Magmatic and Tectonic Structures from the Chimalpahad Layered Complex, Andhra Pradesh, India

Gondwana Research, K 7, No. 3,pp. 887-896. 02004 International Association for Condwana Research, Japan. ISSN: 1342-937X

Magmatic and Tectonic Structures from the Chimalpahad Layered Complex, Andhra Pradesh, India M. Narsimha Reddy and C. Leelanandam Department of Geology, Osmania University, Hyderabad - 500 007, India, E-mail: cleelanandamQredi~ail.com (Manuscript received August 5,2003; accepted April 10,2004)

Abstract The Chimalpahad Layered Complex (CLC) is the largest (1200 sq km) metamorphosed anorthosite complex in the Precambrian shield of Peninsular India. It exhibits spectacular magmatic layering together with superposed tectonic structures. Modal layering, with primary cumulus texture, is the most prominent magmatic structure displayed by the complex. A variety of magmatic structures (rhythmic mineral-graded layering, cyclic and cross layering, and troughs), practically unaffected (or weakly modified) by later tectonic overprinting, form the relict igneous structures in iow strain zones within the CLC; they are described and depicted in sufficient detail, and the possible mechanisms for their formation are enumerated. The structural features in the CLC owe their origin to dual play of magmatic and tectonic processes. The original magmatic structures in high strain zones are variably overprinted by: (1) planar foliation (Sl) and mineral lineation (L1) of the D, deformation, (2) the Fla and Flb folding event of the D, ductile deformation and (3) brittle-ductile fractures/faults of the D, deformation that are inferred to have developed during exhumation of the CLC by transpressional tectonism. Micro-textural features of the CLC provide useful criteria for identification of superimposition of solid-state recrystallization on magmatic fabrics. Simultaneous compression and strike-slip shear mechanism (transpression and transtension) have been proposed to account for the magmato-tectonic evolution of the CLC. Key words: Chimalpahad layered complex, magmatic layering, tectonic structures, shear zone, transpression.

Introduction The Chimalpahad Layered Complex (CLC) is the largest, deformed and metamorphosed high-Ca Archaean anorthosite complex in the Precambrian shield of South India (Leelanandam, 1987; Ashwal, 1993). It consists of predominant anorthositic, subordinate gabbroic and rare ultramafic rocks. The CLC is enclosed by schistose and gneissose rocks of the Khammam Schist Belt (KSB), and cut by mafic sills and dykes (now preserved as amphibolites). All components of the CLC and KSB have been subjected to upper amphibolite-lowergranulite facies metamorphism (Leelanandam and Narsimha Reddy, 1983, 1985; Subba Raju, 1987; Narsimha Reddy and Leelanandam, 1999; Hari Prasad et al., 2000). The structural features of the CLC owe their origin to a combination of igneous and tectonic processes. The rocks were either partly or completely recrystallized during metamorphic and tectonic events which followed the primary crystallization of the igneous complex. The structural features of the CLC are, in general, comparable to those of the Gosse Pile intrusion, Central Australia

(Moore, 1973), and the Fiskenaesset Complex, SW Greenland (Myers, 1985; Ashwal and Myers, 1994). Studies on the deformational and metamorphic structures on the magmatic intrusions are scarce in this country. The cLcoffers an unique opportunity to fill up this deficiency and the present paper is an attempt in that direction.

Geology of the CLC The CLC falls between latitudes 17'18' and 17'34'N, and longitudes 80'23' and 80'35'E. It was syntectonically emplaced as a "sill-like" layered intrusive body within the KSB wherein the foliation is oriented towards NE-SW direction. The NE-SW trending CLC is arcuate in shape for over a length of about 30 km with an average width of 5 km (Fig. 1).Smaller outcrops of the CLC that are spatially separated from the main body by prominent shear zones (Fig. 1)occur as tectonically dismembered and deformed sheets and lenses, in the southern, northern and western parts of the complex. Though anorthosite from the Chimalpahad area was first reported by Ramaswamy

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The CLC is structurally divided into two broad zones : (1) marginal zone, and (2) central zone (Fig. 1). The marginal zone of the CLC forms a thin discontinuous zone of 1-2 km width along the fringe of the complex and is characterized by spectacular igneous layering of different styles and scales in the low strain areas. The central zone of the CLC is occupied by the entire Chimalpahad hill ranges and is characterized by massive cumulates with overprinted tectonic lineation/foliation, defined by penciltype aggregate hornblende crystals. The rocks from the CLC are categorized - on the basis of colour, composition and texture - into three main lithological units: (1)anorthositic rocks, (2) gabbroic rocks and ( 3 ) ultramafic rocks with chromitite and magnetite.

(1962), the Chimalpahad complex as a whole (with its geology, structure and origin) was brought to light by Appavadhanulu et al. (1976). They suggested that the main body was emplaced along the axial zone of an isoclinal anticline, while the minor bodies occur along shear zones parallel or sub-parallel to the fold axis. The host rocks of the CLC are characterized by extensive migmatisation and polyphase deformation, and are comparable to those of the Sargur Group of the Dhanvar Craton (Subba Raju, 1975). The petrology of the CLC was subsequently described by Leelanandam and Narsimha Reddy (1985) and Subba Raju (1987). Rhythmic layering and cross-stratification (or cross bedding) in the CLC were described by Ramamohana Rao and Satyanarayana Raju (1986), and Subba Raju (1987).

INDEX Magnetite Chrornite

n V V

Amphibolite

1/1 Gabbro Ultramafics Massive anorthosite Layered anorthosite Khammam schist belt

I

60- 2s M Z - Marginal zone

---

---

Shear zone

80"/35'

130' CZ - Central zone

/-

CB - Cuddapah Basin

Strike & Dip of Foliation

Fig. 1. Geological map of the Chimalpahad Layered Complex, Khammam district, Andhra Pradesh. Modified after Appavadhanulu et al. (1976) (see also Fig. 5).

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Anorthositic rocks include minor anorthosites, and predominant gabbroic anorthosites and anorthositic gabbros; the last two rock types are collectively referred to as leuco-gabbros. Anorthositic rocks constitute the bulk (>95%) of the CLC, and can be petrographically divided into two varieties: (1)garnet-bearing and (2) garnet-free anorthositic rocks. The garnet-bearing anorthositic rocks are comparatively less abundant than the garnet-free varieties; the former generally occur as lensoid bodies ( < 2 m x 1m) confined to shear zones in the CLC. Garnetfree varieties are widespread over the hill ranges in the central part of the complex. Layering with cumulus texture is the prominent primary feature of the anorthositic rocks and is extremely well preserved in these rocks at different localities of the marginal zone of the CLC. Gabbroic rocks are the distinct mafic rock units of the CLC. They are sparsely distributed and have limited areal extent in the complex. They occur as concordant bands and lenses in the anorthositic rocks within the complex, and also as discordant massifs, lenses and dykes in the schistose formation (KSB) outside the complex. Ultramafic rocks are scanty in the CLC. They occur as pods, lenses and minor bands within and outside the main complex. Chromitites and magnetites occur as small lensoid bodies with float-ores in the CLC. The lithological units of the CLC are distinguished based on their colour, form, texture and mineralogic composition (Table 1).

Structures Magmatic structures The magmatic structures of the CLC comprise of rhythmic layering, cyclic layering, cross layering (or crossbedded layering), trough structures, and zebra banding with relict cumulate textures. The igneous layering is best seen, not persistently, in the marginal zone of the CLC and is made up of different proportions of felsic (calcic plagioclase) and mafic (hornblende+clinopyroxene) minerals (Fig. 2a). The felsic and mafic layers are well preserved with either gradational or sharp contacts. The layering is common in the marginal zone of the complex and is prominently seen at: (1) Gaddigutta (A196) in the northeastern part of the complex; (2) about 2 km ENE of Chimalpahad; and (3) about 2.5 km NE of Yenkur (Fig. 1). Layering is a regular three dimensional planar fabric of cumulate rocks and is manifested by combination of layers and laminations, while banding is a three dimensional structural feature characterized by alteration of planar rock units (bands) of contrasting appearance in the cumulate rocks (see Irvine, 1982). Rhythmic mineral-graded layering is a relict minor Gondwana Research, V. 7, No. 3, 2004

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structural feature in the CLC (Fig. 2b). It consists of repetition of light and dark isomodal layers with gradational contact relationships on different scales from centimetre to decimetre, and the layering is regular with uniform spacing. This layering is akin to the structures described from the layered series of the Skaergaard intrusion (Wager and Brown, 1967) and Fiskenaesset complex (Myers, 1985; Ashwal and Myers, 1994). Several mechanisms have been suggested for the formation of the mineral-graded layering: (1) steady convection and steady cooling by continuous convective circulation of magma, (2) surge-type density-sorted turbidity currents of magma, and (3) compaction, mechanical sorting and recrystallization (Irvine, 1987; Naslund and McBirney, 1996 and references therein). Mineral-graded layering in the CLC can be attributed to slow cooling of magma by continuous convective currents against walls of the magma chamber. Periodic repetition of alternate sets of uniformly spaced plagioclase-rich layers, separated by graded hornblenderich layers, forms the micro-cycliclayering (Fig. 2c). These hornblende-rich layers occasionally form doublets in the host anorthosite. This type of layering is comparable to that of inch-scale layering from the Stillwater complex (Jackson, 1970). Cyclic layering is attributed to a variety of mechanisms: (1) magma chamber recharge; (2) a period of convective overturn in an otherwise relatively stagnant magma; (3) periodic convection and non-convection currents (intermittent convection mechanism); and (4) a combination of solution and reprecipitation during slow cooling and Ostwald ripening (see Naslund and McBirney, 1996; Boudreau and McBirney, 1997). Intermittent convection is the likely mechanism for the development of micro-cycliclayering in the CLC. Cross layering is rare in the CLC and is observed only in the western and northern parts of the marginal zone of the CLC. Cross layering, with angular unconformity and layer truncation, forms a cross-stratification due to dynamic process (magmatic flow) and magmatic deformation (see McBirney and Nicolas, 1997; Naslund and McBirney, 1996). Dynamic process such as magmatic flow and fluctuation of the velocity of currents along the walls of the magma chamber (or across the floor of chamber) may possibly induce the development of crosslayering in the CLC. Trough structures are uncommon and only present in the marginal zone of the CLC (Fig. 2d), and form broad synformal depressions with mineralgrading. Troughs from the CLC are comparable to the principal-type of troughs from the Skaergaard intrusion (Irvine, 1987) and to those of the Fiskenaesset complex (Myers, 1976). These structures have been attributed to magmatic depositions caused by intermittent or surge-

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type density currents (Wager and Brown, 1967; Irvine, 1987). Hornblende-pegmatite veins and different types of tectonically induced pull-apart magmatic structures are occasionally observed from the anorthositic rocks of the CLC, and they are produced by late stage magmatic processes. The hornblende-pegmatite veins of the CLC are in a way comparable to the pegmatitic gabbro pods from the Skaergaard intrusion (cf. Fig. 43 in Irvine, 1987) and to the hornblende-pegmatites from the Fiskenaesset complex (Myers, 1978). Tectonic structures

The CLC is a polydeformed igneous body with three events of deformation. Ductile-brittle deformations (Dl, D, and DJ are recorded in the rocks of the CLC during and after its emplacement. The first deformation D, is represented by strong vertical/sub-vertical planar foliation (Sl) (Fig. 3a), and vertical (or horizontal) stretching mineral lineation (Ll) in the rocks of the CLC (Fig. 3b). The NE-SW trending S1 foliation on map-scale (Fig. 5) is variably overprinted on the primary magmatic structures, and hence it is often difficult to distinguish between the primary igneous layering and metamorphic gneissic banding. The stretching lineation L 1 is defined by the preferred orientation of (1) aggregate crystals of (pencil-type) amphibole (see Fig. 3b), and (2) stretched bands of amphibole-rich layers in the layered leucogabbros. The structural features of the D,

deformation are widespread in the rocks of the CLC and are inferred to have developed by NW-SE coaxial component of shortening (see Fig. 5). The D, deformation of the CLC can be compared with the Neo-Proterozoic deformation at a boundary zone between the NelloreKhammam schist belt and Pakhal basin in the Kinnerasani area which is situated 25 km north of the CLC (Rajneesh Kumar et al., 2000). D, ductile deformation is represented by the folding event of the complex. Folding occurs as two phases of Fla and Flb fold patterns, and is variably overprinted on the structures of the D, deformation of the complex. The D, deformation is inferred to be controlled by the interplay of a zone boundary-parallel non-coaxial dextral strike-slip shearing and a zone boundary-normal coaxial component of shortening (by compression in the NW-SE direction) due to the type B transpression (see Fossen and Tikoff, 1998). Initial phase of folding event of D, deformation is designated as F l a folds having axial surfaces co-parallel to the NE-SW orientation of the compositional layering/ metamorphic banding. Fla folds are described largely as similar-stylefolds akin to the Class 2 and 3 folds of Ramsay (1967, p. 366). In addition to the similar folds (Fig. 3c), rare flame type folds and intrafolial folds belonging to the Fla fold pattern are present in the complex. Final phase of folding of D, deformation is denoted as Flb folds which are generated by minor modification of Fla folds by intense shearing effects. The Flb folds include S type asymmetric fold, parasitic fold and complex steep plunging fold

Table 1. Distinguishing characteristics among the three main lithological units of the CLC (see also Leelanandam and Narsimha Reddy, 1983, 1985). Lithological units

I.

Anorthositic rocks a. Anorthosite (F = >90%; M = < 10%) b. Gabbroic anorthosite (F=90-80%; M=10-20%) c. Anorthositic gabbro

Colour Colourless to greyish white Greyish white

Whitish grey

(F=80-65%; M=20-35%)

Form/Texture Layer (sheet), Band and Lens; Adcumulate Layer (sheet), Band and Lens; Mesocumulate Layer (sheet), Band and Lens; Mesocumulate

Mineral assemblage pl (+hbl+Scp+ zo/cl-zo +gart+ Fe-Ti oxides) pl+ hbl( +cpx+scp + zo/cl-zo +gart +Fe-Ti oxides) pl+ hbl+ cpx( + scp+zo/cl-zo+gart+ Fe-Ti oxides)

11. Gabbroic rocks

(F=65-10%; M=35-90%) a. Biotite gabbro

Dark

b. Orthopyroxene gabbro Dark c. Norite Dark Ill. Ultramafic rocks (F= < 10%; M= >90%) a. Clinopyroxenite Dark b. Hornblendite Dark c. Act-tr-hbl-schist Dark d. Chromitite Black Rock classification after Windley et al. (1973); Mineral

Dyke, Lens, Granular; Ophitic/granoblastic Lens, Granular; Mesocumulate Lens, Granular; Mesocumulate

+

+

pl cpx+ hbl+ gan+ bio ( scp +sp +qtz Fe-Ti oxides) pl+cpx+ opx+ hbl( +gart +scp+qtz+ Fe-Ti oxides) pl +opx+ hbl (+gart +scp+qtz+Fe-Ti oxides)

Lens, Granu1ar;Adcumulate cpx (+hbl+qtz) Dyke, Lens, Granular; Adcumulate hbl (+pl+gart+qtz) hbl+act+tr+(+chl+qtz+tlc+chr) Band, Lens; Schistose Massive, Lens: Mesocumulate chr+ hbl + (+act tr+ath+ zed) abbreviations after Kretz (1983); F-Felsic minerals, M-Mafic minerals.

+

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(Fig. 3d). The axial traces of Flb folds are locally variable, but are generally consistent with those of Fla and confined to the orientations between NNE-SSW and ENE-WSW. Brittle-ductile and brittle deformations (D,) in the CLC are not prominent when compared to the D, and D,ductile deformations. D, deformation is represented by N-S, NW-SE and ESE-WNW oriented open and upright folds (Fig. 3e), brittle-ductile fractures, brittle faults, en echelon tension gashes and different vein fibres with calcite, amphibole and plagioclase (Fig. 30. These mesoscopic faults and fissures cut across the earlier structural features of D, and D, deformations.

Shear zones A mylonitic shear zone is represented by deformed amphibolites and anorthositic rocks in a zone of intense deformation across marginal zone of the CLC (see Figs. 1 and 5). It trends in a NE-SW direction and separates the main body of the CLC in the east from the adjoining KSB and Pakhal Group of Proterozoic meta-sediments in the west (see Fig. 1). Besides, there are several small-scale (mesoscopic) shear zones (see Fig. 3c, 3d and Fig. 3g)

891

affecting amphibolites and anorthosites from the Bethampudi shear zone (see Fig. 5 ) , which is a northeastern extension of crustal-scale shear zone lying western margin of the CLC. A vast majority of mesoscopic shear zones exhibit dextral sense of shear displacement with NE-SW (45/85"E) and NNE-SSW (35/80"E) trends with steeply dipping mylonitic foliation, while the subordinate shear zones with opposite sense of shear (sinistral) have steep mylonitic foliation in the direction of N-S and NNW-SSE. Thus, the subordinate shear zones with sinistral strike-slip sense of shear can be considered as Reidel shears (Ghosh, 1993). The mesoscopic shear zones are either synchronous or post date the D, and D, deformations of the CLC.

Microtextures The rocks of the CLC exhibit a variety of textural and structural features ranging from relict magmatic to metamorphic (and deformational) fabrics (Fig. 4a, b, c and d). They show evidences for the superimposition of solid-state (metamorphic) recrystallization on the early formed magmatic fabric (see Fig. 4a, b and c); these

Fig. 2. (a) Rhythmic layering in the layered leucogabbro (LLG) at Gaddigutta (A196). Note the centimetre-scale dark hornblende-rich and white plagioclase-rich layers. Hammer: 30 cm (b) Mineral-graded layering in the LLG in the Nallavagu section, near Puligundam village. Dark hornblende is enriched at the base of the layer, while white plagioclase is concentrated at the top of each layer. The 'way-up' of the layering is towards left. Plan view. Length of the hammer: 30 cm. (c) Hornblende-rich layers, forming doublets in the host of pure anorthosite, generate micro-cyclic layering. Scale: 10 paisa coin (2 cm).(d) Layered leuco-gabbro exhibiting trough structure in a quarry face near Gaddigutta (A196). Mineral (modal) grading is seen in synformal depressions (troughs). Section view. Scale bar: 5 cm.

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MI NAI<'~lMl-1AREDUY AND C Lhl:J.ANANDAM

Fig. 3. (a) Deformed 1,I.G with srrong planar foliation in the quarry near Gaddigutta ( ~ 1 9 6 ) . Note the vertical/sub-vertical cm-scale rhythmic layers locally grading to gneissic layers towards right. Section view. Hammer: 30 cni. (b) The LLG showing tectonic lineation with aggregates of pencil-type hornblelide crystals. Seciioii view. Scale: 30 cm (Loc : 1.5 ltm NE of Bethampudi). (c) Dextral ductile shear zones synchronous with similar-style tight Fla fold structure in the LLG. Plan view. Hammer: 30 cm. LOC: 1 km NNE of Bethampudi. (d) Complex . view. (e) Open and upright folded steep plunging fold pattern in the strongly deformed LLG in the quarry near Gaddigutta ( ~ 1 9 6 ) Section geometry cut by the sinistral arid dextral ductile shearing in the LLG. Section view. Lens cap: 5 cm. (f) Composite-vein fibre of hornblende (central portion) bordered by plagiocalsc on either side in the sinistral shear zone of the LLG. The fibre is due to the crack-seal mechanism (see Ramsay and Huber, 1983; lip. 235-263). Vertical view. Hammer head: 10 cm. length. (g) Dark hornblende and white plagioclase gneissic layers are stretched to form mylonitic foliation within sinistral ductile shear zone in the Nallavagu section. Plan view. Lens cap: 5 cm.

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Fig. 4. (a) Leucogabbro (B 2) showing post-cumulus recrystallization texture. Note the partially recrystallized post-cumulus hornblende and plagioclase around coarse cumulus plagioclase grains. XPL; scale bar is 1.0 mm. (b) Newly developed strain-free recrystallized fine-grained plagioclase along the grain boundaries, and across the micro-fractures in the coarse plagioclase from anorthosite (L 7 17). XPL; scale bar is 1.0 mm. (c) Anorthosite (L 24) showing typical equilibrium granoblastic texture. Note the polygonal equant grains, planar boundaries and triple junctions in between the plagioclase grains. XPL; scale bar is 1.0 mm. (d) Ainphibolite (L 26) showing the mylonitic foliation. Note the porphyroblastic garnet and lenticular hornblende aligned parallel to the foliation. PPI,; scale bar is 1 .0 mm.

features are inferred to have been formed during plastic deformation and dynamic recrystallisation. Plastic deformation involves in the development of equant grains, planar boundaries and triple junctions in between the grains due to rapid movement (or migration) of the grain boundaries at higher temperatures.Dynamic recrystallisation is witnessed by the development of new (strain-free) grains along the grain boundaries and across the micro-fractures (see Fig. 4a and 4b) due to grain size reduction and subgrain rotation at lower temperatures (see Paterson et al., 1989).

Discussion and Conclusions The CLC is rather unique in the sense that both magmatic and tectonic structural features are well displayed. The rocks of the marginal zone are affected by severe deformational forces as evidenced by numerous folds, fractures and shears; yet, original magmatic structures in localized pockets are rarely preserved. However, the central zone of the CLC has virtually escaped the intense shearing, and hence the rocks from the central zone do not exhibit any severe strain effects. The petrological and structural features of the CLC ate similar Gondwana Research, V. 7, No. 3, 2004

to those of the Kadugannava complex near Kandy from the Sri Lanka basement (Kleinschrodt et al., 1991). However, magmatic features of the CLC are much better preserved than those of the Kadugannava complex, Kandy (Kleinschrodt pers. comm.) . Igneous layering in the CLC is possibly generated by the in situ crystallization of magma against walls or floor of the magma chamber due to: heat-loss; diffusivity; varied nucleation and growth of crystals in gradients of temperature and composition; and, compaction related mechanical sorting of cumulus minerals (see McBirney and Noyes, 1979; Naslund and McBirney, 1996; Boudreau and McBirney, 1997). It is well known that a niechanism for the formation of a single layer cannot explain all or even most of the known features of igneous layering in any complex. Furthermore, tectonically induced striictures such as the pull-apart fibres and (sigmoidal) en-echelon fissure veins represent the surface manifestations of the magma emplacement in a strikeslip regime. The emplacement of large complexes like the CLC may not be possible without magma being overpressed by both buoyancy and tectonic forces during regional transpressive deformation (see Saint Blanquat et al., 1998).

a94

M. NARSIMHA REDDY AND C. LEELANANDAM

00-1 33'

80 z

C E N T R A L Z O N E OF C L C Bethornpudi

1 Km 80'

31'

8oDJ33'

80" 132'

1s

-

80"

Oblique Convergence

2

Dextral Strike-Slip

Shear

S i n i s t r a l S t r i k e - S l i p Shear ( D u c t i l e ) 6

I

Sinistral Strike- S l i p Fault ( B r i t t l e )

3+

F o l d A x i s o f Fla

--+

Vertical / Horizontal Mineral Lineation

4

S t r i k e ond D i p of S l F o l i a t i o n D i r e c t i o n of Younging

In the marginal zone of the CLC, a NE-SW trending 2 km wide deformation zone, north of Bethampudi, exhibits variable strain effects (see Figs. 1 and 5), and this zone of deformation is termed as the Bethampudi shear zone. Vertical/sub-vertical planar foliation with either vertical or horizontal lineation fabric is overprinted on the earlier igneous structures due to simultaneous interplay between co-axial contraction and non-coaxial wrench-dominated strike-slip partition in the Bethampudi shear zone. There is vertical extension (or stretching) along the Z-axis, but no stretching and shortening along the shear direction (X-axis), and hence the transpressional deformation is relegated to the type B transpression of Fossen and Tikoff (1998).

Fig. 5. Structural m a p of t h e Bethampudi shear zone in the northeastern region of t h e CLC.

The rocks from the Bethampudi shear zone have become mechanically unstable and exhibit layer anisotropy inferred to have developed during type B transpression of Fossen and Tikoff (1998). The large-scale D, and D, ductile structures with LS and S fabric, S-type folds and upright-fold patterns of the NE-SW trending Bethampudi shear zone correspond to the first-order dextral strike-slip (oblique)-contractional shear zones. Crystal-plasticdeformation, dynamic recrystallization (see Fig. 4b) and diffusive mass transfer can be invoked as mechanisms for the ductile flow during the D, and D, ductile deformation of the rocks at lower crustal levels. Minor and local brittle-ductile fractures in the N-S, NW-SE and ESE-WNW orientations of D, deformation are caused by second-order sinistral strike-slip shears at Gondwana Research, V. 7, No. 3, 2004

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upper crustal levels. The tectonic scenario of the Bethampudi shear zone is broadly comparable to that of the Rio Paraiba do Sul strike-slip shear belt (RPSSB) of southeast Brazil (Ebert and Hasui, 1998). Simple shear and oblique strike-slip shear mechanisms have played a vital role in the shaping the original igneous structures into deformed structures. The original igneous layering was strongly overprinted by D, (S1 gneissosity) and D, folding, and by D, fracturing (from lower crustal to upper crustal levels respectively), during exhumation of the CLC by transpressional tectonism. It is suggested that the complex folded geometry (together with “finite strain” propagated asymmetric and symmetric fabric of the multi-layered leucogabbros from the Bethampudi shear zone) was controlled by: (1) the composition and rheological properties of the white (plagioclase-rich) and dark (hornblende-rich) layers; (2) the pressure and temperature conditions in relation to the depth of rockdeformation; (3) the strain effects due to simple and oblique shearing in deformation zones between the CLC and KSB (see Fig. 5); and, (4) the medium-grade regional metamorphism. It is interesting to note that the strikeslip tectonism (transpression and transtension) of the CLC is similar in a way to oblique collision tectonism of the Eastern Ghats Belt with Bastar Craton (see Bhattacharya, 2002).

Acknowledgments The authors express their grateful thanks to Dr. T.R.K. Chetty (NGRI, Hyderabad), Prof. A.K. Jain (IIT, Roorkee) and Prof. R. Kleinschrodt (University of Cologne, Koeln, Germany) for their critical and constructive reviews of the manuscript, and also for their valuable suggestions for its improvement. We sincerely thank Ms. Nancy Rajan (NGRI, Hyderabad) for efficientlypreparing the electronic version of the paper. The financial support given by the Department of Science and Technology (ESS/23/172/93), New Delhi is gratefully acknowledged.

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