Anisotropy Of Anhysteretic Remanent Magnetization 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.

Magnetic multi-fabrics as tools for understanding ignimbrite emplacement processes: An example from late-Variscan Tharandter Wald Caldera, Bohemian Massif

Felsic collapse calderas pose a hazard to our society, thus detailed knowledge of collapse mechanisms and behavior of eruptive products could help us to better understand the active calderas. We present a model of the eruption and collapse of Late Carboniferous Tharandter Wald Caldera (Variscan Bohemian Massif) based on field mapping and a magnetic multi-fabric approach. We investigate intra-caldera rhyolitic ignimbrites using a combination of conventional in-phase and relatively new out-of-phase anisotropy of magnetic susceptibility, along with the anisotropy of anhysteretic remanent magnetization. A two-stage caldera evolution model is proposed. (1) The high-energetic and high-temperature ignimbrites filled the paleo-valleys to form a thick ignimbrite sheet with welded to rheomorphosed central portion. Concurrently, eruptions emptied the source magma chamber triggering the piston caldera collapse. (2) The subsidence caused a monoclinal ductile bending of the still-hot and ignimbrite sheet around the caldera rim towards its center. Caldera activity was terminated by the emplacement of subvolcanic ring and radial dikes associated with hydrothermal activity. The latter likely induced abundant magneto-mineralogical alteration and formed composite fabrics. Hence, the application of the multi-fabric approach may serve as a useful tool to extrapolate reliable geodynamic proxies from the magnetic fabric of old, eroded, or altered caldera ignimbrites.

Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts

This paper presents a comprehensive analysis of the anisotropy of magnetic susceptibility (AMS) and paleomagnetic data from Quaternary basalt outcrops along the Hantangang River, Korea. A total of 554 samples were collected from 20 sites, representing three distinct units, Unit I, Unit II, and Unit III. Paleomagnetic data reveal a difference in the timing of eruptions between Units I and II, suggesting distinct periods by volcanic episodes. The mineral magnetic analysis identified titanomagnetite as the dominant magnetic carrier in the samples. AMS results indicated weak anisotropy and scattered AMS directions, indicating a low degree of preferred orientation of grains within the basalt rocks. The inverse AMS fabrics observed at specific sites are attributed to single-domain (SD) grains. Comparing the AMS data with the anisotropy of anhysteretic remanent magnetization (AARM) data, three distinct types of magnetic fabrics (normal, intermediate, and inverse) were discerned. The magnetic fabric was utilized to ascertain the flow direction based on the findings obtained from the AMS results. The findings suggest that the Quaternary basalts in this study’s area were primarily confined to the Hantangang River channel and its immediate vicinity during lava flow. However, distinct flow patterns are observed in the southwestern region, implying the presence of unknown volcanic sources.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Unraveling the emplacement history of a Portuguese post-tectonic Variscan pluton using magnetic fabrics and gravimetry

The Lamas de Olo pluton is a post-tectonic Variscan pluton located in northern Portugal. It is a composite pluton comprising three outcropping biotite granites, similar in mineralogical composition but with different textures and different biotite and magnetite contents: Lamas de Olo, Alto dos Cabeços, and Barragem. Two magnetic fabric techniques, anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanent magnetization, were used to test the coaxiality of magnetite and biotite fabrics. Comparing both magnetic tensors, the fabric of magnetite and paramagnetic minerals was found to be coaxial. The magmas ascended passively through opening fractures and were controlled by the intersection of ENE-WSW, NNW-SSE and NNE-SSW fault systems. The gravimetric study shows that the Lamas de Olo is an asymmetric, wedge-shaped pluton with a thickness of ca. 2.5–3.5 km. The kinematic model points to the presence of a feeder zone following an ENE-WSW direction, with two roots for Lamas de Olo and Alto dos Cabeços granites. Regarding Barragem granite, several conduits were identified along an NNE-SSW fault system, subparallel to the Verín-Régua-Penacova fault. Comparing the shape of three post-tectonic biotite plutons with same age and located along the same regional structure, the Verín-Régua-Penacova fault, we observed that the geometry is mainly conditioned by the nature of the host rocks and characteristics of the crustal structure. Wedge-shaped plutons are intruded into granites, and flat-shaped plutons are hosted in metasediments.

A Practical Approach for Identification of Magnetic Fabric Carriers in Rocks

AbstractMagnetic fabric analyses of rocks by anisotropy of magnetic susceptibility (AMS) are a robust petrofabric tool that has been used in varied geological environments and tectonic settings. A fundamental difficulty of this method is to define the dominant magnetic phases and their resulting geological interpretation. We study the magnetic behavior of rocks by simulating data of mixed magnetic phases (i.e., diamagnetic, paramagnetic, and ferromagnetic). We show that it is possible to recognize the dominant magnetic phases by measuring the mean susceptibility at room temperature (kmRT) and at low temperature (kmLT). Distinct regions of magnetic phase dominancy are demonstrated in kmLT/kmRT versus kmRT and kmLT versus kmRT plots. We present a comprehensive approach by coupling the magnetic phase dominancy with possible magnetic fabrics, which are obtained from AMS measured at room and low temperatures (RT‐AMS and LT‐AMS) and anisotropy of anhysteretic remanent magnetization, into a scenario table. Application of this table allows a robust procedure for determining which magnetic phases are dominant, and permits a fast and reliable geological interpretation in complex settings.

Magnetic anisotropy of an ancient volcanic system: Flow dynamics of post-collisional Ediacaran volcanism in southernmost Brazil

Knowledge about flow dynamics of volcanic sequences is fundamental for understanding their emplacement and consequently the evolution of the associated volcanic terrain. Despite this importance, studies that apply different approaches to ancient volcanic systems are still rare. In this paper, we study the case of silicic volcanic sequences in southernmost Brazil, contributing to the interpretation of the post-collisional Ediacaran volcanic settings of the Sul-riograndense shield. Rock magnetism analyses, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM), were performed on 32 sites of silicic volcanic rocks integrated with fieldwork observations. Magnetic mineralogy data indicate that magnetite or Ti-poor magnetite and high-coercivity phases (e.g., hematite) are the main magnetic carriers for the studied volcanic deposits. AARM results reveal an inverse magnetic fabric when single-domain grains are present, strongly affecting the interpretation of flow directions of lavas and ignimbritic deposits. AMS scalar results integrated with ignimbrite lithofacies analyses showed different fabric imbrication styles between stratified lower units and rheomorphic upper ignimbrites, allowing their separation in the emplacement model. Flow directions based on AMS, AARM data and field observations show a potential correlation of these volcanic deposits with an intrusive complex located on the southeastern border of the ignimbritic plateau. The emplacement of pyroclastic flow deposits was probably associated with a complex fissure system, where discontinuities within the basement plateau border may have served as feed conduits for these deposits. Our results highlight the importance of applying a regionally distributed AMS sampling coupled with a strong mineralogical and field control to the study of ancient volcanic systems.

Interpreting Inverse Magnetic Fabric in Miocene Dikes From Eastern Iceland

AbstractAnisotropy of Magnetic Susceptibility (AMS) is a valid tool to investigate magma flow direction within dikes. However, geometrically inverse magnetic fabric characterized by maximum magnetic susceptibility axis (kmax) perpendicular to the dike wall may complicate the interpretation of flow trajectories. To better understand the nature of this fabric, we present a multiscale study on 19 dikes (383 samples) in the Miocene Alftafjordur volcanic system (Iceland), where 80% of the samples show a geometrically inverse magnetic fabric. We carried out (1) AMS measurements at different magnetic fields and temperatures, along with Anisotropy of Anhysteretic Remanent Magnetization (AARM) analysis; (2) hysteresis loops and FORC diagrams; (3) thin section analysis; (4) structural fieldwork. A variable Ti‐content (0.1 < x < 0.6, Fe3‐xTixO4) titanomagnetite is the main magnetic carrier, and the contribution of the paramagnetic elongated crystals to the magnetic fabric is negligible. Single domain is not the prevailing domain state of the magnetic particles, suggesting that its occurrence cannot be the main cause for the inverse fabric. AMS analysis at different fields and temperatures along with AARM allow us to exclude any mineral phase change of the titanomagnetite across the dike. Nevertheless, kmax is parallel to a diffuse horizontal column‐like fracture pattern perpendicularly oriented with respect to the dike strike. This suggests that the Ti‐magnetite mineral orientation during dike cooling was affected by the fracture network progressively developing columnar basalts. This study demonstrates that the interpretation of AMS data on old and deep volcanic bodies is not straightforward and observations at different scales are required.

Consequences of switching field angular dependence for applications of anhysteretic remanent magnetization

The measurement of anhysteretic remanent magnetization (ARM) is an easy nondestructive means of evaluating many important rock properties. This involves applying an alternating magnetic field (AF) along with a direct biasing field (DF) to magnetize rock samples. Various techniques employing ARM have been widely used with notable success to help solve problems in tectonics, volcanology, sedimentology, environmental and paleoclimate studies and to better understand geomagnetic field behavior. Often, however, too little attention is paid to the effects of switching field angular dependence, which has important influence on the coercivity and angular distributions of grains that actually carry the ARM. As a result, commonplace methods of ARM measurement are not ideally designed for their intended purposes. For example, differentiation of a progressive ARM acquisition or turning on the direct field within a narrow AF window (i.e. partial ARM) are unable to isolate the ARM contribution from a specified coercivity grain fraction. Instead, the optimal method of measuring ARM in a targeted coercivity grain fraction is to differentiate a progressive tumble demagnetization of a total ARM generated with a peak AF high enough to fully activate the coercivity range of interest. Ignoring the activation property can lead to overestimation of relative paleointensity, underestimation of high coercivity grain concentrations, amplification of ARM anisotropy (e.g. error in natural remanence corrections), and unwanted mixing of anisotropy fabric components held in separate coercivity grain fractions.

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.

Anisotropy of Full and Partial Anhysteretic Remanence Across Different Rock Types: 1—Are Partial Anhysteretic Remanence Anisotropy Tensors Additive?

AbstractSeveral types or grain sizes of ferromagnetic minerals can contribute to a rock's remanence and anisotropy of remanence. Each subpopulation may have a different fabric. Measuring anisotropy of partial anhysteretic remanent magnetization (ApARM) allows one to determine the anisotropy contribution of subpopulations with different coercivity distributions. Separating these contributions to remanence anisotropy can provide information about early versus late stages of deformation in fabric studies and is the basis for improved anisotropy corrections in paleomagnetic studies. Unfortunately, collecting multiple ApARM tensors on each specimen is time‐consuming and not often done. Measuring a smaller number of carefully chosen ApARM tensors and obtaining the remaining tensors of interest by tensor calculation would be more efficient. This can only be done, however, when ApARM tensors are additive. Here we investigate the additivity of ApARM tensors in a range of lithologies, by measuring a total of seven ApARM and anisotropy of anhysteretic remanent magnetization (AARM) tensors for each specimen, and comparing the tensors calculated from a combination of ApARM tensors to the corresponding measured AARM. Differences in principal directions between measured and calculated tensors are often smaller than the confidence angles of the measurements. Mean anhysteretic remanences are additive to within ±5%. The anisotropy degree varies by ±30% (k′) or ±0.15 (P), and the shape parameter U by ±0.4. These error limits will help to determine whether or not it is necessary to measure each ApARM tensor in future fabric or paleomagnetic studies, or if these tensors can be calculated from a smaller set of measurements.

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.

New Late Pennsylvanian Paleomagnetic Results From Paraná Basin (Southern Brazil): Is the Recent Giant Gaussian Process Model Valid for the Kiaman Superchron?

AbstractThe most recent Giant Gaussian Process (GGP) model, based on the last 5 Ma, has been used as a reference for directional distribution of paleomagnetic record of older rocks as Cenozoic and Proterozoic. However, for Paleozoic times, its validity has not yet been tested. Here we evaluate the validity of this recent GGP model for the Kiaman superchron. We present new paleomagnetic results from a late Pennsylvanian section of glacial rhythmites (Mafra Formation) from southern Brazil. The 5‐m section sampled spans more than 800 kyr, as evaluated by cyclostratigraphic analysis. Thermal demagnetization revealed a reversed characteristic component carried by single domain magnetite. Anisotropy of anhysteretic remanent magnetization indicated a small shallowing correction of f = 0.97. The final paleomagnetic pole position is located at 51.9°S, 344.3°E (N = 111, R = 109.0, K = 55.9, A95 = 1.8°), with a mean direction of Dec = 144.2°, Inc = 69.5° (N = 111, R = 110.2, k = 134.4, α95 = 1.2°, Paleolat = 53.2°S). The shape of the distribution of magnetization directions (elongation E = and the dispersion of virtual geomagnetic poles ( are incompatible with the recent model. The reduced dispersion, also found in other studies, implies a different shape in directional distributions for any GGP model describing the Kiaman interval. This result alerts us that we should abandon the use of the recent GGP model as a reference for inclination shallowing correction of Carboniferous sedimentary data.

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.

Mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust: The Sondalo magmatic system (N Italy)

Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explore the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a mid-crustal gabbroic to dioritic magmatic system (Sondalo complex, Eastern Central Alps, N Italy). We characterize the structure and Anisotropy of Magnetic Susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: the fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host-rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton resulting in the formation of a structural aureole, i.e. a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the P−T evolution recorded by metasedimentary rocks in the contact aureole of the pluton.

The effect of speleothem surface slope on the remanent magnetic inclination

AbstractSpeleothems are of interest for high‐resolution reconstruction of the Earth's magnetic field. However, little is known about the influence of speleothem morphologies on their natural remanent magnetization (NRM) record. Here we report on a high‐resolution paleomagnetic study of a dome‐shaped speleothem of middle Holocene age from southern Portugal, with special attention to the anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). To assess the potential influence of the slope of the speleothem surface on the recorded remanent magnetization, we compare magnetic directions and AMS and AARM fabrics from subhorizontal to gradually subvertical calcite growth layers collected in a transversal cross section of the speleothem. A linear correlation is observed between magnetic inclinations, calcite laminae slope, and AARM k1 inclination. The AMS fabric is mostly controlled by calcite crystals, with direction of the minimum axes (k3) perpendicular to laminae growth. Magnetic inclinations recorded in inclined and vertical calcite growth layers are underestimated when compared to a global paleosecular variation (PSV) model. After extrapolating magnetic inclinations to the horizontal, the corrected data better fit the PSV model but are still lower than the predicted magnetic inclinations, suggesting that inclination shallowing affects the entire speleothem. We suggest that speleothem morphology exerts a critical role on the magnetic inclination recording, which is controlled by the Earth's magnetic field but also influenced by particle rolling along the sloping surfaces. These observations open new avenues for reconstructing high‐resolution paleomagnetic secular variation records from speleothems and provide new insights into their NRM acquisition mechanisms.

Controls and implications of anisotropy across a strain gradient within granodiorite, Serifos, Greece

Microtextural analyses including electron backscatter diffraction (EBSD), anisotropy of magnetic susceptibility (AMS) and the anisotropy of anhysteretic remanent magnetization (AARM) have been applied across a shear zone within a Miocene granodioritic intrusion on Serifos island, Western Cyclades, Greece. The shear zone records progressive deformation through a ductile to brittle transition. One of the consequences of anisotropic crust is the variation in seismic wave velocity with the direction of propagation, which is largely controlled by the crystallographic preferred orientation (CPO) of anisotropic minerals such as micas and feldspars. CPO data was collected using EBSD and seismic properties were calculated using Voigt-Reuss-Hill averaging of the single minerals' elastic stiffness tensor. The magnetic fabric is used to characterize weakly defined tectonic fabric, thus complementing the CPO data. The structure of rocks exhibiting weak magmatic fabric, as defined by magnetic anisotropy and CPO, appears to reflect the elliptical shape of the pluton. The SW-directed kinematics of the Western Cyclades produced an ENE-striking, SSE dipping low-angle shear zone in the granodiorite that imparted a signature onto the magnetic data of foliated and mylonitic samples, creating a composite fabric from the magmatic emplacement fabric and the top-to-SW directed shear. As for the seismic properties, P-wave anisotropy reaches 11.8% and is consistently controlled by the CPO of plagioclase feldspar, with the direction of fast P-wave propagation subperpendicular to foliation and the direction of slow P-wave propagation subparallel to foliation. The magnetic fabric, coupled with the tectonic fabric inferred from CPO data, gives unique insight into the character of anisotropic minerals in a syntectonic intrusion. This is especially useful for weakly strained rocks where the lineation and foliation (given by the magnetic fabric) can be related to the seismic anisotropy derived from the CPO data. Given the anisotropy caused by the preferred orientation fabrics of major rock-forming minerals, the microstructural analysis of anisotropic minerals is fundamental for incorporating seismic anisotropy into large-scale geophysical models.

Cenozoic magnetostratigraphy and magnetic properties of the southern Issyk-Kul basin, Kyrgyzstan

We present paleomagnetic data from the northern flank of the Tianshan range, southeast of Lake Issyk-Kul (Kyrgyzstan). 613 cores were collected in two parallel sections with a total thickness of 960m (Chon Kyzylsuu, CK) and 990m (Jeti Oguz, JO), as well as 48 cores at six sites in a nearby anticline. Rock magnetic analyses identify both magnetite and hematite in the fluvial–lacustrine sediments. The concentration of both minerals, the magnetite:hematite ratio, and the average magnetite grain size increase upward in both sections. Anisotropy of anhysteretic remanent magnetization defines a tectonic fabric with sub-horizontal maximum axes that parallel the strike direction together with intermediate and minimum axes that streak out about a great circle orthogonal to the maximum axes suggestive of a tectonic fabric emplaced during folding. Stepwise thermal demagnetization isolates interpretable magnetization components in 284 samples that define 26 polarity chrons in CK and 19 in JO. A positive fold test, dual polarities and systematic changes in rock-magnetic parameters with depth suggest that the high temperature magnetization component was acquired coevally with deposition. An age model based on a visual magnetostratigraphic correlation of both sections with the geomagnetic polarity time scale defines absolute ages from 26.0 to 13.3Ma, with a fairly constant sedimentation rate of 9–10cm/ka. A correlation based on a numerical algorithm arrives at a slightly different conclusion, with deposition ages from 25.2 to 11.0Ma and sedimentation rates from 5 to 8cm/ka. In comparison with sedimentation rates found at other magnetostratigraphic sections in the Tianshan realm, we infer that the sedimentary record in this part of the Issyk-Kul Basin precedes the more rapid phase of uplift of the Kyrgyz Tianshan. The onset of deposition and concomitant erosion of the adjacent Terskey Range is in good agreement with independent assessments of the exhumation history of this mountain range, with erosion increasing at 25–20Ma and accelerating after 11–13Ma.

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.