PETIT-SPOT SUBMARINE VOLCANOES AS A GEOLOGICAL PERTURBATION OF SUBDUCTING PLATE

Petit-spot submarine volcanoes erupted during the recent 10 million years on the outer rise at the subducting NW Pacific Plate (Hirano et al., 2006; Machida et al., 2015). The magmas forming these volcanoes originate below the lithosphere and ascend along the concavely flexed zone of the outer rise before plate subduction at the trench (Hirano et al., 2006). Such tiny volcanoes are likely to be ubiquitous in zones of plate flexure, which have been reported from the oceanward slope of the Tonga, Chile, and Java trenches (Hirano et al., 2008; 2013; Taneja et al., 2015). The widespread occurrence of petit-spot prior to plate subduction is supported by the reinterpretation of the origin of alkaline basalts found within accretionary complexes.

A petit-spot melt must provide information on the asthenospheric components located just below the lithosphere. However, recent studies indicate that most of the petit-spots that erupted on a concavely flexed plate highly reacted with the depleted lithosphere during magma ascending to surface. The entrained xenolith from the depleted mantle is geochemically disturbed by the wall–rock interaction (Pilet et al., 2016). Tonegawa et al. (2018) observed some structural changes in the NW Pacific crust and moho caused by the plate flexures and petit-spot volcanic activities. The accreted petit-spot would be a successful candidate to know the subsurface structure of subducting plates.

We examined the “potential petit-spot” in Cretaceous accretionary complexes and forearc basins of the Pacific Rim (Shimanto Belts of SW Japan, Hidaka Belt of central Hokkaido, Tokoro and Nemuro Belts of E Hokkaido, Franciscan Belt of California, and Santa Rosa of Costa Rica) (Buches et al., 2013; Endo et al., 2018; this study). The petit-spot alkali basalts could easily be captured into accretionary complexes because such lavas are floating onto the pelagic sediments prior to the plate subduction. Although alkali basalts in the accretionary complex had traditionally been discriminated by ocean island basalts implying a seamount accretion, we newly report several lavas, sills, and lamprophyres recognized as “accreted” petit-spots prior to plate subduction. They are all characterized by occurrences without limestones, Zr/Hf ratios, and younger eruption ages than seamount OIBs.