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The present structure of Sumatra is dominated by the effects of the current subduction system in which the Indian Plate is being subducted northeastwards beneath the island at a rate of c. 7 cm a1. The structure of Sumatra was described by van Bemmelen (1949) and in terms of plate tectonics by Hamilton (1979). The main structural elements of Sumatra and its surrounding region are defined with respect to the Sumatran subduction system (Fig. 13.1).
(1) Forearc region, which includes the subduction trench, part of the Sunda Trench extending from Myanmar to eastern Indonesia, the developing accretionary complex, composed of ocean floor materials scraped off the Indian Plate, the forearc ridge which rises above sea level to form the forearc islands, and the forearc basins which lie between the ridge, and the volcanic arc on the mainland of Sumatra (Fig. 13.2).
(2) Barisan Mountains and the Sumatran Fault System. The Barisan Mountains are composed of an uplifted basement of Upper Palaeozoic and Mesozoic sedimentary and volcanic rocks, variously metamorphosed, deformed and intruded by granites, overlain by Cenozoic sediments and volcanics, including the products of the volcanoes related to the present subduction system, which form the currently active volcanic arc. The Sumatran Fault system is a complex of dextral strike-slip faults running the whole length of the island through the centre of the Barisan Mountains from NW to SE, with zones of compression and extension, forming areas of uplift and pull apart basins which form grabens along the line of the
