Subduction and Aseismic Ridges

Abstract

WHETHER or not aseismic ridges and seamounts on the ocean floor are due to stationary hot spots^1–7 or some other process, one may ask what happens when an aseismic ridge is forced into a subduction zone. Here I consider one aspect of the problem, namely whether or not the plan view of island arcs and their associated trenches has been influenced by aseismic ridges being subducted. Island arc type subduction occurs primarily in the western Pacific and the Indian Oceans. As indicated in Fig. 1, there are a small number of aseismic ridges in the vicinity of the north and western Pacific and Indian trenches. The bathymetric charts recently published by Chase et al.⁸ were used to obtain the general outlines of the Pacific aseismic ridges. The two most massive Pacific ridges, the Emperor and Caroline, intersect the trench system near prominent cusps, where one arc gives way to the next. In addition, the Marcus–Necker ridge intersects the northern edge of the arcuate portion of the Marianas trench; the Gulf of Alaska seamounts, presumably generated by the Juan de Fuca and/or its neighbour, the Yellowstone plume^5–7, intersect the Aleutians only slightly west of the cuspate eastern end of the Aleutian arc. In the Indian Ocean, the two principal cusps in the boundary between Asian and Indo–Australian plates are now intra-continental. It may reasonably be supposed, from its arcuate character and continuity with the Indonesian arc, that the Himalayan arc replicates the shape of an extinct island arc system lying in the Tethyan Ocean, that lay between India and central Asia until it was closed by subduction in later Tertiary times. Whether or not the Himalayan arc preserves the shape of an old island arc, the Indian Ocean's two prominent aseismic ridges⁹ extend, with comparatively minor extrapolation, into the two major cusps of the Himalayan tectonic arc.