SUBDUCTION INITIATION TERRANES IN THE GEOLOGICAL RECORD

Investigations of the Izu-Bonin-Mariana (IBM) forearc over the past several decades, culminating in IODP Expedition 352, show the forearc between at least 12^oN to 34^oN latitude was built during a large scale but short-lived subduction initiation event that took place at c. 52 Ma in the Western Pacific. Near-trench sea-floor spreading related to this event first produced basaltic then boninitic oceanic crust. Eruption of high-Si boninites followed as the system transitioned from near-trench sea-floor spreading associated with slab rollback to the establishment of the volcanic arc (e.g. Reagan et al., 2017, Internat. Geol. Rev.). This entire sequence was generated over < 1.2 Myr (Reagan et al., 2019, Earth Planet. Sci. Lett.). The scale of this event was large enough to change Pacific Plate motion by trench pull, suggesting that a broad swath of the western Pacific lithosphere was primed to subduct once it was triggered. Similar episodic large-scale subduction initiation events are recorded in accreted terranes stretching back to c. 2 Ga. Terranes older than 2 Ga with boninitic lavas either are related to extensive melting associated with mantle plumes or are related to subduction without the rollback and prior melt depletion required to generate high-Si boninites. Thus, there was a fundamental change in the nature of subduction at 2 Ga (Pearce and Reagan, 2019, Geosphere). Locations presently undergoing subduction initiation (e.g. Puysegur, Foley et al., 2013, J. Petrol.; Matthew and Hunter, Patriat et al., 2019, Earth Planet. Sci. Lett.), are small in scale and lack the organized pattern of basalt to boninite volcanism seen in the IBM forearc. The initiation of subduction in these areas occurred because of local changes in plate motion. We postulate that this form of locally induced subduction initiation may be common, and typically does not lead to ocean-basin-wide plate reorganization.