Pacific‐type orogeny revisited: Miyashiro‐type orogeny proposed

Abstract

Abstract The concept of Pacific‐type orogeny is revised, based on an assessment of geologic data collected from the Japanese Islands during the past 25 years. The formation of a passive continental margin after the birth of the Pacific Ocean at 600 Ma was followed by the initiation of oceanic plate subduction at 450 Ma. Since then, four episodes of Pacific‐type orogeny have occurred to create an orogenic belt 400 km wide that gradually grew both oceanward and downward. The orogenic belt consists mainly of an accretionary complex tectonically interlayered with thin (<2 km thick), subhorizontal, high‐P/T regional metamorphic belts. Both the accretionary complex and the high‐P/T rocks were intruded by granitoids ∼100 million years after the formation of the accretionary complex. The intrusion of calc‐alkaline (CA) plutons was synchronous with the exhumation of high‐P/T schist belts. Ages from microfossils and K‐Ar analysis suggest that the orogenic climax happened at a time of mid‐oceanic ridge subduction. The orogenic climax was characterized by the formation of major subhorizontal orogenic structures, the exhumation of high‐P/T schist belts by wedge extrusion and subsequent domed uplift, and the intrusion‐extrusion of CA magma dominantly produced by slab melting. The orogenic climax ended soon after ridge subduction, and thereafter a new Pacific‐type orogeny began. A single Pacific‐type orogenic cycle may correspond to the interaction of the Asian continental margin with one major Pacific oceanic plate. Ophiolites in Japan occur as accreted material and are not of island‐arc but of plume origin. They presumably formed after the birth of the southern Pacific superplume at 600 Ma, and did not modify the cordilleran‐type orogeny in a major way. Microplates, fore‐arc slivers, intra‐oceanic arc collisions and the opening of back‐arc basins clearly contributed to cordilleran orogenesis. However, they were of secondary importance and served only to modify pre‐existing major orogenic components. The most important cause of cordilleran‐type orogeny is the subduction of a mid‐oceanic ridge, by which the volume of continental crust increases through the transfer of granitic melt from the subducting oceanic crust to an orogenic welt. Accretionary complexes are composed mainly of recycled granitic sediments with minor amounts of oceanic material, which indicate that the accretion of oceanic material, including huge oceanic plateaus, was not significant for orogenic growth. Instead, the formation and intrusion of granitoids are the keys to continental growth, which is the most important process in Pacific‐type orogeny. Collision‐type orogeny does not increase the volume of continental crust. The name ‘Miyashiro‐type orogeny’ is proposed for this revised concept of Pacific‐type or cordilleran‐type orogeny, in order to commemorate Professor A. Miyashiro's many contributions to a better understanding of orogenesis.