Abstract
Abstract In the Wilson cycle, there is a change from an opening to a closing ocean when subduction begins. Subduction initiation is commonly identified as a major problem in plate tectonics and is said to be nowhere observable, yet there are many young subduction zones at the west Pacific margins and in eastern Indonesia. Few studies have considered these examples. Banda subduction developed by the eastwards propagation of the Java trench into an oceanic embayment by tearing along a former ocean–continent boundary. The earlier subducted slab provided the driving force to drag down unsubducted oceanic lithosphere. Although this process may be common, it does not account for young subduction zones near Sulawesi at different stages of development. Subduction began there at the edges of ocean basins, not at former spreading centres or transforms. It initiated at a point where there were major differences in elevation between the ocean floor and the adjacent hot, weak and thickened arc/continental crust. The age of the ocean crust appears to be unimportant. A close relationship with extension is marked by the dramatic elevation of land, the exhumation of deep crust and the spectacular subsidence of basins, raising questions about the time required to move from no subduction to active subduction, and how initiation can be identified in the geological record.
Highlights
A crucial stage in the Wilson cycle is the change from opening to closing of an ocean basin and the development of subduction
Producing new trenches is considered difficult, and subduction initiation is commonly identified as a major problem in plate tectonics (e.g. McKenzie 1977), ‘not answered by direct observation’
Recent work in the region means that we have a significantly improved knowledge of the geology and tectonics of the region from the North Sulawesi trench to the North Banda Sea
Summary
A crucial stage in the Wilson cycle is the change from opening to closing of an ocean basin and the development of subduction. One early proposal was the initiation of subduction at a transform in a major ocean basin (e.g. Uyeda & Ben-Avraham 1972; Casey & Dewey 1984; Stern & Bloomer 1992) with a type area in the Izu–Bonin–Marianas arc. This model has linked forearcs, supra-subduction zone ophiolites, the unusual geochemistry of volcanic rocks such as boninites, and the growth of arc and continental crust. I contend here that different stages in the development of subduction, from the earliest stages of the downward flexure of oceanic crust to the formation of a well-defined Benioff zone, can be observed in the eastern Indonesia and southern Philippines region. This mechanism does not account for the many young subduction zones nearby, which are not fully connected together
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