THE OPHIOLITE PARADOX RESOLVED?

We restrict the term ophiolite to large obducted slabs with the full range (Coleman, 1972), from base to top, of lherzolite/ariegite, harzburgite, dunite, gabbro, sheeted dyke complex, pillow basalts, and sediments, commonly with a two-pyroxene mafic granulite metamorphic sole attached to the base of the complex; they are mainly early Ordovician and mid-Cretaceous. Their principal enigma is that they were generated by sea-floor spreading (Gass, 1959) yet have an arc tholeiitic to calc-alkaline volcanic affinity (Miyashiro, 1978) that may include fore-arc boninites (high-temperature/low-pressure, hydrous high Mg and Si andesites). Many ophiolites have complexly-deformed assemblages that suggest fracture zone geology, which has led to models involving the nucleation of subduction zones on fracture zones. Arc-related sea-floor-spreading has been considered to be either pre/infant-arc or post/syn-arc. We propose a new model, with syn-arc boninites, involving a stable ridge/trench/trench triple junction, the ridge being between the two upper plates. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created, the arc and fore-arc lengthen, and a syn-arc ophiolite complex is generated that ages along arc-strike; a distinctive diachronous boninite/arc volcanic stratigraphy develops. Dikes in the ophiolite are normal to oblique to the trench as are magnetic anomalies in the “back-arc” basin. Boninites are generated in the fore-arc under the aqueous, LP/HT, regime at the ridge above the dehydrating slab. The mafic protolith, garnet/two pyroxene, metamorphic sole is generated in and sliced from the subducting slab and sub-accreted to the base of the overriding lithosphere at about 1000°C, and ten to twelve million years from the ridge axis, where the ophiolite is about twelve kilometers thick, when the thickness of the ophiolite is buffered by the subducting slab. Obduction of the SSZ ophiolite with its subjacent sole occurs whenever the oceanic arc faces and encroaches onto a continental margin edge.