Alteration induced changes of magnetic fabric as exemplified by dykes of the Koolau volcanic range

We studied 93 samples from 8 basaltic dykes of the Koolau volcanic range on the island of Oahu,Hawaii,USA,to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility (AMS) measurements. Rock magnetic as well as microscopic investigations show that only ≈25% of the samples have retained their original magnetomineralogical composition of unaltered Ti-poor titanomagnetite. The remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite, titanomaghemite and hematite. In both sample groups,this magnetic phase occurs in coarse (tens of microns),irregularly shaped particles as well as interstitial clusters of smaller (< 5 μm) grains. Our investigations show that hydrothermal alteration does change the bulk susceptibility and the degree of anisotropy but not the directions of principal axes of the AMS ellipsoid which are predominantly corresponding to normal magnetic fabric. The stability of AMS directions, regardless of the degree of alteration, points towards the model of distribution anisotropy as the controlling factor for the observed magnetic fabric.     

Correlation of strain with anisotropy of magnetic susceptibility (AMS)

Existing correlations between strain and anisotropy of low-field magnetic susceptibility (AMS) have been re-assessed using a single parameter to express both anisotropies. TheP parameter (Hrouda, 1982) shows potential as a powerful single expression of the intensity of strain and of AMS. Previous correlations are improved by use of this parameter. Cautious optimism is justified for correlations between strain and susceptibility in a certain strain window between a lower limit (excluding the incomplete overprint of predeformation anisotropy) and an upper limit (excluding the effects of saturation anisotropy). For successful correlations the influence of stress-controlled recrystallisation should be minimal and the mineralogical sources of susceptibility must predate deformation.     

A brief outline of the magnetic susceptibility anisotropy of various rock types

Summary In the first part of this paper, the main geological and geophysical applications of magnetic susceptibility anisotropy are briefly described. In the second part, the data on the magnetic anisotropy of various rock types are summarized and some conclusions concerning its significance are deduced. For this purpose, all accessible data were used.Dedicated to 90th Birthday of Professor Frantiek Fiala     

Magnetic susceptibility anisotropy,strain, and progressive deformation in Permian sediments from the Maritime Alps (France)

The relationships among magnetic susceptibility anisotropy, finite strain, and progressive deformation have been studied in Permian red shales and slates of the Maritime Alps (southeastern France). These rocks contain deformed reduction spots which serve as finite strain indicators. The magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements. The magnetic fabric changes progressively from oblate to prolate, and back to oblate as deformation increases. The quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined. They differ between the less and more deformed areas, perhaps due to a change in deformation mechanism accompanying an increase in metamorphism. The relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control.     

Magnetic anisotropy of HED and Martian meteorites and implications for the crust of Vesta and Mars

We investigate the petrofabric of crustal rocks from Mars and Vesta through the measurement of the anisotropy of the magnetic susceptibility (AMS) of achondrites. Previous data are integrated with new measurements to obtain a dataset that provide macroscopic information about the magnetic fabric of 41 meteorites of the howardite–eucrite–diogenite clan (HED, falls only) and 16 Martian meteorites. The interpretation takes into account the large contribution of paramagnetism to the magnetic susceptibility of these meteorites. We use a model that allows the computation of the anisotropy degree of the population of ferromagnetic grains and provides a quantitative proxy for the degree of shape preferential orientation of these grains in HED and Martian meteorites. The results also provide quantitative information about the shape of the magnetic fabric (prolate, oblate).In HED achondrites, the ductile FeNi grains are sensitive strain recorders and our magnetic fabric data provide the first quantitative insights to the strain history of the crustal rocks of Vesta. Most HED achondrites are breccias but display a strong and spatially coherent magnetic anisotropy, indicating that intense deformation of FeNi grains took place after brecciation. The average fabric of eucrites, howardites is oblate (i.e. the texture is foliated) whereas the fabric of diogenites is more neutral. The howardite results suggest the existence of an isotropic fraction of ferromagnetic minerals that can be ascribed to the presence of carbonaceous chondrite clasts that have preserved their original magnetic fabric. In this hypothesis, howardites have an intensity of petrofabric very similar to eucrites and diogenites. Thermal metamorphism (itself possibly impact-related) plus lithostatic compaction occurring after brecciation appears as the best candidate to explain the observed petrofabric in eucrites and diogenites, whereas compaction by hypervelocity impacts may be reponsible for the fabric of howardites.Martian meteorites may still possess their primary magmatic fabric. Among Martian meteorites, basaltic shergottites and nakhlites display an oblate fabric (foliated texture) with only limited variations among each group. Olivine–phyric shergottites have a neutral fabric that points to a different petrogenesis. Nakhlites have weaker fabric intensity than shergottites. The fabric intensity is comparable to what is classically observed in terrestrial volcanic and plutonic rocks.     

MAGNETIC SUSCEPTIBILITY ANISOTROPY OF VARIOUS ROCK TYPES AND ITS SIGNIFICANCE FOR GEOPHYSICS AND GEOLOGY*

In the interpretation of magnetic anomalies and in paleomagnetism, the anisotropy of magnetic susceptibility is commonly neglected. Nevertheless, this property has basic significance, because, owing to susceptibility anisotropy, the directions of the vectors of induced and remanent magnetization are deflected from the direction of the Earth's magnetic field. Almost all rock types investigated possess higher or lower degree of the susceptibility anisotropy. Effusive and sedimentary rocks have the lowest degree of anisotropy. For the latter, the “masking effect” of the paramagnetic mineral components has some influence on the anisotropy degree due to the low mean susceptibility of sedimentary rocks. Metamorphic and plutonic rocks usually exhibit a considerable degree of anisotropy. The highest degree of anisotropy has been found in the rocks containing ferromagnetic minerals with mimetic fabric. The dependence of the degree of the susceptibility anisotropy on the degree of metamorphism proved to be very complicated; of the rock sequence from slates to gneisses, the transient rocks (roofing slates and mica-schist-gneisses) showed the highest degree of anisotropy. This result can be used in geology for reliable determination of these rock types.     

Anisotropy of magnetic remanence: A brief review of mineralogical sources,physical origins,and geological applications,and comparison with susceptibility anisotropy

The magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low-field magnetic susceptibility (AMS). However, alternative methods based on remanent magnetization (measured in the absence of a magnetic field) rather than induced magnetization (measured in the applied field) have distinct advantages for certain geological applications. This is particularly true for; (1) adjunct studies in paleomagnetism, in order to assess the fidelity with which a natural remanence records the paleofield orientation; (2) studies of weakly magnetic or weakly deformed rocks, for which susceptibility anisotropy is very difficult to measure precisely; and (3) quantitative applications such as strain estimation. The fundamental differences between susceptibility and remanence (and their respective anisotropies) are due to several factors: (1) susceptibility arises from all of the minerals present in a sample, whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals; (2) ferromagnetic minerals are generally more anisotropic than para- and diamagnetic minerals; (3) for ferromagnetic minerals, remanence is inevitably more anisotropic than susceptibility; and (4) a number of common minerals, including single-domain magnetites, possess an inverse anisotropy of susceptibility, i.e., they tend to have minimum susceptibility parallel to the long axis of an individual particle; remanence is immune to this phenomenon. As a consequence of all these factors, remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample.Contribution number 9102 of the Institute for Rock Magnetism, University of Minnesota.     

非滞后剩磁各向异性

由于非滞后剩磁各向异性和等温剩磁各向异性的参数直接与岩石中的携磁矿物颗粒有关,它比传统的磁化率各向异性更明确地指示岩石组构,在构造地质研究中有较大应用前景．本文介绍了非滞后剩磁及其各向异性的测定方法,并以一个实例阐述了它在构造地质中的应用．     

A study of the magnetic susceptibility of rocks under uniaxial compression

Summary Preliminary experiments conducted on rocks under uniaxial compression seem to indicate that the magnetic susceptibility parallel to the domain wallsK does not play an effective role in stress-induced anisotropy.     

The development of magnetic susceptibility anisotropy through crystallographic preferred orientation in a calcite rock

High-field torque-meter measurements of diamagnetic susceptibility anisotropy of a suite of samples of Carrara marble, axially shortened by amounts up to 50% at (1.5–3.0) · 10⁸ Pa confining pressure and at 20–500°C (mainly 400°C), have been compared with optical measurements of preferred crystallographic orientation. A revised value for the susceptibility anisotropy of calcite has been obtained from studies of single crystals, and it has been shown to be almost independent of the state of intracrystalline plastic strain. From the measured anisotropy of calcite, quantitative comparison of optical and magnetic fabric measurements is possible. It is found that these measurements agree and the implications of the observed progressive development of fabric intensity with strain are discussed.     

Influence of magnetic interactions on the anisotropy of magnetic susceptibility: the case of single domain particles packed in globule aggregates

The influence of magnetic interactions on the anisotropy of magnetic susceptibility (AMS) have been largely studied by several theoretical models or experiments. Numerical models have shown that when magnetostatic interactions occur, the distributions of particles over the volume rather than their individual orientations control the AMS. We have shown recently from a comprehensive rock magnetic study and from a theoretical 2-dimensional (2-D) model that single domain particles closely packed in globule aggregates could produce strong local random interaction magnetic fields which could influence the magnetic susceptibility and decrease the degree of anisotropy. In this paper, we first present in detail this 2-D theoretical model and then we extend it to the 3-D case. The possible distribution function of the magnetostatic interaction fields comprises two extreme states: it is either isotropic or ordered. The former case corresponds to the thermal-demagnetized state while the second case corresponds to the alternating field (AF) demagnetized state. We show that when easy axes of magnetization are not uniformly distributed, the degree of anisotropy decreases as the interaction field increases in both AF- and thermal-demagnetized states in 2-D and 3-D geometry. Thus we conclude that random magnetic fields generated by a random arrangement of magnetic particles over the sample volume decrease the degree of anisotropy of AMS and may alter the magnetic fabric.     

嵊泗岛辉绿岩墙群的侵位方式:来自磁组构的证据

岩墙磁组构能反映岩浆的侵位方式.中国东部嵊泗岛广泛发育了晚白垩世辉绿岩岩墙群.我们对其中8条不同走向岩墙进行了采样,沿岩墙两边部及横跨岩墙剖面获得共273个独立定向岩芯样品.岩石磁学分析表明辉绿岩的主要携磁矿物为多畴贫钛磁铁矿,可能含少量磁赤铁矿.各条岩墙的磁组构均具有低的各向异性度Pj＜1.2,且主轴的空间方位各不相...     

Changes of parameters of magnetic anisotropy of foliated rocks under pressure

Summary The changes of magnetic anisotropy under pressure were studied on a set of rocks with marked macroscopic stratification, collected from the Rjvíz borehole. The anisotropy parameters are relatively very stable under directional pressure, acting parallel with the rocks' bedding, and, on the contrary, significantly unstable under pressure acting perpendicular to the bedding. Systematically different magnitudes of stress sensitivity coefficients, , for parallel directional susceptibility were observed in both cases in the same types of rocks. The main cause is probably the different capability of transferring external stress to ferrimagnetics via the stratified non-magnetic matrix. With regard to magnetomechanical phenomena, therefore, a singel value of the stress sensitivity coefficient is insufficient to characterize rocks with a markedly anisotropic matrix.     

秦祁接合带造山缝合带磁组构特征及其构造意义

结合构造及磁化率各向异性研究详细解剖了秦祁接合带唐藏—关子镇—武山和新阳—元龙造山缝合带的应变及岩组特征.41个采点168个构造岩样品的平均磁化率全部较低,磁化率椭球形态分析表明其以平面和压扁应变为主,磁化率各向异性度普遍较高,属强变形岩石组构类型,结合野外观察认为其与变形强度明显正相关.此外,磁化率各向异性参数T、P′可能受岩石类型一定程度的影响.磁化率椭球主轴方位与变形密切相关,提供了丰富的岩组信息.两构造带具有类似的岩组特征,磁面理大致分为呈共轭形态的两组,暗示高应变剪切带在平面上可能以网格状形态出露;高倾伏角磁面理与占优势的低倾伏角、近水平磁线理表明了构造带明显的走滑特征,部分高角度磁线理可能与构造带的挤压和（或）转换挤压相关;磁组方法不能简单用于判别复杂强变形带的运动指向,糜棱面理的复杂变化及K_min与构造带夹角过高使其判别结果意义不明,而野外及显微构造观察都表明了构造带的右行走滑特征.上述结果表明,沿缝合带大规模的右行转换挤压形成了秦祁接合带反“S”型的平面构造形态,暗示在南北板块拼合过程中,西秦岭诸中、小块体一定程度的向西挤逸.     

The magnetic fabric of some dolerite intrusions,northeast Spitsbergen; implications for their mode of emplacement

The titanomagnetite fabric of some dolerites from two areas of northeast Spitsbergen, Lomfjorden and the Bastian, and Rønnbeck islands have been studied by means of magnetic susceptibility anisotropy measurements. The shape and orientation of the susceptibility ellipsoids were determined with an a.c. bridge. Both areas showed a dominating magnetic foliation in the horizontal plane. A weakly developed preferred orientation of the maximum (K^I) and intermediate (K^II) susceptibility axis within the magnetic foliation plane was found, the K^I-axis striking predominantly E-W and the K^II-axis striking N-S. The magnetic fabric is interpreted as resulting from horizontal movement of magma. It is concluded that the Bastian and Rønnbeck islands are the relics of one or more larger sills.     

The magnetic anisotropy of basalts and its variations under oxidation of titanomagnetites

Summary Using four samples of basaltic rocks from the Bohemian Massif, Nos 201, 202, 206 and 218, the curves of rotational moments in a magnetic field of 4 × 10⁵ A/m were studied as a function of the degree of oxidation of titanomagnetites. The amplitudes of the individual harmonics were determined by harmonic analysis for samples oxidized to various degrees. It was found that the proportion of the harmonic A ₁ withsin increases with the degree of oxidation. Simultaneously, the anisotropy of the magnetic susceptibility changes in a field of 60 A/m and the structure changes from linearly parallel to plane parallel.     

Magnetic fabric of Plio-Pleistocene sediments from the Crotone fore-arc basin: Insights on the recent tectonic evolution of the Calabrian Arc (Italy)

Low-field anisotropy of magnetic susceptibility (AMS) analyses were performed on 532 samples collected in 36 (mostly lower Pliocene to lower Pleistocene) marine clay sites from the Crotone basin, a fore-arc basin located on top of the external Calabrian accretionary wedge. The Crotone basin formed since mid-late Miocene under a predominant extensional tectonic regime, but it was influenced thereafter by complex interactions with NW–SE left-lateral strike-faults bounding the basin, which also yielded post-1.2 Ma ∼30° counterclockwise block rotations. The basin is filled by continental to marine sediments yielding one of the thickest and best-exposed Neogene succession available worldwide. The deep-marine facies – represented by blue-grey marly clays gave the best results, as they both preserved a clear magnetic fabric, and provided accurate chronology based on previously published magnetostratigraphy and calcareous plankton (i.e. foraminifers and nannofossils) biostratigraphy. Magnetic susceptibility range and rock magnetic analyses both indicate that AMS reflects paramagnetic clay matrix crystal arrangement. The fabric is predominantly oblate to triaxial, the anisotropy degree low (<1.06), and the magnetic foliation mostly subparallel to bedding. Magnetic lineation is defined in 30 out of 36 sites (where the e12 angle is <35°). By also considering local structural analysis data, we find that magnetic fabric was generally acquired during the first tectonic phases occurring after sediment deposition, thus validating its use as temporally dependent strain proxy. Although most of the magnetic lineations trend NW–SE and are orthogonal to normal faults (as observed elsewhere in Calabria), few NE–SW compressive lineations show that the Neogene extensional regime of the Crotone basin was punctuated by compressive episodes. Finally, compressive lineations (prolate magnetic fabric) documented along the strike-slip fault bounding the basin to the south support the significance of Pleistocene strike-slip tectonics. Thus the Crotone basin shows a markedly different tectonics with respect to other internal and western basins of Calabria, as it yields a magnetic fabric still dominated by extensional tectonics but also revealing arc-normal shortening episodes and recent strike-slip fault activity. The tectonics documented in the Crotone basin is compatible with a continuous upper crustal structural reorganization occurring during the SE-migration of the Calabria terrane above the Ionian subduction system.     

Magnetic fabric and strain in Miocene volcanics in the southern margin of the Bükk Mountains, Hungary

The magnetic anisotropy of two Miocene volcanic horizons south of the Bükk Mountains, northeast Hungary, was investigated (28 geographically distributed sites, over 400 samples). The horizons contain rocks from ignimbrites to porous tuffs. Microscopic and rock magnetic investigations showed that the main carrier of anisotropy is multidomain maghemite with varying composition. The complex magnetic fabric was separated into deformational and flow fabric in the upper horizon. This was carried out by monitoring the distribution of minimum susceptibility directions supposed to be elongated towards intermediates as a response to compressional strain. Jelínek's tensor statistics were used in the computations. Filtering out the effects of volcanic flow and compaction, north-south striking horizontal compressional strain was determined for the upper horizon in the geographic system. Owing to the lack of elongated distribution of minima, the characteristic intermediate susceptibility directions were taken as markers of strain directions in the lower horizon (WNW-ESE and NNE-SSW in the geographic system). The strain directions showed correlation with compressional stress orientations observed by microtectonics, thus a homogeneous stress-strain field was supposed within the area investigated. The tectonic component of the strain can be determined by taking the respective paleomagnetic declination rotations as the rotational components of the deformation.     

Magnetic fabrics of the Bloodgood Canyon and Shelley Peak Tuffs,southwestern New Mexico: implications for emplacement and alteration processes

Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K ₁ axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K ₁ susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K ₁ orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information.     

A deformed Pliocene-Quaternary alluvial and red paleosol succession in the Eastern Betics: Paleomagnetic,rock-magnetic and sedimentological pilot study

Summary An integrated pilot paleomagnetic and sedimentological study has been conducted in the Neogene-Quaternary infilling materials of the Bajo Segura Basin (Eastern Betics, SE Spain). The studied sediments belong to the youngest (late Pliocene-Quaternary) lithostratigraphic unit of the basin (P-Q unit). The statistical analysis of tectonic striations and stylolitic dissolutions on the conglomerate limestone clasts indicates a NNW-SSE maximum compression direction. This is in accordance with the principal susceptibility axes, determined from the anisotropy of magnetic susceptibility measurements (AMS) of the interbedded siltstones where the Kmax axis group in a subhorizontal N 080° E direction.A total of 45 core samples have been collected from 16 stratigraphic siltstone levels encompassing the described section at Crevillente with the aim to establish a magnetochronology. Upwards in the section the NRM intensity and bulk susceptibility vary from 10^–2 to 10^–3 A/m and from 1550 to 100 × 10^–6 SI, respectively. The mean characteristic remanent magnetization (ChRM) after bedding correction (Dec/Inc = 10°/60°, ₉₅ = 8.7°, k = 15.9) is deviated slightly towards the E from the reference N direction, and could reflect a deflection related to the observed magnetic fabric although no clear correlation exists with AMS parameters. The derived magnetostratigraphy reveals only one reversal boundary within the upper third of the section, delimiting an upper reversal magnetozone which has been tentatively correlated with the Olduwai subchron close to the Plio-Pleistocene boundary.Pilot and preliminary rock-magnetic experiments and standard X-ray difraction (XRD) analysis have been performed on typical samples in order to establish the carriers of magnetization and characterize the sedimentological and magnetic-acquisition processes in these sediments. Magnetic carriers seem to be dominated by magnetite with a relative grain size within the PSD state (pseudo single-domain) threshold, but closer to the MD (multi domain) threshold, which favours the detrital origin for the magnetite.