RELATIONSHIP BETWEEN TECTONISM AND VOLCANISM ALONG THE GREAT SUMATRAN FAULT ZONE DEDUCED BY SPOT IMAGE ANALYSES

Satellite images provide evidence for numerous stepovers, pull-apart grabens and volcanic structures along the NW-trending right-lateral Great Sumatran Fault Zone. High-resolution SPOT images permit us to analyze the relationship between volcanic and tectonic structures. This analysis reveals that releasing stepovers and pull-aparts are ephemeral structures along the Great Sumatran Fault Zone and that the geometry of the strike-slip fault evolves permanently through time. The interpretative structural maps obtained from SPOT image analysis reveal that the formation of huge, peculiarly shaped, volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull-apart basins are analogous to the circular ring faults of calderas. In particular, detailed study of the segments at the southernmost, 150-km-long termination of the Great Sumatran Fault Zone, from northwest of Ranau Lake to Semangka Bay, allows us to describe the development of the Ranau collapse caldera. This caldera is located within a pull-apart basin bounded by a stepover. Its peculiar rectangular shape and its relatively large size (about 200 km) are controlled by the evolution of the extinct Ranau releasing stepover. Similarly, the Toba elliptical caldera, one of the largest volcanic caldera in the world, is elongated parallel to the present trace of the Great Sumatran Fault and appears to be related to a releasing stepover associated with a wide pull-apart basin that is not active at present.