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Magnetic Anomalies Over the Andaman Islands and Their Geological Significance

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Magnetic Anomalies Over the Andaman Islands and Their Geological Significance Magnetic anomalies over the Andaman Islands 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 410 218, India. 2Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India. ∗Corresponding author. e-mail: [email protected] 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 with surface geology of the islands. The residual total field magnetic anomaly maps have revealed distinct magnetic anomalies having intermediate to high amplitude magnetic signatures and correlate with the areas over/close to the exposed ophiolite rocks along the east coast of north, middle and the south Andaman Islands. The 2D modelling 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. 1. Introduction 1984). Several earlier investigators have studied the structure and deformation along this active The major morpho-tectonic elements (Weeks et al. plate boundary zone (e.g., Gahalaut et al. 2008; 1967) running N–S along the Andaman arc are Radhakrishna et al. 2008). According to Hamilton the Andaman trench, Andaman–Nicobar ridge (1979), the Andaman arc region forms an impor- (Andaman–Nicobar Islands), Nicobar deep, West tant transitional tectonic link between the Burma– Andaman fault (WAF), Andaman Spreading Ridge Himalayan arc/collision system in the north and and the Mergui terrace (figure 1). The Andaman– the Indonesian arc system in the south. The Nicobar Islands form a major part of the outer- main rock types occurring over the islands are arc sedimentary complex that stretches nearly 1100 either of sedimentary or magmatic. The mag- km along the Andaman trench-arc system (Curray matic rocks constitute the ultramafic plutonites 2005), and separate the Andaman back-arc basin and basalt, together with minor radiolarian chert, from the Bay of Bengal. Along the western mar- and this forms the Andaman ophiolite suite (Pal gin of the islands, the sediments of the Bengal Fan et al. 2003a). Owing to strong magnetization have been deformed below the Andaman trench contrast between the ophiolites and sedimentary (Curray et al. 1982). The Andaman arc is char- rocks, magnetic method would be an effective tool acterized by the presence of east dipping Benioff for investigating the subsurface structure of the zone down to about 200 km depth (Mukhopadhyay ophiolite 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. 359–368 c Indian Academy of Sciences 359 360 PBVSubbaRaoetal. 15 Bay of R 14 Bengal West T Basin mGal CF . h 13 s F c I 120 n n 100 F S e a r D 80 m A T a e 60 c 12 d Alcock a 40 n rise r p r A C e 20 W e n e T 0 D A a r -20 11 a m b a -40 o d c i i -60 n N u -80 A g Invisible Bank 10 r e -100 F M M -120 E O -140 9 -160 -180 R -200 S W -220 8 Ninetyeast Ridge 7 90 91 92 93 94 95 96 97 98 Figure 1. Tectonic map of the Andaman arc and the Andaman Sea region (modified after Curray 2005). The tectonic subdivisions 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: 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. Grey shaded area represents the extent of Burma plate (after Curray 2005). magnetic data was not acquired over the onshore groups in the Andaman Islands (figure 2b), and part of the Andaman Islands. So, for the first they are: time, we carried out a detailed magnetic survey (i) Cretaceous to Paleocene Ophiolite suite, over the main Andaman Islands with an objec- (ii) Lower to Middle Eocene Mithakhari Group, tive to understand the geometry of the ophiolite (iii) Oligocene to Late Eocene Andaman turbidities bodies in the subsurface as well as their associa- (Andaman Flysch Group), and tion with the major fault systems over the islands. (iv) Mio-Pliocene Archipelago Group (Ray 1985; Salient results of this investigation are presented in Ray et al. 1988; Pal et al. 2003a; Bandopadhyay this paper. 2005). The rock types mainly constitute serpentinites, ophiolites with radiolarian cherts, cherty pelagic 2. Geology of the Andaman Islands limestone and the flysch sediments (Chatterjee 1967; Eremenko and Sastri 1977; Roy 1983). Two The topographic shaded relief map of the Andaman prominent N–S trending faults/thrusts have been Islands (figure 2a) using the Shuttle Radar Topog- observed over the Andaman Islands (figure 2c). raphy Mission (SRTM) database (Jarvis et al. Among these two, Jarwa thrust is the most sig- 2008) reveals the sub-areal expression of the nificant morpho-tectonic feature (figure 2a) trend- islands with many geomorphologic features and ing N–S all along the main islands (Eremenko and linear ridges (Narayana 2011). Regional geo- Sastri 1977), and the second important fault is logical mapping of the Andaman Islands was the Eastern Margin Fault (EMF) in the immediate mostly carried out by the GSI and ONGC. The Andaman offshore along its east coast (Roy 1983). Andaman–Nicobar Islands have formed during the The EMF acts as a boundary between the outer-arc Oligo–Miocene times (Rodolfo 1969). The rock for- ridge and the fore-arc basin (Roy 1983; Cochran mations can be broadly classified under four major 2010). 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 and prominent linear ridges, (b) geological map of the Andaman 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–c are indicated on this map, (c) tectonic map of the islands and the adjoining Andaman basin region (modified after Pandey et al. 1992: Curray 2005). The ophiolite rocks in the Andaman Islands ophiolite rocks occur in N–S to NE–SW trending are interpreted as remnants of oceanic crust and bodies. These ophiolite rocks have contact with upper mantle that were obducted by complex tec- underlying sediments and denote thrust-controlled tonic processes during the Miocene times, and emplacement in an accretionary environment (Pal these are observed along N–S trending linear belts et al. 2003a, b). Petrographically, ophiolites con- in structurally weak zones (Roy 1983; Acharyya sist of serpentinite, dunite, harzburgite, pyrox- 2007). The ophiolite group of rocks consisting of enite, olivine gabbro, anorthositic gabbro and metamorphic, m´elange and ophiolitic rocks with gabbroic anorthosite (Dhana Raju and Negi 1984; pelagic sediments occur as dismembered bodies Jafri et al. 2010). (Pal et al. 2003a). The eastern part of Andaman Islands is characterized by highly deformed ophi- olite rocks from the oceanic floor as well as ultra- 3. Magnetic data acquisition and analysis basic/volcanic/pelagic sediments along with older metamorphics, whereas, in the western part, sedi- 3.1 Magnetic surveys ments belong to the accretionary prism consisting of flysch-sandstone/siltstone with conglomerates As mentioned earlier, for the first time, ground and calcareous sediments (Mukhopadhyay et al. magnetic surveys were carried out over the main 2003). Ophiolites are exposed in small isolated Andaman Islands in different field seasons span- patches in the west, whereas, in the east, most ning the period February 2007–December 2009. 362 PBVSubbaRaoetal. Figure 3. Topography (left panel), geology (middle panel) and the magnetic anomaly (right panel) maps prepared for (a) north Andaman Islands (Area A), (b) middle Andaman Islands (Area B), and (c) south Andaman Islands (Area C). Details are discussed in the text. Magnetic anomalies over the Andaman Islands 363 The total magnetic intensity measurements were the exposed ophiolite bodies, the anomaly H2 could made with a portable proton precession magne- be related to mafic/ultramafic rocks surrounded tometer of GEM System, Canada with a sensor by the obducted sediments. The topography map height of 1.6 m from the ground. We have placed shows that height of saddle peak varies between the observation points at intervals of 1000 m, and 600 and 800 m. in some places, where we found bare rock out- crops of the ophiolite; the points were picked up at (ii) Area B: Middle Andaman surrounding Rangat 500 m interval. The total field magnetic anomaly The magnetic anomaly map (figure 3b) covers the was obtained at each observation point by sub- middle Andaman Islands from Bartang to Pan- tracting the International Geomagnetic Reference chavati area. The magnetic anomaly high (H3) is Field (IGRF) 2005 model (Rukstales and Love located near Panchavati over the ophiolite bodies 2007) from the observed data. Diurnal variation of as observed from the surface exposure. Intermedi- the geomagnetic field was corrected by using total ate magnetic highs in this region could be related geomagnetic intensity data computed using the to mafic/ultra mafic rocks within the sediments.
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