Two Major Island Arc-Trench Systems in the Northwest Pacific Ocean and the Philippine Basin - (I)

Abstract

Geological researches of the Philippine Basin have been greatly advanced by works on the islands of frontal arc systems, by deep sea drillings, and by marine geophysical surveys. However, we cannot constract any concrete concept on the basin genesis. We can only suggest some speculative and qualitative indications arising from the synthesis of data, as follows.1 : Since the Middle Eocene or in places since the Oligocene or Early Miocene, the Philippine Basin has been almost entirely under sea, where thick beds of limestone and miscellaneous elastics often rich in fossils, together with volcanic ash, volcanic conglomerates and breccias, and volcanic rocks mostly basic in character were formed.2 : Synthesis of these data also suggests that the basin was subjected to wide spread crustal movements at least three times, namely the age ranging probably from the Late Cretaceous to the Early Eocene, the Mid-Tertiary and lastly the Pleistocene, and that the fundamental features of the submarine physiography as shown in the basin at present were formed most probably in an early stage of the Eocene.3 : It has been announced that the Daito Ridges area is constructed of continental crust (MIZUNO, 1975), extent of which, however, remained almost unexplained. In the writer's conception, areas extending widely from the Izu-Hakone crustal block in Central Honshu, Japan, so far as Izu Is. and adjacent areas of Yap Is., Palao Is., Halmahera Is. and Vogelkopf Penninsula of New Guinea are also constructed almost no doubt of continental crust. It may be rather possible that the basin is constructed entirely of continental crust, and further that the continental crust had been long remained in a state of continental land until it was covered by the Tertiary marine water.4 : The characteristic physiographic pattern shown by the NW-SE parallel arrangement of Daito Ridges, Shikoku Basin, Kinan Sea-mounts Area together with the Izu-Mariana insular arc-trench system is in fact note-worthy feature in the northern part of the Philippine Basin, the arrangement being nearly perpendicular in direction against the arcuate structure of the frontal insular arc-trench system of East Asia in the Angara Land. The characteristic feature may strongly suggests that the Philippine Basin belongs to a global structural unit, quite different from the Angara Land, and that the basin crust is most probably covered by the overthrusted plane of the East Asiatic frontal belt drifting away from the continental Angala Land.5 : In structure, the Philippine Basin extends southeasterly so far as New Zealand area as shown in Fig. 2, sarrounding Austrarian continent of the Gondwana Land, and constructing a frontal tectonic zone of the Gondwana. The tectonic zone is appropriately called the Austro-Philippine Tectonic Zone by the present writer, the tectonic zone being comparable in structural development with the Northwest Pacific Tectonic Zone (TATEIWA, 1976, 1979) of the Angara Land.6 : It is percieved that the Mariana trough has been extended at rates up to 10cm/yr. or more during the last 2-3 m. y. (KARIG, 1973). In general concept, however, these phenomena may be a matter of naturally expect in considering the genetic relationships between arcuate structure of insular arcs and inside marginal sea. Hypothetical setting of sea-floor spreading axis, specially fixed, for elucidation of the phenomena, seems to have no convincing basis.