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Aseismic ridges on underthrusting oceanic plates often trend into cusps or irregular indentations in the trace of the subduction zone. For example, the Hawaii-Emperor Ridge trends into the Kuril-Aleutian cusp, and the Marianas arc is bounded by the Marcus-Necker Ridge on the north and the Caroline Ridge on the south. The association between ridges and cusps is too common to be due to chance; it is proposed that the extra buoyancy of the plate with its aseismic ridge gives the plate greater resistance to sinking. This would inhibit back-arc extension and thereby produce a notch in the subduction zone. Island arcs may, therefore, acquire their curvature by additional constraints than the Earth’s curvature. The geology of about 15 such cusp areas is examined for evidence to test the hypothesis that cusps were caused by subducted aseismic ridges. This hypothesis applies only to cases where extensional basins lie behind the arcs. There also appear to be cases where the trace of the subduction zone has been modified not by inhibited back-arc spreading but by splintering of the overthrusting and possibly the underthrusting plate as well. Extremely high, massive aseismic ridges might induce arc polarity reversals and thereby assume the role of protocontinental nuclei.

Seismicity and volcanism are examined where aseismic ridges are being subducted; there are several examples of reduced seismicity that cannot be explained by insufficient sampling time. By modifying the geometry of the subduction zone, the downgoing ridges necessarily affect seismicity. In addition, the plate containing the ridge may be thinner and hotter and more likely to deform by creep. There is no systematic increase or decrease in the number of andesite volcanoes where the ridges are subducted. However, lines of volcanoes and sometimes other kinds of geologic and seismic provinces may stop or start at the arc-ridge intersections. This is attributed to segmenting of the lithosphere into distinct tongues, each tongue acting more or less independently. Aseismic ridges would act as lines of weakness along which the downthrust slab becomes detached.

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