Structure and tectonics of the Sumatra Fault Zone-Sundra Trench junction

Abstract

The Sunda French marks the subjection zone of Indian-Australian plate beneath the Eurasian plate. Due to the geometry of the Eurasian plate, the convergence is normal towards Java and oblique towards Sumatra. A major zone of dextral strike slip displacement along Sumatra Island, called the Sumatra Fault, partially accommodates the oblique convergence off Sumatra. It extends from north Sumatra south- eastward to the southern tip of Sumatra. It has been hypothesized that the Sumatra Fault extends offshore from the southern tip of Sumatra southward across the shore-ward trench slope to the trench axis. However, little evidence has been published to show the connection. The Sunda Strait forearm region, where the subjection direction changes from normal to oblique, is the focus of this study. The Sunda Strait fore-arc region is subject to northwest - southeast extension related to the motion of the Sumatra Fault and northeast - southwest compression because of subjection along the trench. The objective of this study is to determine the structures of the Sunda Strait forearm region, the south - southeast continuation of the Sumatra Fault, and model the tectonic development. All available geophysical data for the Sunda Strait forearm region have been compiled. These data consist of topography, free-air gravity, seismic refection profiles, and seismicity. Gravity analysis and modeling reveals that the Sunda Strait forearm region lacks accretionary prism development and instead consists of lower inner trench slope accretion and a broad basin beneath the upper inner trench slope and shelf. Topography, seismic reflection and earthquake data indicate a zone of mixed right-lateral and extensional tectonic across this forearm region from Sumatra to the trench. These results are interpreted as showing ongoing separation of the Sunda Strait forearm region as the Sumatra forearm plate has moved northwest, bounded by the Sumatra Fault, in response to oblique convergence. The Sumatra Fault does, therefore, extend across the inner trench slope to the trench where it forms a Trench-Trench-Fault triple junction.