SUPRA-SUBDUCTION ZONE OPHIOLITES: THE SEARCH FOR MODERN ANALOGUES

Abstract

the development of ideas on modern analogues for ophiolite complexes can be divided into three parts. In the first (1963 to 1972), ophiolites were mainly viewed as on-land analogues of the crust and upper mantle formed at mid-ocean ridges. The second (from 1972 to 1984) is a period of paradigm shift to the view that the majority of ophiolites forms above subduction zones, and can be grouped as the class known as supra-subduction zone ophiolites. The third (from 1984 onward) is a period that has led to a much better understanding of what these supra-subduction zone settings represent. The realization that many ophiolites were linked to subduction resulted from a combination of advances in geochemical discrimination methods, exploration through dredging and drilling of arc-basin systems of the Western Pacific and South Atlantic, and further field studies in ophiolite terranes. The transition in ideas from a mid-ocean ridge to a subduction origin was prolonged and much-debated, primarily because many of the best-developed ophiolites had the geochemical characteristics of subduction environments, yet none of the geological characteristics (arc volcanics and overlying volcanogenic sediments) thought to characterize those environments. This in turn led to a need for a non-generic term, supra-subduction zone, to describe this ophiolite type. Since the supra-subduction zone concept became established, precise analogues continued to be debated. Subduction initiation and ridge-trench intersections are probably the most important settings, with back-arc rifting and spreading and oblique subduction also significant.