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Magnetic Anomalies over the and their geological significance

P B V Subba Rao1, M Radhakrishna2*, K Haripriya2, B Someswara Rao2 and D Chandrasekharam2 1Indian Institute of Geomagnetism, Kalamboli Highway, Panvel, Navi Mumbai- 410218 2Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076 (*correspondingCorresponding author: [email protected])

Abstract

The Andaman Islands form part of the outer-arc accretionary sedimentary complex belonging to the Andaman-Sumatra active subduction zone. The islands are characterized by thick cover of Neogene sediments along with exposed ophiolite rocks at few places. A regional magnetic survey was carried out for the first time over the Andaman Islands with a view to understand the correlation of anomaly signatures over the exposed ophiolites as well as delineate their subsurface geometry.with surface geology of the islands. The residual total field magnetic anomaly maps have revealed major lows over the thick sedimentary regions of the south Andaman islands, whereas, in the middle and north Andaman islands,distinct magnetic anomalies showhaving intermediate to high positive anomalies over the mafic / ultramafic intrusives along with amplitude magnetic signatures and correlate with the areas over / close to the exposed ophiolite bodies.rocks along the east coast of north, middle and the south Andaman Islands. The 2-D modeling of magnetic anomalies along selected E-W profiles across the islands indicate that the ophiolite bodies extend to a depth of about 5-8 km and spatially correlate with the mapped fault / thrust zones.

Key Words: Magnetic SurveysSurvey; Ophiolites; Magnetic Anomalies; Andaman Islands; Andaman-Sumatra Arc; northeast

1. Introduction

The major morpho-tectonic elements (Weeks et al., 1967) running N-S along the Andaman arc are the Andaman trench, Andaman-Nicobar ridge (Andaman-Nicobar Islands), Nicobar deep, West Andaman fault (WAF), Andaman Spreading Ridge and the Mergui terrace (Fig. 1). The Andaman-Nicobar Islands forms a major part of the outer arc sedimentary complex

that stretches nearly 1100 km along the Andaman trench-arc system (Curray, 2005), and separate the back-arc Andaman backarc basin from the . Along the western margin of the islands, the sediments of the Bengal Fan have been deformed below the Andaman trench (Curray et al., 1982). The Andaman arc is characterized by the presence of east dipping Benioff zone down to about 200 km depth (Mukhopadhyay, 1984). Several earlier investigators have studied the structure and deformation along this active plate boundary zone (e.g., Gahalaut et al., 2008; Radhakrishna et al., 2008). According to Hamilton (1979), the Andaman arc region forms an important transitional tectonic link between the Burma-Himalayan arc/collision system in the north and the Indonesian arc system in the south. The main rock types occurring over the islands are either of sedimentary or magmatic. The magmatic rocks constitute the ultramafic plutonites and basalt, together with minor radiolarian chert, and thesethis forms the Andaman ophiolite suite (Pal et al., 2003a). Owing to strong magnetization contrast between the ophiolites and sedimentary rocks, magnetic method would be an effective tool for investigating the subsurface structure of the ophioliterocks.ophiolite rocks. However, till date the gravity and magnetic data was not acquired over the onshore part of the Andaman Islands. So, for the first time, we carried out detailed magnetic survey over the main Andaman Islands with an objective to understand the geometry of the ophiolite bodies in the subsurface as well as their association with the major fault systems over the islands. Salient results of this investigation are presented in this paper.

2. Geology of the Andaman Islands

The topographic shaded relief map of the Andaman Islands (Fig. 2A) using the Shuttle Radar Topography Mission (SRTM) database (Jarvis et al., 2008) reveals the sub-areal expression of the islands with many geomorphologic features and linerlinear ridges (Narayana, 2011). Regional geological mapping of the Andaman Islands was mostly carried out by the GSI and ONGC. The Andaman-Nicobar islands have formed during the Oligo-Miocene times (Rodolfo, 1969). The rock formations can be broadly classified under four major groups in the Andaman Islands (Fig. 2B), and thesearethese 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; Bandopadhyay, 2005). The rock types mainly constitute of serpentinites, ophiolites with radiolarian cherts, cherty pelagic limestone and the flysch sediments (Chatterjee,

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1967; Eremenko and Sastri, 1977; Roy, 1983). Two prominent N-S trending faults / thrusts have been observed over the Andaman Islands (Fig. 2C). Among these two, Jarwa thrust is the most significant morpho-tectonic feature (Fig. 2A) trending N-S all along the main islands, (Eremenko and Sastry, 1977), and the second important fault is the Eastern Margin Fault (EMF) in the immediate Andaman offshore along its east coast. (Roy, 1983). The EMF acts as a boundary between the outer-arc ridge and the fore-arc basin (Roy, 1983,; Cochran, 2010).

The ophiolite rocks in the Andaman Islands are interpreted as remnants of oceanic crust and upper mantle that were obducted by complex tectonic processes during the Miocene times, and these are observed along N-S trending linear belts in structurally weak zones (Roy, 1983; Acharyya, 2007). The ophiolite group of rocks consisting of metamorphic, mélange and ophiolitic rocks with pelagic sediments occur as dismembered bodies (Pal et al., 2003a). The eastern part of Andaman Islands is characterized by highly deformed ophiolite rocks from the oceanic floor as well as ultrabasic / volcanic / pelagic sediments along with older metamorphics, whereas, in the western part, sediments belong to the accretionary prism consisting of flysch- sandstone / siltstone with conglomerates and calcareous sediments (Mukhopadhyay et al., 2003). Ophiolites are exposed in small isolated patches in the west, whereas, in the east, most ophiolite rocks occur in N–S to NE–SW trending bodies. These ophiolite rocks have contact with underlying sediments and denote thrust-controlled emplacement in an accretionary environment (Pal et al., 2003a,b). Petrographically, ophiolites consist of serpentinite, dunite, harzburgite, pyroxenite, olivine gabbro, anorthositic gabbro and gabbroic 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, ground magnetic surveys were carried out over the main Andaman Islands in different field seasons spanning the period during February 2007 to December 2009. The total magnetic intensity measurements were made with a portable proton precession magnetometer of GEM System, Canada with a sensor height of 1.6 meters from the ground. We have placed the observation points at intervals of 1000 m, and in some places, where we found bare rock outcrops of the ophiolite; the points were picked up at 500 m interval. The

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total field magnetic anomaly was obtained at each observation point by subtracting the International Geomagnetic Reference Field (IGRF) 2005 model (IAGA, 1985Rukstales and Love, 2007) from the observed data. Diurnal variation of the geomagnetic field was corrected by using total geomagnetic intensity data measured everycomputed using the Magson fluxgate magnetometer that measures north-south (X), east-west (Y) and vertically downward (Z) components at a sampling interval of 1 minute at the. The temporary magnetic observatorybase station was established at the Department of Science and Technology (DST), Port Blair, Andaman-Nicobar Division. In certain inaccessible areas such as the reserved Jarwar forest area in the middle-south Andaman, we could not collect the magnetic data. during the survey period. A total of 418 magnetic measurements were made and the data coverage is shown in Fig. 2B. Though the coverage is mainly along the N-S oriented grand trunk road covering the entire island, wherever possible we have collected data along many E-W profiles as well as other minor roads. We could not collect the magnetic data in certain inaccessible areas such as the reserved Jarwa forest of the middle-south Andaman Islands.

3.2 Magnetic Anomaly Maps

Based on theThe distribution of data points, we have selectedmeasured magnetic stations described above gave rise to three areas of the Andaman Islands (see Fig. 2B), one in the north (Area A), second in the middle (Area B) and the third in south (Area C) Andaman Islands.having reasonable spatial coverage to prepare the magnetic anomaly maps. For these three areas, magnetic anomaly maps have been prepared along with the topography and surface geology maps (Fig. 3A-C). The topography of the islands vary between 0-600 m and most of the elevated areas lie in the eastern part of Andaman Islands and correlate with ophiolite rocks. The magnetic data acquired over the islands reveal that the anomalies range between -600 nT and +600 nT. Details of these anomalies for the three selected areas are discussed below:

(i) Area A: North Andaman surrounding

The magnetic anomalies in this area (Fig. 3A) reveal twomagnetictwo magnetic anomaly highs, one over the saddle peak (H1) and the other intermediate high (H2) in the eastern part, and two magnetic anomalylowsanomaly lows L1 and L2 over the Mithakhari group of rocks. While, the anomaly H1 is observed over the exposed ophiolite bodies, the anomaly H2 could be related

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to mafic/ultramafic rocks surrounded by the obducted sediments. The topography map shows that height of saddle peak varies between 600 to 800 m.

Formatted: Line spacing: single

(ii) Area B: Middle Andaman surrounding

The magnetic anomaly map (Fig. 3B) covers the middle Andaman Islands from Bartang to Panchavati area. The magnetic anomaly high (H3) is located near Panchavati over the ophiolite bodies as observed from the surface exposure. Intermediate Magnetic highs in this region could be related to mafic/ultra mafic rocks within the sediments. L3 is observed in the sedimentary region.

(iii) Area C: South Andaman surrounding Port Blair

The magnetic anomaly map (Fig. 3C) in this area shows prominent magnetic low (L4) located near Shoal Bay region and could be due to thick sediments.. The magnetic highs (H4, H5 and H6) are located over the ophiolite/mafic intrusive bodies near chidiyatopChidiyatapu. Another high (H7) is located near Tirur within the sedimentary region and may be due to the localized emplacement of ophiolite body in the subsurface.

3.3 Magnetic profiles

Four magnetic profiles (AA' through DD') oriented in ~ NE-SW direction were selected across the islands from the anomaly mapsbasedmaps based on the availability of dense coverage of magneticstationsmagnetic stations and are presented in Fig. 4. The surface geology is also shown along these profiles for correlation with magnetic anomalies. As can be seen, the profile AA’ is located in north Andaman region and passes through Mithakhari group of rocks and ophiolites. The anomalies along the profile show a broad low overWhile, the magnetic signature is broader and devoid of significant anomaly variation within the sedimentary formations, and a high locatedtowards east, over the ophiolite rocks knownasknown as the saddleSaddle peak ophiolite hill. Profile; a distinct magnetic high is observed. The profiles BB’ passesand CC’ pass through athe sedimentary formations belonging to Mithakhari group of rocks and low is

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observed in the eastern margin indicating that the thicknessregion of the basin increases towards east. Profile CC’ Mithakhari group of rocks. The magnetic anomalies are broader all along the profile BB’ except for minor low observed in the eastern margin. Whereas, along profile CC’ which is located in middle Andaman Islands and passes through Mithakhari group of rocks. A, a sharp magnetic high and low anomaly signature can be correlated with the contact between the ophiolite bodies (as exposed near Panchavati) and the sedimentary rocks. ProfileIt can be seen from the surface geological map (Fig. 3B), the eastern part of the profile is in close proximity to the exposed ophiolite rocks near Panchavati. On the other hand, profile DD’ is located in the south Andaman Islands and passes through Andaman flysch and Mithakhari group of rocks. In the eastern part of this profile, a similar magnetic anomaly high could be related to ophiolite bodies as surface exposures are observed in and around Chidiya TopChidiyatapu.

3.4 2-D magnetic modeling

From the foregoing description of magnetic anomaly maps and profiles, a clear correlation of strong magnetic anomalies can be seen associated with the emplaced ophiolite rocks over the islands. In order to delineate their subsurface geometry and magnetic characteristics, we selectedhave chosen three profiles, the AA’ in the north, CC’ in the middle and DD’ in the south Andaman Islands (location in Fig. 4).4) for the purpose of modeling as these three profiles show distinct anomaly correlation with the emplaced ophiolites. Profile BB’ do not reveal any distinct magnetic anomaly variation, hence not considered for modeling. The magnetic anomalies along each of these profiles were modeled using the GM-SYS software, which is based on the 2-D polygon method of Talwani (1965). For the purpose of modeling, the sedimentary strata and the basement structure is considered known from the published seismic sections across the islands.Roy (1983) and Roy (1992) have been used. Based on the seismic and well correlations, two E-W seismo-geologic sections have beenwere prepared by earlierthese workers. These across the Andaman Islands, and relevant parts of these sections arewere projected along profile CC’ (after Roy, 1992) and profile DD’ (after Roy, 1983) and restricted). The modeling is carried out along CC’ and DD’ by partly constraining these sections with minor modifications in order to their respective lengths. The fit the observed magnetic anomalies. Below the Andaman Islands, the basement rocks are considered as is composed of oceanic crust (Curray, 2005) with ophiolite suite of rocks emplaced along the faults or thrust sheets. within the

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outer arc accretionary prism. It is known that the ophiolites were emplaced as a result of scrapping of oceanic crust during late Cretaceous – Paleocene time (Roy et al., 1988; Pal, 2011). This tectonic scenario is used for structural delineation through magnetic anomalies. For this purpose, we considered present-day age of the oceanic crust below the Andaman Islands as ~85- 90 Ma, and the age of oceanic crust (ophiolites) at the time of emplacement as ~120 Ma. For these two ages, we obtained paleo latitude of (Klootwijk et al., 1991; Radhakrishna, 2003; Radhakrishna and Mathew, 2012) which then provided us the remnant magnetization (paleo inclination and declination) required for magnetic modeling. The parameters requiredused for the magnetic modeling that include the magnetization, inclination and declination are given in Table 1. We assumed that oceanic crust below the Andaman Islands is ~ around 100 Ma and the ophiolite rocks were emplaced subsequently. Magnetic anomaly highs having amplitude of about 100-300 nT have been interpreted as ophiolite / mafic rocks that are constrained to be at the ground surface as well as extending/subsurface and extend to a depth of about 5-8 km. Magnetization intensities were adjusted to match the observed magnetic anomaly. High magnetization is observed in the middle Andaman Island region that could be related to mafic / ultramafic intrusions in this region. The resultant models are shown in Fig. 5.

4. Results and Discussion

The present study clearly brought out strong magnetic anomaly signatures over the ophiolite bodies along the eastern margin of Andaman Islands and modeling of these anomalies revealed their extension to depths of about 5-8 km. Seismic studiesThe seismo-geologic sections across themiddle and south Andaman Islands (prepared by Roy, (1983,; 1992) brings outrevealed an accretionary prism (outer arc ridge) structure formed by imbricate thrusting of east-dipping fault slices (along with Ophiolites intrusions); linked to westward shifting of the subduction zone. ElectromagneticThe electromagnetic induction (Magnetotelluric and Geomagnetic Depth soundingSounding) studies over the islands also brought out anomalous conductivity zones in the eastern part of Andaman Islands and were related to emplacement of N-S trending ophiolites (Gokarn et al., 2006; Subba Rao, 2011).

Previous geological mapping of the Andaman Ophiolites revealed that they occur as rootless, sub horizontal bodies with overlying Eocene-Oligocene flysch sedimentary rocks

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uplifted and emplaced by a series of east dipping thrusts (Sengupta et al., 1990; Pal et al., 2003a; Srivastava et al., 2004; Ghosh et al., 2013). These ophiolite slices show thrust contacts with underlyingthe surrounding sediments and occasionally with overlying sediments.(Roy, 1992). While, the ophiolites are exposed as small patches in the western part of Andaman Islands, in the east, they occur as N–S to NE–SW-trending discontinuous slices having tectonic contacts with underlying sediments indicating thrust-controlled emplacement in an accretionary environment (Pal et al., 2003a; Pal, 2011). It is suggested that these ophiolites were emplaced as a result of subduction; ocean crust (ophiolite) of the subducting slab was scraped off and emplaced as a thrust slice. Our modeled profiles in this study support such emplacement. With continuous subduction from the Cretaceous onwards, the ophiolites were emplaced along a series of thrust slices (Sengupta et al., 1990; Pal et al., 2003a). The convergence of India and Burma during late Eocene possibly enhanced uplift of ophiolite and metamorphic rocks and caused extensive deformation in the accreted sediments (Pal et al., 2003a). However, SHRIMP age data of the plagiogranites in the Andaman ophiolites gave rise to 93.6 Ma (Sarma et al., 2010), which suggest that arc magmatism was initiated much earlier than the Eocene accretion of the ophiolites (Sengupta et al., 1990). These ophiolites preserve the mantle sequence, layered ultramafic–mafic rocks, intrusive, and extrusive rocks (Pal, 2011). Geochemical studies indicate that the ophiolite bodies show interplay of mantle-crustal processes and were derived from a boninitic parental melt (Pal et al., 2003b; Jafri and Sheikh, 2013). This melt represents the crystallization of ophiolites through differentiation and assimilation reactions of andesitic rich magma containing iron rich material and derived from multiple parental magmas. Thus magnetic highs are observed over ophiolite bodies. High magnetization observed in the middle Andaman region along Profile CC’ may be due to highly magnetized thick mafic/ultra mafic rocks (volcanic sediments) present in this region. Dhana Raju and Negi (1984) opined that these ophiolite rocks maymight have been emplaced through the EMF.

5. Conclusions

Total magnetic field measurements carried out for the first time over the Andaman Islands revealed the following geological information: i) Magnetic anomaly lows occur over thick anomalies are in general smooth in the sedimentary regions in the region, however, along the east coast of north, middle and

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south Andaman islands, whereas, in the middle and north Andaman islands,significantly high amplitude magnetic anomalies show intermediate to high positive anomalies over occur over the mafic / ultramafic intrusives along withcumulates of ophiolite bodies. ii) 2-D modeling of magnetic anomalies along selected profiles across the islands indicate that the ophiolite bodies extend to a depth of about 5-8 km and spatially correlate with the mapped fault / thrust zones. Acknowledgements

The authors are grateful to the Department of Science and Technology (DST), New Delhi for the financial support under the research project (ESS/16/346/2008) sanctioned to Prof. M Radhakrishna, IIT Bombay. We also thank Mr. Rishikesh, Mr. Rajubabu and the officers of the DST, Andaman & Nicobar Administration for the excellent logistic support. Critical comments from two anonymous reviewers helped to improve the manuscript.

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Figure Captions

Fig.1. Tectonic map of the Andaman arc and the Andaman Sea region (modified after Curray, 2005). The tectonic sub-divisions of the region (after Roy and Das Sharma, 1993; Kamesh Raju et al., 2004) are indicated in 1-65. These are: 1. Outer slope, 2. Inner slope, 3. Trench slope Break, 4. Forearc basin, 5. Volcanic arc, 6. Backarc. and the backarc basin, EMF-Eastern Margin Fault, DF-Diligent Fault, CF- Cocos Fault, WAF-West Andaman Fault, WSR- West Sewell Rise, ASR- Andaman Spreading Ridge, (after Kamesh Raju et al., 2004), OCT- Ocean Continent Transition. The grey shaded area represents the extent of Burma plate (after Curray, 2005). Fig.2. (Aa) Shaded relief map of the Andaman Islands from the SRTM data showing the topographic variations and prominent linear ridges, (Bb) Geological map of the Andaman 13

Islands (after Pal et al., 2003a) along with locations of magnetic measurement points occupied in three different phases of field surveys. The magnetic anomaly maps prepared for the three areas A-Ca-c are indicated on this map, (Cc) Tectonic map of the Islands and the adjoining Andaman basin region (modified after Pandey et al., 1992: Curray,

2005). Formatted: Highlight Fig.3. Topography (left panel), geology (middle panel) and the magnetic anomaly (right panel) maps prepared for the A) north Andaman Islands (Area A), B) middle Andaman Islands (Area B), and C) the south Andaman Islands (Area C). Details are discussed in the text. Fig.4. The topography and magnetic Magnetic anomalies plotted along four selected profiles (AA’ through DD’) across the Andaman Islands. Surface geology along these profiles is presented along with the topography in the bottom panel for each profile. Fig.5. 2-D modeling of magnetic anomalies along the profiles located in the north Andaman (profile AA’), middle Andaman (profile CC’)), and the south Andaman (profile DD’) Islands region. The models reveal the presence of highly magnetized bodies related to ophiolites and ultramafic rocks along the east coast of Islands. Table 1. Magnetic parameters used in the 2-D magnetic modeling for the oceanic crust and the emplaced ophiolites. Details are discussed in the text.

14 Revised MS-R2 Click here to view linked References

1 2 3 4 5 Magnetic Anomalies over the Andaman Islands and their geological 6 significance 7 8 * 9 P B V Subba Rao1, M Radhakrishna2 , K Haripriya2, B Someswara Rao2 and D 10 Chandrasekharam2 11 1Indian Institute of Geomagnetism, Kalamboli Highway, Panvel, Navi Mumbai- 410218 12 2 13 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076 14 (*Corresponding author: [email protected]) 15 16 17 18 Abstract 19 20 The Andaman Islands form part of the outer-arc accretionary sedimentary complex 21 22 belonging to the Andaman-Sumatra active subduction zone. The islands are characterized by 23 24 thick cover of Neogene sediments along with exposed ophiolite rocks at few places. A regional 25 26 magnetic survey was carried out for the first time over the Andaman Islands with a view to 27 understand the correlation of anomaly signatures with surface geology of the islands. The 28 29 residual total field magnetic anomaly maps have revealed distinct magnetic anomalies having 30 31 intermediate to high amplitude magnetic signatures and correlate with the areas over / close to 32 33 the exposed ophiolite rocks along the east coast of north, middle and the south Andaman Islands. 34 35 The 2-D modeling of magnetic anomalies along selected E-W profiles across the islands indicate 36 37 that the ophiolite bodies extend to a depth of about 5-8 km and spatially correlate with the 38 mapped fault / thrust zones. 39 40 41 Key Words: Magnetic Survey; Ophiolites; Magnetic Anomalies; Andaman Islands; Andaman- 42 43 Sumatra Arc; northeast Indian Ocean 44 45 46 47 48 1. Introduction 49 50 The major morpho-tectonic elements (Weeks et al., 1967) running N-S along the 51 52 Andaman arc are the Andaman trench, Andaman-Nicobar ridge (Andaman-Nicobar Islands), 53 54 Nicobar deep, West Andaman fault (WAF), Andaman Spreading Ridge and the Mergui terrace 55 56 (Fig. 1). The Andaman-Nicobar Islands forms a major part of the outer arc sedimentary complex 57 that stretches nearly 1100 km along the Andaman trench-arc system (Curray, 2005), and separate 58 59 the Andaman backarc basin from the Bay of Bengal. Along the western margin of the islands, the 60 61 sediments of the Bengal Fan have been deformed below the Andaman trench (Curray et al., 62 63 1 64 65 1 2 3 4 1982). The Andaman arc is characterized by the presence of east dipping Benioff zone down to 5 6 about 200 km depth (Mukhopadhyay, 1984). Several earlier investigators have studied the 7 8 structure and deformation along this active plate boundary zone (e.g., Gahalaut et al., 2008; 9 10 Radhakrishna et al., 2008). According to Hamilton (1979), the Andaman arc region forms an 11 12 important transitional tectonic link between the Burma-Himalayan arc/collision system in the 13 north and the Indonesian arc system in the south. The main rock types occurring over the islands 14 15 are either of sedimentary or magmatic. The magmatic rocks constitute the ultramafic plutonites 16 17 and basalt, together with minor radiolarian chert, and this forms the Andaman ophiolite suite (Pal 18 19 et al., 2003a). Owing to strong magnetization contrast between the ophiolites and sedimentary 20 21 rocks, magnetic method would be an effective tool for investigating the subsurface structure of 22 23 the ophiolite rocks. However, till date the gravity and magnetic data was not acquired over the 24 onshore part of the Andaman Islands. So, for the first time, we carried out detailed magnetic 25 26 survey over the main Andaman Islands with an objective to understand the geometry of the 27 28 ophiolite bodies in the subsurface as well as their association with the major fault systems over 29 30 the islands. Salient results of this investigation are presented in this paper. 31 32 33 34 2. Geology of the Andaman Islands 35 36 The topographic shaded relief map of the Andaman Islands (Fig. 2A) using the Shuttle 37 38 Radar Topography Mission (SRTM) database (Jarvis et al., 2008) reveals the sub-areal 39 40 expression of the islands with many geomorphologic features and linear ridges (Narayana, 2011). 41 42 Regional geological mapping of the Andaman Islands was mostly carried out by the GSI and 43 44 ONGC. The Andaman-Nicobar islands have formed during the Oligo-Miocene times (Rodolfo, 45 46 1969). The rock formations can be broadly classified under four major groups in the Andaman 47 Islands (Fig. 2B), and these are (i) Cretaceous to Paleocene Ophiolite suite, (ii) Lower to Middle 48 49 Eocene Mithakhari Group, (iii) Oligocene to Late Eocene Andaman turbidities (Andaman Flysch 50 51 Group), and (iv) Mio-Pliocene Archipelago Group (Ray, 1985; Ray et al., 1988; Pal et al., 2003a; 52 53 Bandopadhyay, 2005). The rock types mainly constitute of serpentinites, ophiolites with 54 55 radiolarian cherts, cherty pelagic limestone and the flysch sediments (Chatterjee, 1967; 56 Eremenko and Sastri, 1977; Roy, 1983). Two prominent N-S trending faults / thrusts have been 57 58 observed over the Andaman Islands (Fig. 2C). Among these two, Jarwa thrust is the most 59 60 significant morpho-tectonic feature (Fig. 2A) trending N-S all along the main islands (Eremenko 61 62 63 2 64 65 1 2 3 4 and Sastry, 1977), and the second important fault is the Eastern Margin Fault (EMF) in the 5 6 immediate Andaman offshore along its east coast (Roy, 1983). The EMF acts as a boundary 7 8 between the outer-arc ridge and the fore-arc basin (Roy, 1983; Cochran, 2010). 9 10 11 12 The ophiolite rocks in the Andaman Islands are interpreted as remnants of oceanic crust 13 and upper mantle that were obducted by complex tectonic processes during the Miocene times, 14 15 and these are observed along N-S trending linear belts in structurally weak zones (Roy, 1983; 16 17 Acharyya, 2007). The ophiolite group of rocks consisting of metamorphic, mélange and 18 19 ophiolitic rocks with pelagic sediments occur as dismembered bodies (Pal et al., 2003a). The 20 21 eastern part of Andaman Islands is characterized by highly deformed ophiolite rocks from the 22 23 oceanic floor as well as ultrabasic / volcanic / pelagic sediments along with older metamorphics, 24 whereas, in the western part, sediments belong to the accretionary prism consisting of flysch- 25 26 sandstone / siltstone with conglomerates and calcareous sediments (Mukhopadhyay et al., 2003). 27 28 Ophiolites are exposed in small isolated patches in the west, whereas, in the east, most ophiolite 29 30 rocks occur in N–S to NE–SW trending bodies. These ophiolite rocks have contact with 31 32 underlying sediments and denote thrust-controlled emplacement in an accretionary environment 33 34 (Pal et al., 2003a,b). Petrographically, ophiolites consist of serpentinite, dunite, harzburgite, 35 pyroxenite, olivine gabbro, anorthositic gabbro and gabbroic anorthosite (Dhana Raju and Negi, 36 37 1984; Jafri et al., 2010). 38 39 40 41 3. Magnetic Data Acquisition and Analysis 42 43 44 3.1 Magnetic Surveys 45 46 47 As mentioned earlier, for the first time, ground magnetic surveys were carried out over 48 the main Andaman Islands in different field seasons spanning the period during February 2007 to 49 50 December 2009. The total magnetic intensity measurements were made with a portable proton 51 52 precession magnetometer of GEM System, Canada with a sensor height of 1.6 meters from the 53 54 ground. We have placed the observation points at intervals of 1000 m, and in some places, where 55 56 we found bare rock outcrops of the ophiolite; the points were picked up at 500 m interval. The 57 total field magnetic anomaly was obtained at each observation point by subtracting the 58 59 International Geomagnetic Reference Field (IGRF) 2005 model (Rukstales and Love, 2007) from 60 61 the observed data. Diurnal variation of the geomagnetic field was corrected by using total 62 63 3 64 65 1 2 3 4 geomagnetic intensity data computed using the Magson fluxgate magnetometer that measures 5 6 north-south (X), east-west (Y) and vertically downward (Z) components at a sampling interval of 7 8 1 minute. The temporary base station was established at the Department of Science and 9 10 Technology (DST), Port Blair, Andaman-Nicobar Division during the survey period. A total of 11 12 418 magnetic measurements were made and the data coverage is shown in Fig. 2B. Though the 13 coverage is mainly along the N-S oriented grand trunk road covering the entire island, wherever 14 15 possible we have collected data along many E-W profiles as well as other minor roads. We could 16 17 not collect the magnetic data in certain inaccessible areas such as the reserved Jarwa forest of the 18 19 middle-south Andaman Islands. 20 21 22 23 3.2 Magnetic Anomaly Maps 24 25 The distribution of measured magnetic stations described above gave rise to three areas 26 27 of the Andaman Islands (see Fig. 2B), one in the north (Area A), second in the middle (Area B) 28 29 and the third in south (Area C) having reasonable spatial coverage to prepare the magnetic 30 31 anomaly maps. For these three areas, magnetic anomaly maps have been prepared along with the 32 33 topography and surface geology maps (Fig. 3A-C). The topography of the islands vary between 34 35 0-600 m and most of the elevated areas lie in the eastern part of Andaman Islands and correlate 36 with ophiolite rocks. The magnetic data acquired over the islands reveal that the anomalies range 37 38 between -600 nT and +600 nT. Details of these anomalies for the three selected areas are 39 40 discussed below: 41 42 43 44 (i) Area A: North Andaman surrounding Diglipur 45 46 The magnetic anomalies in this area (Fig. 3A) reveal two magnetic anomaly highs, one over 47 48 the saddle peak (H1) and the other intermediate high (H2) in the eastern part, and two magnetic 49 50 anomaly lows L1 and L2 over the Mithakhari group of rocks. While, the anomaly H1 is observed 51 52 over the exposed ophiolite bodies, the anomaly H2 could be related to mafic/ultramafic rocks 53 surrounded by the obducted sediments. The topography map shows that height of saddle peak 54 55 varies between 600 to 800 m. 56 57 58 59 60 61 62 63 4 64 65 1 2 3 4 (ii) Area B: Middle Andaman surrounding Rangat 5 6 7 The magnetic anomaly map (Fig. 3B) covers the middle Andaman Islands from Bartang 8 9 to Panchavati area. The magnetic anomaly high (H3) is located near Panchavati over the 10 ophiolite bodies as observed from the surface exposure. Intermediate Magnetic highs in this 11 12 region could be related to mafic/ultra mafic rocks within the sediments. L3 is observed in the 13 14 sedimentary region. 15 16 17 18 (iii) Area C: South Andaman surrounding Port Blair 19 20 The magnetic anomaly map (Fig. 3C) in this area shows prominent magnetic low (L4) 21 22 located near Shoal Bay region. The magnetic highs (H4, H5 and H6) are located over the 23 24 ophiolite/mafic intrusive bodies near Chidiyatapu. Another high (H7) is located near Tirur within 25 26 the sedimentary region and may be due to the localized emplacement of ophiolite body in the 27 subsurface. 28 29 30 31 3.3 Magnetic profiles 32 33 34 Four magnetic profiles (AA' through DD') oriented in ~ NE-SW direction were selected 35 36 across the islands from the anomaly maps based on the availability of dense coverage of 37 38 magnetic stations and are presented in Fig. 4. The surface geology is also shown along these 39 profiles for correlation with magnetic anomalies. As can be seen, the profile AA’ is located in 40 41 north Andaman region and passes through Mithakhari group of rocks and ophiolites. While, the 42 43 magnetic signature is broader and devoid of significant anomaly variation within the sedimentary 44 45 formations, towards east, over the ophiolite rocks known as the Saddle peak ophiolite hill; a 46 47 distinct magnetic high is observed. The profiles BB’ and CC’ pass through the sedimentary 48 49 region of Mithakhari group of rocks. The magnetic anomalies are broader all along the profile 50 BB’ except for minor low observed in the eastern margin. Whereas, along profile CC’ which is 51 52 located in middle Andaman Islands, a sharp magnetic high and low anomaly signature can be 53 54 correlated with the contact between the ophiolite bodies and the sedimentary rocks. It can be 55 56 seen from the surface geological map (Fig. 3B), the eastern part of the profile is in close 57 58 proximity to the exposed ophiolite rocks near Panchavati. On the other hand, profile DD’ located 59 60 in the south Andaman Islands passes through Andaman flysch and Mithakhari group of rocks. In 61 62 63 5 64 65 1 2 3 4 the eastern part of this profile, a similar magnetic anomaly high could be related to ophiolite 5 6 bodies as surface exposures are observed in and around Chidiyatapu. 7 8 9 10 3.4 2-D magnetic modeling 11 12 13 From the foregoing description of magnetic anomaly maps and profiles, a clear 14 correlation of strong magnetic anomalies can be seen associated with the emplaced ophiolite 15 16 rocks over the islands. In order to delineate their subsurface geometry and magnetic 17 18 characteristics, we have chosen three profiles, AA’ in the north, CC’ in the middle and DD’ in 19 20 the south Andaman Islands (location in Fig. 4) for the purpose of modeling as these three profiles 21 22 show distinct anomaly correlation with the emplaced ophiolites. Profile BB’ do not reveal any 23 24 distinct magnetic anomaly variation, hence not considered for modeling. The magnetic 25 anomalies along each of these profiles were modeled using the GM-SYS software, which is 26 27 based on the 2-D polygon method of Talwani (1965). For the purpose of modeling, the 28 29 sedimentary strata and the basement structure known from Roy (1983) and Roy (1992) have 30 31 been used. Based on the seismic and well correlations, two E-W seismo-geologic sections were 32 33 prepared by these workers across the Andaman Islands, and relevant parts of these sections were 34 35 projected along profile CC’ (after Roy, 1992) and profile DD’ (after Roy, 1983). The modeling is 36 carried out along CC’ and DD’ by partly constraining these sections with minor modifications in 37 38 order to fit the observed magnetic anomalies. Below the Andaman Islands, the basement is 39 40 composed of oceanic crust (Curray, 2005) with ophiolite suite of rocks emplaced along the faults 41 42 or thrust sheets within the outer arc accretionary prism. It is known that the ophiolites were 43 44 emplaced as a result of scrapping of oceanic crust during late Cretaceous – Paleocene time (Roy 45 46 et al., 1988; Pal, 2011). This tectonic scenario is used for structural delineation through magnetic 47 anomalies. For this purpose, we considered present-day age of the oceanic crust below the 48 49 Andaman Islands as ~85-90 Ma, and the age of oceanic crust (ophiolites) at the time of 50 51 emplacement as ~120 Ma. For these two ages, we obtained paleo latitude of India (Klootwijk et 52 53 al., 1991; Radhakrishna, 2003; Radhakrishna and Mathew, 2012) which then provided us the 54 55 remnant magnetization (paleo inclination and declination) required for magnetic modeling. The 56 parameters used for the modeling that include the magnetization, inclination and declination are 57 58 given in Table 1. Magnetic anomaly highs having amplitude of about 100-300 nT have been 59 60 interpreted as ophiolite / mafic rocks that are constrained to be at the ground surface/subsurface 61 62 63 6 64 65 1 2 3 4 and extend to a depth of about 5-8 km. Magnetization intensities were adjusted to match the 5 6 observed magnetic anomaly. High magnetization is observed in the middle Andaman Island 7 8 region that could be related to mafic / ultramafic intrusions in this region. The resultant models 9 10 are shown in Fig. 5. 11 12 13 14 4. Results and Discussion 15 16 The present study clearly brought out strong magnetic anomaly signatures over the 17 18 ophiolite bodies along the eastern margin of Andaman Islands and modeling of these anomalies 19 20 revealed their extension to depths of about 5-8 km. The seismo-geologic sections across middle 21 22 and south Andaman Islands prepared by Roy (1983; 1992) revealed an accretionary prism (outer 23 24 arc ridge) structure formed by imbricate thrusting of east-dipping fault slices (along with 25 Ophiolites intrusions); linked to westward shifting of the subduction zone. The electromagnetic 26 27 induction (Magnetotelluric and Geomagnetic Depth Sounding) studies over the islands also 28 29 brought out anomalous conductivity zones in the eastern part of Andaman Islands and were 30 31 related to emplacement of N-S trending ophiolites (Gokarn et al., 2006; Subba Rao, 2011). 32 33 34 35 Previous geological mapping of the Andaman Ophiolites revealed that they occur as 36 rootless, sub horizontal bodies with overlying Eocene-Oligocene flysch sedimentary rocks 37 38 uplifted and emplaced by a series of east dipping thrusts (Sengupta et al., 1990; Pal et al., 2003a; 39 40 Srivastava et al., 2004; Ghosh et al., 2013). These ophiolite slices show thrust contacts with the 41 42 surrounding sediments (Roy, 1992). While, the ophiolites are exposed as small patches in the 43 44 western part of Andaman Islands, in the east, they occur as N–S to NE–SW-trending 45 46 discontinuous slices having tectonic contacts with underlying sediments indicating thrust- 47 controlled emplacement in an accretionary environment (Pal et al., 2003a; Pal, 2011). It is 48 49 suggested that these ophiolites were emplaced as a result of subduction; ocean crust (ophiolite) 50 51 of the subducting slab was scraped off and emplaced as a thrust slice. Our modeled profiles in 52 53 this study support such emplacement. With continuous subduction from the Cretaceous onwards, 54 55 the ophiolites were emplaced along a series of thrust slices (Sengupta et al., 1990; Pal et al., 56 2003a). The convergence of India and Burma during late Eocene possibly enhanced uplift of 57 58 ophiolite and metamorphic rocks and caused extensive deformation in the accreted sediments 59 60 (Pal et al., 2003a). However, SHRIMP age data of the plagiogranites in the Andaman ophiolites 61 62 63 7 64 65 1 2 3 4 gave rise to 93.6 Ma (Sarma et al., 2010), which suggest that arc magmatism was initiated much 5 6 earlier than the Eocene accretion of the ophiolites (Sengupta et al., 1990). These ophiolites 7 8 preserve the mantle sequence, layered ultramafic–mafic rocks, intrusive, and extrusive rocks 9 10 (Pal, 2011). Geochemical studies indicate that the ophiolite bodies show interplay of mantle- 11 12 crustal processes and were derived from a boninitic parental melt (Pal et al., 2003b; Jafri and 13 Sheikh, 2013). This melt represents the crystallization of ophiolites through differentiation and 14 15 assimilation reactions of andesitic rich magma containing iron rich material and derived from 16 17 multiple parental magmas. Thus magnetic highs are observed over ophiolite bodies. High 18 19 magnetization observed in the middle Andaman region along Profile CC’ may be due to highly 20 21 magnetized thick mafic/ultra mafic rocks (volcanic sediments) present in this region. Dhana Raju 22 23 and Negi (1984) opined that these ophiolite rocks might have been emplaced through the EMF. 24 25 26 5. Conclusions 27 28 29 Total magnetic field measurements carried out for the first time over the Andaman 30 31 Islands revealed the following geological information: 32 33 i) Magnetic anomalies are in general smooth in the sedimentary region, however, along the 34 35 east coast of north, middle and south Andaman islands, significantly high amplitude 36 magnetic anomalies occur over the mafic / ultramafic cumulates of ophiolite bodies. 37 38 ii) 2-D modeling of magnetic anomalies along selected profiles across the islands indicate 39 40 that the ophiolite bodies extend to a depth of about 5-8 km and spatially correlate with the 41 42 mapped fault / thrust zones. 43 44 Acknowledgements 45 46 47 The authors are grateful to the Department of Science and Technology (DST), New Delhi 48 for the financial support under the research project (ESS/16/346/2008) sanctioned to Prof. M 49 50 Radhakrishna, IIT Bombay. We also thank Mr. Rishikesh, Mr. Rajubabu and the officers of the 51 52 DST, Andaman & Nicobar Administration for the excellent logistic support. Critical comments 53 54 from two anonymous reviewers helped to improve the manuscript. 55 56 57 References 58 59 Acharyya S K 2007 Collisional emplacement history of the Naga-Andaman ophiolites and the 60 61 position of the eastern Indian suture; J. Asian Earth Sci. 29 229–242. 62 63 8 64 65 1 2 3 4 5 6 Bandopadhyay P C 2005 Discovery of abundant pyroclasts in the Namunagarh Grit, South 7 8 Andaman: Evidence for the arc volcanism and active subduction during the Paleogene in the 9 10 Andaman Sea; J Asian Earth Sci. 25 95–107. 11 12 13 Chatterjee P K 1967 Geology of the main Islands of the Andaman area; Proc. Symp. Upper 14 15 mantle project, Hyderabad, India, 348 – 362. 16 17 18 19 Cochran J R 2010 Morphology and tectonics of the Andaman Forearc, northeastern Indian 20 21 Ocean; Geophys. J. 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India 14 1 – 13. 44 45 Gahalaut, V K, Jade, S, Catherine, J K, Gireesh, R, Ananda, M B, Dileep Kumar, P Narsiah, M 46 47 Jafri, S H, Ambikapathy, A, Bansal, A, Chadha, R K, Gupta, D C, Nagarajan, B and Kumar, S 48 49 2008 GPS measurements of postseismic deformation in the Andaman-Nicobar region following 50 51 the giant 2004 Sumatra-Andaman earthquake; Journal Geophysical Research 113 B08401 - 52 53 08418. 54 55 Ghosh B, Morishita T and Bhatta K 2013 Significance of chromian spinels from the mantle 56 57 sequence of the Andaman ophiolite, India: paleogeodynamic implications; Lithos, 164-167, 86- 58 59 96. 60 61 62 63 9 64 65 1 2 3 4 Gokarn S G, Gupta G, Dutta S, and Hazarika N, 2006 Geoelectric structure in the Andaman 5 6 Islands using magnetotelluric studies; Earth, Planets and Space 58 259–264. 7 8 9 Hamilton W 1979 Tectonics of the Indonesian region; U.S.G.S Prof. Paper 1078 345. 10 11 12 Jafri, S H., Sarma, D S and Sheikh, J M 2010 Hyaloclastites in pillow basalts, South Andaman 13 Island, Bay of Bengal, India; Current Science 99 1825-1828. 14 15 16 17 Jafri S M and J M Sheikh 2013 Geochemistry of pillow basalts from Bompoka, Andaman– 18 19 Nicobar islands, Bay of Bengal, India; J. Asian Earth Sci. 64 27–37. 20 21 22 23 Jarvis A, H I Reuter, A Nelson, and E Guevara, 2008 Hole-filled SRTM for the globe Version 4, 24 25 available from the CGIAR-CSI SRTM 90m Database: http://srtm.csi.cgiar.org 26 27 Kamesh Raju, K A, Ramprasad, T, Rao, P S, Ramalingeswara Rao, B, and Varghese, J 2004 28 29 New insights into the tectonic evolution of the Andaman basin, northeast Indian Ocean; Earth 30 31 Planet. Sci. Lett. 221 145-162. 32 33 34 35 Klootwijk C T Gee J S Peirce W and Smith G M 1991 Constraints on the India-Asia 36 convergence: Paleomagnetic results from Ninetyeast ridge; Proc. Ocean Drilling Program, 37 38 Scientific Results 121 771-882 39 40 41 42 Mukhopadhyay M 1984 Seismotectonics of subduction and back arc rifting under the Andaman 43 44 Sea; Tectonophys. 108 229 – 239 45 46 Mukhopadhyay B, Chakraborty P P, and Paul S 2003 Facies clustering in turbidite successions: 47 48 case study from Andaman Flysch Group, Andaman Islands, India; Gondwana Research 6 918– 49 50 924. 51 52 53 Narayana A C 2011 Tectonic geomorphology, tsunamis and environmental hazards: reference to 54 55 Andaman-Nicobar Islands; Nat Hazards. 57 65–82 56 57 58 59 60 61 62 63 10 64 65 1 2 3 4 Pal T, Chakraborty P P, Gupta T D and Singh C D 2003a Geodynamical evolution of the outer- 5 6 arc-forearc belt in the Andaman Islands, the central part of the Burma and Java Subduction 7 8 complex; Geol. Mag.140 289-307. 9 10 11 12 Pal T, Chakraborty P P and Gosh R N 2003b PGE distribution in chromite placiers from 13 Andaman ophiolite and its boninitic parentage; Jour. Geol. Soc. India 62 671-680. 14 15 16 17 Pal T 2011 Petrology and geochemistry of the Andaman ophiolite: melt–rock interaction in a 18 19 suprasubduction-zone setting; J Geol Soc. Lond 168 1031–1045 20 21 22 23 Pandey J, Agarwal P P, Dave A, Maithani A, Trivedi K B, Srivastava A K and Singh D N 1992 24 Geology of Andaman; Bull. ONGC India 29 19–103. 25 26 27 28 Radhakrishna M, Lasitha S, and Mukhopadhyay M 2008 Seismicity, gravity anomalies and 29 30 lithospheric structure of the Andaman arc, NE Indian Ocean; Tectonophysics 460 248-262. 31 32 33 34 Radhakrishna T 2003 Modeling of Lithospheric Evolution Using Geophysical Method: The 35 evidenece from Paleomagnetic Studies; Mem Geol.Soc. India 53 247-278 36 37 38 39 Radhakrishna T and J Mathew 2012 Geochemistry and Paleomagnetism of late Cretaceous 40 41 mafic dykes in Kerala, southwest coast of India in relation to large igneous provinces and mantle 42 43 plumes in the Indian Ocean region; Geol. Soc. America Bulletin 124 240-255 44 45 46 Ray K K 1985 East Coast Volcanics – A new suite in the ophiolite of Andaman Islands; Rec. 47 48 Geol. Surv. India 116 83–87. 49 50 51 52 Ray K K, Sengupta S, and Van Den Hul H J, 1988 Chemical characteristics of volcanic rocks 53 54 from Andaman ophiolite, India; J. Geol. Soc. Lond. 145 393–400. 55 56 57 Rodolfo K S, 1969 Bathymetry and marine geology of the Andaman basin, and tectonic 58 59 implications for Southeast Asia; Geol. Soc. America Bulletin 80 1203–1230. 60 61 62 63 11 64 65 1 2 3 4 Roy D K, Acharyya S K, Ray K K, Lahiri T C, and Sen M K 1988 Nature of occurrence and 5 6 depositional environment of the oceanic pelagic sediments associated with the Ophiolite 7 8 assemblage, South Andaman Island; Indian Minerology 42 31-56 9 10 11 12 Roy S K 1992 Accretionary prism in Andaman Forearc; Geol. Surv. Ind. Spl pub. 29 273-278 13 14 15 Roy S K and Das Sharma S 1993 Evolution of Andaman foprearc basin and its hydrocarbon 16 17 potential; Proc. 2nd Seminar on Petroliferous basins of India, Vol. 1 409-435. 18 19 20 21 Roy T K 1983 Geology and hydrocarbon prospects of Andaman–Nicobar Basin; Petroleum Asia 22 23 Jour 1 37-50. 24 25 Rukstales, K.S., and Love, J.J., 2007, The International Geomagnetic Reference Field, 2005: 26 27 U.S. Geological Survey Scientific Investigations Map 2964, 5 pls., scale 1:30,000,000. 28 29 30 Sarma, D S, Jafri, S H, Fletcher, I A and McNaughton, N J 2010 Constraints on the tectonic 31 32 setting of the Andaman ophiolites, Bay of Bengal, India, from SHRIMP U-Pb Zircon 33 Geochronology of plagiogranite; Journal of Geology 118 691-697. 34 35 36 Sengupta S, Ray K K, Acharyya S K and de Smeth J B 1990 Nature of ophiolite occurrences 37 38 along the eastern margin of the Indian plate and their tectonic significance; Geology 18 439-442. 39 40 41 42 Srivastava R K, Chandra R, and Shastry A 2004 High-Ti type N-MORB parentage of basalts 43 from the south Andaman ophiolite suite, India; Proc. Indian Acad. Sci. (Earth Planet. Sci.) 113 44 45 605–618. 46 47 48 49 Subba Rao P B V 2011 Frequency Characteristics of Geomagnetic Induction Anomalies in 50 51 Andaman Islands; Mem. Geol. Soc. India 77 531-547. 52 53 54 Talwani 1965 Computation with the help of a digital computer of magnetic anomalies caused by 55 56 bodies of arbitrary shape; Geophysics 30(5) 797-817. 57 58 59 60 Weeks L A, Harbison R N, and Peter G, 1967 Island arc system in the Andaman Sea; American 61 62 Association of Petroleum Geologists Bulletin 51 1803–1815. 63 12 64 65 1 2 3 4 5 Figure Captions 6 7 Fig.1. Tectonic map of the Andaman arc and the Andaman Sea region (modified after Curray, 8 9 2005). The tectonic sub-divisions of the region (after Roy and Das Sharma, 1993; 10 11 Kamesh Raju et al., 2004) are indicated in 1-5. These are: 1. Outer slope, 2. Inner slope, 12 13 3. Trench slope Break, 4. Forearc basin, 5. Volcanic arc and the backarc basin, EMF- 14 Eastern Margin Fault, DF-Diligent Fault, CF- Cocos Fault, WAF-West Andaman Fault, 15 16 WSR- West Sewell Rise, ASR- Andaman Spreading Ridge (after Kamesh Raju et al., 17 18 2004), OCT- Ocean Continent Transition. The grey shaded area represents the extent of 19 20 Burma plate (after Curray, 2005). 21 22 Fig.2. (a) Shaded relief map of the Andaman Islands from the SRTM data showing the 23 24 topographic variations and prominent linear ridges, (b) Geological map of the Andaman 25 Islands (after Pal et al., 2003a) along with locations of magnetic measurement points 26 27 occupied in three different phases of field surveys. The magnetic anomaly maps prepared 28 29 for the three areas a-c are indicated on this map, (c) Tectonic map of the Islands and the 30 31 adjoining Andaman basin region (modified after Pandey et al., 1992: Curray, 2005). 32 33 Fig.3. Topography (left panel), geology (middle panel) and the magnetic anomaly (right panel) 34 35 maps prepared for the A) north Andaman Islands (Area A), B) middle Andaman Islands 36 (Area B), and C) the south Andaman Islands (Area C). Details are discussed in the text. 37 38 Fig.4. Magnetic anomalies plotted along four selected profiles (AA’ through DD’) across the 39 40 Andaman Islands. Surface geology along these profiles is presented along with the 41 42 topography in the bottom panel for each profile. 43 44 Fig.5. 2-D modeling of magnetic anomalies along the profiles located in the north Andaman 45 46 (profile AA’), middle Andaman (profile CC’), and the south Andaman (profile DD’) 47 Islands region. The models reveal the presence of highly magnetized bodies related to 48 49 ophiolites and ultramafic rocks along the east coast of Islands. 50 51 Table 1. Magnetic parameters used in the 2-D magnetic modeling for the oceanic crust and the 52 53 emplaced ophiolites. Details are discussed in the text. 54 55 56 57 58 59 60 61 62 63 13 64 65 Figure 1-5 R2

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Table 1

Table 1

Oceanic Crust Ophiolites

Profile Susceptibility Remanent Magnetic Magnetic Susceptibility Remanent Magnetic Magnetic (cgs) Magnetization Inclination Declination (cgs) Magnetization Inclination Declination (emu/cc) (emu/cc) 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