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.

Tomographic images of the mantle beneath the region extending from the Molucca Sea eastward to Tonga, and from the Australian craton north into the Pacific, reveal a number of distinctive high seismic-velocity anomalies. The anomalies can be interpreted as subducted slabs and the positions of the slabs can be compared to predictions made by tectonic models for the region. Several strong anomalies are due to present-day subduction and the slab lengths and positions are consistent with Neogene subduction at the Tonga and the New Hebrides Trenches, where the anomalies suggest rapid rollback of subduction hinges since about 10 Ma, and beneath the New Britain and Halmahera Arcs. There are several generally flat-lying deeper anomalies which are not related to present subduction. Beneath the Bird's Head and Arafura Sea is an anomaly which we interpret to be the result of north-dipping subduction beneath the Philippines-Halmahera Arc between 45 and 25 Ma. A very large anomaly, which extends from the Papuan peninsula to the New Hebrides and from the Solomon islands to the east Australian margin, is interpreted as the result of south-dipping subduction beneath the Melanesian Arc between 45 and 25 Ma. Our interpretation implies that a flat-lying slab can survive for many tens of millions of years at the bottom of the upper mantle. There is a huge anomaly in the lower mantle which extends from beneath the Gulf of Carpentaria to Papua. We suggest this is a slab subducted before 45 Ma, which may be correlated with a Cretaceous slab beneath the Australian-Antarctic Discordance or an early Cenozoic slab sub-ducted north of Australia. The anomaly is located above the position where there must have been a change in polarity in subduction at the boundary between the north- and south-dipping subduction zones north of Australia between 45 and 25 Ma. All of these have been overridden by Australia since 25 Ma. One subduction system predicted by the tectonic models, the Marumuni Arc of Papua New Guinea, is not seen on the tomographic images.

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.

The Appalachian-Caledonian orogen records a complex history of the closure of the Cambrian-Ordovician Iapetus Ocean. The Dunnage Zone of Newfoundland preserves evidence of an Ordovician arc-arc collision between the Red Indian Lake Arc, which forms part of the peri-Laurentian Annieopsquotch accretionary tract (ca. 480–460 Ma), and the peri-Gondwanan Victoria Arc (ca. 473–453 Ma). Despite the similarity in age, the coeval arc systems can be differentiated on the basis of the contrasts that are apparent across the suture zone, the Red Indian Line. These contrasts include structural and tectonic history, stratigraphy, basement characteristics, radiogenic lead in mineral deposits, and fauna. The arc-arc collision is considered in terms of modern analogues (Molucca and Solomon Seas) in the southwest Pacific, and the timing is constrained by stratigraphic relations in the two arc systems. The Victoria Arc occupied a lower-plate setting during the collision and underwent subsidence during the collision, similar to the Australian active margin and Halmahera arcs in the southwest Pacific. The timing of the subsidence is constrained by three new ages of volcanic rocks in the Victoria Arc (457 ± 2; 456.8 ± 3.1; 457 ± 3.6 Ma) that immediately predate or are coeval with deposition of the Caradoc black shale. In contrast the Red Indian Lake Arc contains a sub-Silurian unconformity and a distinct lack of Caradoc black shale, suggesting uplift during the collision. The emergent peri-Laurentian terranes provided detritus into the newly created basin above the Victoria Arc. The evidence of this basin is preserved in the Badger Group, which stratigraphically overlies the peri-Gondwanan Victoria Arc but incorporated peri-Laurentian detritus. Thus the Badger Group forms a successor basin(s) over the Red Indian Line. Following the collision, subduction stepped back into an outboard basin, the Exploits-Tetagouche backarc, closing the Iapetus Ocean along the Dog Bay Line in the Silurian. Correlative tracts in the Northern Appalachians and British Caledonides support the Ordovician arc-arc collision; however, the evidence is less obvious than in Newfoundland.

The Indonesian region includes several volcanic island arcs that are highly active at the present day, and also contains a record of Cenozoic volcanic activity owing to subduction of oceanic lithosphere at the margins of SE Asia. As a result of long-term subduction, there is a high regional heat flow, and a weak crust and lithosphere, as identified in other subduction zone backarcs. The stratigraphic record in the Indonesian region reflects a complex tectonic history, including collisions, changing plate boundaries, subduction polarity reversals, elimination of volcanic arcs, and extension. The arcs have not behaved as often portrayed in many arc models. They mark subduction but were not continuously active, and it is possible to have subduction without magmatism. Subduction hinge retreat was accompanied by significant arc volcanism, whereas periods of hinge advance were marked by reduction or cessation of volcanic activity. Growth of the region occurred in an episodic way, by the addition of ophiolites and continental slivers, and as a result of arc magmatism. In Indonesia, relatively small amounts of material were accreted from the downgoing plate during subduction, but there is also little evidence for subduction erosion. During collision the arc region may fail, resulting in thrusting, and the weakest point is the position of the active volcanic arc itself. Volcanic arcs shift position suddenly, and arcs can disappear during collision by overthrusting. Arcs are geologically ephemeral features and may have very short histories in comparison with most well-known older orogenic belts. The stratigraphic record of the basins within arc regions is complex. Because of a weak lithosphere the character of sedimentary basins may be unusual, and basins are commonly very deep and subside rapidly. There is a high sediment flux. The volcanic arc itself influences the stratigraphic record and basin development. The load imposed by the volcanic arc causes flexure and provides accommodation space. The volcanic arc thus can form the basin and supply most of its sediment. Tropical processes influence the mineralogy and apparent maturity of the sediment, especially volcanogenic material. A complex stratigraphy will result from the waxing and waning of volcanic activity.

Abstract The Eurasian margin in SE Asia is a geologically complex region situated at the edge of the Sundaland continent, and is mainly within Indonesia. The external margins of Sundaland are tectonically active zones characterized by intense seismicity and volcanic activity. The region is an obvious modern analogue for older orogens, with a continental core reassembled from blocks rifted from Gondwana, and surrounded by subduction zones for much of the Mesozoic and Cenozoic. It is a mountain belt in the process of formation, and contains many features typically associated with older Pacific margin orogens: there is active subduction, transfer of material at subduction and strike-slip boundaries, collision of oceanic plate buoyant features, arcs and continents, and abundant magmatism. The orogenic belt surrounds Sundaland and stretches from Sumatra into eastern Indonesia and the Philippines. The orogen changes character and width from west to east. Its development can be tectonically described only in terms of several small plates and it includes several suture zones. The western part of the orogenic belt, where the Indian plate is subducted beneath continental crust, is a relatively narrow single suture. Further east the orogenic belt includes multiple sutures and is up to 2000 km wide; there is less continental crust and more arc and ophiolitic crust, and there are several marginal oceanic basins. The orogen has grown to its present size during the Mesozoic and Cenozoic as a result of subduction. Continental growth has occurred in an episodic way, related primarily to arrival of continental fragments at subduction margins, after which subduction resumed in new locations. There have been subordinate contributions from ophiolite accretion, and arc magmatism. Relatively small amounts of material have been accreted during subduction from the downgoing plate. In eastern Indonesia the wide plate boundary zone includes continental fragments and several arcs, but the arcs are most vulnerable to destruction and disappearance. Rollback in the Banda region has produced major extension within the collision zone, but future contraction will eliminate most of the evidence for it, leaving a collage of continental fragments, similar to the older parts of Sundaland.

Abstract SE Asia lies at the convergence of the Eurasian, Pacific and Australian plates. The region is made up of many active arcs, extensional basins, and the remnants of similar tectonic environments developed throughout the Cenozoic. There are many important hydrothermal mineral deposits and prospects in SE Asia but their formation is often poorly understood due to the complicated tectonic history of this region and the knowledge of relationships between mineralization and tectonics. Plate reconstruction offers a framework to integrate geological and geochemical data that can be used to unravel the large-scale tectonic processes that affected mineralized provinces. We present examples of the information that can be derived from this approach and discuss the implications for understanding the origin of some hydrothermal mineral deposits in SE Asia.