Flexural and Buckling Analysis along the Java-Sumatra trench: Implications for Plate Coupling

The bathymetry and free-air gravity data offshore Java - Sumatra trench are utilised to analyse the flexural fore-bulge and bending moment variations along the Southeast Asian subduction zone. The observed bathymetry is corrected for various isostatic effects such as the sediment loading, seamount and ridge topography and thermal subsidence due to lithosphere age variations. The gravity-derived isostatically compensated topography was then interpreted for the flexural features in the subducting Indo-Australian plate. To model the flexural bending, 28 across-trench sections were constructed along the Sunda trench. We observed that, except in the northern Sumatra part, rest of the trench is in agreement with the simple flexural model explained by bending moment applied by the slab. In the northern Sumatra part of the trench, additional horizontal stresses of 30-40 MPa are required for better match of the flexural fore-bulge thereby increasing the coupling with the upper plate. The outcome of this analysis in comparison with the slab depth variation, which abruptly reduces from 600 km in Java to 200 km towards northern Sumatra, suggests that horizontal stresses are anti-correlated with the slab depth. The short slab at northern Sumatra does not effectively pull the incoming plate into the mantle, and therefore plate convergence is accommodated at shallow depth, increasing the coupling with the upper plate. We propose that horizontal stresses are the result of the lateral propagation of the stronger slab pull from the neighbouring deeper southeastern Java slab.