Syn-emplacement ophiolites and relationship to supercontinent cycle

Ophiolites are fragments of oceanic lithosphere now entrained within continents. Some ophiolites, such as the Bay of Islands ophiolite in the Appalachian-Caledonian Orogen and the Semail ophiolite in the Alpine-Himalayan Orogen, are characterized by a metamorphic sole along their structural base. The sole records metamorphism along a subduction zone interface between upper and lower plates with structural imbrication resulting in an inverted metamorphic assemblage of higher grade over lower grade. The sole is juxtaposed with the ophiolite, which forms through extension in the upper plate of the subduction zone, during attempted subduction of, and subsequent emplacement onto, a continental margin. We argue that such ophiolites form in small interior ocean basins during supercontinent breakup with generation and expansion of these oceans compensated by lithospheric compression and initiation of multiple subduction zones. Subduction initiation is induced adjacent to a continental margin, likely along lithospheric discontinuities such as transform systems that formed during continental lithospheric extension and interior ocean basin formation. In such newly formed subduction zones, the subducting plate has limited lateral extent enabling its rapid evolution through trench rollback leading to ophiolite generation in the upper plate. This infant subduction system consumes the narrow tract of oceanic lithosphere adjacent to the continental margin leading to ophiolite obduction through attempted margin underthrusting. These convergent boundaries are secondary to already established and ongoing subduction zones within small interior ocean basins. They correspond temporally with the termination of convergent plate interaction and plate reorganization elsewhere in the global plate network, which we argue is the likely primary driver, triggering a cascading sequence of events, that induces the initiation of these transient secondary systems.