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Preferred orientation of ferromagnetic phases in rock-forming minerals: insights from magnetic anisotropy of single crystals
Abstract The Sakarya Zone and the Kırşehir Block of northern Turkey are separated by the İzmir–Ankara–Erzincan Suture (IAES) Zone which is the remnant of the northern branch of the Neotethys Ocean. During the closure of the IAES in the Late Cretaceous, northwards drift of the Kırşehir Block and its eventual indentation into the Sakarya Zone produced crustal deformation defined by thrusts and reverse faults, mainly between the indenting Kırşehir Block and the Sakarya Zone. Previous palaeomagnetic studies in the eastern part of the Pontides and the Sakarya Zone showed that palaeomagnetic declinations could record the deformation that resulted in the curvature of the IAES. In order to define the tectonic deformation of the northern part of the Kırşehir Block, we present new palaeomagnetic data from 57 different sites that include Mesozoic–Cenozoic sedimentary and volcanic rocks. The results from Late Cretaceous rocks (40 sites) indicate that large clockwise rotations of c. 140–165° occurred in the eastern limb of the bend, while anticlockwise rotations progressively decreased from c. 80° to 55° from SW to NW in the western limb of the bend. In contrast, small clockwise and anticlockwise rotations are observed in the flat-lying segment of the suture zone. These rotation patterns are consistent with the geometrical trends of the IAES in northern Turkey. Declinations of seven different Middle Eocene sites within the Kırşehir Block are rotated anticlockwise by c. 30–10°. This indicates that the deformation in the Sakarya Zone and the Kırşehir Block continued in the Middle Eocene.
Magnetic anisotropy in natural amphibole crystals
Paramagnetic metamorphic mineral assemblages controlling AMS in low-grade deformed metasediments and the implications with respect to the use of AMS as a strain marker
Magnetic anisotropy of carbonate minerals at room temperature and 77 K
Abstract The low- and high-field magnetic anisotropy (AMS, HFA) of the Rhenohercynian mudstones and greywackes is compared to the theoretical anisotropy calculated from neutron texture goniometry measurements. The magnetic anisotropy is predominantly carried by the paramagnetic phyllosilicates in the form of chlorite/mica stacks and the ferromagnetic contribution is insignificant. The respective principal directions of the theoretical anisotropy and the AMS and HFA are sub-parallel; magnetic foliation reflects the orientation of the maximal concentration of phyllosilicate basal planes, magnetic lineation is subparallel to the intersection axis of those planes. For the purpose of quantitative comparison, the infrequently used standard deviatoric susceptibility as a measure of the HFA degree is employed. A very good linear correlation of the degree of theoretical anisotropy and the measured AMS and HFA is found. The prolate and oblate shapes of the respective fabric ellipsoids are reasonably well correlated. Neutron texture goniometry justifies the use of the conventional magnetic anisotropy technique for the assessment of the mineral fabric of studied rocks. When compared with other works relating the magnetic anisotropy to the mineral preferred orientation (examined by e.g. U-stage or X-ray texture goniometry) neutron texture goniometry seems to be a preferable and very precise method fabric analysis.
Abstract The Variscan, magnetite-bearing Soultz-sous-Forêts granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility (κ) of the granite is between 10 and 80 × 10 −3 SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced κ (< 1 × 10 −3 SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.