Evolution of the Western Sunda Arc: A Model Derived From Study of the Andaman Sea

The Andaman Sea is an active backarc basin lying above and behind the western Sunda Arc, where convergence between the Indian and Sunda plates is highly oblique. I accept the premise that prior to the collision of India with Asia, perhaps in the Paleocene, the alignment of this western part of the arc was about 120°-300°. As India plowed its way into Asia this part of the arc rotated clockwise to form the east limb of the eastern Himalayan syntaxis. Initially convergence with the Indian plate was about normal to the arc, but with rotation it became highly oblique. The result of this increasing obliquity was, first, formation of an arc-parallel strike slip fault behind the subduction zone and formation of a sliver plate between the fault and the trench. Second, a series of separate extensional basins opened, starting in the Oligocene. The total of the northward components of opening of these basins suggests a total offset along the Sagaing Fault of Myanmar of about 330 km, and the total of the westward components explains the unusual double bulge in the alignment of the arc off the Andaman Sea. With increasing obliquity both northward and with time, and as the convergence rate decreased with time, the component of subduction has decreased. Thus the frequency of subduction earthquakes that could generate tsunamis would also decrease from Sumatra northward toward Myanmar. The total width of sea floor subducted since collision started is greater beneath Java, less beneath Sumatra and even less beneath the Andaman Sea and Myanmar.