Abstract The Molucca Sea Collision Zone in eastern Indonesia is the site of an orthogonal collision between two active subduction systems. Both the Halmahera subduction zone, to the east, and the Sangihe subduction zone, to the west, have subducted oceanic lithosphere of the Molucca Sea Plate, which has now been completely consumed. Both volcanic arcs were active since the Neogene and provide a means of probing the element fluxes through the two systems. The geochemistry of Neogene and Quaternary lavas from each volcanic arc is compared to constrain changes in the mass fluxes through the systems and the processes controlling these fluxes at different times during their history. Both arcs show increased evidence for sediment recycling as the collision progressed, but for contrasting reasons. In Halmahera this may represent an increased sediment flux through the arc front, while in Sangihe it may simply reflect a greater opportunity for melting of sediment-fluxed portions of the mantle wedge. In both cases the change in arc geochemistry can be related to the evolving architecture of the particular subduction zone. The Halmahera lavas also record a temporal change in the chemistry of the mantle component that resulted from induced convection above the falling Molucca Sea Plate drawing compositionally distinct peridotite into the mantle wege.

This paper quantifies the flexural subsidence expected from loading by a volcanic arc. The resulting mathematical model shows that the arc width should grow with time and that the subsidence beneath the load can be estimated from the observed arc width at the surface. Application of this model to the Halmahera Arc in Indonesia shows an excellent fit to observations if a broken-plate model of flexure is assumed. The model also gives an excellent fit to data from East Java, also in Indonesia, where it is possible to forward model gravity anomalies. In particular, the depth, location, and width of the depocenter-associated gravity low are accurately reproduced, although the model does require a high density for the volcanic arc (2900 kg m −3 ). This may indicate additional buried loads due, for example, to magmatic underplating. Our main conclusion is that loads generated by the volcanic arc are sufficient to account for much, if not all, of the subsidence in basins within ∼100 km of active volcanoes at subduction plate boundaries, if the plate is broken. The basins will be asymmetrical and, close to the arc, will contain coarse volcaniclastic material, whereas deposits farther away are likely to be volcaniclastic turbidites. The density contrast between arc and underlying crust required to produce the Indonesian arc basins means that they are unlikely to form in young intraoceanic arcs but may be common in older and more mature arcs.

Abstract We recognize two types of geothermal areas—volcanic and nonvolcanic. This paper concerns only the volcanic geothermal areas, which show surface manifestations in the main volcanic belts of the Indonesian archipelago (Fig. 1): (1) The Sunda-Banda mountain system, extending from the northern tip of Sumatra to the Banda Sea; (2) the Minahasa-Sangihe-Ragay zone, extending from the northern part of Sulawesi (Celebes) into the Philippines; and (3) the Halmahera-Ternate zone on the southwest border of the Pacific. Within these areas the temperature gradient (3°C/100 m) is above normal. Generally the heat transfer takes place convectionally, and conduction occurs only under special conditions. The convectional heat flow might be caused by intrusion of magma at shallow depth. In these thermal areas, hot springs indicate subsurface temperatures ranging from 100 to 300°C (Healy, 1970). Such phenomena are generally observed in areas of late Tertiary and Quaternary volcanoes, such as New Zealand, Iceland, Japan, Taiwan, the Philippines, and Indonesia. Distinct hydrothermal activity also occurs at older, eroded volcanoes. Such semivolcanic activity is closely related to shallow igneous activity, which causes a temperature increase in the circulating groundwater. These manifestations cover a vast geothermal area, including parts of Sumatra, Java, Sulawesi, and Flores. Judging from the distribution of the geothermal surface manifestations in the volcanic belts, it might be assumed that about 90% of the geothermal fields in Indonesia are of the volcanic type.