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Lithospheric Density Structure of Andaman Subduction Zone from Joint Modelling of Gravity and Geoid Data

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Lithospheric Density Structure of Andaman Subduction Zone from Joint Modelling of Gravity and Geoid Data Indian Journal of Geo Marine Sciences Vol. 47 (05), May 2018, pp. 931-936 Lithospheric Density Structure of Andaman Subduction Zone from Joint Modelling of Gravity and Geoid data R. Yadav1, 2 & V. M. Tiwari1* 1CSIR – National Geophysical Research Institute, Hyderabad, India 2Academy of Scientific &Innovative Research (AcSIR), CSIR-National Geophysical Research Institute, Hyderabad, India [E.Mail: [email protected]] Received 07 September 2016 ; revised 30 October 2016 The Andaman subduction zone is the result of the oblique convergence of Indian plate that dives beneath Eurasian (Sunda) plate and one of the most seismically active regions. Structure and dynamics of the subduction zone are still a matter of investigations, inspite of a several studies were carried out earlier. In the present study, we have delineated the 2D density structure of the Andaman subduction zone through the joint modelling of gravity and geoid data set derived from the satellite altimetry and interpreted it in term of dynamics of the region. The major features of the density structures are : subduction of Indian plate up to ~ 180 km depth; anomalously thicker oceanic crust below the Ninetyeast Ridge, Sewell rise and in the shelf region; comparatively thin lithosphere under the overriding plate. Further, the lowest gravity anomaly marks the interaction of subducting oceanic crust with overriding upper mantle. [Keywords: Andaman, Gravity, Geoid, Density Structure, Seismogenesis] Introduction studies 16 & 17. All these investigations have facilitated Subduction zones are the results of complex to delineate the structural configuration of the geodynamic processes that evolve due to the Andaman subduction zone. While the gravity convergence of two tectonic plates, wherein one of anomaly pattern was used to understand structural them slides beneath the other and takes over its variations along the Andaman arc, sparse distribution journey towards the deep mantle. Such process causes of seismological and GPS stations over the Andaman the tumbling of the plates in the subduction zone Islands makes such studies confined to the analysis of causing significant forces to drive the plates and data obtained at a few stations. This resulted in the leading to several natural hazards like volcanoes and diminished structural image of the subsurface, causing earthquakes. Some of the noted examples of most the structures to be poorly resolved. active subduction zones of the world are Andaman- The present study is based on the analysis of a Sumatra, Andean, and Hellenic etc., which have been profile along 100 N, running perpendicular to the loci of the devastating earthquakes1- 3. Most of the structural trend of the Andaman subduction zone. earthquakes occurred within the Andaman region This zone is a typical example of the oblique (ANR) are related to the subduction of the Indian subduction and formed due to the subduction of plate. AR has not yet experienced any large Indo-Australian oceanic plate beneath the Eurasian 18, 19 & 20 earthquake events of Mw > 8.0 reported till now. The (Sunda) plate . The western part of this zone is only documented seismic event in the ANR is the outlined by Ninetyeast Ridge (NER) that runs 1881 of Mw 7.9 event according to the historical approximately parallel to the trench. The complex records4. However, the Sumatra mega-earthquake of dynamic processes occurring in this region are 26th December, 2004 caused a chain of aftershock in evidenced by the presence tectono-morphological the rupture area of about 1300 km including the ANR5 features which form parallel to the Islands like the & 6 and prompted geoscientists to re-look into the fore-arc, back-arc, volcanic-arc (Sewell rise) according detailed structural architecture below the region. to Weeks et al21. The work of Radhakrishna et al,8 in Most of the earlier studies in the Andaman this region is based on 2D density structural region are based on gravity 1, 7 & 8, seismology 9, 10, 11 & 12, modelling computed from gravity anomaly observed Global Positioning System (GPS) 13, 14 & 15 and seismic over five different profiles running perpendicular to 932 INDIAN J. MAR. SCI., VOL. 47, NO. 05, MAY 2018 the structural trend and have unveiled significant characterized by the sediments of Bengal Fan (Fig. 1), variations in crustal thickness of ~ 7 km along the which are subducted and deformed below the west of NER; ~ 40 km below the Andaman-Nicobar Andaman trench. The east dipping Benioff Zone ridge; ~ 13km below the Sewell rise. Further, within the Andaman arc exhibits depth extents 24 of Subrahmanyam et al, 7 reported on the crustal approximately 200 km. The Andaman trench thickness of NER and suggessted the crustal roots to witnesses thrust motion with a convergence rate 25 of follow the airy isostasy compensation model. These about 1.4 cm/year. The Andaman back-arc spreading results are based on the computation of 2D gravity centre accommodates the remaining plate motion and models prepared along the available seismic profile 22. makes its journey towards the Sumatra fault in the Recent results of Rao et al, 12 have added new south. Such oblique motions between the Indo- structural information utilizing variation of gravity Australian and Eurasian plates result in a sliver plate, gradient along N-S direction, computed from often referred as the Burma micro plate25. Several sediment corrected Bouguer gravity anomaly. north-south trending faults and thrusts are known to All these studies are focused on crustal thickness prevail within the Andaman-Nicobar ridge and in the variations within the Andaman subduction zone using adjacent offshore areas. Among these structural the gravity technique as an exploring tool for features, the most significant are the Jarwa thrust 26 revealing the crustal and lithospheric mantle structure. developed on the main island, West Andaman Fault However, using a single geophysical technique and (WAF) to the east of Andaman-Nicobar ridge 27 etc. relying on one single data set for exploring the Some of these faults are still active sources of subsurface structure is always an issue of debate. seismicity 24. Moreover, the reliability on the structural image of the subsurface obtained from such analysis is a major questionable issue. Thus, our workflow and analysis provide answers to such debates and allows us to move forward in addressing these issues by performing 2D joint modelling of the gravity and geoid data set for obtaining better constraint structural image of the subsurface. In this joint modelling approach, density model is computed by forward modelling approach, in which gravity and geoid responses of the geometries are compared with observed gravity and geoid data. The results of such modelling approach provide an improved model of the subsurface structure as compared to the earlier work of different researchers in this area. It is a general fact that the gravity anomaly decays at a faster rate with increase in depth as compared to that of the geoid anomaly. In this paper, we have presented a clear picture of the tectonic setting of the study area and the data sets used for modelling the subsurface. Finally, we summarize the 2D modelling technique used for addressing proposed objective, demonstrate the superiority of these techniques and workflow for imaging structural complexities in such a complex Fig.1 — Topography/Bathymetry map is superimposed by the tectonic regime. Tectonics. NER is showing the Ninetyeast Ridge, Andaman Trench, Andaman Is (Island), Nicobar Is(Island), IB(Invisible Tectonic setting of the Andaman Subduction Zone Bank) , SR (Sewell Rise), AR (Alcock Rise), CB(Central Basin), Owing to its evolution during Oligocene-Miocene EAB (East Andaman Basin), CAB (Central Andaman Basin), times, the sedimentary islands of Andaman and WSR (West Sewell Ridge), NSR (North Sumatra Ridge), Gulf of Martaban, And MR (Mergui Ridge) Red colour east-west line is Nicobar form a part of the fore-arc sedimentary the 2D profile, and white line is the seismic line which is used for 23 complex . The western part of this island system, is upper crustal constraint. YADAV & TIWARI: STRUCTURE OF ANDAMAN SUBDUCTION ZONE 933 The Andaman basin lies between the Burma and Martaban, lying to the NE part of the study area Sumatra exhibiting an average width of 650 km from (Fig.1), indicated by yellow color represents shallow the Malay Peninsula to the Andaman and Nicobar bathymetry. Islands (Fig. 1). Predominance of oblique subduction below the Andaman arc resulted in strike-slip faulting Methodology parallel to the trench; back-arc extension and basin The workflow adopted for this study is shown in formation in the Andaman Sea26. However, the (Fig. 2). Gravity and geoid data are taken as input for geology of Andaman back-arc spreading is related to performing this operation. Using these data sets, an the leaky transform tectonics 28 inferred from the initial model is prepared with an aim to match the collision of Ninety East Ridge with that of the model from the observed anomalies. The model is Andaman trench during the middle or late Miocene29. constructed using a priori information obtained from Such ridge trench collision caused the opening of the controlled source seismic, seismic tomography and Andaman Sea. The geological age of this event is other geophysical and geological published studies documented to be of about 13 m.y or Mid-Miocene 30. along the 1000 km long east-west profile up to a Kamesh Raju et al 31 have reported the sea floor depth of 200 km. For modelling, the first layer (sea) is spreading event that got started in the Andaman back- taken from the bathymetry data. Thereafter, two arc basin around 4 m.y ago. Such tectonic events are sediment layers are constructed out of which the the consequence of extrusion tectonics that triggered upper layer (upper sediment) is prepared from extension and rifting along the plane joining the the global sediment data and the lower layer (lower Sagaing and Sumatran fault systems.
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