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

Satellite images provide evidence for numerous stepovers, pull-apart g rabens and volcanic structures along the NW-trending right-lateral Gre at Sumatran Fault Zone. High-resolution SPOT images permit us to analy ze the relationship between volcanic and tectonic structures. This ana lysis reveals that releasing stepovers and pull-aparts are ephemeral s tructures along the Great Sumatran Fault Zone and that the geometry of the strike-slip fault evolves permanently through time. The interpret ative structural maps obtained from SPOT image analysis reveal that th e 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 fa ults of calderas. In particular, detailed study of the segments at the southernmost, 150-km-long termination of the Great Sumatran Fault Zon e, from northwest of Ranau Lake to Semangka Bay, allows us to describe the development of the Ranau collapse caldera. This caldera is locate d within a pull-apart basin bounded by a stepover. Its peculiar rectan gular shape and its relatively large size (about 200 km) are controlle d by the evolution of the extinct Ranau releasing stepover. Similarly, the Toba elliptical caldera, one of the largest volcanic caldera in t he world, is elongated parallel to the present trace of the Great Suma tran Fault and appears to be related to a releasing stepover associate d with a wide pull-apart basin that is not active at present.