Late cenozoic clockwise rotation of Sumatra |
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| Authors: | Dragoslav Ninkovich |
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| Institution: | Lamont-Doherty Geological Observatory, Palisades, N.Y. USA |
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| Abstract: | A clockwise rotation of Sumatra of about 20° about an axis located in or near the Sunda Strait has been inferred on the basis of the following data:(1) The portion of the Indonesian volcanic arc between the Sunda Strait and the island of Timor lies along a small circle whose center is located about 32°N, 119°E. The volcanic chain of Sumatra makes an angle of 20° with this portion of the arc.(2) The Benioff zone of Indonesia has a maximum depth of 600 km to the east of the Sunda Strait, but the maximum depth decreases to 200 km northwestward along the island of Sumatra.(3) The age of the present phase of volcanic activity in Indonesia is proportional to the maximum depth of the Benioff zone; rhyolitic tuffs of the Sunda Strait range in age from Late Miocene to Pleistocene, while ignimbrites of north Sumatra are about 70,000 years old.It is suggested that the increase in sea-floor spreading rate since 10 m.y. B.P. pushed north Sumatra and Malaya northeastward for about 500 km along the system of presently inactive faults, causing a clockwise rotation of both Sumatra and Malaya about an axis located in or near the Sunda Strait. Only when this rotation ceased did the underthrusting of north Sumatra begin, producing a shallow and short Benioff zone, and delayed volcanic activity. |
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