Grain-shape controlled strain in quartz grains in high ductile flow regime: Observations from the Main Central Thrust Zo
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Ó Indian Academy of Sciences (0123456789().,-vo lV)( 0123456789() .,-vol V)
Grain-shape controlled strain in quartz grains in high ductile Cow regime: Observations from the Main Central Thrust Zone of the Kumaun Himalaya, India A R BHATTACHARYA*
and A K VERMA
Centre of Advanced Study in Geology, University of Lucknow, Lucknow 226 007, India. *Corresponding author. e-mail: [email protected] MS received 26 July 2019; revised 24 June 2020; accepted 11 July 2020
In ductile shear zones, the strain shown by the rocks depends much on the composition and shape of the mineral constituents. Under simple shear, quartz grains commonly reorient themselves in the direction of tectonic transport or Cow. In ductile shear zones, quartz grains are elliptically stretched in the direction of mylonitic foliation to accommodate the imposed ductile strain. Our observations on the rocks of a crustal scale shear zone, the Main Central Thrust (MCT) of the Himalaya, however, reveal that at several places of the shear zone the quartz grains are polygonal and show planar boundaries. The fabric of rocks at such places is not compatible with that of the prevailing fabric of rocks, and can be described as strain insensitive fabric. Following the Panozzo (J. Struct. Geol. 6:215–221, 1984) method, we have estimated strain from quartz grains that show planar boundaries. Our results show that in the MCT zone, the areas of high ductile strain, as existing near the trace of the MCT, the amount of strain shown by such grains of quartz is low, while in areas of low strain, as existing in areas away from the MCT, the amount of strain is relatively higher. As such, the method holds importance in those cases where grain shapes (i.e., planar boundaries) put constraint on estimation of strain because the conventional methods of strain estimation require elliptical shape of objects. This is possibly the Brst application of the Panozzo method on deformed rocks from India. Keywords. Strain in quartz; Ductile Cow regime; Panozzo method; Main Central Thrust; Himalaya.
1. Introduction It is a known fact that the nature of deformation of a rock depends much on the nature of the original constituents of the rock, especially the grains and various mineral constituents. In crustal rocks, quartz is one of the most common mineral constituents and it is also highly sensitive to deformation under most conditions. For this reason, presence or absence of quartz is an important factor of rock deformation.
Quartz commonly occurs as single grains or as aggregate of grains. Under simple shear (Mukherjee 2012), quartz grains tend to realign themselves in the direction of tectonic transport or Cow. Individual quartz grains show a variety of shapes and sizes. In quartz-rich rocks, the pattern of deformation, including the Bnite strain acquired, depends to a large extent on the shapes and sizes of quartz. This is however not so for dynamically recrystallized grains of quartz. A lot of work has been done on the nature and behaviour of quartz during deformation (e.g., Mukhopadhya
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