Tectonic features of the southern Sumatra‐western Java forearc of Indonesia

Abstract

Multichannel reflection seismic profiles along the active Sunda Arc, where the Indo‐Australian plate subducts under the overriding Eurasian margin revealed two accretionary wedges: The inner wedge I is of assumed Paleogene age, and the outer wedge II is of Neogene to Recent age. The inner wedge I is composed of tectonic flakes stretching from southeast Sumatra across the Sunda Strait to northwest Java, implying a similar plate tectonic regime in these areas at the time of flake development during upper Oligocene. Today, wedge I forms the outer arc high and the backstop for the younger outer wedge II. The missing outer arc high of the southern Sunda Strait is explained by a combination of Neogene transtension due to a clockwise rotation of Sumatra with respect to Java and by arc‐parallel strike‐slip movements. The rotation created transtensional pull‐apart basins along the western Sunda Strait (Semangka Graben) as opposed to transpression and inversion on the eastern Sunda Strait, within the new detected Krakatau Basin. The arc‐parallel transpressional Mentawai strike‐slip fault zone (MFZ) was correlated from the Sumatra forearc basin to the northwest Java forearc basin. Off the Sunda Strait, northward bending branches of the MFZ are connected with the right‐lateral Sumatra fault zone (SFZ) along the volcanic arc segment on Sumatra. It is speculated that the SFZ was attached to the Cimandiri‐Pelabuhan Ratu strike‐slip fault of Java prior to the presumed rotation of Sumatra, and that since the late lower Miocene the main slip movement shifted from the volcanic arc position to the forearc basin area due to increasingly oblique plate convergence.