A marine geophysical survey of the Andaman sea (southeast Asia) delineated the foredeep, the outer sedimentary island arc, the interdeep, the inner volcanic arc and rift valley, and the backdeep of a region 600 miles (1110 km) in length. The differences in the structural grain between the Malay peninsula and the trends of the island arc on the west probably result from a large fault extending seaward from Burma into the Martaban gulf. The petroliferous sedimentary basins of Sumatra and Burma in the backdeep zone are interconnected through the Andaman sea whose present configuration probably dates back to the end of the Cretaceous. The Cretaceous folded belt to the west extends from central Burma through the Irrawaddy delta into the volcanic Barisan range of Sumatra. Still farther westward, beyond the intervening interdeep, lies the nonvolcanic outer island arc traced from the eastern Himalayan arc through Burma and the Andaman and Nicobar islands to the islands west of Sumatra. The foredeep, or Java trench, still farther to the west is buried north of Simalur island. The principal orogeny, at the end of the Cretaceous, was characterized by folding and southwestward thrusting, uplift, and batholithic emplacement. The Barisan range and the Semangko rift graben developed during the Plio-Pleistocene.

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