Island Arc Features and the Japanese Islands

Abstract

The modern island arc displays a zonal arrangement in distribution of gravity, earthquakes, volcanoes, heat flow, as well as in topography and geologic structures. The island arcs are to be classified into following three structural provinces ; that is, N, P, and I Belts from the oceanic side.
N Belt-the region of the negative gravity anomaly, high intensity of shallow earthquakes, and long-continued subsidence of the crust. Geologically the N Belt is a place of (1) the geosynclinal thick accumulation of sediments, (2) the basic-ultrabasic magmatism (“ophiolite”) and (3) the regional metamorphism of the low temperature-high pressure type.
P Belt the region of positive gravity anomaly, low heat flow and the non-volcanic geanticlinal uplift without significant folding. According to the seismic wave surveys, a high velocity layer (Vp 6.0 ± km/sec) of the crust rises up to a few kilometers below the surface.
I Belt-the region of the acidic-intermediate magmatism, the high heat flow and the block movement of the crust. Features of “the inner volcanic arc” and “the idio-geosyncline” by UMBGROVE are of this Belt. Intrusion of granitic batholith and metamorphism of high temperature-type seem to be significant characters of this Belt.
The Northeast Honshu is made up of two belts, P and I. The N Belt is occupied by the continental slope to the Japan trench. Thus, P, I and N Belts form the East Japan arc.The I Belt of the East Japan arc made its appearance in the middle of the Cenozoic time, while P and N Belts of the same arc date from the end of the Mesozoic time. The N Belt may be older than the P Belt.
The N, P and I Belts must be genetically related, to each other and these three should be considered altogether as one set of geologic development which reflects an orogenic process operating in the Earth's interior.
The N, P and I Belts of the East Japan arc are designated tentatively as “the Mizuho orogeny”. Present knowledge about the Mizuho orogeny suggests that the magmatism tends to decrease in its intensity as a whole, while the tectonism has tended to increase since the Early Miocene when the I Belt appeared.
The Southwest Honshu (including Shikoku), on the other hand, has no distinct features of the island arc, except for the zonal arrangement of the geological terrains. Here, an attempt is made to apply the concept of N-P-I Set of the island arc to the old orogenic belts such as Southwest Honshu. Thus, the Southwest Japan is composed of two sets of the N- (P) -I, being active in the late Paleozoic time (the Sangun-Hida Set) and active in the Mesozoic to Early Tertiary time (the Sanbagawa, Shimanto-Ryoke Set).
The writer proposes to apply his concept of N-P-I Set of the modern island arc to the old orogenic belts in the peripheral zones of oceans, because the modern island arc features are considered to be essentially same as those of the old orogenic belt of the Circum-Pacific Area.