DIVERSITY OF ISLAND ARCS: JAPAN, PHILIPPINES, NORTHERN MOLUCCAS

Abstract

Modern bathymetric, geophysical, and volcanic studies have revealed a remarkably consistent principal profile for island arcs of the Western Pacific. Examination of surface geology, however, suggests great diversity in the geological development of island arcs. Three island arc complexes show the diversity well: Japan, the Philippines, and the Northern Moluccas.

Although Japan appears to have been an island arc in roughly its present configuration only since Early Miocene time, the surface geology indicates that Japan has formed part of the continental margin of Asia at least since Permian and probably Devonian time. Subduction polarity, as displayed in paired metamorphic belts; juxtaposition of continental rise prisms with contemporaneous oceanic suites; and overall tectonic fabric strongly supports belief in the existence of the Pacific Ocean since the Devonian. Hokkaido is a reversed island arc segment which together with Sakhalin is probably a relatively recent addition to the northeast Asian continental perimeter.

The Philippines were formed by the Late Oligocene coalescence of at least four island arc-subduction zone complexes into a single archipelagic basin which has remained more or less coherent throughout Neogene time. Although the oldest Philippine fossils are Permian, the island arcs probably did not start to form until Late Cretaceous time. Neogene volcanism, intrusion and sedimentation are thought to be creating a new small continent out of materials that were originally entirely oceanic.

Island arc systems flanking the Molucca Sea were born in Late Cretaceous time and now appear to be in a stage of coalescence similar to that of the Philippines in Late Oligocene time. The convergence of two west-dipping and one east-dipping Benioff zones, the existence of a very large negative isostatic gravity anomaly, the outcrop of fossil subduction zones and the existence of short topographic trench segments indicate an active, youthful region of crustal construction.