Offshore Southeastern Extension of the Sumatran Dextral Fault: A New Discovery in Indonesian Marine Geology and Implications on the Tectonics of the Sunda Strait – Southwest Java waters*

Y.S. Djajadihardja1), A.H. Satyana2), Won Soh3) C. Gaedicke4), T. Eko1), Sasaki5), R. Riza6), S. Neben4)

1) Agency for the Assessment and Application of Technology (BPPT), Jakarta; 2)PERTAMINA, Jakarta; 3)Japan Marine Science and Technology Center (JAMSTEC), Yokosuka, Japan; 4)Bundesanstalt für Geowiessenschaften und Rohstöffe (BGR), Hannover, Germany; 5)The University of Tokyo, Tokyo; 6)Marine Geological Institute (PPGL), Bandung



Sumatra-Java Trench borders the southeastern margin of the Southeast Asia making a total length of about 6500 kms from Aceh in the northwest of Sumatra to the Sumba Island in the eastern Indonesia. The trench accommodated the subduction of the Indian oceanic plate beneath the Southeast Asia continental plate. Oblique subduction occurs along the western part of Sumatra, while normal subduction occurs to the south of Java. The junction of the two models of subduction occurs in the waters of the Sunda Strait and the Southwest Java. The oblique subduction to the west of Sumatra coincides with the existence of the Sumatran Fault, a dextral strike-slip fault extending from the Andaman Sea to the north of Sumatra to the Sunda Strait of about 1650 kms long. Another dextral fault is the Mentawai Fault, running parallel with the Sumatran Fault in the forearc region.

Previous publications described that the Sumatran Fault ends somewhere in the Sunda Strait area around the Semangko Bay. However, recent marine geological and geophysical research carried out jointly in 2001 by the BPPT (The Agency for the Assessment and Application of Technology) from Indonesia and the JAMSTEC (Japan Marine Science and Technology) from Japan revealed another phenomena. The marine cruises using “Yokosuka” research vessel were conducted on the waters of the southern Sunda Strait and southwestern Java. New multibeam data shows clearly the existence of extension or prolongation of the Sumatran Fault zone running southeasterly as long as 400 kms from the previously end point in the Semangko Bay. The new discovered extended Sumatran Fault stops just about 30 kms to the north of the Java Trench in the southwestern Java waters.

The discovery of the offshore southeastern extension of the Sumatran Fault has provided one of explanations of why the Java Trench to the southwest of the Java Island bends slightly to the north from the normal trend. This extended Sumatran Fault has formed a triangle zone with the Java Trench by which the drag of the portion of the trench took place. The origin of the extended fault is discussed in the paper considering the rifts in the Sunda Strait and the presence of the Cimandiri Fault in the southwest Java Island. This discovery will be further detailed by the BPPT and the JAMSTEC in October 2002 by descending a manned submarine research vessel called “Shinkai 6500” down to the fault. This will become the first deep-water expedition in Indonesia.


* Proceedings Indonesian Association of Geologists, 31st Annual Conference, September 30 – October 2, 2002, Surabaya


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