EVOLUTION OF THE SUPRA-SUBDUCTION ZONE (SSZ) OPHIOLITE CONCEPT: FROM MIYASHIRO (1973) TO THE PRESENT-DAY (2023) (Invited Presentation)

The concept of Supra-Subduction Zone (SSZ) ophiolites essentially began when Miyashiro (in 1973) famously initiated a debate on the tectonic setting of ophiolite complexes by proposing that ‘the Troodos ophiolitic complex was probably formed in an island arc’. This assertion questioned the consensus at that time, that ophiolite complexes were fragments of oceanic crust formed at mid-ocean ridges. Miyashiro’s arguments were based on similarities in lava composition between the Troodos ophiolite and the Izu Island arc. Miyashiro’s paper stimulated a decade of articles, some acrimonious, debating the relative importance of geology and geochemistry in assigning ophiolites to their correct tectonic setting. Not surprisingly, perhaps, both geology and geochemistry proved significant and essential, with geological features (such as crustal thickness, dyke swarms and tectonized peridotites) establishing formation at a spreading axis, and geochemical features (such as selective enrichments in subduction-mobile and redox-sensitive elements) establishing the presence of a spreading axis underlain by a subduction zone. This led Pearce and co-workers, in 1984, to formally define ophiolites with ocean crustal geology but subduction-related geochemistry as SSZ ophiolites. Although this term is now in common use, it was controversial because of the lack of precise modern analogues until Stern and Bloomer, in 1992, were able to convincingly explain that many SSZ ophiolites were related to slab rollback following subduction initiation, a process that last took place on a global scale in the Eocene. Their model was subsequently supported by (in particular) submersible mapping and sampling, dredging and drilling of in situ SSZ oceanic lithosphere in the Eocene outer forearc of the Izu-Bonin-Mariana system. Presently (in 2023), detailed tectonic and petrogenetic models have been developed for this type of SSZ setting and will be summarized (together with other types of SSZ ophiolite) in this presentation. In retrospect, much of the initial controversy emanating from Miyashiro’s assertion was the result of geologists and geochemists working independently and basing their conclusions solely on their evidence from their own disciplines, and so this is a good historical reminder of the value of interdisciplinary research in solving geologic problems.