Awang H. Satyana 1) Edward Erwanto 1) C. Prasetyadi 2)
A major wrench zone, left-lateral slip in nature, strongly deformed a series of Late Oligocene to Pleistocene rocks in the northern coastal line of East Java and its eastern islands and offshore areas. The fault zone trends west – east forming a deformed zone of 15 to 40 km wide and 675 km long from Rembang area in the west through Madura Island and Kangean Islands to Sakala offshore area in the east. The deformed zone is called the Rembang-Madura-Kangean-Sakala (RMKS) Fault Zone. Based on the regional setting of East- and Southeast Sundaland, it is known that the RMKS Fault Zone occurred at the hinge belt or shelf edge to slope area of a geologic transition from the stable Eastern Sunda Shelf to the north (the Northern Platform) to the deep-water area to the south. There is a contrast of sedimentary facies to the north and south of the RMKS Fault Zone. Tectonically, the stable Eastern Sunda Shelf is considered to overlie the expected micro-continent called the Paternoster-Kangean. Therefore, the RMKS Fault Zone is located at the southern margin of the micro-continent. Basement lithology and configuration to the north and south of the RMKS Fault Zone are different. A number of mechanisms are considered to origin the RMKS Fault Zone. These include: the westward stress driven by the collision of the Buton-Tukang Besi and Banggai-Sula to the east of Sulawesi, westward stress due to the collision of Australia with Timor and anticlockwise bending of the Banda Arc, and northward stress due to the subduction of the Indian oceanic crust beneath Java. The initiation of the RMKS Fault Zone was in the upper Early Miocene in Sakala area and younger westward until the Middle Miocene in Rembang area. Along the RMKS Fault Zone, flower structures are definitely identified on seismic sections, showing basement-involved, deeply-rooted vertical master faults with upward diverging splays/strands that have mostly reverse separations. In map view, these splays are mapped as fold and fault belts trending west-east and west northwest-east southeast. Extensional component of the wrench zone subsided the Paleogene rifted blocks such as the Central Deep and formed a number of normal faults. Tectonic inversion related with both pure and simple shear deformations is observed along the fault zone. Shale diapirism commonly occurs to the south of the fault zone and its occurrence is related to wrench tectonism in thick shale sequences deposited rapidly to the south of the RMKS Fault Zone.
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* Indonesian Association Of Geologists, 33rd Annual Convention, Bandung 29 November – 1 December 2004