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Flanking microstructures

Published online by Cambridge University Press:  02 March 2009

SOUMYAJIT MUKHERJEE  and
HEMIN A. KOYI
SOUMYAJIT MUKHERJEE*
Affiliation:
Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
HEMIN A. KOYI
Affiliation:
Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, Uppsala SE-75236, Sweden
*
*Author for correspondence: e-mail: [email protected]
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Abstract

Ductile sheared rocks of the Higher Himalayan Crystalline unit (HHC) in micro-scale reveal flanking microstructures defined by nucleated minerals (the cross-cutting elements, CEs), and deflected cleavages and grain margins (the host fabric elements, HEs) of other minerals. Depending on different or the same senses of drag across the cross-cutting elements, the flanking microstructures are grouped into Type 1 or Type 2 varieties, respectively. Cross-cutting elements of Type 2 flanking microstructures connote post-tectonic directional growth. The cross-cutting elements of the Type 1 flanking microstructures consistently demonstrate top-to-SW non-coaxial shearing in the Higher Himalayan Crystalline unit. Here the external host fabric elements bounding the cross-cutting elements act as the C-planes. These cross-cutting element minerals are usually parallelogram-shaped, underwent crystal-plastic deformation and their nucleations are pre- or syntectonic. The facts that the host fabric elements are dragged even in absence of rheological softening at the boundaries of the cross-cutting elements, and that the cross-cutting elements are non-rigid, indicate strong bonds between the host fabric elements and the cross-cutting elements. Salient morphological variations in the flanking microstructures are: (1) variable intensity and senses of drag along the single and the opposite cross-cutting element margins; (2) host fabric elements defined only at one side of the cross-cutting elements; and (3) presence of a thin hazy zone at the HE–CE contacts. The observed cross-cutting element minerals are either of nearly the same or of greater competency than the mineral grains which host them.

Keywords

flanking microstructureflanking structuremicrostructureductile shearHigher Himalayan Crystallines
Type
Original Article
Information
Geological Magazine , Volume 146 , Issue 4 , July 2009 , pp. 517 - 526
Copyright
Copyright © Cambridge University Press 2009

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