Kinematic analysis using AMS data from a deformed granitoid |
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| Affiliation: | 1. Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur – 721302, India;2. Lehrstuhl für Strukturgeologie und Tektonophysik, Institut für Angewandte Geowissenschaften, Karlsruher Institut für Technologie, Hertzstrasse 16, 76187 Karlsruhe, Germany;1. Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA;2. Department of Geological Sciences, California State University, Long Beach, CA 90840, USA;3. Department of Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, QC H2T 2R8, Canada;1. School of Earth, Atmosphere and Environment, Monash University, Melbourne VIC 3800, Australia;2. Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, Amsterdam, Netherlands;3. Earthquake Research Institute, University of Tokyo, Tokyo, Japan;1. Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague 12843, Czech Republic;2. Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, Prague 16500, Czech Republic;3. Czech Geological Survey, Klárov 3, Prague 11821, Czech Republic;4. Institute of Applied Mathematics and Information Technologies, Faculty of Science, Charles University, Albertov 6, Prague 12843, Czech Republic;5. Environmental Geology, Natural Resources Management Department, New Mexico Highlands University, Las Vegas, NM 87701, USA |
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| Abstract: | This study highlights the usefulness of anisotropy of magnetic susceptibility data from a deformed granitoid in deciphering its kinematic evolution vis-à-vis shear zone. Data are presented from the Chakradharpur Granitoid (CKPG) that lies to the north of the northerly dipping, ENE–WSW striking Singhbhum Shear Zone (SSZ; eastern India). Whilst the foliation recorded in the field in some parts of the granitoid is parallel to the SSZ, the magnetic foliation is N54°E/90° (mean orientation). It is suggested that the magnetic fabric provides a window into an evolutionary stage prior to the final shearing/thrusting event, the evidence of which is preserved on the mesoscopic scale. It is envisaged that during the initial stages of deformation there was simple shear along the evolving SSZ that resulted in sinistral strike-slip movement; the vorticity axis at this stage was steeply plunging and sense of rotation was anticlockwise. Space was generated in a direction ∼N25°E (perpendicular to maximum-Instantaneous Stretching Axis) into which CKPG emplaced synchronously with regional deformation and evolving SSZ. With continued deformation, there was thrusting along the SSZ. The vorticity axis flipped to a sub-horizontal orientation, thus leading to the development of down-dip stretching lineations and sheath folds within the SSZ. However, at the same time, the vorticity axis responsible for fabric evolution within the syntectonically crystallizing/cooling CKPG was steeply plunging with clockwise rotation. The magnetic foliation (mean orientation N54°E/90°) developed during the final stage of syntectonic crystallization. However, deformation in the region and thrusting along the SSZ continued even after the CKPG had fully crystallized and solidified, which led to the development of the ENE–WSW striking mesoscopic foliation that is parallel with the SSZ. We propose that the angle between the magnetic foliation and the SSZ/foliation recorded in the field, enables to decipher the kinematic vorticity number of flow responsible for fabric evolution of the CKPG. It is concluded that transpression was an important mechanism, and during regional deformation, whilst the SSZ developed structures by dominantly simple shear, the CKPG underwent dominantly pure shear. |
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