Journal of Geosciences, Osaka City UniversityVol. 41, Art. 7, p.109-122, March, 1998

Some structural observations in the Eastern Ghats Mobile

Belt surrounding Visakhapatnam, South India

Yasutaka TANI*I, R.S. DIVI*2, Yukari MIYASHITA*3,Masaru YOSHIDA *1, Shin-ichi YOSHIKURA *4 , and A.T. RAO*s

*1) Department of Geosciences, Faculty of Science, Osaka City University•2) CNS Geomatics, Ottawa, Canada*3) Geological Survey of Japan, Tsukuba, Japan* 4) Departmant of Geology, Faculty of Science, Kochi University, Japan* 5) Department Geology, Andhra University, Visakhapatnam, India

AbstractThe Eastern Ghats Mobile Belt is an arcuate belt of the high-grade terrain located along the

east coast of Peninsular India. This mobile belt is considered to lie between the ArcheanIndian Craton and the East-Antarctic Craton in Gondwanaland. Therefore, this mobile beltplayed an important role in the construction of Gondwanaland. However, geologic studies, es­pecially structural ones in the Eastern Ghats Mobile Belt, are still insufficient.

Detailed work was carried out in the Eastern Ghats Mobile Belt area around Visakha­patnam in order to evaluate the possible deformational history in the light of recent studies.Deformational structures occurring along the route from Kottaplem to Gollapalem, north ofVisakhapatnam, are described in this short note. This selected route is situated in the northernmarginal area of the macroscopic domal structure known as the 'Madhuravada Dome'. Well­developed compositional banding, foliation parallel to the compositional banding, pinch-and­swell structures, lineations and several type of folds can be observed on this route. Thesestructures can be classified into the DL, DM , and DN deformational stages, from the earliest tolatest. The DL deformation is characterized by a flattening event, identified by the foliationwhich is composed of garnet grains (or aggregates) showing the oblate strain ellipsoid. TheDM deformation is considered to form the mesoscopic, intrafolial, isoclinal to tight folds, pinch­and-swell structures, and some stretching lineations as the result of a compression perpendicu­lar mostly to the axial planes of the mesoscopic intrafolial folds. The DN structures, such aspossible macroscopic close folds with overturned axial planes and related mesoscopic struc­tures, may also be due to a compression event.

The preliminary structural interpretations in this study area appear to be dissimilar tothose of previous works. It is emphasized, therefore, that regional and local structural data arenecessary for a better understanding of structural evolution of the Eastern Ghats Mobile Beltaround Visakhapatnam.

Key words: deformational structures, Eastern Ghats Mobile Belt, Visakhapatnam, India,Proterozoic.

1. Introduction

The Eastern Ghats Mobile Belt (EGMB) is ahigh-grade terrain distributed along the eastcoast, facing the Bay of Bengal, of Peninsular In­dia (Fig. 1). On a reconstruction map of EastGondwanaland, the EGMB lies between the Indi­an cratons of Dharwar and Bastar, and the EastAntarctica Craton during the middle Proterozoicto Paleozoic and the Mesozoic Eras (Fig. 2).Thus, the EGMB might have played a significant

role in the formation of East Gondwanaland.This mobile belt is also considered to preservestructural and metamorphic evidences for theirbehaviour during the exhumation of granulitesfrom deep crustal levels (Chetty, 1995). How­ever, geologic studies of the EGMB are insuffi­cient, and the characteristics of this mobile beltare still indistinct.

Some contributions concerning the lithologyand deformation of the rocks in the EGMB a­round Visakhapatnam have been published (e.g.,

110 Yasutaka TANI et al.

Fig. 1. Geological outline of India (A) and the EasternGhats Mobile Belt (B), showing location of thestudy area (modified after Yamamoto, 1996).

Sriramadas and Rao, 1979; Fonarev et aI., 1995).The petrological features were well described inthese works, but deformational and kinematic in­terpretations, on the basis of detailed structuraldata, as well as investigation of the deformationmechanisms of the structures, were not exam­ined. Considering the importance of the Visakha­patnam area in evaluating the structural andtectonic evolution of the EGMB, a detailed geo­logical survey much needed. The present studyaims to establish detailed lithological and struc­tural maps, to characterize the lithological andstructural features of the rocks in this area, andto discusses the structural and tectonic evolu­tions of the EGMB in and around Visakha­patnam.

This note introduces the structural outline ofthe area around Visakhapatnam and some obser­vations on the deformational structures in thearea.

~ Normal fault

....-- ... - Shear ZoneE83 Archean gmnilc greenslOllc terrain

I±§ Archean granulite facies terrain

["] Mid-Proterozoic granulite facies terrain

EEJ Mid-Proterozoic amphibolite facies tcmllll ---"""' Thrust Zone

liliilll Mid-Proterozoic superposed granulite facIes· amphibolite facies terrain

~ Archean graJlUlitel:,cics terrain reworked by mid-Proterozoic granulite facies mC~'lmorphisrn

~ Thrust movement directions during 0.7 - 0.5 Ga

~ Strike-Slip movement directions during 0.7·0,5 Ga

~ '111ruSt movement directions during 1.3 - 1.0 Ga orotdcr

Fig. 2. The juxtaposition map of India-Sri Lanka­Antarctica in East Gondwana, showing themega-faults and shear zones (sites from Taniand Yoshida, 1995). Abbreviations are EGMB:Eastern Ghats Mobile Belt; KBB: Kabaldurga­Biligirirangan Belt; PMB: Periyar-MaduraiBlocks; SGS: Southern Granulite Segment; DC:Dharwar Craton; SC: Singhbhum Craton; HC:Highland Complex; WC: Wanni Complex; VC:Vijayan Complex; AG: Arena Gneiss; VH:Vestfold Hills; SPC: Sou thern Prince CharlesMountains; RC: Rayner Complex; NC: NapierComplex; POC: Prince Olav Complex; OSG:Ongul and Skallen Group; TVG: Teltet VengenGroup; NLG: Nils Larsenfjellt Group.

2. General features of the Eastern Ghats MobileBelt

The EGMB is a curvilinear mobile belt of lessthan 60 km width and more than 700 km length(Fig. 1). The western margin of this belt is incontact with the Archean Bastar Craton in thesouthern part and the Dharwar Craton in thenorthern part (Fig. 1). The Archean SinghbhumCraton is adjacent to the northern margin of theEGMB (Fig. 1). It is considered that the bound­ary between the EGMB and Bastar-Dharwar cra­tons is marked by a thrust, with the former's su­perposition on the latter (Fig. 2).

In the EGMB, metamorphosed supracrustalsshowing granulite-facies grade, granites, charno­ckites, anorthosites and alkaline rocks occurwith minor amoun ts of calc-silicate rocks, mar-

• Schist belt

~ Gondwana sediments

D CharnockltelGrt-SIl gneiss

~Thrust200km

o Indo-Gangetic and Indian Peninsularsimilar alluvium .)rccambrian

lS'S:l Himalayas ~ Archean CratonsGill Proterozoic Mobile BeltS Proterozoic Sediments

Phanerozoic~ Gondwana to Cenozoic covers_ Deccan Flood Basalts

BC: Baster craton

DC: Dharwar craton

SC: Singhbhum craton

AD: Aravalll-De/hi Belt

EGMB: Eastern GhatsMob/leBelt

SI: South IndianGranulite Belt

wj:

+-10'N

A

Some structural observations in the Eastern Ghats III

ble and quartzite. Migmatitic rocks are also ob­served in some regions (e.g., the Tekkali area,Yamamoto et aI., 1998). Pressure-temperature (P­T) path studies from the different areas of theEGMB generally show a near-isobaric coolingpath, followed by a near-isothermal decompres­sion, from a peak temperature-pressure conditionaround .-...,950°C at '-""9 kbar (e.g., Dasgupta et aI.,1995; Shaw and Arima, 1997). Sanyal and Fuku­oka (1995) have suggested a near-isothermal de­compression path, followed by a near-isobariccooling path from the P-T co-ordinates of 8.4kbar and 900°C.

Grew and Manton (1986) indicate 979 Mafor zircon from charnockites of Visakhapatnamand a similar age for sapphirine-bearing gran­ulites from Anakapalle. Paul et a1. (1990) alsoreport U-Pb zircon and monazite ages of990~950 Ma from the charnockites of theVisakhapatnam and Phulbani areas. Aftalion eta1. (1988) considered that the charnockitesmight have formed between 1100-950 Ma in theEGMB.

The obtained ages, ranging from 1500 to1400 Ma, appear to be the ages for an alkalinemagmatism or an anorthositic activity (Sarkaret aI., 1981; and Sarkar et aI., 1994). An U-Th-Pbage of 2600 Ma, and a Nd model age of TDM2600 Ma for the charnockites of Vsakhapatnam,are also reported (Vinagradov et a1., 1964; andPaul et aI., 1990). Unnikrishnan-Warrier et a1.(1993) compiled the geochronological data ofsouthern India and concluded that the EGMB ischaracterized by charnockite formation at ca.1000 Ma over precursor granulites of 2000 Maand/or 2500 Ma. On the other hand, minor datashowing a ~500 Ma Pan-African thermal eventare also known (see Fig. 4 in Yoshida, 1995).Thus, geochronological works have proposed~1500-1400 Ma and ~1000 Ma for the majorevents of the EGMB, while the relict ages reflectArchean and minor amounts of ~500 Ma Pan­African activi ties.

Structural and tectonic studies based on fieldstructural analysis are only a few and are gener­ally concentrated in the northern part of theBel t (the Angul area by Halden et aI., 1982; theChilka Lake area by Bhattacharya et a1., 1993and 1994; and the Ryaganda area by Shaw,1996). Chetty and Murthy (1994) investigatedthe mesoscopic to satellite-scale structures of the

EGMB occurring near the River Godavari withLandsat Thematic Mapper image data as well asa field structural analysis of a selected area.They focussed on the structural fabrics relatedto the mesoscopic and macroscopic shear zones,and proposed two essential major tectonic e­vents: an earlier, horizontal tectonic regime, resulting in thrust systems with westward vergence;and a later, strike-slip tectonic regime characterized by high strains, showing dextral shear sense.They also considered that the structuralfeatures mentioned above might have been at­tribu ted to a collision process during the Proter­ozoic period. However, it is apparent that thedescriptions of the deformational structures andtheir sequences so far observed in the studiedarea are insufficient for an understanding of thestructural evolution of the area.

3. Structural outline of the present study areain and around Visakhapatnam

The study area is mostly composed of garnet­sillimanite (-biotite) gneiss (Khondalites) andgarnet-biotite or biotite gneisses (Leptynites), aswell as minor charnockite-enderbite rocks andbasic pyroxene granulites (Fig. 3). Quartzite lay­ers are often interbedded with the garnet­sillimanite (-biotite) gneiss. The lithological bou­ndaries between the quartzite and garnet­sillimanite (-biotite) gneiss are generally parallelto the major compositional banding (Fig. 4). Basicpyroxene granulite and enderbite-charnockiterocks are distri bu ted within the leptyni tes. Ende­rbite-charnockite rocks are characteristically ex­posed in the crestal and axial parts of themacroscopic overturned folds (Fig. 4, Sriramdasand Rao, 1979).

Macroscopic, overturned isoclinal synformsare well reported in the EGMB around Visakha­patnam by Sriramadas and Rao (1979) (Fig. 3).The axial traces of the macroscopic overturnedfolds trend E-W and plunge towards the east inthe coastal area. On the other hand, the over­turned isoclinal folds in the inland area showNE-SW-trending axial traces (Fig. 3). Accordingto Sriramadas and Rao (1979), the arcuatedshape of the axial traces of the macroscopicoverturned folds are considered to be the resultof the superposition of later folding on the mac­roscopic overturned folds (Figs. 3 and 4). In

112

Legend of lithology

I: ~::: I Garnet-sillimanite gneiss

•Charnockites and pyroxenegranulites

.....

N o Elamanchili

j )a 2 4 6kmI I I I

Yasu taka TANI et a1.

Legend of structures -----t

~ I.-.- Axial trace of overturnedV isoclinal synclinal fold

~ Axial trace of cross fold

f - - FaultA Strike and dip of foliation

Fig. 3. Structural outline around Visakhapatnam in the Eastern Ghats Mobile Belt (modified after Sriramadasand Rao, 1979; Fonarev et al., 1995).

spite of the development of the macroscopicoverturned isoclinal synforms, overturned isocli­nal anti forms, which should have been formedsimultaneously with the synforms, were not re­ported by Sriramadas and Rao (1979). A macro­scopic domal structure with upright axial planecan be recognized in the Madhuravada area (Figs.3 and 4). This domal structure is well known asthe 'Madhuravada Dome' (Rao et aI., 1994) andis are occupied mostly by quartzo-feldspathicrocks (Fig. 4).

Fonarev et a1. (1995) discussed the tectono­thermal history of the granulite terrain around theVisakhapatnam area and identified at least fourdifferent phases of ductile deformation (Table 1).However, their description of the structural datais not enough for the establishment of the re­gional structural evolution of this area. Hence,their interpretation of the deformational partneeds to be reinterpreted, asing additional de­tailed structural and kinematic analyses.

4. Deformational structures in the area be­tween Kottapalem and Gollapalem, northernVisakhapatnam district

The selected route is situated in the north­eastern marginal region of the MadhuravadaDome (Fig. 4). It is thought that this macro­scopic domal structure is reflected in the map asa topographic basin (Fig. 5). In this area, garnet­sillimanite (-biotite) gneiss constitutes the domi­nant rock type. Compositional banding, foliation,folding structures, pinch-and-swell structures andminor linear structures are well observed in thisarea. However, features indicative of a ductileshear zone, such as a mylonite zone and a brittleshear zone, could not be found. The deforma­tional structures in the area between the Kot­tapalem and Gollapalem villages are describedbelow.

Some structural observations in the Eastern Ghats 113

Legena---------lo Garnet-sillimanite (-biotite)

gneiss (Khondalites)

• Basic pyroxene granulite

EJ Quartzite

[J Quartzo-feldspathic rocks(Leptynites)

urrn Charnockite

.-"... Strike and dip of foliation

~ Overturned isoclinal synclinalfold+ Domal structure

Axial trace of F2 fold

-----<> Axial trace of F3 fold

~ - Axial trace of F4 fold

N

j2kmo

!

....... : 7~

···'t7~

..... : : :56~ . ,'0: : .: : : \'5.0'. '..: : :}s.\;~:.

: :fi.~ ::dGundreddipalem::••••~0)7~.::·

. :: : : :: : p;o}.o

........ ::::: :F2: .

.:::::::::::: :~~:

::: :::::::: :/10:::::::::::: ::: :e~:

. ~ ~ ~ ;;:;;~...4'. ;4,0 10!~ ..... 50 'SO" 50.

:::::: A,,;: : :.<: .A.·.'9'..: ./:.'5: : : : : : ~o: . . -...

~. .-?70: : : : .~ :

Fig. 4. Geological map around Visakhapatnam (modified after Rao, A.T. et aI., 1993).

Table 1. Structural evolution zn and around the Visakhapatnam area (Fonarev et al., 1995).

DeformationDeformational structures

stage

D,Strong foliation (S,) parallel the compositional banding

Strong stretching lineation

D2 Tight to isoclinal fold (F2) with axial planar cleavage (S2)

D3

Macroscopic open to tight fold (F3) showing often doubly plunging

shape (domal structure)

D,SE-pl unging open uprigh t fold (F,) characterized by prominent axial

planar shears

NNE-trending kink bandsPost-D,

Brittle shear on a minor scale

Planar structuresCompositional bandings, identified by the al­

ternation of garnet-rich layers and quartzo­feldspathic layers, are well developed within thegarnet-sillimanite (-biotite) gneiss (Fig. 6A). Thiscomposi tional banding is distinguishable from

other minor compositional banding structures inthis area and is termed the 'major compositionalbanding'.

Although the garnet grains often haverounded shapes, lenticular garnet grains (or gar­net aggregates) can be observed both on the

114 Yasutaka TANI et al.

N

lo 2km-=

~ Major compositional banding structureand lithological booundary

Fig. 5. Map showing the simplified topographic features of the area around Madhuravada, and the selected routeillustrated in Fig. 7. Some structural data, collected during the present survey, are also given.

horizontal and vertical rock surfaces in some lo­calities (Fig. 6B). This observation indicates thatthese garnet grains (or aggregates) are theshape of the oblate strain ellipsoid which mighthave been formed by flattening (Davis, 1984).This preferred orientation of lenticular garnetgrains form a foliation, generally paralleling themajor compositional banding. On the other hand,this foliation is deformed by an isoclinal folding,as explained below.

The major planar structures, consisting of themajor compositional banding and foliation paral­lel to the major compositional banding, trendNW-SE and have moderate to high NE dippingin the outcrops located near Kottapalem (Fig.7). On the other hand, the N-S-trending and E­dipping planar structures, with moderate dip an­gles, are common in the outcrops situated nearGollapalem (Fig. 7). On the basis of the majorplanar structures, the survey area is divided intotwo structural domains (Structural domains Aand B, Fig. 7).

The major planar structures are deformed bypinch-and-swell structures in some localities(Fig. 6C). The direction of the necks of thepinch-and-swell structures show various orienta­tions, but the data are not enough to confirmthis (Fig. 8).

Steeply dipping cleavages associated withleucocratic veins are locally observed in thisarea (Fig. 7). These cleavages cut the majorcompositional banding and trend E-W (Fig. 6D).As the outcrops are highly weathered, the min­erals in these leucocratic veins are indistinct.

Fold structuresDeformed, major compositional banding struc­

tures often show mesoscopic intrafolial folds ofisoclinal to tight type, with asymmetric or sym­metric types in some outcrops. These fold struc­tures are generally non-rootless and show nearlysimilar shapes. However, the intrafolial tightfolds, observed at outcrop No. P96091011, arenearly of the chevron type (Fig. 6E). An axial

Some structural observations in the Eastern Ghats 115

Fig. 6. Field photographs showing the deformational structures in the area between Kottapalem and Gollapalem.A: intrafolial isoclinal to tight fold, showing rootless type. B: Foliation formed by oblate strain ellipsoid.C: pinch-and-swell structure. D: leucocratic veins. E: intrafoJial tight fold showing nearly chevron style. F:isoclinal to tight fold showing asymmetric shape.

planar foliation, composed of weakly flattenedgarnets, is rarely observed in some intrafolial folds(Fig. 9A and 9B). The equal-area stereographicprojections illustrate that the hinges of theintrafolial, isoclinal to tight folds have variousdirections (Fig. 8).

At the outcrop No. P96091011, it can be ob­served that the foliation, composed mainly of

shape preferred orientation of garnet aggregatesshowing an oblate strain ellipsoid, is deformedby the isoclinal folding event (Fig. 9C and 9D).The foliation marked by garnet is observed inthe limbs as well as in the hinge areas of theisoclinal folds (Fig. 9C and 9D). This indicatesthat the isoclinal folding might have occurredafter the formation of the garnet foliation, which

116 Yasutaka TA I et at.

\ - '\

\\

" -

/

/

I

N', .

,\

\

\

I/

i\

t-,

- - "

-. -.. --, ,

" \

l::.

Structural domain'S'

,.1>,;'l?i~'- -,/........: '-35

3

f'7: "

o ~91(~),_/

69 37 I 97(37)

43 I----_/112(40) 11(11)

8 193 / 136(31)(40) \

Reddilapalem.,.,. ..Gollapalem

Lo~ No. P96091011

, ",,-"

64 78(75) ,

~~9,(51) ., '0. ,

\\ "~,-"Lo;' No. P96090902 ? \

•

- -,

Structural domain A

\ l::.,.- " \

/ \ \\ ,

55 "-

~35) -. ,,'",\

\ ,I \

1 _ .- '-' .

Intrafolial isoclinal foldshowing asymmetric shape

.---:----l,--------.

Gentle fold

"'\ Fold axial plane

~ Long axis of sillimanite

Crenulation lineation

\,

1

1I

\ ~61 113

r'~i

I\ ...

--- .~

\

\

\

Road

Exposures

Khondalite

Khondalite (graphite rich)

Peak

"-

Lac. No. P96090901

Kottapalem

CJo~L.:.:J

Legend of symboles

I '_ I Contour line

/ Elongation of garnet

+-- Neck of pinch-and­swell structures

HInges of folding structures

'd- Intrafolial isoclinal rootless fold ~\ showing symmetric shape -\

~ Intrafolial isoclinal fold\ showing symmetric shape

Lineations

/Y' Elongation of quartz

Planar structuresCompositional bandingand major foliation

Minor foliation

\

I

1\

\

o-- . 250m

Fig. 7. Route map showing structural data in the area between Kottapalem and GolJapaJem.

Some structural observations in the Eastern Ghats 117

+

o

•

B

•

+

tion of these asymmetric folds are almost simi­lar with that of the NNW-SSE-trending macro­scopic close fold (Fig. 8). Their vergence, de­fined by the fold asymmetry, is concordant withvergence of the parasitic folds of the macro­scopic close fold. Hence, it is possible that theseasymmetric, non-rootless folds are parasitic foldsof the macroscopic close fold mentioned above.

c

+

+ Phai (TT)-pole of major compositional banding (MCS) and major foliation (MF).

o Minor foliation cutting the MCS.

t::. Neck direction of the boudinaged and pinch-and-swell structures.

• Hinge of intrafolial isoclinal fold (symmetric shape).

• Hinge of intrafollal isoclinal- tight fold (asymmetric shape).

<> Long axis of elongated quartz.

* Long axis of elongated garnet.

o Long axis of sillimanite and crenulation lineation.*B-maximam oi TT-poles of the MCB and MF.

Fig. 8. The equal-area stereographic projections (lowerhemisphere) of Land S structures in the areabetween Kottapalem and Gollapalem. A: struc­tural domain A; B: structural domain B; C:major planar structures in domains A and B.

was probably associated with the flatteningevent.

Any shear-zone features, such as mylonite andfault zones, which could change the structuralorientation, were not identified in this area (Fig.7). The variation of the orientation of the majorplanar structures suggests that a macroscopicfold structure possibly lies in this area (Fig. 7).Structural data of the major planar structuresare plotted on an equal-area stereographic pro­jection (Fig. 8e). This shows that the macro­scopic fold is a NNW-SSE-trending close fold,with overturned axial plane. The hinge of thismacroscopic fold is moderately dipping towardENE and moderately plunging toward ESE.However, more structural data are necessary toestablish the macroscopic fold completely.

Two asymmetric non-rootless folds (Fig. 6F)of isoclinal to tight type can be observed instructural domain A (Fig. 7). The hinge direc-

LineationsAlthough lineation constitutes a very minor

structural element in this area, crenulation line­ation, mineral lineation and mineral-stretchinglineation can rarely be observed in a few locali­ties. Mineral lineation is defined by the long axisof sillimanite. This lineation is generally associ­ated with the crenulation lineation parallel tothe mineral lineation, and trends ENE.

In an exposure situated near the outcrop No.P9609l011, garnets show granular or weaklyelongated shapes on the vertical section, in spiteof the lenticular shape on the horizontal surface(Fig. 9E and 9F). This observation may indicatethat these garnets probably give an outline ofthe prolate strain ellipsoid, related to a stretch­ing deformation (Davis, 1984). Elongated quartzgrains, showing the prolate strain ellipsoid, arealso observed in some localities where the pinch­and-swell structures can be well identified (lo­cality No. P96090902 and No. P9609l0ll, Fig.7). These elongated shapes of minerals are con­sidered to be the stretching lineations. Althoughstretching lineation is locally developed, almost allthese lineations trend E-W in this area (Fig. 8).These are, in general, eastward-trendinglineations.

5. Discussion

A schematic illustration of the outcrop No.P9609l0ll is given in Figure 10, together withan equal-area stereographic projection. Judgingfrom the geometrical relationship and the fieldobservations, the following interpretations canbe considered: (1) the intrafolial, isoclinal totight folds were formed after the formation ofthe major compositional banding; (2) the miner­al-stretching lineation and the pinch-and-swellstructures might have been formed by extension,parallel mostly to the hinges of the isoclinal totight folds. This extension may be due to the

118 Yasutaka TANI et a1.

Fig. 9. Field photographs showing the deformational structures in the area between Kottapalem and Gollapalem.A: intrafolial isoclinal to tight fold in khondalites. B: the hinge area of the fold shown in A. C: isoclinalto tight fold affecting the foliation marked by garnet. D: the hinge area of the fold shown in C. E: garnetshowing granular or weakly elongated shapes on the vertical section in khondalites. F: photograph of thehorizontal surface showing garnet of the prolate strain ellipsoid.

post-buckling compression of the intrafolial,isoclinal to tight folds.

On the other hand, the major foliation, com­posed of the preferred orientation of lenticulargarnet grains or aggregates showing the oblatestrain ellipsoid, is deformed by the isoclinal foldsin a few localities. This observation may give

rise to the following interpretation: (3) the intra­folial, isoclinal to tight folds might have oc­curred after the formation of the garnetfoliation. A mineral grain or a mineral aggregateshowing the oblate strain ellipsoid is generallyformed by a flattening mechanism (Davis, 1984).Thus, it is possible to consider that, (4) a flat-

Some structural observations in the Eastern Ghats 119

Table 2. Preliminary interpretation of the relationship between the structures in the area between Kottapalem andGollapalem (this study).

Deformation

stage Structures Remarks

Major compositional bandingDL Foliation composed of lenticular garnet grains or Flattening

garnet aggregates

a Mesoscopic intrafolial isoclinal to tight fold

DM---------------------------------------------------------------

bPinch-and-swell structure Post-bucklingSome stretching lineations compression

DNMacroscopic close fold wi th overturned axial plane (?) possibleMescoscopic isoclinal to tight fold compression

tening deformation might have affected the rocksin this area before the isoclinal folding event.

The intrafolial structures between the majorcompositional banding are deformed by the mac­roscopic close fold wi th overturned axial plane(Fig. 4). Hence, it is possible to consider thatthe macroscopic close fold and its associatedmesoscopic fold might have been formed afterthe formation of the intrafolial, isoclinal to tightfolds and their associated structures, mentionedbefore. However, the mechanism and kinematicsof this macroscopic folding event is not clear. Tosum up, a preliminary interpretation showingthe relationship among the deformational struc­tures in the area between Kottapalem andGollapalem is given in Table 2.

The DL stage is characterized by a flatteningdeformation, which is identified by the major fo­liation composed of garnet grains (or aggre­gates) showing the oblate strain ellipsoid. TheDM deformation is also considered to form themesoscopic intrafolial, isoclinal to tight folds,pinch-and-swell structures, and some stretchinglineations, as the result of a compression perpen­dicular mostly to the axial plane of the meso­scopic intrafolial folds. Moreover, the DN structures,such as the possible macroscopic close fold withoverturned axial plane and related mesoscopicstructures, may also be due to a compressionevent. Thus, the structures observed in the areabetween Kottapalem and Gollapalem are possi­bly the resul t of three stages of compression, in­cluding the first flattening deformation.

The first stage of deformation (DL ) is possi­bly equivalent to the D1 stage of Fonarev et al.(1995). They concluded the strong foliation wasaccompanied by a strong stretching lineation.However, some stretching lineation is developedafter the formation of the strong foliation, as dis­cussed in this paper. On the other hand, it seemsthat the DM deformation is not comparable tothe D2 deformation of Fonarev et al. (1995), be­cause the hinges of mesoscopic intrafolial foldsgenerally show random orientation, which is en­tirely different from the linear features of themacroscopic F2 folds, illustrated in Figures 3 and4. The DN deformation may be equal to the D2

deformational stage of Fonarev et al. (1995).

6. Concluding remarks

The results of the structural analysis carriedout locally are not harmony with the structuralhistory proposed by previous contributions (e.g.,Sriramadas and Rao, 1979; and Fonarev et aI.,1995). Such a disharmonic interpretation canalso be recognized at the macroscopic scale. Raoet al. (1993) and Fonarev et al. (1995) illustratethe clear domal structure in and around theMadhuravada area (see Figs. 3 and 4). It is truetha t the topographic characteristics and someplanar structural data of the Madhuravada Domearea suggest the presence of the domal structurein the southeastern part of the Dome area (Fig.5). However, it appears that the northwesternpart of the Dome area shows a plunging syn-

120 Yasutaka TAN I et al.

Intrafolial isoclinal totight fold

Major compositional banding and major foliation

Axial planar foliation

BNA

Legend+ Phai (n) -pole of the major compositional banding and the major foliation.

• Phai (n)-pole of the axial plane of the intrafolial isoclinal to tightfold and axial planar foliation composed of garnet folia.

o Hinge of the axial plane of the intrafolial isoclinal to tight fold.

o Direction of stretching lineation composed of elongated quartz.

6. Direction of neck of the pinch-and swell structure.

Fig. 10. The equal-area stereographic projection (lower hemisphere) of the structural data (A) and interpretativeblock diagram (B) of the locality No. P96091011.

form rather than a plunging antiform, althoughthe data are insufficient for a macroscopic anal­ysis of this Dome area (Fig. 5).

It is emphasized that we need more micro-,meso-, and macroscopic structural data collectedregionally from the study area, for a better un­derstanding of structural evolution of the EGMBaround Visakhapatnam.

Acknowledgments

We wish to express our sincere gratitude toDr. K. Srinivasa Rao. Mr. S. Srinivas, and Mr. P.Saradhi of Andhra University, India, for their as­sistance and hospitality during the field work.We would like to thank Prof. V. Glebovitsky andDr. A.B. Kovach of Institute of Precombrian Geo­logy and Geochronology, St Petersburgh, Russiafor their fruitful discussions in the field. We arethankful to Mr. H.M. Rajesh of the Departmentof Geosciences, Osaka City University, for valu­able review and discussions. This paper is a con-

tribution to IGCP 368.

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