Structural pattern and kinematic framework of deformation in the southern Nallamalai fold–fault belt, Cuddapah district, Andhra Pradesh, Southern India

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: [email protected] (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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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

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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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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).

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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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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

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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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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

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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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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

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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

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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.

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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.

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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. References Balakrishna, S., Chistopher, G., Ramana Rao, A.V., 1967. Regional magnetic and gravity studies over Cuddapah basin. Proceedings of the Symposium on Upper Mantle Project. GRB & NGRI Publ. No. 8., pp. 309±319. Butler, R.W.H., 1992. Structural evolution of the western Chartreuse fold and thrust system, NW French Subalpine Chains. In: McClay, K.R. (Ed.), Thrust tectonics. Chapman & Hall, New York, pp. 287±297. Cosgrove, J.W., 1976. The formation of crenulation cleavage. Journal of the Geological Society, London 132, 155±178. Ghosh, S.K., 1993. Structural Geology; Fundamentals and modern developments. Pergamon Press, Oxford, p. 598. Jamison, W.R., 1987. Geometric analysis of fold development in overthrust terrains. Journal of Structural Geology 9, 207±219. Kaila, K.L., Bhatia, S.C., 1981. Gravity study along Kavali-Udipi Deep Seismic Sounding Pro®le in the Indian Peninsular Shield: Some inferences about the origin of anorthosites and Eastern Ghats orogeny. Tectonophysics 79, 129±143. Kaila, K.L., Tewari, H.C., 1985. Structural trends in the Cuddapah Basin from Deep Seismic Soundings (DSS) and their tectonic implications. Tectonophysics 115, 68±86. King, W., 1872. The Kadapah and Karnul Formations in the Madras Presidency. Geological Survey of India, Memoir 8 (pt-1), 1±346. Knipe, R.J., 1985. Footwall geometry and the rheology of thrust sheets. Journal of Structural Geology 7, 1±10. Matin, A., Guha, J., 1996. Structural geometry of the rocks of the southern part of the Nallamalai Fold Belt, Cuddapah Basin, Andhra Pradesh. Journal Geological Society of India 47, 535±545. Meijrink, A.M.J., Rao, D.P., Rupke, J., 1984. Stratigraphy and structural development of the Precambrian Cuddapah Basin, S. E. India. Precambrian Research 26, 57±104. Morley, C.K., 1994. Fold-generated imbricates: examples from the

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