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The sea floor inside island arcs characteristically is less deep and has higher heat flow than the ocean floor outside the arc and trench system. Direct evidence from drilling and indirect evidence based on thin sediment cover, interrupted geologic trends, paleomagnetic studies, and fitting of pre-drift continental margins show that the lithosphere behind island arcs is young and commonly did not form on the mid-oceanic ridge system. The slab of dense lithosphere that flexes and sinks spontaneously through the asthenosphere under arcs is shown to sink at an angle that is steeper than the plane of the earthquakes. As a consequence, the trench and arc migrate seaward against the retreating line of flexure of the suboceanic lithosphere. Part of the warm asthenosphere pushed aside by the plunging slab migrates up by creep and as magma, then cools and forms new lithosphere in the extensional region behind the advancing island arc. Extension is favored where the lithospheric plate behind the arc is moving tangentially or away from the plate outside the arc.

A series of maps shows the tectonic development of the western Pacific from mid-Eocene to the present. The maps are based on concepts developed from sea-floor spreading and the new global tectonics, and incorporate the postulate that new lithosphere can form behind advancing island arcs. The origin and later deformation of arcs and basins are shown as resulting mainly from the great shear between the northward-moving Australian plate and the northwestward-moving Pacific plate.

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