Kinematics of the Palu-Koro Fault, Sulawesi
Ian Watkinson and Robert Hall, completed project
This project aims to understand the timing, role and kinematic history of the Palu-Koro Fault, a prominent N-S-trending structural feature of central Sulawesi. The fault is seismically active and regionally significant, lying near, and being partly controlled by the convergent triple junction between Eurasia, Australia and the Pacific. The fault has a rapid displacement rate of 3.4 cm/yr determined from 5 years of GPS measurements (Walpersdorf et al., 1998). The historical record of relatively low magnitude earthquakes (<magnitude 6) (Bellier et al., 2001) caused by the Palu-Koro fault is discordant with predictions of a magnitude 7 event every 100 years based on its slip rate (Wells and Coppersmith 1994). Since the last large earthquake, over 3 m of elastic strain has accumulated across the fault (Walpersdorf et al., 1998), which may ultimately be released by brittle failure.
Satellite imagery clearly shows the fault cutting a straight, steep-sided valley 170 km long from the Gulf of Palu through the mountainous interior of the island. Its remarkable topographic expression suggests important historical activity. Many major faults in SE Asia have a prolonged history of repeated reactivation under changing stress regimes (e.g., Lacassin et al., 1997; Imtihanah, 2000; Wang et al., 2000). Knowledge of the Palu-Koro Fault’s origins and evolution is essential to explaining its recent activity. This is particularly true since modern records cover only a fraction of the fault’s lifetime, and are therefore poorly representative of its long-term behavior.
Major fault strands are exposed along the Palu valley. They dip steeply, have an en echelon geometry with respect to the main fault trace and are associated with slickensides, foliated gouge and extensive damage zones. Faults bound lenticular slivers of country rock, sometimes including serpentinised peridotite which is smeared throughout the fault zone and may reduce friction. In (meta) basic rocks west of Palu pseudotachylites are well developed. Dyke displacement, slickenside ‘steps’ and gouge fabrics indicate a sinistral shear sense, often with an extensional component. Although the fault cuts through and has exhumed parts of a sheared basement consisting of amphibolites, mica schists, gneisses, granulites and garnet peridotite, no mylonites related to strike-slip along the fault have been discovered.