The Evolution of the Sumatran Fault System, Indonesia
Andy McCarthy, completed PhD project
Subduction of Indian Ocean lithosphere beneath the Sundaland continental margin is strongly oblique in the region of Sumatra, western Indonesia. Deformation is partitioned between nearly orthogonal subduction at the Sunda Trench and dextral transcurrent motion along the parallel Sumatran Fault System (SFS). This arrangement probably dates from the Middle Miocene and the opening of the Andaman Sea, although the relative motions of the major plates have changed little since the Middle Eocene. The SFS runs the length of the Barisan Mountains, a range of uplifted basement blocks, granitic intrusions, and Tertiary sediments, topped by Tertiary-Recent volcanics.
The study employed detailed structural mapping along approximately 1000km of the SFS and adjacent basement and Tertiary basin areas, and the analysis of Synthetic Aperture Radar (SAR) images. Cenozoic deformation is superimposed on basement structures related to the pre-Tertiary accretion and deformation of terranes along a long-lived active continental margin dating from at least the Permian.
A series of discrete tectonic phases have characterised the Sumatran margin during the Tertiary. Eocene initiation of the present day backarc basins took place along N-S trending rifts, strongly oblique to the NW-SE trending margin. Previous workers have proposed a regional dextral simple shear couple for their formation. However, contemporaneous rifting resulting in the formation of the Ombilin Basin near the site of the present day volcanic arc indicates extension perpendicular to the margin. A clear transition from rifting to dextral transtension in the Ombilin Basin occurred during the late Oligocene/early Miocene. Frontal compression followed in the Middle Miocene and is particularly well recorded in the forearc basin of southern Sumatra. From this time strike-slip deformation was concentrated along the SFS. Post-Middle Miocene deformation has been characterised by a combination of contemporaneous pure strike-slip, pure frontal compression, and oblique-slip. Backarc and forearc basins have been inverted. Quaternary basin formation has been restricted to the SFS itself, along which several small graben have formed. Few have classic strike-slip basin geometries and localised extension related to thermal doming along the line of the arc is partly responsible for their development. Some graben contain structures indicative of frontal compression within the rift-fill sequence.
Studies of Mesozoic outcrops in central Sumatra suggest that the SFS has a displacement of approximately 150km in this area. It should however be noted that strike slip deformation is distributed over a geographically wide area outside the present active trace of the SFS such that calculations of offset across the SFS may be inaccurate when based on measurements of offsets across individual fault strands.
Since the plate motion vector has increased gradually in magnitude but changed little in orientation since the Middle Eocene, other factors must have controlled the development of the margin. Extrusion of parts of SE Asia following India/Asia collision may have resulted in rotation of the margin and changes in the obliquity of subduction. Alternatively, collision of oceanic fragments with the margin may have triggered phases of compression. The SFS illustrates the complexities which may occur within subduction-related transcurrent systems, and their sensitivity to changes in boundary conditions.