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Flanking microstructures

Published online by Cambridge University Press:  02 March 2009

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: soumyajitm@gmail.com

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.

Type
Original Article
Copyright
Copyright © Cambridge University Press 2009

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