Tectonic Controls of Volcanism and Complex Movements Along the Sumatran Fault System

Recent geological work within the Contract Area of P.T. Riotinto Bethle­ hem Indonesia in West Sumatra, including aerial photographic and aeromagnetic surveys by Lockwood Survey Corporation, have yielded much new data on the Sumatran Fault Zone and have revealed its intimate connection with volcanism. Three major fault zones each characterized by complex wrench, thrust and subordi­ nate normal faulting can be recognized from aerial photographic interpretation. During the Quaternary significant di splacements along the Sumatran Fault Zone occurred and are sti ll continuing, and in consequence morphological features and especially drainage patterns are in the closes t correspondence to the fracture systems. Strong E-W magnetic lineaments thought to represent major intermediate intrusives at depth are found associated with volcanic centres. Where these lineaments are cut by strands of the fault zones vol­ canoes are located. Igneous activity a long original E-W a lignments and dextral tranSCUlTent faulting along NNW trending zones interact to produce a barrier range of volcanic rocks along the active fault zone with evenly spaced active volcanic centres at 75-100 km. distances along it. The total horizontal shift along the fault is approximately 130 km. since the inception of the present volcanic cycle. The fault system is thought to be a fundamental fracture whose full explanation is to be sought within the concept of a plate tectonic interpretation of the geology of South­ east Asia.