Fig. 2: Tectonic background and key observations of the Izu–Bonin-Mariana (IBM)-type subduction zone.
From: Horizontally forced initiation of the Izu-Bonin-Mariana subduction zone
a Tectonic background for SZI in the Izu–Bonin-Mariana-type subduction system3,32,34. At ~52 Ma, the Pacific plate is suggested to be separated from the Philippine plate by a weak transform fault8,9,14,62,63, the motion direction of the Pacific plate rapidly changed32,33, and the continental plates with gray colors were roughly drifting towards the IBM, i.e., Australia, India, Arabia, and the terranes in present-day Southeast Asia. The brown region marks one possible reconstruction of the subducted block(s) between India and Eurasia34. b Map of the IBM sample localities in (c) (after Ishizuka et al.30). The subduction evolution is nearly synchronous throughout the >2500 km long trench (see inset)31. c Pb isotopes of the IBM igneous rocks reveal changing subduction inputs with time30,46. △8/4 (Pb) is the displacement from the Northern Hemisphere Reference Line (NHRL) in 208Pb/204Pb–206Pb/204Pb space67. The forearc basalts’ mantle source is similar to the Philippine Sea Mid-Ocean Ridge Basalts (MORB), without subduction inputs. Later Expedition 352 low silica boninites bear clear subduction inputs from slightly altered Pacific Oceanic crust, probably the lower crust29,46. Subsequent Expedition 352, Chichijima and Mukojima high silica boninites contain a substantial contribution from the marine sediments30,46,68. Marker meanings are the same as in (b).
