Present day continental margins are of two types: those of 'Atlantic type' on the aseismic non volcanic margins of oceans expanding by ocean-floor spreading, and those of 'Pacific type' where the lithosphere of a spreading ocean descends beneath oceanic trenches. A new model for the evolution of orogenic belts is proposed whereby spreading-expanding oceans are converted into spreading-contracting oceans by the development of marginal oceanic trenches. Orthotectonic orogens on 'Pacific type' continental margins result from the deformation and metamorphism of continental rise accumulations built on 'Atlantic type' margins. Paired metamorphic belts characterize orthotectonic orogens and consist of a high P T blueschist belt developed in the region of the oceanic trench and a high T P belt on the continental side. The thick intermediate layer forming a basaltic root to orthotectonic orogens is developed by the intrusion of massive volumes of andesitic and basaltic magmas produced by melting and/or partial melting of the descending lithosphere plate. The orthotectonic orogens forming island arcs such as Japan lie entirely within ocean basins and have an ensimatic origin. The crust of island arc complexes is built from volcanic accumulation and by the 'scraping up' of oceanic sediments. During the final stages of contraction of an ocean basin, paratectonic orogens are developed by the thickening and deformation of marginal sedimentary wedges and oceanic sediments during the 'collision' between opposing continental margins. It is suggested that cycles of oceanic expansion and contraction related to the growth, movement, and destruction of lithosphere plates are the fundamental cause of all major global tectonic features from at least late Pre-Cambrian times to the present day. © 1969.
