Suprasubduction

Supra-subduction zone (SSZ) refers to a tectonic setting in which magmatism and crustal growth occur in the mantle wedge above a subducting slab, rather than at a mid-ocean ridge. The term is used to describe rocks and ophiolites that record subduction-zone–related melting and mantle-wedge processes in a zone directly above an actively subducting plate. SSZ magmatism is often associated with subduction initiation or slab rollback under hot mantle conditions and fluid-fluxed melting.

Geology and petrology in SSZ settings typically involve melts generated in a hydrated mantle wedge above a subducting slab. Early magmatic products commonly include boninite, high-magnesium andesite, and primitive arc basalts, which differ from classical island-arc tholeiites. The resulting ophiolitic sequences may expose ultramafic mantle blocks, cumulates, a sheeted-dike complex, and volcanic rocks, reflecting rapid crustal creation in a subduction-related regime.

Geochemistry of SSZ rocks often shows elevated Mg content and volatile-rich signatures, with trace-element patterns influenced by slab-derived fluids and melts. These rocks can exhibit enrichments in fluid-mobile elements and distinctive HFSE and REE patterns that set them apart from MORB or mature island-arc rocks, providing a diagnostic contrast for supra-subduction zone processes.

Examples and significance: Classic SSZ ophiolites are represented by belts such as Oman/Semail and Troodos, where rock sequences and geochemistry are interpreted as forming in the forearc mantle wedge above an active subduction zone. The SSZ concept has been influential in models of subduction initiation, arc evolution, and the diversity of oceanic crust.