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.q2001 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; Venkata-krishnan and Dotiwalla, 1987). Detailed documentation and analysis of the structural geometry and deformation kine-matics 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 struc-tural styles and interpreting the kinematic framework of deformation in the southern NFB close to the relatively undeformed lower Cuddapah succession in the west. The
study area, about 570 km2 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 Nalla-malai 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;
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Nagaraja Rao et al., 1987). In the study area, the Nagari Quartzite (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 (Cumbum Formation) which consists of shales, dolomites, and graded siltstones capped M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
2
Fig. 1. Generalized geologic map of the Cuddapah Basin with the study area shaded.
Table 1
Lithostratigraphy of the Cuddapah Basin (after Nagaraja Rao et al., 1987)
Group Formation Thickness (m) Lithology
Kurnool group Nandyal Shale 50±100 Shale
Koilkuntla Limestone 15±50 Limestone
Paniam Quartzite 10±35 Quartzite
Owk Shale 10±15 Shale-ocherous
Narji Limestone 100±200 Limestone
Banaganapalli Quartzite 10±50 Conglomerate, quartzite
Unconformity
Srisailam Quartzite 300 Quartzites & shale
Unconformity Nallamalai group Cumbum: Phyllite, slate,
quartzite, dolomite
Cumbum (Pullampet) Formation 2000 Pullampet: shale, dolomite, quartzite Bairenkonda (Nagari) Quartzites 1500±4000 Bairenkonda: quartzite and shale
Nagari: conglomerate, quartzites and shales with intrusives Angular unconformity
Chitravati group Gandikota Quartzite 300 Quartzites and shale
Tadpatri Formation 4600 Shale, ash fall tuffs, quartzite, dolomite with intrusives
Pulivendla Quartzite 1±75 Conglomerates and quartzite
Disconformity
Papaghni group Vempalli Formation 1900 Stromatolitic dolomite, dolomite mudstone, chert breccia and quartzite Ð with basic ¯ows and intrusives
Gulcheru Quartzites 28±210 Conglomerate arkose, quartzite and shale Non-conformity
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 character-istics 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 com-bination 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 M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
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 shaleshave 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 sandstoneshave 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 ordercontractional faults are characterised by displacements of tens of metres and strike lengths up to 2±3 km.Third orderfaults 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 458and 308towards the ENE with dominant dip-slip 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 strike-slip components occur near Vontimitta. Development of closely spaced minor third order faults bounded on either side by second order faults, is common (Fig. 3c). Some-times, 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 4
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 rela-tively 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
with interlimb angles ranging between 208and 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 sand-stones 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 over-turned (Fig. 4a±c; Fig. 3) with axial planes dipping between 458and 608towards 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 detach-ments (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).
4.3. Cleavage
Cleavage occurs both ascontinuousandspacedcleavages (Powell, 1979). Continuous cleavage is developed in argil-laceous 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 anasto-mosing 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 disposi-tion of the two sets of cleavage in shales near Marayi-garipalle 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 158and 808towards the NE (Fig. 5b±d).
Crenulation cleavage also strikes NNW±SSE and dips M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
708to 808towards the NE (Fig. 5i l). The strike of crenula-tion cleavage is statistically parallel to the axes of meso-scopic 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 inter-bedded calcareous siltstones in some places (Fig. 8b). 2. Cleavage in overlying dolomites is absent while
well-developed 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 Budugunta-palle and BalarajuBudugunta-palle. On the other hand, argillaceous rocks east of the line are strongly cleaved. Selective occur-ences of cleavage are also observed in dolomites, interca-lated 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
6
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) (N500), (b) poles to cleavage (S1) in shales (N175)
(c) poles to cleavage (S1) in interbedded sandstones and shales (N58), (d) poles to cleavage (S1) in dolomites and dolomitic limestones (N48), (e)
orientation of fold axes of small folds (N60), (f) cleavage (S1) and bedding (S0) intersection lineation (L1) (N60), (g) slickensides (N54), (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 circlesÐ poles to bedding (S0),open circlesÐ poles to cleavage (S1),squaresÐ poles to cleavage (S2),inverted trianglesÐ axes of small folds,right trianglesÐ slickensides) around
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-uous cleavage in shales, interbedded shales and sandstones, and spaced disjunctive cleavages in dolomites and dolomi-tic limestones (compare Fig. 5b±d with h). Cleavage-bedding 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 clea-vage 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
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 inter-beds. 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 tight-ness of the folds increases with increased fold±fault interac-tions 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.
M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
Fig. 6. Graphic log constructed at location 6F/1-95. Note that small folds and cleavage occur above D±D0but are absent below it even though the gross lithology remains the same. D± D0 is interpreted to represent a decollement.
Table 2
Variation of structures across the belt in different sectors of the study area in the southern Nallamalai fold-fault belt
Structures Sectors
± 1158±608(Quartzites and siltstones of
Cumbum Formation)
608±208(Interbedded sandstones and shales and silty units)
2. Wave length of folds
Only a monocline is observed (in Nagari Quartzites)
400±500 m ,0.5±70 m
3. Dip of axial plane
± 608±708(ENE) 458±558(ENE)
4. Hinge ± Well rounded Angular
5. Fold±fault interaction
Minor decollements in isolation
Decollements with transported folds in some; blind thrusts
Intense fold±fault interaction with development of tight to isoclinal folds cut by faults
6. Cleavage Sporadic (develops only in micaceous siltstones)
Generally slaty in argillites and disjunctive spaced in dolomites. Spaced to no cleavage in quartzites
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
8
Fig. 7. (a) Slaty cleavage (trace at about 458to 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
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 short-ening 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 struc-tures 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.
M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
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 struc-tures 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 homoge-neous 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 short-ening 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
10
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 defor-mation 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 decol-lements 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).
8.2. Fold±fault interactions
Superposition of a ®nite top-to-WSW simple shear inten-si®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 decolle-ment 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
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 contrac-tion 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
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; Trea-gus, 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 M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
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 M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
12
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.
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 asym-metric 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 I and Type-II are mutually exclusive in the sense that the latter requires higher viscocity contrasts compared to Type-I. Type-I asym-metry 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 asso-ciation with folds, top-to-WSW simple shear and the rota-tion of fold limbs together control the rotarota-tion 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 208and 458, towards the NE with the bedding plane sub-horizon-tal. 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, theXaxis along with theXYplane of the incremental strain ellipsoid makes an angle of 458with 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 M.K. Mukherjee / Journal of Asian Earth Sciences 19 (2001) 1±15
Fig. 13. Development of a (a) break-thrust subsequent to folding and (b) detachment fold in the hanging wall of a bedding-parallel thrust.
shear is required to bring about a very small rotation of theXYplane towards the plane of shear (Fig. 15a). Hence if cleavage is assumed to track theXYplane of the strain ellipsoid, then the rate of rotation of cleavage will gradu-ally 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 betweenu values of 458and 208(Fig. 15a), whereu is the angle between theX-axis of the strain ellipsoid and the shear direction in a progressive simple shear defor-mation. 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 458with the horizontal shortening direction will be buckled (Cosgrove, 1976). If the earlier cleavage makes an angle slightly less than 458with 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.
8.5. Kinematic framework
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 non-coaxial progressive deformation.
Although the general trend of structural elements is NNW±SSE, the fold axis at a few localities exhibit an obli-que relationship of up to 408 with the regional trend (Fig. 5j). This obliquity may be explained by envisioning an obli-que 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 curva-ture of the axial plane of the folds (as exemplied by the curved anticlinal axial trace immediately north of Naraya-narajupalle in Fig. 2).
The kinematic framework of deformation in the southern Nallamalai fold±fault belt is thus marked by a heteroge-neous 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
contrac-tional 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
14
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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