DEM of central Seram, showing the prominent
WNW-trending fault valley
Thin section of mylonite from the eastern end of the shear zone. Rolled garnet porphyroclast and asymmetric folds indicate top-to-the-left (NE) shear
This project aims to constrain the kinematics and timing of Neogene deformation and exhumation in Seram, Eastern Indonesia. Seram lies in an area of tectonic complexity, whose history includes Mesozoic rifting of Palaeozoic basement, and late Neogene uplift and thrusting, resulting in mountains 3 km high. High rates of convergence between the Australian and Pacific plates are partly absorbed below Seram (e.g. Hill, 2005), and the island is presently sandwiched between the Bird’s Head peninsula to the north, and the Banda Sea to the south. It is dominated by shortening and uplift, expressed by allochthonous thrust sheets which overlie an imbricated para-autochthon (Audley-Charles et al., 1979), intruded by granites interpreted to be the product of obduction-induced metamorphism and anatexis (Linthout and Helmers, 1994).
Exhumed metamorphic rocks in central Seram have been deformed by a prominent shear zone which has been described as a dextral strike-slip structure (Linthout et al., 1991). The shear zone lies in a steep valley almost 100 km long, which marks the edge of a narrow mountain belt. Gunung Murkele-Ketjil, the highest peak in the belt, rises to 3027 m only 15 km from the coast. It is unclear what the history of this major structure is, how it relates to the northern thrusts, what its role in the uplift of the mountains is, and whether it is active today.
View north into the major fault valley, showing the high mountains bounding the valley
View west towards Pulau Kelang from west Seram. Strongly foliated paragneisses in foreground
Recent reconnaissance fieldwork on Seram has located a zone of intense mylonitisation at the eastern end of the shear zone. A large, (>>100 m wide) shear zone, and several smaller, parallel (<10 m wide) shear zones have been identified cutting through regionally metamorphosed slates, phyllites and garnet-mica schists. The latter lithology is the protolith for spectacular mylonites bearing rounded garnet porphyroclasts with well developed asymmetric tails, and rolled ‘snowball’-type morphologies. Shallow-dipping to sub-vertical mylonitic foliations strike broadly parallel to the host valley, and dip both north and south. Kinematic indicators show a consistent top-to-the-NE vergence. Generally this is obliquely up-dip, therefore there is a sinistral strike-slip component. On north-dipping surfaces, the vergence is down-dip, associated with a dextral component. This indicates that the observed shear zone has been folded, and may not be directly related to the deformation responsible for the linear valley.
