Anisotropy Of Magnetic Susceptibility Axes Research Articles

Strain decoupling across the décollement in the region of large slip during the 2011 Tohoku-Oki earthquake from anisotropy of magnetic susceptibility

The Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) drilled and cored across the plate-boundary décollement at Site C0019 near the Japan Trench, where large slip occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Anisotropy of magnetic susceptibility (AMS) data obtained from core samples show a striking change in magnetic fabric across the décollement. In the frontal prism above the décollement, the maximum AMS axes display a strong preferred orientation in the northeast–southwest direction, with the intermediate and minimum AMS axes distributed in the vertical plane. In the underthrust sediments below the décollement, the maximum and intermediate AMS axes are subhorizontal but variable in direction, and the minimum AMS axes display an approximately vertical preferred orientation. The AMS ellipsoids for all samples have an oblate component, but the AMS ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism. The magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the Japan Trench, while that in the underthrust sediments represents a vertical, uniaxial strain. The magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the décollement at shallow depths, and imply that large co-seismic slip occurred along a weak plate-boundary décollement that is mainly decoupled over the long-term.

Internal structure of basalt flows: insights from magnetic and crystallographic fabrics of the La Palisse volcanics, French Massif Central

We present a new interpretation of anisotropy of magnetic susceptibility (AMS) fabrics in basaltic lava flows based on the detailed study of magnetic mineralogy and silicate crystallographic fabric of a Quaternary lava flow from the French Massif Central (La Palisse). We consider the model of AMS fabric imbrication between magnetic foliation and flow surface, as initially proposed for dykes. At the two sampling sites, the concordance between the flow direction deduced from the AMS foliation and that deduced from field observations indicates that the imbrication model could apply to the lava flows. However, the flow senses inferred fromAMSare systematically opposed between the two sampling sites suggesting permutations between K1 and K3 AMS axes, a configuration referred to as inverse fabric. Electron backscatter diffraction (EBSD) measurements show strong lattice-preferred orientations (LPO) for plagioclase, especially the (010) plagioclase plane, which tends to be parallel to the flow. Clinopyroxene LPO remains less marked than plagioclase LPO, whereas titanomagnetite does not display a significant LPO. Comparison between magnetic and crystallographic fabrics suggests that the AMS fabric of the lava flow results from the distribution of titanomagnetite grains, which is in turn controlled by the fabric of the silicate framework. Magnetic hysteresis parameters and anisotropy of remanent magnetization (ARM) measurements exclude a significant contribution from single-domain grains, often called upon to explain inverse magnetic fabrics. The origin of the observed inverse magnetic fabric may relate to the dip of the palaeosurface, which is the only remarkable difference between the two sampling sites. AMS appears as a good tool to determine the direction of basaltic lava flows and coupling with local crystallographic fabric data provides a valuable control of relationships between magnetic fabrics and flow and thus contributes to better constrain the AMS signature of lava flows.

Variations in magnetic properties of target basalts with the direction of asteroid impact: Example from Lonar crater, India

Abstract– The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (approximately 65 Ma) at approximately 570 ± 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately 30–45o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east–west plane of impact (Misra et al. 2010). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g., low‐field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM)/bulk susceptibility (χ), and high‐coercivity and high‐temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K3 (minimum) AMS axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2–3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact. The biaxial distribution of AMS axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts, although transitional type distribution also exists. The degree of anisotropy (P′) of AMS ellipsoids of the shocked basalts decreases by approximately 2% when compared with those of the unshocked target (approximately 1.03). The NRM/χ (Am−1) values of the shocked basalts on the rim of the Lonar crater do not show much change in the uprange or downrange direction on and close to the east–west plane of impact, and the values are only approximately 1.5 times higher on average over the unshocked basalts around the crater. However, the values become approximately 1.4–16.4 times higher for the shocked basalts on the crater rim, which occur obliquely to the plane of impact. The target basalts at approximately 2–3 km west of the crater center in the downrange also show a significant increase (up to approximately 26 times higher) in NRM/χ. The majority of the shocked basalt samples (approximately 73%) from around the crater rim, in general, show a lowering of REM, except those from approximately 2–3 km west of the crater center in the downrange, where nearly half of the sample population shows a higher REM of approximately 3.63% in average. The shocked target basalts around the Lonar crater also acquired an HC_HT magnetization component due to impact. These HC_HT components are mostly oriented in the uprange direction and are symmetrically disposed about the east–west plane of impact, making an obtuse angle with the direction of impact. The low‐coercivity and low‐temperature (LC_LT) components of both the unshocked and shocked basalts are statistically identical to the present day field (PDF) direction. This could be chemical and/or viscous remanent magnetization acquired by the target basalts during the last 570 ± 47 ka, subsequent to the formation of the Lonar crater. The shocked Lonar target basalts appear to have remagnetized under high impact shock pressure and at low temperature of approximately 200–300 °C, where Ti‐rich titanomagnetite was the main magnetic remanence carrier.

Fabrica Magnetica y Microestructuras a traves de los Andes de Tierra del Fuego, Argentina.

An anisotropy of magnetic susceptibility (AMS) transect was carried out across the Fuegian Andes, in Argentina, with the aim of studying its tectonic evolution. Two-hundred and forty oriented samples were collected from 27 sites distributed between the Paso Garibaldi to the north and the Canal Beagle to the south. The study was restricted to the Upper Jurassic Lemaire Formation, with a single site located in the Lower Cretaceous Yahgan Formation. Studied rocks comprised basaltic, andesitic, volcaniclastic and sedimentary rocks affected by low-grade metamorphism. AMS measurements were complemented with thin section analyses of representative samples in order to characterize the microstructures and metamorphic assemblages. In general, the magnetic fabric shows dominant oblate shapes and a large variation in the anisotropy degree from 1.04 up to 2. The anomalously high values were observed to be associated to growth of secondary pyrrhotite, which was identified by rock magnetic tests. Magnetic foliation was generally consistent with slaty cleaveage as observed in the field, confirming the tectonic origin of the magnetic fabric. Three geographic domains were distinguished in the study region on the basis of the pattern of the AMS axes distribution. In the northern domain, from Paso Garibaldi to Valle Carbajal, the orientation of the maximum susceptibility axis (k1), or magnetic lineation, is N-S to NE-SW with moderate plunge towards the S-SW and coincides with previous determination of mineral lineations associated with the Andean deformation and very low grade metamorphism. The magnetic fabric pattern can be correlated with the main deformational phase responsible for the development of slaty cleavage (main Andean deformational phase) and the tectonic transport due to progression of the Fuegian fold and thrust belt in the Late Cretaceous. A different character is shown along the Valle Carbajal domain, where subvertical E-W magnetic foliation planes and roughly E-W to ESE-WNW subhorizontal magnetic lineations are more difficult to correlate with the main folding phase and suggest its relation to an E-W, possibly localized, strike-slip regime during the main deformational and metamorphic phase. The magnetic fabrics in the third domain, close to the Canal Beagle, displays a more heterogeneous character with both E-W and N-S striking foliations; in this case a population of subhorizontal E-W magnetic lineation (k1) suggests the existence of a significant component of strike-slip deformation.

Low-latitude Oxfordian position of the Oravic crustal segment (Pieniny Klippen Belt, Western Carpathians): Palaeogeographic implications

The Bajocian and the Oxfordian–Kimmeridgian crinoidal limestones of the Pieniny Klippen Belt (PKB), Western Carpathians and the Bathonian–Callovian neptunian dykes which cut them were sampled for rock magnetic and palaeomagnetic study. The sampled sections were: Babiná, Mestečko, Vršatec, Bolešovská dolina and Szaflary. SIRM(T) curves show that the limestone and the dykes from Mestečko contain hematite with blocking temperature T b about 650–670 °C. Babiná limestone and dykes contain magnetite with T b between 470 and 570 °C, usually accompanied by small hematite tail. Sometimes goethite with T b about 80–100 °C is seen. Dykes and limestone from Vršatec contain magnetite with T b of 570 °C or hematite with unblocking temperature of 670 °C. After cleaning of recent soft component by a temperature of 200 °C and a field of 10–20 mT, the hard component was revealed. After tilt correction the locality-mean directions found in limestones have scattered declination but similar reversed inclination between −35° and –40° for the Bajocian and between −35° and −49° for the Oxfordian–Kimmeridgian. An exception was the mean direction for the Bajocian limestone from Bolešovská dolina which has normal polarity. Fold test for inclination only showed pre-folding inclinations for the Bajocian and Oxfordian–Kimmeridgian limestones and scattered inclinations for the in situ and tilt corrected directions for the dykes. The mean palaeoinclination calculated for Bajocian and Oxfordian–Kimmeridgian by means of “inclination-only” test are −38.5° +/−3° and −42.5° +/−11.2°, respectively, which corresponds to 21.7 +/−1.5°N and 24.6 +/−5.6°N of palaeolatitude. This is about 10°–15° lower than the value of inclination expected for that age for the Pieniny Klippen Belt from the Jurassic poles for Stable Europe. As positive inclination only fold test proves pre-folding age of isolating directions other possibility — flattening of inclination should be considered. Anisotropy of magnetic susceptibility (AMS) and magnetic unhysteretic remanent magnetisation (AARM) can be used for the evaluation of origin and for the deviation of natural remanent magnetisation NRM. The axes of AMS are scattered for all localities. The degree of anisotropy is low. The shape parameter T shows equal distribution of oblate and prolate fabric. On the contrary, the ARM maximum axes are well clustered, especially for Mestečko and Vršatec. The maximum axes are distributed in a vertical plane for Mestečko and in horizontal plane for Babiná and Vršatec. This evidences that the AARM fabric is not related to sedimentation and compaction. The anisotropy axes are not close to the NRM directions. For Mestečko they are steeper. This excludes an effect of shallowing of NRM. On this basis we can assume that more southern palaeolatitude of the Pieniny Klippen Belt than expected is a demonstration of palaeogeography and tectonics and points to a close vicinity of the PKB to the African plate in the Jurassic. The results from the Polish part of PKB which emerged in the latest years showed the same palaeolatitude for the Callovian–Kimmeridgian limestones. The low palaeomagnetic inclinations measured in the Bajocian crinoidal limestones show that the southward drift may have started earlier than previously expected.

Magnetic fabrics in L–S tectonites: How many specimens?

Single large blocks (10–30 kg) of homogeneously strained, fine-grained slate or schist with homogeneous fabrics reveal subtle but sometimes significant variations in magnetic fabric from large numbers (up to 100) of closely spaced cylindrical cores. Traditional samples of three to six cores per block or per site may suffice for low precision, regional determinations of fabric orientations, if suitably fine-grained and homogeneous. However, for the most precise definition of anisotropy of magnetic susceptibility axes (AMS), small sample sizes (<15 cores) yield inconsistent orientations and shapes of the mean tensor. Coarse grain size would exacerbate these shortcomings since individual cores fail to sample the fabric representatively and inter-specimen variation may exceed inter-site variation in AMS. Fine-grained homogeneous rocks most successfully yield reproducible AMS orientations, and more especially AMS shapes. The optimum number of cores is best determined by experimentation since it depends on fabric heterogeneity, mineral proportions and grain-size variations at the inter-site and intra-site level.

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 .

Sampling strategies and the anisotropy of magnetic susceptibility of dykes

Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that susceptibility axes should define a unique orientation relative to the dyke walls and magma flow direction. Theoretical considerations, however, predict systematic variations of AMS as a function of the amount of shear experienced by the flowing magma. Although this feature of AMS might seem undesirable at first sight, based in a theoretical model it has been suggested that actually we can take advantage of such variation in the orientation of the AMS axes to infer magma flow directions more confidently than until now if proper attention is given to the scheme followed during sample collection. In this work we report the results of a pilot study made to test those theoretical predictions. We resampled two dykes from the Ko'olau dyke complex in O'ahu and for which magma flow direction was previously inferred albeit with some uncertainty. The resampling was made following the guidelines suggested by the theoretical work as closely as possible (i.e., collecting more than one profile parallel to the dyke walls and across the dyke). The results of our measurements provide the first empirical evidence supporting the occurrence of a cyclic fabric acquisition during the emplacement of at least some dykes. The procedure described in this paper therefore constitutes a self-contained approach that can be followed to test in a case by case basis the occurrence of such fabric behavior. This in turn should provide enough elements to increase the reliability of the interpretation in the obtained results.

Scattering of magnetic fabrics in the Cambrian alkaline granite of Meruoca (Ceará state, northeastern Brazil)

Anisotropy of magnetic susceptibility (AMS) applied to an alkaline granite from Meruoca (NE Brazil) recorded weak anisotropies, typically below 4%, and a considerable dispersion of the AMS axes. Red-clouded feldspars and clots of metasomatic minerals enclosed in magmatic crystals indicate that hydrothermal fluids altered the granite. U–Pb isotopic data show high-common Pb on zircons but allowed the calculation of a mean SHRIMP age of 523 ± 9 Ma attributed to the magmatic crystallization. Growth of fine oxides by late fluid–rock interactions was responsible for the scattering of AMS. Rock magnetic data indicate they consist mainly of an oxidized magnetite and (titano)hematite. Shape preferred orientation of mafic aggregates measured in granite quarries shows that the pluton preserves a gently dipping magmatic foliation. AMS in some quarries with a well-defined magmatic fabric, however, remains highly dispersed. When AMS mimics the mafic shape fabric, only magnetic foliations share a common orientation. Locally, AMS grounded in coarse Ti-poor magnetite associated with titanite develops a consistent subhorizontal oblate fabric that agrees with tectonic models suggesting that the cupola of the pluton has been exposed by erosion.

Evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the Main Central Thrust Zone (Garhwal Himalaya, India): Inferences from finite strain and AMS analyses

The footwall of the Main Central Thrust (MCT) Zone along the Bhagirathi valley comprises a wide zone of mylonitic quartzite and deep-level tectonites. The systematic variation of finite strain parameters (Es, k and v) in the mylonites indicates heterogeneous deformation, which is determined to vary between, simple shear and non-coaxial flattening type. In such a strain regime the outer boundary of the quartz clasts are no longer preserved thus leading to an error in finite strain measurement. In order to supplement the finite strain studies, Anisotropy of Magnetic Susceptibility (AMS) analyses were carried out on the mylonitic quartzites. A systematic variation in degree of anisotropy ( P′) with distance from the MCT is documented and is interpreted to be tectonic in origin. Based on these results it is concluded that P′ can be used as a strain-intensity gauge at least on an outcrop scale, where a systematic variation in P′ values from one part of the outcrop to the other can be established. However, the quantitative relation between principal axes of finite strain ellipsoid and AMS axes, magnitude of principal susceptibility difference (Δ K 1 and Δ K 3) and finite strain magnitude ( ε 1=ln 1 + e 1 and ε 3=ln 1 + e 3) were related by a logarithmic relationship with a correlation coefficient of 0.844.

Paleocurrent patterns of the sedimentary sequence of the Taitao ophiolite constrained by anisotropy of magnetic susceptibility and paleomagnetic analyses

Determination of paleocurrent directions is a robust approach to establish the mechanism of sediment transport and of deposition. However, in tectonically controlled (or affected) basins where rotations are suspected, it is of critical importance to correct previously calculated paleocurrent directions to establish the original (pre-tectonic) transport and deposition patterns. In order to illustrate this, we determined the orientations of the anisotropy of magnetic susceptibility (AMS) fabrics from 14 sites distributed along the sedimentary deposits interbedded within the Late Miocene Main Volcanic Unit (MVU) of the Taitao ophiolite, Southern Chile. Data were used to estimate characteristics (directions and relative velocities) of the sediment transporting paleocurrents and orientations of the depositional slope. Sedimentary deposits consist of a rhythmic sequence of conglomerate (base), sandstone and mudstone (top) beds that present a systematic decrease in grain size up-sequence as a whole, and within individual beds. Sedimentary deposits of the MVU, as well as overlaying effusive volcanic products, are disrupted by small and discrete dextral/normal brittle faults. The distribution of fault planes and their kinematic characteristics suggest the development of a dextral negative flower-like fault zone in the area. Previously reported NW-ward paleocurrent directions obtained from the orientations of sedimentary structures roughly agree with our calculations based on the orientations of AMS fabrics. Calculated paleocurrent directions are sub-parallel to the NW-SE trend of Estero Cono channel, the main lineament in the area. The spatial distribution of AMS axes between and within sites suggest that coarse grain deposits were transported by relatively high velocity flows whereas fine grained deposits were by low to nearly null velocity flows. Previously reported paleomagnetic analyses indicate that counterclockwise rotations affected the whole Taitao ophiolite, including the MVU. By applying the necessary corrections we obtained paleocurrent directions yielding mostly NNW-ward directions. Restoration of the orientations of beddings indicates that the depositional surface dipped steeply at its easternmost edge and shallowly at the westernmost end. We propose here that sediments were transported by turbidite flows generated by slope instabilities in the innermost part of the Estero Cono. These turbidites followed the Estero Cono lineament while they filled the channel and transported sedimentary material (eroded from the upper portions of the ophiolite and nearby units) towards the Pacific Ocean. Sediments were deposited on a NW- to W-facing slope and sorted in decreasing grain size along the channel. The instability of the slope was most probably triggered by the activity of a transtensional fault zone developed along the mid-axis of the Estero Cono channel.

Magnetic fabric studies of the Nipissing sill province and Senneterre dykes, Canadian Shield, and implications for emplacement

The 2.21 Ga Nipissing diabase sills intrude the Huronian Supergroup of the Southern Province. Coeval Senneterre dykes in the adjacent portion of the Superior Province are potential feeders for the sills. Anisotropy of magnetic susceptibility (AMS) data were obtained from both of these units to study flow patterns and test emplacement models. The sills exhibit three distinct paleomagnetic directions that are likely primary and represent a time-correlation tool within the magmatic event. Two of the three directions are also found in Senneterre dykes. Sites in the northern Cobalt Embayment carry an N1 (reverse) paleomagnetic signature. They possess maximum AMS axes that trend north-northwest–south-southeast, interpreted as the direction of magma emplacement. In one area, the data rule out a cone sheet emplacement mechanism. The N3 paleomagnetic signature is restricted to a sill and (or) dyke-like bodies near Lake Timiskaming, where the AMS axes are dominantly north-northwest–south-southeast, similar to that of the nearby N1 (reverse) sites. Sites in a geographically restricted area farther south carry an N1 (normal) paleomagnetic direction. The AMS patterns in these sites exhibit very poor within- and between-site consistency, compatible with the magma having crystallized statically in this volumetrically minor magmatic pulse. In the Sudbury – Elliot Lake area, the Nipissing diabase is deformed, and outcrops exhibit east–west elongation parallel to the fold axes of the enclosing Huronian sedimentary rocks. AMS fabrics are interpreted as reflecting this deformation rather than primary magmatic flow. Three of eight Senneterre dyke sites show horizontal flow patterns, whereas the remainder have more complicated AMS patterns.

The magnetic anisotropy of mantle peridotites: Example from the Twin Sisters dunite, Washington

The Twin Sisters dunite massif, Washington State, provides unweathered peridotites for magnetic analysis. These rocks have been chosen to test a new magnetic fabric based on measuring the anisotropy of magnetic susceptibility (AMS) in high-field with a vibrating sample magnetometer. Although the dunites are fresh, they host small quantities of secondary magnetite formed along cracks. Under low-field conditions magnetite masks the magnetic fabric of olivine, and thus the magnetic fabric does not reflect mantle flow deformation. Above the magnetic saturation, at a field of 1 T, the ferrimagnetic component is removed and the high-field slope represents the paramagnetic AMS only. This high-field AMS originates from the magnetocrystalline anisotropy of olivine. The high-field AMS are compared with lattice preferred orientation (LPO) fabric data obtained by electron backscatter diffraction. LPO point distributions are formed by high-temperature plastic flow, and match the high-field measurements. In contrast, girdle LPOs present notable departures from the high-field AMS axes.

Archaeomagnetic studies in central Mexico—dating of Mesoamerican lime-plasters

For the first time results of an archaeomagnetic study of unburned lime-plasters from Teotihuacan and Tenochtitlan in central Mesoamerica are presented. Plasters made of lime, lithic clasts and water, appear during the Formative Period and were used for a variety of purposes in floors, sculptures, ceramics and supporting media for mural paintings in the Oaxaca and Maya area. In Central Mexico, grinded volcanic scoria rich in iron minerals is incorporated into the lime-plasters mixture. Samples were selected from two archaeological excavation projects in the Teopancazco residential compound of Teotihuacan and the large multi-stage structure of Templo Mayor in Tenochtitlan, where chronological information is available. The intensity of remanent magnetization (natural remanent magnetization (NRM)) and low-field susceptibility are weak reflecting low relative content of magnetic minerals. NRM directions are well grouped and alternating field demagnetization shows single or two-component magnetizations. Rockmagnetic experiments point to fine-grained titanomagnetites with pseudo-single domain behavior. Anisotropy of magnetic susceptibility (AMS) measurements document a depositional fabric, with normal to free-surface minimum AMS axes. Characteristic mean site directions were correlated to the paleosecular variation curve for Mesoamerica. Data from Templo Mayor reflect recent tilting of the structures. Teopancazco mean site declinations show good correspondence with the reference curve, in agreement with the radiocarbon dating. Dates for four stages of Teotihuacan occupancy based on the study of lime-plasters range from ad 350 to 550. A date for a possible Mazapa occupation around ad 850 or 950 is also suggested based on the archaeomagnetic correlation. The archaeomagnetic record of a plaster floor in Teopancazco differed from the other nearby sites pointing to a thermoremanent magnetization; comparison with the reference curve suggests dates around ad 1375 or 1415. The burning of the stucco floor likely occurred during a late re-occupation of the site by the Aztecs. Our results suggest that archaeomagnetic dating can be applied to lime-plasters, which are materials widely employed in Mesoamerica.

Statistical significance of magnetic fabric data in studies of paramagnetic granites

AbstractThe low anisotropies of paramagnetic granites, due to magnetocrystalline anisotropy, require a statistical treatment of the anisotropy of magnetic susceptibility (AMS) data when systematic fabric studies are performed. Absence of statistical information on these data makes evaluation of their quality difficult. The statistical significance of magnetic fabric in granites is evaluated in this paper. Jelinek’s elliptical confidence angles for the three principal susceptibility axes (E13, E12, E23) of a specimen are used as markers of the quality of the AMS data. Comparing these markers at sample-, site- and massif-scale with the mean AMS axes that result from spherical statistical models helps clarify the reliability of the AMS data. This analysis is presented in detail for the plutons of Veiga and Trives (Spain). It is then applied to seven other massifs from the Pyrenees. We propose the following guides: (1) fabrics with E13 between 10° and 20° tend to isotropy; the directional data and the shape parameter should be considered with great care; (2) lineation is not reliable when E12 &gt; 25°, i.e. when Kmax is almost the same as Kint; (3) similarly, foliation is considered as not reliable when E23 &gt; 25°, i.e. Kmin does not easily differentiate from Kint. Errors attached to the mean Kmax and Kmin axes should always be produced, thus allowing further interpretation. In Trives and Veiga, ‘perfect’ triaxiality cannot be automatically assumed since foliation and lineation could be defined simultaneously in only 53% of the cases. Finally, a minimum of three cores (9 specimens) per site would considerably increase the proportion of reliable orientation data.

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 magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite, Italy

The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions.

Theoretical models of intermediate and inverse AMS fabrics

Multi‐domain magnetites display a normal anisotropy of magnetic susceptibility (AMS) fabric where grain shape axes coincide with AMS axes. By contrast, single‐domain magnetite has an inverse magnetic fabric where magnetic axes are interchanged. The mixing of normal and inverse magnetic fabrics results in intermediate fabrics. Theoretical models for intermediate fabrics consider all combinations of normal and inverse fabrics. The minimum amount of inverse component required for intermediate fabrics to form is about 20% in the case of prolate normal (T = −0.50) and prolate inverse (T = −0.42) components. Such a small amount of inverse component may not be noticed. The anisotropy resulting from intermediate fabrics is lower than that of the normal or inverse contribution to AMS. This suggests that whenever intermediate fabrics occur neither the shape factor nor the degree of anisotropy relate to strain in a simple way.

Magnetic and structural fabrics of the melange in the Shimanto accretionary complex, Okinawa Island: Implication for strain history during decollement‐related deformation

The magnetic fabrics of the melange in the Shimanto accretionary complex of Okinawa Island were obtained from the anisotropy of magnetic susceptibility (AMS) data. The magnetic fabrics show a marked contrast between the structurally lower and upper parts of the melange; the lower part records deformation in the footwall of the decollement (D1), whereas the upper part documents that in the hanging wall of the decollement (D2). In the lower part of the melange the magnetic fabrics show good correlations with S1 layer‐parallel foliation, reflecting strain geometries under layer‐perpendicular flattening. As S‐C fabrics were locally developed by heterogeneous shear during D1, some of the previous magnetic fabrics were rotated with S1. In this case, the resultant magnetic fabrics reflect “S” surfaces, and maximum AMS axes are reoriented parallel to shear directions without clear change in shape and anisotropy degree of the AMS ellipsoids. Thus “S” surfaces did not act as flattening planes but may represent the deformation through independent particulate flow and mechanical rotation. The lower part of the melange was pervasively sheared during D2, reflecting that it laid immediately above the decollement. However, D1 magnetic fabrics were not modified because shear strain was accomplished by brittle microfaults without development of penetrative strain. In contrast, penetrative strain was significant in the upper part of the melange, resulting in the magnetic fabrics recording incremental strain associated with D2. Therefore deformation mechanism and proximity to a decollement play an important role on development of magnetic fabrics in a melange.

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.