Evidences of Large-Scale Shearing from
South-Eastern Extension of The Mahakoshal
Belt, Covering Parts of Sidhi & Singrauli
Districts of Madhya Pradesh and Sonbhadra
District, Uttar Pradesh, India by Banerji DC in Integrative Journal of Conference Proceedings_integrative journal of conference proceedings impact factor
Abstract
The WNW-ESE trending arm of the Mahakoshal belt, extending towards south-east in the state of
Jharkhand, through U.P, exposes variety of structural features those are uncommon in main Mahakoshals
having ENE-WSW trend. The turbidites, said to be characteristic of the Parsoi Formation of the WNW
trending arm, appear to be shear generated structures, having imprints across the length and width of
the arm. Evidences of bulk flow along the pervasive foliation make it a mylonitic foliation, restricting
possibility of survival of primary structures.
Overwhelming presence of anticlockwise rotated fabric elements, producing S-C structure in phyllite/
phyllonite and in meta-greywacke, along with frequent presence of ‘alpha’ and ‘delta’ type structures,
strongly asymmetric ‘S’ and at places ‘Z’ shaped folds and the sheath folds suggest that the ductile shearing
with a sinistral shear sense is spread over the entire WNW trending sinuous arm of Mahakoshals. This
belt has a strong manifestation on the imagery, maintaining a cross cutting relationship with the ENEWSW
trending fabric of the main Mahakoshals and appears to be superposed over it. Evidences suggest
it to be a wide and extensive shear zone.
Keywords: Shears; Progressive shearing; Sinistral shear sense; Rotation; Mylonitic Foliation; Turbidites
Introduction
The Mahakoshals have a WNW-ESE trending arm in the south-eastern corner, especially
south of Chitrangi. East of Chitrangi, the main Mahakoshals, continuing with an ENE-WSW
trend from Narsighpur district of Madhya Pradesh, continues within the state of Uttar Pradesh
and abuts against the Son River (Figure 1). The WNW-ESE trending arm, however, continues
much to ESE and traceable even within the state of Jharkhand after crossing over the state of
Uttar Pradesh.
Figure 1:Geological sketch map of parts of Sidhi and Singrauli districts
of M.P. and Sonbhadra district of U.P.
Explanation to this change of trend from ENE to WNW, and the
nature of contact between the two arms of Mahakoshals, is barely
available in the existing literature. Banerji [1,2] reported large scale
non-coaxial shearing of Mahakoshal rocks, resulting into a WNWESE
trending sinuous belt, having imprints of superimposition
over the ENE trending main Mahakoshals. The superimposition
of the WNW fabric is well supported by the satellite image of the
area. Sharma [7], however, has reported the presence of turbidites
restricted within the WNW trending arm of the Mahakoshals.
Present work further analyses the ground evidences to
ascertain if the differently oriented arm of Mahakoshals is a result
of ductile flow, producing a WNW trending mylonitic foliaton.
Geologic setting
The ENE-WSW trending narrow belt of Mahakoshal rocks extends
along the Narmada and Son rivers between Barmanghat
(Narsinghpur, Madhya Pradesh) in the west to Palamau (Jharkhand)
in the east. This belt is represented by rocks like quartzite, phyllite,
dolomite, meta basics and banded ferruginous chert. The lower division
of the Mahakoshals, occupying the western and central part
of the belt is known as Agori Formation and is represented by a
sequence of basal quartzite interbedded with meta basics and overlained
by dolomites with chert inter bands. The eastern part of the
belt is dominated by meta basics, banded ferruginous chert, phyllite
and meta-greywacke with near absence of carbonates. Nair et
al. [3] have considered the meta basics / ultramafic flows occurring
near Chitrangi to be the lowermost part of Mahakoshals and named
them ‘Chitrangi Formation’. Sharma et al. [8] however, have considered
these meta basics / ultramafic flows as Post Jungel igeous suit.
The argillite-meta greywacke association occupying central part of
the WNW trending arm of the Mahakoshals (Figure 1) is flanked
on either side by rocks of Agori Formation and has been considered
{Nair et al. [3]} as uppermost ‘Parsoi Formation’. The Parsoi
rocks also include turbidites, restricted only to this part of the belt
[7]. The southern margin of the eastern Mahakoshal belt is affected
by intrusions of granitic to granodioritic and syenitic plutons [9].
These igneous rocks are generalized as Dudhi granitoid in the existing
geological maps.
Banerji [2] reported the presence of a major shear zone in the
south-eastern part of Mahakoshal belt and considered the WNWESE
trending tract of Agori and Parsoi Formation of the south-east
as mylonites, representing his ‘Dudhmania Shear Zone’. Ductile
shearing along the southern boundary of the WNW trending Mahakoshal
belt of south-east, is also discussed by Roy & Devrajan [6].
Structural features
In the WNW trending arm of the Mahakoshals, the trend of the
foliation is N80° W-S80° E in the western end, swerving to N60°
W-S60° E in the central part and again swinging to N80° W-S80° E
in the eastern end of the belt. This change in the trend of foliation
imparts a sinuous appearance to the belt. The dip of the foliation is
sub-vertical.
Northern margin of Dudhi Granitoid, located south of the
Songarh-Kasar-Dudhi (SKD) fault bordering WNW trending belt
of Mahakoshals, record ample evidences of ductile shearing. The
rock locally acquires gneissosity because of alignment of stretched
feldspars alternating with layer lattice minerals. The granitoid
shows imprints of two obliquely running planes, having an angle
of about 100 to 120 between them (Figure 2). While, the feldspar
phenocrysts are extremely stretched along pervasive ‘C’ planes
representing mylonitic foliation, the obliquely oriented grains along
‘S’ planes show regular offsest by the mylonitic foliation, imparting
an ophthalmic shape to the phenocrysts. Crude rotation of the ‘S’
planes gives impression of a strain sensitive fabric.
Figure 2:Sheared granitoid (pink gneiss) from
Kanhana, north of Singrauli.
North of the SKD fault, exposures of greywacke/phyllite/
phyllonite are common, representing the WNW trending
Mahakoshal belt. Development of a strain sensitive fabric with ‘S’
planes rotated both clockwise and anticlockwise (Figure 3a & 3b)
directions, in mesoscopic scale, is common all along the rocks of
bordering zone. The ‘C’ planes define mylonitic foliation, running
parallel to the boundary of the WNW trending Mahakoshal belt. The
angle between these two planes, near the flank, is about 15°. This
type of rotational feature persists across the length and width of the
WNW trending Mahakoshal belt. The angle between the two planes
reduces to less than 10° in the central part of the belt.
Figure 3:Strain sensitive fabric developed in phyllite/phyllonite. The coin is placed on ‘C’ plane dominated area,
while above and below the coin ‘S’ planes show rotation.
Also, the ubiquitously present Quartz veins and the chert bands
of the peripheral area exhibit variety of shortening and extensional
features. The bands which are oriented parallel to the pervasive
foliation (‘C’ planes) show stretching and boudinaging. Boudins,
thus created within the stretched part, lying parallel to the flow
planes, are further rotated during the progressive movement and
play the role of excellent shear sense indicators (Figure 4a & 4b).
On the contrary, veins/bands having oblique orientation to the flow
plane display shortening by compression, producing short hinges.
With the progressive movement, the limbs of the short hinges
acquire parallelism with the flow direction and get considerably
stretched. This phenomenon is quite corroborative with the
instantaneous extensional and shortening regimes, conforming
flow along mylonitic foliation. The extent of stretching of the limbs
along the ‘C’ planes, during the progressive deformation, also
indicates the amount of flow involved.
Figure 4: Shortened and boudinaged (extended) quartz veins oriented oblique and parallel to mylonitic foliation
respectively. Note the anticlockwise rotation of the boudins, and extended limbs and short hinges in obliquely
oriented quartz veins. The extent of stretching of the limbs in Figure b is also noteworthy. Sense of shear is
sinistral
Formation of ‘alpha’ and ‘delta’ structures (Figure 5a & 5b),
within the stretched bands are again a common phenomenon
throughout the belt. The boudins formed during stretching are
rotated either clockwise or anticlockwise, depending upon the
prevailing dextral or sinistral movement. In most of the cases,
however, the shear sense deduced from such rotated boudins
indicate a sinistral sense of movement. One of the chert bands from
east of Dudhmania exhibit a complex structure, which appears to
have resulted from change in direction of non-coaxial movement,
from dextral to sinistral, and consequent reverse rotation of the
boudins (Figure 6).
Figure 5:(a) Quartz vein showing ‘alpha’ structure. (b) Quartz vein showing ‘delta’ structure. Sense of shear is
sinistral.
Figure 6:Complex structure showings reverse
rotation in pre-existing rotated boudins. The over
tightening is caused probably by reverse rotation of
the earlier rotated boudins in chert band.
The over tightening of ‘Z’ fold, represented by a chert band
observed near Dudhmania and its reverse rotation (Figure 7) also
suggest that initial sense of asymmetry of folds (i.e. dextral as
indicated by ‘Z’ folds) was subjected to reverse shearing (sinistral)
during the progressive non-coaxial deformation [5].
Figure 7:Compex structure produced by reverse
rotated ‘Z’ fold in the chert band. Because of
reverse rotation, the lower hinge of ‘Z’ fold is rotated
upward, and the upper hinge has come closer to the
lower hinge. The extended / boudinaged fragments
of the upper and lower limbs have moved inward
(towards center).
Chert bands of this area also preserve a variety of folds, tight to
isoclinal and asymmetric in nature. Both ‘S’ and ‘Z’ shaped folds are
represented; however, ‘S’ shaped folds predominate over the other
type. These mesoscopic folds have low (<30°) easterly plunges.
However, steep (50°-70°) easterly plunges are also observed in the
obliquely trending (NNE-SSW to NNW-SSE) shortened chert bands.
Anticlockwise rotation of the axial planes of near isoclinal folds
and their amplification (Figure 8) is also a regular phenomenon
in this part of the belt. Sheath folds with an eyed outcrop pattern
and the textbook type plane non-cylindrical sheaths (Figure 9) are
also represented in the area having proximity to the central belt of
Parsoi Formation of Nair et al. [3].
Figure 8:Anticlockwise rotated axial planes of
near isoclinal ‘S’ folds in chert band. Shear sense
is sinistral.
Figure 9:Sheath fold. Of the two white colored
chert bands, the one present to the right of the coin
has produced plane non cylindrical sheath fold.
Further towards the central part of the belt, i.e. near the belt
of Parsoi rocks of Nair et al. [3], the greywacke exhibit presence of
numerous, thin, siliceous lamellae, concentrated as discontinuous
bands. Extent of such bands is restricted to a few tens of meters,
tapering at either ends before disappearance. These siliceous
lamellae, in fact, get dissipated within the greywacke, leaving
behind still traceable faint markings. Within the bands, the lamellae
are stretched, rotated and shortened and displaced along the long
axis, depending upon the orientation of the lamellae with respect to
the direction of flow. The rotational component of flow is exhibited
by rotation of the siliceous lamellae having obliquity to the flow
plane (Figure 10). Rotation of these quartzose lamellae very often
produce a pseudo cross bedding. Nevertheless, these pseudo cross
bedded structures occur in close association with tight rootless
miniature folds (Figure 11), indicating that such structures are
the products of intense deformation [4]. These structures are
likely to be mistaken for turbidites. However, as discussed earlier,
evidences of bulk flow along the pervasive foliation, ubiquitously
present in the belt, reduces the possibility of existence of any
primary structure. Such structures, therefore, represent secondary
structures, produced by progressive shearing of the rocks.
Figure 10: Anticlockwise rotation of siliceous
lamellae having obliquity to the flow plane (bottom
right) in metagreywacke. Note the systematic
partitioning of the rotational component by discrete
shear planes and presence of rootless folds (below
coin). Shear sense is sinistral.
Figure 11: Pseudo cross bedding associated with
rootless folds (above and below the coin). Shear
sense is sinistral.
Close to the central belt of Parsoi Formation, these siliceous
lamellae are present only as small discontinuous patches within
the meta-greywacke, mostly as hinges of small folds with markedly
oblique relationship with the flow plane. Continuity of these
obliquely oriented and highly folded lamellae are conspicuously
broken along the discrete planes representing foliation. The rock
itself acquires a crudely banded nature with alternate light and
darker bands (Figure 12).
Figure 12:Crudely banded greywacke with traces
of obliquely running siliceous lamellae.
Remarkably, adjacent to Parsoi belt, the chert bands, both
ferruginous and nonferrous types, drastically reduce in number.
They, however, continue displaying rotation of the axial planes by a
dominant sinistral flow (Figure 13). Greywacke of this zone attains
a well-defined banded nature, very likely to be confused with
turbidites. The alternate light and dark bands are arranged parallel
to the foliation and show shortening features, wherever there is an
obliquity to the flow plane, differentiating them from the turbidites,
the rock becomes a classic banded mylonite (Figure 14), with a
marked sinistral shear sense.
Figure 13: Banded ferruginous chert showing
anticlockwise rotation.
Figure 14:Banded mylonite. Alternate light and
dark bands are arranged parallel to the foliation
and show shortening/rotational features, wherever
there is an obliquity.
In the central belt of Parsoi Formation, the banded mylonite
passes into ultra mylonite with still retaining the banded nature.
Rocks are friable and show faster denudation. The alternate dark
and light bands, however, become thinner with minimum obliquity
between them. Rotational features continue as pressure shadows
(Figure 15), very helpful in differentiating them from a turbidite.
A very strong and close spaced flow plane representing mylonitic
foliation is preserved all through the Parsoi belt. Near-complete
transposition and rotation of oblique planes produce localized
pseudo cross bedding (Figure 16). However, without a holistic
approach, differentiating them from a normal turbidite is difficult.
Rarely, small, rounded, relatively stiff mass of rock (isotropic) appear
as rotated segments wrapped within the intensely foliated
mass. These rounded segments show a crude sigmoidal trail of the
internal foliation, oblique to the external foliation i.e. the foliation
present in the wrapping mass (Figure 17). This represents the
presence of a component of simple shear flow along the foliation
present in the wrapping mass, causing rotation to the relatively stiff
and isotropic material.
Figure 15:Layered ultramylonite with rotated
pressure shadow (near left margin).
Figure 16:Near complete transposition and
rotation of planes / siliceous lamellae in phyllite/
phyllonite. Note the presence of pseudo cross
bedding (below lens cap).
Figure 17: Small, rounded, relatively stiff mass,
present as rotated segment, wrapped within the
intensely foliated ultramylonite. The rounded segment
shows a crude sigmoidal trail of the internal
foliation, oblique to the external foliation. Shear
sense is sinistral.
The Parsoi belt, having a width of about 2km, is remarkably
devoid of chert bands. Innumerable thin quartz veins, sub-parallel
to the mylonitic foliation, however, traverse this belt. These
quartz veins are also affected by shearing and rendered as quartz
mylonites.
The commonest micro shape fabric in meta-graywacke,
near the margin of the WNW trending arm of Mahakoshals, is a
porphyroclastic texture where the ‘porphyroclasts’ are wrapped
around by quartz ribbons and muscovite flakes (Figure 18). Quartz
veins present in the entire belt are rendered quartz mylonite
showing a micro fabric predominantly of S-C type (Figure 19).
Presence of synthetic fractures/ slips in chloritoid porphyroclast of
phyllonite/ultramylonite (Figure 20) also represents a component
of simple shear (sinistral) affecting the rocks.
Minor faults with sinistral shift of the trail of vein quartz
boudins/blocks (Figure 21), present near the southern margin,
further confirm continuity of shear deformation till much late, even
after hardening of the deforming mylonite.
Figure 18: Quartz porphyroclast wrapped by
quartz ribbon and muscovite flakes. Crossed
nicols, 32.
Figure 19:S-C fabric shown by quartz mylonite.
Crossed nicols, 3X. Shear sense is sinistral.
Figure 20:Synthetic fracture/slip in chloritoid
porphyroclast within phyllite. Crossed nicols, 3X.
Shear sense is sinistral.
Figure 21: Fault affecting the quartz vein resulting
in trail of quartz boudins/blocks. A sinistral shift.
Supportive evidences from remote sensing
In satellite imagery, the WNW trending arm of the Mahakoshal
is represented as a sinuous belt hitting against the ENE-WSW
trending arm of the main Mahakoshals. The belt has a characteristic
broom stick fabric. This uncommon, very close spaced, fabric is
different from rest of the Mahakoshals. The close spaced fabric has
a cross cutting relationship with the ENE-WSW trending fabric of
the main Mahakoshals, as is seen west of Obra (Figure 22). In the
strike continuity, south of Chitrangi, the WNW-ESE fabric of the
sinuous belt abuts / merges into the main Mahakoshal belt. This
fabric appears to be superposed over the fabric present in the
northern main Mahakoshals.
Figure 22: Satellite image showing the sinuous belt of the Sidhi and Singrauli districts of M.P. and Sonbhadra
district of U.P. The northern Obra-Amsi-Jiawan (OAJ) fault (F) and the southern Songarh-kasar-Dudhi (SKD)
fault is marked with broken white lines.
Discussion and Conclusion
The main belt of Mahakoshal, extending between Narsinghpur
in the west to Sidhi in east, has a dominant ENE-WSW trend,
representing the axial traces of D1 and D2 structures. However,
the southeastern extension of the belt, lying between Obra-Amsi-
Jiawan fault (OAJ) and the Songarh-Kasar-Dudhi (SKD) fault, shows
a WNW-ESE trend. This trend clearly is superimposed on the main
Mahakoshal trend, well depicted in imagery, and does not appear to
have resulted by folding.
The overwhelming presence of rotation of fabric elements all
along the length and width of the WNW trending arm, frequent
presence of ‘alpha’ and ‘delta’ type structures, strongly asymmetric
‘S’ and at places ‘Z’ shaped folds and the sheath folds suggest that
the belt has undergone strong ductile deformation. Transposition
of fabric along the WNW-ESE trending foliation is also amply clear
all through the belt. This foliation, therefore, represents a ‘mylonitic
foliation’. The stretching and boudinaging of chert bands and
quartz veins along this foliation and shortening (tight folding) of the
obliquely oriented features, is suggestive of bulk flow along the
foliation. Rotation of the stretched boudins and of the axial planes
of near isoclinal folds, however, indicates that the deformation
was non-coaxial and progressive in nature. The dominance of
anticlockwise rotated fabric and shape symmetry indicate existence
of a sinistral sense of movement, though, at places, clockwise
rotated fabric along with ‘Z’ folds are also observed.
Finally, it is obvious that the shear generated structures present
in the sinuous belt were mistaken for turbidites in earlier literature.
Close association of these structures with shear sense indicators
further helps in differentiating them from normal syn-sedimentary
structures (turbidites) formed in tectonically active basins.
Overwhelming presence of rotational features in chert bands and
in quartz veins, further ascertains that the terrain is representative
of a flow regime. Splendid representation of the belt in satellite
imagery, clearly showing a superposed trend, leaves no ambiguity
in mind in differentiating the sinuous belt from the ENE-WSW
trending main Mahakoshals. The name DUDHMANIA SHEAR ZONE
{Banerji [1]} was proposed for this remarkable belt.
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