Accretion of ancient oceanic crust as in ophiolites

Fossil oceanic crust preserved in ophiolites represents relics of different stages of the Wilson cycle evolution of ancient ocean basins & has been incorporated into continental margins through collisional and/or accretionary orogenic events. Suprasubduction zone (SSZ) ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Mantle flow & slab rollback result in one or more episodes of arc splitting and basin opening, producing a collage of ’proto-arc and forearc oceanic lithosphere’ in suprasubduction zone settings. Unusual occurrence of fertile peridotites and high-Mg andesites in forearc oceanic crust results from the injection of high-temperature asthenospheric material into the mantle wedge in these rollback cycles. Tethyan ophiolites in the eastern Mediterranean region range from Ligurian-type ’continental margin’ ophiolites to SSZ ophiolites. Continental margin ophiolites consist of Hess-type oceanic crust with MORB affinities and represent ancient oceanic crust developed in embryonic seaways and para-rift basins. SSZ ophiolites (i.e. Mirdita, Troodos, Kizildag, Oman) generally have Penrose-type oceanic crust and contain well-developed sheeted dyke complexes developed due to robust magmatic extension beneath narrow rift zones during their seafloor spreading history. Igneous accretion of these SSZ Tethyan ophiolites involved upper plate extension & advanced melting of previously depleted asthenosphere in host basins, showing a progressive evolution from MORB-like to IAT to boninitic proto-arc assemblages. Tethyan ophiolites structurally overlie passive continental margins, microcontinents or island arcs, whose collisions with trenches and attempted subduction led to induced subduction initiation in the region and/or resulted in the formation of intra-continental mountain belts. Cordilleran ophiolites in accretionary-type orogenic belts structurally overlie subduction-accretion complexes & are incorporated into active continental margins via progressive underthrusting of oceanic material and/or through ridge-trench interactions. Cordilleran-type ophiolites are commonly polygenetic, developed on and across a deformed, heterogeneous oceanic basement and may include fully developed island arc sequences having island arc tholeiite (IAT) to calcalkaline affinities, pyroclastic rocks, and felsic differentiates. Prolonged history of subduction with variable polarity and kinematics may generate nested Cordilleran ophiolites with different ages and chemical compositions that may have been affected by orogen-parallel wrench faulting due to oblique convergence. The best examples are seen in the California, Japan, and the Philippines. Variations in the structural architecture and chemical compositions of ophiolites are a result of their origin in various tectonic environments and reflect different geodynamic evolutionary paths.