Anisotropy Of Low-field Magnetic Susceptibility Research Articles

Emplacement of the paleoproterozoic Itareru tonalite (NE São Francisco Craton, Brazil) during arc-continent collision revealed by magnetic fabrics

The Itareru tonalite is one of the Paleoproterozoic plutonic rock units that intruded the Serrinha block, an Archean-Paleoproterozoic crustal fragment of the São Francisco Craton (Bahia, Brazil). It is characterized by a ca. 80 km long, NW-SE elongated leaf-shaped body, mainly comprised of tonalite-diorite to granodiorite rocks with numerous dioritic enclaves. Magnetic studies were performed on 26 sites of the Itareru pluton (including country rocks). Anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) are coaxial, indicating that all minerals have the same orientation, partially controlled by regional strain. Rock-magnetism measurements reveal that paramagnetic–matrix minerals contributed significantly to the magnetic susceptibility and AMS while magnetite grains in the range of 2–5 μm are responsible for AARM. Magnetic fabrics in the central region of the pluton are probably primary (magmatic), which was acquired when the rocks were crystallized, reflecting magma flow linked to a magma feeder zone (magma chamber). The moderated magnetic lineation plunges in the northern part of the pluton suggest that this region was close to the feeder zone. In contrast, the southern region should be far from it, since the magnetic lineations are horizontal to sub-horizontal. The Itareru tonalite emplacement was also controlled by regional strains related to the collision of the Paleoproterozoic Rio Itapicuru greenstone Belt with the Archean basement.

On the origin of apparently negative minimum susceptibility of hematite single crystals calculated from low-field anisotropy of magnetic susceptibility

SUMMARY As shown in the literature several times, the calculation of the anisotropy of magnetic susceptibility (AMS) of hematite single crystals using standard linear AMS theory reveals that the calculated minimum principal susceptibility is parallel to the crystallographic c-axis, but is negative, which is however not due to diamagnetism as evidenced by direct measurements of susceptibility along the principal directions. Susceptibility of a few hematite single crystals from Minas Gerais, Brazil, was measured in 320 directions using a special 3-D rotator and the measurements were processed through AMS calculation by means of standard linear theory and through constructing contour diagrams in equal-area projection. In addition, the susceptibility was in detail measured in three perpendicular planes approximately passing through the main crystallographic directions. The crystals show extremely high anisotropy, the susceptibility measured along the basal plane is several hundred times higher than that along the c-axis and the AMS ellipsoids are very oblate. The contour diagrams show relatively simple patterns of directional susceptibilities, similar to those of the second-rank tensor. Fitting tensor to measured hematite grains results in small but negative minimum principal susceptibility. Nevertheless, summation of many oriented grains filters out the non-tensorial parts of the grain susceptibility in multicrystal assemblages, the AMS of which is well represented by a tensor.

Can magnetic fabric indicate the direction of a glacier movement? An example from Itararé Group and Aquidauana Formation, Paraná Basin, Brazil

Magnetic fabrics was determined on Late Paleozoic sedimentary rocks from Aquidauna Formation (50 sites) and Itararé Group (71 sites) that outcrop in Paraná Basin, applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AARM). Rock magnetism analyses reveal that magnetite, hematite, and paramagnetic matrix minerals are responsible for the magnetic susceptibility and AMS, while the magnetite is the carrier AARM. The AARM tensor is coaxial with AMS fabric. The analysis at the individual-site scale defines two AMS fabric types (A and B). The type A shows vertical AMS foliation pole (Kmin perpendicular to the bedding plane), while Kmax and Kint are scattered within the bedding plane itself. This fabric is usually interpreted as sedimentary-compactional. Type B shows the three well-clustered AMS axes with Kmin off-vertical, this fabric is the most important since it was found in the majority of the sites. This fabric is interpreted as indicative of sediment transport that could be inferred by the imbrication of AMS foliation (Kmin). The comparison of magnetic lineation (Kmax) for both Aquidauana Formation and Itararé Group with their respective glacial striae allow inferring that Kmax can be used as an indicator of glacier movement that was from SE towards NW, even though some variation or changes in the movement orientation could happen as found in the Itararé Group (from São Paulo and Paraná states), probably related to glacier advance and retreat or different glacier lobes.

Emplacement of dike swarms from the island of Ilhabela (SE Brazil) and its relationship with the South Atlantic Ocean opening revealed by magnetic fabrics

Magnetic fabric and rock-magnetism studies were performed on 153 dikes from diabase, lamprophyre and alkaline dike swarms that outcrop on the island of Ilhabela, NE São Paulo State. The dikes crosscut Archean and Proterozoic polymetamorphic rocks. Their thicknesses range from a few centimeters to 4 or 5 m. They trend predominantly N30-60E with a very steep (~60°) to vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2–5 μm. For a few dikes, these grains are the carriers of bulk magnetic susceptibility but are not responsible for the AMS, which is due to Fe-bearing minerals, as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax–Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the minority of the dikes, the AMS and AARM tensors are not coaxial. The AARMmax is oriented N30–60 W, approximately perpendicular to the Kmax. The AARMmax orientation is similar to the direction of a fault system located mainly in the Santos marginal basin which, was formed in the Cretaceous rifting during the South Atlantic opening. The AARM fabric is tectonic in origin, and is parallel with the extensional paleostress responsible for the Gondwana break-up. The comparison of AMS and AARM fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening. Later, the alkaline dikes were emplaced in the final stages of the Atlantic opening.

Emplacement of the Anta Granite from Rio de Janeiro State (SE, Brazil); determined by magnetic fabrics

Magnetic fabric and rock magnetism studies were performed on the three facies of the Anta granite (SE of Rio de Janeiro State, southern Brazil). This intrusion is roughly elliptical (∼ 14 km2), NE-SW oriented and composed of granodiorite, monzodiorite, quartz-diorite, syenogranite, and monzogranite. It shows solid-state deformation and tectonic foliation at its borders and is apparently isotropic in the central part where the internal fabrics are impossible or very difficult to measure through classical techniques. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The AMS and AARM fabrics are coaxial for all facies. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect in the AMS fabric. Several rock-magnetism experiments performed on one specimen from each facies show that, for all units, magnetic susceptibility and fabrics are carried by magnetite grains. Most of the magnetic foliations are steeply dipping in all facies, and are roughly parallel or sub-parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for almost the whole pluton, except for one site where it is steep (> 60°). Thin section analysis show that rocks from the borders of the Anta pluton are solid-state deformed, indicating that the borders of the pluton were affected by the regional strain during its emplacement. On the other hand, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the central part of the pluton to be determined, where igneous textures are preserved. This evidence allows us to interpret the magnetic fabrics observed in this part of the pluton as primary in origin (magmatic) and acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlain this area. Our data suggest that the regional deformation partially affected the emplacement of the Anta granite.

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.

An AMS study of different silicic units from the southern Paraná-Etendeka Magmatic Province in Brazil: Implications for the identification of flow directions and local sources

In the Southern portion of the Paraná-Etendeka Magmatic Province in Brazil, extensive silicic (dacite-rhyolite) deposits occur at the top of a sequence of low-Ti pahoehoe to rubbly basalts. The internal architecture of the silicic deposits and their eruptive style, as well as the location of their sources are still unsatisfactorily known. In an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive, and test the hypothesis of local sources, we carried out anisotropy of magnetic susceptibility (AMS) studies on the two main silicic units (Caxias do Sul dacites and Santa Maria Rhyolites) with the best exposures in an area previously mapped in detail.Magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites. Rock magnetism properties indicate that “pseudo-single-domain” magnetite carries the fabrics and the remanence. Magnetic fabrics were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Both AMS and AARM tensors are coaxial, indicating that the AMS fabric is not affected by the effect of magnetite single-domain grains.Magnetic data from several dacitic coulées (Caxias do Sul unit) indicate flows from SE to NW. The location and spatial distribution of these lavas support the hypothesis of local sources, aligned along a NE-SW trend. These data are in agreement with the alignments of structures (dome-shaped hills) observed in field work and DEM images. On the other hand, magnetic data obtained in Santa Maria rhyolites indicate that flow directions in two different areas are distinct (towards NW/NE and W), suggesting that they derived from different emission centers.So, regarding the silicic volcanism in the studied region, our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit. In contrast, we propose the occurrence of local volcanic events, implying in the existence of different sources, possibly characterized by local emission centers.

Magnetic fabrics of the Cretaceous dike swarms from São Paulo coastline (SE Brazil): Its relationship with South Atlantic Ocean opening

Magnetic fabric and rock magnetism studies were performed on 91 dikes from Cretaceous diabase and lamprophyre dike swarms that outcrop side by side on the beaches of NE São Paulo State coastline. The dikes crosscut Archean and Proterozoic poly-metamorphic rocks of the Costeiro Complex. Their thicknesses range from a few centimeters to 2m for lamprophyre and up to about 10m for the diabase. They trend predominantly N30°–60°E with vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that magnetite grains in the range of 2–5μm are the magnetic mineral of both swarms. For most dikes, these grains are the carriers of bulk magnetic susceptibility but, surprisingly, are not responsible for the AMS which is carried by Fe-bearing minerals as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax-Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the majority of the dikes the AMS and AARM tensors are not coaxial. The AARM lineation (AARMmax) is oriented N30–60W, approximately perpendicular to AMS lineation (Kmax) suggesting that magnetite grains were rotated approximately 90° anticlockwise from the dike plane. The AARMmax orientation is similar to the direction of a fault system mainly in the Santos marginal basin which was formed in the Cretaceous rifting during the South Atlantic opening. Therefore the AARM fabric is tectonic in origin, and the comparison of AMS and AARM fabrics suggests that lamprophyre and diabase dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening.

Magnetic fabric (AMS, AAR) of the Santa Marta batholith (northern Colombia) and the shear deformation along the Caribbean Plate margin

Anisotropy of low-field magnetic susceptibility (AMS) and anhysteretic remanence (AAR) were measured in the Santa Marta Batholith formed by subduction of the Caribbean Plate beneath the northern South America. The batholith, elongated in the N-S direction, records multiple pulses of quartzdiorite to tonalite and granodiorite magmas between 58 and 49 Ma. The high mean magnetic susceptibility (4 × 10−3 SI) combined with thermomagnetic and partial magnetic remanence measurements indicate that the magnetic susceptibility depends on Ti-poor magnetite. AMS is defined by ellipsoids that are dominantly oblate. The foliation was used to distinguish a narrow band of E-trending magnetic structures that separate the batholith in two lobes. The southern lobe is characterized by foliations that are broadly parallel to the contact with the wall rocks, while the northern lobe by foliations oblique to the batholith elongation. Late tonalitic magmas dated at c. 50 Ma record, in turn, a fabric apparently controlled by E-trending tectonic events. Partial AAR indicates that the subfabrics of magnetite with different grain sizes are nearly parallel to AMS, therefore discarding the possibility of superposed fabrics with different orientations. The magnetic fabric pattern is consistent with a magma emplaced in an arc setting deformed by a dextral shear. Synthetic extensional shear bands localize the magmatic deformation along East-trending corridors that probably were exploited to emplace the late magmatic pulses. Accretion of the Eocene batholith and the Late Cretaceous metasedimentary host-rocks to the South American continent defines a major strike-slip shear suture that resulted from the oblique convergence of the Caribbean Plate.

Paleoflow directions of a subaqueous lahar deposit around the Miocene Keserűs Hill lava dome complex (North Hungary) as constrained by photo-statistics and anisotropy of magnetic susceptibility (AMS)

A twofold fabric analysis by using photo-statistics on rock surfaces and low-field anisotropy of magnetic susceptibility (AMS) is presented, making it possible to infer paleoflow directions, which in turn helps to constrain the primary volcanic geomorphology of a deeply eroded mid-Miocene field (Keserűs Hill lava dome group, Visegrád Mountains, North Hungary). The analyses were carried out on the Rám Hill Pumiceous Sandstone (RHPS) that, based on its petrographic features, is considered as a subaqueously emplaced laharic deposit.The observed anisotropic frequencies of clast a-axis azimuths, the good agreement of fabric directions obtained for all and most elongated samples (elongation>2.5), the good clustering of the declinations of the K1 susceptibilities largely corresponding with the result of image analysis, allow to infer reliable flow directions. The obtained large-scale paleoflow paths show a quasi-radial pattern around the central part of the Visegrád Mountains, which quantitatively confirms the previous hypothesis on the volcanic structure, namely a central edifice-topped syn-eruptive topography.The RHPS is characterized by a clast fabric (revealed by photostatistics on cutted rock surfaces considering the lapilli-sized fraction) as strong as in subaerial PDC deposits, but by a significantly weaker magnetic fabric. The weak magnetic fabric is supposed to be the result of rock heterogeneity, thus the weakening effect of abundant lithic clasts with strong internal magnetic susceptibility anisotropy, different from the shape anisotropy of the clasts and the flow-related magnetic fabric of the fine-grained matrix.

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

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

Applicability of magnetic fabrics in rocks associated with the emplacement of salt structures (the Bicorb–Quesa and Navarrés salt walls, Prebetics, SE Spain)

This work assesses the applicability of the analysis of the anisotropy of magnetic susceptibility (AMS) to rocks associated with salt structures. Magnetic fabrics from 16 sites sampled on Upper Triassic clays (Keuper facies) interbedded within the salt layers of the Bicorb–Quesa and Navarrés salt walls (Prebetic Zone, SE Spain) are mostly characterized by oblate magnetic ellipsoids with their magnetic foliation parallel to the bedding and magnetic lineation contained in the bedding plane but without a constant relationship with dip direction. To evaluate the quality and significance of the low-field AMS, several experiments were carried out to characterize the magnetic mineralogy of representative samples and enable separation of the contributions from paramagnetic and ferromagnetic (s.l.) minerals. The comparison between the orientation of the observed magnetic lineation and that of the structural elements characterizing the internal geometry of the studied salt structures (antiforms, minor folds, faults and shear zones) and flanks indicates that the magnetic fabric is controlled by deformation processes affecting the salt walls during their growth and/or evolution. Samples were also analyzed from sixteen sites from Miocene rocks outcropping in several syn-diapiric Miocene half-grabens related to the salt walls studied. Their magnetic fabric is interpreted to reflect the dominant stretching direction at the syn-diapiric half grabens during and/or shortly after deposition. Our results indicate that caution is required in interpreting magnetic lineations related to salt flow mechanics from rocks associated with salt structures.

Tectonic and sedimentary evolution of the late Miocene–Pleistocene Dali Basin in the southeast margin of the Tibetan Plateau: Evidences from anisotropy of magnetic susceptibility and rock magnetic data

The Cenozoic Dali Basin, located at the northeast of Diancang Shan and south of the first bend of Yangtze River, is tectonically controlled by the Dali fault system in the southeast margin of the Tibetan Plateau. The basin is filled with late Miocene to Pleistocene fluviolacustrine sediments, which provide invaluable information about the tectonic deformation and drainage network reorganization in this area. In this study, we discuss the anisotropy of magnetic susceptibility (AMS) of the Dasongping section in the Dali Basin, which spans an interval from 7.6 to 1.8Ma. Although rock magnetic experiments indicate that magnetite and hematite are the main remanence carriers, hysteresis loops, low values of bulk susceptibility and low temperature susceptibility suggest that paramagnetic minerals are major contributors to low-field AMS. The rock magnetic parameters indicate that the Dali Basin experienced four stages of infilling and the sediment sources may have changed at 4.2Ma. The clustering of the minimum principle axes (Kmin) nearly perpendicular to the bedding plane and the pronounced N–S magnetic lineation parallel to the bedding plane indicate that the AMS of the Dali Basin is a superimposed fabric consisting of a sedimentary-compaction and a mild initial deformation overprint. The well-defined N–S magnetic lineation is likely due to the NNW and NNE oblique shear caused by the Heqing and Red River faults.

Understanding the Mesozoic kinematic evolution in the Cameros basin (Iberian Range, NE Spain) from magnetic subfabrics and mesostructures

Analysis of anisotropy of magnetic susceptibility (AMS) and brittle mesostructures (hydroplastic synsedimentary faults and tension gashes) is applied in this study in order to characterize the Mesozoic tectonic events in the Cameros basin (NW Iberian Range), formed between Tithonian and Albian times. Low-field AMS at room and low temperature (LF-AMS at RT and LF-AMS at LT, respectively) together with high-field AMS (HF-AMS) measurements allow separating ferro- and paramagnetic fabrics. The combination of LF-AMS at LT and HF-AMS torque measurements confirms the reliability of both procedures in terms of isolating the paramagnetic contribution to the AMS. Magnetic fabric results combined with the analyses of synsedimentary faults indicate a NW–SE extension direction during Aptian (and probably Barremian) times. This extension direction is perpendicular to the main extension direction (NE–SW) prevailing during early and late stages of basin evolution. It is also consistent with extension direction deduced from large-scale bending folds and tension gashes, developed after partial lithification. Cleavage development during Albian enhanced the orientation of the magnetic fabric in lithologies where the previous extensional magnetic lineation is coaxial with the expected one for compression.

Magnetic study of a late Alpine dike crosscutting the regional foliation

Dikes crosscutting existing folds or foliation are one of the classic time markers used in establishing relative geological chronology. We examine the paleomagnetic directions and magnetic fabric in a deformed gneiss from the Southern Steep Belt, which lies immediately north of the Insubric fault, and in an aplite dike that transects this foliation. No clear foliation or other evidence of internal deformation is discernible in the aplite dike in the field and it was originally interpreted as post-tectonic, with its intrusion age placing a young limit on the timing of deformation in the adjacent gneiss. The aplite dike does not carry a stable remanent magnetization and cannot be used to resolve the age of intrusion. The characteristic paleomagnetic remanence in the gneiss is compatible with Eurasian paleomagnetic directions during the last 20Ma. These results suggest that the timing of magnetization postdates dextral motion along the Insubric and Rhine–Rhone faults. The low-field anisotropy of magnetic susceptibility (AMS) in the gneiss is controlled by the regional schistosity. High-field AMS suggests that both paramagnetic and ferromagnetic minerals contribute to the magnetic fabric, whereby the ferromagnetic minerals show a stronger lineation. This is further confirmed by the anisotropy of anhysteretic remanent magnetization (AARM). Rock magnetic experiments show that multi-domain magnetite is the main ferromagnetic mineral in the gneiss. Unexpectedly, the AMS results from the aplite dike show that it also has a magnetic fabric, which is shown by high-field AMS, AARM and rock magnetic experiments to be carried by superparamagnetic magnetite. The presence of magnetic fabric in the dike demonstrates that regional deformation was still on-going in the Southern Steep Belt of the Central Alps at the time of its intrusion.

Deformation in rocks from Itajaí basin, Southern Brazil, revealed by magnetic fabrics

Magnetic measurements were performed on sedimentary and volcanic rocks from the Itajaí basin. Rock magnetism analyses reveal that magnetite and hematite and paramagnetic minerals show different contributions to magnetic susceptibility. Magnetic fabrics were determined in 19 sites by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AARM). The analysis at the individual-site scale defines three AMS fabric types. The first type shows Kmin perpendicular to the bedding plane, while Kmax and Kint are scattered within the bedding plane itself. This fabric is usually interpreted as primary. The second type (77% of the sites) shows the three well-clustered AMS axes with Kmin still perpendicular to the bedding plane. The third type is characterized by well-clustered Kmax in the bedding plane, while Kmin and Kint are distributed along a girdle perpendicular to Kmax. The second and third fabric types are interpreted as combinations of sedimentary-compactional and tectonic contributions at the earliest, and at a slightly later stage of deformation, respectively. AMS represents the contribution of all the rock-forming minerals, while AARM isolates the contribution of remanence-bearing minerals with coercivity <100mT. For some sites, the AMS and AARM fabrics are non-coaxial, mainly with respect to the lineation orientations. The comparison of magnetic lineations and the regional structural data indicates that the rocks, with no mesoscopic or macroscopic deformation, were affected by the two deformation phases that occurred in the region. One of them results from compressional tectonics related to the collisional orogen (D1), and the other is extensional related to post-collisional orogen (D2). After the former tectonic event the volcanic rocks were emplaced at the top of the stratigraphic sequence and, after that the Itajaí basin as a whole was deformed by the extensional tectonic (D2) that occurred after 560Ma which is the age of the intrusion.

Extrusion tectonics inferred from fabric study of the Guanzizhen ophiolitic mélange belt in the West Qinling orogen, Central China

The recently identified Guanzizhen ophiolitic mélange belt (GOMB) in the West Qinling orogen is regarded, by many geochemical and geochronological works, as the westward extension of the Shangdan belt in the East Qinling that separated the North and South China blocks. The GOMB trends generally NW–SE and comprises ophiolites and forearc volcanic–sedimentary complex (the Liziyuan Group). Ductile shear zones with various widths are a striking structural feature of the GOMB and studying them could contribute to a better understanding of the tectonic processes of the collisional orogeny in this area. Here we characterize the deformation style of these shear zones by combining the low-field anisotropy of magnetic susceptibility (AMS) data, microstructural and field observations. The new dataset demonstrates that the deformation of these ductile shear zones is dominated by dextral transpressional shearing under low amphibolite grade condition. Combining the dextral transpression of the GOMB and the coeval sinistral transpression of the Shangdan belt in the East Qinling, we propose a Late Triassic bilateral extrusion tectonics of the Qinling orogen in response to the collision between the Ordos and Sichuan blocks.

Magnetic interactions at the origin of abnormal magnetic fabrics in lava flows: a case study from Kerguelen flood basalts

Anisotropy of low-field magnetic susceptibility (AMS) of basaltic lava flows can give some clues about post-emplacement tilting occurring in volcanic sequences. Such a study has been carried out on a sequence of 19 successive lava flows from Kerguelen Archipelago. Surprisingly, two different patterns were observed. The first one—herein called normal fabric—is consistent with the lava-flow direction inferred from geological observations, whereas the second one observed for about 70 per cent of the analysed samples—herein called abnormal fabric—appears to be unrelated to the shear history of the lava flows during their emplacements. These abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin. The unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high-resolution electron backscattered diffraction measurements. The aim of this study is to propose a physical interpretation of these fabrics. A first step in our understanding was to clearly identify the nature and the magnetic properties of the Fe-Ti oxides assemblage. Thus, we present a comprehensive rock magnetic analysis relying on low- and high-temperature thermomagnetic curves (K–T), crossover diagrams, first-order reversal curves, ore-microscopy, Mössbauer measurements and electron backscattered images. We found that in the present case the abnormal fabrics are probably linked to a mixture of prevailing single domain (SD) strongly interacting grains population and a subsidiary multidomain grains population. SD grains are identified as small magnetite crystallized within the interstitial glass in globular aggregates along the silicate framework. Then a question arises: to what extent such SD interacting grains in globular aggregates can affect the magnetic susceptibility and its anisotropy? To answer this question, we developed a physical model in which the total energy density of an SD grain with an uniaxial anisotropy is minimized. In this model, the distribution function of the direction of the magnetic interaction field is comprised between two boundary states. It is either isotropic, which formally corresponds to the thermal demagnetized state, or it is ordered, which formally corresponds to the Alternative Field demagnetized state. We show that in both cases the degree of anisotropy decreases as the interaction field increases. Thus, we conclude that the abnormal fabric encountered in this study can be simply explained by strong magnetostatic interactions that would explain the large scattering of AMS often observed in basaltic lava flows.

Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton, SE Brazil

Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of Sao Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km2), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.

Using AMS combined with mineral shape preferred orientation analysis to understand the emplacement fabrics of the Apiaí gabbro-norite (Ribeira Belt, SE Brazil)

The Apiaigabbro-norite is a massive fine- grained Neoproterozoic intrusion emplaced in a core of synformal structure that deforms low-grade marine metasedimentary rocks of the Ribeira Belt of south-eastern Brazil. The lack of visible magmatic layering or any internal fabric has been a major limitation in deciding whether the emplacement occurred before or after the regional folding. To assist in the tectonic interpretations, we combine low-field anisotropy of magnetic susceptibility (AMS) and silicate shape preferred orientation (SPO) to reveal the internal structure of the mafic intrusion. Mag- netic data indicate a mean susceptibility of about 10 -2 SI and a mean anisotropy degree (P) of about 1.08, essentially yielded by titanomagnetite. The magnetic and silicate foliations for P C 1.10 are parallel to each other, while the lineations tend to scatter on the foliation plane, in agree- ment with the dominant oblate symmetry of the AMS and SPO ellipsoids. For lower P values, the magnetic and silicate fabrics vary from coaxial to oblique, and for P B 1.05, their shapes and orientations can be quite distinct. The crystal size distribution (CSD) of plagioclase for P ( 1.05 is log linear, in agreement with a bulk simple crystallisation history. These results combined show that for a strong SPO, corresponding to a magnetic anisotropy above 1.10, AMS is a reliable indicator of the magmatic fabric. They indicate that the Apiaigabbro-norite consists of sill-like body that was inclined gently to the north by the regional folding.