| Abstract: | Earthquake foci suggest that the India plate has been underthrust in the Sunda Arc to depths increasing from little more than 200 km beneath central Sumatra to well over 600 km beneath the Java Sea. Geological differences between Sumatra and Java do not fully account for the anomaly. The explanation would appear to lie with an oblique India-Eurasia convergence caused by the rotation, relative to Eurasia, of the Sunda backarc area. Sinistral movements on several southeast-trending wrench faults in the region between Yunnan and Java appear to have been responsible. Backarc rotation also explains the pattern of Cenozoic volcanicity in Sumatra, and resolves controversy over the nature of the Andaman Basin, which may be interpreted as a rhombochasm forming behind a locally divergent plate margin. |
