Anisotropy Of Magnetic Susceptibility Orientation Research Articles

Anisotropy of magnetic susceptibility in salt rocks from the German Zechstein Basin, Sondershausen mine

SUMMARY The anisotropy of magnetic susceptibility (AMS) of rocks reflects the alignment of certain minerals, and therefore it can be used to investigate the deformation history of rocks. However, for salt rocks, very few studies on the AMS of salt rocks and the influence of accessory minerals exist. In this study, we analysed the potential to use the AMS of salt rocks with low impurity content for fabric characterization. Samples of rock salt, sylvinite and carnallitite from a salt mine in Sondershausen (Germany) from the Late Permian (Zechstein 2, Stassfurt series) are investigated. The results of low-field AMS (LF-AMS) measurements show a very weak but significant magnetic anisotropy for sylvinite, carnallitite, and rock salt with an elevated content of accessory minerals. The AMS results are consistent in individual layers of the same rock type. In order to identify the magnetic minerals, which cause the magnetic anisotropy, the high-field AMS (HF-AMS) was measured using a torque magnetometer in order to separate ferrimagnetic and paramagnetic contributions to the AMS. A significant paramagnetic subfabrics exists, which reflects the alignment of phyllosilicates. The magnitude of the LF-AMS is considerably greater than that of the paramagnetic subfabric. This indicates the existence of a ferrimagnetic subfabric due to magnetite, which can have a different orientation than the paramagnetic subfabric. Differences in the orientation of the AMS in samples from two sites suggest a relationship of deformation history and AMS. At a site with dipping layers, the AMS orientation is independent of the bedding and shows large differences between individual lithological layers. In a tight fold, the AMS of all rock types has similar shape and orientation. We conclude that AMS in salt rocks can give meaningful information on the mineral fabric, which could be used in the analysis of the deformation history.

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

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

Asian Monsoon variation over the late Neogene–early Quaternary recorded by Anisotropy of Magnetic Susceptibility (AMS) from Chinese loess

This paper discusses the Anisotropy of Magnetic Susceptibility (AMS) records during late Neogene to early Quaternary from the Lantian loess section near the southern margin of the Chinese Loess Plateau. The AMS results for this site are shown to reflect variations in the direction of the paleomonsoon. The Lantian loess sediments are largely aeolian in origin, but also include a thin layer that exhibits secondary aqueous depositional features. The major ellipsoid axis AMS orientations in the loess samples indicate a pervasive Northwest–Southeast directional component since the late Neogene. Since that time, the W–E component gradually overtakes the N–S component, which may explain the trend toward increased aridity during that time. Finally, a local heavy rainfall event seemingly occurred in the Lantian region about 2.6 Ma, which may have been responsible for the aqueous redeposition of sediments observed in L33 layer in this study.

Pliocene shortening direction in Nankai Trough off Kumano, southwest Japan, Sites IODP C0001 and C0002, Expedition 315: Anisotropy of magnetic susceptibility analysis for paleostress

Integrated Ocean Drilling Program Expedition 315 recovered cores from accretionary units, which are overlain by cover sequences, at a site on the hanging wall of the mega‐splay fault (C0001), and at a seaward margin of a forearc basin (C0002) in Nankai Trough off Kumano, Japan. In order to investigate the amount and the style of deformation and shortening direction of the accretionary prism units, anisotropy of magnetic susceptibility (AMS) of cored samples were measured. AMS of the late Pliocene to late Miocene accretionary prism at C0001 reveals a deformed magnetic ellipsoid (prolate type), and a restored direction of the AMS orientation indicates northwest–southeast shortening which is the same as the present‐day stress field measured by borehole breakout. The older Miocene accretionary prism cored beneath the forearc basin at C0002 does not show a typical prolate type but more oblate feature in parallel to plunged bedding planes. This type can be interpreted as an intermediate type between prolate and oblate types, by indicatives of AMS parameters, which was formed under a bedding vertical loading and a relatively weakly lateral compaction. The restored AMS orientation of the accretionary prism beneath the forearc basin also indicates the northwest‐southeast shortening which is different from the present‐day principal horizontal stress orientations at C0002. It is supposed that the shortening direction had been recorded in Pliocene time probably when the sequence was sited in the outer wedge. The northwest–southeast directions of AMS in the Pliocene time agree with the past subducting direction of the Philippine Sea Plate since the Pliocene.

Superposed deformation fabrics in the Precambrian metabasic rocks of the Iron Ore Group, Singhbhum craton, Eastern India: Evidences from anisotropy of magnetic susceptibility studies

Anisotropy of magnetic susceptibility (AMS) data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the Paleo-Mesoarchean greenstone succession of the Iron Ore Group rocks in the western Iron Ore basin, Singhbhum craton, eastern India. The basal lava crops out bordering the regional horseshoe shaped outcrop (the horseshoe synclinorium structure) of the overlying BIF rocks. AMS data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis. AMS orientation data have been analyzed in the light of superposed fold (Type-1 interference) fabrics developed within BIF as a result of successive deformation episodes (D 1 and D 2). It is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub-areas, which have been successfully correlated with the D 1 and D 2 deformation fabrics from the BIF rocks. Moreover, a regional scale variation in intensity and nature of strain is noticed from the AMS data, which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events.

Magnetic fabrics indicating Late Quaternary seismicity in the Himalayan foothills

The anisotropy of magnetic susceptibility (AMS) study was performed on soft sediment samples from a trenched fault zone across the Himalayan frontal thrust (HFT), western Himalaya. AMS orientation of K min axes in the trench sediments is consistent with lateral shortening revealed by geometry of deformed regional structures and recent earthquakes. Well-defined vertical magnetic foliation parallel to the flexure cleavage in which a vertical magnetic lineation is developed, high anisotropy, and triaxial ellipsoids suggest large overprinting of earthquake-related fabrics. The AMS data suggest a gradual variation from layer parallel shortening (LPS) at a distance from the fault trace to a simple shear fabric close to the fault trace. An abrupt change in the shortening direction (K min) from NE–SW to E–W suggests a juxtaposition of pre-existing layer parallel shortening fabric, and bending-related flexure associated with an earthquake. Hence the orientation pattern of magnetic susceptibility axes helps in identifying co-seismic structures in Late Holocene surface sediments.

Regional scale strain variations in Banded Iron Formations of Eastern India: results from anisotropy of magnetic susceptibility studies

Anisotropy of magnetic susceptibility (AMS) data are used as a tool to determine strain variations in different parts of the Banded Iron Formations (BIFs) of the Bonai Synclinorium, eastern India. AMS data of 88 cylindrical cores drilled from 29 samples collected from the limb and hinge parts of mesoscopic scale folds as well as different parts of the entire synclinorium are presented. It is found that the samples from limbs of small-scale folds and also from limbs of the regional scale synclinorium have higher degrees of anisotropy than the hinges. This is inferred to indicate that the limbs accommodated higher strain than the hinges. AMS orientation data are analysed in conjunction with field data. It is concluded that the magnetic fabric developed in the limbs as well as hinges of the BIFs of the study area is related to deformation and is not a manifestation of sedimentary fabric.

Cryptic post-depositional reworking in aeolian sediments revealed by the anisotropy of magnetic susceptibility

Our anisotropy of magnetic susceptibility (AMS) investigation of the Gold Hill Steps (GHS2) and Halfway House (HH3) loess and paleosol profiles in central Alaska confirms that post-depositional reworking of loess at Gold Hill Steps has taken place and, of greater importance, identifies the reworked depth intervals. In GHS2, the majority of the loess below 7.80 m depth has been reworked. The loess at the Halfway House site is, as stated in the literature, a predominantly undisturbed aeolian deposit. However, we have identified two intervals that have been reworked in HH3. We infer from the depth variation of the AMS orientation distribution at GHS2 that the presence of permafrost in the past was the dominant active mechanism producing the observed deformations. Permafrost loess behaved (1) as impermeable layers focusing and channeling flow, most likely that of groundwater, and (2) as rigid bodies undergoing rotations and lateral translations. A modern analogue of focused flow by an impermeable layer is identified at HH3.

Palaeomagnetic evolution of the Çankırı Basin (central Anatolia, Turkey): implications for oroclinal bending due to indentation

Palaeomagnetic data in combination with palaeostress data and anisotropy of magnetic susceptibility orientations are utilized to develop a tectonic evolutionary model for the Early Tertiary part of the Ω (omega)-shaped Çankırı Basin (Turkey). The results reveal clockwise rotations in the northeast and anticlockwise rotations in the west and southeastern corner of the basin. The magnetic inclinations indicate a northward drift of the Çankırı Basin and support an indentation model for the Kırşehir Block. It is proposed that the Ω-shape of the Çankırı Basin was the result of indentation of the Kırşehir Block into the Sakarya Continent during northwards migration accompanying closure of Neotethys. It appears that the indentation started prior to Eocene and ended before Middle Miocene times.

Tectonic significance of magnetic susceptibility fabrics in Plio-Quaternary mudstones of southwestern foothills, Taiwan

Anisotropy of magnetic susceptibility (AMS) was studied in three Plio-Pleistocene turbiditic mudstone sequences accumulated in the foreland basin of southwestern Taiwan. These formations were incorporated in the front units of the collision belt and underwent folding and thrusting during the last 2 Ma. Five types of fabrics were identified from more than 3,000 samples collected in 352 sites, with 251 sites allowing determination of a magnetic lineation. NNE-SSW trends are predominant, minor N-S and NE-SW trends are present. Magnetic lineations are widespread in the lower section where folds are tight, and scarce in the youngest sediments where folds are gentle. The strong correlation between the structural features and the AMS orientations suggests a tectonic origin for most magnetic lineations superimposed on the initial flattening that results from sediment compaction. This is confirmed by tectonic studies based on structural analysis and paleostress tensor reconstructions. The tectonic studies reveal a major WNW-ESE compression, which provide orientations of compressive tectonic regimes consistent for resulting the magnetic lineations. In contrast, the hypothesis of a sedimentary origin can be ruled out in most cases, because the orientations of magnetic lineations and those of depositional fabrics (paleocurrents, sediment supply directions and even slumps) are oblique at a variety of angles. Furthermore, based on magnetostratigraphy, we conclude that this compression culminated about 0.9–1 Ma ago. Earlier minor events, NW-SE and W-E compression, have also been found and we propose that they have occurred in approximately 1 and 2 Ma ago, respectively. Thus, the main cause of AMS trend is thought to be the WNW-ESE Quaternary compression responsible for major folding and thrusting. In addition, the magnetic fabric of tectonic origin is absent, or poorly marked, in formations younger than about 0.9 Ma to the north. However, it is still recognized but decreased after about 0.7 Ma ago to the south. This indicates that the WNW-ESE compression propagated southward between 0.9 and 0.7 Ma ago, consistent with the migration of folding and thrusting during the last Taiwan collision. 527-538

Magnetic fabric of the Barrington Passage pluton, Meguma Terrane, Nova Scotia: a two-stage fabric history of syntectonic emplacement

Structural analysis and the anisotropy of magnetic susceptibility (AMS) are used to document the internal, magmatic to high-temperature subsolidus fabrics of the tonalitic Late Devonian Barrington Passage pluton. The AMS fabric orientations compare favorably with mesoscopic-scale fabrics. In general, the K 1– K 2 principal plane of the AMS is close to the observed mineral foliation defined by biotite and plagioclase, and the K 1 axis is aligned parallel to a mineral lineation defined by the same minerals, and to a locally developed, weak extension lineation defined by quartz. Measurement of the anisotropy of anhysteretic remanence (AAR), and image analysis of biotite preferred orientations confirm that the orientation of the AMS is principally controlled by biotite. Locally anomalous AMS orientations are attributed to the influence of fine-grained magnetite crystals, the presence of which is revealed by measurements of hysteresis and coercivity properties. A two-stage fabric history is revealed. First, horizontal fabrics were formed (foliation, compositional banding) during emplacement of a laccolith-like pluton. The second stage involved synmagmatic folding of the horizontal foliation and banding in response to regional contraction associated with the Acadian Orogeny, with development of a lineation that records stretching parallel to the fold axis. The results demonstrate that the Barrington Passage pluton was emplaced during the main phase of the Acadian Orogeny. A model is proposed that depicts the Barrington Passage pluton as the folded base of a larger syntectonic pluton, that is now largely eroded off.

Strain decoupling across the decollement of the Barbados accretionary prism

The interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate, particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material. Anisotropy of magnetic susceptibility (AMS) data from sediments spanning the basal decollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured. In the accretionary prism, above the decollement, the minimum AMS axes are subhorizontal and oriented nearly east-west, whereas the maximum AMS axes are oriented nearly north-south and shallowly inclined. At the top of the decollement, the minimum AMS axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American plate and the maximum axes are trench-parallel. Because the orientations of the AMS axes in deformed sediments usually parallel the orientations of the principal strains, the AMS results indicate that the incremental strain state in the Barbados prism is one dominated by subhorizontal shortening. In contrast, the AMS axes within and below the decollement are consistent with a strain state dominated by vertical shortening (compaction). This abrupt change in AMS orientations at the top of the decollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sediments from the underthrust sediments. The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of flowing, high-pressure fluids.

Plastic behavior of magnetite and high strains obtained from magnetic fabrics in the Parry Sound shear zone, Ontario Grenville Province

Magnetic anisotropy and quantitative petrology of a transition from relatively undeformed metaanorthosite protolith to highly deformed ultramylonite in the Parry Sound shear zone provide information on magnetite as a kinematic indicator. Rock magnetic experiments show that anisotropy of magnetic susceptibility (AMS) is controlled by magnetite in the mylonites and ultramylonites, and by paramagnetic minerals (hornblende, biotite, ilmenite) in the protolith. Geothermometry in the ultramylonite indicates a temperature of 630 ± 50 °C during deformation. A deformation mechanism map calculated from published experimental work for 630 °C indicates that magnetite in the Parry Sound shear zone deformed plastically rather than via rigid-body rotation. The AMS orientations accurately track a foliation trajectory in a portion of the shear zone, which allows the utilization of orientation-based strain models to determine shear strains. Shear strains (γ) of 1–9 were obtained, and an empirical correlation of strain with the ellipsoid shape resulted in the logarithmic relationships: Mmax = 0.15 ln( X), and Mmin = 0.13 ln( Z), where Mi = ( ki/ kmean) describes the principal AMS axes, or ( k max/k min) = ( X Z )0.14 . These AMS-strain correlations were subsequently applied to mylonites and ultramylonites elsewhere in the shear zone, predicting shear strains as high as γ = 13. These observations also reveal that AMS fabrics do not saturate at high strains, as would be expected for rigid-body rotation, due to the plastic behavior of magnetite during deformation.