| Abstract: | Older concepts of island arcs, prior to the 1960's, were dominated by the Indonesian model and described in terms of geosynclinal theory, but not altogether fruitfully. Platetectonics theory has given new insights into the genesis, mode of formation, behaviour and ultimate fate of island arcs. Subduction with descent of the lithospheric slab is the governing phenomenon. As the slab descends it contributes directly and indirectly to the formation of a thickened rim (the nascent arc) on the adjacent superposed plate. The direct contribution comes from the upper part of the slab (oceanic crust) and varies progressively in type as the slab sinks more deeply. This results in the island arc series, front to rear, of tholeiitic to calcalkaline to high potash rocks, with concomitant geochemical indices. The indirect contribution stems from the slab's interference with the thermal regime of the intervening mantle and both aids and affects the passage of melts to the surface. A melange of surface and near-surface rocks is piled against the arc edge to form a break in the arc-trench slope and is subject to high pressure metamorphism. As the arc grows in bulk and width, the volcanic axis shifts to the rear. The arc components will then be: trench inner wall—trench slope break—arc trench gap—main volcanic arc. A high temperature metamorphic belt will underly the main arc. Because the hanging slab tends to retreat, and the arc tends to follow, the retroarc area may develop extensional features floored by extrusions from mantle diapirs. The arc will become separated from the nearest continent by a sub-oceanic marginal sea. With further rifting, a remnant arc may be detached from the main arc with an interarc basin between. The subduction zone at this or earlier stage may be choked and flip, with reverse dip, to the other side of the arc. Subsequent retreat consumes the marginal sea so that the arc system re-tracks toward the continent possibly to become accreted to it. Many variables are involved in this general process so that the individual character of arcs is to be expected. Models of continental accretion are not wholly convincing. This, and other primary problems can be solved with more data, but drawn from a wide range of disciplines. In particular, results from experimental geochemistry, seismology and theoretical physics are essential to additional understanding of the major problem, the detailed behaviour of the slab and its effect within the asthenosphere. |
