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The Sunda Arc is continuous from east of Java northwest to Burma and has a wide lower trench slope, an outer-arc ridge, and a fore-arc basin with a thick sediment fill. South of Java, a thin cover of hemipelagic sediment is carried into the trench on the subducting plate, whereas northwest of Sumatra up to 6 km of Bengal Fan sediments enter the trench. This change in sediment thickness produces a corresponding change in trench slope structure. Off Java, trench and hemipelagic sediments are accreted in thrust packets with no discernible internal structure. In the north, the fan sediments are deformed into large coherent folds. The outer-arc ridge, which is 1 to 3 km below sea level off Java, becomes emergent to the northwest as Nias, Mentawai, Nicobar, and Andaman Islands. Subduction i oblique west of Sumatra, and the fore-arc region is cut by strike-slip faults. The fore-arc basin is segmented into smaller basins along its strike by transverse structural highs. Fore-arc basin sediments are derived from the old crystalline terranes of Sumatra and the volcanic terrane of Java. Sedimentary sequences in the fore-arc basin reflect several periods of uplift and deformation of the outer-arc ridge and the magmatic arc.

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