Andaman-Nicobar-Sumatra Margin Revisited: Analysis of the Lithospheric Structure and Deformation Based on Gravity Modeling and Distribution of Seismicity

The Andaman-Nicobar-Sumatra subduction margin, with a well-developed Benioff zone down to similar to 200 km depth, extends over 1300 km along strike and has a lateral extent of similar to 200 km. Two-dimensional (2D) profiles based on generalized inversion of free-air gravity anomaly data across different segments of the Andaman-Nicobar sector of the margin were analyzed by reconstructing the geometry of the converging India-Eurasia lithospheric plates. Detailed 2D structures of the Ninetyeast Ridge (NER), fore-arc basin, volcanic arc, back-arc basin, spreading ridge, Sewell Rise, Mergui Ridge and Mergui Basin, and depths of Moho and the lithosphere-asthenosphere boundary were delineated. The volcanic arc is located at a distance of similar to 150-200 km from the trench and is marked by a zone of positive gravity anomaly separated from the trench by the fore-arc basin with a zone of gravity low. The topographic and gravity anomaly patterns show complex geometrical patterns over the volcanic arc due to the presence of a number of regional faults striking parallel to the trench axis developing a series of pull-apart (transtensional) basins as well as the interaction of the spreading ridges on the overriding plate. A narrow zone of negative gravity anomaly (similar to - 185 to - 110 mGal) observed in the gravity model of the margin is interpreted to be associated with fore-arc shear fault above a zone of fractured oceanic crust with a thick sedimentary layer, located above a zone of sharp bending of the eastward converging Indian lithosphere in the fore-arc basin. A high concentration of seismicity and a trench-parallel band of moderate moment energy release along this fault might have been caused by concentrated deformation within the zone of flexing of the descending plate. A wide fore-arc (> 200 km) and the enhanced deformation of the subducting Indian oceanic plate in central sectors possibly resulted from increased interaction between the NER and the Andaman trench and extension in the Andaman back-arc. Appearance of a second moderate energy band and shifting of seismicity toward the trench axis following the 2004 M-W 9.2 mega-event are apparently caused by the migration of the stress field from deeper to shallower part of the Indian lithosphere. A similar type of seismicity migration toward shallower part of the descending plate has been recorded along other subduction margins around the globe.

Automated summary

The Andaman-Nicobar-Sumatra subduction margin has a complex geological structure with the presence of multiple regional faults between the volcanic arc, fore-arc basin, and trench, causing deformation of lithospheric plates. The observed negative gravity anomaly in the margin's gravity model may be associated with a fore-arc shear fault above fractured oceanic crust with a thick sedimentary layer, located in an area of sharp bending of the converging Indian lithosphere in the fore-arc basin.