Subduction of diverging plates and the principles of slab window formation

Abstract

Consumption of an ocean basin by subduction commonly brings a sea-floor-spreading ridge toward a deep-sea trench. If plate divergence and convergence continue after the ridge intersects the subduction zone, a slab window forms between the subducted parts of the diverging oceanic plates, producing anomalous thermal, physical and chemical effects in the surrounding asthenospheric mantle. In turn, these conditions alter the tectonic and magmatic evolution of the overriding plate, usually disturbing ordinary fore-arc and arc regimes. Differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate. Anomalous magmatism from fore arc to back arc may be accompanied by fore-arc metamorphism, strike-slip faulting, uplift, extension and, in extreme cases, rifting. The shape and size of the window are controlled mainly by the pre-subduction ridge-transform-trench configuration, slab dip angles and vectors of plate convergence. Subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates. Subducted transform faults continue to be active, usually as oblique-slip faults, until the plates separate. As transform faults subduct, they become longer on the plate which occupies the acute angle between ridge and trench, and shorter on the other plate. Trains of isolated windows produced by subduction of a segmented ridge-transform system progressively expand during descent, commonly merging together to form a composite slab window. Oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows, one at each site of ridge-trench intersection. Above a slab window, arc volcanism diminishes and may be replaced by volcanism of mid-ocean ridge or rift affinity. The change in chemical character reflects various processes including elevated heat flow, decreasing hydration of the upper mantle, juxtaposition of supra- and sub-slab mantle reservoris, asthenospheric upwelling and melting of the trailing plate edges. If the slab window migrates, the anomalous magmatic regime may be replaced by renewed arc volcanism. Identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications.