Principal Anisotropy Of Magnetic Susceptibility Research Articles

Preliminary anisotropy of magnetic susceptibility studies on 2367 Ma Bangalore-Karimnagar giant dyke swarm, southern India: Implications for magma flow

Anisotropy of magnetic susceptibility (AMS) fabrics have been studied for the first time on a well constrained Paleoproterozoic Bangalore-Karimnagar giant mafic dyke swarm at ca. 2367 Ma in the Dharwar craton, south India. A total number of 43 samples from nine sites representing five N-E trending dykes have been subjected to AMS and conventional rock magnetic studies. The magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are >10−3 SI, which is further supported by Isothermal Remanent magnetization (IRM) studies. Hysteresis loops, Isothermal remanent magnetisation (IRM), thermomagnetic properties and optical observations indicate that pseudo single domain (PSD) low-titanium magnetite particles dominate the low field susceptibility of samples. The magnetic fabrics results correspond well with the comprehensive field studies. However, the degree of anisotropy results ranges from 0.4% to 12.8%, with a mean total anisotropy of 4.7%. The principal AMS axes for samples from all the sites are tightly clustered on equal area plots, and two main fabrics were revealed in the Bangalore-Karimnagar giant dyke swarm. In the first, the maximum susceptibility axes is sub-horizontal and parallel to the dyke walls, minimum susceptibility axes plot near the pole of the dyke. It is interpreted as the primary magma flow direction. A second fabric with maximum susceptibility axis is sub-horizontal and perpendicular to the dyke walls, and is interpreted as a result of vertical compaction at a late stage of crystallization. Preliminary results suggest that dyke propagation in the Bangalore-Karimnagar giant dyke swarm preserve a flow-induced fabric slightly upwards to the NE and magma flow is sub-horizontal.

Relationship between magnetic fabrics and deformation of the Miocene Pohorje intrusions and surrounding sediments (Eastern Alps)

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.

Distinguishing the Mélange-Forming Processes in Subduction-Accretion Complexes: Constraints from the Anisotropy of Magnetic Susceptibility (AMS)

The strong morphological similitude of the block-in-matrix fabric of chaotic rock units (mélanges and broken formations) makes problematic the recognition of their primary forming-processes. We present results of the comparison between magnetic fabric and mesoscale structural investigations of non-metamorphic tectonic, sedimentary, and polygenetic mélanges in the exhumed Late Cretaceous to early Eocene Ligurian accretionary complex and overlying wedge-top basin succession in the Northern Apennines (northwest Italy). Our findings show that the magnetic fabric reveals diagnostic configurations of principal anisotropy of magnetic susceptibility (AMS) axes orientation that are well comparable with the mesoscale block-in-matrix fabric of mélanges formed by different processes. Broken formations and tectonic mélanges show prolate and neutral-to-oblate ellipsoids, respectively, with magnetic fabric elements being consistent with those of the mesoscale anisotropic “structurally ordered” block-in-matrix fabric. Sedimentary mélanges show an oblate ellipsoid with a clear sedimentary magnetic fabric related to downslope gravitational emplacement. Polygenetic mélanges show the occurrence of a cumulative depositional and tectonic magnetic fabric. The comparison of field and laboratory investigations validate the analysis of magnetic features as a diagnostic tool suitable to analytically distinguish the contribution of different mélange forming-processes and their mutual superposition, and to better understand the geodynamic evolution of subduction-accretion complexes.

Anisotropy of magnetic susceptibility in diamagnetic limestones reveals deflection of the strain field near the Dead Sea Fault, northern Israel

To exploit the potential of anisotropy of magnetic susceptibility (AMS) as a tool to estimate the strain field around major faults, we measured the AMS of calcite-bearing diamagnetic rocks that crop out next to the Dead Sea Fault (DSF) in northern Israel. Through integrated magnetic and geochemical methods we found that the rocks are almost pure calcite rocks and therefore the magnetic fabric is primarily controlled by preferred crystallographic orientation (PCO) with the minimum principal AMS axes (k3) parallel to calcite c-axes. We applied a separation procedure in several samples with high Fe content in order to calculate the AMS anisotropy parameters and compare them to pure diamagnetic rocks. AARM, thermo-susceptibility curves and IRM were used to characterize the magnetic phases. We found that for Fe content below 500ppm the AMS is mostly controlled by the diamagnetic phase and showed that differences in the degree of anisotropy P' up to 3% (P'=1.005 to 1.023) and in anisotropy difference Δk (up to ~0.25×10−6 SI) in diamagnetic rocks are related to differences of strain magnitudes. The spatial distribution of the magnetic fabrics indicates ~N–S maximum shortening parallel to the strike of the Hula Western Border fault (HWBF), one of the main strands of the DSF in northern Israel. The anisotropy parameters suggest that the strain magnitudes increase eastward with the proximity to the HWBF. These results suggest that the strain field near the HWBF is locally deflected as a consequence of the DSF activity. In light of the “fault weakness” model and geological setting of the study area, we suggest that the area accommodates dominant transtension during the Pleistocene. The present study demonstrates the useful application of AMS measurements in “iron-free” limestones as recorders of the strain field near plate boundaries.

Distinctive diamagnetic fabrics in dolostones evolved at fault cores, the Dead Sea Transform

We resolve the anisotropy of magnetic susceptibility (AMS) axes along fault planes, cores and damage zones in rocks that crop out next to the Dead Sea Transform (DST) plate boundary. We measured 261 samples of mainly diamagnetic dolostones that were collected from 15 stations. To test the possible effect of the iron content on the AMS we analyzed the Fe concentrations of the samples in different rock phases. Dolostones with mean magnetic susceptibility value lower than −4 × 10−6 SI and iron content less than ∼1000 ppm are suitable for diamagnetic AMS-based strain analysis. The dolostones along fault planes display AMS fabrics that significantly deviate from the primary “sedimentary fabric”. The characteristics of these fabrics include well-grouped, sub-horizontal, minimum principal AMS axes (k3) and sub-vertical magnetic foliations commonly defined by maximum and intermediate principal AMS axes (k1 and k2 axes, respectively). These fabrics are distinctive along fault planes located tens of kilometers apart, with strikes ranging between NNW-SSE and NNE-SSW and different senses of motion. The obtained magnetic foliations (k1–k2) are sub-parallel (within ∼20°) to the fault planes. Based on rock magnetic and geochemical analyses, we interpret the AMS fabrics as the product of both shape and crystallographic anisotropy of the dolostones. Preferred shape alignment evolves due to mechanical rotation of subordinate particles and rock fragments at the fault core. Preferred crystallographic orientation results from elevated frictional heating (>300 °C) during faulting, which enhances c-axes alignment in the cement-supported dolomite breccia due to crystal-plastic processes. The penetrative deformation within fault zones resulted from the local, fault-related strain field and does not reflect the regional strain field. The analyzed AMS fabrics together with fault-plane kinematics provide valuable information on faulting characteristics in the uppermost crust.

Syntectonic emplacement of Late Cretaceous mafic dyke swarms in coastal southeastern China: Insights from magnetic fabrics, rock magnetism and field evidence

Magma flow directions for 6 Late Cretaceous mafic dyke swarms exposed in coastal southeastern China (SE China) were analyzed using anisotropy of magnetic susceptibility (AMS) and field evidence. Normal AMS fabrics are predominant. The AMS of the dyke swarms originates mainly from the distribution anisotropy of intersertal magnetite that crystallized during late stage magma flow or after the magma cooled. The AMS fabrics record tectonic stress combined with magma flow. Sub-vertical to vertical magma flow is inferred from symmetrical imbricated magnetic foliations of dyke walls and field evidence for 5 dyke swarms. The inferred (sub-) vertical flow directions also indicate that the magma chambers were probably just beneath the sampled locations. Low anisotropy degree, different orientations of principal AMS axes, and asymmetrical magnetic foliations of normal fabrics oblique to dyke walls indicate syntectonic emplacement of the Late Cretaceous dyke swarms under an extensional tectonic regime caused by Paleo-Pacific plate subduction.

Magnetic fabrics induced by dynamic faulting reveal damage zone sizes in soft rocks, Dead Sea basin

SUMMARY The anisotropy of magnetic susceptibility (AMS) of soft rocks was measured in order to distinguish between the effect of remote and local strain fields, determine the size of the related inelastic damage zone and resolve the fault-plane solutions of past earthquakes. The AMS fabrics were explored next to late Pleistocene syndepositional normal faults (total displacement up to ∼3.5 m) that cross soft lacustrine rocks within the seismically active Dead Sea basin. ‘Deposition fabrics’ prevail meters away from the fault planes and are characterized by scattered maximum and intermediate principal AMS axes. ‘Deformation fabrics’ are detected up to tens of centimetres from the fault planes and are characterized by well-grouped AMS axes, in which one of the principal axes is parallel to the strike of the nearby fault. Variations in the AMS fabrics and magnetic lineations define the size of the inelastic damage zone around the faults. The results demonstrate that the deformation-driven magnetic fabrics and the associated inelastic damage zones are compatible with coseismic dynamic faulting and the effects of the local strain field during earthquakes. Most of the AMS fabrics show a conspicuous similarity to that of the fault-plane solutions, i.e., the principal AMS axes and instantaneous strain ellipsoids are coaxial. These results suggest a novel application of the AMS method for defining the shape and size of the damage zones surrounding dynamic faults and determining the full tensor of the local strain field.

Degree of magnetic anisotropy as a strain-intensity gauge in a saturated finite-strain zone

Abstract: Anisotropy of magnetic susceptibility (AMS) analyses were carried out on mylonitic quartzites from the footwall of the Main Central Thrust Zone, Garhwal Higher Himalaya, India. A systematic increase in degree of magnetic anisotropy ( P ′) with distance from the Main Central Thrust is documented, even in a saturated finite-strain zone. On the basis of correlation between orientation of principal AMS axes and multidomain magnetite and the mylonitic foliation and lineation direction, strain-related recrystallization was inferred to be the cause of development of the magnetic fabric. Based on these results, it is concluded that if P ′ is not controlled by K m , then the former can be used as a strain-intensity gauge. Supplementary material: Three-dimensional finite-strain parameters and rock-magnetic and AMS datasets are available at http://www.geolsoc.org.uk/SUP18328 .

Magnetic fabrics of the Morcles Nappe complex

Anisotropy of magnetic susceptibility (AMS) measured with high fields, at ambient and low temperatures (77 K), has been used to separate ferromagnetic ( sensu lato), paramagnetic and diamagnetic sub-fabrics for deformed limestones located in a nappe structure from southwest Switzerland. The magnetic fabrics are dominantly controlled by paramagnetic and diamagnetic minerals, and ferromagnetic minerals contribute significantly only at two of the eight study locations. Separation of the magnetic sub-fabrics is possible in the root zone and overturned limb of the Morcles Nappe, which contains a high ratio of diamagnetic matrix calcite to paramagnetic secondary phases. Each of the sub-fabrics displays a relationship with structural cleavage and the crystallographic preferred orientation (CPO) of calcite in the root zone and overturned limb. The maximum axis of the paramagnetic AMS ellipsoid ( k max) lies close to the pole to cleavage, with the intermediate ( k int) and minimum ( k min) axes near the cleavage plane. Conversely, the diamagnetic k min lies close to the pole to cleavage with k max and k int axes near the cleavage plane. Slight offsets in the poles to cleavage and in the principal AMS directions are observed. They are interpreted to be the consequence of a second phase deformation event, affecting only the CPO but not the macroscopic structural elements. The diamagnetic sub-fabric reflects the CPO of calcite, produced through recrystallization during nappe deformation. Iron-rich calcite appears to control the paramagnetic sub-fabric. Susceptibility ellipsoids approach oblate shapes for the diamagnetic sub-fabric, with the most extreme shapes reaching susceptibility differences (Δ k = k max − k min) close to that of single crystal calcite, whereas the paramagnetic sub-fabric is represented mainly by prolate shape ellipsoids. The low-field AMS ellipsoids show mixed oblate and prolate shapes, without consistency within or between sites. It is evident that, when the magnetic sub-fabrics of the root zone and overturned limb are separated, their individual shapes and distribution of principal directions mirror the developed CPO.

Anisotropy of magnetic susceptibility of Hannuoba basalt, northern China: Constraints on the vent position of the lava sequences

Anisotropy of magnetic susceptibility (AMS) was determined for Hannuoba basaltic samples from 17 flows (21.4 ± 0.7Ma) distributed in three sections (Jianshaba, Yuershan and Manjing) around Zhangbei, northern China. The results show normal ‘flow’ fabrics with horizontal foliation planes. The typical ratios of the principal AMS axes are 1.01 for the magnetic lineation (L) and 1.02 for the foliation (F), respectively. Magnetic hysteresis loop, isothermal remanence, thermomagnetic properties, and optical observations indicate that pseudo‐single domain (PSD) low‐titanium magnetite particles dominate the low‐field susceptibility of samples. Thus the AMS of the magma at this area is believed to be originated from the lineation of the low‐Titanium magnetite particles driven by the lava flow. Since the Kmax trends coincide well with the observed flow directions, the intersection of the mean Kmax trends from these three sites perfectly defines the previously known vent position of the lava sequences.

Anisotropy of diamagnetic susceptibility in Thassos marble: A comparison between measured and modeled data

A study of shear zones within the calcite marble complex of the island of Thassos (Greece) shows that the low field anisotropy of magnetic susceptibility (AMS)-technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics. The strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial (neutral) to uniaxial (prolate) shapes of AMS ellipsoids. The maximum susceptibility is oriented perpendicular to the rock foliation, reflecting the preferred orientation of calcite c-axes in the protolith as well as in the mylonites. For three samples that represent different types of calcite fabrics, the AMS was recalculated from neutron and electron backscatter diffraction textural data. A comparison of the measured and modeled data shows a good coincidence for the orientation of the principal AMS axes and for the recalculated anisotropy data. Both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation.

Characteristics of magnetic fabrics of Jiangshan-Shaoxing collision belt and its tectonic implications

The study of anisotropy of magnetic susceptibility (AMS) of rocks from the northern part of Jiangshan-Shaoxing collisional belt demonstrates that the minimum principal AMS axes are the most concentrated one in various types of rocks. The planar projection of the minimum axis shows that the main compressive stress lies on northwest 30°–60°, which trends dominantly in northwest-southeast of Chencai area and arc-like disperse towards both north and south of Chencai. The inclination of the minimum axis illustrates that the Huaxia block is moving northwesterly. Although the maximum axes show slightly dispersal, they are within the confidential circle. The maximum axis intersects with the lineation with a small angle, suggesting that the Jiangshan-Shaoxing collision belt be of a sinistral shear slip. By comparison with magnetic fabrics of various rocks, intensive collision of the Jiangshan-Shaoxing belt was in Late Triassic and sustained to the late Yanshanian period in the form of sinistral shear slip.

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.

Magnetic fabric, flow directions, and source area of the Lower Miocene Peach Springs Tuff in Arizona, California, and Nevada

We have used anisotropy of magnetic susceptibility (AMS) to define the flow fabric and possible source area of the Peach Springs Tuff, a widespread rhyolitic ash flow tuff in the Mojave Desert and Great Basin of California, Arizona, and Nevada. The tuff is an important stratigraphic marker from the Colorado Plateau to Barstow, California, a distance of 350 km; however, the location of its source caldera is unknown. Dated at 18.5 Ma by 40Ar/39 Ar, the tuff erupted during the early stages of Miocene extension along the lower Colorado River. The thicker accumulations (>100 m) occur at Kingman, Arizona, and in the Piute Mountains, California, on opposite sides of the Colorado River extensional corridor. Our AMS studies produced well‐defined magnetic lineations in 30 of 42 sites distributed throughout the tuff. Typical ratios of the principal AMS axes are 1.01 for the magnetic lineation (kmax/kint) and 1.02 for the foliation (kint/kmin); the bulk magnetic susceptibility of the Peach Springs Tuff averages 2.0×10−3 in the SI unit system. The subhorizontal lineations, which presumably parallel the flow directions, form a pattern radiating outward from the approximate center of the outcrop area. Magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff. The lineation and imbrication indicate a source region near the southern tip of Nevada. Defining the best intersection of the AMS lineations required restoration of major extension, strike‐slip faulting, and associated tectonic rotation in the disrupted tuff. The optimum intersection of magnetic lineations lies in the southern Black Mountains of Arizona on the eastern side of the Colorado River extensional corridor. No caldera structures are known from that area, but the area contains thick sections of the Peach Springs Tuff above a silicic volcanic center. The caldera may be buried under younger deposits in the Mohave Valley of Arizona. Tertiary granite in the Newberry Mountains may represent a deeper level of the Peach Springs Tuff vent that has been exhumed by detachment faulting.

Magma flow directions in dikes of the Koolau Complex, Oahu, determined from magnetic fabric studies

We compared the magnetic fabric with the macroscopic surface lineation direction (previously assumed to be parallel with magma flow direction) for 35 dikes of the Koolau complex, Oahu. Anisotropy of magnetic susceptibility (AMS) was determined for 530 samples from 59 dikes (including a sill and a pluglike intrusive body) and two filled lava tubes, and statistically significant AMS clusters were found in all but five dikes. For the 35 lineated dikes, the mean maximum AMS direction (χ1) coincides with the macroscopic lineations to within 25° in 18 and to within 15° in eight. The AMS ellipsoid shape in about half of the samples is prolate (cigar shaped) and in about half is oblate (pancake shaped). We interpret the AMS to be determined by shape anisotropy of ferromagnetic grains. The mean total anisotropy is 2.2%, and total anisotropy (H) ranges from 0.24% to as high as 14.12%. The principal AMS axes for samples from a dike are tightly grouped on equal‐area plots, and for most of the dikes the minimum AMS axes plot near to the pole of the dike. Our AMS measurements enable us to infer absolute magma flow directions for half the dikes studied; the maximum AMS axes of samples from one side of the dike form a tight cluster on an equal‐area plot, and those from the other side of the dike form another tight cluster. The two clusters are close together (separated by 10–30°) for narrow dikes (<100 cm wide), and are farther apart (30–60°) for wide dikes and are symmetrically disposed on either side of the plane of the dike. We infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient, and hence indicate the flow azimuth, i.e., the absolute magma flow direction. Nearly half of the dikes measured (25) gave paired AMS plots; in 12 the magma flowed up at low oblique angles 5–30° from the horizontal, in five it flowed down at 5–20° in four it flowed horizontally (<5°), in three it flowed up at a steep angle (50–75°), and in one it flowed down at a steep angle. The magma flow in general was rarely vertical or horizontal.