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Journal of Asian Earth Sciences 19 (2001) 1±15
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Structural pattern and kinematic framework of deformation in
the southern Nallamalai fold±fault belt, Cuddapah district,
Andhra Pradesh, Southern India
Mrinal Kanti Mukherjee
Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Calcutta, 700035, India

Abstract
An association of westerly verging asymmetric folds, easterly dipping cleavages and contractional faults control the pattern and intensity
of structures at different scales in the southern Nallamalai fold±fault belt, Cuddapah district of Andhra Pradesh, Southern India. Variation in
structural geometry is manifested across the section by the occurrence of relatively low amplitude folds, sometimes only a monocline and by
the near absence of contractional faults in the WSW, but tight to isoclinal folds with frequent fold±fault interactions through the central areas
towards ENE.
The relationships of structural elements in terms of orientation, style, sense of movement and general vergence indicate their development
under a progressive contractional deformation. The structures are interpreted to result from a combination of bulk inhomogeneous shortening
across the belt and a top-to-west, variable simple shear. Localized developments of crenulation cleavage, rotation of cleavage in the shorter
limbs of some mesoscale asymmetric folds and general variation of structural elements in morphology and associations across the belt,
indicate partitioning of deformation and a varying degree of non-coaxiality in discrete domains of the bulk deformation. q 2001 Elsevier
Science Ltd. All rights reserved.

1. Introduction
The Nallamalai fold±fault belt (NFB) is a major tectonic
element in the Proterozoic Cuddapah Basin of Southern
India. The NFB, which is arcuate and convex towards the
west, covers the eastern half of the basin. An understanding
of the gross tectonic features of the Cuddapah Basin and its
evolution through space and time has been achieved through
work spanning over a century (King, 1872; Narayanswami,
1966; Balakrishna et al., 1967; Sen and Narasimha Rao,
1967; Kaila and Bhatia, 1981; Kaila and Tewari, 1985;
Meijrink et al., 1984; Nagaraja Rao et al., 1987; Venkatakrishnan and Dotiwalla, 1987). Detailed documentation and
analysis of the structural geometry and deformation kinematics within the NFB, however, are few (cf. Saha, 1994;
Matin and Guha, 1996) but necessary for constructing
tectonic models of evolution of the deformed parts of the
Cuddapah Basin.
Here, I report a map for part of the Cuddapah Basin (Fig.
1) (1:50,000 scale), with a view to documenting the structural styles and interpreting the kinematic framework of
deformation in the southern NFB close to the relatively
undeformed lower Cuddapah succession in the west. The

E-mail address: res9429@www.isical.ac.in (M.K. Mukherjee).

study area, about 570 km 2 in extent, straddles the
Cuddapah±Chennai highway between Vontimitta and
Rajampeta and also includes stretches along Rajampeta±
Rayachoti road as far as Sanipai (Fig. 2). In this paper, the
results of the above study are documented in terms of
morphology, geometry, orientation, vergence and variation
of different structural elements and their relationships.
Based on this documentation, the kinematic framework of
deformation is analysed and discussed.

2. Stratigraphic framework
The general stratigraphy of the Cuddapah Basin is
outlined in Table 1 showing the Papaghni, Chitravati and
Nallamalai Groups that together constitute the Cuddapah
Supergroup. The rocks of the study area belong to the Nallamalai Group of the Cuddapah Supergroup (Nagaraja Rao et
al., 1987). The Nallamalai Group is subdivided into a lower,
quartzite dominant, Bairenkonda Formation and an upper,
shale dominant, Cumbum Formation, the type sections of

which are present in the northern part of the NFB.
The Nagari Quartzite and Pullampet Formation exposed
in the southern NFB are correlatives of the Bairenkonda and
Cumbum Formation, respectively (Meijrink et al., 1984;

1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00004-3

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M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

Fig. 1. Generalized geologic map of the Cuddapah Basin with the study area shaded.

Nagaraja Rao et al., 1987). In the study area, the Nagari
Quartzite ( ˆ Bairenkonda Formation) consists chie¯y of a
thick succession of sandstones and minor shales. It lies
unconformably over Archaean gneisses near Sanipai village

to the southwest (Fig. 2). Towards the northeast, the Nagari

Quartzite is overlain by relatively younger rocks of the
Pullampet Formation ( ˆ Cumbum Formation) which
consists of shales, dolomites, and graded siltstones capped

Table 1
Lithostratigraphy of the Cuddapah Basin (after Nagaraja Rao et al., 1987)
Group

Formation

Kurnool group

Nandyal Shale
Koilkuntla Limestone
Paniam Quartzite
Owk Shale
Narji Limestone
Banaganapalli Quartzite
Srisailam Quartzite

Nallamalai group

Cumbum: Phyllite, slate,
quartzite, dolomite
Cumbum (Pullampet) Formation
Bairenkonda (Nagari) Quartzites

Chitravati group

Gandikota Quartzite
Tadpatri Formation
Pulivendla Quartzite

Papaghni group

Vempalli Formation
Gulcheru Quartzites

Thickness (m)

Lithology

50±100
15±50
10±35
10±15
100±200
10±50
Unconformity
300
Unconformity

Shale
Limestone
Quartzite
Shale-ocherous
Limestone
Conglomerate, quartzite

2000

1500±4000

Pullampet: shale, dolomite, quartzite
Bairenkonda: quartzite and shale
Nagari: conglomerate, quartzites and shales with intrusives

Angular unconformity
300
4600
1±75
Disconformity
1900
28±210
Non-conformity
Archaean and Dharwar

Quartzites & shale

Quartzites and shale
Shale, ash fall tuffs, quartzite, dolomite with intrusives

Conglomerates and quartzite
Stromatolitic dolomite, dolomite mudstone, chert breccia and
quartzite Ð with basic ¯ows and intrusives
Conglomerate arkose, quartzite and shale

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

3

Fig. 2. Geologic map of the southern Nallamalai fold±fault belt. Locations of photographs, ®eld sketches, cross sections and equal area projection diagrams in
this paper are shown in the map as ®lled circles with symbols like 27D/3-94, 21J/1-95, etc.

by interbedded sandstones and shales and at places by white
medium grained quartzites.

3. Petrography of rocks in the study area
The percent mineralogical composition of grains and
matrix, the grain-matrix ratio, and the range of modal
grain size of representative specimens of different rock
types in the study area are brie¯y described below. The

data re¯ect the range of variation of petrographic characteristics in the study area.
Carbonates are generally impure with framework
grains consisting of quartz (0±36.5%), dolomite (0±
62.5%) and muscovite (1.5±13%). The matrix is
composed of dolomite (15.3±58.3%) and the combination quartz plus muscovite which together
comprise 17.8±27.4% of the rock volume. The grain/
matrix ratios in the carbonates vary between 0.33±
1.76 with modal grain sizes generally ranging between

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M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

Fig. 3. Cross section across the structural trend of major structures. Symbols are similar to those shown in the geologic map of Fig. 2. T. S. Ð Topographic
surface.

0.026±0.089 mm for framework grains and 0.001±0.02 mm
in the matrix.
Siltstones have framework grain sizes ranging between

0.026±0.141 mm consisting mainly of quartz (27±44.5%)
and muscovite (1.3±22.9%). The matrix comprises between
46.2±66.1% of the rock volume and consists mainly of
muscovite and quartz with grain sizes ranging from
0.004±0.076 mm. The grain/matrix ratios in siltstones
range between 0.24±1.158.
Interbedded siltstones and shales have framework grains
consisting of quartz (18±54.2%), muscovite (0±2.3%),
chlorite (0±7.6%) and opaque (0±2.6%). The matrix,
which occupies 45.7±69.4% of the rock volume, is mainly
composed of quartz, muscovite and chlorite. Grain sizes
range from 0.051± 0.106 mm in the framework and between
0.006±0.020 mm in the matrix.
Quartzose sandstones have framework grains consisting
of quartz (66.8±61.8%) and opaque (,8.8%) minerals. The
matrix comprises 24.2±29.2% of the rock volume and is
composed of muscovite, opaque and grains that are too
small to be identi®ed. Grain sizes range between 0.088±
0.708 mm with grain/matrix ratios varying from 2.417±
3.128.

4. Structural elements
Structural elements in the southern NFB are described
under the headings: faults, folds and cleavages. Locations
of the structure sections referred to in this paper are shown
in Fig. 2.

4.1. Faults
The study area is characterized by different orders of
contractional faults (cf. Price, 1968). First order faults,
which are regional thrusts having displacements of tens of
kilometres and strike lengths of hundreds of kilometres, are
not recognized in the area. Second order contractional faults
are characterised by displacements of tens of metres and
strike lengths up to 2±3 km. Third order faults are marked
by displacements of less than 10 m (generally 1±3 m) and
strike lengths less than 0.5 km. Both second and third order
faults occur in the study area (Fig. 2).
The majority of the second and third order faults dip
between 458 and 308 towards the ENE with dominant dipslip components (Fig. 3a,d), although relatively steeply
dipping contractional faults are not uncommon (Fig. 3c,e).
The east±west trending fault in the west central part of the
map near Naryanarajupalle, is an oblique slip high angle
reverse fault with a dominant dip-slip component. This
fault juxtaposes the older Nagari Quartzites against the
relatively younger dolomites and interbedded dolomites
and shales of the Pullampet formation to the south. The
displacement is at least 100 m.
Minor east±west trending faults with dominantly strikeslip components occur near Vontimitta. Development of
closely spaced minor third order faults bounded on either
side by second order faults, is common (Fig. 3c). Sometimes, minor antithetic third order contractional faults also
occur. Third order contractional faults occur in isolation in
relatively undeformed strata in the SW part of the study area
(Fig. 3d).

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

5

with interlimb angles ranging between 208 and 508. Where
adjacent to third order contraction faults, they are tight to
isoclinal. The geometry of these folds approximate class 1B
(Ramsay and Huber, 1987) in the sandstone beds and
conform to class 3 in the shales where interbedded sandstones and shales occur. Folds in dolomites have sharp
hinges with chevron morphology near the third order faults
(Fig. 4f). In the central part of the area, folds in dolomites
approximate an elongate dome-basin morphology. Folds in
relatively strongly deformed areas are asymmetric to overturned (Fig. 4a±c; Fig. 3) with axial planes dipping between
458 and 608 towards the ENE. Some folds near Hastavaram
(Fig. 4d,e), however show westerly dipping axial planes.
Plunge amount and directions for the fold axes are variable
throughout the study area (Fig. 5i).
4.2.2. Group-II
These folds are associated with bedding-parallel detachments (Fig. 6). They are small scale, slightly angular hinged
folds with wavelengths ranging between 10 and 15 cm.
A girdle distribution of poles to bedding and the attitude
of small scale fold axes show that folds in the study area
trend roughly NNW±SSE (Fig. 5a,e). In some places, the
fold axes show obliquity up of a maximum of 408 to the
regional trend (Fig. 5j).
Fig. 4. Field sketches of folds at different locations and with variable lithologies. Locations: (a) 20J/1-96 (North of Isukapalle, looking SSE) (b) 21J/195 (near Hastavaram clay quarry, looking NNW), (c) 27D/3-94 (near Isukapalle, looking NNW) (d) & (e)1J/1-96 (south-east of Hastavaram, looking
SSE), (f) 26D/7-95(near the location of (b), looking NNW). Symbols:
Stippled Ð sandstones, dashed Ð shales, and bricked Ð dolomites.

4.2. Folds
Folds occur on different scales in the area. First order
folds are large structures with wavelengths up to 3 km and
axial trace continuity of up to 8 km. Second order folds have
wavelengths on the order of several metres and axial trace
continuity of about 2 to 3 km. Higher order folds of relatively smaller dimensions occur within domains of second
order folds.
The observed folds have been classi®ed into two groups
based on associations with other structural elements,
geometry and distribution within the area of study:
4.2.1. Group-I
This group comprises both isolated folds in relatively
undeformed strata and also fold complexes associated
with third order contraction faults. Fold styles vary with
lithology. Broad open folds with wavelengths varying
between 400±500 m and amplitudes up to 50 m, with
round hinge and interlimb angles of 808±1108, occur in
the south central part of the area within the shales and
quartzites of the Cumbum Formation. Minor folds in the
sandstone±shale intercalations in the eastern part have
wavelengths of 0.3±1 m and amplitudes of around 1 m

4.3. Cleavage
Cleavage occurs both as continuous and spaced cleavages
(Powell, 1979). Continuous cleavage is developed in argillaceous rocks where it is de®ned by preferred orientation of
platy minerals distributed evenly throughout the rock
rendering it morphologically similar to slaty cleavage.
Spaced cleavage occurs both as disjunctive and crenulated
types (Powell, 1979), the former being con®ned mainly to
dolomites and the latter to argillaceous rocks in which the
spaced cleavage transposes an earlier slaty cleavage. Spaced
disjunctive cleavages in dolomites are frequently associated
with profuse solution seams. Bedding laminae are offset
against cleavage seams where the two planes make an
acute angle. Without offset, the cleavage is orthogonal to
the bedding laminae. These cleavages are frequently anastomosing to rough in dolomites (Fig. 8a) but tend to be smooth
in calcareous mudrocks.
Crenulation cleavage develops as a second set cleavage
exclusively in the argillaceous rocks where they transpose
the earlier slaty cleavage. Fig. 7a and b shows the disposition of the two sets of cleavage in shales near Marayigaripalle and Buduguntapalle villages, respectively. They
also occur near Ellamrajupalle and Yerracheruvupalle.
Sometimes crenulation of ®ne primary lamination occurs
within spaced cleavage domains.
The dominant strike of slaty cleavage and disjunctive
spaced cleavage in the area is NNW±SSE with dips varying
between 158 and 808 towards the NE (Fig. 5b±d).
Crenulation cleavage also strikes NNW±SSE and dips

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M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

Fig. 5. Equal area projection for orientation data of different structural elements. (a)±(g) are synoptic diagrams from the entire study area (data from
Narayanarajupalle are not included in this synoptic diagram; see (j)). (a) poles to bedding (S0) (N ˆ 500), (b) poles to cleavage (S1) in shales (N ˆ 175)
(c) poles to cleavage (S1) in interbedded sandstones and shales (N ˆ 58), (d) poles to cleavage (S1) in dolomites and dolomitic limestones (N ˆ 48), (e)
orientation of fold axes of small folds (N ˆ 60), (f) cleavage (S1) and bedding (S0) intersection lineation (L1) (N ˆ 60), (g) slickensides (N ˆ 54), (h) poles to
axial planes of folds in different locations in the study area; for example around Vontimitta and Nadimpalle (squares), Isukapalle (upright triangles),
Hastavaram (inverted triangles) and Attirala & Gollapalle (circles). (i)±(l) Orientation data of various structural elements (®lled circles Ð poles to bedding
(S0), open circles Ð poles to cleavage (S1), squares Ð poles to cleavage (S2), inverted triangles Ð axes of small folds, right triangles Ð slickensides) around
(i) Marayigaripalle, (j) Narayanarajupalle, (k) Buduguntapalle and (l) Yerracheruvupalle.

708 to 808 towards the NE (Fig. 5i l). The strike of crenulation cleavage is statistically parallel to the axes of mesoscopic folds in the study area.
Cleavage development in the area is strongly controlled
by lithological contrasts as suggested by the following
observations:
1. Refraction of cleavage with relatively steep dips are
common in dolomites while gentle dips occur in interbedded calcareous siltstones in some places (Fig. 8b).
2. Cleavage in overlying dolomites is absent while welldeveloped cleavage occur in the underlying calcareous
siltstones (Fig. 8c).
The morphology and development of cleavage is variable
across the study area from WSW to ENE. There is sporadic

development of cleavage in shales, but quartzites are usually
devoid of any cleavage west of a line joining Buduguntapalle and Balarajupalle. On the other hand, argillaceous
rocks east of the line are strongly cleaved. Selective occurences of cleavage are also observed in dolomites, intercalated sandstones and shales from the eastern part.

5. Interrelationship of mesoscopic structures
Faults, folds and cleavages described in the previous
section interactively de®ne the structural make up of the
NFB. Small scale folds occur in the hanging wall of the
third order contraction faults. Intense fold±fault interactions
lead to development of anticlines stacked over anticlines
with synclines faulted out (Fig. 9a,b, Fig. 3a,c) leading to

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

7

uous cleavage in shales, interbedded shales and sandstones,
and spaced disjunctive cleavages in dolomites and dolomitic limestones (compare Fig. 5b±d with h). Cleavagebedding intersection lineation (L1) also conforms to the
orientation of the fold axes of small folds (compare Fig.
5e with f). These together indicate that, in general, the cleavage just mentioned are axial planar (S1) to the mesoscopic
folds.
In some places where decollements occur, cleavage
develops in the hanging wall (Fig. 6). Also in some areas,
such as near Gollapalle (Fig. 8c) and Marayigaripalle
villages, cleavages are reoriented. Here cleavage strikes
nearly parallel to the trend of the fold axes but dips vary
across the folds as follows: cleavage is horizontal in the
steep to overturned limb of the asymmetric folds and thus
is oblique to the easterly dipping axial planes of the folds.
On the gently dipping normal limbs of the folds, cleavage is
nearly parallel to the axial planes of the folds.

6. Variation of mesoscopic structures across the belt

Fig. 6. Graphic log constructed at location 6F/1-95. Note that small folds
and cleavage occur above D±D 0 but are absent below it even though the
gross lithology remains the same. D± D 0 is interpreted to represent a
decollement.

an imbricate geometry that de®nes the contractional fault
zones.
The equal area projection of poles to the axial planes of
the mesoscopic folds lie close to the poles to the contin-

The spatial variation of mesoscopic structures is well
displayed in the area when traced from ENE to WSW. In the
WSW part of the area, the Nagari Quartzites come into contact
with the Archaean peninsular gneisses and granites. Here the
sedimentary cover is relatively undeformed, except for local
development of a monocline or cleavage in thin shaly interbeds. Minor folds appear near Balarajupalle village on the
eastern bank of the Cheyyeru river. Sporadic cleavage is
con®ned to thin micaceous siltstone horizons within the
Nagari Quartzites that outcrop on the western side of the
Cheyyeru river in the southwestern part of the area. The tightness of the folds increases with increased fold±fault interactions across the central part of the study area towards the ENE.
Cleavage is also well developed in the central and eastern part
of the study area. Table 2 summarizes the spatial variation and
association of structural features from the area.

Table 2
Variation of structures across the belt in different sectors of the study area in the southern Nallamalai fold-fault belt
Structures

1. Fold interlimb
angle
2. Wave length of
folds
3. Dip of axial
plane
4. Hinge
5. Fold±fault
interaction
6. Cleavage

Sectors
Rachapalle-BalarajupalleBuduguntapalle sector (WSW)

Balarajupalle-SR palem-GundlapalleHastavaram sector (CENTRAL)

Hastavaram-Isukapalle-Attirala-Golapalle
sector (ENE)

±

1158±608 (Quartzites and siltstones of
Cumbum Formation)
400±500 m

608±208(Interbedded sandstones and shales and
silty units)
, 0.5±70 m

608±708 (ENE)

458±558 (ENE)

Well rounded
Decollements with transported folds in
some; blind thrusts
Generally slaty in argillites and
disjunctive spaced in dolomites. Spaced
to no cleavage in quartzites

Angular
Intense fold±fault interaction with development
of tight to isoclinal folds cut by faults
Slaty in siltstones and slaty to spaced in dolomites.
Spaced cleavage tending towards slaty in
sandstones in some.

Only a monocline is observed
(in Nagari Quartzites)
±
±
Minor decollements in
isolation
Sporadic (develops only in
micaceous siltstones)

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M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

Fig. 7. (a) Slaty cleavage (trace at about 458 to the horizontal bedding laminae) and steeper, spaced crenulation cleavage (trace parallel to hammer handle) in
shales near Marayagaripalle (location- 5J/4-97; section looking towards NNW); (b) Bedding and cleavage in shales near Buduguntapalle (location- 7F/1-97).
Bedding (S0) is horizontal, slaty cleavage (S1) is gently dipping and crenulation cleavage (S2) is steep; (c) Nearly horizontal orientation of cleavage in the steep
(nearly vertical) limb of a westerly verging asymmetric anticline in dolomites with intercalated calcareous shales near Gollapalle (location- 28J/4-97; looking
towards SSE, pencil points towards west).

7. Chronology of development of structural elements
Based on observed relationships, it is apparent that slaty
cleavage in mud rocks and slaty to disjunctive-spaced clea-

vage in dolomites were the ®rst structures to form. In some
places (such as around Yerracheruvupalle, Marayigaripalle,
Buduguntapalle), the early cleavage is crosscut by a
relatively younger and steeper crenulation cleavage. Slaty

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

9

Fig. 8. (a) Disjunctive-spaced cleavages in bedded dolomites. Beds dip from the upper left to lower right. Cleavage is parallel to the pencil (location- 25J/1-97;
looking towards NNW); (b) Cleavage refraction in contrasting lithologies. Steep to vertically oriented spaced cleavage occurs in dolomites that overlie
intercalated calcareous siltstones having relatively gently dipping slaty cleavage. Bedding is horizontal (location- 28J/1-97; section looking towards SSE;
scale bar is 7 cm); (c) Cleavage develops in underlying calcareous siltstones but no cleavage occurs in overlying dolomites (location- 24J/5-97; looking
towards NNW).

cleavage in argillites and disjuntive-spaced cleavage in
dolomites, which are in general axial planar to folds, are
sometimes modi®ed in their orientation, in the short and
steep limb of asymmetric mesoscale folds. Here cleavage
becomes almost horizontal (Fig. 8c) and no longer remain
axial planar to the folds. These features are interpreted to be
indicative of an earlier origin of slaty cleavage prior to the
development of mesoscale folds.
Within fault zones, folds have developed in the hanging
wall of contractional faults in order to accommodate shortening during fault propagation. Here cleavages are truncated
by faults and are intensi®ed in the vicinity of the faults,
suggesting that the cleavages developed earlier than the faults
and have been modi®ed concurrently with fault development.
The distribution of folds and cleavages helps in certain
cases to identify small scale decollements internal to the
Cumbum formation (Fig. 6). In these areas, cleavage
probably develops very early with the initiation of the
decollements.

8. Kinematic framework of deformation
8.1. Progressive deformation and its elements in the study
area
The relationships of the structural elements, their
distribution and style, indicate that they developed
during a single progressive contractional deformation.
Progressive deformation is used to imply (Tobisch and
Paterson, 1988):
1. A close relationship between various sets of structures in terms of orientation, sense of movement,
style and prevailing metamorphic conditions.
2. A relatively constant orientation of the regional stress
®eld.
3. The various sets of structures developed during a
relatively continuous sequence of events within a
geologically short period of time.

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M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

Fig. 9. (a) Fold±thrust interactions with anticlines stacked over anticlines along moderately steep ramps whereas synclines are apparently faulted out (location12J/2-97; looking towards NNW). (b) Sketch from the photograph (a) shows the nature of the ramps.

The variable lithological composition and competence
as well as inherent inhomogeneities in layer boundary
surfaces (for example: large scale trough cross bedding,
and wavy bedding) provide a layered anisotropy suitable
for the establishment of buckling instability under
contractional deformation. The axial planes of buckle
folds and axial planar cleavage showing an easterly
dip, combined with differences in the limb lengths of
the folds, indicates asymmetry in the mesoscopic structures with an overall vergence towards the WSW. This
vergence conforms to the easterly dipping contractional
faults that occur in the area. In addition, slickensides,
which are present dominantly on the gentle limbs of
asymmetric mesoscopic folds (Fig. 5k), indicate an
apparent top-to-west sense of movement as determined
using criteria outlined by Petit (1987). All these features
together suggest a WSW directed movement.

The inter-relationship of mesoscopic structures, their
distribution and spatial variation in the study area, re¯ects
strain heterogeneity on a variety of scales. The association
and general facing of structures indicates a combination of
inhomogeneous shortening in an ENE±WSW direction and
a top-to-WSW directed tectonic transport that results in
heterogeneous deformation. Heterogeneous deformation
can be assumed to involve discrete domains of homogeneous deformation of different types. The smaller domains
of homogeneous deformation appear to involve either pure
shear or simple shear.
In order to explain the kinematics of deformation in the
study area, pure shear is assumed with the principal shortening direction along the horizontal in an ENE±WSW
direction. At the outset, this shortening acts parallel to the
bedding or primary layering of sedimentary rocks and is
referred to here as layer-parallel shortening (LPS). The

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

11

Fig. 10. Model structures that develop under pure shear with horizontal shortening parallel to the layers. (a) Decollements, when A is much more competent
than B; (b) Buckle folds, where layers A and B have nearly equal competence; (c) Penetrative cleavage in a single layer.

pure shear component may vary in different layers to give
rise to a bulk inhomogeneous shortening. A top-to-WSW
directed simple shear is also assumed in a similar manner.
Under pure shear, as de®ned above, three different deformation features develop in layered rocks.
(A) Differences in pure shear component in different
layers of varying competence causes the softer layer to
experience greater shortening (higher LPS) relative to the
stiffer layer lying immediately above or below it. This
results in instability along the interface leading to decollements with slickensides on the interface (Fig. 10a) The
stiffer layers represent quartzose sandstones, siliceous
dolomites or calcarenites whereas the softer layers are
made up of argillaceous shales or calcareous mudrocks.
(B) Buckle folds form as a result of buckling instabilities
that stem from internal perturbations in the primary layers
during LPS (Fig. 10b).
(C) Cleavage (S1), as the expression of ¯attening,
assumed to have been oriented normal to the direction
of maximum shortening during LPS. Thus cleavage will
be vertical (Fig. 10c).

might develop on the hanging wall side of the ramp as a
consequence of ramp development (Fig. 12b-i) similar to
the `fault-propagation' folds of Suppe and Medwedeff
(1990). Development of more frontal ramps in an imbricate
style under continued progressive shortening may then give
rise to intense fold±fault interactions (Fig. 12b-ii). In this
model, folding in the footwall is not signi®cant and the
ultimate structural manifestation will be a series of contraction faults with hanging wall anticlines stacked one on top
of another with an apparent absence or faulting out of any
synclines (Fig. 9a,b, Fig. 3a,c). Where footwall deformation
is also signi®cant (Fig. 3e), the fold±fault relationship is
interpreted in terms of the `break-thrust' model (Willis,
1893; Butler, 1992; Morley, 1994). In this model, folding
precedes thrusting (Fig. 13a). Folds may also originate in
the hanging wall of a bedding-parallel thrust (decollement)
without ramp development. These are known as detachment
folds (Jamison, 1987) (Fig. 13b), that develop in order to
accommodate the shortening in the hanging wall during
thrust propagation. Such folds may also be associated with
axial planar cleavage (Fig. 6).
8.3. Asymmetric folds

8.2. Fold±fault interactions
Superposition of a ®nite top-to-WSW simple shear intensi®es the layer-parallel slip in (A). In this process, small
decollements that had already developed under pure shear
(Fig. 11a) and those which are newly initiated (Fig. 11b) are
extended in the slip direction. Where the tip of the decollement encounters a perturbation in the footwall in the frontal
part, a ramp develops that cuts upward through the primary
layering (Knipe, 1985) (Fig. 12a). In this process, folds

Superimposition of simple shear on symmetrical buckle
folds, as in (B), results in westerly-verging asymmetric
folds. The different models of asymmetric buckle folds
need to be discussed here to understand their importance
in the study area.
Asymmetric buckle folds may arise in many different
ways during contractional deformation (Price, 1967; Treagus, 1973; Sanderson, 1979; Ramsay and Huber, 1987, p.
28; Rowan and Klig®eld, 1992). Asymmetric folds are not
only asymmetric in terms of limb dip, but also in limb length

12

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

Fig. 11. Discontinuous deformation features under the effect of superimposed simple shear. (a) Additional displacement on existing decollement (compare
with Fig. 10a); (b) decollements that are initiated, due to failure and displacement at the layer contact, under a shear couple.

where the steep limb is shorter than the gentle limb.
Limb length asymmetry geometrically requires limb dip
asymmetry for development of a train of asymmetric
folds with a uniform enveloping surface. With these
constraints, two separate types of asymmetric folds are
distinguished:
Type I folds: These are generated by modi®cation of
symmetrical buckle folds by superimposed ®nite shear

strain (Ramberg, 1963). These folds are distinguished
by their shortening and thickening of the short limbs
while long limbs are lengthened and thinned. This
relationship implies large and ®nite shear strain and
a relatively low viscosity contrast between layer and
medium.
Type II folds: Here asymmetry develops progressively
during the course of folding without the signi®cant
internal strain required to shorten and lengthen the

Fig. 12. (a) Ramp and ¯at development in the process of layer parallel slip over a decollement (see text); (b) (i) hanging wall anticline formed as a result of fault
propagation and (ii) imbricate geometry due to the stacking of anticlines.

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

13

Fig. 13. Development of a (a) break-thrust subsequent to folding and (b) detachment fold in the hanging wall of a bedding-parallel thrust.

limbs. This implies a migration of the hinge area towards the
shorter limb of the fold. Type-II folds would develop under
conditions of high viscosity contrast.
Type-I asymmetric folding is more common in areas of fold±
thrust interactions (Rowan and Klig®eld, 1992). The asymmetric buckle folds associated with contraction faults in the
study area may be explained in terms of the above alternatives
or a possible combination of them. However Type-I and TypeII are mutually exclusive in the sense that the latter requires
higher viscocity contrasts compared to Type-I. Type-I asymmetry also explains the passive rotation of cleavages in the
shorter limbs of asymmetric folds as the limb rotates under
simple shear.
8.4. Disjunctive and crenulation cleavage
Finally, the consideration of the effect of top-to-WSW
directed simple shear on cleavage, as in (C), will rotate
the vertical cleavage and make it inclined (Fig. 14). This
now conforms with the easterly-dipping cleavage in those
localities in the study area where bedding is horizontal to
sub-horizontal. In such situations, the greater the magnitude
of the shear the gentler the cleavage dip. Variations in the
magnitude of this simple shear from domain to domain
across the structural trend would then account for variations
in the attitude of cleavage. Where cleavages occur in association with folds, top-to-WSW simple shear and the rotation of fold limbs together control the rotation of early
formed cleavage.
The origin of locally developed crenulation cleavages
(S2) in the study area depends on the nature and orientation
of the earlier foliation.
1. Where the earlier cleavage is a primary scaly foliation
parallel or subparallel to the bedding laminae (cf. fabric

by load metamorphism), crenulation is brought about by
buckling of this foliation to form microfolds. The plane
of weakness that develops along the limbs of the folds
and oriented parallel to the axial plane of the microfolds
de®nes the crenulation cleavage.
2. A second type of crenulation cleavage develops where
the earlier cleavage is tectonic and dips between 208 and
458, towards the NE with the bedding plane sub-horizontal. The crenulation cleavage is vertical or dips steeply
towards the NE. This may be explained in terms of a
combination of pure shear and simple shear as follows:
In progressive simple shear alone, the X axis along with
the XY plane of the incremental strain ellipsoid makes an
angle of 458 with the direction of simple shear (Ghosh,
1993, p. 149). Cleavage (S1) may become parallel to this
direction by either, (1) progressive simple shear-induced
rotation of cleavage that developed earlier under pure
shear, or, (2) initiation of cleavage along the incremental
XY plane during simple shear alone. In either case the
cleavage will be rotated gradually towards the shear
plane under progressive simple shear and this implies
that the angle between cleavage and shear direction
gets progressively smaller. When this angle has become
rather small (about 208), a large increment of simple

Fig. 14. The superposition of simple shear on vertical cleavage causes the
cleavage to become inclined. Here the superimposed simple shear is top-towest and thus the resultant cleavage will be easterly dipping.

14

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

8.5. Kinematic framework

Fig. 15. (a) The relationship between shear strain (g ) and angle (u Ð in
degrees) between the X-axis of the strain ellipsoid and the shear direction in
a progressive simple shear deformation. (b) Development of asymmetric
buckles on the early cleavage, when the latter is oriented at nearly 458 to the
direction of horizontal shortening. Crenulation cleavage develops along the
dashed lines.

shear is required to bring about a very small rotation of
the XY plane towards the plane of shear (Fig. 15a). Hence
if cleavage is assumed to track the XY plane of the strain
ellipsoid, then the rate of rotation of cleavage will gradually decrease with progressive simple shear. Although
further rotation by simple shear under very large shear
strain is theoretically possible (Fig. 15a), such a large
effect of simple shear seems unlikely to have occurred
in the study area as evidenced by the apparent absence of
any structures indicative of high-magnitude simple shear.
The cleavage therefore attains a steady orientation
between u values of 458 and 208 (Fig. 15a), where u is
the angle between the X-axis of the strain ellipsoid and
the shear direction in a progressive simple shear deformation. If a pure shear is next superimposed, then this
steadily oriented cleavage, which imparts an anisotropy
to the rock and makes an angle slightly less than 458 with
the horizontal shortening direction will be buckled
(Cosgrove, 1976). If the earlier cleavage makes an
angle slightly less than 458 with the horizontal shortening
direction, then the general displacement pattern will
involve asymmetric buckles (Cosgrove, 1976) (Fig.
15b). A plane of weakness develops along the short
limbs of the asymmetrically-folded earlier cleavage to
de®ne a second, spaced crenulation cleavage (S2) (Fig.
15b) which applies to those areas where locally spaced
asymmetric crenulation cleavage has developed.

From the above discussion, the kinematic framework
should not be regarded as composed of pure shear and
simple shear acting as discrete phases of deformation.
Instead, both components were simultaneously operating
in varying combinations to give rise to a generally noncoaxial progressive deformation.
Although the general trend of structural elements is
NNW±SSE, the fold axis at a few localities exhibit an oblique relationship of up to 408 with the regional trend (Fig.
5j). This obliquity may be explained by envisioning an oblique stretching component on the fold axis within the axial
plane of the folds (Sanderson, 1972) that also results in
variations in both the pitch and plunge of the fold-axes.
The E±W trending second order fault in the map is an
apparent lateral ramp within a fold±thrust system where the
frontal ramp verges WSW. This fault may be a reactivation
of an earlier normal fault. Movement under strike-slip
component along such a transverse fault, where the hanging
wall side moves towards west, apparently results in curvature of the axial plane of the folds (as exemplied by the
curved anticlinal axial trace immediately north of Narayanarajupalle in Fig. 2).
The kinematic framework of deformation in the southern
Nallamalai fold±fault belt is thus marked by a heterogeneous non-coaxial ¯ow with a shear component directed
towards the WSW.
9. Conclusions
1. The dominant structural elements of the southern NFB
are folds, faults, and cleavages that interactively de®ne
the structural setting of the region. The major trend of
structural elements are NNW±SSE.
2. Spatial variations from WSW to ENE are observed in the
fold dimensions, fold±fault interactions and cleavage
development. These variations may re¯ect internal strain
heterogeneity within a bulk, progressively contractional
type of deformation with varying non-coaxiality.
3. The dominantly WSW verging folds along with contractional faults and regionally developed easterly-dipping
cleavages indicate an asymmetry of structure that results
from tectonic transport towards the WSW.
4. Kinematically speaking, the regional deformation is
marked by a combination of pure shear and top-to-west
simple shear across the belt. The magnitude of these
components varies locally in discrete domains within
the bulk deformational framework.

Acknowledgements
The work presented in this paper was completed as a part
of the requirement for a doctoral dissertation, during tenure

M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15

of a research fellowship at the Indian Statistical Institute,
Calcutta. Many ideas discussed in the paper arose during
stimulating discussions with Dr. Dilip Saha. I am indebted
to his constant encouragement, advice, and suggestions. I
thank Prof. Kevin Burke, Dr. Amit Singh and Dr. Jennifer
Lytwyn for critical comments on the manuscript. Field
support was from a research project grant (Acc No- 5624)
awarded to Dr. Dilip Saha, by I.S.I. Thanks are extended to
Khoka Oraon of Geological Studies Unit, Ashok Sill, Ashok
Saha and Swapan Aich of the Transport Unit for their active
cooperation in the ®eld. I acknowledge T.S. Kutty and Sojen
Joy for helping me with their computer programme for
equal area projection of orientation data. Shri A.K. Das of
G.S.U. is thanked for drafting some of the ®gures.
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