SUPRA-SUBDUCTION ZONE (SSZ) HISTORY OF THE SOUTHERN INDIAN OCEAN: IMPLICATIONS FOR OPHIOLITES

Recent discovery of high velocity (lithospheric) slabs in the mid-deep mantle beneath the Southeast Indian Ocean (Simmons et al., 2015) provides new insight into compositional variations in oceanic crust formed at a mid-ocean ridge system, and their possible relations to ophiolites. In the SE Indian Ocean, a large anomalously thick, possibly deformed, north-dipping high-velocity slab extends from the Kerguelen Plateau NE to beneath Indonesia and Australia. Some Indian Ocean lavas differ in composition from "traditional" MORBs and overlap with those of many ophiolitic ones. These compositions suggest derivation of Indian mid-ocean lavas from subduction-modified mantle. The SE Indian Ocean apparently spread across a stagnant pre-existing slab located in the mid-lower mantle.

The idea of SSZ (suprasubduction zone) ophiolites, that is, ones derived from spreading centers above a subduction zone, has been well established for some 30 years. Most interpretations ascribe SSZ ophiolites to spreading in active fore-arc or back-arc Circumpacific-like settings, above an actively descending lithospheric slab. The Indian Ocean may represent another SSZ tectonic setting, that is, crust formed by mid-ocean spreading of a large ocean above extensive, previously subducted mantle. Thus spreading in a mid-oceanic environment can produce SSZ compositions. Emplacement of fragments of ocean crust-mantle formed in this way may have produced ophiolites in, say, Tethyan or similar environments. Understanding these ophiolite tectonic settings no doubt will require field structural and stratigraphic information, as well as geochemical data. The tectonic significance of oceanic crustal rock compositions in ophiolites may depend on the prior tectonic history of the region in question.