AMS Ellipsoids Research Articles

Structural Characterization of the Pan-African Banyo Area (Western Cameroon Domain): Constraints from Field Observations, Structures and AMS

The Banyo area, located in the southern prolongation of the Mayo Nolti shear zone trend, belongs to the western Cameroon domain of the Neoproterozoic Central African Belt (NCAB). It is made of granitic rocks that intrude metamorphic banded rocks. Both are sometimes mylonitized. The pluton is dominantly of paramagnetic behavior, as shown by the hysteresis loops and the Fe-bearing silicates crystals are the susceptibility carriers. AMS ellipsoids are dominantly of oblate shape, pointing to the importance of flattening during pluton emplacement. The anisotropy degree of magnetic susceptibility values (≤1.20) characterize the magmatic fabric flow. The microstructural study of the granite reveals magmatic, sub-magmatic, solid-state and mylonitic deformations. Field and AMS fabrics show evidence of polyphase deformation (D1–D3). The D1 phase is of flattening mechanism (flat-laying foliation). The D2 phase points to sinistral ductile simple shear accommodating moderate to steep dipping and N-S- to NW-SE-oriented foliations in plutonic and country rocks and conjugated E-W mylonitic foliation in country rocks bearing sub-horizontal- to moderate-plunge mineral stretching lineation. The D3 phase is of dextral ductile simple shear. σ- and δ-type kinematic markers in the pluton indicate sinistral top-to-south sense of shear movement, indicating a non-coaxial component of the tectonics. The magnetic fabrics of the pluton are parallel to those of the D2 deformation phase of the study area. The transpressive D2 and D3 events correlate with the D2 and D3 phases of the Pan-African tectonic dated at 613–585 Ma and 585–540 Ma, respectively. The pluton, then, emplaced during regional sinistral D2 deformation under transpressive regime. The emplacement of the NE Banyo granite took place as rock strips sheared in sinistral sense of shear movement.

A transtensional basin model for the Organyà basin (central southern Pyrenees) based on magnetic fabric and brittle structures

SUMMARY The purpose of this paper is to explain the kinematics of the inverted extensional Organy` basin(CentralSouthernPyrenees)duringitsextensionalevolution.Thestudyoftheanisotropy of magnetic susceptibility (AMS) and the study of brittle mesostructures are the instruments used to decipher the extensional kinematics. Separation of magnetic subfabrics (analyses of AMS at low temperature and anisotropy of the anhysteretic remanence – AARM) has been also done to properly interpret the obtained magnetic fabric data. The combination of all magnetic fabric and brittle mesostructures analyses has provided new information to clarify thestructuralevolutionofthebasin,whichsupportanewtectonicmodelfortheAptian–Albian basinal stage. 20 sites for the magnetic fabric analysis were sampled in Aptian–Lower Albian marls. The AMS ellipsoids obtained at room temperature show their kmin axes perpendicular to bedding and kmax axes oriented NW–SE in 70 per cent of the samples, N–S in 15 per cent and NE–SW in 15 per cent. Analyses of AMS at low temperature show a better clustering of the axes of the magnetic ellipsoid pointing out that phyllosilicates constitute the carriers of the AMS, whereas the analyses of the AARM confirm the scattering of the ferromagnetic grains. These results suggest that the obtained AMS in the Organy` a basin was acquired during or shortly after deposition of the Aptian-Lower Albian marls without any subsequent overprint. The direction of the magnetic lineation obtained from the AMS at room and low temperatures, agrees with the orientation of the stress axes obtained from the analysis of brittle mesostructures (mainly faults and calcite-filled tension gashes) indicating its tectonic origin. We interpret the orientation of the magnetic ellipsoids related to the transtensional movements linked to the formation of the Organy` a basin during the Aptian–Albian interval, which are consistent with the eastward displacement and rotation of Iberia with respect to Europe. Analysis oftensiongashesalsoindicatesamainNW–SEextensiondirection,whereaspalaeostressanalysis of striated faults shows a more N–S trending σ3. The transtensional model proposed from magnetic fabric and brittle mesostructures for the Organy` a basin during the Aptian–Albian is consistent with the creation of a strongly subsiding area south of the Axial Zone during the Cretaceous and implies that rotation and separation of Iberia respect to Europe took place simultaneously.

Comparison of Fry strain ellipse and AMS ellipsoid trends to tectonic fabric trends in very low-strain sandstone of the Appalachian fold–thrust belt

Comparison of Fry strain ellipse and AMS ellipsoid trends to tectonic fabric trends in very low-strain sandstone of the Appalachian fold–thrust belt

The magnetic fabric in “undeformed clays”: AMS and neutron texture analyses from the Rif Chain (Morocco)

The magnetic fabric in “undeformed clays”: AMS and neutron texture analyses from the Rif Chain (Morocco)

Multiple magmatic fabrics in the Sázava pluton (Bohemian Massif, Czech Republic): a result of superposition of wrench-dominated regional transpression on final emplacement

Multiple magmatic fabrics in the Sázava pluton (Bohemian Massif, Czech Republic): a result of superposition of wrench-dominated regional transpression on final emplacement

Strike-slip and tectonics granitoid emplacement: an AMS fabric study from the Odenwald Crystalline Complex, SW Germany

AMS fabric studies supported by field and microscopic work were applied to identify the internal structure and possible emplacement processes of the Variscan late-tectonic granodiorite-granite intrusions of the Unit III in the Odenwald Crystalline Complex. This Unit is bounded towards NW and NNE by steeply inclined shear zones, the southern part is unexposed. The magnetic susceptibility ranges between 10−3 and 10−6 SI units and is caused by paramagnetic and subordinately by ferromagnetic components. AMS ellipsoids are typically oblate with gently plunging long axes (lineations). AMS foliations and lineations trend mainly WSW-ENE and NNW-SSE, parallel with the NNW and ENE trending marginal shear zones of Unit III, respectively. As revealed by microstructural studies, a penetrative foliation in the plutons is related to emplacement processes. Therefore the observed AMS foliation and lineation are also interpreted as the result of syn-emplacement deformation which is dominantly strike-slip. Weakly inclined foliations around pluton roof xenoliths point to a component of buoyant rise of magma. It is suggested that the granitoid magma was generated in a low level anatectic zone along a left-lateral transpressive shear zone during local extension at releasing bends. During successive fault movements magma ascended through extensional parts of the shear zone. Local normal faults and the Otzberg zone at the eastern margin of Unit III document mostly brittle extension, which overprinted the strike-slip fabrics after the emplacement of the plutons.

Magnetic minerals and magnetic properties of the Siwalik Group sediments of the Karnali river section in Nepal

Sandstones and siltstones collected from the Siwalik molasse sequence (~ 16 to 5 Ma) of the Karnali river section have been studied for their magnetic properties. Behavior of the specimens during demagnetization (of the NRM and IRM) and magnetic susceptibility vs. temperature data suggest that goethite, maghemite/magnetite, and hematite are the main magnetic minerals in the section. Goethite, carrying a recent component, is the dominant magnetic mineral in the fine-grained lithologies from the lower part. Maghemite and magnetite, which also carry a secondary remanence, occur in the sandstones from the upper part. Hematite, mainly of detrital origin, is present in the whole sequence. The magnetic fabric is defined by mainly oblate AMS ellipsoids and a low degree of anisotropy (P′ < 1.20). The magnetic lineations (declinations: 75°–130° or 245°–310°; peak orientation: 290°/2.8°) are subparallel to the fold axes/bedding strikes/thrust fronts (WNW-ESE). The initial sedimentary-compactional fabric has been overprinted by a secondary tectonic fabric, which was probably induced by mild deformation active in the compressive tectonic setting.

Magnetic fabric and palaeomagnetic analyses of the Plio–Quaternary calc–alkaline series of Aegina Island, South Aegean volcanic arc, Greece

Magnetic fabric and palaeomagnetic analyses of the Plio–Quaternary calc–alkaline series of Aegina Island, South Aegean volcanic arc, Greece

Magnetic susceptibility variation and AMS exchange related to thermal treatment of siderite

Magnetic susceptibility variation and AMS exchange related to thermal treatment of siderite

Fabric, Texture and Anisotropy of Magnetic Susceptibility in High-Grade Iron Ores from the Quadrilátero Ferrifero, Minas Gerais, Brazil

The iron protores of the Quadrilatero Ferrifero in Minas Gerais, Brazil, are quartz- or carbonate-banded laminated rocks (itabirite) which still display primary features similar to those encountered in non-metamorphic jaspilites, such as sedimentary layering, slump structures and porous granoblastic martite fabric with relictic kenomagnetite. Synmetamorphic deformation initiated recrystallization of the platy hematite variety (specularite) and by this gave rise to a variety of secondary grain fabrics and textures portrayed by the intensity of magnetic (low-field) susceptibility and by the geometry of corresponding anisotropy ellipsoids. Folded ores usually have a foliation roughly parallel to the compositional banding, an axial plane schistosity, lepidogranoblastic fabric and textures dominantly resulting from flexural slip. The stereogram of the corresponding (11.0) texture shows a single maximum parallel to the intersection line of banding and schistosity. The AMS is generally low; its ellipsoid is triaxial. Increasing strain and shearing obliterates the primary layering and induced the formation of ore mylonites with lepidoblastic (S-tectonites) or nematoblastic (L-tectonites) grain fabrics which results in varying shapes of the AMS ellipsoids. Schistose ores, finally, have girdle arrangements of hematite-(11.0) poles, high susceptibilities and oblate shapes of the AMS ellipsoids, whereas strongly lineated ores display a single maximum parallel to the stretching direction, low to moderate susceptibilities and AMS ellipsoids with prolate shape. Secondary recrystallization and grain growth does not affect the lattice-preferred orientation.

Tectonic applications of magnetic susceptibility and its anisotropy

Tectonic applications of magnetic susceptibility and its anisotropy