Anisotropy Of Magnetic Susceptibility Research Articles

Anisotropy of Magnetic Susceptibility (AMS) of Bostonite Rocks, Um-Gir and El-Atshan-II Uranium Prospect Areas, Central Eastern Desert, Egypt

Um-Gir and El-Atshan-II areas represent important structurally-controlled radioactive anomalies and have become promising areas for U exploration. The bostonite rocks represent the most promising rock unit in the two studied areas. They form the most promising rocks due to their high radioactivity and, hence uranium mineralization. Anisotropy of magnetic susceptibility (AMS) measurements was carried out for Um-Gir and El-Atshan-II bostonite rocks. The AMS results show that the mean magnitude of magnetic susceptibilities is low, of the order of 10-5 S.I. units. Consequently, the bostonite rocks of the two areas are considered as non-magnetic, reflecting the presence of hematite as the main ferromagnetic mineral. These results are in agreement with the measurements of initial remnant magnetization (NRM) and the results of isothermal remnant magnetization (IRM) for the two areas. Besides, the results of magnetic anisotropy degree (PJ), refer to and concluded that the magnetic fabric of bostonite of two the areas has secondary fabric. AMS results clearly show the differences between the bostonite dyke of Um-Gir and Bostonite sill of El Atshan-II. The lineation (P1) > foliation (P3), and the negative values of the ellipsoid shape (T) of Um-Gir bostonite reflect clearly magnetic lineation (prolate ellipsoid). However, in El Atshan-II, (P3 > P1), the values of T are positive, which indicate the predominance of magnetic foliation (oblate ellipsoid) over lineation. Furthermore, the foliation planes of the bostonite dyke of Um-Gir possess a shallow to moderate inclination and a nearly N-S direction, while, the foliation planes of the bostonite sill of El-Atshan-II show a shallow to moderate inclination and a NNE direction. Finally, the AMS method is a powerful tool for detecting the structures that affect the bostonite rocks of the two studied areas.

Magnetic susceptibility anisotropy in normal appearing white matter in multiple sclerosis from single-orientation acquisition

Quantitative susceptibility mapping (QSM) has been successfully applied to study changes in deep grey matter nuclei as well as in lesional tissue, but its application to white matter has been complicated by the observed orientation dependence of gradient echo signal. The anisotropic susceptibility tensor is thought to be at the origin of this orientation dependence, and magnetic susceptibility anisotropy (MSA) derived from this tensor has been proposed as a marker of the state and integrity of the myelin sheath and may therefore be of particular interest for the study of demyelinating pathologies such as multiple sclerosis (MS). Reconstruction of the susceptibility tensor, however, requires repeated measurements with multiple head orientations, rendering the approach impractical for clinical applications.In this study, we combined single-orientation QSM with fibre orientation information to assess apparent MSA in three white matter tracts, i.e., optic radiation (OR), splenium of the corpus callosum (SCC), and superior longitudinal fascicle (SLF), in two cohorts of 64 healthy controls and 89 MS patients. The apparent MSA showed a significant decrease in optic radiation in the MS cohort compared with healthy controls. It decreased in the MS cohort with increasing lesion load in OR and with disease duration in the splenium. All of this suggests demyelination in normal appearing white matter. However, the apparent MSA observed in the SLF pointed to potential systematic issues that require further exploration to realize the full potential of the presented approach. Despite the limitations of such single-orientation ROI-specific estimation, we believe that our clinically feasible approach to study degenerative changes in WM is worthy of further investigation.

Fracture Frequency and Anisotropy of Magnetic Susceptibility: A Case of Study in the Claromecó Basin (Pang 0003 Well), Southwestern Gondwana Boundary

Fracture Frequency and Anisotropy of Magnetic Susceptibility: A Case of Study in the Claromecó Basin (Pang 0003 Well), Southwestern Gondwana Boundary

СРЕДНЕЮРСКИЕ ТУРБИДИТЫ ЭЛЬГОНСКОЙ СВИТЫ УЛЬБАНСКОГО ТЕРРЕЙНА: СЕДИМЕНТОЛОГИЧЕСКИЕ ОСОБЕННОСТИ И НАПРАВЛЕНИЯ ПАЛЕОПОТОКОВ

The Ulban terrane composed of a thick sedimentary series of Jurassic clastic deposits is located at the junction of the Mongol-Okhotsk and Sikhote-Alin orogenic belts. The results of the study of the deposits assigned to the Middle Jurassic Elgon Formation are presented. The structure of the sedimentary succession and sedimentological features are considered and illustrated, which indicate sand-reach turbidite sedimentation in a setting of sandy lobe of turbidite cones or ramps. The paleocurrent directions measured in outcrops and calculated using the anisotropy of magnetic susceptibility suggest the transport of clastic material from the western or southwestern sources.

Late Triassic successive amalgamation between the South China and North China blocks: Insights from structural analysis and magnetic fabrics study of the Bikou Terrane and its adjacent area, northwestern Yangtze block, central China

ABSTRACT The Bikou Terrane, located at the conjunction of the Longmenshan fold-thrust belt and the west Qinling orogenic belt in centeral China, was involved in the Late Triassic collision between the South China and North China blocks. The Bikou Terrane has preserved crucial information on structural geometry and kinematics of Triassic tectonics, and is therefore of great importance for reconstructing the Paleo-Tethyan evolutionary history. However, multi-phase tectonic events of the Bikou Terrane are unsettled. This work presents detailed structural analysis based on both the field and laboratory works, which reveals three phases of deformation events in Bikou and its adjacent areas, including top-to-the-SW shearing related to SW-ward thrusting (DI) mainly to the north of the Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (DIII) locally developed in the northern Bikou Terrane. Anisotropy of magnetic susceptibility (AMS) study and related structural analysis not only support the multiphase deformation but also reveal a gradual transition from the DII-related magnetic fabrics to the DIII-related magnetic fabrics in the Bikou Terrane. Integrating published geochronological data, it is constrained that DI occurred at ca. 237–225 Ma, DII occurred at ca. 224–219 Ma, and DIII possibly occurred during the Early Cretaceous. Based on regional tectonics, the DI event corresponds to the collision between the South Qinling block and the Bikou Terrane, and the DII event reflects the intracontinental amalgamation between the Bikou Terrane and the Yangtze block, which indicates a Late Triassic successive amalgamation from the North China block to the South China block. Intracontinental adjustment represented by the strike-slip (DIII event) occurred after the final amalgamation between the North China and South China blocks. By applying AMS on deciphering structural geometry and multi-phase deformation, our study suggests that AMS is a useful tool for structural analysis.

Deciphering syn- and post-emplacement processes in shallow mafic dykes using magnetic anisotropy

Dykes form key pathways for the transport and emplacement of magma within the crust. We have identified syn- and post-emplacement processes recorded across a ~2 m thick basaltic dyke on the Isle of Skye, Scotland. We measured the rock magnetic properties, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic magnetisation (AARM) across two dyke-thickness profiles spaced 13 m apart along the dyke strike (sites G5 and G6). At 20–25 cm intervals, our samples are very closely spaced compared to standard sampling protocols. Our results show that the dyke's magnetic fabrics originate from two mineral groups: titanomagnetite, which is abundant in the central dyke region at site G5, and iron sulphides (pyrite and pyrrhotite) that dominates the margin regions at both sites. The titanomagnetite occurs in unaltered dyke rock; its magnetic fabrics are primary, having formed during magma solidification, and record lateral magma flow. The pyrrhotite occurs in jointed and hydrothermally altered dyke rock; its petrological and magnetic fabrics are secondary, having originated from a sulphide-rich fluid which infiltrated cooling joints oriented perpendicular to the dyke margins and locally modified the primary magnetic fabrics. At site G6, pyrrhotite also occurs in the dyke centre, suggesting that locally the post-emplacement sulphide-rich fluid permeated into this region; this site is located close to a branch in the dyke and has increased joint frequency, which could explain the enhanced alteration. The presence of syn- (primary) and post-emplacement (secondary) fabrics was only identified due to our high frequency sampling regime and use of both AMS and AARM techniques. We highlight that future magnetic anisotropy studies of dykes may benefit from high sample frequency combined with sampling along-strike and across-thickness. Using both AMS and AARM techniques to detect more variations in magnetic fabrics can reveal more complete syn- and post-emplacement dyke histories.

Systematic extraction of crystal electric-field effects and quantum magnetic model parameters in triangular rare-earth magnets

A primary goal at the interface of theoretical and experimental quantum magnetism is the investigation of exotic spin states, mostly notably quantum spin liquids (QSLs) that realize phenomena including quasiparticle fractionalization, long-ranged entanglement, and topological order. Magnetic rare-earth ions go beyond the straightforward paradigm of geometrical frustration in Heisenberg antiferromagnets by introducing competing energy scales, and in particular their strong spin-orbit coupling creates multiple split crystal electric-field (CEF) levels, leading to anisotropic effective spin models with intrinsic frustration. While rare-earth delafossites have a triangular-lattice geometry and thus have gained recent attention as candidates for hosting spin-1/2 QSL physics, the reliable extraction of effective spin models from the initial many-parameter CEF spectrum is a hard problem. Using the example of ${\mathrm{CsYbSe}}_{2}$, we demonstrate the unambiguous extraction of the Stevens operators dictating the full CEF spectrum of ${\mathrm{Yb}}^{3+}$ by translating these into parameters with a direct physical interpretation. Specifically, we combine low-field susceptibility measurements with resonant torsion magnetometry (RTM) experiments in fields up to 60 T to determine a sufficiently large number of physical parameters---effective Zeeman splittings, anisotropic van Vleck coefficients, and magnetotropic coefficients---that the set of Stevens operator coefficients is unique. Our crucial identification of the strong corrections to the Zeeman splitting of Kramers doublets as van Vleck coefficients has direct consequences for the interpretation of all anisotropic magnetic susceptibility measurements. Our results allow us to determine the nature and validity of an effective spin-1/2 model for ${\mathrm{CsYbSe}}_{2}$, to provide input for theoretical studies of such models on the triangular lattice, and to provide additional materials insight into routes for achieving magnetic frustration and candidate QSL systems in rare-earth compounds.

Lattice preferred orientation of graphite determined by the anisotropy of out-of-phase magnetic susceptibility

Single crystals of graphite are diamagnetic with a strong crystalline anisotropy. Their magnetic susceptibility measured in alternating fields exhibits a weak, negative in-phase component and a relatively strong out-of-phase component due to electric eddy currents. Consequently, if graphite crystals are oriented preferentially by their crystal lattices (LPO) in graphite ore, one would expect a strong anisotropy of magnetic susceptibility (AMS) of the bulk rock. However, the standard AMS, which is the anisotropy of the in-phase component of susceptibility (ipAMS), reflects not only the LPO of graphite, but also the preferred orientation of the paramagnetic and ferromagnetic admixtures. On the other hand, the anisotropy of out-of-phase susceptibility (opAMS) in rocks such as these should directly indicate the graphite LPO due to its strong electrical conductivity, and can therefore be considered free from the contributions of non-conductive paramagnetic and ferromagnetic minerals. These competing factors were tested on natural metamorphic graphite ores near the South Bohemian town of Český Krumlov in the Moldanubian Zone. The ipMS of the ore is very low, 10−6 to 10−4 [SI], being positive in some specimens and negative in others, which indicates mixtures with variable proportions of graphite and paramagnetic-ferromagnetic minerals. On the other hand, opMS is exclusively positive, on the order of 10−5 to 10−4, and is strongly indicative of graphite. The degree of opAMS is distinctly high at 200–300%, and the opAMS lineation is closely perpendicular to the metamorphic foliation in ores and wall rocks. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation.

Axial longitudinal flow in volcanic materials of the Late Carboniferous-Permian Cadí basin (Southern Pyrenees) determined from anisotropy of magnetic susceptibility

The aim of this work is the application of the anisotropy of magnetic susceptibility (AMS) to the study of volcanic materials from the Late Carboniferous-Permian Cadí basin (southern Pyrenees) in order to (i) explore the limits of this technique in ancient volcanic materials that have undergone strong chemical alteration and a complex tectonic evolution and (ii) provide new insights on the deposition of volcanic materials in this tectonically controlled sedimentary basin. To establish a comparison with the petrofabric of rocks, AMS was complemented with observations and orientation analyses at the outcrop scale, in oriented polished blocks and in thin sections under the microscope. High resolution sampling (620 specimens from 56 sites located in five transects along a sequence 150 m thick) allowed to characterize the volcaniclastic and lava flow series. The AMS results show a fairly constant average magnetic lineation oriented WNW-ESE (in situ and tilt-corrected) and two secondary maxima (N-S to NE-SW in some areas). The magnetic mineralogy analyses indicate the contribution of paramagnetic and ferromagnetic (magnetite, Tc 580 °C) phases to susceptibility. The magnetic fabric has a close relationship with the spatial distribution and orientation of the mineral and grain components of the studied rocks, mainly controlled by depositional processes, with a negligible contribution of the tectonic imprint. Therefore, the magnetic lineation (i.e. K1 axes) can be reliably used as a paleoflow indicator. Nevertheless, it was not possible to establish a clear correspondence between lithological and magnetic fabric types. The results obtained indicate that the AMS approach represents a powerful tool to infer the paleoflow pattern in ancient volcanic and volcaniclastic rocks, even under a priori unfavourable conditions.

Deformation in SW Anatolia (Turkey) Documented by Anisotropy of Magnetic Susceptibility Data

AbstractConvergence between the Eurasian and the African plates in the West Anatolian‐Aegean region results in a trench retreat due to slab roll‐back and tearing of the subducted African lithosphere. The upper plate response of this process gave way to back‐arc extension in the region. We have conducted a very detailed anisotropy of magnetic susceptibility (AMS) study on the Neogene rocks in SW Anatolia to unravel the style and magnitudes of deformation. For this purpose, from 83 sites in 11 structurally homogeneous domains, 1,680 paleomagnetic samples were analyzed. The results show that AMS fabrics are related to the tectonic deformation and that the magnetic lineation (maximum susceptibility axis, k1) is parallel to inferred maximum extension, while minimum susceptibility (k3) is typically normal to the bedding plane, corresponding to a preserved compaction associated with deposition fabric. The intermediate axis (k2) is parallel to a second extension direction and indicates that the region has been under the control of multi‐directional extension during the Neogene. Two main magnetic lineation directions are identified and represent Oligocene to middle Miocene E‐W, and late Miocene to Pliocene NW‐SE oriented extension. The magnetic lineation directions are dominantly parallel or perpendicular to the general strikes of the normal faults. The results show that the deformation in the region resembles two differentially stretched rubber sheets under the influence of SW oriented extension, exerted by the southward retreating Eastern Mediterranean subduction system.

Startlingly strong shape anisotropy of AC magnetic susceptibility due to eddy currents

SUMMARY In order to better understand the anisotropy of AC magnetic susceptibility due to eddy currents (EC susceptibility) in rocks and minerals, we investigated the anisotropy of both in-phase (ipAMS) and out-of-phase (opAMS) magnetic susceptibility. Copper cylinders were chosen as experimental material because copper is diamagnetic, internally isotropic magnetically, and its out-of-phase magnetic susceptibility is no doubt due to eddy currents. Results of laboratory measurements were successfully compared to theoretical models. Both the in-phase and out-of-phase EC susceptibility components are strongly frequency dependent. The principal directions of both ipAMS and opAMS are closely related to the shapes of the copper bodies. While the degree of AMS of elongated cylinders asymptotically approaches 2, which is the value for infinite cylinder, it can be two orders higher in strongly flattened cylinders of the same volume. Analysis of directional susceptibilities has shown that EC susceptibility is well described by the tensor of second rank. These results are useful for the structural interpretation of the opAMS of rocks and ores whose opAMS is due to eddy currents (in electrically conductive minerals, such as graphite, and galena).

Phases of Magmatic Activities in South Wadi Hodein-Shalatein, Egypt: Resolving controversy about their age using Paleomagnetism and AMS

ABSTRACT Several exposures of basaltic intrusions are distributed near Red Sea coast at Wadi Hodein Southern Egypt. There is some disagreement about the palaeomagnetic pole results of these basalts; some claim it is Tertiary in age, while others claim it is of Cretaceous age. To our knowledge, no Anisotropy of Magnetic Susceptibility (AMS) results have ever been published from these rocks. Therefore, palaeomagnetic and AMS studies were performed on these basalts in order to identify phases of magmatism, paleomagnetic pole position and types of magmatic intrusions. In the present study, eight sites (219 oriented core specimens) were sampled from eight location. Rock magnetic studies reflect the presence of magnetite as the chief mineral in these rocks. Alternating field technique in association with Thermal Demagnetisation process revealed presence of one magmatic phase took place in Cretaceous age with VGP (65°N, 250°E; A 95 = 5.3). AMS results showed that the basaltic intrusions in the study area are still holding their primary fabrics.

Spatial-temporal heterogeneity of magma emplacement process and its constraints on localization of associated orebody: A case study in the Shizishan orefield of the Tongling Ore Cluster, East China

We present a study on the Dongguashan quartz diorite stock which is the largest Early Cretaceous intrusion associated with skarn-porphyry polymetallic ore deposit in the Tongling Ore Cluster, Middle-Lower Yangtze Metallogenic Belt in East China. The Dongguashan quartz diorite show massive texture without obvious foliation, and intrusive contact was locally observed inside of the stock. The anisotropy of magnetic susceptibility (AMS) results of the stock show two distinct groups. The Group I (G-I) is dominated by NE-SW-striking magnetic foliation with variably oriented magnetic lineation. The Group II (G-II) that intruded into G-I is characterized by steep NW-SE-striking magnetic foliation and lineation, which are parallel to the vein-like orebody developed in the stock. The 3D geometrical modelling of the stock displays a triangular shape in plan-view with an eastward bulge and irregular stock boundary in the eastern side and the contact surface is steeper in the west than that in the east, denoting that the stock was constructed with an eastward magma accretion trend. Furthermore, the Dongguashan quartz diorite has a wide range of composition and geochronological data, suggesting a multiple magma pulses emplacement model. Accordingly, we propose that the Dongguashan stock was constructed by at least two stages magma pulses. The earlier stage magma pulses intruded along the NE-SW-striking pre-emplacement structures in the country rock and partly intruded into the lithological and mechanical discontinuous interfaces which yielded the stratabound skarn orebody. The eastward magma accretion produced a highly deformed and longer heated country rock on the eastern side of the stock, favoring the orebody development along the eastern stock-country rock contact interface. The late-stage magma pulse and the parallelism among the trend of magnetic foliation of G-II, extensional structure and vein-like orebody suggest that the vein-type orebodies may controlled by both magma emplacement and regional tectonics.

Imaging the roots of a post-collisional pluton: Implications for the voluminous Cambrian magmatism in the Araçuaí orogen (Brazil)

The Cambrian Santa Angélica intrusive complex (SAIC - SE, Brazil), is composed of two lobes with concentric fabric and bull's eyes shapes, each lobe containing a mafic nuclei and granitic borders. It intruded during the post-collisional phase of the Araçuaí orogen (AO). Although SAIC'S internal structure is described as exclusively magmatic, prominent solid-state deformation occurs at its borders, as well as an internal shear zone (ISZ). We studied the mechanisms that caused the different architectures displayed by the post-collisional bodies when emplaced in distinct crustal levels across the AO. We forward-modelled gravity data from the SAIC and its country rocks and performed structural analysis through anisotropy of magnetic susceptibility (AMS) and microstructural data of oriented thin sections in the SAIC border and the intrusion's country rocks. Magnetic measurements indicates that multidomain magnetite and pyrrhotite control the magnetic fabric of SAIC's country rocks. 2D gravity modelling shows that the northeast lobe outcrops its roots and its thickness is less than half of the southwest one. Three zones have been identified based on the occurrence of the solid-state microstructures: the country rocks and the border of the pluton display higher temperatures solid-state microstructures, overlapped by lower temperature ones, while the ISZ only shows high temperature structures. We suggest the SAIC experienced a reverse diapiric mechanism caused by the negative buoyancy of its mafic nuclei during crystallization. As the mafic cores sink through a ductile and hot halo in midcrust conditions, they deform the granitic rocks at the borders, as well as the country rocks, also generating the internal shear zone by the relative motion observed between the lobes. Furthermore, we infer that the buoyancy of the mafic and felsic magmas, together with crustal rheology, were the main constraints responsible to restrain the emplacement depth of the post-collisional plutons in the AO.

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.

Pressure, temperature and lithological dependence of seismic and magnetic susceptibility anisotropy in amphibolites and gneisses from the central Scandinavian Caledonides

As a petrofabric indicator, anisotropy of magnetic susceptibility (AMS) can potentially be used to infer seismic properties of rocks, and in particular seismic anisotropy. To evaluate the link between AMS and seismic anisotropy we present laboratory measurements of elastic wave velocities and anisotropy of magnetic susceptibility (AMS) for eight samples from the deep drilling investigation forming a part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project. The samples consist of a representative suite of mid crustal, deformed rock types, namely felsic and biotite-rich gneisses, and amphibolites (mafic gneisses). Compressional (P) and shear (S) waves were measured at confining pressures from room pressure to 600 MPa and temperature from room condition to 600 °C. Seismic anisotropy changes with increasing temperature and pressure, where the effect of pressure is more significant than temperature. Increasing pressure, considering the range of samples, results in an increase in mean wave speed values from 4.52 to 7.86 km/s for P waves and from 2.75 to 4.09 km/s for S waves. Biotite gneiss and amphibolite exhibit the highest anisotropy with P wave anisotropy (AVp) in the ranges of ~9% to ~20%, and maximum S- wave anisotropy exceeds 10%. In contrast, Felsic gneisses are significantly less anisotropic, with AVp of <7% and AVs of <6%. Up to 20% anisotropy may be generated by microcracks at 600 MPa and 600 °C, which is likely originating from thermal expansion of anisotropic minerals. An agreement is found between AMS and seismic anisotropy, although this is only a case if mean magnetic susceptibility (kmean) ranges between ~1 × 10−5 to ~1 × 10−3 [SI]. Such kmean values are common in rocks dominated by paramagnetic matrix minerals. Based on our results we propose that such samples are the most likely to be useful for the prediction of seismic anisotropy based on their AMS.

Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW–SE, parallel to the fold axes. The Kmin axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.

The onset of the right-lateral strike-slip setting recorded in magnetic fabrics of A-type granite plutons of the Ribeira belt (SE Brazil)

Strike-slip shear zones formed during continental collisions accommodate the convergence and indentation of rigid cratonic blocks. In high-temperature settings of deeply eroded orogens, such as the Neoproterozoic Ribeira belt in southeastern Brazil, the shear deformation is associated with syntectonic granitic magmatism. Anisotropy of magnetic susceptibility (AMS) and zircon U-Pb results from late A-type granites were used to investigate the timing of the strike-slip deformation. Some of these plutons show reddish colors, local rapakive textures and magnetic susceptibilities that are typically lower than older batholiths. Rock magnetic data and SEM analysis indicate that AMS of the A-type plutons is dominated by coarse magnetite and locally (titano)hematite, the latter formed by oxidizing hydrothermal fluids in the final stages of magmatic crystallization. The shape preferred orientation of mafic minerals and feldspars measured in the reddish granite facies , however, indicate that AMS reproduced the magmatic fabric defined by the silicates. Magnetic fabric orientations vary from concordant (Ouro Verde and Capão Bonito plutons) to partially concordant (Sguário pluton) with respect to the host rock shear zones, and define the transition from concentric magnetic lineations and foliations experienced by older I-type batholiths. These A-type plutons constrain the onset of the ductile transpressional deformation at ca. 595 Ma. The magmatism and shear deformation persisted up to 550 Ma in the western Ribeira belt when more brittle transtensional conditions prevailed. These results are consistent with the convergence and indentation of the Congo-São Francisco craton that partitioned the deformation across the orogen and promoted the belt lateral escape mediated by dextral shear zones.

Emplacement and Segment Geometry of Large, High-Viscosity Magmatic Sheets

Understanding magma transport in sheet intrusions is crucial to interpreting volcanic unrest. Studies of dyke emplacement and geometry focus predominantly on low-viscosity, mafic dykes. Here, we present an in-depth study of two high-viscosity dykes (106 Pa·s) in the Chachahuén volcano, Argentina, the Great Dyke and the Sosa Dyke. To quantify dyke geometries, magma flow indicators, and magma viscosity, we combine photogrammetry, microstructural analysis, igneous petrology, Fourier-Transform-Infrared-Spectroscopy, and Anisotropy of Magnetic Susceptibility (AMS). Our results show that the dykes consist of 3 to 8 mappable segments up to 2 km long. Segments often end in a bifurcation, and segment tips are predominantly oval, but elliptical tips occur in the outermost segments of the Great Dyke. Furthermore, variations in host rocks have no observable impact on dyke geometry. AMS fabrics and other flow indicators in the Sosa Dyke show lateral magma flow in contrast to the vertical flow suggested by the segment geometries. A comparison with segment geometries of low-viscosity dykes shows that our high-viscosity dykes follow the same geometrical trend. In fact, the data compilation supports that dyke segment and tip geometries reflect different stages in dyke emplacement, questioning the current usage for final sheet geometries as proxies for emplacement mechanism.

Fabric Analysis in Upper Crustal Post-Collisional Granitoids from the Serre Batholith (Southern Italy): Results from Microstructural and AMS Investigations

The Serre Batholith in Central Calabria (southern Italy) represents the intermediate portion of a continuous cross-section of late Variscan continental crust. The various granitoid units of the batholith were emplaced at depths between 23 and 6 km through an overaccretion mechanism that, at its upper levels, was marked by the emplacement of two-mica granodiorites and granites (MBG) at c. 295 Ma, followed by weakly peraluminous granodiorites (BAG) at c. 292 Ma. These upper crustal granitoid rocks have recorded tectonic stresses, which affected the batholith during cooling of the magmatic bodies, exhibiting a range of deformation microstructures from submagmatic to low-temperature subsolidus conditions, but without developing an evident meso/micro-structural fabric. Anisotropy of magnetic susceptibility (AMS) was employed to identify a possible “internal” fabric of the Serre upper crustal granitoids, revealing a magnetic foliation represented by a mainly oblate AMS ellipsoid. Magnetic foliations and lineations are consistent with a stress field characterized by a shortening axis roughly oriented NW–SE. Further studies are in progress to investigate more in depth the relationships between regional tectonic structures and the emplacement of the late-Variscan Serre Batholith granitoids.