• A R BHATTACHARYA

Articles written in Journal of Earth System Science

• N–S crustal shear system in the Bundelkhand massif: A unique crustal evolution signature in the northern Indian peninsula

The Bundelkhand massif, located in the northern part of the Indian shield, is a poly-deformed and poly-metamorphic terrain. This paper reports a new shear system developed throughout the massif in the form of N–S trending quartz veins that are sometimes quartzo-feldspathic and rarely granitic in composition. The veins are vertical and commonly occur in conjugate sets. This tectono-magmatic event appears to represent the youngest shear system of the massif as it cross-cuts all the earlier shear systems (E–W, NE–SE and NW–SE). Emplacement of this N–S vein system may have taken place due to extensional processes that developed some cracks along which siliceous magma was vertically emplaced. The complete absence of signature of the N–S event from the surrounding sedimentary cover of Vindhyan Supergroup, Bijawar and Gwalior Groups suggests that this shear system is pre-tectonic to the nearly E–W trending passive basins developed at the margins of the Bundelkhand craton. Further, several workers have considered the Bundelkhand massif as a part of the Aravalli craton. However, due to the absence of N–S, as well as the other (i.e., E–W, NW–SE and NW–SE), tectonic fabrics of the Bundelkhand massif in other cratons of the Peninsular India, and vice versa, makes the Bundelkhand block a separate and unique craton of its own and is not part of the Aravalli craton.

• Grain-shape controlled strain in quartz grains in high ductile flow regime: Observations from the Main Central Thrust Zone of the Kumaun Himalaya, India

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 flow. 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 first application of the Panozzo method on deformed rocks from India.

$\bf{Highlights}$

$\bullet$ The general fabric of rocks of ductile MCT zone of Himalaya is dominated by elliptically deformed quartz grains.

$\bullet$ However locally the fabric, not compatible with prevailing ductile fabric, contains polygonal quartz grains with flat boundaries.

$\bullet$ Strain has been estimated for polygonal grains by digitizing their outlines and analysing data by computer software.

$\bullet$ Such grains show lower strains near MCT and higher strains away. This is reversely shown by elliptically deformed grains.

$\bullet$ This suggests that the quartz grains with polygonal shapes remained rather insensitive to ductile strain.

• Journal of Earth System Science

Volume 130, 2021
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