Mantle structure and seismotectonics of the Sunda and Banda arcs

Abstract

We have examined the mantle structure and seismotectonic features of the Sunda and Banda arcs, Indonesia, based on the P-wave tomographic images, focal mechanism solutions, gravity anomaly and heat-flow data. On the basis of slab morphology and seismicity, we can divide the arcs into three parts, the Western Sunda, Eastern Sunda, and Banda arc. The slab-like tomographic image penetrates to a depth of about 500 km below the Western Sunda arc where seismicity does not exceed a depth of 250 km. In the Eastern Sunda arc, where a seismic gap exists between 300 and 500 km depths, the slab appears to be continuous and to penetrate into the lower mantle. Beneath the Banda arc, with seismicity down to a depth of about 650 km, the slab dips gently and does not appear to penetrate into the lower mantle. The positive gravity anomaly shows a systematic pattern, namely, the anomaly along the Eastern Sunda arc is larger than that in the Western Sunda and the Banda arcs. Along the back-arc side of the Sunda and Banda arcs, the heat flow decreases from the west to the east. Seismic strain release from the shallow earthquakes calculated from the CMT solutions show the strain axes to be oblique to the structural trends. The CMT solutions show that the Eastern Sunda arc is characterized by normal earthquakes along the trench and back-arc thrusting earthquakes north of the volcanic line. In the Western Sunda and the Eastern Sunda arcs, earthquakes of the down-dip extension type dominate the slab down to a depth of 200 km while down-dip compression earthquakes occur below 500 km depth. In the Banda arc, deep earthquakes show down-dip extension to a depth of 500 km; below this depth the state of stress is not clearly defined.