Geological and combination of sections to obtainGeological and combination of sections to obtain

Geological strain analysis is common procedure for
quantitative estimation of amount of deformation in the rocks. Finite Strain
determination can be carried out by noting principal strain axis for strain
marker such as conglomerate pebbles.  Many
workers have developed different techniques to quantify strain in conglomerate.
The first 3D strain analysis was carried out by D Flinn in 1956, popularly
known as Flinn’s analysis. In early studies by Hossack (1968), burns and spary
(1969) and gay (1969), it was largely assumed that the ellipsoid pebble of
conglomerate shows relict feature of deformation stages. Ramsay (1967) and
ramsay and hubber (1983) each provide comprehensive review of methods in the
analysis of strain in conglomerates. Probably the most widely used methods is
Rf/? analysis (ramsay, 1967; Dunnet, 1969; Lisle, 1985), making use of aspect
ratios and orientations of the pebble in several planar sections of a
conglomerate outcrop, either from measurements in the field or from filed
photographs. Here we deal from both photographs and field measurements, which gives
Rs the tectonic strain ratio from the various means of axial ratio of pebbles
that are assumed as negligible viscosity contrast with their host rock and an
originally spherical, or subspherical shape shape.

 For the
computation of Rf/? and Fry methods, computer based programme is used called EllipseFit
3.4.0 software (Vollmer, 2011) while for the computation of Flinn’s diagram
Flinn Plot (Roday, 2003) is used. EllipseFit is suitable for
determining two- and three-dimensional strain using various objects including centre
points (Fry analysis), lines, ellipses, and polygons. EllipseFit includes
procedures for complete fabric and strain analyses, including image processing,
digitizing, calculation of two-dimensional sectional ellipses, and combination
of sections to obtain three-dimensional ellipsoids (Vollmer, 2011). For Flinn
diagram, the Flinn plot software is written by Roday, 2003, for Windows 32-Bit
Platform Software for plots to display the finite strain data. This software
can be downloaded from the URL: http://www.structural-geology-portal.com/downloads.html.

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GEOLOGICAL SETTING

The sonakhan greenstone belt of central indian cratonic (CIC)/bastar
block of central india is a classic example late Archean – Paleoproterozoic
mobile belt in india. It covers an area of about 1200 sq km. the sonakhan
granite green stone belt trends NNW-SSE direction for about 40km from sonakhan
in the north to remra (21°17″N: 82°46″E). The sonakhan group of
Paleoproterozoic divided into lower baghmara formation, middle formation as
Arjuni and upper Bilari formation. The lower predominantly consists of volcanic
suites, mainly meta-ultramafites, schistose and massive metabasalt,
meta-gabbro, pyroclastics of intermediates to basics composition, ignimbrite,
rhyolites, acidic tuff, pebbly tremolite-actinolite schist, carbonaceous
argillite and ferruginous sulphide-bearing chert.  The upper formation arjuni

unconformably
overlies the baghmara formation by a thick sedimentary piles and starts with
jonk river conglomerate. The jonk river conglomerate mark the unconformity
between the baghmara formation and arjuni formation, which is polymictic in
nature and demonstrate bimodality in matrix composition. The matrix is mainly
meta – arkosic and chlorite /biotite rich greywacke but at some place near to
rajadevri and upto north arjuni, it is totally replace by volcanic matrerials.
The jonk conglomerate is sandwiched between baghmara and arjuna formation and
constricted to jonk river only (Das et.al 1990). The conglomerate horizon are
marked by ill sorted pebble, cobbles and boulders with preserved striations
marks. The pbbles of granite, gneiss, acidic volcanic rocks, porphyries,
amphibolite, metabasalts, quartzites, quartz veins, BIF,jasper, phyllites and
schists. Since the strain analysis of conglomerates  can give the trur results if the clast matrix
ratio are assumed to be low as 90:10. Here the clast versus matrix ratio varies
with average from 90:10 to 10:90. Bilari Group essentially comprises basic and
acid intrusives and extrusives, (Das, et.al.1990). All three formation rest on
a gneissic basement, the Baya gneissic complex.

 

 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).

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).