnetite and hematite. Temperature-dependent magnetic susceptibility measurements can therefore be used to infer the amounts of both magnetite and hematite. (3) Observations of a strongly depressed Morin transition at approximately −60 to −70 °C (200 to 210 K) are made during low-temperature susceptibility measurements. This anomalous Morin transition is most likely related to trace amounts of V and Ti that substitute for Fe in the hematite. When taken together, these magnetic observations improve the understanding of the magnetic anomaly signature of the Blötberget apatite – iron oxide deposits and may potentially be utilised in a broader context when assessing similar (Paleoproterozoic) apatite – iron oxide systems.

Magnetic fabric development in the Lower Seve thrust from the COSC-1 drilling, Swedish Caledonides

The COSC-1 project drilled the several hundred meters thick basal shear zone of the Lower Seve nappe with mylonites in mica schists, amphibole schists and gneisses. In zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics, and magnetic mineralogy. Borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility (AMS) in relation to tectonic evolution of the Seve thrust. We measured AMS at room temperature, added low-temperature and field-dependent AMS for a subset of samples, and compared magnetic with petrographic fabrics. Triaxial and prolate magnetic fabrics with degree of anisotropy (P′) up to 3.2 together with abundant S-C fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m. Magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and/or magnetic interaction and either contribute to increase or decrease of P′, depending on the dominating rock fabric elements. Field-dependency of pyrrhotite and magnetite in kmax-direction further increases P′. Homogeneous and oblate petrographic and magnetic fabrics in the greenschist-grade overprinted rocks below 2300 m with subhorizontal kmax-kint-girdle distributions indicate dominant flattening. AMS depicts shear fabrics including magnetite and ilmenohematite, and is additionally increased by retrograde magnetite-rutile intergrowth in ilmenohematites. We interpret that shape and degree of AMS are controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c) field-dependency of deformed pyrrhotite and/or magnetite. We observed that all petrographic and magnetic subfabrics are coaxial, and lineations are mainly E-W to SE-NW directed confirming the transport direction of the Caledonian allochthonous. From our microstructural and AMS results we suggest that thrusting of the Lower Seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist-grade conditions.

Breakdown of linear response theory under low-power excitation in NMR. II. The case of "long-lived" signals in homogeneously broadened dipolar spin systems.

In this work, the previous linear response theory developed to describe low-power, radiofrequency (RF) excitation in inhomogeneously broadened spin systems [Z. Gong and J. D. Walls, J. Chem. Phys. 145, 164201 (2016)] is applied to the problem of low-power excitation in homogeneously broadened dipolar spin systems when the strength of the RF pulse, ν RF , is much less than the homogeneous linewidth, . Application of a low-power pulse for a time T p with a nominal flip-angle of Θ generates a broad signal with a "dip" at the RF transmitter frequency that deepens with increasing Θ. When a delay is placed before signal acquisition, only a negative, "long-lived" signal from the narrow "dip" remains. If a π X -pulse is applied after low-power excitation, a "long-lived" signal lasting a time t ≈ T p after the π X -pulse is generated where dephasing due to B 0 inhomogeneity, anisotropic bulk magnetic susceptibility, and chemical shift anisotropy is refocused while dephasing due to nonzero chemical shift differences is only partially refocused. Contrary to previous observations, experiments in powdered hexamethylbenzene demonstrate that these "long-lived" signals can exist even in the absence of nonzero chemical shift differences. Additional experimental demonstrations in powdered and single-crystalline adamantane and ferrocene samples are also presented.

The extent of penetrative Pyrenean deformation in the Ebro foreland Basin: Magnetic fabric data from the eastern sector

We studied Cenozoic sedimentary rocks in the NE Ebro foreland Basin using the Anisotropy of Magnetic Susceptibility (AMS) as a proxy for grain preferred orientation. Our new data, in combination with existing results, reveal that penetrative strain by layer-parallel shortening extends well beyond the Pyrenean thrust-wedge front into the Ebro foreland Basin, challenging the concept of the location of “deformation front” in the Southern Pyrenees. Penetrative strain, as revealed by the magnetic fabrics, seems to reach larger distances into the Ebro foreland Basin where stiffer layers constitute the main detachment level, whereas to the East strain dissipates closer to the deformation front when the detachment is mostly due to salt deposits.

Analysis of the electrical, magnetic and structural properties of A3B6 type layered semiconductor crystals intercalated with metals with reference to their military applications

Magnetic sensors are key elements in many security and military systems. Magnetic sensors are often used for military applications such as navigation, position tracking and antitheft systems. Nowadays sensitive magnetic sensors are used in many technical systems, including modern anti-tank missiles to identify the center of the target area and a minimal armor region.The applications of magnetoresistive structures based on semiconductor crystals of InSe for high precision measurement of the magnetic field are outlined in this article. Possibilities of using magnetic field sensors based on InSe structures for revealing the armour military vehicles are discussed. The impact of metal impurities on the layered structure of the semiconductor material as referred to the strong covalent bond within the layers as well as the weak van-der-Waals bond in the interlayer space is studied. Nyquist diagrams for InSe crystal with the impurities of nickel at different temperatures ranging from liquid nitrogen to room temperature are analyzed. Temperature is shown to be a significant factor for affecting the Nyquist diagrams. The curvature of the Nyquist diagrams for intercalated InSe varies with the temperature as opposed to the pure InSe. Topological images of crystal surface obtained by using atomic force microscopy confirmed the layer structure of nickel-intercalated InSe. Structures with alternating layers of semiconductor and metal provide the fundamental control of magnetic properties. These structures have a sharp anisotropy of magnetic susceptibility. The investigated semiconductor crystals with impurities of 3d-elements can extend the functionality of modern magnetic sensors designed to detect heavy armor.

Tectonic strain recorded by magnetic fabrics (AMS) in plutons, including Mt Kinabalu, Borneo: A tool to explore past tectonic regimes and syn-magmatic deformation

Tectonic strain commonly overprints magmatic fabrics in AMS (Anisotropy of Magnetic Susceptibility) data for plutonic rocks produced by both compressional and extensional regimes. Mt Kinabalu, Borneo, is a composite pluton with an exceptional vertical range of exposure and clearly defined internal contacts. We show that tectonic fabrics are recorded pervasively throughout the intrusion, even near contacts, and present a workflow distinguishing compressive and extensional syn-magmatic deformation. At Mt Kinabalu this reveals a pervasive tectonic fabric indicating NW-SE Miocene extension in Borneo at 7.9–7.3 Ma, later than previously recognised, oriented NW-SE at 319° ±13.1°. Comparing data from Mt Kinabalu with data from globally distributed studies shows that tectonic strain is commonly recorded by plutons. Therefore, AMS fabric can be used to identify the syn-magmatic tectonic setting and combined with both geochronology and evidence for paleomagnetic rotation to provide a powerful tool for accurate determination of syn-magmatic tectonic regimes and strain orientations within temporal frameworks.

Using AMS and Palaeomagnetic Data to Assess Tectonic Rotation: A Case Study from Savannah Nickel Mine, WA

Recent research has shown that Ni-PGE mineralisation is typically associated with specific intrusion types, e.g., chonoliths, bladed-dykes and funnels that acted as high-throughput magma conduits within much larger magmatic provinces. The initial architecture of these systems is very different, but additional structural modification during orogenesis can render such intrusions very difficult to understand structurally, and hence make it very difficult to target the fertile zones within the system. In structural geology it is common to evoke shortening directions, which are assumed to apply to all rocks regardless of their rheology. This is not realistic. Furthermore, resolving the partitioning of strain is not straightforward for intrusive rocks, which tend not to develop visible tectonic fabrics, but act as rigid blocks that rotate to accommodate strain, rather than compress or shear, during deformation. Unfortunately there are few structural techniques that can be used to quantify such rotation. Intrusions near the Savannah Nickel Mine, East Kimberley, WA were observed to have different deformation histories, despite being temporally equivalent. In this study we measured anisotropy of magnetic susceptibility (AMS) and remanent magnetisation in two adjacent, temporally equivalent, intrusions. The observed K3 AMS vectors are typically normal to the magmatic layering in layered intrusions. Where K3 vectors sit along a great circle, the pole to that great circle indicates the rotation axis. Original palaeomagnetic vectors would be expected to be consistently oriented with respect to magmatic layering, and can be used similarly to test the consistency of the inferred rotation analysis. The rotations inferred for the intrusions tested were consistent between the two techniques. Savannah and Savannah North, were subjected to N-S, NE-SW and NW-SE shortening, consistent with the Halls Creek Orogeny. However, N-S shortening was dominant at Savannah and NE-SW dominant at Savannah North. Therefore, despite their equivalent emplacement ages and implied tectonic history, each intrusion has undergone very different deformation.

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.

Neogene Paleomagnetic Study of the Western Baicheng Depression: Implications for the Intensified Deformation of Tian Shan Since the Latest Miocene

AbstractThe fold‐and‐thrust belts in the southern forelands of Tian Shan were tectonically deformed as a result of intracontinental deformation in response to the India‐Asia collision; therefore, they are essential for understanding tectonic evolution and basinward propagation of the Tian Shan Range. Although the reactivation of Tian Shan has long been studied, when and how this deformation was initiated and propagated are still controversial. In this study, we present new paleomagnetic results, anisotropy of magnetic susceptibility, tectonic rotations, and interpretations of subsurface seismic images on the Kalayuergun Structural Belt, western Baicheng Depression. Our new results suggest that the deformation of the Kalayuergun Structural Belt initiated at ~5.3 Ma and became young southeasterly, indicating that the deformation reached to the study area in the latest Miocene and propagated basinward progressively since then. The clockwise rotations, about half amounts of which must have been occurred at around ~5.3 Ma, can be linked to the activity of the Kalayuergun dextral strike‐slip fault on the western end of the anticline. Combined with previous results around Tian Shan, we suggest that the long range has been subjected to an episode of intensified tectonic deformation since the latest Miocene, which was caused by the accelerated northward motion of the Tarim Block and Pamir in response to the India‐Asia collision.

Zebra layers and palaeoenvironment of Late Miocene Stratum in the Linxia Basin, northwestern China

Miocene strata in the Linxia Basin (Gansu, China) are usually interpreted as lacustrine sediments. However, the red–grey inter-beds known as ‘Zebra layers’ commonly tilt with respect to the terrain on the side slopes of the modern valley, which may be due to mantling palaeotopography (similar to aeolian loess). The anisotropy of magnetic susceptibility, which reflects the original arrangement of magnetic particles in sediments, was applied to investigate this phenomenon. The results showed that the tilting of the inter-beds in the side slope was due to mantle palaeotopography rather than soil creep, and that they were not deposited in a subaqueous environment. The grain sizes of sediments showed similar features as aeolian loess. We speculate that Miocene sediments were deposited by mantling the palaeotopography where aeolian materials accumulated. After deposition, flowing water submerged these strata, which caused the side slope to become gradually thinner from top to bottom and stirred the magnetic particles in these sediments. The grey colour of the Zebra layers may not be original, as it may be due to waterlogging and deoxidation after deposition; finally, when the iron oxides in these sediments were transformed or removed, their colours became grey. The formation of Zebra layers indicates that the Late Miocene palaeoenvironment of northwestern China was similar to that in which Quaternary aeolian loess was deposited.

Complex mid‐crustal flow within a growing granite–migmatite dome: An example from the Variscan belt illustrated by the anisotropy of magnetic susceptibility and fabric modelling

AbstractThis study describes anisotropy of magnetic susceptibility (AMS) of migmatites associated with crustally derived granites of the Pelhřimov core complex, Bohemian Massif. In combination with numerical modelling, we use this case example to discuss some of the general complexities related to interpreting flow patterns of anatectic lower and middle crust from magnetic fabric during growth of metamorphic domes. Magnetic lineations, reflecting tiling or zone axis of magnetically oblate grains, commonly tend to develop along the principal stretching direction. Our fabric modelling, however, shows that a lineation may also develop at a high angle to the overall flow (tectonic transport) direction as a relict of a pre‐existing fabric or due to a component of core complex axis‐parallel stretching and may be maintained for a significant period of time. Furthermore, we demonstrate that the finite magnetic fabric orientation is largely independent on the initial migmatite fabric intensity (formed before dome growth) but is rather a function of (a) shear strain, causing magnetic foliation and lineation to rotate towards the shear plane, and (b) magnitude of dome growth, causing their external rotation. Similarly, the degree of anisotropy P is also a function of accumulated shear strain and increases over time. These findings suggest that low fabric intensities may point to rapidly cooling, less sheared domains that are more likely to preserve early formed fabrics. In dome limbs undergoing protracted simple shear, low and high degrees of oblateness (as expressed by the shape parameter T) may discriminate between domains with weak and strong initial fabrics and may thus be used to map out domains with different strain histories and perhaps also with different melt proportions and rheological behaviour within metamorphic domes.