The geology of the southern Mariana fore-arc crust: Implications for the scale of Eocene volcanism in the western Pacific

• We present results of a HOV dive at the crust-mantle boundary in the Mariana fore-arc.
• The first geological map of an ophiolitic fore-arc is presented.
• The igneous rocks here are associated with subduction initiation and are ∼51.5 Ma∼51.5 Ma.
• CO2 degassing from early-arc magmas and large igneous provinces significantly affected Early Eocene climates.

The Mariana fore-arc southeast of Guam consists of ophiolitic lithologies related to subduction initiation and early-arc development. Ages of zircons extracted from gabbroic rocks within this sequence are 51.5±0.7 Ma51.5±0.7 Ma, synchronous with similar rocks from the Bonin fore-arc 1700 km to the north. Basalts collected from the Izu fore-arc are similar to those of the Bonin and Mariana fore-arcs extending the distance of this ophiolitic geology to the entire 3000 km length of the IBM fore-arc. The Tonga fore-arc has similar lithologies and ages, suggesting that subduction began nearly simultaneously along much of the western margin of the Pacific plate. We postulate that closing of the Tethyan suture provided the trigger for subduction initiation in the Western Pacific. The volume of basalt erupted near western Pacific trenches associated with subduction initiation and early-arc development in the early Eocene could rival the volumes of large igneous provinces. The eruption of these basalts corresponds with the height of the Early Eocene Climatic Optimum (EECO), when global atmospheric temperatures were likely at or near their Cenozoic maximum. Therefore, CO2 vented during this volcanism as well as that associated with the North Atlantic Igneous Province, the Siletzia terrane, and slab rollback and detachment beneath central and east Asia were likely responsible for the EECO.