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Magnetic susceptibility anisotropy, an unexploited petrofabric element

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... Por isso, o estudo destas sequências requer o uso de técnicas alternativas para reconstrução do ambiente vulcânico (e.g., Pueyo Anchuela et al., 2014). A anisotropia de susceptibilidade magnética (ASM) constitui uma técnica geofísica que possibilita a determinação da petrotrama em rochas vulcânicas (Graham, 1954;Savian et al., 2021). Especialmente em ignimbritos, a ASM pode ser útil na identificação de sentido e regime de fluxo, processos deposicionais e centros emissores (Cañón-Tapia & Mendoza-Borunda, 2014;Savian et al., 2021). ...
... A ASM consiste em uma técnica geofísica que possibilita a determinação da petrotrama em rochas ígneas, metamórficas e sedimentares (Graham, 1954;Savian et al., 2021). Neste capítulo são apresentados os fundamentos dessa técnica, e como ela se relaciona com minerais presentes em amostras naturais de rochas. ...
... A ASM se utiliza da κ presente em rochas para obter a orientação da trama microscópica dos minerais presente numa amostra (Graham, 1954), por meio do uso de susceptômetro. Este aparelho obtém κ aplicando um campo magnético (H) medindo a magnetização (M) da amostra (Fig. 7). ...
Experiment Findings
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M.B.Haag's MSc dissertation reporting AMS findings in the Riscos Bayos Ignimbrites.
... AMS is a sensitive indicator of magnetic sediment texture and the anisotropy can be determined by the preferred crystallographic and in the case of ferrimagnetic minerals, the dimensional (or shape) orientation of grains in the sediments. Graham (1954) first noted that AMS is a good tool to determine the palaeodirection of sediments. Rees and Woodall (1975) discussed the application of AMS under various hydrodynamic conditions. ...
... In the laboratory, we cut each sample into two sets of smaller parallel specimens (2 Â 2 Â 2 cm), The AMS of each sample was measured with an AGICO KappaBridge KLY-4 S magnetic susceptibility meter, coupled with an automated sample handling system. The lowfield AMS from the sample is calculated by the least-squares methods as an oriented ellipsoid with maximum, intermediate, and minimum axes of MS (K max , K int , and K min ) (Graham, 1954;Hrouda, 1982). The anisotropy parameters of magnetic foliation (F = K int /K min ), lineation (L = K max /K int ), and degree of anisotropy (Pj = K max /K min ) were obtained with Anisoft software using the statistical method of Constable and Tauxe (1990). ...
Article
Oriented samples were collected in the southern Mongolia Plateau and north‐eastern Tibetan Plateau to detect links between microstructural features, which are inferred from magnetic fabrics, and outcrop‐scale sedimentary structures that include slender fossil orientations. Palaeocurrent imprints in particles and their recognition in magnetic fabrics depend on the kinetics of current and any post‐depositional reworking. Here, we restored the flooding palaeocurrent direction through the statistical orientation of slender fossils and imprints indicated by magnetic fabric patterns in the sediments. We found that the tilt response of the minimum susceptibility axes preserves more links to the dynamic orientation of fossil deposits, while the maximum susceptibility axes may be easily reworked by a weak but long‐term tectonic compression. Comparing the two methods reveals the correlation between fossil distribution and magnetic fabric. The dynamic identification of anisotropy of magnetic susceptibility on large quantitative samples contributes to provide an alternative way for further statistics of fine‐grained sediments and fabric arrangement. We compared outcrop scale slender fossil orientations and microstructural features of magnetic fabrics collected from two sections in the north‐eastern Tibetan Plateau and southern Mongolia Plateau. We found palaeocurrent imprints in particles and their recognition in magnetic fabrics depend on the kinetics of current and any post‐depositional reworking including tectonics.
... Since the basic theory of AMS was initially proposed by Graham (1954), this method has been widely applied in constraining emplacement mechanism of granites, infill history of basins, or flow of basaltic lava (Talbot et al., 2005;Gutiérrez et al., 2013;Wei et al., 2014;Caricchi et al., 2016;Ji et al., 2018;Qiu et al., 2020). The magnetic ellipsoid and the strain ellipsoid can be compared after a careful recognition of the magnetic carrier (Borradaile and Henry, 1997;Borradaile and Jackson, 2004). ...
... The magnetic ellipsoid and the strain ellipsoid can be compared after a careful recognition of the magnetic carrier (Borradaile and Henry, 1997;Borradaile and Jackson, 2004). Furthermore, the shape and eccentricity of the magnetic ellipsoid can be used as indicators to delineate multiphase deformation events (Graham, 1954;Ferré et al., 2014;Parsons et al., 2016;Marcén et al., 2018). The Bikou Terrane is characterized by penetrative foliations, few lineations, and multi-stage deformation, so it is a natural laboratory to analyze the tectonic response of AMS fabrics to the macroscopic structures and finite strain characteristics. ...
Article
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The Bikou Terrane, located at the conjunction of the Longmenshan fold-thrust belt and the west Qinling orogenic belt in centeral China, was involved in the Late Triassic collision between the South China and North China blocks. The Bikou Terrane has preserved crucial information on structural geometry and kinematics of Triassic tectonics, and is therefore of great importance for reconstructing the Paleo-Tethyan evolutionary history. However, multi-phase tectonic events of the Bikou Terrane are unsettled. This work presents detailed structural analysis based on both the field and laboratory works, which reveals three phases of deformation events in Bikou and its adjacent areas, including top-to-the-SW shearing related to SW-ward thrusting (DI) mainly to the north of the Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (DIII) locally developed in the northern Bikou Terrane. Anisotropy of magnetic susceptibility (AMS) study and related structural analysis not only support the multiphase deformation but also reveal a gradual transition from the DII-related magnetic fabrics to the DIII-related magnetic fabrics in the Bikou Terrane. Integrating published geochronological data, it is constrained that DI occurred at ca. 237−225 Ma, DII occurred at ca. 224−219 Ma, and DIII possibly occurred during the Early Cretaceous. Based on regional tectonics, the DI event corresponds to the collision between the South Qinling block and the Bikou Terrane, and the DII event reflects the intracontinental amalgamation between the Bikou Terrane and the Yangtze block, which indicates a Late Triassic successive amalgamation from the North China block to the South China block. Intracontinental adjustment represented by the strike-slip (DIII event) occurred after the final amalgamation between the North China and South China blocks. By applying AMS on deciphering structural geometry and multi-phase deformation, our study suggests that AMS is a useful tool for structural analysis.
... Quando os grãos são orientados preferencialmente em uma rocha, ela pode apresentar uma anisotropia de susceptibilidade magnética (ASM). A ASM pode ser investigada, por meio da anisotropia magnética, técnica baseada na variabilidade direcional das propriedades magnéticas, que permite a determinação da trama estrutural (petrofábrica) das rochas (Graham, 1954). Desde a publicação do trabalho pioneiro de Graham (1954), a ASM tem sido amplamente utilizada no estudo de processos geológicos. ...
... A ASM consiste em uma técnica geofísica, capaz de determinar a orientação da fábrica magnética de rochas ígneas, sedimen-tares e metamórficas (Graham, 1954). Fluxos de lava e corpos subvulcânicos frequentemente apresentam escassez de indicadores cinemáticos macroscópicos (como, por exemplo, minerais alongados e estruturas de lineação e de foliação magmática), tornando necessária a aplicação de uma técnica capaz de determinar a orientação da trama microscópica presente nestas rochas. ...
... Magnetic fabrics are commonly used as a proxy for mineral alignment and therefore a powerful tool in all areas of geology investigating dynamic processes, including the emplacement of igneous rocks, sediment transport and deformation (Borradaile and Henry, 1997;Borradaile and Jackson, 2004;Borradaile and Jackson, 2010;Cañón-Tapia, 2004;Graham, 1954;Hrouda, 1982;Jackson and Tauxe, 1991;Jackson, 1991;Owens, 1974;Owens and Bamford, 1976;Parés, 2015;Tarling and Hrouda, 1993). Less common is the investigation of pore fabrics and prediction of preferred flow directions by measuring anisotropy of magnetic susceptibility (AMS) after a sample has been impregnated with ferrofluid (Parés et al., 2016;Hrouda et al., 2000;Jones et al., 2006;Halls, 1990, 1994). ...
... Anisotropy-induced deviations of magnetization directions and intensities have major consequences for paleogeographic reconstructions or the study of the evolution of the geomagnetic field through time (Bilardello and depend on measurement direction, and relate to phenomena known today as shape anisotropy and magnetocrystalline anisotropy, respectively. Comparisons between magnetic fabrics and macroscopic foliation and lineation, or paleoflow and -current directions in rocks revealed directional relationships (Fig. 1b) (Balsley and Buddington, 1960;Graham, 1954Graham, , 1966Hamilton and Rees, 1970;Ising, 1942;Khan, 1962;Rees, 1965;Stacey, 1960;Taira, 1989), as well as correlations between the degree of magnetic anisotropy and strain (Borradaile, 1988;Cogné Fig. 1. Very brief history of the evolution of our understanding of magnetic fabrics (a-d). ...
Article
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Magnetic fabrics are used extensively as proxies for mineral fabrics and to correct paleomagnetic data, and have been proposed to quantify pore fabrics. Today's understanding and interpretation of magnetic anisotropy benefits from decades of carefully observing anisotropic mineral and rock properties, establishing and testing empirical relationships between magnetic and mineral fabrics, and developing models to quantify aspects of anisotropy. Each advance was preceded by some puzzling observations, i.e., data that could not be explained based on the models available at the time, e.g., ‘oblique’ or ‘inverse’ fabrics, or lithology-dependent anisotropy-strain relationships. These observations led to numerous experimental and numerical techniques designed to characterize magnetic fabrics and determine their origin. Despite the successful application of magnetic fabrics in many structural and tectonic problems, there are still phenomena and measurements that cannot be explained based on today's magnetic fabric theory. With the purpose of fostering future development, I will touch on three main areas where I see challenges in magnetic fabric research: (1) Experimental characterization of magnetic anisotropy, including the non-uniqueness of reported tensors, and non-linearity; (2) anisotropy modelling and defining the carrier minerals and origin of the magnetic fabric; (3) selection of adequate (sets of) anisotropy tensors to correct paleomagnetic data. The description of the challenges presented here will hopefully help define directions for future research, inform those that are new to anisotropy, and advance our field.
... Anisotropy of low-field magnetic susceptibility (AMS) was reported as a useful tool to determine paleocurrent or paleodirection by Graham (1954). The application of magnetic anisotropy measurements has increased rapidly since Graham's pioneering study, and this technique is an essential method in a wide range of fields in Earth and planetary science (e.g., Hrouda, 1982;Tarling and Hrouda, 1993;Parés, 2015;Bilardello, 2016). ...
... Following the study of Graham (1954), the first studies on the connection between depositional processes observed sedimentary structures and the magnetic fabric (e.g., Granar, 1958;Rees, 1961). Such works and subsequent studies in the 1960's laid the foundation of magnetic anisotropy studies of sediments, i.e., the use of the magnetic fabric characteristics to determine the orientation of (paleo)currents responsible for sediment transportation (e.g., Rees, 1965) (Fig. 1). ...
Article
A summary of approximately three decades and greater than thirty loess magnetic fabric studies is presented here. The revised studies cover various loess regions from the Chinese Loess Plateau across the European Loess Belt to Alaska. Although there is still an ongoing argument about the feasibility of the magnetic fabric of loess in paleowind reconstructions, the determination of prevailing wind direction during various periods of the Pleistocene is the main goal of magnetic anisotropy analysis of the revised loess successions. The magnetic fabric analysis of loess from Chinese Loess Plateau provided significant information about the characteristics of paleomonsoon in East Asia, and the results from other loess regions, such as Alaska, the European Loess Belt, and Siberia, are also promising. As it is shown in this review, the synthesis of the paleowind direction results from the studied profiles may already provide a significant foundation for future climate models by the reconstruction of key climate centres and main continental level wind tracks. Besides the reconstruction of prevailing paleowind directions, there are numerous loess magnetic fabric studies using magnetic anisotropy parameters in the reconstruction of the characteristics of long-term climate trends, climate transitions and glacial- interglacial cycles. There are some lesser known aspects contributing to magnetic fabric of loess, such as the influence of various types of magnetic contributors on the overall fabric (i.e. the study of sub-fabrics) and their role in environment reconstruction. Besides the identification of aeolian magnetic fabric, not so many studies focus on the magnetic anisotropy characteristics of materials, possibly developed by water-lain sedimentation, pedogenesis, mass movements and permafrost activity. Novel results from Hungarian loess, especially from Paks, connected to some of the latter topics are also presented. Such topics includes the analysis of the nanofabric in paleosols, developed by pedogenesis, the comparison of magnetic fabric, formed during high energy transportation by aeolian or aquatic agents, and the periodicity analysis of magnetic parameters during the early Middle Pleistocene. New research lines, introduced in this review, may inspire new researches, and provide new perspectives for the next generation of magnetic anisotropy studies of loess successions.
... The AMS can be described by the sizes and directions of the maximum, intermediate and minimum axes of the AMS ellipsoid. As a geological analysis method, magnetic fabric analysis was first used by Graham [1] to study the fabric of rocks, and has since been widely applied in research on sedimentary processes and to distinguish the transport power direction of sediments [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. ...
... Because our samples were taken from the basset section, we can correct the D VRM value of the basset section itself. For instance, when we take the VRM direction as the north direction for re-calculating the D AMS of the samples, we can directly use the average D VRM value as the correction term δ, using the following formula: (1) where: D AMS0 is the declination of the maximum axes of the measured AMS ellipsoid, using geographic north as the benchmark direction. D VRM is the VRM declination. ...
Article
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The loess formation sampling method on the Chinese Loess Plateau generally involves the acquisition of samples from the basset section and the drilling core. Constraints imposed by the precision of the drilling machine operation make it difficult to determine the orientation of the samples due to the rotation of the core. Although researchers have proposed solutions for reconstructing the north direction of the samples by adopting the viscous remanent magnetization (VRM) orientation, it remains uncertain whether this approach can be adopted in studies that use the magnetic fabric to trace the palaeomonsoonal direction, and the degree to which this approach will change the magnetic fabric results. Based on the achievements of other researchers, we adopted the VRM orientation of the basset section samples oriented in the field. By determining how the VRM orientation changes the magnetic fabric of loess over different demagnetization temperature ranges, we can draw the preliminary conclusion that there is no significant difference between the magnetic fabric information of the loess obtained at 100–150°C VRM orientation and that obtained from the field orientation (the statistical bin size is 22.5°, significance level α = 0.05). This indicates that the VRM orientation approach is feasible for studying non-oriented drilling core samples to determine the prevailing surface paleowind direction with appropriate precision.
... Shortly after the first geological pioneering application of the AMS (Ising 1942), Graham (1954) suggested to open a new way based on this approach. For more than 10 years, AMS use was very limited, due to the heavy conditions of measurement and to the necessary use of large samples. ...
...  henry@ipgp.fr A b s t r a c t Shortly after the first geological pioneering application of the AMS (Ising 1942), Graham (1954) suggested to open a new way based on this approach. For more than 10 years, AMS use was very limited, due to the heavy conditions of measurement and to the necessary use of large samples. ...
... The AMS signal is particularly sensitive to igneous rocks and other relevant deposits, due to the relative abundance of magnetic minerals. Graham (1954) was the first to address geological processes through AMS; subsequently, this approach was adopted in volcanological studies worldwide to estimate the flow directions of ancient pyroclastic flow deposits (e.g., ignimbrites), lava flows, and dikes, as well as source vent locations (e.g., Ellwood, 1978Ellwood, , 1982Cagnoli and Tarling, 1997;Palmer and MacDonald, 1999;Cañón-Tapia, 2004;Agrò et al., 2015;Ort et al., 2015a;Porreca et al., 2015). The azimuth of the K max axis inferred from AMS analysis (usually indicating magnetic lineation, as K max > K int ≈ K min in the magnetic susceptibility intensity) is generally parallel to the flow direction of a unit of lava, dike, or pyroclastic flow deposit (Fig. 3b). ...
Article
Multidisciplinary studies of volcanic processes and their timescales are vital for the assessment and mitigation of the risk of future volcanic disasters. Paleomagnetic and rock magnetic approaches have long been applied in diverse volcanological studies worldwide. In this review, we provide conceptual and methodological explorations of a representative suite of paleomagnetic and rock magnetic applications, and discuss their future perspectives in Korean volcanological research. These paleomagnetic and rock magnetic applications will contribute to a better understanding of volcanic and associated processes during the pre-, syn-, and post-eruption periods, and their timing and duration, and also contribute to the improvement of spatiotemporal correlations of volcanic and associated units in Korea. Thus it will be connected to meaningfully lead future volcanic hazard analyses.
... Sedimentary rocks, and in particular continental sediments, such as siltstones and claystones, were the starting point for the development of AMS from its beginning (Graham, 1954) and its application was later extended to igneous rocks, either intrusive or extrusive (Khan, 1962;Knight & Walker, 1988). Some of the most common minerals in sedimentary rocks are phyllosilicates, that in a nonhydrodynamically energetic environment are expected to be oriented with their largest surface parallel to the ground due to their platy shape. ...
Article
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A magnetic fabric study was carried out in Permo-Triassic continental sedimentary red beds from the Cadí Basin (Eastern Pyrenees). These rocks were deposited on the top of a volcanic sequence when the regional tectonic regime changed from transtension to extension. Standard paleomagnetic cores (cylinders), cubes and irregular fragments were obtained for this study in 25 sites distributed throughout the basin. The “fragment methodology”, using oriented irregular fragments in claystones, is demonstrated to be successful and replicable. The aims of this study are (i) to interpret the magnetic fabrics in red beds according to sedimentary or tectonic models; (ii) to establish the relationship between the magnetic fabrics in the sedimentary red beds and the magnetic fabrics previously obtained in the underlying volcanic series, and (iii) to compare magnetic fabrics in continental sediments with other basins of the same age to interpret the tectonic scenario in the Iberian plate during Permian and Triassic times. The results obtained reveal a primary magnetic fabric that recorded a syn-sedimentary stretching in two nearly perpendicular directions, WNW-ESE and N-S, linked to basin development. Other sedimentary and tectonic factors, such as paleocurrents and deformation derived from the Alpine compression did not have a significant influence on the development of magnetic fabrics. The geodynamic scenario of the Iberian plate during Late Permian to Triassic times can be explained according to regional trajectories of extensional stress due to the activity of the North-Pyrenean fault zone.
... Cuando un campo magnético es aplicado en una roca, la intensidad de magnetización no es siempre uniforme en todas las direcciones, sino que ésta varía según la orientación de sus minerales constituyentes. Este método es usualmente utilizado para estudiar la petrofábrica de las rocas, inclusive cuando la deformación es débil (Graham, 1954;Tarling y Hrouda, 1993). La técnica es no-destructiva y puede utilizarse en todo tipo de rocas o sedimentos inconsolidados, incluso cuando los minerales ferromagnéticos están ausentes. ...
Thesis
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Claromecó Basin (Buenos Aires province, Argentina), considered as a frontier basin, has a relevant economic-energetic interest due to the presence of coal layers in subsurface, assigned to the Tunas Formation (Pillahuincó Group, Permian). During this thesis, diagenetic and magnetic susceptibility anisotropy (AMS) studies were performed in cores samples from the PANG 0001 and PANG 0003 wells to reconstruct the postdepositional history of the Tunas Formation and to define its potential to generate and storage hydrocarbons. Two main levels composed of organic-rich mudrocks and coals were defined: an upper level, at the top of the succession, with 20 meters of thickness, and a lower one, at the base, with 36 meters. Total organic carbon (TOC%) values of 0.5-2% to 53.9% assign them a good to excellent quality as source rocks. The organic matter is of terrestrial origin (type III kerogen), its maturity indicates a late catagenesis to metagenesis stage within the wet-condensate to dry gas (methane) window. The organic matter quantity, type, and maturity suggest that the formation has a good potential as gas-prone source rocks. These results, in addition with compaction, porosity, mineral association and fluid inclusion studies confirm that the formation is in a mesogenesis stage, during which it could reached palaeotemperatures of 140 to 200°C and burial depths of at least 3 km. Regarding the reservoir point of view, porosity (0.1 to 4%) and permeability (10 -3 to 10 -6 mD) values suggest that naturally fractured sandstones and carbonate-cemented sandstones near carbonaceous levels have potential as unconventional tight gas sandstones. Porosity is secondary type, by fracture of the rock and grains and dissolution. It was generated during mesogenesis as a consequence of the alteration and dissolution of instable grains and cement and fracturing, product of the compaction during burial, diagenetic fluids overpressure and/or tectonic stresses. Coal and carbonaceous mudrocks strata have potential as unconventional coal gas reservoirs (Coal Bed Methane, CBM), given their thickness, TOC values and maturity (rank). Fine lithologies (mudrocks/tuffs) and sandy levels with low porosity and permeability above the carbonaceous deposits could act as seal rocks, constituting a stratigraphic trap. Obtained AMS parameters determine that the petrofabric of the sequence is mainly sedimentary type in the upper and middle layers, with tectonic imprints towards basal levels. These results, in addition with previous AMS outcrop data confirm that deformation degree of the area was gradually attenuated toward the foreland basin and to the younger strata, deposited during the early Late Permian. Based on the data obtained, the Tunas Formation could be considered as a speculative petroleum system: Tunas-Tunas (?), with potential as source/reservoirs rocks of gas.
... Despite the obvious practical complications of precisely interpreting AMS axes in terms of mineral orientations, early investigators confirmed that induced magnetization was easier in certain significant directions, e.g. parallel to bedding or schistosity (Ising 1942;Graham 1954). In isotropic material there is no variation in total induced magnetization with direction indicating a random orientation of minerals. ...
Thesis
The first and foremost information generated during this study was about the nature of magnetic mineralogy. The routine rock magnetic method applied here clearly indicate the overall ferrimagnetic mineralogy to add to the previously known work inferring magnetite (M) of different magnetic domain size (SP- SD, MD-PSD) and that too within a single sample facilitating multiple analyses like AMS and Paleomagnetism. The petrographic analyses reveal a fairly high concentration of magnetite(s) within the studied slides. hereby revealing the carrier mineral responsible for the ChRM. Also, olivine megacrysts are abundant among the finer grained plagioclase crystals which form the groundmass along with Opaques. There are a few clinopyroxenes macrocrysts present with the Olivine. Within the Lamproites the major mica is biotite, which is observed as yellowish flakes and dispersed within the groundmass. Altogether the samples show a fairly high concentration of ferrimagnetic minerals facilitating paleomagnetic and rock magnetic studies. The presence of plagioclase as small crystals probably microlites implies rapid rate of cooling or high number of nucleation sites thereby enriching the groundmass in plagioclase nuclei rather than well developed crystals. A variety of directions and magnitudes are observed in the AMS results of the studied dykes. The distinct fabric observed is prolate with K1 parallel to the dyke wall. However, many other varieties including the oblate fabrics with K1 near horizontal is observed. Apart from this the well clustered K1 and K2-K3 girdle can also be observed. This indicates that each dyke has different style of fabric formation and indicated the instantaneous stress regime recorded by the AMS. The well clustered K1 parallel to the dyke walls and shallow but well clustered K3 and K2 suggest higher stress regime vis-a-vis rapid cooling experienced by the dyke. The shallow K1, high angle K3 and mixed K2 with oblate fabrics suggest slow cooling and relatively low stresses from the walls. Similarly, the random fabrics too indicate slow cooling and low stresses. The high angle K1 shows directions in different quadrants indicating the intrusive directions. It is proposed that the dykes from Deccan traps can be classified based on their AMS characters that are influenced by the intensity and directions of intrusion and the stress regime penultimate to cooling. From the Paleomagnetic results it is observed that the samples obtained from the margins of the dike show stable directions while those obtained from the core (central portions) show scattered results. This could be in fact due to higher rate of cooling along the dike walls, on account of contact with the country rocks leading to chilling and recording of a point event. While, the central portions require time to cool thereby forming multidomain grains of varying grain sizes these could lead to the scatter observed in the data. The results from the Declination show a large scatter while inclinations show fairly good consistency. The ‘α95’ and ‘k’ are fairly good to produce a broad idea of the paleomagnetic distribution. The VGP pole lat/long are distributed in both hemispheres due to normal and reversals, whereas the paleolatitudes are normalised to indicate the true geographic positions of the dykes. The mean paleolatitude is 16° South with a standard deviation of 7.79 varying between 37° and 9.38°. The median value is 16.64°. When these values are compared with the published data on Deccan Trap lava flows and dykes, the above paleolatitude fall in some of the youngest regimes of Deccan volcanism or even later to it. However, considering the large statistical variation accounted to mineralogical reasons (domain size), more detailed sampling and analysis is required. From the present paleomagnetic attempt following two alternative hypotheses can be derived: a) The studied dykes represent one of the youngest event in the Deccan volcanic episodes OR b) There is a significant Southward dipping tectonic tilt related to the West Coast tectonics. These two hypotheses need to be tested with more detailed studies in line with the present thesis.
... The AMS of sedimentary strata are dominated by the dimensional orientations of ferromagnetic and paramagnetic minerals grains (Graham, 1954;Parés, 2015;Tarling & Hrouda, 1993), and thus, it is essential to reveal the magnetic carriers of the nine sampling sections. Representative samples from the sampled strata have been subjected to isothermal remanent magnetization acquisition followed by reverse field demagnetization curves, and thermal demagnetization of three-component IRMs (DC fields of 2.5T, 0.4T, and 0.12T successively applied to the three orthogonal axes of representative samples; Lowrie, 1990), as described in previously published paleomagnetic and magnetostratigraphic studies (Gao et al., 2014;Tong, Yang, Mao, et al., 2017;H. ...
Article
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The steps of the southeastward stepwise‐descending topography of the Chuandian Fragment (CDF) in the southeastern edge of the Tibetan Plateau coincide with N‐S and NE‐SW trending faults that changed from thrusting to normal activity, indicating that since the Middle‐Late Miocene the CDF maintained an E‐W oriented extensional tectonic environment. However, the anisotropy of magnetic susceptibility (AMS) data sets presented herein, from the Cretaceous and Cenozoic sedimentary strata of the southeastern Tibetan Plateau and its edge, show that since ∼28.0 Ma the CDF remained within a stabilized compressional stress field, oriented NEE‐SWW. A comprehensive analysis of the AMS results, topography, fault systems, and geophysical observations indicate that the northward‐advancing eastern Himalaya syntaxis impeded the southeastward‐flowing viscous lower crust and correspondingly decreased its flux, which could have reduced the thickness of the lower crustal channel beneath the CDF. The original low‐gradient topography of the CDF formed by the viscous lower crustal flow could no longer be supported by the thinned lower crustal channel, and thus gravity drove the collapse of the uplifted CDF. This process reconstructed the topography and remodeled the preexisting fault systems of the CDF, resulting in the coexistence of the E‐W oriented extensional tectonic environment and the stabilized NEE‐SWW oriented compressional stress field of the western part of the CDF, since the Middle‐Late Miocene. Thus, the change in the status of the lower crustal channel related to block interactions provided a major geodynamic setting for shaping the topography of the southeastern edge of the Tibetan Plateau.
... It is possible by this method to describe and measure different characteristics as cleavage and lineation in some minerals, lattice-preferred orientations, deformation mechanisms and 60,80 . Moreover, the anisotropy of magnetic susceptibility (AMS) is an effective technique used to measure the primary or tectonic petrofabric of the rocks 81 . The method is based on measuring the intensity of magnetization and the direction of magnetic minerals in the rock 82 . ...
Article
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At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant K max axes trending NW–SE, parallel to the fold axes. The K min axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.
... Besides, the large number of dykes associated with most of the Continental Flood Basalt (CFB) provinces causes added inconveniences. Graham (1954), using the Anisotropy of Magnetic Susceptibility (AMS) technique, postulated the reconstruction of the shape and orientation of the magnetic fabric from the three susceptibility axes (K 1 , K 2, and K 3 ) from which the magma flow direction and its sense can be determined. This method has already been applied previously several times to investigate the emplacement mechanism of dyke swarms in different tectonic domains, viz.: Hawaian dykes (Knight and Walker, 1988), Proterozoic mafic dyke swarm of the Canadian shield (Ernst, 1990), the Troodos ophiolite (Staudigel et al., 1992), Makhtesh Ramon dykes, Israel (Baer, 1995), the Independence dyke swarm, California (Dinter et al., 1996), Cretaceous mafic dykes in the Moyar Shear Zone (MSZ) area (Pratheesh et al., 2011), radiating dyke swarm in the Eastern Dharwar Craton, Southern India (Kumar et al., 2015) and others (see Cañón-Tapia, 2004 for detailed references) and flow-induced strain in mafic rocks (e.g., Kodama, 1995;Martín-Hernández et al., 2004;Ort et al., 2015;Rochette et al., 1992;Tarling and Hrouda, 1993). ...
Article
The Deccan volcanic province (DVP; ~65 Ma old) of India is one of the classic examples of continental flood basalts. One of the most favoured hypotheses for DVP emplacement is “eruption through fissures” facilitated by major pre or syn-Deccan crustal extension. Determination of magma flow direction in its feeder dyke system provides clues on its possible association with a mantle plume, depth and number of the feeder chambers and other important geodynamic information on magma emplacement relevant in arguing about such hypotheses. In this paper, we have studied Dhule-Nandurbar Deccan (DND) dyke swarm (~210 mappable dykes) from Western India, that intruded compound basaltic (older than dykes) Deccan lava flows. Magnetic fabric which is commonly used as a proxy for magma flow fabric, was analysed in multiple oriented dyke samples using Anisotropy of Magnetic Susceptibility (AMS) technique. AMS analysis suggests that the studied dykes display dominantly subvertical to inclined flow and occasional sub-horizontal/lateral flow. The cumulative flow geometry further suggests the presence of multiple sub-surface magma centres (polycentric flow) from which magma pulses got injected through crustal fissures forming this magnificent dyke swarm that ultimately fed significant volume of the DVP.
... The anisotropy of magnetic susceptibility (AMS) was studied together with mesoscopic structural measurements at 33 sites. The AMS was used for the study of the preferred orientation of magnetic minerals in rocks (Graham, 1954;Borradaile and Henry, 1997), particularly in granites (Bouchez et al., 1990;Bouchez, 1997Bouchez, , 2000Hrouda, 1999). Magnetic susceptibility can be visualized as an ellipsoid with three principal axes, usually with different lengths ( k 1 ≥k 2 ≥k 3 ; Tarling and Hrouda, 1993). ...
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Tectonic development of the Variscan belt in Central Europe included, besides important compression, also an extensional phase related to gravitational collapse, which governed the origin of many sedimentary basins and magmatic bodies. One of these bodies is the Benešov pluton, featuring primary magmatic fabrics as well as deformational fabrics, related to subsequent extensional stages. Recognition of these fabrics and their links to other significant extension-induced structures in the Bohemicum and Moldanubicum not only sheds new light on the pluton itself but also extends a general knowledge of deformational stages, accompanying gravitational collapse of the Variscan orogen. The authors found that this pluton was strongly strained in a normal-faulting regime under brittle-ductile conditions. The age of deformation is constrained by a magmatic age of 347 ±3 Ma and by the age of Carboniferous sedimentary cover. New data indicate a three-stage extensional history during the phase of gravitational collapse: (1) Tournaisian extension (~350–345 Ma) within arc-related tonalitic intrusions; (2) late Viséan to Serpukhovian extension (~332–320 Ma), connected to the brittle-ductile unroofing and origin of a NE–SW basin system; and (3) Gzhelian to Cisuralian extension (~303–280 Ma), related to normal faulting and sedimentation in “Permo–Carboniferous” troughs, elongated NNE–SSW. Consequently, the gravitational collapse studied involved a complex succession of individual extensional stages, rather than a simple process.
... Anisotropy of magnetic susceptibility (AMS) has been commonly used as a tool for petrofabric studies since the midtwentieth century (e.g., Khan, 1962;Tarling and Hrouda, 1993;Borradaile and Henry, 1997;Cullity and Graham, 2011). This technique investigates the results of magnetic grain alignments or crystallographic lattice orientation, commonly represented by an ellipsoid comprising three (K 1 : maximum; K 2 : intermediate; and K 3 : minimum) principal axes of magnetic susceptibility (Graham, 1954;Nye, 1957). In particular, AMS is useful for those rocks in which mesoscopic fabrics are poorly developed or absent (e.g., igneous rocks and dikes; Khan, 1962;Caňόn-Tapía et al., 1995;Caňόn-Tapía, 2004), in order to assess their petrofabrics and emplacement mechanisms (Tarling and Hrouda, 1993;Borradaile and Henry, 1997;Hrouda et al., 2005). ...
Chapter
This volume represents the proceedings of the homonymous international conference on all aspects of impact cratering and planetary science, which was held in October 2019 in Brasília, Brazil. This volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures—on Earth and Mars. These contributions are authored by many of the foremost impact cratering researchers. Many contributions report results from state-of-the-art investigations, for example, several that are based on electron backscatter diffraction studies, and deal with new potential chronometers and shock barometers (e.g., apatite). Established impact cratering workers and newcomers to this field will both appreciate this multifaceted, multidisciplinary collection of impact cratering studies.
... Our structural and AMS data provide information about the magnitude and style of tectonic deformation processes active in the Tarom Basin, and hence on the strain state of rocks (e.g., Graham, 1954;Tarling and Hrouda, 1993;Parés and van der Pluijm, 2002;Soto et al., 2009;Alimohammadian et al., 2013;Cifelli et al., 2015;Caricchi et al., 2016). AMS can also provide insights into sedimentary processes, when tectonic deformation is limited or absent (Dall'Olio et al., 2013;Parés, 2015). ...
Article
Abstract The upper plate of the Arabia-Eurasia collision zone experienced orogen-perpendicular to orogen-parallel extension from 25-22 to 10-9 Ma. Although such an extension occurred during widespread collisional deformation, it is not clear if it is a local feature or if represents a major phase of upper plate extension. In this study we combine anisotropy of magnetic susceptibility (AMS) with fault kinematic analysis and sedimentologic data from 16.2- to 7.6-My-old deposits of the Upper Red Formation of the intermontane Tarom Basin (NW Iran). These strata present syndepositional, normal faults and offer the possibility to gain new insights into the spatial extent of such a Miocene extension. AMS data from the central and northern sectors of the basin document a tectonic fabric with a magnetic lineation parallel to the strike of the orogen, suggesting a compressional tectonic overprint. Conversely, the southern margin of the basin presents a purely sedimentary magnetic fabric despite a ∼ NE–SW orogen-perpendicular extension. This suggests that basin formation was not driven by extensional tectonics. Rather, the normal faults are gravity instabilities induced as also documented by coeval landslide deposits. This allows concluding that the orogen-perpendicular extension observed in few sectors of the collision zone is not regionally pervasive and hence it is not controlled by large-scale processes. Combined, our results indicate that if orogen-parallel extension associated with tectonic denudation and metamorphic core complex development occurred in certain sectors of the collision zone (Takab complex), it must have ended before 19-16 Ma, when widespread upper plate contractional deformation started. Keywords orogen-perpendicular extension, orogen-parallel extension, Arabia-Eurasia collision zone, Anisotropy of Magnetic Susceptibility (AMS), synsedimentary normal faults, Tarom intermontane basin basin
... The AMS signal consists of a superposition of diamagnetic, paramagnetic and ferromagnetic minerals, depending on their intrinsic anisotropy and spatial distribution within a rock sample (Tarling and Hrouda, 1993). This technique is based on the measurement of the magnetic susceptibility in different directions to resolve the magnetic susceptibility tensor (K), which ultimately represents the shape and orientation of the particles in the sample (represented by the principal axes K 1 ≥ K 2 ≥ K 3 ), allowing several interpretations related to flow direction and regime in volcanic rocks (Graham, 1954;Cañón-Tapia and Mendoza-Borunda, 2014). In the laboratory, samples were cut into standard specimens (25.4 mm in diameter, 22 mm in thickness), totaling 144 specimens. ...
Article
Pyroclastic deposits can cover significant areas and register major geological events. Despite their importance, understanding depositional dynamics of pyroclastic density currents (PDCs) and linking explosive deposits to their emission centers is still a challenge, especially in the case of non-welded, massive ignimbrites. Located in the Southern Andes, the Caviahue Copahue Volcanic Complex (CCVC) comprises one of the most active volcanic centers in the Andean Belt. This volcanic complex hosts massive ignimbrites with both source emplacement poorly constrained, currently grouped in the Riscos Bayos Ignimbrites (RBI). In this contribution, we perform a full magnetic characterization and anisotropy of magnetic susceptibility (AMS) study on the massive RBI of the CCVC. The magnetic characterization was performed using magnetic experiments including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves, and scanning electron microscopy. Magnetic experiments indicate primary, multi-domain, high Curie temperature titanomagnetites as the AMS carriers. Ellipsoids are predominately oblate, with a low degree of anisotropy and east-southeastward imbrication. This fabric arrangement is consistent with PDC sedimentary fabrics deposited under laminar flow conditions. Despite RBI massive structure AMS data reveals changes in transport capacity of the PDC and particle organization. These changes are marked by increasing AMS dispersion and decreasing degree of anisotropy up-section within flow units. Directional statistics of AMS data implies the Las Mellizas Caldera as the emission center of RBI. The reconstructed flow path also suggests the PDC overrun of the Caviahue Caldera topographic rim. This study highlights the application of AMS to the identification of emission centers of explosive deposits, featuring its application to massive ignimbrites.
... The Anisotropy of low-field magnetic susceptibility (AMS) allows the determination of the petrofabrics of rocks, through the measurement of the orientation of the minerals present in the samples (e.g., Graham, 1954;Tarling and Hrouda, 1993). The AMS technique has been extensively applied in flow direction studies, considering the limitations in defining magmatic fabrics and flow structures using only compass field measurements. ...
Article
Rhyolites compose an important record in the volcanic history of Earth, with significant occurrences in volcanic arcs, large igneous provinces and post-collisional terranes, often associated with explosive events. In several geologic provinces, rhyolites dominate as the most expressive geologic units (e.g., Silicic Large Igneous Provinces-SLIPs). Despite their importance, several aspects related to the emplacement of rhyolite flows are still enigmatic. Recent studies in modern rhyolite lavas suggest similar emplacement mechanisms to basaltic lavas, implying a more dynamic growth model for silicic flows, including outbreak lobes and outpour structures. Despite these advances, studies related to the recognition of these features in ancient flows are still rare. In this work we perform a multi-proxy study of an ancient (Neoproterozoic) rhyolitic lava flow combining fieldwork, petrography, geochemistry, rheology and magnetic fabric analysis. The Cerro do Perau outcrop (CP, southern Brazil) consists of a natural laboratory for the study of rhyolite lavas, presenting excellent exposure of a partially preserved flow with distinct flow features and folds. CP flow consists of a high-silica and low-crystal content rhyolite, suggesting its emplacement as an obsidian flow. Rheology data indicates high liquidus temperatures (>957 • C), with maximum viscosities of 10 8.5 Pa s and glass transition temperatures (T g) of 750 • C. The absence of brittle features suggests little to none displacement below T g. Structural analysis indicates the predominance of sub-vertical foliation planes, including axial planes of folds, indicative of proximal (near-vent) regions in rhyolite flows. The absence of lineations favors a predominantly planar accommodation of the flow-induced deformation, which is confirmed by the shape of the magnetic fabric ellipsoids. Several of these ellipsoids display a high degree of anisotropy, mostly related to an oblate fabric, indicative of the development of high-strain zones within the flow. Our data suggest that CP flow presents some similarities with recently proposed field-based emplacement models for rhyolitic flows, highlighting the significant data that can be extracted from a combination of magnetic fabrics and rheological analyses.
... Anisotropy of Magnetic Susceptibility (AMS) methods have been used to study shear zones, and deformed rocks that have problematic textures to quantify, but lack strain markers ( Goldstein, 1980 ;Goldstein and Brown, 1988 ;Ruf et al., 1988 ;Hrouda and Lanza, 1989 ;Housen et al., 1995 ;Pare´s and van der Pluijm, 2002 ). It was first recognized by Graham ( Graham, 1954 ), and subsequently, it has been used as a tool to analyze fabrics in a variety of rocks and materials ( Hrouda, 1982 ). Among the varieties of susceptibilities, the lowfield anisotropy of magnetic susceptibility is used most frequently ( MacDonald and Ellwood, 1988 ;Bina and Henry, 1990 ;Lagroix and Borradaile, 2000 ;Ferré et al., 2005 ) in the study of rock deformation. ...
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The magnetic fabric of the meta-ultramafic rocks and the associated gneisses in the Wayanad district, in the Moyar Shear Zone of the Southern Granulite Terrain of the state of Kerala, in India has been investigated using Anisotropy of Magnetic Susceptibility (AMS) and its low-field variation, as well as temperature variation of magnetic susceptibility vs temperature (Tc). The dominant magnetic minerals that control and contribute to the magnitude of the AMS were quantified and the regional deformation in the area and its effects on the rock types deciphered. The mean bulk susceptibility of the two rock types indicates the ferromagnetic character in the area. Ferromagnetic and ferrimagnetic minerals – magnetite and maghemite – are the major magnetic mineral contributors of AMS in the meta-ultramafic rocks of Wayanad whereas the mafic minerals – garnet, hornblende, biotite, and pyroxene – influence the magnetic bulk susceptibility in the gneiss. The development of fabric in the meta-ultramafics of Wayanad has an intimate evolutionary relationship with the gneisses and the shear zone itself. This has been demonstrated by magnetic and field-based studies. Both the magnetic foliation of the meta-ultramafic and the gneiss have orientations that fit well with the occurrence of the meta-ultramafic body in the field within the orientation of the Moyar Shear Zone.
... Many authors have used indirect methods, such as anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) measurements, to assist in these studies (e.g., Incoronato et al., 1983;Ort, 1993;Fisher et al., 1993;Cagnoli and Tarling, 1997;Le Pennec et al., 1998;Palmer and MacDonald, 1999;Ort et al., 1999;LaBerge et al., 2009). AMS is a powerful tool for volcanic studies because it permits determining the preferred orientation of the magnetic minerals present in the rocks during their formation (e.g., Graham, 1954;Tarling and Hrouda, 1993). Despite its strong potential, case studies using AMS and AARM in Precambrian volcanic systems are still rare. ...
Article
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.
... The anisotropy of magnetic susceptibility (AMS), or magnetic fabric, is a useful approach for directly measuring paleowind directions from aeolian deposits. AMS is a physical property of sediments, reflecting differences in the magnetic susceptibility measured in different sample orientations, and it can be graphically expressed as a three-dimensional ellipsoid with maximum (κ max ), intermediate (κ int ), and minimum (κ min ) axes of susceptibility (Ising, 1943;Graham, 1954). AMS is dominantly controlled by the preferred crystallographic and dimensional orientations of anisotropic magnetic minerals within a sample (Owens and Bamford, 1976;Hrouda, 1982;Lowrie, 1989;Rochette et al., 1992). ...
Article
The aeolian loess-paleosol sequences in the Chinese Loess Plateau (CLP) are an excellent archive of variations in atmospheric circulation in the geological past. However, there is no consensus regarding the roles of the East Asian winter monsoon and westerly winds in transporting the dust responsible for loess deposition during glacial and interstadial periods. We conducted detailed measurements of the anisotropy of magnetic susceptibility (AMS) on two parallel loess profiles covering the most recent 130 ka in the western CLP to determine paleowind directions. Results show that the magnetic lineations of the loess and paleosol units in both sections are significantly clustered along the northwest to southeast direction. These observations demonstrate that the prevailing wind system responsible for dust transport in the western CLP was the northwesterly winter monsoon, rather than the westerly winds. The AMS-derived dust-bearing wind direction was relatively stable during the last glacial and interglacial cycle in the western CLP, consistent with sedimentary and AMS evidence from the eastern CLP. Accordingly, it is reasonable to conclude that large areas of deserts and Gobi deserts areas located in the upwind direction were the dominant sources for the aeolian deposits of the Loess Plateau.
... Additionally, we thank: Dr. Rodrigo Fernandez for his assistance in the paleomagnetic laboratory of the Department of Geology, Universidad de Chile, during AMS data acquisition; the free access to Faultkin 8 software for kinematic analysis, developed by Dr. Richard Allmendinger. Chapple, 1978, Coutand et al., 2001, Dahlen et al., 1984, Davis et al., 1983, DeCelles et al., 2011, Ehlers and Poulsen, 2009, Garzione et al., 2008, Graham, 1954, Hunt et al., 1995, Iriarte et al., 1999, Martí nez et al., 2015, Mpodozis et al., 1995, Mpodozis et al., 2019, Quade et al., 2015, Wolc et al., 1989 ...
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We present an integrated structural and Anisotropy of Magnetic Susceptibility (AMS) study focused on the characterization of the spatio-temporal variation of the strain field during the main Neogene deformation, of the Argentine Precordillera and the northern Sierras Pampeanas, immediately to the south of the Puna plateau (27°30′S). The AMS from 43 sites and fault-slip data analysis from mesoscale faults (n = 540) were carried out in Lower Miocene to Middle Pliocene sedimentary and volcaniclastic rocks. The AMS results show moderate values of anisotropy degree near to deformation zones, and low and high values of bulk susceptibility. The most predominant magnetic fabric is sedimentary type I and II, with a weak magnetic lineation. In areas where the rocks are deformed, the incipient magnetic lineation are subparallel to the regional structural trend and present a main N–S orientation in the Precordillera and Fiambalá basin, while in the Sierras Pampeanas they are mostly NE-oriented. Combining these results with kinematic analysis from fault-slip data, we identify, in the Precordillera and Fiambalá basin areas, a contractional event with E-W main direction, from ~23 Ma to 5 Ma, mostly active from the onset of deposition of Neogene sequences. During this period, in the NW Sierras Pampeanas, the contraction directions follow a NW orientation. The onset of N–S to NNW contraction, mostly recognized in rocks younger than 5 Ma, exposed in the northern areas of Precordillera and Sierra Pampeanas, documents a change of the strain field during the Early-to-Middle Pliocene, associated with a shift in the contractional direction from E-W to N–S. We relate the change to the rapid uplift of the southern Puna, generating a juxtaposition of an area under N–S extension, affecting the topographically-higher Puna, with the other area under N–S contraction in the adjacent Precordillera and Sierras Pampeanas. We associate these two events as part of the evolution of this segment of the Andes, where the broken foreland setting is active during this time. By combining the AMS and kinematic analysis results, we obtain a timing for the spatio-temporal change of contraction directions, which allows us to compare both the broken-foreland and foreland basins to understand spatio-temporal strain variations.
... Anisotropy of magnetic susceptibility (AMS) is a well-established technique for petrofabric analysis of less deformed rocks, where mesoscopic foliations and lineations are not very well developed (Hrouda, 1982;Tarling and Hrouda, 1993;Borradaile and Jackson, 2004;Zak et al., 2008;Sen, Mukherjee and Collins, 2014). AMS is often used to investigate the fabric pattern in plastically deformed rocks that can be correlated with meso-microscopic fabric (Graham, 1954;Owens and Bamford, 1976;Borradaile, 1991;Jackson and Tauxe, 1991;Nakamura and Nagahama, 1997;Mukherjee et al. 2004;Mamtani and Greiling, 2005;Agarwal et al. 2010;Mamtani and Vishnu, 2012;Pant et al. 2012;Ferre et al., 2014). Some workers have also used AMS to understand deformation and tectonic setup of the Himalayan region (Jayangondaperumal and Dubey 2001;Tripathy et al. 2012;Sen et al. 2012;Agarwal et al. 2016). ...
Article
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In this paper, Anisotropy of Magnetic Susceptibility (AMS) data from the rocks of Almora crystalline in the vicinity of Rameshwar is presented. The study integrates field, micro-structural and Anisotropy of Magnetic Susceptibility (AMS) studies. Field foliation strike shows NW-SE orientation with moderate to high dip in rocks of Almora Crystalline, whereas near North Almora Thrust rocks of Almora Crystalline are steeply dipping and litho-units are intensely mylonitized due to NE-SW regional compression. The magnetic foliations are recorded to be parallel to the field foliation of the study area. Variation in orientation of magnetic lineation is inferred to imply superposed deformation in the study area. AMS study also reveals that the shape of susceptibility ellipsoid is oblate which is inferred to be due to compression.
... In addition, magnetic fabric studies have proven useful in evaluating remanence carriers and/or deformation. These include anisotropy of magnetic susceptibility (AMS; Graham, 1954Graham, , 1957, anisotropy of isothermal remanence (AIR; McCabe et al., 1985) and anisotropy of anhysteretic remanence (AAR). Microscopic investigations of magnetic carriers using polished thin sections under reflected light; scanning electron or transmission electron microscopes help identify possible magnetic carriers and potential alteration of the the original petrology (Poldervaart and Gilkey, 1954;Pichamuthu, 1959;Halls and Zhang, 1995;Halls et al., 2007;Sun and Jackson, 1994). ...
Article
Thirty years ago, Rob Van der Voo proposed an elegant and simple system for evaluating the quality of paleomagnetic data. As a second-year Ph.D. student, the lead author remembers Rob waxing philosophical about the need to have an appropriate, but not overly rigid evaluation system. The end result was a 7-point system that assigned a (1) or (0) for any paleomagnetic result based on objective criteria. The goal was never to reject or blindly accept any particular result, but merely to indicate the degree of quality for any paleomagnetic pole. At the time, the global paleomagnetic database was burgeoning and it was deemed useful to rank older paleomagnetic results with the newer data being developed in modern laboratories. Van der Voo's, 1990 paper launched a silent revolution in paleomagnetism. Researchers began to evaluate their data against those seven criteria with the anticipation that reviewers would be similarly critical. Today, paleomagnetism is a mature science. Our methods, analyses, and results are more sophisticated than they were 30 years ago. Therefore, we feel it is appropriate to revisit the Van der Voo (1990) criteria in light of those developments. We hope to honor the intention of the original paper by keeping the criteria simple and easy to evaluate while also acknowledging the advances in science. This paper aims to update the criteria and modernize the process. We base our changes on advances in paleomagnetism and geochronology with a faithful adherence to the simplicity of the original publication. We offer the “Reliability” or “R” index as the next generation of the Van der Voo “Quality” or “Q” index. The new R-criteria evaluate seven different information items for each paleomagnetic pole including age, statistical requirements, identification of magnetic carriers, field tests, structural integrity, presence of reversals and an evaluation for possible remagnetization.
... The analysis of magnetic fabrics (i.e., AMS) is a powerful technique for studying rock fabrics (Graham, 1954;Hrouda, 1987Hrouda, , 1993Borradaile and Henry, 1997;Borradaile and Jackson, 2004). We applied the technique at 38 sites across the Jiufeng ductile shear zone (Fig. 1c) using a portable drill. ...
Article
The Jiufeng Sn deposit is located in the outer contact zone southwest of the Neoproterozoic Motianling granitic pluton, in northern Guangxi, South China. The deposit is characterized by a NNE-striking, 30–200-m-wide ductile shear zone, named the Jiufeng ductile shear zone. Majority of the Sn orebodies, including four large ones, are hosted by the shear zone. Field and microstructural observations, electron backscatter diffraction measurements, and magnetic fabric analyses show that the shear zone is characterized by sinistral thrust shearing. The shear zone has also slightly modified the Sn orebodies. Quartz in the shear zone exhibits core-mantle structure and undulose extinction. Quartz has experienced significant sub-grain rotation and grain-boundary migration recrystallization. Lattice preferred orientation analysis reveals primarily basal , rhomb , and prism slips. Cassiterite grains in the shear zone, however, demonstrate brittle deformation. Therefore, the deformation temperature during the ductile shearing is estimated lower than 650°C and below U-Pb closure temperature of cassiterite. As such, the U-Pb isotopic composition of the cassiterite was not disturbed by the shearing and the U-Pb age of the cassiterite could be used for the age of the Sn mineralization. LA-ICP-MS U-Pb dating of cassiterite grains selected from a cassiterite-bearing tourmalinite sample performed in this study yielded a Tera–Wasserburg U-Pb lower intercept age of 834.5 ± 5.3 Ma. This age is consistent within error with that of the neighboring Neoproterozoic Motianling granitic pluton, which suggests a close genetic relationship between the Sn mineralization and the granitic magmatism. Studies show that the Jiufeng ductile shear zone was produced by a widespread Caledonian (or Kwangsian) tectonic event in South China, indicating that the shearing was much later than the Sn mineralization and has no generic relationship to the Sn deposit.
... This is a petrofabric tool able to determine the preferred orientation of particles under an external field (e.g., gravity, wind, magnetic field, flow, and strain) during deposition and diagenesis of rocks. The AMS method has been widely employed as an easy, fast, economic, and sensitive rock strain indicator (e.g., Borradaile, 1988;Borradaile & Henry, 1997;Graham, 1954;Kissel et al., 1986) and has been widely successfully applied in sedimentary basins to detect regional deformation (e.g., Gilder et al., 2001;Gong et al., 2009;Li et al., 2014;Maffione et al., 2015;Mattei et al., 1997;Parés et al., 1999;Soto et al., 2009;Z. Tang et al., 2012;van Hinsbergen et al., 2005). ...
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Plain Language Summary How the Tibetan Plateau evolved during India‐Asia convergence and collision is notoriously challenging to decipher. Use of sedimentary records to date periods of tectonic activity is a popular approach, yet distinguishing between tectonic versus climate signals in sediment records can be challenging. The anisotropy of magnetic susceptibility (AMS) is an effective and sensitive technique that reveals tectonic stress fields during sedimentation and changes therein, even in weakly deformed clastic sedimentary rocks. We report a detailed record of AMS data from a ~3 km thick section of redbeds from the Gonjo Basin in eastern Tibet. Previous magnetostratigraphy dated deposition from 69 to 41.5 Ma; during this time period marked sedimentation rate increases and simultaneous vertical‐axis rotations were interpreted to reflect shortening pulses. Our new AMS data indicate an increased shortening strain at ~52 Ma, demonstrating that the sedimentation rate changes are tectonic rather than climatic in origin, showing that a pulse in crustal shortening of Tibet occurred simultaneous with a marked ~52 Ma onset of deceleration of India‐Asia convergence. We show that applying a suite of paleomagnetic and rock magnetic techniques, including magnetostratigraphy and sedimentation rate calculation, vertical‐axis rotation analysis, and AMS analysis, provides a powerful tool to differentiate tectonic versus climatic influence on a sediment archive, allowing the precise dating of discrete deformation phases in intensely deformed regions that evolved over long periods of time.
... AMS is an excellent tool for studying small-scale deformation of different rock types (Graham 1954;Borradaile and Henry 1997). This method is particularly useful in granitoid intrusions and related dykes emplaced under varying Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0053 ...
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The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso- and microscale structures and anisotropy of magnetic susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps–Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N–S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE–SW to E–W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE–NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.
... In the past few decades there is increasing use of various magnetic methods, particularly anisotropy of magnetic susceptibility (AMS) for petrofabric analysis that provides greater information about rock deformation, strain and tectonics (e.g., Graham, 1954;Tarling and Hrouda, 1993;Bouchez, 1997;Borradaile and Jackson, 2010;Mamtani, 2014 amongst others). Magnetite is a common accessory mineral in many crustal rocks on earth and an important ore mineral. ...
Article
Although experimental studies have shown dislocation creep to be an important deformation mechanism in magnetite at medium to high temperature, evidence of intracrystalline deformation remains to be established in natural tectonically deformed rocks. In this study we investigate intracrystalline deformation features and nanostructures in elongated magnetite from a naturally deformed rock (mylonitized mica schist deformed in a large-scale shear zone of the Seve nappe, Swedish Caledonides). The magnetite grains have very high aspect ratios (up to 10.40) that result in very high degree of magnetic anisotropy in the rock. We show low and high angle grain boundaries (LAGB and HAGB) in magnetite using a combination of electron backscatter diffraction and high-resolution transmission electron microscopy (HRTEM) analysis. HRTEM studies on lamellae excavated perpendicular to the LAGB and HAGB reveal translational and rotational (twisted) Moiré fringes, respectively. Dislocations, slip bands, stacking faults, twins and recrystallized domains are observed in the vicinity of the grain boundaries, thus providing unequivocal evidence of intracrystalline deformation of magnetite. Our study also reveals the presence of biotite inclusions intergrown epitaxially with magnetite that show no evidence of lattice defects, thus suggesting that the intracrystalline deformation of magnetite took place under wet conditions. The movement at the grain boundaries is interpreted as a response to regional tectonics with a top-to-NW transport direction. It is established that at the nanoscale, the LAGB and HAGB were favourably oriented to accommodate strain dominantly by translation and rotation, respectively. Thus, the nanotectonic processes are consistent with the regional tectonic reference frame. The importance of evaluating ductile behaviour of magnetite from deformed polymineralic rocks in petrofabric analysis and modeling the relation between strain and rock magnetic anisotropy is discussed.
... Anisotropy of magnetic susceptibility (AMS) has been used as a petrofabric approach in a wide range of geological applications (e.g. Graham 1954;Hrouda 1982;MacDonald & Ellwood 1987;Rochette et al. 1992;Kodama 1995;Borradaile & Henry 1997;Martín-Hernández et al. 2004;Borradaile & Jackson 2010). In particular, AMS is used to evaluate flow direction in lava and pyroclastic flow deposits especially in volcanic systems with restricted outcrops (e.g. ...
Article
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Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolution. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing 'scattered' fabrics when late Ti-rich titanomagnetite phases dominate the fabric, and 'inverse' or 'intermediate' fabrics when single-domain grains are present. 'Normal' fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.
... Also similar to MPF, exceptions to those empirical relationships have been observed; for example, different correlations exist between AMS orientation and flow direction in lava (Khan, 1962;Wing-Fatt & Stacey, 1966), and certain minerals or grain sizes produce "inverse" or "anomalous" fabrics (Borradaile et al., 1993;Rochette, 1988;Rochette et al., 1999). Careful and systematic investigations of factors contributing to anisotropy (magnetocrystalline, shape and distribution anisotropy; Graham, 1954;Grégoire et al., 1995;Hargraves, 1959;Hargraves et al., 1991;Mainprice & Humbert, 1994;Stephenson, 1994) together with the characterization of single crystal properties (Biedermann, 2018, and references therein) now allows one to model and understand even these anomalous and complex fabrics . ...
Article
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Pore fabrics define physical properties of a rock, such as permeability and elasticity, both of which are important to many geological, hydrological, and environmental applications. Minerals and hence pores are often preferentially aligned, leading to anisotropy of physical properties and preferred flow directions. Preferred flow paths are defined by the shape and arrangement of pores, and a characterization of this pore fabric forms the basis for prediction of fluid flow directions. Magnetic pore fabrics (MPFs), that is, magnetic anisotropy measurements on ferrofluid‐impregnated samples, are a promising and fast way to characterize the pore fabric of connected pores in 3‐D, while analyzing a large number of pores with sizes down to 10 nm, without the need for any a priori knowledge about fabric orientation. Empirical relationships suggest that the MPF is related to the pore shape and orientation and approximates permeability anisotropy. This study uses models including shape and distribution anisotropy to better understand and quantify MPFs, using simple pore shapes and pore assemblies measured in previous studies. The results obtained in this study show that (1) shape anisotropy reliably predicts the MPF of single pores, (2) both shape and distribution anisotropy are needed to predict MPFs of pore assemblies, and (3) the anisotropy parameters P, L, and F are affected by the intrinsic susceptibility of the ferrofluid in addition to pore geometry. These findings can help explain some of the variability in empirical relationships and are an important step toward a quantitative understanding and application of MPFs in geological and environmental studies.
... The relative difficulty of material magnetization in different directions can be represented by anisotropy of magnetic susceptibility (AMS). Anisotropy of magnetic susceptibility is always used to study fabric characteristics and structural deformation of rock, which was called analysis of rock magnetic fabric, first proposed by Graham [1]. The magnetic minerals in granitic magma shows a certain optimum arrangement orientation when forces act on them during magma emplacement and condensation. ...
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The Keziletage pluton which plays an important role in studying the nature of continental lithosphere and restraining the tectonic evolution of the Southern TianShan orogenic belts is located in the middle of the Keziletage synclinorium, a secondary tectonic unit of the South TianShan syncline fold belt. Emplacement mechanism of the Keziletage pluton in Xinjiang is studied through detailed analyses of magnetic fabrics. Data from 28 sampling sites within the pluton show that the value of the average susceptibility (K m ) is high. The corrected anisotropy degree (P j ) is less than 1.2, which can be an indicator of flow magnetic fabrics. The shape parameter ( T ) of magnetic susceptibility ellipsoid displays that the Keziletage pluton is dominated by oblate compression fabric. Although the magnetic fabrics show NE-SW compression as a whole, the magnetic lineations are little over magnetic foliations. We suggest that the collision of Tarim blocks to YiLi-mid TianShan micro-plate became much weaker during the emplacement when the regional tectonic setting was changing to extension environment.
... The analysis of Anisotropy of Magnetic Susceptibility (AMS or magnetic fabrics) represents a powerful technique to identify the rock petrofabric (e.g. Graham, 1954;Hrouda, 1987Hrouda, , 1993Borradaile and Tarling, 1981;Tarling and Hrouda, 1993;Borradaile and Henry, 1997;Borradaile and Jackson, 2004). This technique is especially useful in scenarios of low-strain, brittle fault zones because of its proved ability to average the orientations of the whole set of petrofabric elements and, therefore, to provide a finite strain ellipsoid of the sheared rock (Solum and van der Pluijm, 2009;Mertanen and Karell, 2012;Levi et al., 2014;Casas-Sainz et al., 2017;Casas-Sainz et al., 2018;Marcén et al., 2018a;Vernet et al., 2019;Román-Berdiel et al., 2018). ...
Article
AMS and structural analysis are here applied to study the deformed zone associated with a large-scale, active normal fault in the central Betic Cordillera (Spain), namely the Baza fault system, to determine: i) the kinematics of structures and their relation with fault zone architecture and segmentation degree, ii) the correlation between deformational structures and the different types of magnetic fabrics and iii) the evolution of magnetic fabrics patterns, from sedimentary to shear-related, associated with normal faults. Five outcrops (969 samples) were analysed along the fault trace, which shows different degrees of segmentation along strike and strong localization of deformation along narrow fault zones. A first, main set of magnetic fabric data corroborates the normal kinematics of the Baza fault, showing magnetic lineations parallel to the dip-slip, transport direction. A second, secondary set of magnetic lineations, is parallel to the intersection lineation, and can be related to less intense deformation in the fault rocks. Furthermore, a detailed study (523 samples) of a trench excavated across the fault zone, where two fault splays tend to coalesce in a linkage relay zone indicates that i) lithology and distance to fault planes are two factors that control the development of extension-related magnetic fabrics in weakly deformed sediments, ii) the development of shear-related fabrics in fault zones entails the mechanical rotation of minerals, iii) different orientation of magnetic lineations are related to different intensity of bulk deformation and iv) magnetic lineation is useful to define local deviations of deformation axes produced by changes in the local extension direction (from fault-perpendicular to fault-parallel extension) in the linkage zone between adjacent fault splays.
... Since then, texture and AMS have been qualitatively compared in numerous studies (e.g. Balsley and Buddington, 1960;Cifelli et al., 2005;Graham, 1954;Hirt et al., 2004;Kligfield et al., 1983;Lüneburg et al., 1999;Schmidt et al., 2007), but attempts to quantitatively link a measured texture with intrinsic mineral AMS have been shown to be complex and dependent on a number of factors, e.g., mineralogy and deformation process (cf. Borradaile and Jackson, 2010). ...
Article
Anisotropy of magnetic susceptibility (AMS) has been shown to be a good proxy for crystallographic preferred orientation (texture). However, it is not clear in detail how different factors, e.g. modal composition and preferred orientation, define the total AMS in a rock. Black shale samples from a drill core on the Appalachian Plateau in western Pennsylvania were analyzed with respect to their microfabric as determined by texture and microstructure, and AMS. Low- and high-field AMS, which was measured at room temperature, reveals that the AMS of the samples is dominated by the para- and diamagnetic phases. Synchrotron diffraction was applied to determine the texture of all relevant mineral phases incorporated in the samples. Muscovite and chlorite pole figures show single maxima perpendicular to the foliation, reflecting dominant flattening strain. From these textures and the modal compositions, AMS models were calculated using the intrinsic magnetic anisotropy of the single crystals. The modeled and measured AMS are comparable in terms of their principal directions and shapes of the anisotropy ellipsoid for the dominantly paramagnetic samples, with the maximum susceptibility axes oriented subhorizontal to the NE-SW. Both, textures and AMS, indicate that the samples have undergone largely bedding compaction with a weak tectonic overprint linked to the Alleghenian orogeny. Our results suggest that the microfabric and the magnetic fabrics on the plateau, 200 km away from the Allegheny Front, detect the tectonic shortening. The AMS modeling demonstrates how the contribution of the intrinsic AMS of each mineral controls the AMS of the whole rock.
... Susceptibility Some structures in igneous bodies cannot be observed or measured directly, and measurement of anisotropy of magnetic susceptibility (AMS) can be of great help in unravelling them. This method, although first highlighted more than 60 years ago (Graham 1954), has remained a somewhat specialist tool despite the advances in measurement precision, accuracy and efficiency (Borradaile and Henry 1997;Martín--Hernández et al. 2004;O'Driscoll et al. 2015;Tarling and Hrouda 1993). The principle behind AMS measurements relies on the physical property of magnetic susceptibility, K, which is the relationship between the magnetization, M, of a sample in an externally applied magnetic field of strength H. ...
... Traditionally, magnetic fabrics have been interpreted based on empirical relationships. For example, AMS has been shown to reflect the macroscopic foliation and lineation in many rocks, i.e., the maximum susceptibility (k 1 ) indicates lineation, and the minimum susceptibility (k 3 ) is normal to foliation [8][9][10][11][12][13][14][15][16]. However, the magnetic and mineral fabrics are not always parallel. ...
Article
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Empirical relationships between magnetic fabrics and deformation have long served as a fast and efficient way to interpret rock textures. Understanding the single crystal magnetic properties of all minerals that contribute to the magnetic anisotropy of a rock, allows for more reliable and quantitative texture interpretation. Integrating information of single crystal properties with a determination whether or not mineral and magnetic fabrics are parallel may yield additional information about the texture type. Models based on textures and single crystal anisotropies help assess how the individual minerals in a rock contribute to the rock’s anisotropy, and how the individual anisotropy contributions interfere with each other. For this, accurate and reliable single crystal data need to be available. This review paper discusses magnetic anisotropy in single crystals of the most common rock-forming minerals, silicates and carbonates, in relation to their mineralogy and chemical composition. The most important ferromagnetic minerals and their anisotropy are also discussed. This compilation and summary will hopefully lead to a deeper understanding of the sources of magnetic anisotropy in rocks, and improve the interpretation of magnetic fabrics in future structural and tectonic studies.
... The magnetic susceptibility (K) is a parametercounts on the magnetic properties of the full composition of a rock. The AMS technique is a rapid and precise tool to determine the petrofabric controlled by ferromagnetic, paramagnetic and diamagnetic minerals in rocks [32]. A systematic study of the fabric permitted by AMS method is resulted in directional data such as foliation and lineation, as well as quantitative parameters represent the mineralogical composition and deformation state of rocks [71]. ...
Research
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... distribution anisotropy) of Fe-bearing silicate and oxide phases (e.g. Voigt & Kinoshita, 1907;Graham, 1954;Hrouda, 1982;Tarling & Hrouda, 1993;Dunlop & Ö zdemir, 2001). The principal axes of the magnetic fabrics measured by AMS can thus be related to the orientation, shape and distribution of individual grains (i.e. the petrofabric) (e.g. ...
Article
Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry and electromagnetic data can identify contemporary melt zones, magma reservoirs and/or crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs) and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.
... The anisotropy of magnetic susceptibility (AMS) technique was early recognized as a powerful tool for structural studies in rocks (Graham, 1954). The AMS efficiency in determining the internal structures of plutonic rocks, especially the lineations that are difficult to determine in the field, has been demonstrated in plutons throughout the world (e.g. ...
Article
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The anisotropy of magnetic susceptibility is a physical property of the rocks widely used in petrofabric studies and other applications. It is based on the measurement of low-field magnetic susceptibility in different directions along the sample. From this process several scalar properties arise, defining the magnitude and symmetry of the AMS ellipsoid, along with the magnetic foliation and lineation, namely the magnetic fabric. A case study is presented, dealing with the deformation of the Mont-Louis-Andorra pluton. Finally, the method was applied in Tertiary magmatic rocks from the Rhodope Massif, revealing their magnetic character and internal structures.
Article
We investigate the relationships among magnetic, mylonitic and strain fabrics from the Pelling-Munsiari thrust (PT), a major shear zone in the Sikkim Himalaya, that records varying mylonitic fabrics and strain geometry along its transport direction. The fabric changes from an SL-tectonite with oblate strain ellipsoids in its main exposures, to a local, transport-parallel L-tectonite with prolate strain ellipsoids, forming a synformal klippen. Magnetic fabric is related to the PT deformation, and is affected by the local folding of the klippen. The bearings of magnetic and stretching mineral lineation vary more than the orientations of the planar fabrics. Irrespective of their strain geometry, most of the fault rocks, including SL mylonites of the structurally overlying Main Central thrust, record oblate susceptibility ellipsoids. The paramagnetic phases tracked an incremental deformation stage that froze as oblate susceptibility ellipsoid in the hinterlandmost fault rocks, and was not realigned by successive higher magnitude, constrictional strain, associated with a local, lateral ramp, followed by flattening strain in the forelandmost exposure during translation of the PT. Additionally, strain partitioning between quartz, tracking the finite strain fabric, and micas primarily contributing to the magnetic fabric explain the deviations in the studied fabrics in the fault rocks.
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Pyroclastic deposits can cover significant areas and register major geological events. Despite their importance, understanding depositional dynamics of pyroclastic density currents (PDCs) and linking explosive deposits to their emission centers is still a challenge, especially in the case of non-welded, massive ignimbrites. Located in the Southern Andes, the Caviahue Copahue Volcanic Complex (CCVC) comprises one of the most active volcanic centers in the Andean Belt. This volcanic complex hosts massive ignimbrites with both source emplacement poorly constrained, currently grouped in the Riscos Bayos Ignimbrites (RBI). In this contribution, we perform a full magnetic characterization and anisotropy of magnetic susceptibility (AMS) study on the massive RBI of the CCVC. The magnetic characterization was performed using magnetic experiments including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves, and scanning electron microscopy. Magnetic experiments indicate primary, multi-domain, high Curie temperature titanomagnetites as the AMS carriers. Ellipsoids are predominately oblate, with a low degree of anisotropy and east-southeastward imbrication. This fabric arrangement is consistent with PDC sedimentary fabrics deposited under laminar flow conditions. Despite RBI massive structure AMS data reveals changes in transport capacity of the PDC and particle organization. These changes are marked by increasing AMS dispersion and decreasing degree of anisotropy up-section within flow units. Directional statistics of AMS data implies the Las Mellizas Caldera as the emission center of RBI. The reconstructed flow path also suggests the PDC overrun of the Caviahue Caldera topographic rim. This study highlights the application of AMS to the identification of emission centers of explosive deposits, featuring its application to massive ignimbrites.
Article
Characterising deformable substrates of modern and past ice sheets is a key factor in understanding ice mass behaviour and dynamics. This is challenging when field-scale signatures of deformation in soft sedimentary beds are weakly developed or absent. To overcome these difficulties, in this study, we combine micro- and macroscale studies and quantitative analyses to decipher the nature of subglacial sediment deformation under a Pleistocene fast-flowing (streaming) ice sheets. We investigated a complex sedimentary succession at Nieszawa in central Poland. A multi-proxy approach involving till micromorphology in thin sections, ripple-cross lamination orientation, till fabric, clast microfabric and the anisotropy of magnetic susceptibility (AMS) data in a closely sampled vertical profile throughout the fluvial sand and till was used to reveal subglacial bed deformation mechanisms and kinematics. This records subglacial deformation of the sedimentary succession at Nieszawa under low basal shear stresses, where simple shear was the dominant bed deformation mechanism. Intergranular advection, decoupling at the ice-bed interface related to high subglacial water pressure, clast ploughing, and basal sliding also took place. The main finding of this study is that AMS reveals the shear strain development and ice flow direction in the structurally and texturally complex deposits under weak coupling conditions at the ice-bed interface beneath a fast flowing/streaming ice sheet, where strain and kinematic indicators other than AMS fabrics are vague or absent.
Article
Integration of remotely-sensed data, field work in addition to geochemical and mineral chemistry analyses, indicates that the Um Naggat granite pluton (UNP) has distinctive lithologic and structural characteristics. Image transformation techniques of using Landsat-8 Operational Land Imager (OLI) data successfully discriminated lithologic characteristics and highlighted hydrothermally altered areas. Shuttle Radar Topography Mission (SRTM) and OLI data allowed highlighting the structural contacts and revealed that the dominant structural trends are NW-SE, NE-SW, and N-S. Petrographic examination, field, and structural survey results revealed that the (UNP) pluton consists of biotite alkali feldspar granite (BAFG) and alkali granite (AG) in addition to a hybrid granite zone (HGZ). The BAFG phase is dominated by an alkaline suite with composition ranging from granite to alkali feldspar granite and pertains to post-orogenic granite (POG) and within-plate granites (WPG). The AG represents the youngest alkali granite phase of magmatism in the studied pluton. Various enclaves from diorites and microgranites have been recorded within the HGZ. Field relationships confirmed with quantitative analysis of magmatic fabric measurements indicate that the AG granitic phase was emplaced as subparallel-elongated sheets or dike-like masses trending E-W and associated with an extensional regime overprinted by the right-lateral strike-slip regime. This phase of “within plate” hypersolvus granite has excessive of large-ion lithophile (LIL) and high field strength (HFS) elements such as Zr, Nb, and Ga that belongs to A-type granites, being depicted by elevated alkali composition and high Fe2O3t/(Fe2O3t +MgO) ratios, and excessive amount of Zr, Y, Nb, and Ga. Microprobe analysis reveals that the compositions of amphibole mineral in the HGZ extent from magnesio-hornblende to actinolite and the temperature of the plagioclase range from 900 to 1050 °C, while the plagioclase of the BAFG is extremely pure albite in composition.
Chapter
The Pan-African belt (PAB) of Egypt outcrops mainly in the Eastern Desert and Southern Sinai. It occupies the northwestern sector of the Arabian-Nubian Shield (ANS) and lies at the northern continuation of the East African Orogen (EAO). The EAO comprises the low-grade greenschist facies lithologies of the ANS to the north and the deeper crustal equivalents of the Mozambique belt (MO) to the south. It is regarded by many workers to form a major suture zone delineating the oblique convergence (transpression) between East and West Gondwanaland. The PAB represents a complete succession of the Neoproterozoic basement encountered elsewhere in the ANS. It is recently interpreted as composed of a series of Neoproterozoic intra-oceanic arcs amalgamated during the climax of the Pan-African Orogeny c. 630 Ma. It consists of four major rock groups: high-grade gneisses and migmatites, arc-type volcanic/volcano–sedimentary sequence and dismembered ophiolites, Hammamat Sediments and Dokhan Volcanics and voluminous granitoids. These rock groups have undergone a prolonged history of deformation involving three ANS-scale deformation events (D1–D3) (Hamimi et al. 2019). The D1 resulted from arc accretion and led to the formation of S-, SE-, SW- and NE-directed thrusts, as well as N-directed escape of the ANS, via N- to NNW-directed thrusts and thrust-propagation folds. The D2 deformation was a post-accretion shortening phase, that produced transpressive structures, including NNW–SSE trending sinistral transcurrent shears (e.g. Nugrus and Atalla Shear Zones), the dextral transcurrent shearing along with NE-directed mega shears (e.g. Idfu-Mersa Alam and Qena Safaga Shear Zones) and the post-accretionary shear zone-related gneiss domes. The D3 deformation was an extensional long-lasting phase associated with the crustal relaxation following the Gondwana assembly. This chapter addresses the application of the anisotropy of magnetic susceptibility (AMS), as an appropriate petrofabric technique, in unravelling and deciphering the tectonic evolution of the PAB. Emphasis will be given to some key areas where such a technique has been used in an adequate manner.
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SUMMARY The rockmagnetic properties of granitic rocks of the northern Bergell massif which is situated at the Swiss-Italian frontier are due to their magnetite and ilmenohematite content. The magnetite is homogeneous and nearly pure, whereas the ilmenohematite is exsolved into antiferromagnetic titanhematite, ferrilmenite which is paramagnetic at room temperature and rutile. The magnetite only possesses unstable components of remanent mag¬ netization, but the titanhematite displays a very stable magnetization of different polarity. In the south of the sampling area the titanhematite is magnetized normally, i.e. parallel to the present geomagnetic field; in the adjoining area to the north it is magnetized reversely. The change in polarity is based on a self-reversal mechanism probably caused by decrea¬ sing oxidation and exsolution of ilmenohematite to the north during coo¬ ling of the granitic rocks. The rocks have a distinguished anisotropy of susceptibility giving magnetic fabric which is in good agreement with the macroscopic structure of the Oligocene - Miocene intrusive rocks. The anisotropy strongly influences the direction of the stable component of remanent magnetization. Having eliminated this influence, the following palaeomagnetic pole position is computed: long. 263,6°E; lat. 71,5°N.
Article
One of the main aims in the field of structural geology is the identification and quantification of deformation or strain. This pursuit has occupied geologists since the 1800’s, but has evolved dramatically since those early studies. The quantification of strain in sedimentary lithologies was initially restricted to lithologies of known initial shape, such as fossils or reduction spots. In 1967, Ramsay presented a series of methods and calculations, which allowed populations of clasts to be used as strain markers. These methods acted as a foundation for modern strain analysis, and have influenced thousands of studies. This review highlights the significance of Ramsay’s contribution to modern strain analysis. We outline the advances in the field over the 50 years since publication of the ‘Folding and Fracturing of Rocks’, review the existing limitations of strain analysis methods and look to future developments.
Chapter
Paleomagnetism is a technique very useful in structural geology providing that the rocks keep a stable record of the paleoearth magnetic field information. Paleomagnetism has been used in deciphering the evolution and geometry of orogens and extensional basins. The evolution of an orogen has many points of view, but the use of paleomagnetism allows (i) calculating vertical and/or horizontal rotations during basin and orogen evolution and hence providing actual data to calculate out-of-plane movements particularly important when constructing cross-sections to calculate shortening, (ii) dating syntectonic sediments at foreland basins, which in combination with exhumation rates, for example, allow for a complete reconstruction of the evolution of the orogen, and (iii) deducing chemical interactions when remagnetization occurs. At larger scale, paleomagnetism has been fundamental for paleogeographic reconstructions and evolution of tectonic plates.
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