K-Ar age of the Ranau Tuffs: implications for the Ranau caldera emplacement and slip-partitioning in Sumatra (Indonesia)

摘要

The Sumatran subduction is an example of oblique convergence which is partitioned into a component normal to the plate boundary and a wrench component taken up by strike-slip deformation within the overriding plate. Indeed, off Sumatra, the approximately NNE-trending convergence is mechanically accommodated both by subduction processes and strike-slip deformation partly localised on the Great Sumatran dextral Fault (GSF). The GSF parallels the trench and follows approximately the magmatic arc, where major calderas are installed. The Ranau caldera is one of those located along the GSF in south Sumatra. A Ranau Tuff sample yielded ~(40)K-~(40) Ar ages of 0.55 +- 0.15 Ma for its separated feldspars, which places the major Ranau caldera collapse between 0.7 and 0.4 Ma, a period of paroxysmal calderic activity along the Sumatran Arc. Geomorphic features affecting the Ranau Tuff and offset by the GSF yield a long-term dextral slip rate of 5.5 +- 1.9 mm/yr at 5 deg S. Consequently, south Sumatra represents an intermediate case between complete slip-partitioning and purely oblique thrusting, where the leading edge is characterised by a low convergence obliquity (<20 deg) accommodated by strike-slip deformation in the overriding plate. This demonstrates that even for low obliquity, slip-partitioning can exist.