ABSTRACT Ophiolite complexes represent fragments of ocean crust and mantle formed at spreading centers and emplaced on land. The setting of their origin, whether at mid-ocean ridges, back-arc basins, or forearc basins has been debated. Geochemical classification of many ophiolite extrusive rocks reflect an approach interpreting their tectonic environment as the same as rocks with similar compositions formed in various modern oceanic settings. This approach has pointed to the formation of many ophiolitic extrusive rocks in a supra-subduction zone (SSZ) environment. Paradoxically, structural and stratigraphic evidence suggests that many apparent SSZ-produced ophiolite complexes are more consistent with mid-ocean ridge settings. Compositions of lavas in the southeastern Indian Ocean resemble those of modern SSZ environments and SSZ ophiolites, although Indian Ocean lavas clearly formed in a mid-ocean ridge setting. These facts suggest that an interpretation of the tectonic environment of ophiolite formation based solely on their geochemistry may be unwarranted. New seismic images revealing extensive Mesozoic subduction zones beneath the southern Indian Ocean provide one mechanism to explain this apparent paradox. Cenozoic mid-ocean-ridge–derived ocean floor throughout the southern Indian Ocean apparently formed above former sites of subduction. Compositional remnants of previously subducted mantle in the upper mantle were involved in generation of mid-ocean ridge lavas. The concept of historical contingency may help resolve the ambiguity on understanding the environment of origin of ophiolites. Many ophiolites with “SSZ” compositions may have formed in a mid-ocean ridge setting such as the southeastern Indian Ocean.

Convergent margin tectonic settings involving accretion of large turbidite fans represent important sites of growth and regeneration of continental crust. The newly accreted continental crust consists of an upper crustal layer of recycled crustal detritus (turbidites) underlain by a lower crustal layer of tectonically imbricated oceanic crust, and/or rifted and thinned continental crust, along with underplated magmatic materials; the new lower crust represents additions to continental crustal volume differentiated from the mantle. This two-tiered crust is of average continental crustal thickness and is isostatically balanced near sea level, resulting in remarkable stability. The Paleozoic Tasman orogen of eastern Australia is the archetypal example of this style of orogeny, representing continental growth rates of cubic kilometers per year of material that does not return to the mantle by oceanic plate-tectonic recycling. The Neoproterozoic Pan-African Damara orogen of SW Africa is a similar orogen, whereas the Mesozoic Rangitatan orogen or Rakaia wedge of New Zealand illustrates the transition of the convergent margin from a Lachlan-type to more recognizable “ring of fire”-type orogen. These orogens illustrate continental growth from the shortening of deep marine successions and their oceanic crustal basement involving subduction-accretion. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of monotonous turbidites and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism reflect the crustal thickening phase, whereas the posttectonic granitoids and surficial volcanic deposits give the timing of cratonization. The turbidites represent fertile sources for crustal melting and are the main sources for the S-type granites.

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