Introduction

Andaman-Nicobar-Sumatra subduction process mainly involves three major plates such as India-Australia, Burmese and Sunda Plate. Andaman Sea is referred as an extensional backarc basin located in the Andaman-Nicobar-Sumatra subduction zone in the Indian Ocean, which formed due to the extensional force associated with the subduction (Kamesh Raju et al. 2004, 2020; Curray 2005). Andaman backarc region consists of main tectonic features such as sliver fault system, volcanic arc and Andaman Backarc Spreading Centre (ABSC, Fig. 1) (Kamesh Raju et al. 2004; Curray 2005). The volcanic arc chain extending from inactive volcanoes in the Myanmar, Narcondam and Barren Island in the north to the Sumatra arc in the south (Curray 2005; Kamesh Raju et al. 2012a; Tripathi et al. 2017). The subduction of the Indo-Australian Plate beneath the Sunda plate which led to the melting of the subducted crust and melted magma rises to the surface through fissures and fractures, resulting in the formation of chain of volcanoes in the Andaman-Nicobar-Sumatra subduction zone (Curray 2005; Kamesh Raju et al. 2012a, 2020).

Fig. 1
figure 1

Satellite derived free-air gravity anomaly map of Andaman Nicobar Subduction Zone and the adjacent areas (Sandwell et al. 2021). The dashed black lines represent the major faults (Kamesh Raju et al. 2004, 2007; Tsutsumi and Sato 2009; Cochran 2010; Singh et al. 2013). The black bordered red triangles represent the subaerial volcanoes (Sheth et al. 2009; Bandopadhyay and Carter 2017). The thick red lines indicate the Andaman Backarc Spreading Centre (Kamesh Raju et al. 2004). The black stars represent the locations of major earthquakes of 2004 and 2012. ABSC: Andaman Backarc Spreading Centre; NI: Narcondam Island; BI: Barren Island; EMF: Eastern Margin Fault; DLF: Diligent Fault; ANF: Andaman-Nicobar Fault; WAF: West Andaman Fault; ACF: Aceh Fault; SF: Seulimeum Fault; GSF: Great Sumatra Fault; MNF: Mentawai Fault; ATF: Andaman Transform Fault; SGF: Sagaing Fault. Thick black dashed rectangle represents the study area

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The trench-parallel motion occurring due to the oblique subduction between the Indo-Australian plate and Sunda plate is largely accommodated by the sliver fault system (Fitch 1972; Curray 2005; McCaffrey 2009; Singh et al. 2013; Moeremans and Singh 2015). The submarine volcanic arc and sliver fault system present in the Andaman Sea play a major role in the stress distribution and occurrence of earthquakes in the backarc region. The sliver fault system includes the Sagaing Fault (SGF) in the Myanmar, which is connected through Andaman Transform Fault (ATF) to ABSC in the middle, and Andaman Nicobar Fault (ANF), West Andaman Fault (WAF) and the Great Sumatra Fault (GSF) in the south (Fig. 1) (Curray 2005; Diehl et al. 2013; Singh et al. 2013). In the backarc of the Andaman Sea, the boundary between the sliver fault, Burma Plate, and Sunda Plate comprises a network of arc-parallel transforms and arc-normal ridges. This boundary extends to the right-lateral Sumatra fault along the volcanic arc in the southwest. The association with the Sagaing Fault to the northeast appears less definitive, with various proposed geometries (Rangin et al. 1999; Curray 2005). As the Burma Plate is pulled northward relative to the Sunda Plate by the subducting Indian-Australian plate, "pull-apart" basins emerge along the plate boundary, leading to northeast-southwest extension of the Andaman Sea (Curray 2005; McCaffrey 2009). While "pull-apart" typically denotes intracrustal extension along a strike-slip system, the extension in the Andaman Sea involves the genesis of new crust (Kamesh Raju et al. 2004), supporting the identification of the sliver as a distinct plate. This mode of transtensional backarc opening is also known as "rhombochasm" (Rodolfo 1969; Curray 2005) or "leaky-transform" (Thompson and Melson 1972; Uyeda and Kanamori 1979; Taylor et al. 1994). SGF is mainly a continental right lateral strike slip fault extended from Myanmar to the northern Andaman Sea (Bertrand and Rangin 2003; Htet et al. 2022; Diehl et al. 2013). Southern end of this SGF connected to the ABSC with a large transform fault ATF (Singh and Moeremans 2017; Yatheesh et al. 2021). ANF is the main strike-slip fault lying north–south direction referred as a boundary between Andaman forearc and backarc basins (Cochran 2010; Singh et al. 2013). GSF is approximately 1900 km long strike-slip fault that extends from the northwestern part of Sumatra, Indonesia, to the Nicobar Islands in the Andaman Sea (Sieh and Natawidjaja 2000; Ghosal et al. 2012). GSF has indeed been recognized as an active fault since 2 Ma (Sieh and Natawidjaja 2000). WAF is situated south of the Nicobar Island considered as a back thrust fault (Kamesh Raju et al. 2007; Singh et al. 2013). In the off Nicobar region, the ANF, WAF and GSF meet at a junction where the closely spaced seamount chain is seen forming a part of the volcanic arc. This region is witnessing intense seismicity with the frequent occurrence of earthquake swarms in 2005, 2014, 2015 and 2019. Recent studies suggested that the magma movement from deeper reservoir to shallow magma chamber, triggered the reactivation of sliver fault system and cause the occurrence of earthquake swarm (Aswini et al. 2020, 2021, 2022).

Earlier research indicated that while there are active volcanoes on land in Sumatra and Java, as well as extinct and dormant volcanoes in Myanmar (Chhibber 1934; Rock et al. 1992; Zaw et al. 2017; Acocella et al. 2018; Belousov et al. 2018; Hariyono and Liliasari 2018), the Andaman Sea was considered to have a lack of active submarine volcanoes. Recent studies identified the presence of submarine volcanoes in the Andaman Sea (Kamesh Raju et al. 2004, 2012a; Tripathi et al. 2017). Tripathi et al. 2017 identified 22 submarine volcanoes in the Andaman Sea. A combined morphotectonic analysis of the entire volcanic arc and its adjacent regions are still awaiting. In this present study, we have attempted to compile all the available high resolution bathymetry data which covered the entire inner volcanic arc of the Andaman Sea. The data allow us to identify more localized geomorphological features such as submarine volcanoes, fault systems, basins, and ridge-like features in the Andaman backarc region. Thereby we provide detailed morphotectonic analysis of these elements to improve our understanding of the dynamics of the Andaman arc volcanism.

Data and methodology

The high-resolution multibeam data acquired by CSIR- National Institute of Oceanography are used for the morphological interpretation of the study area. The presented data is the compilation all the data available with CSIR-NIO. New data are from 4 cruises, SK-89 in 1994, SSK-33 in 2012, SSK-49 in 2013 carried out onboard RV Sindhu Sankalp and SSD-046 in 2018 carried out on board RV Sindhu Sadhana are included. RV Sindhu Sankalp is equipped with Kongsberg EM302 multibeam echosounder operating at a frequency of 30 kHz. The multibeam system onboard RV Sindhu Sadhana was operated at 15 kHz frequency during the acquisition. Navigational errors resulting from the vessel movement during the survey were corrected. We collected sound velocity profiles during the survey to correct the variation of sound velocity in seawater. The multibeam data of the Andaman backarc basin were from the SK89 expedition (Kamesh Raju et al. 2004) and SSD046 expedition (Yatheesh et al. 2021), and the data from the Sumatra region were sourced from Graindorge et al. 2008. The multibeam raw data was processed using MB system. The final processed data was extracted into ASCII format and then converted into 200 m spatial resolution bathymetric grid using Generic Mapping Tool (GMT) software. This high-resolution bathymetric grid was used for the interpretation. In addition, a ~ 90 km long multi-channel seismic reflection profile (AN08Z06) in the off Nicobar region, provided by Directorate General of Hydrocarbon (DGH), India has been interpreted to understand the sub-seafloor characteristics.

Results

The multibeam bathymetry data collected in the volcanic arc region has been compiled to make a high-resolution bathymetry map of the study area (Fig. 2). The map covers an area of ~ 1,40,000 km2 between 4°N and 14°N and depicts detailed geomorphology of the study area. The major features comprise of regional fault systems, submarine volcanic arc defined by a chain of volcanoes, and narrow basins. These are discussed in detail in the following sub-sections.

Fig. 2
figure 2

a High-resolution multibeam bathymetry data of Andaman Backarc Basin and Sumatra region. The multibeam bathymetry data provided in Kamesh Raju et al. 2004, Graindorge et al. 2008, Yatheesh et al. 2021 are also compiled for this map. The dashed black lines represent the major faults, and the black outlined triangles indicate the submarine volcanoes identified from the bathymetry. The thick red lines indicate the Andaman Backarc Spreading Centre. ABSC: The black bordered red triangles represent the subaerial volcanoes (Sheth et al. 2009; Bandopadhyay and Carter 2017). ANF: Andaman-Nicobar Fault; WAF-1 and 2: two strands of West Andaman Fault; ACF: Aceh Fault; SF-1, 2 and 3: three strands of Seulimeum Fault. b Enlarged bathymetry image depicts the Andaman Nicobar Fault (ANF) located in the deep basin. c Enlarged bathymetry image of Aceh Basin. d Enlarged bathymetry image of Weh Basin. e Enlarged bathymetry image of Breueh Basin

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Fault systems, basins and ridge-like features

The configuration of Andaman forearc and backarc system is mainly modulated by the formation of sliver fault systems present in this region. The SGF, ATF, ABSC, ANF, WAF and GSF are the major fault and spreading center systems in the Andaman-Nicobar-Sumatra Subduction and backarc region, played a significant role in modulating the strain partitioning along the trench parallel direction. The high-resolution bathymetric map of the study region (Fig. 2) shows a notable contrast in the orientations of fault systems and the spatial layout of narrow basins. While the fault systems e.g., WAF, ANF and ATF are predominantly oriented in a near N-S trend, whereas GSF and its offshore branches exhibit a distinct SE-NW orientation. The bathymetry data revealed the fault patterns that present in the off Nicobar region, where Seulimeum Fault (SF) and Aceh Fault (ACF) of the Sumatra Fault joined with the WAF and ANF creating a triple junction approximately around 7°N (Fig. 2a). The convergence of these fault systems give rise to an intricate pattern of deformation in the surrounding area of Nicobar Island. The triple junction area is characterized by the presence of uplifted ridges at the basin's centre, making it challenging to precisely determine the connections of the strike-slip faults.

The ANF, has a generally north–south orientation to the north of Nicobar Island and is located in a narrow basin in the south that is 7 km wide around 8°N, and the rift basin deepens to approximately 4000 m and widens to about 10–20 km towards the northern portion between 8.6°N to 10°N (Fig. 2b). Its western boundary is defined by a steep marginal fault, while its eastern boundary is marked by a sedimented ridge (Fig. 2a).

A north–south trending, 50 km long, 7 km wide, elongated ridge-like feature, with its summit area reaching up to 500 m from nearby region of 2000 m water depth, is observed between ANF and the northern part of WAF at around 7°30’N (black rectangular block A in Fig. 2a). Serpentinites recovered from the eastern cliff of this block suggest mantle origin for this feature (Kamesh Raju et al. 2012b). Similar kind of two elongated ridge-like features are seen north of the above mentioned block, which is linearly configured, but not continuous (black rectangle boxes B and C in Fig. 2a).

The seismic profile (Figs. 3 & 4) passing through ~ 8°N (Fig. 3a) with NNE-SSW trend shows 7 km wide narrow basin present at the base of a 1200 m wide deep scarp. The sedimentary layers in the basin reach a maximum thickness of 600 m and display characteristics of deformation. Towards the northeastern part of this basin, a sedimented small push-up ridge (Figs. 3c & d) is present. Below the ridge feature, exists a thrust fault that dips to northeast, forming the boundary of the narrow basin. This basin, in turn, is bordered on its northeast side by a normal fault that dips to the southwest. The gravity profile shows low gravity up to -40 mGal along the deep scarp and narrow valley (Fig. 3b). Seen together with the bathymetry data, we can interpret that the southern tip of the ANF is lying in this narrow basin. Further northeast a volcanic cone and then a small valley bounded by thin sediment capped ridges are seen (Fig. 3c).

Fig. 3
figure 3

a Location map of seismic section AN08Z06. Black dashed line is the seismic line location and pink line represent the location of zoomed seismic section shown in Fig. 4. b Gravity anomaly(red) and bathymetry (blue) over the seismic profile. c Seismic reflection image of AN08Z06. d Enlarged image of push-up ridge and basin in the west. e Enlarged seismic image of deep valley in the east

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Fig. 4
figure 4

a Seismic reflection section of AN08Z06 (Location shown as pink line in Fig. 3a), (b) Interpreted Seismic reflection section of AN08Z06

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South of 7°N the WAF is bifurcated into two faults WAF-1 and WAF-2, and these strands make a deep graben like feature which is called as Aceh Basin. The Aceh Basin, which has a triangle shape and a length of around 270 km, is bounded to the east by the Sumatra platform and to the west by the fore-arc high of the inner accretionary wedge (Figs. 2a & c). This basin has a width of approximately 10 km in the northern region adjacent to Nicobar Island. As one progresses southward, the width of the basin expands to about 50 km (Fig. 2c).

The GSF originates from the Sumatra mainland and splits into two branches in the offshore region: the western ACF and the eastern SF. Two strands of SF make a deep graben like feature, termed as Weh Basin, and it is approximately 80 km in its width and 115 km in length (Figs. 2a & d). The deeper region in the Weh Basin reaches up to 3600 m water depth and consists of submarine volcanoes. The rhombic shaped Weh Basin is considered to have formed by a releasing step fault related to SF (Ghosal et al. 2012). A rugged elevated topographic feature referred as ‘push-up ridge’ following the same trend of Weh Basin is located between Weh Basin and Aceh fault. Another narrow basin called Breueh Basin (Figs. 2a & e), that is shallow compared to the other basins, located east of the Aceh fault widens towards south.

Sumatra platform is ~ 60 km wide, extends from the southeast of Nicobar Island, located between WAF and ACF, have a gentle topography between 1000 and 2000 m water depth (Fig. 2a). This terminates around 7°N where the WAF and ACF converge. This platform is considered as the offshore continuation of the Sumatra continental block (Singh et al. 2012).

Submarine volcanic arc

The submarine volcanic arc is a major feature in the Andaman Sea that provides a link between the Barren Island and the volcanic system in Sumatra. The ~ 564 km stretch of this submarine volcanic arc between 6°24'N and 11°N has been examined in this study. Within this region, thirty-three different volcanoes of varying heights and sizes have been mapped (Figs. 5, 6, 7, Table 1 and Supplementary figures S1-S5). V1 is the northernmost volcano, with a conical shape and a height of 2155 m (Figs. 5a & b, Table 1, Supplementary figure S1a). Volcano V2, located 26 km to the southeast of V1, has a collapsed top area (cratered) with a total height of 1435 m (Figs. 5a & b, Table 1, Supplementary figure S1b). Volcano V3 has an uneven shape and a SSW-NNE general trend (Figs. 5a & b, Table 1, Supplementary figure S1c). The conical-shaped volcano V4 is located on the flat ocean floor and has a height of 1485 m overall (Figs. 5a & b, Table 1, Supplementary figure S1d). A 2.3 km-diameter formation that resembles a caldera can be found over V4's northern flank. The volcanoes V1 to V4 are situated north of the spreading centre in the Andaman Backarc Basin (Figs. 5a & b). Volcano V5 is a conical-shaped volcano with a height of 1350 m that is situated across an irregular surface, south of the spreading centre (Figs. 5a & b, Table 1, Supplementary figure S2a). Volcano V6 has a roughly conical shape and is in a flat seabed basin (Figs. 5a & b, Table 1, Supplementary figure S2b). V7 is near to V6, which covers an area of 728 km2 (Figs. 5a & b, Table 1, Supplementary figure S2c). V7 is the highest of the identified volcanoes, with a total height of 2455 m and an erratic shape with several summits. Volcanoes V1 to V7 line up with ANF (Fig. 5a). The V1-V4 are located on the northern part of the Segment A of the ABSC and V5, V6 and V7 are in the southern part of the spreading centre.

Fig. 5
figure 5

a High Resolution multibeam bathymetry map of submarine volcanoes V1-V7. b 3D image of the volcanic arc in the north

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Fig. 6
figure 6

a High Resolution multibeam bathymetry map of submarine volcanoes V8-V20. b 3D image of the volcanic arc in the off Nicobar region

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Fig. 7
figure 7

a High Resolution multibeam bathymetry map of submarine volcanoes V21-V33. b 3D image of the volcanic arc in the south towards Sumatra Island

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Table 1 Characteristics of volcanoes (V1-V33) in the Andaman Sea. Table 1 provides the location, height, basal length, basal width, basal area and height-width ratio of each volcano, and distance between adjacent volcanoes
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From the V7 towards the south, there is a conspicuous gap of 169 km in the occurrence of arc volcanoes between 8.4°N to 9.9°N. After this gap, 26 volcanoes are documented in the Off Nicobar region (Figs. 6 & 7, Table 1). This part of the volcanic arc chain has a deep graben shaped structure towards the east of WAF and west of step-like fault system of SF, and it is the northward extension of the Weh Basin. Volcano V8, is located off the Nicobar region at a water depth of 2350 m and rises to 680 m above the surrounding terrain (Figs. 6a & 6b, Table 1, Supplementary figure S3a). V8 exhibits a well-defined crater having a depth of 300 m and a maximum width of ~ 2.5 km (Figs. 6a & 6b, Table 1, Supplementary figure S3a). South of this V8, rugged elevated topographic feature about 15 km in length is present that resembles a push-up ridge like feature. Volcano V9 has a roughly conical shape and is located about 40 km south of volcano V8 and rises to a height of 740 m (Figs. 6a & 6b, Table 1, Supplementary figure S3b). Volcanoes V10 and V11, which are very close by were named cratered seamount (CS) (Kamesh Raju et al. 2012a). These volcanoes are of 1180 m and 1005 m height, respectively, and occupy an area of ~ 90 km2 (Figs. 6a & 6b, Table 1, Supplementary figure S3c). The well-defined breach in the crater (approximately 800 m width and 100 m depth) and the seabed samples from the crater indicate that these volcanoes erupted in the recent geological past.

The seismic line nearest to the crater seamount is depicted in Fig. 4, passing approximately 15 km from the seamount's center. The seismic section near to cratered seamount (V10) shows an existence of wide conical feature spanning 20 km in width, with an elevation reaching up to 800 m in water depth. The sides of this cone display sediment layers up to 100 m in thickness, while the remainder of the cone lacks sediment deposits. The seismic data suggest (Fig. 4b) that the volcanic rocks are buried beneath 50–250 m thick sedimentary layers appearing as parallel semi-continuous layers that warp the volcanic rocks. Notably, a mass-transport deposit, characterized by acoustically transparent layers, is evident between CDPs 43,600 and 44,100, while a small basin fill deposit is observed between CDPs 44,400 and 44,600 (Figs. 4a & 4b). The seismic data also reveal numerous near-vertical dyke intrusions within the volcanic rocks alongside some semi-horizontal intrusions or sills.

The volcanoes V12 to V15 are smaller in size and closely spaced to each other with a distance of ~ 4–5 km and exhibits similar morphology (Figs. 6a & 6b, Table 1). V12, V13, and V14 are all aligned in the same direction, while V15 is located on the eastern side of V14. These 4 volcanoes are situated on the rugged and elevated topography (Figs. 6a & 6b, Table 1, Supplementary figure S3d, S3e). South of V15, four volcanoes (V16-V19) are evenly distributed with a distance of ~ 10–11 km apart from each other. V16's summit is irregularly shaped; unlike other volcanoes, it lacks an overall conical shape. Its flanks likewise have an uneven morphology (Fig. 6a, Table 1, Supplementary figure S3f). The heights of V17 and V18 are 760 m and 820 m, respectively, and both exhibit morphology with a conical shape that is identical (Figs. 6a & 6b, Table 1, Supplementary figure S4a, S4b). V19, which is located at a sea depth of 2500 m and has a height of 210 m, has the shape of a collapsed crater (Figs. 6a & 6b, Table 1, Supplementary figure S4c). The maximum width of the crater is ~ 2.5 km.

Further south, we have identified fourteen distinct volcanoes (V20-V33) in the graben-like depression between 6°24' N and 7°24' N (Figs. 7a & 7b, Table 1). Compared to V8-V19 volcanoes which are situated in the elevated topography in the off Nicobar region, these southern 16 volcanoes are situated in the deeper basin. The volcanoes V20-V23 are conical in shape with steep flanks on either side, situated in the bathymetric depression i.e., the Weh Basin (Fig. 7a). Volcanoes V20-V23 are located at smooth seafloor around 3080 m to 3300 m water depth and possess height ranging from 375 m (V20) to 915 m (V23) with an averaging spacing of 12 km (Figs. 7a & 7b, Table 1, Supplementary figure S4d-g).

Closely spaced volcanoes V24, V25 and V26 are nearly conical in shape having rugged summit area with a total relief of 765 m, 600 m and 900 m respectively (Figs. 7a & 7b, Table 1, Supplementary figure S4h). V27 and V28 are associated with the escarpment of SF and exhibit similar morphology. These exhibit an elongated shape in the NNW-SSE direction with irregular flanks. These volcanoes are separated at a distance of 13 km and exhibit a height of 715 m and 1050 m respectively for V27 and V28 (Figs. 7a & 7b, Table 1, Supplementary figure S4h, S5a). V29 is located in the Weh Basin and has a height of 755 m (Figs. 7a & 7b, Table 1, Supplementary figure S5b). Volcanoes V30-V33 are located close to each other (~ 4–7 km apart) with conical shape situated in the smooth seafloor of Weh Basin (Figs. 7a & 7b, Table 1). V30 having a total height of 690 m exhibits a crater shape at the summit with a maximum width of 630 m which is less compared to the cratered seamounts in the northern region (Supplementary figure S5c). Volcanoes V31-V33 occupies an area of ~ 60 km2 and possess height of 560 m, 855 m and 495 m respectively for V31, V32 and V33 (Figs. 7a & 7b, Table 1, Supplementary figure S5d).

A distinct large gap (169 km) of distance in active volcanoes V7 and V8 between 8.6°N and 9.8°N is quite prominent (Figs. 2 & 8). We have also observed distinct variations in the spacing between the volcanoes and their physical dimensions (Fig. 8). The distance from volcano V1 to V33 versus height and area of the volcanoes clearly shows that the height and base area of the volcanoes V1-V7 are relatively high when compared to the V8-V33 volcanoes. The volcanoes (V8-V33) are closely spaced and smaller in physical dimension (area) when compared to volcanoes (V1-V7) present in the northern sector of the Andaman Sea (Fig. 8). The spacing between the volcanoes is 17 to 38 km for the northern volcanoes (V1-V7) and it is about 9 to 19 km for the southern off Nicobar volcanoes.

Fig. 8
figure 8

The graph showing the distance of volcanoes measured with respective to V1, versus height (red circle indicates height > 1000 m and yellow circle indicates height < 1000 m) and area (green square indicates area > 300 km2 and blue square indicates area < 300 km.2) of each volcano (V1-V33) from north (93.6°E, 11.04°N) to south (94.9°E, 6.43°N)

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Discussion

The Neogene Inner Arc volcanic system in the Andaman-Nicobar-Sumatra subduction zone encompasses a volcanic arc extending from northern extinct volcanoes like Mt. Popa and Mt. Wuntho in Myanmar to southern active volcanoes in the Sumatra through subaerially exposed dormant Narcondam Island, active Barren Island and submarine volcanoes in the Andaman Sea (Haldar et al. 1992; Curray 2005), the total span is over 2000 km long.

Myanmar, located at the northern end of the volcanic arc, has a complicated tectonic setting where highly oblique subduction takes place. This region is characterised by the presence of the Burmese Microplate which separates the Indian plate from the Burma plate and is surrounded by important fault systems, including Churachandpur‐Mao (CM) Fault to the west (e.g., Gahalaut et al. 2013) and Sagaing Fault to the east (Bertrand et al. 1998; Curray 2005; Maurin and Rangin 2009). This section of the subduction system is characterized by nearly trench parallel subduction and rate of convergence varies from 14 mm/yr to 34 mm/yr. From the geochemical analysis the quaternary volcanoes Wuntho, Monwya, and Popa (Wuntho-Monywa-Popa arc) present in the central Myanmar basin are thought to have developed by melting a previously metasomatized mantle wedge and these volcanoes are distinct for their short life and limited magmatic productivity (Lee et al. 2016). The distance between these volcanoes varies from 166 to 223 km. The volcanic activity in Myanmar is attributed to the region's deep geodynamics and its position above a slab tearing window due to the oblique subduction. This window, located beneath the Burmese plate at mantle depths, allows Indian oceanic lithosphere to detach from continental lithosphere, influencing Myanmar's volcanic activity's spatial distribution and characteristics (Lee et al. 2016).

Moving southward into the Andaman Sea region, the obliquity in the subducting plate decreases when compared to north Myanmar. The Sagaing Fault, believed to have originated in the middle to late Eocene, around 44 Ma, was initially identified as the sliver fault (Curray 2005; Singh et al. 2013). This fault system in Myanmar is intricately linked to the Andamana Nicobar Fault (ANF), West Andaman Fault (WAF) and Great Sumatra Fault (GSF) systems further south, forming a network of transform faults and connecting to the spreading center (Andaman Backarc Spreading Center) in the central Andaman Sea basin (Curray 1979; Curray 2005). Sieh and Natawidjaja (2000) proposed that the GSF, located on the mainland of Sumatra, started its activity around 2 Ma ago. It is suggested that the sliver fault in the Andaman Sea could have also shifted to become the ANF around the same time (Singh and Moeremans 2017). Through in-depth studies involving seismic reflection and refraction, Singh et al. (2012) proposed that the WAF is likely a back thrust, but later studies suggested that WAF is considered as strike-slip fault (Martin et al. 2014; Ghosal et al. 2021).

In the Andaman Sea, the recent volcanic activity has been recorded on Barren Island, which has frequent eruptions, while Narcondam is inactive. The Andaman Backarc Spreading centre and Andaman Nicobar Fault shapes the geological topography of the Andaman Sea, allowing for relative motion between the Indian plate and the Burma microplate. The inner arc volcanic chain of the Andaman Islands, situated between the east of Andaman and Nicobar Island and the west of Sewell and Alcock rises, exhibits distinctive geomorphological features. This volcanic arc chain comprises two sub-aerial volcanic islands, Barren Island and Narcondam Island, along with thirty-three prominent submarine seamounts and numerous smaller seamounts of varying heights and diameters. Geodetic measurements suggest that southwest convergence at Barren Island and northeast motion at Narcondam, indicating the diverse tectonic processes at work in the area. Out of 33 distinct volcanoes, 5 volcanoes are characterized as cratered volcanoes (V2, V8, V10, V19 and V30). The cratered volcanoes having rugged topography and seems to comprise of hard substratum depict their eruptive history in the recent past. In particular, the region off Nicobar is dominated by several cratered seamounts and fissure fracture features, which were most likely created by volcanic activity in the recent geological past. In the seismic profile (Fig. 4), numerous near-vertical dyke intrusions within the volcanic rocks, alongside some semi-horizontal intrusions or sills, resemble observations in the Romney volcanic field offshore New Zealand (Bischoff et al. 2021). Earthquake studies also indicate significant activity in the off Nicobar region, notably the occurrence of very long period hybrid earthquake swarms, attributed to magma movement (Aswini et al. 2020, 2021). These observations suggest the region between 6ºN to 8ºN is the most active part of the volcanic arc during the recent past. Recent study by Sriram et al. 2023 observed for the first time two gas flares originating from the outer flanks of cratered seamount (V10) in the water column imaging data during the expeditions of 2018 and 2021. It is probable that the observed gas flares are induced by the perturbations in the subsurface shallow magma chamber. Detailed analysis of the gas emanations is required to ascertain their origin.

There is distinct gap in between the volcanoes V7 and V8 when compared to all the submarine volcanoes in the inner volcanic arc region. Earlier studies suggested that the subduction of base of the Ninetyeast Ridge was one of the possible reasons for the gap in volcanic activity in this region. Thick, buoyant roots underlying the Ninetyeast Ridge basement highs tend to flatten the subducting plate, inhibiting the subduction process and resulting in gaps in volcanism and seismicity, and deformation of the forearc (Subrahmanyam et al. 2008; Singha et al. 2019). Here, the detailed multibeam bathymetry helped us to confirm the gap in the volcanoes in support of the inferences of earlier studies.

Further south, along the islands of Sumatra and Java, the direction of subduction changes from approximately 60° near Sumatra to 90°off Java, indicating a reduction in the obliquity and an increase in the rate of convergence of subduction off Java (Cattin et al. 2009). The GSF fault system stretches along the western border of Sumatra and is connected with intense seismic activity and volcanic activity. A chain of volcanoes lines the western edge of Sumatra. The focal points of these volcanoes form a curvilinear pattern, unexpectedly shifting back and forth, altering the trajectory of the primary GSF (Sieh and Natawidjaja 2000). In Sumatra the quaternary volcanoes are unevenly dispersed, with the majority located north of the main volcanic arc. Some of these volcanoes are associated with structural components, such as the Seulimeum fault (Pacey et al. 2013). Java's volcanic arc, located within the larger tectonic framework of the Sunda Arc, saw significant volcanic activity during the Tertiary and Quaternary periods. Although not directly related to a major fault system, Java's volcanic landscape is influenced by regional tectonic pressures and interactions with other geological structures (Soeria-Atmadja and Noeradi 2005).

Volcanoes in normal subduction zones, such as the Izu-Bonin-Mariana volcanic arc or Aleutian arc in the Philippine Sea (Usui and Nishimura 1992; Ishizuka et al. 2006), the Tonga-Karmadec-Lau arc in the southwest Pacific (Ewart et al. 1998), and the Java-Sumatra arc system, are about 60 to 120 km apart. The volcanoes in oblique subduction zones, such as the Kuril Island arc in the NW Pacific (DeMets 1992; Glasby et al. 2006), the Lesser-Antilles volcanic arc (Calais et al. 2002), and the Andaman-Sumatra arc (Kamesh Raju et al. 2012a and current study), are more closely spaced, ranging from 3 to 30 km apart. This finding implies that oblique subduction and back arc rifting in the Andaman Sea most likely resulted in the creation of a zone in the westernmost part of the Andaman backarc region where the submarine volcanoes are very closely spaced. Volcanoes in the northern sector of the Andaman Sea are spaced differently probably due to local characteristics such as distance from the trench, age of the subducting plate from south to north, existence of active spreading centres, and magma supply. These variables could potentially explain the variations in volcano spacing. The size and spacing of the submarine volcano can be controlled by distribution of magma generated by subduction processes and related tectonic activities (England et al. 2004). The large submarine province identified in the Mariana Arc resembles a closely spaced characteristic (overall volcanic center density is 4.4/100 km of arc) while in the case of the Tonga-Kermadec arc bear volcanic density of 2.9/100 km (Baker et al. 2008). Spacing and distribution of the Andaman arc volcanoes shows different characteristics from some of the mapped submarine volcanoes which are unevenly distributed with varying physical dimension from small to large, similar to south New Hebrides volcanic arc (Patriat et al. 2015), south Aegean Active Volcanic Arc (Foutrakis and Anastasakis 2018) volcanism observed in this area. The dip angle of the subducting Indian plate increases progressively from the Nicobar segment (NS) to the Andaman segment (AS), with values of approximately 26°, 47°, and 53° respectively (Singha et al. 2019). This trend is supported by tomographic inversion data beneath the Andaman and Nicobar Islands (Kennett and Cummins 2005). Age plays a significant role in governing this variation, with the younger Indian plate in the Nicobar segment (60–80 million years old) exhibiting a lower dip angle due to greater buoyancy, compared to the older plate in the Andaman segment (∼90–100 million years old) which subducts at a higher angle (Müller et al. 2008). Previous studies have indicated that the deformation modes in overriding plates of convergent margins are significantly influenced by both the dip angle and age of the subducting slab. Shallow dip angles (< 30°) and young ages (< 60 Ma) tend to promote compressional deformation, while steeper dip angles (> 50°) and older ages (> 60 Ma) favor extensional deformation (Dasgupta et al. 2021). These relationships dictate the tectonic styles observed at convergent margins, where compressional stresses typically result in prominent forearc wedges and narrow backarc basins, whereas tensile stresses lead to wide extensional backarc basins and minor forearc wedges. In the context of the Andaman Sea region, the presence of backarc spreading indicates extensional deformation. Furthermore, the higher dip angle observed in the Andaman Segment compared to the Nicobar Segment could contribute to the formation of a backarc basin in the northern part of the region. This suggests that the variation in dip angles along the subduction zone plays a significant role in shaping the tectonic features and deformation styles observed in the Andaman Sea area. According to McCaffrey (2009), the convergence rate off the coast of Sumatra is estimated to be approximately 40 to 50 mm/yr. However, this rate decreases to less than 20 mm/yr off the Andaman-Nicobar subduction zone. This variation in convergence rates is primarily attributed to oblique convergence, where the relative motion between the two converging plates is not purely perpendicular to the subduction trench. Given that the subduction rate is higher in the Nicobar region compared to the Andaman region, this could potentially lead to increased dehydration and release of fluids from the subducting slab. The presence of abundant magma can lead to the formation of a larger number of volcanoes in the Nicobar region compared to the Andaman region. The availability of more magma increases the volcanic activity in the region. Therefore, the higher subduction rate in the Nicobar region may play a role in the geological activity and volcanic hazards observed in this area. In contrast to the continental part of the overriding plate found in northern Myanmar and southern Sumatra, the Andaman-Nicobar segment is characterized by an ocean-ocean collision setting. This distinction is significant and may contribute to the abundance of volcanoes observed in the vicinity of the Nicobar region.

Conclusion

This study presents an examination of the morphotectonic features of the Andaman Backarc region, with a special emphasis on the inner volcanic arc, sliver fault system, and small oceanic basins in the Andaman backarc region. The discovery of 33 submarine volcanoes and fault networks in the Andaman Sea was made possible by the high-resolution multibeam bathymetry data.

When compared to other volcanic arcs throughout the world, volcanoes in the Andaman-Sumatra arc region, vary in size and distribution, influenced by factors such as subduction direction, distance from the trench, age of the subducting plate, and magma supply. This results in distinct patterns of volcanic activity, with closely spaced submarine volcanoes in off Nicobar region and uneven distribution towards the north. Also, the presence of an ocean-ocean collision setting in the Andaman-Nicobar segment contributes to the abundance of volcanoes observed in the vicinity of the Nicobar region, distinguishing it from adjacent regions with continental overriding plates. The existence of well-developed crater morphology, frequent occurrence of earthquake swarms and gas emanations through the flanks of the crater seamounts suggest intense active volcanism in the off Nicobar region.

The Andaman backarc region is characterised by several narrow basins and graben structures that are impacted by sliver fault systems such as ANF, WAF and GSF. The presence of fault networks and basins reflect the Andaman Sea's complicated tectonic processes and stress distribution.