Magnetic anomalies over the Andaman Islandsand their geological significance

P B V Subba Rao1, M Radhakrishna

2,∗, K Haripriya2,

B Someswara Rao2 and D Chandrasekharam

2

1Indian Institute of Geomagnetism, Kalamboli Highway, Panvel, Navi Mumbai 410 218, India.2Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.

∗Corresponding author. e-mail: mradhakrishna@iitb.ac.in

The Andaman Islands form part of the outer-arc accretionary sedimentary complex belonging to theAndaman–Sumatra active subduction zone. The islands are characterized by thick cover of Neogenesediments along with exposed ophiolite rocks at few places. A regional magnetic survey was carriedout for the first time over the Andaman Islands with a view to understand the correlation of anomalysignatures with surface geology of the islands. The residual total field magnetic anomaly maps haverevealed distinct magnetic anomalies having intermediate to high amplitude magnetic signatures andcorrelate with the areas over/close to the exposed ophiolite rocks along the east coast of north, middleand the south Andaman Islands. The 2D modelling of magnetic anomalies along selected E–W profilesacross the islands indicate that the ophiolite bodies extend to a depth of about 5–8 km and spatiallycorrelate with the mapped fault/thrust zones.

1. Introduction

The major morpho-tectonic elements (Weeks et al.1967) running N–S along the Andaman arc arethe Andaman trench, Andaman–Nicobar ridge(Andaman–Nicobar Islands), Nicobar deep, WestAndaman fault (WAF), Andaman Spreading Ridgeand the Mergui terrace (figure 1). The Andaman–Nicobar Islands form a major part of the outer-arc sedimentary complex that stretches nearly 1100km along the Andaman trench-arc system (Curray2005), and separate the Andaman back-arc basinfrom the Bay of Bengal. Along the western mar-gin of the islands, the sediments of the Bengal Fanhave been deformed below the Andaman trench(Curray et al. 1982). The Andaman arc is char-acterized by the presence of east dipping Benioffzone down to about 200 km depth (Mukhopadhyay

1984). Several earlier investigators have studiedthe structure and deformation along this activeplate boundary zone (e.g., Gahalaut et al. 2008;Radhakrishna et al. 2008). According to Hamilton(1979), the Andaman arc region forms an impor-tant transitional tectonic link between the Burma–Himalayan arc/collision system in the north andthe Indonesian arc system in the south. Themain rock types occurring over the islands areeither of sedimentary or magmatic. The mag-matic rocks constitute the ultramafic plutonitesand basalt, together with minor radiolarian chert,and this forms the Andaman ophiolite suite (Palet al. 2003a). Owing to strong magnetizationcontrast between the ophiolites and sedimentaryrocks, magnetic method would be an effective toolfor investigating the subsurface structure of theophiolite rocks. However, till date the gravity and

Keywords. Magnetic survey; ophiolites; magnetic anomalies; Andaman Islands; Andaman–Sumatra Arc; northeast Indian

Ocean.

J. Earth Syst. Sci. 125, No. 2, March 2016, pp. 359–368c© Indian Academy of Sciences 359

360 P B V Subba Rao et al.

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Figure 1. Tectonic map of the Andaman arc and the Andaman Sea region (modified after Curray 2005). The tectonicsubdivisions of the region (after Roy and Das Sharma 1993; Kamesh Raju et al. 2004) are indicated in 1–5. These are: 1.Outer slope, 2. Inner slope, 3. Trench slope break, 4. Forearc basin, 5. Volcanic arc and the backarc basin, EMF: EasternMargin Fault, DF: Diligent Fault, CF: Cocos Fault, WAF: West Andaman Fault, WSR: West Sewell Rise, ASR: AndamanSpreading Ridge (after Kamesh Raju et al. 2004), OCT: Ocean Continent Transition. Grey shaded area represents theextent of Burma plate (after Curray 2005).

magnetic data was not acquired over the onshorepart of the Andaman Islands. So, for the firsttime, we carried out a detailed magnetic surveyover the main Andaman Islands with an objec-tive to understand the geometry of the ophiolitebodies in the subsurface as well as their associa-tion with the major fault systems over the islands.Salient results of this investigation are presented inthis paper.

2. Geology of the Andaman Islands

The topographic shaded relief map of the AndamanIslands (figure 2a) using the Shuttle Radar Topog-raphy Mission (SRTM) database (Jarvis et al.2008) reveals the sub-areal expression of theislands with many geomorphologic features andlinear ridges (Narayana 2011). Regional geo-logical mapping of the Andaman Islands wasmostly carried out by the GSI and ONGC. TheAndaman–Nicobar Islands have formed during theOligo–Miocene times (Rodolfo 1969). The rock for-mations can be broadly classified under four major

groups in the Andaman Islands (figure 2b), andthey are:

(i) Cretaceous to Paleocene Ophiolite suite,(ii) Lower to Middle Eocene Mithakhari Group,(iii) Oligocene to Late Eocene Andaman turbidities

(Andaman Flysch Group), and(iv) Mio-Pliocene Archipelago Group (Ray 1985;

Ray et al. 1988; Pal et al. 2003a; Bandopadhyay2005).

The rock types mainly constitute serpentinites,ophiolites with radiolarian cherts, cherty pelagiclimestone and the flysch sediments (Chatterjee1967; Eremenko and Sastri 1977; Roy 1983). Twoprominent N–S trending faults/thrusts have beenobserved over the Andaman Islands (figure 2c).Among these two, Jarwa thrust is the most sig-nificant morpho-tectonic feature (figure 2a) trend-ing N–S all along the main islands (Eremenko andSastri 1977), and the second important fault isthe Eastern Margin Fault (EMF) in the immediateAndaman offshore along its east coast (Roy 1983).The EMF acts as a boundary between the outer-arcridge and the fore-arc basin (Roy 1983; Cochran2010).

Magnetic anomalies over the Andaman Islands 361

Figure 2. (a) Shaded relief map of the Andaman Islands from the SRTM data showing the topographic variations andprominent linear ridges, (b) geological map of the Andaman Islands (after Pal et al. 2003a) along with locations of magneticmeasurement points occupied in three different phases of field surveys. The magnetic anomaly maps prepared for the threeareas a–c are indicated on this map, (c) tectonic map of the islands and the adjoining Andaman basin region (modifiedafter Pandey et al. 1992: Curray 2005).

The ophiolite rocks in the Andaman Islandsare interpreted as remnants of oceanic crust andupper mantle that were obducted by complex tec-tonic processes during the Miocene times, andthese are observed along N–S trending linear beltsin structurally weak zones (Roy 1983; Acharyya2007). The ophiolite group of rocks consisting ofmetamorphic, melange and ophiolitic rocks withpelagic sediments occur as dismembered bodies(Pal et al. 2003a). The eastern part of AndamanIslands is characterized by highly deformed ophi-olite rocks from the oceanic floor as well as ultra-basic/volcanic/pelagic sediments along with oldermetamorphics, whereas, in the western part, sedi-ments belong to the accretionary prism consistingof flysch-sandstone/siltstone with conglomeratesand calcareous sediments (Mukhopadhyay et al.2003). Ophiolites are exposed in small isolatedpatches in the west, whereas, in the east, most

ophiolite rocks occur in N–S to NE–SW trendingbodies. These ophiolite rocks have contact withunderlying sediments and denote thrust-controlledemplacement in an accretionary environment (Palet al. 2003a, b). Petrographically, ophiolites con-sist of serpentinite, dunite, harzburgite, pyrox-enite, olivine gabbro, anorthositic gabbro andgabbroic anorthosite (Dhana Raju and Negi 1984;Jafri et al. 2010).

3. Magnetic data acquisition and analysis

3.1 Magnetic surveys

As mentioned earlier, for the first time, groundmagnetic surveys were carried out over the mainAndaman Islands in different field seasons span-ning the period February 2007–December 2009.

362 P B V Subba Rao et al.

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Magnetic anomalies over the Andaman Islands 363

The total magnetic intensity measurements weremade with a portable proton precession magne-tometer of GEM System, Canada with a sensorheight of 1.6 m from the ground. We have placedthe observation points at intervals of 1000 m, andin some places, where we found bare rock out-crops of the ophiolite; the points were picked up at500 m interval. The total field magnetic anomalywas obtained at each observation point by sub-tracting the International Geomagnetic ReferenceField (IGRF) 2005 model (Rukstales and Love2007) from the observed data. Diurnal variation ofthe geomagnetic field was corrected by using totalgeomagnetic intensity data computed using theMagson fluxgate magnetometer that measuresnorth–south (X), east–west (Y) and verticallydownward (Z) components at a sampling intervalof 1 min. The temporary base station was estab-lished at the Department of Science and Technol-ogy (DST), Port Blair, Andaman–Nicobar Divisionduring the survey period. A total of 418 magneticmeasurements were made and the data coverageis shown in figure 2(b). Though the coverage ismainly along the N–S oriented Grand Trunk Roadcovering the entire island, wherever possible wehave collected data along many E–W profiles aswell as other minor roads. We could not collect themagnetic data in certain inaccessible areas suchas the reserved Jarwa forest of the middle-southAndaman Islands.

3.2 Magnetic anomaly maps

The distribution of measured magnetic stationsdescribed above gave rise to three areas ofAndaman Islands (see figure 2b), one in the north(Area A), second in the middle (Area B) and thethird in south (Area C) having reasonable spatialcoverage to prepare the magnetic anomaly maps.For these three areas, magnetic anomaly mapshave been prepared along with the topography andsurface geology maps (figure 3a–c). The topogra-phy of the islands vary between zero and 600 m andmost of the elevated areas lie in the eastern partof Andaman Islands and correlate with ophioliterocks. The magnetic data acquired over the islandsreveal that the anomalies range between –600 and+600 nT. Details of these anomalies for the threeselected areas are discussed below:

(i) Area A: North Andaman surrounding Diglipur

The magnetic anomalies in this area (figure 3a)reveal two magnetic anomaly highs, one overthe saddle peak (H1) and the other intermediatehigh (H2) in the eastern part, and two magneticanomaly lows L1 and L2 over the Mithakhari groupof rocks. While, the anomaly H1 is observed over

the exposed ophiolite bodies, the anomaly H2 couldbe related to mafic/ultramafic rocks surroundedby the obducted sediments. The topography mapshows that height of saddle peak varies between600 and 800 m.

(ii) Area B: Middle Andaman surrounding Rangat

The magnetic anomaly map (figure 3b) covers themiddle Andaman Islands from Bartang to Pan-chavati area. The magnetic anomaly high (H3) islocated near Panchavati over the ophiolite bodiesas observed from the surface exposure. Intermedi-ate magnetic highs in this region could be relatedto mafic/ultra mafic rocks within the sediments. L3is observed in the sedimentary region.

(iii) Area C: South Andaman surrounding PortBlair

The magnetic anomaly map (figure 3c) in this areashows prominent magnetic low (L4) located nearShoal Bay region. The magnetic highs (H4, H5and H6) are located over the ophiolite/mafic intru-sive bodies near Chidiyatapu. Another high (H7) islocated near Tirur within the sedimentary regionand may be due to the localized emplacement ofophiolite body in the subsurface.

3.3 Magnetic profiles

Four magnetic profiles (AA’ through DD’) ori-ented in ∼NE–SW direction were selected acrossthe islands from the anomaly maps based on theavailability of dense coverage of magnetic stationsand are presented in figure 4. The surface geol-ogy is also shown along these profiles for correla-tion with magnetic anomalies. As can be seen, theprofile AA’ is located in north Andaman regionand passes through Mithakhari group of rocksand ophiolites. While, the magnetic signature isbroader and devoid of significant anomaly variationwithin the sedimentary formations, towards east,over the ophiolite rocks known as the Saddle peakophiolite hill; a distinct magnetic high is observed.The profiles BB’ and CC’ pass through the sedi-mentary region of Mithakhari group of rocks. Themagnetic anomalies are broader all along the pro-file BB’ except for minor low observed in the east-ern margin. Whereas, along profile CC’ which islocated in middle Andaman Islands, a sharp mag-netic high and low anomaly signature can be cor-related with the contact between the ophiolitebodies and the sedimentary rocks. It can be seenfrom the surface geological map (figure 3b) thatthe eastern part of the profile is in close prox-imity to the exposed ophiolite rocks near Pan-chavati. On the other hand, profile DD’ locatedin the south Andaman Islands passes through

364 P B V Subba Rao et al.

Figure 4. Magnetic anomalies plotted along four selected profiles (AA’ through DD’) across the Andaman Islands. Surfacegeology along these profiles is presented along with the topography in the bottom panel for each profile.

Andaman flysch and Mithakhari group of rocks. Inthe eastern part of this profile, a similar magneticanomaly high could be related to ophiolite bodiesas surface exposures are observed in and aroundChidiyatapu.

3.4 2D magnetic modelling

From the foregoing description of magneticanomaly maps and profiles, a clear correlation ofstrong magnetic anomalies can be seen associatedwith the emplaced ophiolite rocks over the islands.In order to delineate their subsurface geometryand magnetic characteristics, we have chosen three

profiles, AA’ in the north, CC’ in the middleand DD’ in the south Andaman Islands (figure 4)for the purpose of modelling as these three pro-files show distinct anomaly correlation with theemplaced ophiolites. Profile BB’ do not reveal anydistinct magnetic anomaly variation, hence notconsidered for modelling. The magnetic anomaliesalong each of these profiles were modelled usingthe GM-SYS software, which is based on the 2Dpolygon method of Talwani (1965). For the pur-pose of modelling, the sedimentary strata and thebasement structure known from Roy (1983, 1992)have been used. Based on the seismic and well cor-relations, two E–W seismo-geologic sections wereprepared by these workers across the Andaman

Magnetic anomalies over the Andaman Islands 365

Table 1. Magnetic parameters used in the 2D magnetic modelling for the oceanic crust and the emplaced ophiolites. Detailsare discussed in the text.

Oceanic crust Ophiolites

Remanent Remanent

Susceptibility magnetization Magnetic Magnetic Susceptibility magnetization Magnetic Magnetic

Profile (cgs) (emu/cc) inclination declination (cgs) (emu/cc) inclination declination

AA’ 0.004 0.005 −57 310 0.014 0.002 −61 300

CC’ 0.004 0.008 −58 310 0.016 0.0065 −62 319

DD’ 0.004 0.006 −57 310 0.014 0.008 −63 319

Figure 5. 2D modelling of magnetic anomalies along the profiles located in the north Andaman (profile AA’), middleAndaman (profile CC’), and south Andaman (profile DD’) Islands region. The models reveal the presence of highlymagnetized bodies related to ophiolites and ultramafic rocks along the east coast of Islands.

366 P B V Subba Rao et al.

Islands, and relevant parts of these sections wereprojected along profile CC’ (after Roy 1992) andprofile DD’ (after Roy 1983). The modelling iscarried out along CC’ and DD’ by partly con-straining these sections with minor modificationsin order to fit the observed magnetic anomalies.Below the Andaman Islands, the basement is com-posed of oceanic crust (Curray 2005) with ophiolitesuite of rocks emplaced along the faults or thrustsheets within the outer-arc accretionary prism. Itis known that the ophiolites were emplaced as aresult of scrapping of oceanic crust during lateCretaceous–Paleocene time (Roy et al. 1988; Pal2011). This tectonic scenario is used for struc-tural delineation through magnetic anomalies. Forthis purpose, we considered present-day age ofthe oceanic crust below the Andaman Islands as∼85–90 Ma, and the age of oceanic crust (ophi-olites) at the time of emplacement as ∼120 Ma.For these two ages, we obtained paleo-latitudeof India (Klootwijk et al. 1991; Radhakrishna2003; Radhakrishna and Mathew 2012) which thenprovided us the remnant magnetization (paleoinclination and declination) required for magneticmodelling. The parameters used for the modellingthat include magnetization, inclination and dec-lination are given in table 1. Magnetic anomalyhighs having amplitude of about 100–300 nT havebeen interpreted as ophiolite/mafic rocks that areconstrained to be at the ground surface/subsurfaceand extend to a depth of about 5–8 km. Mag-netization intensities were adjusted to match theobserved magnetic anomaly. High magnetization isobserved in the middle Andaman Island region thatcould be related to mafic/ultramafic intrusions inthis region. The resultant models are shown infigure 5.

4. Results and discussion

The present study clearly brought out strongmagnetic anomaly signatures over the ophiolitebodies along the eastern margin of AndamanIslands and modelling of these anomalies revealedtheir extension to depths of about 5–8 km.The seismo-geologic sections across middle andsouth Andaman Islands prepared by Roy (1983,1992) revealed an accretionary prism (outer-arcridge) structure formed by imbricate thrusting ofeast-dipping fault slices (along with ophiolitesintrusions); linked to westward shifting of thesubduction zone. The electromagnetic induction(magnetotelluric and geomagnetic depth sounding)studies over the islands also brought out anomalousconductivity zones in the eastern part of AndamanIslands and were related to emplacement of

N–S trending ophiolites (Gokarn et al. 2006; SubbaRao 2011).

Previous geological mapping of the AndamanOphiolites revealed that they occur as root-less, subhorizontal bodies with overlying Eocene–Oligocene flysch sedimentary rocks uplifted andemplaced by a series of east dipping thrusts(Sengupta et al. 1990; Pal et al. 2003a; Srivastavaet al. 2004; Ghosh et al. 2013). These ophioliteslices show thrust contacts with the surroundingsediments (Roy 1992). While, the ophiolites areexposed as small patches in the western part ofAndaman Islands, in the east, they occur as N–Sto NE–SW-trending discontinuous slices havingtectonic contacts with underlying sediments indi-cating thrust-controlled emplacement in an accre-tionary environment (Pal et al. 2003a; Pal 2011).It is suggested that these ophiolites were emplacedas a result of subduction; ocean crust (ophiolite) ofthe subducting slab was scraped off and emplacedas a thrust slice. Our modelled profiles in thisstudy support such emplacement. With continu-ous subduction from the Cretaceous onwards, theophiolites were emplaced along a series of thrustslices (Sengupta et al. 1990; Pal et al. 2003a).The convergence of India and Burma during lateEocene possibly enhanced upliftment of ophioliteand metamorphic rocks and caused extensive defor-mation in the accreted sediments (Pal et al. 2003a).However, SHRIMP age data of the plagiogranitesin the Andaman ophiolites gave rise to 93.6 Ma(Sarma et al. 2010), which suggest that arc mag-matism was initiated much earlier than the Eoceneaccretion of the ophiolites (Sengupta et al. 1990).These ophiolites preserve the mantle sequence,layered ultramafic–mafic rocks, intrusive andextrusive rocks (Pal 2011). Geochemical studiesindicate that the ophiolite bodies show interplayof mantle-crustal processes and were derived froma boninitic parental melt (Jafri and Sheikh 2013;Pal et al. 2003b). This melt represents the crys-tallization of ophiolites through differentiation andassimilation reactions of andesitic rich magma con-taining iron rich material and derived from mul-tiple parental magmas. Thus magnetic highs areobserved over ophiolite bodies. High magnetiza-tion observed in the middle Andaman region alongProfile CC’ may be due to highly magnetizedthick mafic/ultra mafic rocks (volcanic sediments)present in this region. Dhana Raju and Negi (1984)opined that these ophiolite rocks might have beenemplaced through the EMF.

5. Conclusions

Total magnetic field measurements carried out forthe first time over the Andaman Islands revealedthe following geological information:

Magnetic anomalies over the Andaman Islands 367

(i) Magnetic anomalies are, in general, smooth inthe sedimentary region, however, along the eastcoast of north, middle and south AndamanIslands, significantly high amplitude magneticanomalies occur over the mafic/ultramaficcumulates of ophiolite bodies.

(ii) 2D modelling of magnetic anomalies alongselected profiles across the islands indicate thatthe ophiolite bodies extend to a depth of about5–8 km and spatially correlate with the mappedfault/thrust zones.

Acknowledgements

The authors are grateful to the Departmentof Science and Technology (DST), New Delhifor the financial support under the researchproject (ESS/16/346/2008) sanctioned to Prof. MRadhakrishna, IIT Bombay. They also thank Mr.Rishikesh, Mr. Rajubabu and the officers of theDST, Andaman & Nicobar Administration, for theexcellent logistic support. Critical comments fromtwo anonymous reviewers helped to improve themanuscript.

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MS received 17 May 2015; revised 29 October 2015; accepted 5 November 2015