structural study of meghalaya plateau through aeromagnetic data

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.79, January 2012, pp.11-29

0016-7622/2012-79-1-11/$ 1.00 © GEOL. SOC. INDIA

Structural Study of Meghalaya Plateau through Aeromagnetic Data

RAJENDRA SHARMA, H. C. GOUDA, R. K. SINGH and B. V. NAGARAJU

RSAS, Geological Survey of India, Vasudha Bhavan, K.S. Layout, Bangalore – 560 078Email: [email protected]

Abstract: The study of aeromagnetic data over parts of Assam-Meghalaya has delineated major discontinuities representingfaults/ fractures/ contacts/ shears and magnetic body axes, which helped in understanding the regional and structuralsetup of the area. The known Barapani and Dapsi Thrusts, Dudhnai Fault and three strong magnetic anomalies withreverse magnetisation have been delineated. The quantitative analysis of aeromagnetic data brought out the depths ofmagnetic interfaces and the magnetic picture of different layers, which also helped in understanding the regional andstructural setup. The contact modeling across few profile sections showed the depths to the top of contacts and the mapof magnetic-basement-depth-model revealed the undulations of the basement. Magnetic body axes of several magneticanomalies and also several magnetic discontinuities representative of faults/ fractures/ contacts/ shears system in thearea along with five potential blocks for mineral prospecting are the main outcome of this exercise. The seismotectonicactivity associated with the magnetic features has also been studied to understand the structural setup.

Keywords: Structural study, Aeromagnetic data, Meghalaya.

INTRODUCTION

The North-Eastern Council (NEC) engaged the NationalGeophysical Research Institute (NGRI) for carrying out highaltitude airborne magnetic survey over Meghalaya Plateauand parts of Assam over three blocks covering 43 toposheetsat two different heights of 4600 ft and 7000 ft above m.s.l.over an area of about 24,040 km2. The area was flown duringNovember-December, 1977 for assessment of naturalresources in the region.

The NGRI interpreted the aeromagnetic maps andmarked four magnetic regions (Region-I, II, III, IV), threecategories of magnetic zones (High anomalous zones: HA1-HA15, Moderate anomalous zones: MZ1-MZ15, Lowanomalous zones: LA1, LB1-LB4), seven categories ofmagnetic lineaments trending in NE-SW, WNW-ESE, E-W, ENE-WSW, N-S, NW-SE and NNE-SSW directions andfew magnetic bands. Thereafter the North-Eastern Region(NER) and North-Eastern Zone (NEZ) of AMSE Wing,Geological Survey of India, Shillong took up the job ofground check up in the area during different field seasons.

The AMSE Wing, Bangalore and North-Eastern Zonaloffice, Shillong, Geological Survey of India compiled theaeromagnetic data of different blocks, applied correction ofInternational Geomagnetic Reference Field (IGRF), broughtthe data to a common level at 7000 ft =2.134 km abovem.s.l. and prepared a combined aeromagnetic contour mapof the study area. This aeromagnetic data has been

interpreted with an objective of studying structural setup inthe study area and to identify structural patterns favourablefor mineral prospecting.

REGIONAL GEOLOGY

The Meghalaya (Shillong) plateau in the northeasternpart of India constitutes Archaean Gneissic Complex withnon porphyritic granitoid rocks, Proterozoic rocks ofShillong group with Khasi greenstones, Proterozoic to earlypalaeozoic porphyritic granitic plutons, Sylhet traps ofJurassic-Cretaceous period and Cretaceous-Tertiarysediments. (Fig.1a)

The gneissic complex is subjected to metamorphism andexhibits green schist to amphibolite facies (Mazumder,1976). It is also affected twice by the mafic intrusions. Thenon porphyritic and migmatised granitoids are found withinthe Gneissic Complex. The Gneissic Complex with the nonporphyritic granitoids show several phases of deformation(Murthy et al. 1976; Barooah, 1976). The first deformationhas oriented the axes of the fold along E-W direction; thesecond deformation has resulted in the development of foldswhose axes are found along NE-SW direction. The thirdgeneration of fold is developed with the fold axes alongN-S direction. The E-W and NE-SW trending deformationsare very prominent. In addition to this, there are brittle typesof shallow fractures noticed along NW-SE direction and

JOUR.GEOL.SOC.INDIA, VOL.79, JAN. 2012

12 RAJENDRA SHARMA AND OTHERS

Fig

.1. I

GR

F co

rrec

ted

tota

l int

ensi

ty a

erom

agne

tic im

age

map

.


STRUCTURAL STUDY OF MEGHALAYA PLATEAU THROUGH AEROMAGNETIC DATA 13

the deformations due to folding are plastic type and are deep(Murthy et al. 1976).

The Shillong group of rocks lies unconformably overthe Gneissic Complex in an intra cratonic basin extendingalong NE-SW direction. The Western contact of the basinwith the Gneissic Complex is tectonic in nature and ischaracterized by the shear known as Barapani shear andhas been emplaced by the younger porphyritic granites viz.Nongpoh granites and South Khasi Batholith. (Mazumder,1976; Murthy et al. 1976; Mazumder, 1986). Themetamorphosed form of argillaceous and arenaceous typeof rocks forming Shillong group show the trends mainlyalong NE-SW direction and are folded in “crest like” type(Belouassov, 1962; Mazumder, 1976). Later these rockshave been intruded by the Khasi greenstones along thegeneral trend of Shillong group of rocks as sills and dykes.The Khasi green stones are also metamorphosed along withthe Shillong group of rocks. There are four generations offold that have affected the Shillong group of rocks (Mitra,1998). The first phase of deformation is the major one withthe fold system whose fold axes are plunging in NE-SWdirection. The other phases of deformation havesuperimposed over the earlier phases.

The porphyritic granites of Late Proterozoic toPalaeozoic are the post tectonic plutons, namely South KhasiBatholith, Mylliem granites, Nongpoh granites and Kyrdemgranites intruded the gneissic complex and the Shillonggroup of rocks (Ghosh et al. 1991). These intrusives asdiapirs form the final stage of orogenic episode (Mazumder,1986). The most of the plutons are found within the gneissiccomplex eg. Nongpoh Pluton. Myllieum and Kyrdem plutonsoccur within the Shillong group of rocks. The South KhasiBatholith intruded both gneissic complex and Shillonggroup is interesting and helps in understanding the otherplutons which occur alone either in Gneissic Complex or inShillong group of rocks as it may genetically connect them(Mazumder, 1986). The distribution of the plutons arecontrolled by the pre existing NE-SW and NW-SE trendingtectonic contacts, fault and fracture systems. The straightnature of edges of Nongpoh and the South Khasi Batholithplutons sharing with the Shillong group trending in theNE-SW direction indicates that they are emplaced alongthe earlier tectonic contact (Murthy et al. 1976). Similarlythe western margin of the South Khasi Batholith, wherethe NW-SE trending fault is noticed, seems to control thepluton.

Fig.1a. Regional geological map of Shillong plateau and adjacent areas (after Mazumder, 1976; Ghosh et al. 1991).

FF

FF F

FF

F

F

F

F

F

F

F

F F

F

FF

F

LUMDING

DAWKI

TURA

F

F F

F

GAUHATI

F

SHILLONG

Cretaceous Tertiary sediments

Shylet Traps

Porphyritic granite

Shillong Group

Mafics

Precambrian Gneissic Complex

Fault

Boundary of Meghalaya

10 0 10 30 Km

N

4) South Khasi pluton3) Nongpoh pluton2) Kyrdem pluton1) Mylliem pluton

3

4

2

1

I N D E X



The rifting of Gondwanic India from Australia andAntarctica during Jurassic-Creataceous period has resultedin vast extrusion of Rajmahal and Sylhet traps followed bythe domal rise of the Shillong plateau which lead to its breakup along the southern margin by developing E-W trendingfault systems known as the Dauki fault system through whichthe Sylhet traps made their passage (Gupta and Sen, 1988).Eventually the effect of upliftment of the Shillong plateauinitiated the development of N-S trending lineaments throughwhich alkaline-ultramafic-carbonatite suites are emplaced.These suites may be the continental intrusive equivalents ofSylhet traps and possess the same phase of deep-seatedtectonic igneous activity like Sylhet traps (Gupta and Sen,1988). The Sung valley ultramafic complex of Cretaceousage (Chattopadhyay and Hashimi, 1984) and the pyroxinitebody (Prasad, 1983) near Jowai are associated with the N-Strending long lineament zone termed as UM NGOTlineament zone (Gupta and Sen, 1988). Another suchlineamnet (Gupta and Sen, 1988) observed betweenDurgapur and Dalgoma is found to be associated with thecarbonatite affinity rocks (Kumar et al. 1984). The sedimentsbelonging to the Cretaceous, Tertiary and Quaternary timesoccur over and towards south of Dauki fault zone. Apartfrom the Dauki fault, the area has been affected by faultsand thrusts. The well-known faults in the area are N-Strending Dudhnai and Kulsi faults (GSI, 2000) in theGneissic Complex. The Dapsi thrust zone (Murthy et al.1976) is traced between Tertiary sediments and GneissicComplex near Tura.

Many people have traced the above described

lineaments, structural trends, fault and fracture systemsbased on the field studies, photo geological studies (aerialphotographs) and landsat imageries (Krishnan, 1960; Evans,1964; Valdiya, 1976; Mazumder, 1976; Parthasarathi, 1978;Gupta and Sen, 1988; GSI, 2000). The correlation ofgeophysical features with these structural features andtheir importance in seismotectonic activity and associatedmineralisation is discussed herein.

ANALYSIS OF AEROMAGNETIC DATA

The study of aeromagnetic data through a variety ofderived maps e.g. reduced to pole (RTP) map, maps ofdifferent layers (shallow layer, middle layer, deep layer),magnetic-basement-depth-model map and also by modelingover selected features as contacts has been carried out. Thestudy included quantitative as well as qualitative approachof analysis.

Estimation of Radial Power Spectrum: Computerprogram for radial and angular spectrum estimation (Naiduand Mathew, 1988a) was used to calculate the radial powerspectrum of IGRF corrected aeromagnetic data (Fig.1). Theplot of log of radially averaged power spectrum againstfrequency (Fig.2) indicated three interfaces and their depthsare computed as 1.986 km, 3.72 km and 24.336 km belowthe observation plane (7,000 ft = 2,134 m) above m.s.l.representing shallow, middle and deep interfacesrespectively. This indicated that magnetic susceptibility haschanged at these levels. These depths seem to be of regional

Fig.2. Radially averaged power spectrum of IGRF corrected total intensity aeromagnetic data.

nature because of the surveyparameters of 2 km line spacingand flight height of 7,000 ft abovem.s.l., which enabled to pick up themagnetic response of deeper and/or regional features.

Magnetic Layer Maps: Thematched filtered technique wasused to separate the magneticresponse of features falling inbetween different magnetic inter-faces. In other words the magneticeffect of shallow layer (Observa-tion plane -1.986 km), middle layer(1.986 km-3.72 km) and deep layer(3.72 km - 24.336 km) werecomputed from IGRF correctedaeromagnetic data by matchedfiltering. This helped in under-

Shallow magnetic interface (1.986 km below the observation plane)

Middle magnetic interface (3.720 km below the observation plane)

Deep magnetic interface (24.336 km below the observation plane)



standing the depth continuity of magnetic features from onelayer to another. The software used for matched filteredtechnique was an in-house development (Mathew, 1988).

The magnetic map of shallow layer (Fig.3) indicates thepresence of magnetic features similar to that of observedmap excepting the reduction in amplitudes of anomalies.The magnetic map of middle layer (Fig.4) indicates thecontinuation of the magnetic features noticed in the shallowlayer map, with smoothened and broadened quality. Themagnetic map of deep layer (Fig.5) indicates deep crustalfeatures and thus all features observed in top two layers arefound to disappear in this map excepting at few places. Itcan be inferred that most of the magnetic features noticedin the observed map (Fig.1) continue up to the maximumextent of 3.72 km below observation plane.

In the deep layer map the prominent E-W trendingmagnetic feature from east of Mojam up to west of Barapani,has been intersected by other trends around Nongkhlaw inthe west and around Mojam in the east forming junctions.The junction at Nongkhlaw, which is of deeper nature, isshown in the interpreted map (Fig.8). Only the northern partof NE-SW trending magnetic discontinuity F1F1 (knownBarapani Thrust) truncated by an E-W gradient passingthrough Barapani is noticed in the deep layer map, whereasits southern part is not noticeable. In other words the northernpart of F1F1 discontinuity is of deeper nature compared toits southern part and both the parts are separated by a gap,as visible in the interpreted map. The NW-SE trendingmagnetic discontinuity F2F2 (part of known Dapsi Thrust)in the southwestern part passing through south of Tura isobserved to continue in the deep layer map suggesting itsdeeper nature.

The NE-SW trending magnetic feature towards west ofDarugiri and a junction towards north of Darugiri noticedin the interpreted map (Fig.8) is also observed in the deeplayer map indicating its deeper nature. The deep layer mapshows only the magnetic response of deep crustal features,whereas the IGRF corrected map, shallow and middle layermaps show mainly the magnetic response of shallowsubsurface regional structures and tectonic features. Sincethe data was acquired at an altitude of 7000 ft above mslwith 2 km line spacing, the magnetic response of lowfrequency, broad wavelength representing regional andstructural features were recorded and the high frequencyresponse of near surface magnetic features could not bepicked up due to high survey altitude and large flight linespacing. Also, the deep layer map indicates almost similarstructural blocks, as discussed in the basement depth modelsection e.g. western, central, eastern and northern blocks.

The ground magnetic (VF) survey in the areas towards

northwest and northeast of Tura (Venkata Ramana andSawaiyan, 1993) gave the results of modeling by assumingas horizontal cylinder across the four profiles in the areatowards northwest of Tura with radii of 1.49 km, 2.33 km,1.71 km and 1.92 km. This anomaly zone represented in theaeromagnetic map by a strong anomaly towards NW ofDanangiri was noticed in the top two layers and disappearedin the deep layer map i.e. below 3.7 km from observationplane. The modeling results of ground magnetic profilesthus corroborate mostly with the inference drawn thatthis anomaly zone is confined to the top two layers ofaeromagnetic data and not extending to the deep layer.

Modelling: Magnetic anomalies of regional nature arenoticed all over the study area. The modelling of suchanomalies was avoided after studying the depths of magneticinterfaces and the maps of different layers. The contacttype of features were observed in the magnetic map andhence modeling over selected features was taken up along14 profile sections C1 to C14 (Fig.7) in the RTP map ascontact model (Stanely John, 1977). The profiles werepresented in graphical form (Fig.9) along with the results.The depths computed to the top of the contacts were foundto vary between 1230 m - 3672 m from the observation planeand susceptibility contrast varying between 0.00106 to0.01659 cgs units (Table 1). The depths to the top of thecontacts are in agreement with the depth-range of toptwo layers indicating the depth variance and nature ofcontacts.

Ground gravity survey data along E-W profile over highmagnetic anomaly near SW of Jowai indicated an anomalyof 18 mGal with density contrast of 0.6 g/cc (3.3-2.7=0.6),which was modeled as semi horizontal cylinder of radius

Table 1. Results of quantitative interpretation by contact modelling

Sl. Profile Depth Dip SusceptibilityNo (metres) (degrees) contrast

(cgs units)

1 CC1 3672 123 0.009622 CC2 1971 151 0.006363 CC3 1821 143 0.003254 CC4 2176 166 0.016595 CC5 3272 123 0.002706 CC6 2080 131 0.003607 CC7 1230 127 0.003448 CC8 1554 115 0.001749 CC9 2643 132 0.0038410 CC10 2288 136 0.0049611 CC11 1343 126 0.0014112 CC12 1508 129 0.0024613 CC13 1407 111 0.0013814 CC14 1965 098 0.00106

Note: Depths are below the observation plane (7000 ft above msl)



Fig

.3. M

atch

ed f

ilter

ed im

age

map

(Sh

allo

w la

yer)

.



Fig

.4. M

atch

ed f

ilter

ed im

age

map

(M

iddl

e la

yer)

.



Fig

.5. M

atch

ed f

ilter

ed im

age

map

(D

eep

laye

r).



0.8 km showing a thickness of 0.8 km and breadth of 2 km(Rama Rao et al. F.S.1981-1982).

A ground magnetic (VF) profile in the Sung Valley wasmodeled by half-width formula (Hasan, 1985) and the depthof the causative body was calculated to be 40 m belowground surface. The result of inclined borehole drilled inthe area confirms the presence of pyroxenite with bands ofcarbonatite and magnetite with apatite up to the depth of193.1 m (Hasan, 1985).

These results of ground survey corroborate the fact thatmost of the magnetic features fall within the depth range oftop two layers. Since the aeromagnetic data is of regionaltype, the depths to the top of contacts thus calculated indicatethe regional depths only.

Basement Depth Model: The magnetic basement depthmodel map was generated by choosing a moving windowof 15 km (50 x 300 m) with a sliding of 6 km (20 x 300 m)of the aeromagnetic data with grid size of 300m. The meandepth was obtained from each sliding window with the helpof radial power spectrum and these values were contoured.

The magnetic basement depth model map (Fig.6) hasindicated the general background depth in the range of2,850m - 3,050 m below observation plane. Though thesedepths are estimated depths only, but are very useful inunderstanding the relative vertical movements of varioussegments of magnetic basement. The study of this map showsthree prominent uplifted areas: (i) within a broad depressedblock in western part near Tura and east of Danangiri, (ii) incentral part near Nongkhlaw and Mairang and (iii) in easternpart near Mawryngkneng and Nongkynrih. The throw ofuplifted areas around Tura in the western depressed block,varies between 300 m -650 m, in central block aroundMairang varies between 400 m -500 m and in the easternblock in between Umsning and Garampani is observed tobe of the order of 1000 m, which includes the strongmagnetic anomaly of reverse magnetization nearMawryngkneng in Sung valley. In the northeastern andeastern part of the area, the uplifts of about 650 m aroundLengri and 550 m around Nongirong are noticed. Theincomplete northern uplifted block, which is covered partlyby airborne survey, has shown an uplift varying between400 m - 800 m from Kuhikuchi to Dudhnai. In addition,there are few isolated uplifted zones in the area, but majoruplifted blocks are discussed here along with the results ofground geophysical surveys carried out earlier duringdifferent years.� In the western side an area from north of Tura towards

Rengdim is noticed to be uplifted in NW-SE directionwithin a broad depressed block. The ground magnetic

survey (Hasan and Thakuria, 1993) delineated a basinlike feature, which corroborates well with thedownthrown feature towards SE of Tura with a throw of400 m. The ground magnetic survey also delineated alitho-contact between the Precambrian and the Tertiaries,which in general corroborates well with the thrust planetowards SE of Tura in the magnetic basement depthmodel map and also with the F2'F2' magnetic discontinuityin the interpreted map (Fig.8). Towards SSW edge ofthe map from south of Tura and SE of Rengdim, anupliftment is indicated with incomplete image, whichmay fall over a gravity high closure of +40 mGal (Khanand Chakraborty, 2007), indicating an uplifted block inthe SSW corner of the area.

� In the western part of the depressed block, few upliftedregions are noticed towards immediate west, east andnorth of Dananggiri. The ground magnetic (VF) survey(Venkata Ramana and Sawaiyan, 1993) in this part ofthe areas indicated few faults, which corroborate well,as discussed below, with the results of study of magneticbasement depth model map.a) Ground survey indicated a fault towards NW of

Dananggiri with a throw of 408 m - 460 m, which isalmost in corroboration with the depth model mapshowing a throw of 500 m.

b) Ground survey indicated two faults towards NE ofDananggiri with a throw in the ranges of 250 m-322m and 252 m-262 m, which indicate the northerncontact of the down thrown block in the depth modelmap. These faults are also reported by groundmagnetic survey corroborate with the magneticdiscontinuities shown in the interpreted map (Fig.7)towards NW, NE and east of Dananggiri.

� The central uplifted block occurs around Nongkhlaw andMairang and is separated from the western depressedblock by a moderate NNE-SSW trending uplift overPatharknang. The southern part is downthrown fromMawsynram to Cherrapunji.

� The eastern uplifted block occurs around Mawryngknengand Nongkynirh, which is separated from the centraluplifted block by N-S trending depression towards eastof Umsning - Barapani. This corroborates with theground geophysical results (Jawahar and Raghuramaiah,1993). The eastern margin of the uplifted blockterminates by a prominent low of Mojam-Garampani-Lobang.

� The central and the eastern uplifted blocks are terminatedin the north by a long E-W depression through Bardur-Nongpoh-Khongmasi-Sarmang Mabai. Indication ofgravity low in these parts (Pathak and Syiem, 2006a)



Fig

.6. M

agne

tic b

asem

ent

dept

h m

odel

im

age

map

.



Fig

.7. R

educ

ed to

Pol

e im

age

map

.



Fig

.8. I

nter

pret

ed a

erom

agne

tic m

ap.



corroborates with the Bardur-Makakuki-Sarmang Mabaidownthrown block.

� A prominent E-W trending uplift around Lengri has beenobserved within the northern downthrown block.

� There is an incomplete indication of another upliftmentall along the northern boundary of surveyed area, asfurther northern part of the area was not flown.

� In Sung Valley, a ground magnetic (VF) profile wasmodeled by half-width formula (Hasan, 1985) and thedepth of the causative body was calculated to be 40 mbelow ground surface. The results of inclined boreholedrilled in the area confirm the presence of pyroxenitewith bands of carbonatite and magnetite with apatite upto a shallow depth of 193.1 m (Hasan 1985). Thisindicates the shallow nature of magnetic basement andcan be corroborated with the central uplifted block, asSung Valley is part of this uplifted block.The study has shown that the magnetic basement of the

Shillong Plateau is full of undulations with variable depthsof uplifts/ downthrows indicating its heterogeneous nature.Similar conclusion was drawn by Khan and Chakraborty(2007) based on Bouguer gravity anomalies and seismic b-values and also by Kayal and Zhao (1998) based on seismictomographic results.

Reduced To Pole (RTP) Map: The reduction ofaeromagnetic data to pole helps in marking the location ofanomalies correctly and also the magnetic discontinuities,as the bipolar nature of anomalies is reduced to unipole.There are three locations noticed in the aeromagnetic mapshowing the reverse magnetization. The aeromagnetic datain these locations was not considered at the time of reducingthe aeromagnetic data to pole and hence these three locationshave been left blank in RTP map (Fig.7). The RTP map hasbeen generated by using the in-house developed software(Mathew, 1988; Naidu and Mathew, 1998) mainly forqualitative interpretation. The high value closure in RTPmap is considered to fall over a magnetic body. Hence, byusing the same logic RTP map was studied and the axes ofmagnetic bodies have been marked along the center of thehigh magnetic closures indicating the magnetic linears. Themagnetic discontinuities have been marked along gradientsin the RTP map, which indicate faults/ fractures/ contacts/shears.

The map has been divided into three magnetic domains,based on criteria of shape and amplitude of anomalies,spacing of magnetic lineations and so on (Reeves, 1985).The high amplitude magnetic anomaly domains were markedas ‘A’, the moderate anomaly domains as ‘B’ and the lesseranomaly domains as ‘C’. Three high magnetic anomalies

of domain ‘A’ with reverse magnetization have been noticedin the map, one around Mawryngkneng (Sung Valley), othertowards southwest of Jowai and another in the northeasternpart in between Lengri and Laopani. Most of the area isrepresenting magnetic domain ‘B’ excepting lesser anomalydomains ‘C’ in SSW part around Darugiri-Rengdim, in theNW corner, south of Gauhati, in the central part aroundUmpyrtha-Umsning and around Nongirong in the easternpart of the study area. The prominent anomaly was noticedtowards NW of Dananggiri in the western part in domain‘B’. The axes of magnetic bodies, magnetic discontinuitiesand magnetic zones have been shown in the interpretedmagnetic map (Fig.8).

Two major magnetic discontinuities F1F1 representingBarapani Thrust and F2F2 , F2

’F2’ indicating Dapsi Thrust

were delineated in the RTP map. The N-S trending magneticdiscontinuity towards south of Dudhnai was noticed torepresent the Dudhnai Fault.

The ground geophysical surveys in Sung Valleyconfirmed the presence of ultramafic bodies with highmagnetic anomalies and reverse magnetization(Bhattacharya et al., F.S.1977-1978; Hasan, 1985; Das etal. F.S.1989-1991).

The ground magnetic and gravity surveys in Jowai area,Jaintia Hills District, Meghalaya confirmed the presence ofultramafic bodies with high magnetic anomalies and reversemagnetization and gravity high towards SW of Jowai (RamaRao et al. F.S.1981-1982; Das et al. F.S.1989-1991).

The gravity map generated by ground survey in Sungvalley (Das et al. F.S.1989-1991) indicated high gravityclosure of 31 mGal (without terrain correction) whichremains open in the northern side, corroborates with the highmagnetic anomaly there.

DISCUSSION OF RESULTS

Magnetic features and mineral prospecting- acorrelation: The qualitative interpretation of IGRFcorrected aeromagnetic data was carried out after studyingthe various derived maps and mainly the RTP map. Themagnetic domain ‘A’,’B’,’C’, axes of magnetic bodies andmagnetic discontinuities observed in RTP map have beenshown in the interpreted aeromagnetic map (Fig.8). Thestudy of this map was tried by incorporating the analysis ofother maps and also by corroborating the results of groundgeophysical surveys carried out earlier.

The interpreted map shows that magnetic body axes andmagnetic discontinuities trend mainly in NE-SW, NW-SE,E-W and few in N-S, NNE-SSW, NNW-SSE directions.Presence of a prominent E-W trending magnetic body has



Fig

.9. P

rofi

le m

odel

ling

by c

onta

ct m

etho

d.



been noticed over Nongpoh. The ground geophysicalsurveys (Pathak et al. 2006) indicated gravity and magnetic(V.F) highs corresponding to Nongpoh Pluton intrusive inarchaean gneiss. The ground survey along Guwahati-Shillong road (Jawahar and Raghuramaiah, 1993) estimatedthe width of Nongpoh Pluton as 16 km. A few moreprominent magnetic bodies near Nongpoh in Ribhoi districtand around Kyllang (near Mairang) have been observed.The ground gravity-magnetic survey (Pathak and Syiem,2006a) interpreted these anomalies to represent the plutonsof porphyritic granites.

A major NE-SW magnetic discontinuity F1F1 passingthrough Bargang in the northeastern part to the south ofPambriew in the southern part passing through Barapanihas been observed which corroborates with the knownBarapani Thrust. The ground geophysical surveys over apart of magnetic discontinuity F1F1 in Barapani-Mawiongarea were conducted by employing Magnetic (VF), SP, IP(Time Domain) and resistivity (Nayak et al. F.S.1975-1976)methods. The results of ground survey showed thosemagnetic anomalies at places trend in NE-SW direction andthe resistivity data indicated weathering up to 50m fromground level over the metabasic rocks. The inclined borehole(inclination 40°) drilled on the basis of ground geophysicalresults intersected sulphide mineralisation between 44.10m -160 m with rather conspicuous sulphide mineralisationbetween 92.85 m to 130.65 m. Since the area falls along themagnetic discontinuity F1-F1, the results indicate thepotentiality of the discontinuity as far as the mineralisationis concerned.

Other major magnetic discontinuity F2F2 and F2'F2'trending in NW-SE direction passing around Tura to southof Rengdim, have been observed in the southwestern part,which corroborates with the known Dapsi Thrust. The Microseismicity map of the region shows intense crustal (10 km -40 km) acitivity in an area near Tura corresponding to Dapsiand Barapani thrust zones respectively (Reddi et al. 1995).It may be inferred at this stage that magnetic discontinuitiesF1F1, F2F2 and F2'F2'

corroborating with Barapani and Dapsithrust zones respectively are indicative of intense crustalactivity.

The ground Magnetic (VF) survey carried out (Hasan etal. 1993) towards southeast of Tura, delineated a basin likefeature confirming the lineament feature (WNW) markedby NGRI and a litho-contact between the Precambrian andthe Tertiaries. The ground magnetic (VF) surveys in areastowards NW and NE of Tura (Venkata Ramana andSawaiyan, 1993) delineated a magnetic high anomalyshowing traces of pyrites, chalcopyrite and bornite in calciteveins within a basic dyke in the gneissic complex and

magnetic low anomalies over sandstones wherein coaloccurrences have been noticed. The ground magnetic highanomaly was found to corroborate well with the strongaeromagnetic anomaly towards NW of Dananggiri whereasground magnetic low was found to fall over the junction ofN-S trending and NE-SW trending discontinuities towardsNE of Dananggiri (Fig.8).

In the eastern part, majority of magnetic features werefound to trend in E-W direction and also ending abruptlyaround Jowai area where high magnetic closures of reversemagnetization have been observed. A prominent E-W featurehas been noticed passing from east of Mojam to northeastof Mawryngkneng. In the central part of the area majorityof the features were found to trend in NE-SW and NW-SEdirections, indicating major tectonic disturbances.

The study has helped in demarcating the five junctionsnamed as Blocks B-I to B-V (Fig.8), the boundary of theseblocks is drawn tentatively around the junction of features.These Blocks are called, one towards north of Darugiri orsouth of Dudhnai forming junction with Dudhnai Fault asBlock B-I, others around Hamka as Block B-II, aroundUmpyrtha as Block B-III, around Nongkhlaw as Block B-IV and around Mojam-Nongirong as Block B-V. The areaencompassing Nongkhlaw, Mairang, west of Shillong andwest of Barapani (Block B-IV) is a zone of confluence ofnumber of magnetic features indicative of highly disturbedarea due to tectonic activity and hence assume importancefor mineral prospecting. Similarly, the other four Blocksalso are important for mineral prospecting.

Magnetic discontinuities and seismotectonic activity-a correlation: The northeastern part of Indian plate thatincludes Shillong plateau and the Assam-Brahmaputra valleyin the form of foredeep is caught between the Himalayancollision zone in the north and northeast and the Burmesesubduction zone in the east. This tectonic unit is alsobounded by the E-W trending Dauki Fault system in thesouth and N-S trending Yamuna lineamnet in the west. Thedrag experienced by the Indian lithosphere under theHimalayan and Burmese arcs makes this tectonic unit pronefor the seismic activity (Mukhopadhyay, 1984). The Shillongplateau is affected by several lineaments, faults, fracturesand thrusts at different geological times and these featuresare very well reflected in the aeromagnetic data. Themagnetic discontinuities (Fig.8) show their trends alongE-W, N-S, NE-SW, NW-SE, ENE-WSW and WNW-ESEdirections.

The E-W and NW-SE trending magnetic discontinuitiesobserved in the gneissic complex reflect its deformationalpattern. Some of the magnetic discontinuities trending in



NE-SW, NW-SE and nearly E-W directions aroundNangpoh-Mawlong, Umpyrtha, Nongkhlaw, Pambriew andeast of Darugiri are reflecting the fracture pattern occurringover the porphrytic granititc intrusives into the gneissiccomplex and also the Shillong group. These discontinuitiescontrol the emplacement of granites as is evident in themaping that the discontinuity F1-F1 between Pambriew inthe south and Bargang in the north tracing the tectonic zonebetween the Shillong group and gneissic complex alongNE-SW direction, controls the emplacement of Nongpohpluton in the north and “South Khasi Batholith” in the southaround Pambriew. The presence of NW-SE trendingdiscontinuity associated with the fault is noticed whichcontrols the southwestern boundary of the South Khasibatholith (Fig.1a and Fig.8). The magnetic features due tothe Myllieum and Kyrdem plutons within the Shillong groupare not noticeable in the aeromagnetic map, as these are notprominently picked up by airborne survey.

As regards seismotectonic activity, there are othermagnetic discontinuities trending in E-W, N-S, NE-SW andNW-SE directions appearing in the rest of the area that arevery important. The relationship possessed by the E-W andN-S trending lineaments and faults formed during Jurassic-Cretaceous period has already been discussed in the section“Regional Geology”. There are magnetic discontinuities,which can be corroborated with the above-mentionedlineaments and faults. Though the magnetic data towardssouth covers only a part of the area of Dauki fault system,there are several E-W, ENE-WSW trending magneticdiscontinuities parallel or sub parallel to this fault towardssouth and southeast of the area representing the part of Daukifault system which are formed as a result of uparching ofShillong plateau (Gupta and Sen, 1988). Khan andChakraborty (2007) studied the seismic ‘b’ values and theirrelationship with the seismicity. Bouguer gravity values andstress level accumulation explains how these values areindicative of consistently the thinner crustal roots andinferred the inverse relationship of the seismic ‘b’ valueswith the Bouguer gravity values in the area. The low levelof seismicity associated with the low seismic ‘b’ values isobserved towards south along Dauki fault system indicatingthe high level of stress accumulation, which is alsocharacterized by the consistent thinner crust with upliftedMoho and high Bouguer gravity anomaly value of +40 mGal(Khan and Chakraborty 2007; Bouguer Gravity AnomalyMap of India 2006). This is the most anomalous area inIndia with a strong positive Bouguer anomaly recordedinspite of its high elevation (GSI, 2000) suggestive of thepresence of the high density mantle derived matter belowthe Shillong plateau (Verma and Mukhopadhyay, 1977),

raised as a result of its E-W compression due to itssubduction under the Indo-Burmese arc and N-Scompression due to its collision with the Himalayas (Khanand Chakraborty, 2007). The Dauki fault system is havingan arcuate trend towards east and terminates into theNaga thrust zone.

There are several N-S trending magnetic discontinuitiespassing towards west of Rengdim to Dudhnai throughDarugiri, which can be correlated with the prominent N-Strending lineament between Dalgoma and Durgapur tracedby Landsat data (Gupta and Sen, 1988) termed as Dudhanaifault (GSI, 2000) that is found to be associated withcarbonatites (Kumar et al. 1984). A low velocity zone wasidentified (Kayal and Zhao, 1998) over the magneticdiscontinuity associated with the Dhudanai fault. The highlevel of seismicity (Khan and Chakraborty, 2007) withmoderate seismic ‘b’ values recorded along this fault zoneindicates the high level of stress accumulation. This activitymay be due to shallow movements along the fault zone ormovements initiated by deep-seated source (Khan andChakraborty, 2007). Another nearly N-S trending magneticdiscontinuity passing towards east of Bhotpur andBardur may be correlated with the Kulsi fault (GSI, 2000).Another important magnetic feature along N-S directionnoticed towards north of the Mawrynkneng extendingtowards south into the area of pole reversed anomaly zone(Fig.8) is found corroboratable with the N-S trending UmNgot lineamnet zone (Gupta and Sen, 1988), which isassociated with the alkaline–ultramafic–carbonatitie(Chattopadhyay and Hashimi 1984) and pyroxinite (Prasad,1983) rocks.

Apart from the above-mentioned major N-S trendingdiscontinuities, there are minor N-S trending discontinuitiesseen in the area, whose association with the ultramaficrocks hold importance, if these are correlated with the N-Strending lineaments and faults formed as a result ofuparching of the plateau during Jurassic-Cretaceous timeshaving concurrent relationship with the E-W trendingDauki fault system (Gupta and Sen, 1988). These featuresare associated with the neotectonic seismic activitycaused by the northward movement of the Shillong plateauwith its lower edge colliding against the Himalayas (Guptaand Sen, 1988). Therefore the E-W and N-S trendingmagnetic discontinuities as described above become veryimportant from the point of seismotectonic activity in thearea.

There is a major Magnetic discontinuity F1-F1 trendingin NE-SW direction from Pambriew in the south passingthrough Barapani to Bargang in the north with breaks inbetween. This discontinuity forming the tectonic contact is



characterized by shear and igneous activity in the form ofgranite intrusions. Many minor NE-SW and ENE-WSWtrending magnetic discontinuities are observed towards northand on either side of F1-F1 magnetic discontinuity. High levelof seismicity and high seismic ‘b’ values were recorded overthis region and further towards NE of this region over Mikirhills, which is not covered by this survey. This activity isattributed to the tectonic adjustment between the Shillongplateau and Mikir hills separated by the NW-SE trendingKopili Lineament (Khan and Chakraborty, 2007). Theseismic activity in the tectonic unit along with the Shillongplateau is mainly due to the northeasterly trending lineamentslike Brahmaputra lineament and Jorhat lineament, whichtraverse across the Assam-Brahmaputra foredeep(Mukhopadhyay, 1984). In this context it is pertinent tomention that the NE-SW trending magnetic discontinuitiestraced up to the available magnetic data, if extended furthertowards NE across the Kopili lineament into the Assam-Brahmaputra foredeep, become important from the point oftheir association with the seismic activity.

The magnetic discontinuities F2-F2 and F2'-F2' trendingin WNW-ESE direction between Tura and Rengdimrepresent the Dapsi Thrust Zone (GSI, 2000) between thecontacts of Gneissic complex and tertiary sediments. Thiszone shows moderate seismicity with high seismic ‘b’ values(Khan and Chakraborty, 2007). The low velocity zone (Kayaland Zhao, 1998) is identified along NW-SE trending zoneindicating the weaker crust characterized by the presenceof thrust between the Archaean and Tertiary sediments. Thestress accumulation in this case is attributed to Himalayancollision (Kayal, 1987).

CONCLUSIONS

The study of aeromagnetic data has delineated regional/structural features e.g. faults/fractures/shears/contacts etc.These include the known NW-SE trending Dapsi Thrust inthe southwestern part and NE-SW trending Barapani Thrustin the central part. The study revealed that Barapani Thrustis deeper in the northern side compared to its southern part.Three strong magnetic anomalies near Mawryngkneng,Jowai and Lengri have been identified with reversemagnetization. Most of the magnetic features are found tobe emplaced in top two layers e.g. up to the maximum extent

of 3.72 Km below observation plane (7000 ft above msl),which is corroborated by the results of the groundgeophysical surveys carried out in the area earlier.

An indication of upliftment in the SSW part of the areais noticed, which would have been corroborating well withthe high Bouguer gravity, if that part had been covered byaeromagnetic surveys. The study also shows that magneticbasement of the Shillong Plateau is full of undulations withvariable depths of uplifts/ downthrows indicating itsheterogeneous nature.

The analysis and interpretation of aeromagnetic data hashelped in understanding the regional setup in the area. TheE-W, N-S trending magnetic discontinuities along with majorNE-SW trending discontinuity F1-F1 and WNW-ESEtrending F2-F2 and F2'F2' magnetic discontinuities havehelped in understanding the structural setup of the area, asthese corroborate with the known lineaments, faults andthrusts.

The study has brought out five potential Blocks B-I -to-B-V marked tentatively around junction of features, onetowards north of Darugiri, others around Hamka, Umpyrtha,Nongkhlaw and Mojam-Nongirong respectively for mineralprospecting. The strong anomalies with reversemagnetization near Mawryngkneng, Jowai and Lengri andthe fault/fracture/contact/shear system delineated in the areaincluding known thrusts e.g. Dapsi and Barapani areimportant for mineral prospecting. The prominent E-Wtrending features passing through Mojam also appearimportant for mineral prospecting, as these indicate weakzones.

Acknowledgements: The authors are thankful toDr.A.K.Sinha, Dy.Director General & HOD, RSAS (AMSEWing) and Sri S.G. Gaonkar, Dy.Director General (Geo-physics), Mission 1B for encouragement and support.Authors thank N.K. Dutta, V.P. Mishra, former Dy. DirectorGeneral & HODs of AMSE Wing and Shyamal De, formerDy. Director General (Geophysics) for guiding and providingnecessary facilities in carrying out this project. The authorsexpress their gratitude to M.P.Mathew, Rtd. Director-SG(Geophysics) for guiding and facilitating the use of thesoftware package developed by him for interpreting the data. The authors are thankful to Murthy, draughtsman, forassisting in finalization of maps and plates.



References

KRISHNAN, M.S. (1960) Geology of India and Burma. HigginBothams, Madras, 553p.

KUMAR, S., DHANARAJU, R., VARMA, H.M. and DOUGALL, N.K.(1984) Cancrinite-Tinguite and K-rich trachyte in theNongcharam-Darugiri area of East Garo Hills District,Meghalaya: A preliminary study. Jour. Geol. Soc. India, v.25pp.528-532.

MATHEW, M.P. (1988) Computer programs developed at AMSEWing, GSI, Bangalore. Geol. Surv. India, Unpubld. Report

MAZUMDER, S.K. (1976) A summary of the Precambrian geologyof the Khasi hills, Meghalaya. Geol.Surv. India, Misc. Publ.,No.23(Part II), pp.311-334.

MAZUMDER, S.K. (1986) The Precambrian framework of part ofthe Khasi Hills, Meghalaya. Rec. Geol.Surv.India, v.117(2),pp.1-59.

MITRA, S.K. (1998) Structural history of the rocks of the Shillonggroup around Sohiong, east Khasi hills,Meghalaya. IndianJour. Geol., v.70, pp.123-131.

MUKHOPADHYAY, M. (1984) Seismotectonics of transverselineaments in the eastern Himalaya and its foredeep.Tectonophysics, v.109, pp.227-240.

MURTHY, CH.V.V.S., GOUDA, H.C.,SINGH, R.K. and NAGARAJU, B.V.(2007) Report on Interpretation of Aeromagnetic Data in partsof West Khasi Hills and Kamrup Districts of Meghalaya andAssam, F.S. 2006-2007.

MURTHY, M.V.N., MAZUMDER, S.K. and BHAUMIK, N. (1976)Significance of tectonic trends In the geological evolution ofthe Meghalaya uplands since Precambrian. Geol. Surv. India,Misc. Publ., No.23, pp.471-484.

NAIDU, P.S. and MATHEW, M.P. (1988a) Computer program forradial and angular spectrum estimation. Geol. Surv. India,Unpubld. Report.

NAIDU, P.S. and MATHEW, M.P. (1998) Analysis of GeophysicalPotential Fields: A Digital Signal Processing Approach.Advances in Exploration Geophysics, v.5, Elsevier,Netherlands.

NAYAK, P.N., ROY, J.C. and PRAKASH, D. (F.S. 1975-76) Geophysicalinvestigations for detecting zones of sulphide mineralisationin Barapani-Mawiong area, District Khasi hills, Meghalaya,F.S. 1975-76. Geol. Surv. India, Unpubld. Report.

NGRI Map (1977) Total Intensity aeromagnetic map of Assam-Meghalaya, generated and interpreted after aerial surveyduring Nov-Dec1977 by NGRI.

PARTHASARATHI, E.V.R. (1978) Lineament Map of N.E. India, GSIMap.

PATHAK, B., SYIEN, S.M. and ZAHIDI, A. (2006) Report on gravity–magnetic survey along Guwahati-Shillong Road, in parts ofAssam and Meghalaya, outside F.S.P. 2003-04. Geol. Surv.India, Unpubld. Report.

PATHAK, B. and SYIEM, S.M. (2006a) Report on gravity-magneticmapping in parts of Ribhoi, East and West Khasi Hills District,Meghalaya, F.S. 2003-2004. Geol. Surv. India, Unpubld.Report.

PRASAD, R.K. (1983) Geological Report on Ground Evaluation of

BAROOAH, B.C. (1976) Tectonic pattern of the Precambrian rocksaround Tyrssad, Meghalaya. Geol. Surv. India Misc. Publ.,No.23 (Part II), pp.485-496.

BELOUASSOV, V.V. (1962) Basic problems of Geotectonics.McGraw-Hill.

BHATTACHARYA, R.S., KAR, N. and GHOSH, D. (F.S. 1977-78) AShort note on test magnetic and gravity traverse for detectionof the ultrabasic body in Sung valley, Meghalaya. F.S. 1977-78. Geol. Surv. India, Unpubld. Report.

CHATTOPADHYAY, N. and HASHIMI, S. (1984) The Sung Valley alkalineultrmafic carbonatite complex, East Khasi and Jaintia Hillsdistricts, Meghalaya. Rec. Geol. Surv. India, v.113(IV), pp.24-33.

DAS, A.P., PATHAK, B. and BAISHYA, R.C. (F.S. 1989-91) Report onregional gravity and magnetic surveys for delineation ofultramafic/ ultra-basic bodies with special emphasis for thesearch of platinoids in Sung Valley and adjoining areas, EastKhasi and Jaintia Hills Districts, Meghalaya. F.S. 1989-91.Geol. Surv. India, Unpubld. Report.

EVANS, P. (1964) The tectonic frame work of Assam. Jour. Geol.Soc. India, v.5, pp.80-96.

GHOSH, S, CHAKRABORTY, S., BHALLA, J.K., PAUL, D.K., SARKAR

AMITABHA, BISHUI, P.K. and GUPTA, S.N. (1991) Geochronologyand Geochemistry of Granite Plutons from East Khasi Hills,Meghalaya. Jour. Geol. Soc. India, v.37, pp.331-342.

GSI (2000) Seismotectonic Atlas of India and its Environs, Seisat-14.

GSI, NGRI, ONGC, O.I.L. (2006) Bouguer Gravity Anomaly(Terrain Corrected) Map of India with Station Distribution.

GUPTA, R.P. and SEN, A.K. (1988) Imprints of the Ninety-East Ridgein the Shillong Plateau, Indian Shield. Tectonophysics, v.154,pp.335-341.

HASAN, K. (1985) Report on the geophysical surveys for Magnetite,Apatite occurrences in the Sung Valley Complex, Jaintia HillsDistrict, Meghalaya, F.S.1984-85. Geol. Surv. India, Unpubld.Report.

HASAN, K. and THAKURIA, N.C. (1993) Report on magnetic (VF)surveys as ground follow-up work of aeromagnetic anomalousband E-2 and other magnetic features, south east of Tura, Eastand West Garo Hills District, Meghalaya, F.S.1988-89. Geol.Surv. India, Unpubld. Report.

JAWAHAR, G. and RAGHURAMAIAH, K. (1993) Report on the regionalgravity and magnetic surveys along Guwahati-Shillong Road,F.S. 1989-90. Geol. Surv. India, Unpubld. Report.

KAYAL, J.R. (1987) Microseismicity and source mechanism study:Shillong Plateau, northeast India. Seismological Soc.Amer.Bull., v.77, pp.184-191.

KAYAL, J.R. and ZHAO, DAPENG (1998) Three-Dimensional seismicstructure beneath Shillong Plateau and Assam Valley, NortheastIndia. Seismological Soc. Amer. Bull., v.88, pp.667-676.

KHAN, P.K. and CHAKRABORTY, P.P. (2007) The seismic b-valueand its correlation with Bouguer gravity anomaly over theShillong Plateau area: Tectonic implications. Science Direct.Jour. Asian Earth Sci., v.29, pp.136-147.



Aeromagnetic Features in toposheet Nos. 83C/3,7 and 8 JaintiaHills District, Meghglaya. Geol. Surv. India, Unpubld. Reportfor F.S.1982-84.

RAMA RAO, M.S.V., BHATTACHARYA, R.S., SAHA, K. and RAO, C.M.(F.S. 1981-82) Report on geophysical investigation as groundfollow up of the aeromagnetic anomaly (HA3) inPynthorlangteine area, Jaintia Hills District, Meghalaya, State,F.S. 1981-82. Geol. Surv. India, Unpubld. Report

RAVINDRA, R., PRASAD, R.K., DUARA, B.K. and MISHRA, S.N. (1994)On the Efficacy of Integrated Study of Aeromagnetic data : ACase History from Meghalaya Plateau. Geol. Surv. India Spec.Publ. No.35, pp. 147-160.

REDDI, A.G.B., MURTHY, B.S.R. and KESAVAMANI, M. (1995) Acompendium of four decades of geophysical activity inGeological Survey of India, pp.91-94, Plates: NE-1 and NE-2.

REEVES, C.V. (1985) Airborne Geophysics for Geological mapping

and regional exploration. ITC Jour, v.3, pp.147-161.STANELY JOHN, M. (1977) Simplified magnetic interpretation of

the geologic contact and thin dyke. Geophysics, v.42(6),pp.236-1240.

VALDIYA, K.S. (1976) Himalayan transverse faults and folds andtheir parallelism with subsurface structures of North Indianplains. Tectonophysics, v.32, pp.353-386.

VERMA, R.K. and MUKHOPADHAYAY, M. (1977) An analysis of thegravity field in northeastern India. Tectonophysics, v.142,pp.283-317.

VENKATA RAMANA, M. and SAWAIYAN, S.C. (1993) Report ongeophysical surveys for ground evaluation of aeromagneticanomalous band E-1 and other magnetic features, Northwestand Northeast of Tura in the total intensity aeromagnetic(NGRI) map of Assam-Meghalaya West and East Garo hillsDistricts, Meghalaya, F.S.1989-90. Geol. Surv. India, Unpubld.Report.

(Received: 3 February 2011; Revised form accepted: 30 August 2011)

structural study of meghalaya plateau through aeromagnetic data

Documents