Tectonic and crustal understanding of the Lesser Himalayas ... · Stanvac Petroleum Project, and the Oil and Natural Gas Commission (ONGC). The borehole stratigraphic studies (DST

Bollettino di Geofisica Teorica ed Applicata Vol. 57, n. 4, pp. 301-312; December 2016

DOI 10.4430/bgta0180

301

Tectonic and crustal understanding of the Lesser Himalayas along the Purnea-Sevoke transect through integrated geophysical studies

D.C. Naskar

Geological Survey of India, Salt Lake City, Kolkata, India

(Received: July 31, 2015; accepted: July 1, 2016)

ABSTRACT Geophysicalinvestigationemployingdeepelectricalresistivity,gravity,andmagnetictechniques was carried out along Purnea-Sevoke (NH-31) transect. The basementdepthvariesfrom2917to4450m,indicatinghugerelief.Threebasementfaultshavebeen mapped over the transect. The Siwalik floor is quite undulatory in this part of the frontal foredeep region of the Himalayas. The gravity profile along NH-31 from Purnea to Sevoke brought out two basinal structures. Spectral analysis of the saidgravity profile brought out the Conrad discontinuity and the basement depth at 18.4 and 3.2,kmrespectively.2DgravitymodellingalongtheabovetransectindicatesgradualdeepeningofthebasementtowardstheNE.Thisisduetosaggingofthecrustalblockin the foothills of the Himalayas. The magnetic anomaly is of fairly high order (800-1600nT)inthearea,whichmaybeduetothepresenceoftrapsbelowthesedimentcover.

Key words: LesserHimalaya,Bougueranomaly,2Dmodelling.

© 2016 – OGS

1. Introduction

ThestudyofcrustalstructureintheSub-HimalayaandLesserHimalayaregionshasalwaysattracted theattentionofgeoscientistsbecauseof itsacademicandeconomic importance.Thearea was surveyed using different geophysical methods by Standard Vacuum Oil Company, Indo Stanvac Petroleum Project, and the Oil and Natural Gas Commission (ONGC). The borehole stratigraphic studies (DSTExpertGroup,1995)conducted foroil andnaturalgasprospectingshowabout2500-3500m thickSiwalik sediments and a total sedimentary cover thicknessofmorethan6000mincertainpartsoftheSiwalikregion.ReddyandArora(1993)havereportedahighconductivelayerinthecrustatdepthsvaryingfrom10to15kmbelowtheHimalayancollision region. Intensive geophysical surveys and deep drilling in the alluvial plains of the west Bengal (Sengupta, 1966) revealed a thick section of Cretaceous and Tertiary sediments lyingon a basement of basalt lava flows.Todelineate the thicknessesof the lowerSiwaliks/Gondwana formations above the basement and to prepare a meaningful crustal model, thearea bounded by latitudes 25°52’ to 27° 00’ N and longitudes 87°28’ to 88°28’ E (Fig. 1) wassurveyed using gravity, magnetic, and deep electrical resistivity techniques. Deep electricalsurveyswere undertaken in the area at 10-15 km intervals along the Purnea-Sevoke transect,alongwithgravityandmagneticobservationateverykilometrefromPurneatoSevoke.

302

Boll. Geof. Teor. Appl., 57, 301-312 Naskar

2. Geology and tectonics of the study area

The survey area (Fig. 1) is totally covered by recent alluvium.Tectonically, the area isassociatedwith theExtraPeninsularmountainous terrainandpiedmontplainofnorthBengal,which is also covered by alluvium.The Quaternary deposits of the Extra Peninsular regionoccurringjustsouthoftheSiwalikGroupconsistofboulders,gravels,pebbles,sands,andsiltsinthehigherreaches,formingalluvialfansandfluvialdepositionalterraces,whilesand,silt,andclaysinthelowerreachesformfluvialterracesoffloodplainfacies.

The Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT) separate SiwalikGroup of the Sub-Himalayas in the foreland from the Lesser Himalaya (Fig. 2). Gondwana Supergroup, comprising pebble/boulder beds, quartzites, sandstone, and slates with anthracitecoal seams, is found near Purnea in a borehole. Normally, lower Siwaliks (Tertiary) areunderlain by Gondwanas and are overlain by pre-Quaternary and Quaternary alluvium (Yin,2006; Bhattacharya, 2008).

3. Geophysical survey

In this region, geophysical surveys involving gravity, magnetic, and deep resistivity methods were initiated by the Geological Survey of India from 1993 to 2009 (Fig. 1). It was expected that sedimentssettled in thebasementdepressionswouldbe indicatedbybroadgravity“lows”.Themagneticmethodsupplementedthegravitymethodinbringingoutpossiblebasementstructures.Thedeepresistivitysoundingsweretakenatsuitablelocationstodeterminedepthstothebasementrockandtoobtaincharacterandthicknessesofthedifferentlayersoverlyingthebasement.

The gravity survey was conducted employing Sodin (Canada) brand gravimeters with a reading accuracy of 0.01 mGal, and the magnetic survey was conducted employing Scintrex(Canada) brand digital-grade vertical force magnetometers with a reading accuracy of 1 to 10 nT. A Scintrex 10 KVA time domain resistivity unit was used for field surveys. An RDC-10 receivermadebyScintrexwasusedformeasuringthepotentialdifferencebetweenthetwonon-polarisableelectrodes(copperdippedinsaturatedcoppersulphatesolution).

Fig. 1 - Location map of study area showing the locations of electrical sounding.

Tectonic and crustal understanding of the Lesser Himalayas Boll. Geof. Teor. Appl., 57, 301-312

303

Fig. 2 - Tectonic map of the study area.

304

Boll. Geof. Teor. Appl., 57, 301-312 Naskar

4. Deep resistivity sounding

Resistivity of rocks is an important parameter for mapping crustal structures, which mayprovide a clue on the processes of crustal evolution (Das et al., 1993). Subsurface resistivitydistribution is directly related to the physical character of the lithological units.Variation inresistivityof therocks is themainsourceofelectricalanomaly.Alpinet al. (1966)haveusedthis technique extensively for crustal mapping in Europe. Van Ziji (1977, 1978) effectively used the technique in South Africa. Webster (1997) successfully utilized it in Canada. In the presentareaofinvestigation,alongthePurnea-Sevoketransect,14geoelectricsoundingshavebeen carried out with Schlumberger configuration in which maximumAB/2=3 km (half thecurrent electrode spread) and bilateral/unilateral azimuthal dipole sounding with R=10 km(maximum separation of transmitting and receiving dipoles).The length of the current andpotentialdipolesareincreased,keepingAB<1/3RandMN(potentialdipolelength)<1/5AB,inordertogivearesistivityvalueaccuratetowithinacoupleofpercentagepoints.AtanyAB/2,readings should be made at several MN/2 values in order to indicate the presence of lateralinhomogeneity.TheshallowerpartuptoAB/2=3kmisusedinSchlumbergerelectrodearraytoavoidlateralinhomogeneity.Azimuthaldipolesoundingcanbeinterpreteddirectlyintermsoflayerparametersbyusing two/multilayermastercurveswhentheazimuthangle issmall (lessthan 10-15°). Inter-convertibility of dipole-apparent resistivities is discussed in Bhattacharya (1968) and Koefoed (1979). The resistivity sounding curves were initially interpreted using Orellana and Mooney (1966) master curves. A standard inverse modelling program ‘RESIST’ (Vander Velpen, 1988) has been used to get the layer parameters, i.e., resistivity and thickness of different subsurface layers having distinct resistivity values.A four-layer subsurface set-up, i.e., theArchaean overlain by the Gondwana, the Siwalik, and the Quaternary alluvium,wasnormallyexpectedinthisarea.Duetohighconductivityof theQuaternaryaswellas theTertiaryformations,thepenetrationhasbeenpoorinmostofthecases.

5. Resistivity interpretation along the transect Purnea-Sevoke

The interpreted geological subsurface layers are characterized by a distinct range inresistivity values (Figs. 3 and 4). Thus, the very high order of resistivity, varying 1100-2400 Ω·m, is interpreted as a boulder bed at the surface (ES-10, ES-11, ES-12, where AB/2=3000 m is taken for Schlumberger sounding and ES-14) around Sonapur, Chopra, Siliguri, and Sevoke. The lowest resistivity order is ranging 10-39 Ω·m around the Dalkhola, Purnea, Chopra, Sonapur areas, which vary in thickness from 2495 to 3150 m, and appears at depths rangingfrom922to2495m,withaminimumnearPurneaandmaximumnearDalkhola.ThisisinterpretedasthesignatureofSiwaliksinthestudyarea(ES-1,ES-2,ES-3,ES-4,ES-5,ES-10,andES-11).SoundingnumberES-1hasbeenconductedwithSchlumbergeranddipolemethodsjointly,withanoverlapof200mtoavoidlateralinhomogeneity,becausethedipolesoundingismoresensitivetolateralvariationandtheinhomogeneitiesofthesurfacelayerscanconsiderablydistort the resistivity values for the deeper layers. In ES-2, bi-directional azimuthal dipole-dipole sounding is observed up to 3 km towards Dalkhola and 6 km towards Purnea.A continuousdecreaseinresistivityisobservedinbothdirections.Theresistivityvaluethenincreasesinboth

Tectonic and crustal understanding of the Lesser Himalayas Boll. Geof. Teor. Appl., 57, 301-312

305

Fig. 3 - Dipole-dipole resistivity sounding ES-1, centred at km stone 435 along NH-34, and ES-2, ES-3, ES-4, ES-5, ES-6, ES-7, and ES-8, centred at km stone 423, 433, 452, 462, 476, 488, and 498 along NH-31, respectively.

306

Boll. Geof. Teor. Appl., 57, 301-312 Naskar

directions.A notable lowering is observed towards Purnea after attaining a depth of 922 m.Presence of lateral inhomogeneities are observed around Kishanganj and Islampur areas, which maybeduetothepresenceoffaultedstructure(ES-6whereR=10kmisused,ES-7whereR=10km is used, and ES-8). Some spike-like features observed in some of the soundings may be due topoorsignalornoise.

AnotablefeatureisobservedbetweenSiliguriandSevoke(ES-14),wherearesistivityvalueof 1100 Ω·m near the surface persists with depth. However, after 40 m of depth, this resistivity

Fig. 4 - Schlumberger resistivity sounding ES-9, ES-10, ES-11, ES-12, ES-13, and ES-14, centred at km stone 508, 513, 524, 529, 538, and 588 along NH-31.

Tectonic and crustal understanding of the Lesser Himalayas Boll. Geof. Teor. Appl., 57, 301-312

307

valuesuddenlyfalls.ThisisattributedtothepresenceofSiwaliksatdepth.Thehigh-resistivitybasement is encountered at a depth varying from 2917 to 4450 m around the Dalkhola andKishanganj areas (ES-1 and ES-5). An ONGC borehole (500 m from ES-2) towards Purnea, encounteredSiwaliks(around2km)followedbyupper(369m)andlower(1253m)GondwanasovertheArchaeanbasement.

6. Apparent resistivity pseudo-depth section along Purnea-Siliguri

Anapparentresistivitypseudo-sectionplotting(withAB/2asordinateandresistivityvaluesof each sounding location as abscissa) is shown in Fig. 5. The section shows four distinct breaks (faults).These faults are located near km stone 452, 493, and north of km stone 524.Therelativedisplacementsofthebasementareshownbyarrowsdepictedonthefigurearoundtheinterpretedfaults.Ofparticularinterestisthezonebetweenkmstones452and493,whereresistivities to the tune of 1000-2000 Ω·m are mapped at a comparatively shallower level. This isinterpretedasthebasementresistivityandtheup-archedzone.Thiszonemaybeofinterestfor oil and gas. In the rest of the area north of km stone 493, the basement deepens further, with anotherfaultatkmstone524.

Fig. 5 - Apparent resistivity pseudo-depth section along NH-31.

308

Boll. Geof. Teor. Appl., 57, 301-312 Naskar

7. Gravity survey

As the overlying lithic fill (alluvium, the Quaternary sediment, the Siwaliks, and theGondwanatogether)hasaloweraveragedensityvalue(2.17g/cm3)thantheArchaeanbasement(averagedensity2.67g/cm3)anditscompositethicknessiswellover2km,substantialgravityanomaly was expected. With this idea in mind, gravity observations were taken at everykilometre stone fromPurnea toSevoke (NH-31).BetweenPanjiparaandSonapur (NH-31), asystematic gravity survey was also done, covering 5-8 km on either side of the highway, just to havealookattheregionalgradientofthepotentialfields.

The gravity values were correlated with a Survey Of India (SOI) base (Gulatee, 1956) in this area. Station elevations were obtained from close-loop levelling, correlated with an SOI benchmark at Islampur. Position control for the stations was guided by SOI topographic sheets (1:50,000scale).TheBougueranomaliespresentedherewerecomputedaccordingtothe1967International Gravity Formula. A density factor of 2.67 gm/cm3 for the slab above mean sealevelwasassumed for combinedelevationcorrection.Terraincorrectionswerenot applied totheanomalies.TheoverallaccuracyoftheBougueranomaliesinthepresentsurveyisestimatedtobe±0.3mGalorbetter.

8. Frequency analysis

In the absence of any other collateral subsurface data which may help in the interpretation ofBouguergravityanomaly,frequencyanalysisofthegravitydatawasdonealongtheprofile

Fig. 6 - Spectral analysis of the Bouguer anomaly profile from Purnea to Sevoke along NH-31.

Tectonic and crustal understanding of the Lesser Himalayas Boll. Geof. Teor. Appl., 57, 301-312

309

of the Purnea-Sevoke Road (NH-31) in order to map the different crustal inhomogeneities(Fig. 6). For this purpose, the gravity value at every 2.5 km interval along the above profile has beenrecorded.ThemeanofthelogarithmofenergyassociatedwithparticularfrequencieshasbeencomputedfollowingSpectorandGrant(1970),toobtainanaverageradialspectrum.Theaverage logarithmic energy has been plotted against the wave number. The plot (Fig. 6) reveals

Fig. 7 - Bouguer anomaly profile and 2D gravity model from Purnea to Sevoke along NH-31.

Fig. 8 - Bouguer anomaly map from Panjipara to SonapuralongNH-31.

310

Boll. Geof. Teor. Appl., 57, 301-312 Naskar

the presence of two major interfaces at average ensemble depths of 18.4 and 3.2 km. These average depths are interpreted as the depths to the Conrad interface and basement (granite).

9. 2D gravity model

In order to understand the variations of Bouguer anomaly in terms of crustal structure, i.e.,massdistribution,2Dgravitymodellingwasdoneover thePurnea-Sevokegravityprofile(Fig. 7). A four-layer crustal model was assumed, taking into account the local geology and theinterceptsfromthefrequencyanalysisdata.ThegravityvaluewascalculatedaccordingtoTalwaniet al. (1959). Values of individual crustal layers are shown in Fig. 7. The thickness of the composite sedimentary column has been varied from 0.5 to 5.5 km from SW to NE (Fig. 7). Two basinal features associated with equal numbers of basement rise are interpreted. Severalfaults at km stones 425, 440, 455, 480, and 520 have also been interpreted from the 2D gravity modelling.

ApartofthegravitytransectfromPanjiparatoSonapuratNH-31ispresentedasacontourmap with intervals of – 2 mGal (Fig. 8). The gravity values vary from – 69.6 to – 130 mGal fromSWtoNEofthearea.ThegravitymaprevealsthatcrustalthicknessincreasesfromSWto NE, with a break north of Chopra and east of Sonapur. The rate of descent decreases here, suggestingasaddle-likehumpstructureinthebasement.

10. Magnetic survey

Sincethemagneticsusceptibilityofthecompositelithicfill(alluvium,Quaternary,Siwalik,and the Gondwana) in the study area is much less than theArchaean basement, substantiallow magnetic relief was expected over this transect, except, however, where some trap rocks(equivalent to theRajmahals)havinghighermagneticsusceptibilityoccurover theGondwanasediment.Proceedingwith thispremise,magneticobservationswere takenat everykilometrestone from Purnea to Sevoke Road (NH-31) (Fig. 9). The magnetic vertical force anomaly profilebetweenPurnea andSevoke shows twobasinal structures, onebetween station457 to470 km and the other SW of it (Fig. 9). This finding is supported by gravity survey. After km stone470,magneticanomalyshowshighplateau,suggestingthepossibilityoftrapunderneath.

Fig. 9 - Magnetic profile from Purnea to Sevoke along NH-31.

Tectonic and crustal understanding of the Lesser Himalayas Boll. Geof. Teor. Appl., 57, 301-312

311

Fluctuations in magnetic anomaly are seen beyond station 550 km, possibly due to the presence ofburiedintrusives.

11. Concluding remarks

Duetothehighconductivityoftheoverlyingsediments,basementdepthcouldbeascertainedonly at a few resistivity-sounding locations.The pseudo-depth section of resistivity indicatedfourfaultsnorthofkmstones452,493,510,and526.Theinterveningzonebetweenkmstones452and493showsahighlyresistivebedlyingunconformablyoveraconductivehorizon.Thismay be a basement slice thrusted over the Siwaliks. The FFT of gravity anomaly revealed two interfaces at ensemble depths of 18.4 and 3.2 km, which are interpreted as Conrad discontinuity andthebasementdepth.Thethicknessofthecompositelithicfillisinterpretedfrom2Dgravityanomalytobe0.5to5.5kmfromthesouth-westernpartof theareato thenorth-easternpart.The crust becomes progressively thicker towards the NE, following isostatic consideration.Themagneticanomalyprofile shows twobasinal structures,onebetweenstation457kmandtheotherSWofitalongthePurnea-Sevoketransect.Beyondstation470kmstone,amagneticanomaly shows a high plateau, suggesting the possibility of trap underneath. Fluctuations in magneticanomalyareseenbeyondstation550km,possiblyduetothepresenceofintrusions.At least three prominent deep-seated faults and a gravity saddle between Chopra and Sonapur havebeeninterpretedfromthepresentstudy.Thesestructuresmayprovetobeofsignificancewithregardtooilexplorationinthisarea.

Acknowledgements. The Director General of the Geological Survey of India is gratefully acknowledged for hiskindpermissiontopublishthispaper..

REFERENCES

AlpinL.M.,BerdichevskiiM.N.,VendrintsevG.A.andZagarmisterA.M.;1966:Dipole methods for measuring earth conductivity. Translated by Keller G.V., A special research report, Consultants Bureau, New York, NY, USA.

Bhattacharya P.; 1968: Direct current geoelectric sounding, 1st ed.: principles and interpretation.ElsevierPublishingCompany, Amsterdam, The Netherlands, 144 pp.

Bhattacharya A.R.; 2008: Basement rocks of the Kumaun-Garhwal Himalaya: some implications for the Himalayan tectonics. Earth Sci. India, 1,1-11.

Das L.K., Saha D.K., Rai M.K., Naskar D.C., Ghosh B. and Chakravorty K.; 1993: A report on the integrated analysis of deep resistivity, gravity and magnetic data over the Singhbhum-Orissa-iron ore craton, India. GSI, Unpublished Report.

DSTExpertGroup;1995:Report of the expert group on the NW Himalayan geotransect programme (NWHGP).Earthsystem science division, Dept. Sci. Tech., Govt. India, (unpublished).

GulateeB.L.;1956:Gravity data in India. Surv. India., Tech. Paper, 10,1-95.Koefoed O.; 1979: Geosounding principles, 1: resistivity sounding measurements (Methods in geochemistry and

geophysics).ElsevierScientificLtd,Amsterdam,TheNetherlands,290pp.Orellana E. and Mooney H.M.; 1966: Master tables and curves for vertical electrical sounding over layered

structures. Interetencia, Madrid, Spain.Reddy C.D. and Arora B.R.; 1993: Quantitative interpretation of geomagnetic induction response across the thrust

zones of Himalaya along the Ganga Yamuna valley.J.Geomagn.Geoelec.,45, 775-785.SenguptaS.;1966:Geological and geophysical studies in western part of Bengal basin, India.Am.Assoc.Pet.Geol.

Bull.,50,1001-1017.Spector A. and Grant F.S.; 1970: Statistical method for interpreting aeromagnetic data.Geophys.,35,293-302.

312

Boll. Geof. Teor. Appl., 57, 301-312 Naskar

TalwaniM.,WorzelJ.L.andLandismanM.;1959:Rapid gravity computations for two dimensional bodies, the mid-oceanic submarine fracture zone.J.Geophys.Res.,64,49-59.

Vander Velpen B.P.A.; 1988: A computer processing package for DC resistivity interpretation for an IBM compatibles.ITC J., 4,TheNetherlands.

Van Ziji J.S.V.; 1977: Electrical studies of the deep crust in various tectonic provinces of southern Africa. In: Headcock J.G. (ed), The Earth’s crust, Am. Geophys. Unions, Washington, DC, USA, Geophys. Monogr. Ser., 20,470-500.

Van Ziji J.S.V.; 1978: The relationship between deep electrical resistivity structure and tectonic provinces in southern Africa, Part 1.Trans.Geol.Surv.SouthAfr.,81,129-142.

WebsterB.;1997:Logistical and interpretive report on spectral IP/resistivity survey conducted on Harker/Holloway townships, northeastern Ontario for Franco-Nevada Mining Corporation. JVX Ltd., GeoscienceAssessmentOffice, JVX Ref: 9762, Canada.

YinA.; 2006: Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation.EarthSci.Rev.,76,1-131.

Corresponding author: Dulal C. Naskar Eastern Region, Geophysics Division, Geological Survey of India Bhu-Bijnan Bhavan, DK-6, Sector-II, Salt Lake City, Kolkata-700091, India Phone: +91 xxx xxx; fax: +91 xxx xxx; e-mail: [email protected]