Tectonic implications for the occurrence of ocean floor, hotspot, and island arc materials within accretionary prisms: Examples from the Mesozoic-Cenozoic NW Pacific Rim

On-land Mesozoic-Cenozoic accretionary prisms exposed in Japan commonly have basaltic rocks incorporated as blocks into melanges or fault zones during a prolonged history of subduction and/or obduction. Chemical signatures of these basaltic rocks and their mode of occurrence with sedimentary covers and/or associated sedimentary rocks indicate that most of these isolated small basaltic blocks consistently display a WPB chemistry, whereas large slabs of basaltic rocks around the Izu Arc collision zone show MORB chemistry with rare examples of IAT, BABB, and/or WPB affinities. Comparing with the present uniformitarian examples of convergent plate boundaries in the western Pacific that we know through the DSDP and ODP projects and submersible and seismic surveys, we can interpret some of the basaltic material with WPB affinity in the Japanese accretionary prisms as relict edifices of seamounts with hotspot origin. These hotspot-related basaltic rocks are commonly associated with reefal limestones and were incorporated into continental margin melanges either by submarine sliding from the downgoing oceanic plate or by shallow-level offscraping along decollement surfaces during the subduction of oceanic plates. Older, uplifted parts of the fossil accretionary prisms on the continent side further inward from the trench where the deeper levels of accreted material are exposed include larger amounts of basaltic blocks. This observation suggests that significant amount of underplating might have occurred in the deeper levels of oceanic crust along decollement zones at structurally lower depths. The metamorphic belts (e.g.Sambagawa, Chichibu, Shimanto etc.) have commonly alkaline rocks or plateau-type E-MORB basalts without any trace of N-MORB rocks with rare special exceptions. Besides these ordinary accretionary prism examples formed by a simple plate subduction system, another type of accretion resulting from island arc or ridge collision is observed to have occurred in both the eastern and western Izu Arc collision zone since the Miocene. The arc/ridge collision caused the incorporation of a particular assemblage of basaltic rocks in this tectonic accretion system which we interpret as an ophiolite. These _gophiolitic_h rocks are composed of various types of basaltic to rhyolitic, effusive and intrusive, dismembered, disrupted, sheared and faulted rocks that are locally associated with some hotspot and island arc igneous rocks and pelagic sedimentary rocks. This ophiolite assemblage is widely distributed particularly in the trench-slope break or within the forearc sliver boundary in the Circum Izu region. Deformation and metamorphism in these settings are weaker at shallower levels than those in the accretionary prisms, other than the Izu Arc collision zone. Based on these examples from Japan, we infer that ocean floor, hotspot, and island arc rocks become accreted into active continental margins either through ordinary subduction-accretion processes in a non-collisional subduction system or by obduction-accretion processes in a collisional island arc system.