Abstract

Although the reality of subduction has been greatly strengthened by recent investigations, there is little information dealing with the mechanisms by which material is subducted or accreted to the upper plate. An attempt to determine the gross evolution of subduction zones has been made, assuming that geographic variations in morphologic and geophysical characteristics of trenches can be transformed into temporal trends. Deformation associated with subduction extends across the lower trench slope, from the trench axis to the trench-slope break. This region is a rising tectonic element, but the upper slope is a subsiding region of sediment accumulation. An upper slope discontinuity separates this zone of subsidence from the rising frontal-arc block. Examination of very young trenches indicates that the upper-slope discontinuity marks the upper section of the continental or insular slope that existed before a subduction pulse began. As material is fed to the subduction zone, the distance between the upper slope discontinuity and the trench increases, and an accretionary prism develops, but its shape depends on the relative rates of sediment feed from the arc and from the offshore basin.

The lower boundary of the accretionary prism is the upper section of the seismic zone, which apparently widens and flattens as one or more accretionary prisms accumulate. The sediment cover on the downgoing plate and some of the igneous crust appears to be stripped off the plate before it reaches a point beneath the volcanic chain. Turbiditic sediments deposited in the trench axis are preferentially sheared off the underlying pelagic sediments and are accreted to the lower trench wall. The pelagic sediments and crustal material are probably accreted at deeper structural levels.

Where turbidites overlie pelagic sediments in the trench axis, the turbidites are stripped off in fold packets with axial surfaces having very low dips. These dewatered and rigidified structural units move up the lower slope, as subsequent packets are accreted. In trenches that subduct lithosphere carrying very thin pelagic sediment covers, accretion and uplift of crustal slabs seem to occur as topographic irregularities enter the trench.

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