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 DEFORMATION PATTERN

The jonk river conglomerate are conentrated
along the river jonk trending NW-SE extended for a length about 28 km with huge
boulder, pebbles and cobbles shapes, before disappearing beneath the
Mesoproterozoic Chhattisgarh super group cover sediments. The sonakhan green
stone belt trending the NW-SE forms a broad synformal basin with steep dipping
NNW-SSE trending axial surface. The aforesaid belt shows two phases of
deformation. The first deformation produces (F1) NNW-SSE trending subvertical
schistosity, and associated steeply plunging isoclinal fold, mineral lineation
having almost downdip alignment on the schistosity surface. The F2 folding has
produced broad open flexures with NE-SW axial planes, and has resulted into culminations
and depressions due to interference of fold patterns. Most of the large-scale folds
developed in the region are synformal (GSI, Unpublish report, 2000). Some
of the shear sense have been marked out by outlining the alignment of mean
orientation of long axes of pebble on schistosity surface is nearly downdip. In
the vertical section pebble are sub perpendicular to the schistosity the
pebbles are sub-elliptical, and mean orientation of the long axes is parallel
to the schistosity trace indicating normal compression (Fig 2a). On the
horizontal surface,  the effect of
sub-horizontal dextral simple shear is seen. These features indicates that
shear are, long axes of pebble are aligned with oblique to the schistosity with
counter – clockwise sense (Fig. 2b), the matrix around the pebbles are asymmetrically
deflected of schistosity trace (Fig. 2c and d).

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It is commonly found
in the jonk river conglomerate that some pebbles shows distinctive shape
characteristics of an asymmetry with diagonally opposite angular corners and
other two opposite corner in round shape (Fig. 2c and d). Treagus and Lan
(2000, 2003) have shown that similar shapes are developed in incompetent
objects in both pure shear and simple shear if the initial shapes are squares
with their sides askew to either the elongation and shortening directions (pure
shear) or the shear direction (simple shear). Their model is applicable in the present
case because the granitic pebbles are expected to be more or less same competent
than the volcanic matrix. We propose that this shape was formed by simple shear
deforming an initial superellipse (Gardner, 1965; Lisle, 1988) formed by
earlier pure shear. A superellipse has the general formula, (x/a)n +(y/b)n
= 1. Where n is >2, the shape is a rectangle with rounded corners. Fig. 2e
illustrates a superellipse (n = 4) deformed by simple shear with movement direction
parallel to x-axis (? = 2.5). The
resultant shape has the characteristic angular and rounded corners (Dasgupta.
et.al, 2013).

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