Helga de Wall

Post-collisional shortening in the late Pan-African Hamisana high strain zone, SE Egypt: field and magnetic fabric evidence

Abstract The Hamisana zone (HZ) is one of the major high strain zones of the Pan-African (Neoproterozoic) Arabian–Nubian shield (ANS). It trends broadly N–S from northern Sudan into southeastern Egypt and meets the present Red Sea coast at ≈23°N. The HZ has been the subject of controversy with regard to its importance for the Pan-African structural evolution. Interpretations range from a suture zone, a regional shear zone, or a large-scale transpressional wrench fault system. In this study, we characterize the nature of the high strain deformation by applying the anisotropy of magnetic susceptibility method along with field and microstructural investigations. These investigations demonstrate that deformation in the HZ is dominated by pure shear under upper greenschist/amphibolite grade metamorphic conditions, producing E–W shortening, but with a strong N–S-extensional component. This deformation also led to folding of regional-scale thrusts (including the base of ophiolite nappes such as Gabal Gerf and Onib). Consequently, the high strain deformation is younger than ophiolite emplacement and suturing of terranes. A weak subsequent overprint was mostly non-coaxial. It took place under considerably lower temperature and led to a minor NE–SW-trending, dextral wrench fault. Although it is of only local importance this fault may be itself a conjugate relative to the prominent NW–SE-trending sinistral Najd faults in the northern ANS. Therefore, the HZ is dominated by late orogenic compressional deformation and cannot be related to either large-scale transpressional orogeny or major escape tectonics.

Mineralogy and magnetic behavior of pyrrhotite from a 260 °C section at the KTB drilling site, Germany

Abstract The ultradeep bore hole of the German Continental Deep Drilling Program (KTB) reached a depth of 9100 m and in situ temperatures of about 260 °C, offering an unique opportunity to study natural pyrrhotite. An integrative approach using optical methods, electron microprobe analysis, X-ray diffraction, transmission electron microscopy (see Pósfai et al. 2000), and temperature-dependent magnetic susceptibility measurements were used to characterize pyrrhotite types as a function of lithology and depth. We found a lithology-controlled distribution of pyrrhotite types to a depth of 8080 m, with ferrimagnetic, monoclinic 4C pyrrhotite (metal content 46.0 to 47.2 at%) as the dominant magnetic phase in gneisses and metabasic rocks. In the gneisses, a second pyrrhotite type with higher metal concentrations (46.9 to 48.2 at%) and antiferromagnetic behavior also occurs. At depths greater than 8080 m (in situ temperature > 230 °C) antiferromagnetic pyrrhotite, predominates in all lithologies. That 4C pyrrhotite does not occur below 8080 m, suggests that 4C is unstable above 230 °C in these rocks. Instead of 4C, a 5C type with a ferrimagnetic structure occurs below 8080 m. Thermomagnetic experiments indicate that the metal-poor Weiss-type pyrrhotite is stabilized by oxygen that causes the formation of magnetite during heating. From our observations on natural pyrrhotites we suggest that the magnetic λ-transition is related to the growth of ordered nA pyrrhotite domains to single domain size.

Anisotropy of diamagnetic susceptibility in Thassos marble: A comparison between measured and modeled data

Abstract A study of shear zones within the calcite marble complex of the island of Thassos (Greece) shows that the low field anisotropy of magnetic susceptibility (AMS)-technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics. The strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial (neutral) to uniaxial (prolate) shapes of AMS ellipsoids. The maximum susceptibility is oriented perpendicular to the rock foliation, reflecting the preferred orientation of calcite c -axes in the protolith as well as in the mylonites. For three samples that represent different types of calcite fabrics, the AMS was recalculated from neutron and electron backscatter diffraction textural data. A comparison of the measured and modeled data shows a good coincidence for the orientation of the principal AMS axes and for the recalculated anisotropy data. Both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation.

Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forêts granite from the EPS-1 borehole, Upper Rhine Graben

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.

Magnetic susceptibility zonation of the melilititic Riedheim dyke (Hegau volcanic field, Germany): evidence for multiple magma pulses?

The olivine–melilitic Riedheim dyke complex is related to the Tertiary alkaline magmatism within the Hegau volcanic field. Magnetic susceptibility measurements and the mineral composition of oxides together with petrographic observations imply two phases of magma intrusion for the emplacement of this dyke. The first intrusion forms the eastern and western margins of the dyke section with high magnetic susceptibility values of up to 75×10−3 SI which are related to ferrimagnetic Mg–Al(–Cr)-bearing titanomagnetites with a magnetite component between 0.59 and 0.75 mol%. A generally lower magnetic susceptibility of up to 45×10−3 SI and a significantly lower and homogeneous magnetite component of the titanomagnetites (XMt=0.38 mol%) are recorded for the second intrusion into the central part of the dyke. Furthermore, a dependence of magnetic susceptibility on the amplitude of the ac-field was measured, which is more pronounced in the second intrusion (χHd=25%) than in the first one (6%<χHd<10%) confirming the magneto-mineralogical data. Secondary pervasive alteration is more pronounced in the first intrusion phase, characterised by a very fine-grained and glassy matrix that is more receptive to hydrothermal overprint than the well crystallised matrix of the second intrusion. Localised, thermally induced alteration at the intimately fractured contact between the first and second intrusion phase has oxidised the titanomagnetite of the first intrusion phase but did not significantly effect the second intrusion. Differences between the dyke sections are also documented by the anisotropy of magnetic susceptibility (AMS), which reflects a magmatic flow fabric with long axis of the AMS-ellipsoids (magnetic lineation) parallel to the flow direction and short axis normal to the N–S trending dyke boundary. The orientation of the magnetic fabric for the individual magma pulses is different, with a more moderate plunge of the magnetic lineation within both the western (177°/57°) and eastern parts (181°/50°) of the first dyke intrusion compared to a nearly vertical orientation in the second, central dyke intrusion (180°/78°). We suggest that the primary crystallisation history of the two intrusion phases caused the magnetic susceptibility zonation in the Riedheim dyke complex.

AMS–NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic fabrics in flood basalts

The evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies. A statistically significant sample set of titanomagnetite-bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India, has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar anisotropy of magnetic susceptibility (AMS) parameter and the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an alternating field (AF) tumbling demagnetizer (peak fields of 100 mT) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic fabrics, carrying a higher portion of single-domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic fabrics) and therefore a more precise and better reflection of the “actual” mineral fabric.

Rocks, fabrics and magnetic anisotropy: an introduction to the issue in honour of František Hrouda

All rocks—igneous, sedimentary as well as metamorphic or tectonically deformed—develop a spatial and geometrical arrangement of their constituent minerals commonly referred to as fabric. The genesis of rock fabric, its orientation and intensity can be directly linked to certain geological processes or, on a larger scale, to tectonic phenomena. For this reason, fabric analysis plays a pivotal role in geological investigation. While in some rocks the fabric can be seen with the naked eye, in the others it can be recognized only with the help of instruments and special techniques. A very effective technique, among others, is the analysis of magnetic fabric based on the measurements of anisotropic magnetic properties. Due to its rapidity, accuracy, low cost and non-destructiveness, magnetic fabric analysis has been successfully applied in all branches of earth sciences for more than five decades. Because of his pioneering role in the study of magnetic properties of rocks, this issue is dedicated to Professor František Hrouda. It contains 19 papers dealing with magnetic anisotropy measurements in a variety of rocks— granite, gabbro, mica schist, quartzite, obsidian, monzodiorite, syenite, as well as tuffs, clays and Quaternary deposits—and in minerals, such as haematite. The first few papers are of a more fundamental nature, which are methodology-based and provide the reader with an insight into the variety of problems in rocks and minerals that can be handled using magnetic fabric. These are followed by several papers dealing specifically with the magnetic fabric analysis in igneous rocks. Finally, there are papers addressing problems related to alteration of rocks and highlighting the application of magnetic fabric analysis in understanding various processes in sediments and sedimentary rocks. The issue starts with a paper by Hirt and Almqvist, on the interference of dia-, paraand ferromagnetic subfabrics in polycrystalline rocks. The paper gives an overview on existing techniques for the isolation of magnetic subfabrics and shows examples for geological application. Martı́nHernández along with Guerrero Suarez presents two back-to-back papers dealing with magnetic anisotropy of haematite natural crystals. The papers underline the complex behaviour of natural haematite under the influence of different field amplitudes and illustrate the variation in principal directions, shape and degree of anisotropy of magnetic susceptibility (AMS) ellipsoids with changing field magnitude. Cañón-Tapia and Cárdenas report detailed AMS, hysteresis and thermomagnetic experiments on the magnetic properties of obsidian and their implications for measuring AMS. Particularly important is the indication of submicroscopic ferromagnetic mineral constituents, which control the AMS ellipsoid. This result suggests that similar submicroscopic fabrics may be present in further volcanic rocks, but have been overlooked as yet. Kontny et al. present AMS data from measurements taken in low field at room and liquid nitrogen temperatures, as well as in high field, along with Electron Back Scattered Diffraction (EBSD) data on magnetite bearing mylonitic garnet mica schists from the relatively HP-HT metamorphic Seve Nappe Complex (Scandinavian Caledonides). M. A. Mamtani (&) Indian Institute of Technology Kharagpur, Kharagpur, India e-mail: mamtani@gg.iitkgp.ernet.in

Oblique magnetic fabric in siderite-bearing pelitic rocks of the Upper Carboniferous Culm Basin, SW England: an indicator for palaeo-fluid migration?

Abstract A zone of siderite dominated magnetic fabrics is recognized within clastic argillaceous rocks of the southern part of the Upper Carboniferous Culm foreland basin of SW England. This zone was identified by measuring the anisotropy of magnetic susceptibility (AMS) before and after heat treatment of samples. A detailed investigation of a recumbent fold structure within this zone (at the well-known Crackington Haven locality) reveals the pre-folding nature of siderite formation. The restored κmax axes of AMS-ellipsoids plot on a segment of a small circle, with a mean inclination of c. 45° to the pole of the sedimentary bedding planes. This oblique magnetic fabric geometry is considered to reflect substrate-controlled siderite growth within a migrating fluid medium, which crystallized during diagenesis and the early stages of Variscan compression. The regional distribution of siderite growth, in combination with the directional information from the AMS, is discussed as an indicator for the palaeo-flow direction of diagenetic fluids within a foreland basin setting.

Continental rift-setting and evolution of Neoproterozoic Sindreth Basin in NW-India

The Neoproterozoic Sindreth Basin, NW India, and its surrounding area represent a half graben structure situated between the undeformed Malani Igneous Suite (MIS) in the west and a corridor of coeval Cryogenian ductile deformation, anatexis and granite intrusion in the east. The main lithologies observed in the basin are conglomerate, fanglomerate, debris flow and lake deposits derived from a nearby continental provenance, intercalated with concurrent mafic and felsic lava flows. Based on geological traverses across the strike of the basin, we propose a three-fold classification comprising Lower Clastic Unit and an Upper Clastic Unit and a Bimodal (basalt–rhyolite) Volcanic Unit separating the two. Tilting due to basin inversion and faulting has been observed; however, the rocks are unmetamorphosed and show undisturbed primary sedimentary features. The stratigraphic record of the basin is characteristic for deposition and magmatism in a fault-related continental setting. Implications of the findings have been discussed in the context of Neoproterozoic crustal dynamics in NW India. This study provides conclusive evidence for a continental setting for Sindreth Basin evolution and contests the recent models of active subduction setting (either back-arc basin or accretionary sediments over a subduction zone).

Tectonic evolution of the Central Steep Zone, Axum area, northern Ethiopia: inferences from magnetic and geochemical data

Abstract Northern Ethiopia is marked by a fanning system of thrust planes with NW-dipping structures in the east and southeast-dipping in the west. The central zone of this large-scale (200 km long) structure is formed by a c. 10 km wide zone of localized strain and amphibolites facies metamorphic conditions (680 °C and 3.4 kbar) referred to as the Central Steep Zone (CSZ). The CSZ comprises a mafic rock assemblage of amphibolite, serpentinite showing ocean-floor characteristics and calc-silicate schist. A monzonite intrusion in the central part of the CSZ post-dates the deformation and is related to partial melting of the mafic rocks. Magnetic fabric measurements reveal NE-trending (043°) steep foliations in the CSZ with vertical orientation of lineation, parallel to the axes of micro-folds. This high-strain zone is interpreted as central zone of a positive flower structure on the basis of simultaneous flattening and shear movement, typical for transpressive kinematics. The CSZ has a northern continuation into the Nafka terrane of Eritrea where it can be traced over a distance of 200 km. This high-strain belt forms a major structure in the context of Arabian–Nubian Shield (ANS) collision tectonics during the closure of the Mozambique Ocean and assembly of Gondwana.