Magnetic Anisotropy vs. Shape Preferred Orientation in Quartzites with Negative Susceptibility — Implications for Analysing Strain Intensity Variations
- 8 Downloads
Through this study the robustness of using anisotropy of magnetic susceptibility (AMS) data is tested as a gauge of intensity of shape preferred orientation (SPO) in pure quartzites that have a low mean magnetic susceptibility (Km). AMS of eight quartzite samples from the Rengali province (eastern India) is measured, and the degree of magnetic anisotropy Pj, which is a measure of the intensity of magnetic fabric is calculated. Quartz grain size, shape as well as orientation data are obtained based on SEM-EBSD analysis of each sample. Using these microstructural data, intensity of SPO of quartz grains in each sample is quantified by measuring (i) the concentration parameter (κq) and (ii) the azimuthal anisotropy of fractal dimension (AAD). Magnitude of 2D strain (E) is also estimated for each sample. Based on these data the statistical relationship between the various parameters is evaluated viz. Pj vs. κq, Pj vs. AAD, Pj vs. E, κq vs. AAD, κq vs. E, AAD vs. E. A strong linear relationship is established in each case. It is argued that quartz aspect ratio, dominant slip systems/recrystallization mechanisms cannot explain the strong linear correlations between magnetic anisotropy, SPO and strain. To further support findings of the above determined relationships, positive Km quartzites were also investigated. It is found that the latter do not show a strong relationship between the intensity of AMS, SPO and strain. It is therefore, established that whilst the variation in intensity of magnetic fabric can be used to gauge variation in intensities of strain as well as SPO in the investigated negative Km quartzites, the same is not true for positive Km quartzites, where the AMS is controlled the para/ferromagnetic phases present in the rock.
Unable to display preview. Download preview PDF.
This paper is a part of ARR’s doctoral research carried out at the Indian Institute of Technology (IIT) Kharagpur, India. The authors thank Biswajit Mishra, Manoj Kumar Ozha and B. Govindarao for SEM-EDS analysis in Department of Geology & Geophysics (IIT Kharagpur). Niloy Bhowmik is thanked for technical support in carrying out SEM-EBSD analysis at the Central Research Facility (CRF, IIT Kharagpur). Temperature variation of magnetic susceptibility measurements were made at the Karlsruhe Institute of Technology (Karlsruhe, Germany) by MAM during a research visit that was funded by the Alexander von Humboldt Foundation (Germany). Discussions with Agnes Kontny are gratefully acknowledged. Thanks are due to Koushik Sen for a thoughtful review.
- Borradaile, G.J., Jackson, M. (2004) Anisotropy of magnetic susceptibility (AMS): magnetic petrofabrics of deformed rocks. In: Martín-Hernandez, F., Lüneburg, C.M., Aubourg, C., Jackson, M. (Eds.), Magnetic Fabric: Methods and Applications. Geol. Soc. London, Spec. Publ., no. 238, pp.299–360.Google Scholar
- Burmeister, K.C., Harrison, M.J., Marshak, S., Ferré, E.C., Bannister, R.A., Kodama, K.P. (2009) Comparison of Fry strain ellipse and AMS ellipsoid trends to tectonic fabric trends in very low-strain sandstone of the Appalachian fold-thrust belt. Jour. Struc. Geol., v.31, 1028–1038.CrossRefGoogle Scholar
- Cañón-Tapia, E., Chávez-Álvarez, M.J. (2004) Rotation of uniaxial ellipsoidal particles during simple shear revisited: the influence of elongation ratio, initial distribution of a multiparticle system and amount of shear in the acquisition of a stable orientation. Jour. Struc. Geol., v.26, pp.2073–2087.CrossRefGoogle Scholar
- Gerik, A. (2009) Modification and automation of fractal geometry methods: new tools for quantifying rock fabrics and interpreting fabric-forming processes. Unpublished Ph.D. thesis, Technische Universität München, 126 pp.Google Scholar
- Ghosh, G., Bose, S., Das, K., Dasgupta, A., Yamamoto, T., Hayasaka, Y., Chakrabarti, K., Mukhopadhyay, J. (2016) Transpression and juxtaposition of middle crust over upper crust forming a crustal scale flower structure: Insight from structural, fabric, and kinematic studies from the Rengali Province, eastern India. Jour. Struc. Geol., v.83, pp.156–179.CrossRefGoogle Scholar
- Goswami, S., Mamtani, M.A., Virendra, R. (2018) Quartz CPO and kinematic analysis in deformed rocks devoid of visible stretching lineations: an integrated AMS and EBSD investigation. Jour. Struc. Geol., in-press ( https://doi.org/10.1016/j.jsg.2018.04.008).
- Hrouda, F. (2004) Problems in interpreting AMS parameters in diamagnetic rocks. In: Martin-Hernández F, Lüneburg CM, Aubourg C, Jackson M (eds.) Magnetic fabric: methods and applications, Geol. Soc. London, Spec. Publ., no. 238, pp.49–59.Google Scholar
- Mamtani, M.A., Greiling, R.O. (2010) Serrated quartz grain boundaries, temperature and strain rate: testing fractal techniques in a syntectonic granite. In: Spalla, I., Marotta, A.M. and Gosso, G. (Eds.), Advances in Interpretation of Geological Processes: Refinement of Multi-Scale Data and Integration in Numerical Modelling. Geol. Soc. London, Spec. Publ., no. 332, pp.35–48.Google Scholar
- Mamtani, M.A., Greiling, R.O., Karanth, R.V., Merh, S.S. (1999) Orogenic deformation and its relation with AMS fabric—an example from the southern Aravalli mountain belt, India. In: Radhakrihsna, T., Piper, J.D. (Eds.), The Indian subcontinent and Gondwana: a palaeomagnetic and rock magnetic perspective. Mem. Geol. Soc. India, no. 44, pp.9–24.Google Scholar
- Mondou, M., Egydio-Silva, M., Vauchez, A., Raposo, M.I.B., Oliveira, A.F. (2012) Complex, 3D strain patterns in a synkinematic tonalite batholith from the Araçuaí Neoproterozoic orogen (Eastern Brazil): Evidence from combined magnetic and isotopic chronology studies. Jour. Struc. Geol., v.39, pp.158–179.CrossRefGoogle Scholar
- Nagata, T. (1961) Rock magnetism. Maruzen Tokyo.Google Scholar
- Naha, K. (1960) Granite emplacement in relation to thrusting in south Dhalbhum and northeastern Mayurbhanj. Quart. Jour. Geol. Min. Metall. Soc. India, v.32, pp.115–122.Google Scholar
- Nye, J. F. (1957) Physical Properties of Crystals. Clarendon Press, Oxford.Google Scholar
- Owens, W. H., Bamford, D. (1976) Magnetic, Seismic, and Other Anisotropic Properties of Rock Fabrics. Phil. Trans. Roy. Soc. London, A283, 55p.Google Scholar
- Tarling, D.H., Hrouda, F. (1993) The Magnetic Anisotropy of Rocks. Chapman and Hall, London, 217p.Google Scholar
- Tripathy, N.R., Srivastava, H.B., Mamtani, M.A. (2009) Evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the Main Central Thrust Zone (Garhwal Himalaya, India): Inferences from finite strain and AMS analyses. Jour. Asian Earth Sci., v.34, pp.26–37.CrossRefGoogle Scholar