Elastic-plate thinness deduced from subtle inner-arc appearance in the Andaman Sea

The Andaman Sea is located in the northernmost part of the Sunda (Java–Sumatra–Andaman) subduction zone. The topography and free-air gravity anomaly in the Java and Sumatra segments exhibit typical double-arc features comprising a trench low, an outer (non-volcanic) arc high, an inter-arc oceanic basin low and an inner (volcanic) arc high. However, no evident inner arc high appears in the Andaman segment. This study provided a mechanical explanation for the inner-arc topographic inactivity in the Andaman Sea under the continuous condition of the potential double-arc characteristics along the entire Sunda subduction zone. To compute the long-term surface uplift rates of the northern Sumatra and Andaman segments, the trench-perpendicular plate-interface profiles were determined from the hypocentre distribution and incorporated into a dislocation-based two-dimensional plate subduction model. The computed surface uplift rates were dependent on the plate-interface curvature distribution within the thickness of the elastic lithosphere. The observed topographic and gravitational double-arc profiles in the northern Sumatra segment were explained by 50–100-km-thick elastic layers, whereas the outer single-arc profiles (trench low–outer-arc high–oceanic-basin low) in the Andaman segment were explained by 20–30-km-thick elastic layers. This contrast implies that overriding-plate thinning is responsible for the single-arc appearance in the Andaman Sea.